investigation of dielectrophoresis and its applications in...
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Investigation of Dielectrophoresis and Its
Applications in Micro/nano Fluidics,
Electronics and Fabrications A thesis submitted in fulfilment of the requirements for the
degree of Doctor of Philosophy
Chen Zhang
B.Eng. (Electronics), RMIT University, Melbourne, Australia
School of Electrical and Computer Engineering
RMIT University, Melbourne, Australia
July, 2010
ii
AUTHOR’S DECLARATION
April, 2008.
I certify that except where due acknowledgement has been made, the work is that of the author
alone; the work has not been submitted previously, in whole or in part, to qualify for any other
academic award; the content of the thesis is the result of work which has been carried out since
the official commencement date of the approved research program; and, any editorial work, paid
or unpaid, carried out by a third party is acknowledged.
Chen Zhang
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Acknowledgements
I would like to thank my senior supervisor Associate Professor Kourosh Kalantar-Zadeh for
providing me such an opportunity to conduct my PhD research program; I also appreciate for his
financial and technical supports, creative advices and helpful feedbacks during my study in RMIT
University. There are special thanks to my secondary supervisor, Professor Wojtek Wlodarski, for
his encouragements and positive suggestions which helped and lead me to such completion point.
I wish to thank current and former students within the Sensor Technology Group: Dr. Abu Sadek,
Dr. Samuel Ippolito, Dr. Glenn Matthews, Dr. Rashidah Arsat, Ms Joy Tan, Mrs. Mahnaz Safei,
Mr. Michael Breedon, Mr. Laith Al-Mashat, Mr. Hanif Yaacob, Mr. Rick Zheng and
Mr. Aminuddin Ahmad Kayani for providing a friendly and relaxed atmosphere for conducting
research.
At this moment, I would like to thank several people who had contributed and participated in this
research program. I would appreciate Mr. Khashayar Khoshmanesh of the Department of
Enigneerin, Deakin University, Geelong, Australia for his extensive assistance and calibrating
activities in the design and simulations of microfluidic and dielectrohporetic systems. I would
also like to thank Ms Veronica Strong of the Department of Chemistry and Biochemistry, UCLA,
Los Angeles for her supports on Polythiophene/CdTe quantum dots composites for the
development of field effect devices. There are many thanks for Prof. Hu Zheng of the Key
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Laboratory of Mesoscopic Chemistry of MOE and the School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing, China, for the assistance of his group on providing
nitrogen doped multi-walled carbon nanotubes with different metal catalysts. Additionally, I
would thank Mr. Francisco Tovar-Lopez of School of Electrical and Computer Engineering,
RMIT University, for his helps on the measurements with inverted microscope. There are also
thanks to Mr. Philip Francis of the Department of Applied Physics, RMIT University, for his
helps in obtaining SEM images.
I acknowledge the RMIT University providing me with financial support trough RMIT PhD
scholarship. I also acknowledge the faculty and staff of the School of Electrical and Computer
Engineering at RMIT University for their support on tuitions, causal employment and the
presentation in 2010 Micro and Nano Scaled Heat and Mass Transfer International Conference
(MNHMT), Shanghai, China. I am grateful to the technical staff of the school and MMTC for
their assistance in fabrication technology, design and measurements systems and general
technical advice.
I would like to thank Australian Research Council Nanotechnology Network (ARCNN) for
proving financial support to present my papers at the 2007 Society of Photo-Optical
Instrumentation Engineers (SPIE) conference in Canberra, Australia.
I would also like to take this opportunity to express my thanks to the supports obtained from my
parents, my wife and close friends and for their encouragements and understanding during my
candidature.
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Abstract
In this thesis, the author investigated the phenomenon of dielectrophoresis and its applications in
electronics, microfluidics and optofluidics. Different configurations of microelectrodes, which
were employed for such applications, were designed and fabricated. Their functionalities in
producing dielectrophoretic (DEP) forces were evaluated through the simulation of electric fields.
It is believed that dielectrophoresis is one of the best approaches for depositing nanomaterials for
device applications, since dielectrophoretically assembled materials are highly aligned and
concentrated in desired locations; such characteristics ensure intimate contacts between interfaces
and hence guarantee exceptional device reliability and functionality. To develop nanostructured
electronic devices using dielectrophoresis, finger-paired electrodes were first employed to reveal
the ability of AC electric fields in aligning and assembling nanomaterials. After that, micro-tip
electrode arrays were utilized to develop electronic devices such as conductometric sensors and
field effect transistors using dielectrophoretically deposited nanomaterials.
To realize the ability of DEP forces for manipulating particles in stationary liquid, a DEP-
microfluidic device with micro-tip electrodes was utilized to comprehensively investigate the
DEP behaviors of polystyrene microparticles using different AC electric fields. In a further study,
the separation of carbon nanotubes (CNTs) from polystyrene particles was demonstrated using
the same system. In order to achieve the most efficient DEP manipulation within liquid flows,
curved electrodes were developed. With devices that use such a design, the author successfully
conducted the following tasks in flowing liquid: the separation of polystyrene microparticles from
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CNTs, controllable focusing of polystyrene microparticles, sorting of polystyrene particles
according to their dimensions, sorting of live and dead cells, trapping of polystyrene particles
using pre-patterned CNTs, and the development of optofluidic waveguide using
dielectrophoretically controlled silica particles.
Within this PhD research, the author achieved the following outcomes:
• The DEP separation of nanostructures from microstructures was demonstrated for the
first time. The results suggested that the polystyrene particles and multi-walled carbon
nanotubes can be distinguished using DEP forces at the frequencies higher than 100 kHz.
This task showed the possibility of using dielectrophoresis related techniques in novel
applications, such as the purification of nano-scaled impurities, separation of organelles
from tissues and in-vivo manipulations of nanostructures.
• For the first time, curved electrodes were developed to manipulate particles in flowing
liquid using dielectrophoresis. Such an electrode configuration can produce strong and
homogenously distributed electric field over a large portion of microfluidic channel.
Therefore, the particles do not experience an abrupt increase of DEP forces over the tips,
avoiding spiral or chaotic motions. The functionality of the curved electrodes was
characterized through the DEP concentration and separation of the polystyrene particles
with different dimensions. Additionally, the curved electrodes were also employed as a
cell sorter to distinguish the live and dead cells.
• The DEP trapping of polystyrene microparticles using pre-patterned CNTs was
demonstrated for the first time. In this work, pre-patterned CNTs served as the extensions
of the electrodes and produced sharp electric field to capture polystyrene microparticles.
Using such a technique, the accurate patterning of particles into desired formations is
possible.
• An optofluidics waveguide, which was achieved using dielectrophoretically controlled
silica particles, was demonstrated for the first time. This work revealed new concepts of
optofluidic systems with configurable functional elements that are formed using
suspended particles. It is also suggested that other optical devices such as, tunable
couplers, multiplexers, isolators, lenses, switches, and polarizers can be also developed in
liquid using dielectrophoresis.
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• Gas sensors, which are fabricated using dielectrophoretically assembled and aligned
NCNTs and NCNTs/Pt:Ni nanoparticles, were developed for the first time. The results
suggested that NCNT/Pt:Ni sensors have higher sensitivities in comparison with NCNT
sensors due to the catalytic properties of the metallic nanoparticles. In addition, Pt
nanoparticles exhibited a better catalytic property compared with Pt-Ni alloyed
nanoparticles as the highest sensitivity, fastest response and recovery were all measured
for Pt/NCNT sensors.
• Field effect transistors were developed using dielectrophoretically deposited hybrid
organic-inorganic materials and demonstrated for the first time. The developed
polythiophene/CdTe QDs field effect transistor illustrated exceptional performances: a
current on/off ratio of more than 3×104 and a carrier mobility of approximately
2000 cm2/Vs at the room temperature. This work promised a new route to the
understanding and development of efficient hybrid organic-inorganic devices.
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Contents
Chapter 1 ......................................................................................................................................... 1
Introduction ................................................................................................................................. 1
1.1 Motivation ......................................................................................................................... 1
1.2 Major works included in this research............................................................................... 3
1.3 Author’s achievements ...................................................................................................... 7
1.4 Thesis organization............................................................................................................ 9
Chapter 2 ....................................................................................................................................... 19
Theory and Literature Review................................................................................................... 19
2.1 Introduction ..................................................................................................................... 19
ix
2.2 Dielectrophoresis............................................................................................................. 20
2.3 Dielectrophoretic forces .................................................................................................. 21
2.4 DEP spectrum and crossover frequency.......................................................................... 25
2.5 Conductivity of particles in liquid suspension ................................................................ 27
2.6 Configurations of DEP systems....................................................................................... 30
2.7 Implementation of DEP systems ..................................................................................... 35
2.8 Literature review ............................................................................................................. 40
2.9 Summary ......................................................................................................................... 50
Chapter 3 ....................................................................................................................................... 68
Design and Simulation .............................................................................................................. 68
3.1 Introduction ..................................................................................................................... 68
3.2 Designs of DEP platforms ............................................................................................... 69
3.3 Simulations of designed DEP platforms.......................................................................... 73
3.4 Designs of microfluidic channels .................................................................................... 82
3.5 Summary ......................................................................................................................... 83
Chapter 4 ....................................................................................................................................... 85
Fabrication................................................................................................................................. 85
4.1 Introduction ..................................................................................................................... 85
4.2 Fabrications ..................................................................................................................... 86
4.3 Assembling of the DEP-microfluidic devices ................................................................. 91
4.4 Summary ......................................................................................................................... 92
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Chapter 5 ....................................................................................................................................... 94
Particles Manipulation in DEP-microfluidic Systems............................................................... 94
5.1 Introduction ..................................................................................................................... 94
5.2 DEP manipulation of polystyrene microparticles in stationary liquid environment ....... 96
5.3 DEP separation of polystyrene microparticles from multi-walled carbon nanotubes ... 100
5.4 DEP manipulation of polystyrene microparticles in continuous flowing liquid
environment......................................................................................................................... 104
5.5 The controllable focusing of micro particles................................................................. 111
5.6 Optical waveguide based on suspended sub-micron scaled SiO2 particles ................... 116
5.7 Other collaborative tasks ............................................................................................... 121
5.8 Summary ....................................................................................................................... 135
Chapter 6 ..................................................................................................................................... 140
Development of Nanostructured Electronic Devices .............................................................. 140
6.1 Introduction ................................................................................................................... 140
6.2 DEP assembling of MWCNTs using parallel finger paired electrodes ......................... 141
6.3 Conductometric hydrogen gas sensor based on graphene with Pd nanoparticles.......... 145
6.4 Nitrogen-doped MWCNTs with uniformly doped Pt-Ni nanoparticles for hydrogen gas
sensing ................................................................................................................................. 153
6.5 Field effect transistor based on polythiophene with CdTe quantum dots...................... 162
6.6 Summary ....................................................................................................................... 171
Chapter 7 ..................................................................................................................................... 181
Conclusion and Future Works ................................................................................................. 181
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7.1 Conclusions ................................................................................................................... 182
7.2 Future works.................................................................................................................. 187
Appendix A ................................................................................................................................. 189
List of Author’s Publications................................................................................................... 189
A.1 Journal publications...................................................................................................... 189
A.2 Conference publications ............................................................................................... 190
1
Chapter 1
Introduction
1.1 Motivation
The ability to accurately control and manipulate micro and nano scale particles in liquid is
fundamental for many applications including biological and medical studies, environmental
protections, the development and fabrication of micro/nano electronic and optical devices. Most
of such applications involve processes such as placing and patterning the particles into desired
locations, as well as detecting and sorting them according to their properties. Since the particles
that are required to be manipulated range from organic to inorganic, it is significantly important
to develop compatible systems that can be used for universal tasks; microfluidics enables such
systems.
2
Since Manz et al [1] have introduced the concept “micro total chemical analysis systems” (µTAS)
to address the issue of automation in analytical chemistry in 1990, networks of interconnecting
microchannels have been used for manipulating particles [2]. Because of the significant
advantages offered by µTAS, such as minimized consumption of reagents, versatility in design,
reduced manufacturing costs and the potential to enable parallel operations, further development
of such systems was continuously performed in the past decades and resulted in a new
terminology: microfluidics [2-4].
Microfluidic systems refer to set-ups which are employed for the manipulation of liquids and
gases in channels that have cross-sectional dimensions generally on the order of micrometers [5].
They can be employed for the manipulations of micro/nano scaled particles in both stationary and
flowing liquid environments for different applications. More importantly, it is possible to
incorporate them with other techniques and components to achieve different system
functionalities. In recent decades, optical [6], thermal [7, 8], magnetic [9], and electrical [10, 11]
systems have been reported to be integrated with microfluidics for manipulating particles.
Amongst the electrical systems that can be integrated with microfluidics to manipulate particles,
two methodologies are commonly implemented. The first one is known as electrophoresis, which
utilizes applied electric fields to control the motions of charged particles [2, 12]. The other is
known as dielectrophoresis, which utilizes the interaction between self polarized particles and
non-uniform electric fields to achieve the particles manipulation [12-14]. In this work, the author
has focused on the research of dielectrophoresis associated microfluidic systems.
Dielectrophoresis occurs when a neutral particle is placed in a non-uniform electric field and
experiences a translational force due to the polarization effect induced in the body of the particle
[14]. Such forces are known as the dielectrophoretic (DEP) forces which have properties that
strongly depend on the non-uniformity of both magnitude and phase of the electric fields. DEP
forces can be either positive or negative which depends on the polarizability of the particles and
the suspending medium. Positive DEP forces attract particles into regions with large gradients of
electric fields; negative DEP forces repel them from such regions.
There are many advantages when dielectrophoresis is employed for particle manipulations. First
of all, dielectrophoresis utilizes non-uniform electric field to manipulate self-polarized particles;
therefore, generally no prior treatments are required for particles before applications. Second,
DEP forces are accurately controlled using applied electric fields that can be adjusted by tuning
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the amplitudes, frequencies and phases of the applied electric potentials and enables accurate
controlling of particles. Moreover, the magnitudes and directions of DEP forces, which are
related to the particles’ properties, are suitable to be employed for manipulating particles in
varying applications, such as sorting and trapping [15]. In addition, DEP-microfluidic systems are
low cost experimental platforms that can be conveniently fabricated. Photolithographic
techniques, which have been developed and used by semiconductor industry for many years, can
be employed to fabricate both interdigitated electrodes (to generate DEP forces) and microfluidic
elements [15]. Finally, since dielectrophoresis enables accurate placement of particles, electronic
and optical devices can be developed by placing nanomaterials into desired locations through
such technique.
Dielectrophoresis have been employed for a wide range of lab-on-a-chip applications and the
fabrication of devices. These includes the separation and sorting [11, 12, 14, 16-45], trapping and
assembling [11, 46-74], patterning [75-80], purification [81, 82], and characterization [83-85] of
micro/nano particles as well as the development of nanostructured electronic and optical devices
[62, 86-90].
1.2 Major works included in this research
In this PhD research program, the author has demonstrated the separation of carbon nanotubes
(CNTs) from polystyrene microparticles for the first time using a DEP-microfluidic system [91].
The separation of nanostructures from microstructures is rarely reported, such a technique can be
implemented in novel applications including: purification of particle suspensions with nanoscale
impurities, separation of organelles from cell suspensions, in vivo drug delivery as well as the
detection of single molecules. In this task, the author has designed and fabricated interdigitated
electrodes with sharp tips placed in arrays (micro-tip electrode arrays) which maximize the
difference between strong and weak electric field regions and clearly distinguish between positive
and negative DEP behaviors. To ensure the functionalities of the designed system, simulations of
electric fields were performed; the results indicated positively behaved particles would be
attracted to electrode tips, while negatively behaved particles would be repelled to the regions
between the electrodes arrays. In order to obtain the suitable operational conditions for the
separation experiments, the author has also calculated the magnitudes of DEP forces affecting
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both CNTs and polystyrene microparticles in a wide range of frequencies. To perform such
calculations, the surface conductance (induced when a particle is suspended in liquid medium) of
polystyrene microparticles was also analyzed and estimated.
Another task included in this research program was regarding to the concentrating of polystyrene
microparticles into streams using dielectrophoresis at different flow conditions [92]. Such a
feature can be implemented as building blocks of configurable electronic and optical devices in
liquid since particles can be either focused or scattered with appropriate applied electric fields. To
perform such a task, the author has modified the micro-tip electrodes into curved shape to
improve the focusing of particles. The simulations of electric fields clearly indicated that the
curved electrodes provided a more homogenously distributed electric field and effective area to
accumulate the trapped particles, which is ideal for focusing particles suspended in the flowing
liquid. The author has comprehensively studied DEP focusing of 1 and 3 µm polystyrene particles
under different experimental conditions including electric potentials, frequencies and liquid flow
rates. The results suggested both sizes of particles can be well focused at 100 kHz for AC
potentials higher than 10 V peak to peak, the thicknesses (cross-sectional diameters) of such
focused particle streams could be further tuned by adjustments of liquid flows.
Using the curved electrodes, a novel optofluidic system, which utilized dielectrophoretically
concentrated sub-micron particles to operate as an optical waveguide, was developed [93]. One of
the most significant advantages of such an optofluidic system is its great configurability, since
functional 3D elements can be formed, tuned, and disabled in liquid using the
dielectrophoretically controlled nanoparticles. In this work, silica particles (230 and 450 nm in
diameters) were applied to the system and concentrated into focused streams, their waveguiding
properties were comprehensively investigated. The results indicate that the light transmission was
achieved when 230 nm particles were applied into the system, but was not observed for 450 nm
particles. This is because the inter-particle spacing between the 230 nm particles (tightly in
contact) is slightly smaller than a quarter of the applied laser wavelength (635 nm); conversely,
the concentrated 450 nm particles blocked the light due to scattering. The task was carried out in
collaboration with Mr Aminuddin Kayani, from the School of Electrical and Computer
Engineering, RMIT University, Melbourne, Australia.
Three additional works, which were conducted by the author in collaboration with Mr Khashayar
Khoshmanesh, from the Centre for Intelligent Systems Research, Deakin University, Australia,
are also included in this thesis.
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1. The curved electrodes were evidenced to be capable of concentrating micro/nano scaled
particles in previous works [93, 94]; however, they had not been employed in other
applications. In order to reveal the functionality of the curved electrodes in sorting
particles, they were employed to separate polystyrene microparticles of different
dimensions [95]. In this work, negative DEP forces were utilized to repel the particles
from the centerline of the microchannel; the particles with different dimensions were
hence re-distributed to different locations since they experienced different magnitudes of
DEP forces.
2. In the previous work, curved electrodes were utilized to develop a particle sorter for the
separation of microparticles in different dimensions; however, the performance of the
device was not quantitatively measured. In order to evaluate the sorting efficiency of the
particle sorter, an additional assessing electrode array was added to the system [96]. In
this work, the live and dead cells were distinguished by DEP forces due to the large
differences in their dielectric properties. The sorting efficiency of the system was
evaluated using the assessing electrode array by capturing and counting the escaped live
cells. The cell counting analysis suggested that the efficiency of the sorting system is
approximately 80% when separating live and dead cells.
3. Carbon nanotubes can be dielectrophoretically patterned between microelectrodes [97]; it
is believed that such patterned nanotube networks can operate as the extension of the
electrodes to place microparticles into desired locations. Therefore, nitrogen doped
carbon nanotubes with Pt nanocomposite were dielectrophoretically pre-patterned
between curved electrodes and polystyrene microparticles were applied to characterize
the DEP trapping effect achieved by such pre-patterned nanotubes [98]. The experimental
observation and SEM characterization suggested that the particles were trapped by the
pre-patterned nanotubes networks even at the frequency of 20 MHz. Since polystyrene
particles are generally repelled by DEP forces at such a high frequency, the trapping of
such particles is achieved due to a change in the dielectric properties caused by the
coating of carbon nanotubes at the particle’s surfaces.
In order to facilitate the DEP manipulations of nano scaled particles for device fabrications, the
author has investigated the self-assembly of carbon nanotubes (CNTs) using dielectrophoresis
[97]. In this work, different magnitudes and frequencies of AC potentials were utilized to align
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CNTs between interdigitated electrodes. The developed samples were characterized using
current-voltage measurements and scanning microscopy to evaluate the effectiveness of DEP
forces that produced by different AC potentials.
To evaluate the functionality of the nanostructured devices fabricated using the designed
interdigitated microelectrodes and the catalytic properties of metal nanoparticles on carbon
surfaces, hydrogen gas sensors based on graphene with Pd nanocomposite were developed. In this
work, the author evidenced that the incorporation of Pd nanoparticles onto graphene surface can
improve the sensing performances of the devices due to their catalytic properties which promoted
chemisorption of hydrogen even at room temperature. The work is currently under submission to
the journal “Chemical Physics Letters”.
Additionally, dielectrophoresis was utilized to assemble nitrogen doped carbon nanotubes
(NCNTs) with Pt/Ni nanocomposite onto Si/SiO2 substrates. Such assembled material were
employed as the sensing elements of hydrogen gas conductometric sensors [99]. According to
previous experimental results, the author believes dielectrophoresis is one of the best approaches
to deposit nanomaterial for device applications. Dielectrophoretically assembled nanomaterials
are highly aligned and concentrated in desired locations, ensuring proper contacting between
interfaces and exhibiting exceptional device reliability and functionality. More importantly,
NCNTs and NCNT/metal nanocomposite are new materials, which have never been reported for
gas sensing applications; the analysis of catalytic properties of different metal nanocomposites
could also reveal the methodologies for improving sensitivities of other chemical and gas sensors.
In this task, micro-tip electrodes were adopted for device fabrication as they have been proved to
have the capability for assembling nano scaled materials. The fabricated sensors were tested
towards different concentrations of hydrogen gas; the sensing characteristics were investigated as
a function of operating temperature and Pt/Ni compositions. The testing results suggested the
sensors have an optimum operational temperature that lies between 50 and 70 °C; the highest
sensitivity, fastest response and recovery were all measured for the NCNT/Pt nanocomposite
sensors. Therefore, Pt has a better catalytic property than Ni and the performances can be further
improved by increasing the adhering density of Pt nanoparticles.
Polythiophene/CdTe quantum dots (QDs) composite have also been assembled through
dielectrophoresis to fabricate a field effect transistor (FET) [100]. Such a material is a compound
of conductive polymer and semiconducting nanocomposite, the author believes it is suitable to
operate as the conduction channels of a field effect transistor. In this project, micro-tip electrode
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was again adopted for device fabrication; the performances of the developed device were
evaluated through measurements of current-voltage (I-V) transfer characteristics. The obtained
results suggested the device has an excellent current on/off ratio which is more than 3×104.
Further analysis also revealed that polythiophene/CdTe QDs composite has a exceptional charge
carrier mobility which is as high as 2000 cm2/(Vs) at room temperature.
1.3 Author’s achievements
This PhD research program has successfully produced a number of significant contributions with
regard to the field of manipulation of micro/nano scaled particles using dielectrophoresis in the
applications of microfluidics and device development. The major outcomes from this PhD
research program can be concluded as below:
• To the best of the author’s knowledge, the separation of metallic multi-walled CNTs and
polystyrene micro particles were separated for the first time using dielectrophoresis and
presented in this thesis. The work demonstrated the ability of DEP-microfluidic systems
in discriminate and separate nanostructures from microstructures. This work was
published in journal “Microfluidics and Nanofluidics” [91].
• The controllable focusing of polystyrene microparticles using dielectrophoresis is
presented in this thesis. The work was published in the proceeding of Micro/ Nano scaled
Heat and Mass Transfer International Conference, Shanghai, China, 2009 [94].
• A novel system which utilized dielectrophoretically concentrated sub-micro scaled silica
particles to operate as an optical waveguide is also included in this thesis [101]. This
work was conducted in collaboration with Mr Aminuddin Kayani, from the School of
Electrical and Computer Engineering, RMIT University, Melbourne, Australia.
• Three collaborative works with Mr Khashayar Khoshmanesh, from the Centre for
Intelligent Systems Research, Deakin University, Australia:
o First, the sorting of polystyrene microparticles using dielectrophoresis [95]. In
this work, negative DEP forces were utilized in this work to distinguish the
8
particles in different dimensions.
o Second, the sorting of live and dead cells using a dielectrophoretic-activated cell
sorter [96]. In this work, the live and dead cells were distinguished by
positive/negative DEP forces at the pre-determined optimum operating frequency.
The sorting efficiency of the device was evaluated using an assessing electrode
array.
o Third, Particle trapping using dielectrophoretically pre-patterned carbon
nanotubes [98]. In this work, polystyrene microparticles were successfully
trapped using pre-patterned carbon nanotubes even at the frequency of 20 MHz.
• Hydrogen gas sensors based on graphene with Pd nanoparticles were demonstrated in this
thesis. In this work, the interactions between metallic nanoparticles and sp2 carbon
networks were evidenced by Raman spectroscopy. It is suggested that the attaching of Pd
nanoparticles onto graphene surface can improve the sensing performances of the devices
due to their catalytic properties which promoted chemisorption of hydrogen even at room
temperature. The work is currently under submission to the journal “Chemical Physics
Letters”.
• The assembly of NCNT/Pt:Ni nanocomposite were carried out using dielectrophoresis.
Hydrogen gas sensors, utilizing such assembled materials as sensing elements, were
successfully fabricated for the first time and presented in this thesis. This work was
conducted in collaboration with Dr Abu Z Sadek, from School of Electrical and
Computer Engineering, RMIT University, Melbourne, Australia. The work was published
in “Journal of Physical Chemistry C” [99].
• A field effect transistor was developed using dielectrophoretically assembled
polythiophene/CdTe QDs composites for the first time and presented in this thesis. The
developed device exhibits an excellent current on/off ratio that more than 3×104, the
charge carrier mobility within the material was measured to be approximately 2000
cm2/(Vs) at the room temperature. The work was conducted in collaboration with
Ms Veronica Strong, from the Department of Chemistry and Biochemistry, University of
California Los Angeles, California, US and is currently under submission [100].
9
• The author has also published a critical review entitled “Dielectrophoresis for the
Manipulation of Micro/Nano Particles in Microfluidic Systems” on the journal of
“Analytical and Bioanalytical Chemistry” [92]. In this review, the author has presented a
conclusive summary for the configurations of DEP systems used in microfluidics. The
most impacted examples of DEP-microfluidic systems were presented regarding to the
classification of manipulating organic and inorganic particles.
To summarize, from these achieved works, there have been several key outcomes that have been
published in referred journals and presented at international conferences. These include:
• Two first author and six co-authored publications in referred journals which include
Microfluidics and Nanofluidics, Analytical and Bioanalytical Chemistry, Electrophoresis
and Journal of Physical Chemistry C.
• Three first author publications in the proceedings of international conferences.
The author’s work has been presented both personally and on his behalf at several international
conferences that the author has attended as follow:
• Society of Photo-Optical Instrumentation Engineers (SPIE) Conference, Canberra,
Australia, December, 2007.
• International Conference on Nanoscience and Nanotechnology (ICONN), Melbourne,
Australia, February, 2008.
• Micro and Nano scaled Heat and Mass Transfer (MNHMT) International Conference,
Shanghai, P. R. China, December, 2009.
1.4 Thesis organization
This thesis is primarily dedicated to investigate the manipulations of micro/nano particles using
dielectrophoresis and its applications in microfluidics and device development. The major
sections of this thesis are listed as below:
10
Chapter 2 introduces the theory for dielectrophoresis and includes the literature review on the
applications of dielectrophoresis in both microfluidics and device fabrications. In this chapter, the
terms “classical DEP forces”, “travelling wave DEP forces”, “DEP spectrum” and “crossover
frequency” will be defined and explained in detail. The most impacted examples of applications
for DEP systems will be also illustrated.
Chapter 3 describes the designs and simulations for the DEP-microfluidic systems used in this
research program. In this chapter, three different configurations of interdigitated microelectrodes
are illustrated; their performances in producing DEP forces are revealed by simulations.
Chapter 4 presents the fabrication process for both DEP platforms and microfluidic channels. In
this chapter, the procedures of photolithography and soft lithography are introduced.
Chapter 5 presents DEP manipulation of micro scaled particles in microfluidic systems. Six
related works, which involve the applications of DEP-microfluidic systems, are presented in this
chapter.
Chapter 6 presents the development of nanostructured devices using dielectrophoresis and other
material deposition techniques. In this chapter, the works which involve the fabrication of gas
sensors and field effect transistors using different nanomaterials are presented.
Finally, Chapter 7 comes to the conclusions and suggests possible future works.
11
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19
Chapter 2
Theory and Literature Review
2.1 Introduction
As mentioned previously, the author has published a critical review on dielectrophoresis which is
one of the most comprehensive reviews found in literature. In this review, an overall summary for
the configurations of DEP systems which are used in microfluidics are presented with detailed
explanations on the approaches to implement such systems. In addition, the most significant
contributions in the development of DEP-microfluidic systems were presented based on the
classification that considers the manipulation of organic and inorganic particles.
In this chapter, the author presents the theories of dielectrophoresis and the literature review
based on his published review article. The equations of DEP forces are derived through the vector
calculations of a dipole moment induced in a non-uniform electric field. The key factors of
20
dielectrophoresis: “DEP spectrum” and “crossover frequency” are introduced with example
calculations; the mechanisms for the induction of “particle surface conductance” are also
investigated.
In this literature review, DEP systems with different electrode configurations are presented. The
approaches and strategies to implement such systems for the applications in microfluidics and
device fabrications are explained in detail. The DEP systems which are utilized to manipulate
organic particles (cells, DNA, virus and protein) and inorganic particles (CNTs, metal oxides,
sulphides and nitrides) are reviewed.
2.2 Dielectrophoresis
When a dielectric particle is suspended in an electric field, polarized charges are induced on its
surface, which establishes electric dipoles. The magnitude and direction of this induced dipole
depend on the frequency and magnitude of the applied electric field, morphology of particles, and
the dielectric properties of particle and medium [1]. The interaction between the induced dipole
and the electric field can generate a force affecting the particle, which depends on the non-
uniformity of both magnitude and phase of the electric field.
In a uniform electric field (Figure 2.1-A), Coulomb force are generated on both sides of the
particle which are equal in magnitude and opposite in direction, resulting in zero net force on the
particle. However, when the electric field is non-uniform (Figure 2.1-B), the Coulomb forces on
either side of the particle can be different, and the overall force results in particle motion. Since
the direction of the force is determined by the spatial variation of the field, the particle always
moves toward/against the direction of the electric field maxima. The phenomenon can be
observed under the influence of either alternating current (AC) or direct current (DC) electric
fields. In an AC electric field the direction of the induced force does not change and generally the
time average is used for the calculation of the DEP force. The dielectrophoresis which is
produced by the electric field magnitude gradient is called “classical dielectrophoresis”.
21
Figure 2.1: (A): A particle experiences zero net force while suspended in uniform electric field.
(B): A particle experiences the DEP force due to magnitude gradient of electric field. (C): A
particle experiences the DEP force due to electric field phase gradient.
Travelling electric waves can also be used for the generation of the DEP force. Electric fields
with phase gradients can be realized using arrays of electrodes with applied phase shifted AC
signals (Figure 2.1-C). The dielectrophoresis resulting from such an electric field phase gradient
is called “travelling wave dielectrophoresis”. As there is no phase gradient existing in DC electric
fields, travelling wave dielectrophoresis can only be produced by AC electric fields.
2.3 Dielectrophoretic forces
When a dipole is induced within a particle by a locally non-uniform electric field, the net DEP
force on the particle is given by the expression [2, 3]:
)())(()(DEP tEtmtF ∇⋅= , (2.1)
where m(t) and E(t) are the time dependant dipole moment and electric field, respectively.
In an AC electric field, E(t) is a harmonic function of time described by [2, 3]:
zzyyxx atEatEatEtE )()()()( ++= , (2.2)
where, ax, ay and az are unit vectors, Ex, Ey, Ez are the electric field components along x, y and z
directions. Along the x-axis:
22
)],,(cos[),,( )( 0 zyxtzyxEtE xxx ϕω += , (2.3)
where, ω is the angular frequency of the electric field, Exo and φx are the magnitude and phase
components of the electric field along x direction. The field components Ey(t) and Ez(t) have a
similar form to Ex(t) with subscript x being replaced by y and z, respectively.
The dipole moment m(t) can be represented as [2, 3]:
zyx )()()()( atmatmatmtm zyx ++= , (2.4)
where, mx(t), my(t) and mz(t) are the components of the dipole moment along x, y and z directions,
respectively. Along the x-axis:
,)](sin)(Im
)(cos)(Re[2)(
0xxCM
xCMmx
Etωf -
tωfεΓtm
ϕ
ϕ
+⋅
+⋅⋅= (2.5)
where, Γ is the geometric factor of the particles, εm is the dielectric constant of suspending
medium, and fCM is the Clausius-Mossotti (CM) factor. The terms Re(fCM) and Im(fCM) refer to the
real and imaginary parts of fCM, respectively, which are described later in this section. The dipole
components my(t) and mz(t) have a similar form to mx(t) with subscript x being replaced by y and z
respectively.
The time-dependent DEP force calculated according to Equation (2.1) can be resolved into its
three orthogonal components [2, 3]:
zzyyxx atFatFatFtF )()()()( ++= , (2.6)
where the Fx(t) component can be presented as:
z
tEtm
y
tEtm
x
tEtmtF x
z
x
y
x
xx∂
∂+
∂
∂+
∂
∂=
)()(
)()(
)()()( . (2.7)
Again, the force components Fy(t) and Fz(t) have a similar form with appropriate subscripts.
Averaging Equation (2.7) over a time which is much longer than the electric field period, xF can
be obtained [2, 3]:
23
].)()Im(
2
)()[Re(
20
20
20
20
20
20
x
zE
yE
xEf
x
EEEfεΓF
z
z
y
y
x
xCM
zyx
CMm
∂
∂+
∂
∂+
∂
∂+
∂
++∂⋅=
ϕϕϕ (2.8)
Following the same procedure, similar expressions for the force components along y and z
directions can be obtained. The complete time-averaged DEP force can be obtained by summing
all three force components [2, 3]:
].)()Im(
)()Re(21
[
20
20
20
20
20
20
zzyyxxCM
zyxCMm
EEEf
EEEfεΓF
ϕϕϕ ∇+∇+∇+
++∇⋅= (2.9)
As can be seen, the DEP force has two major terms: the first term represents the “classical DEP
force” which can be further simplified into [2-12]：
2DEP-C )Re(
2
1EfεΓF CMm ∇⋅⋅= ; (2.10)
the second term represents the “travelling wave DEP force”. When the travelling electric field has
a wavelength of λ, the phase gradient of the electric field is -2π/λ, and the travelling wave DEP
force can be written as [2, 3]:
2DEP-TW ][Im
2EfεΓ
λ
π-F CMm⋅= .
(2.11)
The behavior of dielectrophoresis can be described by the direction of the DEP force. In a
dielectric medium, the direction of the DEP force is influenced by the polarizability of the particle,
which depends on the permittivities of the particle and the suspending medium. This behaviour is
expressed by the fCM: when the fCM is positive, a positive DEP force is generated. On other hand,
when the fCM is negative, a negative DEP force is induced. As fCM is a complex function of
permittivities, conductivities and frequency of the applied field, a particle can experience both
positive and negative DEP forces at different combinations of the above variables [13].
24
In classical dielectrophoresis, the positive DEP force attracts particles into the regions of strong
electric fields while negative DEP force repels them from those regions. In travelling wave
dielectrophoresis, the positive DEP force pushes the particles along the direction of the travelling
wave while the negative DEP force pushes them in the opposite direction [13, 14]. The fCM
depends on the dielectric properties of particles and medium as well as the geometry of particles.
It is expressed differently for cylindrical [12, 15-19] and spherical [20] structures:
*
m
*
m
*
p
CM-ε
εεf
−=lcylindrica ,
*
m
*
p
*
m
*
p
CMεε
εεf
2spherical-
+
−= ,
(2.12)
(2.13)
where, ε*m and ε*p are the complex permittivities of suspending medium and particles,
respectively, and these complex permittivities are defined as:
. ω
σiεε* −=
(2.14)
where, σ is the electric conductivity and i=√-1.
Similar to the fCM, the geometry factor Γ also depends on the morphologies of the particles that is
expressed distinctly for spherical [20-22] and cylindrical structures [15-19] as:
3spherical 2π rΓ = ,
2
2
lcylindrica
lπrΓ = ,
(2.15)
(2.16)
where, r indicates the radius of the sphere or the cross section of the cylinder and l represents the
length of the cylindrical structure.
Moreover, for DC dielectrophoresis, as the frequency of the applied electric field is zero, the
equation for Re(fCM) can be further simplified into [12]:
25
. m
mp
CM σ
σσf
−=)Re( ,
(2.17)
and as a result, DC DEP force can be calculated by using Equations (2.10) and (2.17).
2.4 DEP spectrum and crossover frequency
To explore the DEP force dependence on fCM at various electric field frequencies, the terms “DEP
spectrum” and “crossover frequency” are used. The DEP spectrum describes the variations of the
fCM at different frequencies while the crossover frequency refers to the frequency at which the fCM
becomes zero.
In this section, a series of calculations were conducted using Equations (2.12) and (2.14). The
calculations are purposed to illustrate the influence of change in the conductivities of the particles
and the medium on both the DEP spectrum and crossover frequency.
Assuming there are two types of particles and media for comparison purposes: particles type A
with a conductivity of σp = 1 mS/m (typical value for polymer particles when suspended in water)
and particles type B with a ten-time lower conductivity of σp = 0.1 mS/m; medium type A with a
conductivity of σm = 1 mS/m (close to the conductivity of DI water) and medium type B with a
hundred-time lower conductivity of σm = 0.01 mS/m. The relative permittivities of particles were
assumed to be 2 (close to a typical polymer) and the permittivity of media was assumed to be 100
(close to DI water), respectively.
Two sets of calculations were performed. Case one: for particles type A and B which are
suspended in media type B. Case two: for particles type A which are suspended in media type A
and B. The values of the fCM (real and imaginary parts) were calculated for frequencies in a range
from 1 kHz to 100 MHz as shown in Figure 2.2 (the values of conductivities were utilized to
name the curves in figures in order to provide better comparison).
26
Figure 2.2: (A): CM factor vs frequency for specific particle conductivities. (B): CM factor vs
frequency for specific medium conductivities [23].
In Figure 2.2-A (case one), we can clearly observe the shift of the DEP spectrum when the
conductivity of particles is changing. When the conductivity increases from 0.1 mS/m to 1 mS/m,
27
the crossover frequency of the system shifts to larger frequencies. Therefore, the crossover
frequency can be tuned by adjusting the particles conductivity. This can be achieved by a variety
of techniques such as surface coating and doping.
In Figure 2.2-B (case two), significant change in the magnitude of the fCM can be observed when
the conductivity of the medium is changing. In contrast, the shift of the spectrum is much smaller
comparing with Figure 2.2-A. When the medium conductivity is 0.01 mS/m, the particle
experiences positive DEP force before 100 kHz and negative DEP force after 100 kHz; when it is
increased to 1 mS/m, the particle experiences negative DEP force for the whole calculated range
of frequencies. As a result, adjustment of the conductivity of liquid media can reverse the
direction of the DEP force in certain frequency ranges.
2.5 Conductivity of particles in liquid suspension
The conductivity of particles can be expressed as a sum of two terms:
SurfaceBulk −− += ppp σσσ , (2.18)
where σp−Bulk is the bulk conductivity (or core conductivity) of the particles and σp−Surface is the
surface conductivity induced due to the charges in the diffused double layer and in the Stern layer
[24], when particles are suspended in liquid medium. These induced shell-liked layers have
different dielectric properties from the particles [25], and can influence the DEP behaviour [24].
For particles made of conductive materials, the σp−Bulk is large, and σp−Surface can be negligible.
However, for particles made of non-conductive materials, the surface conductivity dominates
over bulk conductivity. In this case, the effective conductivity σp can be expressed as [26, 27]:
r
Kσσ S
pp
2Surface == − ,
(2.19)
where Ks represents the surface conductance and r is the radius of the particles. Such an
expression of surface conductivity can be employed to model the properties of micorparticles
larger than 1 µm [28].
28
However, in a more accurate approach the term KS is expressed as
diffstern KKK S += , (2.20)
where Kstern and Kdiff are the Stern layer conductance and the diffuse layer conductance,
respectively [28].
The Stern layer conductance is given by
sternsternq,stern µρ=K , (2.21)
where ρq,stern and µ stern are the equivalent surface charge density and ion mobility in the Stern layer
[28]. The diffuse layer conductance is given by
)1]2
(cosh[)/31(4 222
diff −+
=kT
zq
RT
zmDczFK
ζ
κ,
(2.22)
where ζ is the zeta potential (the potential at the surface of shear), D is the diffusion coefficient of
the excess free charge in the diffuse layer, z is the valence of the counterion, F is the Faraday
constant, R is the gas constant, c is the electrolyte concentration, k is the Boltzmann’s constant, q
is the charge on the electron and T is the temperature [28, 29].
In Equation (2.22), the inverse Debye length κ and the dimensionless parameter m (describes the
contribution of the electroosmotic ion flux to the diffuse layer surface conductance) are given by
)/()2( 2RTczF mεκ = ,
DF
RTm m
η
ε
3
2)( 2= ,
(2.23)
(2.24)
where η is the viscosity of suspending medium [28, 29].
Based on the results reported by Cui et al [26], the surface conductance Ks of a latex bead has a
proportional relationship with its diameter, as illustrated in Figure 2.3. In the work, latex beads
with diameters from 2 to 10 µm were characterized in a DEP system to measure their crossover
frequencies, and the values of Ks were calculated thorough the equation:
29
)])2)(2)([(9(4
20
2f
rK mpmpmmS πεεεεσσ +−−+−= ,
(2.25)
where, f0 refers to the measured crossover frequencies. Since particles which have diameters
smaller than 2 µm were not adopted in the experiments in the reported work, the value of
conductivity of 1 µm particles used for the calculations of fCM in Chapter 6 was obtained through
the extrapolation.
Figure 2.3: Surface conductances for carboxylate latex beads of different dimensions. [26]
Since the crossover frequency can be shifted by adjusting particles conductivity, small changes in
surface conductance can give rise to significant changes in the DEP spectrum [24]. For many
chemical and biochemical applications, the change of surface conductivity is the easiest way to
alter the influence of the DEP forces on particles.
According to the work reported by Ermolina & Morgan [28], Equation (2.19) can be utilized to
calculate surface conductivity for particles larger than 1 µm. In this PhD research, all the nano
scaled structures (i.e. carbon nanotubes) are conductive, and the calculation of surface
conductance of such particles is not required. Also, the smallest nonconductive particles used in
this PhD research are 1 µm; therefore, the used of Equation (2.19) in predicting the surface
conductance of non conductive particles is appropriate throughout this thesis.
30
2.6 Configurations of DEP systems
DEP manipulation can be achieved using both positive and negative DEP forces. Such forces can
attract or repel particles from regions with large electric field gradients (magnitude and phase).
Since the distribution of the electric fields strongly depends on the configurations of the
interdigitated electrodes, the optimization of electrode configurations is the first priority in the
system design procedures for the purpose of achieving the most efficient manipulation systems.
In this section, a review on the commonly implemented electrode configurations for DEP systems
is conducted to aid the designs of appropriate DEP-microflidic systems.
2.6.1 Configurations of classical DEP systems
Many different configurations can be employed for the design of electrodes in classical DEP
systems. One of the most popular designs is the “parallel finger paired electrodes” as illustrated in
Figure 2.4-A. The electrodes generate the greatest electric field gradient directly above their
surface, and hence can effectively trap or repel the particles nearby. This design offers a large
trapping area in microfluidic systems by adjusting the ratio of the fingers length to the
micro-channel width. This design is widely implemented for the fabrication of devices such as
sensors and field effect transistors [30-35], and for particles trapping [36, 37].
Another popular design is the “micro tip electrodes” as shown in Figure 2.4-B. The electrodes
generate the maximum electric field gradient at the tips, and hence generate a large trapping DEP
force. As the triangular micro tips produce a relatively low effective trapping area, multiple pairs
of electrode tips can be patterned to enhance the trapping efficiency, as shown in Figure 2.4-C.
This design is efficiently employed for capturing single nano particles [38], and fabricating field
effect transistors [39-42] and sensors [43-45].
31
Figure 2.4: Electrode design and electric field distribution for different configurations: (A)
Finger paired electrodes, (B) Single paired micro tips electrode, (C) Cross-electrode paired micro
32
tips, and (D) Polynomial electrodes. The polarity of electrode is presented by “+” and “-” signs.
[23]
“Polynomial electrode” design is mostly considered as a transformed version of the two-paired
micro tip electrodes design, as illustrated in Figure 2.4-D. However, polynomial electrodes
operate in an opposite way of two-paired micro tip electrodes. The polynomial electrodes produce
the strongest electric field gradient along the pads, and the weakest at the centre of the pattern.
Therefore, polynomial electrode configurations provides multiple functionalities in particle
separation applications [46].
Figure 2.5: (A): Micro tip electrode array, (B and C): Castellated electrode arrays with normal
and shifted configurations [23].
Arrays of micro tip electrodes provides a satisfactory approach for improving the effective
manipulation area as shown in Figure 2.5-A. These arrays can be further transformed into
“castellated electrode” configurations, as illustrated in Figure 2.5-B and Figure 2.5-C. Such
configurations can complete the tasks such as trapping [47] and separation [48] of particles within
microfluidic channels, as well as for the fabrication of micro-electronic devices [47, 49, 50].
“Post arrayed electrodes” (or elevated electrodes) as shown in Figure 2.6 enable 3D
manipulations of particles. A particle can be trapped in virtual cages generated by the DEP forces
from post array electrodes. However, the fabrication procedure is more complicated. A typical
design of such a system was demonstrated by Voldman et al [51] (Figure 2.6).
33
Classical DEP systems can also be configured to operate without electrodes. In such a system,
insulator posts are employed to locally change the distribution of the electric field and hence
generate the field gradient. As there is no applied voltage to the posts, the electrolysis of water
and the exposure of bio-particles to the electrodes are avoided. The design is mechanically robust
and less expensive as less fabrication stages are involved. A typical design of electrodeless DEP
system was demonstrated by Lapizco-Encinas et al [52] as seen in Figure 2.7.
Figure 2.6: The post arrayed electrode configuration developed by Voldman et al. [51], the circles
are represented as post cylindrical shaped electrodes, the polarity of electrode is presented by “+”
and “-” signs.
Figure 2.7: The setup configuration of an electrodeless DEP system which was developed by
Lapizco-Encinas et al [52]. The electric field is applied externally (at the two electrodes at the
inlet and the outlet), and insulating posts in the channel were used for producing field gradient.
34
2.6.2 Configurations of travelling wave DEP systems
The travelling wave DEP systems utilise the phase shift of electric fields to induce particle
motions. Since the travelling wave DEP force only exists when the electric field has phase
gradient, the system design for travelling wave dielectrophoresis generally consists of multiple
electrodes with applied phase shifted AC signals as shown in Figure 2.8-A. Such a configuration
is typically utilized for the transporting particles within a microfludic channel or chamber without
the assistance of any flow. It can also employed for the manipulation of particles: particles
experiencing negative and positive DEP forces can be separated. Particles can also be transported
along or against the direction of the travelling wave at different velocities.
Figure 2.8: (A): Configuration of traveling wave DEP system with 120° phase shifted applied
signals. (B and C): The electrode array designed for a three-phase travelling wave DEP system to
produce electric field phase gradient [23].
35
One of the practical configurations for travelling wave DEP systems is an array of spiral electrode
elements. Figure 2.8-B and Figure 2.8-C show two similar designs of three-phase-electrode array
utilized to produce electric field phase gradient. The looping electrodes are connected to three
alternating signals which are of the same magnitude and frequency but with a 120° phase shift.
As the electrodes loop around one another, a continuous array of electrodes phased 120° apart is
formed to generate a travelling field moving towards the centre of the pattern [13]. Particles then
move towards to or away from the centre region, according to their polarizabilities.
2.7 Implementation of DEP systems
2.7.1 Implementation in microfluidic system.
Particle separation and sorting
Since DEP forces can be either positive or negative, particles with different dielectric properties
can be attracted and accumulated into different locations according to the strength and gradient of
applied electric field. This unique property for DEP forces facilitated accurate sorting and
separation for different types of particles. For instance, metallic and semi-conductive CNTs can
be sorted using dielectrophoresis [53]. Because of the immense difference in conductivities,
metallic CNTs were trapped at the edge of electrodes (region with high gradient of electric field)
while semi-conductive CNTs were repelled from those regions. Additionally, the magnitude of
DEP forces is strongly dependent on the dimension and geometry of manipulated particles. DEP
forces can also be employed in the sorting of particles which have similar dielectric properties but
with different shapes and sizes. As a result, cells of different dimensions can be effectively sorted
and recollected using dielectrophoresis within a microfluidic system [21].
There are many strategies for sorting particles in microfluidic systems. First of all, the DEP forces
can be utilized to trap and release the target particles in microfluidic systems. As shown in Figure
2.9, in which the target particles are trapped using positive dielectrophoresis while the other
particles are flushed out of the channel and collected in a reservoir. The trapped particles are later
released by switching off the applied signal and collected by providing the liquid flow in the
reverse direction [54].
36
Figure 2.9: DEP trapping and releasing for particle separation in microfluidic system [23].
In another strategy, a combination of the DEP, sedimentation, and hydrodynamic drag forces is
utilized for the separation of target particles, as shown in Figure 2.10. The negative DEP force
repels the particles from the electrodes and levitates them within the channel. Different particles
experience various magnitudes of DEP force and levitate at different heights. The velocity profile
inside the channel follows a parabolic distribution with a maximum at the central of the channel.
Hence, different particles travel at different velocities and can be fractionated along the channel.
The technique is known as field flow fractionation (FFF) [55].
Figure 2.10: Field flow fraction (FFF) method for particle sorting [23].
37
DEP barriers can also be used for sorting particles as illustrated in Figure 2.11 [56]. Particles are
exposed to a hydrodynamic drag force that guides them along the channel and a negative DEP
force that repeals them from the electrodes. Particle type 1 experiences a weak negative DEP
force that is not enough to overcome the drag force and remains in the flow. Conversely, particle
type 2 experiences a strong negative DEP force that deflects it and makes it follow the zig-zag
pattern of the electrodes.
Figure 2.11: Particles are separated by DEP barriers which are generated by appropriate designed
electrodes [23].
Particle assembling and patterning
DEP particles manipulation can be used for placing particles in desired locations with desired
formations. One approach is to utilize negative dielectrophoresis to achieve particle patterning, as
shown in shown in Figure 2.12. In such systems, an electrode array is placed on the top surface of
a microfluidic channel. Particles settle to the bottom surface of the channel under the influence of
gravitational force, and then are attracted to the regions of weak electric field gradient by the
negative DEP force. In this case, different particle formations are achieved by adjusting the
geometry of electrode patterns and arrays. Micro and nano particles can be formed into chains,
bridges and other formations [57, 58] by using DEP patterning and positioning techniques.
Depending on the electrode geometry, different particle formations can be achieved for various
applications.
38
Figure 2.12: Patterning of particle achieved using negative dielectrophoresis [23].
Transportation of particles
In microfluidic systems, the transportation of a particle is normally achieved by hydrodynamic
(mechanical) forces. Travelling wave DEP forces can be employed to transport particles, as they
attract the particles either towards or against the direction of the phase gradient applied to the
electrodes, as illustrated in Figure 2.13. When a microchannel is integrated with such a system, it
can be used for transporting particles within the micro channel [59].
Figure 2.13: Travelling wave DEP force for transporting particles along the direction of the
travelling waves in a three phase arrayed system [23].
Characterization of particles
DEP characterization can be used for the investigation of dielectric properties of biological and
non-biological materials. In such applications the dielectric properties of the particles can be
explored by adopting different approaches such as optical absorption, crossover frequency, and
electro-rotation measurements.
The DEP trapping rate of particles at different frequencies can be obtained through the
measurement of optical absorption. In such systems, the electric field is applied to accumulate the
39
particles in a desired location, then the localized change in the optical absorption of the
suspension is measured to characterize the DEP spectrum of the particles [60].
The measurement of crossover frequency can be used for obtaining the surface conductance of
non-conductive particles when suspended in liquid media and the bulk conductance of
semiconducting particles [26].
Electro-rotation is another method for exploring the dielectric properties of particles, and
travelling wave DEP forces are utilized in such systems. Depending on the dielectric properties,
suspending medium and the frequency of the applied electric field, the particles can rotate in
either the same direction or in the opposite direction of the rotating electric field, as shown in
Figure 2.14. The rotation rate can be measured and used for obtaining the DEP spectrum [61].
Figure 2.14: A particle suspended between four electrodes which is subjected to a rotating electric
field created using four sinusoidal electrical signals with a 90° phase difference [23].
2.7.2 Implementation in device development
Dielectrophoretically assembled nanostructures with proper designed micro electrodes can be
employed to develop electronic devices such as, chemical and photo sensors [31, 34, 50, 62] and
field effect transistors [39]; they may also be used as periodical structures of optical devices.
As shown in Figure 2.15, nanowires can be assembled using two electrodes providing electric
potentials. Since the magnitude gradient of applied electric field is non-uniform above the
electrodes, positive DEP forces can be produced for manipulating the nanostructures that are
40
suspended above the substrate. Because the field magnitude gradient is at its maximum on the
surface of electrodes, nanostructures are trapped by the electrodes and bridging in between them
to form conduction. If the materials of such nanowires are capable for electronic and optical
applications, the system can be used as devices for electronic and optical purposes.
Figure 2.15: DEP assembling of nanostructures for device development.
2.8 Literature review
The considerations for the deployment of dielectrophoresis strongly depend on the properties of
particles, and an important distinction can be made between organic and inorganic particles. In
this section, the reported approaches for the DEP manipulations of such particles are reviewed.
2.8.1 DEP manipulation of organic particles
Organic particles such as cells, DNA fragments, proteins and viruses are of great interests for
researchers in biological, medical and biochemical studies. The application of DEP techniques
enables efficient and accurate manipulations of such particles within microfluidic systems.
2.8.1.1 Cell and its organelles
Cell is the smallest structural and functional unit of all known living organisms [63]. Cell
manipulations are extensively used in biology and biochemical applications for the transportation,
41
sorting, pattering and positioning of cells [64]. Additionally, cell manipulations are extremely
important in the study of blood deceases since the blood cells can be manipulated within
microfluidic systems to simulate their real behaviour in particular occasions such as the blood
aggregation [65].
The DEP techniques offer unique features for medical applications. For example, cancer and
healthy cells can be separated using a DEP system [66], facilitating a label-free and non-invasive
diagnosis of cancer. Moreover, the DEP techniques are used in the characterization of cell
organelles such as mitochondria and cytoplasm [67].
The DEP separation of live and dead cells was firstly demonstrated by Pohl et al [68] in 1966
within stationary systems, and it was in 1970s that the DEP techniques were applied within
continuous flow systems. Since then, a lot of research has been conducted to integrate the DEP
systems with the microfluidic systems to achieve higher functionality, efficiency and accuracy.
Dielectrophoresis has been employed for the separation of blood cells from different particles
such as polystyrene beads [10, 48, 69], Bacillus cereus (B. cereus) [54], Escherichia coli (E. coli)
[54, 69]，Listeria monocytogenes (L. monocytogenes) [54] and blood platelets [55]. In the blood
sciences, dielectrophoresis has also been utilized for the characterization of cells to determine the
blood type [70].
The use of dielectrophoresis to sort cells has also been reported [9, 21, 69, 71-84]. The DC
electrodeless DEP separation of cells with different dimensions was demonstrated by Kang et al
[21]. In this work, a DC non-uniform electric field was generated within a polydimethylsiloxane
(PDMS) microchannel. The cells experience negative DEP forces with different magnitudes
according to their size, and hence were continuously separated as shown in Figure 2.17. The
castellated microelectrodes array was employed in the separation of the live and dead cells by
Pethig et al. [85, 86]. They have also successfully demonstrated the sorting of cells using a
traveling wave dielectrophoresis based system (Figure 2.16) [87]. Additionally, a device which
enabled electrodeless dielectrophoresis was also developed by Demierre et al [80] and a cell
sorting system based on the use of DEP barriers was demonstrated by Kim et al. [88].
42
Figure 2.16: A traveling-wave DEP junction, fabricated by laser ablation, for the separation of
cells [27, 87].
Figure 2.17: DC electrodelss DEP separation of cells according to their dimensions, the non-
uniform electric field is generated by the triangular hurdle in micro-channel [21].
DEP techniques are also widely applied for the trapping [7, 11, 89-95], concentrating [8, 81-83,
96-98] and patterning [99, 100] of cells within microfluidic systems. DEP trapping can also be
used for developing operational devices in biological studies. Hunt et al [92] has developed DEP
tweezers for tweezing single cells (saccharomyces cerevisiae). The tweezers consist of a sharp
glass tip with electrodes on either side. These tweezers were capable of trapping a single cell as
illustrated in Figure 2.18. In addition, Jang et al. [101] have developed a single-cell trapping
device using the quadrupole polynomial electrode configuration.
43
Figure 2.18: A cell is trapped and held by the DEP tweezers [92].
The transportation of cells can also be achieved through the use of travelling wave
dielectrophoresis. Wang et al [102] has demonstrated the manipulation of cells with spiral
electrodes using such a DEP system. In this work, a four-phase electrode array was developed,
and electric fields with 90º phase shift were applied to the electrode arrays to produce phased
field gradient. A DEP system based on 3D travelling wave dielectrophoresis was developed by
Huang et al [59] for catching, transporting and separating cells from other particles. Additionally,
Goater et al. [103] developed a device to concentrate and assay the viability of microorganisms
using traveling wave dielectrophoresis.
2.8.1.2 DNA and RNA
There are two types of nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA)
which are different from each other in chemical composition and structure. In cells, DNA serves
as the storehouse for genetic information, and this genetic information usually proceeds from
DNA through RNA to proteins [104].
As the most popular researched bio-material in the past few decades, the study and manipulation
of DNA segments have drawn considerable attention from researchers. Having unique structures,
DNA has a number of applications in the development of bio-devices. For instance, DNA
fragments are used for decoding gene expressions with micro-arrays and can be used as selective
layers in protein detection [105]. By using DEP methods, DNA can be sorted, separated and
trapped for different applications including the collection, sorting and deposition of DNA
segments according to their size.
44
Figure 2.19: Fluorescence image of DNA molecules attached on selected electrodes using the
DEP trapping [47].
In 1995, Washizu et al [106] achieved DNA positioning by using a DEP trapping technique.
Since then, as the most efficient DNA collection methodology, DEP trapping and positioning has
been extensively studied [37, 47, 107-119]. In 2002, the DEP trapping of DNA was successfully
demonstrated by Chou et al [115] using an electrodeless system. As no metallic electrodes were
used in their system, water electrolysis was avoided at low frequencies, and the DNA structures
were not damaged during the trapping process. In 2003, Germishuizen et al [47] demonstrated a
surface immobilization procedure to enable the controlled attachment of very long DNA
molecules onto gold electrodes using dielectrophorsis, as illustrated in Figure 2.19. In 2006,
Tuukkanen et al [109] demonstrated DNA trapping using carbon nanotubes as the electrodes. In
such a system, relatively low applied voltages can generate high field gradients due to field
enhancement at the very fine tips of the carbon nanotubes.
2.8.1.3 Protein
Proteins are made of amino acids which are joined by peptide bonds, they carry out processes
within cells and are essential for any living organism [120]. Proteins can be used in applications
such as the development of sensitive materials, pores with triggers, switches, self-assembled
arrays and motors [105].
In many medical and biochemical applications such as the development of the vaccines, proteins
must be purified away from other cellular components. With the aid of the DEP manipulation
techniques, proteins can be effectively manipulated.
45
Figure 2.20: The SEM image of trapped protein structures, which are attracted to the high electric
field regions by positive DEP force [46].
Washizu et al [121] were one of the first to demonstrate the DEP manipulation of protein
nanoparticles within microfluidic systems. In their work, avidin nanoparticles were separated
from mixed suspension. In 2003, Asokan et al [46] demonstrated two dimensional manipulation
of protein using a DEP trapping technique; quadrupole electrodes were employed to control the
separation of protein structures from polystyrene beads. Figure 2.20 illustrates the
dielectrophoretically trapped protein structures. The trapping of single protein molecules of R-
phycoerythrin has been reported by Holzel et al [38] in 2005, and DEP trapping of protein on the
tip of a carbon nanotube was reported by Maruyama et al [122] in 2008. In the same year, DEP
manipulation of protein particles using an electrodeless system was also successfully
demonstrated by Lapizco-Encinas et al [52].
2.8.1.4 Viruses
Viruses are simple, acelluar entities consisting of one or more molecules of either DNA or RNA
enclosed in a coating of protein. They can reproduce only within living cells and are generally
intracellular parasites [104]. Viruses are the focus of important research in biological,
biochemical and medical studies and applications, as they can be used in developing vaccines and
medicines to prevent and cure diseases.
In 1997, Green et al developed a DEP system to manipulate latex beads and the tobacco mosaic
virus (TMV), by which they characterised the properties of the virus [123]. Morgan et al [124]
46
achieved controlled separation of TMV and herpes simplex virus in 1999. Hughes et al
investigated the manipulation of herpes simplex virus type-1 [125, 126] and characterised its
dielectric properties [126, 127]. In 2006, Grom et al [128] successfully demonstrated the
accumulation and trapping of the hepatitis A virus, and in the same year, Ermolina et al [129]
investigated the dielectric properties of cow pea mosaic and TMV by measuring their crossover
frequencies.
2.8.1.5 Polymeric particles
Polymers are large molecules composed of repeating structural organic units [130]. Polymers can
be categorized into conductive and non-conductive groups according to their electrical properties.
Conductive polymers such as polypyrrole and polyaniline can be utilised in the development of
sensors [130, 131]. A nanowire gas sensor based on conductive polymer (poly (3,4-
ethylenedioxythiophene)/poly(styrenesulfonate)) has been developed by Dan et al [132] using the
DEP assembling technique. Non-conductive polymers such as polystyrene particles are widely
used for the calibration and testing of DEP and microfluidic systems [133-139]. The DEP
manipulations of polystyrene particles using castellated [140], polynomial [141] and angled [142-
144] electrode configurations have been extensively reported. Polystyrene particles were also
employed as simulants in the DEP separation of cells [48, 59], protein [46] and carbon nanotubes
[145].
2.8.2 DEP manipulation of inorganic particles
Semiconducting materials are fundamental elements of smart and functional devices and systems
[146]. Nanostructured semiconducting materials, such as silicon, metal oxide, nitride and
sulphide nanoparticles as well as semiconducting carbon nanotubes have been widely used in the
development of micro and nano electronic and optical devices. Dielectrophoresis can be
effectively used for placing nanoparticles in desired configurations for the fabrication of such
devices.
Carbon nanotubes (CNTs) are long, slender fullerenes with hexagonal carbon walls (graphite
structure) [147]. CNTs can be classified into single-walled or multi-walled groups according to
their structure, or into metallic or semiconducting groups according to their electronic properties
47
[147]. CNTs have unique thermal, mechanical and electrical properties. For example their
thermal conductivity is higher than diamond and their stiffness, strength and resilience exceeds
any known material [147]. Due to these unique characteristics, CNTs have been widely applied as
key elements of micro/nano electronic devices such as sensors (force, thermal and chemical),
diodes and field effect transistors. CNTs are also used in medical and biological applications. For
example, CNTs have been employed as carriers for drug transportations into target cells [148,
149].
In order to deploy CNTs onto electronic platforms for the fabrication of devices, or to separate
them from mixed suspensions, DEP techniques can be used. In 2003, the separation of metallic
and semiconducting CNTs was successfully demonstrated by Krupke et al [53], and in the same
year, Suehiro et al [62] demonstrated the fabrication of a CNT based field effect transistor by
using dielectrophoresis. Since these initial demonstrations, DEP trapping and positioning of
CNTs has been extensively reported [49, 150-182], and Makaram et al [170] demonstrated 3D
assembling of single-walled CNTs (SWNTs) as shown in Figure 2.21. Jung et al [153] fabricated
CNT enhanced electric field emitters by attaching a single-walled CNT onto an atomic force
microscope (AFM) tip, and the CNT AFM tips were also developed for ultra high resolution
imaging [163, 178-181]. The DEP trapping technique has been utilized to create field effect
transistors [161, 165, 174, 183] and sensors [31, 34, 42, 44, 62, 155, 157, 160, 169, 184]. Since
different types of CNTs can be used in different applications, the sorting of CNTs according to
their size and electric properties has become an active field of research [53, 185, 186].
Figure 2.21: (A) 3D assembly of SWNTs on a multiple electrodes’ structure. (B) A close-up SEM
image of one of the fingers. [170]
48
Metal oxides, sulphides and nitrides such as zinc oxide, tin oxide, gallium oxide and nitride are
important semiconducting and piezoelectric materials. They have also numerous applications in
the fields of optoelectronics, piezoelectric sensors, field effect transistors, transducers and
resonators [146, 187].
In 2005, using dielectrophoresis, Kumar et al [188] developed nanosensors based on tin oxide
nanobelt structures. One year later, a field effect transistor based on gallium nitride nanowires
was demonstrated by Kim et al [15]. The DEP assembly of silicon oxide nanowires was reported
by Wissner-Gross [189] in the same year. In 2007, Wang et al [190] demonstrated controlled
DEP assembly of zinc oxide nanowires as illustrated in Figure 2.22. In their work, the assembly
effects of nanowires were investigated under different frequencies and magnitudes of applied
voltages and gap distances. Other types of metal oxide, sulphide and nitride nanostructured
materials were also been widely employed in micro and nano fabrications [16-19, 30, 35, 50, 191-
193].
Micro and nano particles of metals such as gold, silver and palladium can be patterned into chains
to form bridges between electrodes. Such bridges can show one dimensional ohmic properties and
enable forming electrical connections between electrodes and conductive islands which have the
potential to form microelectronic circuits [194].
In one pioneering work of the year 2001, Hermanson et al [194] demonstrated the assembly of
micro-wires from gold nanoparticle suspensions. As shown in Figure 2.23, gold nano particles
were formed into bridges between electrodes and islands which were deposited between electrode
pairs. The DEP manipulation of gold, silver, palladium and other types of metallic nanostructures
were also reported in [195-199].
49
Figure 2.22: The SEM images of assembled ZnO nanowires between electrodes with different
gap distances: (a) The four gap distances are 2, 4, 6, and 10 µm respectively and are enlarged in
(b) to (e). (f) to (i) illustrates assembling effects using different magnitudes and frequencies of the
applied electric field. The magnitudes and frequencies were (f) 1Vpp at 0.1 MHz, (g) 1Vpp at 10
MHz, (g) 20Vpp at 0.1 MHz, and (i) 20Vpp at 10 MHz. [190]
50
Figure 2.23: (A) A composite optical micrograph of a gold micro-wire spanning a 5 mm gap
between planar gold electrodes. (B) Two wires that are connected to the opposite sides of a
conductive carbon island, and (C) schematics of the B configuration. [194]
2.9 Summary
There are many reviews on the topic of dielectrophoresis published during the last decade;
however, are all incomplete compared to the author’s work. For instance, both Gascoyne et al [14]
and Hughes et al [13] have reviewed the strategies that dielectrophoresis can be employed for
51
particle separation; however, other applications such as particle patterning, transportation, and
characterization are not included. Also, Zheng et al [200] have published a review article focusing
on the manipulation of organic particles without mention the works to manipulating inorganic
materials. Moreover, Wong et al [201] has concluded the methodologies to implement electro-
kinetics for manipulating particles, however, the configurations of the reviewed systems are all
absent.
In this chapter, the author presented the theory of dielectrophoresis and the literature review
based on his published review article which includes a conclusive summary for the configurations
of DEP systems used in microfluidics and the most impacted examples of DEP-microfluidic
systems. The author has introduced the key factors such as “DEP spectrum” and “crossover
frequency” to describe the properties of DEP forces in colloid systems and highlighted the
importance of the surface conductance of particles induced in colloid systems. Calculations of
CM factors were also conducted to illustrate the influences of the conductivities (of both particles
and medium) on the DEP forces.
Based on the literature review presented in this chapter, dielectrophoresis has been widely
employed in the manipulation of particles and the development of nanoelectronic devices.
However, the applications of dielectrophoresis in optofluidics have rarely been reported. Using
dielectrophoresis, functional 3D elements can be formed, tuned, and disabled in liquid by
applying appropriate AC electric fields. Such systems provide great controllability and hence can
be employed to develop configurable devices such as optical waveguides, couplers, multiplexers,
isolators, lenses, switches, and polarizers. In addition, dielectrophoresis has been employed to
develop electronic devices using organic and inorganic materials; however, the functionalities of
dielectrophoretically fabricated organic/inorganic composited devices have not been investigated.
Therefore, in the author’s opinion, it is of significant importance to: first, develop functional
optofluidic devices formed using dielectrophoretically controlled particles; and second, develop
functional electronic devices using dielectrophoretically assembled organic/inorganic composite
materials. To achieve such goals, the investigation of DEP manipulations of particles and DEP
assembly techniques is also essential and desirable in this research.
52
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Chapter 3
Design and Simulation
3.1 Introduction
To conduct any of the experiments that were described in Chapters 1 and 2, the author designed
experimental platforms with different electrode configurations: micro-tip electrode arrays, curved
electrode array and finger paired electrode. Micro-tip electrode arrays were designed for the DEP
separation experiments since they maximize the difference between strong and weak field regions
and clearly distinguish between positively and negatively behaved particles. Curved electrodes
were modified from micro-tip electrodes to manipulate particles in flowing liquid environments,
such a design provided extra area for the accumulation of particles to be held against the liquid
flows. Finger paired electrodes were designed to investigate the DEP alignment of nanostructures
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for the fabrication of electronic devices and employed in the development of conductometric
sensing devices in this PhD research.
In this chapter, the DEP platforms designed for the predetermined tasks are introduced in details.
In addition, the author presents the simulations of the electric fields produced by micro-tip and
curved electrodes to predict the device functionality, the results indicate that the electrodes
configuration are suitable for corresponding experimental uses. The designs for microfluidic
channels elements are also included in this chapter.
3.2 Designs of DEP platforms
Based on the literature review presented in Chapter 2, the author decided to utilize three different
electrode configurations to produce DEP forces for the experiments included in his PhD research.
• Micro-tip electrode arrays
o For the manipulation of polystyrene microparticles within stationary liquid
environment [1].
o For the separation of polystyrene microparticles from multi-walled carbon
nanotubes (MWCNTs) [1].
o For the development of hydrogen gas sensors using nitrogen doped carbon
nanotubes (NCNTs) with metal nanocomposite [2].
o For the development of a field effect transistor using polythiophene/CdTe
quantum dots (QDs) composite [3].
• Curved electrode array
o For the manipulation of micro particles within liquid flows [4].
o For the demonstration of focused particle streams with controllable thicknesses
[4].
• Parallel finger paired electrodes
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o For the study of the alignment and assembly of carbon nanotubes [5].
All the designs included in this PhD research were completed using advanced design system
(ADS) software package. Before the patterns were sent to be printed, the designed files were
exported as GDSII stream format. In order to have a longer usage life, the patterns were printed
on quartz substrates (4 inches).
3.2.1 Micro-tip electrode arrays
Figure 3.1 illustrates the DEP platforms employed for the separation of microparticles from
nanoparticles and the development of electronic devices by assembly of nano structures. The IDT
consists of 8 arrays of microelectrodes, each containing 20 pairs of micro-tip electrodes with a
gap separation of 10 µm and a displacement of 100 µm from each other.
Figure 3.1: Design of the micro tip electrode arrays (A): top overview of the DEP platform; (B):
the arrays of micro electrode gap; and (C): the close view of a gap section [1, 2, 6].
In this design, a gap distance of 10 µm was used because any smaller gap distance resulted in
damages to the electrodes when an AC potential of 10 V (peak to peak) was applied. The purpose
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to separate each pairs of micro-tip by 100 µm is to maximize the difference between strong and
weak field regions, in order to clearly distinguish between positive and negative DEP behaviors.
The distance between arrays was designed to be 260 µm to minimize the field interference
between arrays. Such a design was used by the author in demonstrating the separation of
polystyrene microparticles from MWCNTs [1], the fabrication of H2 gas sensor based on
NCNTs/metal composite [2] and field effect transistor using polythiophene/CdTe QDs
composites [6].
3.2.2 Curved electrode array
The micro-tip electrodes configuration has the ability to dielectrophoretically manipulate
micro/nano particles; however, it also has its own limitations. Micro-tip electrode generates large
field gradients at its sharp tips and weak field gradients at all other locations, such a field
distribution results in a very limited area that is under the influences of DEP forces. Therefore,
the micro-tip electrode configuration is not capable of particles manipulation when liquid flows
are applied.
Figure 3.2: (A) The schematic of curved electrode array. (B) Close view for the electrode features.
(C) Dimension of electrode features [4].
In order to address the issue for manipulating particles in flowing liquid environments, the author
has modified the micro-tip electrodes into curved shape as illustrated in Figure 3.2. The electrode
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array was set to be 18 mm in length to fit in a 20 mm microfluidic channel. In such a design, the
platform had an original gap distance of 80 µm which shrinks to 20 µm at the end of the electrode
tips. The function of this feature is to gather the particles which are suspended in the flowing
liquid and guide them to move along the curved electrodes. Such a design has been employed in
the demonstration of controllable focusing of polystyrene microparticles [4].
3.2.3 Finger pair electrode
Figure 3.3 illustrates the experimental platform prepared for the assembly of nanostructures. The
DEP experiment platforms consist of 40 Au IDT electrode pairs, where each finger has a width of
9 µm and a length of 1 mm, and the inter-spacing between adjacent fingers is 11 µm. Since such
IDT generate the greatest electric field gradient directly above their surfaces, it is capable of
trapping nanostructures in suspension. The Author has utilized such a system to study the
alignment of CNTs using dielectrophoresis [5].
Figure 3.3: The finger pair electrode IDT utilized for assembling carbon nanotubes. The pattern
contains 40 pairs of fingers with 9 µm inter-space, and each finger measuring 1 mm in length and
11 µm in width [5].
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3.3 Simulations of designed DEP platforms
The simulations of electric fields and DEP forces presented in this section were completed in
collaboration with Mr Khashayar Khoshmanesh, from Centre for Intelligent Systems Research,
Deakin University, Geelong, Australia.
The term 2rmsE∇ , which generates the DEP force, exclusively depends on the configuration of
microelectrodes and is of great interest for designers of DEP devices. In order to calculate this
term, first the electric field Erms and electric potential φrms needs to be calculated. Assuming that
the medium filling the microchannel has a constant electric conductivity, the electric potential
field is governed by the Laplace’s equation:
02 =∇ rmsϕ , (3.1)
where ∇ is the gradient operator. Consequently, the induced electric field is obtained by taking
the derivative of the electric potential field:
rmsrmsE ϕ−∇= . (3.2)
Applying proper boundary conditions, the value of 2rmsE∇ can be obtained throughout the field
and substituted into the DEP force equation which has been presented in Chapter 2:
2DEP-C )Re(
2
1rmsCMm EfεΓF ∇⋅⋅= . (3.3)
However, obtaining the exact solution of those equations is generally cumbersome due to the
complex geometry of microelectrodes, especially in a 3D field, and is only limited to very simple
configurations such as parallel one.
To remedy the problem, the CFD method was applied to simulate the DEP field and analyze the
distribution of desired variables within the microchannel. Basically, the CFD method is applied to
simulate the fluidic systems and predict the velocity, pressure and other flow variables by solving
the governing differential equations of the flow known as continuity and Navier-Stokes equations.
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Other differential equations such as the Laplace equation of the electric potential field can also be
solved using this method.
In doing so, the geometry of the field was created and divided into small elements, using Gambit
2.3 Software Package (Fluent, USA, Lebanon, NH). A combination of structured and non-
structured elements was applied in the numerical model. The density of elements was increased
around the microelectrode surface, and especially at the tips to calculate the gradients of electric
potential and electric filed. Taking micro-tip electrodes as the example, the inner and outer edges
of microelectrodes were surrounded by a structured boundary layer (Figure 3.4-A and B). The
boundary layer consisted of 4-6 rows of rectangular elements starting from a thickness of 0.7 µm
and increasing with an expansion ratio of 1.07 to reach a final thickness of 2 µm. The areas
between the boundary layer elements were divided by unstructured quadrilateral elements, which
had an adequate flexibility to adopt with complex geometries while keeping the number of
elements as low as possible (Figure 3.4-A and B). Along the z-axis, the microchannel was divided
to 30 elements starting from a thickness of 1 µm at the bottom surface and increasing with an
expansion ratio of 1.07 to reach a final thickness of 4 µm at the top surface of the chamber
(Figure 3.4-C). In this case, a total number of 681,120 quadrilateral elements were applied in the
simulations. After that, the size, density and number of elements were varied several times to
achieve the optimized pattern of elements and ensure that the simulation results are independent
from elements.
Next, the governing Equations (3.2) and (3.3) were solved using Fluent 6.3 software package
(Fluent, USA, Lebanon, NH). The software applies the finite volume method to descritise the
governing equations across each element of the field. The computational capabilities of this
software facilitates the three dimensional modeling of the problem. The microelectrodes were
excited at 5 V while for the other surfaces of the model, including the bottom, top and sidewalls
the zero electrical flux condition was applied. Taking advantage of the user defined scalar (UDS)
module of the software, the equations were solved and the values of φrms, Erms and 2rmsE∇ were
obtained at different locations within the microchannel. The simulation was continued until a
convergence of 5×10−8 was achieved for the electric field.
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Figure 3.4: The microchannel was divided into 681,120 quadrilateral elements. The density of
elements increased at microelectrode edges: (A-B) along the x- and y- axes, and (C) z- axis.
3.3.1 Micro-tip electrode arrays
Figure 3.5, Figure 3.6 and Figure 3.7 illustrate the gradient of the electric field square along x, y
and z directions. Such simulations have been published in the work of separating MWCNTs from
polystyrene microparticles [1].
Figure 3.5 and Figure 3.6 show that the field has a much higher gradient at the corners of
electrode tips. Figure 3.7 illustrates that the field gradient is positive in the region between the
electrode tips and is negative at electrode edges. Therefore, particles with a positive CM factor
are trapped when they are close to the electrode tips, and are strongly levitated when are located
within the intermediate region.
The simulation results assert that the design of this DEP platform is appropriate for the separation
and assembly experiments. The regions around the electrode tips produce a sharp gradient of
electric fields, which are suitable for producing positive DEP forces. The regions that are far
away from the electrode tips have weak and uniformly distributed electric fields, which are
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suitable for producing negative DEP forces. Hence spatial separation of the particles will occur if
some of the particles experience negative DEP forces while the others experience positive DEP
forces.
In DEP trapping applications, the patterns and intensities of the assembled particles are important
considerations for evaluating system performances. Therefore, vectors of both electric fields and
DEP forces created by the electrodes are illustrated in Figure 3.8. Such simulations have been
published in the work of fabricating field effect transistors using polythiophene/CdTe QDs
composite [6].
Figure 3.8-A shows the vectors of electric field created between the electrode tips, which
suggested the fields strength reached a maximum value of 2.06×106 V/m at the sharp corners of
the tips and the electric field lines followed an arc-shaped trend between the tips. Therefore, the
DEP assembled particles would also follow the same arc-shaped pattern during the deposition
processes.
Figure 3.8-B shows the vectors of 2E∇ induced between the tips; which have the same directions
and proportional magnitudes of the generated DEP forces. In this case, it is clear that the DEP
forces generated between the electrode tips rotate in the opposite directions and tend to push the
particles towards the electrode tips. Since the particle distribution after the deposition depends on
the intensity of the DEP force [7], the simulations predicted most of the particles would be
patterned on the edges of electrode tips and the region between them. As a result, the designed
system is capable for particle manipulations.
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Figure 3.5: The gradient of the electric field square along x direction, which depicts the intensity
of the DEP forces. [1]
Figure 3.6: The gradient of the electric field square along y direction, which depicts the intensity
of the DEP forces. [1]
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Figure 3.7: The gradient of the electric field square along z direction, which depict the intensity of
the DEP forces. [1]
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Figure 3.8: The simulation results of (A): vectors of electric field E (V/m), shows the pattern of
deposited particle, and (B): vectors of 2E∇ (V2/m3), shows the possible particle distribution after
the DEP deposition [6].
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3.3.2 Curved electrode arrays
Figure 3.9 shows the distributions of electric field produced by curved electrodes, the terms
“sidewall” and “centerline” refer to the features of the microfluidic channel. It is clear that the
regions with strong field gradient are larger for curved electrodes compared with micro-tip
electrodes as the field strength along the electrode elements are all on the order of 104 to 106. In
this case, when the liquid flows are applied to the systems, the curved electrodes would gather
more particles into the area which is under the influence of DEP trapping forces. more
importantly, the simulation results also suggested the feature would also provide a large area to
accumulate the trapped particles and hold them against the liquid flow.
Figure 3.9: The simulation result of the electric field produced by curved electrodes.
Figure 3.10 illustrates the simulation results for 2E∇ along x, y and z directions for different
locations and heights within a microfluidic channel. Two locations were selected for simulations:
location one, the positions close to the starting points of the curved shaped electrode elements;
location two, the positions close to the tips of the curved shaped electrode elements. Also, the
simulations were conducted at the height of 10 and 40 µm above the electrodes for both locations.
During the analysis of the obtained results, the DEP forces along the x direction are not
considered because the hydrodynamic forces are dominant along this direction.
For location one, at the height of 10 µm (Figure 3.10-B), the magnitude of DEP-y forces has a
negative bottom (at y-distance of 30 µm) and two positive peaks (at y-distance of 40 and 70 µm).
The negative bottom indicates the particles would be attracted from the centerline to the inner
edge of electrodes (at y-distance of 30 µm). The two positive peaks are generated by the two
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edges of the electrodes, which would attract the particles from the sidewall to the electrodes. The
DEP-z forces have two positive peaks at the both edges of the electrode, which indicates the
particles suspended above would be trapped. At the height of 40 µm (Figure 3.10-C), both of the
DEP-y and DEP-z forces are much weaker due to the decrease of the field gradient. Thus, the
function of the system at location one is to gather particles and guide them to move along the
curved electrode.
For location two, at the height of 10 µm (Figure 3.10-D), both DEP-y and DEP-z have only one
positive peak, this is because location two is very closed to the electrode tip where both edges are
not separated for large distance. It is also clear that, the magnitudes of the DEP forces at such a
location are much larger than those at location one, which indicates particles are more likely to be
trapped near location two. Therefore, the designed system is capable of experimental uses.
Figure 3.10: Simulation results for the values of 2E∇ along x, y and z directions for different
locations and height within a microfluidic channel. (A): The locations and height selected for
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simulations. (B) and (C): The simulated values of 2E∇ near the starting points of curved
electrode elements at the height of 10 and 40 µm, respectively. (D) and (E): The simulated values
of 2E∇ near the tips of curved electrode elements at the height of 10 and 40 µm, respectively.
3.4 Designs of microfluidic channels
In this thesis, the microfluidic channels were fabricated using polydimethylsiloxane (PDMS). As
shown in Figure 3.11, the microfluidic channels used in this research are straight lines with a
width of 1 mm and a length that varies from 8 to 20 mm in order to fit with different DEP
platforms. The solid circles represent the reservoirs where liquid suspension can be filled. Holes
are punched into PDMS element to connect tubing and pumping systems.
Figure 3.12 illustrates different ways that a microfluidic channel is assembled with interdigitated
electrodes. For the separation of CNTs from polystyrene particles and the demonstration of
controllable focusing of polystyrene particles, the particle suspensions were conducted within the
PDMS microfluidic elements. For the fabrications of gas sensors and field effect transistors,
microfluidic elements were not used.
Figure 3.11: Two of the designs for the microfluidic channels adopted in this thesis. These
channels have a width of 1000 µm which were fabricated within the PDMS blocks.
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Figure 3.12: Combined DEP platforms with microfluidic channels for different applications.
3.5 Summary
Based on the literature review presented in chapter 3, the author made a decision to utilize three
different electrode configurations to conduct his predetermined tasks. Micro-tip electrode arrays
were designed for the manipulation of particles in stationary liquid and fabrication of
nanostructured devices. Curved electrode array was fabricated to manipulate within liquid flows
and demonstrate the controllable focused particles. Parallel finger paired electrodes was
employed for investigating DEP assembling of nanomaterials. In order to ensure the functionality
of the designed systems, simulations of electric fields and DEP forces were performed. The
simulation results indicated the designed systems are capable of predetermined experimental
applications.
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Reference
[1] C. Zhang, K. Khoshmanesh, F. J. Tovar-Lopez, A. Mitchell, W. Wlodarski, and K. Klantar-Zadeh, "Dielectrophoretic separation of carbon nanotubes and polystyrene microparticles," Microfluidics and Nanofluidics, vol. 7, pp. 633-645, 2009.
[2] A. Z. Sadek, C. Zhang, Z. Hu, J. G. Partridge, D. G. McCulloch, W. Wlodarski, and K. Kalantar-zadeh, "Uniformly Dispersed Pt-Ni Nanoparticles on Nitrogen-Doped Carbon Nanotubes for Hydrogen Sensing," Journal of Physical Chemistry C, vol. 114, pp. 238-242, 2009.
[3] V. Strong, C. Zhang, K. Khoshmanesh, R. Kaner, and K. Kalantar-Zadeh, "Field Effect Transistor Based on Dielectrophoretically assembled Polythiophene/CdTe Quantum Dots Composite," (under submission), 2009.
[4] C. Zhang, K. Khoshmanesh, A. A. Kayani, F. J. Tovar-Lopez, W. Wlodarski, A. Mitchell, and K. Kalantar-zadeh, "Dielectrophoretic Manipulation of Polystyrene Micro Particles in Microfluidic Systems," in Micro/Nanoscale Heat & Mass Transfer International
Conference, Shanghai, China, 2009.
[5] C. Zhang, M. Breedon, W. Wlodarski, and K. Kalantar-zadeh, "Study of the alignment of multiwalled carbon nanotubes using dielectrophoresis " in Society of Photo-Optical
Instrumentation Engineers (SPIE) Conference, Camberra, Australia, 2007.
[6] V. Strong, C. Zhang, K. Khoshmanesh, R. Kaner, and K. Kalantar-Zadeh, "Field Effect Transistor Based on Dielectrophoretically assembled Polythiophene/CdTe Quantum Dots Composite," (under submission), 2010.
[7] K. Khoshmanesh, C. Zhang, F. J. Tovar-Lopez, S. Nahavandi, S. Baratchi, K. Kalantar-Zadeh, and A. Mitchell, "Dielectrophoretic manipulation and separation of microparticles using curved microelectrodes," Electrophoresis, vol. 30, pp. 3707-3717, 2009.
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Chapter 4
Fabrication
4.1 Introduction
The DEP-microfluidic devices used in this research consist of substrates with gold
microelectrodes and microfluidic channels fabricated using polydimethylsiloxane (PDMS) to
connect with a fluidic pumping system. Such DEP microelectrodes were fabricated using
standard photolithography and etching processes; the PDMS microfluidic elements were
fabricated using soft lithography technique. The DEP-microfluidic devices were then assembled
for experimental uses.
In this chapter, the fabrication processes of DEP microelectrodes and microfluidic elements are
presented. The main procedures involved in such fabrication processes include: mask design,
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metal layer deposition, and photolithography, chemical etching, PDMS mixing and curing, and
assembling of DEP-microfluidic devices.
4.2 Fabrications
There are two methods developed to implement the fabrication of interdigitated electrodes:
etching and lift-off. In this thesis, most of the author’s works were conducted using the devices
that fabricated from the etching process. Soft lithography was employed for the development of
PDMS blocks: a photoresist mould was pre-fabricated on silicon wafer, the PDMS polymer and
curing agent were mixed with an appropriate ratio and cured at the optimized temperature to
achieve the required softness or toughness.
All the fabrication activities included in this thesis was conducted in the Microelectronics and
Materials Technology Centre (MMTC), clean-room and vacuum laboratory facilities of RMIT
University.
4.2.1 The fabrications of microelectrodes
4.2.1.1 Photolithographic mask preparation
There are two types of masks which are applied in photolithography technique: clear field and
dark field masks, which can be applied with positive or negative photoresist to image desired
patterns. Since the etching methodology was adopted in this thesis for the fabrication of IDTs, the
author decided to use AZ1512 photoresist (a positive resist, purchased from MicroChemicals) and
clear field masks to pattern IDT electrodes onto the metalized substrates. The IDT patterns were
designed using Advanced Design System (ADS) software package. Before the patterns were sent
to be printed, the designed files were exported as GDSII stream format. In order to have a longer
usage life, the patterns were printed on quartz substrates (4 inches).
4.2.1.2 Substrate preparation
Because of the dimension of IDT patterns, any small particles remained on the surface of the
substrate before the fabrication process would result in irreversible faults. As a result, cleaning of
the substrates with standard processes is necessary to ensure the quality of the fabricated devices.
To do so, the substrates were first cleaned in acetone for surface degreasing and rinsed in
87
Isopropyl Alcohol (IPA) or methanol; after bath in deionised (DI) water, the substrates were dried
using nitrogen blow. In order to ensure that the substrates were properly cleaned, a visual
inspection was conducted using an optical microscope prior to subsequent fabrication stages.
4.2.1.3 Substrate metallization and dicing
In this PhD research, the substrates for applications were metalized with Cr and Au (normally 50
and 100 nm, respectively) using the electron beam evaporation technique. The purpose to have a
Cr layer prior to the deposition of Au is to ensure its good adhesion to the surface. For the
fabrication of nanostructured devices, the substrates were diced into designed dimensions after
the metallization process using a dicing saw (DAD 321) with a diamond blade. To protect the
surface of the substrates during the dicing process, a layer of photoresist was spin-coated on the
substrate. For the development of DEP-microfluidic systems, the metalized glass substrates
(microscope cover slips) were directly used for the fabrication process without dicing.
4.2.1.4 Photolithograph
The photolithographic processes were conducted in a class-1000 clean room environment with a
constant temperature of approximately 22°C and a relative humidity of 40%. The AZ 1512
photoresist was spin-coated at 3000 rpm (acceleration of 1000 rpm/sec) for 25 seconds to form a
layer of approximately 1 µm. After the substrates were soft-baked for 20 minutes at 90 °C, a
mask aligner (Suss MJB3) was used to expose the mask pattern onto the resist layer. The UV-
exposed samples were subsequently developed in a mixture of AZ-400 developer (mixed with DI
water in a ratio of 1:4) for 15 seconds. After the substrates were rinsed in DI water, visual
inspection was conducted to prevent under/over developing of the resist patterns. The samples
were then hard-baked at 110 °C for 20 minutes to achieve sufficient hardness before chemical
etching.
4.2.1.5 Chemical etching
At this stage, the etching of unwanted metals was performed. Firstly, the unwanted Au coating
was etched using aqua regia, which is a mixture formed by concentrated nitric acid (HNO3) and
hydrochloric acid (HCl) in a volumetric ratio of 1:3, respectively. Etching time is very critical to
the developed device as over etching can result in the lift-off of interdigitated electrodes.
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Normally, the samples were immersed in the gold etchant for 10 to 20 seconds and obvious color
change can be observed due to the appearance of Cr layer.
After rinse in DI water for 60 seconds, Cr etching was performed using a solution of 4% nitric
acid, 11% ceric ammonium nitrate (Ce(NH4)2(NO3)6) and 85% water. The etching process can
last from 30 to 60 seconds and followed by rinsing of DI water. The photoresist layer was then
striped by acetone bath for 30 seconds and the samples were again inspected under an optical
microscope to ensure the complete removal of the resist. If necessary, a plasma stripping process
would be performed to remove remaining resist.
Figure 4.1: The schematic of photolithography fabrication processes.
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4.2.1.5 Fabrication of dissimilar metal electrode
In addition to conventional electrodes, the author has also developed a two stage procedures to
fabricate dissimilar electrodes in a single device, which was published in the proceedings of
International Conference On Nanoscience and Nanotechnology (ICONN) 2008 [1]. This device
was formed by interchanging finger pairs of two different types of metals (Au and Cr) and was
employed as a moisture sensor. The dissimilar metal electrodes can generate electric potential
when the surface media contains sufficient charge carriers and hence improve the device
sensitivity. The device was coated by a TiO2 layer which was employed as the sensitive media to
provide extra charge carriers when the sensor is exposed to moisture.
Figure 4.2: The fabrication procedures for dissimilar metal electrodes.
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For device development, the first stage of fabrication was described in the Sections 4.2.1.4 and
4.2.1.5 and illustrated in Figure 4.1. In the second stage of fabrication, as illustrated in Figure 4.2,
an additional mask with half of the designed pattern was required. When the prefabricated
electrodes were spin-coated with photoresist, the second mask was applied to align with the
electrodes. Since such a mask only covered single side of the electrode pair, the photoresist would
remain on half of the pattern after the developing process. Consequently, the gold layer on one of
the electrode elements can be removed and the fabricated device consists of one Cr electrode and
one Au electrode.
4.2.2 The fabrications of PDMS microfluidic elements
In the process of fabricating PDMS microfluidic blocks, KMPR1025 resist (purchased from
MicroChem) was applied to build micro-channel moulds onto Si wafers. Since KMPR1025 is a
negative photoresist, a dark field mask is required. The Mask design follows a same procedure as
the author mentioned in Section 4.2.1.1.
Si wafers were first cleaned through standard cleaning process as presented in Section 4.2.1.2.
KMPR1025 resist was then spin-coated onto the Si wafer at a speed of 600 rpm and an
acceleration of 200 rpm/s to achieve a layer of approximately 80 µm in thickness. After that, the
wafer was soft-baked at 100 °C for 20 minutes on a hot plate. When the resist layer became solid,
UV exposure of 1 minute was performed using Suss MJB3 mask aligner. The wafer was then
placed back onto hotplate for post exposure bake until an image of the mask was visible in the
resist coating. The developing of the resist layer was conducted using SU-8 developer for
5 minutes, followed by isopropanol rinsing for 10 seconds and dried using nitrogen blow.
For PDMS block fabrication, the polymer and curing agent were first mixed with the weight ratio
of 10:1. After the proper mixing of the material, the mixture was placed into the vacuum oven for
15 minutes to clean the bubbles within the liquid PDMS. A pre-milled plastic frame was then
placed on the Si wafer and liquid PDMS was applied into the mould. After cured on the hot plate
at 100 ºC for 2 minutes, the PDMS block was removed and 2 holes were punched to function as
liquid inlet and outlet. The complete fabrication process is illustrated in Figure 4.3.
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Figure 4.3: The process in the fabrication of PDMS microfluidic block.
4.3 Assembling of the DEP-microfluidic devices
As shown in Figure 4.4, the microelectrodes adopted for the generation of DEP forces were
patterned on a glass substrate. The PDMS block with a micro channel was assembled onto the
substrate such that the IDTs were sealed within the channel but with contacting pads exposed.
For measurements, the device was placed onto an inverted microscope; the inlet and outlet holes
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were then connected to a micro-liter range pumping system and the contacting pads were
connected to a function generator.
Figure 4.4: Schematic of the assembled microfluidic system consists of a substrate with
electrodes patterned on a glass slide and sealed by a PDMS micro channel block (reversible
sealing) [2].
4.4 Summary
In this PhD research, the author has successfully fabricated experimental platforms with designed
interdigitated electrodes on both glass slides for the application in DEP-microfluidic systems and
Si/SiO2 substrates for the development of nanostructured devices. Both clear/dark field masks and
positive/negative photo resist were employed for such fabrications.
In this chapter, the fabrication procedures for both DEP microelectrodes and PDMS microfluidic
blocks are presented. The fabrication process can be summarized as follow: mask design,
substrate preparation and metallization, photolithography, etching and PDMS curing etc.
Additionally, the use of both clear field and dark field masks, positive and negative photoresists
for photolithographic patterning were introduced.
93
Reference
[1] C. Zhang, A. Z. Sadek, M. Breedon, S. J. Ippolito, W. Wlodarski, T. Truman, and K. Kalantar-zadeh, "Conductometric Sensor based on Nanostructured Titanium Oxide Thin Film Deposited on Polyimide Substrate with Dissimilar Metallic Electrodes," in 2010
International Conference On Nanoscience and Nanotechnology, Melbourne, Australia, 2008.
[2] C. Zhang, K. Khoshmanesh, F. J. Tovar-Lopez, A. Mitchell, W. Wlodarski, and K. Klantar-Zadeh, "Dielectrophoretic separation of carbon nanotubes and polystyrene microparticles," Microfluidics and Nanofluidics, vol. 7, pp. 633-645, 2009.
94
Chapter 5
Particles Manipulation in DEP-microfluidic
Systems
5.1 Introduction
In Chapter 1, the author described his main tasks included in this PhD research. Two of the tasks,
that involve the manipulation of particles within DEP-microfluidic systems: “dielectrophoretic
separation of polystyrene particles from multi-walled carbon nanotubes” and “controllable
focusing of polystyrene microparticles using dielectrophoresis”, are presented in this chapter. In
addition, the results of the collaborative works with colleagues on the applications of curved
electrodes for creating advanced functional systems are presented at the final stage of this chapter.
95
To separate polystyrene microparticles from multi-walled carbon nanotubes (MWCNTs), the
author has theoretically and experimentally investigated the DEP behaviors of polystyrene
microparticles and compared them with the DEP spectrum of the metallic MWCNTs. The
separation experiments were then performed using micro-tip electrode arrays at the optimal
operation frequency that was obtained during the comparison.
To demonstrate the controllable focusing of polystyrene microparticles, the author investigated
the DEP behaviors of polystyrene microparticles in flowing liquid environments. Curved
electrode array was used in such a task since micro-tip electrode arrays were not capable of
particle manipulation in liquid flows. The operation frequency was set to be 100 kHz ensuring the
generation of strong positive DEP forces during the focusing processes. Different AC potentials
and liquid flow rates were applied to the system to achieve focused particle streams in various
thicknesses (diameters).
Using the curved electrodes, a novel system, which utilized dielectrophoretically concentrated
sub-micron particles to operate as an optical waveguide, was developed. In this work, silica
particles (230 and 450 nm in diameters) were applied to the system and concentrated into focused
streams, a laser (635 nm in wavelength) was employed to characterize the waveguiding properties
of the system. This work was carried out in collaboration with Mr Aminuddin Kayani, from the
School of Electrical and Computer Engineering, RMIT University, Melbourne, Australia.
Three additional works, which were conducted by the author in collaboration with Mr Khashayar
Khoshmanesh, from Centre for Intelligent Systems Research, Deakin University, Australia, are
also presented in this chapter. First, curved electrodes were employed as a sorter to separate
polystyrene microparticles of different dimensions. In this work, negative DEP forces were
utilized to repel the particles from the centerline of the microchannel; the particles with different
dimensions were then distributed to different locations since they experienced different
magnitudes of DEP forces. Second, the same configuration of electrodes was utilized to sort live
and dead cells; the sorting efficiency of the system was evaluated by an additional assessing
electrode array. In this work, the live cells and dead cells were distinguished by DEP forces due
to the large differences in their dielectric properties. At the pre-determined operating frequency,
the live cells were experienced positive DEP forces and trapped by the curved electrodes; while,
dead cells experienced negative DEP forces and repelled to the sidewalls of the microfluidic
channel. At last, the trapping of polystyrene particles were demonstrated using pre-patterned
nitrogen doped carbon nanotubes with Pt nanocomposite using the electric field with an
96
alternating frequency of 20 MHz. Since polystyrene particles are generally repelled by DEP
forces at such a high frequency, the trapping of such particles is achieved due to a change in the
dielectric properties caused by the coating of carbon nanotubes on the surfaces of the particles.
5.2 DEP manipulation of polystyrene microparticles
in stationary liquid environment
In this section, the manipulation of polystyrene microparticles in stationary liquid is presented.
The micro-tip array electrodes were applied for the generation of DEP forces; a PDMS micro
channel element was utilized to seal particle suspensions. Polystyrene particles suspension (Duke
Scientific, 1µm in diameter, and original concentration of 1.8×1010) was first mixed with DI water
in a volume ratio of 1:1, and treated in ultrasonic bath (Branson, 125W) for 45 minutes before the
experiments. After the liquid suspension was injected into the microfluidic channel, alternating
voltages with amplitudes of 10 V peak-to-peak and frequencies from 100 Hz to 1 MHz were
applied across the electrodes. An inverted microscope (Nikon Eclipse FE 2000H) was used to
observe the behavior of the particles. The contend which is presented in this section has been
published in the journal “Microfluidics and Nanofluidics” [1].
5.2.1 Theoretical calculations
As mentioned in Chapter 2, the property of DEP forces can be described by the terms “DEP
spectrum” and “crossover frequency” which can be revealed through the calculations of Re(fCM),
as shown in equations (5.1) and (5.2).
*
m
*
p
*
m
*
p
CMεε
εεf
2spherical-
+
−= ,
ω
σiεε* −= ,
(5.1)
(5,2)
97
where εp* and εm
* are the complex permittivities of the particles and suspending medium,
respectively; ε is the dielectric constant, σ is the conductivity and ω is the angular frequency of
the electric field.
To perform such calculations, the values for the conductivities and permittivities of polystyrene
and DI water are required. The relative permittivity of polystyrene and water are assumed to be
2.5 and 78, respectively; their dielectric constants are: εP=2.21×10-11 and εm=6.90×10-10 F/m. The
conductivity of DI water used in the experiments is measured to be 2 mS/m. Since the dimension
of the used polystyrene particles is large (1 µm), the conductivity of suspended polystyrene
microparticles can be estimated using [2]
r
Kσσσσ S
pppp
2SurfaceCoreSurface ==+= −−− ,
(5.3)
where σp refers to the overall conductivity of the polystyrene particles, σp-Surface and σp-Core are the
surface conductivity and core (bulk) conductivity of the polystyrene particles, respectively. KS is
the surface conductance of the polystyrene particles induced in liquid medium, r is the radius of
the polystyrene particles. The core conductivity is ignored in the calculation because polystyrene
is a nonconductive material.
According to the experimental results that was reported by Cui et al [3], the value for surface
conductance KS is proportional to the diameter of the suspended particle. For 1 µm particles (r =
0.5 µm), KS=0.3 nS and σp-Surface=1.2 mS/m.
The DEP spectrum of polystyrene particles suspension is illustrated Figure 5.1 It is suggested that
such particles would experience positive DEP forces before the crossover frequency (~200 kHz)
and negative DEP forces for the frequencies higher than the crossover frequency.
98
Figure 5.1: Re(fCM) vs. frequency for 1 µm polystyrene microparticles which are suspened in DI
water.
5.2.2 Experimental results
To verify the value of the crossover frequencies obtained in Section 5.2.1, DEP manipulation of
polystyrene microparticles in DI water were performed for the frequencies from 100 Hz to 1 MHz.
The AC potential was set to 20 V peak-to-peak during the experiments.
Figure 5.2 presents the DEP behaviors of polystyrene particles at different frequencies:100 Hz
(A), 500 Hz (B), 1 kHz (C), 5 kHz(D), 10 kHz (E), 50 kHz (F), 150 kHz (G), 500 kHz (H) and 1
MHz (I). Positive DEP behavior can be observed in Figure 5.2-A to D, as the microparticles are
trapped at the edge of microelectrodes. The decreasing in the number of trapped particles was
observed while the frequency of the applied field was increased from 100 Hz to 5 kHz; this is
caused by the decreasing of the of DEP forces. Figure 5.2-E and F show the particle behaviors at
the frequencies close to the crossover, as the microparticles lost the tendency to be trapped by the
microelectrodes. At 150 kHz (as shown in Figure 5.2-G), a typical negative DEP behavior is
evident, since the microparticles are repelled from the microelectrodes and clustered in the spaces
between them; chaining of microparticles is caused by the polarization effect of the electric field.
99
There is not much difference in the particle behaviors in Figure 5.2-H and I (500 kHz and 1 MHz)
compared to that of Figure 5.2-G (150 kHz) due to the stabilization of DEP forces.
According to the theoretical calculations that is presented in Section 5.2.1, the crossover
frequency is approximately 200 kHz; however, the experimental results as shown in Figure 5.2
suggested the actual crossover frequency is between 10 kHz to 50 kHz. This mismatching might
be due to the fact that the assumed value for the surface conductance KS was not accurate.
Figure 5.2: Particle behaviours at frequencies from 100 Hz to 1 MHz. (A-D): positive DEP
observed; (E & F): close to crossover frequency. (G): typical negative DEP behaviour. (H & I):
DEP behaviour of particles at 500 kHz and 1 MHz is similar to that at 150 kHz [1].
100
5.3 DEP separation of polystyrene microparticles
from multi-walled carbon nanotubes
The separation of particles using dielectrophoresis is generally based on distinguishing of
different particle behaviors under the influence of electric fields. For instance, positively behaved
particles can be separated from negatively behaved ones; strongly positive or negative behaved
particles can also be separated from weakly behaved ones.
There are many reports on the separation of particles within microfluidic systems utilizing DEP
forces: the application of dielectrophoresis to particle discrimination, separation, and field-flow
fractionation (FFF) was reviewed in [4]. Cells with different size were separated using direct
current dielectrophoresis such as the work by Kang et al. [5]. The separation of polystyrene beads
and cells by different dielectric fields was reported by Demierre [6], where opposite DEP forces
were used for accurately focusing a stream of beads and yeast cells at different locations across
the channel by adjusting potentials and frequencies. A microparticle filter was reported to be
successfully developed and characterized by Li et al [7] where DEP force was employed to
capture desired particles. In addition, live and nonviable cells were separated due to their
significant differences in dielectric properties [8-11]; and the sorting of normal and Babesia bovis
infected erythrocytes was reported using dielectrophoresis by exploiting the higher ionic
membrane permeability of Babesia Bovis infected cells [12]. Separating and sorting of other
bioparticles including DNA [13, 14], proteins [15] and viruses [16-18] were also reported.
Additionally, the separation of metallic and semi-conducting single-walled carbon nano-tubes
(CNTs) employing DEP forces was also demonstrated [19].
In this section, the DEP separation of polystyrene micro particles from MWCNTs using micro-tip
electrode arrays is presented. The significant difference in conductivities was utilized to
distinguish polystyrene microparticles from MWCNTs using AC electric fields. The included
content has been published in the journal of “Microfluidics and Nanofluidics” [1].
5.3.1 DEP spectrum of MWCNTs
In order to pick the optimum frequency for separating polystyrene microparticles and MWCNTs,
the comparison of DEP behaviors for both types of particles at different frequencies was required.
101
Since MWCNTs are cylindrical structures, its DEP spectrum is calculated using Error!
Reference source not found.
*
m
*
m
*
p
CM-ε
εεf
−=lcylindrica .
(5.4)
The relative permittivity of metallic MWCNTs was taken as 2.5 while the conductivity was
assumed between 1000 S/m and 150 S/m according to ref. [20, 21]. The DEP spectra for
MWCNTs, as shown in Figure 5.3, suggests MWCNTs experience positive DEP forces for the
whole range of investigated frequencies. Therefore, the separation of polystyrene microparticles
from MWCNTs is possible. The optimum separation frequency is approximately 100 kHz as
MWCNTs experience positive DEP (trapping) forces with maximum magnitude, while the
polystyrene particles experience negative DEP (repelling) force at such a frequency.
The DEP trapping of nanostructures with high aspect ratios has been previously investigated. For
example, Dimaki and Boggild have theoretically studied the DEP trapping and assembling of
carbon nanotubes [20]. Zhou et al. has reported that CdSe nanowires can be self-assembled using
symmetric ac dielectrophoresis due to the large induced dipoles within the nanowires [22].
Krupke et al. have also successfully demonstrated the separation of semiconducting and metallic
carbon nanotubes using DEP trapping based techniques [19, 23].
102
Figure 5.3: The calculated real part of CM factor vs frequency for MWCNTs with conductivities
of 150 and 1000 S/m [1].
5.3.2 Experimental and results
MWCNTs (Sigma Aldrich, 636649), 20-50 nm in diameter, 0.5-2 µm in length, with single-wall
thickness of 1-2 nm were dispersed in DI water with an initial concentration of 0.1 mg/ml. The
suspension was stabilized with the aid of Triton X-305 surfactant (Sigma Aldrich, approximately
40 µl surfactant per 10 ml suspension). The liquid was then mixed with polystyrene micro
particles suspension (Duke Scientific, 1µm in diameter, concentration: 1.8×1010 in DI water) in
the volume ratio of 1:1. The mixture was then placed in a 125-W ultra-sonicator (Branson) bath
for 45 minutes and left intact for 6 hours to allow visible agglomerates to precipitate from the
suspension. The separation experiments were performed using AC potentials with amplitude of
10 V (peak to peak) at frequencies of 100 Hz, 1 kHz, 100 kHz and 1 MHz. The separation
progress in the stationary liquid is shown in Figure 5.4 and Figure 5.5.
At 100 Hz (Figure 5.4-A), strong positive DEP behaviors were observed, large numbers of
polystyrene microparticles and MWCNTs were trapped in the region near the electrode tips.
When the frequency of applied AC potential was increased to 1 kHz (Figure 5.4-B), less micro
particles were remained trapped because this frequency is very close to the crossover frequency
and the DEP forces became weak. Figure 5.4-C illustrates the initial separation of micro particles
from MWCNTs at 100 kHz. At this frequency, polystyrene micro particles were repelled from
strong electric field regions (electrode tips and surfaces), while MWCNTs remained at their
positions; therefore, the separation of MWCNTs and polystyrene micro particles was complete.
Since the magnitude of DEP forces on both MWCNTs and polystyrene micro particles stabilized
for the frequencies larger than 100 kHz, similar DEP behavior was observed at the frequency of
1MHz.
After the polystyrene microparticles were removed and the liquid has evaporated, the substrate
was characterized using a scanning electron microscope (SEM) to prove the trapping effect of
MWCNTs. As can be seen in Figure 5.5-B, MWCNTs were trapped on the tips and edges of
electrodes, and hence the DEP separation was successful.
103
Figure 5.4: Particle separation progress observed from the frequency of 100 Hz to 100 kHz.
(A): at 100 Hz, both microparticles and MWCNTs were experiencing strong positive DEP forces;
(B): at 1 kHz, positive DEP forces were weaker, (C): at 100 kHz, MWCNTs were still
experiencing positive DEP forces, while microparticles were influenced by negative DEP forces
and repelled from electrode tips. (D): at 1 MHz, particle behaviors are similar to that at 100 kHz
[1].
Figure 5.5: SEM image of assembled MWCNTs at the tip and edge of electrodes [1].
104
5.4 DEP manipulation of polystyrene microparticles
in continuous flowing liquid environment
The investigation on continuous flow DEP manipulation has been demonstrated using different
electrode configurations including the castellated, finger paired, angled and funnel-shaped
electrodes [24-27]. Additionally in a very complex system, Cheng et al. have integrated all the
mentioned electrode configurations into one single DEP system to achieve the filtering, focusing,
sorting and trapping of particles simultaneously [25]. In such a system, finger paired electrodes
were utilized to filter debris from the sample by removing particles that exhibit positive
dielectrophoresis, the funnel shaped electrodes were then employed to concentrate the particles
into a narrow band. After that, three pairs of angled electrodes were activated using different AC
potentials to selectively deflect the particles according to their negative DEP motilities and hence
distribute them into different channel outlets. At last, the trapping electrodes were employed to
assess the system sorting efficiency.
In this section, the DEP manipulation of microparticles in continuous liquid follows achieved
using curved electrodes is presented. Polystyrene microparticles (Duke Scientific, red fluorescing,
1 and 3 µm in diameters, concentration: 1.8×1010 in DI water) were adopted in the experiments to
assess the functionality of the curved electrodes. A pump (Harvard Apparatus PHD 2000) was
used to refill the liquid suspension of micro particles into the microchannel. AC potentials with
the magnitudes of 1 to 30 V peak-to-peak and frequencies from 100 kHz to 20 MHz were applied
across the electrodes using a function generator (Etabor Electronics 8200). An inverted
microscope (Nikon Eclipse FE 2000H) was utilized to observe the DEP behaviors of micro
particles and the performances of the DEP platforms.
The content which is included in this section was presented at the 2010 International Conference
on Micro/nano scaled Heat and Mass Transfer (MNHMT) [28].
5.4.1 Hydrodynamic forces in microchannels
The hydrodynamic forces in microfluidic channels involve the drag, lateral lift and sedimentation
forces. The drag force, which pushes particles along the microchannel, is approximated by the
Stokes law, as follows:
105
UπrµF 6drag = , (5.5)
where µ is the dynamic viscosity of the medium and U is the average velocity of the medium
inside the microchannel.
The lateral lift force, which repels particles from the walls and pushes them towards the centreline
of the microchannel, is calculated as [29-31]
22
lift 2rUCF m
L ⋅= πρ
,
dLd
LfC Re)(=
,Reµ
ρ dUm
d =
(5.6)
(5.7)
(5.8)
Where CL is the wall-induced lift force coefficient, ρm is the medium density, L is the distance
between the particle and the sidewalls of the microchannel, d is the diameter of the particle and
Red is the Reynolds number based on the particle diameter. The Reynolds number is a
dimensionless number, which is proportional to inertial force divided by viscous force. For most
microfluidic flows, the Reynolds number is less than 100, which corresponds to a purely laminar
behavior. For (L / d) Red < 0.2, CL = 1.125 - 0.38 (L / d)2Red
2, therefore, the CL starts from 1.125
for zero L values and drops to zero at large L values [30].
The sedimentation force applied on the particle is the interaction of the gravity and buoyancy
given as
grF mp )(43 3
ionsedimentat ρρπ −= , (5.9)
where ρp is the density of particles and g is the gravitational acceleration.
106
5.4.2 Theoretical prediction of DEP spectrum
Since the particles used in the experiments are different in dimensions, their surface
conductivities induced in DI water were not the same according to ref. [3]. Using the same
method that was presented in Section 5.2.1, the surface conductivities for 1 and 3 µm polystyrene
particles were calculated to be 1.20 and 1.07 mS/m, respectively. As illustrated in Figure 5.6, the
DEP spectra for both sizes of particles indicate that the crossover frequencies for 1 and 3 µm
particles are approximately 200 and 100 kHz, respectively. Since positive DEP forces are
employed to concentrate micro particles at the centre of a microchannel, the frequency of the
applied electric filed should be lower than the crossover frequency.
The magnitude of DEP forces are proportional to the volume of particles and can be illustrated by
the values of r3×Re(fCM). As shown in Figure 5.7, the values of r3
×Re(fCM) suggest that the DEP
forces on 3 µm particles are much larger than 1 µm particles.
Figure 5.6: The calculated real part of CM factor vs frequency for both 1 and 3 µm polystyrene
particles [28].
107
Figure 5.7: The values of r3×Re(fCM) for both 1 and 3 µm polystyrene particles at different
frequencies [28].
5.4.3 Experimental results
5.4.3.1 DEP Manipulation using micro tip electrode array
The micro-tip electrode arrays were proved to be capable of manipulating polystyrene particles in
a stationary liquid; therefore, the author decided to use such a design for particles manipulation in
flowing liquid environment. 3 µm particles were applied to the system as they can be affected by
larger DEP forces. During the experiments, the flow rate of liquid suspension was set to be 1
µL/min (corresponding to a fluid velocity of 200 µm/s), as any higher flow rate resulted in
significant decrease in trapping efficiency. The DEP behaviors of polystyrene microparticles
obtained with AC potentials of 30, 20, 10 and 1 V (peak-to-peak) at the frequency of 100 kHz are
illustrated in Figure 5.8.
108
Figure 5.8: The DEP concentration of micro particles with a diameter of 3 µm at the flow rate of
1 µL/min using AC potentials at 100 kHz with the magnitude of (A) 30 V peak-to-peak, (B) 20 V
peak-to-peak, (C) 10 V peak-to-peak and (D) 1 V peak-to-peak [28].
As shown in Figure 5.8-A, particles were found to be trapped by micro-tip electrode pairs with a
limited trapping rate when the AC potential is at its maximum value. When the potential was
reduced to 20 V, releasing of particles can be observed as shown in Figure 5.8-B; however, the
released particles were scattered. When the potential was further reduced to 10 V peak-to-peak
(Figure 5.8-C), a thick particles stream was observed but lacked focus. In Figure 5.8-D, most of
particles were released when the potential was reduced to 1 V peak-to-peak; individual particles
were found remaining on the electrode tips due to the extremely sharp field gradients produced at
such locations. Therefore, it is evidenced that micro-tip electrode configuration is not suitable for
particles manipulation within continuously flowing liquid environment.
5.4.3.2 DEP Manipulation using curved electrode array
Since the manipulation of polystyrene microparticles using micro-tip electrode array was not
successful, the author modified and altered the micro-tip electrodes into curved shape. By using
109
curved electrode array, the DEP behaviors of 1 and 3 µm polystyrene microparticles were both
investigated with applied AC potential of 30 V (peak-to-peak) at of frequencies of 20 MHz,
1 MHz, 500 kHz and 100 kHz. During the experiments, the liquid flow rate was set to be 1
µL/min to ensure well focused particle streams.
As shown in Figure 5.9, the DEP behaviors of 1 µm polystyrene particles under the described
experimental conditions are illustrated. Figure 5.9-A shows a typical negative DEP behavior of
polystyrene microparticles at 20 MHz: the particles were repelled from the central line of the
microchannel (region with high electric field gradient). In Figure 5.9-B, the particles were
partially trapped and a stream of particles was formed between the electrode tips. It is suggested
that particles were experiencing positive DEP forces at the frequency of 1 MHz as trapping
process was observed; however, such positive DEP forces are clearly not strong enough to hold
all the particles against the liquid flow and hence resulted in the observed particle stream along
the central line. Figure 5.9-C illustrates trapped micro particles which were accumulating in the
gap between curved electrode pairs, indicating an enhancement of the positive DEP forces while
the frequency dropped from 1 MHz to 500 kHz. It seems the trapping forces has reached its
maximum at 100 kHz, since the accumulating process is not observed in Figure 5.9-D.
Additionally, according to the DEP behaviors illustrated by Figure 5.9-A and B, the crossover
frequency should be lower than 20 MHz but higher than 1 MHz. After tuning the frequency of the
applied AC potential, the crossover frequency is measured to be approximately 5 MHz, which
does not match the calculated results in Section 5.4.1. This mismatching is mainly due to the
misjudgments in predicting the value of surface conductance, as the polystyrene microparticles
used in this work are with red fluorescing coatings which have a higher surface conductance
compared to plain polystyrene microparticles.
110
Figure 5.9: The DEP behaviour of micro particles with a diameter of 1 µm for AC potentials of
30 V peak-to-peak at the frequencies of (A) 20 MHz, (B) 1 MHz, (C) 500 kHz and (D) 100 kHz
[28].
As shown in Figure 5.10, the 3 µm polystyrene particles were behaving similarly as 1 µm
particles. A typical negative DEP behavior was observed at 20 MHz (Figure 5.10-A). At 1 MHz
(Figure 5.10-B), the particles were uniformly distributed in the channel indicating weak DEP
forces were produced at such a frequency; therefore, the crossover frequency for 3 µm particles is
believed to be very close to 1 MHz. In Figure 5.10-C and D, enhancement of the positive DEP
forces were also observed while the frequency of the applied AC potential dropped from 1 MHz
to 500 and 100 kHz.
In this case, the curved electrode array is practically evidenced to be capable of manipulating
polystyrene particles in flowing liquid environments and can be employed for concentrating such
particles into focused streams.
111
Figure 5.10: The DEP behaviour of micro particles with a diameter of 3 µm for AC potentials of
30 V peak-to-peak at the frequencies of (A) 20 MHz, (B) 1 MHz, (C) 500 kHz and (D) 100 kHz
[28].
5.5 The controllable focusing of micro particles
In this section, the controllable DEP focusing of polystyrene microparticles (1 and 3 µm in
diameters) were demonstrated. AC potentials with the magnitudes from 1 to 30 V (peak-to-peak)
at the optimum frequency (100 kHz) were applied across the curved electrode array to produce
positive DEP forces; liquid flow rates from 1 to 10 µL/min were also applied to the system to
evaluate their influences. The content which is included in this section was presented at the 2010
International Conference on Micro/nano scaled Heat and Mass Transfer (MNHMT) [28].
5.5.1 DEP focusing of 1 µm particles
1 µm polystyrene particles were first adapted for the manipulation; the magnitude of AC potential
was first varied from 1 to 30 V peak-to-peak to characterize the effect of DEP forces at the flow
112
rate of 5 µL/min (constant hydrodynamic force). After that, the flow rate of liquid suspension was
varied from 1 to 10 µLl/min to characterize the effect of hydrodynamic force when a constant
magnitude of AC potential (30 V peak-to-peak) was applied across the electrodes.
Figure 5.11: The DEP concentration of micro particles with a diameter of 1 µm at the flow rate of
5 µL/min using AC potentials (100 kHz) with the magnitude of (A) 30 V peak-to-peak, (B) 20 V
peak-to-peak, (C) 10 V peak-to-peak and (D) 1 V peak-to-peak [28].
In Figure 5.11-A, the particles were held by the electrodes when an AC potential of 30 V (peak-
to-peak) was applied across the electrodes. When the AC potential was decreased to 20 V (peak-
to-peak), a thin stream of particles (approximately 10 µm in diameter) was observed as shown in
Figure 5.11-B. This is because the DEP forces generated by AC potential of 20 V (peak-to-peak)
were not strong enough to hold all of the polystyrene particles. In Figure 5.11-C, the thickness of
particle stream increased to approximately 20 µm due to the further weakened positive DEP
forces at 10 V (peak-to-peak). In Figure 5.11-D, when the AC potential was set to be 1V peak-to-
peak all the particles were released as the DEP forces were close to zero. Since a constant liquid
flow rate was used in this set of experiments, the hydrodynamic drag force induced in the
microfluidic channel was also constant. Therefore, the focused particle streams were formed
113
when the positive DEP forces were weaker than the hydrodynamic drag forces, the diameter of
such focused particle streams can be adjusted by tuning the magnitude of the applied AC
potential.
Figure 5.12: The DEP concentration of micro particles with a diameter of 1 µm using an AC
potential (100 kHz) with the magnitude of 30 V peak-to-peak at the flow rate of (A) 1 µL/min,
(B) 5 µL/min, (C) 8 µL/min and (D) 10 µL/min [28].
In Figure 5.12 the influences of liquid flow rates were investigated when a constant AC potential
(100 kHz, 30 V peak-to-peak) was applied. As shown in Figure 5.12-A and B, the particles were
held steadily by the electrode pairs when the flow rate was slower than 5 µL/min. In Figure 5.12-
C, when the flow rate was increased to 8 µL/min, a thin stream of particles was observed. Such
particle stream increased to approximately 5 µm in diameter when the flow rate was set to be
10 µL/min as shown in Figure 5.12-D. Since a constant AC potential was used in such set of
experiments, the magnitude of the generated positive DEP forces were stable. When the flow rate
was increased from 1 to 10 µL/min, the hydrodynamic drag force produced by liquid flow
increased; at the moment that the hydrodynamic drag forces exceeded the DEP trapping forces,
the polystyrene microparticles were forced to be released and formed focused particle streams.
114
5.5. 2 DEP focusing of 3 µm particles
Same strategies were used to characterize the controllable focusing of 3 µm polystyrene particles.
First, the flow rate of the liquid suspension was set to be 5 µL/min, AC potentials (100 kHz, 1-
30 V peak-to-peak) were applied across the electrodes to change the magnitude of DEP forces,
the concentrated particle streams were observed as shown in Figure 5.13. Then, a constant AC
potential (100 kHz, 30 V peak-to-peak) was applied, the liquid flow rate was increased from
1 µL/min to 15 µL/min to characterize the effect of hydrodynamic drag forces (Figure 5.14).
Figure 5.13: The DEP concentration of micro particles with a diameter of 3 µm at the flow rate of
5 µL/min using AC potentials (100 kHz) with the magnitude of (A) 30 V peak-to-peak, (B) 20 V
peak-to-peak, (C) 10 V peak-to-peak and (D) 1 V peak-to-peak [28].
115
Figure 5.14: The DEP concentration of micro particles with a diameter of 3 µm using an AC
potential (100 kHz) with the magnitude of 30 V peak-to-peak at the flow rate of (A) 1 µL/min,
(B) 5 µL/min, (C) 10 µL/min and (D) 15 µL/min [28].
Figure 5.13 illustrates a similar trend in the change of focused particle streams when the AC
potential is decreased from 30 V to 1 V peak-to-peak. Interestingly, at 1 V peak-to-peak, two
bands of 3 µm particles can be observed along the curved electrode tip-pairs (Figure 5.13-D).
Also, as shown in Figure 5.14-A, B and C, no particle was observed to leave the trapping area
when the liquid flow is increased from 1 µL/min to 10 µL/min; until the flow rate is further
increased to 15 µL/min, a thin stream of particles occurred. Therefore, it is very clear that the
DEP forces that affected the 3 µm particles were much stronger than those affected the 1 µm
particles.
Based on the results demonstrated in this section, the thickness (width) of the concentrated micro
particle stream can be controlled by a combination of two experimental parameters: the
magnitude of the applied electric field and the flow rate of liquid suspension. It is also suggested
that the particles with larger dimensions can be manipulated more effectively and efficiently. Due
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to this reason, it is also possible to sort particles with different dimensions using the developed
DEP-microfluidic system.
5.6 Optical waveguide based on suspended sub-
micron scaled SiO2 particles
As mentioned in Chapter 1, the dielectrophoretically focused particle streams can be employed as
the building blocks of configurable optical devices. After the controllable focusing of polystyrene
particles was successfully demonstrated using curved electrode array, further developments of the
device was carried out to create a novel system which utilized dielectrophoretically concentrated
sub-micron particles to operate as an optical waveguide.
In such a work, SiO2 (silica) particles (230 and 450 nm in diameters) were applied to the DEP-
microfluidic system and concentrated into focused particle streams. Such focused particle streams
were employed as the core of the optical waveguide due to its higher refractive index compared to
DI water. The performance of the system is analyzed by illuminating the channel with
incandescent and laser light sources, while varying parameters including nanoparticles size, flow
rate, amplitude and the frequency of the applied electrical signal.
Such a work was completed in collaboration with Mr Aminuddin Kayani, from the School of
Electrical and Computer Engineering, RMIT University, Melbourne, Australia and was published
in the journal “Electrophoresis” [32].
5.6.1 Experimental setup
As shown in Figure 5.15, curved electrode array was employed for generating positive DEP
forces and hence concentrate silica particles into focused streams. The multimode fiber used to
couple light into the channel has a 62.5 mm core/125 mm cladding diameter with a numerical
aperture of 0.22. The light detected at the optically transparent output window was imaged
through 5× objective lens using a camera (Philips SPC900NC). The optical source was a Class 2,
1 mW, 635 nm wavelength laser (OZ Optics). A 3 CC syringe (Luer lock Becton D Plastic) was
mounted on a syringe pump (Harvard Apparatus PHD2000) in refill mode to provide liquid flows
(5-10 µL/m) within the microfluidic channel. A function generator (ETABOR electronics) was
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employed to apply the AC potentials (10-30 V peak-to-peak, 100 kHz-5 MHz) across the
electrodes.
Figure 5.15: The schematic of experimental setup [32].
5.6.2 Optical waveguiding response
To obtain the optical waveguiding responses, the laser source was coupled to the microfluidic
channel via a fiber recess in the PDMS using a multimode optical fiber. The optical profiles in the
output were investigated using different particle sizes at various frequencies and flow rates for
different applied AC potentials. As shown in Figure 5.16 the light profiles have provided strong
evidence that DEP forces can change waveguiding properties of the suspended optical waveguide.
Figure 5.16-A and C show the output of the waveguide without DEP forces for 230 and 450 nm
particles, respectively. When the flow rate was set to be 10 mL/min, the scattered light appears to
be well dispersed in the channel indicating that the particles are homogenously dispersed. The
output optical profile without the applied DEP force was fairly rectangular shaped resembling the
geometry of the output of the microfluidic channel.
After applying the AC potential of 30 V peak-to-peak across the electrodes, waveguiding (for
230-nm particles) and scattering (for 450-nm particles) phenomena were observed. The 230 nm
silica particles appeared to show waveguiding properties for frequencies in the range of 500 kHz
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to 2 MHz as the beam was concentrated at the center of the output window (Figure 5.16-B).
However, with the same experimental condition, the behavior of the system with 450-nm
nanoparticles was quite different. It appeared that no light could pass through the center of the
microfluidic channel as the profile was separated in the center by a shadow due to low light
intensity (Figure 5.16-D); such a phenomenon was well observed for all frequencies higher than
1 MHz. In addition, at low flow rates, the assembly of nanoparticles did not resemble a smooth
waveguide but instead produced an array of particle barricades near the tip of electrodes that
scattered the applied light.
Figure 5.16: 635 nm laser output observed when silica nanoparticles were applied to the system at
the flow rate of 10 mL/min: 230 nm nanoparticles (A) before and (B) after applying the DEP
force; 450-nm nanoparticles (C) before and (D) after applying the DEP force [32].
5.6.3 Discussions
At the applied conditions for both 230- and 450-nm cases, when AC potential was increased, the
DEP force focused the nanoparticles forming a dense nanoparticles stream in the centerline.
Figure 5.17 shows an illustration of how the laser light is scattered or guided as a function of
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particles’ separations and dimensions. The center of the channel acts as the core of the optical
waveguide while the adjacent streams are function as cladding bearing a lower concentration of
nanoparticles.
The significant scattering, which was observed when the 450 nm particles are concentrated by the
DEP forces, can be explained qualitatively by noting that when the particles are in contact, their
inter-particle separation is larger than a quarter of the wavelength. In this case, the light will
diffract strongly around these particles and thus strong scattering will be observed. Conversely,
when the 230 nm particles are in direct contact, the inter-particle spacing is slightly less than a
quarter of the wavelength and the light transmission through the particles is possible.
Figure 5.17: Scattering and waveguiding due to the DEP focusing of: (A) 230 nm particles, (B)
450 nm particles [32].
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To quantitatively assess the performance of the developed optofluidic waveguide, the optical
mode intensity profiles was analyzed. As shown in Figure 5.18 and Figure 5.19, an obvious
increase in light intensity (in the center of the channel) can be observed when dielectrophoresis
was applied to the system. This suggests that the suspended silica particles were concentrated in
the center of the channel and enabled the light transmission. In addition, the shifts of intensity
peaks were observed in Figure 5.19 for different DEP conditions indicating that the developed
optofluidic system can be employed in optical sensing of nano and submicron scaled particles.
Figure 5.18: 2-D Colored intensity profiles of the optofluidics waveguide based on suspended
silica particles activated using different AC potentials. The insets are 3-D Colored intensity
profiles.
Figure 5.19: Intensity profiles for the optofluidics waveguide along (a) vertical (b) horizontal,
cross hair axes.
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5.7 Other collaborative tasks
In addition to the tasks that were presented in previous sections, the author also participated in
several tasks conducted in collaboration with Mr Khashayar Khoshmanesh, from Centre for
Intelligent Systems Research, Deakin University, Australia. The outcomes were published in the
journals: “Microfluidics and Nanofluidics” and “Electrophoresis” [31, 33, 34].
5.7.1 DEP sorting of polystyrene particles in different dimensions
There are extensive reports on the sorting of microparticles using DEP-microfluidic systems. For
instance, the DEP microchip introduced in Ref. [35] is one of the first reported designs applied to
separate microparticles. The DEP forces were provided by parallel microelectrodes fabricated on
the bottom surface of a microchannel. Particles were levitated due to the negative DEP forces and
were lifted until reaching an equilibrium height, where the DEP forces, gravity forces and
buoyancy forces reached a balance. Using this system, polystyrene microparticles of different
dimensions and surface coatings were levitated to different heights and subsequently travelled at
different velocities due to the parabolic velocity profile governing in the flow. In addtion, the
DEP system introduced in Ref. [36] utilized planer microelectrodes to trap single microparticles
against high flow rates. Arrays of square cages and straight microelectrodes were patterned on the
bottom surface of a microchannel. The system utilized the negative DEP forces to hold the
particles inside the square cages enabling the user to apply high flow rates to flush away the
untrapped particles at the end of experiment.
The aim of this study was to characterize a DEP system for the separation of microparticles in
liquid flow, using polystyrene microparticles. In this work, curved electrode was employed to sort
polystyrene particles of 1, 5 and 12 µm in diameters (Kisker, PPs.-1.0, PPs.-5.0 and PPs.-12.0).
As shown in Figure 5.20, the device consists of a microchannel that was attached to a glass
substrate. The microchannel was fabricated from PDMS with a width of 1000 µm and a height of
80 µm. The substrate was a glass slide of 75×25.5×1mm that supported five pairs of
microelectrodes on its surface. The microelectrodes had a curved shape with the trace width of 50
µm. The minimum gap between the opposite microelectrodes was 40 µm at the midline of the
microchannel, while the spacing between the sequential microelectrode pairs along the
microchannel was 1000 µm. AC potential of 30 V peak-to-peak was applied across the electrode
pairs, the frequency was set to be 5 MHz to ensure the generation of negative DEP forces. The
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original particle suspensions were first diluted in DI water in a volume ratio of 1:20
(particles/water) before the experiments.
Figure 5.20: The layout of the designed DEP-microfluidic device [31].
The developed system was applied to separate 1 and 5 µm particles suspended in the flow. Equal
volumes of 1 and 5 µm particles were diluted in DI water in a 1:20 (particle/water) volumetric
ratio. The separation performance of the system with a flow rate of 1 µL/min is shown in Figure
5.21. The particles were uniformly distributed in the flow far from the microelectrodes. Reaching
the first pair of microelectrodes, particles joined each other to form pearl chains and then
migrated towards the sidewalls, as shown in Figure 5.21-A. This is because the particles were
self-polarized under the influence of electric field and attracted by each other. As shown in Figure
5.21-B and C, both types of particles demonstrated negative DEP behaviors and were repelled
from the microelectrodes. However, the 5 µm particles experienced a stronger repelling force due
to their larger dimensions and were pushed towards the sidewalls more aggressively. The
particles remained in their positions even after leaving the last pairs of microelectrodes and this
trend continued throughout the microchannel, as shown in Figure 5.21-D. 1 and 5 µm particles
were formed into streams at the locations that are approximately 80 and 160 µm off the centerline,
respectively; therefore, the sorting of 1 and 5 µm polystyrene particles was successful.
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Figure 5.21: The sorting processes of 1and 5 µm polystyrene particles at different locations of the
microfluidic channel: (A) at the first microelectrode pair, (B) at third microelectrode pair, (C) at
the last microelectrode pair and (D) after the last microelectrode pair [31].
The successful separation of 5 and 12 mm particles is depicted in Figure 5.22. Figure 5.22-A
corresponds to the flow rate of 0.5 µL/min. The particles were separated across the microchannel
according to their dimensions. Thin streams of 5 and 12 mm particles were formed at the
locations that are approximately 130 and 180 µm off the centerline, respectively. Since the size
ratio between 12 and 5 µm particles is smaller than the ratio between 5 and 1 µm particles, it was
more challenging to sort 12 and 5 µm particles. Figure 5.22-B corresponds to the flow rate of
2 µL/min. Again, the particles were separated across the microchannel according to their
dimensions. Thin strips of 5 and 12 mm particles formed approximately 95 and 145 µm off the
centerline, respectively. This indicates the increasing of the flow rate strengthened the
hydrodynamic drag forces and enabled particles to penetrate the DEP barriers between the
microelectrodes more easily, therefore, resulted in a decreased in the sorting efficiency.
This work was published in journal “Electrophoresis” [31].
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Figure 5.22: The separation of 5 and 12 mm particles across the microchannel with the flow rate
of (A) 0.5 µL/min and (B) 2 µL/min [31].
5.7.2 Dielectrophoretic-activated cell sorter based on curved
microelectrodes
Since the curved electrodes have been proved to capable of sorting particles in different
dimensions, the system was then employed for sorting dead and live cells. As shown in Figure
5.23, in addition to the curved electrodes, five pairs of boomerang shaped electrodes were
patterned on the glass substrate. The function of such additional electrodes is to assess the
performance of the curved electrodes by capturing untrapped particles. The sorting efficiency of
the developed system was evaluated through microscopic cell counting analysis.
Instant-dried yeast powder (Tandaco, New South Wales, Australia) was used as the source of
Saccharomyces cerevisiae yeast cells. In order to produce the live yeast solution, 150 mg of yeast
powder was mixed with 40 ml of DI water and kept in a water bath for 30 min. Alternatively, to
produce the dead yeast solution, 150 mg of yeast powder was mixed with 40 ml of a 50-50% DI
water–methanol solution and kept in a water bath for 45 min. In order to perform the separation
of the live cells from the dead ones, a mixture of 50–50% live–dead yeast cells with a total
concentration of 5×107 cells/ml was prepared. AC potentials of 30 V peak-to-peak at the
frequency of 20 MHz were applied across the electrodes to generate negative DEP forces. The
live cells were separated from dead ones due to the large difference in their dielectric properties.
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Figure 5.23: The layout of the designed microelectrodes with (A): curved configuration for
sorting array, and (B–C): boomerang shaped configuration for assessing array [33].
Figure 5.24 shows the sorting processes of the live and dead yeast cells at different locations of
the microfluidic channel. The flow rate was set to be 0.75 µL/min, while the magnitude and
frequency of the applied AC potential were 30 Vp-p and 20 MHz, respectively. For more
clarification, the live and dead cells are marked with filled and hollow arrows, respectively.
In the sorting array, the live cells were trapped by the positive DEP forces and accumulated along
the first pair of microelectrodes (Figure 5.24-A); most of the live cells that were moving close to
the centerline and trapped by the upstream pairs. Therefore, the density of trapped cells decreased
significantly at the last pair of microelectrodes, as shown in Figure 5.24-B. Alternatively, the
dead cells were repelled by the negative DEP forces. These cells were decelerated along the
microelectrodes and pushed towards the sidewalls to create an almost cell-free region along the
centerline, as shown in Figure 5.24-B.
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Figure 5.24: The sorting processes of the live and dead cells by the DEP system observed at
different locations of the microfluidic channel: (A): beginning of the sorting array, (B) end of the
sorting array, (C) the first electrode pair of the assessing array, and (D) after the first electrode
pair of the assessing array [33].
After 5 min, when the system reached a stable condition, the assessing array was activated with
the same AC potentials. The live cells that remained in the flow were accumulated between the
boomeranged pairs due to the positive DEP forces. As shown in Figure 5.24-C, the live cells were
only trapped by the assessing electrode array close to the sidewalls, this indicates that all the live
cells moving close to the centerline had been trapped by the sorting array. The density of the
trapped cells over the second array increased very slowly with respect to time, which
corresponded to the effective trapping performance of the sorting array. Alternatively, the dead
cells were repelled from the boomerang-shaped microelectrodes due to the influences of the
negative DEP forces and exhibited two different responses according to their locations across the
microchannel. The dead cells that were moving close to the sidewalls were blocked by the
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assessing electrode pairs as the negative DEP forces were function as a barrier (Figure 5.24-C).
For the dead cells, which were moving close to the centerline, were levitated by the negative DEP
forces and passed through the microelectrodes due to the higher flow rate at the center of the
microfluidic channel. Therefore, a portion of dead cells, which had passed through the first pair
before activating the assessing array, were retained behind the consequent boomerang-shaped
pairs (Figure 5.24-D).
The separation efficiency of the system was further evaluated by conducting Trypan blue
exclusion assay. In this process, first, the assessing array was inactivated and the outlet flow of
the system, which was supposed to contain dead cells, was collected in a syringe (BD plastic, 1Ø).
Second, the AC signal was inactivated and the live cells trapped by the microelectrodes were
released and collected in another syringe. Third, the samples were introduced to the wells of a
standard cell counting chamber (Neubauer hemocytometer) using a pipet. The cover slip of the
chamber was divided into nine grids of 0.04 mm2. The number of non-stained (viable) and stained
(non-viable) cells was counted in five different grids of the cover slip, under the microscope to
calculate the percentage of the stained cells. According to the results, approximately 85% of the
live sample cells were not stained (viable) while approximately 75% of the dead sample cells
were stained (non-viable). Combining those two figures, the overall separation efficiency of the
system was estimated as ~80%.
This work was published in the journal “Microfluidics and Nanofluidics” [33].
5.7.3 Particle trapping using dielectrophoretically patterned
carbon nanotubes
This study presents the dielectrophoretic (DEP) assembly of nitrogen doped carbon nanotubes
with Pt nanocomposite (NCNTs/Pt) between curved microelectrodes for the purpose of trapping
polystyrene microparticles within a microfluidic system. Under normal conditions, polystyrene
particles exhibit negative DEP behavior and are repelled from microelectrodes. Interestingly, the
addition of NCNTs/Pt to the system alters this situation in two ways: first, they coat the surface of
particles and change their dielectric properties to exhibit positive DEP behavior; second, the
assembled NCNTs/Pt are highly conductive and serve as extensions to the microelectrodes after
the deposition. The established NCNTs/Pt network can effectively trap the NCNTs/Pt-coated
particles, since they cover a large portion of the microchannel bottom surface and also create a
much stronger electric field than the primary microelectrodes.
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In this work, the curved electrodes, which were illustrated in Figure 5.20, was employed in the
experiments. Due to its large electric conductivity, NCNTs/Pt were utilized to cover the
polystyrene particles and hence to change the overall dielectric properties of such particles. AC
potential of 30 V (peak-to-peak) at the frequency of 20 MHz was applied across the electrodes to
produce the DEP forces.
To evaluate the DEP behaviors of the coated particles, the DEP force induced on polystyrene
particles, Pt/N-CNTs and Pt/N-CNT-coated particles was calculated, as shown in Figure 5.25.
The DEP force imposed on microparticles fluctuated between positive and negative values, with a
crossover frequency of approximately 55 kHz, above which the particles exhibited negative DEP
behavior (curve a). Alternatively, the Pt/N-CNTs exhibited positive DEP response at all
frequencies due to their high conductivity and permittivity but experienced different DEP forces
according to their dielectric properties (curves b-f). Finally, the Pt/N-CNT-coated particles
experience positive DEP forces for the investigated frequencies due to their large overall
conductivity (curves g and h).
Figure 5.25: The variations of scaled DEP forces induced on polystyrene particles, Pt/N-CNTs
and Pt/N-CNT-coated particles at different frequencies.[34].
The original NCNT/Pt suspensions (10 mg/mL) was diluted with DI water in a 1:20 volume ratio
and ultrasonicated in a Branson 1200W sonicator for 30 min to break up large clusters formed in
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the solution. The polystyrene particles (6 µm in diameter, Kisker, PPs-6.0) were first diluted with
DI water in a volume ratio of 1:15 (particles/water) and ultrasonicated for 15 min to make a
homogenous suspension of the particles. The two suspension were mixed in a 1:1 volume ratio
for experimental uses.
To perform the trapping of polystyrene microparticles (6 µm in diameter), NCNTs/Pt were pre-
patterned using the AC potential. After such nanotubes have established a network between the
curved electrodes, as shown in Figure 5.26, the mixed suspension of NCNTs/Pt and polystyrene
microparticles was applied to the system. As illustrated in Figure 5.27, polystyrene microparticles
were trapped using the electric field produced by the curved electrodes at the frequency of
20 MHz. According to the studies which were presented in Section 5.7.1, such particles should
experience negative DEP forces at such a high frequency. The trapping of polystyrene particles,
which was achieved suing such a frequency, can be explained by a change in the dielectric
properties due to the coating of NCNTs/Pt on the particles’ surfaces. Such an assumption is
evidenced by the SEM images illustrated in Figure 5.28.
This work was published in the journal “Electrophoresis” [34].
Figure 5.26: The pre-patterned NCNTs/Pt network between the curved electrodes [34].
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Figure 5.27: The response of particles to the DEP system. (A): polystyrene particles were
patterned between the microelectrodes under the negative DEP force while (B): the Pt/N-CNT-
coated particles were patterned along the microelectrodes under the positive DEP force.[34].
Figure 5.28: The SEM images for the trapped polystyrene particles [34].
5.7.4 Size based separation of polystyrene particles using
dielectrophoresis
In this section, the DEP separation of polystyrene particles of different dimensions (1, 6, and
15 µm) was demonstrated using the curved electrodes. The trapping efficiency of the system was
qualitatively and quantitatively investigated for different particles using different AC potentials.
Qualitative assessing
131
As shown in Figure 5.29, Figure 5.30 and Figure 5.31, the DEP behaviors of polystyrene particles
in different dimensions at different DEP conditions are illustrated.
Figure 5.29: Separation of 1, 6, and 15µm particles at 20 MHz (A)-(C): the response of particles
at the first, middle, and last microelectrode pairs and (D): the schematic dynamics of particles
[37].
In Figure 5.29, all the particles were observed to experience negative DEP forces and were
repelled from the microelectrodes. The 1 µm particles were pushed toward the sidewalls and
levitated by the negative DEP forces (Figure 5.29-A1 and D: Path 2). After passing the
consequent pairs, the particles reached a stable condition and marched as bright strips along the
both sides of the centerline (Figure 5.29-B, C1 and D: Path 5). Alternatively, the 6 µm particles
were pushed toward the sidewalls more insistently since they have larger radius and hence
experience stronger DEP forces (in y direction). These particles had a lower levitation height than
their 1 µm counterparts since they were farther from the microelectrode tips and experienced a
weaker DEP force in z direction. After passing the next electrode pairs, the particles reached a
stable condition and marched as thick strips very close to the sidewalls (Figure 5.29-B, C1 and D:
Path 6). Finally, the 15 µm particles were initially repelled toward the sidewalls and were
moderately levitated to a limited height due to their large dimension. The hydrodynamic drag
force, which pushed the particles along the microchannel, was very weak at this height and could
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hardly haul the particles. Moreover, the DEP forces in the x direction, which opposed to the
movement of the particles, were considerable at this low height (close to electrode surface) and
further decelerated the particles. Under this combination, the 15 µm particles stayed for a long
time behind the second microelectrode pair (Figure 5.29-A1and D: Path 7). As a result, the
density of the 15 µm particles decreased gradually along the microchannel since most of them
had been retained by the upstream microelectrodes (Figure 5.29-B and C1). Hence, the 15 µm
particles can be filtered at the frequency of 20 MHz.
Figure 5.30: Separation of 1, 6, and 15µm particles at 200 kHz (A)-(C): the response of particles
at the first, middle, and last microelectrode pairs and (D): the schematic dynamics of particles
[37].
At 200 kHz, as shown in Figure 5.30, the 1 µm particles experienced positive DEP forces and
were trapped along the microelectrodes; most of them were accumulated at the tips due to the
presence of strong electric field gradients (Figure 5.30-A and D: Path 2). The particles, which still
remained in the flow, gradually lost their heights under the positive DEP force (in z direction)
until being trapped by the next electrode pairs (Figure 5.30-B and D: Path 5). As a result, the
density of trapped particles decreased consistently at the consequent electrode pairs since most of
them had been trapped by the upstream pairs. Alternatively, the 6 µm particles exhibited a similar
response as in Figure 5.29, they were repelled toward the sidewalls and simultaneously levitated
(Figure 5.30A and D: Path 3). The consequent microelectrode pairs stabilized the location of such
particles and led to the formation of thick strips close to the sidewalls (Figure 5.30-B and D: Path
6). Finally, the 15 µm particles also exhibited a similar response to Figure 5.29; most of them
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were retained by the microelectrodes (Figure 5.30- B). Therefore, both 1 and 15 µm particles
were filtered at the frequency of 200 kHz.
Figure 5.31: Separation of 1, 6, and 15µm particles at 100 kHz (A)-(C): the response of particles
at the first, middle, and last microelectrode pairs and (D): the schematic dynamics of particles
[37].
At 100 kHz, as shown in Figure 5.31, both 1 and 6 µm particles experienced positive DEP forces
and were trapped by the microelectrodes. The 6 µm were large enough to cover the area between
the pair while the 1 µm particles were interwoven within the larger particles and could not be
observed distinguishably (Figure 5.31-A and D: Path 2). The 15 µm particles still demonstrated
negative DEP behavior and were retained by the microelectrodes (Figure 5.31-B1). Therefore, the
particles of all dimensions were filtered from the fluid system at the frequency of 100 kHz.
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Quantitative assessing
The system was further evaluated quantitatively in terms of its trapping efficiency defined as
(ninlet − noutlet / ninlet) × 100% (n is the number of target particle), yield defined as (noutlet /ninlet) ×
100%, and enrichment factor defined as (foutlet / finlet) × 100% (f is the fraction of target particle
with respect to all particles). The above evaluations were conducted using a standard cell
counting chamber slide (Neubauer hemocytometer), as follows. First, the inlet suspension was
diluted and applied to the chamber of hemocytometer. Next, the average number of 1, 6, and 15
µm particles per milliliter of inlet (ninlet) was obtained by counting the particles in five squares of
the slide under the microscope, to be used as the reference. Then, the outlet of the DEP system
was collected in a syringe, diluted, and applied to the hemocytometer to be counted. This
procedure was repeated three times and the data presented as mean ± standard error.
Figure 5.32: The trapping efficiency of particles obtained at different frequencies [37].
At 20 MHz, the 15 µm particles were retained by microelectrodes (Figure 5.29) and therefore,
were considered as the target particles. In doing so, the average number of released 15 µm
particles per milliliter of the outlet (noutlet) was counted, revealing the trapping efficiency of these
particles as 88 ± 4%. Consequently, the retained 15 µm particles were released and applied to the
hemocytometer, revealing the yield and enrichment factor of these particles as 86 ± 4% and 21,
respectively. Alternatively, at 200 kHz, the 1 and 15 µm particles were trapped and retained,
respectively, by microelectrodes (Figure 5.30) and their trapping efficiencies were evaluated as
79 ± 3% and 85 ± 4%, respectively. The 6 µm particles freely passed the microelectrodes, were
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collected at the outlet with a yield, and enrichment factor of 93 ± 3% and 2.5, respectively.
Finally, at 100 kHz, the 1, 6, and 15 µm particles were trapped, trapped, and retained,
respectively, by microelectrodes (Figure 5.31) and their trapping efficiencies were evaluated as
82 ± 4%, 86 ± 5%, and 84 ± 3%, respectively, as given in Figure 5.32. However, the yield and
enrichment factor of particles could not be evaluated at this frequency, since the particles were
mixed.
The work was published in Journal of Applied Physics [37].
5.8 Summary
In this chapter, the manipulations of polystyrene microparticles using DEP-microfluidic systems
were presented. The DEP behaviors of polystyrene microparticles were investigated in both
stationary and continuously flowing liquid environments. In addition, the separation of
polystyrene microparticles from MWCNTs was successfully demonstrated using the developed
DEP-microfluidic system. In this work, the significant difference in conductivities was utilized to
distinguish polystyrene microparticles from MWCNTs using AC electric fields.
The controllable focusing of polystyrene particles was also presented. Curved electrode array was
used in such an application since micro-tip electrode array was not capable of particle
manipulation in liquid flows. Different experimental conditions, including liquid the flow rates
and AC potentials, were applied to control the width of the concentrated particle streams. In
addition, since 3 µm particles can be more efficiently manipulated than 1 µm particles using DEP
forces, the designed system can also be used in sorting particles with different dimensions.
A novel system, which utilized dielectrophoretically concentrated sub-micron particles to operate
as an optical waveguide, was demonstrated in this PhD research. Silica particles with diameters of
230 and 450 nm were concentrated into focused streams using dielectrophoresis. A laser (635 nm
in wavelength) was employed to evaluate the waveguiding properties of the suspended optical
waveguide. The results suggest that the laser transmission is possible when 230 nm silica
particles were adopted in the system because the inter particle spacing is slightly smaller than a
quarter of the laser wavelength.
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Three additional publications, which were conducted by the author in collaboration, are also
presented in this chapter. In these works, the curved microelectrodes were employed for sorting
polystyrene particles with different dimensions and live/dead cells. The sorting efficiency of the
device in separating live and dead cells was quantitatively measured. In addition, the
dielectrophoretically pre-patterned nitrogen doped carbon nanotubes with Pt nano composite were
employed as the extensions of the electrodes to capture polystyrene microparticles. It is suggested
that, the trapping effect can be observed even at the frequency of 20 MHz. Since polystyrene
particles are generally repelled by DEP forces at such a high frequency, the trapping of such
particles is achieved due to a change in the dielectric properties caused by the coating of carbon
nanotubes at the particle’s surfaces.
137
Reference
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Chapter 6
Development of Nanostructured Electronic
Devices
6.1 Introduction
In this chapter, the development of nanostructured electronic devices using dielectrophoresis and
other material deposition techniques is presented. To perform the DEP deposition of
nanostructures, the author first investigated the alignment and assembling of multi-walled carbon
nanotubes (MWCNTs) using AC electric fields.
141
In order to evaluate the functionality of nanostructured devices fabricated on the designed
microelectrodes and the catalytic properties of metal nanoparticles on carbon surfaces, the author
developed a conductometric sensor based on graphene with Pd nanoparticles. In this work, an
interaction between metal nanoparticles and sp2 carbon network was evidenced by Raman
spectroscopy. The results suggested that the adhesion of Pd nanoparticles onto graphene surface
can improve the sensing performance of the devices due to their catalytic properties which
promoted chemisorption of hydrogen even at room temperature.
Since metal nanoparticles were evidenced to be capable of improving gas sensing performances
of carbon based materials, nitrogen-doped carbon nanotubes (NCNTs) with different
compositions of metal nanoparticles (Pt:Ni) were deposited onto the micro-tip electrode arrays
using dielectrophoresis to function as gas sensors. Such sensors were tested towards different
concentrations of hydrogen gas; their performances were compared to evaluate the catalytic
properties of Pt and Ni nanoparticles.
The fabrication of field effect transistors using dielectrophoretically assembled polythiophene
with CdTe quantum dots is also presented in this chapter. The current-voltage transfer
characteristics of the devices suggest that the developed transistor has a current on/off ratio of
more than 3×104 and the carrier mobility of the material is approximately 13 cm2/Vs at room
temperature.
6.2 DEP assembling of MWCNTs using parallel finger
paired electrodes
The DEP assembly and alignment of nanomaterials with high aspect ratios have been previously
reported. For example, Zhou et al. have successfully demonstrated the DEP assembly of CdSe
nanowires using AC dielectrophoresis [1]. In order to develop electronic devices using DEP
assembly and alignment techniques, the author decided to comprehensively investigate the
alignment and assembly of nanostructures (MWCNTs) using AC electric fields with different
amplitudes and frequencies.
CNTs have been widely applied as key elements in micro/nano electronic devices due to their
unique thermal, mechanical and electrical properties [2], and dielectrophoresis can be employed
142
in such fabrications. For instance, Suehiro et al. [3] demonstrated the fabrication of a CNT based
field effect transistor by using dielectrophoresis, Jung et al. [4] fabricated CNT enhanced electric
field emitters by attaching a single-walled CNT onto an atomic force microscope (AFM) tip. In
addition to such device demonstrations, DEP trapping and positioning of CNTs have also been
extensively reported [4-37] and utilized to create field effect transistors [15, 19, 28, 38] and
sensors [3, 9, 11, 14, 23, 39-42].
In this section, finger paired electrodes were utilized to reveal the ability of electric fields on
assembling and alignment of MWCNTs (Sigma Aldrich, 20-50 nm in diameter, 0.5-2 µm in
length). During the DEP assembling process, droplets of MWCNT suspension (concentration:
0.1 mg/ml) were placed onto the microelectrodes. AC potentials with the magnitudes of 2V, 4V,
8V and 10V (peak-to-peak) at the frequencies of 1k Hz and 150 kHz were applied across the
electrode pairs. The developed samples were then characterized via I-V curves and scanning
electron microscope (SEM).
6.2.1 The selection of experimental parameters
According to the DEP spectrum of metallic MWCNTs which was presented in Section 5.3.1,
MWCNTs experience positive DEP forces in the frequency range from 1 kHz to 100 MHz, the
magnitude of such DEP forces are at their maximum level for the frequencies lower than 10 kHz
and are at their minimum level for the frequencies higher than 150 kHz. Therefore, AC potentials
with the alternating frequencies of 1 kHz and 150 kHz were applied to characterize the ability of
electric field in assembling MWCNTs.
6.2.2 Results
I-V curves were measured to evaluate the performance of DEP assembling of MWCNTs, as
illustrated in Figure 6.1and Figure 6.2. Since the slope of an I-V curve represents the conductance
of the characterized device, it is very clear that the samples developed using electric field at
1 kHz (Figure 6.1) have lower impedances than those developed at 150 kHz (Figure 6.2) for all of
the attempted AC potentials. Since the nanotubes used in this task are metallic MWCNTs, a
higher conductivity indicates lager numbers of nanotubes were assembled between the electrodes.
It is suggested that more MWCNTs were assembled with the AC potentials at 1 kHz due to the
stronger DEP forces produced at this frequency. This fact is also evidenced by the SEM images
which are shown in Figure 6.3 and Figure 6.4. It is obvious that more MWCNTs were
143
dielectrophoretically assembled between the microelectrodes at the frequency of 1 kHz compared
with at 150 kHz. As a result, nano materials such as MWCNTs, can be more effectively
assembled at lower frequencies using dielectrophoresis.
The content which is presented in this section was presented on the Society of Photo-Optical
Instrumentation Engineers (SPIE) 2007 international Conference, Canberra, Australia and was
published in the proceedings of the conference [43].
I-V Transfer Characteristics (1 kHz)
-6.00E-03
-4.00E-03
-2.00E-03
0.00E+00
2.00E-03
4.00E-03
6.00E-03
-6.00E+00 -4.00E+00 -2.00E+00 0.00E+00 2.00E+00 4.00E+00 6.00E+00
voltage (v, peak to peak)
cu
rre
nt
(mA
)
2v
4v
8v
10v
Figure 6.1: I-V curves of the DEP platform after the assembly of MWCNTs with applied AC
potentials of 2 V, 4 V, 8 V and 10 V peak-to-peak, at1 kHz [43].
144
I-V transfer characteristic (150 kHz)
-6.00E-03
-4.00E-03
-2.00E-03
0.00E+00
2.00E-03
4.00E-03
6.00E-03
-6.00E+00 -4.00E+00 -2.00E+00 0.00E+00 2.00E+00 4.00E+00 6.00E+00
voltage (v, peak to peak)
cu
rre
nt
(mA
)
2v
4v
8v
10v
Figure 6.2: I-V curves of the DEP platform after the assembly of MWCNTs with applied AC
potentials of 2 V, 4 V, 8 V and 10 V peak-to-peak, at 150 kHz [43].
Figure 6.3: Images taken by NOVA 200 nano-SEM for samples prepared with AC potentials at
1 kHz with the peak-to-peak magnitudes of (A) 2 V, (B) 4 V, (C) 8 V and (D) 10 V [43].
145
Figure 6.4: Images taken by NOVA 200 nano-SEM for samples prepared with AC potentials at
150 kHz with the peak-to-peak magnitudes of (A) 2 V, (B) 4 V, (C) 8 V and (D) 10 V [43].
6.3 Conductometric hydrogen gas sensor based on
graphene with Pd nanoparticles
In order to demonstrate the effect of metallic nanoparticles in improving the gas sensing
performances of carbon based materials, hydrogen gas sensors fabricated using graphene sheets
with Pd nanocomposite are presented in this section. The devices were fabricated using traditional
material deposition techniques including spin-coating and drop-casting. The catalytic properties
of the Pd nanoparticles and the interactions between metal nanoparticles and sp2 carbon networks
were analyzed.
Graphene is the name given to a monolayer of sp2-bonded carbon atoms that tightly pack into a
two-dimensional lattice [44-46], and is a basic building block for other graphitic materials such as
carbon nanotubes and fullerenes [47]. Because of its single-atom-thick structure, graphene
possesses a zero electronic bandgap and exhibits exceptional charge carrier mobility at room
146
temperature [45]. Since graphene demonstrates excellent ballistic transport properties, it can be
utilized as the conducting channels of FET devices [45]. As a result, one of the current research
interests in graphene is the potential to replace silicon-based integrated circuit (IC) technology
which is rapidly approaching its theoretical limits, with graphene based ICs [46].
In addition, graphene has also been used in the development of chemical, mass, and bio-sensors
[47-52], with demonstrated sensitivity to gas species including: nitrogen dioxide [47-49, 53],
nitrogen monoxide [49], ammonia [47, 49], hydrogen [47, 51], carbon dioxide [49], carbon
monoxide [49, 51, 54], nitrogen [50] and oxygen [49, 50]. However, methodologies that enhance
graphene reactivity are still required to achieve higher sensing performance and the
commercialization of such devices.
According to a study of hydrogen chemisorption on sp2-bonded carbon surfaces that was
conducted by Ruffieux, et al. [55], the chemical binding of hydrogen to an sp2-bonded carbon
network requires a local rehybridization from sp2 to sp3 that has a large adsorption energy barrier.
Such energy barrier can be lowered by the formation of surface defects and curvatures [55]. The
surface defects can be induced by attaching metal catalysts which also enhance the reactivity of
the material [56]. Such a mechanism has been demonstrated by an improvement in nitrogen
doped carbon nanotube sensors using Pt/Ni metal composites [56].
In this work, Pd nanoparticles are deposited on graphene sheets to enhance their sensing
performance towards hydrogen. Sheets of graphene were obtained through hydrazine reduction of
graphene oxide and subsequently deposited onto interdigitated transducers (IDTs); Pd
nanocomposites were prepared by drop-casting ethanol/Pd nanoparticles from solution and drying
under vacuum. The material is characterized by transmission electron microscopy (TEM),
scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. The
developed sensors are tested towards different concentrations of hydrogen gas. The sensing
performance of the graphene/Pd devices are compared with graphene devices.
6.3.1 Material synthesis
Graphite oxide (GO) was prepared from graphite powder via the reduction of GO, which was
performed in anhydrous hydrazine [46, 47]. The quartz substrates (pre-patterned with metallic
microelectrodes) were transferred into the drybox and spin-coated with prepared graphene
dispersions at 1500 rpm for 30 seconds. After the deposition, films were dried under vacuum for
24 hours to remove residual hydrazine.
147
For the preparation of Pd nanoparticles, 6 mg of palladium chloride (PdCl2) was dissolved in
20 ml of 95% ethanol and stirred for 24 hours to achieve a yellow-colored solution. A Beckman-
Coulter Allegra® X-15R was used at 4500 rpm for 30 min for centrifugation of the solution,
saving the supernatant. Using a 20 ml scintillation vial, 3 ml of the supernatant PdCl2 solution
was diluted to 6 ml with Milli-Q water and cooled in an ice bath. The solution was then heated in
an oil bath at 90 °C for 1 hour, followed by reaction quenching immediately thereafter by cooling
the vial in an ice bath. Finally, the dispersion was diluted to 1/5 by a water-ethanol mixture (95%
ethanol: water = 1:1 in volume) and kept at -2 °C until needed. Graphene/Pd were prepared by
drop-casting Pd solution onto the graphene covered substrates and drying the composites under
vacuum.
6.3.2 Material characterization
TEM images, were taken using a Philips CM120 under 120 kV accelerating voltage. The Pd(0)
nanoparticle samples were imaged on carbon film coated Cu TEM grids. Graphene and
graphene/Pd were imaged using silicon dioxide/monoxide film covered Cu TEM grids in the
glove box followed by evacuation over 24 hours to remove residual hydrazine. As shown in
Figure 6.5, most of Pd nanoparticles bundles were observed at the locations with wrinkle/defect,
it appears that Pd nanoparticles either prefer such sites or directly cause them on the surface of
graphene.
XRD characterization was performed using 100 fold scale up of the synthesis procedure. The Pd
nanoparticle dispersion was dried to yield grey powder and characterized on zero background
silicon substrate in a Crystal Logic diffractometer with Ni-filtered Cu K radiation (λ=1.5418 Å).
The obtained pattern of Pd nanoparticles (Figure 6.6) illustrates a perfect match with a reference
Pd pattern, represented by blue lines at 2θ = 40°, 47°, 68°, 82° and 86°. We also observed an
impurity peak at 2θ = 16°, the impurities were believed to be caused by the scaling up of the
synthesis procedure; since the dispersion experienced less time at necessary temperature and not
all palladium chloride was reduced.
Figure 6.7 illustrates the SEM images for graphene/Pd which were deposited on the surface of the
device. Similar to the images taken by TEM, bundles of Pd nanoparticles, which were formed
after the deposition of the material, were observed on the surface of graphene structure.
148
Figure 6.5: TEM image of Pd nanoparticles with the average size of 37nm. Insert is TEM image
of Pd nanoparticles in bundles on top of a sheet of graphene.
Figure 6.6: XRD pattern of Pd(0) nanoparticles illustrates a perfect match with a reference Pd
pattern, blue lines. A small peak at 16° 2θ is due to a small impurity of precursor material.
149
Figure 6.7: SEM images for graphene sheet with Pd nanoparticles taken on the surface of sensing
device. Insert illustrates the aggregated Pd nanoparticles on graphene surfaces.
6.3.3 Results and discussions
In Figure 6.8, the Raman spectra of graphene sheets and graphene/Pd are presented. Both spectra
are dominated by a D band at approximately 1350 cm-1 and a G band at approximately 1600 cm-1.
In addition, the 2D band and D + G combination mode are also observed at approximately 2700
and 2950 cm-1, respectively. Such a Raman fingerprint indicates that the sample is a combination
of graphene and graphite oxide [51, 57].
It is evidenced that Pd nanoparticles have enhanced the surface Raman scattering of graphene
sheets since more counts are received at the spectrum peaks in the presence of Pd nanoparticles.
The surface enhanced Raman scattering (SERS) effect is known to occur on surfaces with high
roughness and metallic nanostructures [58, 59]. The SERS effect caused by metallic nanoparticles
can be explained by either charge transfer theory and/or localized electric field enhancement
theory [60, 61]. According to a study of SERS achieved using roughened Pd surfaces [62], the
electromagnetic enhancement plays an important role in the processes of SERS. Therefore, the
observed SERS effect in our work, which has an enhancement ratio of approximately 5, is
believed to result from the enhancement of the localized electric field.
150
Figure 6.8: Raman spectra of graphene sheet(s) with and without Pd nanoparticles deposited on
conductometric devices obtained using 532 nm laser excitation.
According to the TEM analysis, the Pd nanoparticles either prefer moving to the defect sites or
directly cause such defects in graphene structures. In Raman spectra, a small variation of the ID/IG
(integrated intensity ratio for the D band and G band) from ~1.05 to ~1.18 is observed when Pd
nanoparticles are present; this suggests that such Pd nanoparticles did not induce any additional
structural disorder on the surfaces of the carbon networks [63]. As a result, it is more likely that
Pd nanoparticles prefer such defect sites during the deposition processes.
The dynamic responses of the graphene and graphene/Pd sensors towards different concentrations
of H2 gas are shown in Figure 6.9. The normalized device resistances for both sensors were
utilized to illustrate their sensitivities, which are defined as:
linebas
gas
normalizedR
RR
−
= , (6.1)
151
where Rgas is the real time device resistance measured in gas chamber and Rbase-line is the device
resistance measured when the sensor is stabilized in synthetic air. The testing was conducted at
room temperature to prevent any additional oxidization of the graphene structures.
The responses indicate that the graphene sensor has a sensitivity of approximately 0.12%
(resistance change) towards 0.06% H2 gas, while the graphene/Pd sensor has a sensitivity of
approximately 0.2% towards the same concentration of H2 gas (Figure 6.9-C). Similarly, the
sensitivity of graphene/Pd sensors towards 0.12% and 0.25% H2 gas is also higher than graphene
sensor. However, when the H2 concentration has increased to 0.5% or higher, the sensitivities for
both types of sensors are almost similar. In addition, it seems the graphene sensor has a faster
response time compared to graphene/Pd sensor.
Figure 6.9: Dynamic responses (change in normalized resistance) of the developed
conductometric sensors towards different concentrations of H2 at room temperature: (A) graphene,
(B) graphene/Pd. The devices were placed in a computerized multi-channel gas calibration
system and five pulses (0.06%, 0.12%, 0.25%, 0.5% and 1%) of H2 gas in synthetic air were
applied to the gas chamber. (C) illustrates the comparison of the responses for both sensors
towards 0.06% H2 gas.
152
The obtained sensing responses of the developed devices are attributed to a combination of
physisorption and chemsisorption of H2 [64, 65]. Physisorption involves the adsorption of
molecular hydrogen on the graphene surfaces, while chemisorption involves the adsorption of
atomic hydrogen graphene surfaces [64-68]. The improvement of device sensitivity (at 0.06%,
0.12% and 0.25% H2 concentrations) can be caused by a combination of two effects. First, Pd
nanoparticles have catalytic properties, which help dissociate hydrogen molecules into atomic
hydrogen and promote surface chemisorption of hydrogen [69]. Second, the atomic hydrogen
dissolves into the Pd nanoparticles and consequently lowers the work function of Pd, such a
process causes the electron transfer from Pd to the carbon networks and improves the device
sensitivity [69].
Since the sensors were tested at room temperature, the response of the graphene sensor is
believed to be dominated by physisorption of molecular hydrogen as there would be insufficient
thermal energy to promote chemisorptions [56, 64]. For the graphene/Pd sensor, the
chemisorption of atomic hydrogen is promoted because of the Pd nanoparticles presence,
however, with a low efficiency due to the lack of thermal energy. Therefore, the graphene/Pd
sensor illustrates a higher sensitivity towards H2 but with a slower response time. As a result,
when the sensors were exposed to high concentrations of H2 (0.5 % and 1%), there is insufficient
time for all of the hydrogen molecules to be dissociated into atomic hydrogen and both sensors
demonstrate similar sensitivities due to the domination of physisorption.
The comparison of graphene/Pd and Pt/NCNT hydrogen gas sensors (presented in Section 6.4.6)
indicates that the deposition coverage of Pd nanoparticles on graphene sheets (less than 10% in
Figure 6.7) is much lower than Pt nanoparticles on NCNTs (more than 50% in Figure 6.11). Such
a low metal load ratio in the developed graphene/Pd material can be the major reason for the
sensors to have an insignificant improvement (achieved using Pd nanoparticles) in its sensitivities.
Moreover, the particle size of metal nanocomposite in the graphene/Pd material (~ 10 nm) is also
larger than in the Pt/NCNTs composite (~4 nm). Generally, smaller particle size indicates higher
surface-volume ratio and hence results in better catalytic properties. Therefore, it is believed that
the developed graphene/Pd sensors can be further improved by increasing the metal load ratio (in
other words, deposition coverage) and decreasing the size of Pd nanoparticles.
This work was conducted by the author in collaboration with Mr Sergey Dubin, from the
Department of Chemistry and Biochemistry, University of California Los Angeles, California, US.
153
The content which is included in this subsection is currently under review by the journal
“Chemical Physics Letters”.
6.4 Nitrogen-doped MWCNTs with uniformly doped
Pt-Ni nanoparticles for hydrogen gas sensing
Since the dielectrophoretic assembly of MWCNTs was successfully demonstrated, the author
started to develop electronic devices using this method. In this section, hydrogen gas sensors
based on nitrogen-doped MWCNTs with Pt-Ni nanoparticles fabricated using dielectrophoresis
are presented. Micro-tip electrode arrays were employed in this work because they can assemble
nanostructures more efficiently in comparison with finger paired electrodes due to the enhanced
electric fields near the tips.
CNTs have been employed as the sensing element for chemical and bio-sensors and
demonstrated sensitivity to gas species such as H2 [70-72], methane [73], oxygen [74], carbon
dioxide [75] and ammonia [76]. However, methods that enhance the reactivity of CNTs are
required to produce higher sensitivity and develop commercial applications.
According to the study of hydrogen chemisorptions on sp2-bonded carbon surfaces that was
conducted by Ruffieux et al [55], the chemical binding of hydrogen to an sp2-bonded carbon
network requires a local rehybridization from sp2 to sp3 that has a large adsorption energy barrier.
Such energy barrier can be lowered by the formation of surface defects and curvatures [55] and
the incorporation of such defects and/or metallic catalysts can improve the surface reactivity [77].
Nitrogen doped multi-walled carbon nanotubes (NCNTs) have recently been synthesized from a
heterocyclic pyridine precursor with chemical vapour deposition (CVD) growth technique [78-
80]. Pt nanoparticles were deposited on such multi-walled NCNTs at defects associated with the
incorporated nitrogen, resulting in hybrid Pt/NCNTs [81]. Moreover, Pt-Ni alloyed nanoparticles
have also been deposited onto NCNTs with a similar process [82].
In this section, Pt-Ni/NCNTs and MWCNTs have been assembled using dielectrophoresis to form
conductometric devices for hydrogen gas sensing. The sensing characteristics of such developed
samples were investigated as a function of operating temperature and Pt:Ni composition of the
nanoparticles to evaluate the catalytic properties of such nanoparticles.
154
6.4.1 Material synthesis
In this task, NCNTs with nitrogen-content of 3~5% were synthesized at 650 °C as mentioned in
Ref. [79]; a dehydrogenated bimetallic catalyst of Fe-Co/γ-Al2O3 (01 mmol/g Fe, 01 mmol/g
Co/γ-Al2O3) was used in the synthesis according to Ref. [80]. The as-prepared NCNTs were
flushed in 6 M NaOH and in 6 M HCl aqueous solution at 110 °C for 4 h to remove the Al2O3 and
metals. Afterwards, the purified NCNTs were thoroughly washed with distilled water until the pH
value of the filtrate reached 7, and then dried at 70 °C.
Pt-Ni alloyed nanoparticles with Pt:Ni molar ratios of 3:1 and 1:1 were deposited onto the
NCNTs using a microwave-polyol method [83]. First of all, 20 mg of the NCNTs was placed in
50 mL ethylene glycol (EG) and treated in ultra sonic bath for 20 min to ensure proper dispersion.
At the same time, H2PtCl6 and Ni(CH3COO)2 were mixed with EG into solutions with
concentrations of 7.5 and 1.2 mg/mL, respectively. After the solutions were mixed with NCNT
suspension, the mixture was magnetically stirred and 2 mL of NaOH/EG solution (2 mol/L) was
added. After stirring for 10 min, the mixture was placed in a microwave oven (700 W) for 90 s,
and the NCNTs were filtered out from the suspension. The NCNTs were then rinsed in ethanol
and vacuum-dried at room temperature.
According to the load ratio of Pt and Ni, the author denoted prepared materials as Pt3Ni1/NCNTs
(75% Pt - 25% Ni) and Pt1Ni1/NCNTs (50% Pt - 50% Ni). In addition, Pt/NCNTs were also
prepared with the same method for experimental comparisons.
6.4.2 Material characterizations
As shown in Figure 6.10, the XRD diffractograms of the (a) Pt/NCNTs, (b) Pt3Ni1/NCNTs,
(c) Pt1Ni1/NCNTs and (d) pristine NCNTs are presented [84]. The diffraction peak at 26.1°
observed from all samples results from the (002) graphitic planes in the NCNTs. The three peaks
between 30 and 80° from the Pt/NCNTs sample (a) can be indexed to face centre cubic crystalline
Pt (JCPDS-ICDD, Card No. 04-802). The XRD peaks exhibited from the Pt3Ni1/NCNT sample (b)
are similar to those of (a) but are shifted to slightly higher values of 2θ due to alloying and an
associated reduction in the lattice constant [85]. This trend is continued in the peaks from the
Pt1Ni1/NCNT sample (c). The broadening of the peaks suggests that the crystallite size is
decreasing as the Ni content in the nanoparticles is increased.
155
C(0
02)
Pt(
111
)
(degree)
Pt(
200
)
Pt(
22
0)
C(0
02)
Pt(
111
)
(degree)
Pt(
200
)
Pt(
22
0)
Figure 6.10: XRD diffractograms taken from NCNTs supporting nanoparticles of (a) Pt/NCNTs,
(b) Pt3Ni1/NCNTs, (c) Pt1Ni1/NCNTs and (d) NCNTs [77].
6.4.3 DEP assembling
NCNTs with different metallic nanoparticles were suspended in deionised (DI) water with a
concentration of approximately 10 mg/ml. The suspensions were then placed in an ultra-sonic
bath for 10 minutes and then left for 2 hours allowing agglomerates to precipitate from the
suspension. After placing a droplet of the suspension onto the electrodes, an AC potential (20 V
peak-to-peak, 5 kHz) was applied across the electrodes for 20 seconds. After such processes,
NCNTs formed connections between the electrodes as shown in Figure 6.12.
Figure 6.12 illustrates the SEM images of the NCNTs that patterned on the DEP platform after
application of the AC potentials. The images clearly suggested the nanotubes have selectively
assembled between the electrode pairs due to DEP trapping forces induced by the non-uniform
electric field. All samples showed similar assembly properties after application of the AC
potentials.
Higher magnification SEM images shown in Figure 6.11-A, B, C and D represent the NCNTs,
Pt1Ni1/NCNTs, Pt3Ni1/NCNTs and Pt/NCNTs between micro-tip electrode pairs, respectively.
The accompanying insets illustrated corresponding TEM images. Nanoparticles with diameters of
3.0−4.0 nm can be observed homogeneously distributed onto the NCNTs irrespective of their Pt-
156
Ni composition. Further TEM analysis of such Pt-Ni nanoparticles can be found elsewhere [79,
82].
Figure 6.11: SEM images of (a) NCNTs, (b) Pt1Ni1/NCNTs, (c) Pt3Ni1/NCNTs and (d) Pt/NCNTs.
Insets are the corresponding TEM images [77].
50 µm
(a)
50 µm50 µm
(a)
20 µm
(b)
20 µm20 µm
(b)
Figure 6.12: SEM image of the NCNTs assembled on the DEP platform [77].
157
6.4.4 Gas sensing setup
The sensors were mounted inside an enclosed environmental cell. Four mass flow controllers
(MFCs) were connected to form a single output that supplies gas to the cell. In this work, two
channels of MFCs were employed: one for synthetic air and one for low-concentration (1%) high-
purity H2 gas balanced in synthetic air. The concentration of the gas was adjusted by changing the
mixed volume ratio from the two gas cylinders while maintaining a constant flow rate of
200 SCCM. The sensor was exposed to a sequence of H2 gas pulses for a fixed period of time and
the cell was purged with synthetic air between each pulse to enable recovery of the sensor. The
variation of sensor resistance was measured using a Keithley 2001 multimeter. Lab-VIEW-based
software controlled the experimental setup and received real-time measurement data.
6.4.5 Sensors’ performances towards H2 gas
Figure 6.13 to Figure 6.17 represent the dynamic responses of the sensors based on CNTs,
NCNTs, Pt1Ni1/NCNTs, Pt3Ni1/NCNTs and Pt/NCNTs towards hydrogen gas of different
concentrations. The sensor response is defined by normalized impediance, which can be
expressed as:
%100×−
=gas
gasair
R
RRS , (6.2)
where Rair is the sensor resistance in synthetic air and Rgas is the sensor resistance in the
environment where H2 gas exists.
The sensitivities to 0.06% H2 in synthetic air for each sensor at 60 °C are given in Table 6.1 to
provide comparisons on sensors’ performance. It is suggested that, the sensitivity of the NCNTs
towards hydrogen gas was not significantly higher compared with MWCNTs. However, for
NCNTs that include metal catalysts, higher sensitivities can be observed; therefore, the
composition of the nanoparticles affected the sensitivity of sensors. It is clear that higher Pt load
resulted in better sensing performance, since the highest sensitivity, fastest response and recovery
were all measured for the Pt/NCNT sensor. In this case, the catalytic activity of the Pt
nanoparticles was higher than the Pt-Ni nanoparticles.
158
In Figure 6.13 to Figure 6.17, two 0.06 % H2 pulses can be observed in each testing sequence,
which were applied to the sensors for the evaluation of the repeatability in their responses. The
magnitude of the second resistance change was almost identical to the first for all of the samples,
except for the Pt1Ni1/NCNT sensor (Figure 6.15) where a slightly decreased resistance variation
(~ 3%) was observed for the second pulse. After the pulse sequence was completed, the
stabilizing of resistance baseline was exhibited for the sensors, but a drift towards lower
resistance was observed for sensors with metal catalysts (Figure 6.15 to Figure 6.17). Such
reduced sample resistance in synthetic air must be caused by a relatively slower desorption rate of
H2 from the surface of the catalytic particles compared to the tubes.
The developed sensors were tested in a range of temperatures up to 100 °C and the optimum
operating temperature for H2 sensing was found to lie between 50 and 70 °C. In the investigated
temperature range, the responses of the sensors to the 0.06% H2 pulse were ranged from
approximately 0.13 % to 1.53%. At room temperature, the responses to the same concentration of
hydrogen were lower (less than 0.05%), and the responding and recovery time were longer. At
100 °C, the speed of response was significantly increased but without recovery; therefore, the
response was not reproducible.
0.999
1.000
1.001
1.002
1.003
1.004
1.005
0 1000 2000 3000 4000
Time (s)
Resis
tan
ce (
no
rmali
zed
)
0.06% H2
0.25% H2
0.5% H2
0.06% H2
Figure 6.13: Dynamic response of the DEP assembled CNTs (multi-walled) towards different H2
gas concentrations at 60° C [77].
159
0.986
0.988
0.990
0.992
0.994
0.996
0.998
1.000
1.002
0 5000 10000 15000 20000 25000
Time (s)
Resis
tan
ce (
no
rmali
zed
)
0.06% H2
0.25% H2
0.5% H2
0.06% H2
Figure 6.14: Dynamic response of the DEP assembled NCNTs towards different H2 gas
concentrations at 60° C [77].
0.960
0.965
0.970
0.975
0.980
0.985
0.990
0.995
1.000
1.005
0 5200 10400 15600 20800
Time (s)
Resis
tan
ce (
no
rmali
zed
)
0.06% H2
0.25% H2
0.5% H2
0.06% H2
Figure 6.15: Dynamic response of the DEP assembled Pt1Ni1/NCNTs towards different H2 gas
concentrations at 60° C [77].
160
0.94
0.95
0.96
0.97
0.98
0.99
1
1.01
0 4000 8000 12000 16000 20000
Time (s)
Resis
tan
ce (
no
rmali
zed
)
0.06% H2
0.25% H2
0.5% H2
0.06% H2
Figure 6.16: Dynamic response of the DEP assembled Pt3Ni1/NCNTs towards different H2 gas
concentrations at 60° C [77].
0.96
0.96
0.97
0.97
0.98
0.98
0.99
0.99
1.00
1.00
1.01
0 2000 4000 6000 8000 10000 12000 14000
Time (s)
Resis
tan
ce (
no
rmali
zed
)
0.06% H2
0.06% H2
0.5% H2
0.25% H2
Figure 6.17: Dynamic response of the DEP assembled Pt/NCNTs towards different H2 gas
concentrations at 60° C [77].
161
Table 6.1: Responses of CNTs, NCNTs, Pt1Ni1/NCNTs, Pt3Ni1/NCNTs and Pt/NCNT sensor to
0.06% H2 gas concentrations at 50-70° C temperature range [77].
Samples Sensitivity (For 0.06% H2)
CNTs 0.13%
NCNTs 0.14%
Pt1Ni1/NCNTs 0.38%
Pt3Ni1/NCNTs 1.3%
Pt/NCNTs 1.53%
6.4.6 Discussions
According to the sensing response of developed devices towards H2 gas, MWCNT based senor
(Figure 6.13) has the opposite response compared to NCNT based sensors (Figure 6.14 to Figure
6.17). As shown in Figure 6.13, the conductivity of the MWCNT based sensor decreased upon
exposure to hydrogen, which is in agreement with previous measurements [69-71, 86]. Since
hydrogen gas is a type of reducing gas which contributes electrons, p-type materials would have a
lower conductivity when exposed to H2 while n-type material would have a higher conductivity
after the exposure [87]. Therefore, MWCNTs are p-type materials, while NCNTs and the ones
with metal catalyst are n-type materials.
The sensing responses of CNT sensors have been explained by a combination of physisorption
and chemisorption of hydrogen on their surfaces [65]. Physisorption and chemisorption refer to
the adsorption of molecular hydrogen and atomic hydrogen on the material surfaces, respectively
[55, 65]. The response of the NCNTs (Figure 6.14) is expected mainly due to physisorption effect
since there would be insufficient thermal energy (at 60 °C) to promote chemisorption. However,
162
with the existence of metal catalysts, the performances of the sensors were improved as the metal
nanoparticles dissociate hydrogen molecules into atomic hydrogen and thus promoted the
chemisorption [69, 71, 74, 88, 89]. Therefore, such attached metallic nanoparticles have resulted
in the improvement of the sensitivity of the devices.
The sensing responses shown in Figure 6.14 and Figure 6.17 suggest that hydrogen adsorption
and desorption to/from the Pt takes place on a shorter time scale than from the nitrogen defects.
The sensing responses in Figure 5.15 to Figure 6.17 therefore result from a combination of
responses (electron injection) from the nitrogen defects (slower) and the adhered nanoparticles
(faster). Since the highest sensitivity, fastest response and recovery were all measured for the
Pt/NCNT sensor, the catalytic activity of the Pt nanoparticles was higher than the Pt-Ni
nanoparticles; therefore, it is suggested that if the density of the adhered Pt nanoparticles were
increased, the sensing performances can be further improved.
The presence of Pt nanoparticles on NCNTs results in an improvement of sensitivity from
~0.14% to ~1.53% towards H2 gas, approximately 10 times enhancement in sensing performance.
Such a result is much more significant when compared with the graphene/Pd H2 sensor
(performance enhancement of 3 times) presented in Section 6.3. Such a difference in the
improvement of gas sensing property can be attributed to the difference in metal load ratios. The
metal load ratio in Pt/NCNT is approximately 27.3% [90] and the deposition coverage of Pt
nanoparticles on nanotubes surface is more than 50% (Figure 6.11), this is much higher when
compared with the graphene/Pd material (deposition coverage less than 10% as shown in Figure
6.7).
The work, which is presented in this section, was conducted by the author in collaboration with
Dr Abu Z Sadek, from School of Electrical and Computer Engineering, RMIT University,
Melbourne, Australia. This work was published in “Journal of Physical Chemistry C” [77].
6.5 Field effect transistor based on polythiophene
with CdTe quantum dots
Semiconducting nanocrystals, known as quantum dots (QDs), have unique properties that can
make them potentially useful when incorporated into such novel electronic devices. Due to
163
quantum size effects, they exhibit size-dependent optical and electrical properties [91], which
differ both physically and chemically from their bulk counterparts [92]. In recent years, II-VI
semiconducting CdS, CdSe and CdTe QDs have been successfully synthesized for electrical and
optical applications [93-95]. Several problems that must be overcome with QDs in these devices
are their lack of structural robustness and their instability in air. Reactivity with oxygen and
moisture generally requires hermitic sealing in order to prevent serious deterioration in their
performance. Another approach to increase stability and processability, is to embed the QDs in
polymer matrices [96, 97]. However, embedding quantum dots has not been an easy process,
given that QD aggregation often occurs. Therefore, bonding the polymer directly onto the QDs
surfaces has been attempted to improve their stability and in certain cases enhance electron
transport between the two materials [98, 99].
In this section, the development of a field effect transistor (FET) using dielectrophoretically
assembled polythiophene/CdTe QDs composite is presented. Micro-tip electrode arrays, which
are patterned on a Si/SiO2 substrate, were utilized as the assembling platform. The fabricated
devices were characterized by scanning electron microscope (SEM) and current-voltage (I-V)
measurements with different gate voltages (-12 V to 12 V) applied at the silicon back gate. The
experiments and calculations suggest that the developed transistor has a current on/off ratio of
more than 3×104, and the carrier mobility of the material is approximately 13 cm2/Vs at room
temperature.
6.5.1 Material synthesis
6.5.1.1 CdTe QD synthesis
The synthesis of polythiophene have been reported previously in [100]. A trioctylphosphine-
telluride solution (TOP-Te) was synthesized by adding 55.8 mg of tellurium (0.43 mmol) to 5 mL
of TOP (16.2 mmol) and 18 mL of octadecene (a non-coordinating solvent). Elemental tellurium
was gently dissolve over low heat (~30 - 45 ºC) for approximately 2 to 3 hrs or until the solution
turned a clear yellow. The dissolved TOP-Te solution was kept at approximately 45 ºC until a
coordinating solution containing cadmium was prepared. A combination of 26.8 mg CdO (0.21
mmol) and 1.2 mL of oleic acid (3.7 mmol) were dissolved in 20 mL of octadecene at 160 ºC and
heated to a temperature of 240 ºC at which point a 3.7 mL volume of TOP-Te solution was added
via a syringe. The CdTe solution was quenched immediately by pouring it into either a beaker
filled with ice or liquid nitrogen. The solution was then allowed to come to room-temperature. If
164
ice was used to quench the reaction, then the water was removed using a separatory funnel. The
final CdTe QDs colloidal solution was treated with a 2:1 CH3OH/HCCl3 mixture to remove any
un-reacted metal and capping ligands. Note that the reaction was not carried out under air-
sensitive conditions, since experiments with air excluded produced similar results.
6.5.1.2 Oligo-polythiophene/CdTe composite synthesis
3 mL of a 3:1 molar ratio of CdTe QDs were mixed with 21.3 mg of 3-thenoic acid (3-TA)
previously dissolved in 6 mL of 1,2-dichlorobenzene. After 2-3 hrs, 2.0 mg of terthiophene as the
initiator was added directly to the 3-TA/CdTe solution. In another clean vial, 100.0 mg of FeCl3
as the oxidant was dissolved in 3 mL of acetonitrile and added to the fuctionalized CdTe
QD/initiator solution. Upon the addition of the oxidant, there was an immediate color change
from a clear yellow to a black solution, indicating polymerization had commenced. The solution
was allowed to polymerize overnight, after which the polymer composite was washed and
centrifuged to remove any un-reacted material and then stored in 1,2-dichlorobenzene.
6.5.2 Device development
The FET device was developed using alternating current (AC) dielectrophoresis to assemble the
polythiophene/CdTe QDs composite between interdigitated electrodes on the Si/SiO2 substrate. In
order to have a positive value for fCM, and hence DEP trapping force can be produced, the
frequency of applied AC potential was limited to 5 kHz.
After a droplet of material suspension (approximately 1 µl) was placed onto the DEP platform, an
AC potential of 20 V peak to peak was applied across the electrodes at the frequency of 5 kHz
using a function generator (ETABOR Electronics, Model 8200). After 60 seconds, the AC
potential was removed and the sample was kept untouched to let the remaining liquid evaporate.
The FET device was then characterized by a source-meter (Keithley-2602) to obtain the current-
voltage transfer characteristics (voltage range: -15 V to 25 V) with different DC potentials (-12 V
to 12 V) applied at the back Si gate. The sample was also characterized by a scanning electron
microscopy (FEI Nano SEM) to evaluate the DEP assembling performance.
165
6.5.3 Characterizations
6.5.3.1 Microscope characterization
A chemical structure representation of the resulting polythiophene/CdTe QD composite is
presented in Figure 6.18. The benefit of this particular synthesis is that an intimate contact
between polythiophene and the QDs is achieved, in addition to maintaining planarity and thus
enhancing the conjugation length of the polymer. SEM was used to give a visual understanding of
the morphology of the polythiophene/CdTe QD composite, which shows long crystalline fibers as
well as small spherically shaped material (Figure 6.18-A).
Figure 6.18: An illustration of the chemical structure of the polythiophene/CdTe QD composite
(inset) with (A): a scanning electron microscope (SEM) (scale bar = 1 µm) image showing both
the long thin fibers with amorphous polymeric regions and (B): a high resolution transmission
electron microscope (HRTEM) (scale bar = 5 nm) image with an emphasis on the amorphous
regions found within the composite. Lattice fringes (d-spacing of 0.23 nm) correspond to CdTe
quantum dots.
Figure 6.18-B is a HRTEM image of a representative amorphous region of the composite
showing the polythiophene polymer with CdTe QDs nanocrystals embedded throughout the
polymer matrix. The high-resolution image shows regions of highly defined lattice fringes, which
are dispersed within the polymer, indicating highly nanocrystalline material. The lattice fringes
confirm the presence of CdTe QDs with the 0.23 nm d-spacing corresponding closely to the cubic
166
CdTe 220 (hkl) plane. Although, the nanocrystals were expected to disperse well within the
polymer, obvious aggregation of crystallites can be seen in the HRTEM image. Regardless of
some nanocrystal aggregation, it is clear that the two components experience a direct interfacial
contact, which should help in electron transport and support higher mobility values.
Figure 6.19: The SEM images for DEP assembled CdTe QDs with polymer composite. (A) and
(B): overview of assembled material on electrode arrays. (C): close view for the assembled
material between a pair of micro-tips. (D): the distribution of assembled particles indicates the
distribution of electric field.
The SEM was utilized to verify the DEP assembling is successful for the device development. As
shown in Figure 6.19-A and B, the particle shaped structures were successfully assembled on the
edges of the electrodes and between the tip pairs; this is because the electric field gradient is
much higher at such locations and hence generated large positive DEP forces during the
assembling process. In contrast, the micro-wire strictures were not aligned between electrodes
due to their large dimension compared to the distance between electrodes. Figure 6.19-C provides
us a close view on the assembled particle structures and we can clearly observe that the electrode
167
gap has been bridged by DEP assembled material. Figure 6.19-D illustrated a curve pattern in the
distribution of assembled particles indicating the electric field lines between electrodes. Since the
materials were assembled bridging between electrodes, the development of device was successful
and the device was later characterized for FET performance.
6.5.3.2 Performance of the developed FET device
FET devices were tested using a Keithley-2602 source-meter to obtain current-voltage
characteristics. The voltage range of the devices between drain and source was investigated from
−15 to +25 V, in 1 V steps at 1 second per step. An ETABOR 8200 function generator was set
into direct current (DC) mode to produce potentials between the back Si gate and the source
electrode of the FET device, the schematic of the testing set-up is presented in Figure 6.20.
Figure 6.20: Schematic of testing setup for current-voltage characterization of the developed FET
device.
The I-V transfer characteristics obtained suggested the developed FET device was a typical P-
FET, as shown in Figure 6.21. Figure 6.21-A suggested the device can achieve an increased
transconductance “gm” by decreasing the gate-source voltage (VGS), and has a current on/off ratio
as high as 3.2×104. The value of gm for the device at different values of VDS can be obtained from
the linear part of the curves in Figure 6.21-B, and at VDS= 19 V (current limited for VDS>20 V) the
value of gm is obtained as ~0.64 µS.
In order to further evaluate the performance of the material, the carrier mobility µ is calculated
using following equation [101]:
168
DSSiO
m
V
thLg
⋅=
22
)/2ln(
πεµ , (6.3)
where, L is the length of the conducting channel, ε is the permittivity of SiO2, h and t refers to the
thickness of SiO2 insulating layer and the assembled material, respectively. For our device, the
length of the channel is 10 µm, which corresponds to the gap distance between electrode tips; the
thickness of deposited SiO2 layer is 200 nm and the value of t was assumes to be the same as the
thickness of metal electrode (150 nm); the permittivity of SiO2 is 3.91×10-11 F/m (relative
permittivity of 4.42 [102]). The calculated value of the carrier mobility is approximately
13 cm2/Vs, which is superior to current literature reported carrier mobility values, such as
organic-thin film transistors (< 3 cm2/Vs), hybrid organic-inorganic composites thin film
transistors (0.6 cm2/Vs), and semiconducting nanocrystal based FETs (< 1 cm2/Vs) [101, 103-
108].
Previous reports indicate that increasing the crystallinity of either oligothiophene or
polythiophene leads to an improvement in charge carrier mobility [109]. Polythiophene
experiences extensive π-conjugation when there is significant co-planarity from the thienylene
moieties, so by increasing the polymer’s crystallinity more regions of un-disturbed electron
pathways will be created. Since the organic/inorganic composite is composed of highly
crystalline polythiophene with CdTe nanocrystals, it was expected that this could affect the
carrier mobility in a positive manner. Fortunately, this is the case and thus the high carrier
mobility can be attributed to both the relatively high crystallinity of the polythiophene and the
direct interfacial contact between the organic-inorganic composite.
Due to some oxidative degradation of the organic/inorganic composites, the FET device
performance begins to suffer after about four weeks. Figure 6.22-A and B illustrates the I-V
characterization of the same device after four weeks. It is clear that the drain-source current IDS
could only reach ~0.6 mA when VGS = −20 V, the value of gm at VDS = 20 V is measured to be
0.16 µS, which is only 25% of the previous value. Although, the composite was designed to be
more resilient to environmental pressures, it may still be affected and susceptible to deterioration
through slow interaction with moisture and/or oxygen. We believe that the problem stems from
the physical interaction between the polymer and its weak attachment to the CdTe QD surface,
where the method of binding is not strong enough to keep the CdTe QDs from reacting with
oxygen. This would result in a slow deterioration of the components attachment and the eventual
169
decrease in device performance. Overall, the results promise a new route to the understanding and
development of efficient hybrid organic-inorganic devices.
The work was conducted by the author in collaboration with Ms Veronica Strong, from the
Department of Chemistry and Biochemistry, University of California Los Angeles, California, US.
The content which is included in this section is currently under submission.
Figure 6.21: I-V Characteristics of developed FET device. (A): drain-source current versus drain-
source voltage at different gate-source voltages. (B): drain-source current versus gate-source
voltage at different drain-source voltages.
170
Figure 6.22: I-V Characteristics of developed FET device after four weeks has past. (A): drain-
source current versus drain-source voltage at different gate-source voltages. (B): drain-source
current versus gate-source voltage at different drain-source voltages.
171
6.6 Summary
In this chapter, MWCNTs based sensors were developed to study the ability of aligning and
assembling of nanostructures using AC electric fields. After that, the author developed
conductometric sensors based on graphene with Pd nanoparticles to evaluate the functionalities of
the devices fabricated on the designed interdigitated electrodes and the catalytic properties of
metal nanoparticles on carbon surfaces. The results suggested that the incorporation of Pd
nanoparticles onto graphene surface can improve the sensing performance of the devices due to
their catalytic properties, which promoted chemisorption of hydrogen even at room temperature.
Since metallic nanoparticles improved the gas sensing performances of carbon based
nanomaterials, NCNTs with Pt/Ni catalysts in different load ratios were employed to build
conductometric sensing devices using dielectrophoresis. The sensitivities of developed sensors
towards H2 gas were assessed and analyzed. It is suggested that, the highest sensitivity, fastest
response and recovery were all obtained for the Pt/NCNT sensor. Therefore, the catalytic activity
of the Pt nanoparticles was higher than that of Pt-Ni alloyed nanoparticles. The results also
suggested that an increased density of Pt catalysts could result in an improvement of device
sensitivity.
Polythiophene/CdTe composite was successfully assembled in between interdigitated electrodes
using dielectrophoresis to form field effect transistors. The experiments and calculations suggest
that the developed transistor has a current on/off ratio of more than 3×104, and the carrier
mobility of the material is approximately 13 cm2/Vs at room temperature. However, a decreasing
of the device performance was observed after four weeks of operation which is mainly due to the
interactions of the material with moisture and/or oxygen.
172
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Chapter 7
Conclusion and Future Works
One of the most significant advantages of the dielectrophoresis related techniques is their great
controllability. Using dielectrophoresis, the accurate manipulations of particles can be achieved.
As a result, nanotechnology enabled devices can also be formed by placing nanomaterials into
desired locations. In addition, the incorporation of dielectrophoresis and other techniques can be
used for developing advanced tunable systems which are employed in novel applications in
diverse disciplines. Therefore, this PhD research program was commenced with the aim of
investigating the phenomenon of dielectrophoresis and its application in microfluidics,
optofluidics and electronics.
182
To realize such applications using dielectrophoresis, three different configurations of
microelectrodes were designed and fabricated; their functionalities in producing DEP forces were
evaluated through the simulations of electric field distributions. The finger-paired electrodes were
utilized in the investigation of DEP assembling and aligning nanomaterials using different
alternating potentials. The micro-tip electrodes, which can produce sharp electric fields near their
tips, were designed to form electronic devices using dielectrophoresis. Such an electrode
configuration was successfully employed to fabricate gas sensors and field effect transistors
(FETs) using dielectrophoretically assembled nitrogen doped carbon nanotubes (NCNTs) with
metallic (Pt and Ni) nanoparticles and polythiophene/CdTe quantum dots (QDs), respectively. In
addition, due to their exceptional performance in particle trapping applications, micro-tip
electrodes were also utilized to separate polystyrene microparticles from carbon nanotubes in
stationary liquid. The curved electrodes, which are capable of producing strong electric fields
over a large proportion of the microfluidic channel, were designed to manipulate particles within
liquid flows. Using such a design, the DEP behavior of micro particles under different flow
conditions were comprehensively investigated; tasks such as, controllable focusing of polystyrene
microparticles, sorting of polystyrene particles according to their dimensions, sorting of live and
dead cells, trapping of polystyrene particles using pre-patterned carbon nanotubes, were
successfully demonstrated. In addition, the curved electrodes were also employed in the
development of tunable optofluidic waveguides using silica particles.
In the following sections, the major findings in the author’s PhD research are summarized and the
recommended future works are presented.
7.1 Conclusions
The outcomes of the author’s PhD research are as follows:
• The DEP separation of polystyrene microparticles from multi-walled carbon nanotubes
(MWCNTs) was successfully demonstrated in stationary DI water. To the author’s best
knowledge, the separation of nanostructures from microstructures was demonstrated for
the first time. In this work, polystyrene microparticles were distinguished from
MWCNTs by the DEP forces at frequencies higher than 100 kHz. The work illustrated
the possibility of using DEP techniques in novel applications, such as the purification of
183
nano-scaled impurities, separation of organelles from tissues and in-vivo manipulations
of nanostructures.
• Curved electrodes were developed for the manipulation of particles within liquid flows.
The unique advantage of such an electrode configuration is that the produced electric
fields are homogenously distributed along the microfluidic channel, such that the
particles do not experience an abrupt increase of DEP forces over the electrode tips,
avoiding spiral or chaotic motions. Three tasks were conducted to characterize the
functionality and effectiveness of such an electrode configuration:
1. The controllable focusing of polystyrene particles was successfully demonstrated
using the curved electrodes. In this work, polystyrene particles were concentrated
into narrow bands, the widths of such bands can be controlled by tuning the
applied AC potential and liquid flow rate.
2. DEP separation of polystyrene particles of different dimensions was successfully
demonstrated using the curved electrodes. In this work, the polystyrene particles
were redistributed in the microfluidic channel according to their sizes using
negative DEP forces.
3. A DEP-microfluidic device with the curved electrodes was also employed as a
DEP cell sorter to separate live and dead cells. The live and dead cells were
distinguished by DEP forces due to the large difference in their dielectric
properties. The sorting efficiency of the system was quantitatively assessed using
an additional electrode array by capturing and counting the escaped live cells.
• The trapping of polystyrene microparticles using pre-patterned carbon nanotubes was
demonstrated for the first time. Carobon nanotubes were pre-patterned between the
electrodes to serve as the electrode extensions; due to their small dimension, such
nanotubes can produce much stronger electric fields compared with the original
electrodes. Therefore, such a technique can be employed to improve the performances in
particle trapping applications. In addition, it will be possible to deposit micro/nano
particles into desired formations using pre-patterned nanotubes.
• A tunable optofluidic waveguide, which was formed using dielectrophoretically
controlled sub-micron silica particles, was demonstrated. To the author’s best knowledge,
184
it is the first time for the suspended particles to be utilized in optofluidic applications.
The integration of microfluidics and microphotonics facilitates the development optical
devices with characteristics of fluids and offers great device configurability. Suspended
nanoparticles in liquid can be brought into intimate contacts with each other, while still
remaining detached. This means configurable functional 3D elements can be formed,
tuned, and disabled using dielectrophoretically controlled particles. Using such features,
optical devices such as tunable couplers, multiplexers, isolators, lenses, switches, and
polarizers can also be developed in liquid.
• Nitrogen-doped carbon nanotubes (NCNTs) with/without different compositions of metal
nanoparticles (Pt and Ni) were deposited onto micro-tip electrode arrays to form
hydrogen gas sensors. To the author’s best knowledge, it is the first time for such
materials to be employed in gas sensing applications. DEP deposition technique was
employed in the work to ensure the device reliability and functionality. The sensors
fabricated with NCNTs with metal nanoparticles have higher sensitivity and faster
response compared with NCNT sensors. This is due to the fact that metal nanoparticles
dissociate hydrogen molecules into atomic hydrogen and thus promote the chemisorption
of hydrogen. Also, since the highest sensitivity, fastest response and recovery were all
measured for the Pt/NCNT sensor, the catalytic activity of the Pt nanoparticles was
higher than the Pt-Ni alloyed nanoparticles. The results therefore suggest that if the
density of the adhered Pt nanoparticles were increased, higher sensitivity would be
achieved.
• Polythiophene/CdTe quantum dots composites were dielectrophoretically assembled to
form FET devices. To the author’s best knowledge, it is the first time that an organic-
inorganic hybrid material was employed in the fabrication of electronic devices. The
FETs based on such materials promise a combination of useful characteristics such as
increased carrier mobility, easy processability, and inexpensive device fabrication. The
developed device illustrated a typical P-FET behavior and an exceptional current on/off
ratio of 3 × 104. The calculation suggested that the carrier mobility for the synthesized
material is approximately 2000 cm2/Vs, which is superior to current literature reported
carrier mobility values. It is suggested that the developed FET devices can also be
employed as the platform for gas sensing applications. In addition, this work promised a
new route to the understanding and development of efficient hybrid organic-inorganic
devices.
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In conclusion, the author’s research program resulted in numerous of novel and significant
contributions to the field of dielectrophoresis, fluidics and nanotechnology enabled systems. The
outcomes of this PhD research were published in peer reviewed scientific journals and
proceedings of international conferences. A complete list of publications by the author is as
follows:
Journal publications
1. C. Zhang, K. Khoshmanesh, A. Mitchell, and K. Kalantar-zadeh, "Dielectrophoresis for
manipulation of micro/nano particles in microfluidic systems," Analytical and
Bioanalytical Chemistry, vol. 396, pp. 401-420, 2009.
2. C. Zhang, K. Khoshmanesh, F. J. Tovar-Lopez, A. Mitchell, W. Wlodarski, and K.
Klantar-Zadeh, "Dielectrophoretic separation of carbon nanotubes and polystyrene
microparticles," Microfluidics and Nanofluidics, vol. 7, pp. 633-645, 2009.
3. K. Khoshmanesh, C. Zhang, F. J. Tovar-Lopez, S. Nahavandi, S. Baratchi, K. Kalantar-
Zadeh, and A. Mitchell, "Dielectrophoretic manipulation and separation of microparticles
using curved microelectrodes," Electrophoresis, vol. 30, pp. 3707-3717, 2009.
4. K. Khoshmanesh, C. Zhang, F. J. Tovar-Lopez, S. Nahavandi, S. Baratchi, A. Mitchell,
and K. Kalantar-zadeh, "Dielectrophoretic-activated cell sorter based on curved
microelectrodes," Microfluidics and Nanofluidics, vol. 9, pp. 2-3, 2010.
5. A. Z. Sadek, C. Zhang, Z. Hu, J. G. Partridge, D. G. McCulloch, W. Wlodarski, and K.
Kalantar-zadeh, "Uniformly Dispersed Pt-Ni Nanoparticles on Nitrogen-Doped Carbon
Nanotubes for Hydrogen Sensing," Journal of Physical Chemistry C, vol. 114, pp. 238-
242, 2009.
6. A. A. Kayani, C. Zhang, K. Khoshmanesh, J. L. Campbell, A. Mitchell, and K. Kalantar-
zadeh, "Novel tuneable optical elements based on nanoparticle suspensions in
microfluidics," Electrophoresis, vol. 31, pp. 1071-1079, 2010.
7. K. Khoshmanesh, C. Zhang, S. Nahavandi, F. J. Tovar-Lopez, S. Baratchi, Z. Hu, A.
Mitchell, and K. Kalantar-Zadeh, "Particle trapping using dielectrophoretically patterned
carbon nanotubes," Electrophoresis, vol. 31, pp. 1366-75, 2010.
186
8. K. Khoshmanesh, C. Zhang, J. L. Campbell, A. A. Kayani, S. Nahavandi, A. Mitchell,
and K. Kalantar-zadeh, "Dielectrophoretically Assembled Particles:Feasibility for
Optofluidic Systems," Microfluidics and Nanofluidics, In press, DOI 10.1007/s10404-
010-0590-7, 2010.
Articles under submission
1. V. Strong, C. Zhang, K. Khoshmanesh, R. Kaner, and K. Kalantar-Zadeh, "Field Effect
Transistor Based on Dielectrophoretically assembled Polythiophene/CdTe Quantum Dots
Composite".
2. C. Zhang, S. Dubin, K. Wang, R. Kojima, R. Kaner, W. Wlodorski, K. Kalantar-Zadeh,
"A conductometric sensor based on graphene with Pd nanoparticles".
Conference publications:
1. C. Zhang, M. Breedon, W. Wlodarski, and K. Kalantar-zadeh, "Study of the alignment
of multiwalled carbon nanotubes using dielectrophoresis" proceedings of Society of
Photo-Optical Instrumentation Engineers (SPIE) Conference, Camberra, Australia, 2007,
number: 680213.
2. C. Zhang, A. Z. Sadek, M. Breedon, S. J. Ippolito, W. Wlodarski, T. Truman, and K.
Kalantar-zadeh, "Conductometric Sensor based on Nanostructured Titanium Oxide Thin
Film Deposited on Polyimide Substrate with Dissimilar Metallic Electrodes,"
proceedings of 2010 International Conference On Nanoscience and Nanotechnology,
Melbourne, Australia, 2008, pp. 94-96.
3. C. Zhang, K. Khoshmanesh, A. A. Kayani, F. J. Tovar-Lopez, W. Wlodarski, A.
Mitchell, and K. Kalantar-zadeh, "Dielectrophoretic Manipulation of Polystyrene Micro
Particles in Microfluidic Systems," proceedings of Micro/Nanoscale Heat & Mass
Transfer International Conference, Shanghai, China, 2009, number: 18216.
187
7.2 Future works
This thesis presented the work of the author in the fields of dielectrophoresis and its applications
in microfluidics, optofluidics and electronics. To conduct possible extensions and developments
of the current research topic, two options can be selected. One of the options is to develop
advanced systems which operate in liquid. Such tasks will involve the forming of suspension
based optical and electronic devices using dielectrophoretically controlled particles. The other
direction is to utilize dielectrophoresis as a tool to form nanostructured electronic and optical
devices which can be used after removing the liquid. In these tasks, nanomaterials will be
dielectrophoretically deposited, aligned and patterned onto different platforms with desired
formations to form functional elements.
1. For the development of advanced systems that operate in liquid, there are few points to be
considered:
• In order to accurately evaluate the functionality of the developed DEP-optofluidic
systems, the light coupling has to be improved. One of the approaches is to
incorporate rib or ridge type waveguides with gratings. Having this system, to which
the light can be efficiently coupled, the power losses and transmissions caused by the
suspended particles can be accurately assessed.
• When the light coupling performance is successfully improved, it will be valuable to
attempt developing optofluidic devices other waveguides, such as tunable couplers,
multiplexers, isolators, lenses, switches, and polarizers, using the
dielectrophoretically controlled particles.
• It will be challenging to develop configurable electronic devices in liquid using the
dielectrophoretically controlled particles. In order to form electronic devices in liquid,
the materials of the suspended particles have to be conductive or semi-conductive;
therefore, it is very likely for such particles to experience positive DEP forces for all
the frequencies. In this case, the functional 3D elements can be formed between
electrodes using positive DEP forces, but cannot be disabled using negative DEP
forces and the device will not be configurable. To address this issue, nonconductive
particles with conductive surface coatings may be used. Such core-shell structures
have a tunable DEP spectrum according to the dielectric properties of their
188
single/multiple shells. Therefore, it will be possible to develop configurable
electronic devices such as diodes, transistors and switches in liquid.
• It will be valuable to investigate the applications of travelling wave dielectrophoresis
in micorfluidics, optofluidics and electronics. Since such a technique can be
employed for transporting particles, the integration of classical and traveling wave
dielectrophoresis can form DEP-microfluidic systems without liquid flows. Using
such systems, micro/nano scaled particles can be more accurately and effectively
manipulated as the travelling wave DEP forces offer better controllability and
stability in comparison to hydrodynamic forces.
2. To develop nanostructured electronic and optical devices which can be used after
removing the liquid, following options are valuable to be investigated:
• Different types of particles can be dielectrophoretically assembled to form functional
elements such as P-N junctions and Shottkey diodes. To achieve such applications,
multiple electrodes, which can be activated separately, will be required. Using such
electrodes, P and N type or semiconductive particles can be assembled separately to
form contacts and electronic devices can be developed.
• Patterning of nanostructures, such as quantum dots, may be achieved to form
periodical structures for optical applications. Therefore, dielectrophoresis can be
employed to develop nanostructured optical devices, such as waveguides, filters and
couplers using patterned particles.
189
Appendix A
List of Author’s Publications
The following papers have been published based on the work undertaken in this thesis, in refereed
international journals and in the proceedings of international conferences:
A.1 Journal publications
• C. Zhang, K. Khoshmanesh, A. Mitchell, and K. Kalantar-zadeh, "Dielectrophoresis
for manipulation of micro/nano particles in microfluidic systems," Analytical and
Bioanalytical Chemistry, vol. 396, pp. 401-420, 2009.
• C. Zhang, K. Khoshmanesh, F. J. Tovar-Lopez, A. Mitchell, W. Wlodarski, and K.
Klantar-Zadeh, "Dielectrophoretic separation of carbon nanotubes and polystyrene
microparticles," Microfluidics and Nanofluidics, vol. 7, pp. 633-645, 2009.
• K. Khoshmanesh, C. Zhang, F. J. Tovar-Lopez, S. Nahavandi, S. Baratchi, K.
Kalantar-Zadeh, and A. Mitchell, "Dielectrophoretic manipulation and separation of
microparticles using curved microelectrodes," Electrophoresis, vol. 30, pp. 3707-
3717, 2009.
• K. Khoshmanesh, C. Zhang, F. J. Tovar-Lopez, S. Nahavandi, S. Baratchi, A.
Mitchell, and K. Kalantar-zadeh, "Dielectrophoretic-activated cell sorter based on
curved microelectrodes," Microfluidics and Nanofluidics vol. 9, pp. 2-3, 2010.
• A. Z. Sadek, C. Zhang, Z. Hu, J. G. Partridge, D. G. McCulloch, W. Wlodarski, and
K. Kalantar-zadeh, "Uniformly Dispersed Pt-Ni Nanoparticles on Nitrogen-Doped
190
Carbon Nanotubes for Hydrogen Sensing," Journal of Physical Chemistry C, vol. 114,
pp. 238-242, 2009.
• A. A. Kayani, C. Zhang, K. Khoshmanesh, J. L. Campbell, A. Mitchell, and K.
Kalantar-zadeh, "Novel tuneable optical elements based on nanoparticle suspensions
in microfluidics," Electrophoresis, vol. 31, pp. 1071-1079, 2010.
• K. Khoshmanesh, C. Zhang, S. Nahavandi, F. J. Tovar-Lopez, S. Baratchi, Z. Hu, A.
Mitchell, and K. Kalantar-Zadeh, "Particle trapping using dielectrophoretically
patterned carbon nanotubes," Electrophoresis, vol. 31, pp. 1366-75, 2010.
• K. Khoshmanesh, C. Zhang, J. L. Campbell, A. A. Kayani, S. Nahavandi, A. Mitchell,
and K. Kalantar-zadeh, "Dielectrophoretically Assembled Particles:Feasibility for
Optofluidic Systems," Microfluidics and Nanofluidics, vol. 9, 2010.
• K. Khoshmanesh, C. Zhang, S. Nahavandi, F. J. Tovar-Lopez, S. Baratchi, A.
Mitchell, and K. Kalantar-Zadeh, "Size based separation of microparticles using a
dielectrophoretic activated system," Journal of Applied Physics, vol. 108, 2010.
A.2 Conference publications
• C. Zhang, M. Breedon, W. Wlodarski, and K. Kalantar-zadeh, "Study of the
alignment of multiwalled carbon nanotubes using dielectrophoresis" in Society of
Photo-Optical Instrumentation Engineers (SPIE) Conference, Camberra, Australia,
2007, proceeding number: 680213.
• C. Zhang, A. Z. Sadek, M. Breedon, S. J. Ippolito, W. Wlodarski, T. Truman, and K.
Kalantar-zadeh, "Conductometric Sensor based on Nanostructured Titanium Oxide
Thin Film Deposited on Polyimide Substrate with Dissimilar Metallic Electrodes," in
2010 International Conference On Nanoscience and Nanotechnology, Melbourne,
Australia, 2008, proceeding number: 4639254, pp. 94-96.
• C. Zhang, K. Khoshmanesh, A. A. Kayani, F. J. Tovar-Lopez, W. Wlodarski, A.
Mitchell, and K. Kalantar-zadeh, "Dielectrophoretic Manipulation of Polystyrene
191
Micro Particles in Microfluidic Systems," in Micro/Nanoscale Heat & Mass Transfer
International Conference, Shanghai, China, 2009, proceeding number: 18216.