© 2010 in-kook jun - university of...
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BIOINSPIRED, FUNCTIONAL NANOSCALE MATERIALS
By
IN-KOOK JUN
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2010
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© 2010 In-Kook Jun
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To my family and friends, for their unwavering support
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ACKNOWLEDGMENTS
First and foremost, I thank my mentors and guides, Drs. Henry Hess and Peng
Jiang for their brilliance, humility, and patience. Directly and indirectly, they have taught
me far more things than what can be articulated in this note of acknowledgement. I
would like to thank my committee members Drs. David Norton, Wolfgang Sigmund,
Valentin Craciun, and Yiider Tseng for their time, guidance and constructive comments.
I thank all the Hess group members: Thorsten Fischer, Rob, Parag, Isaac, Ashu,
and Yoli, and former members, Shruti and Krishna. I also thank all the Jiang group
members: Stanley, Nick, William, Erik, Tzung-hua, and Hongta, and visiting professors
Dr. Xuefeng Liu from Jiangnan University and Dr. Satoshi Watanabe from Kyoto
University for their constructive feedback and assistance in research. I was very
fortunate to be a member of both groups.
Finally, and most importantly, I express my thanks and gratitude to my parents
and friends in Korea, who have always supported me in spirit along this journey. My
parents and friends gave me the courage to pursue my dreams, and encouraged me to
be a successful researcher with an aggressive attitude in life and work. I am always
going to be grateful for having them by my side. Right now, I am so missing my Mom in
the Heaven.
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TABLE OF CONTENTS
page
ACKNOWLEDGMENTS .................................................................................................. 4
LIST OF TABLES ............................................................................................................ 8
LIST OF FIGURES .......................................................................................................... 9
LIST OF ABBREVIATIONS ........................................................................................... 12
ABSTRACT ................................................................................................................... 14
CHAPTER
1 INTRODUCTION .................................................................................................... 16
Bioinspired, Engineered Functional Materials ......................................................... 16
Scope of Thesis ...................................................................................................... 17
Self-pumping Membranes for Synthetic Transport Systems............................. 17
Nanoscale SERS Substrates for Effective Detecting Systems ......................... 17
Binary Colloidal Crystals for Optical Systems ................................................... 18
Organic-inorganic Nanocomposites for Mechanical Systems........................... 18
2 A BIOMIMETIC, SELF-PUMPING MEMBRANE ..................................................... 21
Background ............................................................................................................. 21
Electroosmotic Flow ......................................................................................... 22
Applications of Electroosmotic Micropumps ..................................................... 24
Challenges for Electroosmotic Pumps .............................................................. 25
Requirements of Self-pumping Membranes ............................................................ 25
Electrochemical Systems for the Self-pumping Membrane ..................................... 26
Compartment-less Fuel Cell System for the Self-pumping Membrane ................... 27
Results and Discussion........................................................................................... 28
Conclusions ............................................................................................................ 31
Materials and Methods............................................................................................ 32
Membrane Preparation ..................................................................................... 32
Chamber Design .............................................................................................. 32
Flow Measurements ......................................................................................... 33
3 GOLD NANOPARTICLE-NANOHOLE ARRAYS AS SERS SUBSTRATES ........... 41
Background ............................................................................................................. 41
Raman Scattering & Surface Enhanced Raman Scattering (SERS) ................ 41
Mechanisms for SERS Enhancement .............................................................. 42
Various Techniques for SERS Substrates ........................................................ 44
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SERS Applications ........................................................................................... 45
Challenges for SERS Substrates ..................................................................... 46
Raman Scattering Intensity Measurement / SERS EFF Calculation ................ 47
Simulation of Effects of Size and Spacing on Electromagnetic Field ................ 49
Particle Self-assembly Approach ............................................................................ 50
Particle Aggregates for SERS .......................................................................... 50
Self-assembled Particles as a SERS Substrate ............................................... 51
STV/PEG-GNP Arrays...................................................................................... 54
Periodic Nanostructure Approach ........................................................................... 55
Periodic Nanostructures as SERS Substrates .................................................. 55
Periodic Nanostructures from Non-close-packed Particle Arrays ..................... 57
Nanohole Arrays on a Glass Substrate ............................................................ 59
STV/PEG-GNP Arrays on Nanohole Arrays ........................................................... 60
Concept of GNP-nanohole Arrays .................................................................... 60
Fabrication of GNP-nanohole Arrays ................................................................ 60
Optical and SERS Properties of STV/PEG-GNP Arrays on Nanohole Arrays .. 61
Conclusions ............................................................................................................ 62
Materials and Methods............................................................................................ 63
Materials ........................................................................................................... 63
Instrumentation ................................................................................................. 63
Gold Nanohole Arrays on a Glass Slide ........................................................... 63
STV/PEG-GNP Arrays on Gold Nanohole Arrays ............................................ 64
Absorbance of Substrates ................................................................................ 64
Raman Spectra Measurements ........................................................................ 64
Calculation of Enhancement Factors ................................................................ 65
4 BINARY COLLOIDAL CRYSTALS ......................................................................... 77
Results and Discussion........................................................................................... 79
Conclusions ............................................................................................................ 82
Materials and Methods............................................................................................ 82
Materials and Substrates .................................................................................. 82
Instrumentation ................................................................................................. 83
Non-close-packed Colloidal Monolayer ............................................................ 83
Ternary Layer Hexagonally Non-close-packed Colloidal Structure .................. 84
5 BIOINSPIRED, ORGANIC-INORGANIC NANOCOMPOSITES ............................. 90
Assembly of Colloidal Nanoplatelets ....................................................................... 91
Synthesized Gibbsite Nanoplatelets ................................................................. 91
Assembly of Gibbsite Nanoplatelets by Electrophoretic Deposition ................. 92
Mechanical Properties of Nanocomposites ...................................................... 92
Assembly of Surface-roughened Nanoplatelets ...................................................... 93
Silica-coated Gibbsite Nanoplatelets ................................................................ 93
Assembly of Silica-coated Gibbsite Nanoplatelets ........................................... 93
Mechanical Properties of Nanocomposites ...................................................... 94
Conclusions ............................................................................................................ 94
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Materials and Methods............................................................................................ 95
Materials ........................................................................................................... 95
Instrumentation ................................................................................................. 95
Synthesis of Gibbsite Nanoplatelets ................................................................. 96
Surface Modification of Gibbsite Nanoplatelets with TPM ................................ 96
Coating of Gibbsite Nanoplatelets with Silica ................................................... 97
Electrophoretic Deposition ................................................................................ 97
Mechanical Test ............................................................................................... 98
6 CONCLUSIONS AND OUTLOOK ........................................................................ 102
Self-pumping Membrane ....................................................................................... 102
Reproducible and Highly SERS-active Substrates ............................................... 102
Potential Applications as a Hybrid Biosensor ................................................. 102
Binary Colloidal Crystals ....................................................................................... 103
APPENDIX
A ELECTROOSMOTIC FLOW ................................................................................. 105
Electroosmotic Flow in One-wall Channel ............................................................. 105
Electroosmotic Flow in Cylindrical Tube ............................................................... 105
B SELF-PUMPING FLOW ........................................................................................ 110
Flow Rate as a Function of Tracer Velocity .......................................................... 110
Conductivity of Working Fluid ............................................................................... 110
Flow Rate as a Function of Current ...................................................................... 111
Flow Rate at Zero Opposing Pressure .................................................................. 113
C FLOW RATE MEASUREMENT ............................................................................ 115
Length-converting Method .................................................................................... 115
Mass-converting Method ....................................................................................... 115
Current-monitoring Method ................................................................................... 115
Particle Image Velocimetry Method ...................................................................... 115
Concentration-monitoring Method ......................................................................... 116
LIST OF REFERENCES ............................................................................................. 120
BIOGRAPHICAL SKETCH .......................................................................................... 136
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LIST OF TABLES
Table page 3-1 Calculated SERS enhancement factor on the fabricated substrates from
measured data of Raman intensities at 999.2 cm-1 and 1023 cm-1. .................... 66
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LIST OF FIGURES
Figure page 1-1 Functional nanoscale materials in nature: a) moth-eye, b) gecko‟s foot, c)
lotus leaf.. ........................................................................................................... 20
2-1 Electroosmotic flow in a channel.. ...................................................................... 34
2-2 Various types of electroosmotic pumps: a) packed-column EP, b) porous monolith column EP, c) open channel EP, and d) porous membrane-based EP. . .................................................................................................................... 34
2-3 The self-pumping membrane: a) gold and platinum electrodes deposited on the opposing surfaces of a track-etched polycarbonate membrane. b) surface and c) cross-section of the track-etched membrane. .......................................... 35
2-4 The dimensions and photograph of the experimental set up. ............................. 36
2-5 The experimental set up. The membrane is mounted submerged in solution to create a compartment connected to the larger reservoir by the membrane and a narrow channel.. ....................................................................................... 36
2-6 The electroosmotic pumping membrane. Flow rate and electric current as a function of external voltage in an aqueous solution. The inset shows a rescaled plot of the voltage range from -1 V to + 1 V.. ....................................... 37
2-7 Flow rate dependent on current in electroosmotic pumping. In the electroosmotic flow, the measured flow rate increases linearly with increasing measured current. .............................................................................................. 37
2-8 The self-pumping membrane. Flow rate and electric current as a function of time in a 0.01% hydrogen peroxide solution as the gold and platinum electrode are externally connected and disconnected.. ...................................... 38
2-9 Scanning electron microscopy images of the membrane after operation show tracer particles accumulated on the membrane surface and within the pores, possibly leading to a reduction in pumping efficiency. ........................................ 38
2-10 Flow rate dependent on current in the self-pumping. In the self-pumping flow, the measured flow rate increases linearly with increasing measured current. .... 39
2-11 Calculation of tracer velocity at low flow rates. At a low velocity of tracers, individual tracer positions can be accurately determined and tracers can be followed from frame to frame. ............................................................................. 39
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2-12 Calculation of tracer velocity at high flow rates. At a high velocity of tracers, tracers moving in the center of the channel appear as streaks while tracers adsorbed to the channel surface appear as dots.. .............................................. 40
3-1 Various characteristic energies: a) Rayleigh scattering, b) Stokes Raman scattering, and c) anti-Stokes Raman scattering. ............................................... 67
3-2 Schematic diagrams of a) a surface plasmon polariton (or propagating plasmon) on a flat surface and b) a localized surface plasmon on a nanostructured surface.. ..................................................................................... 67
3-3 Various SERS substrates: a) Ag film on nanospheres, periodic nanostructures by b) nanosphere lithography and c) electron-beam lithography, and d) colloidal aggregates.. ........................................................... 68
3-4 SERS applications: a) in vivo glucose sensing equipment consisted of SERS spectroscopy, implanted substrate, beam directing optics, and collection lens and b) identification of cancer genes by Raman labels.. .................................... 69
3-5 Representative Raman spectrum of benzenethiol adsorbed on a SERS substrate.. ........................................................................................................... 69
3-6 The SERS enhancement calculated based on finite element methods in the case of nanoparticle arrays.. .............................................................................. 70
3-7 Schematic diagram depicting self-assembly of gold nanoparticles using a flow cell. .............................................................................................................. 71
3-8 SEM images of 30 nm gold nanoparticle arrays with a gap between particles on glass substrates. ............................................................................................ 72
3-9 Schematic diagram depicting the fabrication procedures for making GNP-nanohole arrays. ................................................................................................. 73
3-10 SEM images of nanohole arrays: a) 330 nm nanohole arrays and b) 400 nm nanohole arrays. ................................................................................................. 74
3-11 SEM images of nanohole-GNP arrays: a) 330 nm nanohole arrays covered by 30 nm gold particles and b) 400 nm nanohole arrays covered by 30 nm gold particles. ..................................................................................................... 74
3-12 Absorption spectra on a) gold nanoparticle (GNP) arrays and nanohole arrays and b) nanohole arrays covered by gold nanoparticles. .......................... 75
3-13 Raman spectra of benzenethiol absorbed on a) flat god surface, b) gold nanoparticle (GNP) arrays, c) 330 nm nanohole arrays covered by gold nanoparticles, and d) 400 nm nanohole arrays................................................... 76
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4-1 Photonic crystals: a) photonic crystals in 1-D, 2-D, and 3-D and b) a photonic band gap compared to an electronic band gap................................................... 85
4-2 Schematic illustration of the procedure for fabricating binary hexagonal arrays of silica spheres by using monolayer nonclose-packed colloidal crystals as substrates. ........................................................................................ 86
4-3 Monolayer of nonclose-packed silica particles (300 nm) fabricated by the spin-coating technique. ....................................................................................... 86
4-4 Binary hexagonal arrays of silica spheres: a, b) 300 nm particles array on 300 nm particles array (300/300), and c, d) 400 nm particles array on 300 nm particles array (400/300). .................................................................................... 87
4-5 Pressure effects on the ordering of particles in the second layer: a, b) 0.2 MPa for 2 min, and c, d) 0.33 MPa for 2 min in 300 nm particles arrays on 300 nm particles arrays (300/300). ..................................................................... 88
4-6 Cross-sections of hexagonal arrays of silica spheres: a, b) binary layer of 400 nm particles array on 345 nm particles array (400/345) and c) ternary layer. ..... 89
5-1 Hard biological tissues and their microstructures: a) tooth, b) vertebral bone, c) shell, d) Enamel made of long needle-like crystals with soft protein matrix, e) dentin and bone made of plate-like crystals. .................................................. 99
5-2 A model of biocomposites: a) a schematic diagram of staggered mineral crystals embedded in protein matrix and b) a simplified model showing the load-transfer mechanism in the mineral–protein composites.. ............................ 99
5-3 Nanocomposite of colloidal nanoplatelets: a) TEM image of gibbsite nanoplatelets, b) SEM image of a free standing gibbsite-ETPTA nanocomposite, and c) tensile strain-stress curves. ......................................... 100
5-4 Nanocomposite of surface-roughened nanoplatelets: a) TEM image of acid-leached silica-coated gibbsite nanoplatelets, b) SEM image of silica-coated gibbsite-PEI-ETPTA nanocomposite on an ITO electrode. ............................... 101
C-1 Various methods for measuring the flow rate: a) length-converting method, b) mass-converting method, c) current-monitoring method, and d) particle image velocimetry method.. .............................................................................. 118
C-2 The double membrane configuration having compartment 1 and compartment 2 in the concentration-monitoring method. .................................. 118
C-3 Plot of function, f, with time. The diffusion constant, D, and the pumping rate, k, by the electroosmotic pump can be estimated from the slope. ..................... 119
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LIST OF ABBREVIATIONS
ADP Adenosine diphosphate
AFM Atomic force microscopy
APTCS Acryloxypropyl trichorosilane
ATP Adenosine triphosphate
CTAB Cetrimonium bromide
DI Deionized
DMFC Direct methanol fuel cell
DNA Deoxyribonucleic acid
EBL Electronic beam lithography
ETPTA Ethoxylated trymethylolpropane triacrylate
FEG-SEM Field emission gun scanning electron microscopy
FEM Finite element method
FIB Focused ion beam
FITC Fluorescein isothiocyanate
GNP Gold nanoparticle
HIV Human immunodeficiency virus
ICP Inductively coupled plasma
IEP Isoelectric point
ITO Indium tin oxide
LB Langmuir-Blodgett
LBL Layer-by-layer
LSPR Localized surface plasmon resonance
MFON Metal film on nanosphere
NIR Near-infrared
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NSL Nanosphere lithography
ORC Oxidation-reduction cycle
PBG Photonic band gap
PC Photonic crystal
PDMS Poly(dimethylsiloxane)
PEG Poly(ethylene glycol)
PEI Poly(ethylenimine)
PEMFC Proton exchange membrane fuel cell
PML Perfect matched layer
PMMA Poly(methyl methacrylate)
PNIPAM Poly(N-isopropylacrylamide)
PVP Polyvinylpyrrolidone
RIE Reactive ion etching
SEM Scanning electron microscopy
SERS Surface enhanced Raman scattering
SERS EF Surface enhanced Raman scattering enhancement factor
STV Streptavidin
TEM Tranmission electron microscopy
TEOS Tetraethyl orthosilicate
TPM Trimethoxysilyl propyl methacrylate
UV Ultra-violet
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Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
BIOINSPIRED, FUNCTIONAL NANOSCALE MATERIALS
By
In-kook Jun
August 2010
Chair: Henry Hess Cochair: Peng Jiang Major: Materials Science and Engineering
Functional nanomaterials in nature exhibit many unique functions and optical and
mechanical properties. Examples of this include the dry adhesion of a gecko‟s foot, the
reduced drag on a shark‟s skin, the high strength and toughness of nacre, and the
superhydrophobic self-cleaning of a lotus leaf. This dissertation is devoted to creating
unique and enhanced properties by mimicking such functional materials.
We have developed a novel self-pumping membrane, which does not require an
applied voltage. The self-pumping membrane harvests chemical energy from a
surrounding fluid and uses it for accelerated mass transport across the membrane. A
device such as this has promising applications in implantable or remotely operating
autonomous devices and membrane-based purification systems.
Reproducible and highly active surface enhanced Raman scattering (SERS)
substrates were developed using a bottom-up self-assembly technology. With their high
sensitivity and good reproducibility, the developed nanostructures (gold nanoparticle
and nanohole arrays) as SERS substrates are very promising for applications such as
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ultra-sensitive detectors for chemicals and reproducible sensors for chemical and
biological molecules.
Binary colloidal crystals were created using a simple, fast, and scalable spin-
coating technology. Although further investigation of the procedure is needed to improve
the ordering of particles in the individual layers, the developed assembly technology has
a promising outlook in applications such as optical integrated circuits and high-speed
optical computing.
Inorganic-organic nanocomposites were realized by assembling synthesized
gibbsite nanoplatelets using the electrophoretic deposition and infiltration of a monomer
followed by polymerization. Via surface modifications of gibbsite nanoplatelets,
nanocomposites were further reinforced with covalent linkages between the inorganic
platelets and organic matrix.
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CHAPTER 1 INTRODUCTION
In nature, functional nanoscale materials exhibit unique optical, mechanical, and
functional properties, usually obtained from hierarchical structures.[1] For example, a
moth‟s eye has many hexagonally shaped facet lenses on which highly ordered arrays
of nanoscale nipples are located.[2] The hierarchical nanoscale nipple arrays generate
the enhanced light-sensitivity of light-craving moths. The gecko has an exceptional
ability to climb rapidly up smooth vertical surfaces due to the hierarchical toe structure
consisting of a myriad of nanoscale spatulae.[3] The nanoscale hair-like structures on
micro-scale mound-like structures protruding from its leaf are responsible for the
superhydrophobicity in a Lotus-leaf.[4]
Bioinspired, Engineered Functional Materials
The driving force to investigate and mimic nature‟s structures comes from the
belief that nature‟s structures are most efficient. Researchers have tried to mimic
various natural structures, such as the dry adhesion of a gecko‟s foot, the reduced drag
on a shark‟s skin, the iridescent color of a morpho-butterfly, the high strength and
toughness of nacre, and the superhydrophobic self-cleaning of a lotus leaf.[3-6]
Biomolecular motors such as kinesin and myosin are capable of transporting
analytes in a biosensor.[7, 8] They gain mechanical energy from hydrolyzing ATP to
ADP and inorganic phosphate at high efficiency. Biomolecular motors could potentially
replace a pump and a power supply in a smart dust biosensor; they may be integrated
with the housing and communication unit. Biofuel cells, which generate electricity using
glucose as fuel from the fluid, have been developed.[9] They gain electrical energy from
oxidizing fuel and reducing oxygen. Highly active catalysts for the electrochemical
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reactions and electron transfer systems have been investigated to improve the
performance. Hydrogen peroxide, hydrazine, ethanol, and methanol have been
explored as alternative fuels for the electrochemical reactions in fuel cell systems to
enhance their stability, durability, and power density.
Artificial antireflection coatings are used in displays, optical components, and solar
cells; however, current antireflection technologies such as quarter-wavelength multilayer
films and nanoporous coatings are expensive and suboptimal. By mimicking the moth-
eyes, non-close-packed nipples in the sub-300 nm size range, effective antireflection
coatings have been recently developed with a bottom-up self-assembly technique.[10-
12]
Scope of Thesis
Self-pumping Membranes for Synthetic Transport Systems
Hybrid biosensors face the fundamental challenge of limited stability, due to their
biological components and the difficulty of obtaining macroscopic observable signals.
Conventional transporting systems such as electroosmotic pumps are robust and
stable; however, they still require a highly miniaturized pump and power supply. It has
been shown that some electrochemical reactions can induce mass transport by
converting the chemical energy gained from the surrounding fluid into the mechanical
energy with no need of an externally applied voltage. In this work, we design such a
synthetic transport system and investigate the effectiveness and efficiency in mass
transport.
Nanoscale SERS Substrates for Effective Detecting Systems
One of the challenges in biosensors is achieving high sensitivity. Surface
enhanced Raman scattering (SERS) can be utilized to amplify the signal. It is well
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known that Raman signals dramatically increase when metallic nanostructures with
wavelength scale topography are used as SERS substrate. The maximum SERS
enhancement has been reported up to 1014 in nanoparticle aggregates.[13] Moreover,
Raman spectroscopy is a label-free technique, so a tagging procedure is not necessary.
In this work, we fabricate several nanostructures with a bottom-up self-assembly
technique and investigate their capabilities as SERS substrates.
Binary Colloidal Crystals for Optical Systems
The development of integrated optical circuits with photonic crystals has been
greatly impeded by its reliance on expensive and complex nanofabrication techniques
such as electron-beam lithography (EBL) and focused ion-beam (FIB). Bottom-up
colloidal self-assembly and subsequent templating nanofabrication can provide a much
simpler, faster, and inexpensive alternative. However, current colloidal self-assemblies
are limited to a low volume, laboratory-scale production. In this work, we investigate a
bottom-up self-assembly approach via a spin-coating technology to create non-close-
packed binary colloidal crystals.
Organic-inorganic Nanocomposites for Mechanical Systems
The nacreous layer of mollusk shells has an intricate brick-and-mortar
nanostructure which makes the shells exceptionally tough and stiff. Various bottom-up
self-assembly techniques such as layer-by-layer (LBL), ice-templated crystallization,
spin-coating, gravitational sedimentation, and centrifugation, have been explored to
mimic the nacre structure. In this work, we assemble gibbsite nanoplatelets, having high
aspect ratio (diameter-thickness ratio), via a simple, inexpensive, and scalable
elelctrophoretic deposition. We generate organic-inorganic nanocomposites by
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infiltrating monomer between platelet layers and polymerizing monomer, and investigate
their mechanical properties.
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Figure 1-1. Functional nanoscale materials in nature: a) moth-eye, b) gecko‟s foot, c) lotus leaf. Adapted from [2, 4, 14-16].
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CHAPTER 2 A BIOMIMETIC, SELF-PUMPING MEMBRANE
Biological membranes accelerate materials exchange across the membrane by
active, ATP-dependent transport through specialized channel proteins. Similarly, the
integration of “pumping” driven by chemical energy harvested from the fluid into a
synthetic membrane is highly desirable from an engineering point of view, since it would
obviate the need for external devices, such as pumps or centrifuges, to drive flow
across the membrane. Here, a novel pumping-membrane with no need of applied
voltage is described. Instead of the externally applied voltage, electrodes deposited on
the opposing surface of a membrane generate a transmembrane potential from the
electrochemical reactions (the hydrolysis of hydrogen peroxide in aqueous solution).
Short-circuiting the electrodes permits an ionic current to flow between the electrodes,
which in turn, creates a flow of about 100 nL cm-2 min-1. Future applications of such self-
pumping membranes may include implantable or remotely operating autonomous
devices and membrane-based purification systems.
Background
Microfludic devices have advantages for mixing, separating, and sensing reagents
due to their laminar flows and small diffusion distances compared to macro-scale
devices.[17] Microfluidic devices require small volumes of samples (from 100 nL to 10
μL), are disposable due to low production costs, and are amenable to high throughput
due to processing assays in parallel. Micropumps as fluidic transport system are
essential in operating microfluidic devices.
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Various Types of Micropumps
Micropumps can be divided into two categories according to their operating
mechanisms: (1) displacement pumps, which exert forces on the working fluid through
moving surfaces and (2) dynamic pumps, which add energy to the working fluid by
increasing momentum or pressure directly.[18] The displacement pumps usually rely on
moving parts such as check valves, oscillating membranes, or turbines for the constant
delivery of a fluid. Diaphragm pumps, which make up the majority of reported
displacement micropumps, are actuated by piezoelectric, thermopneumatic,
electrostatic, and electromagnetic mechanisms.[19-22] On the other hand, dynamic
pumps generate the kinetic energy of the fluid by electroosmosis, electrowetting,
thermocapillary, and electrochemical, electrohydrodynamic, and magnetohydrodynamic
mechanisms without moving parts.[23-28] Dynamic pumps are advantageous in
micrometer-scale devices where the high surface-to-volume ratio is favorable.
Electroosmotic Flow
Electroosmotic micropumps are of significant promise for a wide variety of
applications.[17, 29, 30] Briefly, surface charges within a channel attract counter ions
which experience a force directed along the channel axis when an electric field is
applied across the channel (Figure 2-1). The viscous drag between the counter ions and
fluid in turn exerts a force on the fluid that is localized at the channel wall, inducing a
plug-like flow profile.[31]
In electrophoresis, the solids (the particles) are moving in the applied electrical
field. Moving particles can drag fluid (water) molecules, generating an electrophoretic
fluid flow through channels. On the other hand, in electroosmosis, the solid (the channel
wall) is stationary, while the fluid is moving in the applied electrical field. When the solid
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contacts the liquid, the ions having charge opposite to that of the solid are attracted to
the solid forming an electric double layer.[30, 32, 33] Electroosmotic flow is induced by
an applied electric field to the electric double layer.
Various Types of Electroosmotic Micropumps
Based on the pumping elements, there are various types of electroosmotic
micropumps such as packed column, porous monolith pumps, open-channel pumps,
and membrane-based pumps.[29, 34-44] Packed-column electroosmotic pumps can
produce high pressures (Figure 2-2a).[29, 34, 35] The interstitial spaces between the
packed particles are parallel passages for fluid flows. The pumping power closely
depends on the size of the packed particles. In the pumps with large particles, high flow
rates can be achieved, while pumping pressures decrease. Physical geometries of the
channel such as tortuosity, porosity, and effective pore size and properties of the
working fluid are controlled to optimize the performance of electroosmotic pumps.
A porous monolith column is prepared by polymerizing the column of a monomer,
a crosslinker, a free radical initiator, and a porogenic solvent (Figure 2-2b).[36, 37] The
materials can be an organic polymer or an inorganic silica monolith. The advantage of a
porous monolith column is the elimination of frits or filters which are used in packed-
column electroosmotic pumps. The pore size and porosity as well as chemistry and
crosslinking density can be easily controlled to achieve the desired flow rate and
pressure.
Channel electroosmotic pumps use narrow capillaries or microchannels as
pumping elements (Figure 2-2c).[38-40] By consisting of hundreds of parallel and small
microchannels, both high pumping rates and high pressures can be achieved. Channel
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electroosmotic pumps can be easily integrated on a microfluidic platform by standard
microfabrication techniques and are robust and reliable due to the simplicity of design.
The pump rate is controlled by the electric field and the size and number of channels.
Porous membrane pumps have many channels through the thin membrane
(Figure 2-2d).[41-44] Since the channels are short, a high electric field is achieved with
a low applied voltage. Due to the large number of microchannels, it is possible to
generate high pressures at high flow rates. Supporting frames are often used to secure
the robustness of the membrane and to apply voltage as electrodes. Various
membranes such as glass, alumina, and polymer are reported to show excellent
pumping performances. Alumina membranes with highly aligned nanochannels are
reported to show high flow velocities at low voltage.
Applications of Electroosmotic Micropumps
Electroosmotic pumps can transport liquid samples with significant flow rates and
pressures. Without moving parts, electroosmotic pumps generate plug-like and laminar
fluid flows, which can be controlled by applied voltages. The electroosmotic pumps are
favorable in microfluidic devices where the surface-to-volume ratios are high.[45]
Due to the unique characteristics of electroosmotic micropumps, many potential
applications have been suggested. Electroosmotic micropumps have great potential in
liquid drug delivery and biological sample assays.[30] Electroosmotic micropumps have
been integrated into proton exchange membrane fuel cells (PEMFCs) to remove water
from cathodes.[46] Electroosmotic pumps have also been used as fuel delivery systems
in direct methanol fuel cells (DMFCs).[47] Compact micropumps having high heat
dissipation rates are essential in microelectronics as cooling systems. Electroosmotic
micropumps made of glass frits have been reported as effective cooling systems.[48]
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Challenges for Electroosmotic Pumps
Although electroosmotic pumps have great potentials as transport systems, there
are still challenges to overcome for real applications. Nonpolar liquids such as oil are
difficult to transport by the electroosmotic pump and the pumping function degrades
over time due to the interaction with working fluids. The bubbles generated on the
electrodes can interfere with the pumping function, especially in closed-loop fluidic
devices. The efficiency of the pump should also be improved.
Requirements of Self-pumping Membranes
To construct a self-pumping membrane which generates a fluid flow by harvesting
chemical energy from a fluid and converting the chemical energy into the kinetic energy,
there are several requirements to be satisfied.
First, the electrochemical reactions on the electrodes deposited on the opposite
surfaces of a membrane should generate a transmembrane potential. From this point of
view, battery systems such as primary cells (e.g. Daniell cell) and secondary cells (e.g.
lead-acid cell) and fuel cell systems can be candidates.
Second, the electrochemical reactions should generate new ions or protons as
products. For example, the protons generated on the anode will drag the fluid flow in the
electric field. On the other hand, the transition from Fe3+ to Fe2+ doesn‟t generate new
ions or protons for dragging the fluid flow.
Third, the electrodes should not be changed by the reduction-oxidation reactions.
In battery systems, the anode themselves dissolve into the solution, while metal ions
reduce on the cathode. In this view point, fuel cell systems are preferable since
electrodes in fuel cell systems can continue working by being fed with fuels without a
change of electrode.
26
Fourth, the fluid flow should not interfere with the electrochemical reactions. The
reactants for the electrochemical reaction (oxidation) on the anode and their products
should not interfere with the electrochemical reaction (reduction) on the cathode and
vice versa. Thus, “compartment-less” electrochemical systems are most promising
candidates for the self-pumping membrane.
Electrochemical Systems for the Self-pumping Membrane
Electrochemical systems utilized in the research on chemotaxis or catalytic
nanomotors can offer promising candidate systems for the self-pumping membrane
satisfying the above requirements. Metal rods or surfaces (Pt, Au/Ni, catalyst, Pt/Au,
Ag) in the hydrogen peroxide solution show autonomous movement, rotational motion,
and transportation of cargos by ejecting small oxygen bubbles or generating an
interfacial tension force or a difference in diffusion coefficient due to the catalytic
decomposition of hydrogen peroxide.[49-53] However, it was proved later that the
autonomous movements of Pt/Au nanorods were due to the self-electrophoresis.
Protons generated by the electrochemical reaction of hydrogen peroxide flow in the
catalytically induced electric field inducing autonomous movements of nanorods.[54, 55]
In further research, Pt/Au or Au/Ag rods in hydrogen peroxide solution have shown an
autonomous movement and the speed and direction of movement can be controlled.[56,
57] A convective fluid flow was shown on the Ag/Au patterned surface in hydrogen
peroxide solution.[58]
Other electrochemical systems have also been investigated for autonomous
movement. Glucose oxidase/bilirubin oxidase fibers were propelled in the interface
between electrolyte and air due to the electrochemical reactions of glucose and
27
oxygen.[59] A Pd/Au patterned surface in hydrazine solution showed electroosmotic
flow in a catalytically induced electric field.[60]
Compartment-less Fuel Cell System for the Self-pumping Membrane
The electrochemical systems developed in the investigations of chemotaxis and
catalytic nanomotors have been applied to compartment-less fuel cell systems. In
compartment-less fuel cells, a differential in the ability of the two electrodes to catalyze
the anodic and cathodic reaction enables the creation of an electric potential and
removes the need for an ion-exchange membrane. A compartment-less hydrogen
peroxide fuel cell with Au and Ag as catalytic electrodes was developed generating the
maximum current density of 2.9 mA cm-2. [61] In this fuel system, the bubble generation
on the anode interfered with further electrochemical reactions. While Ni electrodes
generated less bubbles, they were also less-active for the electrochemical reaction of
hydrogen peroxide.
Compartment-less glucose/oxygen fuel cells have been developed.[9, 62-65] A
biofuel cell having an anode functionalized with surface-reconstituted glucose oxidase
and a cathode modified with cytochrome c and cytochrome oxidase generated a
maximum power of 4 μW.[62] The current density generated in biofuel cells having an
anode immobilized with glucose oxidase and a cathode immobilized with a bilirubin
oxidase reached up to 10 mA cm-1.[9] The maximum power density from the biofuel cell
having an anode functionalized with glucose dehydrogenase complex and a Pt cathode
was 930 nW cm-2.[64]
Compartment-less fuel cells using methanol or ethanol as fuels were also
reported.[66, 67] Fuel cells with a nickel hydroxide anode and a silver oxide cathode
using a fuel mixture of methanol and hydrogen peroxide achieved the maximum power
28
density of 28.73 μW cm-2.[66] The maximum power density from fuel cells having an
anode of alcoholdehydrongenase associated with carbon nanotubes and a cathode of
bilirubin-Pt nanoparticle composite was recorded at 200 μW cm-2.[67] Although there
are membrane-less fuel cells, these fuel cells separate fuels by laminar flow instead of
ion-exchange membranes, so they don‟t satisfy the requirements of an electrochemical
system for self-pumping membrane.[68, 69]
While glucose is the fuel of choice for most studies, and significant advances have
been achieved in the design of enzyme-based biofuel cells,[70] in this work, hydrogen
peroxide is adopted as the fuel to simplify the electrode design and to utilize a
combination of platinum and gold electrodes known as effective catalysts for the
electrochemical decomposition of hydrogen peroxide to drive electroosmotic flows.[56]
Results and Discussion
To construct the self-pumping membrane, platinum and gold films of 25 nm
thickness (and a 5 nm titanium adhesion layer) are sputter-deposited on the opposing
surfaces of track-etched polycarbonate membranes with a pore diameter of 0.96 μm, a
thickness of 18 μm and a porosity of 12% (Figure 2-3, 2-4). The platinum electrode and
the gold electrode are electrically connected by an external switch.
In the proposed self-pumping membrane, on the platinum electrode, hydrogen
peroxide decomposes into oxygen, protons, and electrons. The generated electrons
flow along the electrical connection, while the generated protons flow through the pores
of the membrane under the catalytically induced electric field. These electrons and
protons are consumed on the gold electrode by combining with hydrogen peroxide to
produce water.
29
The membrane is then integrated into a fluid chamber designed to facilitate the
measurement of nL s-1 pumping speeds at near-zero backing pressure (Figure 2-5).
Flow rates are measured by microparticle image velocimetry,[71] using fluorescent
microspheres (1 μm diameter) as tracers. To amplify the flow velocity, the flow is
monitored in a narrow channel of 51 μm height, 320 μm width, and 6 mm length.
The proper functioning of the experimental setup was validated by providing an
external voltage to the membrane submersed in a solution of water and tracer
microspheres. The dependence of electric current and particle velocity as a function of
external voltage (Figure 2-6) followed the behavior expected for the hydrolysis of water,
which has a decomposition potential difference of 1.23 V.[72] Positive numbers in flow
rate and current correspond to flow/current from the gold electrode to the platinum
electrode, while negative numbers signify the opposite direction. The flow rate is
calculated from the measured velocities of tracers based on the relationship between
maximum velocity and the flow rate in a rectangular channel.[73] The flow rate
increases linearly with increasing voltage across the membrane up to about 1.2 V. At an
applied voltage of 1V the flow rate is -0.94 nL s-1 and the current is -1.9 A. For applied
voltages above 1.4 V, flow rate and current increase linearly with increasing voltage with
slopes of -43 nL s-1 V-1 and -120 μA V-1, respectively. This implies a conductivity of the
working fluid (water and tracer particles) of 2.0 μS cm-1, which is close to the
conductivity of water.
Using the Helmholtz-Smoluchowski equation to calculate the electroosmotic flow
through the membrane while assuming the zeta potential of polycarbonate[71] to be -27
mV and considering the pressure-induced reverse flow caused by the resistance of the
30
small outlet channel, the flow rate as a function of current can be calculated. While the
calculation shows the observed linear dependence of flow rate on current with a slope
of 3 nL μA-1 s-1, it also shows that the high resistance of the small detection channel
relative to the membrane resistance reduces the net flow through the membrane about
30-fold relative to the expected electroosmotic flow at zero pressure. In the
electroosmotic pump, the observed pumping efficiency varies from 0.4 to 0.5 nL μA-1 s-1
(Figure 2-7), which is lower than the calculated 3 nL μA-1 s-1.
Pumping in the absence of an external voltage is activated by the addition of
hydrogen peroxide to the aqueous solution at a concentration of 0.01 wt%. At this low
concentration of hydrogen peroxide, the formation of gas bubbles at the electrodes is
avoided. The platinum and gold electrodes are connected to a switch and an
amperemeter. Fluid flow is dependent on the state of the switch: when the switch is
closed, flow across the membrane commences from platinum to gold (Figure 2-8); when
the switch is open, the flow rate is near zero, initially with a small flow resulting from
small initial pressure differences. In less than 30 s, the flow rate reaches 0.9 nL s-1 and
the current reaches 0.26 μA. When the switch is opened, the flow rate rapidly ceases. In
subsequent switching cycles, the “closed switch” flow rates decreased by 20% after 270
min. This reduction is likely to be the result of a falling hydrogen peroxide concentration
due to its consumption at the electrodes or the result of clogging of the pores with tracer
particles (Figure 2-9).
The observed flow direction (from platinum to gold) is consistent with the proposed
pumping mechanism. The observed pumping efficiency of 3 nL μA-1 s-1 matches the
above calculated electroosmotic pumping efficiency (Figure 2-10). This agreement
31
supports the hypothesis that the pumping results from electroosmosis and not from the
formation of gas bubbles at the electrodes. The observed current density on the order of
3 mA m-2 at a hydrogen peroxide concentration of 0.01 wt% is in good agreement with
Paxton et al.‟s observation of 1 mA m-2 at a concentration of 0.006 wt%.[56] Using our
model of the system, we can estimate a flow rate at zero opposing pressure of 25 nL s-1
and a stall pressure of 1 Pa in the self-pumping membrane; both parameters are typical
for microfluidic pumps.[72]
Future improvements to such self-pumping membranes can focus on the
performance of the fuel cell component, the pump component, or their interaction. The
fuel cell can be improved by the use of better electrodes and alternative fuels, in
particular glucose. Compartment-less biofuel cells have reached current densities on
the order of 10 A m-2,[9, 73] which means that an improvement of four orders of
magnitude is possible. The electroosmotic pump can be optimized by increasing the
zeta potential, and tuning the hydrodynamic resistance of the pump and outlet, e.g. by
adjusting the pore diameter. Miao et al. described an electroosmotic pump which
creates a more than ten-fold higher flow rate for a given amount of current and
membrane area.[43] Finally, the current-voltage characteristics of the fuel cell power
source and the electroosmotic pump should be matched to maximize power conversion
efficiency. In the present design, the voltage drop across the electroosmotic pump is on
the order of 2 mV. Thus, the pump uses only a small fraction of the electromotive force
provided by the fuel cell.
Conclusions
Despite its performance limitations, the self-pumping membrane described here is
a first step towards replicating the ability of biological membranes to harvest chemical
32
energy from the surrounding fluid and use it for accelerated mass transport across the
membrane. Micro and macroscopic devices for drug delivery, sensing and purification,
as well as oil recovery and removal may benefit from this technology.
Materials and Methods
Membrane Preparation
A titanium adhesion layer (5 nm) and platinum and gold films (25 nm) were
deposited on the opposing surfaces of a polycarbonate membrane (pore diameter of
0.96 μm porosity of 12% and thickness of 18 μm determined from analysis of SEM
images, Isopore membrane filter, Millipore, Ireland) with a multi-target sputtering system
(Kurt J. Lesker CMS-18, Clairton, PA). The membrane was glued to a polycarbonate
frame (instant Krazy glue, Elmer‟s Product Inc., Columbus, OH). The platinum and gold
electrodes were connected to metal wires using silver paste (Leitsilber 200 Silver Paint,
Ted Pella Inc., Redding, CA). The resistance between the electrodes after
manufacturing was measured to be 0.2 MΩ.
Chamber Design
A narrow channel (51 μm high, 320 μm wide, 6 mm long) between two open
chambers was patterned with Polydimethylsiloxane (PDMS, Sylgard 184 silicone
elastomer kit, Dow Corning Corporation, MI) using a mold. The prepared chamber-
channel layer and the membrane layer were assembled on glass to form the
experimental set-up as shown in Figure 2-4.
For electroosmotic pumping, the working fluid is DI water (18 kΩ cm, Millipore Inc.,
Billerica, MA) with yellow-green fluorescent microspheres (1 μm diameter, 0.002% solid
loading, FluoSphere amine-modified microsphere, Molecular Probes, Eugene, OR). An
external voltage is applied between the electrodes with a DC regulated power supply
33
(EXTECH instruments, Waltham, MA). For the self-pumping membrane, 0.01 wt% H2O2
solution (Aldrich, St. Louis, MO) is added. The platinum and gold electrodes are
electrically connected by clamping the metal wires connecting platinum and gold
electrodes together.
Flow Measurements
The narrow channel between the two chambers was imaged with an Eclipse
TE200U epi-fluorescence microscope (Nikon, Melville, NY) equipped with an X-cite 120
lamp (EXFO, Ontario, Canada), a 10X objective, a cooled CCD camera (Andor iXon,
Andor Technology, Windsor, CT), and a FITC filter cube (no. 48001, Chroma
Technology Corp, Rockingham, VT). The focal plane was set to the center of the
channel where moving tracers were clearly visible. At low flow rates, images were
acquired at 100 ms intervals with 20 ms exposure times (Figure 2-11). Flow rates were
calculated by measuring the particle displacement after 1 s. At high flow rates, images
were acquired at 1 s intervals with 100 ms exposure times (Figure 2-12). Flow rates
were calculated by measuring the length of the streak generated by the moving particle
and dividing by the exposure time. The current was measured with an amperemeter
(Digital Multimeter 34410A, Agilent Technologies, Santa Clara, CA).
34
Figure 2-1. Electroosmotic flow in a channel. Adapted from [74].
Figure 2-2. Various types of electroosmotic pumps: a) packed-column EP, b) porous monolith column EP, c) open channel EP, and d) porous membrane-based EP. Adapted from [32, 36, 39, 43].
35
Figure 2-3. The self-pumping membrane: a) gold and platinum electrodes deposited on the opposing surfaces of a track-etched polycarbonate membrane. b) surface and c) cross-section of the track-etched membrane imaged by scanning electron microscopy enable measurements of pore diameter, porosity and thickness of the membrane.
36
Figure 2-4. The dimensions and photograph of the experimental set up.
Figure 2-5. The experimental set up. The membrane is mounted submerged in solution to create a compartment connected to the larger reservoir by the membrane and a narrow channel. Flow through the membrane forces fluid through the narrow channel where the flow velocity can be measured by particle-tracking microscopy.
O2 + 2H+ + 2e-
flu
id f
low
H2O2
2H2OH2O2 + 2H+ + 2e-
e-H+
channel
51 μm x 320 μm
25 nm Pt
25 nm Au
5 nm Ti
5 nm Ti
H+
Tracer particles
objective
H2O2 solution
O2 + 2H+ + 2e-
flu
id f
low
H2O2
2H2OH2O2 + 2H+ + 2e-
e-H+
channel
51 μm x 320 μm
25 nm Pt
25 nm Au
5 nm Ti
5 nm Ti
H+
Tracer particles
objective
H2O2 solution
37
Figure 2-6. The electroosmotic pumping membrane. Flow rate and electric current as a function of external voltage in an aqueous solution. The inset shows a rescaled plot of the voltage range from -1 V to + 1 V. Error bars show the standard deviation of tracer particle velocities.
Figure 2-7. Flow rate dependent on current in electroosmotic pumping. In the electroosmotic flow, the measured flow rate increases linearly with increasing measured current.
Pt → Au
Au → Pt
-2 -1 0 1 2
-20
0
20
40
60
-2 -1 0 1 2-100
-50
0
50
100
150
Voltage (V)
Flo
w r
ate
(nL s
-1)
flow rate
Curr
ent (
A)
currentflow ratecurrent
Au → Pt
Pt → Au
-1.0 -0.5 0.0 0.5 1.0
-1.0
-0.5
0.0
0.5
1.0
-1.0 -0.5 0.0 0.5 1.0
-2
-1
0
1
2
Voltage (V)
Flo
w r
ate
(nL s
-1)
Curr
ent (
A)
Pt → Au
Au → Pt
-2 -1 0 1 2
-20
0
20
40
60
-2 -1 0 1 2-100
-50
0
50
100
150
Voltage (V)
Flo
w r
ate
(nL s
-1)
flow rate
Curr
ent (
A)
currentflow ratecurrentflow ratecurrent
Au → Pt
Pt → Au
-1.0 -0.5 0.0 0.5 1.0
-1.0
-0.5
0.0
0.5
1.0
-1.0 -0.5 0.0 0.5 1.0
-2
-1
0
1
2
Voltage (V)
Flo
w r
ate
(nL s
-1)
Curr
ent (
A)
Au → Pt
Pt → Au
-1.0 -0.5 0.0 0.5 1.0
-1.0
-0.5
0.0
0.5
1.0
-1.0 -0.5 0.0 0.5 1.0
-2
-1
0
1
2
Voltage (V)
Flo
w r
ate
(nL s
-1)
Curr
ent (
A)
38
Figure 2-8. The self-pumping membrane. Flow rate and electric current as a function of time in a 0.01% hydrogen peroxide solution as the gold and platinum electrode are externally connected and disconnected. Error bars show the standard deviation of tracer particle velocities.
Figure 2-9. Scanning electron microscopy images of the membrane after operation show tracer particles accumulated on the membrane surface and within the pores, possibly leading to a reduction in pumping efficiency: a) Au surface of membrane and b) cross-section of membrane.
connect connect connect connect connect
disconnect disconnect disconnect disconnect
Pt → Au
Au → Pt
0 50 100 150 200 250
-0.4
0.0
0.4
0.8
1.2
0 50 100 150 200 250
-0.1
0.0
0.1
0.2
0.3
0.4
F
low
rate
(nL s
-1)
Time (min)
flow rate
Curr
ent (
A)
currentflow rate currentconnect connect connect connect connect
disconnect disconnect disconnect disconnect
Pt → Au
Au → Pt
0 50 100 150 200 250
-0.4
0.0
0.4
0.8
1.2
0 50 100 150 200 250
-0.1
0.0
0.1
0.2
0.3
0.4
F
low
rate
(nL s
-1)
Time (min)
flow rate
Curr
ent (
A)
currentflow rate current
39
Figure 2-10. Flow rate dependent on current in the self-pumping. In the self-pumping flow, the measured flow rate increases linearly with increasing measured current.
Figure 2-11. Calculation of tracer velocity at low flow rates. At a low velocity of tracers, individual tracer positions can be accurately determined and tracers can be followed from frame to frame (time between frames: 100 ms; exposure time: 20 ms). Each velocity data point is the average of the velocity of 10 tracer particles, which is determined by dividing the distance advanced after 10 frames by 1 s.
20 μm20 μm20 μm
40
Figure 2-12. Calculation of tracer velocity at high flow rates. At a high velocity of tracers, tracers moving in the center of the channel appear as streaks while tracers adsorbed to the channel surface appear as dots. The tracer velocity is calculated by dividing the streak length by the exposure time (100 ms). Each velocity data point is the average of 10 tracer particle velocities.
10 μm10 μm10 μm
41
CHAPTER 3 GOLD NANOPARTICLE-NANOHOLE ARRAYS AS SERS SUBSTRATES
Raman spectroscopy is a noninvasive technology that enables label-free detection
of molecules. However, the Raman signal is very weak due to the small inelastic Raman
scattering cross section. Surface enhanced Raman scattering (SERS) can greatly
increase the Raman signal by electromagnetic enhancement and chemical
enhancement. The SERS enhancement factor was reported to be 1014 with silver or
gold nanoparticle aggregates as SERS substrates.[13] In this range of SERS
enhancement, even single molecules can be detected.
The SERS substrates should have high enhancements of Raman signal and show
reproducible enhancements for sensing or detecting applications. Particle aggregates or
fractals show high SERS enhancements, but the reproducibility is poor due to their
irregular structures. On the other hand, periodic nanostructures fabricated by
lithographical techniques exhibit better reproducibility, but their SERS enhancements
reported in literature are lower than that of particle aggregates.
Background
Raman Scattering & Surface Enhanced Raman Scattering (SERS)
Raman spectroscopy is a critical technique for structural analysis of molecules
which relies on inelastic scattering of visible light. Raman scattering is attributed to the
excitation and relaxation of vibrational modes of a molecule (Figure 3-1). Because
different functional groups have different characteristic vibrational energies, the
molecular structures of every molecule can be probed by the inelastic Raman
scattering. However, since Raman scattering cross sections are typically 14 orders of
magnitude smaller than those of fluorescence, the Raman signal is several orders of
42
magnitude weaker than fluorescence emission. Thus, the applicability of Raman
scattering is restricted to structural analysis.[75]
However, a dramatically enhanced Raman signal has been obtained with a
technique called surface enhanced Raman scattering (SERS). When the scatterer is
placed on or near roughened noble-metal substrates, the magnitude of the Raman
scattering signal can be greatly enhanced. This SERS enhancement of the signal
transforms Raman spectroscopy from a structural analytical tool to a structural probe
with single-molecule sensitivity.[76]
Mechanisms for SERS Enhancement
The mechanism of SERS enhancement remains an active research topic. There
are two mechanisms which contribute to the SERS effect: an electromagnetic
enhancement and a chemical enhancement. In the chemical enhancement, new
electronic states are created from chemisorption between the metal and adsorbate
molecules. They serve as resonant intermediate states in Raman scattering. Charge
transfer excitations can occur at about half the energy of the intrinsic intramolecular
excitations of the adsorbate. The existence of charge-transfer state increases the
probability of a Raman transition by providing a pathway for resonant excitation. This
mechanism is site-specific as well as and analyte-dependent and contributes an
enhancement factor of about 100.[77]
The electromagnetic enhancement arises from focusing an electromagnetic field
via plasmon resonance of the metallic substrate on the metal surface. Surface plasmon
polaritons propagate along the metallic surface and are trapped on the surface because
of the resonant interaction between the surface charge oscillation and the
electromagnetic field of the light (Figure 3-2a).[78] When light interacts with
43
nanostructures much smaller than the incident wavelength, surface plasmon polaritons
are localized (Figure 3-2b). When the localized surface plasmon resonance (LSPR) of
nanostructures on a sliver or gold substrate is excited by visible light, strong
electromagnetic fields are generated due to selective absorption and scattering of the
resonant electromagnetic radiation. When the analyte molecule is subjected to these
intensified electromagnetic fields, the intensity of the inelastic Raman scattering
increases. Electromagnetic enhancement contributes an average enhancement factor
of over 10,000.[75]
The resonant frequency of the conduction electrons in a metallic nanostructure
depends on the size, shape, and material of the structure. In the case of a spherical
nanoparticle of radius a that is irradiated by z-polarized light of wavelength λ (where a is
much smaller than λ), by solving Maxwell‟s equations using a quasi-static
approximation, the resulting solution for the electromagnetic field outside the particle is
given by
zzyyxxr
z
r
zEazEzyxE
outin
outinout 530
3
0
3
2),,(
(3-1)
where εin is the dielectric constant of the metal nanoparticle, and εout is the dielectric
constant of the external environment. When the dielectric constant of the metal is
roughly equal to -2εout, the electromagnetic field is enhanced relative to the incident
field. In the case of silver and gold, this condition is met in the visible region of the
spectrum.[79]
The enhancement factor for SERS is calculated as
44
volvNRS
surfvSERSvoutout
vSERSNI
NI
E
EEEF
/)(
/)()()()(
4
0
22
(3-2)
The Raman enhancement effect is a result of enhancing both the incident
excitation, Eout(ω), and the resulting Stokes‟ shifted Raman, Eout(ω- ω0),
electromagnetic fields. The overall enhancement scales roughly as E4. A small increase
in the local field produces large enhancements in the Raman scattering. The
enhancement factor from experimental measurements is given by the right-hand side of
equation. It is the SERS enhanced Raman intensity, ISERS(ωv), normalized by the
number of molecules bound to the enhancing metallic substrate, Nsurf, divided by the
normal Raman intensity, INRS(ω0), normalized by the number of molecules in the
excitation volume, Nvol.
Various Techniques for SERS Substrates
Various SERS substrates have been developed: electrodes roughened by an
oxidation-reduction cycle (ORC), island films, colloidal nanoparticles, and surface-
confined nanostructures. ORC-roughened electrodes provide reproducible, in situ SERS
substrates with moderate (~ 106) enhancement factors.[75]
Metal island film substrates are easy to fabricate and the LSPR wavelength can be
tuned by varying the film‟s thickness and confluence. However, the enhancement
factors achieved with these films are generally smaller (~ 104 – 105) than those
observed with other SERS substrates. Surface-confined nanostructures can be
produced by several fabrication schemes, including electron-beam lithography, colloid
immobilization, and soft lithography (Figure 3-3). With substrates fabricated via electron
45
beam lithography, enhancement factors as large as 108 have been achieved by
controlling the inter-particle spacing.[80]
Colloidal nanoparticle substrates are well suited to solution-phase SERS studies.
The Kneipp research group probed small (100 - 150 nm) silver colloid aggregates dosed
with crystal violet molecules. The large (1014) single-molecule enhancement was
attributed to large electromagnetic fields generated by fractal-pattern clusters of silver
colloid nanoparticles.[13] In colloidal aggregates substrates, there are a small number of
hot spots, which occur at the junction between nanoparticles. Theoretical modeling
shows the strong electromagnetic field between nanoparticles separated by < 1 nm is
due to the surface junction excitation and the efficient interaction of the molecular wave
function with the wave function of the excited metal surface.[81]
SERS Applications
SERS holds great potential as an ultra-sensitive and selective tool for the
identification of biological or chemical agents. The narrow and well-resolved bands
allow simultaneous detection of multiple analytes. As water has a very weak Raman
signal, investigation of biological samples can also be carried out. SERS offers a
method for multicomponent or multiplexed detection of low-concentration analytes,
either by directly revealing the target analyte or by indirectly detecting the fingerprint of
a molecular label.
SERS has been applied to the signal transduction mechanism in a prototype for an
implantable glucose sensor (Figure 3-4). Glucose was detected and quantified in the
physiological range with an accuracy approaching the requirements as a biomedical
device. SERS has also been applied in the detection of trace levels of chemical warfare
agents. Silver nanowires have been used as a substrate to detect 2,4-dinitrotoluene, the
46
most common chemical indicator of buried landmines and explosives, at a sensitivity of
0.7 pg.[82, 83]
SERS-active molecules have been implemented as labels on the analyte of
interest. Raman labels have been used to identify cancer genes, thus avoiding the
introduction of undesirable radioactive DNA labels.[84] ssDNA coupled with a gold
nanoparticle and a SERS label has been used to detect DNA hybridization events. Also,
multiplexed detection has been used to distinguish hepatitis A, hepatitis B, HIV, Ebola,
smallpox, and anthrax with a detection limit of 20 fM.[85]
Challenges for SERS Substrates
The Raman spectroscopy using SERS provides much more information about
molecular structure and the local environment in condensed phases than electronic
spectroscopy techniques like fluorescence. Minor changes in the orientation of an
adsorbate can be detected as slight variations yields measurable shifts in the locations
of Raman spectral peaks. The abrupt decay of the electromagnetic fields ensures that
only adsorbate molecules on or near noble-metal substrate are probed. This technique
is well suited for analyses performed on molecules in aqueous environments, because
water has an extremely weak Raman signal intensity.[75]
However, the inherent limitation of the technique is that the substrates should be
made of silver, gold, or copper. Other materials are not usable unless they are applied
as thin coatings on SERS-active materials. SERS has limited applicability when the
molecule of interest is not adsorbed directly onto the substrate.
The primary bottleneck has been the reproducible preparation of well-defined,
reliable, and stable substrates with a high SERS-activity.[86] Colloidal substrates tend
to aggregate and thus the molecular surface coverage changes with time. Variation in
47
the fabrication processes used to produce SERS-active substrates leads to inconsistent
optical properties and discrepant enhancement factors. In the currently used
nanofabrication techniques, the SERS enhancement factors can fluctuate by up to an
order of magnitude for substrates fabricated with seemingly identical methodology.
Regularly arranged monodispersed colloidal gold and silver particles on
functionalized metal or glass substrates or well-ordered nanostructured surfaces
produce SERS with good reproducibility and stability.[87] As the SERS intensity
depends on the excitation of the localized surface plasmon resonance (LSPR), it is
important to control all of the factors influencing the LSPR, namely the size, shape,
particle interspacing, and the dielectric environment to maximize signal and ensure
reproducibility.
Reproducibility can be improved by creating a long-range pattern with sub-
micrometer periodicity on the substrate. Nanosphere lithography (NSL) has excellent
control over nanometer scale features by utilizing self-assembled polymer nanospheres
as vapor deposition masks [88]. Electron-beam lithography (EBL) on a scanning
electron microscope has also been used to produce regular elongated particle arrays for
SERS with optimization control based on center-to-center spacing.[89]
However, these nanofabrication techniques have relatively lower enhancements
than colloidal aggregates. These techniques are also limited by higher cost of
fabrication, the availability of equipment, the substantial expertise required and the low
surface area of the metal structure.
Raman Scattering Intensity Measurement / SERS EFF Calculation
To evaluate the performance of prepared metal nanoparticles array as a SERS
substrate, benzenethiol is used as a model compound. In addition to the excellent
48
affinity to gold surfaces, benzenethiol molecules have a large Raman cross-section,
presumably as a result of chemical enhancement alongside electromagnetic
enhancement.[82] Each peak in the Raman spectra corresponds to the vibrational
specific mode: the peak at 1572 cm-1 to C-C stretching, the peak at 1072 cm-1 to C-S
stretching, the peak at 1021 cm-1 to in-plane ring deformation, and the peak at 997 cm-1
to S-H bending, as shown in Figure 3-5.[90]
Gold nanoparticles arrays are placed in a 5 mM solution of benzenethiol in 200-
proof-ethanol for 45 min and then rinsed in 25 ml of 200-proof-ethanol for several
minutes. The samples are allowed to air-dry for 20 min, after which the Raman spectra
are measured.[91] A flat gold film is sputter-deposited on a glass slide using the same
deposition condition used for the control sample for Raman spectra. Raman spectra are
measured with a Renishaw inVia confocal Raman microscope using a 785 nm diode
laser at 15 mW with an integration time of 10 s and a 40 μm2 spot size.
Raman scattering intensity is also measured in an aqueous environment.[13] The
excitation source is an argon-ion laser pumped Ti:sapphire laser operating at 830 nm
with a power of about 200 mW at the sample. Dispersion is achieved using a Chromex
spectrograph with a deep depletion CCD detector. A water immersion microscope
objective (363, NA 0.9) is brought into direct contact with a 30 ml droplet of sample
solution for both excitation and collection of the scattered light. Scattering volume is
estimated with a cylinder of diameter derived from the applied power and the intensity
and length derived from depth-of-field consideration.
The SERS enhancement factors are calculated from data collected using confocal
Raman spectrometers in which the surface enhancement, G, is defined as follows:
49
bulk
surfA
RI
hINcG
(3-3)
The intensity of the Raman peak obtained at the SERS surface, Isurf, is compared to that
obtained for a solution, Ibulk, of concentration c∞. NA is Avogadro‟s number, σ is the
surface area occupied by the adsorbate, R is the roughness factor of the surface, and h
is a parameter defined by the confocal volume of the spectrometer.[92]
Simulation of Effects of Size and Spacing on Electromagnetic Field
If SERS substrates with various parameters are assessed and optimized by
modeling their electromagnetic characteristics, time and effort can be saved in
laboratory preparation and experimental testing.
The field enhancement is strongly dependent on physical parameters such as the
surface morphology, the dielectric constants used to perform the modeling, and the
excitation conditions. Finite element electromagnetic modeling was applied to predict
the Raman enhancement with a variety of SERS substrates with differently sized,
spaced, and shaped morphologies with nanometer dimensions.[93]
The electromagnetic waves must satisfy Maxwell‟s equations within the modeling
domain, and adhere to the boundary equations at the interface between the media and
the scatterer. Finite element methods (FEM) using Comsol Multiphysics software have
been employed to provide numerical solutions for each substrate. For a radiation
condition, the perfect matched layers (PML) boundaries method and a low-reflection
boundary condition are applied. The output of the modeling process is a two-
dimensional map of the electric field intensity, which can be used to calculate the
Raman enhancement G(r, ω). When the polarization of the scattered light is the same
50
as that of the incident light, the expected electromagnetic enhancement of the Raman
signal may be expressed to a first approximation as
422
)(
),(
)(
)(
)(
)(),(
incfree
loc
Lfree
Lloc
E
rE
E
E
E
ErG (3-4)
where E(r, ω) is the total predicted electric field at position r, and Einc(ω) is the electric
field associated with the incoming electromagnetic radiation.
Particle Self-assembly Approach
Particle Aggregates for SERS
Kneipp et al reported that the SERS enhancement of 1014 was achieved with
colloidal silver solution, where silver nanoparticles slightly aggregated.[13] The
enhancement was calculated from comparison between the concentration of analyte in
the colloidal silver solution and the concentration in control solution (without colloidal
silver particles) having the same level of Raman signal. The general method to calculate
the enhancement factor (G) will be discussed in the next section. The possibility of
single molecule detection was demonstrated by the change in the distribution of Raman
signal when the average number of analyte is one.[13] Single hemoglobin molecules in
a junction between silver nanoparticles could be detected and the maximum SERS
enhancement in the hot spot was estimated to be 1010.[94]
The SERS enhancement has an advantage in the applications in an aqueous
solution due to very weak Raman signal of water. The SERS imaging in a living cell was
successful by using colloidal gold particles.[95] The biological molecules including
adenine, L-cysteine, L-lysine, and L-histidine, were successfully detected by using gold
nanoparticle aggregates as SERS substrates, where the enhancement was 107-109 in
bulk solution.[96]
51
The hot spot of high enhancement, which is generally located in a junction
between particles, is sensitive to the spacing between particles as well as the
wavelength and polarization of the excitation laser, as shown in Figure 3-6.[76, 93, 97]
The SERS enhancement up to 1014 in a junction between Ag-coated particles was
reported by calculation based on the extended Mie theory (the classical electromagnetic
theory of spherical particles).[94]
According to the mechanism for the electromagnetic enhancement, the localized
surface plasmon resonance (LSPR) is excited to generate a high SERS enhancement
when electromagnetic radiation is incident upon substrates with the same
wavelength.[75] To generate a high SERS enhancement, the wavelength of
electromagnetic radiation was tuned to the excitation of LSPR with different laser
sources. Alternatively, the excitation of LSPR, which is indicated as a peak in the
extinction spectrum, was tuned to the wavelength of electromagnetic radiation with
using various size and shape of particle and controlling the interparticle spacing. The
plasmon absorption was tuned by using different sizes of gold nanoparticles in aqueous
solution.[98] By assembling silver nanoparticles with DNA bases (adenine, guanine,
cytosine, and thymine) or surface-modifying gold nanoparticles with ATP, the
interparticle spacings between particles in particles aggregates were controlled so that
the plasmon absorption could be tuned.[99, 100]
Self-assembled Particles as a SERS Substrate
For a SERS substrate, a three-dimensional multilayered film with assembled gold
nanoparticles was prepared by using the Langmuir-Blodgett (LB) method.[90] The
Raman signals from self-assembled particles as a SERS substrate were ~ 107 higher
52
than those from the control (a bare substrate). The enhancements depended on the
Raman peak as well as the particle size and the film thickness.
Periodically ordered arrays of nanoparticles were devised to produce reproducible
SERS enhancement. To predict or estimate the SERS enhancement on the ordered
array of nanoparticles and understand the effects of parameters such as the particle
size, shape, and interparticle spacing, several models were designed to solve the
electromagnetic field satisfying the Maxwell‟s equations. Finite element methods (FEM),
T-matrix method, RLC circuit analogy were employed to solve the electromagnetic field
on nanoparticle arrays.[93, 101, 102]
It is worthwhile noting that the maximum value of enhancement (Gmax) obtained
from the hot spot should be distinguished from the average value of enhancement (Gave)
calculated over the entire surface of the substrate. The maximum enhancement factor
was used to emphasize the capability of single-molecule detection, for example, in
Kneipp‟s paper,[13] while the average enhancement factor was used to demonstrate the
effectiveness of the nanostructure as a SERS substrate, for example, in Jung‟s
paper.[90] Since the portions of hot spot in the entire surface of substrate are much less
than 1 %, the average enhancement (Gave) is several orders lower than the maximum
enhancement factor (Gmax).[103]
The enhancement factors are maximized at the wavelengths of the incident light
which are close to the excitation of the localized surface plasmon resonance (LSPR).
The maximum enhancement factor (Gmax) on periodic arrays of silver nanoparticles
calculated based on the Finite element methods was increased up to 108 with
decreasing separation between particles.[93] Based on the T-matrix theory, the
53
maximum calculated enhancement factor (Gmax) on one-dimensional arrays of silver
nanoparticles was calculated to be 109.[101] The average enhancement factor (Gave) on
two-dimensional square arrays of gold nanospheres calculated based on the RLC circuit
model is about 108, where Gave depends on the ratio of interparticle spacing and particle
size.[102]
Controlling the interparticle spacing and particle size is important to achieve a high
enhancement factor (Gave) in terms of generating large area of hot spots in a junction
between particles. Periodically ordered arrays of nanoparticles functionalized with
surfactant molecules were assembled to control the interparticle spacing between
particles. Gold nanoparticles of 50 nm diameter functionalized with
cetytrimethylammonium bromide (CTAB) were self-assembled to generate hexagonally
close-packed monolayer with the interparticle spacing of 8 nm.[104] The average
enhancement factors (Gave) were calculated to be up to 108 at the near-infrared (785
nm) excitation by comparing ratios of the SERS peak intensities to the corresponding
unenhanced signals from neat analyte films. Close-packed arrays of silver nanoparticles
of 20 nm capped with surfactant molecules of oleic acid and oleylamine were
assembled on the poly (N-isopropylacrylamide) (PNIPAM) film, where the interparticle
spacing could be controlled by altering the temperature since PNIPAM is a temperature-
sensitive polymer.[105] Controlling the interparticle spacing from less than 4 nm to 24
nm, brought the plasmon resonance peak closer to the laser excitation wavelength,
generating larger Raman signals of an analyte. Two-dimensional hexagonal close-
packed arrays of gold nanoparticles functionalized with resorcinarene tetrathiol had
interparticle spacing less than 1 nm.[106, 107] The average enhancement factors (Gave)
54
were determined using peak integration ratios of the SERS peak intensities to the
corresponding unenhanced signals from neat analyte films. The SERS enhancement
factors (Gave) of 107 with NIR excitation were shown in large particle arrays.
Most investigation has been carried out based on the direct relationship between
extinction/absorption and SERS enhancement. On the other hand, Lu et al claim that
the connection between extinction/absorption and SERS enhancement is indirect and
qualitative at best since the spatial distribution of collective resonances should be
considered.[108] Bulk-like resonances (proportional to the volume) have a large
contribution to absorption, while surface-like resonances (proportional to the surface)
have a large contribution to SERS enhancement. For example, a high SERS
enhancement was observed at the wavelength of excitation laser where there is no
resonance in the absorption/extinction.
STV/PEG-GNP Arrays
In this work, gold nanoparticle (30 nm) functionalized with streptavidin and
polyethylene glycol (PEG) chain are assembled into hexagonally close-packed arrays
using a flow cell, as shown in Figure 3-7. The size of streptavidin immobilized on the
nanoparticles is about 4 nm.[109] Double-sided tapes are assembled as spacers on a
clean glass slide and a thin glass slip is covered to make a flow cell.
Streptavidin/PEGylated gold nanoparticles (STV/PEG-GNP) solution monodispersed in
distilled water is flown through the flow cell. Solutions are pipeted from one side of the
flow cell and sucked out from the other side by capillary action. After 5 minutes, rinsing
is done by flowing enough volume of DI water to remove excessive gold particles and
the remaining salts in the solution. Solvent is then evaporated at the room temperature
generating STV/PEG-GNP arrays on a glass slide.
55
The self-assembled STV/PEG-GNP arrays are shown in Figure 3-8. They are non-
close-packed arrays due to spacers such as streptavidin and PEG. The interparticle
spacing corresponds to two-fold of the streptavidin size (8 nm). Most part of the surface
are covered with a monolayer of particle arrays, though some areas are not covered
with particles.
In the absorption spectrum, there is a peak at the excitation wavelength of 544
nm indicating the excitation of localized surface plasmon resonance (LSPR), as shown
in Figure 3-12a. The SERS spectrum at the excitation wavelength of 785 nm is shown in
Figure 3-13a. Benzenethiol is used as an analyte as it has a good affinity to gold and
forms a monolayer on gold surface. The SERS enhancement factor (Gave) is about
5×105 which is calculated from the SERS peak intensity at the Raman shift of 999.2 cm-
1 (S-H bending + in-plane ring deformation mode).[110] Although the wavelength of the
excitation laser (785 nm) is not close to the excitation of LSPR (544 nm), there is a high
SERS enhancement of 5×105.
Periodic Nanostructure Approach
Periodic Nanostructures as SERS Substrates
To generate reproducible SERS substrates, periodic nanostructures have been
fabricated by various micro/nanofabrication techniques. Silver nanoparticle arrays were
produced by electro-beam lithography.[111] The important factors in SERS
enhancements such as particle size, shape, and interparticle spacing could be
controlled. Triangular silver nanoparticle arrays were fabricated by nanosphere
lithography (NSL).[112] The excitation wavelength of localized surface plasmon
resonance (LSPR) was controlled with various dimensions of the nanoparticles and the
56
wavelength of the excitation laser was tuned with tunable laser systems. According to
the systematic investigation, the SERS enhancement was maximized when the
excitation wavelength of LSPR was located between the wavelength of the excitation
laser and the wavelength of the Raman scattered photon by the analyte molecules. The
SERS enhancement factor (Gave) was calculated up to be 108.
Hexagonally ordered arrays of nanovoids as SERS substrates were fabricated by
self-assembly of sacrificial nanospheres and electrochemical deposition of gold.[113-
115] The fabricated nanostructures had either gold flat surfaces or corrugated surfaces
depending on the thickness of gold film. Surface plasmon polaritons propagate along
the flat gold surface and scatter at the rims of shallow dishes forming delocalized Bragg
modes. On the highly corrugated surface with nanovoids, surface plasmon polaritons
were localized in the nanovoids forming localized Mie modes. The Bragg plasmon mode
depends on the incident angle and sample orientation, while the Mie plasmon mode
depends on the geometry of nanostructure.
Maximum SERS enhancements occurred when the excitation laser is incident at
the wavelength of the excitation of LSPR on the substrate and the Raman scattered
photon is also coincide with the excitation of LSPR.[92, 115] The measured SERS
enhancement factor (Gave) was 7×108 on a highly corrugated substrate with nanovoids
of 350 nm diameter, where the wavelengths of both excitation and Raman scattered
photons were in the absorbance peak (corresponding to the excitation of LSPR). It was
claimed that it was not possible to achieve completely uniform SERS enhancements
over whole Raman scattering modes, because the wavelengths of Raman scattered
were all different and could not be matched to the excitation of LSPR all together.
57
The excitation of localized surface plasmon resonance (LSPR) could be tuned to
near infrared (NIR) wavelength, by controlling the nanovoid size and the film
thickness.[116] The use of NIR laser sources can be favored since the photochemical
reactions can be activated and the fluorescence from adsorbed molecules can interfere
with the Raman signals. The SERS enhancement factor of 3×106 was obtained at the
NIR of 1064 nm.
Three-dimensional nanostructures were fabricated for reproducible and highly
SERS-active substrates.[117, 118] Inverse opal films fabricated by self-assembling a
binary mixture of sacrificial latex microbeads and gold nanoparticles showed stable and
reproducible SERS enhancements. The alumina membranes decorated with gold
nanoparticles as SERS substrates had advantages of efficient light interaction on the
wall of cylindrical pores with minimal absorption and scattering. The SERS
enhancement factor (Gave) was calculated to be 106.
Periodic Nanostructures from Non-close-packed Particle Arrays
Various non-close-packed arrays of nanopillars, nanodots, nanoholes, nanovials,
and nanovoids, could be achieved based on the hexagonally non-close-packed
nanoparticle arrays fabricated by a colloidal self-assembly.[119-123] Concentrated silica
nanoparticles (20 vol%) dispersed in ethoxylated trimethyllolpropane triacrylate
(ETPTA) monomer were spin-coated on a silicon wafer. Then, ETPTA monomer is
rapidly photopolymerized to immobilized silica particles on the substrate. After removing
the polymer matrix by an oxygen plasma etching, colloidal monolayer of hexagonally
non-close-packed silica particles is fabricated. The interparticle spacing between
particles is about 1.4 times of the particle diameter, which is explained by keeping a
58
minimal volume fraction of silica particle in ETPTA matrix due to the centrifugal force
during spin-coating.[119]
Periodic nanopyramid arrays are fabricated using the non-close-packed
nanoparticle arrays as deposition masks during Cr deposition.[91] The silica particles
are removed by dissolving in hydrofluoric acid aqueous solution, generating Cr
nanohole arrays on silicon wafer. Inverted pyramid arrays of silicon are produced by
wet-etching in KOH solution, where Cr nanohole arrays play the role of etching masks.
After removing Cr layer with a Cr etchant and depositing gold with thickness of 500 nm,
the deposited gold layer is transferred onto a glass substrate using a polyurethane
adhesive. The SERS enhancement on the fabricated nanopyramid arrays is 7×105.
Instead of depositing gold and transferring onto a glass substrate, polymer replication
can be used to generate nanopyramids of ETPTA and, by deposition of a gold layer,
gold coated nanopyramid arrays are fabricated.[124] The SERS enhancement is
improved to 108 due to the unbroken tips of nanopyramid. The strong concentration of
electromagnetic field near sharp tips of nanopyramids is contributed to the high SERS
enhancement. The charged analytes can be concentrated to the surface of
nanopyramid arrays under externally applied electric field, strengthening the Raman
signals.[125]
The metal film over nanosphere (MFON) as a SERS substrate is fabricated by
depositing gold on the hexagonally non-close-packed array of silica nanoparticles
without a polymer etching process.[126] Contrary to conventional MFON substrates, the
fabricated MFON consists of gold islands of 10 nm and gaps of less than 10 nm. Gold
islands form only on the polymer wetting layer during the deposition of gold layer. The
59
SERS enhancement is up to 108 due to the delocalized Bragg plasmon modes along
the periodic nipple structure and of the localized Mie plasmon modes in gold islands.
Disordered arrays of gold half-shells as SERS substrates are fabricated by
depositing gold on non-close-packed arrays of silica nanoparticles, transferring to a
glass substrate, and removing the silica particles in HF solution.[127] The strong
concentration of electromagnetic field near the sharp edge of half-shell and hot spots
between half-shells contribute to the measured high SERS enhancement (Gave) of 1010.
On the other hand, a maximum SERS enhancement factor (Gmax) of larger than 1010 in
the hot spot near sharp tips of gold nanocrescent moons was reported.[128]
Nanohole Arrays on a Glass Substrate
In this work, nanohole arrays are fabricated based on the colloidal self-assembly
for hexagonally non-close-packed arrays and the nanosphere lithography, as shown in
Figure 3-9. To generate nanoparticle arrays on glass slides instead of silicon wafers,
PMMA is spin-coated as a sacrificial layer. The concentrated silica nanoparticles
dispersed in ETPTA monomer are spin-coated on a PMMA coated glass slide
generating hexagonally non-close-packed arrays of nanoparticles. After
photopolymerization of ETPTA to immobilize particles on the substrate, ETPTA and
PMMA are etched by using nanoparticles arrays as an etching mask. Silica
nanoparticles are removed by ultrasonication in ethanol and the remaining ETPTA and
PMMA are removed in acetone, generating nanohole arrays on a glass substrate.
The fabricated gold nanohole arrays with different sizes (330 and 400 nm) are
shown in Figure 3-10. They are hexagonally non-close packed arrays of nanoholes. In
the absorption spectrum of 330 nm nanoparticles arrays, there is a peak at the
excitation wavelength of 722 nm, indicating the excitation of localized surface plasmon
60
resonance, as shown in Figure 3-12a. In the case of 400 m nanoparticles arrays, a peak
is present at the excitation wavelength of 816 nm. These excitations of LSPR are close
to the used excitation wavelength of 785 nm.
STV/PEG-GNP Arrays on Nanohole Arrays
Concept of GNP-nanohole Arrays
In the absorption spectra of STV/PEG-GNP arrays, 330 nm gold nanohole arrays
fabricated in our study, and 400 nm nanohole arrays, the excitation of localized surface
plasmon resonance are located at the wavelengths of 544 nm, 722 nm, and 816 nm,
respectively. To maximize the SERS enhancement, the excitation of LSPR should be
between the wavelengths of the excitation and the Raman scattered photons, according
to literatures. Thus, there are two ways to maximize the SERS enhancement such as
tuning the excitation of LSPR to the wavelengths of excitation and the Raman scattered
photons by controlling the geometry of the substrates and tuning the laser excitation
wavelength by using tunable laser systems.
The laser excitation wavelength of 785 nm is favored in biological sensing
applications since the photochemical reactions can be activated and the fluorescence
from adsorbed molecules can interfere the Raman signals in the visible excitation
wavelength.[116] In this work, we try to tune the excitation of LSPR to the wavelength
range around the laser excitation wavelength of 785 nm by combining the STV/PEG-
GNP arrays and the gold nanohole arrays.
Fabrication of GNP-nanohole Arrays
The STV/PEG-GNP arrays on gold nanohole arrays are fabricated by colloidal
self-assembly and metal deposition, as shown in Figure 3-9. First, the gold nanohole
arrays are prepared on a glass substrate based on the colloidal self-assembly for
61
hexagonally non-close packed arrays and nanosphere lithography. Then, gold
nanoparticles (30 nm) functionalized with streptavidin (STV) and polyethylene glycol
(PEG) chains are assembled on the prepared gold nanohole arrays instead of bare
glass substrate to form hexagonally close-packed arrays by using a flow cell, as shown
in Figure 3-7.
The fabricated STV/PEG-GNP arrays on gold nanohole arrays with different sizes
(330 and 400 nm) are shown in Figure 3-11. Gold nanohole arrays with different sizes
(330 nm and 400 nm) are fully covered with gold nanoparticles (30 m) forming non-
close-packed arrays due to streptavidin (STV) and PEG chains.
Optical and SERS Properties of STV/PEG-GNP Arrays on Nanohole Arrays
The absorption spectra of STV/PEG-GNP arrays with different sized nanoholes
are shown in Figure 3-12b. In the case of 330 nm nanoholes, the excitation of LSPR is
at the wavelength of 551 nm and 746 nm, which are red-shifted from those in the
STV/PEG-GNP arrays (544 nm) and the nanohole arrays (722 nm). In the case of 400
nm nanoholes, the excitation wavelengths of LSPR are also red-shifted to 563 nm and
875 nm. There are delocalized surface plasmon polaritons (SPP) along the flat surface
of periodic nanohole arrays. The localized surface plasmon in gold nanoparticles is
indicated as a Raman peak around the wavelength of 550 nm and the localized surface
plasmon in nanoholes is indicated as a Raman peak from 722 nm to 875 nm. The red-
shifts in the excitation of LSPR indicate the interactions between the excitation of LSPR
in gold nanoparticles and the excitation of LSPR in gold nanoholes. At the wavelength
of 785 nm, STV/PEG-GNP arrays on 330 nm nanohole arrays show a high absorption
plateau, while the STV/PEG-GNP arrays on 400 nm nanohole arrays show a high and
still increasing absorption.
62
The Raman spectra of STV/PEG-GNP arrays with different sized nanoholes at the
excitation wavelength of 785 nm are shown in Figure 3-13b,c. There is no Raman peak
around 2600 cm-1 (corresponding to v(S-H) stretching and vibration modes), indicating
that there is no analyte (benzenethiol) unbound to the substrate. Both substrates show
much stronger Raman signals than those from nanoparticle arrays. Moreover, the
signal-to-background ratios in both cases are larger than the reported ratios in the
literature, which is favorable in sensing and detecting applications. The STV/PEG-GNP
arrays on nanohole arrays show the good reproducibility in the SERS enhancement in
terms of low standard deviations (~ 10 %) of Raman signals (Table 3-1). The higher
SERS enhancement in 400 nm nanoholes is consistent with the higher absorption in
400 nm nanoholes.
The SERS enhancement factors at different Raman peaks are different even in
the same substrate, as shown in Table 3-1. Reported empirical results show that the
high SERS enhancement occurs when the excitation wavelength of LSPR is between
the excitation and the Raman scattered photons. The difference in the SERS
enhancement can be explained by different wavelengths of Raman scattered photons at
different Raman peaks.
Conclusions
In conclusion, we have developed a self-assembly technology for fabricating gold
nanoparticle arrays on gold nanohole arrays as reproducible and high SERS-active
substrates. The excitation of localized surface plasmon resonance (LSPR) in
nanoparticle arrays and nanohole arrays around the wavelength of excitation laser of
785 nm contribute to high SERS enhancements. Due to a high sensitivity (high signal-
to-background ratios) as well as a good reproducibility (low variations of Raman signal
63
on different spots and samples), this simple and scalable technology for fabricating gold
nanoparticle arrays on gold nanohole arrays is promising for developing ultra-sensitive
detectors for chemicals and reproducible sensors for chemical and biological molecules.
Materials and Methods
Materials
Monodispersed silica colloids with less than 10 % diameter variation are
synthesized by the Stober method.[129] Ethoxylated trimethylolpropane triacrylate
(ETPTA) monomer is obtained from Sartomer (Exton, PA). The photoinitiator, Darocur
1173 (2-hydroxy-2-methyl-1-phenyl-1-propanone), is provided by Ciba Specialty
Chemicals. Streptavidin PEGylated gold nanoparticles (30 nm) are purchased from
Polysciences (Warrington, PA).
Instrumentation
A standard coater (WS-300B-6NPP-Lite Spin Processor, Laurell) is used to spin-
coat colloidal suspensions. The polymerization of ETPTA monomer is carried out on a
Pulsed UV Curing System (RC 742, Xenon). A Unaxis Shuttlelock RIE/ICP reactive-ion
etcher is utilized to remove polymerized ETPTA and PMMA. Scanning electron
microscopy is carried out on a JEOL 6335F FEG-SEM. Raman spectra are measured
with a Renishaw inVia confocal Raman microscope.
Gold Nanohole Arrays on a Glass Slide
Glass slides were cleaned using a standard RCA1 cleaning[130]. A resist PMMA
(MicroChem 950 PMMA A4) was spin-coated on a 1 inch2 glass slide (4000 rpm for 1
min) as a sacrificial layer and then was baked for hardening (180 °C for 3 min).
Monolayer non-close-paced silica particles dispersion (400 or 330 nm) was then spin-
coated on PMMA.[131] The final step of the spin coat process is 8000 rpm for 5 min.
64
After UV-curing poly(ethoxylated trimethylolpropane triacrylate), reactive ion etching
(Unaxis Shuttlelock operating at 5 mTorr oxygen pressure, 15 sccm oxygen, 5 sccm
argon, and 100 W RIE power and 300 W ICP power for 90 s) was performed to etch
both ETPTA and PMMA by using silica nanoparticles as etch masks. A 100 nm thick
gold film was deposited by an electron beam evaporator. Silica nanoparticles were
removed by ultrasonication in ethanol. The PMMA/ETPTA layers were then lifted off in
acetone resulting in gold nanohole arrays on a glass substrate.
STV/PEG-GNP Arrays on Gold Nanohole Arrays
Flow cell was constructed by placing two strips of double-side sticky tape on the
Au nanohole arrays about 7 mm apart and covering with a cover glass. To provide
enough number of gold particles to cover whole surface area, 3 layers of double-side
sticky tape (~ 300 ㎛) were used to increase the volume of solution (~ 60 μL).
Streptavidin PEGylated gold nanoparticles dispersed in distilled water was flown
through a flow cell. Solutions are pipeted on one side and sucked out the other side by
capillary action. Solvent was then evaporated at room temperature. To remove crystals
formed due to salts in the solution, Au nanoparticles array was rinsed by flowing ample
amount of distilled water through the flow cell.
Absorbance of Substrates
The absorbance of the GNP arrays on gold nanohole arrays is evaluated using
visible-near-IR absorbance measurement with an Ocean Optics HR4000 high resolution
fiber optic UV-visible-near-IR spectrometer.
Raman Spectra Measurements
STV/PEG-GNP arrays on gold nanohole array samples are placed in a 9 mM
solution of benzenethiol in 200-proof-ethanol for 45 min and then rinsed in roughly 25
65
mL of 200-proof-ethanol for several minutes. The samples are allowed to air-dry for 20
min, after which the Raman spectra are measured. A gold nanoparticles array on bare
glass is used as the control sample for Raman spectra measurements. Raman spectra
are measured with a Renishaw inVia confocal Raman microscope using a 785 nm diode
laser at 6.4 mW with a 20× objective and an integration time of 10 s and a 170 μm2 spot
size. Raman spectra were also measured in 9.77 M benzenethiol contained in a flow
cell at 6.4 mW with an integration time of 10 s.
Calculation of Enhancement Factors
The SERS enhancement factor, Gave, is calculated from data collected using the
confocal Raman microscope. Gave is defined as follows:
bulk
surfA
bulkbulk
surfsurf
aveRI
hINc
NI
NIG
/
/ (3-5)
The intensity of the Raman peak obtained at the SERS surface, Isurf, is compared to that
obtained for a solution, Ibulk, of concentration C∞. NA is Avogadro‟s number, σ is the
surface area occupied by one adsorbate molecule, R is the roughness factor of the
surface, and h is a parameter defined by the confocal volume of the microscope.[132] h
is measured to be 400 μm with the 20× objective.
66
Table 3-1. Calculated SERS enhancement factor on the fabricated substrates from measured data of Raman intensities at 999.2 cm-1 and 1023 cm-1.
SERS substrate ~ 999.2 cm-1 ~ 1023.2 cm-1
GNP array 4.59×105 ± 1.93×105 1.56×106 ± 6.52×105 330 nm nanohole-GNP 2.15×106 ± 2.06×105 7.90×106 ± 8.55×105 400 nm nanohole-GNP 2.98×106 ± 2.64×105 1.15×107 ± 1.07×106
67
Figure 3-1. Various characteristic energies: a) Rayleigh scattering, b) Stokes Raman scattering, and c) anti-Stokes Raman scattering.
Figure 3-2. Schematic diagrams of a) a surface plasmon polariton (or propagating plasmon) on a flat surface and b) a localized surface plasmon on a nanostructured surface. Adapted from [79].
Vibrational
Energy States
Virtual Energy States
Electronic States
3
2
1
0
a b c
Vibrational
Energy States
Virtual Energy States
Electronic States
3
2
1
0
a b c
68
Figure 3-3. Various SERS substrates: a) Ag film on nanospheres, periodic nanostructures by b) nanosphere lithography and c) electron-beam lithography, and d) colloidal aggregates. Adapted from [80, 99].
69
Figure 3-4. SERS applications: a) in vivo glucose sensing equipment consisted of SERS spectroscopy, implanted substrate, beam directing optics, and collection lens and b) identification of cancer genes by Raman labels. Adapted from [82].
Figure 3-5. Representative Raman spectrum of benzenethiol adsorbed on a SERS substrate. Adapted from [90].
1000 1200 1400 1600
1572
1072
1021
997
Raman shift (cm-1)
1000 1200 1400 1600
1572
1072
1021
997
Raman shift (cm-1)
70
Figure 3-6. The SERS enhancement calculated based on finite element methods in the case of nanoparticle arrays. Adapted from [93].
71
Figure 3-7. Schematic diagram depicting self-assembly of gold nanoparticles using a flow cell.
72
Figure 3-8. SEM images of 30 nm gold nanoparticle arrays with a gap between particles on glass substrates.
73
Figure 3-9. Schematic diagram depicting the fabrication procedures for making GNP-nanohole arrays.
74
Figure 3-10. SEM images of nanohole arrays: a) 330 nm nanohole arrays and b) 400 nm nanohole arrays.
Figure 3-11. SEM images of nanohole-GNP arrays: a) 330 nm nanohole arrays covered by 30 nm gold particles and b) 400 nm nanohole arrays covered by 30 nm gold particles.
75
Figure 3-12. Absorption spectra on a) gold nanoparticle (GNP) arrays and nanohole arrays and b) nanohole arrays covered by gold nanoparticles.
76
Figure 3-13. Raman spectra of benzenethiol absorbed on a) flat god surface, b) gold nanoparticle (GNP) arrays, c) 330 nm nanohole arrays covered by gold nanoparticles, and d) 400 nm nanohole arrays covered by gold nanoparticles.
77
CHAPTER 4 BINARY COLLOIDAL CRYSTALS
The electronics revolution sparked by the invention of transistors and the
miniaturization of integrated electronic circuits has affected almost every aspect of our
daily lives. In an effort toward further high-density integration and system performance,
scientists are now turning to light as the information carrier. The travel speed of light in a
dielectric material is greater than that of an electron in a metallic wire.[133] The
bandwidth of dielectric materials is about 3 to 4 orders of magnitude larger than that of
metals.[133] Moreover, light particles (photons) are not as strongly interacting as
electrons, which helps to reduce energy losses. Unfortunately, our ability to control
photons in miniaturized volumes is in many ways in its infancy, compared with our
ability to manipulate electrons.
A new class of optical materials known as photonic crystals (PCs) may hold the
key to continued progress towards all-optical integrated circuits and high-speed optical
computing.[134-137] PCs are periodic dielectric structures (Figure 4-1) with a forbidden
gap (or photonic band gap) for electromagnetic waves, analogous to the electronic band
gap in semiconductors. Photons with energies lying in the photonic band gap (PBG)
cannot propagate through the medium, providing the opportunity to control the flow of
light for photonic information technology. The lattice constant of the artificial crystal must
be comparable to the wavelength of the light passing through the crystal.[136] For
optical communication systems operating at near-infrared wavelengths, the lattice
constant must have dimensions on the submicrometer scale.[138, 139]
Unfortunately, the development and implementation of integrated optical circuits
with photonic crystals have been greatly impeded by expensive and complex
78
nanofabrication techniques. Electron-beam lithography (EBL) and focused ion-beam
(FIB) are two popular methods in fabricating photonic crystals with arbitrary
geometries.[140, 141] However, attaining high-throughput and large-area fabrication
continues to be a major challenge with these top-down techniques. By contrast, bottom-
up colloidal self-assembly and subsequent templating nanofabrication provide a much
simpler, faster, and inexpensive alternative to nanolithography.[138, 139, 142] A variety
of methods, such as gravity sedimentation,[138, 143] electrostatic repulsion,[144-147]
template-assisted assembly,[148-150] and capillary force induced convective self-
assembly,[139, 151-153] have been developed to create colloidal photonic crystals.
However, current colloidal self-assemblies are only favorable for low volume, laboratory-
scale production. It usually takes days or even weeks to grow a centimeter-size colloidal
crystal.[139, 152, 153] In addition, most of current colloidal self-assembly technologies
are not compatible with mature semiconductor microfabrication, limiting the mass-
production and on-chip integration of practical photonic crystal devices.
Another major issue of current colloidal self-assembly is the limitation on
achievable crystal structures. Although calculations show that nonclose-packed
photonic crystals (e.g., diamond-structured crystal) facilitate the opening of wider
PBGs,[154, 155] the realization of open-structured crystals by self-assembly is
challenging.[156] Binary colloidal photonic crystals composed of particles of two
different sizes have attracted a great deal of recent interest as they are promising to
open wider PBGs.[157-182] For instance, van Blaaderen et al. developed a convective
self-assembly technology to assemble small silica particles on a pre-assembled
colloidal array consisting of larger particles.[176] Similar to the limitations of traditional
79
colloidal assembly, all available bottom-up methodologies in creating binary colloidal
photonic crystals suffer from the scalability and microfabrication-compatibility issues.
We have recently developed a simple and scalable spin-coating technology that
combines the simplicity and cost benefits of bottom-up colloidal self-assembly with the
scalability and compatibility of standard top-down microfabrication.[183, 184] The spin-
coating technique enables mass-fabrication of wafer-scale (up to 8 inch) colloidal
crystals, which is a length scale nearly two-orders of magnitude larger than that
currently available through other methods. Additionally, the entire crystal is formed
within minutes, as compared to days or even weeks needed to produce a centimeter-
size crystal using other self-assembly techniques. Most important, the spin-coating
technique is compatible with standard microfabrication, allowing for the creation of
complex micropatterns for optical on-chip integration. In this chapter, we developed a
new bottom-up approach to create non-close-packed binary colloidal crystals by using
spin-coated colloidal crystals as structural template.
Results and Discussion
Figure 4-2 shows a schematic outline of a procedure for achieving binary colloidal
crystals. The established spin-coating technique is utilized to generate a wafer-scale
monolayer of hexagonally ordered nonclose-packed silica nanoparticles. The resulting
nanocomposite of silica particles and polymer matrix has a thin polymer wetting layer
(~100 nm) next to the Si substrate, which can still immobilize the silica particles on the
substrate after partial etching of the polymer.[185] The fabricated monolayer of silica
particles (300 nm diameter) is shown in Figure 4-3. In the literature, the close-packed
colloidal crystals were achieved via the spin-coating technique, where the evaporation
rate of solvent in the colloidal dispersion was very high.[186] In the established spin-
80
coating technique for nonclose-packed colloidal crystals, on the other hand, silica
particles are dispersed in non-volatile monomer (ETPTA). The nonclose-packed
monolayer has the particle center-to-center distance of 1.41 (of particle diameter), which
corresponds to the minimal volume fraction of silica particle in the silica-polymer
nanocomposite.[183]
The dispersions of silica particles in different sizes are subsequently spin-coated
on the prepared monolayer. The prepared monolayers of silica particles are used as
templates for guiding the silica particles in the dispersions into the interstices between
the three neighbor particles in the template layers. The fabricated binary colloidal
crystals are shown in Figure 4-4. In the colloidal crystal consisting of a 300 nm particle
monolayer on a 300 nm particle monolayer, the particle in the second layer is located in
the trap which is the center of interstice between three neighboring particles in the first
template layer as shown in Figure 4-4b. In the colloidal crystal consisting of a 400 nm
particle monolayer on a 300 nm particle monolayer, on the other hand, the particles in
the second layer are not confined in the traps, as shown Figure 4-4d. The larger
particles (400 nm) in the second layer rather form a nonclose-packed array independent
of the first template layer.
In the literature, attempts at localizing small particles in the traps between large
particles were successful. However, attempts at localizing larger particles in the traps
which are the interstices between small particles in the templating layer were not
successful with the convective assembly (but, they were successful in the LS2 close-
packed binary crystal),[176] although it is thermodynamically favorable. In our spin-
coating process with a template layer, the “trapping effect” due to the geometry of
81
template layer is competing with the centrifugal force due to spinning. In the case of
localizing larger particles, the trapping effect is reduced because the depth of interstice
between particles in the template layer is shallow. Thus, the particle arrays in the
second layer are affected by the centrifugal force, where the minimal volume fraction of
silica particles is achieved.
Several strategies have been reported to improve the ordering in the colloidal
crystals. Enhanced orderings of colloidal particles were successful by steady shearing,
ultrasonication, and oscillatory shearing.[151, 187, 188] Defect-free arrays of nanoholes
in the block-copolymer were achieved by solvent-induced ordering.[189] In this work,
pressure is applied on the surface of shear-aligned silica particles in a ETPTA monomer
matrix to increase the trapping effect. The resulting binary colloidal crystals are shown
in Figure 4-5. In the binary colloidal crystals consisting of a 300 nm particle layer on a
300 nm particle layer, the orderings of particles in the second layer into the traps, which
are interstices between the three neighbor particles in the template layer, are improved,
as shown in Figure 4-5a-d. In addition, the domain boundaries are clearly consisting of
vacancies, while the domains in the binary crystals fabricated without an applied
pressure show gradual change across the domain boundaries. This is due to the
rearrangement of particles in monomer matrix under the applied pressure. There is no
significant difference between 0.2 MPa and 0.33 MPa of pressure. On the other hand,
the binary colloidal crystals consisting of a large particle (400 nm) layer on a small
particle (300 nm) don‟t exhibit the pressure effect. The large particles form a non-close-
packed array, while they are independent of the first template layer.
82
The particle size and the thickness of layer in colloidal crystal are controllable.
Figure 4-6a shows the cross-section of the binary colloidal crystals consisting of 400 nm
particles on 345 nm particles. The individual layers are monolayers of 400 nm particles
or 345 nm particles. By repeating the spin-coating process with different sized particles,
the colloidal crystal consisting of many monolayers of different size of particles can be
achieved. The thickness of individual layers can also be controlled with spin-coating
parameters such as spinning speed and time. Binary colloidal crystals consisting of a
400 nm particle monolayer on a 345 nm particle double layer and ternary colloidal
crystals consisting of a 300 nm double layer on a 400 nm double layer which in turn is
on a 345 nm monolayer are shown in Figure 4-6b,c.
Conclusions
Binary colloidal crystals are achieved with a simple, fast, and scalable spin-coating
technique. The thickness of individual layers is easily controlled with spin-coating
parameters. Although the pressure is helpful for the ordering of particles, further
improvement for better orderings of particles in the individual layers is still needed. The
characterization of optical properties in the fabricated binary colloidal crystals is in
investigation.
Materials and Methods
Materials and Substrates
Monodispersed silica colloids with less than 10% diameter variation are
synthesized by the Stober method.[190] Ethoxylated trimethylolpropane triacrylate
(ETPTA) monomer is obtained from Sartomer (Exton, PA). The photoinitiator, Darocur
1173 (2-hydroxy-2-methyl-1-phenyl-1-propanone), is provided by Ciba Specialty
Chemicals. The silicon-wafer primer, 3-acryloxypropyl trichorosilane (APTCS), is
83
purchased from Gelest (Morrisville, PA). Silicon wafer (test grade, n-type, (100)) are
obtained from Wafernet (San Jose, CA) and primed by swabbing APTCS on the wafer
surfaces using cleanroom Q-tips (Fisher), rinsed and wiped with 200 proof ethanol three
times, spin coated with a 200 proof ethanol rinse at 3000 rpm for 1 min, and baked on a
hot plate at 110 °C for 2 min.
Instrumentation
A standard coater (WS-300B-6NPP-Lite Spin Processor, Laurell) is used to spin-
coat colloidal suspensions. The polymerization of ETPTA monomer is carried out on a
Pulsed UV Curing System (RC 742, Xenon). A Unaxis Shuttlelock RIE/ICP reactive-ion
etcher is utilized to remove polymerized ETPTA for releasing shear-aligned colloidal
crystals. Scanning electron microscopy is carried out on a JEOL 6335F FEG-SEM.
Non-close-packed Colloidal Monolayer
The fabrication of wafer-scale, monolayer, non-close-packed colloidal crystal-
polymer nanocomposites is performed according to reference.[119] Monodispersed
silica colloids (300 nm) are dispersed in non-volatile ethoxylated trimethylolpropane
triacrylate (ETPTA, Sartomer) monomer (20% volume fraction) and 2 wt% Darocur 1173
is added as photoinitiator. The silica-ETPTA dispersion is dispensed on a 3-
acryloxypropyl trichlorosilane (APTCS)-primed (100) silicon wafer and spin-coated at
8000 rpm for 3 min on a standard spin coater, yielding a hexagonally ordered colloidal
monolayer. The monolayer is then photopolymerized for 4 s using a Pulsed UV Curing
System. To utilize this monolayer as a substrate for spin-coating of the second layer,
the polymer matrix between particle arrays is partially removed using a reactive ion
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etcher operating at 40 mTorr oxygen pressure, 40 sccm flow rate, and 100 W RIE
power for 2 min 30 s.
Binary Layer of Hexagonally Non-close-packed Colloidal Structure
20 vol% silica colloid (400 nm)-ETPTA dispersion (including 2 wt% photoinitiator)
is prepared. The silica-ETPTA dispersion is dispensed on a pre-fabricated substrate,
non-closed packed colloidal monolayer and spin-coated at 8000 rpm for 3 min on a
standard spin coater, yielding a hexagonally ordered colloidal binary layer. The binary
layer is then photopolymerized for 4 s using a Pulsed UV Curing System.
For better ordering of silica particles in the second layer by applying a pressure, a
glass slide is placed on top of the binary layer before photopolymerizing and then a
weight (2 or 3.2 kg) is placed on this glass slide for 2 min. The pressure is calculated
from the weight and the area of binary layer. The ETPTA monomer in the binary layer is
then photopolymerized for 4 s using a Pulsed UV Curing System.
Ternary Layer Hexagonally Non-close-packed Colloidal Structure
To utilize the binary layer as a substrate for spin-coating of third layer, the polymer
matrix is partially removed using a reactive ion etcher operating at 40 mTorr oxygen
pressure, 40 sccm flow rate, and 100 W power for 2 min 30 s. The 20 vol% silica-
ETPTA dispersion (including 2 wt % photoinitiator) is dispensed on the prepared
substrate and spin-coated at 8000 rpm for 3 min on a standard spin coater, yielding a
hexagonally ordered colloidal ternary layer. The ETPTA monomer in the ternary layer is
then photopolymerized for 4 s using a Pulsed UV Curing System.
85
Figure 4-1. Photonic crystals: a) photonic crystals in 1-D, 2-D, and 3-D and b) a photonic band gap compared to an electronic band gap. Adapted from [136].
86
Figure 4-2. Schematic illustration of the procedure for fabricating binary hexagonal arrays of silica spheres by using monolayer nonclose-packed colloidal crystals as substrates.
Figure 4-3. Monolayer of nonclose-packed silica particles (300 nm) fabricated by the spin-coating technique.
87
Figure 4-4. Binary hexagonal arrays of silica spheres: a, b) 300 nm particles array on 300 nm particles array (300/300), and c, d) 400 nm particles array on 300 nm particles array (400/300).
88
Figure 4-5. Pressure effects on the ordering of particles in the second layer: a, b) 0.2 MPa for 2 min, and c, d) 0.33 MPa for 2 min in 300 nm particles arrays on 300 nm particles arrays (300/300), and e, f) 0.2 MPa for 2 min in 400 nm particles arrays on 300 nm particles arrays (400/300).
89
Figure 4-6. Cross-sections of hexagonal arrays of silica spheres: a, b) binary layer of 400 nm particles array on 345 nm particles array (400/345) and c) ternary layer of 300 nm particles array on 400 nm particles array on 345 nm particles array (300/400/345).
90
CHAPTER 5 BIOINSPIRED, ORGANIC-INORGANIC NANOCOMPOSITES
In nature, hard biological tissues show excellent mechanical properties due to their
unique microstructures (Figure 5-1).[191] Scientists have been inspired by mechanical
design principles found in nature to generate new materials of high mechanical
performance. The nacreous layer of mollusk shells has an intricate brick-and-mortar
nanostructure consisting of 95 vol% brittle aragonite platelets and 5 vol% of soft
biological macromolecules, making the shells exceptionally tough and stiff.[191-196]
Mimicking the unique structure in the nacreous layer has been tried by various bottom-
up self-assembly techniques. Layer-by-layer (LBL) assembly of ceramic-polymer
alternative layers is successful in generating reinforced nanocomposites with aligned
structures.[197, 198] Ice-templated crystallization, spin-coating, gravitational
sedimentation, and centrifugation have been explored to assemble ceramic platelets
into ordered structures.[199-202]
Electrophoretic deposition is a simple, inexpensive, and scalable process that
enables rapid production of thick films over large areas, while LBL assembly is a slow
process.[203-205] In addition, Nanocomposites can be assembled in a single step by
electrophoretic codeposition of colloids and polymers.[206] For the layered
nanocomposites of nanoclays and polymer, the agglomeration of nanoclays should be
avoided for highly aligned structures.
To achieve nanocomposites with a high level of strength and toughness, inorganic
platelets should have high aspect ratio, small thickness, high shear stress or adhesion
with polymer matrix, and high volume fraction (Figure 5-2).[191] Uniform gibbsite
(Al(OH)3) nanoplatelets can be synthesized by hydrolysis of aluminium alkoxide.[207,
91
208] The aspect ratio of the synthesized gibbsite nanoplatelets (~ 10) is close to that of
natural aragonite (CaCO3) platelets in nacre.[193] The diameter and thickness can be
controlled by seeded growth.[209] Due to the hydroxyl groups on the surface, different
functionalities can be rendered by the chemical modification.[208]
In this work,[210, 211] synthesized gibbsite nanoplatelets are aligned and
assembled by electrophoretic deposition. Inorganic-organic nanocomposites having
nacreous microstructures are achieved by infiltrating monomer into the interstitials
between the assembled nanoplatelets and polymerizing the monomer. The resulting
nanocomposites exhibit significantly improved mechanical properties. By the surface
modifications of gibbsite nanoplatelets, covalent linkages between the inorganic
platelets and organic matrix are facilitated to create further reinforced nanocomposites.
In this study, Huang synthesizes gibbsite nanoplatelets and Lin modifies the surfaces of
gibbsite nanoplatelets and fabricates gibbsite-polymer nanocomposites. My contribution
is characterizing the mechanical properties of the fabricated nanocomposites.
Assembly of Colloidal Nanoplatelets
Synthesized Gibbsite Nanoplatelets
The gibbsite nanoplatelets synthesized from aluminium alkoxides in an acidic
aqueous solution have a hexagonal shape and uniform size, as shown in Figure 5-3a.
The diameter is about 188 ± 40 nm and the measured thickness ranges from 10 to 15
nm, as measured by atomic force microscopy (AFM). The hydroxyl groups on the
surface of gibbsite nanoplatelets can be modified by reacting with 3-
(trimethoxylsily)propyl methancrylate (TPM) via the silane coupling reaction.[212] This
TPM-modification forms dangling acrylate bonds which can be cross-linked with
acrylate-based ethoxylated trimethylolpropane triacrylate (ETPTA) matrix.
92
Assembly of Gibbsite Nanoplatelets by Electrophoretic Deposition
Gibbsite nanoplatelets are assembled in a parallel-plate sandwich cell under an
electric field. Ethanol is added to the aqueous dispersions to promote colloidal
coagulation on the ITO electrode by reducing the electric double layer thickness of the
particles. In the resulting film, gibbsite nanoplatelets are densely packed and aligned
(Figure 5-3b). It is known that the isoelectric point (IEP) of the gibbsite edges (pH ~ 7)
differs from the IEP on the gibbsite faces (pH ~ 10).[207] Thus, in the suspension, the
nanoplatelets have positively charged faces and neutral edges. This charge distribution
is favorable in alignment of nanoplatelets in parallel to the electrode surface.
Gibbsite-polymer (ETPTA) nanocomposites are made by infiltrating photocurable
monomers into the interstitials between the nanoplatelets in the assembled gibbsite film
and photopolymerizing the monomer matrix.
Mechanical Properties of Nanocomposites
The tensile stress-strain curves for ETPTA, gibbsite-ETPTA, and TPM-modified
gibbsite-ETPTA films are shown in Figure 5-3c. The gibbsite-ETPTA nanocomposite
shows 2-fold higher tensile strength and 3-fold higher Young‟s modulus compared to
those of pure ETPTA polymer. The TPM-modified gibbsite-ETPTA nanocomposite
displays 4-fold higher tensile strength and 1 order of magnitude higher Young‟s modulus
due to cross-linking with ETPTA matrix. It is known that the covalent linkage between
the inorganic fillers and the organic matrix determines the mechanical properties of the
nacres composites.[197]
93
Assembly of Surface-roughened Nanoplatelets
Silica-coated Gibbsite Nanoplatelets
Silica-coated gibbsite nanoplatelets are synthesized by coating a thin shell of sol-
gel silica on the surface of gibbsite nanoplatelets.[213] The amphiphilic
polyvinylpyrrolidone (PVP) macromolecule can be adsorbed onto a broad range of
colloids stabilizing them in water and various nonaqueous solvent and acts as a
coupling agent during this coating process. The TEM image in Figure 5-4a shows
hollow silica nanoplatelets after leaching out gibbsite parts. The silica shell has a
thickness of 10 nm. The silica shells driven by the sol-gel process are much rougher
than the single-crystalline gibbsite nanoplatelets.
Assembly of Silica-coated Gibbsite Nanoplatelets
Silica-coated gibbsite nanoplatelets are assembled in a parallel-plate sandwich
cell under an electric field. Cracks easily form on the silica-coated gibbsite film during
the drying process. Polyethyleneimine (PEI) is added to the bath solution of silica-
coated gibbsite nanoplatelets to solve the cracking issue by increasing the adherence
and strength of the electrodeposited films. PEI is known to act as a particle binder by
adsorbing strongly onto silica at various pH.[214]
Unlike gibbsite nanoplatelets, silica-coated gibbsite-PEI nanoplatelets have
positive charges on both faces and edges due to the PEI macromolecules adsorbed on
the uniform silica shell. There is no electric-field-induced reorientation of silica-coated
gibbsite-PEI nanoplatelets due to the uniform distribution of surface charge. The
formation of polymer-bridges between neighboring particles also leads to the imperfect
alignment of nanoplatelets.[214] Nevertheless, the nanoplatelets still preferentially
94
aligned since the orientation is energetically favorable under electric field (Figure 5-
4b,inset).
Silica-coated gibbsite-polymer (ETPTA) nanocomposites are made by infiltrating
photocurable monomers into the interstitials between the nanoplatelets in the
assembled gibbsite film under vacuum for a few hours and photopolymerizing the
monomer matrix.
Mechanical Properties of Nanocomposites
The tensile stress-strain curves for ETPTA, gibbsite-ETPTA, and silica-coated
gibbsite-PEI-ETPTA films are shown in Figure 5-4c. The silica coated-gibbsite-PEI-
ETPTA nanocomposite shows 2.5-fold higher tensile strength. This is due to the
presence of PEI macromolecules, which strongly adsorbed on the negatively charged
surface of silica-coated gibbsite. The PEI macromolecules can also interlock with cross-
linked ETPTA backbone.
The silica-coated gibbsite-PEI-ETPTA nanocomposite shows 5-fold larger
elongation compared to that of pure ETPTA. The large elongation is due to the strong
ionic bonding between the PEI macromolecules and the nanoplatelets, and the natural
elasticity of PEI. The surface roughness of nanoplatelets and the rotation of misaligned
nanoplatelets under an applied tensile mode can also be a reason for the large
elongation observed.
Conclusions
We developed a simple and rapid electrodeposition technique for assembling
gibbsite nanoplatelets into organized multilayers. Nanoplatelets with high aspect ratio
(diameter-thickness ratio) are aligned under electric field and the interstitials between
nanoplatelets are infiltrated with polymer generating organic-inorganic nanocomposites
95
with significantly improved mechanical properties. For further improvement in
mechanical properties of nanocomposites, gibbsite nanoplatelets are surface-treated
(TPM or silica) and assembled into aligned multilayers. This technique is promising to
achieve oriented deposition of a wide range of materials including ceramics, metals,
ceramic-metal, or ceramic-conducting polymer nanocomposites.
Materials and Methods
Materials
Ultrapure water (18.2 M cm-1) was used directly from a Barnstead water system.
200-proof ethanol is purchased from Pharmaco Products. Hydrochloric acid (37%),
aluminum sec-butoxide ( 95%), aluminum isopropoxide ( 98%), polyvinylpyrrolidone
(PVP, Mw ~ 40,000), polyethylenimine (PEI, 50 wt% in water, Mw ~ 750,000), and
sodium hydroxide ( 98%) were obtained from Sigma Aldrich. Tetraethyl orthosilicate
(TEOS, 99%) was purchased from Gelest. Ammonium hydroxide (14.8 N) was
obtained from Fisher Scientific. Ethoxylated trimethylolpropane triacrylate (ETPTA,
SR454) monomer was provided by Sartomer (Exton, PA). The photoinitiator, Darocur
1173 (2-hydroxy-2-methyl-1-phenyl-1-propanone), was obtained from Ciba Specialty
Chemicals. Two-part polydimethylsiloxane (PDMS, Sylgard 184) was provided by Dow
Corning. Indium tin oxide (ITO) coated glass substrates with sheet resistance of 8 ohms
were purchased from Delta Technologies. Silicon wafers (test grade, n type, (100)) were
purchased from University Wafer.
Instrumentation
An EG&G Model 273A potentiostat/galvanostat was used for electrophoretic
deposition. Scanning electron microscope (SEM) was carried out on a JEOL 6335F
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FEG-SEM. Transmission electron microscope (TEM) and selected area electron
diffraction awere performed on a JEOL TEM 2010F. Atomic force microscope (AFM)
was conducted on a Digital Instruments Dimension 3100 unit. A standard spin-coater
(WS-400B-6NPP-Lite spin processor, Laurell) was used to spin-coat ETPTA monomer.
The polymerization of ETPTA was carried out on a pulsed UV curing system (RC 742,
Xenon). A kurt j. Lesker CMS-18 Multitarget Sputter was used for the deposition of Ti
and Au.
Synthesis of Gibbsite Nanoplatelets
The gibbsite nanoplatelets were synthesized by following a published
method.[207] Hydrochloric acid (0.09 M), aluminum sec-butoxide (0.08 M), and
aluminum isopropoxide (0.08 M) were added to 1 L ultrapure water. The mixture was
stirred for 10 days and then heated in a polyethylene bottle in a water bath at 85 C for
72 h. After cooling to room temperature, dispersions of gibbsite nanoplatelets were
centrifuged at 3500 g for 6 h and the sediments are redispersed in deionized water. For
completely removing the unreacted reactants and concentrating the nanoplatelets, this
process was repeated for five times.
Surface Modification of Gibbsite Nanoplatelets with TPM
Gibbsite nanoplatelets were surface-modified with 3-(trimethoxysilyl)propyl
methacrylate (TPM).[215] Prior to adding gibbsite nanoplatelets, 10 mL TPM was mixed
with a 100 mL water-methanol solution (water/methanol volume ratio of 3:1) for 1 hour
in order to fully hydrolyze TPM. Surface modification was then accomplished by adding
100 mL of gibbsite dispersion (ca. 1 vol% aqueous solution) into the hydrolyzed TPM
solution. The suspension was stirred at 40 °C for 30 min. The modified nanoplatelets
97
were washed by repeated centrifugation-redispersion cycles with pure ethanol and
finally concentrated to a stock suspension of 0.045 and 0.035 (g/g) in ethanol.
Coating of Gibbsite Nanoplatelets with Silica
Purified gibbsite nanoplatelets were coated with a thin layer of silica by a two-step
procedure: adsorption of PVP and growth of silica shell via Stober method.[129] PVP
was dissolved in DI water by ultrasonication. Subsequently, 200 mL aqueous solution of
gibbsite nanoplatelets (1 wt%) was mixed with 300 mL PVP solution (10 wt%). Then,
the mixture was stirred for 1 day for the complete adsorption of PVP on the gibbsite
surface. PVP-coated gibbsite nanoplatelets were transferred into ethanol by repetitively
centrifuging the mixture and redispersing the sediments in ethanol three times. The
PVP-modified gibbsite nanoplatelet suspension of 500 mL was mixed with 33 mL
ammonium hydroxide and 1 mL TEOS for the growth of silica shell. After stirring for 4-6
h, silica-coated gibbsite nanoplatelets were transferred into water by centrifuging the
dispersion and redispersing the sediments in DI water.
Electrophoretic Deposition
Electrophoretic deposition of nanoplatelets was performed in a horizontal
sandwich-cell. The bottom and the top of the cell were an ITO working electrode and a
gold counter electrode, respectively, with a PDMS spacer. The active area and cell gap
were 1.5×1.5 cm2 and 2.2 mm, respectively. The bath solutions for gibbsite-ETPTA and
TPM-modified ETPTA were nanoplatelet dispersions in water-ethanol mixtures. The
volumetric ratio of ethanol to the aqueous suspension was 2. The bath solutions for
silica-coated gibbsite-PEI-ETPTA were prepared by mixing 9 mL of 1.5 wt% silica-
coated gibbsite nanoplatelet aqueous solution with 1 mL 1.5 wt% PEI aqueous solution
and ultrasonicating the mixture. A constant voltage of -2.5 V (ITO vs. Au) was applied to
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deposit the positively charged nanoplatelets onto the ITO cathode. The electroplated
gibbsite films were rinsed with 200-proof ethanol.
Mechanical Test
Tensile strengths were measured using an Instron model 1122 load frame
upgraded with an MTS ReNew system and equipped with a 500 g load cell at a
crosshead speed of 0.5 mm/min. Testing samples with width of 1.5 mm and thickness
ranging from 30 to 80 m were adhered on home-made sample holders with a 10 - 20
mm gap using polyurethane monomer (NOA 60, Norland) as an adhesive and then UV-
cured. The thickness of the tested samples was measured by cross-sectional SEM to
calculate the final tensile strength.
99
Figure 5-1. Hard biological tissues and their microstructures: a) tooth, b) vertebral bone, c) shell, d) Enamel made of long needle-like crystals with soft protein matrix, e) dentin and bone made of plate-like crystals embedded in a collagen-rich protein matrix, and f) nacre made of plate-like crystals with a very small amount of soft matrix in between. Adapted from [191].
Figure 5-2. A model of biocomposites: a) a schematic diagram of staggered mineral crystals embedded in protein matrix and b) a simplified model showing the
load-transfer mechanism in the mineral–protein composites. Adapted from
[191].
100
Figure 5-3. Nanocomposite of colloidal nanoplatelets: a) TEM image of gibbsite nanoplatelets, b) SEM image of a free standing gibbsite-ETPTA nanocomposite, and c) tensile strain-stress curves for plain ETPTA film, gibbsite-ETPTA nanocomposite, and TPM-modified gibbsite-ETPTA nanocomposite. Adapted from [210].
101
Figure 5-4. Nanocomposite of surface-roughened nanoplatelets: a) TEM image of acid-leached silica-coated gibbsite nanoplatelets, b) SEM image of silica-coated gibbsite-PEI-ETPTA nanocomposite on an ITO electrode, and c) tensile strain-stress curves for plain ETPTA film, gibbsite-ETPTA nanocomposite, and silica-coated gibbsite-PEI-ETPTA nanocomposite. Adapted from [211].
102
CHAPTER 6 CONCLUSIONS AND OUTLOOK
Functional nanoscale materials mimicking nanostructures found in nature are
investigated in a broad range of fluidic, optical, and mechanical systems.
Self-pumping Membrane
The developed self-pumping membrane harvests chemical energy from the
surrounding fluid and uses it for accelerated mass transport of 0.9 nL cm-2 s-1 across the
membrane. Micro- and macroscopic devices for drug delivery, sensing and purification,
as well as oil recovery and removal may benefit from this technology.
The self-pumping membrane still needs to be improved by optimizing the
electrochemical reaction, the materials of membrane and electrodes, and the geometry
of the membrane. The removal of oxygen gas generated on the electrode surfaces
should be studied to improve pumping efficiency.
Reproducible and Highly SERS-active Substrates
Gold nanoparticle and nanohole combined arrays are developed as reproducible
(low variation of Raman signal on different sites) and highly active SERS (high SERS
EFs and high signal-to-background ratios) substrates via the cost-effective technology.
Due to a high sensitivity as well as a good reproducibility, these SERS substrates are
promising in applications of ultra-sensitive detectors for chemicals and reproducible
sensors for chemical and biological molecules.
Potential Applications as a Hybrid Biosensor
Robust and stable synthetic transport systems and effective detection systems are
essential to solve the problems of poor stability and weak fluorescence in hybrid
biosensors. A biomimetic, self-pumping membrane can be integrated as a robust
103
transport system. Reproducible and highly SERS-active substrates can be integrated as
effective detection systems. SERS sensors are known as label-free detecting systems.
Thus, a simpler configuration of biosensor device is feasible without an analyte tagging
process for detection.
Binary Colloidal Crystals
Binary colloidal crystals consisting of colloidal particles of different sizes are
achieved with a simple, fast, and scalable spin-coating technology. The thickness of
individual layer is easily controlled with spin-coating parameters. Although the pressure
is helpful for ordering of particles, further improvement for better orderings of particles in
the individual layers is still needed. The characterization of optical properties in the
fabricated binary colloidal crystals is in progress.
Organic-inorganic Nanocomposites
Gibbsite-polymer nanocomposites are developed via a simple and rapid
electrodeposition. Gibbsite nanoplatelets having high aspect ratio (> 10) are aligned
under electric field and the interstitials between nanoplatelets. The assembled
nanoplatelets are infiltrated with monomer, and monomer is cured by UV generating
organic-inorganic nanocomposites. The developed gibbsite-ETPTA composite shows 2-
fold higher tensile strength than pure polymer. TPM-modified gibbsite-ETPTA composite
has 4-fold higher tensile strength and silica-coated gibbsite-PEI-ETPTA composite has
5-fold larger elongation.
Although this technique is promising to achieve oriented deposition of a wide
range of materials including ceramics, metals, ceramic-metal, or ceramic-conducting
polymer nanocomposites, the alignment of nanoplatelets and the complete infiltration of
104
polymer matrix between aligned nanoplatelets are still under investigation for better
mechanical performance.
The self-pumping membrane designed and demonstrated in this dissertation will
stimulate the research of converting chemical energy into kinetic energy directly with
high efficiency, especially in micro- and nanodevices. The simple, scalable, and cost
effective spin-coating technique utilized in the fabrication of binary colloidal crystals and
hole-particle arrays in nanoscale can replace expensive and slow top-down techniques.
On the other hand, the simple and fast electrophoretic deposition technique employed in
the assembly of nanoplatelets can be substituted for slow bottom-up techniques. The
functional nanomaterials and techniques discussed in this dissertation represent a
contribution to achieve and control nanostructures via simpler, cheaper, and faster
technologies.
105
APPENDIX A ELECTROOSMOTIC FLOW
Electroosmotic Flow in One-wall Channel
In the one-wall case, if the ionic concentration is C0 and electrical potential, φ,
goes to zero and the potential takes the value, ζ (zeta potential), at the surface, the ionic
concentration are given by standard expression from statistical physics.[216]
TkZewherezTk
ZeCz
Tk
ZeCC B
BB
,exp 00 (A-1)
The assumption (Zeφ << kBT) is known as the Debye-Hückel approximation.
The charge density, ρel, is,
z
Tk
CZe
B
el 0
22
(A-2)
With the Poisson equation ( /" el ), the electrical potential, φ, is,
D
zz
exp , where the Debye-length
0
22 CZe
TkBD
(A-3)
Electroosmotic Flow in Cylindrical Tube
The Navier-Stokes equation for steady-static cylindrical tube is,
rr
rrE
r
vr
rr
z
11 (A-4)
1Cr
rE
r
vr z
(A-5)
With the boundary conditions ( 0,000
rr
z
rand
r
v , C1 = 0), the fluid
velocity is,
2CE
vz
(A-6)
106
With boundary conditions (
ECRrandRrvz 2,,0)( ), the fluid
velocity is,
r
Ezvz
(A-7)
In the analogy with the one-wall case, the electric potential, φ(r), is,
D
D
DD
DD
R
r
RR
rr
r
cosh
cosh
expexp
expexp
(A-8)
The fluid velocity, vz(r), is,
1
cosh
cosh
D
D
zR
r
Erv
(A-9)
The average fluid velocity, <vz>, is,
ER
R
RR
R
Erdrrv
Rv
D
DD
D
D
R
zz
22
2
202 2
1
cosh
tanh2
)(2
(A-10)
where RD .
The charge density, ρel, is calculated from the Poisson equation.
107
D
D
D
D
D
D
D
D
D
D
D
D
el
R
r
rR
r
R
r
rR
r
rrrr
cosh
sinh1
cosh
cosh
cosh
sinh1
cosh
sinh1
2
2
2
(A-11)
The average charge density, <ρel>, is calculated,
DD
R
DDD
D
D
R R
D
D
D
D
D
D
elel
R
R
drrr
rR
R
drR
r
rdrR
r
Rrdr
R
tanh2
sinhcosh1
cosh
2
cosh
sinh
cosh
cosh22
02
0 0 2
22
(A-12)
The volume occupied by proton flowing through the cylindrical tube for time t, V, is,
2RtvV proton (A-13)
The total charge of proton flowing through the tube for time t, Ctotal, is,
elprotontotal RtvC 2 (A-14)
Then the flowing current, I, is,
elprotontatal Rvt
CI 2 (A-15)
The velocity of proton, vproton, is calculated from the balance between the electric
force and the drag force of proton in the electric field,
108
proton
protonr
eEv
6 (A-16)
In the membrane having cylindrical pores, the porosity of membrane, p, is,
A
NR
A
Ap
porepore
2 (A-17)
where Npore is the number of pore and A is the total area of membrane.
The total current flowing through membrane, Itotal, is,
DDproton
pore
DD
proton
proton
poreelprotontatal
R
Rr
NReER
RNR
r
eE
NRvI
tanh6
2tanh
2
6
2
2
2
(A-18)
The fluid velocity, vfluid, is,
ReA
RrI
RAe
RrI
R
Rr
eA
I
A
NRE
A
NRvpvv
D
Dprotontotal
D
Dprotontotal
DDproton
total
porepore
zzfluid
3
tanh
3
tanh6
2
22
(A-19)
The maximum pumping pressure (counter pressure) is calculated by applying zero
velocity in the Navier-Stokes equation.
01
EP
r
vr
rrel
z (A-20)
EdP el (A-21)
where ρel is not constant, but a function of r.
The average pressure, <P>, is calculated by <P> = < ρel>Ed.
109
pore
totalproton
pore
totalproton
pore
proton
total
poreproton
total
eA
dIr
eNR
dIr
NRr
eE
EdIEd
NRv
IP
66
6
22
2 (A-22)
110
APPENDIX B SELF-PUMPING FLOW
Flow Rate as a Function of Tracer Velocity
According to Holmes and Vermeulen,[217] the fluid velocity in the narrow
rectangular channel, v(x, y) is,
v
vmax 1
2
Bx
1.54
1
2
Hy
2
, for0
H
B2
3 (B-1)
where vmax is the maximum velocity in the center of the channel and equals the
measured tracer velocity, and the width, B, and the height, H, of the narrow channel are
3.2×10-4 m, 5.1×10-5 m, respectively.
The flow rate through the channel, Qchannel, is calculated from the measured tracer
velocity, vmax.
dxdyyH
xB
vdxdyyH
xB
vQBHB
B
H
Hchannel
2
0
254.1
2
0max
2
2
254.1
2
2
max
21
214
21
21
(B-2)
For a channel of the given dimensions, this yields
max
29 )106.6( vmQchannel (B-3)
Conductivity of Working Fluid
The measured slope of 120 μA V-1 for the I-V curve for electroosmotic pumping
(Figure 2-7) implies an ohmic resistance of 8.4 kΩ by
pA
l
kR
1 (B-4)
where the length of the pores, l, is 1.8×10-5 m, the membrane area, A, is 9×10-5 m2, and
the porosity of the membrane, p, is 12%.
111
We obtain a conductivity, k, of 2.0 ×10-4 S m-1 which compares well with the
conductivity of de-ionized water of 9.9×10-5 S m-1.[218]
Flow Rate as a Function of Current
The flow rate through membrane, Qpore, equals to the flow rate through channel,
Qchannel. The flow through the pore is composed of an electroosmotic and counter-
pressure component
pressureconteroticelectroosmporechannel QQQQ (B-5)
According to Lazar and Karger,[40] the flow rate due to electroosmosis,
Qelectroosmotic, is,
poreoticelectroosm Ndl
UQ 2
4
(B-6)
with ε as the permittivity and η as the viscosity of the working fluid, ζ as the zeta-
potential, d as the diameter and l as the length of the pore, and U as the applied voltage
and Npore as the number of pores.
Utilizing U = IR together with (B-4) and pA = πd2Npore /4 we obtain
Ik
Q oticelectroosm
(B-7)
Using ε = 7.08×10-10 C V-1 m-1 (the permittivity of water at 20 ℃),[218] ζ -27
mV,[71] η = 1.002×10-3 N m-2 s (the viscosity of water at 20 ℃),[218] and k = 2.0×10-4 S
m-1, we obtain
IsAnLQ oticelectroosm )97( 11 (B-8)
According to Lazar and Karger,[40] the flow rate due to counter pressure, Qcounter-
pressure, is,
112
pore
pore
pressurecounter Ndl
PQ 4
128
(B-9)
with ΔPpore as the pressure differential across the membrane.
Since this pressure is balanced by the pressure differential across the small
channel, ΔPchannel, we can write
pore
channel
pressurecounter Ndl
PQ 4
128
(B-10)
According to Holmes and Vermeulen,[217] the flow rate through a rectangular
channel can be approximated by
Qchannel MPchannelBH
3
12Lc (B-11)
with the channel length Lc = 6 mm, the channel width B = 320 μm, the channel height H
= 51 μm and the correction factor M given by
M 1 0.630H
B
0.052
H
B
5
0.9 (B-12)
Inserting (B-11) into B-(10), and the resulting expression into (B-5), we obtain
channelc
channel QlMBH
ApLd
k
IQ
3
2
8
3
(B-13)
which simplifies to
I
lMBH
ApLdk
Q
c
channel
3
2
8
31
(B-14)
Using d = 0.96 μm and l = 18 μm, we obtain
113
IsAnLIsAnL
Qchannel
)9.2()331(
)97( 1111
(B-15)
Flow Rate at Zero Opposing Pressure
The flow rate in the absence of the counter pressure is equal to the flow rate due
to electroosmosis calculated in (B-7) and (B-8). Given the maximum available current of
0.26 μA this translates into a maximum flow rate of 25 nL s-1.
Stall pressure at zero flow. The stall pressure can be calculated by equating (B-7)
and (B-9) to be
Pstall 32l
d2pAkI (B-16)
For a current of 0.26 μA, this yields a stall pressure of 1.4 Pa.
Consumption of H2O2 and Decreasing Flow Rate
The O2 generation rate per electrode area, kO2, is
kO21
2
I
fAF (B-17)
where F is the Faraday constant, and f is the fraction of oxygen which originates from
the electrochemical reaction H2O2 → O2 + 2 H+ + 2 e- (and not from 2 H2O2 → H2O +
O2). Paxton et al estimated f = 40 % based on their measurements of O2 generation and
current density for gold/platinum microelectrodes,[56] which implies
128
251
7
104)109()96485(4.02
)106.2(2
smmol
mmolC
AkO (B-18)
Since 60% of the oxygen molecules require 2 hydrogen peroxide molecules to be
produced and 40% of the oxygen molecules require only one hydrogen peroxide
molecule, the hydrogen peroxide consumption rate is given by
114
kH2O21.6kO
2 (B-19)
The fraction of the initially available hydrogen peroxide which is consumed after
time t is approximately given by
Vc
Atk
OH
OH Ot
2
6.1
022
22 (B-20)
where c is the hydrogen peroxide concentration (0.01 wt%), r is the density of the
solution (1 g cm-3) and V is the volume of the solution (4.3 mL), and M is the molecular
weight of hydrogen peroxide (34 g mol-1).
Inserting (B-17) into (B-20) yields
VfFc
MIt
OH
OHt
5
4
022
22 (B-21)
At the end of the experiment described in Figure 2-8, 270 minutes have passed,
so that
%7.0)0043.0()1000()10()96485(4.05
)34(60270)1026.0(4141
16
022
22
LLgmolC
molgsA
OH
OHt (B-22)
In contrast, the measured decrease in the flow rate and the current is about 20 %.
However, the fraction of oxygen originating from the electrochemical reaction may be
sensitive to the experimental conditions, and only a fraction of the total hydrogen
peroxide in the cell may be locally available to the membrane. This may cause us to
underestimate the hydrogen peroxide depletion. Alternatives, which cannot be ruled out
at this time, are a reduction in the catalytic efficiency of the membrane and partial
clogging of the pores with tracer particles.
115
APPENDIX C FLOW RATE MEASUREMENT
Length-converting Method
The flow rate is measured by tracing the flow front in test section with a ruler
(Figure C-1a).[74] The flow rate is calculated by multiplying the velocity of the flow front
with the cross-section of channel. The error in the flow rate measured by this method is
within 50 nL min-1.
Mass-converting Method
The flow rate is measured with a digital balance (Figure C-1b).[42, 219] The flow
rate is calculated by dividing the change in mass of the reservoir with the density of
fluid. Heads of the head tank and the weighing reservoir are set at the equal height for
equilibrium pressure. The measured flow rate is in the range from1 to 100 μL s-1.[219]
Brask et al reported the accuracy of the flow rate within 1 μL min-1 with a balance of 0.1
mg precision.[42]
Current-monitoring Method
The electroosmotic flow rate is determined by the current-monitoring method
(figure C-1c).[220] When the fluid is driven through a capillary tube from reservoir 1 to
reservoir 2 by the electroosmotic flow, the change in measured current corresponds to
the volume of driven fluid from reservoir 1 to reservoir 2.
Particle Image Velocimetry Method
The flow rate is measured by tracing the marker particles (Figure C-1d).[221, 222]
The two syringes connected to the ends of the channel in the micropump are set at the
equal height for equilibrium pressure. The speed of an air slug in the channel is
measured. The pressure difference due to electroosmotic pumping is almost zero since
116
the liquid level change in the syringe is negligible due to large diameter of the syringe.
This method can measure the flow rate in the range of nL min-1.
Concentration-monitoring Method
The flow rate is estimated based on the change in concentrations of dye in
compartment 1 and compartment 2 (Figure C-2). This method is similar to the current-
monitoring method. The concentration is measured with the absorbance or fluorescence
of dye instead of current. To remove back-pressure problems, double membrane-based
electroosmotic pumps are used.
A flow is driven by the first electroosmotic pump from the compartment 1 having
volume, V1, and concentration, C1, of dye to the compartment 2 having volume, V2, and
concentration, C2, of dye. At the same time, a flow is also driven by the second
electroosmotic pump from the compartment 2 to the compartment 1.
It is assumed that the pumping rates of the first and second electroosmotic pumps,
kc1→c2 and kc2→c1, are identical (k= kc1→c2= kc2→c1) and the diffusion constant of dye, D, is
constant.
The dye flux through the membrane (electroosmotic pump), Jdye, is,
At
VCJ dye
22 (C-1)
where Δt is the elapse time and A is the area of membrane.
According to the Fick‟s second law, the dye flux through the membrane
(electroosmotic pump), Jdye, is,
kCCdx
dCDkC
dx
dCDkC
dx
dCDJ dye 2121 2 (C-2)
By combining (C-1) and (C-2), the following relationship is obtained.
117
tkl
D
V
A
V
V
C
C
C
C
V
V
V
V
C
C
21
1
11
ln21
2
0,1
0,2
0,1
2
1
2
1
2
0,1
0,2
(C-3)
where l , t, C1,0, C2,0 are the thickness of membrane, time, initial concentration in
compartment 1, and initial concentration in compartment 2, respectively.
From the slope in a plot drawn with experimental data, the diffusion constant, D,
and the pumping rate, k, by the electroosmotic pump can be estimated (Figure C-3).
118
Figure C-1. Various methods for measuring the flow rate: a) length-converting method, b) mass-converting method, c) current-monitoring method, and d) particle image velocimetry method. Adapted from [42, 74, 220, 221].
Figure C-2. The double membrane configuration having compartment 1 and compartment 2 in the concentration-monitoring method.
119
Figure C-3. Plot of function, f, with time. The diffusion constant, D, and the pumping rate, k, by the electroosmotic pump can be estimated from the slope.
120
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BIOGRAPHICAL SKETCH
In-kook Jun grew up in Seoul, Republic of Korea. He received his bachelor„s
degree at the Seoul National University in 2003 and his master‟s degree at the Seoul
National University in 2005. He started his military service in November 1998, and he
was discharged in January 2001. He joined the group of Dr. Henry Hess in the Materials
Science and Engineering Department and the group of Dr. Peng Jiang in the Chemical
Engineering department at the University of Florida, Gainesville. He graduated in the
Summer 2010 after spending four years being educated in materials science and
chemical engineering.