nano materials in biosensor application [autosaved]11

8/4/2019 Nano Materials in Biosensor Application [Autosaved]11

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Content

1) Introduction to Biosensor2) Nano materials

3) Nanoscale cantilevers behave anomalously

4) Nanotube

i. Carbon nanotubeii. The Chicken Wire Tube

5) Single Walled and Multiwalled

6) Myriad Applications

7) Nanowire

i. Fabrication of Nanowires at Surfaces

ii. Atom Chains, the Ultimate Nanowires

8) Spin Chains for Single Spin Electronics

9) Summary


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Introduction

In the most common biosensor implementation, a probe molecule isaffixed to a sensing platform and used to recognize or detect a targetmolecule which is complementary to the probe- it is this feature ofbiosensors which provides high specificity and a low false-positiverate in qualitative sensing applications (Prasad, 2003).

Other parameters, such as the acoustic properties of surface-acousticwave devices or the mass of a resonant structure may be altered byprobe-target binding, and these parameters may also serve totransducer a binding event into a detectable signal. This signal can

then be further processed to provide a qualitative or quantitativemetric of the presence of the target biomolecule. In the followingsections, specific biosensor implementations are discussed, based onthe material systems.


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NanomaterialsThe term nanomaterials has been applied to materials that incorporate structures

having dimensions in the range 1-100 nm, and whose electrical and /or chemical

properties are also influenced by their small dimensional scale. These materials

have a wide variety of morphologies, including nanotubes, nanowires, nanoparticles

(also termed quantum dots), and sheet-like two-dimensional structures (Vollath

2008). The unique optical, electrical, mechanical and chemical properties of

nanomaterials have attracted considerable interest- these properties are influenced

by quantum mechanical effects, and may vary from those of the individualconstituent atoms or molecules, as well as those of the corresponding bulk material.

Material systems based on combinations of

nanomaterials (so-called hybrid nanomaterials) have also

received a great deal of attention in the research

community based on the proposed synergistic effects ofnanomaterials of different compositions and

morphologies in close proximity. Hybrid nanomaterial

systems may exhibit great sensitivity to variations in the

local electrochemical milieu, and this has led to the

design of novel sensing devices for biological and

chemical applications.


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Nanoscale cantilevers behave

anomalously

Normally a cantilever's resonant

frequency decreases when molecules

attach to ita finding that is thebasis of nanomechanical sensing

devices. But now researchers from

Purdue University, US, have found

that the resonant frequency of some

nanoscale cantilevers may actuallyincrease on the addition of

molecules.

Nano cantilever


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Nanotube

A carbon molecule that resembles a cylinder made out of chicken

wire one to two nanometers in diameter by any number of

millimeters in length. Accidentally discovered by a Japanese

researcher at NEC in 1990 while making Buckyballs, they have

potential use in many applications. With a tensile strength 10

times greater than steel at about one quarter the weight, nanotubes

are considered the strongest material for their weight known to

mankind.


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Carbon nanotube

Carbon nanotubes (CNTs; also known asbuckytubes), not to be confused with carbonfiber, are allotropes of carbon with acylindrical nanostructure. Nanotubes have

been constructed with length-to-diameterratio of up to 132,000,000: significantly larger

than any other material. These cylindricalcarbon molecules have novel properties,making them potentially useful in manyapplications in nanotechnology, electronics,optics, and other fields of materials science,as well as potential uses in architectural

fields. They may also have applications in theconstruction of body armor. They exhibitextraordinary strength and unique electrical

properties, and are efficient thermalconductors.


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The Chicken Wire Tube

At the molecular level, a single-walled carbon nanotube looks a lot like rolled up chickenwire with hexagonal cells. The number of applications that may ultimately benefit from

carbon nanotubes is enormous.


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A Single Walled and

Multiwalled

Single-walled nanotubes (SWNTs) use a single sheath of graphite one atom

thick, called "graphene." Multiwalled nanotubes (MWNTs) are either

wrapped into multiple layers like a parchment scroll or are constructed of

multiple cylinders, one inside the other. See Buckyball, nanotechnology

and NRAM.

SWNTs MWNTs


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Myriad Applications

Currently used to strengthen plasticsand carbon fibers, nanotubes have the

potential for making ultra-strong

fabrics as well as reinforcing structural

materials in buildings, cars and

airplanes. In the future, nanotubes may

replace silicon in electronic circuits,and prototypes of elementary

components have been developed. In

1998, IBM and NEC created nanotube

transistors, and three years later, IBM

created a NOT gate using twonanotube transistors. Nanotubes are

already used as storage cells in

Nantero's non-volatile memory chips

(see NRAM), and they are expected to

be used in the construction of sensors

and display screens.


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Nanowire

Nanowires are especially attractive for Nanoscience studies

as well as for nanotechnology applications. Nanowires,

compared to other low dimensional systems, have two

quantum confined directions, while still leaving one

unconfined direction for electrical conduction. This allowsnanowires to be used in applications where electrical

conduction, rather than tunneling transport, is required.

Because of their unique density of electronic states,

nanowires in the limit of small diameters are expected toexhibit significantly different optical, electrical and magnetic

properties from their bulk 3D crystalline counterparts.


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Driven by:

1. These new research and

development opportunities

2. The smaller and smaller

length scales now being used

in the semiconductor, opto-

electronics and magnetics

industries


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Fabrication of Nanowires at

Surfaces

The design of artificial materials that consist of ultrafine wires or linear arrays

of dots, ten to hundred times finer than those produced with commercial

micro-structure fabrication techniques. In fact, we have gone all the way down

to atom chains which may be viewed as the ultimate nanowires (scroll to the

bottom for those). These patterns are formed by self-assembly, where atoms

arrange themselves naturally at stepped silicon surfaces. The figure below

shows the preparation of calcium fluoride masks in schematic form (top),

together with actual data (bottom).


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To start along this pathway, we determine the conditions for obtaining highly-

regular step structures on silicon. The images below demonstrate the range of

step arrays that can be formed on silicon surfaces by self-assembly. Typically,

the step spacing is comparable to the size of a virus. These images are takenwith a scanning tunneling microscope (STM). They show the derivative of the

tip height. That gives the impression of a surface illuminated from the left,

with the steps casting dark shadows to the right.


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The stripe width of 7 nm achieved

here is well below the resolution of

180 nm achieved in commercial

lithography for chip fabrication.

The picture below shows that

molecules can be deposited

selectiveley in the CaF1 grooves

between CaF2 stripes.


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Atom Chains, the Ultimate

Nanowires

Self-assembly can reach atomic precision

for very small structures (up to 10 nm in

size). It is possible to go all the way to the

ultimate limit for nanowires, i.e. chains of

single atoms with a single set of orbitalsconnecting them. Such atomic wires are

obtained by depositing a fraction of a

monolayer of metal atoms onto a stepped

silicon surface. An example is the Si(557)-

Au surface shown below. It contains a

step every five silicon atom rows and arow of gold atoms in the middle of the

terrace. The STM image below shows two

rows of fine white dots, which are

magnified in the inset. They correspond to

silicon atoms with dangling bonds.


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Some of the atomic wires are found to be metallic (above). The metallic

behavior is deduced from the observation that the bands extend all the

way up to the Fermi level EF for Si(557)-Au, as in a metal. By way of

contrast, the flat Si(111)-Au surface in the panel on the right shows a bandthat does not reach EF. Even though the metal atoms are strongly coupled

to the substrate, metallic electrons do not interact with the silicon

substrate because their energy lies in the band gap of silicon.


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Currently, we are exploring silicon surfaces with a variety of step spacings.

That makes it possible to vary the dimensionality between 2D and 1D. For

example, the coupling ratio parallel/perpendicular to the chains can be

varied from 10:1 to >70:1


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Spin Chains for Single Spin

Electronics

The ultimate limit for making electronic

circuits smaller and smaller is reached when

one bit of data is shrunk to a single electron,

carrying a single spin and sitting on a single

atom. Self-assembly of silicon and gold

atoms produces a very unusual structure at

the step edge. The silicon atoms form a stripe

of graphitic silicon at the step edge (green),

and the gold atoms (yellow) form two rows in

the middle of the terrace. Broken bonds at the

step edge contain a single electron each, andwith it comes a single spin (up or down

arrow). These unpaired spins occur exactly at

every third edge atom, while the two atoms in

between pair up their spins and are non-

magnetic.


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Summary

nano materials in biosensor application [autosaved]11

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