Overview of carbon nanotubes cnts novelof applications as microelectronics optical communications biological biomedicine and biosensing

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 10, October (2014), pp. 104-133 © IAEME
104
OVERVIEW OF CARBON NANOTUBES (CNTS) NOVEL
OF APPLICATIONS AS MICROELECTRONICS, OPTICAL
COMMUNICATIONS, BIOLOGICAL, BIOMEDICINE AND
BIOSENSING
Jafaar Fahad A. Rida, A. K. Bhardwaj, A. K. Jaiswal
1, 3
Dept. of Electronics and Communication Engineering, SHIATS - DU, Allahabad, India
2
Dept. of Electrical and Electronics Engineering, SHIATS - DU, Allahabad, India
ABSTRACT
This review explores the state-of-the-art applications of various kinds of carbon nanotubes.
The uniqueness of nanotubes that makes them better than their competitors for specific applications
The last decade of research in this field points to several possible applications for these materials;
electronic devices and interconnects, field emission devices, electrochemical devices, such as
supercapacitors and batteries, nanoscale, sensors, electromechanical actuators, separation
membranes, filled polymer composites, and drug-delivery systems are some of the possible
applications. The combination of structure, topology, and dimensions creates a host of physical
properties in carbon nanotubes that are unparalleled by most known materials. After a decade and a
half of research efforts, these tiny quasione-dimensional structures show great promise for a variety
of applications areas, such as nanoprobes, molecular reinforcements in composites, displays, sensors,
energy-storage media, and molecular electronic devices. There have been great improvements in
synthesis and purification techniques, which can now produce good-quality nanotubes in large
quantities Carbon nanotubes exhibit many unique intrinsic physical and chemical properties and
have been intensively explored for biological and biomedical applications in the past few years
Ultra-sensitive detection of biological species with carbon nanotubes can be realized after surface
passivation to inhibit the non-specific binding of bio-molecules on the hydrophobic nanotube
surface. Electrical nanosensors based on nanotubes provide a label-free approach to biological
detections. Thus exploitation of their unique electrical, optical, thermal, and spectroscopic properties
in a biological context is hoped to yield great advances in the therapy of disease and detection
biomolecules such as DNA, antigen–antibody, cells, and other biomolecules. special attention has
been drawn into promising orthopaedic use of CNT for improving tribological behaviour and
material mechanical properties. However, and considering the conductive properties of CNT the
range of orthopaedic application may broaden up, since it is known that electrical fields as small as
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ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
Volume 5, Issue 10, October (2014), pp. 104-133
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0, 1 mV/cm may enhance osteoplastic proliferation locally. CNT based electrodes could be
considered for integrating implantable orthopaedic devices.
Keywords: CNTs, Microelectronics, Optical Systems, Biological, Biomedicine, and Biosensing.
INTRODUCTION
Carbon nanotubes have attracted the fancy of many scientists worldwide. The small
dimensions, strength and the remarkable physical properties of these structures make them a very
unique material with a whole range of promising applications [1].The important materials science
applications of carbon nanotubes are specially the electronic and electrochemical applications of
nanotubes, nanotubes as mechanical reinforcements in high performance composites, nanotube –
based field emitters, and their use as nanoprobes in metrology and biological and chemical
investigations, and as templates for the creation of other nanostructures with electronic properties
and.device applications of nanotubes. The discovery of fullerenes [2] provided exciting insights into
carbon nanostructures and how architectures built from ‫݌ݏ‬ଶ
carbon units based on simple
geometrical principles can result in new symmetries and structures that have fascinating and useful
properties [3]. There have been great improvements in synthesis techniques, which can now produce
reasonably pure nanotubes in gram quantities. Studies of structure topology- property relations in
nanotubes have been strongly supported, and in some cases preceded by theoretical modeling that
has provided insights for experimentalists into new directions and has assisted the rapid expansion of
this field [4], [5]. Carbon Nanotubes are structures from the fullerene family consisting of a
honeycomb sheet of ‫݌ݏ‬ଶ
bonded carbon atoms rolled seamless into itself to form a cylinder. Single –
walled carbon nanotubes are nearly one dimensional (1D) materials with a diameter ranging 1nm to
3nm, and a length that can go from of nanometers to centimeters [6]. The former can be considered
as a mesoscale graphite system, whereas the latter is truly a single large molecule. However, Single
Walled Carbon Nanotubes (SWCNTs) also show a strong tendency to bundle up into ropes,
consisting of aggregates of several tens of individual tubes organized into a one – dimensional
triangular lattice. One point to note is that in the most applications, although the individual nanotubes
should have the most appealing properties, one has to deal with the behavior of aggregates (Multi-
Walled Carbon Nanotubes (MWCNTs) or Single Walled Carbon Nanotubes (SWCNTs)), as
produced in actual samples as shown figure 1.
Figure 1: illustrates all stages fabricated for carbon nanotubes (CNTs) from carbon atoms,
graphite sheet, and rolled as form tube [14]

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The best presently available methods to produce ideal nanotubes are based on the electric arc
[7], [8] and laser ablation processes [9]. The material prepared by these techniques has to be purified
using chemical and separation methods. None of these techniques are scalable to make the industrial
quantities needed for many applications. Chemical Vapor Deposition (CVD) techniques using
catalyst particles and hydrocarbon precursors to grow nanotubes [10 - 13]; such techniques have
been used earlier to produce hollow nanofibers of carbon in large quantities. The drawback of the
catalytic CVD-based nanotube production is the inferior quality of the structures that contain gross
defects (twists, tilt boundaries etc.), particularly because the structures are created at much lower
temperatures (600– 1000௢
C) compared to the arc or laser processes (∼2000௢
C). Since their
discovery in 1991, several demonstrations have suggested potential applications of nanotubes. These
include the use of nanotubes as electron field emitters for vacuum microelectronic devices,
individual MWNTs and SWNTs attached to the end of an Atomic Force Microscope (AFM) tip for
use as nanoprobe, MWNTs as efficient supports in heterogeneous catalysis and as microelectrodes in
electrochemical reactions, and SWNTs as good media for lithium and hydrogen storage. Some of
these could become real marketable applications in the near future, but others need further
modification and optimization. Areas where predicted or tested nanotube properties appear to be
exceptionally promising are mechanical reinforcing and electronic device applications. The lack of
availability of bulk amounts of well-defined samples and the lack of knowledge about organizing
and manipulating objects such as nanotubes (due to their sub-micron sizes) have hindered progress in
developing these applications. The last few years, however, have seen important breakthroughs that
have resulted in the availability of nearly uniform bulk samples. Electron field emission
characteristics of nanotubes and applications based on this, nanotubes as energy storage media, the
potential of nanotubes as fillers in high performance polymer and ceramic composites, nanotubes as
novel probes and sensors, and the use of nanotubes for template based synthesis of nanostructures.
There are two types for fabrication first, chemical (chemical vapor deposition (CVD)) and second,
other physical methods (Arc discharge, Laser ablation). Carbon nanotube belongs to polymer
electronic Nano system. It is a tube – shaped material, made of carbon, having a diameter
measuring on the nanometer scale that means one- billionth of a meter or about one ten –
thousand of the thickness of a human hair. The graphite layer appears somewhat like a rolled up
chicken wire with a continuous unbroken hexagonal mesh and carbon molecules at the apexes of the
hexagons [14]. They have two conduction bands ܿଵ andܿଶ and two valence bands ‫ݒ‬ଵ and‫ݒ‬ଶ , these
are called Van Hove Singularities observed in their electronic density of state (DOS) of
these carbon nanotubes (CNTs). The direct electronic band gap proportional to diameter for
semiconducting carbon nanotubes, while the direct band gap equal zero for metal carbon nanotubes
so, they use in high electrical current. It has typically have diameters range (1-2) nm for single
walled nanotubes and (2-25) nm for multi-walled nanotubes as well as the length of nanotubes may
be (0.2 - 5) µm or some centimeters, and the spacing distance between walls is 0.36nm [14].
Carbon nanotubes exhibit many unique intrinsic physical and chemical properties and have been
intensively exploredfor biological and biomedical applications in the past few years. Ultra-sensitive
detection of biological species with carbon nanotubes can be realized after surface passivation to
inhibit the non-specific binding of bio-molecules on the hydrophobic nanotube surface. Electrical
nanosensors based on nanotubes provide a label-free approach to biological detections.
Nanomaterials have sizes ranging from about one nanometer up to several hundred nanometers,
comparable to many biological macromolecules such as enzymes, antibodies, Deoxyribose Nucleic
Acid (DNA) plasmids. Applications of CNTs span many fields and applications, including
composite materials, nano-electronics, field-effect emitters, and hydrogen storage. In recent years,
efforts have also been devoted to exploring the potential biological applications of CNTs as
motivated by their interesting size, shape, and structure, as well as attractive, unique physical
properties [15]. Photovoltaic device is a device that converts the energy of light directly into

107
electricity by the photovoltaic effect. It is a crucial part of solar cells. Currently, wafer-based silicon
(single crystal, poly crystalline and multicrystalline) solar cells and thin film solar cells based on
amorphous silicon, CdTe, CuInGaSe2, and III–V semiconductors dominate photovoltaic
manufacturing. However, they are low-efficient and expensive, which have limitations for
replacement of current energy sources. There is a clearly need to look for low-cost and high-efficient
solar cells. Many new kinds of solar cells have been proposed, such as p-n junction solar cells,
dyesensitized solar cells and organic solar cells. Nanomaterials have been widely used in above
proposed solar cells. The advantages of using nanostructure-based solar cells are, on one hand,
reducing manufacturing costs as a result of using a low temperature process similar to printing
instead of the high temperature vacuum deposition process typically used to produce conventional
cells made with crystalline semiconductor material, and on the other hand, improving quantum
efficiency by using multiple electron-hole pair generation in nanostructures, like quantum dots and
carbon nanotube [16]. Nanotechnology is a most promising field for generating new applications in
medicine. However, only few nanoproducts are currently in use for medical purposes. A most
prominent nanoproduct is nanosilver. Thus exploitation of their unique electrical, optical, thermal,
and spectroscopic properties in a biological context is hoped to yield great advances in the therapy of
disease and detection biomolecules such as DNA, antigen–antibody, cells, and other biomolecules
[17]. Most of the biological sensing techniques rely largely on optical detection principles. The
techniques are highly sensitive and specific, but are inherently complex; require multiple steps
between the actual engagement of the analyzed and thegeneration of a signal, multiple reagents,
preparative steps, signal amplification, and complex data analysis.
Several Interesting Applications of Carbon Nanotubes (CNTs)
Several interesting applications of carbon nanotubes based on some of the remarkable
materials properties of nanotubes. Electron field emission characteristics of nanotubes and
applications based on this, nanotubes as energy storage media, the potential of nanotubes as fillers in
high performance polymer and ceramic composites, nanotubes as novel probes and sensors, the use
of nanotubes for template based synthesis of nanostructures, optical communication systems, solar
cell systems, biological systems, biomedicine and biosensor systems, and microelectronic
applications.
2.1 Carbon Nanotubes in Microelectronic Applications
Many of the problems that silicon transistor technology is or will be confronted with do not
exist for CNT transistors. The strictly one - dimensional transport in CNTs results in a reduced
phase space, which allows almost ballistic transport and reduced scattering, especially at reduced
gate length and low voltages. The direct band structure of CNTs is completely symmetric for hole
and electron transport and allows for symmetrical devices and optically active elements. As there are
no dangling bonds in CNTs, the use of high - kmaterial as gate dielectrics is simple. In fact, the
application ofܶܽ௫ܱ௬, ‫݂ܪ‬௫ܱ௬and ‫݈ܣ‬௫ܱ௬gate material has produced superior CNT transistors with low
sub - threshold slopes and low hysteresis. Both n - type and p - type conduction is possible, enabled
by charge transfer doping or different work functions for gate, source or drain. CNTs are created in a
“self -assembling” process and not by conventional top - down structuring methods. The scalability
has been shown down to an 18 - nm channel length recently. CNTs are chemically inert and due to
the covalent bonds mechanically very stable. Therefore, they would allow integration even in a high
- temperature process. The device performance is considered to be more robust against process -
induced fluctuations than their silicon counterpart. Transistor devices made of semiconducting
SWCNTs can be considered as simple silicon CMOS field - effect transistors with the silicon
material replaced by the carbon nanotube structure. The source and drain contacts in conventional
silicon devices are made by highly doped silicon regions, which in turn are contacted by metals to

108
form low - resistance contacts. Contacting a piece of silicon with metals leads to the formation of a
Schottky contact and results in a Schottky barrier transistor if the source – drain areas have not been
doped. The doping of the source – drain areas makes the Schottky barrier thin enough so that charge
carriers can easily tunnel through the barrier and at an interface doping level of ∼2 ∗ 10ଶଵ
ܿ݉ିଷ
, a
contact resistance of the order of 10ି଻
ܿ݉ଶ
should be achievable. Therefore, a low - resistance
contact to a MOSFET - type transistor can be formed with metal contacts if the contact regions are
highly doped. The same approach can be applied to the contact formation of a nanotube transistor.
The metal contacts can be formed on highly doped CNT regions, where the doping can also be
introduced by electrostatic doping of a nearby gate voltage, or the intrinsically doped nanotube is
contacted directly by the metal, In the latter case, a Schottky barrier field effect transistor (SBFET) is
formed. The height of the Schottky barrier is basically determined by the differences in the work
function of the CNT and the metal contact. Therefore, the Schottky barrier can be considerably
reduced and a quasi - MOSFET transport behavior established if the right work function material is
chosen. For a typical CNT, the mid - gap work function is 4.5 eV. The ambipolar behavior is
characterized by hole andelectron transport in the channel depending on the polarity of the gate
voltage. The on/off ratio of the current is severely affected by the ambipolar behavior, which
therefore should be avoided in logic devices. The whole Si substrate is then acting as a gate
electrode. Another approach is the top - gate approach, also shown in Figure 2.
Figure 2: Schematic of two different gate contacts for nanotube transistor. A top - gate is
shown on the image in (a), where a gate dielectric needs to be deposited on the CNT before the
metal gate is formed. A cross - section through a bottom - gate (back - gate) device where the
CNT is grown on top of the silicon oxide and the gate - electrode is depicted on the image in (b)
Here, the nanotubes are covered with the gate dielectric prior the top metal - gate deposition.
In the following, it will be shown that a combination of top and bottom gates achieves the best
performance. The capacitance of the gate is a critical issue for future high - performance transistors.
A high - kdielectric is, therefore, unavoidable since the thickness of a silicon oxide or an oxy -
nitride gate dielectric cannot be reduced below a certain value without causing an intolerable
increase in the gate leakage by direct tunneling. In addition, encapsulation of nanotubes is necessary
in order to protect the dopants from desorption and to allow further integration. Therefore, it is
necessary to evaluate different processes and high - kmaterials for the encapsulation of nanotubes.
While the application of high - kstacks to silicon transistors is still cumbersome due to severe
mobility degradation of the Si device, the use of high – k dielectrics for CNT transistors is relatively
easy. The scaling properties of every rival technology to silicon need to be explicitly demonstrated
before the new technique can be taken seriously. Successful n - type doping has been achieved with
functionalization of the SWCNTs with amine - rich polymers the completely altered characteristics
of an SWNT transistor after doping with polyethylenimine (PEI). The device was submerged in a 20
wt.% solution of PEI (average molecular weight 500 Da) in methanol for various times.
Subsequently, the sample was rinsed with methanol and 2 - propanol to remove non - specifically

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adsorbed PEI on the sample, leaving approximately a monolayer of PEI adsorbed on the device. A 1
- min anneal at 50 ° C evaporated the remaining solvent. Prior to PEI adsorption, the semiconducting
SWNT exhibits p - type FET characteristics revealed by the decreasing current as a gate voltage is
stepped to more positive values. The p - type behavior is due to adsorbed O 2 from the ambient.
After PEI adsorption, the SWNT exhibits clear n - type FET characteristics. The current of the
device increases when Vgateis stepped to more positive values. The current of the device is
completely undisturbed, remaining at around 400 nA at0.1 V source – drain voltage after n - doping.
This is indicative of the low number of scattering centers introduced in the device by this doping
scheme. This is achieved by covering the CNT device with PMMA resist and exposing only a small
area of the channel tothe electron beam. After dissolution of the exposed PMMA area the device is
locally n - doped with PEI. The diode - like current voltage characteristic is the off - current cannot
be determined exactly and is limited by the measurement setup. However, an extrapolation from the
positive exponential behavior would yield a value of ∼2 pA. The forward current grows
exponentially and is limited by the overall serial resistance of ∼1 M . If one applies the ordinary
diode equation for the exponential forward current an ideality factor of the diode of n ≈2.1 can be
fitted to the curve. The device behaves like a gated diode if operated with the Si substrate as gate.
The palladium source and drain regions were defi ned on the SWCNT layer using electron beam
lithography, metal deposition and lift - off. These transistors initially display an on/off ratio of about
3 due to the parallel connection of metallic and small band gap SWCNTs together with the
semiconducting nanotubes. As progressively higher burn pulses are applied at high positive gate
voltage, which turns the semiconducting CNTs off, first the metallic and then the small band gap
SWCNTs are eliminated. The promising properties of carbon nanotubes have sparked a huge world -
wide activity to investigate these objects in many technical areas – not only in microelectronic
applications. Implementations, which rely on the statistical averaging of material properties, i.e.
CNTs as additives in plastics, polymers and epoxies or as transparent conductive coatings, are closer
to or already in the market. For microelectronic applications, the attractiveness has been already
verified experimentally on the laboratory scale; however, a detailed strategy for large - scale
integration of carbon nanotubes is still lacking. Integrated CNTs have to fulfill a whole range of
requirements simultaneously – the most stringent demand being the precise placement of only one
kind of CNT. The placement might be solved by localized growth of CNTs in vertical structures and
the yield of semiconducting CNTs increased by special growth methods which favor the occurrence
of only semiconducting CNTs. However, and if one looks back and recognizes the tremendous
progress which has been achieved in nanotube technology during the past decade, one is certainly
looking forward to what the future might bring [18-30].
2.2 Potential Application of CNTs in Vacuum Microelectronics
Field emission is an attractive source for electrons compared to thermionic emission. It is a
quantum effect. When subject to a sufficiently high electric field, electrons near the Fermi level can
overcome the energy barrier to escape to the vacuum level. The basic physics of electron emission is
well developed. The emission current from a metal surface is determined by the Fowler–Nordheim
equation: ࡵ = ࢇࢂ૛
ࢋ࢞࢖(ି࢈ ∅૜ ૛⁄ /࡮ࢂ)
whereI, V, φ, β, are the current, applied voltage, work function,
and field enhancement factor, respectively. Electron field emission materials have been investigated
extensively for technological applications, such as flat panel displays, electron guns in electron
microscopes, microwave amplifiers. For technological applications, electron emissive materials
should have low threshold emission fields and should be stable at high current density. The current-
carrying capability and emission stability of the various carbon nanotubes, however, vary
considerably depending on the fabrication process and synthesis conditions. The I–V characteristics
of different types of carbon nanotubes have been reported, including individual nanotubes, MWNTs
embedded in epoxy matrices, MWNT films, SWNTs and aligned MWNT films. Typical emission I–

110
V characteristics measured from a random SWNT film at different anode-cathode distances and the
Fowler–Nordheim plot of the same data is shown as the inset. Turnon and threshold fields are often
used to describe the electrical field required for emission [31-40].
2.2.1 Prototype Electron Emission Devices Based on Carbon Nanotubes
2.2.1.1 Cathode-Ray Lighting Elements
Cathode ray lighting elements with carbon nanotube materials as the field emitters have been
fabricated by Ise Electronic Co. in Japan [49]. As illustrated in Figure 3, these nanotube-based
lighting elements have a triode-type design. In the early models, cylindrical rods containing
MWNTs, formed as a deposit by the arc discharge method, were cut into thin disks and were glued
to stainless steel plates by silver paste. In later models, nanotubes are now screen-printed onto the
metal plates. A phosphor screen is printed on the inner surfaces of a glass plate. Different colors are
obtained by using different fluorescent materials. The luminance of the phosphor screens measured
on the tube axis is 6.4 ∗ 10ସ
ܿ݀/ܿ݉ଶ
for green light at an anode current of 200µAwhich is two times
more intense than that of conventional thermionic CathodeRay Tube (CRT) lighting elements
operated under similar conditions as shown in figure 3.
Figure 3: Demonstration field emission light source using carbon nanotubes as the cathodes
2.2.1.2 Flat Panel Display
Prototype matrix-addressable diode flat panel displays have been fabricated using carbon
nanotubes as the electron emission source [46]. One demonstration (demo) structure constructed at
Northwestern University consists of nanotube-epoxy stripes on the cathode glass plate and phosphor-
coated Indium-Tin-Oxide (ITO) stripes on the anode plate [46]. Pixels are formed at the intersection
of cathode and anode stripes, as illustrated in Figure 4. Ata cathode-anode gap distance of 30µm, 230
V is required to obtain the emission current density necessary to drive the diode display
(∼76µmA/݉݉ଶ
). The device is operated using the half-voltage off-pixel scheme. Pulses of±150 V
are switched among anode and cathode stripes, respectively to produce an image. Recently, a 4.5
inch diode-type field emission display has been fabricated by Samsung as shown in figure 6, with
SWNT stripes on the cathode and phosphor-coated ITO stripes on the anode running orthogonally to
the cathode stripes [47]. SWNTs synthesized by the arc-discharge method were dispersed in
isopropyl alcohol and then mixed with an organic mixture of nitro cellulose.

111
Figure 4 Left: Schematic of a prototype field emission display using carbon nanotubes
(adapted from [50]). Right: Aprototype 4.5 field emission display fabricated b Samsung using
carbon nanotubes (image provided by Dr. W. Choi of Samsung Advanced Institute of
Technologies)
2.2.1.3 Gas-Discharge Tubes in Telecom Networks
Gas discharge tube protectors, usually consisting of two electrodes parallel to each other in a
sealed ceramic case filled with a mixture of noble gases is one of the oldest methods used to protect
against transient over-voltages in a circuit [48]. They are widely used in telecom network interface
device boxes and central office switching gear to provide protection from lightning and ac power
cross faults on the telecom network. They are designed to be insulating under normal voltage and
current flow. Under large transient voltages, such as from lightning, a discharge is formed between
the metal electrodes, creating a plasma breakdown of the noble gases inside the tube. In the plasma
state, the gas tube becomes a conductor, essentially short circuiting the system and thus protecting
the electrical components from overvoltage damage. These devices are robust, moderately
inexpensive, and have a relatively small shunt capacitance, so they do not limit the bandwidth of
high frequency circuits as much as other nonlinear shunt components. Compared to solid state
protectors, GDTs can carry much higher currents. However, the current Gas Discharge Tube (GDT)
protector units are unreliable from the stand point of mean turn-on voltage and run-to-run variability.
Prototype GDT devices using carbon nanotube coated electrodes have recently been fabricated and
tested by a group from UNC and Raychem Co.[49].Molybdenum electrodes with various interlayer
materials were coated with single-walled carbon nanotubes and analyzed for both electron field
emission and discharge properties. A mean dc breakdown voltage of 448.5 V and a standard
deviation of 4.8 V over 100 surges were observed in nanotube-basedGDTs with 1 mm gap spacing
between the electrodes. The breakdown reliability is a factor of 4–20 better and the breakdown
voltage is∼30% lower than the two commercial products measured. The enhanced performance
shows that nanotube-based GDTs are attractive over-voltage protection unitsin advanced telecom
networks such as an Asymmetric-Digital-Signal-Line(ADSL), where the tolerance is narrower than
what can be provided by the current commercial GDTs.
2.3 Energy Storage
Carbon nanotubes are being considered for energy production and storage. Graphite,
carbonaceous materials and carbon fiber electrodes have been used for decades in fuel cells, battery
and several other electrochemical applications [46]. Nanotubes are special because they have small
dimensions, a smooth surface topology, and perfect surface specificity, since only the basal graphite
planes are exposed in their structure. The rate of electron transfer at carbon electrodes ultimately
determines the efficiency of fuel cells and this depends on various factors, such as the structure and

112
morphology of the carbon material used in the electrodes. Several experiments have pointed out that
compared to conventional carbon electrodes, the electron transfer kinetics take place fastest on
nanotubes, following ideal Nernstian behavior [48]. Nanotube microelectrodes have been
constructed using a binder and have been successfully used in bioelectrochemical reactions (e.g.,
oxidation of dopamine). Their performance has been found to be superior to other carbon electrodes
in terms of reaction rates and reversibility [48]. Pure MWNTs and MWNTs deposited with metal
catalysts (Pd, Pt, Ag) have been usedto electro-catalyze an oxygen reduction reaction, which is
important for fuelcells [49, 50, 51]. It is seen from several studies that nanotubes could be excellent
replacements for conventional carbon-based electrodes. Similarly, the improved selectivity of
nanotube-based catalysts have been demonstrated in heterogeneous catalysis. Ru-supported
nanotubes were found to be superior to the same metal on graphite and on other carbons in the liquid
phase hydrogenation reaction of cinnamaldehyde [51]. The properties of catalytically grown carbon
nanofibers (which are basically defective nanotubes) have been found to be desirable for high power
electrochemical capacitors.
2.3.1 Electrochemical Intercalation of Carbon Nanotubes with Lithium
The basic working mechanism of rechargeable lithium batteries is electrochemical
intercalation and de intercalation of lithium between two working electrodes. Current state-of-art
lithium batteries use transition metal oxides (i.e., LixCoO2or LixMn2O4) as the cathodes and carbon
materials (graphite or disordered carbon) as the anodes [50]. It is desirable to have batteries with a
high energy capacity, fast charging time and long cycle time. The energy capacity is determined by
the saturation lithium concentration of the electrode materials. For graphite, the thermodynamic
equilibrium saturation concentration is LiC6 which is equivalent to 372 mA h/g. Higher Li
concentrations have been reported in disordered carbons (hard and soft carbon) and metastable
compounds formed under pressure. It has been speculated that a higher Li capacity may be obtained
in carbon nanotubes if all the interstitial sites (inter-shell van der Waals spaces, inter-tube channels,
and inner cores) are accessible for Li intercalation. Electrochemical intercalation of MWNTs and
SWNTs have been investigated by several groups. Representative electrochemical intercalation data
collected from an arc-discharge-grown MWNT sample using an electrochemical cell with a carbon
nanotube film and a lithium foil as the two working electrodes. A reversible capacity (Crev) of 100–
640 mA h/g has been reported, depending on the sample processing and annealing conditions
[52, 53, 54]. In general, well-graphitized MWNTs such as those synthesized by the arc-discharge
method have a lower Crev than those prepared by the CVD method. Structural studies have shown
that alkali metals can be intercalated into the inter-shell spaces within the individual MWNTs
through defect sites. Single-walled nanotubes are shown to have both high reversible and irreversible
capacities. Two separate groups reported 400–650 mA h/g reversible and ∼1000 mA h/g irreversible
capacities in SWNTs produced by the laser ablation method. The exact locations of the Li ions in the
intercalated SWNTs are still unknown. Intercalation and in-situ TEM and EELS measurements
onindividualSWNT bundles suggested that the intercalants reside in the interstitial sites between the
SWNTs. It is shown that the Li/C ratio can be further increased by ball-milling which fractures the
SWNTs. A reversible capacity of 1000 mA h/g was reported in processed SWNTs. The large
irreversible capacity is related to the large surface area of the SWNT films (∼300 m2 /g by BET
characterization) and the formation of a solid-electrolyte-interface. The SWNTs are also found to
perform well under high current rates. For example, 60% of the full capacity can be retained when
the charge-discharge rate is increased from 50 mA/h to 500 mA/h .The high capacity and high-rate
performance warrant further studies on the potential of utilizing carbon nanotubes as battery
electrodes. The large observed voltage hysteresis is undesirable for battery application. [1],
[50-51], [55-57].

113
2.3.2 Hydrogen Storage
The area of hydrogen storage in carbon nanotubes remains active and controversial.
Extraordinarily high and reversible hydrogen adsorption in SWNT containing materials
[58, 59, 60, 61] and graphite nanofibers (GNFs) [62]has been reported and has attracted considerable
interest in both academia and industry. Materials with high hydrogen storage capacities are desirable
for energy storage applications. Metal hydrides and cryo-adsorption are the two commonly used
means to store hydrogen, typically at high pressure and/or low temperature. In metal hydrides,
hydrogen is reversibly stored in the interstitial sites of the host lattice. The electrical energy is
produced by direct electrochemical conversion. Hydrogen can also be stored in the gas phase inthe
metal hydrides. The relatively low gravimetric energy density has limited the application of metal
hydride batteries. Because of their cylindrical and hollow geometry, and nanometer-scale diameters,
it has been predicted that the carbon nanotubes can store liquid and gas in the inner cores through a
capillary effect [76]. A Temperature-Programmed Desorption (TPD) study on SWNT-containing
material (0.1–0.2 wt% SWNT) estimates a gravimetric storage density of 5–10 wt% SWNT when H2
exposures were carried out at 300 torr for 10 min at 277 K followed by 3 min at 133 K. If all the
hydrogen molecules are assumed to be inside the nanotubes, the reported density would imply a
much higher packing density of H2 inside the tubes than expected from the normal H2–H2 distance.
.The potential of achieving/exceeding the benchmark of 6.5 wt% H2 to system weight ratio set by
the Department of Energy has generated considerable research activities in universities, major
automobile companies and national laboratories. At this point it is still not clear whether carbon
nanotubes will have real technological applications in the hydrogen storage applications area. The
values reported in the literature will need to be verified on well characterized materials under
controlled conditions. What is also lacking is a detailed understanding on the storage mechanism and
the effect of materials processing on hydrogen storage. Perhaps the ongoing neutron scattering and
proton nuclear magnetic resonance measurements will shed some light in this direction. In addition
to hydrogen, carbon nanotubes readily absorb other gaseous species under ambient conditions which
often leads to drastic changes in their electronic properties [1], [52-54], [63-64].
2.4 Filled Composites
The mechanical behavior of carbon nanotubes is exciting since nanotubes are seen as the
“ultimate” carbon fiber ever made. The traditional carbon fibers have about fifty times the specific
strength (strength/density)of steel and are excellent load-bearing reinforcements in composites.
Nanotubes should then be ideal candidates for structural applications. Carbon fibers have been used
as reinforcements in high strength, light weight, high performance composites; one can typically find
these in a range of products ranging from expensive tennis rackets to spacecraft and aircraft body
parts. NASA has recently invested large amounts of money in developing carbon nanotube-based
composites for applications such as the futuristic Mars mission. Early theoretical work and recent
experiments on individual nanotubes (mostly MWNTs) have confirmed that nanotubes are one of the
stiffest structures ever made. Since carbon–carbon covalent bonds are oneof the strongest in nature, a
structure based on a perfect arrangement of these bonds oriented along the axis of nanotubes would
produce an exceedingly strong material. Theoretical studies have suggested that SWNTs could have
a Young’s modulus as high as 1 TPa, which is basically the in-plane valueof defect free graphite. For
MWNTs, the actual strength in practical situations would be further affected by the sliding of
individual graphene cylinders with respect to each other. In fact, very recent experiments have
evaluated the tensile strength of individual MWNTs using a nano-stressing stage located within a
scanning electron microscope. The nanotubes broke bya sword-in-sheath failure mode. This failure
mode corresponds to the sliding of the layers within the concentric MWNT assembly and the
breaking of individual cylinders independently. Such failure modes have been observed previously in
vapor grown carbon fibers. Although testing of individual nanotubes is challenging, and requires

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specially designed stages and nanosize loading devices, some clever experiments have provided
valuable insights into the mechanical behavior of nanotubes and have provided values for their
modulus and strength. The main problem is in creating a good interface between nanotubes and the
polymer matrix and attaining good load transfer from the matrix to the nanotubes, during loading.
The reason for this is essentially two-fold. First, nanotubes are atomically smooth and have nearly
the same diameters and aspect ratios (length/diameter) as polymer chains. Second, nanotubes are
almost always organized into aggregates which behave differently in response to a load, as compared
to individual nanotubes. There have been conflicting reports on the interface strength in nanotube-
polymer composites. Depending on the polymer used and processing conditions, the measured
strength seems to vary. In some cases, fragmentation of the tubes has been observed, which an
indication of a strong interface bonding is. In some cases, the effect of sliding of layers of MWNTs
and easy pull-out are seen, suggesting poor interface bonding. Micro-Raman spectroscopy has
validated the latter, suggesting that sliding of individual layers in MWNTs and shearing of individual
tubes in SWNT ropes could be limiting factors for good load transfer, which is essential for making
high strength composites. To maximize the advantage of nanotubes as reinforcing structures in high
strength composites, the aggregates needs to be broken up and dispersed or cross-linked to prevent
slippage. In addition, the surfaces of nanotubes have to be chemically modified (functionalized) to
achieve strong interfaces between the surrounding polymer chains. There are certain advantages that
have been realized in using carbon nanotubes for structural polymer (e.g., epoxy) composites.
Nanotube reinforcements will increase the toughness of the composites by absorbing energy during
their highly flexible elastic behavior. This will be especially important for nanotube-based ceramic
matrix composites. By using high power ultrasound mixers and using surfactants with nanotubes
during processing, good nanotube dispersion may be achieved, although the strengths of nanotube
composites reported to date have not seen any drastic improvements over high modulus carbon fiber
composites [1], [54],[65-69].
2.5 Nanoprobes and Sensors
The small and uniform dimensions of the nanotubes produce some interesting applications.
With extremely small sizes, high conductivity, high mechanical strength and flexibility (ability to
easily bend elastically), nanotubes may ultimately become indispensable in their use as nanoprobes.
One could think of such probes as being used in a variety of applications, such as high resolution
imaging, nano-lithography, nanoelectrodes, drug delivery, sensors and field emitters. The possibility
of nanotube-based field emitting devices [70]. Since MWNT tips are conducting, they can be used in
STM, AFM instruments as well as other scanning probe instruments, such as an electrostatic force
microscope. The advantage of the nanotube tip is its slenderness and the possibility to image features
(such as very small, deep surface cracks), which are almost impossible to probe using the larger,
blunter etched Si or metal tips. Biological molecules, such as DNA can also be imaged with higher
resolution using nanotube tips, compared to conventional STM tips. MWNT and SWNT tips were
used in a tapping mode to image biological molecules such as amyloid-b-protofibrils (related to
Alzheimer’s disease), with resolution never achieved before. In addition, due to the high elasticity of
the nanotubes, the tips do not suffer from crashes on contact with the substrates. Any impact will
cause buckling of the nanotube, which generally is reversible on retraction of the tip from the
substrate. Attaching individual nanotubes to the conventional tips of scanning probe microscopes has
been the real challenge. Bundles of nanotubes are typically pasted on to AFM tips and the ends are
cleaved to expose individual nanotubes. These tip attachments are not very controllable and will
result in vibration problems and in instabilities during imaging, which decrease the image resolution.
However, successful attempts have been made to grow individual nanotubes onto Si tips using CVD,
in which case the nanotubes are firmly anchored to the probe tips. Due to the longitudinal (high
aspect) design of nanotubes, nanotube vibration still will remain an issue, unless short segments of

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nanotubes can be controllably grown. In addition to the use of nanotube tips for high resolution
imaging, it is also possible to use nanotubes as active tools for surface manipulation. It has been
shown that if a pair of nanotubes can be positioned appropriately on an AFM tip, they can be
controlled like tweezers to pick up and release nanoscale structures on surfaces; the dual nanotube tip
acts as a perfect nano-manipulator in this case. It is also possible to use nanotube tips in AFM nano-
lithography. Ten nanometer lines have been written on oxidized silicon substrates using nanotube
tips at relatively high speeds, a feat that can only be achieved with tips as small as nanotubes. Since
nanotube tips can be selectively modified chemically through the attachment of functional groups,
nanotubes can also be used as molecular probes, with potential applications in chemistry and
biology. Open nanotubes with the attachment of acidic functionalities have been used for chemical
and biological discrimination on surfaces. Functionalized nanotubes were used as AFM tips to
perform local chemistry, to measure binding forces between protein-ligand pairs and for imaging
chemically patterned substrates.These experiments open up a whole range of applications, for
example, as probes for drug delivery, molecular recognition, chemically sensitive imaging, and local
chemical patterning, based on nanotube tips that can be chemically modified in a variety of ways.
The chemical functionalization of nanotubes is a major issue with far-reaching implications [27].
The possibility to manipulate, chemically modify and perhaps polymerize nanotubes in solution will
set the stage for nanotube-based molecular engineering and many new nanotechnological
applications. Electromechanical actuators have been constructed using sheets of SWNTs. It was
shown that small voltages (a few volts), applied to strips of laminated (with a polymer) nanotube
sheets suspended in an electrolyte, bends the sheet to large strains, mimicking the actuator
mechanism present in natural muscles.
Figure 5: Use of a MWNT as an AFM tip (after Endo). At the center of the Vapor Grown
Carbon Fiber (VGCF) is a MWNT which forms the tip. The VGCF provides a convenient and
robust technique for mounting the MWNT probe for use in a scanning probe instrument
2.6 Templates
Since nanotubes have relatively straight and narrow channels in their cores, it was speculated
from the beginning that it might be possible to fill these cavities with foreign materials to fabricate
one-dimensional nanowires. Early calculations suggested that strong capillary forces exist in
nanotubes, strong enough to hold gases and fluids inside them. The first experimental proof was
demonstrated in 1993, by the filling and solidification of molten lead inside the channels of MWNTs.

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Wires as small as 1.2 nm in diameter were fabricated by this method inside nanotubes. A large body
of work now exists in the literature, to cite a few examples, concerning the filling of nanotubes with
metallic and ceramic materials. Thus, nanotubes have been used as templates to create nanowires of
various compositions and structures. The critical issue in the filling of nanotubes is the wetting
characteristics of nanotubes, which seem to be quite different from that of planar graphite, because of
the curvature of the tubes. Wetting of low melting alloys and solvents occurs quite readily in the
internal high curvature pores of MWNTs and SWNTs. In the latter, since the pore sizes are very
small, filling is more difficult and can be done only for a selected few compounds. It is intriguing
that one could create one-dimensional nanostructures by utilizing the internal one-dimensional
cavities of nanotubes. Liquids such as organic solvents wet nanotubes easily and it has been
proposed that interesting chemical reactions could be performed inside nanotube cavities. Hence,
during oxidation, the caps are removed prior to any damage occurring to the tube body, thus easily
creating open nanotubes. The opening of nanotubes by oxidation can be achieved by heating
nanotubes in air (above 600௢
C) or in oxidizing solutions (e.g., acids). It is noted here that nanotubes
are more stable to oxidation than graphite, as observed in Thermal Gravimetric Analysis (TGA)
experiments, because the edge planes of graphite where reaction can initiate are conspicuous by their
absence in nanotubes. Laser ablation also produces heterostructures containing carbon and metallic
species. Multi-element nanotube structures consisting of multiple phases (e.g., coaxial nanotube
structures containing SiC, SiO, BN and C) have been successfullysynthesized by reactive laser
ablation. Similarly, post-fabrication treatments can also be used to create hetero junctions between
nanotubes and semiconducting carbides. It is hoped that these hybrid nanotube based structures,
which are combinations of metallic, semiconducting and insulating nanostructures, will be useful in
future nanoscale electronic device applications. Nanocomposite structures based on carbon
nanotubes can also be builtby coating nanotubes uniformly with organic or inorganic structures.
These unique composites are expected to have interesting mechanical and electrical properties due to
a combination of dimensional effects and interface properties. Finely-coated nanotubes with mono
layers of layered oxides have been made and characterized (e.g., vanadium pentoxide films). The
interface formed between nanotubes and the layered oxide is atomically flat due to the absence of
covalent bonds across the interface. The carbiderods so produced (e.g., SiC, NbC) should have a
wide range of interesting electrical and mechanical properties, which could be exploited for
applications as reinforcements and nanoscale electrical devices [1], [54], [59-62], [65-70][71 – 74].
2.7 Carbon Nanotubes – Optical Communications Systems
This strong dependence of the electronic structure on geometrics observed in fullerenes
should be generally the case in nanostructured carbon materials including carbon nanotubes since the
interaction between valence electrons and the lattice should be much stronger in stiff C_C covalent
– bond materials bond. The one dimension (1D) electronic energy bond structure for carbon
nanotube is related to the energy band structure calculated for two dimensions (2D) graphite
honeycomb sheet used to form the nanotube. These calculations electronic structure for carbon
nanotube shows about 1/3 of carbon nanotube is metallic and 2/3 is semiconducting, depending on
the nanotube diameter (݀௧) and chiral angle (ߠ) Another classification for carbon nanotubes
depending on chiral vectors (‫ܥ‬௛), they are Zigzag nanotube, armchair nanotube, and chiral
nanotube. All these carbon nanotubes have relationship between electronic density of state and
energy band gap. There are three parameters to develop carbon nanotubes optical proprieties to
work in optical system, Electronic structure of carbon nanotubes, Saturable Absorption of carbon
nanotubes, and Third order nonlinear for carbon nanotubes. The high pressure carbon mono-
oxide (HiPCO) has been one of the fabrication methods for the mass production of carbon
nanotubes. They are often seen as straight or elastic bending structures individually or in ropes,
by transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force

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microscopy (AFM), and scanning tunneling microscopy (STM). It can be great potentials towards
the nano -scale photonic devices which can be utilized for optical filtering or nanofiltering,
waveguide, switching, and wavelength multiplexing but it expresses nanoscale devices. The optical
absorption of CNTs is of saturable, intensity-dependent nature, it is a suitable material to employ for
passively mode-locked laser operation. Passive mode locking is achieved by incorporating an
intensity -dependent component into the optical system. The typical absorption of a suspension of
CNT fabricated by the high pressure carbon monoxide method (HiPCO) and measured by a
spectrometer. This is generally a saturable absorber which absorbs the light which is incoming
linearly up to a given threshold intensity, after which is saturates and becomes transparent optical
power intensity for output with losses 5% from input incident optical power intensity. Such saturable
absorbers discriminate in favor of pulse formation over continuous wave lasing. This is one of the
key advantages of carbon nanotube – based devices as has been achieved passive in mode – locked
operation not only in the C (1530nm – 1565nm) and L (1565nm –1625nm) bands. Optical Code
Division Multiple Access (OCDMA) with Carbon Nanotubes (CNTs) to improve three parameters
very important in any communication system as data rate (R), bit error rate (BER), and signal to
noise ratio (SNR). Carbon nanotubes based optical integrated circuit to support high speed optical in
passive optical network. The OCDMA encoders and decoders are the key components to implement
OCDMA based system. It can be divided into broad categories based on the way in which a
particular user’s code is applied to the optical signal. These classifications include coherent optical
CDMA and incoherent optical CDMA approaches. The increasing demand for bandwidth forces
network infrastructures to be large capacity and reconfigurable. The efficient utilization of
bandwidth is a major design issues for ultra-high speed photonic networks, also it increases data
rate (R), and decreases bit error rate (BER) so as to perform with improved signal to noise
(SNR).Silicon optical devices has band gap 1.12eV, called silicon photonics, has attracted much
attention recently because of its potential applications in the infrared spectral region in optical
system having refractive index. Optical code division multiple access with carbon nanotubes
having band gap 2.9 eV and the refractive index optical photonic, brought in the improved best
performance. Next generation of optical communication system may preferably incorporate carbon
nanotubes based devices so as to achieve much higher data rate up to Tb/s in comparison to present
systems using silicon optical devices giving data rate upto Gb/s. Besides, such systems with
advanced energy source power realize in much longer life Nevertheless, future requirements of
ultrahigh speed internet, video, multimedia, and advanced digital services, would suitably be met
with incorporation of carbon nanotubes based devices providing optimal performance [14],[75],[76],
[77] .The Optical Wireless Communications (OWC) is a type of communications system that uses
the atmosphere as a communications channel. The OWC systems are attractive to provide broadband
services due to their inherent wide bandwidth, easy deployment and no license requirement. The idea
to employ the atmosphere as transmission media arises from the invention of the laser. The visible
light communication (VLC) based on Li-Fi (Light Fidelity)-The future technology in optical wireless
communication refers to the communication technology which utilizes the visible light source as a
signal transmitter, the air as a transmission medium, and the appropriate photodiode as a signal
receiving component. The system develops with carbon nanotubes (CNTs) to improve for space
communications but applied for indoor networks. Indoor optical wireless systems face stiff
competition from future WiFi.
2.8 Carbon Nanotubes – Interactions with Biological Systems
Carbon nanotubes (CNT) are highly versatile materials, with an enormous potential for
biomedical applications. Their properties are dependent upon production process and may be
modified by subsequent chemical treatment. Carbon nanotubes can be used to improve polymers’
composites mechanical properties. Its tailoring allows for the creation of anisotropic

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nanocomposites. Due to their semi - conductive behaviour, its usage may provide electrical
stimulation. The use of CNT as translocators in drug-delivery systems or in image diagnosis has also
been suggested. Hightumour accumulation of single-walled CNT (SWCNT) has been described,
anticipating the possibility of further therapeutic uses. There are several studies on gas, temperature,
pressure, glucose, chemical force and resonator mass sensors based on CNT.In face of recent studies,
special attention has been drawn into promising orthopaedic use of CNT for improving tribological
behaviour and material mechanical properties. However, and considering the conductive properties
of CNT the range of orthopaedic application may broaden up, since it is known that electricalfields
as small as 0,1 mV/cm may enhance osteoplastic proliferation locally. CNT based electrodes could
be considered for integrating implantable orthopaedic devices. CNT have been reported to have
direct and distinct effects on osteoblasts and osteoclasts metabolic functions .CNT have been
discovered in 1991, but seem to have been around for quite a long time, since they were detected in
gas combustion streams like the ones in normal households stoves The fact that CNT are small
enough to be inhaled has raised the question of lung reaction to their presence. The impact on the
skin of handlers and the environmental consequences of mass production are also pertinent
interrogations, as it is the possibility of secondary organ dissemination [19], [78], [79].
2.8.1 Health hazards
2.8.1.1 Respiratory toxicity
Some authors described strong cytotoxic effects on guinea pig alveolar macrophages of
SWCNT and, at a smaller extent, of multi-walled carbon nanotube (MWCNT), when compared to
fullerenes (C60). The same authors also describe impairment of phagocytic activity. Cytotoxicity
comparable to asbestos-particles induced on murine macrophages has been described by Soto.
Experiments conducted by Magrez on three lung-tumor cell lines suggest CNT led to proliferation
inhibition and cell death, although CNT showed less toxicity than carbon black nanoparticles and
carbon nanofibers, assessed SWCNT cytotoxicity on a distinct lung-carcinoma cell line (A549) and
describe SWCNT concentration - dependent toxicity and the protective effect of serum . Another
study, conducted by Sharma, concluded that SWCNT induced oxidative stress in rat lung cells. The
same oxidative stress related changes are described by Herzog et al. in primary bronchial epithelial
cells and A549 cells but the study points out that the length of the response is strongly dependent on
the dispersion medium used. Pulskamp also describes oxidative stress in two cell lines (rat
macrophages NR8383 and human A549) cultured in contact with CNT. However, when comparing
purified SWCNT and commercial CNT their findings suggested the biological effects were
associated with the metal traces. They also describe puzzling divergent results between MTT (3-
(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) and WST (water soluble tetrazolium
salt, 2-(4-Iodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2Htetrazolium, monosodium salt)
viability assays, both dependent on the activity of mitochondrial dehydrogenases. These authors
described dose-dependent persistent inflammation and granuloma formation, more significant with
MWCNT than with carbon black but less extensive than with asbestos. Described unusual acute
inflammatory response, early granulomatous reaction and progressive fibrosis in mice exposed to
SWCNT, leading to the conclusion of CNT intrinsic toxicity. This study used a technique of
pharyngeal aspiration instead of the intratracheal instillation used in the previous studies, and
allowed aerosolization of fine SWNCT particles. These particles were associated with fibrogenic
response in the absence of persistent local inflammation, suggesting health risks for workers.
However, a more recent study describes significant changes in deposition pattern and pulmonary
response when SWCNT are more evenly disperse in the suspension prior to pharyngeal aspiration.
More recently, inhaled MWCNTs migration to the subpleura and associated increased number of
pleural mononuclear cells and subpleural fibrosis was described in mice, further advising caution and
appropriate security measures when handling CNT. Presented a study with dispersed SWCNT

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(DSWCNT) supporting data from previous reports, in the sense that they describe invitroand in
vivostimulation of lung fibroblasts proliferation and collagen deposition, and metalloproteinase 9
increased expressions, in the absence of inflammation. It has also been hypothesized, and
demonstrated for other types of nanoparticles, that following inhalation, nanoparticles may reach the
central nervous system (CNS). Nanoparticles enter the nervous system by transcytosis and are
presented to neuron cells. Studies showing that inhaled gold nanoparticles accumulate in factory bulb
of rats and reach the cerebral cortex, as well as the lung and thereof other organs such as esophagus,
tongue, kidney, aorta, spleen, septum, heart and blood. These observations suggest that if there are
high doses of nanoparticles in the air they can enter into the CNS via the olfactory nerve during
accidental or prolonged environmental or occupational exposure to humans, and that nanoparticles
may exert their effects not only on respiratory tract and neighboring organs but spread to distant
organs[80].
2.8.1.2 Epidermal/dermal toxicity
Several studies have also been conducted on epidermal/dermal toxicity of CNT.
Functionalized 6-aminohexanoic acid-derivatized SWCNT may cause dose-related rise in
inflammatory cytokines. MWCNT induction inflammatory pathways may be similar to those of
combustion-derived metals and cause decreased cell viability, changes on metabolic, cell signaling,
stress and cytoskeletal protein expression. Other authors report presence of chemically unmodified
MWCNT in cytoplasmic vacuoles of cultured human keratinocytes and induction of the release of
interleukin 8 in a time dependent manner and SWCNT inhibition of HEK9293 cells growth through
induction of apoptosis and decreased cell adhesion has also been described. Describe dose and time-
dependent cytotoxicity, genotoxicity and induction of apoptosis by purified MWCNT in normal
human dermal fibroblasts cells. The MWCNT used in this study had been treated for extraction of
metal (Fe) impurities and then, by treatment with sulfuric/nitric acid, functionalized in very high
degree. The authors report that 2 to 7% of final weight was due to carboxyl groups.
2.8.1.3 Biological response and mechanisms of toxicity
Whilst assessing in vitrocytotoxicity of SWCNT on fibroblasts and trying to bring some light
on the issue of how the removal of catalytically metal would influence the toxicity. Concluded that
the refined SWCNT were moretoxic, inducing significant changes on cytoskeleton and cell
morphology, probably because of the enhancement of the hydrophobic character by the refinement
treatment, the toxicity seemingly directly related to surface area. Decreased SWCNT cytotoxicity in
dermal fibroblasts with higher functionalization density. However, other authors compared pristine
and oxidized MWCNT effects on human T lymphocytes and described increased toxicity of oxidized
CNT, with high doses, even if oxidation increased solubility [104]. Time-dependent changes in T
lymphocytes by measuring CD4 and CD8, associated with local granuloma formation after
subcutaneous implantation in mice, although overall toxicological changes were inabsolute lower
then with asbestos. These results might seem somehow inconflict with the findings by Dumortier that
concluded that functionalized SWCNT did not affect B and T lymphocytes viability. However, the
authors emphasized that absence of functional changes was only observed in the CNT functionalized
via the 1,3-dipolar cycloaddion reaction, in non - oxidized nanotubes . Brown et al. conducted in
vitrostudies that suggested monocytic cells’ response is strongly dependent of morphology and state
of aggregation of the CNT. Long, straight well-dispersed nanofilaments induced the production of
more TNF-αand ROS than highly curved and entangled aggregates; incomplete uptake or frustrated
phagocytosis of CNT was also described. Barillet et al. showed that short (0.1-5 nm) and long (0.1-
20 nm) CNT, and the presence of metal residues, induced different cell response and toxicity. The
same mechanisms of frustrated phagocythosis, increased production of proinflammatory cytokines
and oxidative stress apparently justified the in vivo findings described by several authors. They

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conducted studies with longer implantation times and these effects may eventually lead to
carcinogenesis[80-81], [81-84].
2.8.1.4 Mechanisms of interaction of CNT
The questions related to possible interactions between CNT and various dye markers,
pointing out the difficulties in the interpretation of the obtained results are raised by several authors,
pointing out the need for careful interpretation The commonly used MTT assay, used to assess cell
viability and proliferation, has been described to falsely lower results due to attachment of insoluble
formazan to CNT. SWCNT dose-dependent adsorption and depletion of over 14 amino acids and
vitamins from RPMI cell culture medium. This implies that indirect mechanisms of toxicity may
influence the results of in vitrostudies, since some of these molecules are essential for cell viability
and proliferation. SWCNT cause dose-dependent adsorption of culture medium amino acids and
vitamins, showing higher affinity for planar aromatic or conjugated structures, and for positively
charged solutes. Functionalization of SWCNT and MWCNT with terminal or surface specific groups
alters solubility and protein adsorption, including of cytokines IL6 and IL8, in a dose-dependent
manner. In the absence of specific chemical affinity between the nanotube surface and the protein,
one cause of interference would be the seizing of the molecule inside the nanotube, dependent on
molecule size, unless CNT are functionalized with specific groups that promote chemical binding.
CNT’s active surface issues are equally important, as in a composite CNT surface available for
interaction is reduced because nanotubes are embedded in a matrix. There are several possible
mechanisms of interaction. Molecule adsorption is probably strongly dependent on charge and
molecule size, and also on the CNT surface available for interaction. The authors explored protein
adsorption to non-functionalized and functionalized multiwalled CNT (MWCNT) and to ultra-high
molecular weight polyethylene (UHMWPE)/ MWCNT composite and with UHMWPE polymer
alone. Two different proteins were chosen, bovineserum albumin (BSA, Promega) and histone.
Histones are a group of small proteins, with molecular weights varying from around 21 500 Dalton
to 11 200 Dalton; at neutral pH, histones are positively charged. Bovine serum albumin (BSA) has a
molecular weight of around 66 700 Dalton and its isoelectric point, thus being negatively charged at
pH 7, due to the domination of acidic groups over amine groups. Solutions of both proteins
(concentration 200 µg/mL) were prepared through agitation in PBS (without calcium and
magnesium), and the pH adjusted to 7 with HCl 1 N. MWCNT (range of diameter 60-100 nm, length
of the tubes 5–15 µm), non-functionalized and functionalized with carbonyl, carboxyland hydroxyl
groups were added to the solutions (n=6), with a concentration of 100 µg/mL, and mixed by
vortexing. Bulk samples of composite (0.2% MWCNT) and polymer were also incubated in the
solutions, maintaining the same weight/volume rate. After 12 hours at room temperature, solutions
were filtered using 0.2 µm polyethersulfone low protein binding syringe filters (VWR). Initial
albumin and histone solutions were also filtered. Protein content in the filtrates was assessed,
intriplicate, by the bicinchoninic acid assay (BCA Protein Assay, Calbiochem), accordingly to the
manufacturer’s instructions [101]. PBSwas used as blank. The protein content in solutions incubated
with the materials is expressed in percentage of histone and BSA filtered solutions, assumed as
100%. The normal distribution was verified by the Kolmogorov-Smirnov test, homogeneity of
variance by the Levene test and means compared ANOVA (Tukey test). The statistical analysis was
done using software OriginPro 8 [19], [78], [80] [85 – 89].
2.9 Carbon Nanotubes in Biomedicine and Biosensing
CNTs have been used as efficient electrochemical and optical sensors, substrates for directed
cell growth, supporting materials for the adhesion of liposaccharides to mimic the cell membrane
transfection and controlled drug release. Some researches have shown the ability of single-walled
carbon nanotubes (SWNTs) to cross cell membranes and to enhance deliver peptides, proteins, and

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nucleic acids into cells because of their unique structural properties. For this reason, carbon
nanotubes could serve as an excellent vehicle to administer therapeutic agent providing effective
utilization of drug and less elimination by the macrophage [102]. One key advantage of carbon
nanotubes is their ability to translocate through plasma membranes, allowing their use for the
delivery of therapeutically active molecules in a manner that resembles cell-penetrating peptides.
Moreover, utilization of their unique electrical, optical, thermal, and spectroscopic properties in a
biological context is hoped to yield great advances in the detection, monitoring, and therapy of
disease [85], [90].
• Advantage
Unique mechanical properties offer in vivo stability.
Extremely large aspect ratio, offers template for development of multimodal devices.
Capacity to readily cross biological barriers; novel delivery systems.
Unique electrical and semiconducting properties; constitute advanced components for in vivo
devices.
Hollow, fibrous, light structure with different flow dynamics properties; advantageous in vivo
transport kinetics.
Mass production – low cost; attractive for drug development.
• Disadvantage
Nonbiodegradable
Large available surface area for protein opsonization.
As-produced material insoluble in most solvents; need to surface treat preferably by covalent
functionalization chemistries to confer aqueous solubility (i.e. biocompatibility).
Bundling; large structures with less than optimum biological behavior.
Healthy tissue tolerance and accumulation; unknown parameters that require toxicological
profiling of material.
Great variety of CNT types; makes standardization and toxicological evaluation cumbersome.
Advantage and Disadvantage of using CNTs for biomedical applications
2.9.1 Functionalization of CNTs
For biological applications, the improvement of solubility of CNTs in aqueous or organic
solvents is a major task. Great efforts have devoted to search cost-effective approaches to
functionalize CNTs for attachment of biomolecules as recognition elements. Generally, this
procedure can be performed by noncovalent and covalent functionalization strategy.
2.9.2 Noncovalent interaction
The noncovalent approach via electrostatic interaction, Van der Waals force, or π–πstacking
is a feasible immobilization method for biomolecules. Particularly, it is promising for improving the
dispersion proteins of CNTs without destructing of the nanotube structure. Generally, this route can
be performed by physical adsorption or entrapment.
2.9.2.1 Physical adsorption
A variety of proteins can strongly bind to the CNTs exterior surface via physical adsorption.
When the ends of the CNTs are open as a resultof oxidation treatment, smaller proteins can be
inserted into the tubular channel (~5–10 nm in diameter). The combined treatment of strongacids and
cationic polyelectrolytes is known to reduce the CNTs length and enhance the solubility under
physiological. After this treatment, cationic polyelectrolytes molecules adsorb on the surface of the

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nanotubes by van der Waals force to produce the distribution of positive charges, which prevents the
aggregation of CNTs.
2.9.2.2 Entrapment
Another method for immobilizing biomolecules on CNTs is to entrap them in biocompatible
polymer hydrogen and sol–gel. Single strand DNA (ssDNA) can wrap around SWCNTs through
aromatic interaction to form a soluble DNA–SWCNT complex, which has been used for construction
of effective delivery for gene therapy. Sol–gel chemistry has paved a versatile path for the
immobilization of biomolecules with acceptable stability and good activity retention capacity [91].
2.9.3 Covalent interaction
Since the as-produced CNT contain variable amounts of impurities, such as amorphous
carbon and metallic nanoparticles, the initial efforts in their purification focused on the selective
oxidation of the impurities with respect to the less reactive CNT. The combined treatment of strong
acids and sonication is known to purify the CNTs and generate anionic groups (mainly carboxylate)
along the sidewallsand ends of the nanotubes. Also, dangling bonds can react similarly, generating
other functions at the sidewalls.
2.9.4 CNTs for biomedical applications
2.9.4.1 CNTs for protein delivery
Various low molecular weight proteins can adsorb spontaneously on the sidewalls of
acidoxidized single-walled carbon nanotubes. The proteins are found to be readily transported inside
mammalian cells with nanotubes acting as the transporter via the endocytosis pathway. This research
was reported by Dai group. Streptavidin (SA) and cytochrome c (Cyt-c) could easily transport into
the cytoplasm of cells by the CNTs and take effect of their physiological action in the cell. Carbon
nanotubes could become new class of protein transporters for various in vitro and in vivo delivery
applications [92-93], [17].
2.9.4.2 CNTs for gene delivery
One of the most promising concepts to correct genetic defects or exogenously alter the
cellular genetic makeup is gene therapy. Some challenges have existed in gene therapy. Primary
concerns are the stability of molecules, the amount of intracellular uptake, their susceptibility to
enzyme degradation, and the high impermeability of cell membranes to foreign substances. To
overcome this problem, the CNTs are used as vector able to associate with DNA, RNA, or another
type of nucleic acid by self-assembly and assist its intracellular translocation. These systems offer
several advantages, including easy upscaling, flexibility in terms of the size of nucleic acid to be
delivered, and reduced immunogenicity compared with viruses. The Kostas group reported CNT-
mediated gene delivery and expression leading to the production of marker proteins encoded in
double-stranded pDNA . The delivery of pDNA and expression of ǃ-galactosidase (marker gene) in
Chinese hamster ovary (CHO) cells is five to ten times higher than naked pDNA alone. The concept
of gene delivery systems based on CNTs has also been reported by Liu group. They report a
noncovalent association of pDNA with PEI–CNTs by electrostatic interaction. They have tested
CNT–PEI:pDNA complexes at different charge ratios in different cell lines. The levels of expression
of luciferase (marker gene) are much higher for the complexes incorporating CNTs than pDNA
alone and about three times higher than PEI alone [94], [95].
2.9.4.3 CNTs for chemical delivery
Recently, Dai group reported that using supramolecular π–πstacking to load a cancer
chemotherapy agent doxorubicin (DOX) onto branched polyethylene glycol (PEG) functionalized

123
SWNTs for in vivo drug delivery applications. It has been found that the surface of PEGylated
SWNTs could be efficiently loaded with DOX by supramolecularπ–πstacking. These methods offer
several advantages for cancer therapy, including enhanced therapeutic efficacy and a marked
reduction in toxicity compared with free DOX [89].
2.9.4.4 CNTs for cancer therapy
More interestingly, CNTs can be used as platforms for multiple derivatizations by loading
their surface with therapeutic agents (treatment), fluorescent, magnetic or radionuclide probes
(tracking), and active recognition moieties (targeting). A strategy for using SWNTs as intracellular
vectors for delivery of ASODNs modified with gold nanoparticles. This strategy allows intracellular
delivery and localization to enhance the therapeutic efficiency of the ASODNs by the conjugations
of SWNTs and GNPs compared with the naked ASODNs in this experiment. Recently, Jia et al, have
explored a novel double functionalization of a carbon nanotube delivery system containing antisense
oligodeoxynucleotides (ASODNs) as a therapeutic gene and CdTe quantum dots as fluorescent
labeling probes via electrostatically layer-bylayer assembling . With this novel functionalization, it
has demonstrated efficient intracellular transporting, strong cell nucleus localization and high
delivery efficiency of ASODNs by the PEI –MWNTs carriers. Furthermore, the ASODNs bound to
PEI-MWNTs show their effective anticancer activity. Another strategy to achieve this is used CNTs
covalently bound to Pt (IV) to deliver a lethal dose of an anticancer drug and to a noncovalently
bound (via a lipid coating of the CNTs) fluorescein to track the system.
2.9.4.5 CNTs for HIV/AIDS therapy
Recently, the delivery of siRNA molecules conjugated to CNT to human T cells and primary
cells. That nanotubes are capable of siRNA delivery to afford efficient RNAi of CXCR4 and CD4
receptors on human T cells and peripheral blood mononuclear cells (PBMCs).The siRNA sequences
used in these studies are able tosilence the expression of the cell-surface receptors CD4 and
coreceptors CXCR4 necessary for HIV entry and infection of T cells.
2.9.5 Nanotubes in biosensing
Carbon nanotubes (CNTs) have recently emerged as novel electronic and optical biosensing
materials for the detection of biomolecules such as DNA, antigen–antibody, cells, and other
biomolecules. Among widespread nanoscale building blocks, such as organic or inorganic nanowires
and nanodots, CNTs are considered as one of the most versatile because of their superior mechanical
and electrical properties and geometrical perfection. DNA analysis plays an ever-increasing role in a
number of areas related to human health including diagnosis of infectious diseases, genetic
mutations, drug discovery, food security, and warning against biowarfare agents. etc. And thus make
electrical DNA hybridization biosensors has attracted considerable research efforts due to their high
sensitivity, inherent simplicity and miniaturization, and low cost and power requirements.
2.9.5.1 Optical DNA sensors
Alternatively, an effective sensing platform has been presented via the noncovalent assembly
of SWCNTs and dye-labeled ssDNA. The signaling scheme. When the SWCNTs are added to the
dye labeled ssDNA solution, the ssDNA/SWCNT hybrid structure can be formed, in which the dye
molecule is in close proximity to the nanotube, thus quenching the fluorescence of dye Molecule.
The dye-labeled ssDNA can restore the fluorescence signal to an initial state in the presence of the
target. It illustrates no significant variation in the fluorescence intensity of fluoresce in derivative
(FAM)-labeled oligonucleotides (P1) in the absence of CNTs. In the presence of SWCNT, a
dramatic increase of the fluorescence intensity at 528 nm can be observed in the DNA concentration
range of5.0–600 nM, suggesting that the SWCNT/DNA assembly approach is effective in biosensing

124
target DNA. Furthermore, a visual sensor has been designed to detect DNA hybridization by
measuring the light scattering signal with DNA modified MWCNT as recognition element . This
sensor can be reused for at least 17 times and is stable for more than 6 months [96].
2.9.5.2 Antigen–antibody immunoreactions
There are two different types of detection patterns for CNT-based immunosensors: label free
immunosensors and immunosensors that employ labels and mediators. The label-free immunosensor
shows a convenient fabricating and detection procedure. Several label-free peptide-coated CNTs
based immunosensors has been proposed for the direct assay of human serum sample using square
wave stripping voltammetry, quartz crystal microbalance measurements, and differential pulse
voltammetry (DPV) ). Based on CNT-FET, a label free protein biosensor was also prepared for
monitoring of a prostate cancer marker. As one of the most popular tracer labels, enzymes, including
ALP, HRP, and GOD have been immobilized on CNTs for enhancing the enzymatic signal.
Typically, a novel immunosensor array was constructed by coating layer-by-layer colloidal Prussian
blue (PB), gold nanoparticles (AuNPs) and capturing antibodies on screen-printed carbon electrodes
and coupling with a new tracer nanoparticle probe labeled antibody (Ab2) that was prepared by one-
pot assembly of GOD and the antibodies on AuNPs attached CNTs. Applications and detection of
low-abundant proteins. In addition, a sensitive method for the detection of cholera toxin (CT) using
an electrochemical immunosensor with liposomic magnification. The sensing interface consists of
monoclonal antibody against the B subunit of CT that is linked to poly (3, 4-ethylenedioxythiophene)
coated on Nafion-supported MWCNT caste film on a glassy carbon electrode.
2.9.5.3 Sensing of cells
To achieve biocompatible interactions between CNTs and living cells, a strategy to
functionalize CNTs with biomolecules such as peptide. A novel electrochemical cytosensing strategy
was designed based on the specific recognition of integrin receptors on cell surface to arginine–
glycine–aspartic acid–serine (RGDS)-functionalized SWCNT. The conjugated RGDS showed a
predominant ability to capture cells on the electrode surface by the specific combination of RGD
domains with integrin receptors.
2.9.5.4 Detection of other biomolecules
2.9.5.4.1 NADH
The electrochemical oxidation of NADH at the electrode surface has received considerable
interest due to the need to develop amperometric biosensors for substrates of NAD+ dependent
dehydrogenases. Dihydronicotinamide adenine dinucleotide (NADH) and its oxidized form,
nicotinamide adenine dinucleotide (NAD+), are the key central charge carriers in living cells.
However, the oxidation of NADH at a conventional solid electrode surface is highly irreversible with
considerable overpotentials, which limits the selectivity of the determination in a real sample. CNTs
have been devoted to decreasing the high overpotential for NADH oxidation on carbon paste
electrodes and microelectrodes. By integrating the hydrophilic ion-conducting matrix of CHITn with
electron mediator toluidine blue O and CNTs, the produced NADH sensor shows very low oxidation
overpotential and good analytical performance.
2.9.5.4.2 Glucose
The detection of glucose in blood is one of the most frequent performances for human
healthy, since some diseases are related to the blood glucose concentration. However, the direct
electron transfer for oxidation of FADH2 or reduction of FAD is hard to realize at conventional
electrodes, because the FAD is deeply seated in a cavity and not easily accessible for conduction of
electrons from the electrode surface.

125
2.9.5.4.3 Organophosphate pesticides
The rapid detection of these toxic agents in the environment and public places has become
increasingly important for homeland security and health protection. The flow injection amperometric
biosensor for OPs has been developed by assembling AChE on CNTs modified GCE. Under optimal
conditions, the biosensor has been used to measure paraoxon as low as 0.4 pM with a 6-min
inhibition time.
2.9.5.4.4 H2O2
H2O2is a product of the enzymatic reactions between most oxidases and their substrates.
This detection is very interesting for the development of biosensors for oxidase substrates. The
earlier work on the electrocatalytic action of CNTs toward H2O2was reported at an apparently
decreased overvoltage using the CNTs/Nafion-coated electrode. With the introduction of MWCNT,
the polyaniline-PB/MWCNT hybrid system showed the synergy between the PANI-PB and
MWCNT, which amplified the sensitivity greatly.
2.9.5.5 Near-IR fluorescent based CNTs biosensor
Generally, the change modes of SWCNT NIR can be modulated to uniquely fingerprint
agents by either the emission band intensity or wavelength. CNTs have been found to be useful
optical materials with high photostability and efficiency for sensing applications because of their
NIR fluorescence properties from 900 to 1600 nm. Other than optical detection, SWCNTs as sensing
elements have a particular advantage due to the fact that all atoms are surface atoms causing the
nanotube to be especially sensitive to surface adsorption events.
2.9.5.5.1 Sensing with change of emission intensity
Quenching of SWCNT fluorescence by means of oxidative charge transfer reactions with
small redox-active organic dye molecules has been demonstrated by suspending in SDS solution and
biotin–avidin test system. The NIR optical properties of SWCNT have attracted particular attention
for nano biosensors based on the redox chemistry. At the most sensitive band of 1270 nm, the
detection limit for H2O2is found to be 8.8, 0.86, and 0.28 ppm by three different methods based on
the concentration dependent rate constant, pectral intensity change, and signal-to-noise ratio.
Another NIR optical protein assay based on aptamers wrapped on the sidewall of SWCNT was
designed.
2.9.5.5.2 Sensing with shift of emission wavelength
The shift of emission wavelength has also beena useful way to make sensing in addition to
emission intensity-based sensing, When actions adsorb onto the negatively charged backbone of
DNA, DNA oligonucleotides transform from the native, right handed B form to the left-handed Z
form, which modulates the dielectric environment of SWCNT and decreases their NIR emission
energy up to 15 meV. The change of the emission wavelength results in an effective ion sensor,
especially for mercuric ions. These NIR ion sensors can operate in strongly scattering or absorbing
mediator to detect mercuric ions in whole blood, black ink, and living mammalian cells and tissues.
2.9.5.5.3 Single-molecule detection
Nanoscale sensing elements offer promise for single molecule detection through NIR
fluorescence in physically or biologically constrained environments. A single-molecule detection of
H2O2has been demonstrated by stepwise NIR photoluminescence quenching of surface-tethered
DNA–SWCNT complexes. The time trace of SWCNT quenching was obtained by measuring the
intensity of four-pixel spots in movies recorded at one frame per second (Figure 5(b)), resulting in
multiple traces that exhibited single-step attenuation upon perfusion of H2O2. These measurements

126
demonstrated single molecule detection of H2O2 and provided promise for new classes of biosensors
with the single-molecular level of sensitivity.
2.9.5.5.4 SWCNT-based field-effect biosensor
Currently, four possible mechanisms have been proposed to account for the observed changes
in the SWCNT conductance: electrostatic gating, Schottky barrier effect, change in gate coupling,
and carrier mobility change, among which the electrostatic gating and Schottky barrier effect are
dominant in the SWCNT based FET biosensing device. The label-free CNTs-based filed-effect
sensor offers a new approach for a new generation of DNA biosensing. For example, a synthetic
polymer is well adsorbed to the walls of CNTs and carries activated succinimidyl ester groups to fix
the NH2-ssDNA probes for constructing a large array of CNTs-FETs. Furthermore, a simple and
generic protocol for label-free detection of DNA hybridization is demonstrated by random
sequencing of 15 and 30 mer oligonucleotides. DNA hybridization on gold lectrodes, instead of on
SWCNT sidewalls, is mainly responsible for the acute electrical conductance change due to the
modulation of energy level alignment between SWCNT and gold contact. Aptamer is
artificialoligonucleotides (DNA or RNA) that can bind to a wide variety of entities with high
selectivity, specificity and affinity, equal to or often superior to those of antibodies. The
firstSWCNT-FET-based biosensor comprising aptamer was proposed by Lee’s group. Briefly,
aptamer immobilization was performed by modifying the side wall of the CNTs with
carbodiimidazole-activated Tween 20 through hydrophobic interaction, and covalently attaching the
3-amine group of the thrombinaptamer [103]. The conductance dropped sharply upon addition of
1.5µmol thrombin. The sensitivity became saturated around protein concentration of 300 nM, where
the linear response regime of the sensor was expected to occur within the 0–100 nM range. The
addition of elastase did not affect the conductance of the thrombin aptamer functionalized SWCNT-
FET. Again, adding thrombin to the thrombin aptamer functionalized SWCNT-FET surface caused a
sharp decrease in conductance, thereby demonstrating the selectivity of the immobilized thrombin
aptamers. The aptamer modified SWCNT-FETs are another promising sensor for the development of
label-free protein detection.
2.9.5.5.5 Electrochemical sensors
Electrochemical DNA sensors can convert the hybridization event into an electrochemical
signal. DNA sensing approaches include the intrinsic electroactivity of DNA, electrochemistry of
DNA-specific redox indicators, electrochemistry of enzymes, and conducting polymers. The direct
electrochemical oxidation of guanine or adenine residues of ssDNA leads to an indicator-free DNA
biosensor. For example, Wang’s group used CNTs for dramatically amplifying alkaline phosphatase
(ALP) enzyme-based bioaffinity electrical sensing of DNA with a remarkably low detection limit of
around 1 fg mL−1(54 aM). Professor Kotov and collaborator (Professor Xu) had demonstrated that
CNT/cotton threads can be used to detect albumin, the key protein of blood, with high sensitivity and
selectivity. In this method, cotton yarn has been coated with CNTs and polyelectrolytes. This method
provides a fast, simple, robust, lowcost, and readily scalable process for making e-textiles,
reminiscent of layer-by-layer assembly processes used before. The resulting CNT/cotton yarns
showed high electrical conductivities as well as some functionality due to biological modification of
inter-nanotube tunneling junctions. When the CNT/cotton yarn incorporated anti-albumin, it became
an etextile biosensor that quantitatively and selectively detected albumin, the essential protein in
blood. The low electrical resistance of CNT/cotton yarn allows for convenient sensing applications
which may not require any additional electronics or converters. It also reduces the power necessary
for sensing. PSS is more hydrophilic than NafionTM, and, thus, CNT-NafionTMis more
advantageous for dry-state sensing while CNT-PSS will be more advantageous in humid conditions.
For intelligent fabric demonstrations, the CNT-Nafion TMyarn was tested as a humidity sensor in a

127
dry state while CNT-PSS yarn served as a wet-state bio-sensor platform. As the humidity was raised,
the resistance increased. This effect is clearly absent whenno antibody was incorporated between the
nanotubes. This finding also correlates well with the general sensing scheme outlined above. The
suggested signal transduction mechanism implies one-time sensing upon complete removal of the
antibodies, or cumulative sensing of the protein until it has been completely removed. From a
fundamental standpoint, it would be interesting to engineer a coating with reversible sensing
functionality. From a practical standpoint, however, which must consider (1) the limited life-time of
antibodies and (2) the actual circumstances that can result in the appearance of blood, the multiple
use of this sensor is unlikely. SEM images of theSWNT-coated paper indeed indicate the typical
paper morphology, presence of the finely integrated nanotubes, and excellent physical integrity of
the material. As expected the conductivity of the produced material increases with increasing the
SWNT contents and the number of layers of SWNT/PSS dispersion deposited. The gradual increase
of conductivity is quite important because in perspective the conductivity of the paper electrode
needs to be within a specific range of values depending on the parameters of electrical circuit being
used in order to get the best noise-to-signal ratio and the detection linearity for sensing in aqueous
environments. For sensing, we employed the standard three-electrode electrochemical station to
measure changes in electrical properties of the SWNT-paper strips, which were used as work
electrodes. Pt wire and the saturated Hg2Cl2were used as a counter and referenced electrodes,
respectively. The standard electrochemical set-up gives more accurate results than a simple clamping
of the SWNT-papermaterial between two electrodes due to interfacial potential drops at electrode-
SWNT interfaces of different nature including the Schottky barrier. This corresponds to the
reduction of the conductivity of SWNT-paper composite, which is quite different than the
observations made for SWNT and anti-albumin Ab on cotton, where the resistivity decreased when
antigen was present in solution. It was explained by the removal of Ab from the SWNT layers,
resulting in shrinking of nanotubes-nanotube gaps and improvement of charge transport [97-101].
CONCLUSION
This review has described several possible applications of carbon nanotubes with emphasis
on materials science-based applications. Hints are made tothe electronic applications of nanotubes
which are discussed elsewhere. The overwhelming message we would like to convey that the unique
structure, topology and dimensions of carbon nanotubes have created a superb all-carbon material,
which can be considered as the most perfect fiber that has ever been fabricated. The remarkable
physical properties of nanotubes create a host of application possibilities, some derived as an
extension of traditional carbon fiber applications, but many are new possibilities, based on the novel
electronic and mechanical behavior of nanotubes. Nanotubes truly bridge the gap between the
molecular realm and the macro-world, and are destined to be a star in future technology. The
promising properties of carbon nanotubes have sparked a huge world - wide activity to investigate
these objects in many technical areas – not only in microelectronic applications. This is one of the
key advantages of carbon nanotube – based devices in passive mode locking operation. It continues
to work even in high temperature environments without affecting in its performance in the system.
The Absorption in carbon nanotube is very low 0.02 dB/km that means very accurate in transfer
the optical signal, their devices offer a very high nonlinear coefficient and fast response time to
reach 100s of femto second, ultrafast work optical switches used in many in communication system
applications, information technology, and sensors system. It has two methods to support work in
nonlinear optical switching by polarization rotation and four wave mixing (FWM) to develop
Wavelength Division Multiplexing (WDM) and Optical Code Division Multiple Access (OCDMA)
to increase high data rates to support increasing transferring a lot of data information in
communication system application carbon nanotubes are observed to be highly efficient providing

Overview of carbon nanotubes cnts novelof applications as microelectronics optical communications biological biomedicine and biosensing

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