Overview of Carbon Nanotubes Cnts Novelof Applications as Microelectronics Optical Communications Biological Biomedicine and Biosensing

8/21/2019 Overview of Carbon Nanotubes Cnts Novelof Applications as Microelectronics Optical Communications Biological Bi

1/30

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, 3Dept. of Electronics and Communication Engineering, SHIATS - DU, Allahabad, India

2Dept. 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 insynthesis 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, antigenantibody, 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 therange of orthopaedic application may broaden up, since it is known that electrical fields as small as

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: www.iaeme.com/IJARET.asp

Journal Impact Factor (2014): 7.8273 (Calculated by GISI)

www.jifactor.com

IJARET

I A E M E


2/30



105

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 carbon units based on simplegeometrical 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 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]


3/30



106

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 havebeen 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 (001000C) compared to the arc or laser processes (2000C). Since theirdiscovery 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, thepotential 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 polymerelectronic 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 and and two valence bands and, theseare 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) m 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 physicalproperties [15]. Photovoltaic device is a device that converts the energy of light directly into


4/30



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 IIIV 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-efficientsolar 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 ofdisease and detection biomolecules such as DNA, antigenantibody, 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 areno dangling bonds in CNTs, the use of high - kmaterial as gate dielectrics is simple. In fact, the

application of, andgate material has produced superior CNT transistors with low

sub - threshold slopes and low hysteresis. Both n - type and p - type conduction is possible, enabledby 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 conventionalsilicon devices are made by highly doped silicon regions, which in turn are contacted by metals to


5/30



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 10, a

contact resistance of the order of 10should be achievable. Therefore, a low - resistancecontact 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, whichtherefore 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


6/30



109

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 thedevice 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 growsexponentially and is limited by the overall serial resistance of 1 M . If one applies the ordinarydiode 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 aquantum 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 FowlerNordheim

equation: ( /)whereI, V, , , 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 IV characteristics

of different types of carbon nanotubes have been reported, including individual nanotubes, MWNTsembedded in epoxy matrices, MWNT films, SWNTs and aligned MWNT films. Typical emission I


7/30



110

V characteristics measured from a random SWNT film at different anode-cathode distances and the

FowlerNordheim 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 Nanotubes2.2.1.1 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 .4 10/for green light at an anode current of 200Awhich is two timesmore 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 30m, 230

V is required to obtain the emission current density necessary to drive the diode display

(76mA/). The device is operated using the half-voltage off-pixel scheme. Pulses of150 Vare 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.


8/30



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 ofTechnologies)

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 protectingthe 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 420 better and the breakdownvoltage is30% lower than the two commercial products measured. The enhanced performanceshows 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 ultimatelydetermines the efficiency of fuel cells and this depends on various factors, such as the structure and


9/30



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 electrodesin 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 LithiumThe 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 400650 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 theintercalated 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 BETcharacterization) 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].


10/30



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 desirablefor 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 (0.10.2 wt% SWNT) estimates a gravimetric storage density of 510 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 amuch higher packing density of H2 inside the tubes than expected from the normal H2H2 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 recentexperiments on individual nanotubes (mostly MWNTs) have confirmed that nanotubes are one of the

stiffest structures ever made. Since carboncarbon 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 Youngs 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 invapor grown carbon fibers. Although testing of individual nanotubes is challenging, and requires


11/30



114

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 nearlythe 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 preventslippage. 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 higherresolution 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

Alzheimers 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 (highaspect) design of nanotubes, nanotube vibration still will remain an issue, unless short segments of


12/30



115

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 wasdemonstrated in 1993, by the filling and solidification of molten lead inside the channels of MWNTs.


13/30



116

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 ofthe 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 00C) or in oxidizing solutions (e.g., acids). It is noted here that nanotubesare 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 theirabsence 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 carbonnanotube 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 () and chiral angle () Another classification for carbon nanotubes

depending on chiral vectors (), they are Zigzag nanotube, armchair nanotube, and chiralnanotube. 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


14/30



117

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 anintensity -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 inpassive 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 users 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 visiblelight 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 polymerscomposites mechanical properties. Its tailoring allows for the creation of anisotropic


15/30



118

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 authorsdescribed 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 andappropriate security measures when handling CNT. Presented a study with dispersed SWCNT


16/30



119

(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 arepresented 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 ininflammatory 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 aftersubcutaneous 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-and 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 cytokinesand oxidative stress apparently justified the in vivo findings described by several authors. They


17/30



120

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.

CNTs 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 g/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 515 m), non-functionalized and functionalized with carbonyl, carboxyland hydroxyl

groups were added to the solutions (n=6), with a concentration of 100 g/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 m 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

manufacturers 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-walledcarbon nanotubes (SWNTs) to cross cell membranes and to enhance deliver peptides, proteins, and


18/30



121

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 toxicologicalprofiling 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 stacking

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 (~510 nm in diameter). The combined treatment of strongacids and

cationic polyelectrolytes is known to reduce the CNTs length and enhance the solubility underphysiological. After this treatment, cationic polyelectrolytes molecules adsorb on the surface of the


19/30



122

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 biocompatiblepolymer hydrogen and solgel. Single strand DNA (ssDNA) can wrap around SWCNTs through

aromatic interaction to form a soluble DNASWCNT complex, which has been used for construction

of effective delivery for gene therapy. Solgel 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, generatingother 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 -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 PEICNTs by electrostatic interaction. They have tested

CNTPEI: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 stacking to load a cancerchemotherapy agent doxorubicin (DOX) onto branched polyethylene glycol (PEG) functionalized


20/30



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 supramolecularstacking. 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 highdelivery 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, antigenantibody, 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 highsensitivity, 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 concentrationrange of5.0600 nM, suggesting that the SWCNT/DNA assembly approach is effective in biosensing


21/30



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 Antigenantibody immunoreactionsThere 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 fabricatin

Overview of Carbon Nanotubes Cnts Novelof Applications as Microelectronics Optical Communications Biological Biomedicine and Biosensing

Documents