2010-5-14 Group Review_Carbon Nanotubes_Final

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MSE/MSNT 505 GROUP REVIEW PAPER

Carbon NanotubesThe material of the future

Bam Aryal, Gadhadar Reddy, James McNamara, Shathabish Gowda

5/14/2010

Carbon nanotubes (CNTs) are one of the most popular and promising nano-materials everdiscovered. They have potential applications in virtually every area of technology. The

versatility of CNTs arises from their phenomenal properties that include strength, high surface

area, high aspect ratio, mechanical and chemical stability, high temperature and electric

conductivity, optical properties and super-capacitance to name a few. Recent studies also reportthat some of the most common (eyeliners) as well as revered (Damascus Sabre) commodities in

the past contained CNTs. Ever since its discovery in 1991, CNTs have been mentioned as the

future of materials by almost every field of science and technology. This article reviews theexpanse of these materials including their synthesis and some of the most talked-about

applications (both fictional and real).


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TABLE OF CONTENTS

TABLE OF CONTENTS .................................................................................................. ii

LIST of FIGURES ......................................................................................................... iii

LIST of TABLES ........................................................................................................... iv

INTRODUCTION ........................................................................................................... 1

HISTORY ..................................................................................................................... 1

STRUCTURE ................................................................................................................ 2

SYNTHESIS .................................................................................................................. 4

APPLICATIONS ............................................................................................................. 6

ELECTRONICS ...................................................................................................... 6

ENERGY ............................................................................................................... 7

ENVIRONMENT ..................................................................................................... 8

SPORTS EQUIPMENTS ........................................................................................... 8

MILITARY .............................................................................................................. 9

CONSTRUCTION/ARCHITECTURE ......................................................................... 9

ORGANIC / BIOMEDICAL APPLICATIONS .............................................................. 10

CONCLUSION ............................................................................................................ 12

REFERENCES ............................................................................................................. 14

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LIST of FIGURES

Figure 1: Multi walled CNTs (left) and single walled CNTs (right)[3]...........................1

Figure 2: Arm chair arrangement (source:

http://en.wikipedia.org/wiki/Carbon_nanotube)..........................................................3

Figure 3: Zigzag arrangement (left) and chiral arrangement (right) (source:

http://en.wikipedia.org/wiki/carbon_nanotube)...........................................................3

Figure 4: Laser vaporizing apparatus [9]....................................................................5

Figure 5: Flame synthesized carbon naotubes [12]....................................................6

Figure 6: Illustration of a CNT network directed toward thesurface, making pathways

for heat removal [15].................................................................................................7

Figure 7: Some sports equipment made with CNT technology. Source:

http://www.composites.ugent.be/home_made_composites/composites_in_daily_life_.

html............................................................................................................................9

Figure 8: 1,3 Dipolar addition to carbon naotube reaction.......................................11

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LIST of TABLES

Table 1: Young's Modulus, Tensile Strength, and Density of CNTs compared with

other materials [3]......................................................................................................2

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INTRODUCTION

Carbon nanotubes (CNTs) are nano-scale cylindrical tubes of graphitic carbon

constituting an important part of nanotechnology. The nano-tubes can be single-walled (SWNT)or multi-walled (MWNT) [1]. CNTs are some of the strongest nano-fibers known and have

remarkable electronic properties in addition to many other unique characteristics [2]. CNTs are

simply a sheet of carbon tubes rolled into a cylinder. The diameter of a CNT is about a fewatomic diameters wide. One centimeter long CNT is ten millions times long than it is wide.

They are stronger than steel and their properties suggest that they can be one of the best

semiconductors. CNTs are cylindrical carbon molecules with novel properties such as

outstanding mechanical, electrical, thermal, and chemical properties: 100 times stronger thansteel, best field emission emitters, thermal conductivity comparable to that of diamond which

make them potentially useful in a wide variety of applications such as optics, nanoelectronics,

composite materials, conductive polymers and sensors [3]. CNTs are compared with other

materials such as steel and wood in Table 1.

HISTORY

L. V. Radushkevich and V.M. Lukyanovich published a paper in Soviet Journal of

Physical Chemistry regarding carbon nanotubes in 1952. It was not until 1991, that research inCNTs took full swing. The discovery of buckminsterfullerene, C60, and other fullerenes in 1985

proved that carbon could also form other stable, ordered structures other than graphite and

diamond which stimulated researchers worldwide to search for other new forms of carbons

ultimately leading to the current state of CNTs [2]. Japanese scientist Sumio Iijima discoveredCNT in 1991. His CNTs contained at least two layers and the outer diameter ranged from about

3-20 nm [2]. In 1993, much narrower single walled nanotubes that tended to flex rather than

remain straight were discovered by Iijima and their diameters ranged from 1-2 nm [2].

Figure 1: Multi walled CNTs (left) and single walled CNTs (right)[3].


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CNTs are also called buckytubes. They are member of the fullerene structural family.

They are made by sp bonds, similar to those observed in graphite and they naturally align

themselves into ropes held together by van der Waals forces [3].

Table 1: Young's Modulus, Tensile Strength, and Density of CNTs compared with other materials [3].

Material Youngs Modulus (GPa) Tensile Strength (GPa) Density (g/cm3)

SWNT 1054.0 150.0 -

MWNT 1200.0 150.0 2.600

Steel 208.00 0.400 7.800

Epoxy 3.500 0.005 1.250

Wood 16.00 0.008 0.600

STRUCTURE

Fullerenes exist in different structures such as sphere, cones, and tubes. There are also

some very complicated shapes. CNTs are bonded by sp bond where each atom is joined to three

others just like in graphite. They have very high length to diameter ratio; so, they can be calledone dimensional. A layer of graphite layer is called grapheme. CNTs can be considered as rolled-

up graphene sheets. There are 3 explicit ways of rolling a grapheme sheet into a tube [2]. SWNT

consists of two distinct regions with distinct physical and chemical properties. The first region isthe sidewall of the tube and the second region is the mouth of the tube which is similar to

fullerene such as C60 [4]. Figures below show three different arrangement of single walled

CNTs.Eulers theorem can be used to derive that a closed cage structure consisting of only

pentagons and hexagons need twelve pentagons for completion. The desired curvature of the

surface to enclose a volume is obtained by the collective arrangement of a pentagon surrounded

by five hexagons [4]. An additional rule, called the isolated pentagon rule states that thedistance between pentagons on the fullerene shell is maximized in order to obtain a minimal

local curvature and surface stress, resulting in a more stable structure [4]. C60 is the smallest

structure that can be made in conformity with both these rules.

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Figure 2: Arm chair arrangement (source: http://en.wikipedia.org/wiki/Carbon_nanotube).

Figure 3: Zigzag arrangement (left) and chiral arrangement (right) (source:

http://en.wikipedia.org/wiki/carbon_nanotube).

The three different types of arrangements of the CNTs are arm-chair, zigzag, and chiral

as shown in Figure 2 and 3. The arrangement of CNTs can be represented by a pair of indices (n,m) which are called the chiral vector. How n and m are related can be used to define the

three different arrangements of CNTs. For arm-chair arrangement n equals m and chiral angle

equals to 30 and for zigzag arrangement either n or m equal to zero and chiral angle equals to0. When the values of n and m are such that they do not fall under either arm-chair or zigzag

arrangement, such arrangement is called chiral arrangement; chiral angle for this arrangement is

between 0-30 [2].MWNT can be envisaged as a concentric assembly of SWNTs with sequentially larger

diameters. The tube length and diameter of MWNTs are different than those of SWNTs and so

are their properties [4].

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SYNTHESIS

Carbon Nanotubes have been produced and used by humans since time immemorial; it

was just that we were not aware of their presence. The world famous Damascus Blades of the

medieval ages got their extraordinary strength and sharpness because of the Carbon Nanotubesthat were accidentally introduced into the steel that was manufactured in India [5, 6]. Carbon

Nanotubes are a natural byproduct of combustion, soot contains about 0.1% of Carbon

nanotubes. The main objective of the chemical synthesis methods is to increase this

concentration of nanotubes to as large a value as possible.Carbon Nanotubes are manufactured artificially by vaporizing carbon atoms and then

letting them condense on a catalyst. This basic principle is used in all the current techniques of

nanotube synthesis. Carbon Nanotubes were first manufactured artificially by Sumo Ijima usingan Arc Discharge device. The arc discharge was carried out under a high pressure of 100torr and

the nanotubes were deposited at the negative end of the graphite electrode. The discharge was

carried out in an environment filled with the noble gas argon. The arc discharge provided enoughheat to vaporize the graphite anode and the carbon atoms condensed to form nanotubes. The

structural characterization of the newly formed carbon material was carried out using a

transmission electron microscope, the images for the first time showed a structure that looked

like it had been formed by rolling a grapheme sheet over itself. Ijima also proposed that thenanotubes grew in a helical manner [7].

Shortly after Ijimas announcement Ebbesen and Ajayan empirically optimized the arc

discharge process to produce large quantities of carbon nanotubes. The nanotube yield was foundto depend linearly on the gas pressure inside the crucible and a pressure of 500 torr always gave

the best yield. The process requires a DC voltage of 18v at 100 Amperes [8]. The arc discharge

technique allows us to produce large quantities of nanotubes. But the nanotubes tend to be of theMultiwalled variety and cannot be produced to be of very long lengths. This is because the arc

discharge is not under the control of the user. This led to the development of a second technique

for the production of nanotubes using Laser.Among all the different varieties of Carbon Nanotubes; the single walled nanotubes

exhibited the best material properties and are the only nanotubes capable of being

semiconductors. The production of single walled nanotubes in large quantities had been sought

after because of this. In 1995, Smalley et.al showed that carbon nanotubes could be grown inlarge quantities using Laser ablation. Laser ablation is similar to an arc discharge except for the

fact that the heat to vaporize the carbon atoms is produced by a laser beam. A powerful laser is

shined on a specially prepared Graphite-catalyst substrate. The Substrate is formed by finely

grounding graphite and a transition metal catalyst and then heating the mixture in an oven to800C for 3-4 hours. This produced a uniform dispersion of carbon and catalyst. The laser beam

vaporizes the graphite along with the catalyst particles. The apparatus used for laser ablation issimilar to the equipment used in the production fullerenes. Since Smalley had discovered

fullerenes earlier using the same equipment; it was natural for him to use a similar apparatus to

study and create carbon nanotubes. In this process a 300mJ, 0.532m pulsed LASER beam is

focused on a small 6-7mm diameter spot as shown in the figure below.

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Figure 4: Laser vaporizing apparatus [9].

The yield of Single walled nanotubes in this process is around 70% but the overall yield

of carbon nanotubes is less than the 25% obtained via arc discharge. The carbon nanotubes are

thought to grow by a method of solvation of carbon vapor on the metal particle followed by precipitation of nanotubes. If the metal catalysts are removed; the process would lead to

production of C60 fullerenes by the condensation of carbon particles on themselves. The metal

particles inhibit the fullerenes from forming by taking over the dangling bonds and hence preventing the closure of the fullerene shell. Smalley hypothesized that the nanotubes were

formed by a process of lengthening of the fullerenes. The catalytic particles are 1-2nm diameter

in size and are formed by quenching of 300-400 metal atoms [9].

Laser Ablation solves the problem of producing large quantities of single wallednanotubes but it suffers from the same deficiency as Arc Discharge in that the nanotubes

produced are not very long. This problem is solved by growing nanotubes using chemical vapor

deposition.

Chemical Vapor Deposition is carried out by passing vapors of a hydrocarbon gas on asubstrate embedded with catalyst particles. The substrate is usually made of silica as it is easy to

create aligned pores on it. The catalyst is prepared by a sol gel process from tetraethoxysilanehydrolysis in an iron nitrate aqueous solution. A gelatin of the mixture is obtained and calcined

for 10 hrs at 450oc at low pressure. This leads to the formation of a silica network with relatively

uniform pores. A mixture of 9% Acetylene in Nitrogen is then introduced into the chamber at a

flow rate of 110cm3/min. Aligned carbon nanotubes are formed by the decomposition ofacetylene on the catalyst substrate. This technique produces perfectly aligned nanotube arrays on

the surface [10]. Since the nanotubes are stuck on a substrate there is no need for expensive

purification steps. There are several variations of the Chemical Vapor deposition and the mostrecent version is called the Odako technique; named after the Japanese word for Kite. In this

technique iron nanoparticles function as catalysts on a silica substrate and are free to move. Asthe carbon vapor is flown on the catalyst they rise above the surface like kites and the nanotubesappear to be like the threads of a kite. The yield of this process is 4 times that of the substrate

materials and nanotubes of up to 1 millimeter can be grown can easily [11].

Chemical Vapor Deposition is by far the most superior technique for manufacture ofCarbon nanotubes but it is not the cheapest. Another simple technique for nanotube synthesis is

the flame synthesis. It uses some aspects of the chemical vapor deposition but does not require

the expensive substrate. Nanotubes are grown by passing a hydrocarbon flame over a stainless

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steel grid with a holding wire of made up of 0.4mm diameter Ni-Cr to collect the soot from the

burnt methane flame. The Nickel-Chromium wire acts as a catalyst on the fly and produces multi

walled nanotubes. The production of nanotubes using this method is cheap but the nanotubesneed to be purified as they are contaminated with soot [12].

Figure 5: Flame synthesized carbon naotubes [12].

APPLICATIONS

Carbon-nanotubes (CNTs) hold great promise, as a material of the future, since they haveprospective applications in virtually every field of science. Carbon nanotubes are extremely

famous among scientists because of their excellent properties that yield high functionality at anextremely small length-scale. It is also a rarity that a lineage of good properties such as strength,specific surface area, thermal/electrical conductivity, high aspect ratio, etc are all available in a

single material. This section relays some of the most interesting applications of CNTs.

ELECTRONICS

Downsizing the electronic circuits without any compromise on performance appears to be

the trend in the electronics industry. One of the major challenges here is that conventional metals

when nano-sized breakdown at much lower current loads. CNTs, in contrast adopt a non-contact,

ballistic electron transfer which essentially reduces the intrinsic resistance to zero. This makesCNTs ideal for such applications where devices that operate at high currents need to be

condensed into smaller devices.

Extensive research is being done in the field of flexible electronic materials, wherein bothmechanical flexibility and high conductivity are the targets desired. Presently, polymers and

composites are the materials used for such applications, but these materials can be considerably

improved by incorporating randomly distributed CNT fibers on their substrates, owing to better

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network conductance [13]. The ease of fabrication and the compatibility of CNTs with polymer

materials are critical factors that make CNTs promising for these applications.

CNTs are used in the fabrication of field effect transistors, where they form theconductive channels in the transistor layout. The apparent advantages by using CNTs are that

such devices possess extremely high sensitivity compared to conventional transistors. Further,patterned growth of CNTs is being probed for integrating multiple transistors and devices [14].

CNTs are as good as diamond in heat conduction. This exceptional thermal conductivity

in CNTs has proven useful in making heat sinks for electronic devices such as computer chips

and MEMS devices. Xu, et. al. reports a novel concept for the heat sink by developing a

hierarchically branched network of CNTs that easily dissipate heat from the centre of a device tothe surroundings [15]. There are many other patented designs for similar heat dissipation

concepts for other devices as well, that use CNTs.

Figure 6: Illustration of a CNT network directed toward thesurface, making pathways for heat removal [15] .

ENERGY

Alternative energy is, by far, todays only true trillion dollar industry. CNTs find a lot

of prospective applications in this field such as materials for energy storage, energy generation,

scaffolds/supports for energy conversion devices etc.One of the most recent breakthrough involving CNTs as energy sources describes a

mechanism by which electric current is produced by creating thermal waves on the surface of

CNTs [16]. In simple words, a device incorporating such a concept could possibly store electrical

energy indefinitely, unlike batteries which lose their charge density when idle. While this is anextremely new finding, the endless possibilities using CNTs are highlighted.

Another area where CNTs are thoroughly investigated is hydrogen storage. The

motivating factor behind storing hydrogen in CNTs is the light weight and high surface area

offered for physically adsorbing hydrogen [17]. Although a lot of high storage capacitiesreported using CNTs have been controversial, and the concept of storing hydrogen by physical

adsorption has been unproductive all these years, CNTs are still a class of compounds alive inthis area of research. One other related application is in fuel cells, where in CNTs are being used

as supports for the cathode (usually platinum, which is very expensive) to reduce the costs of

fuel cells [18].

Electrochemical devices are yet another area of application for CNTs, wherein they areused as electrode materials. Capacitors, in electrochemical devices, made of CNTs have a much

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higher capacitance compared to the conventional capacitor. Super-capacitance would mean that

they can be ideal for applications where there is a requirement for very high power as well as

energy storage. One example could be hybrid electric vehicles, where a higher acceleration willbe possible (power output) as well as more efficient energy storage during braking [14].

Another area of application in hybrid electric vehicles is batteries. Lithium ion batteriesare the current frontrunners in this technology and CNTs are among the best materials that can beused as electrodes. The need of the hour is to develop batteries which can reversibly charge and

discharge at extremely high discharge rates. CNTs deliver almost 1000mA.hour/g of energy

storage capacity and are almost twice that of other materials such as milled graphite [14].

Thin film organic photovoltaic devices, as materials for solar cells are another extremelypopular area with tremendous prospects for CNT applications. The photovoltaic device operates

by generating electron hole pairs by photon absorption that is subsequently collected at their

respective electrodes, and the electron transport produces electricity. CNTs have high electronicconductivity, due to which they are used to provide a quick diffusion path for the electrons to

reach the electrode, with minimal resistance [19]. Another application of CNTs in solar cells is to

replace the expensive silicon that is used to design solar cells.

ENVIRONMENT

There are primarily two areas of concentration for the environmental applications of

CNTs as sorbent materials for preferential adsorption of gases (eg CH4) and as chemical andbiological sensors.

CNTs are used as preferential sorbents in a number of purification processes. CNTs were

found to adsorb dioxins in HNO3 purification [20]. CNTs are extremely sensitive even at room

temperature, which makes them extremely suitable for industrial purification processes. CNTs

have been studied for adsorption of CH4, NO2, and N2O4 etc [20]. The potential applications ofCNTs in CO2 capture (greenhouse gas capture, in general) are being probed.

The electronic properties of CNTs are very sensitive to surface alterations, making themvery excellent sensor materials. By altering the structural defects or doping with certain metals,

the CNTs can be used as very sensitive gas sensors, which can detect certain gases in the ppb

range. They are also used as sensors for bio-hazardous materials at room temperature. SinceCNTs can be synthesized by adopting a variety of processing conditions, their sensing ability is

largely dependent on the conditions they were synthesized in.

SPORTS EQUIPMENTS

The high mechanical strength coupled with low mass makes the CNTs a highly sought-after material in the sports and general consumer goods manufacturing industry. Tennis and

badminton racquets made of CNTs and so are the golf clubs. The equipments are thus lighter

than before without any compensation in power/strength. Other sports goods include bicycles,

helmets and body parts of vehicles used for motor sports, where weight plays a very importantrole in time sensitive sports. In addition to speed, CNTs also contribute to the safety of the

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sportsmen involved in these motor sports. In addition, CNTs are used to make equipments for

water-sports such as surf boards.

Figure 7: Some sports equipment made with CNT technology. Source:

http://www.composites.ugent.be/home_made_composites/composites_in_daily_life_.html.

MILITARY

Research on nanotechnology applications in military are highly funded and aretransforming the way nations are securing themselves. CNTs are, in particular, attractive for

military applications such as safety harnesses, explosion proof blankets, bullet proof body

armors, and smart wireless capabilities in remote areas. One of the most famous applications ofCNTs is in body armor. Of late, a lot of products with CNTs are released including bullet proof

vests. Typical materials used for such products are ceramic materials like boron carbide. The use

of CNTs is justified by the advantage of having a lower weight with the same toughness.

CONSTRUCTION/ARCHITECTURE

Composite materials developed by using CNTs have garnered huge interest for their

mechanical stability, good aspect ratios and applications in architectural and constructionindustry. CNTs are widely used in reinforcing materials and as fillers. CNTs can be used in the

matrix of a wide range of construction materials, including glass, cement, metals, polymers etc

[21]. The thermal conductivity of CNTs is another property that finds use in making heat-resistant structures, as well as insulating materials for boiler and air conditioning equipments

used in the construction industry. The properties of cement are also being enhanced by

reinforcing it with CNTs.

Another fancy application of CNTs in construction materials is the space elevator. Thespace elevator is a fictional concept, which is a long tube structure that connects with the outer

space. The challenge of such a structure is that the tube must be extremely strong to remain in a

stretched position forever, owing to earths rotation. While it is highly speculative that such astructure could ever be built, but if it does, carbon nanotubes are definitely the materials that will

make up these structures.

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ORGANIC / BIOMEDICAL APPLICATIONS

Applications for carbon nanotubes seem limitless when considering the advantages of

weight savings and increased strength as compared to structural materials such as steel. Add to

this the semiconductor properties, and other electrical properties, and one can see a bright futurefor applications of carbon nanotubes, unless of course, they are used in living things.

Carbon nanotubes, as stated earlier, can be found in very old items, and being a part of

lampblack can be obtained quite easily, but never in the quantities and purities available today.These substances were literally unknown and uncharacterized thirty years ago, and now, one can

find them all over, in the average daily life. What does this exposure do to animals? What does

this exposure do to the environment? These questions are the beginnings of regulatory

requirements. But who is to regulate these carbon nanotubes? Are they a drug? Are they adevice? Under which governmental agency do they fall?

After all of these questions were asked when carbon nanotubes were coming to the

forefront, initial results for toxicology studies did not look promising. All the research showeddeadly effects. Even as recently as 2004, evidence of pulmonary toxicity was being brought up.

According to Lam, et al, when carbon nanotubes reach the lungs of a mouse, they kill the mouse,

even in small doses [22]. This is supposing that due to carbon nanotube weight to area ratio, thelikelihood of aerosolization is high. There are many reports of immune cells being damaged by

carbon nanotubes, as well as lung epithelium. The evidence seems to be insurmountable that

these will not be able to be used as other nanoparticles are being tried: as drug delivery systems,as tissue culture scaffolding, and others. As recently as 2008, comparisons of carbon nanotubes

to the dreaded asbestos fiber have shown mesothelioma type damage to lung tissue [23]. Still,

work to find use in living systems continues.

The basis for all of these toxicity problems is the imminent fact that carbon nanotubes areinsoluble in any solvent including organic solvents and water. This also owes to the famous

properties mentioned earlier. But the non-wetting barrier may be bridged. By using azo-ylides, or

nitrogen- carbon compounds where the carbon adjacent to the nitrogen moiety holds a negativecharge, while the nitrogen is in a positively charged state, allows addition of one of these

molecules across one of the rings of the shell of the carbon nanotube. This is known as 1,3

dipolar cycloaddition. The following figure from bianco, et al, shows the general reaction [24].

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Figure 8: 1,3 Dipolar addition to carbon naotube reaction.

By using this principle with an organic solvent yields functionalized carbon nanotubes.These functionalized carbon nanotubes (fCNTs) can then have their solubility tuned by

modifications to the alkane side chains (R groups in the figure), or by using different R groups to

begin with. This change to the solubility now adds a degree of safety, as insoluble objects are a

difficult problem for living systems to deal with. Solubility allows for the living entity to removeunwanted objects through various methods based on solute gradients, and active transport.

This advance has opened up the CNT field. There are many reports being generated by

researchers all over the world that tout the non-toxic advantages offCNTs. By varying theaddition reactants to vary the solubility, French and Italian scientists have shown that Immune

cells can not only survive fCNTs, but produce inflammatory cytokines. This may just be the

result offCNTs acting on the cells, but this opens the possibility of using the tubes as

inflammatory modulators [25].With this hurdle of solubility cleared there have been some animal trials of drug delivery

systems as well. By varying the addition method mentioned before, researchers collaborating atStanford University were able to add phospholipids to the single walled carbon nanotubes.

Phospholipids are easily substituted chemically at the oxygen rich end of the hydrocarbon chain,

and by using an anticancer drug, they have created a very specific targeted delivery system [26].

This group was able to substitute Taxol on the end of the hydrocarbon functionalisation via anester bond. Many nanoparticles and nanoshells sequester a drug inside, and then the leak rate

determines delivery. These particles are often functionalized to optimize the enhanced

permeability and retention effect of nanoparticles in biological systems. This refers tomembrane permeability enhancements into the cell, while exhibiting enhanced retention in the

cell as well. This is observed in many nanoparticle applications. The main difference here is thatthe drug will not be freed unless the particle enters the cell. Naturally occurring esterases in thecells will cleave the ester bond, and release the drug into the target cell. As cancerous cells

exhibit this increased permeability, and increased retention, more of the drug will be delivered to

cancer cells.

These particularfCNTs are also PEGylated, that is , coated with polyethylene glycolmoieties. This additional chemistry makes these soluble in water at physiological pH, yet another

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advancement. The treatment is carried out on live mice with good results, making this group

hopeful for a better chemtoxic cancer treatment.

This work opens the door to making fCNTs of specific dimensions to be ion channelblockers. By using physiological parameters of signaling channels on neurons and other cells, we

can then manufacture agonists or antagonists from these fCNTs to modulate signaling, evenpossibly memory.Using this functionalisation technique groups are using the fCNTs as delivery units for proteins

as well as drugs. The human protein erythropoietin (EPO) has been loaded into fCNTs and

delivered into intestines of live animals, showing increased absorption of EPO. This is important

as this protein regulates production of blood cells, which are often downregulated by chemotoxicanti cancer therapeutics. This would be an adjuvant treatment for cancer.

Even without the functionalisation, CNTs can be used as tips on the tips of AFM to poke

holes in cells. This approach is being called a nanoinjector [27], and may have many uses inbiological research, while not ever being circulated through any living system.

Another study has recently found that carbon nanotubes are broken down by an enzyme

(neutrophil myeloperoxidase) found inside the white blood cells [28]. The nanotubes have anatural biological material that can degrade them but more studies will need to be carried out in

order to learn all aspects of this exciting new man-made material.

As can be seen from this discussion of organic applications, the range is broad and onlylimited by imagination.

CONCLUSION

Carbon Nanotubes were discovered accidentally in the 90s and are now on the threshold

of revolutionizing every aspect of our lives. The nanotubes have a wide lineage of applications ina wide variety of fields ranging from energy to transistors to super materials of the future.

Carbon nanotubes are the only material to have such a wide range of application. The

semiconducting nature of single walled nanotubes allows for fabrication of single molecule

transistors which can keep Moores law continuing for a long time. The tensile strength of singlenanotubes exceeds that of steel by orders of magnitude with a reduction in weight by a similar

margin. The applications of such materials are unparalleled. The electrical conductivity of

nanotubes is four orders of magnitude higher than that of copper. The ease of alteration ofproperties such as chirality, aspect ratio etc makes carbon nanotubes extremely versatile as novel

application-materials. There has been speculation that carbon nanotubes could be poisonous to

human beings. However, these rod-like structures are similar to that of asbestos and symptomssimilar to asbestosis have been produced in lab mice. But the results were shown to be

controversial as the studies failed to consider the effects of iron nanoparticles which were also

present in the nanotubes. As was illustrated in this article, properties of CNTs such as high

surface area, super capacitance, light weight, extrordinary thermal and electronic conductivitymake these materials very promising for the future. Even ambitious projects like the space

elevator count on CNTs as the only materials that are capable of realizing something as fictional.

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REFERENCES

1. S. Reich, C. Thomsen, and J. Maultzsch, Carbon Nanotubes: Basic Concepts and Physical Properties,Weinheim: Wiley-Vch Verlag GmbH & Co. KGaA, (2004).

2. P. Harris, Carbon Nanotube Page, The complete article can be accessed at www.personal.reading.ac.uk/~scsharip/tubes.htm, Last accessed on 05/14/2010.

3. J. Seetharamappa, S. Yellappa, and F. DSouza, Carbon Nanotubes, Next Generation of ElectronicMaterials, The Electrochemical Society Interface, 15(2) pp. 23-25, (2006).

4. M. Daenen, R. D. de Fouw, B. Hamers, P. G. A. Janssen, K. Schouteden, and M. A. J. Veld, "TheWondrous World of Carbon Nanotubes: A Review of Current Nanotube Technologies, Eindhoven

University of Technology (2003). The complete text can be accessed at

http://students.chem.tue.nl/ifp03/Wondrous%20World%20of%20Carbon%20 Nanotubes_Final.pdf. Last

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Group Review Paper

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2010-5-14 Group Review_Carbon Nanotubes_Final

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