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Fullerenes, Nanotubes andCarbon NanostructuresPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/lfnn20

Carbon Nanotubes: Synthesis,Properties and PharmaceuticalApplicationsInderbir Singh a , Ashish K. Rehni a , Pradeep Kumar a

, Manoj Kumar a & Hassan Y. Aboul‐Enein b

a Chitkara College of Pharmacy, Punjab, Indiab Pharmaceutical and Medicinal ChemistryDepartment, National Research Centre, Cairo, EgyptPublished online: 22 Jul 2009.

To cite this article: Inderbir Singh , Ashish K. Rehni , Pradeep Kumar , ManojKumar & Hassan Y. Aboul‐Enein (2009) Carbon Nanotubes: Synthesis, Properties andPharmaceutical Applications, Fullerenes, Nanotubes and Carbon Nanostructures, 17:4,361-377, DOI: 10.1080/15363830903008018

To link to this article: http://dx.doi.org/10.1080/15363830903008018

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Carbon Nanotubes: Synthesis, Properties andPharmaceutical Applications

Inderbir Singh,1 Ashish K. Rehni,1 Pradeep Kumar,1 Manoj Kumar,1 and

Hassan Y. Aboul-Enein2

1Chitkara College of Pharmacy, Punjab, India2Pharmaceutical and Medicinal Chemistry Department, National Research

Centre, Cairo, Egypt

Abstract: Carbon nanotubes (CNTs) are unique cylindrical forms of carbon that

have carved a niche in the field of nanomedicine. The possibility of incorporating

functionalized carbon nanotubes into cells and the biological milieu offers

numerous advantages for potential applications in biology, pharmacology and

drug delivery. One of the most promising is the utilization of CNTs as a new

carrier system for the delivery of therapeutic molecules. Furthermore, recent

findings of improved cell membrane permeability for carbon nanotubes would

expand medical applications to therapeutics using carbon nanotubes as carriers in

gene delivery systems. This review discusses the synthetic methods, properties and

potential pharmaceutical application of CNTs. Toxicological considerations of

CNTs have also been delineated.

Keywords: Applications, Carbon nanotubes, Properties, Synthesis, Toxicology

1. INTRODUCTION

Carbon nanotubes (CNTs) are tubular forms of carbon that can be

envisaged as graphitic sheets rolled into cylindrical form. Thesenanotubes have diameters in the range of few nanometers and their

lengths are up to several micrometers. Each nanotube is a single molecule

made up of a hexagonal network of covalently bonded carbon atoms.

The structure of the nanotube influences its properties, including

electrical and thermal conductivity, density and lattice structure.

Address correspondence to Professor Hassan Y. Aboul-Enein, Pharmaceutical

and Medicinal Chemistry Department, National Research Centre, Dokki, Cairo

12311, Egypt. E-mail: enein@gawab.com

Fullerenes, Nanotubes and Carbon Nanostructures, 17: 361–377, 2009

Copyright # Taylor & Francis Group, LLC

ISSN 1536-383X print/1536-4046 online

DOI: 10.1080/15363830903008018

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Iijima reported for the first time preparation of multi-walled carbon

nanotubes (MWCNTs) by the arc-discharge of graphite electrodes in

1991 (1). Carbon nanotube samples are always contaminated with

impurities including amorphous carbon, residual metal catalyst and

graphitic nanoparticles. Thus the purification and chemical processing of

carbon nanotubes remains as a key step in any application.

Carbon nanotubes are of two types (as shown in Figure 1): single-

walled and multi-walled. A single-walled carbon nanotube (SWCNT)

consists of a single graphene cylinder, whereas a MWCNT comprises

several concentric graphene cylinders.

Strong covalent bonding, unique one-dimensional structure and

nanometer size together impart unusual properties, which includes

exceptionally high tensile strength, high resilience, electronic properties

ranging from metallic to semi-conducting, the ability to sustain high

current densities and high thermal conductivity. Thus carbon nanotubes

could be used as fillers in super-strong composite materials, as wires and

components in nano-electronic devices, as tips of scanning probe

microscopes and in flat panel displays and gas sensors (2).

2. SYNTHESIS (1, 3)

2.1. Arc Method

The carbon arc discharge method, initially used for producing C60

fullerenes, is the most common and perhaps easiest way to produce

CNTs, as it is rather simple. However, it is a technique that produces a

Figure 1. Conceptual diagram of single-walled carbon nanotube (SWCNT) (A)and multi-walled carbon nanotube (MWCNT) (B) showing typical dimensions of

length, width and separation distance between graphene layers in MWCNTs.

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complex mixture of components and requires further purification to

separate the CNTs from the soot and the residual catalytic metals

present in the crude product. This method creates CNTs through arc-

vaporization of two carbon rods placed end to end, separated by

approximately 1mm, in an enclosure that is usually filled with inert gas

at low pressure. Recent investigations have shown that it also is

possible to create CNTs with the arc method in liquid nitrogen. A

direct current of 50–100 A, driven by a potential difference of

approximately 20 V, creates a high temperature discharge between the

two electrodes. The discharge vaporizes the surface of one of the

carbon electrodes and forms a small rod-shaped deposit on the other

electrode. Producing CNTs in high yield depends on the uniformity of

the plasma arc and the temperature of the deposit forming on the

carbon electrode.

2.2. Laser Method

CNTs were first synthesized in 1996 using a dual-pulsed laser and

achieved yields of .70wt% purity. Samples were prepared by laser

vaporization of graphite rods with a 50:50 catalyst mixture of cobalt

and nickel at 1200uC in flowing argon, followed by heat treatment in

a vacuum at 1000uC to remove the C60 and other fullerenes. The

initial laser vaporization pulse was followed by a second pulse to

vaporize the target more uniformly. The use of two successive laser

pulses minimizes the amount of carbon deposited as soot. The second

laser pulse breaks up the larger particles ablated by the first one and

feeds them into the growing nanotube structure. The material

produced by this method appears as a mat of ‘‘ropes,’’ 10–20 nm in

diameter and up to 100 mm or more in length. Each rope is found to

consist primarily of a bundle of single-walled nanotubes aligned along

a common axis. By varying the growth temperature, the catalyst

composition and other process parameters, the average nanotube

diameter and size distribution can be varied. Arc-discharge and laser

vaporization are currently the principal methods for obtaining small

quantities of high quality CNTs. However, both methods suffer from

drawbacks. The first is that both methods involve evaporating the

carbon source, so it has been unclear how to scale up production to

the industrial level using these approaches. The second issue relates to

the fact that vaporization methods grow CNTs in highly tangled

forms, mixed with unwanted forms of carbon and/or metal species.

The CNTs thus produced are difficult to purify, manipulate and

assemble for building nanotube-device architectures for practical

applications.

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2.3. Chemical Vapor Deposition (CVD)

Chemical vapor deposition of hydrocarbons over a metal catalyst is a

classical method that has been used to produce various carbon materials

such as carbon fibers and filaments for more than 20 years. Large amounts

of CNTs can be formed by catalytic CVD of acetylene over cobalt and iron

catalysts supported on silica or zeolite. The carbon deposition activity

seems to relate to the cobalt content of the catalyst, whereas the CNTs’

selectivity seems to be a function of the pH in catalyst preparation.

Fullerenes and bundles of single-walled nanotubes were found among the

multi-walled nanotubes produced on the carbon/zeolite catalyst as well.

Some researchers are experimenting with the formation of CNTs from

ethylene. Supported catalysts such as iron, cobalt and nickel, containing

either a single metal or a mixture of metals, seem to induce the growth of

isolated single-walled nanotubes or single-walled nanotube bundles in the

ethylene atmosphere. The production of single-walled nanotubes, as well as

double-walled CNTs, on molybdenum and molybdenum-iron alloy

catalysts has also been demonstrated. CVD of carbon within the pores of

a thin alumina template with or without a nickel catalyst has been achieved.

Ethylene was used with reaction temperatures of 545uC for nickel-catalyzed

CVD and 900uC for an uncatalyzed process. The resultant carbon

nanostructures have open ends with no caps. Methane also has been used

as a carbon source. In particular, it has been used to obtain ‘‘nanotube

chips’’ containing isolated single-walled nanotubes at controlled locations.

High yields of single-walled nanotubes have been obtained by catalytic

decomposition of an H2/CH4 mixture over well-dispersed metal particles

such as cobalt, nickel and iron on magnesium oxide at 1000uC. It has been

reported that the synthesis of composite powders containing well-dispersed

CNTs can be achieved by selective reduction in an H2/CH4 atmosphere of

oxide solid solutions between a nonreducible oxide such as Al2O3 or

MgAl2O4 and one or more transition metal oxides. The reduction produces

very small transition metal particles at a temperature of usually .800uC.

The decomposition of CH4 over the freshly formed nanoparticles prevents

their further growth and results in a very high proportion of single-walled

nanotubes and fewer multi-walled nanotubes.

2.4. Ball Milling

Ball milling and subsequent annealing is a simple method for the

production of CNTs. Although it is well established that mechanical

attrition of this type can lead to fully nanoporous microstructures, it was

not until a few years ago that CNTs of carbon and boron nitride were

produced from these powders by thermal annealing. Essentially the

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method consists of placing graphite powder into a stainless steel

container along with four hardened steel balls. The container is purged,

and argon is introduced. The milling is carried out at room temperature

for up to 150 hours. Following milling, the powder is annealed under an

inert gas flow at temperatures of 1400uC for six hours. The mechanism of

this process is not known, but it is thought that the ball milling process

forms nanotube nuclei, and the annealing process activates nanotube

growth. Research has shown that this method produces more multi-

walled nanotubes and few single-walled nanotubes.

2.5. Other Methods

CNTs can also be produced by diffusion flame synthesis, electrolysis, use

of solar energy, heat treatment of a polymer, and low-temperature solid

pyrolysis. In flame synthesis, combustion of a portion of the hydrocarbon

gas provides the elevated temperature required, with the remaining fuel

conveniently serving as the required hydrocarbon reagent. Hence the

flame constitutes an efficient source of both energy and hydrocarbon raw

material. Combustion synthesis has been shown to be scalable for high-

volume commercial production.

3. PHYSICAL PROPERTIES (4)

3.1. Electrical Conductivity

CNTs can be highly conducting and hence can be metallic. Their

conductivity has been shown to be a function of their chirality, the degree

of twist and their diameter. CNTs can be either metallic or semi-

conducting in their electrical behavior. Conductivity in MWCNTs is quite

complex. Some types of ‘‘armchair’’-structured CNTs appear to conduct

better than other metallic CNTs. Furthermore, interwall reactions within

multi-walled nanotubes have been found to redistribute the current over

individual tubes nonuniformly. However, there is no change in current

across different parts of metallic single-walled nanotubes. The behavior of

the ropes of semi-conducting single-walled nanotubes is different in that

the transport current changes abruptly at various positions on the CNTs.

3.2. Strength and Elasticity

The carbon atoms of a single sheet of graphite form a planar honeycomb

lattice in which each atom is connected via a strong chemical bond to

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three neighboring atoms. Because of these strong bonds, the basal plane

elastic modulus of graphite is one of the largest of any known material.

For this reason, CNTs are expected to be the ultimate high-strength

fibers. Single-walled nanotubes are stiffer than steel and are resistant to

damage from physical forces. Pressing on the tip of a nanotube will cause

it to bend, but without damage to the tip. When the force is removed, the

nanotube returns to its original state. This property makes CNTs useful

as probe tips for very high-resolution scanning probe microscopy.

Quantifying these effects has been rather difficult, and an exact numerical

value has not been agreed upon.

Using atomic force microscopy, the unanchored ends of a free-

standing nanotube can be pushed out of their equilibrium position, and

the force required to push the nanotube can be measured. The current

Young’s modulus value of single-walled nanotubes is about 1 TeraPascal

(Tpa), but this value has been widely disputed, and a value as high as 1.8

Tpa has been reported. Other values significantly higher have also been

reported. The differences probably arise through different experimental

measurement techniques.

3.3. Thermal Conductivity and Expansion

CNTs have been shown to exhibit superconductivity below 20uK(approximately 2253uC). Research suggests that these exotic strands,

already heralded for their unparalleled strength and unique ability to

adopt the electrical properties of either semiconductors or perfect metals,

may someday also find applications as miniature heat conduits in a host

of devices and materials. The strong in-plane graphitic carbon-carbon

bonds make them exceptionally strong and stiff against axial strains. The

almost zero in-plane thermal expansion but large inter-plane expansion

of single-walled nanotubes implies strong in-plane coupling and high

flexibility against nonaxial strains.

Many applications of CNTs, such as in nanoscale molecular

electronics, sensing and actuating devices, or as reinforcing additive

fibers in functional composite materials, have been proposed. Reports

of several recent experiments on the preparation and mechanical

characterization of CNT-polymer composites also have appeared.

These measurements suggest modest enhancements in strength char-

acteristics of CNT-embedded matrixes compared to bare polymer

matrixes. Preliminary experiments and simulation studies on the

thermal properties of CNTs show high thermal conductivity. It is

expected, therefore, that nanotube reinforcements in polymeric materi-

als may significantly improve the thermal and thermomechanical

properties of the composites.

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3.4. Field Emission

Field emission results from the tunneling of electrons from a metal tip

into vacuum, under application of a strong electric field. The small

diameter and high aspect ratio of CNTs is favorable for field emission.

Even for moderate voltages, a strong electric field develops at the free end

of supported CNTs because of their sharpness.

3.5. High Aspect Ratio

CNTs represent a small, high aspect ratio conductive additive for plastics

of all types. Their high aspect ratio means that a lower loading of CNTs

is needed compared to other conductive additives to achieve the same

electrical conductivity. This low loading preserves more of the polymer

resins’ toughness, especially at low temperatures, as well as maintains

other key performance properties of the matrix resin. CNTs have proven

to be an excellent additive to impart electrical conductivity in plastics.

Their high aspect ratio, about 1000:1, imparts electrical conductivity at

lower loadings compared to conventional additive materials such as

carbon black, chopped carbon fiber or stainless steel fiber.

3.6. Highly Absorbent

The large surface area and high absorbency of CNTs make them ideal

candidates for use in air, gas and water filtration. Much research is being

done to replace activated charcoal with CNTs in certain ultra-high purity

applications.

4. PHARMACEUTICAL APPLICATIONS

The nanotube’s versatile structure allows it to be used for a variety of

tasks in and around the body. The nanotube allows for the drug dosage

to be lowered by localizing its distribution, as well as significantly cut

costs to pharmaceutical companies and their consumers. The nanotube

commonly carries the drug one of two ways: the drug can be attached to

the side or trailed behind or be placed inside the nanotube. Both methods

are effective for the delivery and distribution of drugs inside the body (5).

Important characteristics, such as the ability to easily cross-cellular

membranes, greatly enhance the potential of CNTs for therapeutic uses.

A recent study found that water soluble SWCNTs translocated easily into

the cytoplasm or nucleus of a cell through its cell membrane, without

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producing any toxic effect. Research confirmed that carbon nanotubes

could be effectively used to deliver drugs into tumor cells, enhancing

treatment as it allows for targeted drug delivery to the site of action. New

approaches are being investigated, for example, controlled release drug

reservoirs using microchips and CNTs (6).

4.1. Drug Delivery

Drug delivery has been a major area of focus for researchers aiming to

improve the efficacy of therapeutic molecules. Some obstacles that the

researchers are trying to overcome include poor drug distribution among

cells, unwanted damage to healthy tissue, toxicity and lack of the ability to

select a particular cell type for treatment. Many beneficial molecules can be

bonded to the walls and tips of these soluble CNTs, including peptides,

nucleic acids and various drug molecules for a better-targeted delivery (5).

CNTs have several advantages for drug delivery: i) size in the range

of 10–40 nm, ii) ability to provide a rod-like scaffold, iii) increased

capacity to carry drugs, iv) ability to deliver drugs to the nucleus and v)

inert and nontoxic in nature (43). Wu et al. (2005) postulated that

delivery of amphotericin B by means of CNTs would reduce the amount

of antibiotic necessary resulting improved potency and reduced toxicity.

One major obstacle in traditional delivery of this drug lies in the fact that

it has a low solubility and causes membrane leakage in eukaryotic cells.

This obstacle has hindered the usefulness of the drug when delivered

through traditional methods, such as encapsulation, due to the low

therapeutic index and the need for slow delivery (7).

Hampel et al. (2008) demonstrated CNTs as feasible carriers for

carboplatin, a therapeutic agent for cancer treatment. The drug was

introduced into CNTs to elucidate that they are suited as nanocontainers

and nanocarriers and can release the drug to initialize its medical virtue.

In-vitro study data suggested that carboplatin-filled CNTs inhibited

growth of bladder cancer cells whereas unfilled, opened CNTs barely

affected cancer cell growth (8).

Pastorin et al. (2006) developed a novel strategy for the functiona-

lization of CNTs with two different molecules using the 1,3-dipolar

cycloaddition of azomethine ylides and explored two alternative routes,

which allowed introduction of a fluorescent probe and an anticancer

agent around the CNT sidewalls. They concluded that controlled

multifunctionalization of CNTs and the attachment of molecules that

will target specific receptors on tumor cells would help improve the

response to anticancer agents (9).

Yang et al. (2008) researched the technology of magnetic carbon

nanotubes (MCNTs) for the targeted delivery of drugs in the lymphatic

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tissue. Functionalized MCNTs synthesized with a layer of magnetite

nanoparticles on the inner surface of the nanotubes followed by

incorporation of the chemotherapeutics agent in the MCNT. To improve

drug delivery to cancer cells in the lymph nodes, individualized MNTs

were noncovalently functionalized by folic acid (FA). By using an

externally placed magnet to guide the drug matrix to the regional targeted

lymph nodes, the MNTs can be retained in the draining targeted lymph

nodes for several days and continuously release chemotherapeutic drugs.

Selective killing of tumor cells overexpressing the folate receptors (FRs)

in the lymph nodes can be achieved, as FR is overexpressed across a

broad spectrum of human tumors (10).

Feazell et al. (2007) demonstrated that by combining the ability of

platinum (IV) complexes that resist ligand substitution with the proven

capacity of SWCNTs to act as a longboat, shuttling smaller molecules

across cell membranes, we have constructed a SWNT tethered platinum

(IV) conjugate that effectively delivers a lethal dose of cis-[Pt(NH3)2Cl2]

upon reduction inside the cell (11).

Liu et al. (2007) prepared a solution of SWCNTs wrapped in

poly(ethylene glycol) (PEG) with a tumor-targeting cyclic arginine-

glycine-aspartic acid peptide to the end of the PEG chains. This solution

was injected into mice bearing tumors and it was observed that the

targeted SWCNTs accumulated in tumors. Thus potential drug delivery

applications have been achieved with efficient in-vivo accumulation of

SWCNTs in mice tumors (12).

Zeineldin et al. (2009) demonstrated the dispersion of SWCNTs by

ultrasonication with phospholipid-polyethylene glycol (PL-PEG) frag-

ments, thus interfering with its ability to block nonspecific uptake by

cells. However, unfragmented PL-PEG promoted specific cellular uptake

of targeted SWNTs to two distinct classes of receptors expressed by

cancer cells. Since fragmentation is a likely consequence of ultrasonica-

tion, a technique commonly used to disperse SWNTs, this maybe a

concern for certain applications such as drug delivery (13).

Tumor-targeting CNT constructs were synthesized from sidewall-

functionalized, water-soluble CNT platforms by covalently attaching

multiple copies of tumor-specific monoclonal antibodies, radiometal-ion

chelates and fluorescent probes. The key achievement in the study was

the selective targeting of tumor in-vitro and in-vivo by the use of specific

antibodies appended to a soluble, nanoscale CNT construct (14).

A novel SWNT-based tumor-targeted drug delivery system (DDS)

has been developed, which consists of a functionalized SWNT linked to

tumor-targeting modules as well as prodrug modules (15).

Liu et al. (2008) showed in-vivo SWNT drug delivery for tumor

suppression in mice by conjugating paclitaxel, a widely used cancer

chemotherapy drug, to branched polyethylene glycol chains on SWNTs.

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Nanotube drug delivery is promising for high treatment efficacy and

minimum side effects for future cancer therapy with low drug doses (16).

4.2. Gene Delivery

CNTs are used not only in the delivery of medicinal molecules but also to

deliver genes directly into the cell and across the nuclear membrane. The

lipophilic nature of biological membranes restricts the direct intracellular

delivery of potential drugs and molecular probes and makes intracellular

transport one of the key problems in gene therapy. Because of their

ability to cross cell membranes, SWCNTs are of interest as carriers of

biologically active molecules including genes, DNA, RNA and many

more. CNTs could be fictionalized at their terminal ends with single

stranded DNA or peptide nucleic acid, and hybridized with the

complementary DNA sequences to form supramolecular nanotubes

based structure (17–20).

Vanhandel et al. (2009) studied uptake and toxicity of MWCNTs in

the GL261 murine intracranial glioma model. Within 24 hours of a single

intratumoral injection of labeled MWCNTs (5 microgram), nearly 10–

20% of total cells demonstrated CNT internalization. Furthermore they

suggested that MWCNTs could potentially be used as a novel and

nontoxic vehicle for targeting MP in brain tumors (21).

Kateb et al. (2007) visualized in-vitro ingestion, cytotoxicity and

loading capacity of MWCNTs in microglia. They demonstrated that

MWCNTs do not result in proliferative or cytokine changes in-vitro, are

capable of carrying DNA and siRNA and are internalized at higher levels

in phagocytic cells as compared to tumor cells. Their study furthermore

suggested that MWCNTs could be used as a novel, nontoxic and

biodegradable nano-vehicles for targeted therapy in brain cancers (22).

Singh et al. (2005) studied the physicochemical interactions between

cationic functionalized carbon nanotubes and DNA toward construction

of carbon nanotube-based gene transfer vector systems. The interactions

of ammonium-functionalized single-walled and multi-walled carbon

nanotubes (SWCNT-NH3+; MWNT-NH3

+) and lysine-functionalized

single-walled carbon nanotubes (SWCNT-Lys-NH3+), with plasmid

DNA was investigated. The results indicated that all three types of

cationic carbon nanotubes are able to condense DNA to varying degrees,

indicating that both CNT surface area and charge density are critical

parameters that determine the interaction and electrostatic complex

formation between f-CNTs with DNA. All three different f-CNT types in

this study exhibited up regulation of marker gene expression over naked

DNA using a mammalian (human) cell line. Differences in the levels of

gene expression were correlated with the structural and biophysical data

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obtained for the f-CNT:DNA complexes to suggest that large surface

area leading to very efficient DNA condensation is not necessary for

effective gene transfer. However, it will require further investigation to

determine whether the degree of binding and tight association between

DNA and nanotubes is a desirable trait to increase gene expression

efficiency in-vitro or in-vivo (23).

Li et al. (2007) used the commercialized SWCNT to combine with

two different kinds of biocompatible molecules after purification. The

molecules are putrescine and polyoxyethylene bis-amine to form complex

with plasmid DNA, EGFp-C1 is selected for direct expression, and

evaluate the in-vitro properties (24).

4.3. Peptide Delivery

Carbon nanotubes are researched extensively in directed and targeted

delivery of peptides. Moreover modification of nanotubes by adding

certain functional groups enabled delivery of small peptides into the

nuclei of fibroblast cells. Although the mechanism of how tubes enter and

leave cells is unclear, they appear to be nontoxic (25). Researchers are

continually investigating new ways to deliver macromolecules that will

facilitate the development of new biologic products such as bioblood

proteins and biovaccines. Similarly, the success of DNA and RNA

therapies will depend on innovative drug delivery techniques (26).

Venkatesan et al. (2005) investigated CNTs as a drug delivery tool

for the administration of erythropoietin (EPO) to the small intestine. The

use of CNTs as Liquid-filled Nano Particulate System (LFNPS),

improved the bioavailability of EPO to 11.5% following intra-small

intestinal administration (27).

CNTs are becoming highly vulnerable molecules for applications in

medicinal chemistry. Biologically active peptides can easily be linked

through a stable covalent bond to CNTs. Panrarotto et al. (2003)

demonstrated the conjugation of peptide from the foot-and-mouth

disease virus to CNTs. The peptide-CNT conjugate system could be

greatly advantageous for diagnostic purposes and could find future

applications in vaccine delivery (28).

Kumar et al. (2005) researched to develop functionalized SWCNT

complexed with nanochitosan and used for delivery of DNA encoding

EGFP reporter protein or FITC-labeled peptide. f-SWCNT-chitosan

significantly increases DNA and peptide delivery to the cells (29).

Krajcik et al. (2008) developed a strategy for chemical functionaliza-

tion of SWNTs with hexamethylenediamine (HMDA) and poly(diallyl-

dimethylammonium)chloride (PDDA) to obtain a material that was able

to bind negatively charged siRNA by electrostatic interactions and

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concluded PDDA-functionalized SWNTs to be an effective carrier

system for applications in siRNA-mediated gene silencing (30).

5. TOXICOLOGICAL CONSIDERATIONS

Prior to the safe use of CNTs as a means of targeted drug delivery, a full

understanding of their toxicological properties is needed as well. Thus it is

of utmost importance to study the toxicological properties of nanotubes,

preferentially at a physiologically significant dosage to fully understand

their potential for targeted drug delivery. Toxicity studies on engineered

nanomaterials such as fullerenes, SWCNTs, MWCNTs and nanoscale

metal oxides such as TiO2 and nanometer-diameter low-solubility particles

support the need to carefully consider how nanomaterials are characterized

when evaluating their potential biological activity (31–36).

Mouchet et al. (2008) investigated the ecotoxicological potential of

double-walled carbon nanotubes (DWNTs) in the amphibian larvae

Xenopus laevis. Acute toxicity and genotoxicity were analyzed after 12

days of static exposure in laboratory conditions. They concluded no

genotoxicity in erythrocytes of larvae exposed to DWNTs in water but

acute toxicity at every concentration of DWNTs studied, which was

related to physical blockage of the gills and/or digestive tract. Black

masses suggested the presence of CNTs inside the intestine using optical

microscopy and TEM and confirmed by Raman spectroscopy analysis

(37).

Monteiro-Riviere et al. (2005) assessed the skin absorption and

toxicity of carbon nanotubes by conducting in-vitro studies, which

showed that multi-walled carbon nanotubes are capable of localizing

within and initiating an irritation response in human epidermal

keratinocytes, which are a primary route of occupational exposure (31).

Jia et al. (2005) examined the in-vitro effects of carbon black

nanoparticles on alveolar macrophage phagocytosis. Both the single-

walled and multi-walled CNTs displayed significant in-vitro toxicity;

specifically, single-walled CNTs exhibited toxicity at very low concentra-

tions (around 1.41 g/cm2), whereas multi-walled CNTs only inhibited

alveolar phagocytosis at relatively high concentrations (around 22.60 g/

cm2). However, when presented at the same dosage levels, the single-

walled nanotubes displayed higher levels of toxicity than the multi-walled

nanotubes (38).

In an in-vivo study, Lam et al. (2004) administered carbon nanotubes

to rats and mice through an intratracheal delivery system. Post-

administration pulmonary inflammation was seen in both groups of

rodents; however, the dosage level was fairly high, and results of the

study may not be physiologically relevant as rats were administered

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1–5 mg/kg of nanotubes and mice were administered 3.3–16.6 mg/kg. The

subsequent results of dosage levels included granuloma formation,

oxidative stress and mortality (39). Evidence for oxidative stress included

the formation of free radicals within the cells and a depletion of anti-

oxidants normally found within cells. The researchers have also pointed

out that the mortality rates may have been attributed to the obstruction

of the respiratory airways rather than the actual administration of the

CNTs themselves (40).

Shvedova et al. (2003) investigated adverse effects of SWCNTs

using a cell culture of immortalized human epidermal keratinocytes

(HaCaT). After 18 hours of exposure of HaCaT to SWCNT, oxidative

stress and cellular toxicity were indicated by formation of free radicals,

accumulation of peroxidative products, antioxidant depletion and loss

of cell viability. Exposure to SWCNT also resulted in ultrastructural

and morphological changes in cultured skin cells. These data indicate

that dermal exposure to unrefined SWCNT may lead to dermal

toxicity due to accelerated oxidative stress in the skin of exposed

workers (41).

Zhu et al. (2007) assessed the DNA damage response to MWCNTs in

mouse embryonic stem (ES) cells. A mutagenesis study using an

endogenous molecular marker, adenine phosphoribosyltransferase

(Aprt), showed that MWCNTs increased the mutation frequency twofold

compared with the spontaneous mutation frequency in mouse ES cells.

These results suggested that careful scrutiny of the genotoxicity of CNTs

demonstrated a limited or no toxicity at the cellular level (42).

6. CONCLUSION

Basic research in nanomedicine using the carbon nanotubes for the

delivery of the pharmaceutical and other biological agents remains in a

nascent stage. However, more work is required to characterize the

molecular mechanics and chemistry before the concept may turn

assimilable by the huge pharmaceutical industry. Nevertheless, initial data

suggest that pharmaceutical applications are technically feasible and may

be done safely, but the present information also warns us about taking into

serious consideration the need for development of methods to avert the

toxicology elicited by the otherwise wonderful carbon nanotubes.

7. ACKNOWLEDGMENTS

The authors are grateful to Dr. Madhu Chitkara, director, Chitkara

Institute of Engineering and Technology, Rajpura, Patiala, India, and

Pharmaceutical Applications of Carbon Nanotubes 373

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Dr. Ashok Chitkara, chairman, Chitkara Educational Trust,

Chandigarh, India, for support and institutional facilities.

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