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Sonali Jayronia et al. World Journal of Pharmaceutical Research
CARBON NANOTUBE - A BURGEONING FIELD IN
NANOTECHNOLOGY
*Sonali Jayronia, Anu Hardenia, Sanjay Jain
Assistant Professor, Manavta Nagar, Kanadiya Road, Indore.
ABSTRACT
The increased current interest in carbon nanotubes is a direct
consequence of the development of buckminsterfullerene’s and its
derivatives thereafter. The result of research that carbon could form
stable, ordered structures other than graphite and diamond stimulated
many researchers in the world to search for other allotropes of carbon.
The small dimensions, proper physical properties of these structures
make them very unique with many promising applications. The
properties and characteristics of Carbon Nanotubes are still being
researched heavily and scientists have barely begun to tap the potential
of these structures. They can pass through membranes, carrying
therapeutic drugs, vaccines and nucleic acids deep into the cell to
targets previously unreachable. Overall, recent studies regarding Carbon Nanotubes have
shown a very promising glimpse of what lies ahead in the future of medicines. In this review
the Properties and Structure of carbon Nanotubes is discussed. So as the applications are
given.
KEYWORDS: Carbon Nanotubes, Allotropes, Membranes.
INTRODUCTION
Carbon is a versatile element and it can form various allotropes, including graphite, diamond
and fullerene-like structures. In the well-known layered structure of graphite, there is a large
difference in chemical bonding within the layers and between them. The σ-bonds connecting
each C atom with its three neighbors within the layers are created by sp2 hybridization. They
are possibly the strongest existing chemical bonds. The remaining fourth delocalized electron
in graphite is responsible for the very weak π-bonds acting between the layers. It also
determines the “semimetallic” character of graphite.1 The difference in the bond strength is
World Journal of Pharmaceutical research
Volume 3, Issue 1, 295-310. Review Article ISSN 2277 – 7105
Article Received on 20 October2013 Revised on 24 November 2013, Accepted on 30 December 2013
*Correspondence for
Author:
Sonali Jayronia,
Assistant Professor, Manavta
Nagar, Kanadiya Road, Indore.
jayroniasonali5@gmail.com
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Sonali Jayronia et al. World Journal of Pharmaceutical Research
reflected in the different bond distances: 0.142 nm between the neighboring C atoms within
the layers, as opposed to 0.335 nm between the layers. Due to the large difference in the bond
strength, the layers behave in a rather “independent “way and are termed “graphene” layers.
With the study of C60 and C70, it was soon realized that an infinite variety of closed
graphitic structures could be formed, each with unique properties. All that was necessary to
create such a structure was to have 12 pentagons present to close the hexagonal network (as
explained by the Euler theorem). Since C70 was already slightly elongated, tubular fullerenes
were imagined.
CNTs are allotropes of carbon. They are tubular in shape, made of graphite. CNTs possess
various novel properties that make them useful in the field of nanotechnology and
pharmaceuticals. They are nanometers in diameter and several millimeters in length and have
a very broad range of electronic, thermal, and structural properties. These properties vary
with kind of nanotubes defined by its diameter, length, chirality or twist and wall nature.
Their unique surface area, stiffness, strength and resilience have led to much excitement in
the field of pharmacy.
HISTORY
Carbon Nanotubes were discovered long before researchers even imagined that carbon may
exist in such a form. It was in 1952 when Radushkevichand Lukyanovich reported the
discovery of “worm-like” carbon formations1. These were observed during their study of the
soot formed by decomposition of CO on iron particles at 600 ˙C. On the basis of many
experiments and TEM images, the authors conclude that the formed product consisted of long
filamentary or needle-like carbon crystals with diameters of about 50 nm (the narrowest seen
in their TEM images were about 30 nm), probably hollow, growing from iron compound
(probably carbide) particles. In other words, the products were carbon nanotubes. The
discovered carbon nanotubes (CNTs), as we know today, were in fact multi-walled
(MWNTs), but the resolution of their TEM was too low (5–6 nm) to enable the authors to see
the arrangement of graphenes in then a no tube walls. It is interesting that the authors also
found couples of intertwined nanotubes, resembling at the first sight the DNA double helix
structure. The discovery of nanotubes passed almost unnoticed, like a number of other studies
before and after it.2
A paper by Oberlin, Endo, and Koyama published in 1976 clearly showed hollow carbon
fibers with nanometer-scale diameters using a vapor-growth technique3. Furthermore, in
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1979, John Abrahamson presented evidence of carbon nanotubes at the 14th Biennial
Conference of Carbon at Penn State University. The conference paper described carbon
nanotubes as carbon fibers which were produced on carbon anodes during arc discharge4. In
1981 a group of Soviet scientists published the results of chemical and structural
characterization of carbon nanoparticles produced by a thermocatalytical disproportionation
of carbon monoxide. Using TEM images and XRD patterns, the authors suggested that their
“Carbon multi-layer tubular crystals”were formed by rolling graphene layers into cylinders.
In 1987, Howard G. Tennent of Hyperion Catalysis was issued a U.S. patent for the
production of "cylindrical discrete carbon fibrils" with a "constant diameter between about
3.5 and about70 nanometers, length 10² times thediameter, and an outer region of multiple
essentially continuous layers of ordered carbon atoms and a distinct inner core"5. A large
percentage of academic and popular literature attributes the discovery of hollow, nanometer
sized tubes composed of graphitic carbon to SumioIijima of Nippon Electric Company in
1991. A 2006 editorial written by Marc Monthioux and Vladimir Kuznetsov in the journal
Carbon has described origin of the carbon nanotube.
CARBON STRUCTURE
Carbon has various elemental strutures.
CARBON
Graphite Fullerenes Nanotubes carbyne{chaoite} Diamond
Para crystalline Rhombic Hexagonal Cubic Hexagonal
Figure 1: Carbon elements
The arrangement of carbon atoms in each crystal is the fundamental difference between the
various structures. The three most important carbon structures are cubic diamond, fullerene
(C60) and hexagonal graphite Diamond is a stable form of carbon and can be synthesized
from graphite at high temperatures and pressures. Graphite is characterized by double bounds
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between sp2 hybridized carbon atoms. Fullerene, discovered in 1985, was the first discovered
form of carbon that had a cage like structure. The main derivatives of fullerene include buck
onions and nano tubes.
Structure of CNTs (Carbon Nanotubes)
CNTs are sheets of graphite rolled into tubes and tubes and have excellent due to their
symmetric structure. The two limiting cases, the armchair and zig-zagnanotubes, can be
obtained depending on the direction in which the sheet is rolled, as illustrated in Figure 2.
The structure of a nanotube is described by the chiral vector (ch), which is often referred to as
the rolled-up vector, and the chial angle (C). The chiral vector (n, m) is defined by the
following equation:
Ch= na1 +ma2
Figure 2: Roll-up a graphene sheet leading to the three different type of CNTs2
If the graphite sheet is rolled in all direction, then zig-zag nanotubes will be obtained. A
rolled graphite sheet in chiral vector (Ch) direction results in an armchair nanotubes.
Thesetwo structure are the limiting cases which occur which at chiral angles of the (zig-
zag)and 300 (armchair). The chiral vector of the zig-zagnanotubes is (n, 0) while the chiral
vector of the armchair nanotube is (n, n) (Thotenson et al, 2001). A SWCNT can be classified
as either metallic or semi-conducting based on its chiral vector (n, m).ASWCNT will be
metallic if the difference n-m is a multiple of three. Unlike MWCNTs, the atoms of SWCNTs
from of a singal covalently bound arry. Consequently, SWCNTs have more distinctive
electrical and optical properties when compared to MWCNTs and are typically used in
electronic components.
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METHODOLOGY
Types of carbon nanotubes and related structures
Single-walled
Most single-walled nanotubes (SWNT) have a diameter of close to 1 nanometer, with
a tube length that can be many millions of times longer. The structure of a SWNT can be
conceptualized by wrapping a one-atom-thick layer of graphite called graphene into a
seamless cylinder. The way the graphene sheet is wrapped is represented by a pair of indices
(n, m). The integer’s n and m denote the number of unit vectors along two directions in the
honeycomb crystal lattice of graphene. If m = 0, the nanotubes are called zigzag nanotubes,
and if n = m, the nanotubes are called armchair nanotubes. Otherwise, they are called chiral.
The diameter of an ideal nanotubes can be calculated from its (n, m) indices as follows
Where a = 0.246 nm.
SWNTs are an important variety of carbon nanotubes because most of their properties
change significantly with the (n,m) values, and this dependence is non-monotonic. In
particular, their band gap can vary from zero to about 2 eV and their electrical conductivity
can show metallic or semiconducting behavior. Single-walled nanotubes are likely candidates
for miniaturizing electronics. The most basic building block of these systems is the electric
wire, and SWNTs with diameters of an order of a nanometer can be excellent
conductors6. One useful application of SWNTs is in the development of the first
intermolecular field-effect transistors (FET). The first intermolecular logic gate using
SWCNT FETs was made in 20017. A logic gate requires both a p-FET and an n-FET.
Because SWNTs are p-FETs when exposed to oxygen and n-FETs otherwise, it is possible to
protect half of an SWNT from oxygen exposure, while exposing the other half to oxygen.
This results in a single SWNT that acts as a NOT logic gate with both p and n-type FETs
within the same molecule. Single-walled nanotubes are dropping precipitously in price, from
around $1500 per gram as of 2000 to retail prices of around $50 per gram of as-produced 40–
60% by weight SWNTs as of March 2010
Figure 3: Single wall carbon nanotube7
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Multi-walled
Multi-walled nanotubes (MWNT) consist of multiple rolled layers (concentric tubes) of
graphene. There are two models that can be used to describe the structures of multi-walled
nanotubes. In the Russian Doll model, sheets of graphite are arranged in concentric cylinders,
e.g., a (0,8) single-walled nanotubes (SWNT) within a larger (0,17) single-walled nanotubes.
In the Parchment model, a single sheet of graphite is rolled in around itself, resembling a
scroll of parchment or a rolled newspaper. The interlayer distance in multi-walled nanotubes
is close to the distance between graphene layers in graphite, approximately 3.4 Å. The
Russian Doll structure is observed more commonly. Its individual shells can be described as
SWNTs, which can be metallic or semiconducting. Because of statistical probability and
restrictions on the relative diameters of the individual tubes, one of the shells, and thus the
whole MWNT, is usually a zero-gap metal.
Double-walled carbon nanotubes (DWNT) form a special class of nanotubes because their
morphology and properties are similar to those of SWNT but their resistance to chemicals is
significantly improved. This is especially important when functionalization is required (this
means grafting of chemical functions at the surface of the nanotubes) to add new properties to
the CNT. In the case of SWNT, covalent functionalization will break some C=C double
bonds, leaving "holes" in the structure on the nanotubes and, thus, modifying both its
mechanical and electrical properties. In the case of DWNT, only the outer wall is modified.
DWNT synthesis on the gram-scale was first proposed in 2003 by the CCVD technique, from
the selective reduction of oxide solutions in methane and hydrogen8.
The telescopic motion ability of inner shells and their unique mechanical properties permit to
use multi-walled nanotubes as main movable arms in coming nanomechanical devices9.
Retraction force that occurs to telescopic motion caused by the Lennard-Jones
interaction between shells and its value is about 1.5 nN.
Figure 4: Triple-walled armchair carbon nanotube8
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Torus
In theory, a nanotorus is a carbon nanotube bent into a torus (doughnut shape). Nanotori are
predicted to have many unique properties, such as magnetic moments 1000 times larger than
previously expected for certain specific radii Properties such as magnetic moment, thermal
stability, etc. vary widely depending on radius of the torus and radius of the tube.10
Figure 5: A Torus stable structure8
Nanobud
Carbon Nanobud is a newly created material combining two previously discovered allotropes
of carbon: carbon nanotubes and fullerenes. In this new material, fullerene-like "buds" are
covalently bonded to the outer sidewalls of the underlying carbon nanotubes. This hybrid
material has useful properties of both fullerenes and carbon nanotubes. In particular, they
have been found to be exceptionally good field emitters. In composite materials, the attached
fullerene molecules may function as molecular anchors preventing slipping of the nanotubes,
thus improving the composite’s mechanical properties
Figure 6: A nanobud stable structure9
Graphenated carbon nanotubes (g-CNTs)
Graphenated CNTs are a relatively new hybrid that combines graphitic foliates grown along
the sidewalls of multiwalled or bamboo style CNTs. Yu et al. reported on "chemically
bonded graphene leaves" growing along the sidewalls of CNTs11. Stoner et al. described
these structures as "graphenated CNTs" and reported in their use for enhanced
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supercapacitor performance12. The foliate density can vary as a function of deposition
conditions (e.g. temperature and time) with their structure ranging from few layers
of graphene (< 10) to thicker, more graphite-like.13
The fundamental advantage of an integrated graphene-CNT structure is the high surface area
three-dimensional framework of the CNTs coupled with the high edge density of
graphene. Graphene edges provide significantly higher charge density and reactivity than the
basal plane, but they are difficult to arrange in a three-dimensional, high volume-density
geometry. CNTs are readily aligned in a high density geometry (i.e., a vertically aligned
forest) but lack high charge density surfaces—the sidewalls of the CNTs are similar to the
basal plane of graphene and exhibit low charge density except where edge defects exist14.
Depositing a high density of graphene foliates along the length of aligned CNTs can
significantly increase the total charge capacity per unit of nominal area as compared to other
carbon nanostructures.
Peapod
A Carbon peapodis a novel hybrid carbon material which traps fullerene inside a carbon
nanotube. It can possess interesting magnetic properties with heating and irradiating. It can
also be applied as an oscillator during theoretical investigations and predictions.15-18
Cup-stacked carbon nanotubes
Cup-stacked carbon nanotubes (CSCNTs) differ from other quasi-1D carbon structures,
which normally behave as quasi-metallic conductors of electrons. CSCNTs exhibit
semiconducting behaviors due to the stacking microstructure of graphene layers.
Extreme carbon nanotubesS
Figure 7: Cycloparaphenylene9
The observation of the longest carbon nanotubes (18.5 cm long) was reported in 2009. These
nanotubes were grown on Si substrates using an improved chemical vapor deposition (CVD)
method and represent electrically uniform arrays of single-walled carbon nanotubes.
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The shortest carbon nanotube is the organic compound cycloparaphenylene, which was
synthesized in early 2009.
The thinnest carbon nanotube is armchair (2,2) CNT with a diameter of 3 Å. This nanotube
was grown inside a multi-walled carbon nanotube. Assigning of carbon nanotube type was
done by combination ofhigh-resolution transmission electron microscopy (HRTEM), Raman
spectroscopy and density functional theory (DFT) calculations.19
The thinnest freestanding single-walled carbon nanotube is about 4.3 Å in diameter.
Researchers suggested that it can be either (5, 1) or (4, 2) SWCNT, but exact type of carbon
nanotube remains questionable. (3, 3), (4, 3) and (5, 1) carbon nanotubes (all about 4 Å in
diameter) were unambiguously identified using more precise aberration-corrected high-
resolution transmission electron microscopy. However, they were found inside of double-
walled carbon nanotubes.
PROPERTIES
Hardness
Standard single-walled carbon nanotubes can withstand a pressure up to 24GPa without
deformation. They then undergo a transformation to superhard phase nanotubes. Maximum
pressures measured using current experimental techniques are around 55GPa. However, these
new superhard phase nanotubes collapse at an even higher, albeit unknown, pressure.
The bulk modulus of superhard phase nanotubes is 462 to 546 GPa, even higher than that of
diamond (420 GPa for single diamond crystal) 20.
Kinetic properties
Multi-walled nanotubes are multiple concentric nanotubes precisely nested within one
another. These exhibit a striking telescoping property whereby an inner nanotube core may
slide, almost without friction, within its outer nanotube shell, thus creating an atomically
perfect linear or rotational bearing. This is one of the first true examples of molecular
nanotechnology, the precise positioning of atoms to create useful machines. Already, this
property has been utilized to create the world's smallest rotational motor. Future applications
such as a gigahertz mechanical oscillator is also envisaged.
Optical activity
Theoretical studies have revealed that the optical activity of chiral nanotubes disappears if the
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nanotubes become larger21.Therefore, it is expected that other physical properties are
influenced by these parameters too. Use of the optical activity might result in optical devices
in which CNTs play an important role.
Electrical conductivity
Depending on their chiral vector, carbon nanotubes with a small diameter are either semi-
conducting or metallic. The differences in conducting properties are caused by the molecular
structure that results in a different band structure and thus a different band gap. The
differences in conductivity can easily be derived from the graphene sheet properties.It was
shown that a (n,m) nanotubes is metallic as accounts that: n=m or (n-m) = 3i, where i is an
integer and n and m are defining the nanotubes. The resistance to conduction is determined by
quantum mechanical aspects and was proved to be independent of the nanotube length. For
more, general information on electron conductivity is referred to a review by Ajayan and
Ebbesen.
Mechanical strength
Carbon nanotubes have a very large Young modulus in their axial direction. The nanotube as
a whole is very flexible because of the great length. Therefore, these Compounds are
potentially suitable for applications in composite materials that need anisotropic properties.
Chemical reactivity
The chemical reactivity of a CNT is, compared with a graphene sheet, enhanced as a direct
result of the curvature of the CNT surface. Carbon nanotube reactivity is directly related to
the pi-orbital mismatch caused by an increased curvature. Therefore, a distinction must be
made between the sidewall and the end caps of a nanotube. For the same reason, a smaller
nanotube diameter results in increased reactivity. Covalent chemical modification of either
sidewalls or end caps has shown to be possible. For example, the solubility of CNTs in
different solvents can be controlledthis way. Though, direct investigation of chemical
modifications on nanotube behavior is difficult asthe crude nanotube samples are still not
pure enough.
Thermal properties
All nanotubes are expected to be very good thermal conductors along the tube, exhibiting a
property known as "ballistic conduction", but good insulators laterally to the tube axis.
Measurements show that a SWNT has a room-temperature thermal conductivity along its axis
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of about 3500 W·m−1·K−1, Compare this to copper, a metal well known for its good thermal
conductivity, which transmits 385 W·m−1·K−1. A SWNT has a room-temperature thermal
conductivity across its axis (in the radial direction) of about 1.52 W·m−1·K−1,which is about
as thermally conductive as soil. The temperature stability of carbon nanotubes is estimated to
be up to 2800 °C in vacuum and about 750 °C in air.22, 23
Defects
As with any material, the existence of a crystallographic defect affects the material
properties. Defects can occur in the form of atomic vacancies. High levels of such defects can
lower the tensile strength by up to 85%. An important example is the Stone Wales defect,
which creates a pentagon and heptagon pair by rearrangement of the bonds. Because of the
very small structure of CNTs, the tensile strength of the tube is dependent on its weakest
segment in a similar manner to a chain, where the strength of the weakest link becomes the
maximum strength of the chain.
Crystallographic defects also affect the tube's electrical properties. A common result is
lowered conductivity through the defective region of the tube. A defect in armchair-type
tubes (which can conduct electricity) can cause the surrounding region to become
semiconducting, and single monoatomic vacancies induce magnetic properties.
Crystallographic defects strongly affect the tube's thermal properties. Such defects lead
to phonon scattering, which in turn increases the relaxation rate of the phonons. This reduces
the mean free path and reduces the thermal conductivity of nanotube structures. Phonon
transport simulations indicate that substitutional defects such as nitrogen or boron will
primarily lead to scattering of high-frequency optical phonons. However, larger-scale defects
such as Stone Wales defects cause phonon scattering over a wide range of frequencies,
leading to a greater reduction in thermal conductivity24.
SYNTHESIS
CNTs are generally produced by three main techniques, arc discharge, laser ablation and
chemical vapor deposition.
Arc discharge
Arc dischargeinitially used to for producing C60 fullerenes, is the most common and easiest
way to produce CNTs. This method creates CNTs through arc- vaporization of two carbon
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rods placed end to end, separated by approximately 1mm, in an enclosure that is usually filled
with inert gas at low pressure. A direct current of 50 to 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 other electrode (Figure 8).
Figure 8: Arc Discharge setup for synthesis and growth of nanotube12
Laser ablation
In Laser ablation laser vaporization pulses were 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 nanotubes structure (Figure 9). The material
produced by this method appears as a mat of “ropes”, 10-20 nm in diameter and upto 100µm
or more in length.
Figure 9: Laser Ablation setup for nanotube 19
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Chemical Vapor Deposition
Chemical Vapor Deposition of hydrocarbons over a metal catalyst is a classical method that
has been used to produce various carbon materials like carbon fibers and filaments. Large
amount of CNTs can be formed by catalytic CVD (Figure 10) of acetylene over cobalt and
iron catalysts supported on silica or zeolite. High yields of single walled nanotubes have been
obtained by catalytic decomposition of an H2/CH4 mixture all over well dispersed metal
particles such as cobalt ,nickel and iron on magnesium oxide at 10000C.The reduction
produces very small transition metal particles at a temperature of usually >8000C. The
decomposition of CH4 over the freshly formed nanoparticlesprevents their further growth,
and thus results in a very highproportion of SWNTsand few MWNTs.
Figure 10: Diagrammatical representation of Chemical vapor deposition (CVD) setup
for nanotubes synthesis and growth25
APPLICATIONS25, 26
Various applications of CNTs are as follows:
1) Carrier for Drug delivery: Carbon nanohorns (CNHs) are the spherical aggregates of CNTs
with irregular horn like shape. Research studies have proved CNTs and CNHs as a potential
carrier for drug delivery system.
2) Functionalized carbon nanotubes are reported for targeting of Amphotericin B to Cells.
3) Cisplatin incorporated oxidized SWNHs have showed slow release of Cisplatin in aqueous
environment. The released Cisplatin had been effective in terminating the growth of human
lung cancer cells, while the SWNHs alone did not show anticancer activity.
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4) Anticancer drug Polyphosphazeneplatinum given with nanotubes had enhanced
permeability, distribution and retention in the brain due to controlled lipophilicity of
nanotubes.
5) Antibiotic, Doxorubicin given with nanotubes is reported for enhanced intracellular
penetration
6) The gelatin CNT mixture (hydro-gel) has been used as potential carrier system for
biomedical.
7) CNT-based carrier system can offer a successful oral alternative administration of
Erythropoietin (EPO), which has not been possible so far because of the denaturation of EPO
by the gastric environment conditions and enzymes.
8) They can be used as lubricants or glidants in tablet manufacturing due to nanosize and
sliding nature of graphite layers bound with van der Waals forces.
TOXICITY OF CNT
Harmful effect of nanoparticles arises due to high surface area and intrinsic toxicity of the
surface. CNT, in the context of toxicology, can be classified as ‘nanoparticles’ due to their
nanoscale dimensions, therefore unexpected toxicological effects upon contact with
biological systems may be induced. The nanometer-scale dimensions of CNT make quantities
of milligrams possess a large number of cylindrical, fibre-like particles, with a concurrent
very high total surface area. The intrinsic toxicity of CNT depends on the degree of surface
functionalization and the different toxicity of functional groups. Batches of pristine CNT
(non-purified and/or non-functionalised) readily after synthesis contain impurities such as
amorphous carbon and metallic nanoparticles (catalysts: Co, Fe, Ni and Mo), which can also
be the source of severe toxic effects. On the basis of recent studies with carbon derived
nanomaterials showed that platelet aggregation was induced by both single and multi-wall
carbon nanotube but not by the C60- fullerenes that are used as building blocks for these
CNT.
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