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Functionalization of Carbon Nanotubes
A
TECHNICAL SEMINAR REPORT ON
Functionalization of Carbon Nanotubes
By
Shehaab Savliwala
TE- B- 01 (Gr. No. 101357)
DEPARTMENT OF CHEMICAL ENGINEERING
VISHWAKARMA INSTITUTE OF TECHNOLOGY(An Autonomous Institute, Affiliated to Pune University)
Academic Year 2012-13
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CERTIFICATE
Bansilal Ramnath Agarwal Charitable Trusts
VISHWAKARMA INSTITUTE OF TECHNOLOGY
(An Autonomous Institute affiliated to the University of Pune)
(Academic Year 2012- 13)
Department of Chemical Engineering
It is certified that the seminar work entitled
Functionalization of Carbon Nanotubes
Submitted by
Shehaab Savliwala (T.E.-B- 01), Gr. No. 101357
Is the original work carried out by him under the supervision of Prof.
Mrs. Shachi Nigam for the partial fulfillment of the requirement of the
University of Pune, for the award of the Bachelor of Chemical
Engineering.
Prof. Mrs. Shachi Nigam, Prof. Dr. D.S. Bhatkhande,Guide He ad of De pa r t me nt ,
Chemical Engineering Dept. Chemical Engineering Dept.
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Acknowledgements
It is matter of great pleasure for me to submit this seminar report on
Functionalization of Carbon Nanotubes, as a part of curriculum for award
ofBachelor ofChemical Engineering.
I am thankful to my seminar guide Prof. Mrs. Shachi Nigam, Chemical
Engineering Department for her constant encouragement and able guidance.
I am also thankful to Prof. D.S. Bhatkande, Head of Chemical
Engineering Department for his valuable support.
I take this opportunity to express my deep sense of gratitude towards
those, who have helped me in various ways, for preparing my seminar. At
the last but not least, I am thankful to my parents, who had encouraged &
inspired me with their blessings.
Shehaab Savliwala.
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Table of Contents
1. Abstract2. Introduction
2.1. Structure of Carbon Nanotubes (CNTs).2.2. Synthesis Methods.2.3. Reactivity of CNTs.2.4. Need for Functionalization: Current, Potential Applications.
3. Functionalization of Carbon Nanotubes3.1. End Functionalization3.2. Modification of the Carbon Nanotube Outerwall3.3. Functionalization of The Innerwall3.4. Physical Chemistry of CNTs3.5. Non-covalent Chemistry.
4. Conclusion.5. References.
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Functionalization of Carbon Nanotubes
1. Abstract.
Carbon Nanotubes have a unique molecular structure that results in extraordinary
macroscopic properties, including high tensile strength, high electrical
conductivity, high ductility, high heat conductivity, and relative chemical
inactivity. Owing to these properties, carbon nanotubes have myriad potential
applications as structural materials, in electronics, optics and other fields of
materials science and technology. However, these applications are limited by our
capability to mold CNTs according to our requirements. Due to the difficulty in
synthesis and their low reactivity, it is difficult to synthesize CNTs with the
surface and bulk properties demanded for specific applications (e.g.
Biocompatibility for nanotube biosensors or interfacial strength for blending with
polymers). Therefore, chemical modification (including surface and interfacial
modification) of CNTs becomes essential.
Judicious application ofsite-selective reactions to non-aligned and aligned carbon
nanotubes has opened a rich field of carbon nanotube chemistry. The tips of carbon
nanotubes are more reactive than their sidewalls, allowing a variety of chemical
reagents to be attached at the nanotube tips. Recently, some interesting reactions
have also been devised for chemical modification of both the inner and outer
nanotube walls, though the seamless arrangement of hexagon rings renders the
sidewalls relatively unreactive. This report provides a brief summary of the recent
progress made in the research on chemistry of carbon nanotubes.
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2. Introduction.
Since their discovery by Iijima in 1991, carbon nanotubes have received
considerable attention. Carbon Nanotubes (CNTs) are allotropes of carbon with a
cylindrical nanostructure. They can be thought of as cylindrical fullerenes.
Nanotubes have been constructed with length-to-diameter ratio of up to
132,000,000:1[1]
, significantly larger than for any other material. Carbon nanotubes
can be metallic or semi-conductive, depending on their diameters and arrangement
of hexagon rings along the tube length. Apart from the interesting electronic
characteristics, carbon nanotubes exhibit excellent mechanical and thermal
properties. These interesting physicochemical properties make carbon nanotubes
very attractive as electron emitters in field emission displays, reinforcement fillers
in nanocomposite materials, scanningprobe microscopy tips, actuators and sensors,
as well as molecular-scale components in micro- or nano-electronic devices.[1]
However, it is difficult to synthesize carbon nanotubes with the surface
characteristics demanded for specific applications (e.g. strongly interfacing with
polymers in nanocomposites, or good biocompatibility for nanotube sensors).
Therefore, surface modification and interfacial engineering are essential in
making advanced carbon nanotubes of good bulk and surface properties.
Due to the seamless arrangement of hexagon rings without any dangling bonds,
carbon nanotube walls are rather unreactive. Like C60 fullerene, the fullerene-like
tips of nanotubes are known to be more reactive than the cylindrical nanotube
walls, and hence certain reagents can more readily react with the tips[2.3]
. However,
this does not impact significantly on the chemistry of nanotubes as a whole due to
the relatively smallproportionofthetipsinthestructure.Although research on the
chemical modification of carbon nanotubes is still in its infancy, some interesting
work on carbon nanotube chemistry has recently been reported in literature. I shall
now attempt to present an overview on carbon nanotube chemistry, covering both
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the covalent and non-covalent reactions at the tips, outerwalls, and innerwalls of
SWNTs.
2.1. Structure of Carbon Nanotubes (CNTs).
Nanotubes are categorized as single-walled nanotubes (SWNTs) and multi-walled
nanotubes (MWNTs). These elongated tubularmacromolecules consist of carbon
hexagons arranged in a concentric manner with both ends normally capped by
fullerene-like structures.
The structure of a SWNT can be conceptualized by wrapping a one-atom-thick
layer of graphite called graphene into a seamless cylinder. These sheets are rolled
at specific and discrete (chiral) angles. The way the graphene sheet is rolled is
represented by a pair of indices (n, m). The integers n and m denote the number of
unit vectors along two directions in the honeycomb crystal lattice of graphene. A
tube (n,0) has carbon-carbon bonds that are parallel to the tube axis, and form, at
an open end, a zigzag pattern; these tubes are referred to as zigzag tubes. Tubes
with n=m have carbon-carbon bonds that are perpendicular to the tube axis, and
are often called "armchair" tubes. All others, where m does not equal n, and
neither is 0, are chiral, and have left-and right-handed variants.
The (n, m) values decide the radius and the rolling angle of the SWNT, thus
influencing its properties significantly.
The diameter of an ideal nanotube can be calculated from its (n, m) indices as
follows: a = 0.246 nm.
If there are additional graphene tubes around the core of a SWNT the nanotube is
called a Multi-walled carbon nanotube or MWNTs. We shall be focusing only on
SWNTs in this report.
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The figures shown above illustrate the (n, m) coordinate system and the structure
of a typical SWNT.
2.2. Synthesis Methods.
The three main methods of Carbon Nanotube synthesis are Arc Discharge, Laser
Ablation and Chemical Vapor Deposition. Each of them is described briefly
below.
In Arc Discharge, two graphite electrodes are placed millimeters apart and a 100 A
(or more) current is passed through them. The carbon contained in the negative
electrode sublimates in the form of nanotubes because of the high-discharge
temperatures. The yield for this method is up to 30% by weight and it produces
both single- and multi-walled nanotubes with lengths of up to 50 micrometers with
few structural defects. [4]
In laser ablation, a pulsed laser vaporizes a graphite target in a high-temperature
reactor while an inert gas is bled into the chamber. Nanotubes develop on the
cooler surfaces of the reactor as the vaporized carbon condenses. The laser
ablation method yields around 70% and produces primarily single-walled carbon
nanotubes with a controllable diameter determined by the reaction temperature.
However, it is more expensive than either arc discharge or chemical vapor
deposition.[4]
In CVD, a substrate with a layer of metal catalyst particles is heated to 600 oC in
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an oven, with slow addition of a carbon-bearing gas such as methane. The
vaporized carbon recrystallizes in the form of CNTs. Easiest to scale to industrial
production; long length MWNTs obtained but riddled with defects.[4]
2.3. Reactivity of CNTs.
The Orbital hybridization theory best describes chemical bonding in nanotubes.
The chemical bonding in graphene sheets, ergo in nanotubes, is composed entirely
ofsp2
bonds. As we already know, the strain of a carbon framework is reflected in
the pyramidalization or strain angle (p) of the carbon constituents. Chemical
reactivity is directly proportional to the strain caused by misalignment of pi-
orbitals. Trigonal carbon atoms (sp2
hybridized) prefer a planar orientation with
p=0 (i.e. graphene). Thus, the reactivity of nanotubes with respect to addition
chemistries is strongly dependent on the curvature of the carbon framework. The
(5, 5) SWNT has p~6 for the sidewall.[5] (This is a very moderate curvature).
Values for other (n, n) nanotubes show a trend of increasing p (sidewall) with
decrease in n. Therefore generally the chemical reactivity of SWNT increases with
decrease in diameter (or n, diameter increases with n)[5]
. However, on a
comparative scale, CNT sidewalls are rather unreactive. This low reactivity drops
even further if we consider any reaction occurring on the innerwall of a CNT, due
to hindrance (or blocking). This leaves the terminal ends of the CNT. As
mentioned earlier, the ends of carbon nanotubes are capped by fullerene- like
(football shaped) structures. Over here, the strain due to misalignment is much
higher, and consequently, the hemispherical (roughly) ends of a CNT are the area
where most chemical bonding takes place.[5]
2.4 The Need for Functionalization: Current and Potential Applications.
SWNTs are an important variety of carbon nanotube because most of their
properties change significantly with the (n, m) values, and this dependence is non-
monotonic (see Kataura plot). In particular, their band gap can vary from zero to
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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 excellentconductors.[6,7] One useful application of SWNTs is in the development of the
intermolecular field-effect transistors (FET). The first intermolecular logic gate
using SWNT FETs was made in 2001.[8]
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.
Apart from the interesting electronic characteristics, carbon nanotubes exhibit
excellent mechanical and thermal properties. In particular, owing to their
extraordinary thermal conductivity and mechanical and electrical properties,
carbon nanotubes find applications as additives to various structural materials.[9]
These interesting physicochemical properties make carbon nanotubes very
attractive as electron emitters in field emission displays, reinforcement fillers in
nanocomposite materials, scanning probe microscopy tips, actuators and sensors,
as well as molecular-scale components in micro- or Nano-electronic devices.[2]
However, it is difficult to synthesize carbon nanotubes with the surface
characteristics demanded for specific applications (e.g. strongly interfacing with
polymers in nanocomposites, or good biocompatibility for nanotube sensors). This
is because synthesis methods are not developed enough yet to produce a uniform
product. Therefore, surface modification and interfacial engineering are essential
in making advanced carbon nanotubes of good bulk and surface properties. This
requires a detailed study of their chemistry; especially in light of the fact that they
are highly resistant to most common reactions. Although research on the chemistry
of carbon nanotubes is still in its infancy, some interesting work on both covalent and
non-covalent modification has recently been reported in literature.
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3. Functionalization of Carbon Nanotubes.
3.1. End Functionalization
This section describes the reaction schemes that have been performed on the
terminals of CNTs.
3.1.1. Oxidation of Carbon Nanotubes
Early work on carbon nanotube chemistry can be traced back to the oxidation of
carbon nanotubes at high temperature in air or oxygen. [10] Oxidation of carbon
nanotubes at temperature above 700 C in the presence of air for 10 min results in
the hemispherical end-caps opening, indicating that the hemi- spherical tips are
more reactive than the graphite sidewall. This work has led to the prospect of
filling foreign materials (e.g. metal oxide nanoparticles, biosensors, enzymes) into
the hollow tubes.
The oxidation may be followed by gas-phase reactions with CO2, N2O, NO, NO2,
O3, and ClO2.[11]
Solution-state chemical oxidation, however, is found to be more efficient for the
purification and/or modification of
carbon nanotubes. Tsang et al.[12]
reported the liquid-phase oxidation of
carbon nanotubes in HNO3 in 1994.
Since then, various oxidants have been
shown to react with carbon nanotubes.
Oxygen-containing acids, including
HNO3, HNO3 + H2SO4, HClO4, H2SO4
+K2Cr2O7 , and H2SO4 +KMnO4[13-20]
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have been extensively used, though several other oxidants (e.g. OsO 4 , H2O2) can
also been used.
The oxidation reactions discussed above often generate various functional groups
(e.g. -COOH, -OH, -CO) at the opened end or defect sites of the carbon nanotubestructure (Scheme 1). Other groups may be also introduced due to side reactions.
For example, a small amount of sulfur-containing groups may be introduced onto
the H2SO4 /HNO3 -oxidized carbon nanotubes, as shown in Scheme 2.[17]
3.1.2. Covalent Coupling via the Oxidized Nanotube Ends
The carboxylic acid and hydroxyl
groups of the oxidized nanotubes
can be further used to covalently
connect other small and polymeric
molecules through reactions
characteristic of the -COOH and -
OH functionalities. In this context,
the reactions of molecular
fluorine, rhodamine B, and p-
carboxytetraphenylporphine (TPP) with oxidized nanotubes have been carried out,
following Scheme 3.[22]
Alkyl chains may also be covalently attached onto the oxidized nanotubes through
a simpler reaction between the carboxylic acid and amine groups, Scheme 5, using
a carboxylateammonium salt, which also improves the solubility of the
nanotubes. [23,24]
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Similarly long alkyl chains and/or polymers have been chemically attached onto
the oxidized nanotubes through, for example, an amidation reaction, Scheme 4. In
this case, the carboxylic acid functionalized nanotubes were firstly converted into
alkyl chloride by treatment with SOCl2; aryl amine then reacts with the alkyl
chloride to form an amide bond between the nanotube and the aryl group. The
attachment of long hydrocarbon chains onto carbon nanotubes improves the
nanotubes solubility significantly,[25,23] allowing the modified SWNTs to be
purified by conventional chromatographic techniques.[26]
Polymers with amino terminal groups, such as poly(propionylethylenimine-co-
ethylenimine) (PPEI-EI), have also been grafted onto oxidized nanotubes through
amide formation, Scheme 6. The resulting polymer-grafted SWNTs and MWNTs
are highly soluble in most common organic solvents and water.[27-29]
Apart from the amide linkage, the etherification reaction has also been used to
covalently attach alkyl groups and polymers onto oxidized carbon nanotubes. For
instance, octadecylalcohol [30] and poly (vinyl acetate-co-vinyl alcohol) (PVA-VA)
[27]have been successfully grafted onto oxidized SWNTs to improve solubility,
Scheme 7.
Interestingly, Sano et al.[31] reported the ring formation from the acid-oxidized
SWNTs through the esterification between the carboxylic acid and hydroxyl end-
groups at the nanotube tips in the presence of a condensation reagent, 1, 3-
dicyclohexylcarbodiimide (Scheme 9). Carbon nanotube rings, with an average
diameter of 540 nm and a narrow size distribution, were obtained.
Nanotube hetero-junctions with two carbon nanotubes joined in either an end-
to-side or end-to-end configuration have been constructed through a bi-
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functionalized amine linkage, as schematically shown in Scheme 10 and Figure
2.[32]
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3.2 Modification of the Carbon Nanotube Outerwall
3.2.1. Sidewall Fluorination of Carbon Nanotubes.
Although carbon nanotubes are generally known to be inert towards fluorine at
room temperature, chemical fluorination of carbon nanotubes has been achieved at
relatively high temperatures. The chemical fluorination of the carbon nanotube
wall at temperatures ranging between 250 and 400C has been achieved.[33]
Fluorination of SWNTs has been carried out in the same manner as fluorination of
graphite,[34]
indicating that there is considerable room for chemical
functionalization of carbon nanotubes under appropriate conditions. Fluorination
above 400C is found to cause the decomposition of the nanotube structure.
Fluorination between 250 and 400C does not damage the structure to that extent,
but it reduces conductivity of the SWNTs due to the partial destruction of the
graphitic structure. Thus, electronic properties of fluorinated SWNTs differ
dramatically from those of their unmodified counterparts. Covalently bonded
fluorine is removable from the fluorinated surface by treatment with hydrazine.
This fluorination reaction is of considerable interest because further
functionalization of carbon nanotubes can be carried out through the fluorine
intermediate. The covalently bonded fluorine can be replaced by alkyl groups
through the alkylation reaction [35], with alkyl lithium or Grignard reagent used as a
reagent. Interestingly enough, these alkyl groups can also be removed by heating
the alkylated nanotubes at about 250C, leading to the recovery of the pure
SWNTs. various physicochemical properties of the fluorinated SWNTs, including
high-resolution electron energy loss[36], thermal recovery behavior[37], and
solvation in alcoholic solvents[38], have been investigated.
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3.2.2 Dichlorocarbene.
As is well known, 1, 1-dichlorocarbene can attack C- C bonds connecting two
adjacent six-membered carbon rings to produce 1, 1-dichlorocyclopropane,
Scheme 11. This reaction has been investigated for both SWNTs and MWNTs
[24]
using 1, 1- dichlorocarbene generated from different sources, including
NaOH/CHCl3 and a mercury complex.
3.2.3 Aniline and Azomethine.
Reactions with aniline (Scheme 13)[39]
, and 1, 3-Dipolar Cycloaddition of
Azomethine Ylides (Scheme 12) [40] have also been carried out.
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3.3 Functionalization of the Carbon Nanotube Innerwall.
Compared with the outerwall modifications discussed above, the functionalization
of the nanotube innerwall is much less discussed in literature. This is becauseinnerwall modifications often require opening the nanotube tip(s) and protection of
the outerwall; these processes are complicated and tedious. In this regard, carbon
nanotubes synthesized by the template technique possess advantages towards
innerwall modification.[41]
The template technique involves direct deposition of carbon nanotubes, or their
precursor polymer nanotubes, followed by high-temperature graphitizing, within
the pores of a Nano porous template (e.g. an alumina membrane or a semi porous
zeolite). The template synthesis often allows the production of monodispersed
carbon nanotubes[42, 43]
with opened ends as well as controllable diameters, lengths,
and orientations.
Based on the template method for the nanotube growth, Kyotani et al.[41]
have
successfully carried out the nitric acid oxidation of nanotube innerwalls within the
pores of Nano porous
alumina, in which the
template acts as a protective
layer for the outerwall. Upon
the completion of the
innerwall oxidation, the
template was removed by
dissolving it in aqueous HF,
thereby releasing the
innerwall-modified
nanotubes (Scheme 15).
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3.4 Physical Chemistry of CNTs.
3.4.1. Mechanochemical Reactions
Mechanochemistry refers to processes in which mechanical motions control
chemical reactions.[44]
Specifically, the mechanochemical reaction involves highly
reactive centers generated by mechanical energy (e.g. ultra sonication or ball-
milling) imparted to the reaction system. Certain organic reactions have been
demonstrated to take place efficiently in the solid state [45]. Ultra sonication has
been demonstrated to significantly enhance many chemical reactions, including the
acid oxidation of carbon nanotubes[46]
. It was envisaged that the strong vibration
arising from the ultrasonication created many defect sites on the SWNT sidewalls
(both the inner and outer ones), which could not only facilitate the oxidation
reaction but also cleaved the SWNTs into short segments. Ultrasonication has also
been found to induce a chemical reaction between SWNTs and
monochlorobenzene in the presence of poly (methyl methacrylate) (PMMA)[47]
. In
particular, SWNTs could be separated from carbonaceous impurities and degraded
to short segments by ultrasonicating SWNTs and PMMA in chlorobenzene [47].
Apart from ultrasonication, various other physicochemical processes, including
microwave irradiation [48] and ball-milling[49], have also been investigated. In
particular, ball-milling of carbon nanotubes in the presence of acid was reported to
cleave the nanotubes into 200300 nm long ropes. [49]
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3.4.2. Electrochemical Reactions.
Owing to the good
electronic propertiesintrinsically associated with
carbon nanotubes [50],
electrochemical
modification of individual
carbon nanotube bundles
has been attempted. To
demonstrate this, Kooi et al.[51] electrochemically
polymerized a thin polymer
layer onto the individual
SWNT bundle (Scheme 16), which was pre-coated on a microelectrode. The
resultant polymer layer was directly observed to be of 6 nm thickness, by means of
in situ AFM imaging (Fig. 6).
3.4.3. Photochemical Reactions
Despite of the well-known fact that carbon nanotubes possess unusual
optoelectronic properties, the recent report on ignition of some dry and fluffy
SWNTs in air by the camera flash is rather intriguing[52]
. In view of the highly
efficient absorption of light by carbon nanotubes, the observed ignition may have
resulted from a rapid temperature increase within the nanotube sample upon
exposure to the flash (estimated to heat to 1500C in a very short time), which is
well above the temperature required for ignition when oxygen is present. While the
mechanism of ignition by a camera flash in this system deserves further
investigation, such an interesting photochemical reaction may find its application
in the remote light-triggered combustion or explosives[52]
.
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3.5. Non-covalent Chemistry of CNTs.
Non-covalent chemistry involves self-assembly of molecules or macromolecules to
thermodynamically stable structures that are held together by weak, non-covalentinteractions. These weak non-covalent interactions include hydrogen bonding,
stacking, electrostatic forces, van der Waals forces, and hydrophobic and
hydrophilic interactions. Non-covalent chemistry offers an important advantage in
not changing the carbon nanotubes structure, and hence its electronic and
mechanical properties are largely retained. A few methods have been devised
which are demonstrated below.
3.5.1. Non-Covalent Attachment of Small Molecules onto the Sidewall of
CNTs.
Chen et al. [53] have recently found that certain aromatic molecules with a planar -
moiety (e.g. pyrenylene) could strongly interact with the basal plane of graphite on
the nanotube sidewall via -stacking. The interaction is so strong that the aromatic
molecule is irreversibly adsorbed onto the hydrophobic surface of carbon
nanotubes, leading to a highly stable, self-assembled structure in aqueous
solutions. As schematically shown in Scheme 17, the long alkyl chains, end-
adsorbed onto the nanotube surface via the aromatic anchors, could significantly
improve the solubility of the nanotubes.
The above approach could be used to attach bio- chemically active molecules, such
as DNA and proteins, on the sidewall of SWNTs through appropriate anchoring
molecules. For example, some amine-containing bioactive molecules, includingferritin, streptavidin, and biotinyl-3, 6- dioxactanediamine, have been attached
onto the sidewall of SWNTs through nucleophilic substitution of the amino
functionality by N -hydroxysuccinimide group, which was pre-anchored onto the
nanotube surface through a succinimidyl ester linkage by pyrenylene (Scheme 18).
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3.5.2 Plasma Activation of Aligned Carbon Nanotubes
Dai and coworkers [54-56] have developed
a novel approach for chemical
modification of aligned carbon nanotubes
by carrying out radio-frequency glow-
discharge plasma treatment, and
subsequent reactions characteristic of
plasma- induced surface groups. They
successfully immobilized polysaccharide
chains onto acetaldehyde plasma
activated carbon nanotubes through the
Schiff-base formation, followed by
reductive stabilization of the Schiff- base
linkage with sodium cyanoborohydride
in water under mild conditions (Scheme
21).
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4. Conclusion.
This ends our discussion on the reaction chemistry involved in synthesis,
purification, and application-specific modifications of Single Walled Carbon
Nanotubes (SWNTs).
The various covalent and non-covalent chemistries that have been devised for
making sophisticated carbon nanotube materials of good bulk and surface
properties as demanded for specific applications. Judicious application of these
site-selective reactions to nonaligned and aligned carbon nanotubes has opened up
the rich field of carbon nanotube chemistry.
With so many covalent and non-covalent methods already reported, and more to be
developed, the possibility of actually producing functionalized carbon nanotubes
of the exact physicochemical properties suitable for practical applications will
soon be in sight. Continued research in chemistry of carbon nanotubes should
overcome some of the major hurdles that carbon nanotubes are facing in the race to
the technological marketplace.
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5. References.
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