Biomedical Engineering Reference
In-Depth Information
circumference. Because the deformation energy of the sp
2
bond is inversely
proportional to the diameter of the CNT, nanotubes with a smaller diameter
have higher reactivity (Hirsch, 2002).
However, whereas the nanotube end-caps are quite reactive due to their
fullerene-like structure, even taking into account the folding of the graphene
sheet, the reactivity of CNT sidewalls remains low and sidewall-functiona-
lization is only successful if a highly reactive reagent is used. As mentioned,
an additional constraint on sidewall functionalization is the tendency of
CNTs to form bundles, which limits the available nanotube surface for the
grafting of chemical reagents.
Typically, the covalent sidewall functionalization is carried out in organic
solvent, which allows the utilization of the sonication process to improve the
dispersion of CNTs and, thus, to increase the available surface of carbon
nanotubes. However, precipitation immediately occurs when this process is
interrupted (Tasis et al., 2006). The required reactive species such as
carbenes, nitrenes or radicals are in general made available through
thermally activated reactions (Balasubramanian and Burghard, 2005).
Normally, the grafting reaction can be initiated exclusively on the intact
sidewall or in parallel at defect-sites. The most common sidewall
functionalizations using organic solvents, such as carbene (Lee et al.,
2001) or nitrene (Holzinger et al., 2004) [2+1] in cycloaddition reactions or
radical additions via diazonium salts (Bahr et al., 2001), are shown in
Fig. 14.4.
The first sidewall functionalization studied was the fluorination of CNTs.
Carbon nanotubes were fluorinated using fluorine in the range between
room temperature and 600
C (Mickelson et al., 1998). This reaction is very
useful because further nucleophilic substitutions can easily be accomplished
and make possible a flexible approach to provide the CNT sidewalls with
various types of functional groups (Khabashesku et al., 2002).
Among these reactions, several diamines are reported to react with
'fluoronanotubes' via nucleophilic substitution reactions (Stevens et al.,
2003), leading to the formation of amino-functionalized CNTs. However, if
bifunctional reagents such as diamines with sufficiently long carbon chains
are used, the nanotubes can be covalently cross-linked with each other.
Modification with amino-containing substituents was developed using
photolysis of acetonitrile (Nakamura et al., 2008). The 1,3-dipolar
cycloaddition reaction can be also used to graft linkers, with amino groups
at their ends, uniformly distributed along the sidewalls (Pantarotto et al.,
2003).
As discussed before, the carbon nanotube covalent functionalizations
carried out in organic solvent present some drawbacks. The need to agitate
carbon nanotubes by sonication to improve their dispersion in organic
solvents can trigger severe damage to the tube walls and the average length
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