Biomedical Engineering Reference
In-Depth Information
14.2 Carbon nanotube surface chemistry
and solution-based functionalization
As mentioned above, even though chemical functionalization displays some
drawbacks, it still represents a very important method investigated by
several research groups. Hereafter are discussed some important results
recently reported in the literature.
Actually, the chemical functionalization of CNTs via covalent grafting
reaction can be divided into: (a) end-caps and 'defect-groups' chemistry; and
(b) sidewall functionalization.
14.2.1 End-caps and defect-sites chemistry
Functionalization via 'end-caps and defect-site' chemistry consists of
grafting functional groups directly on the already existing defects in the
structure of CNTs using wet chemistry. Carbon nanotubes are in general
described as perfect graphite sheets rolled into nanocylinders. In reality,
CNTs present structural defects: typically, ca. 1-3% of the carbon atoms of
a nanotube are located at a defect site (Hu et al., 2001).
The end-caps of nanotubes are composed of highly curved fullerene-like
hemispheres, which are much more reactive when compared to the sidewalls
(Sinnott, 2002). The sidewalls themselves contain defect-sites such as
pentagon-heptagon pairs called Stone-Wales defects, sp
3
-hybridized defects
or holes in the carbon sheet (Charlier, 2002). The most frequently
encountered type of defect is the Stone-Wales (or 7-5-5-7) defect, which
leads to a local deformation of the nanotube curvature. Grafting reactions
are most favored at the carbon-carbon double bonds in these positions. The
different defects commonly observed in CNTs are shown in Fig. 14.1
(Hirsch, 2002).
Frequently, the techniques applied for the purification of the raw
material, such as acid oxidation, induce the opening of the tube caps as
well as the formation of holes in the sidewalls. These vacancies and tube
extremities are therefore functionalized with oxygenated functional groups
such as carboxylic acid, ketone, alcohol and ester groups (Chen et al., 1998).
The introduced carboxyl groups can present useful sites for further
modifications in organic solvents such as the coupling of molecules through
the creation of amide or ester bonds. Figure 14.2 represents common
functionalization routes on single-wall CNT ends and defect-sites through
solution-based chemistry (Banerjee et al., 2005).
Using solution-based chemistry, nanotubes can be grafted with a wide
range of functional moieties via amidation or esterification reactions, for
which bifunctional molecules are often utilized as linkers/coupling agents.
This method was used to graft amine moieties onto carbon nanotubes via
