Chemistry Reference
In-Depth Information
Fig. 22 Dendrimer-functionalized SWCNT. The dendrimers are linked directly to the SWCNT
surface using a divergent methodology
Highly functionalized CNTs can be obtained via 1,3-dipolar cycloaddition of
azomethine ylides (Fig.
21
), generated by thermal condensation of alfa-aminoacids
and aldehydes [
87
]. This strategy leads to the attachment of a wide variety of
substituents on the sidewalls, according to the nature of the aldehyde and the
aminoacid chosen for the generation of the ylide. In this way, covalent linkage
of several molecular species has been described, such as triethylene glycol chains,
ferrocene moieties, phenol groups, or polyamidoamine dendrimers directly
synthesized onto the surfaces of pyrrolidino-functionalized tubes (Fig.
22
)[
49
,
88
-
91
].
Due to the lower toxicity of functionalized CNTs as compared to pristine CNTs,
these systems have shown great potential in nanomedicine. Thus, using azomethine
ylide chemistry, CNTs have been modified with aminoacids and peptides, anti-
bodies, and other biological entities, suggesting that functionalized tubes have
potential as drug delivery vehicles [
92
,
93
].
Organic functionalization via azomethine ylides has also been exploited for
different purposes, such as the purification of the raw material from the byproduct
impurities, which is performed by annealing the functionalized tubes at high
temperatures (300
C) yielding a final material free of amorphous carbon and almost
free of metal content [
94
].
Despite the high versatility of sidewall functionalization methods, a severe
limitations are the great amount of solvents needed to disperse CNTs and the
long reaction times necessary to perform the reactions. An alternative approach is
the use of microwave irradiation to activate CNT reactivity. Strong absorptions
observed after CNTs exposure to microwave radiations have been explored by
different groups in order to assist CNT functionalization [
95
]
.
Langa and co-workers described the 1,3-dipolar cycloaddition of nitrile imines
and nitrile oxide to the sidewalls of modified SWCNTs using microwave activation
[
96
]. Our group has shown that a solvent-free technique, combined with microwave
irradiation, produces functionalized nanotubes by using 1,3-dipolar cycloaddition
of aziridines in just 1 h of reaction, an extraordinary time reduction when compared
