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Fig. 29 SWCNT covalently attached to an anticancer drug and biotin as tumor-targeting system
In this way, two distinct functionalities coexist on the CNT surface. This approach
enables the simultaneous link of fluorescent probes to CNTs for tracking the uptake
of the material as well as an antibiotic moiety such as the active molecule. For
this purpose, our group chose fluorescein and amphotericin B (AmB), respectively.
AmB is considered to be a most effective antibiotic in the treatment of chronic
fungal infections [
132
]. The results were very promising as it was observed that
appropriate conjugation can increase the effectiveness of AmB, while decreasing its
toxicity to mammalian cells. Using a similar approach a biotin-SWCNT-linker
taxoid drug delivery system has been prepared. SWCNTs were covalently attached
to an anticancer drug and biotin as the tumor-targeting module (Fig.
29
).
The anticancer drug, taxoid, is readily released because of the linker that possesses
a disulfide bond that can be cleaved by endogenous thiols [
133
].
The covalent approach permits other interesting possibilities. Our group has
prepared multifunctionalized CNTs, using a combination of two different addition
reactions: the 1,3-dipolar cycloaddition of azomethine ylides and the addition of
diazonium salts both via a simple, fast, and environmentally friendly method. The
two reactions can be performed in series [
134
]. Results show that the radical arene
addition saturates more reactive sites than cycloaddition. Therefore, to attach two
different functional groups to SWCNTs, the order of reaction should be first the
cycloaddition and then the arene addition. This gives rise to comparable functiona-
lization degrees for the two additions. If only a few functional groups are required
along with an excess of other organic/inorganic groups, then the functionalization
order can be reversed. This latter could be the case of some biomedical applications,
where, for example, many more drug molecules are needed with respect to a
contrast agent (Fig.
30
).
Finally, it was shown that nucleophilic additions to the sidewalls of carbon
nanotubes are reversible and thus retrofunctionalization of these CNT derivatives
can be achieved under mild conditions. This equilibrium can be used in transfunc-
tionalization reactions, where one functional entity is partly exchanged for another,
giving access to CNTs with different functionality profiles [
135
,
136
] (Fig.
31
).
