Chemistry Reference
In-Depth Information
4.2 Noncovalent Methods
Noncovalent functionalization of CNTs is based on their indirect modification with
different chemical species, mainly by means of
and hydrophobic interactions.
The exploration of alternative approaches to the covalent modification arises
from the necessity to exfoliate CNTs bundles. In the search for a non-invasive and
efficient way to exfoliate bundles, a large number of molecules has been considered
and classified into four different groups: (1) surfactants, (2) small aromatic
molecules, (3) biomolecules, and (4) polymers.
(1) Nanotubes can be suspended in aqueous media in the presence of surfactants,
such as sodium dodecylsulfate (SDS) or sodium dodecylbenzene sulfonate (SDBS),
whose hydrophobic tail is believed to interact with the tubular graphene surface.
The interaction with the amphiphile is even stronger when the hydrophobic portion
is aromatic [
118
,
119
]. As a matter of fact, (2) pyrene and porphyrin derivatives
do interact with the sidewalls of CNTs by means of
ˀ
-
ˀ
interactions. In addition,
it was demonstrated how the noncovalent association opens the door to a novel
family of electron donor-acceptor nanohybrids [
120
].
(3) Biomolecules can be associated supramolecularly with CNTs in both
aqueous and organic solutions. A remarkable increase of CNT water-solubility is
produced due to the formation of wrapped complexes in which nanotubes are
embedded.
The presence of hydrophobic regions on the proteins plays a fundamental role in
their adsorption on nanotubes, as shown for the immobilization of metallothionein
and streptavidin [
121
,
122
].
An important proof of concept was given by Erlanger et al. who reported the
spontaneous recognition between a monoclonal antibody against C
60
and SWCNTs,
efficiently characterized by the analysis of the antibody binding sites, made by a
cluster of hydrophobic aminoacids [
123
].
(4) Polymers have also been employed to wrap CNTs. The choice of using a
polymer is motivated for several reasons. First, the formation of supramolecular
complexes with CNTs occurs almost spontaneously in organic solvents such
as CHCl
3
, avoiding time-consuming procedures. Second, atomic force microscopy
(AFM) has shown not only that polymers homogeneously coat the tubes all
over their surface but also that they mostly isolate each nanotube from the
others, breaking up the bundles. Last, water-stable solutions of SWCNT-polymer
complexes
ˀ
-
ˀ
are
simply achievable by virtue of
the
ability to append
polar side-chains on the polymer
(such as polyvinylpyrrolidone -PVP- or
polystyrenesulfonate -PSS-) [
124
].
Among the polymers, one of the most widely used for wrapping CNTs is poly
(
m
-phenylvinylene) (PmPV), since this polymer contains aromatic repetition units
that strengthen the interaction with the graphitic layer [
125
].
Noncovalent functionalized SWCNTs with lipopolymers covalently attached
to siRNA with a cleavable disulfide bond (Fig.
27
) were used to transfect human
cells in order to silence the expression of HIV specific cell surface receptors [
126
].
