DATA: THE NEW CORPORATE RESOURCE - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 14

were then subjected to oxidation, which led to the formation of carboxylated
CNTs; these were further modiied to obtain reactive acyl chloride groups,
used for the functionalisation with chitosan of 9 kDa 62 and 100 kDa. 63
Successful covalent bonding between the polysaccharide and the fullerene
pipes was demonstrated by Fourier transform infrared spectroscopy (FTIR),
X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance
(NMR). 62 The data collected showed that both the chitosan amino group
in C2 and the hydroxyl group in C6 reacted with the carboxylic groups on
the CNTs' surface when chitosan with a degree of deacetylation of 84.7%
was used, 62 while only the amino groups reacted when a completely
deacetylated chitosan was employed. 63 Furthermore, H-bonding between
the free amino groups on the polymer and the free carboxylic groups on
the nanotubes' sidewalls has been shown to increase the stability of the
binding as well as the crystallinity of the polymer. TGA data showed that 58%
wt of the novel composite 62 was formed by the polysaccharidic polymer as
opposed to the 11 - 17% wt yield obtained by non-covalent complexation. 58
From the studies carried out so far, it is evident that covalent modiication
of CNTs with chitosan can lead to higher modiication degrees. However,
further studies would need to be undertaken to verify whether the extra
time and cost involved in the covalent binding process is justiied by a real
advantage in the characteristics of the inal product, as non-covalent binding
has shown to provide very stable suspensions by employing a much simpler
method. On the other hand, it is also worth investigating new improved and
more convenient methods for the covalent modiication of CNTs - for example,
the method described by Yu et al. 64 that employs microwaves. Carboxylated
MWCNTs were obtained by a 1 h procedure involving 30 min of sonication
and 30 min of reaction under microwaves of a suspension of MWCNTs in 70%
HNO 3 ; the so-obtained MWCNTs-COOH were then reacted with a solution of
chitosan for 20 min in a microwave oven affording a composite with a chitosan
content higher than 25% wt.
Besides the chosen method of preparation, the molecular weight of the
polymer used affects the characteristics of the composite. In literature we
ind evidence of the fact that increasing the molecular weight of chitosan
from 20 to 200 kDa can improve the suspension eficiency from ~36% to
~47%, according to UV absorption measurements. 51 Long et al. 58 reported
that when using a water-soluble chitosan derivative, the carboxymethylated
chitosan, of increasing molecular weight, they obtained MWCNTs with
increasing thickness of coating, respectively 1 - 2 nm for a 7 kDa polymer and
3 nm for a 17 kDa polymer. These data are in good agreement with those
obtained for the thickness of coating using other chitosan derivatives, namely
trimethyl chitosan chloride (TMC) 57 and N -octyl- O -sulphate-chitosan (NOSC)