TOXICITY OF CARBON NANOTUBES - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 235

of adhesion and grooming can lead to active ly-borne nanoparticle transport.
This also conirms our indings concerning the MWCNTs' better dispersibility
proile and the lower cell loss in comparison with SWCNTs.
Interesting preliminary epidemiological data were also provided by Murr
and collaborators, 55 who indicated a correlation between asthma (or related
lung inlammations) incidence and exposure to “gas stoves”, especially
because indoor number concentrations for MWCNT aggregates (of about 1-2
μm in dimension) is at least 10 times the outdoor concentration, and virtually
all gas combustion processes are variously effective sources. These results
also raise concerns for manufactured CNT aggregates and related fullerene
nanoparticles.
A crucial aspect to consider is that in most of the cases just reported,
the observations do not correspond to samples chemically modiied with
different chains and molecules, since they mainly consist of unreined or as-
produced material. Therefore, as we have already mentioned, the tubes that
have undergone further functionalisations display remarkable differences
in comparison with raw material, but their toxicological impact has been
poorly investigated. As a conirmation of this, Kostarelos and collaborators 56
have recently shown that intravenously administrated pristine SWCNTs
accumulated mainly in lung, liver and spleen, while the functionalised tubes
tended to persist much less in tissues and organs (Fig. 8.5 and Table 8.3) ,
with an accumulation proportional to the degree of functionalisation but
independent of the characteristic of the attached groups. 57 For the purpose,
water-soluble SWCNTs were functionalised with the chelating agent
diethylenetriaminepentaacetic acid (DTPA) and labelled with indium ( 111 In)
for imaging purposes. Radioactivity tracing indicated that the tubes were not
retained in any of the reticuloendothelial system organs (liver or spleen) of the
mice. Nearly all the tubes were excreted, through the normal renal pathway,
as intact tubes in the urine, without any remarkable tissue damage even
at high concentrations (max 400 μg/mouse) of tubes. The results could be
explained by the fact that unfunctionalised CNTs are extremely hydrophobic
and dificult to disperse in aqueous milieu owing to the van der Waals forces
leading to aggregation in bundles, while functionalisation allows to obtain
more individual tubes.
On the whole, despite some encouraging results and the lack of absolute
substantiation of toxicity directly associated with CNTs, speciic precautions
should always be adopted while handling such nanomaterial.