TOXICITY OF CARBON NANOTUBES - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 228

8.2.3 CNTs' Dispersibility
Due to their hydrophobic nature, CNTs tend to aggregate in large bundles
especially in media commonly used for toxicological testing. 29 Dispersion
in aqueous solvents can be improved, 3,30 for example, through sonication,
stabilisation with surfactants and CNTs' functionalisation. In previous studies
it has been shown that functionalisation is more effective than surfactants
in reducing CNTs' cytotoxicity, 11 while prolonged sonication can cut the
tubes excessively and thus decrease their aspect ratio, which is a desirable
physicochemical property of CNTs. 31 Functionalisation of CNTs, besides
improving water dispersibility compared with their pristine counterparts, 24
offers the advantage of incorporating biologically active moieties such
as antibodies and drug molecules. 32 Physiological solutions are the most
appropriate, but sometimes the low solubility of carbon material requires
organic solvents. It has already been reported that human keratinocytes,
once exposed to nanotubes, displayed cell death. 33 This was attributed
to oxidative stress within cells and to activation of NF- k B factor. However,
dimethylformamide (DMF) was used to dissolve the samples, and it might
have induced some alterations, since it is a toxic solvent, which should
be avoided in cell manipulation. Similarly, in our investigations we have
maintained di-methyl sulfoxide (DMSO) in concentrations as low as 1% (vol/
vol) of the inal volume, to avoid any deceptive result. DMSO, together with
CNTs' functionalisation, improved the tubes' dispersibility, indicating the
maximum weight of nanotubes that could be added with minimal formation
of aggregates. To reiterate, aggregates of f -CNTs should be kept to a minimum
as they could cause cell death. 34
Moreover, we observed better dispersibility and loading for f -MWCNTs in
comparison with f -SWCNTs (this was statistically signiicant, with p < 0.05);
this could be attributed to the wider surface exposed in the case of multi-
walled tubes, suggesting that further sidewall functionalisation caused an
increase in f- CNTs concentration that could form stable suspensions.
8.2.4 Length and Diameter
Besides the above-mentioned factors, length could also be involved in the
intrinsic toxicity of CNTs. It has been demonstrated that shorter CNTs (220
nm) displayed lower inlammatory effects compared with longer CNTs
(825 nm). 4 Clusters of both samples were surrounded by macrophages,
due to activation of innate immunity. 4 The shorter tubes displayed a lower
inlammatory response, suggesting that tubes' length plays a remarkable role
in toxicity. In both cases, no severe effects, such as necrosis or degeneration,
were observed around CNT throughout the experimental period of 4 weeks.