TOXICITY OF CARBON NANOTUBES - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 230

Moreover, the inluence of diameter on cell viability is not clear. In
previous studies it was reported that MWCNTs with diameters of 60-100 nm
induced more serious cytotoxicity on alveolar macrophages in comparison
with smaller-diameter MWCNTs (<60 nm). 37 Conversely, in another study,
toxicity of SWCNTs with a mean diameter of 1.4 nm was much higher than
MWCNTs of 10-20 nm, with serious damage of alveolar macrophages at doses
as low as 0.38 μg/mL. 38 In our experiments, the effect was strictly dependent
on the cells under investigation (Fig. 8.2), although it was not particularly
determinant ( p > 0.05): HL60 and U937 cells displayed a slightly better cell
viability than MCF7, especially in case of SWCNTs. One explanation could be
that HL60 and U937 cells are suspended cells of smaller dimensions (2-10
μm) than MCF7 cells (30-40 μm), and thus they could accommodate SWCNTs
much better than MWCNTs. On the other hand, the adherence of MCF7 cells
and their bigger dimensions seemed more inluenced by the better dispersion
of MWCNTs. Another possibility could be that MWCNTs, being more easily
suspended, have a more prolonged contact with suspended cells (HL60 and
U937) than with adherent cells (MCF7). Hence, we could conclude that the
tubes' diameter does not play a crucial role in the cytotoxic proile.
Such difference could also be attributed to the method employed for
the cell detection; that is, MTT assay could be more sensitive towards HL60
cells. However, cell viability evaluation through an alternative Cyquant assay
demonstrated the same proile ( p > 0.05), 7 thus excluding any signiicant
interference deriving from the method of analysis employed.
8.2.5 Purity
Several methods adopted for the production of CNTs involve the use of metals
as catalysts 39 which, if present in high amount, enhance cytotoxicity. 38 In fact,
free iron or nickel and transition-metal complexes are known to originate
reactive radicals, which could induce radical oxidation and enhance oxidative
stress to the cells.40
In order to investigate this aspect, Shvedova et al. identiied and quantiied
the reactive species involved during the manipulation of carbon-based
nanomaterials; more precisely, they evaluated the effects of pristine SWCNTs
on human epidermal keratinocytes (HaCaT). 41 Images under scanning and
transmission electron microscopy clearly indicated that oxidative stress,
induced by treatment with SWCNTs, caused a change in the morphology of
cells and altered surface integrity. The same hazardous effects were reduced
by the subsequent use of an iron chelator that displayed a protecting role
towards HaCaT cells and, thus, conirmed that cytotoxicity of SWCNTs was
mainly correlated with iron catalytic effects.