TOXICITY OF CARBON NANOTUBES - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 231

Jia et al. investigated the cytotoxicity determined by some nanostructures
such as SWCNTs, MWCNTs and fullerenes on alveolar macrophages (AM). 38
The different carbon particles were stably suspended in RPMI medium and
sonicated for 20 minutes with a Dounce homogenizer before usage. Although
all the samples had a purity superior than 90%, they showed remarkably
different effects on the phagocytic ability of AM after 6 hours' exposure
to carbon materials. More precisely, at low doses (1.41 μ g/cm 2 ) SWCNTs
displayed a high cytotoxic effect, corresponding to >20% inhibition of cell
growth in a dose-dependent trend. MWCNTs and fullerenes were instead
much less toxic. One possible explanation for such a deep variation could
be attributed to the purity level of SWCNTs that was lower than the other
samples. Indeed, residual amorphous carbon and trace amount of metallic
catalysts, such as Fe, Ni and Y, were present in SWCNT samples. Similarly,
another investigation demonstrated that lower puriied CNTs induced
cytotoxicity, while homogeneously suspended CNTs with high purity did not
alter cell growth. 42
Interestingly, one of our samples, devoid of functional groups,
without catalysts (the initial MmNi 3 was not detected once subjected to
thermogravimetric analysis) and negligible amorphous carbon, 43 showed no
sign of toxicity (Fig. 8.3) in the concentrations normally used for cell viability
assays (10-50 μ g/mL); for that reason, doses were increased, and the irst
evidence of toxicity was detected only at concentrations above 150 μg/mL.
To our knowledge, very few articles have ever reported cytotoxic proiles
testing such high doses, analogue compatibility with cells in vitro and high
purity by energy-dispersive X-ray spectroscopy (EDS) (further data available
in Ref. 7), suggesting that further functionalisations could be introduced in
this last sample for a better cell targeting but without harmful consequences.
This result not only allowed us to identify the sample's purity as the most
crucial parameter that is responsible for the toxicity of CNTs, but it also
encourages further application of these non-toxic nanomaterials in the ield
of nanomedicine.
8.3 ENVIRONMENTAL EXPOSURE
Carbon nanotubes' needle-like fibre shape has been recently compared to
asbestos 44 : previous studies in populations exposed to this material showed
that the main body of the lung was a target for asbestos fibres, resulting
in both lung cancer and scarring of the lungs (asbestosis). Therefore, the
analogy between CNTs and asbestos has resulted in huge concerns since
CNTs' widespread use may lead to analogue inflammation and formation of
lesions known as granulomas.