TOXICITY OF CARBON NANOTUBES - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 219

properties often inluenced by several variables, including size, chemical
composition, solubility and aggregation. 1 These parameters could affect
cellular uptake and protein binding and cause tissue damage. At a more
general level, one severe drawback is represented by the inability to fabricate
structurally and chemically controlled CNTs with identical characteristics in
terms of properties and contaminants amount, and this has also limited their
clinical and pharmacological applications. Moreover, differently from mono-
dispersed particles, their shape belongs to both ibres and nanoparticles,
thus triggering some unpredictable effects. The potential risk associated
with CNTs includes the toxicity of eventual impurities (e.g., metal catalysts)
in the samples, and the possibility, due to their small dimensions, that CNTs
escape from the normal phagocytic defences and deposit into organ and
tissues, with hazardous effects on the body. CNTs commonly show different
levels of purity, which are strictly dependent on the methods employed for
their production. The impurities are essentially made up of residual catalysts
and amorphous carbon. If present in high amount, they might enhance the
toxicity 2 and determine unwanted effects.
Many articles have indicated that CNTs are toxic, proposing some
valuable reasons, which range from reduced dimensions (which enable them
to be subtracted from common scavengers in our body) and hydrophobic
nature (incompatible with physiological luids) to impurities (dificult to be
eliminated even after CNTs manipulation), problems of dispersion (which
might form sediments on the cell culture and therefore reduce cell viability)
and functionalisations. 3-6 Nevertheless, in the most of the cases, data showing
toxicity had been obtained from non-functionalised samples, rather than
ultrapure or chemically modiied nanotubes; therefore, we have recently
reported on the investigation of crucial parameters that might be responsible
for their cytotoxicity. 7 These factors have been summarised in the following
paragraphs, and they include surface functionalisation, concentration, purity
and so forth.
8.2 PARAMETERS RESPONSIBLE FOR THE TOXICITY OF CNTS
8.2.1 Surface of CNTs
Up to now, the size cut-off below which particles are surely toxic has not been
deined. However, it has been observed that the smaller the particles, the more
toxic they become. This is due to the fact that there is more surface area per
mass unit. As a consequence, any intrinsic toxicity of the surface will deeply