Human Toxicology and Effects of Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 364

9.3.1 Fullerenes

The Buckminster fullerene, also known as C 60 , was fi rst discovered by Kroto et al.
(1985). This nanoparticle consists of 60 linked carbon atoms in a highly stable ico-
sahedron structure, with 60 vertices and 32 faces (12 pentagonal and 20 hexagonal).
The term fullerene now applies to molecules composed entirely of carbon that form
spheres or tubes. The use of C 60 is being investigated for use in optics and super-
conductors (Da Ros and Prato, 1999) and for drug delivery (Vogelson, 2001).
Cosmetic products such as face creams containing C 60 nanoparticles have also
entered the market (Halford, 2006), with the suggestion that they contain antioxi-
dant properties (see below). C 60 particles are made on a large scale; for example
Mitsubishi has opened its fi rst fullerene plant in Japan which aims to produce
fullerenes by the ton (Tremblay, 2003).
A number of studies provide evidence that C 60 can act as an antioxidant. For
example, Gharbi et al. (2005) found that C 60 was able to prevent carbon tetrachlo-
ride (CCl 4 ) induced liver toxicity in the rat. In a different study, lipid and water
soluble C 60 derivatives prevented superoxide and hydroxyl radical initiated lipid
peroxidation to a greater extent than the natural antioxidant, vitamin E (Wang
et al. , 1999). Using electron spin resonance (ESR), Xiao et al. (2006) also demon-
strated that chemically generated hydroxyl radicals were quenched by polyethylene
glycol (PEG) - modifi ed and hydroxyl fullerenes. C 60 fullerenes and single-wall
carbon nanotubes (SWCNT) have also been shown not to induce measurable pro-
duction of the reactive nitrogen species, nitric oxide, by a mouse macrophage cell
line (Fiorito et al. , 2006). These studies together suggest that C 60 and its derivatives
could actually be of benefi cial health effect, rather than induce toxicity.
However, toxicity depends upon the dose (as stated in the introduction) as well
as the environment in which the ' antioxidant ' is investigated. Many antioxidants
have the capacity to act as oxidants in the right conditions (e.g. vitamin C). Therefore,
it is not surprising that other studies have presented data suggesting that C 60 and
its derivatives have pro-oxidant and toxic effects. Sayes et al. , (2005) looked at the
effects of nano - C 60 prepared in tetrahydrofuran (THF) on three cell lines. The
nano - C 60 was cytotoxic to all three cell types at doses above 50 ppb, with LC 50
ranging from 2 to 50 ppb depending on the cell type. In this study nano-C 60 induced
membrane damage prevented by the addition of an antioxidant, confi rming a role
for ROS. Note that a number of ecotoxicology studies have also used THF as a
solvent to disperse C 60 (Oberdorster, 2004), but it has been suggested that the THF
may infl uence toxicity (Brant et al. , 2005), and so this data requires careful consid-
eration. Markovic et al. (2007) compared C 60 prepared in THF, ethanol and water
in terms of their ability to generate ROS, cause mitochondrial depolarisation and
necrotic cell death in a variety of cell lines. The THF preparation was found to be
most potent, followed by the ethanol preparation, while the water suspension was
least potent.
Some researchers have suggested that the concentration of THF remaining in
such a C 60 preparation would deliver a fi nal dose in an organism that is below
concentrations normally associated with THF toxicity. However, this suggests
that the concentration of THF is distributed throughout the body as if it were