Human Toxicology and Effects of Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 374

chemical surface coating have been demonstrated to be less toxic than positively
charged, amine coated quantum dots. The amine coated quantum dots appeared to
be less stable over time than the carboxylated quantum dots (Hoshino et al. , 2004b ).
Shiohara et al. (2004) also found that positively charged CdSe/ZnS quantum dots
exhibited reduced stability. It was further observed by Shiohara et al. (2004) that
CdSe/ZnS quantum dots were highly toxic irrespective of coating and induced cell
death even at low concentrations. Lovric et al. (2005b) also investigated unmodifi ed
quantum dots with a cadmium telluride (CdTe) core. In a breast cancer cell line,
they identifi ed damage to the plasma membrane, mitochondria and nucleus. Pre-
treatment of the cells with antioxidants prevented the quantum dot induced toxic-
ity, suggesting a role for ROS. The same group also identifi ed that the localisation
of the CdTe quantum dots in neuronal cell lines was dependent upon particle size
(Lovric et al. , 2005a). In addition, the smaller quantum dots were more toxic as
associated by membrane blebbing and chromatin condensation. Again antioxidants
provided protection against the quantum dot induced cytotoxicity (Lovric et al.
2005a ).

9.4 Relating Physico - Chemical Properties to Toxicity:
Structure - Activity Relationships

The question 'what is so special about nanoparticles that makes them toxic?' is
one that toxicologists working in this area have heard many times. Nanoparticles
are manufactured because they possess properties that are not present in the bulk
material or larger particles. Such properties include surface reactivity, light emission
and electrical conductivity. These properties occur for a number of reasons,
including the high proportion of atoms at the particle surface (Chapter 2). The
more atoms there are at the particle surface, the greater the number to participate
in reactions with other molecules in the environment or in the body. For example,
a 100

m particle has 0.0001% of its atoms at the surface, while a particle of the
same chemical composition but just 10 nm in size has 10% of its atoms at the
particle surface (Stone et al. , 2007). Furthermore, as the particle size becomes
smaller there is less room for electrons to orbit around each atom within the
particle; restriction of this movement means that the atoms change their behaviour
and therefore the particle characteristics. In addition, bond angles between atoms
within a nanoparticle may be restricted, so that they are not optimum for stability,
especially below 5 nm in diameter. This again results in instability and the potential
for increased reactivity (Stone et al. 2007). Therefore, the very properties for
which nanoparticles are manufactured and exploited are also the factors that are
likely to drive their biological reactivity. In order to determine the relationship
between physico-chemical characteristics it is essential that particles used and
published in toxicology studies are well characterised in terms of size, surface area
and composition.
Our own work has shown that surface area is important in driving the ability of
low solubility, low toxicity nanoparticles to induce infl ammation in the rat lung.
Nanoparticles and larger particles of carbon black, polystyrene beads and TiO 2

µ