Ecotoxicology of Manufactured Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 278

Metals such as copper and zinc are extremely toxic to aquatic organisms and effects
on aquatic biota start to become apparent at dissolved metal concentrations of
only a few g/l. Franklin et al. , (2007) compared the toxic effects of nanoparticulate

ZnO, bulk ZnO and ZnCl 2 to the freshwater alga Pseudokirchneriella subcapitata .
Chemical investigations using equilibrium dialysis demonstrated rapid dissolution
of the ZnO nanoparticles in a freshwater medium (pH 7.6), with a saturation solu-
bility in the milligramme per litre range, similar to that of bulk ZnO. Growth
inhibition experiments revealed comparable toxicity for nanoparticulate ZnO, bulk
ZnO, and ZnCl 2 , with a 72 h IC50 value around 60

µ

g/l zinc, which was attributable
solely to dissolved zinc. Consideration of published species sensitivity distributions
(Bodar et al. , 2005) indicates that 5 mg/l zinc is suffi cient to cause adverse effects
to a majority of aquatic species, including algae, invertebrates and fi sh. Thus, the
most likely cause of nano (or bulk) ZnO aquatic toxicity is via dissolution and not
necessarily through any specifi c particulate effects. Brunner et al. (2006) also noted
the importance of rapid ZnO dissolution and the toxicity of Zn 2+ in cytotoxicity
studies using human mesothelioma and rodent fi broblast cell lines.
Conversely, whilst 80 nm copper particles have been shown to be acutely toxic
to zebra fi sh ( Danio rerio ), with a 48 h LC 50 concentration of 1.5 mg/l in dechlori-
nated tap water (pH 8.2) (Griffi tt et al. , 2007), dissolution of the nanoparticles was
insuffi cient to explain the observed mortality compared to a dissolved metal control.
Furthermore, sub-lethal concentrations of nanocopper produced different morpho-
logical effects and gene expression patterns in the gill than soluble copper, demon-
strating that the toxic effects were not mediated solely by dissolution. Thus,
dissolution is a potentially important mechanism of nanoparticle toxicity which
must be considered on a case by case basis, with appropriate controls included in
every study of sparingly soluble nanoparticulate materials.

µ

7.3.3.4 Generation of Reactive Oxygen Species ( ROS )

Due to their large surface area to volume ratio and high chemical reactivity,
nanoparticles have a greater propensity to generate reactive oxygen species (ROS:
oxygen ions, peroxides and free radicals) than bulk materials (Oberdorster et al. ,
2005 ; Nel et al. , 2006). Reactive oxygen species (ROS) may be generated by a
number of mechanisms (Figure 7.7) which are not fully understood (Oberdorster
et al. , 2005 ; Nel 2006 et al. ,) but may include: (i) material composition, for example
discontinuous crystal planes and defects generate active electronic confi gurations
on the particle surface which can react with molecular oxygen (O 2 ) to generate
superoxide ( O 2 ); (ii) interaction with particular environmental conditions, for
example UV activation; or (iii) the presence of redox active chemicals, either as
impurities on the particle surface or of environmental or biological origin (e.g.
transition metals or quinones), may lead to radical formation (Nel et al. , 2006 ).
ROS generation has been demonstrated for materials as diverse as fullerenes,
carbon nanotubes, quantum dots and metal oxides (Oberdorster et al. , 2005 ; Sayes
et al. , 2005 , Derfus et al. , 2004 ; Sawai et al. , 1996) and has been shown to occur both
in cell free systems (Brown et al. , 2001 , Sayes et al. , 2004 , Foucaud et al. , 2007 ) and
in vivo (Li et al. , 2003 ).