Ecotoxicology of Manufactured Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 289

in the gill mucous. The implication was that long term exposure (1-2 weeks) effects
on gills and the associated respiratory distress would result in mortality. Cheng et
al. (2007) suspended SWCNTs by stirring in the absence of dispersants in their
study of the impact on embryos of zebrafi sh, Danio rerio . Not surprisingly, there
was considerable aggregation with clumps greater than 3 m in size, so clearly not

nanoparticles. While there was no mortality or embryonic developmental malfunc-
tions up to 360 mg/l, there was a hatching delay that may be attributable to the
presence of cobalt and nickel catalysts.
Tests on zebrafi sh embryos using 1.5 mg/l of THF - derived nano - C 60 showed only
45% survival after 96 hours compared to solvent controls. Interestingly, the addition
of glutathione signifi cantly reduced toxicity, as a consequence of its antioxidant
properties alleviating oxidative stress (Zhu et al. , 2007). Average aggregate size here
was considerably smaller than that of SWCNTs above, at 100 nm.

µ

7.5.3 Metal Oxides

Metal oxide nanoparticles are fi nding increasing application in a wide range of
applications and represent about one-third of the consumer products nanotechnol-
ogy market (Maynard, 2006). Applications for these materials include pigments in
paints (TiO 2 ), as sunscreens (TiO 2 , ZnO), in ceramics and catalysts (CeO 2 ) and as
antimicrobial agents (MgO). Concern over the toxicity of metal oxide nanoparticles
is related to their assumed persistence as small particles in the environment but in
fact their solubility is highly pH dependant and at pH 7.6 ranges from moderate
( 0.15 mg/l) for TiO 2 and CeO 2 (Rogers

et al. , 2007). Additionally, they can exist in different crystalline forms, for example
anatase or rutile TiO 2 , which may affect their toxic potential. Thus any studies
concerned with the toxicity of metal oxide nanoparticles must ensure that appropri-
ate controls for particle size and solubility are included in the experimental design
and, ideally, include information on the crystal structure and surface coating of the
particles.
A number of studies have examined the toxic effects of metal oxide nanoparticles
to bacteria, algae, daphnids and fi sh, although toxicity data for terrestrial species is
currently lacking or very limited (Table 7.2). The need for comprehensive charac-
terisation of the nanoparticle being tested and, more especially, for appropriate
controls is an issue for some of these studies. However, as standard ecotoxicological
test protocols are developed and measurement techniques become more available
these diffi culties should be overcome. To date, most ecotoxicological studies have
concerned TiO 2 and ZnO, due to the widespread use of these materials as pigments
and sunscreens. Additionally, the bacterial toxicity of SiO 2 , CeO 2 and MgO has been
explored by a number of authors (Adams et al. , 2006a, 2006b ; Thill et al. , 2006 ;
Makhluf et al. , 2005), due to the potential application of these nanoparticles as
bactericides.
Warheit et al. (2007) undertook to develop a base set of toxicity test for fi ne
(380 nm) and ultrafi ne (140 nm) TiO 2 in the rutile phase with an average size of
140 nm measured by dynamic light scattering (DLS). These tests included the
aquatic test species Pseudokirchneriella subcapitata (alga), Daphnia magna (water

> 80 mg/l) for MgO to extremely insoluble ( <