Biomedical Engineering Reference
In-Depth Information
of the test NMs (e.g., Ag, ZnO, multiwall CNTs), suggesting the use of longer-
term assays such as biomass changes. In addition, testing issues regarding
the genotoxicity of NMs have been raised as well [141,142]. Unexpected inter-
actions with test reagents (e.g., colorimetric and fluorimetric dyes) generate
misleading data sets. Similarly, a nanogenotoxicity test of NMs such as met-
als and their oxides on bacteria indicates that the current microbial assays
(e.g., Ames) may not be suitable, possibly owing to the presence of the bacte-
rial cell wall that prevents the entry of NM into the cells [141]. The most sen-
sitive assays were the comet and the micronucleus assays, using eukaryotic
cells. The use of a battery of standard genotoxicity tests is recommended,
including in vivo assays to correlate in vitro r e s u lt s [141].
10.4 Mechanisms of Ecotoxicity
On the basis of available studies, NPs may act on biological systems by
known pathways (as described below) depending on their chemical surface
characteristics and chemical reactivity related to their increased surface area.
Physical effects, such as shading, have been postulated as a mechanism of
growth inhibition, exemplified by adsorption of nano-TiO 2 to algal cells [10].
Several authors demonstrated the relation between NM size (surface area)
and magnitude of the toxic outcome, generally using binary physico-chemical
comparisons (e.g., nanosized vs. bulk, or ultrafine vs. fine). For example, Van
Hoecke et al. [88,95] recently demonstrated a relation between surface area
and toxic effect, using SiO 2 and CeO 2 NP (all <30 nm diameter). Algal toxici-
ties differed when expressed on a mass basis, but these effects were identical
among materials of different sizes when corrected for surface area.
Differences in toxicity between bulk and nanosized materials have shed
some light on modes of toxic action. In metals (Ag, Cu, Zn) and metal oxides
(CuO, ZnO), toxicity could be related to particle solubility (or solubilization
rate). Owing to the high solubility, all forms of ZnO have the same toxicity
for a given ecoreceptor [5,90,110]. For less soluble materials, nanosized par-
ticles are generally more toxic than the bulk counterpart, because of their
increased specific surface area, leading to increased chemical surface reac-
tivity and possibly ROS generation. Auffan et al. [143] reviewed in depth
the size effect and suggested that the size limit for the change from bulk
to nano properties could lie around 30 nm, causing the difference between
small particles and NPs. The atypical surface structure (e.g., crystallinity for
inorganic compounds) and reactivity of NPs may enhance chemical pro-
cesses including dissolution and redox reactions (such as ROS generation)
compared with bulk materials. Also, environmental parameters such as dis-
solved organic carbon and ionic strength can affect the toxicity of metals for
aquatic receptors [35].
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