Biomedical Engineering Reference
In-Depth Information
Interestingly, prokaryotes (e.g., microbial fauna), in contrast to eukaryotes,
may be protected against the uptake of many types of NPs because they do not
have mechanisms for colloidal transport across their cell wall; they are never-
theless susceptible to toxic effects mediated by ROS production (C
60
+ UV light,
TiO
2
), metal ion leakage (Ag
+
, Cu
2+
, Zn
2+
, Cd
2+
, Ni
2+
), or even to the generation
of by-products [14,39,144,145]. Because the release of Cd
2+
did not explain the
observed toxicity of quantum dots, it was suggested that other possible major
contributors to bacterial toxicity could include TeO
2
and CdO (or in some cases
SeO
2−
ions) [144]. Further studies indicated that in intact quantum dots, the main
mechanism of bactericidal action is ROS production, and not Cd
2+
release [78].
Algal nanotoxicity is also related in part to metal dissociation or leakage (e.g.,
ZnO) and/or to ROS production (e.g., TiO
2
in the presence of light), or to specific
particle-membrane interactions (Ag), depending on the type of NM [10].
It is possible that some invertebrates and other animals may share com-
mon modes of toxicity, except that exposure may be increased through the
biomagnification phenomena, as discussed later. Filter-feeding invertebrates
(e.g.,
Daphnia
) were found to be more sensitive to nanometals than zebra-
fish. This may be explained by the separate feeding strategies employed,
daphnids being particulate filter feeders that would be expected to be more
intimately exposed to large numbers of particles during exposure [30]. This
difference in sensitivity could not be verified for other NMs because of a lack
of data. Soluble forms of the metals (salts) were more toxic to
Daphnia
species
(e.g.,
D. pulex
) than the particulate metals using mass-based concentrations
of exposure. In contrast, the same metal salts were less toxic to
Ceriodaphnia
sp. (e.g.,
C. dubia
). In the latter case, nano-Ag or nano-Cu may exert their tox-
icity through ROS production or other unidentified mechanisms.
In some cases in plants, the cell wall limits the passage of NMs; however,
the NMs may be translocated from roots to shoots depending on the type
of material [56]. Similar to bacteria, plant cells are sensitive to the released
metal ions (Ag
+
, Cu
2+
), as well as to ROS generation [56]; however, the uptake
of toxic NPs appears to be limited. For instance, zucchini exposed to 1000 mg
L
-1
nanosilver in hydroponic conditions contained approximately 9.5 mg Ag
kg
-1
tissue [56]. In another study, a 2.5-fold increase in aluminum levels in
ryegrass leaves (but not in red kidney beans) was measured at the highest
concentration of 10,000 mg kg
-1
in soil [71].
The mode of toxicity of organic NMs and polymers may differ com-
pared with those for inorganic NMs. For interpretation of toxicity studies
on carbon-based NMs, one should consider the effects of the concentra-
tion of functional groups, the hydrophilicity of functionalized NPs (nano-
tubes or fullerenes), and the sequential nonspecific interactions between the
NMs and the organisms or constituents in the culture medium (113). The
importance of the cotransporting activity of C
60
and multiwalled NTs that
could lead to detrimental effects on ecological receptors has been studied
[13,113]. In higher animals, C
60
(and perhaps other carbon-based materials) has
been shown to have hazardous consequences, due to inflammatory response
