Biomedical Engineering Reference
In-Depth Information
accompanied by release of nitric oxide, ultimately resulting in immune sup-
pression and cancer [117].
10.4.1 Photoreactivity
Light-triggered ROS production or redox reactions are viewed as important
modes of toxicity for some NMs, such as quantum dots, several metals and
metal oxides, fullerenes, and possibly dendrimers. While redox reactions
might occur in the dark for NMs such as zero-valent iron, the toxicity of
most other NMs is strongly enhanced in the presence of light (e.g., the bac-
tericidal effects of photoactivated TiO
2
) [86]. Differences in testing methods
(i.e., carrying out the experiment in the presence or absence of light, or light
intensity) may explain some of the different outcomes reported in the litera-
ture for the same NP (e.g., TiO
2
) tested in the same species (e.g.,
D. magna
).
The photoactivity of some NMs is a relevant mode of toxicity for photosyn-
thetic organisms [99], and also for bacteria. Dumas et al. [78] reported that
a 30-min exposure of blue light to bacteria simultaneously exposed to CdTe
quantum dots totally inhibited bacterial growth, whereas only weak bacte-
riostatic effects were observed following exposure in the dark. Interestingly,
nanosized TiO
2
can exhibit a specific toxicity on algal cells, without affecting
the photosynthetic bacteria [99]. This differential sensitivity was explained
by the fact that alga can release oxygen, which is then activated by the nano-
TiO
2
(in the presence of UV light) to produce toxic superoxide ions [99].
Light-induced genotoxicity has also been observed in studies using TiO
2
[146].
In the absence of visible or UV light, TiO
2
is not (or only weakly) genotoxic to
mammalian cells or to
Salmonella typhimurium
; however, coexposure to UV light
causes DNA damage as detected by the comet assay (12.5-200 mg
-1
for nano-
TiO
2
; 50-3200 mg
-1
for microscale TiO
2
), as well as chromosomal aberrations
in Chinese hamster cells (3-50 mg
-1
n a no -TiO
2
). Negative results were found
for nano-TiO
2
(microscale TiO
2
not tested) using the mouse lymphoma L5178Y
mutation assay (250-2000 mg
-1
). At concentrations from 5000 to 40,000 mg
-1
,
S. typhimurium
bacteria showed decreased mutant yield upon exposure to nano-
TiO
2
(21 nm average diameter, mixture of anatase and rutile nano-TiO
2
) rela-
tive to UV light illumination only (TiO
2
added), suggesting that photo-activated
n a no -TiO
2
can cause toxicity to bacteria without genotoxicity [146].
10.4.2 Interactions with Xenobiotics: A Carrier Effect
Even if a given NM is not toxic to living organisms, its presence in the environ-
ment can modify the toxicity of other toxicants. Either increased or decreased
toxicities have been reported depending on particular combinations. Highly
hydrophobic compounds that are poorly available to organisms might be deliv-
ered more efficiently when transported by NMs. For instance, a 60% increase
in phenanthrene toxicity was found in
D. magna
that are coexposed to C
60
[147].
In addition, Cd
2+
toxicity to algae can be increased in the presence of 2 mg L
-1
