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<p>The rapidly increasing use of silver nanoparticles (Ag NPs) in consumer products
and medical applications has raised ecological and human health concerns. Significant
progress has been made in understanding the toxicity of silver nanoparticles (Ag NPs)
under carefully controlled laboratory conditions. The goals of this dissertation were
to investigate the mechanism of Ag NP toxicity under both laboratory conditions and
environmental backgrounds, using <italic>Caenorhabditis elegans</italic> (<italic>C.
elegans</italic>) as a model system. A key question for addressing these concerns
is whether Ag NP toxicity is mechanistically unique to nanoparticulate silver or if
it is a result of the release of silver ions. Ag NPs are produced in a large variety
of monomer sizes and coatings, and since their physicochemical behavior depends on
the media composition, it is important to understand how these variables modulate
toxicity.</p><p> In order to test the hypothesis of a particle-specific
effect, multiple techniques were used, including analytical chemistry, pharmacological
rescue, and genetic analysis. Results suggested that dissolution was important for
all tested Ag NPs and oxidative stress (a particle-specific effect) was important
only for some Ag NPs, especially the citrate-coated Ag NPs (CIT-Ag NPs). The hypothesis
of the particle-specific effect was further tested by investigating the cellular uptake
and damage co-localization upon exposures to CIT-Ag NPs. I found that Ag NPs crossed
all layers, including the pharynx, gut, and also embryos through trans-generational
transfer. Sites of damage were examined through transmission electron microscopy (TEM),
and CIT-Ag NPs showed a more severe and deeper level of damage compared to ionic Ag.
In addition, pharmacological inhibitors in parallel with genetic mutants (deficient
in both endocytosis and lysosomal function) were used to explore the impact of those
pathways on Ag NP uptake and associated toxicity. I found that endocytosis was important
for CIT-Ag NP uptake and toxicity. Most intriguingly, one of the lysosomal deficient
mutants was much more sensitive than wild type to reproductive inhibition after exposure
to CIT-Ag NPs but not ionic Ag, constituting a clear nanoparticle-specific toxic effect.</p><p>
These laboratory mechanistic studies, however, cannot be directly extrapolated to
complicated environmental conditions, including variable amounts of natural organic
matter (NOM), different temperatures and salinities, surface sulfidation, etc. My
general hypothesis was that complex environmental medium would reduce Ag NP toxicity.
In support of this, the environmental conditions present in mesocosms resulted in
a loss of toxicity one week after dosing/spiking. In laboratory studies, I found that
that increasing temperature and salinity tended to increase Ag NP toxicity, while
sulfidation reduced Ag NP toxicity, acting as a &ldquonatural antidote&rdquo. I studied
two types of NOM, Suwannee River and Pony Lake fulvic acids (SRFA and PLFA respectively).
PLFA rescued toxicity more effectively than SRFA. Therefore, CIT-Ag NP-NOM interactions
were explored in depth using PLFA. Using hyperspectral dark field microscopy, I was
able to detect the formation of Ag NP-PLFA complexes and the limited tissue uptake
of Ag NPs (with and without PLFA). Consistent with the reduced acute toxicity of Ag
NPs by PLFA, I also found a rescue effect of PLFA on Ag NP-induced ultrastructural
damage.</p><p> In conclusion, Ag NP toxicity resulted largely from dissolution
and in some cases also from a particle-specific effect. However, Ag NP toxicity was
strongly altered by environmental matrices. Continued in depth elucidation of Ag NP
behavior, cellular uptake pathways and trafficking, and their interactions with other
environmental factors will be invaluable in predicting, designing, and remediating
the potential/existing environmental implications of silver-related nanotechnology.</p>
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