Abstract:
Engineered nanomaterials (ENMs) have become increasingly popular for use in
both industrial and consumer settings in recent years. Of the myriad of materials used to
synthesize ENMs, silver is one of the most common. Silver nanoparticles (AgNPs)
undergo dynamic transformations in biological environments, leading to modifications in
their reactivity, surface interactions, and speciation. In particular, release of ionic silver
(Ag(I)(aq)) through AgNP dissolution can have significant impacts on ecological and
human health. Thus, quantifying AgNP dissolution rates in biologically relevant matrices
can provide valuable information regarding their potential cytotoxic effects. Linear sweep
stripping voltammetry (LSSV) is a technique that provides high throughput, in situ
measurements of the concentration of Ag(I)(aq) dissolved from AgNPs. These
measurements are better time-resolved and have comparable sensitivities to those
obtained using atomic spectroscopic techniques, allowing for more detailed investigation
of AgNP dissolution kinetics.
LSSV was first used to investigate the dissolution of AgNPs in the presence of
bovine serum albumin (BSA), a point-of-use system used to model medical and
consumer applications of AgNPs. Dissolution of AgNPs was enhanced in the presence of
BSA in a concentration-dependent manner. This effect was also dependent on AgNP
diameter, with smaller particles exhibiting a greater degree of BSA-enhanced dissolution
than larger particles. These findings were supported by AgNP-BSA binding assays,
which showed a subtle decrease in AgNP-BSA binding strength with decreasing AgNP
size. These data point to a model in which Ag(I)(aq)-loaded BSA is displaced by BSA in
the bulk solution in a more facile manner at the surface of smaller AgNPs. LSSV was
subsequently used to probe the dissolution of AgNPs in the presence of peptone yeast
extract growth medium (PYE) and metabolites isolated from the bacterium Caulobacter
crescentus, referred to as "spent medium". There is significant overlap between common
AgNP waste repositories and the habitats of C. crescentus, making this a relevant end-of-use
model. Dissolution of AgNPs was mediated by spent medium in a culture density-dependent
manner. DLS size data show increasing hydrodynamic diameter of AgNPs
when incubated with spent medium from increasingly dense cultures, supporting these
findings. We hypothesize that glutathione (GSH) levels in spent medium vary with the
density of their derivative culture, and that GSH is responsible for both this dissolution
trend and surface layer formation on AgNPs.
Both the point-of-use and end-of-use models investigated here allowed us to
characterize AgNP dissolution in a wide range of environments. Further, the novel
application of LSSV to study AgNP dissolution kinetics can be expanded to encompass
other model proteins and bacterial species. In this way, the present work both advances
our understanding of complex AgNP transformations and provides quantitative analytical
tools that can be easily accessed and more broadly applied by other researchers in the
nanotechnology field.