Transformations of AgS nanoparticles in simulated human gastrointestinal tract: Impacts of the degree and origin of sulfidation.

Engineered silver sulfide nanoparticles (e-AgS-NPs) are used in industry and can be released into the environment. Besides e-AgS-NPs, transformed silver sulfide nanoparticles (t-AgS-NPs) from silver nanoparticles are more likely to be the form that is widely distributed in the environment. Both e-AgS-NPs and t-AgS-NPs may be ingested and get into human gastrointestinal tract (GIT) through trophic transfer, posing a potential threat to human health. Nevertheless, knowledge of chemical stability of t-AgS-NPs and e-AgS-NPs in the human GIT is very limited. Herein e-AgS-NPs and a series of t-AgS-NPs with different degrees of sulfidation were selected as models for exposure to the simulated human GIT including mouth, stomach and small intestine phases under fed and fasted conditions. Silver ions were detected in the simulated saliva, gastric and small intestine fluids when t-AgS-NPs or e-AgS-NPs were incubated in the simulated GIT, but the amount (e.g., < 20 μg) of silver ion in each phase accounted for < 0.2‰ (w/w) of the silver added (i.e., 100 mg). Silver species of the residual particulate from each phase of the simulated GIT with t-AgS-NPs or e-AgS-NPs were thus analyzed through a developed analytical method that could selectively, successively and efficiently dissolve and quantify AgCl, Ag(0), and AgS in particulates. Both e-AgS-NPs and fully sulfidized t-AgS-NPs were shown to be highly stable in the simulated human GIT. Conversely, partially sulfidized t-AgS-NPs primarily underwent transformations in the mouth phase relative to stomach and small intestine phases regardless of fed or fasted status, wherein AgCl and AgS were observed besides Ag(0). The amount of AgS in the mouth phase negatively (r = -0.99, p <  0.001) correlated with the sulfidation degree of initial t-AgS-NPs. This work improved our understanding of potential transformations of t-AgS-NPs in the simulated human GIT, providing valuable information for future researches on evaluating health risks of ingested AgS-NPs.