Intracellular Dissolution of Silver Nanoparticles: Evidence from Double Stable Isotope Tracing.
To track transformations of silver nanoparticles (AgNPs) in vivo, HepG2 and A549 cells were cocultured with two enriched stable Ag isotopes (107AgNPs and 109AgNO3) at nontoxic doses. After enzymatic digestion, 107AgNPs, ionic 107Ag+ and 109Ag+ in exposed cells could be separated and quantified by liquid chromatography combined with ICP-MS. We found that ratios of 107Ag+ to total 107Ag and proportions of 107Ag+/ 109Ag+ in cells increased gradually after exposure, proving that the Trojan-horse mechanism occurred, i.e., AgNPs released high contents of Ag+ after internalization. While the presence of 109Ag+ (5 and 100 μg/L) has little influence on the uptake of 107AgNPs (0.1 and 2 mg/L), the presence of 107AgNPs at a high dose (2 mg/L) dramatically increases the ingestion of 109Ag+, even though 107AgNPs at a low dose (100 μg/L) showed negligible effects on the internalization of 109Ag+. Cellular homeostasis may be perturbed under sublethal exposure of 107AgNPs, and thus enhanced uptake of 109Ag+. Our findings suggest that the widely adopted control experiments in toxicology studies, culturing organisms with AgNO3 at the same concentration of Ag+ in the AgNP exposure medium, may underestimate uptake of Ag+ and thus cannot exclude suspected toxic effects of Ag+ at high AgNP exposure doses.