Uptake and Transformation of Silver Nanoparticles and Ions by Rice Plants Revealed by Dual Stable Isotope Tracing

Knowledge on the uptake and transformation of silver nanoparticles (AgNPs) and Ag+ ions by organisms is critical for understanding their toxicity. Herein, the differential uptake, transformation, and translocation of AgNPs and Ag+ ions in hydroponic rice (Oryza sativa L.) is assessed in modified Hewitt (with Cl- ions, HS(Cl)) and Hogland solutions (without Cl- ions, HS) using dual stable isotope tracing ((AgNO3)-Ag-107 and (109)AgNPs). After coexposure to Ag-107(+) ions and (109)AgNPs at 50 mu g L-1 (as Ag for both) for 14 days, a stimulatory effect was observed on root elongation (increased by 68.8 and 71.9% for HS(Cl) and HS, respectively). Most of the Ag+ ions (from Ag-107(+) ions and (109)AgNPs) were retained on the root surface, while the occurrence of AgNPs (from 109AgNPs and Ag-107(+) ions) was observed in the root, suggesting the direct uptake of AgNPs and/or reduction of Ag+ ions. Higher fractions of Ag+ ions in the shoot suggest an in vivo oxidation of AgNPs. These results demonstrated the intertransformation between Ag+ ions and AgNPs and the role of AgNPs as carriers and sources of Ag+ ions in organisms, which is helpful for understanding the fate and toxicology of Ag.