In Vivo Bioimaging of Silver Nanoparticle Dissolution in the Gut Environment of Zooplankton.

Release of silver ions (Ag) is often regarded as the major cause for silver nanoparticle (AgNP) toxicity toward aquatic organisms. Nevertheless, differentiating AgNPs and Ag in a complicated biological matrix and their dissolution remains a bottleneck in our understanding of AgNP behavior in living organisms. Here, we directly visualized and quantified the time-dependent release of Ag from different sized AgNPs in an in vivo model zooplankton ( Daphnia magna). A fluorogenic Ag sensor was used to selectively detect and localize the released Ag in daphnids. We demonstrated that the ingested AgNPs were dissoluted to Ag, which was heterogeneously distributed in daphnids with much higher concentration in the anterior gut. At dissolution equilibrium, a total of 8.3-9.7% of ingested AgNPs was released as Ag for 20 and 60 nm AgNPs. By applying a pH sensor, we further showed that the dissolution of AgNPs was partially related to the heterogeneous distribution of pH in different gut sections of daphnids. Further, Ag was found to cross the gills and enter the daphnids, which may be a potential pathway leading to AgNP toxicity. Our findings provided fundamental knowledge about the transformation of AgNPs and distribution of Ag in daphnids.