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.