In Vivo Bioimaging of Mercury Sulfide Nanoparticles Dissolution in the Gut Environment of Zooplankton

The toxicity of mercury sulfide nanoparticles (HgS-NPs) is likely mediated by their in vivo dissolution into bioavailable Hg²⁺. However, tracking this transformation within complex biological systems has been a major analytical challenge, hindering a mechanistic understanding of the HgS-NPs' toxicology. To address this, we developed an aggregation-induced emission (AIE)-based bioimaging technique for the selective and real-time monitoring of Hg²⁺ in <i>Daphnia magna</i>, enabling the direct visualization and quantification of HgS-NP dissolution in a multicellular organism. Our Hg²⁺-specific AIE probe exhibited a detection limit of 0.52 ng/mL and negligible toxicity at working concentrations. Using this method, we directly visualized and quantified the time-dependent dissolution of ingested HgS-NPs of different sizes (20 and 60 nm). We found that the gut region was the primary site of Hg²⁺ accumulation, with a region-specific distribution showing significantly higher concentrations in the foregut than the hindgut. At dissolution equilibrium, 8.4% of the ingested 20 nm HgS-NPs and 4.9% of the 60 nm NPs were transformed into Hg²⁺. Furthermore, by correlating with pH mapping, we demonstrated that the extent of HgS-NPs dissolution is negatively correlated with the local pH in the gut. This study provides crucial insights into the biotransformation of HgS-NPs in a model aquatic organism, fundamentally challenging the paradigm of their environmental inertness. Our findings highlight the gut as a critical bioreactor for the transformation of low-solubility metal sulfides, with significant implications for accurately assessing their bioavailability and ecological risks.