Spatiotemporal Mapping of the Evolution of Silver Nanoparticles in Living Cells.

Bioaccumulated silver nanoparticles (AgNPs) can undergo transformation and release toxic Ag, which can be further reduced and form secondary AgNPs (AgNPs). However, the intricate interconversions among AgNPs, Ag, and AgNPs remain speculative. Herein, we developed a bioimaging method by coupling the aggregation-induced emission method with the label-free confocal scattering and hyperspectral imaging techniques to quantitatively visualize the biodistribution and biotransformation of AgNPs, AgNPs, and Ag in living cells. We demonstrated that AgNPs were first dissolved in the medium, and the released Ag was converted into AgNPs with the presence of algal extracellular polymeric substances and light. Under these conditions, AgNPs alone accounted for 12.4% of the total AgNP toxicity, a percentage comparable to that of AgNPs (15.6%). However, Ag remained the primary contributor to overall AgNP toxicity. Additionally, we found that about 9.00% of the accumulated AgNPs within the algal cells were transformed after 24 h exposure. Of these transformed AgNPs, 4.70% remained as Ag forms (located in the apical region, nucleus, and pyrenoid), while 4.30% persisted as AgNP forms (located in the cytosol) that were only detectable after a 4 h exposure. We further showed that AgNP exposure upregulated algal glutathione production with a 38.3-fold increase in glutathione reductase activity, which potentially resulted in AgNP formation at the active site. Overall, this study differentiated the toxicity of AgNPs, Ag, and AgNPs and directly visualized the ongoing transformation and translocation of AgNPs, Ag, and AgNPs within living cells, which are critical in unveiling the toxicity mechanisms of AgNPs.