Spatiotemporal Mapping\nof the Evolution of Silver\nNanoparticles in Living Cells

Bioaccumulated silver nanoparticles (AgNPs) can undergo\ntransformation\nand release toxic Ag<sup>+</sup>, which can be further reduced and\nform secondary AgNPs (Ag<sup>0</sup>NPs). However, the intricate interconversions\namong AgNPs, Ag<sup>+</sup>, and Ag<sup>0</sup>NPs remain speculative.\nHerein, we developed a bioimaging method by coupling the aggregation-induced\nemission method with the label-free confocal scattering and hyperspectral\nimaging techniques to quantitatively visualize the biodistribution\nand biotransformation of AgNPs, Ag<sup>0</sup>NPs, and Ag<sup>+</sup> in living cells. We demonstrated that AgNPs were first dissolved\nin the medium, and the released Ag<sup>+</sup> was converted into\nAg<sup>0</sup>NPs with the presence of algal extracellular polymeric\nsubstances and light. Under these conditions, AgNPs alone accounted\nfor 12.4% of the total AgNP toxicity, a percentage comparable to that\nof Ag<sup>0</sup>NPs (15.6%). However, Ag<sup>+</sup> remained the\nprimary contributor to overall AgNP toxicity. Additionally, we found\nthat about 9.00% of the accumulated AgNPs within the algal cells were\ntransformed after 24 h exposure. Of these transformed AgNPs, 4.70%\nremained as Ag<sup>+</sup> forms (located in the apical region, nucleus,\nand pyrenoid), while 4.30% persisted as Ag<sup>0</sup>NP forms (located\nin the cytosol) that were only detectable after a 4 h exposure. We\nfurther showed that AgNP exposure upregulated algal glutathione production\nwith a 38.3-fold increase in glutathione reductase activity, which\npotentially resulted in Ag<sup>0</sup>NP formation at the active site.\nOverall, this study differentiated the toxicity of AgNPs, Ag<sup>+</sup>, and Ag<sup>0</sup>NPs and directly visualized the ongoing transformation\nand translocation of AgNPs, Ag<sup>+</sup>, and Ag<sup>0</sup>NPs\nwithin living cells, which are critical in unveiling the toxicity\nmechanisms of AgNPs.