Deciphering the effects of CeO nanoparticles on Escherichia coli in the presence of ferrous and sulfide ions: Physicochemical transformation-induced toxicity and detoxification mechanisms.

The physicochemical transformations as well as the redox reaction-induced toxicity changes of ceria nanoparticles (CeO NPs) in reducing conditions is extremely lacking. Herein, the behaviors, chemical modifications and toxicity of CeO NPs in the presence of reduction-active ions (namely Fe and S) were investigated, with a particular emphasis on the cytotoxicity mechanism associated with their physicochemical transformations. The presence of Fe and S differently altered the surface properties and toxicity of CeO NPs. Redox reactions with Fe led to form small aggregates, boosted the reduction of CeO and enhanced dissolved Ce concentration. Moreover, CeO NPs possessed a high affinity for Escherichia coli (E. coli) and induced the generation of •OH abiotically after reaction with Fe, provoking serious disruption of cell membranes and causing high toxicity to E. coli. In contrast, the amending of S protected E. coli from direct contact with CeO NPs by creating new CeS precipitated on the surface, accelerating the aggregation of NPs and reducing the concentration of dissolved Ce. This study suggested that the chemical interactions between the reactive surfaces of CeO and reduction-active ions highly determined the stability and cytotoxicity of CeO NPs, which provides fundamental insights into the environmental risks of CeO NPs.