Dynamic cytotoxicity of ZnO nanoparticles and bulk particles to Escherichia coli: A view from unfixed ZnO particle:Zn2+ ratio

ZnO nanoparticles (NPs) form binary mixtures of ZnO particles and released Zn2+ in the environment, and the quantitative contributions of these components to toxicity are still uncertain. Herein, quantitative contribution of ZnO particle and Zn2+ to cytotoxicity of ZnO NPs to Escherichia coli were determined during 48 h bioassay. The cytotoxicity and mechanisms of ZnO NPs were dynamic and affected by ionic strength, Fe3+, humic acid, and temperature due to the unfixed ZnO particle:Zn2+ ratio. ZnO NPs and ZnO bulk particles (BPs) had comparable cytotoxicity but distinct cytotoxic mechanisms. ZnO NPs cytotoxicity arises mainly from ZnO particles for 3 h and from Zn2+ afterwards (8-48 h). The cytotoxicity of ZnO BPs depends predominantly on ZnO particles for 12 h and on Zn2+ from 24 to 48 h. The cytotoxicity of ZnO NPs and BPs is partially attributable to Zn accumulation, and dependent on ZnO particle:Zn2+ ratio. The linear regressions of acute toxicity for ZnO NPs vs. BPs and Zn2+ yielded excellent r(2) (0.9994 and 0.9998) from literature data and good r(2) (>= 0.714) under certain environmental factors, which can be applied to assess environmental risk of ZnO NPs. Furthermore, dynamic cytotoxicity and mechanisms should be seriously considered during the environmental risk assessment of ZnO NPs.