Quantitative Analysis of Reactive Oxygen Species Photogenerated on Metal Oxide Nanoparticles and Their Bacteria Toxicity: The Role of Superoxide Radicals

Ecotoxicity of engineered nanoparticles (NPs) has become the focus of considerable attention because of their wide applications. Reactive oxygen species (ROS) play important roles in the toxicity mechanisms of engineered metal oxide NPs. This work aimed to understand quantitatively the contribution of photogenerated ROS on metal oxide NPs to their toxicity. The dynamic generation of O2•-, •OH, and H2O2 in aqueous suspensions of photoilluminated metal oxide nano- and bulk particles (TiO2, ZnO, V2O5, CeO2, Fe2O3, and Al2O3) was measured by a continuous-flow chemiluminescence (CFCL) detection system. Superoxides were generated on all six nanoparticles as well as bulk TiO2 and ZnO, with nano TiO2 producing the highest concentration (180 nM). Hydroxyl radicals were detected on both nano- and bulk TiO2 and ZnO, whereas H2O2 was detected only on TiO2 and ZnO nanoparticles. The generation of ROS can in general be interpreted by the electronic structures and surface defects of the NPs and the ROS redox potentials. Furthermore, acute toxicity of the six metal oxide particles to a luminescent bacterium, P. phosphoreum 502 was assessed after photoillumination. The toxicity effect was attributed to the long-lived O2•- radicals on the nanoparticlce, and its potency follows the order of TiO2 > ZnO > V2O5 > Fe2O3 > CeO2 > Al2O3, which is the same as the order of the O2•- concentration measured by CFCL. Our work revealed quantitatively the important role superoxide radicals play in the toxicity of various metal oxide nanoparticles after photoillumination.