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.