Mechanism of Photogenerated Reactive Oxygen Species and Correlation with the Antibacterial Properties of Engineered Metal-Oxide Nanoparticles

Oxidative stress induced by reactive oxygen species (ROS) is one of the most important antibacterial mechanisms of engineered nanoparticles (NPs). To elucidate the ROS generation mechanisms, we investigated the ROS production kinetics of seven selected metal-oxide NPs and their bulk counterparts under UV irradiation (365 nm). The results show that different metal oxides had distinct photogenerated ROS kinetics. Particularly, TiO2 nanoparticles and ZnO nanoparticles generated three types of ROS (superoxide radical, hydroxyl radical, and singlet oxygen), whereas other metal oxides generated only one or two types or did not generate any type of ROS. Moreover, NPs yielded more ROS than their bulk counterparts likely due to larger surface areas of NPs providing more absorption sites for UV irradiation. The ROS generation mechanism was elucidated by comparing the electronic structures (i.e., band edge energy levels) of the metal oxides with the redox potentials of various ROS generation, which correctly interpreted the ROS generation of most metal oxides. To develop a quantitative relationship between oxidative stress and antibacterial activity of NPs, we examined the viability of E. coli cells in aqueous suspensions of NPs under UV irradiation, and a linear correlation was found between the average concentration of total ROS and the bacterial survival rates (R-2 = 0.84). Although some NPs (i.e., ZnO and CuO nanoparticles) released toxic ions that partially contributed to their antibacterial activity, this correlation quantitatively linked ROS production capability of NPs to their antibacterial activity as well as shed light on the applications of metal-oxide NPs as potential antibacterial agents.