Metal oxide particles catalyze photo-oxidation in environmental media

Metal oxide particles in the submicron and nanometer range endow a wide range of consumer products with unique properties. The widespread use of such products raises concerns on potential toxicity of these materials to man and the environment. Besides their size, the photo-catalytic properties of metal oxide particles are of particular concern. By utilizing molecular probes with tailored optical properties, we investigated the photo-catalytic properties of seven TiO2 (anatase), ZnO and CeO2 manufactured particles in environmental media. Controlled experiments confirmed that the particles catalyzed photo-oxidation and photo-production of reactive oxygen species (ROS), while no ROS generation was observed when the ionic form of the materials was used in place of the particles. While affecting their aggregation and sedimentation, the type of media was not found to strongly influence the photo-catalytic behavior of the particles. Within the size range that was investigated, ZnO particles resulted in the highest production of ROS, while anatase particles possessed the highest oxidative ability. Possible explanations of such behavior are suggested.