Photochemical behavior of nanoscale TiO2 and ZnO sunscreen ingredients

Most commercial sunscreens that use inorganic pigments (TiO2 and ZnO) employ materials with nanoscale dimensions so that the products are both transparent and smooth upon application. However, certain types of TiO2 and ZnO nanoparticles are well known for their ability to produce reactive oxygen species (ROS) upon UV illumination. Consumers would not be protected from the adverse effects of sun exposures if photoactive nanomaterials were employed in sunscreens. To evaluate whether this is the case, eight different commercial sunscreens, as well as nanoscale pigments derived from these products, were exposed to ultraviolet light and evaluated for ROS production. Redundant and complementary assays for detecting reactive oxygen species included dichlorofluorescein fluorescence, luminol chemiluminescence, and the decolorization of dyes (Congo red and Rose Bengal). Additionally, spin trap (POBN and DMPO) electron paramagnetic resonance spectroscopy provided quantitative measures of ROS generation upon UV illumination. Nanoscale TiO2 from neat sunscreens was relatively inactive upon illumination; inert oxide coatings such as alumina and silica apparently render the titania surfaces non-reactive. In contrast, ZnO derived from sunscreens produced substantial amounts of ROS upon UVA illumination. The photocatalytic activity of nanoscale ZnO suggests that more effective sunscreens would rely on strategies, such as surface coatings, designed to limit its ROS generation under ultraviolet illumination. Finally, the simple chemical assays presented here are ideal screening tools for ensuring sunscreen pigments were inert under ultraviolet illumination.