Photocatalytic production of hydroxyl radicals by commercial TiO2 nanoparticles and phototoxic hazard identification

This study identifies the phototoxic potential of commercial titanium dioxide nanoparticles (TiO2 NPs) used in sunscreens and consumer products by employing a tiered testing approach comprising physicochemical, in vitro and ex vivo tests. Our results revealed that all the test samples of TiO2 NPs, varied in surface coating, crystallinity and primary particle size, produced hydroxyl radicals upon UVA photoexcitation as determined by electron spin resonance (ESR) spectroscopy. Their phototoxic potentials were assessed first by combining the validated 3T3 neutral red uptake phototoxicity test and red blood cell phototoxicity test and subsequently in ex vivo models of chick chorioallantoic membrane (CAM) and reconstructed human 3D skin model (H3D). Crystalline structure and particle size of TiO2 NPs were found to exert a major influence on the photocatalytic activity and the associated phototoxic effects. Besides, a medium-sized sample with silica/alumina also exhibited high phototoxic potency with no obvious relevance to the enhanced hydroxyl radicals and lipidperoxidation. This effect might be taken place through the interaction of harmful metal ions released from the oxide coating. However, no phototoxicity was observed on a H3D skin model probably due to the lack of efficient percutaneous absorption of TiO2 NPs. This study demonstrates the efficacy of a tiered testing strategy for identifying phototoxic hazards of TiO2 NPs and suggests the need for a comprehensive assessment that takes account of the effects of different coating materials and potential interactions between multiple mechanisms.