Impact of Surface Coatings on the Photocatalytic Activity and Cytotoxicity of Titanium Dioxide Nanoparticles in Human Keratinocytes: Implications for Sunscreen Safety

Titanium dioxide nanoparticles (TiO2 NPs) are widely used in sunscreens as UV filters because of their high UV absorption efficiency. However, their potential to generate reactive oxygen species (ROS) under UV irradiation raises concerns about their photocatalytic activity and associated cytotoxicity. This study investigates how surface coatings, specifically SiO2 and Al2O3–SiO2, affect the photocatalytic performance and biological impact of anatase-dominant TiO2 NPs on human HaCaT keratinocytes. Photocatalytic activity was evaluated via methylene blue degradation under UVA (365 nm) and UVB (290 nm) irradiation, showing that uncoated TiO2 NPs (Aeroxide P25) exhibited significantly higher degradation rates (64%) compared to SiO2-coated (59–55%) and Al2O3–SiO2-coated (24–22%) variants. Cytotoxicity assays (XTT) revealed that UVA exposure alone had a minimal effect, but in combination with anatase-rich TiO2 NPs, a dose- and time-dependent reduction in cell viability was observed. Notably, Al2O3–SiO2-coated NPs showed significantly lower cytotoxicity than SiO2-coated or uncoated anatase-based NPs. ROS measurements confirmed elevated oxidative stress in nanoparticle-exposed cells under UVA, although no significant differences were observed between coating types. Raman spectroscopy revealed structural damage to cellular components in cells exposed to both UVA and TiO2 NPs, suggesting membrane disruption and genotoxic potential. These results underscore the importance of coating chemistry in nanomaterial design and call for more realistic testing paradigms involving formulated products and complex skin models to inform regulatory standards and consumer safety.