Plastic-free silica-titania-polyphenol heterojunction hybrids for efficient UV-to-blue light blocking and suppressed photochemical reactivity

Abstract

The photocatalytic activity of ultraviolet (UV) filters, generating reactive oxygen species (ROS), must be minimized in skincare products to avoid adverse biological and chemical effects. Unfortunately, the screening effect of UV filters is intrinsically correlated with their photocatalytic activity in the presence of oxygen and water. Polymeric encapsulation has been widely employed to inhibit photochemical ROS generation; however, it can cause serious microplastic issues because UV filters act as photocatalysts under sunlight irradiation. Here we present plastic-free silica-titania-tannin (SiO2-TiO2-TA) heterojunction hybrids to increase UV-to-blue light blocking effects while suppressing UV-induced ROS generation by altering their photochemical routes. Mesoporous SiO2-TiO2 particles were prepared through the in situ synthesis of TiO2 nanocrystals within SiO2 particles, followed by the spontaneous deposition of TA via Ti-pyrogallol and Ti-catechol coordination. Ligand-to-metal charge transfer (LMCT) from TA to the conduction band of TiO2, demonstrated by theoretical calculations, extends the absorption spectrum of TiO2 over the blue wavelength regime, increasing a specific sun protection factor ∼ 1.8 times. The TA layer efficiently suppressed the UV-induced ROS generation: the SiO2-TiO2-TA produced only < 1 % hydroxyl radical ions compared to pristine TiO2 under simulated solar irradiation. Accordingly, the UV and blue light-induced cytotoxicity of fibroblasts was significantly alleviated by the ROS stabilization effect of TA complexed with TiO2, while pristine TiO2 exacerbated the cytotoxic effect via ROS generation in the cytoplasm. This work demonstrates that polyphenol coatings are a promising method to suppress the ROS generation by inorganic UV filters while increasing their screening effects through LMCT.