Tuning photoactivity and photoprotection of TiO₂ via TiO₂/CeO₂ heterostructure composite engineering

Titanium dioxide (TiO₂) is a widely used UV-shielding excipient for topical pharmaceutical and cosmetic formulations, but its ability to generate reactive oxygen species under UV irradiation limits its safety. Here, we report a MOF-derived TiO₂/CeO₂ heterostructure designed to suppress TiO₂ photoactivity while preserving its UV-protective properties. Under UVA irradiation, CeO₂ incorporation strongly inhibits superoxide radical generation and reduces hydroxyl radical formation by ca. 58% compared with rutile TiO₂. Importantly, this reduced photoactivity translates into improved pharmaceutical performance: in hydrogel formulations, 0.1 wt% TiO₂/CeO₂ protects ketoprofen more effectively than TiO₂ (77.54% vs. 52.56% remaining after irradiation), while 0.5 wt% TiO₂/CeO₂ provides almost complete photoprotection. The composite also has a lower impact on hydrogel matrix stability than pristine TiO₂ and shows no acute toxicity in the Microtox assay. Overall, these results demonstrate that TiO₂/CeO₂ heterostructuring is an effective, safe-by-design strategy for developing photoprotective excipients that stabilize photosensitive drugs in topical formulations.