Encapsulating TiO ₂ Nanoparticles in Chitosan-Based Porous Microspheres as Multifunctional UV Filters

Titanium dioxide nanoparticles (TiO2 NPs) are widely used as inorganic UV filters, yet their photocatalytic activity and resulting phototoxicity raise serious safety issues. Here, we report a Pickering double emulsion templating strategy for constructing chitosan-based porous microspheres encapsulating TiO2 NPs (TiO2@CS-PMs), with controllable microstructures and mass loading of TiO2 NPs. The oil-in-water-in-oil double emulsion template confines TiO2 NPs within a cross-linked porous polymer skeleton. Structural confinement, combined with the antioxidant capacity of chitosan (CS), greatly reduces UV-induced reactive oxygen species generation of TiO2 NPs. In vitro assays reveal that TiO2@CS-PMs have good biocompatibility and significantly lower phototoxicity to HaCaT cells than naked TiO2 NPs. The porous architecture promotes optical confinement, and the encapsulation causes interfacial interactions between TiO2 NPs and the CS matrix, resulting in broadened and enhanced UV attenuation. Sunscreen incorporating TiO2@CS-PMs exhibits higher sun protection factors and cellular photoprotection at lower TiO2 NPs doses, while the CS matrix provides antibacterial activity. This work reveals Pickering double emulsion templating porous microstructure as a versatile platform for managing particle photochemistry, providing insights into the development of safer and multifunctional UV filters.