Encapsulating TiO₂ 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.