Core-shell microcrystalline cellulose and rutile nano-titanium dioxide composite for sunscreen: Synthesis and broad-spectrum UV protection mechanism

Micro-sized core-shell microcrystalline cellulose (MCC) and nano-titanium dioxide (nano-TiO 2 ) composite (MCC@nano-TiO 2 ) is a potentially broad-spectrum sunscreen, is a high-value utilization of MCC extracted from plants. However, the underlying mechanism of its broad-spectrum UV protection remains unclear. The study employed the low-temperature hydrolysis method of TiCl 4 varying the mass ratios of MCC to nano-TiO 2 and the reaction temperatures to synthesize MCC@nano-TiO 2 with different crystal types (pure rutile, mixed-crystal, and pure anatase) nano-TiO 2 shell. Notably, the MCC@nano-TiO 2 with a pure rutile type nano-TiO 2 shell is a novel structure that had not been previously observed. A deep analysis of the broad-spectrum UV protection mechanism of MCC@nano-TiO 2 was conducted. The results show that MCC@nano-TiO 2 with either a pure rutile or a mixed-crystal type nano-TiO 2 shell exhibits a porous structure formed by the stacking of irregular platy rutile clusters. This structure is characterized by a high refractive index and a large effective scattering volume. It significantly enhances UV reflection and scattering, promoting UV light localization in nano-TiO 2 shell, thereby increasing UV absorption efficiency to exhibit outstanding broad-spectrum UV protection. Notably, MCC@nano-TiO 2 with a mixed-crystal type nano-TiO 2 shell demonstrated outstanding broad-spectrum UV protection, with a relative rutile content (W R ) of 89.84 %, an in vitro sun protection factor (SPF in vitro ) of 7.54, a critical wavelength (λ c ) of 373 nm. • MCC@nano-TiO 2 with a pure rutile type nano-TiO 2 shell was successfully synthesized. • The pure rutile shell exhibited a porous structure formed by the stacking of irregular platy rutile clusters. • Both the pure rutile and mixed-crystal phases demonstrated broad-spectrum UV protection. • The broad-spectrum UV protection mechanism of MCC@nano-TiO 2 was analyzed.