Facile and Green Preparation of High UV-Blocking Lignin/Titanium Dioxide Nanocomposites for Developing Natural Sunscreens

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Lignosulfonate (LS), one type of industrial lignin and natural macromolecular sun blocker, was applied to modify TiO 2 via one-pot hydrothermal esterification. The reaction mechanism and composite structure of LS@TiO 2 were investigated by DLS, TEM, AFM, TG, UV, FTIR, XPS, XRD, ESR, and contact angle measurements. The results show that the esterification modification occurred between the carboxyl groups of lignin and the hydroxyl groups on the surface of TiO 2 . Lignosulfonate of 3.4 wt % with a thickness of 13 nm effectively coated the normal TiO 2, and the LS@TiO 2 composites were nanoscale with an average size of 119 nm. Hydroxylated TiO 2 particles were more reactive, and the contents of LS on TiO 2 were processed by 0.1 and 1 M HCl and increased to 5.0 wt % (LS@TiO 2 -0.1M) and 6.0 wt % (LS@TiO 2 -1M), respectively. Chemically coated LS not only improved the dispersibility of TiO 2 in the substrates, but also significantly boosted its UV-blocking ability. Therefore, TiO 2 @LS nanocomposites were applied as the sole active in the pure cream and the sunscreen performance was compared with that of TiO 2 . The sun protection factor (SPF) values of creams containing 5, 10, and 20 wt % LS@TiO 2 were 16, 26, and 48, respectively. They were 30–60% higher than those of the creams containing the same amount of TiO 2 . The sunscreen performance of LS@TiO 2 improved with LS content. The SPF value of pure cream containing 10 wt % LS@TiO 2 -1M reached 50+. The possible UV-blocking enhancement mechanism of LS@TiO 2 was also revealed. Preparation of LS@TiO 2 nanocomposites provides a facile and green way for value-added utilization of lignin biomass and highly efficient UV-blocking of nano-TiO 2 materials.