Fractionation of Functional Light-Colored Lignin from Bamboo via Formic Acid/L-Cysteine Pretreatment for High-Performance UV Shielding and Pickering Emulsion Applications

The development and functional utilization of light-colored lignin are largely hindered by uncontrolled condensation reactions during extraction, which reduces lignin reactivity and compromises its functional performance. Herein, a formic acid/L-Cysteine (FA/L-Cys) pretreatment system was developed to fractionate functional light-colored lignin from bamboo. Under mild conditions (100 ° C, 1 h), this system achieved 95.3% delignification and 71.3% lignin recovery, affording pale beige lignin with a lightness value of 80.9. 2D-HSQC NMR analysis revealed that L-Cys suppressed lignin condensation by nucleophilically trapping reactive intermediates. When incorporated into a sun protection factor (SPF) 15 sunscreen formulation at 5 wt %, the optimized lignin (L-Cys-0.3) increased the SPF to 71.7. Moreover, L-Cys-0.3-derived lignin nanoparticles efficiently stabilized Pickering emulsions and exhibited a 12-fold enhancement in UV absorption capacity compared with their LNPs aqueous dispersion counterparts. This work highlights condensation-controlled fractionation as an effective strategy for producing light-colored lignin with potential applications in sustainable sunscreens and functional Pickering emulsion systems.