Synergistic UV Shielding Enabled by Size-Controlled Lignin Nanoparticles and Curcumin

Converting bulk lignin into lignin nanoparticles (LNPs) offers a promising strategy to improve dispersibility and enable high-value applications. However, the intrinsic polydispersity of industrial lignin presents a major challenge for size-controlled nanoparticle fabrication. Herein, a two-step strategy was developed to fabricate size-controlled LNPs: lignin was self-assembled into mixed-size LNPs via rotary evaporation, followed by centrifugal fractionation to isolate LNPs with diameters ranging from 400 to 800 nm. Structural and size analyses revealed that lignin fractions with lower molecular weight, narrower distribution, and lower syringyl-to-guaiacyl (S/G) ratios preferentially formed smaller LNPs. These LNPs were used to form LNP/Curcumin (Cur) composite emulsions. UV–vis analysis showed that the UV absorption was enhanced by 7.3-fold compared to free LNPs and further increased 1.8-fold upon curcumin loading. 1H NMR confirmed synergistic hydrogen bonding and π–π stacking interactions between lignin and Cur. When incorporated into a poly(vinyl alcohol) (PVA) matrix (3 wt %), the resulting composite films exhibited ∼100% UV blocking in both UVA and UVB regions. This study presents a facile, scalable method for size-controlled LNP fabrication and offers a sustainable strategy for designing biobased, high-performance UV-shielding materials.