Coupling Fractionation and Depolymerization of High-Yield Lignin from Eucalyptus for Uniform Nanoparticles in Biophotonic Materials

Lignin, an abundant yet underutilized renewable aromatic polymer, holds great promise for advanced materials but faces challenges of high heterogeneity in uniform nanoparticle preparation. Herein, we report an innovative green strategy coupling fractionation and depolymerization of eucalyptus lignin using a γ-valerolactone (GVL)/H2O solvent system for high-yield and low heterogeneous lignin fractionation followed by transfer into uniform nanoparticles for tunable structural color materials. The intermolecular force quantification using atomic force microscopy (AFM) indicated an efficient adhesion force decrease in lignin-holocellulose and lignin-lignin from 0.39 to 0.07 mN/m and 0.48 to 0.13 mN/m when GVL content increased from 0 to 50%, respectively, suggesting that the GVL addition facilitated component separation and lignin dissolution, respectively. In the optimized solution of 90% GVL, this approach achieves an exceptional lignin yield of 91.7% while retaining over 95% of cellulose for concurrent valorization. More importantly, mechanistic studies reveal that GVL/H2O selectively cleaves partial β-O-4 linkages in lignin, reducing molecular weight distribution and minimizing structural heterogeneity. The resulting lignin self-assembled into highly uniform nanoparticles (LNPs) with a polydispersity index (PDI) of <0.1, a critical breakthrough for monodisperse nanomaterial fabrication. These LNPs further form ordered arrays via simple centrifugation, yielding vibrant biophotonic materials with tunable structural colors (e.g., pure blue) without additional chemicals. This work provides a sustainable, scalable route to convert nearly all lignin from raw biomass into high-value photonic nanomaterials, addressing key barriers in lignin valorization and advancing the bioeconomy.