From biomass deconstruction to reactive plasticization: A trifunctional l-cysteine deep eutectic solvent for the...

To overcome the structural recalcitrance of lignocellulosic biomass (LCB) via a sustainable green fractionation pathway, this study establishes an integrated 'dual-stream' valorization protocol for Tea Stems (TS) driven by the rational design of a novel trifunctional Deep Eutectic Solvent (DES) composed of Choline Chloride and l-Cysteine (ChCl:l-Cys). Uniquely, this system leverages the intrinsic acidity of l-Cys for efficient biomass deconstruction, its thiol (-SH) functionality to suppress lignin condensation, and its hydrogen-bonding capacity for downstream plasticization. In the fractionation phase, optimized pretreatment conditions (130°C, 1 h, 1:15 S:L) facilitated 54.71% lignin extraction (64.19% delignification) while yielding a cellulose-rich residue with a high glucose conversion yield of 84.43% (with tween-80). Structural analysis revealed that the DES effectively disrupted the crystalline structure of cellulose, as evidenced by altered inter/intramolecular H-bonding, increased accessible fibril surface area (AFS), expanded less-ordered regions, and Cellulose II formation, while simultaneously preserving the native β-O-4 linkages and aromatic integrity of the extracted lignin (S/G ratio: 0.23) compared to native TS lignin (S/G 0.26). Transitioning the DES from a disruptive agent to a constructive plasticizer, a 'DES-mediated re-solvation' strategy was employed to fabricate PVA composite films that circumvent the energy-intensive synthesis of Lignin Nanoparticles (LNPs) by directly dissolving the extracted lignin (l-Cys protected lignin) into the DES solvent to ensure homogeneous nano-dispersion (Avg. particle size 331 nm, and 0.451 PDI). The resulting PVA-based bio-composite films exhibited synergistic enhancements in mechanical properties, UV protection, and antioxidant activity, demonstrating a closed-loop strategy for converting agro-industrial waste into high-value functional materials.