Structural and Functional Divergences of Lignin-Derived Carbon Dots and Their Nucleation Pathways under Varying Alcohol-Solvothermal Treatments

Abstract

Lignin-derived carbon quantum dots (L-CQDs) offer a promising avenue for lignin valorization due to their superior biocompatibility, tunable fluorescence, and diverse applications. Notably, conventional hydrothermal methods for L-CQDs synthesis suffer from low yields, suboptimal fluorescence, and wastewater management issues. This study systematically investigated the structural and functional divergences of L-CQDs synthesized via alcohol-solvothermal treatments and hydrothermal synthesis, with a focus on elucidating their distinct depolymerization and nucleation mechanisms. Key findings revealed that solvothermal treatments using alcohols with varying relative energy differences (REDs of 0.85–1.13) significantly enhanced lignin depolymerization. They improved the photoluminescence of L-CQDs compared to hydrothermal synthesis, with the quantum yields and fluorescence intensities showing increases of 2.31–7.25 times and 1.80–5.21 times, respectively. Moreover, time-dependent depolymerization and repolymerization was significant during the transition occurring at around 3–5 h of solvothermal synthesis; the abundant lignin monomers or oligmers served as effective precursors for carbonization nucleation. Monohydric alcohols were inclined to form a highly graphitized structure, while polyol modification of lignin (i.e., α-etherification, PhOH etherification, –COOH esterification) or its self-polymerization would increase chemically bonded defects introduced synchronously during the nucleation. Among them, glycerol-synthesized L-CQDs demonstrated good productivity (~9.4%) and biocompatibility (> 93% of cell viability rate at below 0.5 mg/mL), enabling targeted imaging of liver cancer cells. Overall, this work not only clarifies the nucleation mechanisms and solvent effects on L-CQDs formation but also provides a scalable and eco-friendly strategy for lignin valorization.