O/C Ratio-Driven Fluorescence Enhancement in Cellulose-Derived Carbon Quantum Dots: Mechanistic Insights into Reaction Pathways.

By regulating cellulose molecular weight through enzymatic hydrolysis to obtain different carbon precursors, three types of carbon quantum dots (CQDs) are synthesized via hydrothermal methods: cellulose enzyme-hydrolyzed solids-based carbon quantum dots (CES-CQDs), cellulose enzyme-hydrolyzed mixtures-based carbon quantum dots (CEM-CQDs), and cellulose hydrothermal degradation products-based carbon quantum dots (CHD-CQDs). By controlling the molecular weight of the cellulose precursor, the O/C ratio of the CQDs is systematically modulated from 0.25 to 0.61, resulting in a more than five-fold increase in fluorescence intensity and an approximately seven-fold improvement in quantum yield (QY). Density functional theory (DFT) calculations indicate that high O/C ratio enhances oscillator strength, thereby boosting fluorescence. Through a combination of experimental and density functional theory analyses, the formation mechanisms of cellulose-derived CQDs are revealed to involve: primarily the auto-etherification of 5-hydroxymethylfurfural (5-HMF), concurrently accompanied by esterification reactions between 5-HMF and formic acid (FA), as well as aldol condensation reactions between 5-HMF and levulinic acid (LA). This study elucidates the fundamental relationship between the molecular structure of cellulose precursors and the fluorescence properties of CQDs, providing a universal strategy for rationally designing high-performance luminescent nanomaterials from renewable biomass.