Graded surface-engineered lignin-derived carbon dots for tunable multicolor anti-counterfeiting films

Abstract

Multicolor luminescence modulation of lignin-derived carbon quantum dots (LD-CQDs) is a key prerequisite for anti-counterfeiting applications; however, achieving this through surface engineering using a single precursor remains challenging. This study develops a stepwise surface chemical modulation strategy using alkali lignin (AL) as the sole carbon source to achieve continuous photoluminescence (PL) tuning of LD-CQDs from blue to yellow. Blue-emitting LD-CQDs (L-CQDs) were first synthesized via choline chloride-assisted hydrothermal treatment. Subsequent synergistic nitrogen/sulfur (N/S)-codoping with 2,4-diaminobenzenesulfonic acid produced green emissive LD-CQDs (N-CQDs) with a markedly enhanced quantum yield of 16.3%. Controlled nitric acid oxidation of N-CQDs further yielded oxygen-doped LD-CQDs (O-CQDs) enriched with electron-withdrawing oxygen-containing moieties, resulting in a pronounced PL red shift to the yellow region. Comprehensive structural characterizations confirmed the progressive incorporation of N/S/O heteroatomic functional groups and elucidated their strong structure–property correlations with the sequential PL red shift. Polyvinyl alcohol (PVA) served as a dispersing stabilizer and film-forming matrix to fabricate carbon quantum dot/polyvinyl alcohol (CQDs/PVA) printed fluorescent films (CPPFs) and composite fluorescent films (CPCFs) via screen printing and template casting, respectively. The resulting films exhibited excellent fluorescence and ultraviolet-shielding performance, providing a robust basis for anti-counterfeiting packaging applications. Overall, this work establishes an efficient and generalizable surface-engineering strategy for precise PL regulation of LD-CQDs, underscoring their potential for rapid-response anti-counterfeiting technologies and functional packaging materials.