Biomass-Derived Red-Blue Solvatochromic Carbon Quantum Dots for UV-Blocking and Active Packaging Applications

In this research, biomass-derived carbon quantum dots (CQDs) with solvent-dependent emission colors were synthesized from mangosteen leaves via a solvent-mediated hydrothermal route, in order to elucidate the relationship between structure and emission and to evaluate their potential for multifunctional packaging applications. Systematic morphological and structural analyses demonstrated that the solvent polarity governed the degree of carbonization, particle size, and surface chemistry. Red-emissive CQDs (R-CQDs) were formed in a less polar medium and exhibited larger sp² domains enriched with hydroxyl groups, whereas blue-emissive CQDs (B-CQDs) synthesized in a polar solvent had smaller sp² domains with higher values for the content of oxygen- and nitrogen-containing functionalities. These distinctions resulted in excitation-independent red emission for R-CQDs and excitation-tunable blue-green emission for B-CQDs. R-CQDs showed enhanced antioxidant activity (IC₅₀ = 8.84 μg mL^-1) relative to B-CQDs (IC₅₀ = 21.03 μg mL^-1), while antibacterial activity against Escherichia coli and Staphylococcus aureus was observed exclusively for R-CQDs, indicating a structure-dependent antibacterial response. When the CQDs were incorporated into poly-(vinyl alcohol) (PVA) matrices, the nanocomposite films demonstrated enhanced UV-blocking performance, with R-CQD/PVA film giving stronger attenuation due to enhanced π-π* transitions and more efficient interfacial energy transfer. Storage tests also showed that when tomatoes were packaged with R-CQD/PVA film, ripening was delayed by approximately 7-14 days compared to the control films. These findings establish clear correlations between solvent, structure, and function for biomass-derived CQDs and suggest a sustainable strategy for designing multifunctional nanocomposite films for UV-protective and active food-packaging applications.