Nitrogen and Sulfur Co‐Doped Fluorescent Carbon Dots Constructing PVA@N, S‐CDs Composite Film: A Next‐Generation Food Packaging Material With Dual Functions of Chlortetracycline Detection and Ultraviolet Shielding

Nitrogen and sulfur co-doped carbon dots (N, S-CDs) were synthesized through a facile one-step hydrothermal approach that employed L-proline as the carbon precursor and methyl orange as the dopant. The as-prepared N, S-CDs exhibited robust cyan photoluminescence with a high quantum yield of 22.5%. Because of their superior optical properties, these N, S-CDs were developed as a sensitive fluorescent probe for the selective detection of chlortetracycline (CTC). The sensing mechanism was attributed to dynamic quenching, yielding a wide linear response from 0 to 25 µmol·L^-1 (R² = 0.998) and a low detection limit of 0.94 µmol·L^-1. The practical utility of this probe was validated in environmental water and milk samples, achieving satisfactory recoveries ranging from 83.67% to 94.46%. Additionally, multifunctional PVA@N, S-CDs composite films were fabricated by incorporating N, S-CDs into a polyvinyl alcohol (PVA) matrix via a solvent-casting method. These flexible and highly transparent films demonstrated exceptional UV protection, effectively shielding over 99.5% of UV-C and UV-B radiation while converting the absorbed energy into visible light through the down-conversion effect. Application trials using grapes as a food model confirmed that the composite films significantly mitigated UV-induced deterioration and moisture loss. Overall, this work presented a versatile carbon dot-based platform that integrates specific chemical sensing with efficient UV-shielding, providing a promising strategy for advanced applications in smart food packaging and environmental monitoring.