Influence of synthesis temperature on the active performance of doped‑carbon dots embedded in polyvinyl alcohol and their potential for active food packaging

Abstract

This study explores how synthesis temperature influences the properties and functionalities of sulfur‑nitrogen doped carbon dots (S,N-doped CDs) derived from citric acid and cysteine. The focus lies on their potential in bionanocomposite films for active packaging. Bionanocomposite films were prepared by incorporating CDs synthesized at different temperatures (160, 180, and 200 °C) into a polyvinyl alcohol (PVOH) matrix. CD160 displayed a crystalline structure below 10 nm and demonstrated the strongest antifungal and antibacterial activity, as evidenced by the largest clear zones. This superior performance is presumably due to their high N and S content, as shown by the larger redshift in their photoluminescence spectra upon longer wavelength excitation and elemental analysis. CD180 lacked well-defined lattice fringes, indicating an amorphous structure with larger cluster sizes ranging from 10 to 100 nm. Interestingly, UV-blocking and antibacterial efficacy of both films remained high (over 95% UVA block and 5 log₁₀ CFU/mL reduction in bacteria) regardless of CD crystallinity and size. However, the PVOH-CD180 film exhibited more photoluminescence, most likely due to larger defect-related emission centers. In contrast, CD200 had the weakest antifungal and antibacterial activity. While the PVOH-CD200 film achieved a 5 log₁₀ CFU/mL reduction in Gram-positive bacteria, it only managed a 4 log₁₀ CFU/mL reduction in Gram-negative bacteria under contact conditions at 37 °C for 24 h. In addition, the PVOH-CD200 film had the highest antioxidant activity, 88% DPPH radical scavenging, whereas the others had 80%. These findings underscore the critical role of synthesis temperature in tailoring S,N-doped CDs for specific applications within active packaging, where the desired properties dictate the optimal temperature. Preliminary studies suggest that PVOH-CD films have the potential to extend the shelf-life of sweet bread, demonstrating their promise as bionanocomposite active food packaging materials.