Green synthesis of nitrogen–sulfur co-doped carbon quantum dots from rice straw: Optical properties, antibacterial activity, and toxicity evaluation in cells and zebrafish embryos

Nitrogen and sulfur co-doped carbon quantum dots (NS-CQDs) were synthesized from rice straw using two sustainable bottom-up approaches, microwave-assisted carbonization (MCW) and hydrothermal treatment (HTC), to systematically investigate how the synthesis route influences their physicochemical properties and biological activity. TEM, FT-IR, XPS, UV–Vis absorption, and photoluminescence analyses confirmed successful N,S co-doping, nanoscale morphology, and strong blue fluorescence. The HTC-derived NS-CQDs exhibited a higher quantum yield (39.7%) than the MCW-derived sample (24.9%), demonstrating that the synthesis route significantly influences the optical properties of the resulting nanomaterials. Biological evaluation revealed high biocompatibility toward human skin fibroblast cells and zebrafish embryos at concentrations up to mL −1 , while a concentration-dependent reduction in viability was observed in HeLa cervical cancer cells. Antibacterial assays against Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus demonstrated synthesis- and strain-dependent antibacterial activity. Statistical analysis (ANOVA/LSD, ) further confirmed a significant enhancement of antibacterial performance under 630 nm irradiation, particularly against S. aureus . Complementary DPBF measurements indicated photoinduced reactive oxygen species (ROS) generation, supporting a ROS-mediated contribution to the observed antibacterial activity. This work demonstrates an effective strategy for converting agricultural waste into multifunctional NS-CQDs and provides new insights into the interplay between synthesis route, heteroatom incorporation, physicochemical properties, antibacterial activity, and biosafety, thereby supporting the rational design of sustainable CQD-based nanomaterials for biomedical applications.