Bright and Stable Carbon Quantum Dots for Optical and Antimicrobial Applications

Carbon quantum dots (CQDs) co-doped with nitrogen and sulfur (N,S-CQDs) have recently attracted attention as sustainable nanomaterials combining optical tunability with biological functionality. In this work, we demonstrate a green solvothermal route for synthesizing highly luminescent N,S-CQDs using citric acid and N-acetyl-L-cysteine as environmentally benign carbon and heteroatom sources. The synthesized nanodots exhibit bright, excitation-dependent photoluminescence, abundant surface functional groups, and near-neutral zeta potential, ensuring good colloidal stability in aqueous media. Spectroscopic analysis confirmed efficient charge transfer between nitrogen- and sulfur-containing moieties, leading to enhanced radiative recombination and fluorescence intensity. Beyond their optical properties, the N,S-CQDs displayed broad-spectrum antimicrobial and antibiofilm activity, effectively inhibiting both Gram-positive and Gram-negative bacterial strains as well as Candida albicans. Biofilm disruption efficiency reached up to 74%, and inhibition of biofilm formation ranged between 75% and 85%. Remarkably, their emission spectra remained unchanged after microbial exposure, revealing outstanding optical stability under biological conditions. These results highlight the multifunctional nature of N,S-CQDs, linking structural design and photophysical behavior to biological performance. The study opens a path toward next-generation carbon-based materials that merge photonic and biomedical capabilities within a single, eco-friendly nanoplatform.