Synthesis of multifunctional sulfur-nitrogen co-doped carbon quantum dots via facile one-pot microwave-assisted synthesis: applications on antioxidant, antimicrobial activities, and Fe3+ ion sensing

In this work, a simple, inexpensive, and environmentally benign method has been developed to synthesize luminescent sulfur and nitrogen co-doped carbon quantum dots (S,N-CQDs) utilizing DL-DOPA, o-phenylenediamine, and sulfuric acid via microwave-assisted synthesis. The optical characteristics of the as-fabricated S,N-CQDs were analyzed using various spectroscopic techniques, including UV–Vis, fluorescence, and TCSPC techniques. For structural characterization, a comprehensive approach was employed, involving HR TEM, FE-SEM coupled with EDX, and XRD. Additionally, the functional groups and surface composition were identified through XPS, FTIR, and Raman spectroscopy. The thermal stability of the as-fabricated S,N-CQDs was assessed using thermogravimetric analysis (TGA), confirming their robust structural properties. The synthesized S,N-CQDs, with an average size of 9.3 nm, demonstrated impressive thermal stability, remarkable biocompatibility, and a high quantum yield of 17%, along with outstanding optical and chemical properties, and promising biological activities. They demonstrated excellent free radical scavenging activity (EC50: 61.26 µg/mL) and effective antimicrobial properties. Moreover, the as-fabricated S,N-CQDs exhibited outstanding selectivity and sensitivity toward Fe³⁺ ions, with a limit of detection (LOD) of 0.15 µM. Their ability to distinguish Fe³⁺ from other metal ions confirms their potential as fluorescent probes for Fe³⁺ detection in environmental and biological samples.

Graphical abstract