Preparation of a Carbon Quantum Dot from Star Anise and Its Applications as an Antibacterial Agent and as a Probe for Sensing of Permanganate Ions

A cost-effective and efficient one-step hydrothermal method was employed to synthesize fluorescent carbon quantum dots (CQDs) by using star anise as the precursor, eliminating the need for chemical reagents. The synthesized CQDs were characterized using multiple analytical techniques, indicating that the synthesized CQDs revealed a mean particle size of 1.9 nm, demonstrating excellent dispersion and a spherical morphology. Under 365 nm UV light, the CQDs emitted blue fluorescence. The fluorescence spectra showed that the CQDs had good luminescence performance and the fluorescence had typical excitation dependence. The prepared CQDs exhibited remarkable antibacterial efficacy against Escherichia coli and Staphylococcus aureus. Additionally, the determination of the minimum bacteriostatic concentration revealed a stronger antibacterial efficacy of CQDs against both bacterial strains. Therefore, it has a good development potential as a green antibacterial agent. The CQDs synthesized in this study demonstrate remarkable solubility in hydrophilic solvents such as water and ethanol, along with excellent stability across different conditions. Furthermore, the CQDs served as a probe with biocompatibility to detect permanganate ions within the concentration range of 200–700 μmol/L. A strong linear correlation (R2 > 0.99) was observed between fluorescence intensity and MnO4– concentration. The detection of MnO4– in actual water samples showed that the recoveries and relative standard deviations of MnO4– were 93.15%–107.06% and 1.91%–4.47%, respectively, indicating that MnO4– could be taken as fluorescent probes for detecting permanganate ions in aqueous solutions.