Synthesis of sulfur-doped carbon quantum dots from <i>Solanum nigrum</i> for ciprofloxacin detection in yogurt samples and antibacterial activity

In this study, S-CQDs were fabricated through a green and secure hydrothermal process, the carbon source was derived from the traditional Chinese medicinal herb <i>Solanum nigrum</i> and thiourea served as the sulfur precursor. The characterization results indicated that the S-CQDs presented a spherical shape, featuring a mean particle diameter of 3.96 nm and a lattice spacing measuring 0.19 nm. Surface functional groups such as amides, amino groups, and carboxyl groups were identified, contributing to their physicochemical properties. The S-CQDs demonstrated high selectivity and sensitivity toward CIP, spanning a linear detection range covering 0-0.4 μmol L^-1. Additionally, these materials displayed substantial antibacterial efficacy against both <i>S. aureus</i> and <i>E. coli</i>, with MIC values of 1.6 mg mL^-1 and 0.8 mg mL^-1 for each bacterium, respectively. Mechanistic investigations indicated that the positively charged S-CQDs electrostatically interacted with negatively charged bacterial membranes, disrupting membrane integrity and ultimately leading to bacterial cell death. This work establishes a novel approach for developing multifunctional S-CQDs, highlighting their promising applications in clinical diagnostics, environmental monitoring and antimicrobial therapies.