Green conversion of a soft commodity into visible light-activated N–S doped carbon quantum dots with antibacterial properties

Abstract

Carbon quantum dots (CQDs), with particle sizes less than 10 nm, have been introduced as a promising alternative for water antimicrobial applications. In this study, the authors proposed an effective, green, and sustainable method for producing nature-based nitrogen–sulphur-doped carbon quantum dots (N–S-CQDs) from sugar and onion through a one-step microwave reaction carbonization process. Results showed that N–S-CQDs processed optimally with a 7 : 3 ratio of sugar and onion, at a pH of 7–8, microwave power of 850 W, and a synthesis time of three minutes, exhibited the best fluorescence characteristics. The synthesized sample exhibited excitation within the visible region, with particle sizes of 4 to 6 nm, and amorphous characteristics. Cytotoxicity tests were performed using HeLa cells, with tetracycline as a positive control. Results indicated that the test material exhibits a non-significant cytotoxic effect on HeLa cells, with an IC50 value below 58 mg mL⁻¹. This cytotoxic effect is relatively low compared to that of carbon dots, which caused significant toxicity at 2 mg mL⁻¹. Zeta potential test demonstrated good stability of electrical charge on the surface of N–S-CQDs. According to SEM results, N–S-CQDs caused bacterial cell surface destruction, likely due to changes in the charge balance of the bacterial surface, leading to membrane disruption in both Gram-negative E. coli and Gram-positive S. aureus strains. A promising antibacterial effect was observed in Gram-negative E. coli strains, comparable to those of commercial antibiotics like tetracycline. For further verification of the antimicrobial effect, Gram-negative bacteria such as Salmonella typhimurium and Vibrio parahaemolyticus were tested, showing significant zones of inhibition at concentrations of 125 mg mL⁻¹, 250 mg mL⁻¹, and 500 mg mL⁻¹.