Effect of amino acid-derived nitrogen and/or sulfur doping on the visible-light-driven antimicrobial activity of carbon quantum dots: A comparative study

The purpose of this study was to investigate the effects of nitrogen and sulfur derived from amino acids on the visible-light-driven (VLD) antimicrobial activity of carbon quantum dots (CQDs) and to establish an optimal doping form. Various CQDs were synthesized by combining malic acid as a carbon source, alanine containing N, and cysteine containing N and S through microwave heating. The results showed that the VLD antimicrobial activity of the CQD against Gram-positive (Staphylococcus aureus and Listeria monocytogenes) and Gram-negative pathogens (Escherichia coli O157:H7 and Salmonella Typhimurium) was improved by N and/or S doping, which increased as the ratio of S decreased. The doping of N and/or S made CQDs have a lower band gap energy, and CQD doped with only N (N-CQDs) exhibited greater specific surface area and fluorescence quantum yields (FLQYs), and longer photoluminescence (PL) lifetimes than CQD doped with N and S (NS-CQDs) even though NS-CQD and N-CQD had similar hand gap energies. Based on these results, the mechanism by which the VLD antimicrobial activity of CQD changes by doping was discussed. Furthermore, this study suggests that for CQD synthesis using food by-products, the more amino acids that can induce the doping of N and/or S, wherein the smaller ratio of S containing amino acids, the better VLD antimicrobial activity the synthesized CQD will have.