Antipathogenic carbon dots synthesized from fluorescent amino acids and their photodynamic activity

Nitrogen-doped carbon dots (CDs) were prepared from essential fluorescent amino acids, such as lysine (Lys), phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp) by two routes, microwave and teflon-lined autoclave. The fluorescent properties of these amino acids were significantly improved in their corresponding CD forms, especially for those prepared via the hydrothermal process. A significantly high fluorescent intensity was measured in the emission range of 420-470 nm at the excitation wavelengths of 300-350 nm. While a negatively zeta potential value of - 9.8 ± 2.6 mV was obtained for Lys CDs suspension, the CDs of Phe, Tyr, and Trp afforded strong positive zeta potentials, + 24.0 ± 2.4, + 12.5 ± 3.1, and + 31.9 ± 3.8 mV, respectively. The Lys CDs were found not to be antimicrobial even at a 10 mg/mL concentration, which is likely due to their negative surface charge that weakens electrostatic interactions with negatively charged microbial membranes. Whereas the Phe, Tyr, and Trp CDs had a 2.5 mg/mL minimum inhibition concentration (MIC) against a Gram-negative bacterium, Klebsiella pneumoniae. The Phe CDs commenced the highest antibacterial effect against Bacillus subtilis (ATCC 6633) Gram-positive bacteria, but the lowest MIC value of 1.25 mg/mL was determined for Tyr CDs against Candida albicans (ATCC 10231) fungus. Furthermore, a UV light exposure, 30 min treatment of UV-A light with a 6.88 mW/cm² irradiance value on amino acids CD exhibited improved photodynamic activity. The natural amino acid-derived CDs show great biocompatibility on L929 fibroblast cells with > 86% cell viability, for all formulations except Tyr CDs, retaining 78 ± 3% viability even at 1000 μg/mL concentration, and blood compatibility at 500 μg/mL concentration. Therefore, these CDs derived from fluorescent amino acids are photoactivated and are of excellent nanosized materials in a variety of biomedical in vitro and in vivo uses, including diagnostic, sensor, and therapeutic applications.