Hydrothermal Synthesis of Water-Dispersible Fluorescent Carbon Dots from Renewable Plant Precursors: Antibacterial Activity and Comparative Characterization

We present here a one-pot hydrothermal synthesis of fluorescent carbon dots (CDs) using olive, rosemary, sage, and thyme leaves as a carbon source. The hydrothermal reaction was conducted at 180 °C for 12 h, thereby converting biomass into water-dispersible CDs without a passivating agent. The average size of the CDs, determined by TEM, was 3–6 nm, with a spherical morphology. All synthesized CDs displayed excitation-dependent fluorescence, with emission maxima at 400–440 nm (blue) with excitation at 340–370 nm. FLIM measurements yielded similar fluorescence lifetimes of approximately 5 ns in aqueous solutions of CDs. FTIR spectra indicated surface hydroxyl and nitrogen-containing functionalities that may contribute to photoluminescence. The antibacterial activity of the as-synthesized CDs against a panel of Gram-positive and Gram-negative bacteria (Staphylococcus aureus, Listeria innocua, Escherichia coli, Pseudomonas aeruginosa, methicillin-resistant S. aureus (MRSA), multidrug-resistant (MDR) E. coli, and Klebsiella pneumoniae (MDR)) was investigated. It was shown that biomass-derived CDs exhibit antibacterial activity, being more effective against Gram-positive bacteria, with MIC values as low as 0.625 mg/mL for olive-, rosemary-, and thyme-derived CDs against S. aureus and MRSA. These findings suggest that the developed CDs are fluorescent materials offering viable possibilities in biomedical research, as well as antimicrobial agents.