Microwave-assisted synthesis of sulfur co-doped carbon dots from 2-thiobarbituric acid: insights into core condensation and antibacterial/antifungal properties.

Abstract

Carbon dots (CDs) have emerged as a versatile class of nanomaterials owing to their tunable optical behaviour, facile synthesis, and low toxicity. Sulfur doping has the potential to impart antimicrobial/antifungal properties to CDs. To avoid issues related to multiple sourcing for sulphur-doping of CDs, 2-thiobarbituric acid (2-TBA) was used as a precursor. Using a rapid, one-step microwave-assisted carbonization, without any additional doping or functionalization, 2-TBA-CDs were synthesised and purified by dialysis. CDs were characterized using fluorescence spectroscopy, FTIR, HPLC, CHNS elemental analysis, XRD, TEM, SAED, and zeta potential measurements. This confirmed carbonization of the precursor with retention of oxygen-nitrogen-sulfur functional groups inherited directly from the 2-TBA. The CDs with an atomic ratio of C27.2N4.2O5S1, displayed bright blue fluorescence with a quantum yield of 13.07%, an average diameter of ∼5.4 nm, and a negative zeta potential of -27.99 ± 0.9 mV, indicating good colloidal stability. Evaluation by broth microdilution against standard strains of Staphylococcus aureus and Escherichia coli returned MIC values of 0.195 µg mL-1 and 1.56 µg mL-1, respectively, showing marked improvement over the precursor. The CDs also showed pronounced antifungal activity against Pleurotus pulmonarius, producing a 29 ± 0.7 mm zone of inhibition that exceeded the reference drug fluconazole.

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