Carbon Quantum Dots and N-Doped Carbon Quantum Dots from Dried Mulberry Aqueous Extract: Preparation, Characterization, and Photodynamic Antibacterial Performance

Carbon quantum dots (CQDs) have attracted significant interest due to their intriguing physicochemical properties, and those derived from natural sources can inherit traits of the source material. Here, we report a facile one-step hydrothermal synthesis of fluorescent CQDs and nitrogen-doped CQDs (N-CQDs) using dried mulberry (fruit of Morus nigra L.) aqueous extract as the carbon precursor, with urea as the nitrogen dopant for N-CQDs. X-ray photoelectron spectroscopy (XPS) confirmed that N-CQDs contained a higher nitrogen and a lower oxygen content, compared with CQDs. Transmission electron microscopy (TEM) revealed spherical nanoparticles with diameters of 0.5–4.5 nm for CQDs and 1.2–2.6 nm for N-CQDs. CQDs exhibited concentration-dependent photoluminescence, shifting from strong blue to green emission, whereas N-CQDs consistently showed intense blue emission. In photodynamic antibacterial tests against Escherichia coli and Staphylococcus aureus, CQDs had a minimum inhibitory concentration (MIC) of 0.48 mg/mL after 1 h of 400–410 nm illumination, whereas N-CQDs had an MIC of only 0.06 mg/mL at 450–460 nm. This markedly enhanced antibacterial performance of N-CQDs is attributed to their higher production of reactive oxygen species (ROS) under illumination, especially superoxide radical (·O₂⁻). Therefore, this process can expand the potential application of mulberry, enabling deeper exploitation.