Carbon dots from natural sources as theranostic agents: integrating fluorescence and ROS generation for photodynamic therapy.

Carbon dots (CDs) have emerged as versatile nanostructures for biomedical applications due to their biocompatibility, tunable fluorescence and capacity to generate reactive oxygen species (ROS). In this work, CDs were synthesized from three fruit-derived natural sources (watermelon, strawberry and blueberry) using both undoped and urea-doped formulations and their physicochemical and photochemical properties were systematically compared. While zeta potential values differed depending on the carbon source, nitrogen doping caused a blue shift in excitation and emission peaks, indicating the formation of smaller optical bandgap states. The doped CDs displayed higher fluorescence quantum yields but reduced singlet oxygen production compared to their undoped counterparts, suggesting an inverse correlation between nitrogen incorporation and ROS generation. Among the undoped systems, blueberry-derived CDs exhibited the highest singlet oxygen yields. Cytotoxicity assays on SK-Mel-28 cells showed no significant effects in darkness. Upon irradiation, undoped CDs induced phototoxicity in a source-dependent manner, while doped CDs showed no intrinsic or photoinduced toxicity under the tested conditions. Overall, nitrogen doping enhances CD fluorescence while diminishing ROS generation efficiency. These results support the strategic use of different fruit-derived CDs to balance fluorescence and photodynamic activity and suggest that combining doped watermelon CDs with undoped blueberry CDs could maximize both optical and therapeutic performance.