Endogenous Carbon Dots in Traditional Korean Beverages: Structural Characterization, Antioxidant Activity, and Biocompatibility

Carbon dots (CDs) are emerging nanomaterials with promising applications in food science. Recent reports have shown that carbon dots are inherently present in heat-treated foods and beverages. However, the occurrence of carbon dots in traditional Korean beverages has not yet been investigated. In this study, carbon dots were isolated from three Korean beverages, Nurungji tea, barley tea, and green tea, and characterized using TEM, DLS, ζ-potential, UV-absorbance, photoluminescence (PL), FTIR and XPS analyses. All three beverages contained quasi-spherical (<10 nm) CDs, consistent with quantum dots whose optical properties depend on their size. DLS and ζ-potential measurements (−40 mV for cereal tea and −14 mV for green tea) confirmed their colloidal stability without aggregation in the beverages. PL spectroscopy exhibited excitation-dependent emission with a bathochromic shift, peaking at 412–438 nm, and FTIR spectra revealed abundant O-H, N-H, C=O, and C-N functional groups, reflecting oxygen and nitrogen doping that modulate redox reactions. Due to these surface functional groups, CDs demonstrated excellent antioxidant activity, with green tea CDs achieving 100% scavenging activity at 12.5 µg/mL, while barley and Nurungji CDs reached 100% and 78% scavenging activities at 100 µg/mL, respectively. In the cytotoxicity test using L929 fibroblast cells, grain tea CDs showed a survival rate of over 90% at concentrations of 6.25–100 µg/mL, and green tea CDs showed a survival rate of over 90% at concentrations up to 25 µg/mL, which is consistent with the literature on the biocompatibility of CDs. These results confirm that beverage-derived CDs are non-toxic and powerful antioxidants, reaffirming the safety and functionality of traditional Korean food.