Green Synthesis of Fluorescent Carbon Quantum Dots from Bearberry Extract via Hydrothermal and Microwave-Assisted Routes: Comparative Physicochemical Characterisation, Antioxidant Activity, and Biocompatibility Evaluation

Sustainable synthesis of photoluminescent nanomaterials with tuneable surface chemistry and defined biological activity remains a central challenge in green nanoscience. Here we show that the energy-input route used to carbonise a single bearberry ( Arctostaphylos uva-ursi ) extract precursor system exerts a decisive and mechanistically coherent influence over the surface chemistry, optical performance, and bioactivity of the resulting carbon quantum dots (CQDs). Hydrothermal processing (160 °C, 6 h) yields particles of 7.13 nm hydrodynamic diameter enriched in surface hydroxyl and carbonyl groups, a higher graphitic sp² carbon fraction (43.06%), and potent DPPH radical scavenging activity. In contrast, microwave-assisted synthesis yields 9.65 nm particles with a higher surface carboxylate content (O-C=O: 19.06%), enhanced fluorescence quantum yield, and increased intracellular uptake. Uptake is statistically significant in retinal epithelial cells at 200 µg/mL (p < 0.001) and shows concentration-dependent accumulation in zebrafish larvae from 100 µg/mL (p < 0.05). Combined XPS C 1s deconvolution and FTIR difference spectroscopy indicate that incomplete decarboxylation under microwave conditions underlies these distinct properties. Both formulations maintained full cytocompatibility across 10–250 µg/mL in both RPE-1 and HeLa cells, with no statistically significant reduction in viability at any tested concentration. These findings define a synthesis-route-encoded structure property relationship that enables rational selection between antioxidant-optimised and imaging-optimised CQD formulations from an identical green precursor system.