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

Producing photoluminescent nanomaterials with controllable surface chemistry and predictable biological activity remains one of the outstanding problems in green nanoscience. The present study shows that, even when the same bearberry (Arctostaphylos uva-ursi) extract precursor is used, the mode of energy delivery during synthesis plays a determining role in shaping the surface composition, photophysical properties, and biological activity of the resulting carbon quantum dots (CQDs). Hydrothermal processing at 160 {degrees}C for 6 h yielded CQDs with an average particle size of 7.13 nm. Surface characterisation indicated abundant hydroxyl- and carbonyl-containing functionalities, while XPS analysis showed a comparatively higher proportion of graphitic sp2 carbon (43.06%). These structural features were accompanied by strong DPPH free-radical scavenging activity. Microwave-assisted synthesis, by contrast, yields 9.65 nm particles carrying a substantially greater surface carboxylate content (O-C=O: 19.06%), a higher fluorescence quantum yield, and enhanced intracellular uptake statistically significant in retinal epithelial cells at 200 {micro}g/mL (p < 0.001) and showing concentration-dependent accumulation in zebrafish larvae from 100 {micro}g/mL onwards (p < 0.05). XPS C 1s deconvolution, interpreted alongside FTIR difference spectroscopy, points to incomplete decarboxylation under microwave conditions as the primary mechanistic origin of these divergent properties. Cytocompatibility was uncompromised for both formulations across the full concentration range tested (10-250 {micro}g/mL) in RPE-1 and HeLa cells, with no statistically significant loss of viability at any concentration. Taken together, these results define a synthesis-route-encoded structure-property relationship that permits rational selection between an antioxidant-optimised and an imaging-optimised CQD formulation from the same green precursor feedstock.