Hamelia patens -Derived Red-Emitting Carbon Quantum Dots: Surface-State Luminescence, Antioxidant Potency, and In Vitro Bioimaging

Red-emitting carbon quantum dots (HP-CQDs) were synthesised for the first time from aqueous leaf extracts of Hamelia patens through single-step, reagent-free microwave-assisted carbonisation (750 W). The resulting nanoparticles displayed a narrow hydrodynamic size distribution centred at 3.9 nm, consistent with atomic force microscopy measurements showing a maximum height of 2.81 nm. Under 400 nm excitation, the CQDs exhibited a characteristic red emission maximum at 675 nm, representing a rare example of long-wavelength-emitting green CQDs derived from plant biomass. UV-Vis absorption bands at 224 and 256 nm were assigned to π-π ^* transitions of aromatic carbon domains and n-π ^* transitions associated with carbonyl-containing surface groups, respectively. X-ray photoelectron spectroscopy (XPS) indicated a carbon-rich composition (C: 67.24%, O: 31.25%, N: 1.52%) with prominent C-O (42.67%) and C-C/C=C (42.64%) contributions. ATR-FTIR further confirmed the retention of hydroxyl, ether, and aliphatic functionalities following carbonisation. The excitation-wavelength-independent emission peak position implicates discrete surface molecular states rather than a heterogeneous distribution of emitters. HP-CQDs exhibit potent DPPH radical scavenging activity (IC ₅₀ = 141.8 µg mL ⁻¹ ), comparable to ascorbic acid (IC ₅₀ = 114.8 µg mL ⁻¹ ), and maintain >95% cell viability in both HeLa and RPE-1 cells up to 250 µg mL ⁻¹ . Confocal microscopy demonstrates concentration-dependent cytoplasmic accumulation and selective perinuclear localization at 300 µg mL ⁻¹ . In vivo biodistribution in zebrafish larvae confirms systemic uptake with statistically significant fluorescence enhancement at 500 µg mL ⁻¹ (p < 0.01), establishing HP-CQDs as biocompatible red-fluorescent probes with dual imaging-antioxidant functionality.