Microwave-Assisted Green Synthesis of Fluorescent Graphene Quantum Dots: Metal Sensing, Antioxidant Properties, and Biocompatibility Insights

Graphene quantum dots (GQDs) are highly valued for their chemical stability, tunable size, and biocompatibility. Utilizing green chemistry, a microwave-assisted synthesis method was employed to produce water-soluble GQDs from Mangifera Indica leaf extract. This approach is efficient, cost-effective, and environmentally friendly, offering reduced reaction times, energy consumption, and uniform particle sizes, and has proven advantageous over other methods. Water-soluble GQDs were synthesized using Mangifera Indica leaf extract, which ranged less than 15 nm in diameter, confirmed by high-resolution transmission electron microscopy with a lattice spacing of 0.34 nm. The GQDs exhibited strong photoluminescence with bright red fluorescence under UV light and excitation-independent emission at 662 nm with excitation wavelengths ranging from 300 to 500 nm, achieving a quantum yield of 10.3%. A peak at 27.2˚ was recorded corresponding to the graphite's (002) plane diffraction peak. Raman spectroscopy confirmed their graphitic nature and sp² crystallinity, with an intensity ratio of D and G peak I<inf>D</inf>/I<inf>G</inf> ratio of 1.12. Biocompatibility assays (MTT and live/dead) showed better results at lower concentrations (1 mg/ml) while higher concentrations (2 mg/ml) showed reduced efficacy. Antioxidant tests revealed increased DPPH scavenging activity with higher GQD concentrations and longer incubation times. The GQDs demonstrated excellent performance as fluorescent biosensors for Ni²⁺ (0.15 ppm) and Fe³⁺ (0.20 ppm), with high selectivity in river water samples, highlighting their potential for environmental and health applications.