Waste-To-Wealth Valorization Of The Invasive Grass Leersia Hexandra Into Fluorescent Carbon Dots Via Microwave-Assisted Synthesis: Multi-Technique Characterization And DPPH Radical-Scavenging Potential

Carbon dots (CDs) are zero-dimensional, quasi-spherical carbon nanostructures that have attracted enormous research interest owing to their tunable photoluminescence, low toxicity, aqueous dispersibility, and facile, low-cost synthesis from renewable precursors. In the present study, fluorescent carbon dots were synthesized in a single step from Leersia hexandra, an invasive perennial grass weed of paddy ecosystems, using a rapid, solvent-free microwave-assisted route (total irradiation time of 8 min, 640 W, pulsed 30 s on/30 s off), thereby converting an agronomic waste biomass into a value-added nanomaterial. The resulting carbon dots (LH-CDs) were purified by sequential filtration through muslin cloth, Whatman No. 1 filter paper, centrifugation and 0.22 µm nylon syringe filtration, and were characterized by UV-Visible absorption spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, fluorescence spectroscopy and dynamic light scattering-based zeta potential analysis. UV-Vis analysis revealed characteristic π→π* and n→π* absorption features, from which direct and indirect optical band gaps of 3.75 eV and 2.13 eV were extracted using Tauc plots. FTIR confirmed a hydrophilic, oxygen/nitrogen-rich surface bearing hydroxyl, carboxyl and amine functionalities, while deconvoluted photoluminescence spectra revealed three overlapping emissive states centred at 527, 558 and 614 nm. Zeta potential measurement indicated a weakly negative surface charge of −3.26 ± 6.68 mV. The LH-CDs exhibited concentration-dependent DPPH free-radical scavenging activity comparable to, and at low concentrations exceeding, that of ascorbic acid, with an estimated half-maximal scavenging concentration below 1 µg/mL. These results revealed L. hexandra as an unexplored, abundant, and cost-free precursor for the green synthesis of antioxidant carbon dots, underscoring a practical waste-to-wealth strategy for agricultural weed management and functional nanomaterial development.