Synthesis of graphene quantum dots from Moringa oleifera seed biomass doped with phosphate

In this work, graphene quantum dots (GQDs) were synthesized from Moringa oleifera seed biomass doped with phosphorus, using a low-cost, renewable hydrothermal method. The biomass was ground and sieved (30 mesh), and the classified powder was transferred to a Teflon capsule, placed in a stainless-steel reactor (210 ºC, 24 h). Afterwards, 20 mL of distilled water were added to the synthesized GQDs, stirred for 30 min, and filtered through Whatman filter paper. Then they were centrifuged (12000 rpm, 10 min) and filtered through a 0.22 μm membrane. The filtrate was dialyzed in a cellulose dialysis bag, with a cut-off size of 3.5 kDa, for 7 days in absolute ethanol (99.5%), with gentle stirring, and the dialysate was exchanged every 24 h. The dialysate was dried at 105 ºC for 24 h. The GQDs were characterized by X-ray energy dispersive spectroscopy (EDX), Fourier-transform infrared spectroscopy (FTIR), UV-vis electronic absorption, fluorescence emission, X-ray Diffraction and high-resolution transmission electron microscopy (HRTEM). The results show the GQDs with an average size of 18.63 ± 4.24 nm and high fluorescence (bright blue) under ultraviolet light (365 nm). Our GQDs are promising for application in bioimaging and bioanalysis.