A glassy carbon electrode (GCE) was polished with fine emery paper and chamois leather containing Al2O3 slurry (first 0.3 μm then 0.05 μm), then ultrasonically cleaned in distilled water, and dried in air for use. As-made GQDs was dispersed in water and then ultrasonicated to create a 1 mg/mL good-dispersion which was used immediately. The GQDs modified electrode (GQDs/GCE) was obtained by first sonicating GCE in nafion/ethanol (1%) solution for 5 min, then coating with GQDs dispersion (6 μL) on the GCE surface, and drying in air at room temperature.