The preparation of nitrogen-doped graphene quantum dots (N-GQDs), by a simple and inexpensive synthetic procedure, is reported with blue emission and a high quantum yield of 78%. This procedure involves synthesis via the pyrolysis of citric acid, as a source of carbon, and tris-hydroxymethyl aminomethane, as a surface passivation agent. These N-GQDs have low toxicity and are photostable. They displayed excitation-independent behavior, although their fluorescence intensity was linearly dependent on pH values ranging from 3 to 10. The quenching of the fluorescence intensity of the N-GQDs increases, in a relatively very small amount of time, with the addition of sunitinib varying from 0.05 to 20.00 mmol L 1 and with a detection limit of 0.03 mmol L 1 . The N-GQDs have low toxicity and are pH-sensitive between 3.00 and 10.00, giving a general pH-sensor with a wide range of applications in real urine and serum samples.
