The carbon quantum dots with bule fluorescence were prepared by hydrothermal method and followed by filtration, centrifugation, steaming and dialysis purification methods using sodium cyanate as carbon source and melamine as nitrogen dopant. Their properties were characterized. TEM showed that the size of the prepared nanoparticles was about 5 nm. UV-Vis spectroscopy showed that there were two absorption peaks for carbon quantum dots. Fluorescence spectroscopy showed that carbon quantum dots had an excitation-independent emission at 436 nm with good stability in concentrated salt solutions and...