To study the effect of the kind of nitrogen source on the structure and fluorescence properties of nitrogen-doped carbon dots (NCDs) and the selectivity and sensitivity of NCDs towards Hg2+ detection,this paper aims at the preparation of NCDs on the basis of preliminary researches in our group,in which NCDs were synthesized by a one-step hydrothermal method using citric acid as carbon source,urea and ammonia solution as nitrogen source,respectively.The effect of nitrogen source on the structure,fluorescence properties of NCDs and the selectivity and sensitivity of NCDs towards Hg2+ detection was investigated by high-resolution transmission electron microscopy, X-ray diffraction, Fourier transform infrared, X-ray photoelectron spectroscopy, elemental analysis, ultraviolet-visible absorption and photoluminescence. Results show that different nitrogen sources have obvious effect on carbonization degree,nitrogen doping degree,surface functional groups,particles size and quantum yield,but less influence on ultraviolet-visible absorption and photoluminescence spectra.The two kinds of NCDs used for fluorescence probe exhibit similar high selectivity towards Hg2+.However,the NCDs prepared by using ammonia solution as nitrogen source,possess lower limit of detection and wider linear range for Hg2+ detection.That is to say,higher nitrogen content of nitrogen source could contribute to higher nitrogen doping degree of NCDs,and more amino and amide bonds on the surface of NCDs,which lead to more active sites of NCDs.When NCDs and Hg2+ are complexed,the NCDs can exhibit highly sensitive detection of Hg2+ because of more complexation sites.