A kind of nitrogen doped carbon quantum dots(N-CQDs),with economy,low toxicity and even strong fluorescence performance,were successfully synthesized by hydrothermal method using L citrulline and urea as precursors.The performance of N-CQDs was characterized by transmission electron microscopy(TEM),Fourier transform infrared spectroscopy(FT-IR) and X-ray photoelectron spectroscopy(XPS),etc.Results showed that the quantum yield of the N-CQDs reached up to 39.2% with an average particle size of 2.8 nm,which could be used for HepG2 cell imaging in view of their good water solubility,stability and biocompatibility.In addition,a fluorescence probe with high selectivity and sensitivity was proposed for the detection of o-nitrophenol(o-NP) based on an experimental phenomenon that the fluorescence of the CQDs could be specifically quenched by o-NP.Results showed that there was a good linear relationship for o-NP in the range of 0.083 3-12.0 μmol/L with a correlation coefficient(r2) of 0.998 and a detection limit(LOD) of 25.2 nmol/L(S/N=3).The proposed method was successfully applied in the detection of o-NP in real water samples from Wanfeng Lake and Dingxiao River in Guizhou province.