Nitrogen-doped carbon quantum dots for fluorescence detection of Cu2+ and electrochemical monitoring of bisphenol A

In this work, water-soluble nitrogen-doped carbon quantum dots (N-CDs) were synthetized at low temperature via a simple hydrothermal strategy, using citric acid as the carbon source and polyethylenimine (PEI) as the nitrogen source. The as-prepared N-CDs with near spherical structure and sizes of 4.5–7.5 nm exhibited blue luminescence and a fluorescence quantum yield of 40.2%. Both X-ray photoelectron spectroscopy (XPS) and FTIR spectroscopy measurements demonstrated the presence of the primary and secondary amines on the surface of the N-CDs. The fluorescence of N-CDs could be effectively quenched by Cu2+ owing to the formation of a copper–amine complex between Cu2+ and the amino groups on the surface of the N-CDs. Since this behavior was quite pronounced the fluorescence quenching was used for Cu2+ detection with high sensitivity and good selectivity. The linear range spanned the concentration of Cu2+ from 0.2 to 10 μM with a detection limit of 2 nM. In addition, the N-CDs could effectively electrochemically catalyze the oxidation of bisphenol A (BPA), which provided a promising method for BPA detection. The calibration range of BPA was 0.01 to 0.21 μM with a detection limit of 1.3 nM.