Carbon dots represent a kind of fluorescent nanomaterial and have broad application prospects in the field of biosensing and optoelectronics. Here, we explored carbon dots with a high-fluorescence quantum yield rate synthesized from L-cysteine and citric acid by the microwave-assisted method. The characteristics of the carbon dots were studied using a transmission electron microscope, an X-ray diffractometer, X-ray photoelectron spectra, a UV-Vis absorption spectrometer, a FTIR spectrometer, and a fluorescence spectrophotometer. The obtained carbon dots exhibited a high-fluorescence quantum yield (up to 85%), which is due to the combination of amidogens and sulfydryl with carbon dots, and hence bring the improved fluorescence property. We used carbon dots for in vitro imaging of CRL-5822 cells and human umbilical vein endothelial cells, which showed the low inhibitory rate (0.8%) of cells for 48 h with good biocompatibility demonstrated by the cell viability assay. The image of cells can be observed clearly under UV light. The Stern-Volmer equation was introduced to describe the quenching effect between the fluorescence intensity of carbon dots and the concentration of aqueous dopamine (DA).

A fluorescent carbon dots probe for the detection of aqueous nitrite was fabricated by a one-pot hydrothermal method, and the transmission electron microscope, X-ray diffractometer, UV–Vis absorption spectrometer and fluorescence spectrophotometer were used to study the property of carbon dots. The fluorescent property of carbon dots influenced by the concentration of aqueous nitrite was studied. The interaction between the electron-donating functional groups and the electron-accepting nitrous acid could account for the quenching effect on carbon dots by adding aqueous nitrite. The products of the hydrolysis of aqueous nitrite performed a stronger quenching effect at lower pH. The relationship between the relative fluorescence intensity of carbon dots and the concentration of nitrite was described by the Stern–Volmer equation (I₀/I − 1 = 0.046[Q]) with a fine linearity (R² = 0.99). The carbon dots-based probe provides a convenient method for the detection of nitrite concentration.