Carbon Dots as “On–Off–On” Fluorescence Sensors for Selective and Consecutive Detection of 4-Nitrophenol and Cerium(IV) in Water Samples

An “on–off–on” fluorescence sensor was designed for rapidly and consecutively detecting 4-nitrophenol and cerium(IV) without the use of any labeling materials. The yellow carbon dots were synthesized by a simple one-step hydrothermal approach, and various techniques were applied to investigate the morphology, structure, and optical properties of the carbon dots. Under the optimal experimental conditions, 4-nitrophenol rapidly quenched the fluorescence of carbon dots as a result of the inner filter effect (IFE). The fluorescence intensity of carbon dots was linear with the concentration of 4-nitrophenol (1–150 μmol/L) and the limit of detection was 0.32 μmol/L. The fluorescence was gradually recovered as the cerium(IV) concentration (0.5–100 μmol/L) increased in CDs/4-NP, and the limit of detection was 0.16 μmol/L. The sensor showed good selectivity and demonstrated high accuracy for the analysis of 4-nitrophenol and cerium(IV) in actual water samples.