Fluorescence Regulation of N,P-CQDs and Their Application in Cr ³⁺ Ion Detection

To address the key issues of traditional carbon quantum dots (CQDs), such as unstable photophysical and photochemical properties and limited carrier transport performance, N,P-CQDs were prepared via a one-step hydrothermal synthesis method in this study. By regulating the mass ratio of carbon-to-nitrogen sources, we systematically explored the fluorescence properties of four N,P-CQDs. The findings demonstrated that N,P-CQDs-2 prepared at 180 °C for 8 h with a citric acid-to-ammonium dihydrogen phosphate molar ratio of 1.15:1.0 exhibited the optimal optical performance. The material showed excellent stability with 76.96% fluorescence retention after 180 days of storage at room temperature and stable fluorescence in pH 1–13, demonstrating favorable acid–base resistance. Structure, morphology, and optical properties were characterized by X-ray powder diffractometer (XRD), Fourier transform infrared spectrometer (FT-IR), X-ray photoelectron spectrometer (XPS), transmission electron microscope (TEM), fluorescence spectrophotometer (PL), ultraviolet–visible (UV–vis), and steady-state/transient fluorescence spectrometer (FLS). All samples emitted blue fluorescence under UV light. The carbon–nitrogen ratio did not change the core structure but obviously affected fluorescence intensity and emission peaks. N,P-CQDs-2 was used as a fluorescent probe for ion detection. After different metal ions (Ni 2+, Fe 2+, Cu 2+, Mg 2+, Cr 3+ ) were added to its aqueous solution, the system’s fluorescence emission intensity showed differential attenuation. Among them, the fluorescence quenching effect induced by Cr 3+ was the most significant. The fluorescence quenching efficiency of N,P-CQDs-2 shows a good linear relationship with the concentration of Cr 3+ in the range of 150–330 μg/mL, with a correlation coefficient R 2 of 0.977 and a limit of detection as low as 0.25 mg/L. Based on the characterization results of FT-IR, UV–vis absorption, and fluorescence lifetime, the fluorescence quenching process is verified to be dominated by the static quenching mechanism. This green and low-cost method provides a valuable reference for efficient Cr 3+ detection.