Study on the Selective Fluorescent Detection of Tetracycline, Cr6+, and Ethylene Glycol Using Nitrogen- and Phosphorus-Co-Doped Carbon Dots

Abstract In this work, two types of nitrogen- and phosphorus-co-doped carbon dots (N,P1-CDs, N,P2-CDs) were successfully prepared via a solvothermal method at 180 °C for 6 h, employing o-phenylenediamine and phosphoric acid, as well as 2-aminoterephthalic acid and phosphoric acid, as the respective precursors. Characterizations including X-ray diffractometer, transmission electron microscope, X-ray photoelectron spectrometer, FT-IR, and zeta potential verify the successful co-doping of nitrogen and phosphorus. The zeta potentials of N,P1-CDs and N,P2-CDs are 8.04 mV and 7.12 mV, with corresponding fluorescence quantum yields of 11.38 and 13.2%, respectively. Both CDs exhibit outstanding pH tolerance and long-term fluorescence storage stability. As fluorescent probes, their sensing performance toward Cr6+, tetracycline (TC), and ethylene glycol (EG) was systematically investigated. Both CDs realize quantitative detection of Cr6+ within the concentration range of 5–140 μg/mL with distinct recognition selectivity: N,P2-CDs display superior performance for TC detection, while N,P1-CDs can specifically recognize EG. All sensing systems exhibit favorable linearity and relatively low limits of detection (LOD), among which the LOD of N,P1-CDs toward EG reaches only 0.1052 μM. Coexisting ion experiments prove the favorable anti-interference capacity of the two probes, and N,P2-CDs possess better stability. Satisfactory detection results are achieved in spiked real water matrices including tap water and river water, where the fluorescence quenching efficiencies for Cr6+ all exceed 93%. Mechanism analysis reveals that the detection of Cr6+ by N,P-CDs follows the photoinduced electron transfer pathway. The fluorescence quenching triggered by EG and TC originates from the synergistic effect of hydrogen bonds and inner-filter effect. The dual CDs system enables simultaneous multitarget detection of heavy metals, antibiotics, and organic solvents, which broadens the application prospects of carbon-based fluorescent materials in environmental pollutant monitoring.