Ratiometric Fluorescent Sensor for Hg ²⁺ and Thiophanate-Methyl Based on Nitrogen-Doped Carbon Dots and Rhodamine B

Mercury (Hg 2+ ) and thiophanate-methyl (TM) are recognized as widespread pollutants in the environment and food. Here, a novel ratiometric fluorescent sensor has been developed for the detection of Hg 2+ and TM using nitrogen-doped carbon dots (N-CDs) and Rhodamine B (RhB). In the presence of Hg 2+, the fluorescence quenching of N-CDs was caused by photoinduced electron transfer, while the fluorescence recovery was triggered by the competitive binding of TM and N-CDs to Hg 2+ . However, the RhB fluorescence in the system was unaffected by Hg 2+ and acted as a reference for internal calibration. Under optimized conditions, the sensor displayed a favorable linear dependence on Hg 2+ and TM within the concentration ranges of 0.50–50 μM and 0.50–3.0 mg/L, with detection limits of 0.15 μM and 0.017 mg/L, respectively. And the sensor exhibited remarkable selectivity, with insignificant interference from coexisting metal ions or pesticides. This work presents a novel dual-functional ratiometric sensor for cascade detection of Hg 2+ and TM in diverse environments, such as water, cucumber, and rice, highlighting its potential in practical applications.