Customized carbon dots as a fluorescent sensor for detecting paraquat through enhanced excited state binding energy

A novel fluorescent sensor utilizing customized carbon dots (CDs) has been developed for the rapid and efficient detection of the herbicide paraquat. The 4,4′-diamino-2,2′-stilbenedisulfonic acid (DSD acid) functionalized CDs (DSD-CDs) were synthesized by a hydrothermal method using citric acid and L-cysteine as precursors, followed by amidation with DSD acid. The resultant DSD-CDs exhibited bright blue fluorescence upon UV excitation. The fluorescence of the functionalized carbon dots was effectively quenched by paraquat, following a static quenching mechanism as confirmed by fluorescence lifetime measurements. Interestingly, the mechanism study revealed that the excited-state of the DSD-CDs could interact effectively with paraquat, resulting in stronger fluorescence quenching compared to the ground-state. The sensor showed high sensitivity, selectivity, and stability, with a wide linear range of 0.01 ∼ 2 mg/L (R 2  = 0.997) and a low detection limit of 0.003 mg/L. The developed fluorescent sensor demonstrated exceptional potential for paraquat detection in actual samples, encompassing blood, urine, soybean, and river water, with recovery rates exceeding 83 % and relative standard deviations (RSD) below 10 %. This underscores its practical utility in environmental protection and clinical testing applications.