Fluorescent Sensing Platform Based on Nitrogen-Doped Carbon Dots for Selective Detection of Sulfite

Nitrogen-doped carbon dots (d-CDs) were prepared and refined through a solvothermal approach, employing malonic acid and barbituric acid as the carbon and nitrogen sources, respectively, in N, N-dimethylformamide. Single-factor experiments established the optimal synthesis conditions: reaction time of 18 h, barbituric acid dosage of 1.5 g, and temperature of 180 °C. Under these optimized conditions, the resulting d-CDs exhibited a quantum yield of 23.2%. In a phosphate buffer (pH 7.0), the d-CDs demonstrated specific fluorescence recognition for sulfite ions (SO₃²⁻), achieving a quenching efficiency of 64.6%. Notably, the sensor exhibited strong anti-interference capability against 17 common anions (e.g., F⁻, Cl⁻, Br⁻, I⁻, CH₃COO⁻, NO₃⁻, SO₄²⁻, HPO₄²⁻, CN⁻, S²⁻, CO₃²⁻, PO₄³⁻, C₂O₄²⁻, K⁺, Ca²⁺, Mg²⁺, and Zn²⁺). Kinetic studies revealed an ultrafast response, with quenching equilibrium reached within 10 s. Quantitative analysis yielded a low detection limit of 0.22 μmol·L⁻¹ for SO₃²⁻. The Stern–Volmer equation and fluorescence lifetime measurements demonstrate that this highly sensitive recognition is governed by a static quenching mechanism. Application to real water samples (lake water and river water) showed excellent recovery rates (98.0%–100.7%) and low coefficient of variation (CV < 4.04%) in spike-and-recovery tests.