Development of sludge-based quantum dots for nitrate-nitrogen detection in agricultural drainage

Nitrate-nitrogen (NO₃ ^--N) in agricultural drainage is an important contributor to nutrient enrichment in receiving waters, and rapid, sensitive and low-cost monitoring methods are needed. In this study, sewage sludge was used as a precursor to prepare nitrogen- and sulfur-co-doped carbon quantum dots (S-CQDs) by a hydrothermal method. The obtained S-CQDs showed blue fluorescence with an emission peak at 450 nm under excitation at 340 nm and a fluorescence quantum yield of 38.5%. Based on fluorescence quenching of S-CQDs by nitrate, a sensing method for NO₃ ^--N was established. Under optimized conditions, the method showed a linear response over 0.1-10 mg L^-1 NO₃ ^--N, with a detection limit of 0.03 mg L^-1. The method was applied to agricultural drainage samples, and the results agreed with those obtained by the standard ultraviolet spectrophotometric method, with standard-addition recoveries of 99.0-101.4%. These results indicate that sludge-derived S-CQDs can serve as a low-cost fluorescent probe for NO₃ ^--N detection. Fluorescence lifetime, UV-vis spectral-overlap, Stern-Volmer, FTIR, zeta-potential and DLS/TEM analyses suggested that PET-related electron transfer may contribute to nitrate-induced fluorescence quenching.