Construction of a novel ^Off-On fluorescence sensor for highly selective sensing of selenite based on europium ions induced crosslinking of nitrogen-doped carbon dots

A highly blue luminescent nitrogen-doped carbon dots (N-CDs) was prepared, using citric acid and l-lysine via one pot hydrothermal treatment, and applied to the determination of selenite. The synthesized N-CDs exhibited a high photostability, excellent optical features with a high quantum yield (21.5%) and average size of 3 nm. The method is based on the competition between oxygen donor atoms in selenite oxyanion with those from carboxylate and hydroxyl groups at the surface of N-carbon dots for europium ions binding. N-CDs could be readily quenched upon the addition of europium ions owing to high affinity of carboxylate and hydroxyl groups at the surface of carbon dots to Eu³⁺ leading to aggregation of N-CDs (state OFF). After selenite addition, disruption of the aggregated N-CDs takes place, leading to restoration of the quenched fluorescence (State ON). The repeatability was less than 3.2% for selenite in both standard and real samples (n = 3). The method provides a simple procedure enabled selective detection of selenite with a linear range of 0.078–21.4 µg mL⁻¹ and a limit of detection of 53.0 ng mL⁻¹ (S/N = 3). The accuracy and precision were evaluated based on the detection of selenite in health care products with satisfactory results.