Development of an "Off-On" Fluorescence Sensing System Based on Nitrogen-Doped Carbon Quantum Dot/Fe³⁺ for Sensitive Detection of Cysteine.

Cysteine, one of the twenty proteinogenic amino acids, is distinguished by its unique sulfhydryl (–SH) functional group, which endows it with distinctive biochemical properties. In this work, nitrogen-doped carbon quantum dots (N-CQDs) were synthesized through a microwave-assisted strategy, with citric acid serving as the carbon source and urea as the nitrogen source. The as-prepared N-CQDs were employed for the detection of cysteine in liquid samples via an “off-on” fluorescence signaling mechanism. The fluorescence of N-CQDs was significantly quenched by Fe³⁺, whereas the subsequent addition of cysteine induced efficient fluorescence recovery. On the basis of this “off-on” fluorescence response, a highly selective, sensitive, and facile sensing strategy for the determination of cysteine was developed. Under optimized conditions, a good linear correlation was observed between cysteine concentration and relative fluorescence intensity over the range of 0–1.2 mmol·L⁻¹, with a detection limit of 10.49 µmol·L⁻¹. The calibration equation can be expressed as (F-F₀)/F₀ = 0.6496Ccys + 0.0475 (R² = 0.9932). This N-CQDs/Fe³⁺ fluorescence sensing system demonstrates considerable potential for practical cysteine determination in honey and aqueous samples.