Fluorescence enhanced sensing of cysteine based on reactive probe of naphthalene-quinoline

Naphthalene-quinoline (NQ), a reactive fluorescent probe, was synthesized based on 6-methoxy-2-acetylnaphthalene and quinoline-2-formaldehyde for highly sensitive recognition of cysteine (Cys). The absorption and fluorescence spectra of NQ showed significant changes toward Cys, suggesting the production of the covalent complex of NQ–Cys under physiological condition. The response mechanism of NQ to Cys was evaluated by nuclear magnetic resonance spectroscopy and high resolution mass spectrometry. It revealed that NQ reacted with Cys in the ratio of 1:1 with the Michael addition reaction, which blocked the effect of the twisted intramolecular charge transfer, resulting in the enhancement of the fluorescence intensity of NQ at 452 nm with variable controlled degrees. The detection of Cys using NQ as probe was achieved with the detection limit of 0.5 μM. Furthermore, the proposed method could be successfully applied to detect l-cysteine in real samples with good results.