An acid-free sensing strategy for detecting nitrite using dihydroquinoline-8-carboxylate as a probe

Nitrite (NO₂^-) has been identified as a typical pollutant harmful to the human body and heavily assayed in the fields of food safety and water quality control. The mainstream sensing strategies for detecting NO₂^- depend on Griess reaction or its improved methods which employ Griess reaction to initiate further inter-or intramolecular interaction to generate readout signals. However, a significant drawback of these methods is the use of strongly acidic media. In this study, we designed and synthesized a new NO₂^--specific fluorescent probe (ethyl 3-cyano-2-hydroxy-5-imino-8-(3-methoxy-3-oxopropyl)-4-(pyridin-2-yl)-5,8-dihydroquinoline-8-carboxylate, DHQC). DHQC exhibited strong green fluorescence in an acetonitrile-PBS (10 mM) mixed system (pH 7.0). In the neutral medium and at room temperature, the fluorescence of DHQC changed from green to blue with the addition of NO₂^-. The preliminary mechanistic investigation reveals that NO₂^- can induce the decarboxylation of the probe DHQC. Based on this finding, a high sensitive and selective method for NO₂^--detection was established, which showed good linearity in a range of 5∼50 μM with a limit detection of 3.5 nM (3σ). Given the unique properties of DHQC, a DHQC-loaded hydrogel bead device was further developed and employed for rapid monitoring of NO₂^-, exhibiting the advantages of simple preparation, high sensitivity, and fast response compared with traditional sensing reagents. In addition, DHQC was also used as a fluorescent probe for cell-imaging in live cells, exhibiting good cell permeability and biocompatibility. This study proposes a potential strategy for constructing smart fluorimetric probes used for NO₂^- detection.