A bis-hydrazone Schiff base chemosensor for highly selective and sensitive colorimetric and fluorometric detection of nitrite for environmental, biological, and food sample applications.

A novel symmetrical bis-hydrazone Schiff-base probe, 2,2'-((1E,1'E)-((1,2-diphenylethane-1,2-diylidene)bis(hydrazine-2,1-diylidene))bis(methaneylylidene))bis(4-nitrophenol) (L), was successfully synthesized and comprehensively characterized using FTIR and NMR spectroscopy, ESI-MS, elemental analysis, and density functional theory (DFT) calculations. The probe L showed very good selectivity and sensitivity to nitrite (NO2 -) with a wide variety of metal ions and anions through colorimetric and fluorescence responses. The colorimetric detection limit was found toward be 1.38 × 10-6 M, whereas the fluorometric detection limit was 1.97 × 10-6 M. From the UV-visible titration data in a CH3OH-H2O (1 : 1, v/v) medium using BindFit the binding constant was determined to be 1.50 × 104 M-2. The proposed sensing mechanism involves hydrogen-bond-assisted host-guest adduct formation, which enhances intramolecular charge transfer (ICT) and hydrogen-bond-induced fluorescence enhancement, leading to pronounced optical responses. Furthermore, the sensor demonstrated excellent analytical performance in spiked water and food samples as well as a real sample spinach extract affording good recoveries, confirming its reliability for practical nitrite determination. In addition, the probe L showed promising applications in bovine serum albumin (BSA) recognition, azithromycin detection, and the development of a reversible molecular logic gate, demonstrating its multifunctional sensing capability.