Chromene Carbohydrazide- Schiff Base as a Highly Selective Turn-Off Fluorescence Chemosensor for In³⁺ Ion and its Application

A new 7-(diethylamino)-2-oxo-2 H-chromene-3-carbohydrazide design to synthesize a simple Schiff-base condition. The synthesized molecules’ (probe L) photophysical properties were investigated in various solvent systems and solvent-poor-solvent assays. Probe L exhibits the absorbance band at 440 nm and the emission band at 488 nm in DMSO: H₂O (7:3, v/v). Further, probe L shows selective turn-off emission recognition of In³⁺ ions in DMSO: H₂O (7:3, pH = 7.4). By Job’s plot and ESI mass analysis, probe L forms a 1:2 stoichiometry complex with an estimated association constant of 4.04 × 10⁴ M^− 2 with In³⁺ ions. Metal induces CHEQ (chelation-caused fluorescence quenching) to reduce the intensity of probe L’s emission, and the estimated quenching constant was 4.52 × 10⁴ M^− 1. The limit of detection was found to be 5.93 nM; the time response of the sensor is instantaneous, and its reversible nature was confirmed using EDTA additions. Solid substrates (test strips) were designed and tested for fast, reliable, user-friendly, and real-time sensing of In³⁺ ions for on-site applications. The binding mechanism of probe L with In³⁺ ions was investigated using ¹H NMR titration and DFT/TD-DFT studies.