A “turn-off” ICT-based optical probe for the selective detection of cyanide ions in real samples

Abstract

A benzothiazolium-derived optical sensor, (E)-3-ethyl-2-(2,4,6-trimethoxystyryl) benzo[d]thiazol-3-ium iodide (ETBI), was synthesized for the selective detection of cyanide ions (CN⁻). Its structure was confirmed through ¹H NMR, ¹³C NMR, HRMS, IR, and single-crystal XRD analysis. ETBI exhibits a dual-mode optical response, displaying significant absorption, fluorescence quenching, and a distinct naked-eye-detectable color change from pale yellow to colorless. This change results from nucleophilic addition of CN⁻ to the benzothiazolium ring, which disrupts the π-conjugated system and inhibits intramolecular charge transfer (ICT). The job plot indicated a 1 : 1 binding stoichiometry between ETBI and CN⁻, with an association constant (K_a) of 2.86 × 10⁴ M⁻¹. The probe ETBI demonstrated high sensitivity, achieving a detection limit of 0.49 μM, well below the World Health Organization (WHO) guideline for CN⁻ in drinking water. Furthermore, ETBI was successfully applied in fluorescence imaging of intracellular CN⁻ in HeLa cells and using Whatman filter paper as a test strip.