A Dual-Mode Colorimetric and Fluorescent Sensor for Rapid Detection of Cyanide Ions: Rational Design, Interaction Studies, and Practical Applications

A calix[4]arene-based fluorescent chemosensor containing two quinolinium units (C4-BisQ) was synthesized and evaluated for the selective detection of cyanide (CN−). C4-BisQ showed a dual-mode optical response with a green-to-blue color change under ambient light and fluorescence quenching upon CN− addition in CH3CN/H2O (9:1, v/v). UV–Vis and fluorescence titration studies indicated a 1:1 interaction between C4-BisQ and CN− with an association constant of 1.08 × 105 M−1. The limits of detection were 0.080 µM by ratiometric UV–Vis analysis and 0.54 µM by fluorescence spectroscopy. C4-BisQ was highly selective for CN− in the presence of common competing anions and metal cations such as F−, Cl−, Br−, I−, ClO4−, NO3−, H2PO4−, HSO4−, Ac−, and Li+, Na+, Mg2+, Ca2+, Mn2+, Fe3+, Co2+, Zn2+, Cd2+, Al3+. Spectroscopic data suggest that cyanide recognition mainly involves deprotonation and is accompanied by fluorescence quenching under the applied conditions. Recovery experiments in tap water and seawater, together with visual detection in apricot and peach kernel extracts, confirmed the applicability of the sensor. Overall, C4-BisQ offers a simple dual-mode approach for cyanide detection in environmental water and food-related samples.