Spectroscopic and Theoretical Studies on the Selective Detection of Cyanide Ions by a Turn-On Fluorescent...

A new chemo-dosimeter AK4 containing quinoline fluorophore has rationally been designed, synthesised and characterized using ¹H and ¹³C NMR and mass spectral techniques. The probe senses explicitly CN^- ion through a dramatic enhancement in fluorescence over other commonly coexistent anions in H₂O:DMSO (9:1 v/v) medium over a broad pH range (4-10). ¹H NMR titration revealed the deprotonation followed by nucleophilic addition reaction of CN^-, which was supported by ¹³C NMR and mass spectral examinations. The Job's continuous variation method indicated the formation of a 1:1 adduct between AK4 and CN^- with a binding constant of 1.62 × 10⁴ M^-1. A limit of detection (LOD) towards CN^- of 0.69 µM has been determined, which is much lower than the World Health Organization (WHO) recommended limit of CN^- in drinking water (1.9 µM). The changes in the optical properties of AK4 upon reaction with CN^- were delineated using Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations. Moreover, fluorescence microscopic studies established that AK4 could be an effective probe for imaging intracellular CN^- in HeLa cells.