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[Display omitted] • A new probe was designed, synthesized and characterized using NMR, mass and single crystal XRD studies. • The probe exhibited high selectivity and sensitivity towards cyanide in an aqueous solution. • A fluorescence turn-on process via deprotonation of phenolic –OH group was proposed. • The proposed sensing mechanism and optical properties were investigated using DFT and TD-DFT calculations. • The probe was utilized to detect cyanide in food materials and HeLa cells and to determine in natural water samples. A simple fluorescent probe (KS4) containing multiple reaction sites (phenolic –OH, imine and C = C bonds) is successfully synthesized and characterized using 1H NMR, 13C NMR, mass and single crystal XRD techniques. KS4 exhibits high selectivity towards CN– over a wide range of common anions in H 2 O:DMSO (1:1 v/v) leading to an amazing turn-on fluorescence at 505 nm via deprotonation of the phenolic –OH group. The limit of detection (1.3 µM) for CN– was much below the standard (1.9 µM) set by the World Health Organization (WHO). Stoichiometry of the interaction between KS4 and CN– was ascertained as 1:1 by the Job's plot method and the binding constant was determined to be 1.5x104 M−1. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) based theoretical insight has been appealed to understand the optical properties of KS4 before and after the addition of CN– ion. The probe shows respectable real-time applicability for qualitative detection of CN– in almond and cassava powder as well as quantification in real water samples with excellent recoveries (98.8 – 99.8%). In addition, KS4 is found to safe towards living HeLa cells and successfully applied to the detection of endogenous cyanide ions in HeLa cells. [ABSTRACT FROM AUTHOR]