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A new colorimetric fluorescent probe (MCy) for CN − is designed and synthesized based on triphenylimidazole-hemicyanine dyad. The probe shows high selectivity towards CN − in 100% aqueous solution even in the presence of other competitive anions such as F − , Cl − , AcO − , NO 2 − , H 2 PO 4 − , I − , SCN − , SO 4 2− , NO 3 − , HCO 3 − , Br − and S 2− . Upon nucleophilic addition of CN − to the indolium group, the probe displays large blue-shift in UV–vis spectrum and remarkable “turn-on” fluorescence owing to the disruption of intramolecular charge transfer. Meanwhile, the probe also undergoes a naked-eye discernible color change from orange to light pink in the sunlight as well as an obvious fluorescence color change from non-emissive to bright blue under a UV lamp. The detection limit of CN − in aqueous solution can be as low as 20.6 nM, which is much lower than the permissible level of CN − in drinking water according to the WHO. In addition, the probe has been successfully used to determine CN − in real water samples as well as to fabricate test paper kit for CN − detection. [ABSTRACT FROM AUTHOR]