[en] A naphthoquinone-imine based fluorescent probe (R) has been synthesized and characterized by NMR and mass spectral techniques. The receptor shows high selectivity towards cyanide in water in the presence of other competitive anions with an instantaneous colour change from non-emissive to green under UV light. The turn-on fluorescence is due to deprotonation of three hydroxyl groups by cyanide ions as evidenced from ¹H NMR titration experiment. The LOD of cyanide by R in water is 0.6 μM, which is much lower than the permissible limit of cyanide in drinking water according to the WHO. (paper)