An easily accessible fluorometric probe for selective cyanide detection via nucleophilic addition: Applications to environmental water samples, smartphone, and bio-imaging of fungi and plants

The monitoring of cyanide (CN⁻) is of great significance due to its extreme toxicity to both humans and the environment. In this study, we report a convenient fluorescent sensor HNIC ((E)-7-hydroxy-8-(((2-((7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)amino)phenyl)imino)methyl)-2H-chromen-2-one) for detecting CN⁻. On addition of CN⁻, HNIC exhibited a blue fluorescence turn-on response in aqueous media. Moreover, HNIC could selectively monitor CN⁻ over other interfering ions, with a detection limit of 4.43 μM. The reaction of CN⁻ with HNIC was evaluated using Job plot at a 1:1 ratio. To elucidate the mechanism of the nucleophilic attack of CN⁻ on HNIC, ESI-MS analysis, ¹H NMR titration, and DFT studies were employed. HNIC showed potential for quantifying CN⁻ in environmental water samples (mineral, tap, river, and drinking water). Furthermore, a smartphone application utilizing HNIC successfully monitored CN⁻ concentrations in environmental samples. Importantly, HNIC could track CN⁻ through bio-imaging in fungi and plants, including white beech mushrooms, onions, and radish sprouts. Therefore, HNIC could be employed as a useful tool for detecting highly toxic CN⁻ by triggering the fluorescent turn-on.