A pyronin-based “covalent-assembly” fluorescent probe for sensitive detection of cyanide in pure aqueous media and real water samples

In order to achieve efficient reaction-based fluorogenic sensing of lethal cyanide ions in aqueous environmental matrices, we have rationally designed a fluorescent probe based on the “covalent-assembly” principle and an intramolecular crossed-benzoin reaction assisted by this nucleophilic analyte. An original synthetic route towards this unprecedented water-soluble fluorescent cyanide-responsive chemodosimeter was devised. The unique structural features of this “covalent-assembly” probe enable its facile conversion into a green-yellow emissive pyronin (λ_max Ex/Em 535/556 nm, and Φ_F = 61 % in neutral buffered aqueous solution) through a water-compatible domino reaction triggered by cyanide anion, thus producing a strong “turn-on” fluorescence response. A set of different fluorescence-based assays have highlighted sensing performances of this probe, especially its ability to detect low concentrations of cyanide ions in phosphate buffer within the range of 100–200 nmol/L, by means of a user-friendly portable fluorometer. From a practical point of view, we have shown that this sensing strategy was also effective in real water samples, thus producing a limit of detection (LOD) range of 1–2 μmol/L in line with World Health Organization recommendations. Furthermore, the capability of the present pyronin caged precursor to detect endogenous cyanide ions in a real food sample (freshly cut cassava slices) was also confirmed. This study is thus an important step towards the popularization of the concept of “covalent-assembly” probes in fluorescence-based environmental and food safety analyses, largely under-explored until now.