Highly selective colorimetric sensing of cyanide based on formation of dipyrrin adducts

Abstract

Cyanide sensing has attracted increasing interest due to its toxicity and wide use in industrial activities. Herein, we developed three colorimetric cyanide sensors by the modification of the α-position of a dipyrrin chromophore with various carbonyl groups, namely, C₆F₅CO, C₆H₅CO and CHO for 1, 2 and 3, respectively. In dichloromethane, these sensors respond to both CN⁻ and F⁻ with distinct colour changes. UV-Vis, ¹H NMR and HRMS measurements imply a two-process interaction between the sensors and CN⁻. Initially, CN⁻ forms a hydrogen bond with the NH moiety, and then it attacks the carbonyl group of the sensors via a nucleophilic addition reaction. In contrast, in aqueous systems, only cyanide induced vivid solution colour changes from light yellow to pink via nucleophilic addition reactions. The CN⁻ detection limits reach a micromolar level of 3.6 × 10⁻⁶ M, 4.2 × 10⁻⁶ M and 7.1 × 10⁻⁶ M for 1, 2 and 3, respectively. In view of the easy synthesis and the highly selective recognition of CN⁻ with vivid colour changes, 1–3 may be developed as a novel and promising prototype of selective and sensitive colorimetric cyanide sensors.