Oxyethylene-substituted bis-indolyl derivatives for enhanced cyanide detection: mechanistic insights and application in food safety analysis

Abstract

In this study, we synthesized highly colored, oxidized bis-indolyl derivatives featuring oligo oxyethylene chains on the central phenyl ring, significantly enhancing the solubility of the probe molecule in water. This improved solubility enabled a pronounced response to cyanide ions, facilitating naked-eye detection via a color-changing response and a turn-on fluorescence response (detection limit of ∼17 ppb). Mechanistic investigations revealed a time-dependent, two-step interaction with cyanide. Initially, hydrogen bonding or deprotonation of the indolyl-NH protons by cyanide caused an immediate color change from red to yellow. Over time, a Michael addition reaction triggered by cyanide further altered the color of the solution to colorless, disrupting the conjugation. The formation of the chemodosimetric adduct was confirmed using ¹H NMR, ¹³C NMR, and mass spectroscopies. The system also successfully detected endogenous cyanide in cassava, both in aqueous extracts and on the vegetable surface (without any pretreatment). In addition, we evaluated conventional washing techniques for cyanide removal from cassava, highlighting the potential of these oxidized bis-indolyl derivatives for cyanide detection and remediation in crops.