Ppb-level, dual channel sensing of cyanide and bisulfate ions in an aqueous medium: computational rationalization of the ion-dependent ICT mechanism

In this report, three oxidized diindolylarylmethane (DIAM)-based chromogenic probes (designated as 1, 2, and 3) have been developed for the simultaneous and dual-channel detection of cyanide (LOD: 6.2 ppb) and bisulfate (LOD: 8.7 ppb) ions at pH 7.4. The orange-colored solution of 1 turns red in the presence of bisulfate ions, while the addition of cyanide leads to yellow. Concurrently, the blue fluorescence of 1 gets quenched in the presence of bisulfate ions, while faint cyan color appears upon the addition of cyanide ions. Thus, it is evident that the present system can achieve naked-eye, ratiometric sensing of both cyanide and bisulfate ions in pure aqueous medium. Furthermore, it is observed that the presence of an electron-withdrawing group on the central phenyl ring improves the sensitivity towards cyanide ions, whereas an electron-donating group favors the interaction with bisulfate ions. Detailed computational investigations have been performed to rationalize such ion-specific distinct chromogenic responses and intramolecular charge transfer (ICT) properties of the probes. Considering the high selectivity and sensitivity, 1 is employed for the determination of excess cyanide in natural water resources and the presence of bisulfate in real-life samples. Finally, low-cost, reusable paper strips have been designed for rapid, on-location detection of both cyanide and bisulfate ions.