Pyridine Appended Poly(Alkyl Ether) Based Ionogels for Naked Eye Detection of Cyanide Ions: A Metal-Free Approach

The present study initiates applications of ionogels (gels from ionic liquids (ILs)) toward selective detection of cyanide ions (CN ) in aqueous media through a metal-free and colorimetric approach. Three low molecular weight ionogels based on poly(alkyl ether) appended pyridinium ILs-containing ethyl (IL-1), butyl (IL-2), and hexyl (IL-3) substituents-were designed, synthesized, and used for cyanide sensing. All ILs formed unique thermoreversible gels in aqueous solutions of dimethylformamide, dimethylsulfoxide, ethanol, methanol, and dioxane. The self-assembled structure of the ionogels has been characterized by temperature-dependent H-1 nuclear magnetic resonance, powder X-ray diffraction, scanning electron microscopy, atomic force microscopy, and rheology. The assynthesized ionogels exhibited a rapid color change from yellow to saffron upon interaction with CN- ions (0.1 equiv). The remarkable features of this detection are that (a) the gel color was changed without losing the physical structural integrity; (b) the color change is reversible in the presence of a proton source, assuring the reusability of the material; and (c) the color change can be identified by the naked eye. The mechanism of the sensing has been investigated using H-1 nuclear magnetic resonance titration experiments. All the ionogels showed a CN- response at micromolar range in the solution media and at 0.1 equiv to the gelator concentration in the gel medium. Results taken together suggest that as-prepared gels are useful for developing environmentally friendly portable kits for testing CN- in aqueous samples with a response time in seconds.