Kaolinite–ionic liquid nanohybrid materials as electrochemical sensors for size-selective detection of anions

Abstract

Functionalized nanohybrid materials based on clay minerals were obtained by grafting ionic liquids (trihydroxyethylmethylammonium iodide (AE1), 1-(2-hydroxyethyl)-3-methylimidazolium chloride (AE2) and 1-benzyl-3-(2-hydroxyethyl) imidazolium chloride (AE3) of different sizes in the interlayer spaces of kaolinite. The grafting was confirmed by several characterization techniques (XRD, ¹³C CP/MAS NMR and thermal analysis). The interlayer distances of the grafted materials were 4.1 Å, 6.0 Å and 8.7 Å for AE1, AE2 and AE3, respectively. These materials have an anion exchange capacity related to the mobility of the counter anions of grafted cations. In aqueous medium, the anion exchange capacity does not allow a variation of the interlayer distance of kaolinite, as confirmed by XRD measurements. The interlayer distance of the nanohybrid materials is controlled by the grafted cations. These properties of anion exchangers have been used to achieve the voltammperometric detection of some anions with different sizes (thiocyanate, sulphite and ferricyanide ions), using glassy carbon electrodes coated with a thin film of the modified kaolinite. There is a good correlation between the size of the analyzed anion and the basal spacing of the material used for the electrode modification: depending on the system, the presence of the thin film induces an increase of the current intensities when the anion can easily diffuse through the interlayer space or acts as a barrier when the basal spacing does not allow its insertion. This work shows that derivatives of kaolinite can be used for the selective detection of anions in aqueous solution, using their size as a differentiating factor.