Ionic liquid functionalized halloysite nanotube applied as electrode modifier for trace level determination of caffeic acid

Abstract

The present work reports the synthesis of an anionic exchanger halloysite obtained from the grafting of an ionic liquid onto the clay mineral surface and the application of the organo-inorganic nanohybrid material for the first time to the best of our knowledge, as an electrode modifier for the electrochemical sensing of caffeic acid (CA) in aqueous solution. Physicochemical characterizations (DRX, FTIR, ¹³C NMR, and TGA) of the synthesized material revealed the grafting of the cation of the ionic liquid (1-methylnaphthyl-3-(2-hydroxyethyl)-imidazolium) into the interlayer space of halloysite. The counter anion of the ionic liquid (chloride) remained free and could be exchanged. The anionic exchange properties of functionalized halloysite were highlighted by electrochemical characterizations (cyclic voltammetry and scanning electrochemical spectroscopy), through interactions with ferricyanide and ferrocyanide ions as anionic probes. This property was subsequently exploited for the electrochemical detection of CA in aqueous solution. To overcome the interference of anions of the electrolytic solution, the open-circuit preconcentration method was applied. After optimization of the experimental conditions (accumulation time and pH of the electrolytic solution), CA calibration curve was plotted for concentrations ranging from 0.05 μM to 1.50 μM, with a sensitivity of 3.17 μA μM⁻¹ and a detection limit of 3.2 nM. The developed sensor was successfully applied for the quantification of CA in commercial black tea samples.