Voltammetric ion-channel sensing of ammonium ion using self-assembled monolayers modified with ionophoric receptors

Thiazole benzo-crown ether ethylamine-thioctic acid (TBCEAT) was evaluated as an ammonium (NH₄⁺) ionophore for the preparation of ion-channel sensors (ICSs) based on self-assembled monolayers (SAMs) on a gold electrode. A SAM film based on TBCEAT on a gold electrode was assessed as a NH₄⁺-selective sensor, particularly the voltammetric response. The working principle of this voltammetric sensor conceptually mimicked that of the ligand gated ion-channel proteins with regard to the chemically stimulated changes in the membrane permeability. The response to various analytes is based on the change in the electron transfer rates of a redox reaction of the reporter ion [Ru(NH₃)₆]^3+/2+ before and after the binding of NH₄⁺ to TBCEAT on the electrode surface, where electrostatic repulsion between the NH₄⁺-TBCEAT complex and [Ru(NH₃)₆]^3+/2+ induced a decrease in the rate constant. The selective ion-recognition properties of TBCEAT were characterized fully in the conventional ion-selective electrode (ISE) configuration using plasticized polymeric membranes. The mixed monolayer formed with alkylthiol and TBCEAT could bind ammonium ion selectively, and effectively control the flux of the reporter ion [Ru(NH₃)₆]^3+/2+, resulting in ammonium selective ICS with comparable performance (in terms of selectivity and detection limits) to conventional ISE.