Metal-ion sensor composed of self-assembled monolayer of amine ligand formed by the use of molecular tripod

Abstract

The electrochemical ion-sensor performance of the self-assembled monolayer (SAM) of 3H-imidazo[4,5-b]pyridine, which is known as a metal ionophore, was examined using a triad composed of ferrocene, an ionophore, and a tripodal trithiol with adamantane core. The voltammetry of the SAM-modified Au electrode showed a large positive change (280 mV) in the ferrocene oxidation potential via the addition of Pb2+, with a detection limit of 10–6 M. A Langmuir adsorption-type analysis of the ion binding equilibrium suggested that as much as 70% of the imidazopyridine ligand was actually functioning as a Pb2+ ionophore on the SAM, whereas the rest of the ligand was uncomplexed because of the crowding of the molecules in the SAM. Control experiments using an analogous triad of monothiol revealed a strong advantage in using trithiol; thus, tripod-shaped anchor is requisite for the ionophore to function well as a sensor.