Determination of Surface pKa of SAM Using an Electrochemical Titration Method

Abstract

A self-assembled monolayer (SAM) of 3-mercaptopropionic acid (HSCH2CH2COOH) was formed on a gold electrode. The effect of the charge of the end group on the electrochemical response of Fe(CN)63– at the SAM modified electrode was studied by using cyclic voltammetry. At high pH, when the –COOH groups are dissociated, Fe(CN)63– current is suppressed; as the solution pH is lowered, Fe(CN)63– current increases. The electrochemical titration curve was obtained by correlating the currents to the different electrolyte pH values, from which the surface pKa was obtained to be 5.2±0.1. Furthermore, a calculation equation was presented to simulate the electrochemical titration. As comparison, the surface pKa was also measured by contact angle titration as 5.6±0.1. The surface pKa values determined by the two methods in our work are consistent and accurate.