Ultra-Sensitive Potentiometric Measurements of Dilute Redox Molecule Solutions and Determination of Sensitivity Factors at Platinum Ultramicroelectrodes

Open circuit potential (OCP) measurements can be very sensitive to small changes in the electrode environment and may allow detection of electron transfer events involving few, and maybe single, electrons. Factors affecting the overall sensitivity of OCP measurements were investigated to achieve the highest sensitivity. The OCP of platinum ultramicroelectrodes (UMEs) was determined in solutions that initially contained only supporting electrolyte where the OCP is a mixed potential governed by background faradaic processes. Then, increasing amounts of a redox couple at equimolar amounts of oxidized and reduced forms were added. In dilute solutions of the redox couple, the OCP deviates from the redox potential because of additional background half reactions occurring at the electrode. These dominate the OCP through their partial current contributions, shifting the OCP to a mixed potential region. The OCP at a platinum UME remains unchanged from the aqueous electrolyte solution mixed potential until ∼10^–6 to 10^–7 M concentrations of redox molecules are reached. At higher concentrations, the OCP moves toward the formal potential of the redox couple and eventually becomes poised at this value. By using a simple surface modification, the sensitivity to changing concentrations can be increased by almost 2 orders of magnitude. Numerical calculations with a Butler–Volmer formalism can estimate the contribution to the OCP mixed potential from background half reaction currents which are used to extract sensitivity factors from the change in potential with respect to current. The relative sensitivity to changing concentrations is shown to increase as the electrode size decreases.