Potential control at bipolar microelectrodes

• Redox buffer enables potential control of one side of bipolar electrode • Study of potential profile along current path by varied feeder electrode connection • Shorter time constant of bipolar setup than corresponding three-electrode setup • Bypass current through capillary allows transition from closed to open setup • Tolerance of closed setup to small bypass currents explored Bipolar electrodes (BPEs) are attractive for studying electrochemistry at micro and nanostructures without wiring them. However, very often the potential control is lost. Here we study possibilities to control the potential on one side of a bipolar electrode similarly to the potential control achieved in a potentiostatic three-electrode setup. The BPE is formed by connecting two microdisk electrodes (MEs), one of them is considered as interrogator electrode for which potential control shall be achieved. The MEs are immersed in two electrolyte reservoirs with two feeder electrodes (FEs), between which a voltage is applied. The potential profile along the current path was reconstructed by the use of reference electrodes in one of the reservoirs. Potential control of the interrogator electrode is achieved when the interrogator electrode contacts the analyte solution and the other ME and its FE are immersed in a redox buffer, that is an electrolyte solution containing a high concentration of the two forms of a reversible redox couple. The voltammetric behavior was quantified by slope analysis. Several improvements in the setup brought the slope analysis result down to 62 mV (compared to 59.1 mV for the three-electrode setup). It was also demonstrated that the bipolar setup has a smaller time constant than the same ME in a three-electrode cell as expected from a serial combination of two capacitors. By placing electrolyte-filled capillaries between the FEs parallel to the BPE, the effect of small bypass currents was studied that may be unavoidable when integrating BPEs into microfluidic setups.