TABLE OF CONTENTS Ultramicroelectrodes (UMEs) are electrodes with characteristic dimensions on the micrometer or sub-micrometer scale. Faster double-layer charging, reduced ohmic loss, and high mass-transport rates have allowed them to revolutionize thermodynamic, kinetic, and electroanalytical measurements (1, 2). These properties have pushed the boundaries of electrochemistry into small length scales, nanosecond timescales, hydrodynamic applications, and environments, such as liquid CO 2 and single biological cells that are not possible using conventional-sized electrodes (3, 4). The currents observed at microelectrodes typically lie in the pA to nA range, which is several orders of magnitude smaller than those observed at conventional macroelectrodes, where the radius is usually several millimeters. These reduced currents are a key element in the successfulapplication of microelectrodes:
Unusual media: Traditionally, electrochemistry suffered relative to spectroscopy because electrochemical measurements could be made only in highly ionic conducting solutions. This restriction arose because resistance between the working or sensing electrode and the reference electrode limited the precision with which the applied potential could be accurately controlled. The small electrolysis currents observed at microelectrodes often completely eliminate these ohmic effects.
Small volumes: Electrochemical methods offer significant advantages over spectroscopy when working with small reactions volumes (e.g., as found in biochips and capillary electrophoresis separation systems). Electron transfer occurs at an electrode surface and the limits...
