An ultramicroelectrode (UME) is a working electrode with a low surface area primarily used in voltammetry experiments.
These features allow UMEs to achieve useful cyclic steady-state conditions at fast scan rates (V/s) with limited current distortion.
Ultramicroelectrodes are often defined as electrodes which are smaller than the diffusion layer achieved in a readily accessed experiment.
The most common UME is a disk shaped electrode created by embedding a thin wire in glass, resin, or plastic.
Carbon-fiber microelectrodes are fabricated with conductive carbon fibers sealed in glass capillaries with exposed tips.
The response to a reversible redox couple in the linear region is a "diffusion controlled peak" which can be modeled with the Cottrell equation.
The linear region of an UME only exists at fast scan rates, which is helpful when studying faster electrochemical processes.
The Rg value obtain either by a rough estimation from a microscope image (as long as the electrode was fabricated with an homogeneous wire with a known diameter) or by a direct calculation based on the steady state current (iss) obtained from a cyclic voltamogram based on the following equation: iss=knFaDC* Where k is a geometric constant (disk, k = 4; hemispherical, k =2π), n is the number of electrons involved in the reaction, F is the Faraday constant (96 485 C eq−1), a is the radius of the electroactive surface, D is the diffusion coefficient of the redox species (Dferrocene methanol= 7.8 × 10−6 ; Druthenium hexamine = 8.7 × 10−6 cm2s−1) and C* is the concentration of dissolved redox species[4]