Fast-scan cyclic voltammetry

Voltammetric detection of dopamine. When sufficient potential is applied to the electrode, dopamine is oxidized to dopamine-o-quinone, donating two electrons that are detected as current. When the potential is returned, any dopamine-o-quinone remaining at the electrode surface is reduced back to dopamine by accepting electrons, producing current in the opposite direction. In the example shown, the potential is applied by fast-scan cyclic voltammetry. With this technique, the resultant current comprises time-resolved peaks that aid analyte identification. These measurements are typically repeated several times per second.

Fast-scan cyclic voltammetry was developed by Julian Millar and colleagues in London in the early 1980s. As is implicit from its name, it is based on cyclic voltammetry, the classic electrochemical techniques that offers the highest chemical resolving power. The classic technique has been accelerated by several orders of magnitude to provide temporal resolution on a subsecond time scale (typically 100 ms). The most common substrate for analyte detection is a carbon surface (usually a carbon-fiber microelectrode) which offers a relatively broad voltage range in aqueous solutions.

Using fast-scan cyclic voltammetry at carbon-fiber microelectrodes, changes in the extracellular concentration of electroactive molecules can be monitored. One such molecule that is of biological interest is the neurotransmitter, dopamine (see figure). The potential at the microelectrode is held at a potential insufficient to oxidize dopamine (e.g., -0.4 V vs a Ag/AgCl reference electrode) and then linearly ramped to an oxidizing potential (e.g., +1.3 V) and back at a high scan rate (e.g., 400 V/s) multiple times each second. When dopamine is present in the solution at the surface of the electrode, it is oxidized during the positive sweep to form dopamine-o-quinone (peak reaction at approximately +0.7 V) which is reduced back to dopamine in the negative sweep (peak reaction at approximately -0.3 V). During the redox reactions electrons are transferred between these molecules and the microelectrode (electrolysis). This flux of electrons is measured (as current) and is directly proportional to the number of molecules that undergo electro-oxidation. For analyte identification, current during a voltammetric scan can be plotted against the applied potential to yield a cyclic voltammogram. The cyclic voltammogram provides chemical information that is fairly unique for each substance and thus allows resolution of dopamine from other electroactive compounds. For quantification of changes in dopamine concentration over time, the current at its peak oxidation potential can be plotted for subsequent voltammetric scans. This approach can be utilized to make rapid chemical measurements in a range of biological preparations and conditions. We have developed this technology to allow us to record dopamine in real time, in awake rodents engaging in a variety of behavioral tasks.

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