A useful method for the study of electroactive neurotransmitters is fast-scan cyclic voltammetry (FSCV), an electrochemical technique that can be used to quantify very small concentrations of an analyte, as well as to monitor neurotransmitter release and reuptake in vivo and in vitro. With this method, a carbon fiber microelectrode is linearly ramped through a range of voltages which causes oxidation of target amines at the electrode surface, thus engendering increased current flow. Specifically, the electrode potential is ramped linearly versus time. When a set potential is reached, the potential ramp is inverted, causing the analyte to be reduced at its characteristic reduction potential. A cyclic voltammogram is then produced, which shows the current at the working electrode plotted versus the applied voltage.
Color plots and resulting Cyclic Voltammograms, with and without analyte present.
This technique is combined with a flow injection system to calibrate electrode response to various analytes. In addition, background subtraction techniques are used in order to eliminate the large background current which can obscure the data being sought.
Carbon FIber Microelectrode (CFME).
A problem with using FSCV to quantify certain analytes, such as octopamine and tyramine, is their tendency to polymerize upon reduction and passivate the electrode surface, a process also known as electrode fouling. This passivation causes the electrode to become unusable for further experimentation, and requires more time spent preparing electrodes.
Passivation.
To counter this, various techniques and waveform parameters have been developed to prevent passivation. One method of interest involves the coating of working electrodes with Nafion. Nafion is a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer that is highly conductive to cations. Many electrode coatings that reduce fouling can also bring about a loss of sensitivity, rendering them unfit for analysis. However, Nafion coated electrodes have been shown to increase electrode sensitivity to catecholamines, as well as to reduce electrode fouling. This is important, as much time and materials can be lost in the production of new electrodes.
Nafion.