Cyclic voltammetry with carbon-fiber electrodes

Figure 8. Data obtained In the caudate nucleus of an anesthetized rat from a Naflon-coated, carbon-fiber electrode with voltammetry (300 V s ) during stimulations of dopamine containing neurons. A Temporal changes observed during electrical stimulations at 60 Hz. Each point represents the current from Individual voltammograms Integrated over the range 400-800 mV vs SCE. The response Is converted to concentration by calibration with dopamine. The solid lines are the modelled response which Involves the use of neurochemical kinetic parameters and diffusion from a distance of 10 ixm. B A subtracted cyclic voltammogram obtained during the 60-Hz stimulation. The shape Is identical to that recorded In dopamine solutions.

One of the earliest CNT-tipped biological sensor reported was an SWCNT-coated carbon fiber nanoelectrode (100-300 nm tip diameter, as shown in Figure 7.2b). Using cyclic voltammetry, the CNT-tipped electrode could detect dopamine, epinephrine, and norepinephrine at concentrations on an order of magnitude lower than noncoated probes. The demonstration was significant because the dimensions of the CNT-tipped probe would make it possible to study the functions of living cells and tissue with a minimal level of intrusion. Additionally, the pencil-like shape of the probe readily fits the standard ceU physiology equipment and facilitated its use. 

The slice is electrically stimulated with an enamel-coated bipolar wire electrode causing action potentials that evoke DA release. In the slice preparation, the stimulation is applied directly at the neuron terminals. This is in contrast to an in vivo experiment, where the stimulation is performed at the cell body of the neuron and the DA release is monitored remotely at the presynaptic terminals. DA concentration in the extracellular fluid rises and quickly returns to baseline at the cessation of the stimulation [3]. Fast scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes is used to detect the resulting concentration changes in the extracellular fluid. This analytical technique provides a method for the determination of uptake kinetics in intact brain tissue. Thus, the secretion and subsequent clearance of DA in the tissue is observed in real time. 

Because of its small size, selectivity, time response, and sensitivity, the carbon fiber microelectrode coupled with fast-scan cyclic voltammetry currently represents the most ideal sensor for the measure of kinetics and mechanisms of DA neurotransmission. Release and uptake sites (neuron terminals) are depicted in Figure 3 along with a typical example of DA release and uptake data monitored with a carbon-fiber microelectrode using FSCV. These sites are generically described as a cartoon merely to give the reader an idea of the relative size of the electrode versus the neuron terminals (release and uptake sites). 

Figure 7 Simple model of the concentration of vesicular events as determined by fast-scan rate cyclic voltammetry. (A) Pictorial demonstration that the area of the electrode used for oxidation/reduction of the DA species is very different for the cellular case compared with that in standard solution. (B) A head-on view depicting the difference in electrode area used in the above two cases. Bevehng a carbon fiber on a 45° angle creates an elliptical surface with major and minor radii of about 3.5 and 2.5 i,m, respectively. It is apparent that a large difference exists between the vesicular area and that of the total electrode. (Reproduced from Ana/. Chem. with permission [13].)...

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