Royce C. Engstrom

Real-time characterization of dopamine overflow and uptake in the rat striatum

The rate of overflow and disappearance of dopamine from the extracellular fluid of the rat striatum has been measured during neuronal stimulation. Overflow of dopamine was induced by electrical stimulation of the medial forebrain bundle with biphasic pulse trains. The instantaneous concentration of dopamine was measured with a Nafion-coated, carbon fiber microelectrode implanted in the brain. The measurement technique, fast-scan cyclic voltammetry, samples the concentration of dopamine in less than 10 ms at 100 ms intervals. Identification of dopamine is made with cyclic voltammetry. Stimulated overflow was measured as a function of electrode position, stimulation duration, stimulation frequency, and after administration of L-DOPA and nomifensine. The observed concentration during a 2-s, 60-Hz stimulation was found to alter with position of the carbon fiber electrode. For stimuli of 3 s or less the amount of overflow was found to be a linear function of stimulus duration at a fixed electrode position. The observed overflow was found to be steady-state at a frequency of 30 Hz, suggesting a balance between uptake and synaptic overflow under these conditions. The experimental data was found to be successfully modelled when the balance of uptake and stimulated overflow was considered. It was assumed that each stimulus pulse releases a constant amount of dopamine (125 nM), and that uptake follows a Michaelis-Menten model for a single uptake site with Km = 200 nM and Vmax = 5 microM/s. The increase in stimulated overflow observed after L-DOPA (250 mg/kg) could be modelled by a 1.6-fold increase in the amount of dopamine release with no alteration of the uptake parameters. The increase in modelled by an increase in Km. In addition, the fit of the modelled data to the experimental data was improved when diffusion from the release and uptake sites was considered.

The Effect of Electrode Material on the Electrogenerated Chemiluminescence of Luminol

This paper reports on the oxidation of luminol and its concomitant electrogenerated chemiluminescence (ECL) which were studied at several electrode materials by voltammetry and chronoamperometry. The ECL intensity (I{sub ECL}) was inversely related to the activity of the electrodes. The lowest I{sub ECL}) was measured when luminol was oxidized to 3-aminophthalate (n {approx equal}4 eq mol{sup {minus}1}) at a nearly mass-transport limited rate at glassy carbon. The ECL kinetics were studied and the order of the reaction with respect to luminol was 3/2 at concentrations to ca. 1 mM when O{sub 2} was the coreactant. In the presence of H{sub 2}O{sub 2}, the ECL reaction was first order with respect to luminol. A reaction mechanism is proposed that is consistent with the inetic data and the inverse relationship between electrode activity and I{sub ECL}. The implications of these results are discussed with respect to imaging the spatial distribution of current density at electrode surfaces, including that of PbO{sub 2} films activated by adsorbed Bi(V). A value of 6.6 {times} 10{sup {minus}6} cm{sup 2} s{sup {minus}1} was determined for the diffusion coefficient of luminol in 0.1M NaOH.

Fluorescence imaging of the heterogeneous reduction of oxygen.

The reduction of oxygen at a variety of solid electrodes was spatially imaged using fluorescence microscopy. Hydroxide produced during electrolysis of oxygen converted the acid-base indicator, fluorescein, into its fluorescent form. Fluorescence intensity was collected as a function of potential at platinum, silver, and glassy carbon disk electrodes and tracked the faradaic current due to oxygen reduction at platinum electrodes. The ability to observe spatial variations in electron-transfer kinetics was demonstrated at a bimetallic electrode prepared from silver and platinum. Fluorescence imaging of oxygen reduction on silver electrodeposited on glassy carbon revealed the location and size of the silver deposits. Imaging of oxygen reduction at a ruthenium-graphite composite electrode demonstrated the ability to identify electrochemically active sites on a spatially complex surface.

Time-resolved imaging of current density at inlaid disk electrodes

Abstract The distribution of current density at platinum disk electrodes was studied by imaging light produced in the electrogenerated chemiluminescence (ECL) reaction of luminol in alkaline peroxide. A sensitive imaging system, consisting of a low-lag vidicon equipped with a microchannel plate image intensifier, and an image-processing computer, were used to collect the light from the ECL reaction. Spatial resolution was found to be approximately 0.3 μm, and temporal resolution was as fast as 30 ms. Current density images taken in real-time during potential step and potential scan experiments were recorded and were found to compare quantitatively with current density distributions predicted by theory.

Microderivatization of Anodized Glassy Carbon

Microelectrodes have been used to modify locally the electrochemical activity on glassy carbon electrodes. Glassy carbon was electrochemically oxidized to form an oxide layer which is inhibitory toward certain electron-transfer reactions. Activity was restored through the application of hydroxide, which was generated electrochemically at the tip of a microelectrode. With the tip positioned in close proximity to the anodized glassy carbon surface, microdomains of electrochemical activity were created in an otherwise inactive matrix. The distribution of electrochemical activity was characterized using electrochemical feedback at the microelectrode, electrogenerated chemiluminescence imaging, and electrodeposition of silver. Spatially directed activation of the glassy carbon surface was accomplished in the micrometer domain.

A Calculator-Controlled Potentiostat for Pulsed Rotation Voltammetry

ABSTRACT A voltammetric technique based on hydrodynamic modulation, pulsed rotation voltammetry (PRV), has been placed under automated control by a desk-top programmable calculator. The circuitry is briefly described and the capabilities of the system are illustrated.

Observation of adsorbate coverage with electrogenerated chemiluminescence imaging

Abstract The spatial distribution of poly(phenylene oxide) on platinum electrodes was studied using the technique of electrogenerated chemiluminescence (ECL) imaging. The polymer was deposited on electrodes through the electrooxidation of phenol in aqueous solution. Electrochemical oxidation of phenol itself was effectively inhibited by the film, as was the electrogenerated chemiluminescen of luminol in alkaline peroxide. Using a sensitive microscopic imaging system, light from the ECL reaction was collected and used to show where film coverage occurred on the surface. At partial coverages of poly(phenylene oxide), some regions of the electrode were still able to support electron transfer and therefore the ECL images exhibited heterogeneity. Images showed that the nature and extent of coverage of poly(phenylene oxide) on the surface was a factor of phenol oxidation potential. The potential dependence of coverage was consistent with an isotherm that accounts for a high degree of attraction between adsorbed molecules, so that coverage increased abruptly over a relatively small potential range.