Carbon fibre micro-electrode and in vitro or in brain slices voltammetric measurement of ascorbate, catechol and indole oxidation signals: influence of temperature and physiological media

Characterisation of carbon paste electrodes for real-time amperometric monitoring of brain tissue oxygen

Tissue O₂ can be monitored using a variety of electrochemical techniques and electrodes. In vitro and in vivo characterisation studies for O₂ reduction at carbon paste electrodes (CPEs) using constant potential amperometry (CPA) are presented. Cyclic voltammetry indicated that an applied potential of -650 mV is required for O₂ reduction at CPEs. High sensitivity (-1.49 ± 0.01 nA/μM), low detection limit (ca. 0.1 μM) and good linear response characteristics (R² > 0.99) were observed in calibration experiments performed at this potential. There was also no effect of pH, temperature, and ion changes, and no dependence upon flow/fluid convection (stirring). Several compounds (e.g. dopamine and its metabolites) present in brain extracellular fluid were tested at physiological concentrations and shown not to interfere with the CPA O₂ signal. In vivo experiments confirmed a sub-second response time observed in vitro and demonstrated long-term stability extending over twelve weeks, with minimal O₂ consumption (ca. 1 nmol/h). These results indicate that CPEs operating amperometrically at a constant potential of -650 mV (vs. SCE) can be used reliably to continuously monitor brain extracellular tissue O₂.

Cyclic and pulse voltammetric study of dopamine at the interface between two immiscible electrolyte solutions

The detection of dopamine by differential pulse voltammetry (DPV) and square wave voltammetry (SWV) at the interface between two immiscible electrolyte solutions (ITIES) has been studied. Voltammetry at the liquid/liquid (water/1,2-dichloroethane) interface provides a simple method for overcoming the major problem associated with dopamine detection by voltammetry at solid electrodes: the co-existence of ascorbate at higher concentrations. Selectivity for dopamine was achieved by the use of dibenzo-18-crown-6 as an ionophore for the facilitated transfer voltammetry of protonated dopamine across the ITIES. Under these conditions, ascorbate is not transferred and hence does not interfere in the ion transfer current for dopamine. By use of DPV and SWV, the lowest concentration detectable can be lowered from ca. 0.1 mM (obtained with cyclic voltammetry) to 2 microM. Evaluation of the effect of some other physiologically important species (acetylcholine, sodium, potassium and ammonium ions) on the dopamine transfer voltammetry has been studied, indicating the need for improved ionophore designs in order to achieve practically useful selectivity.

In vivo nitric oxide sensor using non-conducting polymer-modified carbon fiber

Nitric oxide (NO) is emerging as a very important and ubiquitous gaseous messenger in the body. The response characteristics of NO sensors made of non-conducting polymer modified carbon fiber electrodes are investigated to determine their selectivity, sensitivity, and stability for in vivo use. A composite polymer, comprising Nafion, m-phenylenediamine, and resorcinol, showed the best selectivity and stability to amperometric NO detection. The non-conducting, self-limiting polymer film protects the electrode from interference and fouling by other biochemicals. Although the relative sensitivity to NO of the modified sensor is lower than that of the unmodified carbon fiber electrodes (less than 6%), the composite polymer electrode showed high selectivity against ascorbic acid (> 2000:1), nitrite (> 600:1), and dopamine (> 200:1). The stability of the NO sensor was maintained for at least 1 week. The NO sensitivity after in vivo experiments (n = 8) is 88.1 +/- 5.6% of initial sensitivity data obtained before in vivo experiments. Preliminary in vivo experiments done with this electrode are shown to capture elevated NO levels in brain following an ischemic injury.

Dependence of dopamine calibration factors on media Ca2+ and Mg2+ at carbon-fiber microelectrodes used with fast-scan cyclic voltammetry

Carbon-fiber microelectrodes and voltammetric methods have been used extensively for the detection of dopamine in brain tissue in vivo and in vitro. Voltammetric microelectrodes are often calibrated in non-physiological media, like phosphate-buffered saline, rather than in oxygenated physiological media. Here, we determined dopamine calibration factors (nA microM-1) in several defined solutions for two types of carbon-fiber electrode used with fast-scan cyclic voltammetry. For both electrode types, dopamine calibration factors, and thus electrode sensitivities, were 2-3-fold higher in phosphate- or HEPES-buffered saline than in a bicarbonate-based artificial CSF (ACSF) that reflected that normal ionic composition of brain extracellular fluid. Removal of Ca2+ and Mg2+ from ACSF eliminated this difference. Because extracellular Ca2+ concentration ([Ca2+]o) can fall under stimulation conditions used to elicit dopamine release, we also evaluated the size of stimulated [Ca2+]o shifts in guinea pig midbrain slices using ion-selective microelectrodes. The [Ca2+]o decreases were less than 100 microM, which was well below the mM decreases observed to alter DA sensitivity. Consequently, calibration data obtained in normal physiological solutions should be valid under conditions of mild stimulation. Moreover, calibration in divalent cation-free media will cause calculated DA levels to be underestimated and should be avoided, unless appropriate for a given experimental paradigm.