Evaluation of the basal plane of pyrolytic graphite as an electrochemical detector for liquid chromatography

Electrochemical oxidation of catecholamines and catechols at carbon nanotube electrodes

The differences in the electrochemical oxidation of two commonly known catecholamines, dopamine and norepinephrine, and one catechol, dihydroxyphenylacetic acid (DOPAC), at three different types of carbon based electrodes comprising conventionally polished glassy carbon (GC), nitrogen-doped carbon nanotubes (N-CNTs), and non-doped CNTs were assessed. Raman microscopy and X-ray photoelectron spectroscopy (XPS) were employed to evaluate structural and compositional properties. Raman measurements indicate that N-CNT electrodes have ca. 2.4 times more edge plane sites over non-doped CNTs. XPS data show no evidence of oxygen functionalities at the surface of either CNT type. N-CNTs possess 4.0 at. % nitrogen as pyridinic, pyrrolic, and quaternary nitrogen functionalities that result in positively charged carbon surfaces in neutral and acidic solutions. The electrochemical behavior of the various carbon electrodes were investigated by cyclic voltammetry conducted in pH 5.8 acetate buffer. Semiintegral analysis of the voltammograms reveals a significant adsorptive character of dopamine and norepinephrine oxidation at N-CNT electrodes. Larger peak splittings, DeltaE(p), for the cyclic voltammograms of both catecholamines and a smaller DeltaE(p) for the cyclic voltammogram for DOPAC at N-CNT electrodes suggest that electrostatic interactions hinder oxidation of cationic dopamine and norepinephrine, but facilitate anionic DOPAC oxidation. These observations were supported by titrimetry of solid suspensions to determine the pH of point of zero charge (pH(pzc)) and estimate the number of basic sites for both CNT varieties. This study demonstrates that carbon purity, the presence of exposed edge plane sites, surface charge, and basicity of CNTs are important factors for influencing adsorption and enhancing the electrochemical oxidation of catecholamines and catechols.

An in vivo probe based on mechanically strong but structurally small carbon electrodes with an appreciable surface area

Physically small carbon electrodes were fabricated by pyrolyzing acetylene in a nitrogen atmosphere using pulled quartz capillaries as the supporting substrate. A carbon disk geometry was obtained when a parallel flow of acetylene (50 kPa) and nitrogen (10 mL min(-1)) was introduced into the system. Further, carbon was found to deposit at the tip and on the shank of the quartz capillaries when the nitrogen flow rate was increased (80 mL min(-1)), yielding an approximately cylindrical geometry. A series of electrochemical and spectroscopic analyses was carried out to examine the type of carbon surface obtained by pyrolysis of acetylene. The results suggested that a surface consisting of an almost defect-free highly oriented pyrolytic graphite type structure was formed by the pyrolyzed acetylene. However, this contradicts the kinetically reversible electron transfer observed for dopamine oxidation at these electrodes. Meanwhile, the nonpolar and relatively oxygen-free characteristics indicate that these electrodes also behave similarly to a hydrogenated carbon surface. The formation of a hydrogenated carbon-type surface may be plausible as a result of the attack on the carbon surface by a surplus of hydrogen produced by the pyrolysis of acetylene to form graphitic carbon. These characteristics are expected to aid in reducing electrode fouling, which is often encountered in electrochemical detection of neurotransmitters in vivo. In conjunction with a miniature physical dimension, their appreciable surface area and enhanced mechanical strength make these carbon electrodes well suited to the detection of neurotransmitters in vivo.

Preservation of NADH voltammetry for enzyme-modified electrodes based on dehydrogenase

Minimizing overpotential and generating high faradaic currents are critical issues for fast-scan voltammetry of beta-nicotinamide adenine dinucleotide (NADH) for the sensitivity of enzyme-modified electrodes based on dehydrogenases. Although NADH voltammetry exhibits high overpotential and poor voltammetric peak shape at solid electrode surfaces, modification of the electrode surface can improve the electrochemical response at carbon fibers. However, these improvements are severely degraded upon the covalent attachment of enzyme. The creation of improved electron-transfer properties and the retention of these properties throughout the enzyme attachment process is the focus of this study. A novel polishing and electrochemical pretreatment method was developed which generated a decreased overpotential and a high faradaic current at carbon-fiber electrodes for NADH. Factors that lead to a degradation of voltammetric response during the enzyme fabrication were investigated, and both the aging and the covalent modification of the pretreated surface contributed to this degradation. Attachment processes that minimized the preparation time, in turn, maximized the retention of the facile electron-transfer properties. These attachment processes included varying the surface attachment reactions for the enzyme. Preparation time reduction techniques included modeling existing techniques and then improving kinetic and mass transport issues where possible. Alternate covalent attachment methods included a direct electrochemical amine reaction and an electrochemically reductive hydrazide reaction. The surface attachment and retention of electron-transfer properties of these probes were confirmed by fluorescence and electrochemical studies.

Direct determination of seleno-amino acids in biological tissues by anion-exchange separation and electrochemical detection

Several studies have described the determination of selenium in protein extracts from tissues of marine or terrestrial animals, but have not identified the different chemical forms of selenium that are present. Selenium may be present as seleno-amino acids. Selenocysteine, for example, is a normal component of glutathione peroxidase, an antioxidant enzyme which may behave like other antioxidants, such as vitamin E, protecting tissues against methylmercury toxicity. The present study illustrates a method for the characterization of seleno-amino acids, such as selenocysteine and selenomethionine, in proteins extracted from the liver of marine mammals. The mechanism of detoxification of methylmercury, which involves seleno-compounds, is identified. The analytical determination was carried out using high-performance anion-exchange chromatography coupled with integrated pulsed amperometric detection (HPAEC-IPAD). This method allows the direct determination of underivatized amino acids, eliminating the procedure of pre- or postcolumn derivatization. The chromatographic separation was carried out on an anion-exchange column using a quaternary gradient elution. In order to optimize this method, interferences of amino acids and the influence of pH and ionic strength on the separation and electrochemical detection were studied. The IPAD response for the direct detection of amino acids is optimum at pH > 11. The detection limit (S/N = 3) for selenocysteine was found to be 450 micrograms/l. The application of this method for the identification of seleno-amino acids in protein hydrolysates is also shown.

Real-time monitoring of the secretory function of cultured adrenal chromaffin cells

A system to discriminate the real-time dynamics of the secretory function in cultured adrenal chromaffin cells, using a cell bed perfusion technique and an amperometric detector, was established. Examination of basal conditions revealed that the electrode potential and flow rate are crucial factors for monitoring precise dynamics of the secretory process. Stimulation of the cells either with acetylcholine (ACh) or with high K+ concentration caused a transient current response. The current responses showed concentration dependence for both stimuli, and also showed a high correlation with the amount of catecholamines (CA) in the respective peak fraction of perfusate. Either prolonged cholinergic stimulation or maintained depolarization produced a transient response, which is not attributable to a depletion of releasable storage of CA as indicated by double-stimulation experiments. Stimulation with high K+ concentration evoked an additional release of CA even after the cellular response to prolonged ACh was inactivated, whereas maintained depolarization with high K+ produced both facilitatory and inhibitory effects on the cell responsiveness to ACh. Most probably the transient natures of the secretory responses to ACh and to high K+ are mediated by different mechanisms. All the results suggest that the direct monitoring is profitable for studies on the regulatory mechanisms of the secretory function.

Monitoring the stimulated release of dopamine with in vivo voltammetry. I: Characterization of the response observed in the caudate nucleus of the rat

Microvoltammetric electrodes were employed in the brain of an anesthetized rat to monitor chemical substances in extracellular fluid following electrical stimulation of the medial forebrain bundle. An increase in concentration of an easily oxidized substance is observed in the caudate nucleus and in the nucleus accumbens. A large amount of evidence suggests that the substance that is observed following stimulation is dopamine. (1) The location of the stimulating electrode must be in known dopaminergic tracts to induce release. (2) Release is most easily observed in brain regions that contain significant numbers of dopamine-containing neurons. (3) Two voltammetric electrodes with very different electrochemical responses provide voltammograms of the released species that are unique for catechols in one case and catecholamines in another case. (4) The amount of 3,4-dihydroxyphenylacetic acid found in striatal tissue by postmortem analysis correlates with the calculated amount of dopamine released. (5) Inhibition of tyrosine hydroxylase, and thus dopamine synthesis, decreases the observed release while inhibition of monoamine oxidase, and thus formation of dopamine metabolites, does not. (6) The dependence of release on stimulation parameters agrees with results obtained with perfusion techniques. Thus, a new method has been developed to characterize endogenous dopamine release in the rat brain and can be used on a time scale of seconds.

Determination of gamma-aminobutyric acid by liquid chromatography with electrochemical detection

gamma-Aminobutyric acid (GABA) has been determined in rat brain by derivatization with 2,4,6-trinitrobenzenesulfonic acid. The derivative and an internal standard, 2,4,6-trinitrophenyl-delta-aminovaleric acid, are extracted into toluene and separated by reversed-phase chromatography. Electrochemical reduction of these derivatives permits picomole measurements of GABA in microgram amounts of brain tissue.