Carbon Microelectrodes Research Articles

Ca(2+)-dependent K(+) current and exocytosis in responses to caffeine and muscarine in voltage-clamped guinea-pig adrenal chromaffin cells.

We characterized changes in membrane currents and the cytosolic Ca(2+) concentration, [Ca(2+)](i), in response to caffeine, and compared them with those in response to muscarine using the perforated patch-clamp technique and fura-2 microfluorimetry in guinea-pig adrenal chromaffin cells. Catecholamine release from single voltage-clamped cells was monitored with amperometry using carbon microelectrodes. Caffeine produced a transient outward current (I(out)) at holding potentials over - 60 mV, increasing in amplitude with increasing the potentials. It also evoked a rapid increase of [Ca(2+)](i) at all potentials examined. The current-voltage relation revealed that the activation of K(+) channels was responsible for the I(out) evoked by caffeine. Both current and [Ca(2+)](i) responses were reversibly abolished by cyclopiazonic acid, an inhibitor of Ca(2+)-pump ATPase. At - 30 mV, the caffeine-induced I(out), but not [Ca(2+)](i), was partly inhibited by either charybdotoxin or apamin. In the majority of cells tested, caffeine induced a larger I(out) but a smaller [Ca(2+)](i) increase than muscarine. Caffeine and muscarine increased catecholamine release from voltage-clamped single cells concomitant with the transient increase of [Ca(2+)](i), and there was a positive correlation between them. These results indicate that caffeine activates Ca(2+)-dependent K(+) channels and catecholamine secretion due to the release of Ca(2+) from internal stores in voltage-clamped adrenal chromaffin cells of the guinea-pig. There seems to be a spatial difference between [Ca(2+)](i) increased by Ca(2+) release from caffeine-sensitive stores and that released from muscarine (inositol 1,4,5-trisphosphate)-sensitive ones.

An SECM observation of dissolution distribution of ferrous or ferric ion from a polycrystalline iron electrode

The dissolution of iron as ferrous or ferric ion from a polycrystalline iron electrode during anodic polarization in pH 2.3 sulfate solution was evaluated by using scanning electrochemical microscopy (SECM). A graphite reinforcement carbon (GRC) microelectrode was employed as a probe electrode of SECM to detect ferrous or ferric ions dissolved from the iron electrode in the active-dissolution, passive or trans-passive region. The probe current above the iron electrode surface subjected to active-dissolution showed the dissolution distribution of ferrous ion, depending on the substrate grains. It was found that the active-dissolution rate of iron as ferrous ions from the grain on which the thicker film was formed in the passive region, was lower than that from the grain on which the thinner film was formed in the passive region.

On-Demand Electrochemical Release of Nucleic Acids

Electrochemical protocols for releasing DNA from electrode surfaces are discussed. These protocols make use of the negative potential of nucleic acids for modulating electrically their interfacial properties. The miniaturization, surface-tailoring, safety, simplicity, convenience, automation, and low-cost advantages of electrochemical systems, coupled with the lack of immune reactions, make the electrical route a very attractive gene-delivery alternative. In particular, the use of DNA-modified microelectrodes holds promise for delivering the genetic material to specific locations at selected times. Such on-demand electrochemical removal of DNA should find important applications, besides gene therapy, including the localization of DNA onto tiny surfaces, design of ‘active’ scanning probe tips (e.g., SECM, STM) which release the DNA locally while scanning over the surface, controlled introduction of DNA to various analytical systems, or fundamental studies related to DNA interactions. Several protocols for releasing DNA and DNA-lipid complexes from gold and carbon microelectrodes under potential control are described, along with surface characterization techniques for monitoring the removal process. Fundamental and technological issues facing the development of DNA electrochemical carriers are discussed.

A voltammetric study of direct electron transfer to cytochrome c using a very large assembly of carbon microelectrodes.

A very large assembly of more than 8000 carbon fibre microdisk electrodes was used to study direct electron transfer to cytochrome c. Near steady-state cyclic voltammograms were observed, which exhibited excellent signal-to-noise ratios despite the low concentrations of cytochrome c employed (1-50 microM). The high resolution of the voltammograms allowed the formal potential of the native form of cytochrome c to be determined over a range of solution pH.

Effect of Electrode Density of States on the Heterogeneous Electron-Transfer Dynamics of Osmium-Containing Monolayers

Dense monolayers of [Os(OMe-bpy)2(p3p)Cl]1+, where OMe-bpy is 4,4‘-dimethoxy-2,2‘-bipyridyl and p3p is 4,4‘-trimethylenedipyridine, have been formed by spontaneous adsorption onto clean platinum, mercury, gold, silver, carbon, and copper microelectrodes. These systems have been used to probe the influence of the electrode density of states on the rate of electron transfer across the electrode/monolayer interface. Monolayers on each material exhibit well-defined voltammetry for the Os2+/3+ redox reaction where the supporting electrolyte is aqueous 1.0 M NaClO4. The high scan rate (>2000 V s-1) voltammetric response has been modeled using a nonadiabatic electron-transfer model. The standard heterogeneous electron-transfer rate constant, k°, depends on the identity of the electrode material, e.g., k° is 6 × 104 and 4 × 103 s-1 for platinum and carbon electrodes, respectively. Chronoamperometry, conducted on a microsecond time scale, has been used to probe the potential dependence of the heterogeneous electro...

Ionic liquid modified electrodes. Unusual partitioning and diffusion effects of Fe(CN)64−/3− in droplet and thin layer deposits of 1-methyl-3-(2,6-(S)-dimethylocten-2-yl)-imidazolium tetrafluoroborate

Room temperature ionic liquids exhibit unusual and, for electrochemical applications, promising properties such as ionic conductivity and non-volatility. Microdroplet and thin film deposits of an ionic liquid, 1-methyl-3-(2,6-( S )-dimethylocten-2-yl)-imidazolium tetrafluoroborate (MDIM + BF 4 − ) on electrode surfaces are studied in order to assess the ability of aqueous ions to partition into the ionic liquid. Two complementary methodologies, deposition of ionic liquid onto a 4.9 mm diameter basal plane pyrolytic graphite electrode followed by voltammetric analysis and casting of an ionic liquid film onto a random array of 7 μm diameter carbon microelectrodes (RAM TM electrode) followed by chronoamperometry, are employed. The ability of hydrophilic ions to partition from the water phase into the ionic liquid phase is unexpected and is shown to depend strongly on (i) the type of ion, (ii) specific interaction of the ion with the ionic liquid, and (iii) the concentration of the supporting electrolyte. Voltammetric responses obtained for the reduction of Fe(CN) 6 3− partitioned into microdroplet and thin film deposits of MDIM + BF 4 − indicate selective uptake of ferricyanide into the ionic liquid phase. Chronoamperometric experiments at RAM TM electrodes are used to quantify both the concentration of Fe(CN) 6 3− in the ionic liquid and the diffusion coefficient.

Measurement of the electrochemical oxidation of organic electrolytes used in lithium batteries by microelectrode

The electrochemical oxidation of organic electrolytes was investigated using microelectrodes. The electrolytes we used were LiClO 4/(EC+DEC) and LiPF 6/(EC+DEC) which are widely employed in lithium ion batteries. We measured the oxidation current by the potential step method (the potential is maintained until the current becomes constant) at potentials ranging from 4.5 to 5.5 V versus Li/Li +. The current density–potential relationship of the oxidation of 1 M LiClO 4/(EC+DEC) electrolyte on the carbon microelectrode shows good reproducibility and roughly fits exponential lines. We obtained a similar relationship regardless of the material and size of the microelectrodes. Therefore, the electrochemical oxidation of organic electrolytes can be compared using this method. The different oxidation behavior of LiPF 6 salt and LiClO 4 salt electrolyte suggests that the salt anions may initiate the electrolyte oxidation.

Voltammetric Determination of Heavy Metals in Microvolumes of Rain Water

Analytical methods for the trace determination of heavy metals in microvolumes of real water samples by using stripping voltammetry at microelectrodes and microelectrode arrays were investigated. Voltammetric procedures at gold, platinum and glassy carbon microelectrodes including appropriate measuring cells were developed and validated by the application of electrothermal vaporization-inductively coupled plasma-mass spectrometry (ETV-ICP-MS) as well as stripping voltammetry performed at conventionally sized mercury electrodes. The influence of a piezo actuator on the accumulation efficiency of the analyte at the electrode surface was tested. Detection limits in the low picogram range were achieved with anodic stripping voltammetry and Osteryoung-square-wave or differential pulse detection. The methods were applied to the determination of Pb, Cd, Zn, and Cu in single raindrops and small volumes of rain water.

Influence of Geometry on the Electrochemical Response of Carbon Interdigitated Microelectrodes

Microelectrodes were fabricated by carbonizing photoresist (700–1000°C) that was patterned on a Si wafer by use of a mask and UV photolithography. Two geometric designs of interdigitated carbon microelectrodes were produced with dimensions of about 500 μm length and 50 μm width. The carbon structures were characterized by Raman spectroscopy, atomic force microscopy, and optical microscopy. The electrochemical response of the microelectrodes was investigated by cyclic voltammetry using the redox couple. The collection efficiencies of carbon interdigitated array electrodes (IDAEs) varied from 59 to 90% depending on the cell size, geometry, and generator‐collector arrangement. These collection efficiencies are comparable to those reported with multiband (n > 25 bands) IDAEs. © 2000 The Electrochemical Society. All rights reserved.

Electrocatalytic Reduction of Dioxygen at Carbon Electrodes Modified with Co(II)-Schiff Base Complexes in Various pH Solutions

The electrocatalytic reduction of dioxygen has been studied by cyclic voltammetry and chronoamperometry at a glassy carbon electrode and a carbon microelectrode modified with Schiff base Co(II) complex in a dioxygen saturated 1 M KOH aqueous solution of various pH and temperature. The electrochemical reduction of dioxygen at the modified electrode establishes a pathway of two-electron reduction to form hydrogen peroxide. The reduction potential (EP) of dioxygen shows pH dependence. The electrochemical reduction of dioxygen is irreversible and diffusion controlled. The activation energy obtained from Arrhenius plots for the reduction of dioxygen was 3.64 Kcal/mol at the bare glassy carbon electrode, whereas it was 2.51 Kcal/mol at the modified glassy carbon electrode.

Electrochemical Analysis of Carbon Interdigitated Microelectrodes

*,zEnvironmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USAA procedure to fabricate carbon microelectrodes consisting of a patterned arrangement of interdigitated electrodes is described.Conventional semiconductor processing techniques involving photolithography to pattern photoresist layers on a Si wafer permit aconvenient method to form the interdigitated structure. The photoresist is then converted to carbon by pyrolysis. The iodine-iodideredox reaction was used to demonstrate the performance of the carbon interdigitated electrode.© 1999 The Electrochemical Society. S1099-0062(99)04-071-7. All rights reserved.Manuscript submitted April 19, 1999; revised manuscript received May 10, 1999. Available electronically July 9, 1999.

A combined long-term recording system for single-unit activity and neurotransmitter efflux of a brain slice

A combined long-term measurement and recording system for neurotransmission research of brain slices is presented in this study. This system, based on the IBM PC or compatible computer, is capable of simultaneously measuring and recording both single-unit neural electropotential signals and the electrochemical signals of neurotransmitter efflux from the same neuron in a brain slice for long periods of time (time limited largely by hard disk capacity, 100 h or more not being unreasonable with contemporary hardware) using a single carbon microelectrode for both measurements. The combined long-term recording system uses a simple switching circuit to switch periodically the single microelectrode between two data acquisition subsystems, one for electrochemical data and one for electrophysiological data. The simple switching circuit separates the electrophysiological signals and electrochemical signals, overcoming the traditional interference problem caused by the two different measuring techniques. Software designed for the proposed system allows easy reconstruction of the full time course of the compressed measured data and easy, simultaneous display of both types of signals on the same time scale. On-line and recorded displays are available. Test results of a practical implementation of the proposed system verify that the combined long-term recording system meets actual requirements for electrophysiological and neurochemical research.

Metal oxide-modified carbon paste electrodes and microelectrodes for the detection of amino acids and their application to capillary electrophoresis

Using cyclic voltammetry and constant-potential amperometry, copper, nickel, cobalt, lead, silver, manganese, bismuth and palladium oxides in a carbon paste matrix have been investigated as redox mediators for the electro-oxidation of amino acids in the 0.05 mol/L NaOH medium. CuO and NiO have been found to be the most effective electrocatalysts. The 150 μm I.D. microelectrode with 25% CuO was applied to the amino acids detection in capillary electrophoresis.

Direct electrochemical characterization of superoxide anion production and its reactivity toward nitric oxide in solution

We report in this study, and for the first time the direct and simultaneous electrochemical measurements of both nitric oxide and superoxide anion evolution and reactivity in solution. For this purpose, we have combined the use of two electrodes: a previously developed NO-sensor for the amperometric determination of NO and a carbon microelectrode for the amperometric detection of superoxide anion. The latter electrode displayed a potential use for the selective detection of O2− (generated by the xanthine/xanthine oxidase reaction) with an acceptable selectivity against uric acid. Our electrochemical experiments were validated by the Cytochrome c method and by the use of a different procedure for the generation of the superoxide anions. Our results show that the use of the NO sensor provides the selective detection of NO without any interface of the superoxide enzymatic generator.

Membrane-assembly of carbon microelectrodes for the electrochemical study of redox proteins

A novel membrane-assembly of microelectrodes has been used to study the electrochemistry of several redox proteins (c-type cytochromes, ferredoxins). The solutions of protein (5 μL) were entrapped between a dialysis membrane and the assembly surface. Near-steady-state current-voltage curves were obtained; by combining cyclic and square-wave voltammetry, a detection limit of about 20 pmol of analyte was attained. In addition, it has been demonstrated that direct electron-transfer was observed for cytochrome c and ferredoxins in the presence of a promoter within the electrochemical cell. Low molar-mass agents were able to diffuse through the membrane to develop their promoting effect.

Electrochemical Assessment of the Stability of [Os(NH3)5(OH2)]2+ in Aqueous Solutions

The [Os(NH3)5(OH2)]2+ complex in aqueous acid has been widely regarded as short-lived on the basis of results reported in an early pioneering electrochemical study.1 More recently, a much longer-lived Os(II) species generated by reduction of [Os(NH3)5(OH2)]3+ but of uncertain composition has been reported.6 In the present study the lifetime of [Os(NH3)5(OH2)]2+ in aqueous acid was determined by steady-state voltammetry at a carbon microelectrode and by comparing experimental with simulated cyclic voltammograms obtained for solutions of [Os(NH3)5(OH2)]3+. Rate constants for the oxidation of [Os(NH3)5(OH2)]2+ by H+, CH3COOH (HA), and H2O were evaluated as kH = 17 M-1 s-1, kHA = 1.2 M-1 s-1, and kH2O = 4.1 × 10-4 s-1. These values are shown to be compatible with the data in ref 1 and come close to accounting for the long-lived species described in ref 6 which may be [Os(NH3)5(OH2)]2+ itself. A revised value of 4.85 was measured for the pKA of [Os(NH3)5(OH2)]3+. Adsorption of an Os complex on the surface of mercury, but not glassy carbon, electrodes is shown to enhance greatly the rate with which protons undergo direct electroreduction.

Electrocatalyst for non-enzymatic oxidation of glucose in neutral saline solutions

An electroactive coating was deposited on the surface of carbon microelectrodes from solutions containing Pb(NO3)2, Na2IrCl6 and KNO3 by scanning their potentials from 0.0 to 1.15 V. The coating, believed to be IrO2 entrapped inside a PbO2 matrix, catalyzes oxidation of glucose, and other carbohydrates (e.g. mannose), in a phosphate buffered saline solution at pH 7.4. The microelectrodes coated with iridium and lead were used as amperometric detectors at 0.75 V (versus a sodium saturated calomel electrode) for flow injection analysis of glucose samples. The linear dynamic range extended from 0.1 mM up to at least 20 mM, and the detection limit was 50 μM for glucose. The stability and selectivity of the oxide-modified electrodes were improved by application of Nafion® coating. The response of the Nafion®-coated electrode to 15 mM glucose was stable for four days.

Amperometric Sensors for Simultaneous Superoxide and Hydrogen Peroxide Detection

A two-channel sensor capable of almost instantaneous simultaneous detection of superoxide radical and hydrogen peroxide in the concentration range 10(-)(7)-10(-)(4) M is very important for understanding of a number of rapid kinetics processes. A glassy carbon working microelectrode covered by an electrodeposited polypyrrole/horseradish peroxidase (PPy/HRP) membrane was employed as a H(2)O(2) sensor. Another glassy carbon microelectrode covered by a composite membrane of an inside layer of PPy/HRP and an outside layer of superoxide dismutase was employed as a working electrode for superoxide detection. These two working electrodes with Pt counter and tungsten oxide (WO(3)) reference electrodes were contained in one 6 mm diameter Teflon cylinder. Simultaneous measurements were performed at a potential of -60 mV (vs WO(3) reference, pH 5.1). Additional sensor characterization was performed for pH 5.1-9.0. Superoxide sensor behavior as a function of membrane deposition conditions and coating time is reported. Sensors' mutual influence, selectivity, response times, linearity, stability, and sensitivity for hydrogen peroxide and superoxide are presented and discussed. A mathematical model of sensors' responses is proposed, with model calculation corresponding to experiment within 10%.

Surface Analysis of Porous Carbon Microelectrodes with an Adsorbed Osmium Complex as a Mediator.

Many different forms of carbon have been used for preparing chemically modified electrodes with electrontransfer mediators. In previous reports, we described porous carbon microelectrodes (PCMEs) developed as enzyme sensors.'-3 A platinum disk electrode packed with a porous carbon material supported in osmium complex ([Os(bpy)3]2+I3+) as a redox electron transfer mediator and enzyme. The PCME surface was modified by a Nafion solution. The osmium complex was adsorbed on the PCME dipped in the osmium complex+ 10 mM LiC104 (M=mol dm-3) solution. The osmium complex was electrostatically adsorbed onto the Nafion region on the porous carbon. The construction of the PCME surface with the Nafion and osmium complex is important for the performance of an enzyme sensor. It is of considerable interest to examine how the osmium complex interacts with a modified carbon surface, because the adsorption of this mediator is crucial to the operation and lifetime of an enzyme sensor.'-3 However, there have been few reports concerning surface analyses of PCME with an adsorbed osmium complex. In this paper we discuss the depth distribution of Nafion and an osmium complex on a porous carbon surface using X-ray photoelectron spectroscopy (XPS), and the determined amount of the osmium complex on the PCME by means of chronocoulometry. described.)-3 The platinum microelectrode (diameter, 20 μm) was etched in hot aqua regia (80° C,1 h) to create a cavity; this cavity was then packed with a porous carbon material. To load the osmium complex onto the surface, the PCME was first immersed in a 2.5% Nafion solution (Aldrich) for 24 h in order to impart a negative charge onto the surface. The PCME was dipped into a 0.1 mM [Os(bpy)3] (PF6)2+10 mM LiC104 solution for 24 h. [Os(bpy)3] (PF6)2 (bpy: 2,2'-bipyridine) was synthesized from osmium(III) trichloride (Aldrich).6 All of the other chemicals used were of analytical grade. The buffer solution used was a mixture of 0.2 M disodium hydrogenphosphate and 0.1 M citric acid for pH 8.0. All of the electrochemical experiments were performed using a Bioanalytical System Electrochemical Analyzer (BASl00B/W) with a three-electrode system. An Ag/ AgCI reference electrode and a Pt counter electrode were placed in the cell. The surface modification of sheets of porous carbon for use in XPS followed the same treatments as those mentioned above. The XPS spectra were acquired using a Shimadzu ESCA 1000A instrument and Mg-Ku radiation.

Electrocatalytic reduction of dioxygen by quadruply aza bridged closely interfaced cofacial bis(5,10,15,20-tetraphenylporphyrin)s in various pH solutions

The electrocatalytic reduction of dioxygen is investigated by cyclic voltammetry and chronoamperometry at a glassy carbon electrode and a carbon microelectrode adsorbed with cobalt porphyrins in dioxygen-saturated aqueous solutions of various pH. The reduction potential ( E p) of dioxygen at chemically modified electrodes shows dependence on pH. Electrochemical reduction of dioxygen at monomeric Co-1 or dimeric Co 2-2 modified electrodes establishes a pathway of 2e − reduction to form hydrogen peroxide, but it carries out 4e − reduction of dioxygen to water at the electrode adsorbed with dimeric Co 2-3 or Co 2-4 in all pH solutions. The electrocatalytic pathway of dioxygen reduction by cofacial biscobalt porphyrins is independent of pH, but the E p of dioxygen reduction is dependent on pH. Theelectrochemical reduction of dioxygen is irreversible and diffusion controlled.