Platinum Ultramicroelectrode Research Articles

Individually Addressable Thin-Film Ultramicroelectrode Array for Spatial Measurements of Single Vesicle Release

Thin-film platinum ultramicroelectrode arrays (MEAs) with subcellular microelectrodes were developed for the spatial measurement of neurotransmitter release across single cells or clusters of single cells. MEAs consisting of 16, 25, and 36 square ultramicroelectrodes with respective widths of 4, 3, and 2 μm were fabricated on glass substrates by photolithography, thin-film deposition, and reactive ion etching. The electrodes in each MEA are tightly defined in a 30 μm × 30 μm square, which is potentially useful to measure exocytosis across a single cell or clusters of single cells. These MEAs have been characterized with scanning electron microscopy and cyclic voltammetry and show excellent stability and reproducibility. Culturing PC12 cells on top of the MEAs has been achieved by modifying the array with a poly(dimethylsiloxane) chamber and coating a thin layer of collagen IV on top of the electrode surface. The electrochemical response to dopamine has been characterized after coating the surface with the cell-adhering molecules and then with cells attached. Amperometric detection demonstrates that individual exocytotic events can be recorded at these arrays with spatial resolution for dynamic electrochemical measurements near 2 μm. In contrast to previous single-cell experiments, the effect of dopaminergic drugs on imaging single vesicle exocytotic release from PC12 cell clusters is presented at cell clusters incubated with the dopamine precursor and Parkinson's therapy agent, L-3,4-dihydroxyphenylalanine, and at cell clusters incubated with the vesicular monoamine transport inhibitor, reserpine. The results of electrochemical imaging demonstrate that the drug effect on PC12 cell clusters is consistent with previous single-cell experiments.

Elucidation of the Mechanism of Redox Grafting of Diazotated Anthraquinone

Redox grafting of aryldiazonium salts containing redox units may be used to form exceptionally thick covalently attached conducting films, even in the micrometers range, in a controlled manner on glassy carbon and gold substrates. With the objective to investigate the mechanism of this process in detail, 1-anthraquinone (AQ) redox units were immobilized on these substrates by electroreduction of 9,10-dioxo-9,10-dihydroanthracene-1-diazonium tetrafluoroborate. Electrochemical quartz crystal microbalance was employed to follow the grafting process during a cyclic voltammetric sweep by recording the frequency change. The redox grafting is shown to have two mass gain regions/phases: an irreversible one due to the addition of AQ units to the substrate/film and a reversible one due to the association of cations from the supporting electrolyte with the AQ radical anions formed during the sweeping process. Scanning electrochemical microscopy was used to study the relationship between the conductivity of the film and the charging level of the AQ redox units in the grafted film. For that purpose, approach curves were recorded at a platinum ultramicroelectrode for AQ-containing films on gold and glassy carbon surfaces using the ferro/ferricyanide redox system as redox probe. It is concluded that the film growth has its origin in electron transfer processes occurring through the layer mediated by the redox moieties embedded in the organic film.

Surface interrogation mode of scanning electrochemical microscopy for oxidation study of adsorbed CO generated from serine on platinum

Abstract We report a new method for detection and oxidation of adsorbed carbon monoxide (CO(ads)) generated from serine on a polycrystalline platinum ultramicroelectrode (UME) by bromine (Br2) using in situ surface interrogation (SI) mode of scanning electrochemical microscopy (SECM). In the SI mode, tip and substrate are both Pt UMEs, and CO(ads) on Pt substrate, generated from serine, can be oxidized by the tip-generated Br2 giving a positive response. Dosing CO(ads) from serine instead of purging CO gas expands the newly introduced reaction of Br2 with CO(ads) and further enhances the hope to get rid of CO(ads) on Pt for fuel cells.

Simulation of pH-controlled dissolution of aluminium based on a modified Scanning Electrochemical Microscope experiment to mimic localized trenching on aluminium alloys

Some constituent intermetallic (IMPs) particles at the surface of aluminium alloys are considered as preferential sites for the initiation of structural corrosion resulting in localized trenching around the particles and the surrounding Al matrix. In this work, a modified scanning electrochemical microscope (SECM) experiment was used to induce such phenomena via a local alcalinisation on 200 nm thick aluminium coatings promoting their local dissolution in an aerated 0.1 M NaCl electrolyte. The local alcalinisation was induced by the oxygen reduction reaction on the tip of a SECM which mimics the surface of an isolated IMP. From a phenomenological point of view, reproducible cylindrical damage develops in front of the platinum ultramicroelectrode (UME). Using a 2D finite element modelling to simulate the SECM experiments, the role of the local alcalinisation was validated and the calculated Al dissolution rate was found in agreement with the experimental evaluation.

New Generation of Diazonium based on Heterocyclic Amines : Electrochemical Investigations

This work is devoted to the study of the electrochemical grafting onto platinum ultramicroelectrode (UME) through diazonium electroreduction using a heteroaromatic compound, without phenyl groups. The grafting was performed using the electrochemical reduction of 2-aminoterthiophenyldiazonium cation, generated in situ. It leads to the attachment of an electroactive organic layer onto the UME surface. Following that the electrochemical response of the modified UME in the presence of various reversible redox couples shows that the attached layer acts as a conductive switch. It behaves as a barrier to electron transfer when the standard redox potential is below 0.5 V vs. SCE; in this case the oxidation wave of the redox probe is shifted to more positive values. However, for more oxidizing redox probes the layer can be considered as transparent and no barrier effect is observed. Finally, scanning electrochemical microscopy (SECM) can be performed using this electroswitchable modified platinum.

Impurity Effects on Oxygen Reduction Electrocatalysis at Platinum Ultramicroelectrodes: A Critical Assessment

Impurity effects associated with an increase in the rate of mass transport induced by a decrease in the size of electrodes have been examined with emphasis on species capable of undergoing irreversible adsorption. Calculations show that for hemispherical ultramicroelectrodes 50 nm in radius, the flux of a solution-phase contaminant, such as a small organic molecule, present at ppb levels, is large enough to reach saturation coverage over times on the order of seconds. In the case of dioxygen reduction on Pt in aqueous electrolytes, such adsorbed species are believed to be responsible for marked changes in the mechanism, promoting a two-electron over a four-electrode reduction pathway. Such aspects must be considered in the analysis of results obtained with Pt ultramicroelectrodes, either as single particles or in dilute arrays.

Development of an Analytical Methodology for Quantification of Strong Acid in Diesel Oil

AbstractMany factors can affect the quality of diesel oil, in particular the degradation processes that are directly related to some organosulfur compounds. During the degradation process, these compounds are oxidized into their corresponding sulfonic acids, generating a strong acid content during the process. p‐Toluene sulfonic acid analysis was performed using the linear sweep voltammetry technique with a platinum ultramicroelectrode in aqueous solution containing 3 mol L−1 potassium chloride. An extraction step was introduced prior to the voltammetric detection in order to avoid the adsorption of organic molecules, which inhibit the electrochemical response. The extraction step promoted the transference of sulfonic acid from the diesel oil to an aqueous phase. The method was accurate and reproducible, with detection and quantification limits of 5 ppm and 15 ppm, respectively. Recovery of sulfonic acid was around 90%.

Grafting Oligothiophenes on Surfaces by Diazonium Electroreduction: A Step toward Ultrathin Junction with Well-Defined Metal/Oligomer Interface

The functionalization of electrode materials through diazonium electroreduction using a heteroaromatic compound, without phenyl groups, has been investigated for the first time. The electrochemical reduction of 2-aminoterthiophenyldiazonium cation, generated in situ, coats the electrode (glassy carbon (GC), gold or platinum) with an ultrathin organic layer, shown by X-ray photoelectron spectroscopy (XPS) of that deposited on gold to consist of terthiophene or oligothiophene. The coating is electroactive at potential close to that of terthiophene in solution. The electrochemical response of the modified GC electrode in the presence of various reversible redox couples shows that the attached layer acts as a conductive switch. It behaves as a barrier to electron transfer when the standard redox potential is below 0.5 V/SCE; in this case diode-like behavior is observed. However, for more oxidizing redox probes the layer can be considered as transparent and no barrier effect is observed. The layer deposited on a platinum ultramicroelectrode (UME) behaves similarly to that obtained on the large GC electrode. Scanning electrochemical microscopy (SECM) can be performed using this electroswitchable modified platinum UME which can act as a filter toward competitive redox exchange pathways.

Significance of comproportionation reaction in multi-step electrochemical reduction of fullerene C 60

Voltammetry at platinum ultramicroelectrode in 1,2-dichlorobenzene shows that for a high concentration of supporting electrolyte, THAPF 6, reduction waves 1, 2 and 3 of C 60 are of identical height. When supporting electrolyte concentration is lowered, the second and the third wave become lower than the first one. This feature does not agree with simple reduction mechanism under diffusion-migration transport, but supports the model, by Amatore and coworkers, that assumes very fast comproportionation reaction. According to this model, higher-charged fullerene anions are present only in the proximity of the electrode surface, because they react with lower-charged intermediates and are eventually converted to single-charged fullerene anions. Only single-charged anions are transported into the bulk of the solution.

Microelectrodes modification through the reduction of aryl diazonium and their use in scanning electrochemical microscopy (SECM)

This work is devoted to the study of the electrochemical grafting of nitrophenyl groups onto platinum ultramicroelectrode (UME). The grafting was made using the electrochemical reduction of nitrophenyldiazonium. Our results demonstrate the possibility to reduce the diazonium onto Pt UME. As consequence the electrochemical reduction leads to the attachment of nitrophenyl groups onto the UME surface. Following that, the modified UME was characterized using electrochemical techniques. In addition, the electrochemical response of the modified UME in the presence of reversible redox couple, ferrocene, has been studied. The main remark is that the steady state current observed at the UME is not affected by the presence of the nitrophenyl layers. Finally, from this last point we demonstrate the possibility to achieve scanning electrochemical microscopy (SECM) using modified platinum UME.

Conical Tungsten Tips as Substrates for the Preparation of Ultramicroelectrodes

Here we describe a simple method to prepare voltammetric microelectrodes using tungsten wires as a substrate. Tungsten wires have a high tensile modulus and enable the fabrication of electrodes that have small dimensions overall while retaining rigidity. In this work, 125 microm tungsten wires with a conical tip were employed. For the preparation of gold or platinum ultramicroelectrodes, commercial tungsten microelectrodes, completely insulated except at the tip, were used as substrates. Following removal of oxides from the exposed tungsten, platinum or gold was electroplated, yielding surfaces with an electroactive area of between 1 x 10-6 and 2 x 10-6 cm2. Carbon surfaces on the etched tip of tungsten microwires were prepared by coating with photoresist followed by pyrolysis. The entire electrode was then insulated with Epoxylite except the tip, yielding an exposed carbon surface with an area of around 4 x 10-6 to 6 x 10-6 cm2. All three types of ultramicroelectrodes fabricated on the tungsten wire had similar electrochemical behavior to electrodes fabricated from wires or fibers insulated with glass tubes.

Electroanalysis of Bromate, Iodate and Chlorate at Tungsten Oxide Modified Platinum Microelectrode Arrays

AbstractThis article describes the direct detection of Bromate, but also of chlorate and iodate, using modified arrays of platinum ultramicroelectrodes. The detection limits for these ions are IO$\rm{ {_{3}^{-}}}$=0.76 μM; BrO$\rm{ {_{3}^{-}}}$=2.34 μM and ClO$\rm{ {_{3}^{-}}}$=133.2 μM. The formation of a suitable tungsten oxide layer and the optimum analytical conditions are described. These oxide layers are formed electrochemically from hydrogen peroxide‐tungstanate solutions in acid conditions. Although film stability may be an issue, particularly at pH>3, these modified arrays of microelectrodes can be used for analytical purposes. After the analysis, the modifying oxide is easily removed and the Pt microelectrode array recovers its original properties.

Oxygen Reduction at the Pt/Recast-Nafion Film Interface at Different Temperatures and Relative Humidities

The effects of temperature (20-80{degree sign}C) and relative humidity (~0- 100%) on the oxygen reduction reaction (ORR) on a Nafion®- covered platinum ultramicroelectrode were investigated using cyclic voltammetry and chronoamperometry. The cyclic voltammograms exhibited a significant hysteresis between the forward and reverse scans, which increased with either humidity or temperature increases. No hysteresis was observed for the fully hydrated film equilibrated with liquid water. It is postulated that the oxygen transport may be obstructed by water produced by oxygen reduction, which accumulates at the interface due to its hindered diffusion in the partially hydrated films. The charge transfer kinetics of ORR at both clean and oxide-covered platinum were found to slow down with increases in relative humidity. The effect was stronger for the oxide-covered platinum and was correlated with differences in platinum oxide coverage associated with changes in film hydration.

Monitoring β-galactosidase activity by means of scanning electrochemical microscopy

The activity of β-galactosidase from Escherichia coli was characterized by scanning electrochemical microscopy (SECM). β-Galactosidase microspots were patterned by immobilizing biotin-labelled β-galactosidase on streptavidin-coated paramagnetic beads and depositing the beads as microscopic microspots on a hydrophobic surface. The enzyme activity was mapped with SECM in the generation-collection mode by monitoring the oxidation of p-aminophenol formed in the galactosidase-catalyzed hydrolysis at the surface of the beads. The electrochemical properties of the enzyme substrate and product were studied with a platinum ultramicroelectrode, and the imaging conditions were optimised. The suitability of the procedure for forming β-galactosidase microspot arrays and the capability of SECM to resolve enzyme activity over individual enzyme microspots were demonstrated.

Determination of ranitidine in drugs using a mercury coated platinum ultramicroelectrode and hanging mercury dropping electrode

The voltammetric behavior of ranitidine has been studied in aqueous media with a mercury coated platinum ultramicroelectrode (Hg-UME) and by HMDE. The LSV curves for the electroreduction of ranitidine showed that the compound presents two reduction waves in pH < 4.0 and only one in pH > 4.0, the observed waves being attributed to the reduction of the nitro group to hydroxylamine. A linear relation between the current peak or limiting current and the ranitidine concentration using HMDE or Hg-UME was observed, demonstrating that these ultramicroelectrodes can be used in the analytical determination of ranitidine. An alternative and more sensitive methodology for the analytical determination of ranitidine by SWV was also developed, with a detection limit of 3.5 x 10-8 mol L-1 (or 11 mg L-1). The apparent recovery (AR) studies proved the accuracy and precision of the assay developed. The excipients found in comercial Antak tablets (Glaxo Wellcome) and the generic from EMSâ do not interfere in the determination.

Flow-based amperometric detection of dopamine in an immobilized cell reactor

A protocol is described for immobilizing PC 12 cells on to the lumen of fused silica microbore tubing having an inside diameter of 250 μm and coated with a thin layer of poly- l-lysine. Optimization studies of the immobilization procedure indicated that a 50 μg ml −1 solution of poly- l-lysine was the best material for cell adhesion to the fused silica tubing. In addition, it was found that the cells become attached to the poly- l-lysine in approximately 2 h, after which they could be maintained inside of the tubing for a period up to 5 days. Importantly, the immobilized cells ability to release neurotransmitters was evident by measuring the Ca 2+-induced release of dopamine with an in column amperometric detection scheme involving a Nafion® modified platinum ultramicroelectrode.

Electrochemical oxidation of 6-propionyl-2-(N,N-dimethylamino)naphthalene (prodan) in acetonitrile on Pt electrodes: Reversible dimerization of prodan radical cations

The electrooxidation of 6-propionyl-2-(N,N-dimethylamino)naphthalene (prodan) has been investigated for the first time. It has been performed on both conventional size platinum electrodes and platinum ultramicroelectrodes in acetonitrile by cyclic voltammetry with convolution analysis and controlled-potential electrolysis. The voltammetric responses at room temperature are similar to those corresponding to a kinetically controlled anodic electron-transfer process. Cyclic voltammetry measurements were also carried out at different concentrations of prodan and at different temperatures. Chemical transformation of the radical monocation formed after the electrochemical oxidation of prodan, as deduced from reverse controlled potential bulk electrolysis and cyclic voltammetry measurements, gives evidence for the reversible dimerization of prodan radical cations. Both diagnostic criteria and digital simulation confirm that radical–radical coupling is the reversible chemical reaction coupled to the initial electron transfer step, to give the corresponding dimeric dication. The kinetic and thermodynamic parameters for the first electron transfer reaction and the dimerization process were estimated from digital simulation at 293 K. Values of 8.0 × 106 M–1 and 3.2 × 105 M–1 s–1 were determined for the dimerization reaction equilibrium constant and the dimer formation rate constant, respectively. The diffusion coefficient of prodan was determined from both limiting currents from convoluted cyclic voltammograms as well as limiting currents on ultramicroelectrodes. For such cases where the electron number exchanged in the electrode process is unknown, a method derived from the combination of data previously indicated is also proposed to obtain the diffusion coefficient of the electroactive species.Key words: prodan, cyclic voltammetry, reaction mechanism, reversible dimerization, electron transfer reactions, radical cation.

The effect of concentration on the electrocrystallization mechanism for copper on platinum ultramicroelectrodes

The mechanism of copper electrocrystallization on Pt ultramicroelectrode surfaces has been studied by cyclic voltammetry, chronoamperometry and ex situ atomic force microscopy in acid solutions containing different Cu2+ concentrations. To analyze the potentiostatic transients a new model developed in our laboratories that takes into account spherical diffusion towards a disc-shaped ultramicroelectrode was used. A mechanistic change from progressive to instantaneous nucleation was observed when the Cu2+ concentration was increased from 0.01 to 0.1 mol dm-3. The shape and distribution of the nuclei formed on the surface were investigated by AFM. The dependence of the nucleation rate with potential was analyzed in terms of the atomistic theory of electrocrystallization and the results suggest the presence of only one copper atom in the critical nucleus.

Voltammetric oxidation of dipyridamole in aqueous acid solutions

The electrochemical oxidation of dipyridamole (DIP) has been studied in acidified aqueous solutions at platinum electrodes employing cyclic voltammetry and controlled-potential electrolysis. The progress of the anodic oxidation as a function of time was monitored by cyclic voltammetry with platinum ultramicroelectrodes, absorption and fluorescence optical spectroscopies, the resulting integrated charge being indicative of a two electron process. The cyclic voltammograms registered for low scan speeds are characterized by a single irreversible diffusion controlled anodic wave, the related cathodic wave being also observable for scan speeds higher than 1 V s-1. Oxidation reaction stoichiommetric parameters were obtained through Tafel slopes resulting in unitary reaction orders for DIP and H+.

Electrochemical studies of heterogeneous reduction of tetracyanoquinodimethane in poly(ethylene oxide) electrolytes using ac impedance and cyclic voltammetry at an ultramicroelectrode

Electrochemical studies of the TCNQ0/TCNQ− couple have been carried out using ac impedance spectroscopy and cyclic voltammetry at platinum ultramicroelectrodes (UME). Liquid poly(ethylene oxide) (PEO) CH3–O–(CH2–CH2–O)4–CH3 has been used as the solvent with different concentrations of the TCNQ0/TCNQ− couple and LiClO4 as the supporting electrolyte. On the basis of the ac impedance results at the UME it has been found that the double layer capacitance and standard heterogeneous rate constant are independent of the presence of electroactive species and supporting electrolyte concentrations, indicating that adsorption of electroactive species onto the electrode is not significant. The standard heterogeneous rate constant ks was found to be 0.109 ± 0.005 cm s−1. A similar value of ks = 0.094 cm s−1 was obtained for the second reduction step TCNQ−/TCNQ2−. Diffusion coefficients of both TCNQ and TCNQ− are equal, D = 1.0 × 10−6 cm2 s−1 for a 0.5 M LiClO4 solution. Higher diffusion coefficients are obtained in less concentrated supporting electrolyte. Comparison is made between these results and those reported previously for PEO-400 HO–(CH3CH2O)8–OH. The different end groups significantly influence the viscosity and hence ks.