Rotating minidisk–disk electrodes

Rotating minidisk–disk electrode (RMDDE) was developed by replacing ring electrode of rotating ring–disk electrode (RRDE) with a minidisk electrode. Its applications were demonstrated by studying electrochemical reactions of ferricyanide and divalent copper. The replacement of ring electrode by minidisk electrode results in following advantages. First, the fabrication of RMDDE is easier than that of RRDE with the same electrode material. Second, there is more freedom in choosing electrode materials and sizes, since it is difficult to make thin ring electrodes of RRDE with fragile materials. Third, the replacement of ring electrode by minidisk electrode saves electrode materials, especially rare materials. Finally, the substitution of minidisk electrode for ring electrode allows using multiple minidisks for simultaneous monitoring of multiple components. Therefore, RMDDE is a promising generator–collector system, especially when special generator–collector systems are not commercially available, such as corrosion study and electrocatalysis study of new electrode materials.     

Theoretical Investigation of Generator–Collector Microwell Arrays for Improving Electroanalytical Selectivity: Application to Selective Dopamine Detection in the Presence of Ascorbic Acid

Recessed generator–collector assemblies consisting of an array of recessed disks (generator electrodes) with a gold layer (collector electrode) deposited over the top‐plane insulator reportedly allow increased selectivity and sensitivity during electrochemical detection of dopamine (DA) in the presence of ascorbic acid (AA), a situation which is frequently encountered. In sensor design, the potential of the disk electrodes is set to the wave plateau of DA, whereas the plane electrode is biased at the irreversible wave plateau of AA before the onset of the DA oxidation wave. Thus, AA is scavenged but DA is allowed to enter the nanocavities to be oxidized at the disk electrodes, and its signal is further amplified by redox cycling between disk and plane electrodes. Several different theoretical approaches are elaborated herein to analyze the behavior of the system, and their conclusions are successfully tested by experiments. This reveals the crucial role of the plane‐electrode area which screens access to the recessed disks (i.e. acts as a diffusional Faraday cage) and simultaneously contributes to amplification of the analyte signal through positive feedback, as occurs in interdigitated arrays and scanning electrochemical microscopy. Simulations also allow for the evaluation of the benefits of different geometries inspired by the above design and different operating modes for increasing the sensor performance.     

Square Wave Electroanalysis at Generator–Collector Gold–Gold Double Hemisphere Junctions

Gold–gold double‐hemisphere junction electrodes with an inter‐electrode gap of typically 1 µm are employed for generator–collector square wave voltammetry. Electrochemical data are recorded as a function of both generator and collector potential to give three‐dimensional response “maps”. At sufficiently low frequencies, close‐to‐steady‐state conditions for the collector response are achieved and peak responses complementary to those for the generator electrode are recorded. Due to localised interelectrode gap diffusion (fast) as opposed to peripheral diffusion (slow), information about reaction products and intermediates can be obtained. Local pH gradient effects provide additional “fingerprint” information beneficial for future application in analytical detection.     

Screen‐Printed Contactless Conductivity Detector for Microchip Capillary Electrophoresis

A new method for mass fabrication of silver ink conductivity detector electrodes for poly(methylmethacrylate) (PMMA) microchip electrophoretic systems has been developed based on screen‐printing technology. Printing of silver conductivity electrodes was performed through a patterned stencil on thin PMMA sheets. Following the electrode fabrication, the PMMA sheets are cut into cover sheets, and are aligned and sealed to the channel plate thus establishing a complete microchip separation device. The effects of the electrode width and spacing on the response and resolution have been investigated and the optimized electrode performance was compared to commonly used aluminum electrodes in the determination of ammonium, methyl ammonium, and sodium. The utility of the screen‐printed contactless conductivity detector (SPCCD) electrodes is further demonstrated for the separation and detection of organic acids with excellent reproducibility (RSD values of 3.7% and 4.1% for oxalate and tartrate, respectively). The thick‐film fabrication of the electrode material demonstrates the ability to mass‐fabricate detection devices with total process of device fabrication requiring less than 4 h (including the fabrication of channel plate, cover sheet with the electrodes, and subsequent bonding). The fabrication method described here is convenient and does not compromise the detector performance, hence offers great promise for producing single use field deployable analytical microsystems.     

Electrochemical Detection of Protons Produced in an Electrode Reaction Using Interdigitated Microarray Electrodes

This work describes the use of interdigitated array electrodes (IDAE) for proton detection. Methanol electrooxidation in sulfuric acid solution was exemplified. Reduction currents originating in the reaction product generated by methanol electrooxidation on a Pt generator electrode were observed at the Pt collector electrode, the potential of which was fixed in the hydrogen evolution region. In order to reduce the background current of hydrogen evolution, an Hg‐plated Pt collector electrode was fabricated. Compared to the Pt collector electrode, the reduction current observed at the Hg collector electrode was extremely small. The product detected was found to be a proton from the current responses observed at Pt and Hg collector electrodes.     

Growth and Application of Paired Gold Electrode Junctions: Evidence for Nitrosonium Phosphate During Nitric Oxide Oxidation

A bipotentiostatic gold electrodeposition process is developed to grow gold junctions between two adjacent 100 μm diameter platinum disc electrodes. Gold is electrodeposited simultaneously on both electrodes with an automated termination mechanism close to short‐circuit conditions. Gap junctions (average gap width ca. 4 μm) are obtained reproducibly and the behavior of the resulting generator–collector electrode system is investigated for two relevant redox systems. First, the chemically reversible oxidation of 1,1′‐ferrocenedimethanol in aqueous 0.1 M KCl is studied. Well‐defined feedback currents across the electrode junction in generator–collector mode are recorded down to sub‐micromolar analyte concentration. Electrochemically reversible voltammetric responses suggest fast heterogeneous electron transfer and this allows further gap geometry analysis. Second, the (apparently) chemically irreversible oxidation of nitric oxide in 0.1 M phosphate buffer solution (pH 7) at gold electrodes is re‐investigated and, perhaps surprisingly, generator–collector feedback currents are observed for a solution phase intermediate, here tentatively assigned to nitrosonium phosphate, NO⁺H₂PO . The life time of this intermediate, ca. 10 ms, is surprisingly long, given a typical decay time for free NO⁺ in water of only nanoseconds. The results are consistent with an estimated nitrosonium phosphate association equilibrium constant, K≈10⁷ mol^?1 dm³. Without further optimization of the electrode junction gap geometry, the determination of nitric oxide down to ca. 10 μM concentration is achieved. The benefits of smaller junctions and potential analytical applications of paired nanojunction electrodes are discussed.     

Dual Electrode Ensembles with Core and Shell Nanoelectrodes for Dopamine Sensing Applications

This paper presents the fabrication of dual electrode ensembles for electrochemical sensing of dopamine. A new dual electrode ensemble consists of vertically aligned core nanowire electrodes and a shell electrode with a hemi‐cylindrical nanocavity structure and submicron inter‐electrode spacing between the two working electrodes. By using a 3‐dimensional cavity structure of two working electrodes, collection efficiency in redox cycling of dopamine is enhanced. Initial measurement results show the ability to detect 200 μM concentration of dopamine with 69% collection efficiency on a dual electrode ensemble.     

Dual microband electrodes: current distributions and diffusion layer ‘titrations’. Implications for electroanalytical measurements

The simulation of transport to double microband electrodes in generator–collector mode is reported focusing especially on the ‘titration curve’ approach to electroanalysis in which a titrant is electrogenerated from a redox active precursor on the generator electrode and reacts homogeneously with the target analyte. The current on the detector electrode reflects the amount of titrant ‘surviving’ passage between the two electrodes. The form of the titration curve – plots of detector current as a function of generator current – is shown to be highly sensitive to the electrode kinetics of the redox couple driven at the generator electrode. Accordingly the naïve use of such methodology for analysis without accompanying simulation and kinetic analysis is fraught with danger. Use of the conformal mapping approach in combination with the ADI method for investigation of the ‘titration’ current distributions at the double band system gives fast and precise simulation of this and similar problems. Convergence analysis is described which allows for the automatic selection of the simulation grid size so as to obtain a chosen accuracy (for example 1%) of the current for all experimentally meaningful values of the geometrical and physico-chemical parameters of the system to be investigated.     

Fabrication of catalyst-coated membrane by modified decal transfer technique

A decal transfer method based on colloidal ink was developed for the fabrication of membrane electrode assemblies (MEAs). The new method requires fewer steps and utilizes H⁺ form of membrane compared to conventional decal method based on solution ink utilizing Na⁺ form of membrane. The structural features of the electrodes made by the modified decal method were investigated by scanning electron microscopy (SEM). The performance of fabricated electrode was evaluated for oxygen reduction reaction in a proton exchange membrane fuel cell. The results indicate that the modified decal method has the potential to be a facile method of fabricating electrodes with high performance.     

Detection of dopamine in the presence of excess ascorbic acid at physiological concentrations through redox cycling at an unmodified microelectrode array

The electrochemical behavior of dopamine was examined under redox cycling conditions in the presence and absence of a high concentration of the interferent ascorbic acid at a coplanar, microelectrode array where the area of the generator electrodes was larger than that of the collector electrodes. Redox cycling converts a redox species between its oxidized and reduced forms by application of suitable potentials on a set of closely located generator and collector electrodes. It allows signal amplification and discrimination between species that undergo reversible and irreversible electron transfer. Microfabrication was used to produce 18 individually addressable, 4-μm-wide gold band electrodes, 2 mm long, contained in an array having an interelectrode spacing of 4 μm. Because the array electrodes are individually addressable, each can be selectively biased to produce an overall optimal electrochemical response. Four adjacent microbands were shorted together to serve as the collector, and were flanked on each side by seven microbands shorted as the generator (a ratio of 1:3.5 of electroactive area, respectively). This configuration achieved a detection limit of 0.454?±?0.026 μM dopamine at the collector in the presence of 100 μM ascorbic acid in artificial cerebrospinal fluid buffer, concentrations that are consistent with physiological levels. Enhancement by surface modification of the microelectrode array to achieve this detection limit was unnecessary. The results suggest that the redox cycling method may be suitable for in vivo quantification of transients and basal levels of dopamine in the brain without background subtraction.

Generator‐collector Voltammetry at Dual‐plate Gold‐gold Microtrench Electrodes as Diagnostic Tool in Ionic Liquids

Ionic liquids provide high viscosity solvent environments with interesting voltammetric characteristics and new electrochemical mechanisms. Here, a gold‐gold dual‐plate microtrench electrode is employed in generator‐collector mode to enhance viscosity‐limited currents in ionic liquids due to fast feedback within small inter‐electrode gaps (5 μm inter‐electrode gap, 27 μm microtrench depth) and to provide a mechanistic diagnosis tool. Three redox systems in the ionic liquid BMIm⁺BF₄^? are investigated: (i) ferrocene oxidation, (ii) oxygen reduction, and (iii) 2‐phenyl‐naphthyl‐1,4‐dione reduction. Both transient and steady state voltammetric responses are compared. Asymmetric diffusion processes, reaction intermediates, and solubility changes in the ionic liquid are revealed.     

Boron‐Doped Diamond Dual‐Plate Deep‐Microtrench Device for Generator‐Collector Sulfide Sensing

A BDD‐BDD dual‐plate microtrench electrode with 6 μm inter‐electrode spacing is investigated using generator‐collector electrochemistry and shown to give microtrench depth‐dependent sulfide detection down to the μM levels. The effect of the microtrench depth is compared for a “shallow” 44 μm and a “deep” 180 μm microtrench and linked to the reduction of oxygen to hydrogen peroxide which interferes with sulfide redox cycling. With a deeper microtrench and a fixed collector potential at ?1.4 V vs. SCE, two distinct redox cycling potential domains are observed at 0.0 V vs. SCE (2‐electron) and at 1.1 V vs. SCE (6‐electron).     

Electrochemical Sensor Design Using Coplanar and Elevated Interdigitated Array Electrodes. A Computational Study

The time dependent diffusion equation for an interdigitated array (IDA) of coplanar and elevated electrodes is solved numerically by extrapolation of the fully implicit method using a problem adapted space grid. The simulations are performed for IDA electrodes in generator‐collector mode. The influence of the electrode height and the height of the diffusion space on the sensitivity and the response time is investigated.     

A Self‐contained Two‐electrode Nanosensor for Electrochemical Analysis in Aqueous Microenvironments

We describe the development, fabrication, and characterization of a novel two‐electrode nanosensor contained within the tip of a needle‐like probe. This sensor consists of two, vertically aligned, carbon structures which function as individual electrodes. One of the carbon structures was modified by silver electrodeposition and chlorination to enable it to function as a pseudo‐reference electrode. Performance of this pseudo‐reference electrode was found to be comparable to that of commercially available Ag/AgCl reference electrodes. The unmodified carbon structure was employed as a working electrode versus the silver‐plated carbon structure to form a two‐electrode sensor capable of characterizing redox‐active analytes. The nanosensor was demonstrated to be capable of electrochemically characterizing the redox behavior of para‐aminophenol (PAP) in both bulk solutions and microenvironments. PAP was also measured in cell lysate to show that the nanosensor can detect small concentrations of analyte in heterogenous environments. As the working and reference electrodes are contained within a single nanoprobe, there was no requirement to position external electrodes within the electrochemical cell enabling analysis within very small domains. Due to the low‐cost manufacturing process, this nanoprobe has the potential to become a unique and widely accessible tool for the electrochemical characterization of microenvironments.     

A Compact and Versatile Instrument for Radio Frequency Heating in Nonisothermal Electrochemical Studies

We describe a novel instrument and electrical circuit for sensitive electrochemical measurements at simultaneous direct electrode heating. The new measuring principle can be applied to working electrodes of various designs featuring two end contacts. In our experiments, the contacts were connected to a 100 kHz AC heating power supply and the potentiostat via the new inductor bridge circuit. A compact heating‐generator housing contains all components necessary for sine wave generation as well as amplification and transformation of the heating power. The new arrangement yields high temperature cyclovoltammetric signals for the [Fe(CN)₆]^3?/4? redox system with a noise level superior to the earlier symmetrically branched wire electrode designs. Noise and disturbances are dramatically suppressed especially for high resistance electrodes such as glassy carbon electrodes. Without a center contact, the working electrode design is greatly simplified. This opens new opportunities for the design of a great variety of heated electrodes that may be arranged in arrays or consist of materials with relatively high resistivity such as carbon and conducting polymers.     

Simulation of Redox‐Cycling Phenomena at Interdigitated Array (IDA) Electrodes: Amplification and Selectivity

We present Finite Element Method (FEM) simulations of interdigitated array (IDA) electrode geometries to study and verify redox selectivity and redox cycling amplification factor. The simulations provide an adequate explanation of an earlier found, but poorly understood, high amplification factor (65×) in a 1 μm‐spaced IDA microdevice. Moreover, using the FEM calculations we present selectivity measurements with IDA electrodes in a mixture of two redox species, as for example dopamine and ferricyanide. We show that it is possible to electrochemically detect dopamine in presence of the stronger reductor ferricyanide, which is impossible with direct amperometric detection, with the use of IDA electrodes with proper polarization potential of the collector electrode. Using our simulations, we show that a theoretical selectivity of dopamine over ferricyanide of 11 can be achieved.     

A New Infrared Spectroelectrochemical Cell for the Detection of Species Generated by Platinum and Screen‐Printed Carbon Electrodes

In this paper, we describe a simple and effective infrared (IR) spectroelectrochemical cell for detecting species generated from an electrochemical system featuring low‐IR‐reflectivity electrodes. The IR detection mode of attenuated total reflection (ATR) was employed to construct the spectroelectrochemical cell. Two kinds of electrodes, platinum (Pt) and screen‐printed carbon (SPC), were used to examine the performance of this new cell in detection of electroactive species generated by cyclic voltammetry. Because data generated from highly reflective electrodes are available in the literature, Pt electrode was used to characterize the performances of the developed spectroelectrochemical cell. Results indicated that species generated electrochemically can be observed readily and their responses were comparable to those described in the literature. The cell volume could be lower than 300 μL, which suggests that this approach may be very useful to obtain chemical information during electrochemistry for biological fluids with limited sample volumes. By examining the electrochemical behavior of several amino acids using both Pt and SPC electrodes, the redox behaviors can be readily observed indicating a new spectroelectrochemical cell was successfully developed for the purpose of using of SPC electrode.     

Wettability‐Regulated Extracellular Electron Transfer from the Living Organism of Shewanella loihica PV‐4

C‐type cytochromes located on the outer membrane (OMCs) of genus Shewanella act as the main redox‐active species to mediate extracellular electron transfer (EET) from the inside of the outer membrane to the external environment: the central challenge that must be met for successful EET. The redox states of OMCs play a crucial role in dictating the rate and extent of EET. Here, we report that the surface wettability of the electrodes strongly influences the EET activity of living organisms of Shewanella loihica PV‐4 at a fixed external potential: the EET activity on a hydrophilic electrode is more than five times higher than that on a hydrophobic one. We propose that the redox state of OMCs varies significantly at electrodes with different wettability, resulting in different EET activities.     

Theory and practice of electrochemical titrations with dual microband electrodes

Dual parallel microband electrodes, operated as a generator–collector pair and made by a simple, inexpensive mass-production method, have been used to implement a ‘titration’ method. The solution contains the electro-inactive analyte and a reagent from which the titrant can be generated electrochemically. The galvanostatically generated titrant is detected at the collector amperometrically. The collection efficiency is affected by the reaction between the titrant and the analyte. Determination of ascorbic acid by titration against ferricyanide is given as an example. The measurement is performed as follows: After application of the collector potential, the boundary conditions between measurements are renewed by a quick pulsed motion of the electrode assembly. Then the generator current is applied. Following an initial delay, the collector current increases as t^1/2, with slope and collection efficiency dependent on analyte concentration. This results in a fast and effective method for implementing some standard titration methods without the need for accurate volume measurement and reagent preparation. The accuracy is determined by the reproducibility of electrodes. The present work shows concentration measurements in the mM scale to ≈±10%, in a time of a few seconds. The highly stable extrapolation method used for numerical simulation of the generator–collector experiment, which takes into account the non-uniform current distribution over a microband electrode with a galvanostatic boundary condition, is developed using a conformal map. A good agreement between simulations and experimental results was obtained in voltammetric, potential step and generator–collector measurements. It is shown that a useful approximate calculation can be made rather easily by representing the problem in terms of a reaction layer in the conformal space.     

Fabrication, Characterization and Electrocatalysis of an Ordered Carbon Nanotube Electrode

A method for fabrication of ordered carbon nanotube (CNT) film,which was template-synthesized within the highly ordered pores of a commercially available alumina template membrane,modified glassy carbon(CNT/GC) electrode was established.The CNT/GC electrode showed excellent electrocatalytic activity toward dopamine electrochemical reaction without introducing any electrochemically active group into CNT film or activating any electrochemically active group into CNT film or activating the electrode electrochemically.DA undergoes ideal reversible electrochemical reaction on CNT/GC electrode at low scan rate(≤20mV/s) with an excellent reproducibility and stability.The CNT/GC electrode might be used in biosensors because the highly ordered CNT may present a steric effect on more efficient redox reactions of biomolecules.