Showing papers in "Electroanalysis in 1995"

Sensors based on carbon paste in electrochemical analysis: A review with particular emphasis on the period 1990–1993

Abstract: A review is presented dealing with the use of carbon paste as an electrode maaterial for electrochemical sensors (311 references). It covers mainly publications which appeared during the period 1990–1993; numerous applications demonstrate the widespread applicability of carbon paste in the field of electrochemical analysis, such as voltammetry, amperometry, and potentiometry, but also as an electrode for electrochemical detectors in flow systems.

Carbon paste electrodes modified with enzymes, tissues, and cells

Abstract: A review is presented dealing with the use of carbon paste amperometric electrodes for electroanalytical purposes, with either the surface or the bulk being modified with biologically derived material such as enzymes, tissues, and cells It covers virtually all the publications which have appeared from the very first enzyme-modified carbon paste electrode up to early 1994 and includes 220 references

Use of polymer films in amperometric biosensors

Abstract: Different types of polymeric films implemented in various capacities in amperometric biosensor design are reviewed. Conducting and nonconductinzg polymer films and composite films are considered. Methods of film fabrication such as solvent casting, electropolymerization, and adsorption are presented. Film function, including prevention of interference and fouling problems, immobilization of the enzyme or active biocomponent, extension of linear range, or to address biocompatibility problems are described. The use of polymeric films as a medium to incorporate mediators, while an important function, is too vast for review here. The applications of polymer film-modified biosensors for selected analytical determinations are also presented. This review focuses upon the use of polymer films in amperometric biosensors and emphasizes innovative biosensor designs and unique applications, and omits traditional free-standing films unless they are coupled with new types of polymer films or used innovatively.

Ion-exchange voltammetry at polymer-coated electrodes: Principles and analytical prospects

Abstract: This review describes principles and analytical applications of ion-exchange voltammetry (IEV) at polymer modified electrodes. Results of mechanistic studies which are relevant to the development and optimization of IEV methods are discussed. Useful examples of IEV determinations of traces of inorganic and organic electroactive ions of interest for environmental, biomedical or pharmaceutical analysis are given along with future prospects for this technique.

The electrochemical behavior of methylene blue at a microcylinder carbon fiber electrode

Abstract: The electrochemical behavior of methylene blue (MB) at a carbon fiber microcylinder electrode, was studied by cyclic voltammetry. The charge transfer coefficient α of the electrode reaction with two-electron transfer is 0.5, and the number of H+ participating in the electrode process is 3 at pH 2.2–5.4, 2 at pH 5.4–6.0 and 1 at pH 6.0–10.7, respectively. The standard electron transfer rate constant ko′, and standard formal potential Eo′ of MB at various pH were determined by using the carbon fiber microcylinder electrode. The electrode reaction mechanism of methylene blue at various pH is proposed. The adsorbability of MB at the electrode is discussed and explored by cyclic voltammetry and chronocoulometric technique.

Protein adsorption at glassy carbon electrodes: The effect of covalently bound surface groups

Abstract: A diazonium salt coupling procedure was used to covalently bind various p-substituted phenyls to glassy carbon electrodes. The response of modified electrodes for oxidation of ferrocenemethanol in the absence and presence of bovine serum albumin was monitored to give a measure of protein adsorption. Carboxylate groups close to the surface did not influence protein adsorption (relative to polished glassy carbon electrodes), however, hydrophobic groups increased adsorption. These results are compared to those for similar experiments at electrochemically pretreated glassy carbon electrodes. Hydrophilic groups attached to the surface via sufficiently long linkages reduced protein adsorption and may be useful for analysis of clinical samples.

Electroanalysis with interdigitated array microelectrodes

Abstract: This review discusses electroanalysis with interdigitated array (IDA) microelectrodes. The basic electrochemical properties of IDAs are presented along with a calculation of diffusion-controlled current and an example of generation collection voltammetry to clarify the advantages of IDA for electroanalysis. The fabrication procedures of metal and novel carbon based IDAs are also summarized because the fabrication of very small IDAs is advantageous for electroanalysis. Previously reported electroanalytical applications of IDAs are overviewed. Among them are the measurement of redox electron conduction and molecular diffusion in various films, various types of enzyme sensors and electrochemical switches, an example of highly sensitive and selective generation collection voltammetry, a new stripping analysis using self-induced redox cycling and the detectors for flow cells and liquid chromatography.

Studies on the phase boundaries and the significance of ionic sites of liquid membrane ion‐selective electrodes

Abstract: Attenuated total reflectance infrared spectroscopy (ATR-IR) has been used to study the cation permselectivity of liquid-membrane ion-selective electrodes (ISEs). All spectroscopic evidence was found to agree with the interpretation of permselectivity as being due to complete exclusion of counter-ions from the boundary phase. Compared to ATR-IR, optical second harmonic generation (SHG) has an even more pronounced surface sensitivity. It has been demonstrated to be a promising new technique for the study of ISE response mechanisms, as it allows one to observe the actual phase boundary. On the other hand, further potentiometric evidence for the influence of ionic sites on the emf response was found, both by a study of membranes free of ionic sites as well as by the investigation of leaching processes at poly(viny1 chloride) membranes. Finally, the use of a photoresponsive crown ether ionophore allowed the determination of emf response characteristics in relation to the ionophore concentration without any changes in the membrane composition.

Enzyme‐modified microelectrodes for in vivo neurochemical measurements

Abstract: Various approaches to the production of miniaturized chemical sensors for in vivo applications are reviewed with special regard to the measurement of chemical transients in the mammalian brain. The basic tenants of biosensor principles are discussed in terms of the temporal response of enzyme-modified electrodes and selection of redox enzymes. The effects of electron transfer mediators and differential measurements to gain selectivity are also examined. A broad range of immobilization methods including membrane entrapment, physical and chemical adsorption, cross-linking agents, polymeric entrainment, electropolymerized films and covalent derivatization are examined. The use of polymer layers and redox polymers to gain selectivity are discussed. The discussion is limited to microelectrode surfaces (< 100 μm diameter) and various electrode types are examined, including bare and platinized carbon fibers, platinum microelectrodes, conducting organic salts and carbon paste electrodes.

Calixarenes in electroanalysis

Abstract: The use of calix[n]arenes as recognition reagents in electroanalysis is reviewed. Calix[n]arenes (n = 4 to 14) are macrocylic phenol-formaldehyde condensates which may be derivatized at the phenolic oxygen position to produce macrocycles capable of the selective binding of metal cations. Calix[4]arene derivatives having ester, ketone and amide functionality generally exhibit sodium selectivity and the larger calix[6]arenes show a preference for cesium ions. A number of calixarenes have been successfully incorporated into polymeric membrane ion-selective electrodes (including ISFETs). Calixarene-based sodium selective ISEs exhibit excellent sensitivity and selectivity, especially over potassium ions, which has allowed usage of these ISEs in assays of sodium in blood and serum. Calixarene-based ISEs sensitive to potassium, cesium and organic amine ions have also been reported. More recently ISEs and ISFETs sensitive to heavy metal ions such as silver, lead and copper(II) have been fabricated from various sulfur functional group derivatized calix[4]arenes. Potentiometric analytical uses of calixarenes have received most interest to date. However, voltammetric investigations have shown that carbon paste modified electrodes may be prepared with polymeric calixarenes and allow stripping voltammetric determination of lead, copper and mercury simultaneously. Voltammetric studies of calixquinone/hydroquinone and calixarene inclusion complexes have also been reported. Prospects for the future development of calixarenes as ionophoric reagents in electroanalysis are discussed.

Reproducible, reliable and rugged Hg-plated Ir-based microelectrode for in situ measurements in natural waters

Abstract: Construction of a reproducible, reliable and rugged iridium-based microelectrode is described. Perfect electrical contact, Ir-glass sealing and Ir disk morphology are the key points for obtaining reproducible voltammetric sensors. These points are discussed in detail and optimal fabrication conditions are given. Electron bombardment under vacuum yielded good soldering between Ir and Cu electrical cable. Reproducible polishing of the microelectrode to a mirror like Ir disk surface is obtained with silicon carbide pads and diamond paste using an automatic home made polisher. Cyclic voltammetry and optical microscopy have been used to characterize this microelectrode. Hgplated Ir-based microelectrodes are prepared by electrodeposition of mercury on the iridium disk substrate. Reproducibility and reliability close to 100% have been obtained for Ir-based microelectrode preparation, mercury layer deposition and trace metal measurements by differential pulse anodic stripping voltammetry (DPASV) and square wave anodic stripping voltammetry (SWASV) in synthetic solutions even at low ionic strength (10−3M). Continuous measurements over long periods of time indicate that the Hg-plated Ir-based microelectrodes can be used for several days without renewal of the mercury layer. These microelectrodes were applied to lead and cadmium speciation studies directly in river waters by SWASV without any separation. The results show that free PbII and CdII concentrations as low as 0.5 nM and 0.1 nM respectively can be determined by direct measurements, without perturbing the medium. The lifetime of these microelectrodes in the present state of the art is more than 2 years.

Determination of metal–organic complexation in natural waters by SWASV with pseudopolarograms

Abstract: A novel electrochemical method is described for the examination of metal–organic complexation in natural waters. The technique is based upon the comparison of square wave anodic stripping voltammetry (SWASV) ‘pseudopolarograms’ (current vs. deposition potential plots) for natural samples with those obtained for metal complexation with selected ‘model’ binding ligands at constant ionic strength. For electrochemically active zinc species in seawater, the pseudopolarogram approach allowed the estimation of thermodynamic stability constants (KML) within a stability constant window of approximately 104–106M−1 up to ≥ 1015M−1. For a natural rainwater sample, the pseudopolarogram indicated only one zinc class, probably a mixture of ‘free’ and/or inorganically complexed species. In an application to a coastal seawater sample, three species were identified: (1) weakly complexed, log KZnL = 4.14 ± 0.92 M−1; (2) moderately complexed, log KZnL = 7.77 ± 0.62 M−1; (3) strongly complexed, log KZnL = 11.45 ± 0.95 M−1. Results are compared to those obtained using complexometric titration with SWASV at a thin mercury film–glassy carbon rotating disk (TMF–GCRD) electrode. The latter detected a single strong complexing ligand (or ligand class), with [L′] = 15.25 ± 0.04 nM and log K′ZnL ≥ 10.9 M−1.

Applicability of the phase boundary potential model to the mechanistic understanding of solvent polymeric membrane-based ion-selective electrodes

Abstract: Recent experimental evidence suggests that the equilibrium partitioning of sample ions at the sample/membrane interface is the main parameter governing the potentiometric response of polymeric membrane-based ion-selective electrodes (ISEs). It is shown that the response of a neutral-carrier-based H+-selective electrode can be fully predicted on the basis of equilibrium concentrations measured optically within a thin organic film having the same composition as the ISE membrane. Consequently, using this simple and powerful phase boundary potential model together with mass balances and observed complex formation constants, the response and selectivity of various ISE membranes may be described. In this contribution, the most recent applications of the model are reviewed including: (1) a novel and general selectivity description, which is related to the so-called matched potential method and clearly shows the limitations of the extended Nicolsky–Eisenman equation if ions of different charge are considered; (2) the measuring range of neutral-carrier-based H+-selective ISEs, showing that previous experimental findings can now be explained by theory; (3) prediction of the influence of anionic and cationic sites on the selectivity of charged-carrier-based ISEs demonstrating that such ISE membranes need the incorporation of sites of the same charge type as the analyte ion to induce optimum potentiometric selectivity; and (4) the determination of the concentration of anionic impurities in poly(viny1 chloride) by measuring the influence of the anionic site concentration on the divalent/monovalent ion selectivity of a neutral-carrier-based ISE.

Surface‐modified gold electrodes for electrocatalytic oxidation of NADH based on the immobilization of phenoxazine and phenothiazine derivatives on self‐assembled monolayers

Abstract: Self-assembled monolayers of cystamine and cysteine chemisorbed on gold electrode surfaces have been used as a support for covalent immobilization of some phenoxazine and phenothiazine derivatives. The covalent attachment was achieved after previous functionalization of the free amino groups on the electrode surface with bifunctional reagents (terephthaloyl chloride and 1,6-hexamethylene diisocyanate) able to react with the imino form of the amino group in position 3 of the phenoxazine and phenothiazine derivatives. In every case, a shift of the redox potentials between 130 and 180 mV towards more positive potentials, as well as an increase in the peak-topeak separation between anodic and cathodic peaks was observed after covalent attachment. The surface coverages (10−11 < Γ < 10−10 mol/cm2) obtained from the cyclic voltammograms were those for a not densely packed monolayer and varied depending on the redox compound immobilized. The surface-modified electrodes obtained were afterwards used for the electrocatalytic oxidation of NADH. UV-vis spectroscopy was used to monitor the successive steps of surface modification using gold semitransparent electrodes. The total surface coverage of redox mediator on the electrode surface could also be calculated from the optical difference spectra between the oxidized and the reduced state obtained ‘in situ’ in a spectroelectrochemical cell.

Nitrite‐selective electrode based on an electropolymerized cobalt phthalocyanine

Abstract: Cobalt(II) 4,4′,4″,4″′-tetraaminophthalocyanine was electropolymerized on glassy carbon by cyclic voltammetry. The formed polymeric films were shown to interact selectively with anions and were used in the design of ion-selective electrodes. Different polymerization conditions, as well as different conditioning solutions and buffer systems, were investigated in order to optimize the response of the electrodes. The electrodes were found to be most selective for nitrite. The slopes of the calibration plots were sub-Nernstian and the detection limit for nitrite was 1.7 × 10−4 M.

Stabilized bilayer lipid membranes for flow-through experiments

Abstract: This work reports a technique for the stabilization of solventless bilayer lipid membranes (BLMs), and the use of stabilized BLMS as flow detectors. Microporous filters composed of glass fibers, polytetrafluoroethylene (PTFE) and polycarbonate (nominal pore sizes from 1 to 5 μM) can serve as interfaces that separate two solution compartments. The micropores in the filter media can act as supports for formation and stabilization of BLMs. One of the solution compartments is used to cast lipid films on the filters, while a carrier electrolyte solution concurrently flows through the opposing compartment. Optimization of the flow cell design, and the chemical composition and methods for preparation of stabilized BLMs, are described. Lipid membranes composed of mixtures of phosphatidyl choline and phosphatidic acid could respond rapidly to pH alterations of the carrier electrolyte solution. Signals would reproducibly appear within a few seconds following the injection of an electrolyte of different pH than the carrier. Signals took the form of a single ion current transient with magnitude of tens of picoamperes (pA) and a duration of seconds. The mechanism of signal generation is explored by differential scanning calorimetry. The results show that a phase transition within a lipid membrane can be triggered by pH alterations of the electrolyte solution. Stabilized BLMs which provide artificial ion gating events hold prospects for chemical sensing of process streams.

Catalytic oxidation of methanol on a glassy carbon electrode electrochemically modified by a conductive NiII‐curcumin film

Abstract: This work focuses on the electrochemical synthesis and characterization of a conductive polymeric nickel-curcumin film and its application for electrocatalytic oxidation of methanol in alkaline solutions. It has been found that a glassy carbon electrode modified by this film acts as a very effective catalyst for oxidation of alcohols. Electropolymerization of the nickel-curcumin complex and oxidation of methanol have been studied by cyclic voltammetry. The described modified electrode provides a durable catalytic surface which allows the voltammetric oxidation of methanol: in 0.1 M NaOH electrolyte the resulting anodic peak is at 0.59 V (vs. SCE). Electrooxidation was not possible at bare glassy carbon electrodes, at least before the onset of the electrolyte decomposition. The mechanism of modifying the film formation has been discussed.

Electrode modification with a poly(NiII-tetramethyldibenzotetraaza[14]annulene) film. Electrochemical behavior and redox catalysis in alkaline solutions. I

Abstract: The voltammetric behavior in alkaline solution of a nickel-based chemically modified electrode (Ni-CME) prepared by oxidative electropolymerization of a nickel–tetraazaannulene complex (NiIIL) was investigated. After electrochemical deposition of the (NiIIL)n film in dichloromethane on a conducting substrate, the modified electrode was continuously cycled in alkaline solution until a steady voltammetric profile was obtained. Although upon electrochemical treatment in basic solutions the nickel–nitrogen tetracoordination of the (NiIIL)n film is probably lost, the nickel species remain entrapped inside the polymeric skeleton and exhibit stable redox behavior resembling that of the nickel hydroxide electrode. This makes it possible to prepare very thin films of nickel-based modified electrodes. In cyclic voltammetry experiments the Ni-CME exhibits a pH dependence of the peak potentials typical of the α-Ni(OH)2/γ-NiOOH redox process, and a separation between anodic and cathodic peak potentials of 64mV at pH 13.2 and 10 mVs−1. The redox catalysis at Ni-CMEs of representative compounds that are not easily electrooxidized, such as choline, cadaverine, γ-amino butyric acid (GABA), benzyl alcohol, and glycine, is demonstrated. The good stability of these nickel-based modified electrodes makes them attractive in analytical applications.

Study of the nickel‐based chemically modified electrode obtained by electrochemical deposition of an NiII‐tetramethyl‐dibenzo‐tetraaza [14] annulene complex. Redox catalysis of carbohydrates in alkaline solutions. II

Abstract: The electrochemical treatment of an electropolymerized (NiIIL)n thin film (where L represents 5,7,12,14-tetramethyl-dibenzo[b,i]-1,4,8,11-tetraaza [14] annulene) gives rise to a nickel-based catalytic deposit on conducting substrates [glassy carbon (GC), platinum, or gold electrodes], which shows strong similarities to the nickel hydroxide electrode. The effect of potential cycling on surface composition, chemical status, and morphology of this nickel-based chemically modified electrode (Ni-CME) has been investigated by cyclic voltammetry (CV) [1], X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. XPS analysis was very useful for characterizing the chemical changes in the nickel coordination caused by electrochemical treatments in alkaline solution. The featureless surface of the as-prepared GC/(NiII)Ln, film compared with the fractal nature of the GC/Ni-CME was revealed by electron micrographs. The Ni- CME exhibits high electrocatalytic activity towards the oxidation of carbohydrates in alkaline solution. The electrooxidation capability of the Ni-CME is highlighted by the amperometric detection of sugars after anion-exchange liquid chromatographic separations with alkaline mobile phases.

Study of a cobalt‐based surface modified glassy carbon electrode: Electrocatalytic oxidation of sugars and alditols

Abstract: Sugars and alditols undergo an electrocatalytic oxidation in alkaline solution at cobalt-oxide-based glassy carbon (Co/GC) electrodes. The catalytic deposit was investigated by cyclic voltammetry and scanning electron microscopy (SEM). It is pointed out that, at relatively low coverage of cobalt, two main redox processes, CoII CoIII and CoIII CoIV, are observed in 0.2 M NaOH. The redox behavior of the Co/GC electrode is strongly dependent on hydroxide ion concentration, and no electrocatalytic activity is observed at pH lower than 11.5. Under voltammetric conditions the modified electrode exhibits a non-Nernstian potential/pH shift (−88 ± 3 mV per pH unit). A reasonable explanation of the E/pH behavior in alkaline solution is given and an oxidation mechanism of CoII to CoIII is proposed in terms of hydrous cobalt oxides. No detrimental effects on the electrocatalytic oxidation of polyhydric compounds are observed in the presence of high acetonitrile concentrations (up to 40%). Inspection by SEM of the cobalt-based deposit evidenced the presence of grain-like microcrystallites on the glassy carbon surface.

Electrochemical oxidation of DNA at a gold microelectrode

Abstract: The electrochemical oxidation of DNA at a gold microelectrode has been investigated using cyclic voltammetry with a homemade microliter cell. Denatured DNA is adsorbed and oxidized more easily at the gold electrode than native DNA. The electrochemical oxidation of DNA also depends on its strand length. The DNA oxidation product adsorbs strongly on the gold electrode, so poisoning and inactivating its surface; the adsorbed product also inhibits electrooxidation of the gold surface.

Ultrasonic extraction and field‐portable anodic stripping voltammetry of lead from environmental samples

Abstract: Ultrasonic extraction of lead from environmental samples, followed by anodic stripping voltammetric (ASV) analysis using a field-portable ASV instrument, was evaluated. Representative lead-containing standard reference materials were subjected to ultrasonic agitation in dilute nitric acid, and the lead subsequently determined by ASV. Recoveries of lead were found to be statistically equivalent to those obtained previously by means of hotplate concentrated acid digestion and atomic spectrometric analysis. Laboratory-prepared air filter samples were also analyzed for lead content by using the ultrasonic extraction/portable ASV analytical protocol. For duplicate laboratory-prepared air filter samples, results from microwave digestion and atomic absorption analysis compared well with those from ultrasonic extraction and portable ASV analysis. The lower detection limit for the ultrasound/portable ASV method was found to be less than 1 ppb Pb in solution, or < 0.1 μg Pb/filter. The results suggest that the method may allow for the on-site determination of lead in environmental samples such as paint, dust, soil and workplace air.

Amperometric batch injection analysis: Theoretical aspects of current transients and comparison with wall-jet electrodes in continuous flow

Abstract: Amperometric detection using the batch injection analysis technique has been studied and compared with the current obtained at the wall-jet electrode under similar conditions using the oxidation of potassium ferrocyanide and the injection of samples of volume 10 to 100μL. The form of the current transients has been analyzed and compared with the steady-state response at wall-jet disk electrodes. Good agreement is found at high dispension rate, differences at lower dispension rates being ascribed to radial diffusion effects. The importance of tip–electrode distance and injection volume have been evaluated; it is shown that the optimum distance is 3 mm and that maximum sensitivity requires a minimum injection volume of 14 μL. A detection limit of 50 μM using simple amperometric detection is estimated. Implications of these results for the efficient application of amperometric batch injection analysis are discussed.

All‐solid‐state chloride‐selective electrode with poly(pyrrole) solid contact

Abstract: The responses of all-solid-state potentiometric chloride sensors with an internal poly(pyrrole) contact and a plastic membrane with methyltridodecylammonium chloride were investigated. The influence of the poly(pyrrole) doping anion was studied. Chloride-selective electrodes with a poly(pyrrole) solid contact were characterized by good stability. The selectivity coefficients for this electrode are slightly better than for the internal solution electrode.

Electrochemical behavior of metallothioneins and related molecules. Part III: Metallothionein

Abstract: The electrochemical behavior of Cd–Zn Thionein from rabbit liver and horse kidney with respect to pH of the solution at the dropping mercury electrode was studied, using differential pulse polarography (DPP). At pH 7.5 three well defined peaks were observed at a potential different from the potentials corresponding to pure Cd2+ and Zn2+. Two different electrochemical systems are involved: the oxidation of the mercury electrode in the presence of thiol groups contained in the metallothionein and the reduction of either free (pH 6) metal ions. The apparent acid-base dissociation constant of metallothionein was estimated. The apparent stability constants of the complexes CdT, ZnT and HgT were estimated at different pH values. Cadmium forms at least two complexes with thionein exhibiting different stabilities and probably different structures. Modifications in the polarograms with the addition of Cd, Zn and the peptidic fragment [56–61] MT I, a molecule intrinsic to the metallothionein structure, were studied. A diagram showing the oxido-reduction potentials corresponding to twelve diflerent electrochemical species is given. DPP allows one to distinguish the chemical forms of compounds (speciation).

Electrochemical response of poly(vinylferrocenium)-coated Pt electrods to some anions in aqueous media

Abstract: The electrooxidation behavior of iodide, thiocyanate, and cyanide on poly(vinylferrocenium)-coated Pt electrodes was investigated using cyclic voltammetry and differential pulse anodic stripping voltammetry. IR and UV spectroscopic data, and differential scanning calorimetry (DSC) measurements indicated that these anions were incorporated in the polymeric structure as a result of anion exchange. Anodic stripping of these anions from the film was carried out after the initial preconcentration. Cyanide uptake was found to cause an irreversible change in the physical properties of the film, whereas thiocyanate and iodide are merely incorporated in the film structure as counter ions.

Redox polymers for electrocatalytic oxidation of NADH – A random block methyl‐siloxane polymer containing meldola blue

Abstract: A water-insoluble modified methyl-siloxane polymer which held 9-dimethylaminobenzo[a]phenoxazine-7-ium (Meldola Blue) units, was synthesized, characterized, and used to drop-coat solid graphite electrodes. The electrocatalytic oxidation of the nicotinamide adenine dinucleotide (NADH) in the polymer could be described by the Michaelis–Menten type of kinetics as seen from steady-state measurements in 0.25 M phosphate buffer, pH7, at 0 mV (vs. SCE). The electrocatalytic properties of the redox polymer was evaluated using the film thickness as a major variable. The theoretical model found to describe the NADH oxidation at the redox polymer modified electrode, is presented. A maximum NADH sensitivity of 40 μA cm−2 mM−1 at a coverage of 0.5 μg cm−2 was indicated.

The Liquid/Liquid Micro-Interface for the Amperometric Detection of Urea

Abstract: Reference LEPA-ARTICLE-1995-008View record in Web of Science Record created on 2005-11-07, modified on 2017-05-12

Cation determination in aqueous solution using the methyl viologen-doped zeolite-modified carbon paste electrode

Abstract: Methyl viologen (MV2+) initially exchanged in a zeolite Y modified carbon paste electrode (ZMCPE) is reduced after being exchanged for the supporting electrolyte cation. The use of an electrolyte cation that cannot enter zeolite channels to a great extent does not give rise to appreciable currents while a non-size-excluded cation allows liberating MV2+ from zeolite particles and their subsequent reduction at the electrode–solution interface. Following those observations, the MV2+-based ZMCPE has been used for the constant potential (−0.85 V) amperometric detection of nonreducible:, non-size-excluded cations in aqueous medium. The performance of this electrode was investigated in both batch injection analysis (BIA) and flow injection analysis (FIA). The great advantage of BIA coupled to the amperometric detection is the ability to use electrolyte-free sample solutions. For FIA experiments, dissolved oxygen was eliminated by using an electrochemical scrubber containing porous carbon electrodes. With the MV2+-doped ZMCPE, alkali and alkali-metal cations can be detected at the submillimolar level for both batch and flow injection analysis.

The electrochemistry of hydrogen peroxide on evaporated gold/palladium composite electrodes. Manufacture and electrochemical characterization

Abstract: A thin-film composite metal electrode is described for the amperometric measurement of hydrogen peroxide. By thermally evaporating a 27 μM layer of gold/palladium through a copper mask onto a plastic substrate a number of chemically equivalent sensing elements can be prepared. On these electrodes, hydrogen peroxide can be oxidized at a voltage of 0.4V (SCE). A model for the oxidation of peroxide at these electrodes is described based upon reversible binding of hydrogen peroxide to electrochemically generated palladium oxide sites.