Polymer-coated Electrodes Research Articles

Electrochemical deposition and characterization of conducting polymer polypyrrole/PSS on multichannel neural probes

The conducting polymer polypyrrole (PPy) doped with polystyrene sulfonate (PSS) was electrochemically deposited onto the electrode sites of micromachined neural probes. The surface morphology of the films was found to vary with coating thickness. Examinations of atomic force microscope (AFM) images of the coatings using the power spectral density (PSD) method revealed the spatial frequency dependence of surface roughness. The magnitude of the impedance of the PPy/PSS coated electrode at the biologically relevant frequency of 1 kHz decreased with increasing thickness as the film roughens. The lowest impedance was observed at a thickness of ∼13 μm. This decrease in impedance was correlated to the increase in effective surface area. The power-law behavior of the impedance spectra was quantitatively correlated with the fractal characteristics of the polymer-coated electrode surface morphology. High quality neural signals were recorded acutely from cerebellum of guinea pig through the PPy/PSS coated electrodes.

PH-dependent electrocatalysis for proton reduction by bis(2,2′:6′,2″-terpyridine) cobalt(II) complex embedded in Nafion ® membrane

Electrocatalysis for H + reduction by a bis(2,2′:6′,2″-terpyridine) cobalt(II) complex ([Co(terpy) 2 2+]) dispersed in a Nafion ® membrane coated on a basal-plane pyrolytic graphite (BPG) electrode was studied. It was found that the electrocatalysis for H + reduction varies with the pH conditions employed. Under acidic pH conditions ( pH≲ 4 ), the electrocatalytic H + reduction was induced by [Co(terpy) 2 +] (Co(I)), while under weakly acidic–neutral pH conditions ( pH≳ 4 ), H + reduction took place after forming [Co(terpy) 2 + ] Co(0)). The pH-dependent electrocatalysis was attributable to weak basicity of the electrogenerated Co(I): under low pH conditions ( pH≲ 4 ) Co(I) can coordinate H + resulting in an intermediate (Co(I)–H). However, since the reduction potential of Co(II) is pH-independent above pH∼4, the H + reduction took place by Co(0) via Co(0)–H. The electron transfer to generate catalytically active species at the polymer-coated electrode was also studied by potential-step chronoamperometry. Similar to the concentration dependence of the turnover number of the catalyst, the apparent diffusion coefficient ( D app) of the electrons decreased with increasing the complex concentration. It is most probable that the overall kinetics in the electrocatalytic H + reduction is dominated by the reduction of the complex.

Alignment visualization using electroconvection of planar nematics

In electroconvection experiments with planarly aligned nematic liquid crystals the director orientation is, conventionally, fixed through a mechanical treatment (rubbing) of the polymer-coated electrodes. Without rubbing, on the other hand, the flow direction during sample filling imposes the director orientation. We report atomic force microscopy and x-ray measurements that show an anisotropy in the polymer surface structure on several scales as a result of the rubbing. In particular we observe a fish-bone structure on a 10 nm scale. We visualize the orientation of the director both during and after filling the system using the electroconvection pattern. This is a convenient tool for exploring new director configurations. We confirm for the observed surface structure that when flow and surface designate different orientations, the mechanical surface treatment dominates. We have been able to obtain regions with radial director orientation of millimeter size. Such an alignment renders possible new types of electroconvection experiments.

Metal-dispersed conducting polymer-coated electrode used for oxidase-based biosensors

The reduction current of H 2O 2 which was generated by enzymatic reaction of glucose was measured at a microparticle-loaded polypyrrole film when glucose oxidase was in the solution or when it was immobilized in the film. This technique has the advantage of enabling detection at a low potential (0.0 V versus SCE). The current was measured at various thicknesses of the film, various concentrations of glucose and various potentials. The results were analyzed by use of previously derived diffusion-kinetic equations. It was found that the electrochemical reduction of H 2O 2 was the rate-determining step and that the reaction occurred at the interface between the film and the solution.

Multiple square wave voltammetry of nanomolar and subnanomolar concentrations of europium (III) at polymer-coated electrodes

The ion-exchange voltammetric determination of europium(III) at polymer-coated electrodes is examined. The comparison between the ion-exchange cyclic voltammetric behaviour at glassy carbon electrodes, modified with the poly(estersulfonate) Kodak AQ55 and Nafion, indicates a better suitability of the latter. The use of multiple square wave voltammetry (MSWV) results in a dramatic improvement of the signal-to-noise ratios of the voltammograms recorded at the modified electrodes. Eu (III) determinations by MSWV at Nafion-coated electrodes are characterised by detection limits 2–3 orders of magnitude lower than those obtained with differential pulse voltammetry and the rare earth cation can be analysed at nano- and subnanomolar levels, depending on the experimental conditions adopted.

Electrosynthesis and study of carbazole–acrylamide copolymer electrodes

Polymerisation of N-carbazoles in the presence of acrylamide has been studied by electrochemical polymerisation onto platinum and indium tin oxide (ITO), electrodes from an acetonitrile solution of the monomers and tetraethylamonnium perchlorate-supporting electrolyte. The influence of polymerisation conditions such as electrode potential, monomer concentration, type of solvent and supporting electrolyte on the mechanical and electrochemical properties of final polymers have been studied. The redox behaviour of the polymer-coated electrodes was checked in solution containing ferrocene and dopamine solution by cyclic voltammetry. Depending on the conditions the electrode response may be reversible or quasi reversible. Cationic polymerisation of monomer is initiated either by the monomer cation radical or by the proton generated by the coupling reaction of the cation radical of carbazole. For the characterisation of copolymers Cyclic Voltammetry, UV–vis and FT-IR spectrophotometric, solid state conductivity and SEM measurements were used.

Spectroelectrochemical study of the influence of anions on the behaviour of poly(N-vinylcarbazole) films

Poly( N -vinylcarbazole) (PVCz) films were prepared by potentiostatic deposition onto indium tin oxide-coated glass (ITO) electrodes from acetonitrile solutions containing the monomer. Two different supporting electrolyte solutions, LiClO 4 and LiBF 4 , were used. The electrochemical behaviour of the polymer-coated electrodes was studied in solutions containing successively, both electrolytes, by means of in situ UV–vis spectrophotometry. The experimental results obtained in both electrolytes at the same analytical concentration could not be interpreted completely by considering only the influence of the size of the electrolyte anion. Spectroelectrochemical measurements performed in electrolyte solutions of the same conductivity have shown the importance of the specific interactions which may exist between the anions and the oxidised polymer sites.

Theoretical models for ac impedance of finite diffusion layers exhibiting low frequency dispersion

This paper is concerned with frequency dispersion in the low frequency range of electrochemical impedance measurements in thin layer cells such as electrochromic devices, conducting polymer-coated electrodes, ion exchange membranes, and in general any type of diffusion layer which exerts some hindrance to mass transport at the boundaries. New theoretical models are developed for diagnostic applications and treatment of cases in which systematic deviations from the standard models for spatially restricted diffusion impedances are found. This is done by using a generalized boundary condition in the solution of Fick’s law for a small ac perturbation. The resulting model has several satisfactory properties: (a) it generalizes in effect classical boundary conditions related to absorbing and reflecting boundaries, (b) it provides exact analytical solutions which can be tested experimentally, and (c) it provides a very simple physical picture of the origin of low frequency dispersion in film electrodes in terms of interfacial transfer functions. The properties of the generalized diffusion impedance imply that boundary effects cannot influence the impedance for frequencies in excess of the characteristic diffusion frequency ωd=D/L2. On the other hand, at low frequencies the response is a mixture of ‘volume’ and ‘boundary’ properties of the layer. Several particularized examples of blocking and non-blocking dispersive boundary conditions are studied in detail. An extended discussion is focused on a blocking interface that presents a capacitive dispersion describable as a constant phase element (CPE). Approximating expressions are derived which allow separation of boundary and volume contributions in the extreme low frequency range. This is expected to provide a powerful analytical tool for analysis in those instances where a sloped line is found in the low frequency region of the measured impedance.

Effect of water content in the Nafion matrix and the pH of the electrolyte solution on the charge transfer in the oxidation of the Ru(bpy) 32+ complex

Potential-step chronoamperospectrometry (PSCAS) was employed to investigate charge transport in one-electron oxidation of the Ru(bpy) 3 2+ complex in a Nafion film. The polymer-coated electrode was prepared by two methods: mixture casting and adsorption. The electroactive fraction of Nafion[Ru(bpy) 3 2+] prepared by the former method was lower than that prepared by the latter. If the charge transfer distance between the adjacent complexes is considered to be the same in the two systems, higher electroactivity for the adsorption system can be ascribed to the localization of the complex in the hydrophilic region of the Nafion and the localization factor can be calculated. The localization factor was found to decrease with increasing water ratio in the mixed solvent used for casting the film, which leads to an increase in volume of the hydrophilic region. The lifetime of photoexcited Ru(bpy) 3 2+ became shorter with increasing pH showing that movement of the complex was suppressed at higher H + concentration. This was ascribed to a shrinkage of the hydrophilic region in Nafion morphology. Rapid counterion migration under the potential step at low pH would result in a longer charge transfer distance and a faster charge transfer rate.

Electrochemical sensor for measurement of urea and creatinine in serum based on ac impedance measurement of enzyme-catalyzed polymer transformation.

Enzyme-catalyzed polymer transformation with electrochemical ac impedance detection has been employed for the measurement of urea and creatinine in serum samples. A polymer, based on poly(methylvinyl ether)/maleic anhydride modified by esterification with n-octanol, which is stable at pH 7.4 and which is transformed rapidly in response to alkaline pH changes, was linked to enzymatic reactions between urease and urea or creatinine deiminase and creatinine to produce a disposable sensor system. The polymer was screen-printed onto interdigitated screen-printed carbon electrodes and the electrodes overlaid with absorbent pads containing the relevant enzyme. Application of serum samples, "spiked" with either urea or creatinine, resulted in rapid polymer transformation, and resultant changes in the capacitance of the polymer-coated electrodes were analyte-concentration dependent. Additional information on the mechanisms of polymer transformation was obtained from dynamic quartz crystal microbalance measurements.

Reactive groups on polymer-coated electrodes, 9. New electroactive polythiophenes with epoxy and cyclic carbonate groups

Reactive groups on polymer-coated electrodes, 9. New electroactive polythiophenes with epoxy and cyclic carbonate groups

Reactive groups on polymer-coated electrodes, 9. New electroactive polythiophenes with epoxy and cyclic carbonate groups

Two new functionalized thiophene derivatives bearing epoxy and carbonate groups were synthesized. The electrooxidative polymerization of both monomers led to stable electroactive polymers, while the functional groups remained unchanged during the polymerization process. Electrochemical and spectroscopic data of the resulting homopolymers reveal that these polymers possess an extended conjugated π-electron system comparable to unfunctionalized poly(3-alkylthiophene)s. Both monomers can also easily copolymerize with 3-methylthiophene to form the corresponding conducting copolymers. SEM photographs show that the obtained homopolymers and copolymers exhibit a very compact homogeneous morphology, indicating that the new thiophene monomers have good formability of polymer films. The measurement of electrical conductivity of these polymers by means of two-probe method gave values in the range of 10 2 to 10 -1 S . cm -1 . Using butylamine and 2-aminoanthraquinone as model compounds, it was demonstrated that amino compounds easily react with pendant epoxy and cyclic carbonate groups and are convalently attached to the new electroactive polymer surfaces. Thus, a new type of conducting polymers with pendant reactive groups was developed.

Increased Redox-Reaction Rate of Quinhydrone at N-Substituted Polypyrrole Film Electrodes

Abstract A poly[1-methyl-3-(pyrrol-1-ylmethyl)pyridinium] perchlorate-coated glassy carbon electrode noticeably decreased the charge-transfer resistance (Rt) of quinhydrone relative to a naked glassy carbon electrode. The redox species was not concentrated into a polymer film; this result differs markedly from the very good absorption of anthraquinonesulfonates and indigosulfonates by a polymer film. The dependence of Rt and the interfacial capacitance on the film thickness implies that the charge-transfer process occurs at the film-solution interface, because the polymer film is appreciably electronically conductive at positive potentials. Because of a change in the film conductivities with the potential, the polymer-coated electrode increased the anodic peak current of quinhydrone by three times, but decreased its cathodic peak current by two fifths, relative to that of a naked electrode. Although the decrease in Rt at a polymer-film electrode is somewhat ascribed to an increasing area, which is due to a porous film surface, the electrocatalytic effect of the polymer film can be attributed to an increasing standard rate constant of the charge-transfer process; this increase may be due to the adsorption of quinhydrone on the polymer surface.

Mercury film electrodes coated with negatively charged polymer films in speciation studies of trace amounts of lead

AbstractGlassy carbon electrodes modified with Nafion or poly(3‐thiopheneacetate) and subsequently plated with mercury where used for lead speciation studies in buffered (0.1 M Tris, pH 8) sodium chloride (0.6 M) solutions. The complexing agents studied were nitrilotriacetic acid, ethylenediamine, oxalic acid and the well known electrode ‘foulant’ huinic acid. Most measurements were performed with square wave anodic stripping voltammetry. Films of Nafion were spin‐coated and heat‐cured, and poly(3‐thiopheneacetate) films were produced by base hydrolysis of electropolymerized poly(ethyl 3‐thiopheneacetate). The results are, to a certain extent, supporting the permselective model which has been suggested for Nafion modified electrodes. However, they d o not allow the conclusions that negative species in general are not sensed by these ionomer coated electrodes, because the negatively charged groups of the ionomers may compete successfully for metal cations with chelating agents forming labile species. The nonlabile, negatively charged lead‐NTA, where hardly measurable at the polymer‐coated electrodes, and the lead‐oxalate complexes were also to a certain extent excluded by the ionomers. The response of the Nafion‐coated electrode to the positively charged lead‐ethylenediamine complexes was in accordance with the earlier reported permselective properties of the polymer when the literature values of the stability constant were used to calculate the lead species distribution. At the poly(3‐thiopheneacetate) film, the response to lead‐ethylenediamine complexes were higher than to the lead‐oxalate complexes, but not as high as one could have expected if charge dependent permselectivity prevailed. As a means to understand the partition of ligands between bulk solution and polymer film, stability constants for the lead complexes of these were determined at the polymer‐coated electrodes as well as at the bare mercury film electrode.

Flow injection analysis as a tool for studying polymer modified electrodes

Flow injection analysis with a thin-layer amperometric detector is used for elucidating the transport properties of polymer-coated electrodes. The permeation, selectivity and dynamic properties of films such as cellulose-acetate, Nafion, polypyrrole, polyaniline or polyvinylpyridine are thus explored. The advantages of the flow injection manifold for this task are discussed. Equations are derived to support the discussion.

Mechanism of mediation of the electrochemical oxidation of K4Fe(CN)6 at poly-[tris(3-{ω-[4-(2,2′-bipyridyl)] alkyl}-thiophene)iron(II)]-film modified electrodes in aqueous solutions

The kinetics of the electrochemical oxidation of K4Fe(CN)6 at poly-[tris(3-{ω-[4-(2,2′-bipyridyl)]alkyl}thiophene)iron(II)]-film modified electrodes have been investigated in aqueous solutions, using cyclic voltammetry and rotating disk electrode voltammetry. The retro-electrocatalytic reaction of the substrate at the polymer-coated electrodes is dependent on the alkyl chain length between the thiophene and 2,2′-bipyridine moieties and on the concentration of the substrate. The electrodes coated with the films possessing short alkyl chains exhibit more reversible voltammetric behavior and better catalytic activity to the oxidation of [Fe(CN)6]4− species in aqueous solutions compared with those possessing long alkyl chains. Kinetic parameters, calculated on the basis of modified Levich equations, indicate that under most circumstances the mediated reactions are controlled by the chemical step.

Redox active thin films of polymeric metal-free octacyanophthalocyanine

Octacyanophthalocyanine is found to undergo oxidative polymerization under electrochemical conditions at 1.6 V. The polymer was characterized by infrared and UV-Vis spectra. The polymer-coated electrode undergoes a reversible electrochemical process when cycled in various electrolytes. The coated film acts as an ion exchange membrane specific to cations. This was proved by electrochemically trapping Fe(Phen) 3 2+ ions and monitoring the cyclic voltammetric response of the incorporated cations.

Aspects of the electrochemistry, electropolymerization and electrocatalytic activity of rhodium hydride and related complexes with tertiary phosphines and nitrogen ligands

Electrochemical characterization of a series of [RhIII(X)(Y)(P)2(L)]+ complexes (X, Y  H or Cl) containing a 2,2′-bipyridine derivative as the ligand L and a tertiary phosphine (PPh3 or PPh2Et) as the ligand P is described. Dichloro (X  Y  Cl) and monohydrido (X  Cl, Y  H) complexes are characterized by a bielectronic reduction of the metal centre associated with a chemical reaction liberating chloride anions, followed by one or two reversible nitrogen-ligand-localized one-electron reductions. In contrast, the Rh(III) oxidation state is strongly stabilized in dihydrido complexes which present the only two successive reversible one-electron reductions of the ligand L in the negative potential region. In addition, hydrido complexes undergo an irreversible anodic process with proton loss. Electrochemical and 31P nuclear magnetic resonance data demonstrate the formation of dihydrido complexes upon reduction of the corresponding monohydrido and dichloro complexes in the presence of a proton source (water or formic acid). This behaviour was observed in homogeneous solution as well as in polymer films. In fact, polymer-coated electrodes were readily synthesized by oxidative electropolymerization of complexes containing pyrrole-substituted 2,2′-bipyridine ligands. Rhodium(III) complex films were used for the electrocatalytic hydrogenation of unsaturated organic substrates in aqueous electrolytes.

In-situ impedance measurements during oxidation and reduction of conducting polymers: significance of the capactive currents

Abstract An attempt has been made to model mathematically the capacitive component of the current obtained in cyclic voltammograms of polymer-coated electrodes. The capacitance data used were obtained by means of electrochemical impedance spectroscopy (EIS) measurements performed in situ during the potential sweeps. It was shown that the capactive current flowing during the oxidation and reduction processes strongly depends on both the low frequency capacitance of the polymer and its changes during the sweep.

Simulation of electrochemical slow relaxation at electrically conducting polymer-coated electrodes based on the concept of percolation

Electrochemical switching of electrically conducting polymers from the conducting to the insulating state occurs very rapidly at first, followed by a slow relaxation with a logarithmic time dependence continuing for a few hours. The slow relaxation was simulated in a two-dimensional square lattice model by a Monte Carlo technique in the context of electrical percolation. A key aspect of the simulation is to pay attention to statistically aggregated conducting clusters, some of which are in electrical contact with the electrode. These clusters can participate in the conducting-to-insulating conversion, because they are at equi-potential to the electrode and work as a metal-like electrode. The insulating sites thus converted cut off the electrical path to the electrode and divide the cluster into small groups, most of which have lost electrical connection to the electrode. Clusters are rearranged by diffusion or electron exchange to yield new clusters and new electrical paths to the electrode These processes were iterated by computation until all the conducting species were exhaustively converted. When the reaction rate of the conversion or the cathodic electrolysis was a rate-determining step, the amount of the simulated conducting species was linearly related to the logarithm of the electrolysis time. The simulated result was in agreement with the previous experimental observations with polyaniline films.