Potentiometric response of graphite electrodes coated with modified polymer films

Graphite electrodes coated with chemically-modified polymer films are described. Several different polymers were used, including poly(acrylic acid), poly[triethyl(vinylbenzyl)ammonium chloride], poly[trihexyl(vinylbenzyl)ammonium chloride], and poly[trihexyl(vinylbenzyl)ammonium thiocyanate]. A cation-responsive electrode can be prepared from poly(acrylic acid)-coated graphite. Anion-responsive electrodes can be prepared from graphite coated with polymeric quaternary amines. In these electrodes, the ion-sensing species is irreversibly attached to the polymer (rather than physically entrapped within a polymer matrix); this factor eliminates leaching of the active component, and the addition of a plasticizer is unnecessary. A selective sensor for thiocyanate is described; it yields a Nernstian response over the concentration range 1 × 10^?1–1 × 10^?5 M sodium thiocyanate.     

Quasi-reversible voltammetric response of electrodes coated with electroactive monolayer films

Quasi-reversible voltammetric response of the surface-confined redox species is treated, including both ion pairing and double-layer effects. The Smith–White model and the Butler–Volmer formulation are chosen as the electrode|solution interface model and the electrode kinetics, respectively. An algorithm to simulate the voltammogram is derived.     

Reaction, diffusion and migration in conducting polymer electrodes: analysis of the steady-state amperometric response

The processes of reaction, diffusion and electromigration of a charged substrate within an electronically conducting polymer film deposited on an inert supporting electrode are examined in terms of a quantitative analysis and solution of the pertinent differential equations. An analytical expression for the concentration profiles of substrate within the polymer layer is derived and a theoretical expression for the corresponding steady-state amperometric current response is presented. The transport and kinetics of the substrate are discussed in terms of a dimensionless reaction/diffusion parameter γ and a migration/diffusion parameter β. Received: 13 March 1998 / Accepted: 17 July 1998     

Preparation and electrochemical behaviour of electrodes coated with glow-discharge–polymer films

The electrical parameters of low-pressure glow-discharges are discussed with respect to formation of high-quality organic thin films on conducting substrates. It is demonstrated theoretically and experimentally that ac discharges operated at frequencies of the order of 10 KHz are well suited for producing flawless dielectric films. The produced coatings are characterized by electrochemical methods and by electron microscopy. The permeability of the film, its relationship with the chemical nature, and the associated electrochemical behaviour are discussed.     

Theoretical voltammetric response of electrodes coated by solid polymer electrolyte membranes

A model for the differential capacitance of metal electrodes coated by solid polymer electrolyte membranes, with acid/base groups attached to the membrane backbone, and in contact with an electrolyte solution is developed. With proper model parameters, the model is able to predict a limit response, given by Mott–Schottky or Gouy–Chapman–Stern theories depending on the dissociation degree and the density of ionizable acid/base groups. The model is also valid for other ionic membranes with proton donor/acceptor molecules as membrane counterions. Results are discussed in light of the electron transfer rate at membrane-coated electrodes for electrochemical reactions that strongly depend on the double layer structure. In this sense, the model provides a tool towards the understanding of the electro-catalytic activity on modified electrodes. It is shown that local maxima and minima in the differential capacitance as a function of the electrode potential may occur as consequence of the dissociation of acid/base molecular species, in absence of specific adsorption of immobile polymer anions on the electrode surface. Although the model extends the conceptual framework for the interpretation of cyclic voltammograms for these systems and the general theory about electrified interfaces, structural features of real systems are more complex and so, presented results only are qualitatively compared with experiments.     

The preparation of polyaniline/polypyrrole conductive polymer films on polycarbonate‐coated Pt electrodes

In this study, free‐standing polymer films were obtained first with an electrochemical coating of polyaniline and then with a coating of polypyrrole on an insulating polycarbonate‐coated Pt electrode. The films contained varying amounts of polyaniline and polypyrrole obtained by varying the electrolysis time, and their conductivities were determined. The Raman spectra of the films taken from the electrode side were similar to those of pure polyaniline, whereas the spectra of the solution side were identical to those of pure polypyrrole. The resistance change in the films between −15 and +120°C revealed that the films were sensitive to temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 51–59, 2000     

Electrochemical catalysis with redox polymer and polyion-protein films

Supramolecular redox-active assemblies on electrodes are of fundamental interest and can be used to create functioning devices such as sensors, biosensors, and bioreactors. The ability of redox-active films to mediate electron transfer reactions in 3-D dramatically increases the sensitivity with which target molecules can be determined. Metallopolyion hydrogel films immobilized on electrode surfaces exhibit many properties that are reminiscent of those shown by redox-active proteins. This review discusses the electrochemical properties and applications of such films, including mediating electron transfer between electrodes and oxidase enzymes. In addition, polyion-protein films grown layer by layer have certain advantages in device fabrication, including facilitating direct electron transfer for many proteins, mechanical stability, use of tiny amounts of protein, and control of film architecture. This review presents examples of iron heme proteins in films grown layer by layer by alternate electrostatic adsorption for catalytic reduction of hydrogen peroxide and trichloroacetic acid and for oxidation of styrene.     

Theory for solvent and salt transfer accompanying partial redox conversion of electroactive polymer films under permselective and nonpermselective conditions

A comprehensive thermodynamic model for solvent and salt transfer accompanying a partial redox conversion, i.e., conversion between any two oxidation levels, of an electroactive polymer (EAP) film is presented. We discuss two possible cases, namely, one-phase and two-phase behavior of an EAP film. An expression describing the extent of solvent transfer in these situations is presented. Salt transfer is characterized by the difference in permselectivity indices (Delta R(b,a)) between two oxidation levels of the EAP film. Delta R(b,a) represents the difference in co-ion (salt) exclusion properties of the EAP in the two different oxidation levels. Delta R(b,a) is expressed in terms of the EAP's charge, number of electrons transferred in the redox reaction of an electroactive unit, concentration of the supporting electrolyte, salt partition coefficient between solvent and EAP phases, and salt activity coefficients in both phases. Plots of Delta R(b,a) as a function of the electrolyte concentration allow determining the EAP's phase behavior, ratio of salt partition coefficients, and number of electrons exchanged in the redox process. Delta R(b,a) is an experimentally accessible quantity; it can be obtained from electrochemical quartz crystal microbalance (EQCM) experiments. Delta R(b,a) values can be used as a diagnostic tool to characterize an EAP film.     

Nanostructured modified electrodes: role of ions and solvent flux in redox active polyelectrolyte multilayer films

Electrodes modified layer-by-layer by self-assembly of redox active polyelectrolytes comprised of osmium bipyridine-pyridine derivatized poly(allyl-amine) and poly(vinyl) sulfonate have been studied by EQCM, ellipsometry, cyclic voltammetry and electrochemical impedance spectroscopy in aqueous solutions of different anions and cations. Redox driven swelling by solvent exchange during oxidation, in excess to the hydration number, occurs by perturbation of the equilibrium between the osmotic and elastic forces as a result of the electrochemical injection of charge into the film. The exchanged mass and volume change during redox switching strongly depends on the nature of the anion under anion Donnan permselectivity conditions.     

Properties of semiconductor electrodes coated with living films of cyanobacteria

IntactPhormidium sp. cells, immobilized on a SnO₂ semiconductor electrode, are capable of transferring electrons to SnO₂ in a light-dependent reaction. Drying a “wet” algal electrode at 50°C for 60 min increases photocurrent output capacity by 100-fold. We have studied the effect of various parameters on photocurrent generation. The magnitude of the photocurrent increased with increasing light intensity and depended on the nature of the electrolyte solution. The output, about 8 μA 10 μg Chl^?1 cm⁺², was obtained using 50 mM H₃BO₃?Na₂CO₃?KCl buffer as an electrolyte, an irradiance (>460 nm) of 250 J/m², and potentiostatic conditions (the algal working electrode was poised at +0.6 V vs a saturated calomel electrode). The yield was more than doubled upon addition of an electron carrier, such as methyl viologen, benzyl viologen, or Vitamin K₃, to the electrolyte solution. Maximum photocurrent was obtained at around pH 8 and 45°C, which are optimal conditions for growth of the cyanobacterium. Furthermore, DCMU, an inhibitor of photosynthetic electron flow, drastically decreased the yield, as did heat treatment of the electrode at 110°C for 15 min. The photocurrent action spectrum peak coincided well with the absorption peak of the light-harvesting pigment, phycocyanin. These results support the idea that electron transfer can occur across algal cell walls from the source of the light-induced reactions located within the lamellar membranes to the semiconductor electrode.     

Diamond like carbon coated films for enzyme electrodes; characterization of biocompatibility and substrate diffusion limiting properties

The biocompatibility and substrate diffusion limiting properties for a range of diamond like carbon (DLC) coated microporous polycarbonate and DLC coated dialysis (haemodialysis) membranes have been studied. This characterisation builds upon previous findings where DLC coated membranes imparted enhanced enzyme electrode performance. In this study electrode linear ranges have been extended from 10 mM glucose for a 0.01 μm pore size membrane to 160 mM. These findings correlated with the duration of DLC deposition and associated reductions in permeability for glucose. Permeability coefficient ratios for both microporous and dialysis membranes were also found to be important with low glucose/O₂ permeability ratios imparting extensions in glucose linear response range. DLC coated membranes employed within enzyme electrodes have also been shown to exhibit enhanced haemocompatibility as determined by both sensitivity change and surface deposition of blood components examined by scanning electron microscopy. Correlations are made between the reduced losses in sensor response to biofouling/ working electrode passivation processes, and extended linear ranges that DLC coated membranes may impart to enzyme electrode performance. Particular reference is made to the determination of glucose levels within whole blood.     

Multilayer films of carbon nanotubes and redox polymer on screen-printed carbon electrodes for electrocatalysis of ascorbic acid

Multilayer films containing multiwall carbon nanotubes and redox polymer were successfully fabricated on a screen-printed carbon electrode using layer-by-layer (LBL) assembled method. UV-vis spectroscopy, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy and electrochemical method were used to characterize the assembled multilayer films. The multilayer films modified electrodes exhibited good electrocatalytic activity towards the oxidation of ascorbic acid (AA). Compared with the bare electrode, the oxidation peak potential negatively shifted about 350 mV (versus Ag/AgCl). Furthermore, the modified screen-printed carbon electrodes (SPCEs) could be used for the determination of ascorbic acid in real samples.     

Gravimetric measurement in redox polymer electrodes with the EQCM beyond the Sauerbrey limit

Gravimetric measurements during electrochemical redox switching of an Os-containing hydrogel film in contact with aqueous electrolyte have been obtained beyond the Sauerbrey rigid mass limit of the electrochemical quartz crystal microbalance (EQCM). For an acoustically non-rigid film the frequency-to-mass Sauerbrey relationship is not valid and the mass has been obtained from the experimental electroacoustic impedance data collected simultaneously with electrochemical experiments interpreted using Martin's viscoelastic model.     

Preparation and characterization of electrostrictive polyurethane films with conductive polymer electrodes

All-polymer electrostrictive soft films were developed for the first time by depositing conductive polymer (polypyrrole) directly on both sides of solution-cast electrostrictive polyurethane elastomer films. The final composite films are flexible with strong adhesion between the polyurethane film and the conductive polymer electrode. The conductivity (sheet resistivity ∼1000 Ω/□), of the polymer electrode is appropriate for its intended use. The compatible interface between the polypyrrole electrode polymer and the electrostrictive polyurethane significantly improves the acoustic and optical transparency of these composite films, compared with using a metal electrode film. The all-polymer films also exhibit comparable dielectric properties to gold-electroded polyurethane films in the temperature range from −40^°C to +80^°C. The temperature range covers the soft segment glass transition temperature of the polyurethane elastomers, which is about −20^°C. The films also show large electric field induced strain responses which are dependent on film thickness and measurement frequency. The electrostrictive characteristics in the all-polymer films show similarities to those of the films with gold electrodes under identical measurement conditions. © 1998 John Wiley & Sons, Ltd.     

The wettability of polymer films depends on the polymerization conditions

The polymerization conditions have a strong influence on the properties and time-dependent behavior of a sessile water drop on polymer films. For a given copolymer composition, the recipe components of emulsion copolymer films clearly influence the initial static water contact angle. Common emulsifiers such as sodium dodecyl sulfate lead to lower contact angles than poly(ionic liquid) dispersions which are a new class of stabilizers for emulsion polymerization. In contrast to the static contact angle, the time-dependent properties of the water contact angle reflect the particular conditions during the polymerization. It is a general conclusion of this study that waterborne emulsion copolymers show a distinctly different time-dependent behavior of the contact angle than bulk polymers of the same molecular composition regardless of the initial static contact angle.     

Comparative behavior of electrodes coated with thin films of structurally related electroactive polymers

General methods are described for the synthesis of electroactive polymers and the preparation of uniform, stable, polymer-coated electrodes. The electrochemical behaviour of thin films of eight different functionalized polystyrenes, differing in the identity and amount of attached electroactive species, and polyvinylferrocene is presented. A wide range of electrochemical properties can be observed by varying parameters such as film thickness, nature of the bound redox couple, extent of polymer functionalization and oxidation state. The deviations from ideal surface behavior are investigated in detail. These are shown to be consequences of cooperative electronic interactions, structural reorganization or uncompensated resistance within the film, depending upon the material. The film resistance varies greatly among the polymer films studied and is shown, in some cases, to be a sensitive function of the extent of oxidation and prior treatment of the film. This variable resistance is shown to be a consequence of slow ion transport through the film. The mechanism of electron transport in such materials is considered and a model of the metal/polymer/electrolyte interface is proposed.     

Mediated glucose enzyme electrodes by cross-linking films of osmium redox complexes and glucose oxidase on electrodes

Here, we report on a novel, versatile approach for the preparation of mediated enzyme electrodes, demonstrated using cross-linked films of glucose oxidase and a range of functionalised osmium complexes on graphite electrodes. Response of enzyme electrodes are optimised by evaluation of glucose response as a function of variation in ratios of [Os(2,2′-bipyridine)₂(4-aminomethyl pyridine)Cl]⁺ redox mediator, polyallylamine support and glucose oxidase enzyme cross-linked using a di-epoxide reagent in films on graphite. Lowering of the redox potential required to mediate glucose oxidation is achieved by synthesis of complexes using (4,4′-dimethyl-2,2′-bipyridine) or (4,4′-dimethoxy-2,2′-bipyridine) as a ligand instead of (2,2′-bipyridine). Enzyme electrodes prepared using the complexes based on dimethoxy- or dimethyl-substituted bipyridines provide glucose oxidation current densities of 30 and 70 μA?cm^?2 at 0.2 and 0.35 V applied potential compared to 120 μA?cm^?2 at 0.45 V for the initial enzyme electrode, under pseudo-physiological conditions in 5 mM glucose, with stability of signals proving inadequate for long-term operation. Current output and stability may be improved by selection of alternate anchoring and cross-linking methodology, to provide enzyme electrodes capable for application to long-term glucose biosensors and anodes in enzymatic fuel cells.