Studies on the redox behaviour of some polythiophene derivatives by impedance spectroscopy in symmetrical and asymmetrical configurations

The generation and transport of ionic and electronic charges in three polythiophene derivatives, namely poly(3-methyl thiophene), poly(thiophene-3-methanol) and poly(3-thienyl methacrylate), have been studied using impedance spectroscopy in both asymmetrical (Pt ∣ polymer film ∣ electrolyte) and symmetrical (Pt ∣ polymer film ∣ metal contact) configurations. The symmetrical measurements are to our knowledge reported for the first time in the literature for these polymer films. The homogeneous film model proposed by Vorotyntsev was found to describe adequately the impedance spectra for poly(3-methyl thiophene) in the asymmetrical configuration. For the two other polymers, the use of the ordinary Nernst–Planck equations to describe the charge transport seems to be an oversimplification. The impedance spectra of the systems studied in the symmetrical configuration were found to be described adequately by an extension of the homogeneous film model for the metal ∣ film ∣ metal contact. On the basis of the calculated impedance parameters, estimates for the diffusion coefficient and concentration of ionic and electronic charge carriers in the film as a function of potential are obtained. The results are discussed in connection with previous dc resistance and FTIR/UV–vis spectroscopic measurements of the same materials.     

Methanol oxidation at platinum electrodes in alkaline solution: comparison between supported catalysts and model systems

Methanol oxidation was studied in 0.1 M NaOH at supported Pt electrodes, and compared with the single crystal Pt electrodes, Pt(1 1 1), Pt(1 1 0) and Pt(3 3 2), chosen as model systems, and with a polycrystalline Pt electrode. The supported Pt electrodes were obtained by chemical (Pt–C/GC) and electrochemical (Pt/GC) deposition of the catalyst layer on glassy carbon resulting in the same metal loading of 20 μg_Pt cm^?2. Using STM in air, the average particle size distributions, 3–6 nm at Pt–C/GC and 4–32 nm at Pt/GC were determined. Both supported Pt catalysts were less active than polycrystalline Pt. Negligible differences in the kinetics observed between Pt–C/GC and Pt(1 1 0) and also Pt/GC and Pt(1 1 1) suggested that the activities of supported Pt electrodes could be correlated with the activities of single crystal Pt electrodes oriented as the sites dominating in the Pt particles in catalyst deposits. The electrocatalytic activities of the electrodes studied increase in the sequence: Pt(3 3 2) > polycrystalline Pt > Pt–C/GC ～ Pt(1 1 0) > Pt/GC ～ Pt(1 1 1). On the basis of diagnostic criteria obtained, the chemical reaction between HCO_ad and OH_ad giving formate was proposed as the rate limiting step in methanol oxidation.     

Voltammetric and XPS investigations of nickel hydroxide electrochemically dispersed on gold surface electrodes

A chemically modified electrode composed of mixed hydroxide and oxyhydroxide nickel film (6–8 nmol cm^?2) on the gold substrate (Au ∣ Ni) was characterized by cyclic voltammetry and XPS techniques. The gold substrate electrodes were firstly electrochemically conditioned in 0.2 M NaOH by cycling the potential between ?0.25 and 0.6 V versus SCE, then modified by cathodic electrodeposition of nickel hydroxide films. These nickel films were obtained either by voltage cycling (50 mV s^?1) between 0.0 and ?0.5 V (SCE) or at constant potential of ?0.3 or ?0.5 V using non-deaerated 50 mM Ni(NO₃)₂ solutions. X-ray photoelectron spectroscopy (XPS) characterisation and voltammetric behaviour of Au ∣ Ni electrodes in alkaline solutions are described. Continuous electrochemical cycling of the Au ∣ Ni electrodes induces significant changes of the nickel films in terms of crystallographic structures and chemical composition. Combination of XPS and electrochemical methodologies have demonstrated the ability to follow the morphological and chemical changes in alkaline solutions upon cycling potentials. Angular-dependent XPS measurements have demonstrated that electrochemical treatment induces the formation of a uniform film layer with the following chemical distribution: Au ∣ Ni(OH)₂ ∣ NiOOH. The electrocatalytic activity of the Au ∣ Ni electrodes is investigated in alkaline medium using glucose as a model compound. The favourable combination of active species such as gold and nickel leads to a sensing electrode with strong catalytic activity over a wide range of applied potentials.     

Electrocatalytic and adsorption properties of platinum microparticles electrodeposited into polyaniline films

The catalytic and adsorption properties of platinum microparticles incorporated into polyaniline films were investigated using methanol oxidation in acidic solutions as a test reaction. The potential dependences of the specific reaction rates — the stationary oxidation currents referred to the unit of the true platinum surface were measured for different amounts of incorporated platinum (25–750 μg cm^?2). The methanol adsorption kinetics and the dependence of the surface coverage on methanol concentration were established. The results of adsorption measurements were compared with catalytic effects. It was found that the extent of the catalytic effects and their mechanism depend on the amounts of the microdeposits and their distribution in the polymer layer. For moderate and high platinum loadings (60–750 μg cm^?2) the specific reaction rates are higher than those on platinized platinum and increase with increasing loadings up to the reaction rate on smooth platinum. At the same time the reaction mechanism for these electrodes remains the same. It seems that the catalytic effects on such electrodes are influenced by structural peculiarities of the deposits. For low platinum loadings (25–35 μg cm^?2) a change in the ratio of the rate of different steps can be observed as methanol adsorption becomes markedly inhibited and limits the overall reaction rate.     

Novel solid-state cadmium ion-selective electrodes based on its tetraiodo- and tetrabromo-ion pairs with cetylpyridinium

Two novel cadmium solid-state ion-selective electrodes have been prepared by coating the surface of a graphite rod electrode directly with tetrahydrofuran solution containing PVC, cetylpyridinium–tetraiodocadmate (I) or cetylpyridinium–tetrabromocadmate (II), dioctyl phthalate and sodium tetraphenyl borate. The two sensors exhibit near-Nernstian anionic slopes of ?29.8 and ?25.1 mV/concentration decade, independently of pH over a wide range, with very fast response times of 3 and 7 s, respectively. The tetraiodocadmate (TIC) and tetrabromocadmate (TBC) electrodes posses linear ranges of 1.5 × 10^?6–1 × 10^?1 and 1.0 × 10^?6–1 × 10^?1 M and lower detection limits (LDL) of 6 × 10^?7 and 8 × 10^?7 M, respectively. The effects of membrane composition, type of plasticizer and pH of the sample solution were investigated thoroughly. The TBC electrode is shown to be free of all interference that is common for most of the reported cadmium ISEs except for that of Hg²⁺ ion. The two electrodes were applied for the determination of cadmium in some alloys and polluted water.     

Polymer films as permselective coatings for H2O2-sensing electrodes

The performance of amperometric biosensors based on oxidation of the H₂O₂ generated by an oxidase suffers frequently from interference due to electrooxidizable species in a sample. For example, an effective biosensor of this type for the neurotransmitter, glutamate, in brain extracellular fluid (ECF) should show little or no amperometric response to readily oxidizable dopamine (DA) and ascorbate (AA), which are common constituents of ECF. In this work, platinum electrodes treated with four different polymeric, permselective films are compared quantitatively in terms of sensitivity to H₂O₂, DA and AA; and selectivity for H₂O₂ relative to either interferent. Here, the selectivity is defined as the ratio of the H₂O₂ sensitivity to the interferent sensitivity. Platinum electrodes coated with electrodeposited, overoxidized polypyrrole (OPP) and polyphenylenediamine films display H₂O₂ selectivities relative to AA of ?130 and ?240, respectively, and relative to DA of ?100 and 80, respectively. The impressive rejection of DA by OPP films observed in this study contrasts with published results showing enhanced DA permeability of polypyrrole electrode coatings. Nafion- and polyaniline-coated electrodes give inferior selectivity performance. The OPP-treated electrodes are judged superior overall given the best combination of H₂O₂ sensitivity (390 nA μM^?1 cm^?2), detection limit (0.5 μM), response time (～5 s), and stability (>2 weeks), in addition to high H₂O₂ selectivities.     

Light irradiance through novel CAD–CAM block materials and degree of conversion of composite cements

Objective

To assess light irradiance (LI) delivered by two light-curing units (LCU’s) and to measure the degree of conversion (DC) of three composite cements, when cured through different thicknesses of two novel CAD–CAM block materials.

Electrosyntheses of high quality freestanding polyselenophene films in boron trifluoride diethyl etherate

High quality freestanding polyselenophene (PSe) films with conductivity as high as 2.8 × 10^?1 S cm^?1 were electrochemically deposited on stainless steel electrode from distilled boron trifluoride diethyl etherate (BFEE) containing 20 mmol l^?1 selenophene. As-formed polymer films were homogenous and flexible, and could be easily cut into various desired shapes. Meanwhile, PSe films prepared in this medium showed good redox activity and high thermal stability in comparison with polythiophene and its derivative, poly(3,4-ethylenedioxythiophene). To the best of our knowledge, this is the first case for the preparation of PSe films with high quality.     

Preparation and characterization of electrochemically modified electrodes containing the metal complexing moiety quinolin-8-ol anchored to the polymeric film

This work presents a new electrochemically modified electrode which is chemically and mechanically stable, and can be easily constructed. It is being proposed here for the first time, a synthetic route for the monomer 5-(4-allyloxi-fenilazo)-quinolin-8-ol which is electrochemically polymerized via the allyl ether group resulting on a film that coats the electrode’s surface, preserving the quinolinic group intact for metal complexation. The film was characterized by spectroscopic and voltammetric techniques. A complete passivation of the three electrodes tested (glassy carbon, carbon rod and stainless steel) was obtained from ?0.9 to +0.75 V, implying a possible quantification of transition metal species within this potential range. The successful complexation of copper in water media at open circuit and the resulting redox waves are reported for a home made stainless steel coated electrode, showing the potential use of this modified electrode for analytical applications.     

Influence of thermo-mechanical cycling on porcelain bonding to cobalt–chromium and titanium dental alloys fabricated by casting,milling, and selective laser melting

Purpose

The aim has been to determine the effect of thermo-mechanical cycling on shear-bond-strength (SBS) of dental porcelain to Co–Cr and Ti-based alloys fabricated by casting, computer-numerical-controlled milling, and selective-laser-melting (SLM).

Electrostatic modification of ferritin onto polypeptide-functionalized indium oxide electrode surfaces: Electrochemical and AFM studies

We demonstrated the direct electron transfer reaction of ferritin immobilized onto polypeptide-functionalized indium oxide electrodes using electrostatic interactions between polypeptides and ferritin. Polypeptides such as poly(l-lysine) and poly(l-arginine) were strongly adsorbed onto indium oxide electrode surfaces by electrostatic interactions. The modification of electrode surfaces with poly(l-lysine) was achieved by immersing indium oxide electrode into 1 mg ml^?1 poly(l-lysine) (molecular weight: 80,000 and 84,000 Da) for approximating 10 min. Ferritin molecules were fully immobilized onto poly(l-lysine)-functionalized electrodes at immersion times approximating 30 min. After accounting for the roughness for the electrode surface, the surface coverage of ferritin on the functionalized-indium oxide electrode was evaluated to be 9–13 × 10¹¹ molecules cm^?2, which indicates that ferritin molecules were densely packed like a full monolayer. Ferritin immobilized onto functionalized-electrodes showed the direct electron transfer reaction with the electrode. Potential value dependence of redox peaks on ferritin immersion times was not observed. Poly(l-arginine) (molecular weight: 94,000 Da) also acted as a modifier for immobilization of ferritin by electrostatic interactions. The electrochemical behavior of ferritin immobilized onto poly(l-ariginine)-functionalized electrodes was similar to that observed in poly(l-lysine)-functionalized systems. We obtained direct evidence for electrostatic interactions between ferritin molecules and poly(l-lysine) by tapping-mode AFM measurements; molecular ferritin binding to poly(l-lysine) molecular wires was observed.     

Electrochemical behavior of iodide at a nickel pentacyanonitrosylferrate film modified aluminum electrode prepared by an electroless procedure

The surface of an aluminum disk electrode was modified by a thin film of nickel pentacyanonitrosylferrate and used for electrocatalytic oxidation of iodide. The cyclic voltammogram of the modified Al electrode showed surface redox behavior due to the [Ni^IIFe^III/II(CN)₅NO]^0/1? redox couple. The modifying layer shows excellent catalytic activity toward the oxidation of iodide. Different supporting electrolytes containing different alkali metal cations affected the apparent formal potential of the redox films and thus, changed the thermodynamic tendency and kinetics of the modifying film toward the catalytic oxidation of iodide. This was explained by including the concept of a surface coverage normalized-catalytic current. The kinetics of the catalytic reaction were investigated by cyclic voltammetry and rotating disk electrode voltammetry in a suitable supporting electrolyte. The results were explained using the theory of electrocatalytic reactions at chemically modified electrodes. The heterogeneous rate constant for the catalytic reaction, k, diffusion coefficient of iodide in solution, D, and transfer coefficient, α, were found to be 5.8 × 10² M^?1 s^?1, 1.3 × 10^?5 cm² s^?1 and 0.66, respectively. In addition the effect of electrode surface coverage on the dynamic range of a calibration curve was investigated. Under optimum conditions a linear calibration graph was obtained over an iodide concentration range of 2–100 mM.     

Electrochemical investigation on the preparation of Lu–Co thin films in dimethylsulfoxide

Cyclic voltammetry and chronopotentiometric and chronoamperometric curves were used to study the electroreduction of Lu³⁺ in a LiCl + DMSO system. The electrode process of Lu³⁺ reduced on a Pt electrode occurs in one step: Lu³⁺ + 3e^? → Lu. The transfer coefficient and diffusion coefficient of Lu³⁺ in the 0.1 mol dm^?3 LuCl₃ + 0.1 mol dm^?3 LiCl + DMSO system were calculated as 0.141 and 2.86 × 10^?10 m² s^?1 at 305 K, respectively. The experimental results indicate that a Lu–Co thin film containing 5.65–56.30 wt% Lu was obtained by potentiostatic electrolysis in 0.1 mol dm^?3 LuCl₃ + 0.1 mol dm^?3 CoCl₃ + 0.1 mol dm^?3 LiCl + DMSO. The surface of the Lu–Co thin film observed by scanning electron microscopy (SEM) was uniform, adhesive and had a metallic luster. The Lu–Co thin films obtained were amorphous as proven by X-ray diffraction analysis (XRD).     

Improved platinization conditions produce a 60-fold increase in sensitivity of amperometric biosensors using glucose oxidase immobilized in poly-o-phenylenediamine

Platinization has been used to increase electrode surface area and therefore electrode sensitivity. However there are few studies on platinization conditions and their effects on the reaction diffusion of enzyme biosensors. We report the fabrication of sensitive (192 ± 48 μA cm^?2 mM^?1) amperometric glucose sensors. The repeatability (precision) was at worst 2.32% ± 1.22% of the measured value, the limit of detection was 0.94 mM and the detection was linear up to at least 25 mM. Sensor-to-sensor variability was 24%. The half-life of the sensors at 21 °C was 12 days. Shelf life at 4 °C was at least one month without a decrease in sensitivity. Under continuous operation, sensors performed at least 120 determinations without a large decrease in sensitivity; however; fluctuations in current response indicate that frequent calibration is required. Sensors were used for the determination of glucose in apple juice and white wine. High sensitivity was achieved by improving the conditions of platinization and entrapment of glucose oxidase in poly-o-phenylenediamine. Potentiostatic platinization at ?100 or ?50 mV vs. Ag∣AgCl resulted in mechanically stable deposits unlike those formed at the commonly used ?250 mV. Large concentrations of glucose oxidase did not inhibit the electropolymerization of o-phenylenediamine. H₂O₂ diffusion experiments using polished and platinized, bare and polymer-covered rotating disk electrodes suggest that H₂O₂ diffuses from the bulk of the solution through the polymer film to be oxidized at the platinized surface and the diffusion is apparently one-dimensional with platinization compensating for the coverage of catalytic sites by the polymer. Conversely, when, on a platinized electrode, glucose oxidase is immobilized in a thin poly-o-phenylenediamine film, substrate diffuses rapidly through the porous matrix and H₂O₂ is produced throughout the enzyme/polymer film. Under these conditions, diffusion is apparently multidirectional and platinization results in an effectively large increase in surface area that results in a high amperometric response. A mathematical model of the reaction–diffusion matrix of polished electrodes supports this interpretation of the increased response.     

Interactions between DNA and a water-soluble C60 derivative studied by surface-based electrochemical methods

A novel electrochemical micromethod for the investigation of the interactions between DNA and non-electroactive species is described. The method was developed using the system of double-stranded DNA (dsDNA) modified gold electrodes (dsDNA/Au), a synthesized water-soluble C₆₀ derivative as a model, and [Co(phen)₃]^3+/2+ (phen=1,10-phenanthroline) as an electroactive indicator. Electrochemical studies with dsDNA-modified gold electrodes suggest that the C₆₀ derivative can interact strongly with dsDNA, with binding sites of the major groove of the double helix and phosphate backbone of dsDNA, a binding constant of (1.6 ± 0.2) × 10⁵ M^?1 obtained in 5 mM NaCl in Tris–HCl buffer, and a dissociation rate constant from the dsDNA/Au surface of (1.2 ± 0.1) × 10^?2 min^?1.     

Interpretation of the ultra-fast electropolymerization of pyrrole in aqueous media on zinc in a one-step process: The specific role of the salicylate salt investigated by X-ray photoelectron spectroscopy (XPS) and by electrochemical quartz crystal microbalance (EQCM)

The use of sodium salicylate as an electrolyte makes it possible to deposit polypyrrole (PPy) films on oxidizable metals such as zinc by the electrochemical oxidation of pyrrole. In spite of the very large difference between the pyrrole and zinc oxidation potentials, which thermodynamically should lead to metal dissolution and not polymer formation, PPy films are formed as easily as on a platinum electrode. X-ray photoelectron spectroscopy (XPS) and in situ electrochemical quartz crystal microbalance (EQCM) experiments reveal that a very thin composite passivating zinc salicylate layer is formed prior to pyrrole electropolymerization and prevents zinc dissolution without inhibiting polymer formation. Ex situ XPS analysis of the surface at different potentials (?1 to 0.7 V) at a grazing and normal incidence of the photoelectrons shows that pyrrole is adsorbed on zinc at an early stage of the polarization and remains on the metal surface up to the beginning of electropolymerization. In situ EQCM measurements indicate that this passivation layer is very thin (about 5 nm), and does not desorb at the potential where pyrrole is oxidized. The rapid oxidation of pyrrole through this passivating layer is explained by the presence of conductive pyrrolic paths inside the salicylate layer which make this composite layer as active as a noble metal.     

Surface structures at the initial stages in passive film formation on Ni(1 1 1) electrodes in acidic electrolytes

The adsorption of sulfate or OH species and subsequent Ni(OH)₂ film growth on Ni(1 1 1) single crystal electrodes has been investigated using in situ infrared reflection absorption spectroscopy (IRAS) as well as scanning tunneling microscopy (STM). In a pH 3 sulfuric acid solution, STM images show that a well-defined Ni(1 1 1) surface with a (1 × 1) lattice is exposed at ?300 mV, while hexagonal close-packed images with an atomic spacing of 0.32 nm are grown on this electrode at 300 mV. On the other hand, IRAS results in a sulfuric acid solution (pH 3) reveal that an absorption band at 1116 cm^?1, which can be ascribed to ν(S–O) symmetric stretching of sulfate anion on Ni(1 1 1) surface, starts to appear at ?400 mV and develops its intensity with an electrode potential increase, while an absorption band at 930 cm^?1 begins to develop at 0 mV on Ni(1 1 1), Ni(1 0 0) and Ni(1 1 0) electrodes, which can be assigned to an in-plane δ(Ni–OH) bending vibration in Ni(OH)₂ passive film.     

Aniline electropolymerization on mild steel and zinc in a two-step process

A two-step process for aniline electropolymerization on mild steel and zinc from aqueous electrolytes has been developed. The first step is the electrodeposition of a thin polypyrrole film. It acts as a pretreatment of the surface and completely modifies its electrochemical response to usual acidic solutions suitable for aniline electropolymerization. It is much faster than any chemical pretreatment (less than 3 s) and can be performed with almost no metal dissolution. Pretreated mild steel or zinc substrates can then be used for the electrodeposition of a PANI film of controllable thickness. No or very little metal oxidation occurs in this second step. The films show stable electroactivity in acidic electrolytes, similar to that of PANI deposited on platinum, which indicates that the underlying oxidizable metal is fully protected. The overall system has a bilayer structure, indicating that the electrochemical interface during aniline electrodeposition might be situated at the polypyrrole ∣ solution interface. This suggests a new interpretation for the anticorrosion properties of conductive polymers based on the displacement of the electroactive interface from its usual location (metal ∣ solution) to the polymer ∣ solution interface.     

Determination of tricyclic antidepressants using a carbon paste electrode modified with β-cyclodextrin

Carbon paste electrodes modified with β-cyclodextrin have been investigated for the DP voltammetric determination of the tricyclic antidepressants – imipramine, trimipramine and thioridazine. Host–guest interaction was studied and experimental conditions of the working procedure were optimized. The accumulation ability of β-cyclodextrin leads to detection limits down to nanomolar concentrations for 120 s accumulation. The electrode was applied to the determination of imipramine and thioridazine in pharmaceuticals.     

Electropolymerization of poly(N-methylaniline) on mild steel: Synthesis,characterization and corrosion protection

Poly(N-methylaniline) (PNMA) coatings on a steel disc electrode have been successfully electrodeposited from N-methylaniline (NMA) in aqueous oxalic acid solution using potentiodynamic, potentiostatic and galvanostatic techniques. For the potentiodynamic synthesis, formation of the PNMA coatings was found to be strongly dependent on the upper potential limits and scan rates. Smooth and strongly adherent PNMA coatings were obtained under potentiodynamic conditions during sequential scanning of the potential region between ?0.5 and 1.0 V, at a scan rate of 10 mV s^?1. The PNMA coatings were characterized by cyclic voltammetry, FT-IR and UV–Vis spectroscopy and SEM techniques. It was shown that NMA converts the iron(III) species to iron(II) chemically, causing a decrease in the current value corresponding to the electrochemical conversion (repassivation). The corrosion behavior of steels covered by polymer was investigated by anodic potentiodynamic polarization technique and the Tafel test. The corrosion resistance of the PNMA coated steel was found to be considerably higher and the corrosion rate was much slower than that of bare steel.