Polymer-based bilayer interfaces for electrochemical rectification

In this paper, the electrochemical current rectification phenomenon exhibited at an electrochemical interface constituted by a glassy carbon electrode covered with a bilayer of polymer films is discussed. The authors have shown that Methylene Blue (MB) redox species can be confined to a very thin insulating polymer film formed from orthophenylene diamine. The poly(opd) film exhibited excellent blocking properties to redox molecules in solution. On the other hand, the insulating poly(opd) film trapped with MB could mediate electron transfer between the redox molecules in solution and the electrode. Further, a second polymeric layer (Nafion film) trapped with ferrocene redox species was formed as the outer layer over the inner poly (opd) film containing MB. This bilayer-modified electrode, due to the significant difference in the redox potentials of the MB and ferrocene species immobilized in the inner and outer layers, respectively, exhibits unidirectional current flow and the results of the voltammetric investigations on the modified electrodes are described in this communication.     

Microelectrochemical electronic effects in two-layer structures of distinct Prussian blue type metal hexacyanoferrates

The formation of a rigid bilayer structure from two metal hexacyanoferrates: Prussian blue (PB) and nickel hexacyanoferrate (NiHCNFe), as inner and outer films, respectively, has been demonstrated. To avoid intermingling of the granular cyanometallate microstructures, namely of the outer film (NiHCNFe) into the inner film (PB), the morphology of outer film material was changed by forming a polymeric hybrid (composite) of NiHCNFe with poly(N-methylpyrrole). The outer NiHCNFe film is physically separated from the electrode surface, and it undergoes redox reactions at potentials characteristic of the inner PB film. This arrangement leads to the reversible charge state trapping and bistable switching during voltammetric potential cycling. Under solid-state voltammetric conditions in the absence of contact with the liquid electrolyte phase, when the bilayer structure of PB and the oxidized NiHCNFe was formed between two sandwich-forming carbon electrodes, unidirectional rectifying current flow has been observed.Dedicated to Zbigniew Galus on the occasion of his 70th birthday     

Modeling of electrocatalytic processes at conducting polymer modified electrodes

A mathematical model of electrocatalytic processes taking place at conducting polymer modified electrodes has been developed. The model takes into account the diffusion of solution species into a polymer film, diffusion of charge carriers within the film, and a chemical redox reaction within the film. The space- and time-resolved profiles for reactant and charge carrier concentration within the film, as well as dependencies of electric current on the concentration of solute species, reaction rate constant and thickness of a polymer layer have been obtained and discussed. It has been shown that, even at a relatively fast diffusion of charge carriers within the conducting polymer film, exceeding the diffusion rate of reactant by two orders of magnitude, electrocatalysis of solute species at conducting polymer modified electrodes proceeds within the polymer film rather than at the outer polymer/solution interface, i.e., electrocatalytic conversion follows a redox-mechanism rather than metal-like one. Based on the results obtained, optimization of reaction system parameters could be made for any particular case to get an optimum efficiency or reactant to product conversion.      

Electrocatalytic oxidation of some biologically important compounds at conducting polymer electrodes modified by metal complexes

Conducting poly(3-methylthiophene) electrodes were electrochemically prepared. The resulting polymer films were modified with an inorganic complex, ferrocene. The incorporation of the ferrocene/ferrocenium moiety into the polymer film resulted in enhanced charge transfer towards the oxidation of some organic molecules of biological interest. The electrochemical response of the complex-containing polymer electrode was compared to that of the unmodified polymer electrode and that of the substrate. Apparent diffusion coefficients of the redox species were estimated from the cyclic voltammetric data for different biological molecules at the ferrocene-containing polymer electrode. Infra-red spectroscopic measurements for the “as-grown” films revealed the presence of the inorganic complex within the polymer. The modified polymer electrode showed noticeable enhancement for the charge transfer across the film interface and can be used as an electrochemical sensor for biological compounds. Received: 3 June 1997 / Accepted: 7 July 1997     

Long Distance Electron Transfer Across >100 nm Thick Au Nanoparticle/Polyion Films to a Surface Redox Protein

Glutathione‐decorated 5 nm gold nanoparticles (AuNPs) and oppositely charged poly(allylamine hydrochloride) (PAH) were assembled into {PAH/AuNP}_n films fabricated layer‐by‐layer (LbL) on pyrolytic graphite (PG) electrodes. These AuNP/polyion films utilized the AuNPs as electron hopping relays to achieve direct electron transfer between underlying electrodes and redox proteins on the outer film surface across unprecedented distances >100 nm for the first time. As film thickness increased, voltammetric peak currents for surface myoglobin (Mb) on these films decreased but the electron transfer rate was relatively constant, consistent with a AuNP‐mediated electron hopping mechanism.     

血红素修饰电极及其催化氧还原性质

金属大环络合物（卟啉、酞箐、维生素Ｂ_(１２)等）修饰电极对氧、一氧化氮和其它生物物质的催化作用［１－３］引起了化学工作者的极大兴趣,血红素是一种重要的铁卟啉化合物,是血液中血红蛋白的重要组成部分,承担携氧的任务,由于其特殊的生理功能,研究血红素修饰电极的性质和作用对进一步研究开发燃料电池具有很重大的意义．卟啉在电极上的修饰有多种方法,当卟啉或金属卟啉环侧链上具有苯氨基、苯酚基、乙烯基或吡咯等取代基时［３］,可采用电氧化聚合法制备聚卟啉膜．本文采用循环伏安法在水溶液中制备了聚血红素膜电极,研究了聚…     

Chronocoulometric and rotating disc electrode determination of the charge propagation current through poly-4-vinylpyridine films containing hexachloroiridate anions

Charge propagation currents through polymer films coated on electrodes which contain redox active sites are usually determined by potential step methods. The aim of this study was to compare the charge propagation current obtained from chronocoulometric measurements with charge transport through a poly-4-vinylpyridine/IrCl₆²⁻ film during the mediated oxidation of Fe²⁺ at a rotating disc electrode. The two methods gave identical results for the model system chosen.     

The dynamics of electron self-exchange between nanoparticles.

The rate of electron self-exchange reactions between discretely charged metal-like cores of nanoparticles has been measured in multilayer films of nanoparticles by an electrochemical method. The nanoparticles are Au monolayer-protected clusters with mixed monolayers of hexanethiolate and mercaptoundecanoic acid ligands, linked to each other and to the Au electrode surface with carboxylate-metal ion-carboxylate bridges. Cyclic voltammetry of the nanoparticle films exhibits a series of well-defined peaks for the sequential, single-electron, double-layer charging of the 1.6-nm-diameter Au cores. The electron self-exchange is measured as a diffusion-like electron-hopping process, much as in previous studies of redox polymer films on electrodes. The average electron diffusion coefficient is DE = 10(+/-5) x 10(-8) cm2/s, with no discernible dependence on the state of charge of the nanoparticles or on whether the reaction increases or decreases the core charge. This diffusion constant corresponds to an average first-order rate constant kHOP of 2(+/-1) x 10(6) s(-1) and an average self-exchange rate constant, kEX, of 2(+/-1) x 10(8) M(-1) x s(-1), using a cubic lattice hopping model. This is a very large rate constant, considering the nominally lengthy linking bridge between the Au cores.     

Nafion基氧化还原聚合物在空气中的电荷传输性能

利用三明治电池和伏安法测试了不同制备条件的Nafion基氧化还原聚合物膜在空气中的电荷传输性能. 研究结果表明, 混合适量聚乙二醇(PEG)的Nafion基金属联吡啶配合物{Nafion[M(bpy)2+3, PEG](M=Ru, Fe)}膜的表观电荷传递扩散系数(Dct)达到10-6-10-7 cm2·s-1 , 电子或空穴迁移率(μ)达到10-4-10-5 cm2·V-1·s-1. 在导电玻璃(ITO)电极与Nafion基氧化还原聚合物膜界面引入一层导电聚苯胺(PANI)后, 降低了其接触电阻, 使氧化还原聚合物膜的Dct提高至10-5-10-6 cm2·s-1, μ提高至10-3-10-4 cm2·V-1·s-1, 且工作电流提高了近两个数量级. 该固态氧化还原聚合物膜的性能比较稳定, 在空气中放置30天后其Dct和μ降低得很少.     

Polymer films on electrodes: Part II. Film structure and mechanism of electron transfer with electrodeposited poly(vinylferrocene)

The structure and properties of electrodeposited poly(vinylferrocene) (PVF) films on platinum electrodes (PVF/Pt) were examined by electron microscopy, X-ray photoelectron spectroscopy, various electrochemical techniques and measurements of the film resistance. The data were consistent with a mechanism in which the polymer films are permeable to dis-solved reactants. A theoretical treatment of this situation for chronoamperometry is presented. The oxidation and reduction of a variety of dissolved reactants with redox potentials far removed from that of the PVF/PVF⁺ system at PVF/Pt occurred by diffusion of the electroactive species through the polymer film and subsequent reaction at the platinum surface.     

Polypyrrole Bilayers: Charge Trapping and Application for Amperometric Ions Sensing

Bilayers composed of polypyrrole: doped by perchlorate ions (PPy(ClO₄) – anion exchanging inner layer) and by dodecyl sulfate ions (PPy(DS) – cation exchanging outer layer) are very effective charge trapping systems that are usually not observed for other bilayers comprised of polypyrrole. Chronopotentiometric experiments carried out for oxidation and reduction showed that the trapping effect in the inner layer resulted from different ion exchange properties of the component polymers, leading to a low permeability of the reduced outer layer towards anions. Estimated diffusion coefficients of Cl^? anions in the oxidized and reduced PPy(DS) are in the range of 10^?9 and lower than 10^?10 cm² s^?1, respectively. The presence of the outer layer limiting the ion transfer was found to be beneficial to improve the signal resolution in amperometric mode of ion sensing within wide KCl concentration range, from 10^?5 M up to 3 M. The influence of experimental conditions (film thickness, response time) on optimization of this novel kind of polymeric bilayer ion sensors was studied.     

Effect of Poly(amine)s on the Redox Properties of Carboxymethylcellulose and Ferrocene‐modified Poly(amine) Layer‐by‐Layer Films

Multilayer films consisting of carboxymethylcellulose (CMC) and ferrocene‐modified poly(ethyleneimine) (Fc‐PEI) or poly(allylamine hydrochloride) (Fc‐PAH) were successfully prepared on a gold electrode to examine their redox properties. The redox current of (Fc‐PEI/CMC)_n film‐coated electrodes increased with the number of layers, while the (Fc‐PAH/CMC)_n film‐coated electrodes exhibited increased response only for the first eight bilayers. The (Fc‐PEI/CMC)_n and (Fc‐PAH/CMC)_n films deposited on the surface of Fc‐free multilayer film‐coated electrodes also showed a redox response. The (PEI/CMC)₅ film‐coated electrode showed redox responses in Fc‐PEI and Fc‐PAH solutions, confirming the uptake of the Fc‐polymers in the inner film. In contrast, the uptake of the Fc‐polymers in the (PAH/CMC)₅ film was severely suppressed, suggesting that different permeability of (PEI/CMC)₅ and (PAH/CMC)₅ films.     

Synthesis, characterization and immobilization of Prussian blue nanoparticles. A potential tool for biosensing devices

Prussian blue (PB) particles with the size of ca. 5 nm were synthesized and immobilized in a multilayer structure, as a strategy for the potential development of an amperometric transducer for oxidase-enzyme-based biosensors. Multilayer films composed of PB and poly(allylamine hydrochloride) (PAH) were prepared via layer-by-layer (LbL) sequential deposition. The process was carefully monitored by UV-vis spectroscopy and cyclic voltammetry. The increase of the redox current peaks during the layer-by-layer deposition demonstrated that charge propagation within the film occurs. Linear increase of UV-vis absorbance with the number of deposited bilayers indicates that well-organized systems have been elaborated. ITO electrodes coated with PB/PAH films were used successfully for detecting H2O2, sensitivity being dependent on the number of PB/PAH layers.     

Chemically Laminated 2D Bis(terpyridine)metal Polymer Films: Formation Mechanism at the Liquid–Liquid Interface and Redox Rectification

Recent studies on molecular 2D materials with high tunability of structure and function have focused mostly on the discovery of new precursors. Here, we demonstrate a facile one-pot synthesis of laminated 2D coordination polymer films comprising bis(terpyridine)iron and cobalt at a water/dichloromethane interface. Cross-sectional elemental mapping unveiled the stratum-like structure of the film and revealed that the second layer grows to the dichloromethane side below the first layer. Cyclic voltammetry clarified that the bottom layer mediates charge transfer between the top layer and the substrate in a narrow potential region of mixed-valence states. Furthermore, the bilayer film sandwiched by electrodes in a dry condition shows stable rectification character, and the barrier voltage corresponds to the redox potential difference between the two layers. This study introduces a new strategy for polymer design to explore the materials science of molecular 2D materials.     

Photoactivity Characteristics of a Biodevice Using Primary Photosynthetic Reaction Centers

Films with alternating layers of thylakoid membrane from the cyanobacterium Spirulina platensis and the positively charged polyion poly(ethylenimine) (PEI) were prepared by means of alternate electrostatic layer‐by‐layer assembly. The thylakoid membrane/PEI bilayers were functionally and structurally characterized by visible spectra, electrochemical methods and SEM techniques, respectively. The films deposited onto gold electrodes were molecularly smooth and consistent with thylakoid membrane/PEI bilayers as observed by SEM. The photoactivity of the thylakoid membrane/PEI assembly was dependent on thickness of the film that increased with the number of bilayers. There was an obvious difference in the current responses of 1‐ and 5‐bilayer thylakoid membrane/PEI films under illumination. The photosynthetic electron transfer process in the thylakoid membrane/PEI single‐bilayer film was demonstrated by an inhibition of the photoactivity by herbicide. Biodevices for the detection of phytotoxicity were constructed using the isolated thylakoid membrane from chloroplast as biosensing elements by means of electrostatic layer‐by‐layer assembly.     

Weakly coupled lipid bilayer membranes on multistimuli-responsive poly(N-isopropylacrylamide) copolymer cushions

Polymer-cushioned lipid bilayers are frequently used to mimic the native environment of cellular membranes in respect to the extracellular matrix and intracellular structures. With the aim to actively tune lipid membrane characteristics, we pursue the approach to use temperature and pH responsive polymer thin films of poly(N-isopropylacrylamide-co-carboxyacrylamide) (PNIPAAm-co-carboxyAAM) as cushions for supported lipid bilayers. A cationic lipid bilayer composed of dioleoylphosphatidylcholine (DOPC) and dioleoyltrimethylammoniumpropane (DOTAP) (9:1) was formed on top of the polymer thin film in a drying/rehydration process. Fluorescence recovery after photobleaching (FRAP) yielded higher lipid diffusion coefficients (6.3-9.6 μm(2) s(-1)) on polymer cushions in comparison to solid glass supports (3.0-5.9 μm(2) s(-1)). No correlation of the lipid mobility was found with the swelling state of (PNIPAAm-co-carboxyAAM), which is ascribed to restrained interfacial electrostatic interactions and dispersion forces. The results revealed a minimal coupling of the lipid bilayer with the polymer cushions, and thus, bilayers supported by (PNIPAAm-co-carboxyAAM) provide interesting opportunities for unperturbed lipid diffusion combined with control of transmembrane protein mobility due to the impact of a tunable frictional drag.     

New degenerate metal-oxide electrodes for nearly reversible direct electron transfer to the redox proteins

Heavily doped cadmium tin oxide (CTO) film electrodes were developed for fast electron exchange with redox proteins. The metal oxide films showed nearly reversible electron transfer for the [2Fe–2S] proteins spinach ferredoxin (Sp fd) and putidaredoxin (Pdx), and the well-studied heme protein horse heart cytochrome c. These represent a family of proteins that are of comparable size, but vary significantly in overall charge, formal redox potential, and type of metal center. The unmediated electron exchange was achieved through variation of metal oxide film synthesis parameters that led to an increase of the charge carrier concentration up to the levels typical for degenerate semiconductors. In addition, the flat band potential of the films was shifted close to or more positive of the formal redox potentials of proteins such that the semiconductor electrodes would be utilized in an accumulation mode. The rates and sustainability of electron transfer for the two ferredoxins obtained on these cadmium tin oxide electrodes are as high or higher than previously reported.     

Charge transport through Geobacter sulfurreducens biofilms grown on graphite rods

Biofilms of the electroactive bacterium Geobacter sulfurreducens were induced to grow on graphite-rod electrodes under a potential of 0 V (vs Ag/AgCl) in the presence of acetate as an electron donor. Increased anodic currents for bioelectrocatalytic oxidation of acetate were obtained when the electrodes were incubated for longer periods with periodic electron-donor feeding. The maximum current density for acetate oxidation increased 2.8-fold, and the biofilm thickness increased by 4.25-fold, over a time period of 83-147 h. Cyclic voltammetry in the presence of acetate supports a model of heterogeneous electron transfer, one electron at time, from biofilm to electrode through a dominant redox species centered at -0.41 V vs Ag/AgCl. Voltammetry performed under nonturnover conditions provided an estimate of the surface coverage of the redox species of 25 nmol/cm(2). This value was used to estimate a redox species concentration of 7.3 mM within the 34-μm-thick biofilm and a charge-transport diffusion coefficient of 3.6 × 10(-7) cm(2)/s. This value of diffusion coefficient is greater than that observed in traditional thin-film voltammetric studies with redox polymer films containing much higher surface concentrations of redox species and might be associated with proton transport to ensure electroneutrality within the biofilm upon electrolysis.     

Kinetics of electron transfer reactions of metal complexes at impermeable redox active polymeric films on electrode surfaces and charge transport within the polymer film

At rotated Pt disk electrodes coated with thin films of the redox polymer poly-[Ru(vbpy)₃]²⁺, ruthenium and iron bipyridine complexes dissolved in acetonitrile can become oxidized by two pathways. The first is diffusion of the solute complex through the polymeric film to react at its normal potential E_sub⁰, at the Pt/polymer interface. The second is a mediated electron transfer cross-reaction between the solute complex and poly-[Ru(vbpy) ₃]²⁺ sites generated in the film at adequately positive potentials. The mediated reaction, as judged from the lack of variation of its rate k_crsΓ with the quantity of polymer mediator sites present in the multimolecular layer film, and from other evidence, is confined to the outer few (one?) monolayers of ruthenium polymer film sites. The mediated reaction becomes the dominant pathway for films with Γ_T ～2×10^?9 mol/cm² of ruthenium polymer sites, owing to the low permeability (measured independently) of the solutecomplexes into the film. The rate k_crsΓ could be measured when the solute complex oxidation potential is more positive than that of the redox film, and is too fast to measure when E_sub⁰, is more negative than the redox film E_cal?⁰, New theory is presented and evaluated to describe the rising portions of the voitammetric waves for the nine solute complexes studied. The rate of charge transport through the poly-[Ru(vbpy)₃]²⁺ film becomes controlling under certain conditions and can be thereby measured as well.