Quasi-reversible faradaic depolarization processes in the electrokinetics of the metal/solution interface

Bipolar faradaic depolarization of the metal/solution interface is quantitatively analyzed for the case where the solution is subject to lateral flow and contains a quasi-reversible redox couple. Transversal convective diffusion of the electroactive species and a position-dependent degree of reversibility of the interfacial electron-transfer (e.t.) reaction are among the primary features that govern depolarization. The spatial distributions of species concentrations and electric potential are numerically simulated. The system is characterized by nonlinear coupling between the transport (diffusion and flow) and the electric potential distribution under conditions of finite local currents. The resulting picture is that the reversibility of the e.t. reaction varies with position on the surface, with the highest reversibility downstream. This, in itself, generally leads to strongly asymmetric profiles of the faradaic current density along the surface. The impact on the electrokinetic properties of the interface is huge. For example, the steady-state streaming potential is depressed by the contribution from the bipolar faradaic process to the back current to an extent that varies from insignificant to complete, depending on the e.t. rate constant and concentrations of the electroactive species.     

Electrochemical detection of asparaginase antibodies using bifunctionalized carbon interfaces

The early detection of anti-asparaginase biomarker can facilitate timely modification of asparaginase chemotherapy, thereby avoiding serious complications. Herein we describe the preparation of a novel electrochemical biosensing interface for rapid detection of anti-asparaginase in the picomolar range (1–10 000 pM). Coimmobilization of ferrocene and asparaginase on a carbon interface (via diazonium grafting) facilitates transduction through attenuation of the surface-bound ferrocene redox couple. The limit of detection of 0.8 pM for this point-of-care applicable method compares favourably to that of traditional faradaic assaying (2.0 pM) where transduction occurs by the target blocking the diffusion of the solution redox probe [Fe(CN)₆]^3−/4−.     

On the use of electrokinetic phenomena of the second kind for probing electrode kinetic properties of modified electron-conducting surfaces

The electrokinetic features of electron-conducting substrates, as measured in a conventional thin-layer electrokinetic cell, strongly depend on the extent of bipolar faradaic depolarisation of the interface formed with the adjacent electrolytic solution. Streaming potential versus applied pressure data obtained for metallic substrates must generally be interpreted on the basis of a modified Helmholtz-Smoluchowski equation corrected by an electronic conduction term-non linear with respect to the lateral potential and applied pressure gradient-that stems from the bipolar electrodic behavior of the metallic surface. In the current study, streaming potential measurements have been performed in KNO(3) solutions on porous plugs made of electron-conducting grains of pyrite (FeS(2)) covered by humic acids. For zero coverage, the extensive bipolar electronic conduction taking place in the plug-depolarized by concomitant and spatially distributed oxidation and reduction reactions of Fe(2+) and Fe(3+) species-leads to the complete extinction of the streaming potential over the entire range of applied pressure examined. For low to intermediate coverage, the local electron-transfer kinetics on the covered regions of the plug becomes more sluggish. The overall bipolar electronic conduction is then diminished which leads to an increase in the streaming potential with a non-linear dependence on the pressure. For significant coverage, a linear response is observed which basically reflects the interfacial double layer properties of the humics surface layer. A tractable, semi-analytical model is presented that reproduces the electrokinetic peculiarities of the complex and composite system FeS(2)/humics investigated. The study demonstrates that the streaming potential technique is a fast and valuable tool for establishing how well the electron transfer kinetics at a partially or completely depolarised bare electron-conducting substrate/electrolyte solution interface is either promoted (catalysis) or blocked (passivation) by the presence of a discontinuous surface layer.     

Fast visualization of the mass transfer processes at the electrode/electrolyte interface with a Mach-Zehnder interferometer

A Mach-Zehnder interfeometer is employed to visualize the mass transfer processes at the electrode/electrolyte interface during the potentiodynamic sweep of the Pt electrode in 0.1 mol dm^?3 K₄Fe(CN)₆ with 0.5 mol dm^?3 KCl solution at 20 mV s^?1. The changes of solution??s refractive index, brought about by the mass transfer during the reaction, can be recorded in situ in interferograms. The distributions of the optical path difference are obtained by numerical reconstruction of interferograms to reflect changes of solution??s refractive index and the mass transfer processes. The mass transfer of [Fe(CN)₆]^4? and [Fe(CN)₆]^3? is presented visually. This method provides a new approach to detect the mass transfer processes at the electrode/electrolyte interface in real-time.     

Observation of the marcus inverted region of electron transfer reactions at a liquid/liquid interface

Scanning electrochemical microscopy was used to probe the influence of a driving force on the heterogeneous electron transfer (ET) processes at the externally polarized water/1,2-dichloroethane interface. Being a part of the driving force, the Galvani potential difference at the interface, Deltaowphi, can be quantitatively controlled in a wide range, allowing the precise measurements of the rate constants of the ET reactions. Two opposite systems were chosen in this work. One was 5,10,15,20-tetraphenyl 21H,23H-porphyrin zinc (ZnPor, O)/Fe(CN)64- (W), and the other was TCNQ (O)/Fe(CN)63- (W). For both systems studied, the relations between the rate constant and the Deltaowphi were of parabolic shape; that is, the rate constants increased initially with the Deltaowphi until reaching a maximum and then decreased steadily as the Deltaowphi increased further. This is in accordance with the prediction of the Marcus theory. To our knowledge, this is the first report that the Marcus inverted region can be observed electrochemically at an unmodified liquid/liquid interface with only one redox couple at each phase. The effect of the concentrations of the organic supporting electrolyte has also been discussed in detail.     

Coupling between electroosmotically driven flow and bipolar faradaic depolarization processes in electron-conducting microchannels

A quantitative theory is proposed for the analysis of steady electroosmotically driven flows within conducting cylindrical microchannels. Beyond a threshold value of the electric field applied in the electrolyte solution and parallel to the conducting surface, electrochemical oxidation and reduction reactions take place at the two extremities of the substrate. The spatial distribution of the corresponding local faradaic currents along the bipolar electrode is intrinsically coupled to that of the electric field in solution. The nonuniform distribution of the electric field alters the double layer composition, and in particular the zeta-potential value, along the conducting surface via the occurrence of concomitant electronic and ionic double layer charging processes. The combined spatial dependencies of the lateral electric field and electrokinetic potential considerably affects the distribution of the electroosmotic velocity field in the directions parallel and perpendicular to the surface depolarized by faradaic processes. In this paper, the coupling between bipolar electrodic behavior and electroosmosis is explicitly investigated for the case of irreversible--that is, kinetically controlled--electron transfer reactions. Typical simulation results are presented and illustrate the possibility of controlling and optimizing electroosmotic flows in conducting channels by electrochemical means.     

Faradaic depolarization in the electrokinetics of the metal-electrolyte solution interface

Streaming potentials (E(str)) have been measured in a flat thin-layer cell with gold and aluminum surfaces. The conventional relation between E(str) and the zeta-potential is shown to be applicable only as long as charge transfer reactions at the metal-electrolyte solution interface are insignificant in terms of the ensuing contribution to the overall cell conductivity. Owing to the irreversibility of the reduction/oxidation of water at most metal surfaces, streaming potentials can be obtained over a very broad range of pressure gradients for metallic substrates in electrolytes such as KNO3. The situation changes drastically in the presence of a reversible redox couple like Fe(CN)(6)3-/Fe(CN)(6)4-. Even small streaming potentials are then greatly diminished due to the extensive conduction that results from the bipolar electrolysis at the metal surface. For gold and aluminum in the presence of various electroinactive and electroactive electrolytes, the measured values for E(str) are shown to be consistent with their conventional voltammetric characteristics.     

High frequency faradaic rectification voltammetry at microelectrodes

An experimental setup for carrying out faradaic rectification measurements at micrometer-sized electrodes under potential control is described. A new method of data analysis is proposed that allows the determination of the standard rate constant and the electron-transfer coefficient of a fast charge transfer process without knowing the impedance of the microelectrode. This method is based on the frequency dependence of the shape of the faradaic rectification voltammograms (i.e., the average width of the peaks and the ratio of the peak heights) rather than on the magnitude of the faradaic rectification signal. The method was tested in the determination of heterogeneous electron transfer kinetics of Fe(CN)6(3-/4-) and Ru(NH3)6(2+/3+) in aqueous solutions on a platinum microelectrode (12.5 microm in radius) and ferrocene/ferrocinum redox couple in a dimethylformamide solution on a gold microelectrode (12.5 microm in radius).     

应用扫描电化学显微镜研究极化液/液界面上的电子转移反应

将含有氧化还原电对的水溶液滴涂在铂盘电极表面, 然后将该电极插入到1,2-二氯乙烷溶液中, 形成稳定的油/水界面. 液滴中的K3Fe(CN)6和K4Fe(CN)6氧化还原电对既可以作为水相中的参比电对参与控制液/液界面上的电势差, 同时又可以作为水相的电子授受体参与界面上的电子转移反应. 结合扫描电化学显微镜电化学系统的特点, 利用其双恒电位仪分别控制界面电势差和现场扫描的优点, 通过扫描电化学显微镜的渐进曲线得到了不同界面电势差控制的电子转移反应速率常数. 实验结果表明, 应用此方法获得的液/液界面可以被外加电位极化, 在一定的电势差范围内, 反应速率常数与界面电势差的关系遵守Butler-Volmer公式.     

Synergy of Cu/C3N4 Interface and Cu Nanoparticles Dual Catalytic Regions in Electrolysis of CO to Acetic Acid

Electrochemical reduction reaction of carbon monoxide (CORR) offers a promising way to manufacture acetic acid directly from gaseous CO and water at mild condition. Herein, we discovered that the graphitic carbon nitride (g-C₃N₄) supported Cu nanoparticles (Cu−CN) with the appropriate size showed a high acetate faradaic efficiency of 62.8 % with a partial current density of 188 mA cm⁻² in CORR. In situ experimental and density functional theory calculation studies revealed that the Cu/C₃N₄ interface and metallic Cu surface synergistically promoted CORR into acetic acid. The generation of pivotal intermediate −*CHO is advantage around the Cu/C₃N₄ interface and migrated *CHO facilitates acetic acid generation on metallic Cu surface with promoted *CHO coverage. Moreover, continuous production of acetic acid aqueous solution was achieved in a porous solid electrolyte reactor, indicating the great potential of Cu−CN catalyst in the industrial application.     

聚电解质中阳离子对铁氰化钾还原过程影响的原位显微红外光谱电化学

采用原位显微红外光谱电化学方法研究了聚电解质中不同阳离子（Ｌｉ＋,Ｎａ＋,Ｋ＋和ＴＢＡ＋）存在下铁氰化钾的电化学还原过程．观察到支持电解质阳离子对Ｆｅ（ＣＮ）３－／４－６振动频率的影响,探讨了还原反应机理     

A novel dual-impedance-analysis EQCM system--investigation of bovine serum albumin adsorption on gold and platinum electrode surfaces

Both quartz crystal micro-balance (QCM) impedance and electrochemical impedance spectroscopy (EIS) methods are widely used in interface studies. This paper presents details about a new strategy for simultaneous, mutual-interference-free and accurate measurements of QCM impedance and EI, through connecting a suitable capacitance in series with the piezoelectric quartz crystal (PQC) between QCM impedance and EIS measurement instruments. Combined and individual measurements of QCM impedance and EIS during silver deposition gave results comparable with each other, demonstrating the reliability of the proposed method. Bovine serum albumin (BSA) adsorption on gold and platinum electrodes in Britton-Robinson (B-R) buffers was investigated, and the Fe(CN)6(3-)/Fe(CN)6(4-) couple was used as an electrochemical probe to characterize BSA adsorption. While the reversibility of Fe(CN)6(3-)/Fe(CN)6(4-) couple on bare Au and Pt electrodes changed very slightly with decreasing solution pH from pH approximately 7 to pH approximately 2, the standard rate constant (ks) of this couple increased abruptly with solution pH below pH approximately 4.5 at a BSA-modified Au electrode, but decreased with solution pH at a BSA-modified Pt electrode. By analyzing the QCM impedance data with a modified BVD equivalent circuit and the EI data with a modified Randle's equivalent circuit, inflexion changes at pH approximately 4.5 were all found at pH-dependent responses of the resonant frequency, the double-layer capacitance, the capacitance of the adsorbed BSA layer, the peak-absorbance values of BSA solutions at 277.5 and 224.5 nm, and so on. It was also found that a BSA adsorption layer can effectively inhibit gold corrosion during ferrocyanide oxidation in a ferrocyanide-containing BR solution. Some preliminary explanations of these findings have been given. The proposed method is highly recommended for wider applications in surface science.     

Effect of applied potential on arylmethyl films oxidatively grafted to carbon surfaces

Arylmethyl films have been grafted to glassy carbon surfaces and to pyrolyzed photoresist films (PPFs) by electrochemical oxidation of 1-naphthylmethylcarboxylate and 4-methoxybenzylcarboxylate. Atomic force microscopy (AFM) and electrochemistry were used to characterize the as-prepared films and to monitor changes induced by post-preparation treatments. Film thickness was measured by depth profiling using an AFM tip to remove film from the PPF surface. Surface coverage of electroactive modifiers was estimated from cyclic voltammetry, and monitoring the response of a solution-based redox probe at grafted surfaces gave a qualitative indication of changes in film properties. For preparation of the films, the maximum film thickness increased with the potential applied during grafting, and all films were of multilayer thickness. The apparent rate of electron transfer for the Fe(CN)(6)3-/Fe(CN)(6)4- couple was very low at as-prepared films. After film-grafted electrodes were transferred to pure acetonitrile-electrolyte solution and subjected to negative potential excursions, the response of the Fe(CN)(6)3-/Fe(CN)(6)4- couple changed and was consistent with faster electron-transfer kinetics, the film thickness decreased and the surface roughness increased substantially. Applying a positive potential to the treated film reversed changes in film thickness, but the voltammetric response of the Fe(CN)(6)3-/Fe(CN)(6)4- couple remained kinetically fast. After as-prepared films were subjected to positive applied potentials in acetonitrile-electrolyte solution, the apparent rate of electron transfer for the Fe(CN)(6)3-/Fe(CN)(6)4- couple remained very slow and the measured film thickness was the same or greater than that before treatment at positive potentials. Mechanisms are considered to explain the observed effects of applied potential on film characteristics.     

膜电阻对自组装膜修饰电极电化学行为的影响

应用循环伏安和交流阻抗技术研究了 16烷基硫醇自组装膜修饰的金电极在Fe(CN) 63 - /Fe(CN) 64 - 溶液中的电化学行为 .无“针孔”缺陷的自组装膜对溶液与基底间的界面电子转移具有强烈的阻碍作用 ,当过电位较大时 ,In(I/ η)对 η1/2 之间具有良好的线性关系 .通过对Au/SAM /Hg模拟体系的电流———电压曲线进行测定 ,得到了自组装膜膜电阻的特征 .指出由于膜电阻的存在 ,自组装膜修饰电极在Fe(CN) 63 - /Fe(CN) 64 - 溶液中的行为实质上反映了膜自身的电阻特征     

Dependence of depletion layer thickness on polymer concentration determined by Vincent's pragmatic and Donath's electrophoretic theories

On the interface of a solid surface and a solution of nonadsorbing polymer there exists a depletion layer (DL), where the concentration of polymer segments is lower. Donath's electrophoretic theory, based on the decreased viscosity in the DL region, allows computing DL thickness from the relative (with and without polymer) electrophoretic mobility, the bulk viscosity, and the ionic strength. The aim of this work is to check experimentally Donath's nonlinear electrophoretic (NLE) theory under the most favorable conditions--liposomes in solutions of low-molecular poly(ethylene glycol) (PEG). In order to determine DL thickness, the dependence of mobility on viscosity is chosen instead on ionic strength. The value obtained from NLE theory is compared with the DL thickness calculated by Vincent's pragmatic theory. The conformation-statistical parameters are calculated on the base of viscosimetric measurements of PEG solution. The results indicate a few shortcomings of NLE theory. The main one is that DL thickness does not depend on polymer concentration, a fact that is in discrepancy with the prediction of Vincent's theory. The conclusion is that NLE theory describes well the experimental dependence of the relative mobility on the bulk viscosity, but it is inapplicable to quantitative determination of DL thickness.     

Fundamentals and Applications in Electrochemistry of Liquid-liquid Interfaces

The interface of two immiscible electrolyte solutions (ITIES) has generated great interest since it can be used to understand several processes in chemistry and biology. The transfer process of ions and organic molecules of environmental and pharmacological importance have been study across the ITIES. In the present work, a small introduction to the study of ITIES is given, as well as the interpretation of the cyclic voltammetric experiments regarding to the transport of an organic cation (TEA⁺) across the interface. Finally, examples of application of studies of ITIES: a) the transfer of terbutryne herbicide and b) electron transfer reaction between a redox pair (Fe(CN)₆^4-/Fe(CN)₆³⁾ and isoperezone across the water|1,2- dichloroethane interface are discussed.     

The Influence of Doping Levels and Surface Termination on the Electrochemistry of Polycrystalline Diamond

The influence of surface chemistry and boron doping density on the redox chemistry of Fe(CN) at CVD polycrystalline diamond electrodes is considered. It is demonstrated that for this couple both the doping density and the surface chemistry are important in determining the rate of charge transfer at the electrode/electrolyte interface. For hydrogen terminated CVD diamond metallic electrochemical behavior is always observed, even at boron doping densities as low as 7×10¹⁸ cm^?3. In contrast, the electrochemical behavior of oxygen terminated CVD diamond varies with doping density, a metallic response being observed at high doping density and semiconductor behavior at low doping density. It is shown that the results attained may be explained by a surface state mediated charge transfer mechanism, thus demonstrating the importance of controlling surface chemistry in electroanalytical applications of diamond.     

Voltammetric study of interfacial electron transfer between bis(cyclopentadienyl)iron in organic solvents and hexacyanoferrate in water.

The electron-transfer reaction between bis(cyclopentadienyl)iron(II) ([Fe(II)(C5H5)2]) in nitrobenzene and a hexacyanoferrate redox couple ([Fe(II/III)(CN)6](4-/3-)) in water at the nitrobenzene / water interface was studied using normal pulse voltammetry. The voltammetric results indicate that the electron-transfer reaction takes place by way of a so-called ion-transfer (IT) mechanism, of which the forward and backward rate constants of the homogeneous electron-transfer reaction between [Fe(II/III)(C5H5)2](0/+) and [Fe(II/III)(CN)6](4-/3-) in the water phase have been determined. The electron-transfer reaction between [Fe(II)(C5H5)2] in 1,2-dichloroethane and [Fe(II/III)(CN)6](4-/3-) in water at the 1,2-dichloroethane / water interface was shown to also take place by the IT-mechanism.     

Electrolysis with electrolyte dropping electrode: II. Basic properties of the system

In a theoretical discussion the conditions have been pointed out where an interface of two immiscible electrolyte solution behaves as an equilibrium system metal ion-metallic electrode, as an ideally polarized electrode and as an electrode under faradaic current flow. The basic equations for current-electrical potential difference across the interface have been deduced for the cases of ion as well as electron transfer.Experimentally, various base electrolyte systems were studied, the most advantageous among these are LiCl in water+tetrabutylammonium tetraphenylborate in nitrobenzene and MgCl₂ in water+tetrabutylammonium dicarbollyl cobaltate in nitrobenzene. S-shaped polarographic curves were observed with the tetramethylammonium ion. The limiting current is directly proportional to concentration. The limiting currents are somewhat higher than those predicted by the Ilkovi? equation which has been ascribed to the tangential movement of the interface.