The electrochemical behaviour of cobalt in alkaline solutions part II. The potentiodynamic response of Co(OH)2 electrodes

The potentiodynamic behaviour of Co(OH)₂ hydroxide electrodes is studied in the potential range related to the appearance of CO(III) and CO(IV) species. The corresponding electrochemical reactions involve relatively fast proton transfer processes occurring at potentials close to those predicted from thermodynamics. Sandwich-type structures of the electrode/film/solution interface are assumed in the interpretation of the processes. They probably include configurational changes of reactants and products participating in the various electrochemical reactions.     

Electrochemical examples of multiple-electron transfer

Various electroactive reactants that undergo multipleelectron transfer reactions at electrodes are divided into two broad classes. The first class exhibits a series of single electron steps at different electrode potentials. The second class exhibits multiple-electron transfer in a single voltammetric step. Examples from each class are offered and the reasons for their electrochemical behavior are described.     

Sensitive detection of biomolecules and DNA bases based on graphene nanosheets

High surface area electrode materials are of interest for the application of electrochemical sensors. Currently, chemical vapor deposition (CVD) graphene-sensing electrodes are scarce. Herein, for the first time, a graphene based on a Ta wire support was prepared using the CVD method to form a highly electroactive biosensing platform. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and differential pulse voltammetry (DPV) were utilized to characterize the morphology and investigate the electrochemical properties of the CVD graphene electrodes. The resulting CVD graphene electrode exhibited good electrocatalytic activity and had a prominent response effect on dopamine, uric acid, guanine, and adenine. Standing graphene nanosheets have rich catalytic sites such as the edges, the defect levels of the plane, and porous network structures between the graphene nanosheets. These catalytic sites prompt the adsorption and resolution for the four species and the strong electron transport capability of the CVD graphene, which effectively improved the electrical signals for response to four species. Moreover, the graphene electrode is a promising candidate in electrochemical sensing and other electrochemical device applications.     

SPR银金电极上光电化学反应和EC-SERS理论研究

随着纳米科学的发展,人们再次关注到金属电极上的光电化学研究. 这主要得益于币族金属纳米结构具有强的表面等离激元共振（SPR）效应,它能有效地将光从远场光转化为近场光,汇聚光能到金属表面区域,可以在表面产生强的光电场效应,或产生较长寿命的热电子-空穴载流子效应,或是更长时间尺度的热效应. 因此,SPR效应不仅产生了表面增强拉曼散射(SERS)效应,用于表征吸附分子,而且可能诱发表面化学反应,为在电化学界面实现光与电协同调控化学反应提供新思路. 本文首先回顾了金属电极上光电流理论的发展,然后总结了本研究组近年来将量子化学计算用于光电化学反应和SERS光谱研究的工作,并以在银金纳米结构电极上水合质子还原和芳香胺氧化为例,比较了热电子和热空穴参与光电化学反应的特点,揭示了SPR参与光电化学反应的本质.     

Magneto‐switchable Electrodes and Electrochemical Systems

This article is an overview of extensive research efforts in many laboratories in the last two decades in the area of magneto‐switchable electrochemical systems. Electrochemical reactions, including electrocatalytic and bioelectrocatalytic processes, have been reversibly activated and inhibited upon physical translocation and reorientation of different magnetic micro/nano‐species on electrode surfaces, particularly using magnetic micro/nanoparticles, adaptive nanowires and hybrid‐graphene sheets. These processes were triggered by repositioning an external magnet, thus resulting in changes in magnetic field at the electrode interfaces. Coupling of the magneto‐activated electrochemical systems with enzymatic reactions of various complexity allowed sophisticated bioelectronic devices for tunable biosensing, on‐demand power production, unconventional computing and other novel bioelectronic applications.     

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.     

Molecular-level functionalization of electrode surfaces. An overview

Electrochemical reactions occur at electrode/electrolyte interfaces. Hence, manipulation and design of electrochemical interfaces accompanied by surface modifications have assumed vital importance. Molecular level modification, either at the monolayer or multilayer level of electrode surfaces and leading to functionalization of electrodes, is being actively pursued by researchers. Modification based on the self-assembled monolayer approach has enabled electrodes to acquire molecular recognition and molecular electronic characteristics. Functionalization of electrode surfaces using polymeric materials and enzymes has facilitated electrodes in exhibiting properties like catalysis, molecular recognition, electrochromism and birefringence. The results of such molecular level functionalization studies of electrode surfaces carried out recently in our laboratories are presented in this overview. Besides, some representative results reported from elsewhere are also included.     

Demonstration of the advantages of using bamboo-like nanotubes for electrochemical biosensor applications compared with single walled carbon nanotubes

The modification of glassy carbon electrodes with random dispersions of nanotubes is currently the most popular approach to the preparation of carbon nanotube modified electrodes. The performance of glassy carbon electrodes modified with a random dispersion of bamboo type carbon nanotubes was compared with single walled carbon nanotubes modified glassy carbon electrodes and bare glassy carbon electrodes. The electrochemical performance of all three types for electrode were compared by investigating the electrochemistry with solution species and the oxidation of guanine and adenine bases of surface adsorbed DNA. The presence of edge planes of graphene at regular intervals along the walls of the bamboo nanotubes resulted in superior electrochemical performance relative to SWNT modified electrodes from two aspects. Firstly, with solution species the peak separation of the oxidation and reduction waves were smaller indicating more rapid rates of electron transfer. Secondly, a greater number of electroactive sites along the walls of the bamboo-carbon nanotubes (BCNTs) resulted in larger current signals and a broader dynamic range for the oxidation of DNA bases.     

A facile method for preparation of graphene film electrodes with tailor-made dimensions with Vaseline as the insulating binder

This communication describes a facile but effective method to prepare graphene film electrodes with tunable dimensions with Vaseline as the insulating binder. Cyclic voltammetry (CV) studies reveal that the as-prepared graphene film electrodes have tunable dimensions ranging from a conventional electrode to a nanoelectrode ensemble, depending on the amount of graphene dispersed into the insulting Vaseline matrix. A large amount of graphene (typically, 10.0 μg/mL) leads to the formation of the film electrodes with a conventional dimension, while a small amount of graphene (typically, 1.0 μg/mL) essentially yields the graphene film electrodes like a nanoelectrode ensemble. As one new kind of carbon-based film electrodes with tailor-made dimensions and a good electrochemical activity as well as a high stability, the graphene film electrodes are believed to be potentially useful for fundamental electrochemical studies and for practical applications.     

Ultrafast Electron Transfer Kinetics of Graphene Grown by Chemical Vapor Deposition

High electrochemical reactivity is required for various energy and sensing applications of graphene grown by chemical vapor deposition (CVD). Herein, we report that heterogeneous electron transfer can be remarkably fast at CVD‐grown graphene electrodes that are fabricated without using the conventional poly(methyl methacrylate) (PMMA) for graphene transfer from a growth substrate. We use nanogap voltammetry based on scanning electrochemical microscopy to obtain very high standard rate constants k⁰≥25 cm s^?1 for ferrocenemethanol oxidation at polystyrene‐supported graphene. The rate constants are at least 2–3 orders of magnitude higher than those at PMMA‐transferred graphene, which demonstrates an anomalously weak dependence of electron‐transfer rates on the potential. Slow kinetics at PMMA‐transferred graphene is attributed to the presence of residual PMMA. This unprecedentedly high reactivity of PMMA‐free CVD‐grown graphene electrodes is fundamentally and practically important.     

Non-Faradaic electrochemical activation of catalysis

The use of fuel cells for carrying out oxidation reactions with cogeneration of electrical power and chemicals led, upon cofeeding oxygen and fuel at the anode, to the discovery of the effect of non-Faradaic electrochemical modification of catalytic activity or electrochemical promotion of catalysis. This phenomenon has been studied already for more than 70 catalytic reactions, including oxidations, reductions and isomerizations and using a variety of metal catalysts, and solid electrolytes. In this work we summarize the main features of electrochemical promotion and discuss critically its currently accepted sacrificial promoter mechanism which involves electrochemically controlled migration (spillover-backspillover) of promoting species from the electrolyte to the catalytically active metal-gas interface. It is shown that the spillover ionic species (e.g., O(delta-), Na(delta+)) form an overall neutral double layer at the catalyst-gas interface which alters the catalyst work function and the binding energies of coadsorbed reactants and intermediates, thus causing very pronounced and reversible alterations in the catalytic activation energy and catalytic rate and selectivity. Recent efforts for the practical utilization of electrochemical promotion are also briefly discussed.     

CC Bonding of Graphene Oxide on 4‐Aminophenyl Modified Gold Electrodes towards Simultaneous Detection of Heavy Metal Ions

This paper studied the electrochemical sensors based on C? C bonding of graphene oxide (GO) on π‐conjugated aromatic group modified gold electrodes for simultaneous detection of heavy metal ions. For comparison, another sensing interface Au‐Ph‐NH‐CO‐GO, in which GO was modified to Au‐Ph‐NH₂ interfaces by amide bonding. On the basis of the principle of heavy metal ions complexation with oxygenated species on GO, the fabricated sensing interfaces were used for the simultaneous determination of Pb²⁺, Cu²⁺ and Hg²⁺. The performance of two sensing interfaces for simultaneous detection of three metal ions was compared. Au‐Ph‐GO sensing interface demonstrated higher sensitivity and better repeatability than Au‐Ph‐NH‐CO‐GO sensing interface.     

Geometry-dependent electrochemistry of graphene oxide family

The influence of graphene oxide geometry on electrochemical performance is of great interest, but there are few reports on this subject. Three different members of the graphene oxide family, graphene oxide nanosheets, graphene oxide nanoribbons, and graphene oxide quantum dots were comparatively investigated as electrode materials to systematically study the effect of geometric structure. The results showed that, as the geometric structure varies, the three graphene oxide materials possess different electrical conductivities, various defect densities and oxygen contents, as well as diverse electrode surface chemistry and microstructures, which combine together to result in the distinct electrochemical responses for the modified electrodes, depending on the redox system involved. This work broadens the method of studying the electrochemical performance of many other materials from the perspective of geometry.     

Surface preparation of glassy carbon electrodes

Recent work on glassy carbon electrodes for various applications is reviewed. Activation of glassy carbon electrodes by different types of polishing, heat treatment, and electrochemical methods yields enhanced rates of electron transfer. Characterization of different glassy carbon surfaces by x-ray photoelectron spectroscopy shows that polished and electrochemically pretreated surfaces contain more oxygen on the surface than do unactivated surfaces; much of this oxygen is associated with phenolic groups. Causes of activation, characterization of glassy carbon by spectroscopic methods, and the role of surface cleanliness are summarized. For simple electron-transfer reactions, removal of contaminants from the electrode surface is important. For proton-coupled electrode reactions, specific interactions of reactants with catalytic groups created on the surface during polishing tend to play an important role in electrode activation     

Recent trends in carbon nanomaterial-based electrochemical sensors for biomolecules: A review

Carbon nanomaterials are advantageous for electrochemical sensors because they increase the electroactive surface area, enhance electron transfer, and promote adsorption of molecules. Carbon nanotubes (CNTs) have been incorporated into electrochemical sensors for biomolecules and strategies have included the traditional dip coating and drop casting methods, direct growth of CNTs on electrodes and the use of CNT fibers and yarns made exclusively of CNTs. Recent research has also focused on utilizing many new types of carbon nanomaterials beyond CNTs. Forms of graphene are now increasingly popular for sensors including reduced graphene oxide, carbon nanohorns, graphene nanofoams, graphene nanorods, and graphene nanoflowers. In this review, we compare different carbon nanomaterial strategies for creating electrochemical sensors for biomolecules. Analytes covered include neurotransmitters and neurochemicals, such as dopamine, ascorbic acid, and serotonin; hydrogen peroxide; proteins, such as biomarkers; and DNA. The review also addresses enzyme-based electrodes that are used to detect non-electroactive species such as glucose, alcohols, and proteins. Finally, we analyze some of the future directions for the field, pointing out gaps in fundamental understanding of electron transfer to carbon nanomaterials and the need for more practical implementation of sensors.     

Electrochemical treatment of tumours: a simplified mathematical model

Electrochemical treatment of tumours implies that tumour tissue is treated with a direct current. During electrolysis, electrical energy is converted to chemical energy through electrochemical reactions at the electrodes. The anode is preferably placed in the tumour and the cathode in a blood vessel or in fresh surrounding tissue. The main electrochemical reactions are chlorine and oxygen evolution, at the anode, if platinum is used. Hydrogen evolution takes place at the cathode. The aim of this paper is to show how mathematical modelling can be used as a tool for defining and optimising the operating conditions of electrochemical treatment (ET) of tumours. A simplified mathematical model is presented for direct current treatment of tumours, focusing on tissue surrounding a spherical platinum anode. The tissue is treated as an aqueous solution of sodium chloride and only the major electrochemical reactions are considered. The model is based on transport equations of ionic species in dilute solutions. Kinetic expressions for the electrochemical reactions, at the anode surface, are introduced. Inputs to the model are the applied current density, and sizes of the anode and electrolyte domain. Concentration profiles of the ionic species and potential distribution, as a function of time, are calculated. In addition, current yields of the anode reactions are obtained from the model.     

Square‐Wave Voltammetry of Quasi‐Reversible CE Reactions at Spherical Microelectrodes

A mathematical model for the CE mechanism in which the chemical together with the electrochemical reactions are quasi‐reversible at the surface of spherical macro and micro‐electrodes is presented for the case of square‐wave voltammetry. The analysis of voltammometric responses considers the influence of rate and equilibrium constants, together with the electrode radius, and their dependence on the square‐wave frequency (f). Both kinetics and the sphericity effect act synergistically on the electrochemical response. Also, the apparent electrode sphericity and the reversibility of the chemical as well as the electrochemical reactions are jointly affected by the variation of f. Disregarding the sphericity contribution in the calculation of kinetic parameters at a microelectrode may introduce errors even higher than one order of magnitude. The model allows the analysis of a more realistic and complex electrochemical system that requires not only the dependence of experimental responses on f, but also their fit with theoretical voltammograms, in order to provide some useful mechanistic information. Finally, concentration profiles are also studied to realize how the chemical contribution is buffering the absences of oxidized species at the electrode surface, and how those profiles are modified for the case of spherical macro and micro‐electrodes.     

Nanoparticle films as electrodes: voltammetric sensitivity to the nanoparticle energy gap

Electron-transfer reactions of redox solutes at electrode/solution interfaces are facilitated when their formal potentials match, or are close to, the energy of an electronic state of the electrode. Metal electrodes have a continuum of electronic levels, and redox reactions occur without restraint over a wide span of electrode potentials. This paper shows that reactions on electrodes composed of films of metal nanoparticles do have constraints when the nanoparticles are sufficiently small and molecule-like so as to exhibit energy gaps, and resist electron transfers with redox solutes at potentials within the energy gap. When solute formal potentials are near the electronic states of the nanoparticles in the film, electron-transfer reactions can occur. The electronic states of the nanoparticle film electrodes are reflected in the formal potentials of the electrochemical reactions of the dissolved nanoparticles at naked metal electrodes. These ideas are demonstrated by voltammetry of aqueous solutions of the redox solutes methyl viologen, ruthenium hexammine, and two ferrocene derivatives at films on electrodes of 1.1 nm core diameter Au nanoparticles coated with protecting monolayers of phenylethanethiolate ligands. The methyl viologen solute is unreactive at the nanoparticle film electrode, having a formal potential lying in the nanoparticle's energy gap. The other solutes exhibit electron transfers, albeit slowed by the electron hopping resistance of the nanoparticle film. The nanoparticles are not linked together, being insoluble in the aqueous medium; a small amount of an organic additive (acetonitrile) facilitates observing the redox solute voltammetry.     

Bio-electrocatalysis of NADH and ethanol based on graphene sheets modified electrodes

Characterization and application of graphene sheets modified glassy carbon electrodes (graphene/GC) have been presented for the electrochemical bio-sensing. A probe molecule, potassium ferricyanide is employed to study the electrochemical response at the graphene/GC electrode, which shows better electron transfer than graphite modified (graphite/GC) and bare glassy carbon (GC) electrodes. Based on the highly enhanced electrochemical activity of NADH, alcohol dehydrogenase (ADH) is immobilized on the graphene modified electrode and displays a more desirable analytical performance in the detection of ethanol, compared with graphite/GC or GC based bio-electrodes. It also exhibits good performance of ethanol detection in the real samples. From the results of electrochemical investigation, graphene sheets with a favorable electrochemical activity could be an advanced carbon electrode materials for the design of electrochemical sensors and biosensors.     

Surface-enhanced Raman spectroscopy using gold-core platinum-shell nanoparticle film electrodes: toward a versatile vibrational strategy for electrochemical interfaces

The aim of this work is to further improve the molecular generality and substrate generality of SERS (i.e., to fully optimize the SERS activity of transition-metal electrodes). We utilized a strategy of borrowing high SERS activity from the Au core based on Au-core Pt-shell (Au@Pt) nanoparticle film electrodes, which can be simply and routinely prepared. The shell thickness from about one to five monolayers of Pt atoms can be well controlled by adjusting the ratio of the number of Au seeds to Pt(IV) ions in the solution. The SERS experimental results of carbon monoxide adsorption indicate that the enhancement factor for the Au@Pt nanoparticle film electrodes is more than 2 orders of magnitude larger than that of electrochemically roughened Pt electrodes. The practical virtues of the present film electrodes for obtaining rich and high-quality vibrational information for diverse adsorbates on transition metals are pointed out and briefly illustrated with systems of CO, hydrogen, and benzene adsorbed on Pt. We believe that the electrochemical applications of SERS will be broadened with this strategy, in particular, for extracting detailed vibrational information for adsorbates at transition-metal electrode interfaces.