Platinized carbon nanoelectrodes as potentiometric and amperometric SECM probes

Micrometer-sized platinized carbon electrodes have previously been used for the detection of reactive oxygen and nitrogen species (ROS and RNS) in biological systems. Here, we report the preparation and characterization of quartz-sealed platinized carbon nanoelectrodes. Such electrodes can be employed as tips in the scanning electrochemical microscope (SECM). The prepared electrodes were characterized by steady-state voltammetry, scanning electron microscopy, and SECM. In addition to ROS/RNS detection, the high surface area of a platinized nanoelectrode makes it a useful potentiometric probe. Unlike previously fabricated platinized electrodes, carbon electrodes possess a very thin insulating sheath, which is essential for experiments inside biological cells and high-resolution SECM imaging.     

Visualization and characterization of electroactive defects in the native oxide film on aluminium

Spatial!y localized electrochemical activity at Al/Al2O3 electrodes has been investigated using scanning electrochemical microscopy (SECM) in order to establish the relationship between localized corrosion of Al (and Al alloys) with the defect structure of the native Al2O3 film. Local electron transfer at microscopic defects (2 to 50 microm radius) was visualized in acetonitrile solutions using the nitrobenzene/nitrobenzene radical anion (Eo approximately -1.6 V vs. Ag/Ag+) and tetracyanoquinodimethane/tetracyanoquinodimethane radical anion couples (Eo approximately -0.3 V) as redox mediators for imaging. SECM investigations revealed no significant differences in electrochemical activity at Al/AI203 electrodes in the two mediator solutions, indicating that electrical conduction at the defect sites is weakly dependent on interfacial potential and the electric field across the Al2O3 film. The density of electroactive defects observed by SECM varied by 2 to 3 orders of magnitude between electrodes prepared from the same source of Al (either 99.450% and 99.9995%) suggesting that electrical conduction in the native oxide is very sensitive to surface preparation. Defect densities as low as approximately 3 sites cm(-2) were readily measured by SECM.     

Nanostructuring and nanoanalysis by scanning electrochemical microscopy (SECM)

The generation and application of nanodes in SECM experiments are described. Nanodes are ultramicroelectrodes with an active disk diameter in the submicrometer range. We investigated the behaviour of these electrodes by testing their properties with SECM applications which were previously performed at the micrometer scale. The active diameter of the nanodes was determined using cyclic voltammetry and SECM. The nanoanalysis was conducted at two nano interdigitated arrays. The nanostructuring was demonstrated by galvanic and electroless silver deposition from solution and from the surface, respectively. Experiments with nanodes illustrate that they exhibit the same behaviour as ultramicroelectrodes, but are more sensitive to adsorption and dirt particles in the electrolyte solution.     

Fabricating and imaging carbon-fiber immobilized enzyme ultramicroelectrodes with scanning electrochemical microscopy.

The scanning electrochemical microscope (SECM) is used to image the activity of enzymes immobilized on the surfaces of disk-shaped carbon-fiber electrodes. SECM was used to map the concentration of enzymatically produced hydroquinone or hydrogen peroxide at the surface of a 33-microm diameter disk-shaped carbon-fiber electrode modified by an immobilized glucose-oxidase layer. Sub-monolayer coverage of the enzyme at the electrode surface could be detected with micrometer resolution. The SECM was also employed as a surface modification tool to produce microscopic regions of enzyme activity by using a variety of methods. One method is a gold-masking process in which microscopic gold patterns act as mask for producing patterns of chemical modification. The gold masks allow operation in both a positive or negative process for patterning enzyme activity. A second method uses the direct mode of the SECM to produce covalently attached amine groups on the carbon surface. The amine groups are anchors for attachment of glucose oxidase by use of a biotin/avidin process. The effect of non-uniform enzyme activity was investigated by using the SECM tip to temporarily damage an immobilized enzyme surface. SECM imaging can observe the spatial extent and time-course of the enzyme recovery process.     

Micro ring–disk electrode probes for scanning electrochemical microscopy

The construction and characterisation of ring–disk (RD) microelectrodes suitable for use in scanning electrochemical microscopy (SECM) is reported. Such RD electrodes are proposed as probes for novel generator–collector SECM experiments. In this case, the interaction of both the reactants and products with the substrate under investigation can be followed simultaneously from a single approach curve to the substrate. Examples of such approach curves to conducting and insulating substrates are given to demonstrate the potential of this new mode of SECM operation.     

Localized Visualization of Chemical Cross-Talk in Microsensor Arrays by Using Scanning Electrochemical Microscopy

 An investigation of an array of four Pt microband electrodes 25 μm wide and spaced 25 μm apart was performed with the scanning electrochemical microscope (SECM). Where possible the SECM measurements were confirmed with conventional electrochemical measurements. It is shown how the sensiti- vity of the SECM recycling current to the activity of the underlying surface can be used to probe the homogeneity of enzyme-modified microelectrodes. The diffusion of H₂O₂ between these micro enzyme- electrodes and unmodified electrodes was investigated and it was demonstrated how the SECM can be a powerful tool in the elucidation of the properties of these electrodes. Received June 8, 1998. Revision November 12, 1998.     

Fabrication, characterization, and chemical etching of Ag nanoelectrodes

The previously developed methodologies for fabricating flat, polished nanoelectrodes were extended to produce silver electrodes with the radii from 50 nm to micrometers. The prepared electrodes were characterized by steady-state voltammetry, scanning electrochemical microscopy (SECM), and atomic force microscopy. The protocol was developed for controlled chemical etching of silver in ammonia solutions to produce recessed nanoelectrodes. Voltammograms and SECM approach curves were obtained to evaluate the recess depth and other geometric parameters of the etched electrodes.     

Long-Term Stability of Redox Mediators in Carbonate Solvents

Scanning electrochemical microscopy (SECM) used in the feedback mode is one of the most powerful versatile analytical tools used in the field of battery research. However, the application of SECM in the field of lithium-ion batteries (LIBs) faces challenges associated with the selection of a suitable redox mediator due to its high reactivity at low potentials at lithium metal or lithiated graphite electrodes. In this regard, the electrochemical/chemical stability of 2,5-di-tert-butyl-1,4-dimethoxybenzene (DBDMB) is evaluated and benchmarked with ferrocene. This investigation is systematically carried out in both linear and cyclic carbonates of the electrolyte recipe. Measurements of the bulk current with a microelectrode prove that while DBDMB decomposes in ethyl methyl carbonate (EMC)-containing electrolyte, bulk current remains stable in cyclic carbonates, ethylene carbonate (EC) and propylene carbonate (PC). Ferrocene was studied as an alternative redox mediator, showing superior electrochemical performance in ethyl methyl carbonate-containing electrolytes in terms of degradation. The resulting robustness of ferrocene with SECM is essential for a quantitative analysis of battery materials over extended periods. SECM approach curves depict practical problems when using the decomposing DBDMB for data acquisition and interpretation. This study sheds light towards the use of SECM as a probing tool enabled by redox mediators.     

SECM基本原理及其在金属腐蚀中的应用

扫描电化学显微镜（SECM）是一种具有较高空间分辨率的化学显微镜,在成像和动力学研究已经广泛应用. 本文简要介绍SECM基本原理,综述2009年以来SECM在腐蚀方面的应用,包括扫描成像和异相转移电子化学活性的研究,并简要介绍了作者课题组在SECM方面的研究工作,展望SECM在腐蚀研究的应用.     

Local electron transfer rate measurements on modified and unmodified glassy carbon electrodes

A natural and artificial distribution of electron transfer activity on glassy carbon electrodes can be observed and quantified by the use of scanning electrochemical microscopy (SECM). A large (sevenfold) spread in rate constant is found for randomly sampled sites on polished, untreated glassy carbon surfaces. Direct-mode oxidation with the SECM tip was used to produce small regions of oxidized carbon on a polished surface. A large increase in electron transfer rate for the Fe(II/III) ion is observed on the locally oxidized carbon surface in comparison to the unoxidized region. Rate constant measurements made along a line profiles the transition from unoxidized to oxidized surfaces. SECM images of defect sites show reaction–rate variations. Rate constants measured at several locations of the defective surface allows discrimination between the kinetic and topographic components of the SECM image. Dedicated to the 80th birthday of Keith B. Oldham     

Multiscale electrochemistry of hydrogels embedding conductive nanotubes

The local functionalities of biocompatible objects can be characterized under conditions similar to the operating ones, using scanning electrochemical microscopy (SECM). In the case of alginate beads entrapping carbon nanotubes (CNTs), SECM allows evidencing of the local conductivity, organization, and communication between the CNTs. It shows that the CNT network is active enough to allow long range charge evacuation, enabling the use of alginate/CNT beads as soft 3D electrodes. Direct connection or local interrogation by a microelectrode allows visualization of their communication as a network and eventually the study of them individually at the nanoscale.     

In situ formation and scanning electrochemical microscopy assisted positioning of NO-sensors above human umbilical vein endothelial cells for the detection of nitric oxide release

Pt microelectrodes (50 μm diameter) were positioned by means of scanning electrochemical microscopy assisted z-approach curves and in situ modified with nickel tetrasulfonated phthalocyanine tetrasodium salt as electrocatalytic layer for the specific oxidative detection of nitric oxide. The thus modified electrodes were then moved over a layer of adherently growing human umbilical vein endothelial cells (HUVEC) in order to amperometrically detect nitric oxide (NO) released from the cells upon stimulation with bradykinin. This approach actually takes advantage of the use of SECM to define a sequential procedure that enables the in situ functionalisation of the SECM tip thus allowing to accurately control the separation between the functionalised SECM tip and the cell population.     

扫描电化学显微镜在电催化表面反应分析中的现代应用

能源和环境问题成为制约未来可持续发展的关键问题之一,因此,针对不同电催化反应设计电催化剂变得越来越重要.电催化剂因其能量效率高、制备简单和易操作等优点,而应用于可再生能源的相关反应(如水分解和人工光合作用)中.明确不同反应电催化剂的设计原理,深入理解其在相关反应中的催化机理,可进一步优化催化剂性能.本文综述了扫描电化学显微镜(SECM)应用于电催化反应的历程、关键方法以及一些代表性的工作,阐明了电催化剂的工作机理以推进电催化剂的设计.本文还介绍了为提高SECM的空间分辨率而尝试的纳米尺寸电极方面的新进展,分享了纳米电极在以前研究无法涉及的单一催化实体方面的应用.     

Scanning electrochemical microscopy study of ion annihilation electrogenerated chemiluminescence of rubrene and [Ru(bpy)3]2+

Scanning electrochemical microscopy (SECM) was used for the study of electrogenerated chemiluminescence (ECL) in the radical annihilation mode. The concurrent steady-state generation of radical ions in the microgap formed between a SECM probe and a transparent microsubstrate provides a distance-dependent ECL signal that can provide information about the kinetics, stability, and mechanism of the light emission process. In the present study, the ECL emission from rubrene and [Ru(bpy)(3)](2+) was used to model the system by carrying out experiments with the SECM and light-detecting apparatus inside an inert atmosphere box. We studied the influence of the distance between the two electrodes, d, and the annihilation kinetics on the ECL light emission profiles under steady-state conditions, as well as the ECL profiles when carrying out cyclic voltammetry (CV) at a fixed d. Experimental results are compared to simulated results obtained through commercial finite element method software. The light produced by annihilation of the ions was a function of d; stronger light was observed at smaller d. The distance dependence of the ECL emission allows the construction of light approach curves in a similar fashion as with the tip currents in the feedback mode of SECM. These ECL approach curves provide an additional channel to describe the reaction kinetics that lead to ECL; good agreement was found between the ECL approach curve emission profile and the simulated results for a fast, diffusion-limited second-order annihilation process (k(ann) > 10(7) M(-1) s(-1)). In the CV mode at fixed distance, the ECL emission of rubrene showed two distinct signals at different potentials when fixing the substrate to generate the radical cation and scanning the tip to generate the radical anion. The first signal (pre-emission) corresponded to an emission well before reaching the generation of the radical anion and was more intense on Au than on Pt. The second ECL signal showed the expected steady-state behavior from the second-order annihilation reaction and agreed well with the simulation. A comparison of the emission obtained with rubrene and [Ru(bpy)(3)](2+) to test the direct formation of lower energy triplets directly at the electrode showed that triplets are not the cause of the pre-emission observed. Wavelength selection experiments for the rubrene system showed that the pre-emission ECL signal also appeared slightly red-shifted with respect to the main luminophore emission; a possible explanation for this phenomenon is inverse photoemission, where the injection of highly energetic holes by the oxidized species into the negatively biased tip electrode causes emission of states in the metal that appear at a different wavelength than the singlet emission from the ECL luminophore.     

Photovoltaic characteristics and dye regeneration kinetics in D149-sensitized ZnO with varied dye loading and film thickness

Porous ZnO electrodes on fluorine-doped tin oxide (FTO) were prepared by electrochemical deposition from an O(2)-saturated ZnCl(2) solution in the presence of eosin Y as a structure directing agent (SDA). Sensitization was reached by desorption of the SDA and subsequent adsorption of the indoline dye D149. The influence of film thickness and dye concentration in the films on their photovoltaic characteristics, recombination, and dye regeneration kinetics was investigated. The recombination kinetics was analyzed by time-resolved photovoltage measurements. The dye regeneration by iodide ions in the electrolyte was investigated using scanning electrochemical microscopy (SECM) feedback mode approach curves. Analysis of a SECM kinetic model shows strongly different effective D149 regeneration rate constants k'(ox) for D149-ZnO electrodes of systematically varied film thickness and dye loading. It was found that the short-circuit current density J(sc) and k'(ox) correlated directly with the adsorbed dye concentration. k'(ox) was found to be independent of the dye loading but correlated strongly with the dye concentration in the film or inversely with the film thickness. Furthermore, we discussed the perspective of correlating macroscopic cell characteristics with SECM kinetics data.     

Localized Deposition of Chitosan as Matrix for Enzyme Immobilization

A local electrodeposition method was developed for chitosan by exploiting a pH gradient between a macroscopic electrode (the support) and a much smaller counter electrode. The deposition was confined either by using the direct mode of scanning electrochemical microscopy (SECM) or by performing the deposition in channels of a microfluidic network. The roughness was characterized by noncontact scanning force microscopy. The availability of amino groups at the surface of the microstructures was visualized after labeling by confocal laser scanning microscopy. The enzyme glucose oxidase could be entrapped during the electrochemical deposition and showed activity as seen by SECM images.     

Measurement of apparent diffusion coefficients within ultrathin nafion Langmuir-Schaefer films: comparison of a novel scanning electrochemical microscopy approach with cyclic voltammetry

The use of scanning electrochemical microscopy (SECM) to evaluate the apparent diffusion coefficient, Dapp, of redox-active species in ultrathin Nafion films is described. In this technique, an ultramicroelectrode (UME) tip, positioned close to a film on a macroscopic electrode, is used to oxidize (or reduce) a species in bulk solution, causing the tip-generated oxidant (reductant) to diffuse to the film/solution interface. The oxidation (reduction) of film-confined species regenerates the reductant (oxidant) in solution, leading to feedback to the UME. A numerical model is developed that allows Dapp to be determined. For these studies, ultrathin films of Nafion were prepared using the Langmuir-Schaefer (LS) technique and loaded with an electroactive species, either the ferrocene derivative ferrocenyltrimethylammonium cation, FA+, or tris(2,2'-bipyridyl)ruthenium(II), Ru(bpy)32+. The morphology and the thickness of the Nafion LS films (1.5 +/- 0.2 nm per layer deposited) were evaluated using atomic force microscopy (AFM). For comparison with the SECM measurements, cyclic voltammetry (CV) was employed to evaluate the concentration of electroactive species within the Nafion LS films and to determine Dapp. The latter was found to be essentially invariant with film thickness, but the value for Ru(bpy)32+ was 1 order of magnitude larger than for FA+. CV and SECM measurements yield different values of Dapp, and the underlying reasons are discussed. In general, the Dapp values for these films are considerably smaller than for recast Nafion films, which can be attributed to the compactness of Nafion LS films. Nonetheless, the ultrathin nature of the films leads to fast response times, and we thus expect that these modified electrodes could find applications in sensing, electroanalysis, and electrocatalysis.     

Electrochemical and microscopic studies of surface-confined DNA

We have studied the micropatterning and characterization of the organic monolayers using cyclic voltammetry (CV), scanning electrochemical microscopy (SECM), atom force microscopy, and AC impedance, and have determined the electrochemical parameters, i.e., the apparent reaction rate constant (K _f) and the coverage of the electrode surface (θ). CV and SECM experiments demonstrated that the surface of the modified electrode represents an insulating substrate for ferricyanide. Using the high sensitivity of the electron transfer of ferricyanide to the modification of the gold surface with DNA, we selected this reaction as a probe to study the different modification stages at this modified electrode. SECM images obtained from bare, partially modified, and totally modified electrodes showed very good resolution with different topographies or null according to the extent of modification. Based on a comparison with the results of the experiments, a reasonable agreement can be obtained, which means a conjunction of these techniques.     

金属纳米电极的制备、表征及其在电化学测量中的应用

本文简要介绍了各种金属纳米电极的制备及表征方法。结合我们自己的工作,重点介绍了纳米电极在电化学反应动力学参数测量及扫描电化学显微镜(SECM)中的应用,并对其发展前景进行了展望。引用文献78篇。     

Nickel Coated Graphite Microparticle Based Electrodes for Carbon Dioxide Reduction in MonoethanolamineMedium

A composite electrode that contains home-made nickel coated carbon micro-particles as active materials and epoxy binder has been prepared. The surface morphology, elemental composition and size distribution of microparticles were investigated using SEM, EDS, AAS, micro Raman spectroscopy and appropriate seizer apparatus. The prepared micro-particles appeared monodispers with 33 μm most numerous particle size diameter and with 10.1 w/w % Ni content. Conventional electrochemical methods like impedance spectroscopy and voltammetry as well as scanning electrochemical microscopy (SECM) were used for investigation the properties of the composite electrodes. It has been proved that the electrode can be well used for electro-catalytic reduction of CO₂ directly in aqueous mono-ethanol amine (MEA) solution that frequently are applied for capturing it from power plant flue gases. SECM measurements showed that presence of dissolved CO₂ hinders the hydrogen evolution in aqueous MEA solution.