Quantitative characterization of shear force regulation for scanning electrochemical microscopy

The quest for higher spatial resolution in scanning electrochemical microscopy (SECM) calls for the application of smaller probe electrodes. When electrodes are to be used in the feedback mode, smaller electrodes require higher intrinsic kinetics at the sample. The fabrication of nanoelectrodes, as well as their use as SECM probes at constant distance, are reported. The properties of shear force regulation system are characterized quantitatively. Simultaneous topography and reactivity imaging were demonstrated using gold microstructures on a glass substrate.     

Surface analysis by scanning electrochemical microscopy: resolution studies and applications to polymer samples

Resolution studies of scanning electrochemical microscopy (SECM) have been performed in the feedback mode and in the generator/collector mode at circular model structures. A quantitative correlation of the loss in resolution and the increase in distance between tip and sample is found. Measuring a band electrode of just 500 nm width, the high sensitivity of the SECM in identifying chemically active sites is proven. Applied to polymer samples, the chemical composition was determined in the feedback mode at high lateral resolution. The difference in electrical conductivity allows one to distinguish between doped and undoped parts of a polyaniline film. By scanning above a blend consisting of polypyrrole and polypropylene, a map of the local chemical composition was obtained. In this context, the influence of the tip overpotential on the image is discussed.     

Fabrication of Active Horseradish Peroxidase Micropatterns with a High Resolution by Scanning Electrochemical Microscopy

We used a new reactive species OH^? to fabricate active horseradish peroxidase (HRP) micropatterns with a high resolution by scanning electrochemical microscopy (SECM) coupled with a carbon fiber disk electrode as the SECM tip. In this method, except for active HRP micropatterns predesigned other regions on a HRP‐immobilized substrate were deactivated by OH^? generated at the tip held at ?1.7 V in 1.0 mol/L KCl containing 2.0×10^?3 mol/L benzoquinone (BQ) (pH 8.0). The feedback mode of SECM with a tip potential of ?0.2 V was used to characterize the active HRP micropatterns in 1.0 mol/L KCl containing 2.0×10^?3 mol/L BQ and 2.0×10^?3 mol/L H₂O₂.     

Scanning electrochemical microscopy-development of instrumentation utilizing piezo-bimorph X-Y scanners

The development of the instrumentation of a scanning electrochemical microscopy (SECM) is presented. The core of the SECM sensing system is constructed based on piezobimorph scanners, a mechanical micropositioner of multi-dimensional adjustment and ultramicroelectrodes. The control of the electrochemical cell and the SECM system is realized by a battery powered bipoteniostat and analog control circuits respectively with the control of a microcomputer work station. The demonstrations of SECM experiments are given on both a standard IDA sample and a silver electrode. Discussions on the resolution and quality of SECM image are made.     

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.     

扫描电化学显微镜原位表征pH值对核电蒸汽发生器合金腐蚀行为的影响

用扫描电化学显微镜(SECM)和电化学阻抗谱(EIS)原位表征了溶液pH值对核电蒸汽发生器800 合金溶液中的腐蚀行为的影响. 实验结果表明：在酸性氯化钠溶液中,SECM探针渐进曲线为正反馈,表明800合金为活化阳极溶解,腐蚀电位下的EIS 图呈现完整的单容抗弧特征;而在中性或者碱性溶液中,SECM探针渐进曲线为负反馈,表明800 合金为自钝化,不同阳极电位下的EIS 图均呈现不完整的容抗弧特征,但随着阳极极化电位的增加,EIS 谱容抗弧半径减小,表明钝化膜的耐蚀性下降;SECM二维扫描图像结果显示探针电流增加,表明电极表面活性增加,即钝化膜的溶解速度增加. 而在中性或者碱性溶液中的SECM二维图像中均可观察到若干活性点,这可能与晶界或者金属夹杂物等有关.     

Scanning Electrochemical Microscopy as an In Vitro Technique for Measuring Convective Flow Rates Across Dentine and the Efficacy of Surface Blocking Treatments

Scanning electrochemical microscopy (SECM) is shown to be a powerful technique for both the measurement of local solution velocities through human dentine slices, in vitro, and for assessing quantitatively the effect of surface treatments on the flow process. SECM employs a small ultramicroelectrode (micron dimensions) as an imaging probe to provide information on the topography and transport characteristics of dentine, with high spatial resolution. In these studies the dentine sample is a membrane in a two compartment cell, which contains solutions of identical composition, including a redox active mediator (Fe(CN) . In the absence of an applied pressure, the transport‐limited current response at the probe electrode is due to diffusion of Fe(CN) to the UME, which depends on the probe to sample separation. Under an applied hydrostatic pressure, hydrodynamic flow across the sample enhances mass transport to the UME. With this methodology it was possible to accurately measure effective fluid velocities, by recording tip currents with and without pressure, and assess the efficacy of potential flow retarding agents for the treatment of dentinal hypersensitivity. For native dentine, the solution velocity was found to vary dramatically with location on the sample. The application of a glycerol monooleate ‐ base paste treatment to the surface of dentine was found to lower local flow velocities significantly. This electroanalytical methodology is simple to implement and is generally applicable to assessing the efficacy and mode of action of a wide variety of potential fluid flow retarding agents.     

Label‐Free Electrochemical Imaging of Latent Fingerprints on Metal Surfaces

A label‐free electrochemical method based on scanning electrochemical microscopy (SECM) has been developed to image latent fingerprints with high resolution on five kinds of metal surfaces (platinum, gold, silver, copper and stainless steel), as it could measure the minor conductivity differences of the substrate surface and avoid the interference of the background‐color. The images of sebaceous fingerprints on clean metals were revealed by SECM with ferrocene methanol acting as a redox mediator to detect the topology of the fingerprint deposits in constant‐height feedback mode. Inhibition of electrochemical processes on areas of the surface masked by the insulating fingerprint residues generated a negative image of the fingerprint.     

Imaging of enzyme activity by scanning electrochemical microscope equipped with a feedback control for substrate-probe distance

The enzymatic activity of diaphorase (Dp) immobilized on a solid substrate was characterized using a scanning electrochemical microscope (SECM) with shear force feedback to control the substrate-probe distance. The shear force between the substrate and the probe was monitored with a tuning fork-type quartz crystal and used as the feedback control to set the microelectrode probe close to the substrate surface. The sensitivity and the contrast of the SECM image were improved in the constant distance mode (distance, 50 nm) with the shear force feedback compared to the image in the constant height mode without the feedback. By using this system, the SECM and topographic images of the immobilized diaphorase were simultaneously measured. The microelectrode tip used in this study was ground aslant like a syringe needle in order to obtain the shaper topographic images. This shape was also effective for avoiding the interference during the diffusion of the enzyme substrates.     

Novel strategy for constant-distance imaging using alternating current scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) still lacks reliable means for performing constant-distance imaging experiments. We demonstrate, for the first time, that the same negative alternating current (AC) feedback can be observed on approach to an insulator and an unbiased conductor at optimal experimental conditions. This leads to a novel constant-distance imaging mode. To perform AC-SECM imaging, only minor modifications of the existing SECM set-up were necessary. The new constant-distance AC-SECM imaging was conducted to provide topographical information not affected by variations in sample conductivity and reactivity. Furthermore, simultaneous AC and DC SECM measurements were carried out to demonstrate that both topographical and chemical information could be revealed.     

Quantification of the surface diffusion of tripodal binding motifs on graphene using scanning electrochemical microscopy

The surface diffusion of a cobalt bis-terpyridine, Co(tpy)(2)-containing tripodal compound (1·2PF(6)), designed to noncovalently adsorb to graphene through three pyrene moieties, has been studied by scanning electrochemical microscopy (SECM) on single-layer graphene (SLG). An initial boundary approach was designed in which picoliter droplets (radii ~15-50 μm) of the tripodal compound were deposited on an SLG electrode, yielding microspots in which a monolayer of the tripodal molecules is initially confined. The time evolution of the electrochemical activity of these spots was detected at the aqueous phosphate buffer/SLG interface by SECM, in both generation/collection (G/C) and feedback modes. The tripodal compound microspots exhibit differential reactivity with respect to the underlying graphene substrate in two different electrochemical processes. For example, during the oxygen reduction reaction, adsorbed 1·2PF(6) tripodal molecules generate more H(2)O(2) than the bare graphene surface. This product was detected with spatial and temporal resolution using the SECM tip. The tripodal compound also mediates the oxidation of a Fe(II) species, generated at the SECM tip, under conditions in which SLG shows slow interfacial charge transfer. In each case, SECM images, obtained at increasing times, show a gradual decrease in the electrochemical response due to radial diffusion of the adsorbed molecules outward from the microspots onto the unfunctionalized areas of the SLG surface. This response was fit to a simple surface diffusion model, which yielded excellent agreement between the two experiments for the effective diffusion coefficients: D(eff) = 1.6 (±0.9) × 10(-9) cm(2)/s and D(eff) = 1.5 (±0.6) × 10(-9) cm(2)/s for G/C and feedback modes, respectively. Control experiments ruled out alternative explanations for the observed behavior, such as deactivation of the Co(II/III) species or of the SLG, and verified that the molecules do not diffuse when confined to obstructed areas. The noncovalent nature of the surface functionalization, together with the surface reactivity and mobility of these molecules, provides a means to couple the superior electronic properties of graphene to compounds with enhanced electrochemical performance, a key step toward developing dynamic electrode surfaces for sensing, electrocatalysis, and electronic applications.     

Scanning electrochemical microscopy study of laccase within a sol-gel processed silicate film

The enzyme p-diphenol:dioxygen oxidoreductase (laccase, EC 1.10.3.2) was isolated from Cerrena unicolor fungus and embedded in a sol-gel film obtained by acidic condensation of TMOS. The gel was cast to thin films on glass. The laccase-containing silicate films were inspected by confocal laser scanning microscopy (CLSM), scanning force microscopy (SFM) and scanning electrochemical microscopy (SECM). CLSM images in the reflection mode showed aggregates within the silicate films. SECM images in the substrate-generation/tip-collection mode using 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) as electron donor for laccase showed that the position of aggregates coincides with increased enzymatic activity within the silicate film. The flux from individual aggregates was detected. SECM images in the redox competition mode confirmed the assignment and could exclude that topographic features observed by CLSM and SFM could be the reason for the image contrast. SFM images showed that the aggregates partially dissolve during prolonged exposure to aqueous buffer. The experimental setup allowed following one individual aggregate over time with all three microscopic techniques which enabled the collection of complementing information on morphology and catalytic activity as well as their development over time.     

Imaging of voltage-gated alamethicin pores in a reconstituted bilayer lipid membrane via scanning electrochemical microscopy

Voltage-gated biological ion channels were simulated by insertion of the peptaibol antibiotic alamethicin into reconstituted phosphatidylcholine bilayer lipid membranes (BLMs). Scanning electrochemical microscopy (SECM) was utilized to probe initial BLM resistivity, the insertion of alamethicin pores, and mass transport across the membrane. Acquired SECM images show the spatial location of inserted pore bundles, the verification of voltage control over the pore conformational state (open/closed), and variations in passive mass transport corresponding to different topographical areas of the BLM. SECM images were also used to evaluate overall BLM integrity prior to insertion as well as transport (flux in open state) and leakage (flux in closed state) currents following insertion.     

Scanning electrochemical microscopy of living cells: Part 2. Imaging redox and acid/basic reactivities

Amperometric feedback and potentiometric modes of the scanning electrochemical microscope (SECM) have been used to image the topography and map redox and acid–base activities in single mammalian cells. The topographic images of cells were obtained using hydrophilic redox mediators, which cannot penetrate the cell membrane. In contrast, with a hydrophobic mediator one can map redox reactivity with a micrometer or submicrometer spatial resolution. The images obtained with oxygen used as a redox mediator show the distribution of the diffusion rate of oxygen in the cell membrane and inside the cell. The acid release by the cell was imaged with a Sb tip in a potentiometric mode, and the possibility of redox and pH imaging of the same cell with same tip is demonstrated. Significant differences were detected in the redox and pH images of normal human breast epithelial cells and metastatic breast cancer cells.     

Scanning electrochemical microscopic study of hydrogen oxidation and evolution at electrochemically deposited pt nanoparticulate electrode incorporated in polyaniline.

Scanning Electrochemical Microscopy (SECM) feedback mode and substrate generation-tip collection (SG-TC) mode coupled with chronoamperometric approach were used to investigate H2 oxidation and hydrogen evolution reaction (HER) at a polyaniline (PAni) coated highly oriented pyrolytic graphite (HOPG) electrode. Using the former mode, the heterogeneous electron transfer (ET) kinetics for H2 oxidation was studied, while the latter mode allowed mapping of the distribution of local [H2] at the nanoparticulate/aqueous interface, followed by monitoring the transients at the tip. These preliminary studies demonstrate that SECM is useful in evaluating the activity of nanophase electrocatalysts. Particularly, if one employs nanometer-sized tips or hydrodynamic microjet electrodes where the mass transfer rate is significantly high, it should be possible to investigate the ET kinetics more accurately.     

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.     

Controlling of band width,resolution and sample loading by injection system in a simple preparative free-flow electrophoresis with gratis gravity

In this paper, the controllable band width, resolution and sample loading were investigated by the injection system of free-flow electrophoresis (FFE) with gratis gravity. Two general injection methods were described herein. The first method was the one in which sample injection fluxes were variable with constant background flux, while the second was the one in which the background fluxes were flexible with stable sample flux. With methyl green and crystal violet as two viewable model compounds, a series of experiments were performed, and the experimental results revealed that (1) the sample band width could be under desiring control through the regulation of ratios between sample and background fluxes, (2) the separative resolution could be also adjusted elaborately via the regulation of flux ratios during the separation of methyl green and crystal violet with only one charge disparity, and (3) the sample loading could be conveniently controlled via the flux ratios and an approximate maximum sample loading could be selected under the condition of just completed separation of two adjoining solutes. In addition, it was observed that the flux ratio had soft influence on the separative resolution of two solutes. These results were of significance to the designs on band width, resolution and sample loading in the newly developed FFE device with gratis gravity.     

SECM for Studying the Immobilization and Repartition of a Redox Anti-tetracycline Aptamer on Screen-printed Carbon Electrodes

Scanning electrochemical microscopy (SECM) is discussed as a versatile tool to provide a unique approach of localized electrochemical information in the context of biosensing research. The step-by-step immobilization of DNA aptamer with intrinsic redox activity on screen-printed carbon electrode (SPCE) was successfully monitored using SECM imaging tool. Control experiments were performed with a non-electroactive aptamer. After immobilization of these aptamers, SECM images showed the repartition of the electroactive anti-tetracycline aptamer when comparing with images produced for control and for all modification steps of SPCE. The possibility of tetracycline detection was also proved by causing a decrease in recorded current.     

Constant-distance mode AC-SECM for the visualisation of corrosion pits

The local visualisation of corrosion pits has so far failed to convincingly prove the advantage of scanning microscopy in overcoming Abbey’s Limit. Thus, for imaging pit initiation and instant pit growth the resolution of the methods used for detecting the related changes in local electrochemical activity has to be significantly improved. Progress into this direction has been achieved by combining shear force based constant-distance mode scanning electrochemical microscopy (SECM) with the alternating current mode of this technique (AC-SECM). A very small corrosion pit was generated by inducing active corrosion underneath an accurately positioned 5 μm diameter Pt disk SECM tip that served as counter electrode in the direct mode of SECM. Before and after pit initiation, constant-distance mode AC-SECM images were recorded and subsequently used to identify the area of broken passivity. The results demonstrate the feasibility of the suggested approach to image actively corroding sites with overall dimensions of only a few micrometers. Combination of constant-distance scanning with AC-SECM is an important prerequisite for further improving spatial resolution by employing smaller-diameter micro- or even nano-electrodes as SECM tips.     

Fabrication of Prussian Blue modified ultramicroelectrode for GOD imaging using scanning electrochemical microscopy

A Prussian Blue (PB) film modified disk ultramicroelectrode (UME) was fabricated by electrochemical deposition technique on a Pt-disk UME. The electrocatalytical reductions of hydrogen peroxide derived from glucose oxidase (GOD) on this modified UME were investigated. The enzymatic biochemical reactivity was imaged by scanning electrochemical microscopy (SECM) utilizing the PB film modified UME. It is evident that sensitivity and spatial resolution for hydrogen peroxide measurement were improved obviously. SECM images obtained clearly revealed the concentration profile of the reaction products around the enzymes. The PB film modified microelectrode is in the nature of simple preparation, high catalytic activity on hydrogen peroxide and substrate selectivity for SECM etc.