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This Article reports the use of scanning electrochemical microscopy (SECM) for the quantitative study of acid-induced dissolution. An ultramicroelectrode (UME) is used to generate a flux of protons galvanostatically just above a sample surface, creating controlled acid challenges relevant to acid erosion. The electrochemical technique produces etch features in the sample, which are characterized by white fight interferometry (WLI). The technique has been applied to bovine enamel where understanding the kinetics of dissolution is important in the context of acid erosion. Dissolution has been observed as a fast process, but the high rates of mass transport in SECM allow the surface kinetics of dissolution to be evaluated. Key attributes of SECM for these studies are the ability to deliver high, controllable, and local acid challenges in a defined way and that multiple dissolution measurements can be performed on one sample, eliminating intersample variability effects. A novel moving boundary finite element model has been designed to describe the etching process, which allows the etch kinetics to be evaluated quantitatively, simply by measuring the size and shape of etch features over time. 10.1021/ac101662h

We describe highly localized electrochemical measurements and imaging using a simple, mobile them pipet cell. Each channel (diameter < 500 nm) of a tapered them pipet is filled with electrolyte solution and a Ag/AgCl electrode, between which a bias is applied, resulting in a conductance current across a thin meniscus of solution at the end of the pipet, which is typically deployed in air or a controlled gaseous environment. When the position of the pipet normal to a surface of interest is oscillated, an oscillating component in the conductance current is generated when the meniscus at the end of the probe comes into contact with the surface and undergoes periodic (reversible) deformation, so as to modulate the solution resistance. This oscillating current component can be used to maintain gentle contact of the solution from the pipet cell with the surface and as a set point for high resolution topographical imaging with the pipet. Simultaneously, the mean conductance current that flows between the pipet channels can be measured and is sensitive to the local nature of the interface, informing one, for example, on wettability and ion flow into or out of the surface investigated. Furthermore, conductor or semiconductor surfaces can be connected as a working electrode, with one of the electrodes in the pipet serving as a quasi-reference electrode. This pipet cell then constitutes part of a dynamic electrochemical cell, with which direct voltammetric--amperometric imaging can be carried out simultaneously with conductance and topographical imaging. This provides multifunctional electrochemical maps of surfaces and interfaces at high spatial resolution. The prospects for the use of this new methodology widely are highlighted through exemplar studies and a brief discussion of future applications. 10.1021/ac102191u

A new scanning electrochemical microscopy (SECM) tip positioning method that allows surface topography and activity to be resolved simultaneously and independently is presented. The tip, controlled by a piezoelectric positioner operated in closed loop, is oscillated normal to the substrate surface. Changes in the oscillation amplitude, caused by the intermittent contact (IC) of the tip with the substrate surface, are used as a feedback signal to control the tip height. The method is illustrated with amperometric feedback approach curve measurements to inert (insulating) and active (conducting) substrates using 12.5 and 1 [micro]m radii Pt disk electrodes. Imaging of gold bands on a glass substrate demonstrates the capabilities for simultaneous topography and activity mapping. The prospect for using IC methodology more widely with other types of tips is highlighted briefly. 10.1021/ac101099e

Scanning ion conductance microscopy (SICM) is a scanned probe microscopy technique in which the probe is a fine glass pipet filled with a contact (reference) electrode and an electrolyte solution. The current flow between the reference electrode and a second reference electrode positioned in bulk solution when the two electrodes are biased externally can be used as a feedback signal to maintain a constant separation between the tip and a surface during imaging. In usual practice the tip position is modulated over a small amplitude perpendicular to the surface, and the resulting alternating current (AC) is used as the feedback signal, although the direct current can also be used. A comprehensive model for the current response is reported. Laplace's equation has been solved for the electrolyte solution for a range of tip geometries, enabling the factors controlling the tip current to be identified. The approach developed is shown to represent an improvement over earlier semiempirical treatments. To explore the influence of surface topography on the (AC) current response, various surfaces have been considered, including a lip moved toward a planar surface (in the normal direction) and tips scanned over a pit and a step in the surface. The results have allowed a critical assessment of the SICM response as a means of probing surface topography. Features identified through simulation have been found in experiments through studies of two model substrates for which imaging results are reported. In typical experimental practice, the response of the SICM tip to surface features occurs over much greater lateral distances than the size of the pipet aperture.

A scanning micropipet contact method (SMCM) is described which promises wide-ranging application in imaging and quantifying electrode activity at high spatial resolution. In SMCM, a moveable micropipet probe (diameter 300 nm to 1 [micro]m) containing an electroactive species in electrolyte solution is brought to a sample electrode surface so that the liquid meniscus makes contact. The micropipet contains a reference-counter electrode, and the sample is connected as the working electrode to make a two-electrode voltammetric measurement. SMCM thus makes possible highly localized electrochemical experiments, and furthermore, heterogeneous electrode surfaces may be investigated without the substrate being completely immersed in solution. This opens up the possibility of making measurements on a wide range of electrode materials without having to encapsulate the electrode. Furthermore, the electrode/ solution contact can be made rapidly and briefly, which is useful for situations where the electrode would be unstable for longer periods (e.g., due to corrosion or surface adsorption). For heterogeneously active surfaces the technique is particularly powerful as it allows defined areas to be targeted and individual sites to be probed. To exemplify the approach, the electroactivity of basal plane highly oriented pyrolytic graphite (HOPG) and two types of aluminum alloy were investigated. SMCM measurements indicate that basal plane HOPG shows much greater activity than present consensus. Measurements of chemically heterogeneous aluminum alloy surfaces with SMCM allow variations in redox activity to be mapped with high spatial resolution.

The use of fluorescence confocal laser scanning microscopy (CLSM) for flow visualization is described, with a focus on elucidating the pattern of flow in the microjet electrode (MJE). The MJE employs a nozzle, formed from a fine glass capillary, with an inner diameter of ~100 [micro]m, to direct solution at an electrode surface, using high velocity but at moderate volume flow rates. For CLSM visualization, the jetted solution contains a fluorescent probe, fluorescein at high pH, which flows into a solution buffered at low pH, where the fluorescence is extinguished, thereby highlighting the flow field of the impinging microjet. The morphology of the microjet and the hydrodynamic boundary layer are shown to be highly sensitive to the volume flow rate, with a collimated jet and thin boundary layer formed at the faster flow rates (~ 1 [cm.sup.3] [min.sup.-l]). In contrast, at lower flow rates and for relatively large substrates, an unusual recirculation zone is observed experimentally for the first time. This effect can be eliminated by employing small substrates. The experimental observations have been quantified through numerical solution of the Navier-Stokes equations of continuity and momentum balance. The new insights provided by CLSM imaging demonstrate that flow in the MJE, and impinging jets in general, are more complex than predicted by classical models but are well-defined and quantifiable.

This Article reports the use of scanning electrochemical microscopy (SECM) for the quantitative study of acidinduced dissolution. An ultramicroelectrode (UME) is used to generate a flux of protons galvanostatically just above a sample surface, creating controlled acid challenges relevant to acid erosion. The electrochemical technique produces etch features in the sample, which are characterized by white light interferometiy (WLI). The technique has been applied to bovine enamel where understanding the Idnetics of dissolution is important in the context of acid erosion. Dissolution has been observed as a fast process, but the high rates of mass transport in SECM allow the surface kinetics of dissolution to be evaluated. Key attributes of SECM for these studies are the ability to deliver high, controllable, and local acid challenges in a defined way and that multiple dissolution measurements can be performed on one sample, eliminating inteisample variability effects. A novel moving boundary finite element model has been designed to describe the etching process, which allows the etch kinetics to be evaluated quantitatively, simply by measuring the size and shape of etch features over time. [ABSTRACT FROM AUTHOR]