Probing electrode reactions microscopy

A particularly interesting approach for probing electrode reactions at the micro-level has arisen from the combination of microelectrodes and the accurate control instrumentation associated with scanning probe microscopies. 

The usual objective of scanning probe microscopy techniques [41] is to provide images of a solid surface—normally topographic information— with up to atomic resolution. However, they can also be used to probe local solution composition and electrode reactions, as will be described. 

For the investigation of electrode reaction parameters and chemistry at these dimensions, another approach is necessary, in order to make the system species-selective through monitoring the electrochemistry. This involves making tip and substrate into independent electrodes the tip is thus a microelectrode. The microelectrode tip is scanned over the surface this is known as scanning electrochemical microscopy (SECM) [43] and due to its local probe nature can be used to probe microvolumes. 

The properties and applications of microelectrodes, as well as the broad field of electroanalysis, have been the subject of a number of reviews. Unwin reviewed the use of dynamic electrochemical methods to probe interfacial processes for a wide variety of techniques and applications including various flow-channel methods and scanning electrochemical microscopy (SEM), including issues relating to mass transport (1). Williams and Macpherson reviewed hydrodynamic modulation methods and their mass transport issues (2). Eklund et al. reviewed cyclic voltammetry, hydrodynamic voltammetry, and sono-voltammetry for assessment of electrode reaction kinetics and mechanisms with discussion of mass transport modelling issues (3). Here, we focus on applications ranging from measnrements in small volumes to electroanalysis in electrolyte free media that exploit the uniqne properties of microelectrodes. 

N.C. Rudd, S. Cannan, E. Bitziou, L. Ciani, A.L. Whitworth, P.R. Unwin. Fluorescence confocal laser scaiming microscopy as a probe of pH gradients in electrode reactions and surface activity. Anal Chem. 77 6205 (2005). 

The development of scanning probe microscopies and x-ray reflectivity (see Chapter VIII) has allowed molecular-level characterization of the structure of the electrode surface after electrochemical reactions [145]. In particular, the important role of adsorbates in determining the state of an electrode surface is illustrated by scanning tunneling microscopic (STM) images of gold (III) surfaces in the presence and absence of chloride ions [153]. Electrodeposition of one metal on another can also be measured via x-ray diffraction [154]. 

Scanning electrochemical microscopy (SECM) [196] is a member of the growing family of scanning probe techniques. In SECM the tip serves as an ultramicroelectrode at which, for instance, a radical ion may be generated at very short distances from the counterelectrode under steady-state conditions. The use of SECM for the study of the kinetics of chemical reactions following the electron transfer at an electrode [196] involves the SECM in the so-... 

Surface excesses of electroactive species are often examined by methods sensitive to the faradaic reactions of the adsorbed species. Cyclic voltammetry, chronocoulometry, polarography, and thin layer methods are all useful in this regard. Discussions of their application to this type of problem are provided in Section 14.3. In addition to these electrochemical methods for studying the solid electrode/electrolyte interface, there has been intense activity in the utilization of spectroscopic and microscopic methods (e.g., surface enhanced Raman spectroscopy, infrared spectroscopy, scanning tunneling microscopy) as probes of the electrode surface region these are discussed in Chapters 16 and 17. 

In short, SECM is a scanning probe technique similar to STM or atomic force microscopy (AFM). A tip current arises due to an electrochemical reaction (faradaic process) at an ultra-microelectrode (UME) tip (see Chapter 6). The tip generally consists of a Pt wire of diameter between 1 and 25 pm that is sealed in a glass capillary and polished to get a flat electrode surface. A typical voltammogram recorded on a UME is shown in Figure 9.28b. It is a sigmoidal, steady-state current-potential curve without any hysteresis. 

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