Modes of SECM Operation

Feedback (FB) mode is one of the first developed modes of operation that involves the reacting species directly in the SECM measurements. The term feedback does not refer to the electronic feedback of the instmment to maintain a sample-UME distance. In SECM, feedback refers to the coupling of redox reactions at the UME and the sample. 

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The feedback mode of SECM operation is most suited for probing heterogeneous charge-transfer reactions. Electron transfer at the metal-solution interface was the first chemical reaction probed by SECM. An important advantage of this technique for studies of charge transfers at... 

Figure 9.29 Feedback modes of SECM operations. The UME tip is poised at a potential such that R will oxidize to O. (a) UME is freely hanging in the redox solution, ij = (b) UME...

In this section, we describe corrosion-related applications involving the most common modes of SECM operation (i.e., an amperometric microelectrode in either feedback or generator-collector mode). Traditionally, most of these applications have involved investigations of corrosion on bare metal and metal alloy substrates. But there is also much interest in the further development of coatings for corrosion prevention of metals, and a growing number of reports have appeared describing SECM studies related to these applications. 

Until a few years ago, the TG/SC mode of SECM operation was the most common way to image an enzyme that catalyzes oxygen reduction. TG/SC mode is well suited for imaging activity of surfaces with morphological features because it is relatively insensitive to changes in the tip-substrate distance [14]. The main difference between this mode and classical FB mode is that the feedback diffusion process is not required for TG/SC mode, which enables a direct measurement of activity in acidic solutions. This mode is the converse of SG/TC mode used for the anode catalysts. TG/SC mode has been applied to the study of the kinetics of oxygen reduction reaction (ORR) [14], evaluation of catalytically active nonprecious metal alloy compositions [57,58], optimization of Cu(II) biomimetics [59], thermodynamics-based design of catalysts [60], and analysis of wired enzyme architectures [61]. 

Figure 17.3.14 Schematic diagram of SECM operating in the FIT mode. (Reprinted with permission from (121). Copyright 1997 Elsevier Science).

The development of SECM, which began in the late 1980s, is mainly credited to A. Bard and his coworkers who first described the technique, coined its name, and also developed the early modes of its operation, namely the feedback and the generation-collection modes, which will be described later. In the course of its nearly 30 years of existence, SECM has established itself as the tool of choice for studying spatially resolved local electrochemical reactivity of surfaces, including the quantitative study of the kinetics of both electrochemical and chemical reactions from microscopic to submicroscopic scales [2, 7]. Indeed, the ability of SECM to... 

By recording an electrochemical response, typically the current at the SECM tip, sample, or both, as the tip is scaimed in close proximity to a surface, micro- to nano-spatially resolved chemical reactivity images of the surface, and quantitative data for analyzing heterogeneous electron transfer rates [3,4, 13, 54, 55], as well as diffusion profiles [56] and adsorption/desorption phenomena [57] can be acquired. For visualization of the ORR activity of catalysts, various operation modes of SECM including SG-TC [48, 49, 58], TG-SC [59, 60], and the oxygen competition mode may be used [61]. 

Various operation modes of SECM were systematically developed for the study of electrocatalysis at conductive substrates by Bard and coworkers " (Figure 18.14) and recently applied to the... 

FIGURE 18.14 Schematic representation of different operation modes of SECM for screening electrocatalysts. Ej, tip potential ij, tip current E, substrate potential and i, substrate current. (Reprinted with permission from Amemiya, S., Bard, A.J., Fan, F.-R.F., Mirkin, M.V., and Unwin, P.R., Scanning electrochemical microscopy, An . Rev. Anal. Chem., 1, 95-131, 2008. Copyright 2008 Annual Reviews.)... 

Metal nanoparticles were deposited and patterned on various substrates by using the TG/SC mode of SECM. In this operation mode, either Au or Ag tip was used to oxidatively generate metal ions as... 

In SECM, the UME (which is usually called the SECM tip) can be moved in three dimensions, usually under piezo-control. The most common mode of SECM is the amperometric mode, in which a redox-active species is oxidised or reduced at the SECM tip and the effects of moving the tip close to the surface of a second electrode (which is usually called the substrate) are studied. To describe the operation of the amperometric mode of SECM, we again consider the reduction of species Ox to species Red at a disc-shaped SECM tip (Eq. 4.1), which results in the flow of a steady-state current through the tip (Eq. 4.8). In the SECM literature, the... 

The treatment of the two-phase SECM problem applicable to immiscible liquid-liquid systems, requires a consideration of mass transfer in both liquid phases, unless conditions are selected so that the phase that does not contain the tip (denoted as phase 2 throughout this chapter) can be assumed to be maintained at a constant composition. Many SECM experiments on liquid-liquid interfaces have therefore employed much higher concentrations of the reactant of interest in phase 2 compared to the phase containing the tip (phase 1), so that depletion and diffusional effects in phase 2 can be eliminated [18,47,48]. This has the advantage that simpler theoretical treatments can be used, but places obvious limitations on the range of conditions under which reactions can be studied. In this section we review SECM theory appropriate to liquid-liquid interfaces at the full level where there are no restrictions on either the concentrations or diffusion coefficients of the reactants in the two phases. Specific attention is given to SECM feedback [49] and SECMIT [9], which represent the most widely used modes of operation. The extension of the models described to other techniques, such as DPSC, is relatively straightforward. 

The precise positioning capabilities, which make high spatial resolution possible, give the SECM an important edge over other electrochemical techniques employing UMEs [20]. For example, the SECM can pattern the substrate surface, visualize its topography, and probe chemical reactivity on the micrometer or nanometer scale. Here, we briefly survey the fundamentals of various modes of the SECM operation and then focus on more recent advances in SECM theory and applications. 

Figure 2. Feedback mode of the SECM operation, (a) The UME tip is far from the substrate, (b) Positive feedback species R is regenerated at the substrate, (c) Negative feedback Diffusion of R to the tip is hindered by the substrate.

Figure 4. Charge-transfer processes at the liquid-liquid interface, (a) Probing ET at the liquid-liquid interface with the SECM. The kinetics of ET between two redox couples confined to different immiscible liquid phases can be measured with the SECM operating in the conventional feedback mode. Electroneutrality is maintained by transfer of the common ion (shown as an anion) across the interface (IT). Adapted with permission from Ref. [38]. Copyright 1995, American Chemical Society, (b) Schematic diagram of facilitated ion transfer reaction studied by SECM.

Unlike feedback mode of the SECM operation, where the overall redox process is essentially confined to the thin layer between the tip and the substrate, in SG/TC experiments the tip travels within a thick diffusion layer produced by the large substrate. The system reaches a true steady state if the substrate is an ultramicroelectrode (e.g., a microdisk or a spherical cap) that generates or consumes the species of interest. The concentration of such species can be measured by an ion-selective (potentiometric) microprobe as a function of the tip position. The concentration at any point can be related to that at the source surface. For a microdisk substrate the dimensionless expression is [74, 75]... 

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