Applications of SECM

The chapters that follow illustrate a wide range of applications of SECM that have appeared. Given below is an overview and some examples that might help put the technique in perspective before the detailed treatments. 

As suggested above, by recording an approach curve or voltammogram with the tip close to a substrate, one can study the rates of electron transfer reactions at electrode surfaces (Chapter 6). Because mass transfer rates at the small tip electrodes are high, measurements of fast reactions without interference of mass transfer are possible. As a rule of thumb, one can measure k° values (cm/s) that are of the order of Did, where D is the diffusion coefficient (cm2/s). For example, k° for ferrocene oxidation at a Pt electrode in acetonitrile solution was measured at a 1 /xm radius tip at a d of about 0.1 /xm yielded a value of 3.7 cm/s (24). The use of small tips and small currents decreases any interference from uncompensated resistance effects. 

Rate constants for homogeneous reactions of tip-generated species as they transit between tip and conducting substrate can be determined from steady-state feedback current or TG/SC experiments or by transient measurements (Chapter 7). Generally rate constants can be measured if the lifetime of the species of interest is of the order of the diffusion time between tip and substrate, d2HD. Thus first-order reaction rate constants up to about 105 s and second-order reaction rate constants up to about 10s M 1 s are accessible. 

There is considerable interest in ion and electron transfer processes at the interface between two immiscible electrolyte solutions (ITIES), e.g., water and 1,2-dichloroethane. SECM can be used to monitor such processes (Chapter 8). It allows one to separate ion transport from electron transfer 

Measurements of the rates of surface reactions on insulator surfaces, such as dissolution, adsorption, and surface diffusion, are possible (Chapter 12). For example, proton adsorption on an oxide surface can be studied using the tip to reduce proton and induce a pH increase near the surface (22). Then, by following the tip current with time, information about proton desorption kinetics is obtained. Studies of corrosion reactions are also possible. Indeed, work has been reported where a tip-generated species has initiated localized corrosion and then SECM feedback imaging has been used to study it (28). In these types of studies, the tip is used both to perturb a surface and then to follow changes with time. 

Scanning electrochemical microscopy has been exploited in a large number of applications that are too numerous to review at this point. Hence, only a very small selection of studies will be considered in this section, where the unique properties of SECM have been harnessed in the most elegant ways to reveal hitherto unobserved phenomena. 

2 SECM as Tool to Provide Variable Mass Transfer Conditions 

3 SECM as a Tool to Investigate Lateral Charge Transfer 

Overall, significant advances in experimental and theoretical electroanalytical chemistry have been made as a result of the unique properties of microelectrodes. Martin Fleis-chmann s contribution in this area has been of great importance to these developments. 

Daniele, S., Baldo, M.A., Ugo, P. and Mazzocchin, G.A. (1989) Determination of heavy-metals in real samples by anodic-stripping voltammetry with mercury microelectrodes. 2. Application to rain and sea waters. Analytica Chimica Acta, 219, 19-26. 

---

We begin by reviewing the principles of SECM methods, and present an overview of the instrumentation needed for experimental studies. A major factor in the success of SECM, in quantitative applications, has been the parallel development of theoretical models for mass transport. A detailed treatment of the theory for the most common SECM modes that have been used to study liquid interfaces is therefore given, along with key results from these models. A comprehensive assessment of the applications of SECM is provided and the prospects for the future developments of the methodology are highlighted. 

As with previous kinetic applications of SECM, it should be noted that experimental measurements can be tuned to the kinetic region of interest by varying the radius of the electrode [Eq. (33)] and the separation between the tip and interface. In essence, the smaller the UME, and/or tip-interface separation, the higher the diffusion rates that may be generated and, consequently, the greater the tendency for interfacial kinetic limitations. 

APPLICATION OF SECM IN CHEMICAL AND BIOCHEMICAL SENSOR RESEARCH... 

More recently, another application of SECM detection in DNA and protein chips and in electrophoresis gels has emerged with different detection principles. Wang et al. [81] labeled single-stranded DNA (ssDNA) with gold nanoparticles. After binding to their complementary strand at the chip surface, silver was electroless deposited at the... 

The application of SECM to the study of ECE/DISP1 processes was showed in Ref. [84]. In those experiments, anthracene (AC) was reduced in DMF in the presence of phenol (PhOH). The process steps were... 

The resolution of the technique is limited by the size of the microelectrode tip, at present 200 nm. In the future, reduction of tip size to tens of nanometres by use of novel microelectrode fabrication procedures should increase the applicability of SECM. 

It can be appreciated that, now that the technical aspects are sufficiently developed, many further applications of SECM to study microvolumes (and less) of solution and microscopic characteristics of surfaces can be expected. Combination with non-electrochemical time-resolved techniques such as the quartz crystal microbalance will also be fruitful. 

A series of tip feedback current-distance curves for a range of PhOH concentrations is shown in Figure 29. This shows that a PhOH increases, the current decreases due to the consumption of AC - in the gap, causing a diminution in the extent of positive feedback for the AC/AC - couple. Analysis of all data sets allowed values for the effective first-order rate constants to be determined, from which the second-order rate constant, k2 = C/[PhOH] = (4.4 0.4) X 103 M 1 s, was deduced. This value was found to be in good agreement with that measured earlier by double potential step chrono-amperometry (40), confirming the applicability of SECM in the study of ECE/D1SP1 processes. 

Fe(CN)g4 across the membrane pores. In essence, the SECM images thus provide a means to visualize the flux of Fe(CN)g4 across the membrane, a general concept that is employed in all applications of SECM to study membrane transport. 

A recent application of SECM ideas has been in the study of the resorption of bone by specialized cells known as osteoclasts (19,75). Bone is a complex... 

C Wei, AJ Bard, MV Mirkin. Scanning electrochemical microscopy. 31. Application of SECM to the study of charge-transfer processes at the liquid-liquid interface. J Phys Chem 99 16033-16042, 1995. 

FIG. 11 Schematic illustration of the application of SECM to investigate Ag+ adsorption at a pyrite/aqueous solution interface. 

UME-substrate separations of 0.1 and 0.32. The data relate to a r value of 1.0 which, for the range of kinetics considered, is sufficient for a steady-state to be established at the UME. Figure 14 demonstrates that the shape of the working curves, at constant L, are significantly distinct to allow first-and second-order dissolution processes to be resolved experimentally. Moreover, as observed for other kinetic applications of SECM, dissolution kinetics are measurable with greatest sensitivity at the closest tip-substrate separations (1,30,42,49). 

Through the addition of submicrometer-scale spatial resolution, SECM greatly increases the capacity of electrochemical techniques to characterize interfaces and measure local kinetics. In this way, it has proved useful for a broad range of interdisciplinary research. Various applications of SECM are discussed in this book, from studies of biological systems, to sensors, to probing reactions at the liquid/liquid interface. Although we did not intend to present even a brief survey of those diverse areas of research, each chapter... 

A.L. Barker, J.V. Macpherson, C.J. Slevin, and P.R. Unwin (1998). Scanning electrochemical microscopy (Seem) as a probe of transfer processes in 2-phase systems— theory and experimental applications of secm-induced transfer with arbitrary partition-coefficients, diffusion-coefficients, and interfacial kinetics. J. Phys. Chem. B 102, 1586-1598. 

Dobrzeniecka A, Zeradjanin A, Masa J, Puschhof A, Stroka J, Kulesza PJ, Schuhmann W (2013) Application of SECM in tracing of hydrogen peroxide at multicomponent non-noble electrocatalyst films for the oxygen reduction reaction. Catal Today 202 55-62... 

Section 9.10 discusses the applications of SECM in bioelectrochemistry. Also, the reader is referred to the original... 

---