Voltammetry SECM

UMEs decrease the effects of non-Earadaic currents and of the iR drop. At usual timescales, diffusional transport becomes stationary after short settling times, and the enhanced mass transport leads to a decrease of reaction effects. On the other hand, in voltammetry very high scan rates (i up to 10 Vs ) become accessible, which is important for the study of very fast chemical steps. For organic reactions, minimization of the iR drop is of practical value and highly nonpolar solvents (e.g. benzene or hexane [8]) have been used with low or vanishing concentrations of supporting electrolyte. In scanning electrochemical microscopy (SECM [70]), the small size of UMEs is exploited to locahze electrode processes in the gm scale. 

From Eq. (9.6), mo D/d if L<1. This suggests the usefulness of SECM for the study of rapid heterogeneous electron transfer kinetics, the largest ks values that can be determined being of the order of D/d. The largest ks values measurable by voltammetry at UMEs are of the order of D/a. It is easier to get a small L value with SECM than to prepare a UME of very small a value. For example, ks for the Fc+/Fc couple in AN has been determined by SECM to be 3.7 cm s-1 [29]. 

In this mode, a small SECM tip is used to penetrate a microstructure, for example, a submicrometer-thick polymer film containing fixed redox centers or loaded with a redox mediator, and extract spatially resolved information (i.e., a depth profile) about concentrations, kinetic- and mass-transport parameters [33, 34]. With a tip inside the film, relatively far from the underlying conductor or insulator, solid-state voltammetry, at the tip can be carried out similarly to conventional voltammetric experiments in solution. At smaller distances, the tip current either increases or decreases depending on the rate of the mediator regeneration at the substrate. If the film is homogeneous and not very resistive, the current-distance curves are similar to those obtained in solution. 

Kapui et al. prepared a novel type of polypyrrole films [168]. The film was impregnated by spherical styrene-methacrylic acid block copolymer micelles with a hydrophobic core of 18 nm and a hydrophilic corona of 100 nm. The properties of the micelle-doped polypyrrole films were investigated by cyclic voltammetry and SECM. It was found that the self-assembled block copolymer micelles in polypyrrole behave as polyanions and the charge compensation by cations has been identified during electrochemical switching of the polymer films. 

The chemical stability and electrochemical reversibility of PVF films makes them potentially useful in a variety of applications. These include electrocatalysis of organic reductions [20] and oxidations [21], sensors [22], secondary batteries [23], electrochemical diodes [24] and non-aqueous reference electrodes [25]. These same characteristics also make PVF attractive as a model system for mechanistic studies. Classical electrochemical methods, such as voltammetry [26-28] chronoamperometry [26], chronopotentiometry [27], and electrochemical impedance [29], and in situ methods, such as spectroelectrochemistry [30], the SECM [26] and the EQCM [31-38] have been employed to this end. Of particular relevance here are the insights they have provided on anion exchange [31, 32], permselectivity [32, 33] and the kinetics of ion and solvent transfer [34-... 

ET reactions at the polarizable - interface between two immiscible electrolyte solutions (ITIES) have been studied by cyclic voltammetry [ii], AC impedance [iii], and scanning electrochemical microscopy (SECM) [iv]. A simple method was introduced that allows evaluation of the ET rates at the interface between the thin film of an organic solvent and the aqueous electrolyte solution [v]. 

A recent introduction of scanning electrochemical microscopy (SECM) to this field [16-23] has revitalized the study of ET at the OAV interface. In contrast to the conventional, four-electrode cyclic voltammetry at externally polarized OAV interfaces, the SECM measurements not necessarily require supporting electrolytes, and thus can be carried out over a wide range of driving forces without the limitation of the potential window. This advantage of SECM allowed for an experimental verification of the Marcus theory in the driving-force dependence of the ET rate constant [18,21]. 

The main SECM program is involved in acquiring data as a function of position. However, the SECM program is enhanced by the ability to perform voltammetry (i vs. E) or other time-based routines at a stationary electrode. A DAC on the ADC board can generate a voltage sweep or step. Alternately, a trigger pulse starts an external generator. 

The origins of SECM homogeneous kinetic measurements can be found in the earliest applications of ultramicroelectrodes (UMEs) to profile concentration gradients at macroscopic (millimeter-sized) electrodes (1,2). The held has since developed considerably, such that short-lived intermediates in electrode reactions can now readily be identified by SECM under steady-state conditions, which would be difficult to characterize by alternative transient UME methods, such as fast scan cyclic voltammetry (8). 

SECM feedback measurements have been applied, together with cyclic voltammetry and controlled potential electrolysis (30), to the reduction of the rhenium aryldiazenido complex [Cp Re(CO)2(/7-N2CsH4OMe)] [BF4] (III(BF4) Cp = T -C5Me5). A one-electron reduction results in the 19-elec-tron complex, IV [Eq. (40)], which then decomposes to give products [Eq. (41)]. 

SECM SG/TC experiments were carried out to prove that the product of the initial two-electron oxidation process diffused into the solution, where it would react homogeneously and irreversibly. For these measurements, a 10 /xm diameter Au tip UME was stationed 1 /xm above a 100 /xm diameter Au substrate electrode. With the tip held at a potential of —1.3 V versus saturated mercurous sulfate electrode (SMSE), to collect substrategenerated species by reduction, the substrate electrode was scanned through the range of potentials to effect the oxidation of borohydride. The substrate and tip electrode responses for this experiment are shown in Figure 16. The fact that a cathodic current flowed at the tip, when the substrate was at a potential where borohydride oxidation occurred, proved that the intermediate formed in the initial two-electron transfer process (presumed to be mono-borane), diffused into the solution. An upper limit of 500 s 1 was estimated for the rate constant describing the reaction of this species (with water or OH ), based on the diffusion time in the experimental configuration. This was consistent with the results of the cyclic voltammetry experiments (11). 

Of the three SECM modes that can be used to study electrode reaction mechanisms—the TG/SC, feedback, and SG/TC modes—the former is the most powerful for measuring rapid kinetics. With this approach, fast followup and sandwiched chemical reactions can be characterized under steady-state conditions, which are difficult to study even with rapid transient techniques such as fast scan cyclic voltammetry or double potential step chronoamperometry, where extensive corrections for background currents are often mandatory (44). At present, first- and second-order rate constants up to 105 s 1 and 1010 M 1 s, respectively, should be measurable with SECM. The development of smaller tip and substrate electrodes that can be placed closer together should facilitate the detection and characterization of electrogenerated species with submirosecond lifetimes. In this context, the introduction of a fabrication procedure for spherical UMEs with diameters... 

SECM is used to study an Ej-Ci reaction (O + R R Z). A 10-/xm tip is used to reduce O while being positioned over a Pt electrode where R is oxidized back to O at a diffusion-controlled rate. When the tip is 0.2 jxm from the surface, the approach curve shows the same feedback current as for a mediator where the product is stable. However, when the tip is 4.0 /xm away, the response is close to that for an insulating substrate. Estimate the rate constant for the decomposition of R to Z. If this reaction were studied by cyclic voltammetry, approximately what scan rates would be needed to find a nernstian response What are the advantages of SECM in studying this kind of reaction compared to CV ... 

Another approach involves using implicit methods (28, 30, 31) for obtaining/(y, k + 1) [e.g., the Crank-Nicolson (32), the //y implicit finite difference (FIFD) (33), and the alternating-direction implicit (ADI) (34) methods] rather than the explicit solution in (B.1.9). In implicit methods, the equations for calculation of new concentrations depend upon knowledge of the new (rather than the old) concentrations. There are a number of examples of the use of such implicit methods in electrochemical problems, such as in cyclic voltammetry (35) and SECM (36). 

Recently new methods, based on petturbations on the linear sweep voltammetry response of the mediator in the presence of the protein," a mediated thin-layer voltammetry technique, cyclic voltammetric simulation apphed to an electrochemically mediated enzyme reaction" have been setded to gain information on the protein-mediator interactions. More recendy the Scanning Electrochemical Microscopy (SECM) was used to probe the red-ox activity of individual cells of purple bacteria, by using two groups of mediators (hydrophilic and hydrophobic species) in order to gain information on the dependence of measured rate constant on the formal potential of the mediator in solution. By this technique an evaluation of the intracellular potential was also performed. ... 

FSCV-SECM Fast scan cyclic voltammetry scanning electrochemical microscope... 

The quartz crystal nanobalance (QCN) can be combined with practically any electrochemical methods, such as cyclic voltammetry, chronoamperometry, chronocoulometry, potentiostatic, galvanostatic, rotating disc electrode [11], or potentiometric measurements. The EQCN can be further combined with other techniques, e.g., with UV-Vis spectroscopy [12], probe beam deflection (PBD) [13], radiotracer [14], atomic force microscopy (AEM) [15], and scanning electrochemical microscopy (SECM) [16]. The concept and the instrumentation of... 

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