Tip current voltammetry

This technique, thus named by Carlsson et al. [78], consists in recording the tip current as a function of the potential of the sample. TCV can be applied with the regulation loop of the tunnel current active or not. Though the terminology tip current voltammetry is rather confusing, because the tip potential is fixed, we use it in the following to conform with published work. 

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. 

In cases of complex sample behavior it is advisable to vary the sample potential at slow rates and to monitor the tip current. This method was named T/S (tip/substrate) cyclic voltammetry. 

The normalized steady-state approach curve of tip current versus tip/sub-strate separation for the reduction of III(BF4) is shown in Figure 14. This was obtained with a 25 pm diameter Pt UME, biased at —0.8 V versus AgQRE to affect the diffusion-controlled reduction of III, as the probe was translated towards a 1 mm diameter Pt disk substrate, biased at 0.0 V versus AgQRE to promote the diffusion-controlled oxidation of IV. When analyzed in terms of EQ theory, the approach curve yielded a value k, = 145 s-1, which was in excellent agreement with that determined by cyclic voltammetry at sweep rates between 10 and 50 V s-1. 

The experimental assessment of the ET theory [1] has been the central theme in the SECM study of tip reactions. Kinetic parameters for a heterogeneous ET reaction at the UME tip are determined by tip voltammetry. The tip reaction of a redox mediator that is initially present in the bulk solution (i.e., the oxidized form of a redox couple, O, in Figure 6.2) is monitored as tip current at various tip potentials to obtain a steady-state voltammogram. In tip voltammetry based on the total positive feedback current (Figure 6.2a), the tip-generated species, R, is electrolyzed at the surface of an electroactive substrate so that the original mediator is regenerated at a diffusion-limited rate. When the tip is positioned within a tip diameter away from the substrate, the redox molecules efficiently diffuse between the tip-substrate gap to enhance the tip current in comparison to its... 

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). 

Typical steady-state tip and substrate current approach curves for the oxidation of different concentrations of ArCT are shown in Figure 23. A general observation is that as the concentration of ArCT increases, the tip and substrate currents—at a particular distance—decrease, due to the second-order nature of the follow-up chemical reaction. The experimental approach curves are shown alongside theoretically derived curves for a spread of normalized rate constants, K2, from which it can be seen that there is reasonable agreement between the observed and predicted trends. From measurements of both feedback currents, for all three ArCT concentrations investigated, and collection efficiencies, for the lowest two concentrations, a radical dimerization rate constant of 1.2 ( 0.3) X 10s M 1 s 1 was determined (5), which was in reasonable agreement with that determined earlier using fast scan cyclic voltammetry (36). 

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 ... 

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