Subject SECM

An understanding of the operation of the SECM and an appreciation of the quantitative aspects of measurements with this instrument depends upon an understanding of electrochemistry at small electrodes. The behavior of ultramicroelectrodes in bulk solution (far from a substrate) has been the subject of a number of reviews (17-21). A simplified experimental setup for an electrochemical experiment is shown in Figure 1. The solution contains a species, O, at a concentration, c, and usually contains supporting electrolyte to decrease the solution resistance and insure that transport of O to the electrode occurs predominantly by diffusion. The electrochemical cell also contains an auxiliary electrode that completes the circuit via the power supply. As the power supply voltage is increased, a reduction reaction, O + ne — R, occurs at the tip, resulting in a current flow. An oxidation reaction will occur at the auxiliary electrode, but this reaction is usually not of interest in SECM, since this electrode is placed sufficiently far from the UME... 

To calculate the tip current response, the diffusion equation appropriate to the axisymmetric SECM geometry must be solved subject to boundary... 

Combination with scanning electrochemical microscopy (SECM), the subject of Chapter 3.3, has also been explored and the tip-surface separation was found to influence the crystal impedance [53]. Viewed from the opposite perspective, the observation of stress effects on crystal resonant... 

Enzyme activity has been the subject of numerous studies by SECM [39—42]. The enzymes were usually attached raito a uOTicOTiductive surface, and either the feedback or the generation-collection modes were used (Fig. 4). Yet, there are a few studies that used a potentiometric mode in which the enzymatic activity affected local changes in pH that were measured with a pH microelectrode. DNA, proteins, and antibodies have also been targeted... 

The potential dependence of the rate of the interfacial ET reaction at ITIES has been the subject of debate, especially in the SECM community (see the following text). In other words, how much do the reactants feel the local electric field Part of the answer lies in the position of the reactants vis-h-vis the interface. In 1988, Girault and Schiffrin had considered this problem and derived an equation for the ET rate constant [217]. Based on the classical encounter model for ET in solution, where the two reactants O, and R2 meet at the interface, as illustrated in Figure 1.21, to form the precursor 0,IR2 that reorganizes itself to form a reactant pair O1R2 in which the electron-transfer reaction can take place, thereby forming R1O2 that can relax to form the successor complex R,I02 that finally separate into the products Rj and O2, they obtained... 

Under appropriate conditions, the faradaic current may be used to form images of the electrochemical reactivity of a surface. This is known as scanning electrochemical microscopy (SECM), where the transport and heterogeneous redox activity of species within the junction mediate the tip-substrate interaction. This subject has been thoroughly reviewed [43,44], and an excellent paper demonstrating the transition from STM to SECM is available [45]. The possible contribution of confined redox species to resonant tunneling has also been examined [19,46,47]. 

Enzymatic reactions and patterning substrate with enzymes have been the subject of many SECM studies. Original simulations of enzyme kinetics at the substrate were carried out by Pierce et al. [72]. A more extensive set of simulations and the analysis of limitations of the earlier treatment were presented later by Burchardt et al. [73]. The most comprehensive treatment to date is by Lefrou et al. [74] who visualized different experimental situations in the form of a zone diagram, derived analytical approximations for approach curves, and proposed strategies for the extraction of kinetic... 

These results outline the sophisticated survival responses induced by bacteria when they are subjected to deleterious changes of pH, temperature, or osmotic pressure. Under stress, the bacterial defense mechanisms are tightly coupled to the cell s metabolism and homeostatic mechanisms. When stimulating the metabolism of S. aureus with D-(+)-glucose in the presence of highly concentrated salts, a significant current response is measured by SECM in spite of the low cytoplasmic membrane permeability to hydrophilic mediators, such as ferricyanide. One explanation for the presence of this current enhancement implies that the bacteria to survive the osmotic stress, changes the cytoplasmic membrane structure or membrane-associated protein profile to promote ferrocya-nide production. 

FIGURE12.22 The optical (a and c) and SECM images (b and d) of the single cell array of HeLa-NFkB-SEAP cells with (a and b) and without (c and d) lOOng/mL TNF-a stimulation on the PDMS stencil/polystyrene device not subjected to pDEP. The vacant well was marked as ( ). The SECM scan area was 600 mm x 600 pm. (e) The average current response of the HeEa-NFkB-SEAP cells with (black bar, n= 11) and without TNF-a stimulation (white bar, n= 12). (Reproduced from Murata, T. et al.. Biosens. Bioelectron., 25,913,2009. With permission.)... 

In most SECM experiments, the tip is held at a constant potential in an amperometric mode or scanned in the cyclic voltammetry (CV) mode. The substrate can also be subjected to various potential treatments. Studies involving transients or time-dependent signals are especially useful in obtaining information about adsorbed intermediates or products, as discussed in Chapter 16. Another time-dependent technique involves an AC signal applied to the tip, a form of electrochemical spectroscopy impedance (EIS). Examples of this approach have been discussed in Chapter 14 as applied to studies of the mechanism of corrosion processes. For example, in the... 

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