Redox competition mode

In the transient RC mode, both the UME tip and the enzyme-modified surface sample compete for the same analyte present in the microenvironment between the tip and the sample. Here, both tip and sample are held at the same potential, enabling both of them to electrochemically convert the analyte. When tip and sample are not along the same vertical axis, and are sufficiently separated laterally, the currents at the tip and sample are determined by the bulk analyte concentration and kinetics at the respective electrodes. However, as the tip moves closer to the sample and/or when it is directly above the sample, both electrodes (tip and sample) compete for the limited quantity of the analyte present between them. This results in a reduced tip current, as the analyte concentration available at the tip is reduced by its reaction at the sample. The decrease in the tip current is imaged as enzymatic redox activity in RC mode of SECM [63]. RC mode does not impose any limitation on the quality of the SECM activity image. Therefore, there are no restrictions to both the sample size and the minimum size of the UME tip. 

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RC-SECM Redox competition mode scanning electrochemical microscopy... 

The redox competition mode consists of two subsequent steps (Fig. 6). In the first step, the species of interest is electrochemically generated at the tip increasing the concentration of this species in the gap between tip and sample surface. In the subsequent step, the generated species is collected at the tip. Simultaneously, the generated species is also collected at the sample, which means that both tip and sample compete in the second step for the species generated at the tip in the first step. 

F. 6 Redox competition mode. Left the species of interest is generated at the tip in the first step. Right Both the tip and sample compete to collect the generated species... 

The electrocatalytic properties of an electrode surface can be mapped via the generation/coUection or the redox competition mode of SECM. However, evaluation of the properties of a single nanopaiticle supported on a noncatalytic macroelectrode requires rather high spatial resolution of the SECM, below a few hundreds of nanometers which is highly challenging. Two approaches have shown the potential to resolve the electrocatalytic properties of single nanopaiticles toward ORR, namely SICM-SECM and SECCM. 

Fig. 24 SECM images of local electrocatalytic activity of Pt/C spots with different catalyst loading, 1 droplet left) 2 droplets (right) showing the local O2 consumption obtained using the redox competition mode of SECM. Sample potential = 200 mV versus Ag/AgCl/3 M KCl electrolyte = H2SO4 (200 mM) tip potential = 0 mV (5 s) and -600 mV (0.5 s)...

Eckhard K, Chen X, Turcu F, Schuhmann W (2006) Redox competition mode of scanning electrochemical microscopy (RC-SECM) for visualisation of local catalytic activity. Phys Chem Chem Phys 8(45) 5359-5365... 

SECM. In conventional amperometric SECM an eleetro-ehemical probe, typically a glass-encapsulated microdisk electrode, is brought into close proximity to the interface of interest, and electroehemical reaetions are driven in the small gap between the tip and the surface. Current can be measured at the tip and/or at the substrate and the response is highly sensitive to both the nature of the surface and the tip-surface separation. There are various modes of SECM, such as feedback mode, generation-collection mode and redox competition mode, each of which can... 

Santana, J. J., J. Gonzalez-Guzman, L. Fernandez-Merida, S. Gonzalez, and R. M. Souto, Visualization of local degradation processes in coated metals by means of scanning electrochemical microscopy in the redox competition mode, Electrochimica Acta, 55, 2010, 4488. 

Guadagnini, L., Maljusch, A., Chen, X., Neugebauer, S., Tonelli, D., Schuhmann, W. Visualization of electrocatalytic activity of microstructured metal hexacyanoferrates by means of redox competition mode of scanning electrochemical microscopy (RC-SECM). Electrochim. Acta 2009, 54, 3753-3758. 

The other option in GC is SG/TC mode, which has been used successfully for imaging activity of anode catalysts such as GOx. However, in this mode, the analyte (O2) concentration is significantly altered, and large enzyme sample surface and small UME tips are needed. Moreover, the application of potential pulse to the sample in TG/SC mode is limited by capacitive charging currents, resulting in a poorly defined UME potential. To overcome these drawbacks in GC mode for imaging cathode catalyst activity, anew method called redox competition (RC) mode was developed in... 

Thus, the mechanism of MT antioxidant activity might be connected with the possible antioxidant effect of zinc. Zinc is a nontransition metal and therefore, its participation in redox processes is not really expected. The simplest mechanism of zinc antioxidant activity is the competition with transition metal ions capable of initiating free radical-mediated processes. For example, it has recently been shown [342] that zinc inhibited copper- and iron-initiated liposomal peroxidation but had no effect on peroxidative processes initiated by free radicals and peroxynitrite. These findings contradict the earlier results obtained by Coassin et al. [343] who found no inhibitory effects of zinc on microsomal lipid peroxidation in contrast to the inhibitory effects of manganese and cobalt. Yeomans et al. [344] showed that the zinc-histidine complex is able to inhibit copper-induced LDL oxidation, but the antioxidant effect of this complex obviously depended on histidine and not zinc because zinc sulfate was ineffective. We proposed another mode of possible antioxidant effect of zinc [345], It has been found that Zn and Mg aspartates inhibited oxygen radical production by xanthine oxidase, NADPH oxidase, and human blood leukocytes. The antioxidant effect of these salts supposedly was a consequence of the acceleration of spontaneous superoxide dismutation due to increasing medium acidity. 

At the singlet excited state, ortho and meta photocycloadditions are often competitive processes and physicochemical investigations were carried out to rationalize the modes of cycloaddition of arenes with alkenes. In the context of the study of photochemical electron transfer reactions, it has been proposed that the difference of the redox potentials of the reaction partners might play an important role in this competition [10]. Such a discussion involves the intervention of an exciplex as intermediate. The Rehm-Weller equation [11] was used to quantify the relationship. When an electron transfer process is strongly endergonic (AG>1.5eV), the meta cycloaddition should be favored. When such a process is less endergonic (1 < AG< 1.5 eY), the ortho addition dominate [12]. This means that the... 

I- 2] and [3 -I- 2] photocycloadditions are often competitive, and it has been proposed that the difference of the redox potentials of the reaction partners should play an important role for the control of the modes of cycloaddition [84]. However, treatments based on molecular orbital interactions could not differentiate between the two cycloaddition modes [85]. 

It should be emphasized that classical or quantum-mechanical nature of a system is not an inherent property of the system under any conditions. There is always a competition between two ways in which the process can occur, and the same system may behave as a purely classical system at sufficiently high temperatures and as a purely quantum system at sufficiently low temperatures. Dogonadze and Kuznetsov[217] explained just in this way the transition from the Arrhenius dependence of the reaction rate to its temperature independence at low temperatures observed for some reactions, in particular, for redox transformations of cytochrome C[244]. For a sufficiently low value of kT, the characteristic energy for all vibrational modes essential for the reaction turns out to be greater than kT, and all transformations occur quantum-mechanically and not classically. The temperature independence of the reaction rate is not the only possible peculiarity of the behavior of low-temperature systems. Other unusual temperature dependences are also possible[217]. Of course, there are also intermediate cases (AE kT) when tunneling dominates though not from the ground level but from excited levels. 

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