Diffusional electrochemistry

Diffusional electrochemistry of enzymes functionalized with tethered redox relays... 

One final issue remains to be resolved Of the portion of the AEpi that is due to resistance, what part is caused by solution resistance and what part is caused by film resistance To explore this issue we examined the electrochemistry of a reversible redox couple (ferrocene/ferricinium) at a polished glassy carbon electrode in the electrolyte used for the TiS 2 electrochemistry. At a peak current density essentially identical to the peak current density for the thin film electrode in Fig. 27 (0.5 mV see ), this reversible redox couple showed a AEpi of 0.32 V (without application of positive feedback). Since this is a reversible couple (no contribution to the peak separation due to slow kinetics) and since there is no film on the electrode (no contribution to the peak separation due to film resistance), the largest portion of this 0.32 V is due to solution resistance. However, the reversible peak separation for a diffusional one-electron redox process is —0.06 V. This analysis indicates that we can anticipate a contribution of 0.32 V -0.06 V = 0.26 V from solution resistance in the 0.5 mV sec control TiS2 voltammogram in Fig. 27. 

An -> ideal nonpolarizable electrode is one whose potential does not change as current flows in the cell. Much more useful in electrochemistry are the electrodes that change their potential in a wide potential window (in the absence of a - depolarizer) without the passage of significant current. They are called -> ideally polarized electrodes. Current-potential curves, particularly those obtained under steady-state conditions (see -> Tafel plot) are often called polarization curves. In the -> corrosion measurements the ratio of AE/AI in the polarization curve is called the polarization resistance. If during the -> electrode processes the overpotential is related to the -> diffusional transport of the depolarizer we talk about the concentration polarization. If the electrode process requires an -> activation energy, the appropriate overpotential and activation polarization appear. 

Most of the solvents used in electrochemistry, and particularly water, present strong absorption in the mid-IR range. Therefore the use of external reflection IR spectroscopy for the in-situ observation of electrode processes requires a considerable reduction in the solution thickness in the path of the IR beam. Only a very thin layer of electrolyte between electrode and IR window is allowed in order to have enough energy reaching the electrode surface. Typically, the thickness of the solution layer produced by a well-positioned, flat-polished electrode is of the order of 1 - 5 pm. Within this cavity, which has been described by Yeager et al. as diffusionally decoupled, migration is the predominant form of mass transport [26]. 

Self-assembled monolayers of isomerizable compounds can give nst tc surfaces with switchable permeability, thus resulting in different interfacial electrochemistry for a diffusional redox probe. A self-assembled monolayer of 4-cyano-4 -(10-thiodecoxy)stilbene on a Au-electrode demonstrated a higher blocking effect for the [Fe(CN)g] electrochemistry when it was in... 

One of the issues in macroscopic measurements of HOPG electrochemistry is that sites with substantially different electrochemical activity are diffusionally coupled, making it difficult to draw conclusions about the activity of different types of sites, particularly as most CV studies have used only one scan rate [16,71,72,74,78,82], and the local step density on the same cleaved surface can vary significantly across... 

Abrantes, L., Fleischmann, M. and Peter, L. (1988) On the diffusional impedance of microdisc electrodes. Journal ofElectroanalytical Chemistry and Interfacial Electrochemistry, 256, 229-233. 

Such progresses have obviously opened new frontiers to electrochemistry. Yet, several other areas have also become accessible to the discipline because of the unique properties of ultramicroelectrodes. Among these, and besides the two above examples, we wish to restrict our presentation to electrochemistry in the nanosecond time scale, in the one hand, and to diffusional steady state voltanunetry in the other hand. These topics will be discussed based on examples taken from our research in organic or organometallic reactivity. 

Equilibration of the redox protein is faster in the reflection cell, because the diffusional path length is only half the optical pathlength, which enables a rapid switching of the molecule between different redox states. Although direct electrochemistry is preferable, it turns out that redox cofactors are sometimes deeply buried in proteins and enzymes so that mediators are required to speed up the equilibration [4]. However, they can be kept at very low concentrations and thus do not interfere with the optical measurement. Under these conditions, ampero-metric, coulometric, or cyclovoltammetric experiments in combination with UVA IS/IR... 

AMATORE - This is just to add a comment on your answer to Jonah s question. In electrochemistry, at spherical electrode we obtain the same kind of problem in which reaction is convoluted with diffusion. Yet in such a case we are used to deconvolute the diffusional contribution to obtain a real rate constant independent of time or distance. However this is done for diffusional distances in the micrometric range. Would that be transposable for Angstrom diffusion distances as considered in Smoluchowski s model ... 

Katz E, Baron R, Willner I. Magnetoswitchable electrochemistry gated by alkyl-chain-fiinctionahzed magnetic nanoparticles controlling of diffusional and surface-confined electrochemical process. J Am Chem Soc 2005 127 4060-4070. 

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