Optically Transparent Thin-Layer Electrochemistry

The technique of optically transparent thin-layer electrochemistry (ottle) was first applied to the characterization of the stoichiometry and thermodynamics of horse heart cytochrome c by Heineman et alP This report showed how the special features of ottle can be combined with optical monitoring of a mediated biological electrode response to provide a simple, accurate, and precise means of characterizing the stoichiometry and thermodynamics of a biological molecule. The mechanism of mediation is described by the following equations  

The unique advantages of ottle arise both from the very short distances over which molecules must diffuse to undergo electron transfer at the electrode surface and from the facility for acquiring optical measurements to monitor the redox state of solution resident species which is provided by an optically transparent electrode (OTE)/ The time (t, seconds) required to attain redox equilibrium in an ottle cell may be estimated by 

The major liability in using ottle is the requirement that the optically monitored species exhibits a large difference in its absorption spectra between the oxidized and reduced forms, i.e, the sample must have a large difference molar absorptivity. Fortunately, many biological molecules have difference molar absorptivities in excess of 10 M cm and are therefore amenable to study by ottle. 

From the foregoing discussion, it is apparent that a study of a given biological molecule by mediated ottle will require access to a mediator having the desired formal potential as well as optical properties in both the oxidized and reduced forms which do not interfere with the optical response of the biological sample. Many mediators suitable for use in ottle studies of biological molecules have been described. 

Since the first report of the application of mediated ottle to the study of the redox properties of biological molecules, the technique has been widely used for this purpose. Table 1 summarizes this body of work. A number of novel applications of ottle described in this table have served to provide new insights into the thermodynamics of biological molecule redox reactions. One of these applications is described in some detail in the following discussion. 

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OTTLE optically transparent thin-layer electrochemistry... 

The metal-metal interactions in the polymer network were investigated by controlled potential electrolysis with the aid of an optically transparent thin-layer electrochemistry (OTTLE) cell. In the visible/near-IR spectrum of the fully reduced deep-red/orange gel the lowest-energy visible band is assigned to a d-d transition. Upon oxidation, two new absorption peaks emerge one at 640 nm is due to a li-gand-to-metal charge-transfer (LMCT) of the ferrocenium moiety, whereas the... 

Developments in spectroelectrochem-istry based on TCO electrodes under semi-infinite diffusion conditions were reviewed by Kuwana and Winograd in 1974 [244]. Since 1974, significant developments in absorption spectroelectrochem-istry have occurred under semi-infinite diffusion conditions and in optically transparent thin-layer electrochemistry, several new TCO electrodes have heen characterized, and the indirect coulometric titration technique has been developed and apphed to biological systems [245]. 

Other spectroscopic techniques that have been used with electrochemistry to probe nanoparticles include electronic and vibrational spectroscopies. The spec-troelectrochemistry of nanosized silver particles based on their interaction with planar electrodes has been studied recently [146] using optically transparent thin layer electrodes (OTTLE). Colloidal silver shows a surface plasmon resonance absorption at 400 nm corresponding to 0.15 V vs. Ag/AgCl. This value blue shifts to 392 nm when an Au mesh electrode in the presence of Ag colloid is polarized to —0.6 V (figure 20.12). The absorption spectrum is reported to be quite reproducible and reversible. This indicates that the electron transfer occurs between the colloidal particles and a macroelectrode and vice versa. The kinetics of electron transfer is followed by monitoring the absorbance as a function of time. The use of an OTTLE cell ensures that the absorbance is due to all the particles in the cell between the cell walls and the electrode. The distance over which the silver particles will diffuse has been calculated to be 80 pm in 150 s, using a diffusion coef-... 

To resolve the difficulties described above, spectroelectrochemistry is used for monitoring redox processes in proteins. The method combines optical and electrochemical techniques, allowing the electrochemical signals (current, charge, etc.) and spectroscopic responses (UV-VIS, IR) to be obtained simultaneously. As a result, more information about the oxidation or reduction mechanisms of redox proteins can be acquired than with traditional electrochemical methods. Among the various spectroelec-trochemical techniques, in situ UV-VIS absorption spectroelectrochemistry in an optically transparent thin-layer cell has become one of the most useful methods for studying the direct electrochemistry of redox proteins. 

Figure 1 Schematic diagram of spectroelectrochemical techniques at an optically transparent electrode (OTE). (A) Transmission spectroelectrochemistry (B) transmission spectro-electrochemistry with an optically transparent thin-layer electrode (OTTLE) cell (C) internal reflection spectroscopy (IRS). Reprinted by courtesy of Marcel Dekker, Inc. from Heineman WR, Hawkridge FM and Blount HN (1984) Spectroelectrochemistry at optically transparent electrodes. II. Electrodes under thin-layer and semi-infinite diffusion conditions and indirect coulometric titrations. In Bard AJ (ed) Electroanalytical Chemistry. A Series of Advances, Vol 13, pp 1-113. New York Marcel-Dekker.

Thin-layer electrochemistry with an optically transparent electrode (OTE) enables simultaneous monitoring of both the electrochemical and optical responses of the system [1-3]. The oxidation state of the electroactive species in a cell can be precisely controlled by regulating the potential of the OTE and the species in the cell can be completely electrolysed within a short time (typically 20-120s). The electrochemical technique combined with a gold minigrid OTE was first applied to biological molecules to characterize the thermodynamic parameters of the redox reaction of horse heart cytochrome c in the presence of redox mediators [4]. 

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