Optically transparent thin-layer spectroelectrochemistry

The other popular approach to in situ spectroelectrochemistry is based on the use of an OTE electrode in a thin-layer, optically transparent thin layer electrode (OTTLE), cell. A schematic representation of one design of OTTLE cell is shown in Figure 2.105. 

While the redox titration method is potentiometric, the spectroelectrochemistry method is potentiostatic [99]. In this method, the protein solution is introduced into an optically transparent thin layer electrochemical cell. The potential of the transparent electrode is held constant until the ratio of the oxidized to reduced forms of the protein attains equilibrium, according to the Nemst equation. The oxidation-reduction state of the protein is determined by directly measuring the spectra through the tranparent electrode. In this method, as in the redox titration method, the spectral characterization of redox species is required. A series of potentials are sequentially potentiostated so that different oxidized/reduced ratios are obtained. The data is then adjusted to the Nemst equation in order to calculate the standard redox potential of the proteic species. Errors in redox potentials estimated with this method may be in the order of 3 mV. 

Figure 28. Configurations for spectroelectrochemistry. A) optically transparent electrode B) optically transparent thin-layer electrode (OTTLE) C) Internal reflection spectroscopy, and D) specular reflectance spectroscopy.

The optically transparent thin-layer electrochemical (or OTTLE) cell has caught on to the greatest extent for UV-vis spectroelectrochemistry (Figure l).1-3 The OTTLE also offers a way to measure both the redox potential and the n-value without requiring knowledge of the electron... 

Figure 17.1.2 A Cell for transmission spectroelectrochemistry involving semi-infinite linear diffusion. Light beam passes along vertical axis. [Reprinted from N. Winograd and T. Kuwana, Electroanal. Chem., 7, 1 (1974), by courtesy of Marcel Dekker, Inc.] B Optically transparent thin-layer system front and side views, (a) Point of suction application in changing solutions (b) Teflon tape spacers (c) 1 X 3 in. microscope slides (d) test solution (e) gold minigrid, 1 cm high ...

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. 

ABTS + production has been described by using thin-layer spectroelectrochemistry [46]. Fifty microliters of ABTS in 0.1 M acetate buffer solution (pH 5) was oxidized in a quartz flat cell (0.1 cm width) containing an optical transparent thin-layer electrode (OTTLE system). The radical formation was measured with a potential scan from 0.65 to 0.70 V and returned to 0.65 V at the scan rate of 0.05 mV s . The reactions were monitored spectrophotometrically every 30 s. Figure 31.11 shows the 3D plots obtained for spectral changes during ABTS electrolysis at different intervals. At the beginning, with ABTS as the sole species, two peaks were observed (A = 214 nm, A.2 = 340 nm), but as the... 

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.

These coated glasses can be used as working electrodes [optically transparent electrodes (OTE)] in standard three-electrode arrangements provided that both glass and coating are chemically and electrochemically stable and inert in the used electrolyte solution and the applied range of electrode potentials. The use of a modified infrared spectroscopy transmission cell equipped with quartz windows for UV-Vis spectroelectrochemistry has been described [18]. Platinum layers deposited onto the quartz served as an optically transparent working electrode and an additional platinum layer served as a pseudo-reference electrode. A counter electrode outside the thin layer zone (in one of the tubes used for solution supply) served as a counter... 

A first spectroelectrochemical setup employing an optically transparent (ITO-coated) electrode in a thin layer three-electrode arrangement was described by Daub et al. [136-138]. Two ITO-coated glass sheets were mounted at a distance of 0.1 mm at a PTFE body. This body was used as the head of an electrochemical glass cell. The counter electrode, reference electrode and the purge gas inlet and outlet were also attached to the cell head. The volume of electrolyte solution in the cell was adjusted to maintain an upper level just reaching the lower edge of the two ITO-electrodes. Capillary action sucked the solution into the tiny gap. The cell was placed in the beam of an UV-Vis spectrometer with the ITO-electrodes positioned perpendicularly in the beam. The CD spectroelectrochemistry of the optically active esters depicted in Fig. 5.31 has been studied [139]. 

According to their size, in particular to their typical dimensions, electrodes are called macroelectrodes with typical dimension (e.g., diameter of a disc-shaped electrode, length of the edge of a sheet electrode) in the range of mm or cm, microelectrodes (with pm), and nanoelectrodes (with nm). Electrodes for particular methods are called rotating disc electrodes (see entry Controlled How Methods for Electrochemical Measurements ), optically transparent electrodes (OTL, see entry UV-Vis Spectroelectrochemistry ), and thin-layer electrodes (TLE for electrolysis with a limited solution volume present as a thin layer of liquid). 

Spectroelectrochemistry at Optically Transparent Electrodes, II. Electrodes under Thin-Layer and Semi-Infinite Diffusion Conditions and Indirect Coulometric Iterations, William H. Heineman, Fred M. Hawkridge, and Henry N. Blount Polynomial Approximation Techniques for Differential Equations in Electrochemical Problems, Stanley Pons Chemically Modified Electrodes, Royce W. Murray... 

Spectroelectrochemistry at Optically Transparent Electrodes, n. Electrodes under Thin-Layer and Semi-Infinite Diffusion Conditions and Indirect Coulometric Iterations, William H. Heineman,... 

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. 

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