Spectroelectrochemistry thin-layer cell

Spectroelectrochemistry with a thin-layer cell is used to determine the formal potential of a redox system. The absorption curves in Fig. 9.3 were obtained with a thin-layer cell containing a solution of jTclll(dmpe)2Br2]+ (dmpe = l,2-bis(dimethylphosphine)ethane) in 0.5 M Et4NCl04-DMF [4]. On each curve is... 

Types of electrode/solution interface studied include oxide films on metals, monolayer deposits obtained by underpotential deposition, adsorption, and spectroelectrochemistry in thin-layer cells. 

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. 

Fig. 5.7. Spectroelectrochemical thin layer cell for in situ UV-Vis spectroelectrochemistry with a mini-grid electrode [40]...

Fig. 5.38. Thin layer cell for NIR spectroelectrochemistry in the transmission mode according to [142, 144]...

The field of spectroelectrochemistry dates back to 1964 when the first work was reported by Kuwana, et al. [171]. The authors described the use of indium tin oxide (ITO), coated on glass, as an OTE for UV/Vis-transmission spectroelectrochemical measurements of several inorganic and organic redox analytes. The field has evolved over the years to encompass research on a variety of topics new electrode development, thin-layer cell design, and construction. Additionally, many novel measurement schemes have been developed for material characterization, structure-function studies of biomolecules, and environmental contaminant monitoring [172-175]. Several different OTEs have been used along the way, with the most common type being ITO [173]. 

Multiple minigrids were stacked between spacers to increase the optical pathlength of the cell while retaining thin-layer diffusional distances. This cell was used to demonstrate infrared thin-layer spectroelectrochemistry with ninhydrin. This was chosen for its three carboxyl groups, the reduction of which would be easily observable in the infrared. Another thin-layer cell was constructed with quartz plates in order to illustrate applicability in the UV (40). 

In a conventional luminescence experiment, the detector and excitation source must be maintained at a 90° angle to one another in order to limit the amount of excitation Ught reaching the detector. To achieve this requirement, a square clear sided cuvette is needed as shown in Figure 14.14a. The use of such a square cuvette as a cell for spectroelectrochemistry is not easily adaptable to the thin layer or semi-diffusion cell arrangements conventionally used in UV-vis spectroscopy. Two approaches have been taken to circumvent this problem. First, a thin layer cell comparable to that used in an OTTLE experiment, where the cell is positioned at a 45° angle relative to the excitation source and detector, can be used as shown in Figure 14.14b. 

Raman spectroelectrochemistry has been reviewed in detail (65, 66). The type of cell used for spectroelectrochemistry depends to some extent on the optical layout of the Raman experiment. The main optical layouts in conventional Raman spectroscopy are front incident and collection mode, 180° backscattering, and ATR mode. For most solution phase applications of Raman spectroeleclrochemistry, a three-electrode cell for bulk electrolysis is used and a number of such cells have been described (67). The conventional OTTLE cell described for electronic spectroscopy can be used in Raman spectroelectrochemistry. However, this cell can suffer from solvent interference in non-aqueous media Thin layer cells like those desaibed for IR are also frequendy used (66). 

Provisions are also made in the cell for solution sparging (usually with ultrapure Ni gas). This is because the presence of dissolved air (or O2) causes interference with voltammetric measurements. Thin-layer cells are designed to maximize the electrolysis efficiency and are especially suited for spectroelectrochemistry experiments. These are considered later. 

Figure 12.1 Schematic of the spectroelectrochemistry apparatus at the University of Dlinois. The thin-layer spectroelectrochemical cell (TLE cell) has a 25 p.m thick spacer between the electrode and window to control the electrolyte layer thickness and allow for reproducible refilbng of the gap. The broadband infrared (BBIR) and narrowband visible (NBVIS) pulses used for BB-SFG spectroscopy are generated by a femtosecond laser (see Fig. 12.3). Voltammetric and spectrometric data are acquired simultaneously.

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. 

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 ...

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]. 

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