Optically transparent thin-layer electrodes

An optically transparent thin-layer electrode (OTTLE) study18 revealed that the visible spectra of the reduced forms of [Ru(bipy)3]2+ derivatives can be separated into two classes. Type A complexes, such as [Ru(bipy)3]2+, [Ru(L7)3]2+, and [Ru(L )3]2+ show spectra on reduction which contain low-intensity (e< 2,500 dm3 mol-1 cm-1) bands these spectra are similar to those of the reduced free ligand and are clearly associated with ligand radical anions. In contrast, type B complexes such as [Ru(L8)3]2+ and [Ru(L9)3]2+ on reduction exhibit spectra containing broad bands of greater intensity (1,000 [Pg.584]

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. 

Figure 2.105 Optically transparent thin layer electrochemical (OTTLE) cell. A = PTFE cell body, B = 13 x 2 mm window, (C and E) = PTFE spacers, D = gold minigrid electrode, F = 25 mm window, G = pressure plate, H = gold working electrode contact, 1 = reference electrode compartment, J = silver wire, K = auxiliary electrode and L = solution presaturator. From Ranjith...

Figure 5.10 Redox titration of the Ni-C EPR signal in D. gigas hydrogenase, in the presence of mediators under partial pressure of H2. (A) Titration monitored by EPR spectroscopy (data from Cammack et al. 1982, 1987).The data points were obtained by removing samples from a vessel as shown in Fig. 5.8. Data NiA signal A NiC signal. (B) Titration monitored by FTIR spectroscopy (data from De Lacey et al. 1997).The spectra were recorded directly in a sealed optically transparent thin-layer electrode cell. Note that the oxidized and reduced species, which are undetectable by EPR, can be measured. Data o I946cm (NiB state) 1914+ 1934cm (NiSR state) A 1952cm (NiA state) 1940cm (NiR state).

In situ photoacoustic spectroscopy has been used to study the redox process on the surface of an electrode using copper metal in alkaline solution.1029 The E° values of copper(II) Schiff base complexes1030 absorbed on optically transparent thin-layer electrodes (OTTLE) have been... 

Figure 3.15 Thin-layer chronoabsorptometry of electrogenerated reactant in an optically transparent thin-layer electrode. (A) Variation of the absorbance of the intermediate species (A320 nm) during the electrooxidation of 10 mM 5,6-diaminouracil in Mcllvaine buffer pH 5 at 0.35 V. Arrow indicates time at which all 5,6-diaminouracil has been electrolyzed. (B) Kinetic plot of time versus the absorbance at 330 nm of intermediate species. Absorbance data were taken from curve A. Setting time = 0 at the point where the concentration of 5,6-diaminouracil was zero. [From Ref. 34.]...

Figure 3.16A shows spectra of o-tolidine in an optically transparent thin-layer electrode (OTTLE) for a series of applied potentials. Curve a was recorded after application of +0.800 V, which caused complete oxidation of o-tolidine ([0]/[R] > 1000). Curve g was recorded after application of +0.400 V, causing complete reduction ([0]/[R] < 0.001). The intermediate spectra correspond to intermediate values of Eapplied. Since the absorbance at 438 nm reflects the amount of o-tolidine in the oxidized form via Beer s law, the ratio [0]/[RJ that corresponds to each value of Eapplied can be calculated from the spectra by Equation 3.18. 

Figure 3.16 (A) Spectra recorded during spectropotentiostatic experiment in optically transparent thin-layer electrode on 0.97 mM o-tolidine, 0.5 M acetic acid, 1.0 M HC104. Applied potentials A, 800 B, 660 C, 640 D, 620 E, 600 F, 580 G, 400 mV vs. SCE. (B) Nernst plot at 438 nm. [From Ref. 36.]...

Thin-layer CV is frequently used in conjunction with an optically transparent thin-layer electrode to obtain spectra, E°, and n for redox couples by the spectropotentiostatic technique and thin-layer coulometry, as described earlier in this chapter. CV is used initially to locate the redox couple and give an estimate of E°. Once the CV is obtained, appropriate potentials can be selected for the spectropotentiostatic experiment and potential-step coulometry. A typical thin-layer cyclic voltammogram for the Schiff base complex Co(sal2en) in nonaqueous solvent is shown in Figure 3.34. 

Figure 9.9 Assembly of sandwich-type optically transparent thin-layer electrochemical cell, a, Glass or quartz plates b, adhesive Teflon tape spacers c, minigrid working electrode d, metal thin-film working electrode, which may be used in place of (c) e, platinum wire auxiliary electrode f, silver-silver chloride reference electrode g, sample solution h, sample cup. [Adapted with permission from T.P. DeAngelis and W.R. Heineman, J. Chem. Educ. 53 594 (1976), Copyright 1976 American Chemical Society.]...

A vacuum spectroelectrochemical cell that also contains an optically transparent thin-layer electrode (OTTLE) is shown in Figures 18.16 and 18.17. The cell can function either as a spectroelectrochemical cell employing an OTTLE or as an electrochemical cell for voltammetric measurements. This low-volume cell is useful for UV/Vis spectral studies in nonaqueous solvents when the reduction product is sensitive to traces of molecular oxygen present in the solvent. The cell is physically small enough to fit inside the sample compartment of the spectrophotometer. The performance of such a cell was evaluated from visible spectroscopy and coulometry of methyl viologen in propylene carbonate [45]. 

Figure 18.16 Vacuum electrochemical cells (A) vacuum spectroelectrochemical cell that contains an optically transparent thin-layer electrode (OTTLE) and (B) electrochemical cell assembly. [From Ref. 45, with permission.]...

Methods. All solutions were prepared to be ImM Cytochrome c, 0.1mM DCIP, 0.10M alkali halide, and 0.10M phosphate buffer at pH 7.0 or pD 7.0. The DCIP served as a mediator-titrant for coupling the Cytochrome c with the electrode potential. E° values were measured using a previously described spectropotentiostatic technique using an optically transparent thin-layer electrode (OTTLE) (7,11,12). This method involved incrementally converting the cytochrome from its fully oxidized to fully reduced state by a series of applied potentials. For each potential a spectrum was recorded after equilibrium was attained. The formal redox potential was obtained from a Nernst plot. The n value... 

OTTLE optically transparent thin-layer electrode... 

Fig. 9.13. Construction of two optically transparent thin-layer cells, (a) With minigrid electrode (from Ref. 22 with permission) (b) With semi-transparent tin dioxide electrode, and usable in a flow system.

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. 

Optically transparent electrode — (OTE), the electrode that is transparent to UV-visible light. Such an electrode is very useful to couple electrochemical and spectroscopic characterization of systems (- spectroelectro-chemistry). Usually the electrodes feature thin films of metals (Au, Pt) or semiconductors (In203, SnCb) deposited on transparent substrate (glass, quartz, plastic). Alternatively, they are in a form of fine wire mesh minigrids. OTE are usually used to obtain dependencies of spectra (or absorbance at given wavelengths) on applied potentials. When the -> diffusion layer is limited to a thin layer (i.e., by placing another, properly spaced, transparent substrate parallel to the OTE), bulk electrolysis can be completed in a few seconds and, for -> reversible or - quasireversible systems, equilibrium is reached for the whole solution with the electrode potential. Such OTEs are called optically transparent thin-layer electrodes or OTTLE s. 

Rh( [9]aneS3)2] at -25°C using a UV/vis optically transparent thin-layer electrode confirms the isosbestic interconversion of 3+, 2+, and 1+ cations with loss of intensity of the S M cheirge-transfer bands at... 

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.

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-... 

Figure 3 Spectroelectrochemical cell configurations (1) transmission cell with optically transparent electrode (OTE) (2) transmission optically transparent thin layer electrode cell (OTTLE) with OTE (3) sandwich OTTLE cell with minigrid or reticulated carbon (RVC) electrode (4) long optical path-length cell (LOPTC) with light parallel to electrode surface (5) double transmission reflection cell (6) internal...

Fe -"Fe -") species (NC)4Fe(p-bmtz)Fe(CN)4r 6 ( c= 10 in CH3CN/O.I M Bu4NPF6) from an experiment with an optically transparent thin-layer electrolysis (OTTLE) cell with Pt gauze working electrode is only one form of graphical representation, difference spectra or three-dimensional plots are also being used. ... 

The particular advantage of this optically transparent thin-layer electrode (OTTLE) is that bulk electrolysis is achieved in a few seconds, so that (for a chemically reversible system) the whole solution reaches an equilibrium with the electrode potential, and spectral data can be gathered on a static solution composition. 

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