Optically transparent thin layer cells

Figure 3.13 Thin-layer controlled-potential coulometry of chlorpromazine (CPZ) oxidation in an optically transparent thin-layer cell. Ej = 250 mV Es = 575 mV Ef = 250 mV vs. SCE. 4.8 x 10-4 M CPZ, 3 M H2S04, and 3 M H2S04 alone. [From T. B. Jarbawi, Ph.D. dissertation, University of Cincinnati, 1981.]...

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.

Optically Transparent Thin-Layer Cell (OTTLE)... 

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. 

Method Abs, chemical reduction, monitored by absorption spectroscopy CD, chemical reduction, monitored by CD spectroscopy CD/OTTLE, electrochemical reduction using an optically transparent thin layer (OTTLE) cell, monitored by CD spectroscopy CV, cyclic voltammetry EPR, chemical reduction, monitored by EPR. 

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).

Fig. 6. UV-visible spectra of 0.05 mM oxidized and reduced recombinant Rhodnius NP3 (a) at pH 7.5 without ligand (b) at pH 7.5 bound to NO (c) at pH 5.5 bound to NO. In each case, the spectrum of the oxidized nitrophorin is represented by a solid line and the reduced by a dashed line. Spectra were recorded in an optically transparent thin-layer electrochemical cell of approximate window thickness 0.05 mm. To obtain the fully oxidized and reduced spectra, potentials (vs Ag/AgCl) were applied until no change in optical spectrum occurred, of -1-600 and —400 mV, respectively (a), -1-200 and —400 mV, respectively (b), and 0 and -280mV, respectively (c).

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.]...

The question of whether electrons added to a complex ion become localized or delocalized is important, not only for the type of complex mentioned above, but also for much wider ranges of complexes. For such studies the use of an OTTLE (optically transparent thin layer electrochemical) cell is most appropriate... 

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. 

Heineman WR, Norris BJ, Goelz JF (1975) Measurement of enzyme H° values by optically transparent thin layer electrochemical cells. Anal Chem 47 79-84... 

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-... 

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 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. ... 

OTTLE cell Optically transparent thin-layer electrochemical cell... 

Figure 6.6 UV-Vis spectroscopic changes during electrochemical oxidation of (16-TMC)Ru(C=C-C6H4C1-4)2 in an optically transparent thin-layer electrolysis cell (0.1 M Bu4PF6/ CH2CI2 solution). Reproduced with permission from ref 57.

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 ...

One of the main advantages of the optically transparent thin-layer spectroelectrochemical technique (OTTLSET) is that the oxidized and reduced forms of the analyte adsorbed on the electrode and in the bulk solution can be quickly adjusted to an equilibrium state when the appropriate potential is applied to the thin-layer cell, thereby providing a simple method for measuring the kinetics of a redox system. The formal potential E° and the electron transfer number n can be obtained from the Nernst equation by monitoring the absorbance changes in situ as a function of potential. Other thermodynamic parameters, such as AH, AS, and AG, can also be obtained. Most redox proteins do not undergo direct redox reactions on a bare metal electrode surface. However, they can undergo indirect electron transfer processes in the presence of a mediator or a promoter the determination of their thermodynamic parameters can then... 

Fig. I, Optically transparent thin-layer electrochemical cell shown in (A) front view and (B) side view, a, Point of suction application to change solution b. Teflon tape spacers c, microscope slides (1x3 in.) d, solution e, transparent gold minigrid electrode f, optical path g, reference and auxiliary electrodes h, solution cup. Epoxy is used to hold the cell together.

To obtain satisfactory results by the OTTLSET, the design of the optically transparent thin-layer electrochemical cell (OTTLEC) is very important. The first OTTLEC was constructed by Murray et al in 1967. Figure 1 shows the structure of this type of cell which contains a gold... 

Figure 7. Small-volume optically transparent thin layer electrochemical cell. (A) Quartz cover plate, (B) Teflon spacer, (C) gold minigrid optically transparent electrode, (D) quartz disc, (E) plastic body, (F) inlet syringe port, (G) Pt syringe needle for auxiliary electrode. Adapted from Reference (40) with permission.

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