Sandwich-type electrode

As discussed in section 2.2, a mixture of AMP and AA showed two solid condensed phases above and below about 30 mN m- [5,10]. A loosely stacked structure of two porphyrins was proposed for LB films prepared at higher surface pressures than 30 mNmr1, which was caused by squeezing-out of a monomolecular structure formed at lower surface pressure [5,10]. In this section, photoelectric characteristics of LB films containing AMP and AA deposited at two solid condensed phases will be discussed in relation to multilayer structure and the anisotropic intermolecular tunneling rates [87]. Seven monolayers of 1 5 or 1 10 mixture of AMP and AA were deposited at 20 and 50 mN m-1 on an ITO plate at 18 °C to form stable Y-type LB films. Aluminum was vacuum evaporated onto LB films as sandwich-type electrodes at 10-6 Torr. Steady photocurrents were measured in a similar manner as mentioned above. 

Electro-optical modulators are other examples whose efficiency is enhanced in the presence of ion-radicals. These devices are based on the sandwich-type electrode structures containing organic layers as the electron/hole-injecting layers at the interface between the electrode and the emitter layer. The presence of ion-radicals lowers the barrier height for the electron or hole injection. Anion-radicals (e.g., anion-radicals from 4,7-diphenyl-l,10-phenanthroline—Kido and Matsumoto 1998 from tetra (arylethynyl) cyclooctatetraenes—Lu et al. 2000 from bis (1-octylamino) perylene-3,4 9,10-bis (dicarboximide)s— Ahrens et al. 2006) or cation-radicals (e.g., cation-radicals from a-sexithienyl—Kurata et al. 1998 l,l-diphenyl-2-[phenyl-4-A/,A- /i(4 -methylphenyl)] ethylene— Umeda et al. 1990, 2000), all of them are electron or hole carriers. 

Recently, we have studied a variety of sandwich type electrodes for development of enzyme electrodes based on oxidases. In this paper, we focused on extending linearity and blood-compatibility of the outer membrane, i.e., polycarbonate (PC) membrane, without affecting the selectivity. We modified the outer surface of this membrane by plasma polymerization of hexamethyldisiloxane (HMDS) in a glow-discharge reactor or by coating with polyvinylalcohol (PVAL). The PVAL coated surfaces were further treated with an anticoagulant, i.e., heparin. We studied the linearity, selectivity and blood-compatibility of these membranes and related electrodes. Here, we present the results of these studies. 

Translation of this technology to sol-gel electrochemical sensing faced two challenges. First it was not simple to produce thin silicate films without enzyme denaturation. Two ways were devised to solve this problem. The first involved encapsulation of the enzymes in the interface between a solid electrode support and the sol-gel film—a configuration that is sometimes referred to as sandwich type electrodes. The second involved the production of conposite materials such as carbon CCEs and metal CCEs or by incorporation of latex beads or inorganic or organic polymers in silica, alumina, titania or other metal oxides. 

SCHEME 3 The electrochemical gene sensing system based on the formation of complementary sandwich-type complex, (a) Target DNA combines the ferrocenyl ODN with the probe ODN on the electrode. Redox currents due to the surface-confined ferrocenyl units should reflect the concentration of the target, (b) Ferrocenyl units are not deposited onto the electrode using nontarget DNA. 

Several types of experimental magnesium-air cells were tested. These cells varied in their size (the working area of the air electrodes used) [10]. The current-voltage curves of an experimental Mg-air cell with two air electrodes (Sair = 80 cm2) with pyrolyzed CoTMPP catalyst and sandwich-type Mg anode (MA8M06) operating in NaCl-electrolytes with different concentrations are presented by Figure 2. 

The sensing microzone of the flow-through sensor depicted in Fig. 5.9.B1 integrates gas-diffusion and detection with two analytical reactions [28], viz. (a) the urease-catalysed formation of ammonium ion by hydrolysis of urea (the analyte), which takes places on a hydrophilic enzyme membrane in contact with the sample-donor stream, which contains a gel where the enzyme is covalently bound and (b) an acid-b reaction that takes place at the microzone on the other side of the diffusion membrane and involves Bromothymol Blue as indicator. This is a sandwich-type sensor including a hydrophilic and a hydrophobic membrane across which the sample stream is circulated —whence it is formally similar to some enzyme electrodes. Since the enzymatic conversion of the analyte must be as efficient as possible, deteetion (based on fibre optics) is performed after the donor and acceptor streams have passed through the sensor. Unlike the previous sensor (Fig. 5.9.A), this does not rely on the wall-jet approach in addition, each stream has its own outlet and the system includes two sensing microzones... 

The oxidized forms of these two sandwich-type POMs are stable from pH 3 to at least pH 7. Their characterization by cyclic voltammetry revealed the stepwise reduction of the Gu " " centers within the POMs, before Gu" deposition on the glassy carbon electrode surface [115]. Phenomena are described mainly for the P-derivative and are the same for the As-analog. 

Figure 25 Differential conductance of a sandwich-type tunnel junction of Y-Ba-Cu-0(123) Tc = 60 K, formed with native oxide and a Pb counter electrode. Ref. 89.

Hence, the highest photoelectric sensitivity of dyes can be obtained in (sandwich-type) photoresistor cells in which very thin dye layers (L< 1 mm) are sandwiched between semitransparent electrodes. 

Stability tests performed in sandwich-type cells containing the dyes adsorbed on Sn02/Sb electrodes demonstrated a high stability, with optical density changes lower than 2% after cycling the electrochromic device 20000 times between -0.5 and +0.5 V. 

Figure 9.8 illustrates one practical scheme for using sandwich-type cells. The sample is introduced by capillary action from below or through a side arm from above. Solution contact to the reference and auxiliary electrodes is made... 

Figure 9.7 Three configurations for sandwich-type thin-layer cells. (A) Minigrid suspended between two spacers. (B) Twin-electrode cell using metal films on glass. (C) Single-electrode cell, barrier plate and electrode plate, s, Sample solution.

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.]...

Figure 9.10 Assembly of sandwich-type optically transparent electrochemical cell for extended x-ray absorbance fine structure (EXAFS) spectroelectrochemistry. Cell body is of MACOR working electrode is reticulated vitreous carbon (RVC). [From Ref. 64, with permission.]...

Cell DC-2. Earlier demineralization studies by Lyon (9) employed cell DC-2. This was a sandwich-type cell with Lucite side plates bolted together with two epoxy resin-gasketed graphite electrodes separated by an anion-permeable membrane. The membrane was necessary because a suitable anion-responsive electrode was not then known. The principle of operation is that in the cathode compartment, after several current reversal conditioning cycles, sodium ions are removed by the cathode while chloride ions migrate from the cathode through the membrane to the anion chamber. In the anode chamber, sodium ions, from the previous half cycles, are rejected from the anode. The net result was salt depletion in the cathode chamber and a similar concentration increase in the anode chamber. 

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