Sandwich films, configuration

Immobilized cholesterol oxidase (COD) and HRP on tetraethyl orthosilicate-derived sol-gel films A fast response time (5 s) and a lower limit of cholesterol of 0.5 mM was detected when a physically entrapped enzyme sandwich sol-gel film configuration was used (cholesterol detection between 2 and 10 mM) [11]... 

FIGURE 1.97. Typical voltammetric responses observed for a 1 1 mixed-valent poly-[0s(bpy)2(vpy)2](C104) film in a two-electrode sandwich electrode configuration subjected to slow (---) and fast (---) potential sweeps, surface coverage, 1.3 X 10 molcm. Both the potential bias A and the intersite potential are shown in Fig. 1.97. The curve at fast sweep rates is well-described by Eqn. 450. (Adapted from Ref. 149.)... 

Mainly, three approaches have been used to immobilize the enzyme on transducer or electrode surface, single layer, bilayer, and sandwich configurations [69, 98], In some studies enzymes are covalently linked with sol-gel thin films [99], Sol-gel thin films are highly convenient for fast, large, and homogeneous electron transfer [17]. With an increase in gel thickness the signal decays and diffusion of analytes to biomolecule active site becomes difficult eventually these factors lead to poor response. By employing thin films various biosensors such as optical and electrochemical biosensors have been reported. 

Linear-dichroic spectra of SA monolayers prepared from mixtures of OTS and a cyanine-dye surfactant established the absence of dimerization and the orientation of the chromophore parallel to the substrate [183]. In contrast, the same cyanine dye underwent sandwich-type dimer formation in LB films and had its chromophore oriented perpendicular to the water surface [192]. These results highlight an important difference between LB and SA monolayers. Parameters which determine monolayer formation on an aqueous subphase are also responsible for the orientation and organization of the surfactants therein. Furthermore, the configuration of the surfactants is retained regardless of the structure of the substrate to which the floating monolayer was subsequently transferred to by the LB technique. Conversely, in SA monolayers, surfactant organization is primarily dependent upon the nature of the substrate [183]. 

Fig. 62. Configuration of magnetisation and magnetic moment in the sandwich TbCo/NdCo/TbCo film in zero magnetic field (a) and during the (de)magnetisation processes (b-e).

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.

The devices used for the EL and PL measurements were fabricated in the thin film sandwich configuration anode/polymer/cathode. The electronic structure of alkoxy derivatives of PPV has been studied via internal field emission [42], internal photoemission [76], cyclovoltammetric spectroscopy [159]) and photoelectron spectroscopy [160]. The data indicate that the bottom of the Tr -band and the top of the ir-band are at 3 eV and 5 eV respectively, with respect to the vacuum. Thus, relatively good hole and electron injection can be achieved by using transparent indium/tin-oxide (ITO) as the anode and calcium (Ca) or barium (Ba) as the cathode [42,161]. In practice, however, hole injection is very sensitive to the quality of the ITO. A thin layer (= 300 A) of conducting polymer provides excellent, reproducible hole injecting contacts [70,71,162]. 

Polymer membranes have also been used as a "sandwich". In this configuration, the liquid film is supported between two polymer membranes. Ward (18) used two silicone rubber membranes to contain a solution of ferrous ions in formamide. Ward noted that Bernard convection cells could be maintained if the complex were formed at the upper surface. Ward (19) used this same system and membrane configuration to study electrically-induced, facilitated gas transport. The silicone rubber membranes provided the mechanical support so the electrodes could be placed next to each liquid surface. Otto and Quinn (20) immobilized the liquid film in a horizontal layer between two polymer films. The polymer was described as an experimental silicone copolymer having high CO2 permeability as well as excellent mechanical properties. They were studying CO2 facilitated transport in bicarbonate solutions with and without carbonic anhydrase. 

The emission spectra of non-crystalline tetracene films have been interpreted in terms of emission from a molecular pair in a sandwich configuration.32 Substitutional and surface fluorescence in pentacene-anthracene crystals33 and the changes in luminescence of single crystals of pure anthracene, pure tetracene, and pentacene-doped tetracene as a result of electrode-induced changes in electrical charge carriers 34>35 have been examined. Hot bands in the fluorescence and phosphorescence of coronene have been analysed.38... 

Ferrite thin films can be included in integrated circuits where an inductor is required. A metallic coil is deposited on an epitaxial YIG film by sputtering, with a meander pattern. To increase the effective inductance value, a second ferrite film is placed on top of the coil, in a sandwich configuration (Arai et al, 1991). Plating techniques (see Section 3.6.3) have also been used to deposit ferrite thin films on GaAs integrated circuits (Abe et al, 1988). 

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