Selective layer

Figure 6. Bipolar precipitates consisting of an inner anion-selective layer and an outer cation-selective layer.19 When the electrode is polarized to the more noble side, protons and chloride ions are kept from permeating through the film, so that anodic dissolution of the substrate metal is blocked. (Reproduced from N. Sato, Corrosion, 45 354, 1989, Fig. 24 with permission of NACE International.)...

A second option is to apply the membrane on the particle level (millimeter scale) by coating catalyst particles with a selective layer. As a third option, application at the microlevel (submicrometer scale) is distinguished. This option encompasses, for example, zeolite-coated crystals or active clusters (e.g., metal nanoparticles). Advantages of the latter two ways of application are that there are no sealing issues, it is easy to scale-up, the membrane area is large per unit volume, and, if there is a defect in the membrane, this will have a very limited effect on the overall reactor performance. Because of these advantages, it is believed that using a zeolite... 

The signal from continuous chemical sensors is continuous in time. It follows changes in the concentration of the analyte up and down. The signal often originates from the interaction of the analyte with a chemically selective layer... 

As vibrational spectroscopic sensors have a high inherent specificity, selective layers are usually not necessary. Still, sensor modifications can strongly enhance the sensitivity. At the same time, in particular for complex multi-component samples with spectrally interfering analytes, also the sensor... 

The concept of light addressable potentiometric sensors (LAPS) was introduced in 1988 [67], LAPS is a semiconductor-based sensor with either electrolyte-insulator-semiconductor (EIS) or metal-insulator-semiconductor (MIS) structure, respectively. Figure 4.13 illustrates a schematic representation of a typical LAPS with EIS structure. A semiconductor substrate (silicone) is covered with an insulator (Si02). A sensing ion-selective layer, for instance, pH-sensitive S3N4, is deposited on top of the insulator. The whole assembly is placed in contact with the sample solution. 

The working principle of LAPS resembles that of an ion-selective field effect transistor (ISFET). In both cases the ion concentration affects the surface potential and therefore the properties of the depletion layer. Many of the technologies developed for ISFETs, such as forming of ion-selective layers on the insulator surface, have been applied to LAPS without significant modification. 

Samuel, J.I., Sasikaran, Kv Kourosh, K.-Z., Adrian, T. and Wojtek, W., A layered surface acoustic Wave ZnO/LiTaOj structure with a WOs selective layer for hydrogen sensing, Sensor Letters, 1, 33, 2003. 

Silicalite-1 Selective layer PDMS Support PEI NA Thin-film composite fiat sheet Gas separations (e.g., O2/N2) Liquid separations (e.g., ethanol/water separation) [82]... 

Jia and coworkers prepared thin-film composite zeolite-filled silicone rubber membranes by a dip-coating method [82]. The membranes have a thin silicalite-1/ silicone rubber mixed-matrix selective layer on top of a porous polyetherimide support. 

Dual-layer polyethersulfone (PES)/BTDA-TD1/MD1 co-polyimide (P84) hollow fiber membranes with a submicron PES-zeolite beta mixed matrix dense-selective layer for... 

Table 5.1 Chemical composition and charge characteristics of selected layer silicates (McBride 1994)...

This implies that the selective layer of reverse osmosis membranes may have a different origin from that of the micelles. Such a case is clearly identified by examination of the skin structure of cellulose acetate/poly(bromophenylene oxide phosphonate) alloy membranes (1 ), which exhibit a high flux and high salt separation (Figure 13). The skin rests on an assembly of giant spheres (up to 1 pm in diameter) and is certainly originated by a different coagulation mechanism than that of the spheres. 

Table II. Petrographic Composition of Selected layers of Upper Freeport Coal...

We begin by pointing out that this concept of covering an electrode surface with a chemically selective layer predates chemically modified electrodes. For example, an electrode of this type, the Clark electrode for determination of 02, has been available commercially for about 30 years. The chemically selective layer in this sensor is simply a Teflon-type membrane. Such membranes will only transport small, nonpolar molecules. Since 02 is such a molecule, it is transported to an internal electrolyte solution where it is electrochemically reduced. The resulting current is proportional to the concentration of 02 in the contacting solution phase. Other small nonpolar molecules present in the solution phase (e.g., N2) are not electroactive. Hence, this device is quite selective. 

Here asx is the activity of the bound species and ax and as are the activities of the species in the sample and of the binding site in the sensor, respectively. For the purpose of this discussion, the binding site can be thought of as a defined but separate component of the selective layer, such as in heterogeneous selective layers, or it may be a specific part of the uniform matrix, as in homogeneous selective layers. (More on the origins of selectivity are discussed later.) The free energy of interaction for reaction depicted in (1.1) is... 

The response of a sensor to the primary species (X) is described by the response curve (Fig. 1.2). Besides the analyte (X), many other interfering species (/) also interact with, and competitively bind to, the binding sites in the selective layer. Because such interactions are non-specific, the sites occupied by the interferants are expressed as the sum (XasO- Let us assume that only the occupied sites (ao) in or at the selective layer result in the output signal from the sensor. The total available activity of binding sites ( st) in the layer is... 

First, the number of binding sites is much larger for absorption than for adsorption where the maximum surface density is typically 1014 sites cm-2. If the effective thickness of the selective layer is 1 im and a bound molecule occupies a cube (lnm3), the maximum number of binding sites increases from 10 10 mol cm-2 for adsorption up to 10 7 mol cm3 for absorption. 

There are two general classes of selectivity equilibrium-based and kinetically based. Both types depend on specific interactions of the analyte molecule with the selective layer. In addition, there is also physical selectivity based on the highly specific interaction of the molecule with the electrostatic and electromagnetic fields. The topic of selectivity is broad enough to deserve its own chapter. We examine issues pertaining to selectivity in Chapter 2. 

In Chapter 1, we began the discussion of selectivity. In brief review, when an ideal selective layer is exposed to a mixture of molecules, it interacts with those for which the layer is selective and rejects the other, interfering molecules. The selective layer itself can be homogeneous or can contain specific binding sites embedded in a matrix. An outline of the thermodynamics governing the equilibrium binding was given in Section 1.1. 

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