Layers chemical sensors

Table 10.1 summarizes the characteristics of common ISEs and a number of new sensors in this field. We have not included in this table the liquid or polymer membrane-based electrodes which are selective, but rather fragile (for more details on such membranes see References 58,59). ISEs of the first kind are not very numerous, e.g., F -ISE (monocrystalhne membrane based on LaFj), Ag" -ISE (silver salts), or Na" -ISE (Na alumino-silicate glass or polyciystalline NASICON [Na super ionic conductor] membranes). Most of the ISEs are of the second kind and are based on insoluble silver salts for example, halide ISEs (CE, Br, I"), Cd ", Pb ", Cu ", etc. Such ISEs use mixtures of insoluble salts based on silver sulfide or silver selenide. Recently, Vlasov etal. and Neshkova have proposed several glasses sensitive to transition metals. Typical ISE devices are shown in Figure 10.5. Thin-layer chemical sensors based on chalcogenide glasses have also been developed. ... 

A unique but widely studied polymeric LB system are the polyglutamates or hairy rod polymers. These polymers have a hydrophilic rod of helical polyglutamate with hydrophobic alkyl side chains. Their rigidity and amphiphilic-ity imparts order (lyotropic and thermotropic) in LB films and they take on a F-type stmcture such as that illustrated in Fig. XV-16 [182]. These LB films are useful for waveguides, photoresists, and chemical sensors. LB films of these polymers are very thermally stable, as was indicated by the lack of interdiffusion up to 414 K shown by neutron reflectivity of alternating hydrogenated and deuterated layers [183]. AFM measurements have shown that these films take on different stmctures if directly deposited onto silicon or onto LB films of cadmium arachidate [184]. 

NANOSCALE LANGMUIR-BLODGETT FILMS AS SENSITIVE LAYERS OF CHEMICAL SENSORS... 

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... 

Square-planar ds platinum complexes could perhaps be used as the chemically sensitive layer for a chemical sensor system. These complexes are robust and form colored solid-state materials that respond spectroscopically to a wide range of volatile organic compounds (VOCs) this process has been named vapochromism. The vapochromic shifts occur in the solid-state UV-vis,... 

Figure 3. First fiber optic chemical sensor (from ref. [17] used for sensing oxygen). Alos shown is a cross-sedction of the fiber bundle used. 6 light source 9 photodectectors 16 chemically sensitive layer.

In practice, surface modifications are restricted to sensors of the ATR- or FEWS-type. For other transducer layouts, the sample - radiation interaction is less localised, making a modification difficult to impossible. Depending on the analytes and the environment of the sensor, two basic surface modification strategies can be used to enhance the function of vibrational spectroscopic optical chemical sensors. The functional layers can either be... 

It is worth stressing that in the use of the coated LPG as a refractometer, and not as chemical sensor, the sPS layer acts just as a passive HRI layer and no chemical interaction with surrounding medium occurs. Therefore, any HRI overlay could be used for the refractometry, not necessarily a chemo-sensitive one as the sPS. However, in this work the same material has been used for both applications taking advantage of the experience ripened in its deposition. 

Ksendzov, A. Homer, M. L. Manfreda, A. M., Integrated optics ring resonator chemical sensor with polymer transduction layer, Electron. Lett. 2004,40, 63 65... 

The FPI principle can also be used to develop thin-film-coating-based chemical sensors. For example, a thin layer of zeolite film has been coated to a cleaved endface of a single-mode fiber to form a low-finesse FPI sensor for chemical detection. Zeolite presents a group of crystalline aluminosilicate materials with uniform subnanometer or nanometer scale pores. Traditionally, porous zeolite materials have been used as adsorbents, catalysts, and molecular sieves for molecular or ionic separation, electrode modification, and selectivity enhancement for chemical sensors. Recently, it has been revealed that zeolites possess a unique combination of chemical and optical properties. When properly integrated with a photonic device, these unique properties may be fully utilized to develop miniaturized optical chemical sensors with high sensitivity and potentially high selectivity for various in situ monitoring applications. 

Plots in Figure 6.3 show leaching and response of pH sensor layers chemically doped starting from 3-(trimethoxysilyl)propylisocyanate (ICPS) and (glycidyloxypropyl)trimethoxysilane (GOPS) derivatized with the pH indicator AF, or physically doped in SiC>2 (from tetra-methylorthosilicate, TMOS) and in 50% phenyl-modified silica. 

Surface acoustic wave (SAW)-type chemical sensors exploit the propagation loss of the acoustic waves along layered structures consisting of at least a substrate covered by the CIM. 

Panasyuk TL, Mirsky VM, Piletsky SA, Wolfheis OS. Electropolymerized molecularly imprinted polymers as receptor layers in a capacitive chemical sensor. Anal Chem... 

The hydrogen sensitivity of palladinm-oxide-semiconductor (Pd-MOS) strnctnres was first reported hy Lnndstrom et al. in 1975 [61]. A variety of devices can he nsed as field-effect chemical sensor devices (Fignre 2.6) and these are introdnced in this section. The simplest electronic devices are capacitors and Schottky diodes. SiC chemical gas sensors based on these devices have been under development for several years. Capacitor devices with a platinum catalytic layer were presented in 1992 [62], and Schottky diodes with palladium gates the same year [63]. In 1999 gas sensors based on FET devices were presented [64, 65]. There are also a few publications where p-n junctions have been tested as gas sensor devices [66, 67]. 

The concepts involved in the eddy correlation technique and the factors involved in the design of appropriate chemical sensors are briefly examined in this chapter. The discussion focuses on surface-layer measurements because it is in this layer that the atmosphere-biosphere interaction is most readily examined. 

One weakness of chemical sensors based on adsorption of analyte at their surface is their poor selectivity, in other words, their vulnerability to interference from other species. Contrast this situation with the sensors having binding sites buried in the bulk of the selective layer. Such sites are accessible only by absorption. 

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