Chemically sensitive interfaces

Owing to multi-functionahty, physical properties such as solubihty and the glass transition temperature and chemical functionahty the hyperbranched (meth) acrylates can be controlled by the chemical modification of the functional groups. The modifications of the chain architecture and chemical structure by SCV(C)P of inimers and functional monomers, which may lead to a facile, one-pot synthesis of novel functionahzed hyperbranched polymers, is another attractive feature of the process. The procedure can be regarded as a convenient approach toward the preparation of the chemically sensitive interfaces. 

This chapter provides a brief overview of the design, characterization, and function of chemically sensitive interfaces. Chemical sensing is an interdisciplinary problem involving the synthesis of new materials, the characterization of materials and interfaces, the design of new methods of signal transduction, and the development of techniques for microfabrication and miniaturization. This chapter summarizes current research in each of these areas, referring to contributions in the literature and to specific chapters in this book in which recent advances are described. 

Chemical Sensitive Interfaces on Surface Acoustic Wave Devices... 

Surface acoustic wave (SAW) devices have been studied in detail for chemical sensing applications (1-12). Nearly all this work has relied on some sort of chemically sensitive interface, many of which, however, are not particularly chemically selective. Table I summarizes the different classes of materials that have been examined for SAW-based chemical sensing applications, with a few examples in each category. Bearing in mind that SAW devices respond to changes in mass/area, none of the materials in Table I can be claimed to be entirely immune to interference fi om nonspecific adsorption, particularly for interferants vrith vapor pressures significantly below ambient pressure. 

Two Classes of Chemically Sensitive Interface. It is our ultimate goal to select materials from widely different chemical classes— metals, metal oxides, semiconductors, organic polymers, coordination complexes, and organized thin films— to form a chemically sparate array. Because of our fairly extensive experience with metals, oxides, and ordinary polymers, we are currently focusing our efforts on two newer classes of materials, self-assembled monolayers and plasma-grafted polymer films. 

Multifrequency SAW Devices for Chemical Discrimination (17). Each of the numerous physical perturbations to which SAW devices respond has a particular frequency (f) dependence for velocity shifts and, in general, a different frequency dependence for attenuation changes. Examining the frequency-dependent response of a chemically sensitive interface on a SAW device thus provides spectroscopic information. The data we present here are for a combination of mass-loading and viscoelastic perturbations, which involve energy transfer between the SAW device and a thin polymer film (44). 

Figure 3b is a five-frequency set of isotherms for n-pentane. Note the marked difference in the shape of these curves compared to those for TCE (Figure 3a). The principal cause of the difference is pentane s lower density, less than one-half that of TCE for pentane, the plasticization component of the response is dominant and the mass-loading component plays a minor part, so the isotherms are more nearly semicircular. A set of five isotherms for /-propanol (not shown) have their own distinctive set of (nearly linear) shapes, readily distinguishable from those of TCE and pentane. Thus, for a given concentration of a particular analyte, the set of five response points in the attenuation-frequency plane form a relatively unique signature using a nonselective chemically sensitive interface.

While the development of highly selective chemical interfaces for a few key analytes is important, the use of sensor arrays in combination with pattern recognition offers the promise of more general chemical analysis. Two SAW responses per chemically sensitive interface provide more information than single-parameter measurements. While pattern recognition does not require perfect chemical selectivity, it works best when the elements of the array are chemically diverse. The use of new materials such as self-assembled monolayers and plasma-grafted films, that offer relatively simple means to introduce a wide range of chemical functionalities, is one important means to provide such chemical diversity. 

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