CHAPTER 3 C OMBINATORIAL C ONCEPTS FOR D EVELOPMENT OF S ENSING M ATERIALS R. A. Potyrailo 1. INTRODUCTION Rational design of sensing materials based on prior knowledge is a very attractive approach because it may avoid time-consuming synthesis and testing of numerous materials candidates (Honeybourne 2000; Shtoyko et al. 2004; Njagi et al. 2007). However, to be quantitatively successful, rational design (Newnham 1988; Akporiaye 1998; Ulmer II et al. 1998; Lavigne and Anslyn 2001; Suman et al. 2003; Hatchett and Josowicz 2008) requires detailed knowledge regarding relation of intrinsic properties of sensing materials to a set of their performance properties. This knowledge is typically obtained from extensive experimental and simulation data. However, with the increase of structural and functional complexity of materials, the ability to rationally define the precise requirements that result in a desired set of performance properties becomes increasingly limited (Schultz 2003). Thus, in addition to rational design, a variety of sensing materials, ranging from dyes and ionophores to biopolymers, organic and hybrid polymers, and nanomaterials, have been discovered using detailed experimental observations or simply by chance (McKusick et al. 1958; Pedersen 1967; Bühlmann et al. 1998; Svetlicic et al. 1998; Walt et al. 1998; Steinle et al. 2000; Martin et al. 2001; Hu et al. 2004; Potyrailo and Sivavec 2004). Such an approach in development of sensing materials reflects a more general situation in materials design that is "still too dependent on serendipity" and with only limited capability for rational materials design (Eberhart and Clougherty 2004). Conventionally, detailed experimentation with sensing materials candidates for screening and opti- mization consumes a tremendous amount of time and project cost. Thus, developing sensing materials 159