CHAPTER 3 C ALIXARENE -B ASED M ATERIALS F OR C HEMICAL S ENSORS H. M. Chawla N. Pant S. Kumar D. StC. Black N. Kumar 1. INTRODUCTION Ideally, sensors are considered akin to human sensory perceptions of sight, sound, smell, taste, and touch in more than one way. Both these systems involve receptors (biological, enzymic, or molecular) and evolution of some sort of a relationship between these receptors and the species to be sensed. Such a re- lationship is necessarily a reversible phenomenon to be able to be sustained on a long-term basis (life of a sensor!). Many theories have been proposed for this complex relationship between a sensor and the target species to be sensed, and all of them have their plusses and minuses. Chemical sensors involve molecular receptors that are capable of detecting chemical species and contaminants (cations, anions, and neutral molecules), usually at very low concentrations. Successful sensing is achieved through several modes and materials which form the backbone of sensing operations. This chapter discusses the principles involved in the design, synthesis, and applications of calixarene-based materials for chemical sensors. Analyses of ionic and molecular species using chemicals have been performed since antiquity. The well-known use of phenolphthalein, methyl orange, and other indicators to ascertain the acidic or basic 117