INTRODUCTION TO CHEMICAL SENSOR TECHNOLOGIES 15 Table 1.12. Detection thresholds of human nose for some typical odorants in air C OMPOUND Ether Limonene Benzaldehyde Butyric acid Citral Musk xylene O DOR TYPE Ether Lemon Bitter almond Rancid butter Lemon Musky T HRESHOLD 5.8 mg/ml 0.1 mg/ml 3.0 g/ml 9.0 g/ml 3.0 ng/ml 0.8 pg/ml Source: Data from Gardner and Dartlett 1999. that chemical sensors must be extremely sensitive in order to recognize odors as well as the human sensory system does. 4. WHAT DETERMINES SUCCESS IN CHEMICAL SENSOR DESIGN? Despite the large number of chemical sensors already on the market, selection and design of a suitable sensor for a new application is a diffi cult task for the design engineer. Careful selection of the sensing material, the surface platform, and the sampling system is very important because those decisions can determine the specificity, sensitivity, response time, and stability of the final device. Selective func- tionalization of the sensor is also critical to achieving the required operating parameters. Therefore, in designing a chemical sensor, developers have to answer the following questions: (1) Does the application require high sensitivity or a broader range of detection? (2) Can the application's needs be met by careful choice of the operating parameters of the sensors, or will a combination of technologies be needed to sort out the contributions of various similar analytes? (3) Does the application's operating environment require special materials or fabrication procedures? Designing a new sensor requires good understanding of the functioning of various chemical sen- sors, their design, features of exploitation, and modern trends in their development. Multidisciplinary knowledge and very careful multidisciplinary approaches are also needed to achieve good results (see Figure 1.4). Among the sciences needed for the design of chemical sensors, knowledge of materials science is necessary for elaboration of effective technologies. It is required for fabricating appropriate coatings and membranes, synthesis or deposition of sensitive materials, as well as for conducting surface functionaliza- tion and materials modification. In addition, chemical sciences such as interfacial and interphase chem- istry, electrochemistry, molecular chemistry, analytical chemistry, and so on, are necessary for precise understanding of the processes which are the basis on which sensors function. Without knowledge of the biological sciences, including microbiology and molecular biology, protein chemistry, immunology/ enzymology, bioprocessing, etc., it is impossible to understand the main principles of biosensor function and design. The physical sciences provide spectroscopic detection methods (optical, mass, etc.), as well as knowledge necessary for understanding the mechanisms of interaction at gas­solid or liquid­solid interfaces which determine the sensor's signal.