Chemical Sensors for Environmental Monitoring and Homeland Security

35 Chemical sensors consist of a recognition element that is sensitive to stimuli produced by various chemical compounds (analyte) and a transduction element that generates a signal whose magnitude is functionally related to the concentration of the analyte. Chemical sensors also include a special branch referred to as biosensors for the recognition of biochemicals and bio-reactions. The use of biological elements such as organisms, enzymes, antibodies, tissues, and cells as receptors differentiates biosensors from conventional chemical sensors. In general, the chemical sensors are broadly classified into gas, liquid, and solid particulate sensors based on the phases of the analyte. They are further categorized as optical, electrochemical, thermometric, and gravimetric (mass sensitive) sensors according to the operating principle of the transducer. Detailed sub-classification of the sensing schemes along with a comprehensive review of the merits and challenges of each modality can be found in the book authored by Jiri Janata.1 Chemical sensors have become an indispensable part of our technologydriven society and can be found in chemical process, pharmaceutical, food, biomedical, environmental, security, industrial safety, clinical, and indoor monitoring applications to highlight a few.2-6 Like many fields in science, chemical sensors have benefited from the growing power of computers, integrated electronics, new materials, novel designs, and processing tools. Manifestation of such technological Chemical Sensors for Environmental Monitoring and Homeland Security