Organic gas sensors and electronic noses

Smell is one of the five human senses. It is fundamental to people and to many living creatures and by extension it is of great importance for the quality control of online production processes in many mass-production industries. G. Dodd and K. Persaud published the first work regarding the artificial sensing of smells and odours and a few years later Gardner defined the technological solution to an electronic nose as: 'an instrument, which comprises an array of electronic chemical sensors with partial specificity and an appropriate patternrecognition system, capable of recognising simple or complex odours.' In agreement with the definition, a competent electronic nose should be capable of working continuously providing useful information of the samples under test and presenting no fatigue over the measurements. Two key elements should be the base of every electronic nose: a sensor array of cross-selective electrodes and a pattern recognition tool to statistically analyze the samples, which has to be able to point out the similarities and dissimilarities among all the measured samples. These statistical methods are known as chemometrics, which combined with sensors arrays has been recognized by many as a method to enhance the results of sensor systems. One of the most classic sensor arrays with which the first electronic noses were built, and that are still in use, is based on metal oxides. However they present some problems and soit has been necessary to investigate and develop new technologies. These will permit the characterisation of gases, vapours and aromas in a fast, economic and, if possible, online manner, in order to generalise the use of electronic noses in many distinct areas and applied technologies. Sensors using semiconducting polymers have been used since the 1970s due to their favourable features. Recently, there have been a number of further developments in the field of organic semiconductors, which suggests that devices based on these may become even more relevant in the near future. This chapter discusses about semiconducting polymers, organic polymers used in gas sensors, their recognition principles and about adsorbent organic conducting polymer composites. Three configurations are used for organic sensors: chemiresistors, transistors (and diodes) and optodes. Some applications in which these sensors are used are also described, such as humidity, ammonia, nitrogen oxide, hydrogen chloride and nitroaromatic explosives.