We do it every day… we like it when it is the coffee in the morning and don’t like it when it is a sweating person in the full subway. Our olfactory system detects all the pleasant and unpleasant odours of our environment for 24h every day of our life. However, what seems so simple in our daily life is quite hard to rebuild with electronic-chemical devices, so called electronic noses. In order to smell something, different kinds of odourant molecules needs to be detected and their relative amounts decide about the flavour they are creating. Julian W. Gardner and Philip N. Bartlett published 1994 a review about the history electronic noses. The first electronic noses were already build in the early 60’s but it needed further 20 years in order to improve an intelligent design for odour detection and the first conference for electronic noses was held in 1990. In the human nose, G-binding proteins act as chemosensory receptors in the olfactory cells. The binding of the odour molecules with the receptors triggers a second-messenger signal cascade which ends up in the creation of action potential which are transferred by nerve cells to the brain. In the electronic nose, inorganic semiconducting materials such as oxides and catalytic metals have been used as sensors for odour molecules. For example, the electronic nose of Wilkens and Hatman (1964) was based on redox reactions of the odourants at an electrode. The sensors are enough for odour detection, but for odour classification, the electric signal created by the receptors needs to be analysed by the usage of pattern recognition (PARC) engine or supervised learning artificial networking technique. The main problems in building electronic noses are the conflicts between sensitivity, selectivity and life duration. The sensitivity of the sensors depends on environmental properties such as temperature and humidity. Moreover, it is difficult to distinguish between the different odour molecules and reactive species decrease the life time of the sensors. However, the receptor cells in the human nose also have a low sensitivity, low specificity and short life duration (22 days). It is the subsequent neuronal processing which increases the sensitivity (by three orders magnitude) and offers us the probability to distingue between several thousand odours. So the solution for the electronic nose problems is not located in the odour sensors but the computation network behind it. A brief history of electronic noses
Julian W. Gardner and Philip N. Bartlett Sensors and Actuators B: Chemical 18.1 (1994): 210-211.
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