Electronic Odour Sensing

It is well known that many people have some gaps in their olfactory capability, known as specific anosmias . Furthermore, the human nose tends to tire , or become desensitized, after a short period. Aromas detected initially begin to fade in one s consciousness after varying lengths of time, dependent upon the odours involved. It is not difficult, therefore, to imagine an instrument which is the perfect nose one that detects every odour, does not become desensitized and always give the same description to a specific odour. Unfortunately, it is not that simple  

Human noses do need to tire imagine walking off the busy street into a chemist s shop. One would immediately detect the odour of the perfumes, fragranced toiletries and other goods, but the outside smells would be very much in the background. However, an instrument just inside the door would still be picking up the exhaust and diesel fumes of the passing cars and buses. In many applications of an ideal artificial nose, this needs to be catered for. Currently, the only answer is to ensure no extraneous odours or non-odorants enter the machine. 

Several instruments have emerged in the marketplace and have been popularly described as electronic noses . In reality, these are volatile chemical sensor arrays. To give them more likeness to the human nose (or, more accurately, olfactory system), they are coupled to artificial intelligence systems that require development. The instruments currently available detect most vapours, odorous and non-odorous, including water vapour (to which they are all highly sensitive). Sensitivity to other volatiles, however, is highly variable, dependent 

---

The instruments have been compared to a musician with a manuscript. If we see the musical score of, say, Sibelius s Finlandia, it might be difficult to imagine the associated sound unless we are musicians. Electronic odour sensing seeks to transcribe, as it were, a sensory stimulus for one sense (olfactory) to another (visual). In addition, this transcription is required to be measurable and sufficiently reproducible to allow comparison. The analogy with music continues. Just as with a musical score, the transcription cannot reproduce all of the attributes for example, the precise speed of performance, the balance of the instruments and so on only artificial reproduction in the form of a recording can do that. In the same way, electronic odour sensing can only depict some of the attributes of a fragrance. 

J.W. Gardner and P.N. Bartlett, Pattern recognition in odour sensing. In Sensors and Sensory Systems for an Electronic Nose, Vol. 212, Kluwer Academic Publishers, Dordrecht, 1992, pp. 161-179. 

These electronic noses are for specific purposes and there is presently no universal nose that can solve all odour sensing problems. There is thus a need to develop specific electronic nose technology appropriate for the application. This means developing sensors, materials and appropriate pattern-recognition methods. There is thus a wide scope for the development of an artificial nose, based on conducting polymers, that can mimic the human nose. 

Gas sensors are meant to detect volatile compounds when related to the human body these can be odour of urine, armpit sweat or exhaled breath. Seesaard et al. (2015) report on an embroidered sensor that functions as a wearable electronic nose, also referred to as e-nose . Functionalized single-wall carbon nanombe sensing materials were deposited onto interdigitated electrodes that were previously embroidered with electroconductive yam on a fabric substrate (Fig. 2.9). 

The principles of field effect devices with special regard to chemical sensing will be briefly described in the next section. It will be demonstrated how such devices can be used in sensor arrays eventually together with other types of gas sensors to make electronic noses for the identification, classification and analysis of gas mixtures (or odours) [5-7]. A truly microanalytical system, an "olfactory camera" based on a field effect transistor array is also suggested. 

---