Noses, electronic

At the beginning of the 1990s, the term artificial or e-nose appeared and was defined by Gardner and Bartlett in 1993 as an instrument which comprises an array of electronic chemical sensors with partial specificity and an appropriate pattern recognition system, capable of recognizing simple or complex odors.  

A simple odor is usually represented by only or few types of odor molecules, whereas a complex odor is caused by a mixture of hundreds of different types of molecules. Odors (or, scientifically, odorant molecules) are generally light (molecular mass up to 300 Da), small, polar, and often hydrophobic [4]. 

The two main components of an e-nose are the sensing system and the automated pattern recognition system. The sensing system can be an array of several different sensing elements (e.g., chemical sensors), where each element measures a different property of the sensed chemical, or it can be a single sensing device (e.g., spectrometer) that produces an array of measurements for each chemical, or it can be a combination. Each chemical vapor presented to the sensor array produces a signature or pattern characteristic of the vapor. 

Modern instruments can be easily adapted to particular requirement in process-control applications. By adopting a particular sampling strategy, sensor array can be operated in the laboratory as well as on-line for process-contfol or environmental-monitoring applications. In the laboratory, the detector can be used with a headspace autosampler. Adsorbent trapping units are also available for on-line measurement with a gas chromatographymass spectrometry(GCMS) instrument. The different sensors are built into a very small sensor chamber and usually can be ordered for a particular application. In other words, the gas detection devices can be easily adapted to particular requirement in process-control applications. 

The key principle involved in the e-nose concept is the transfer of the total headspace of a sample to a sensor array that detects the presence of volatile compounds in the headspace and a pattern of signals provided that are dependent on the sensors selectivity and sensitivity and the characteristics of the volatile compounds in the headspace [5]. 

The complicated olfactory system in humans and animals can detect and differentiate the presence of an odour even at trace levels [187]. Sensory evaluation is one of the important parameters for environmental monitoring, quality assessment for food, wine and beverages, and clinical diagnosis, as well as for the control of many cosmetics and fermentation processes [188-190]. Typically, sensory evaluation in odour as well as food/ wine testing is performed by a panel of well-trained professionals based upon their sense of smell, taste, experience and mood. However, the human olfactory system is very sensitive but not selective. 

The human nose neither tries to break the aroma into different constituents nor to quantify the constituents. Unfortunately, there is no effective instrumental analysis to replace the human sense. Analytical instruments such as gas chromatography (GC)/mass spectrometry (MS), high pressure liquid chromatography (HPLC) and NMR spectroscopy could be used to monitor the particular compounds present in a variety of samples. However, their use is 

Conducting polymers are highly suited as odour sensing devices  

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. 

---

The electronic nose and electronic tongue can be considered as a specific branch of the development of artificial intelligence and application of the electronic brain. 

Miettinen, S.M. et al.. Effect of emulsion characteristics on the release of aroma as detected by sensory evaluation, static headspace gas chromatography, and electronic nose, J. Agric. Food Chem., 50, 4232, 2002. 

Ampuero, S., Bogdanov, S., and Bosset, J. O. (2004). Classification of unifloral honeys with an MS-based electronic nose using different sampling modes SHS, SPME, and INDEX. Eur. Food Res. Technol. 218,198-207. 

Albone E. (1997). Mammalian semiochemistry chemical signaling between mammals. In Handbook of Biosensors and Electronic Noses (Kress-Rodgers E., ed.). CRC Press, Boca Raton, pp. 503-519. 

DTGS Deuterated triglycine sulfate EN Electronic nose... 

Neural networks are extensively used to develop nonparametric models and are now the method of choice when electronic noses are used to analyze complex mixtures, such as wines and oils.5 Judgments made by the neural network cannot rely on a parametric model that the user has supplied because no model is available that correlates chemical composition of a wine to the wine s taste. Fortunately, the network can build its own model from scratch, and such models often outperform humans in determining the composition of oils, perfumes, and wines. 

Electronic noses." ANNs can take the electronic signals provided by sensors and use these signals to recognize complex mixtures, such as oils, wines, and fragrances. 

ANNs are the favorite choice as tools to monitor electronic noses,8 where the target response may be less tangible than in other studies (although, of course, it is still necessary to be able to define it). Many applications in which a bank of sensors is controlled by a neural network have been published and as sensors diminish in size and cost, but rise in utility, sensors on a chip with a built-in ANN show considerable promise. Together, QSARs and electronic noses currently represent two of the most productive areas in science for the use of these tools. 

Fu, J., et al., A pattern recognition method for electronic noses based on an olfactory neural network, Sens. Actuat. B Chem., 125, 489,2007. 

The electronic nose is an example of an area in which the complexity of the analysis may make it difficult to replace a human with an ES. Electronic noses combine a sensor array with a neural network to make judgments about the composition of complex mixtures, such as fuels, wines, and natural oils. In such tasks, they often beat human noses in accuracy (Figure 7.8). 

Using the principles of biological olfaction, electronic nose systems contain arrays of different types of cross-reactive vapor-sensitive sensors. While it is difficult to discriminate analytes entirely by their responses to a single type of sensor, using an array of sensors yields response patterns that can readily distinguish many different vapors. Ideally, the response mechanisms of the sensors are highly varied and encompass both physical and chemical phenomena1. 

The fiber bundles used for the electronic nose platform are polished and chemically etched with hydrofluoric acid to create an ordered array of micrometer sized wells on the tip of the fiber (Figure 3)4. The etching process takes advantage of the difference in reactivity of the core and the... 

These results demonstrate the ability of the electronic nose to remember the odor of different analytes and to recognize them over extended periods of time. Therefore, we are able to create an odor memory library that can be maintained from one array to another over time10. 

Particular cases are potassium selective potentiometric sensors based on cobalt [41] and nickel [38, 42] hexacyanoferrates. As mentioned, these hexacyanoferrates possess quite satisfactory redox activity with sodium as counter-cation [18]. According to the two possible mechanisms of such redox activity (either sodium ions penetrate the lattice or charge compensation occurs due to entrapment of anions) there is no thermodynamic background for selectivity of these sensors. In these cases electroactive films seem to operate as smart materials similar to conductive polymers in electronic noses. 

Automated baking Multigas sensor, electronic nose R D... 

Fig. 3.32 A complete electronic nose based on gradient microarray technology does not even take up the space of a credit card and is thus applicable in intelligent user products.

A sensor array named the electronic nose is a rapid and relatively simple technique that can be used for monitoring wastewater odors (Stuetz et al., 2000). The electronic nose uses sensors of varying affinities to characterize an odor without reference to its chemical composition. 

Another way for BOD estimation is the use of sensor arrays [37]. An electronic nose incorporating a non-specific sensor array of 12 conducting polymers was evaluated for its ability to monitor wastewater samples. A statistical approach (canonical correlation analysis) showed a linear relationship between the sensor responses and BOD over 5 months for some subsets of samples, leading to the prediction of BOD values from electronic nose analysis using neural network analysis. 

A chemical sensor array (consisting of eight conducting polymer sensors) derived from an electronic nose [62], for the characterization of headspace gas from a sparged liquid sample... 

Chemical sensors are becoming more and more important in any area where the measurement of concentrations of volatile compounds is relevant for both control and analytical purposes. They have also found many applications in sensor systems called electronic noses and tongues. 

The electronic nose and electronic tongue will be described as systems able to give olfactory and chemical images, respectively, in a variety of applications fields, including medicine, environment, food and agriculture. 

In the last decade much effort has been oriented to the fabrication of artificial olfaction machines able to determine chemical images (also odor images) of complex volatile compounds. Today many different electronic noses and tongues are available for odor detection and classification and for the creation of chemical images of liquids. 

Varieties of polymers are also employed as sensitive material for electronic nose applications, and the operating temperature may reach about 100 ° C. In the case of quartz microbalance-based sensors a large role is played by the chemically interactive material (CIM) on which it is deposited. A rather efficient room temperature operating CIM is the metal-porphirin, by which it is possible to construct varieties of nostrils, just changing the type of coordinated metal. Interesting metals success-... 

Future perspectives for the electronic nose research field are listed below. They concern both expected sensing and technical sensing and performance. Improvement of sensing performance of the instrument ... 

---