Advances in Electronic - Nose Technologies

* G.Sujatha1, N. Dhivya2, K. Ayyadurai3 and D.Thyagarajan4 1Assistant Professor (Electrical Engineering), College of Food and Dairy Technology, Koduvalli, Chennai-52. 2PG Student (M.Tech, Food Technology), College of Food and Dairy echnology,Koduvalli,Chennai-52. 3Retd. Professor and Head, Department of Veterinary Biochemistry, Madras Veterinary College, Chennai 7 4Dean, Faculty of Food Sciences, College of Food and Dairy Technology, Koduvalli, Chennai-52.

ABSTRACT: Food quality analysis is one among the most array of electronic chemical sensors with partial important, complex and challenging discipline in the specificity and appropriate pattern recognition food sector. Due to its strict interaction with the system, capable of recognizing simple or complex quality of life it is extremely important to improve odors”. the performances of the methods in the field of food quality analysis. Now a days electronic noses have 2. HISTORICAL MILESTONES IN provided a plethora of benefits to a variety of DEVELOPMENT OF E- nose commercial industries, including the agricultural, The first real tool for measuring aromas was biomedical, cosmetics, environmental, food, developed by Hartman in 1954. The sensing element manufacturing, military, pharmaceutical, regulatory, was a microelectrode, a simple platinum wire of 0.8 and various scientific research fields using mainly mm in diameter, which measured the flow of current metal oxide(MOS), metal oxide field effect by a sensitive milli voltmeter. Metal oxide transistor(MOSFET), acoustic (bulk (BAW) and semiconductor gas sensors (MOS) were first used surface (SAW) wave) and polymer sensor. These commercially. In 1960‟s MOS were used as sensors has unique advantage of providing fast results household gas alarms in Japan under the names of by identifying, characterizing and quantifying the Taguchi or Figaro. Moncrieff (1961) worked on the target analytes of interests present in the flavor concept that different coatings materials, such as emitted by foods without destructing them. All these polyvinyl chloride, gelatin, and vegetable fats could sensors are highly sensitive and inexpensive. be capable of providing different and complementary However, these sensors face a numerous problems data for the discrimination of simple and complex during operations in different aspects. Among the aromas. His studies were limited to the use of a single four sensors, polymer based on nano material may temperature-sensitive resistor, but postulated that an overcome the problems encountered by the array with six thermistors, provided with six different environment including the temperature and humidity. coatings, could discriminate large numbers of different aromas. The principle of BAW sensors was Key Words: E-nose, metal oxide, acoustic and introduced by King in 1964 with his Piezoelectric polymer sensors, gas sensor, aroma detection. Sorption Detector. Buck et al. (1965) studied the modulation of conductivity as an answer to 1. INTRODUCTION differentiating aromas bouquets, while Dravnieks and Now, many of the food industries are Trotter (1965) used the modulation of contact looking forward for the non destructive food quality potential to monitor aromas. These studies have been analysis techniques. The emerging non destructive considered as a first approach to aromas evaluation food quality analysis techniques are capable of because of the lack of analytical instruments. The evaluating the finished products quality by analyzing hydrogen sensitive PdMOS (palladium metal oxide their sensory outputs which may be in the form of semiconductor) device was developed in 1973 by a flavor, odor, color, texture and taste. Electronic Nose group of Swedish researchers (Lundström et al., (E - nose) seems to be a new electronic device which 1975, 1990 and 1993). The first gas sensor based on a can report on the finished products quality by SAW oscillator was introduced by Wohltjen and analyzing the head space gas which is generally Dessy (1979). Conducting organic polymer (CP) composed of the volatile organic chemicals that sensors have been under development for contributes to the typical flavor for the different food approximately 10 years (Hodgins, 1997) since materials. 1990‟s. CP sensors rely on changes of resistance by The term “electronic nose” was coined in the adsorption of gas. 1988 by Gardner and Bartlett, (1994) who later defined it as “an instrument which comprises an

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G.Sujatha, N. Dhivya, K. Ayyadurai, D.Thyagarajan / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue4, July-August 2012, pp.1541-1546 Table: 1 Sensor Types, sensitivity and their detection principle S.No Sensor Type Sensitive Material Detection Principle

1 Metal Oxides Semi-conducting Doped semi- Resistance change (MOS, Taguchi) conducting metal oxides (SnO2, GaO) 2 Metal Oxide Semiconductor field-effect sensors Catalytic metals Electric field change (MOSFET) 3 Conducting polymer sensors Modified Conducting Resistance change Polymers 4 Acoustic sensors: Surface and Organic or inorganic Mass change (frequency Bulk acoustic wave (SAW and, BAW) film layers shift)

3. BIOLOGICAL AND ARTIFICIAL The E - nose often consists of non-selective OLFACTION sensors that interact with volatile molecules that The human olfactory system is more result in a physical or chemical change that sends a complex and contains thousands of receptors that signal to a computer which makes a classification bind odor molecules and can detect some odors at based on a calibration and training process leading to parts per trillion levels (Breer,1997) and include pattern recognition (Fig. 1). The non-selectivity of between 10 and 100 million receptors (Deisingh et the sensors results in many possibilities for unique al., 2007). Apparently some of the receptors in the signal combinations, patterns or fingerprints olfactory mucus can bind more than one odor (Baldwin et al., 2011).The greatest advantage of molecule and in some cases one odor molecule can using E - nose is that it can be calibrated to be bind more than one receptor. This results in a mind- reliably consistent and can give objective data for boggling amount of combinations that send unique important functions like quality and safety control. signal patterns to the human brain. The brain then These instruments can also test samples that are unfit interprets these signals and makes a judgment and/or for human consumption. The disadvantage of the E - classification to identify the substance consumed, nose is that they are affected by the environment based in part, on previous experiences or neural including temperature and humidity, which can cause network pattern recognition. sensor drift.

Fig: 1 Sequence of events that occurs in order for chemical recognition in air phase compounds to be detected in both biological and artificial olfactory system (Kauer et al., 2003)

and/or chemical interactions occurs when volatile 4. E - nose SENSORS compounds flow over the sensor. A dynamic All types of sensors exhibit interactions with equilibrium develops as volatile compounds are the gas to be measured so that a series of physical constantly being adsorbed and desorbed at the sensor surface (Shiers,P., 1995).The ideal sensors to be

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G.Sujatha, N. Dhivya, K. Ayyadurai, D.Thyagarajan / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue4, July-August 2012, pp.1541-1546 integrated in an electronic nose should fulfill the metal film. To shift the selectivity of a metal oxide following criteria (Demarne, et al. 1992, Mari and film towards different chemical compounds the film Barbi 1992, Bartlett, et al. 1993, Hodgins,1995 and is doped with noble catalytic metals (e.g. platinum or 1997): high sensitivity towards chemical compounds; palladium), or the working temperature is changed low sensitivity towards humidity and temperature; within the range of 50–400°C. Although the selectivity to respond to different compounds present selectivity is also greatly influenced by the particle in the headspace of the sample; high stability; high size of the polycrystalline semiconductor, the MOS reproducibility and reliability; short reaction and sensors are usually less selective than other recovery time; durable; easy calibration; easy to technologies such as CP, BAW, SAW or MOSFET. process data output. Various kinds of gas sensors are However MOS sensors are extremely sensitive to available, but only four technologies are currently ethanol (Mielle, 1996 and Wünsche et al, 1995). used in commercialized electronic noses: metal oxide semiconductors (MOS); metal oxide semiconductor 4.2Metal oxide semiconductor field-effect field effect transistors (MOSFET); conducting transistor sensors (MOSFET) organic polymers (CP); piezoelectric crystals (bulk A MOSFET sensor comprises three layers, a acoustic wave = BAW). Others such as fibre optics silicon semiconductor, a silicon oxide insulator and a (Dickinson, et al.1996, Nakagawa, et al.1997, catalytic metal (usually palladium, platinum, iridium Eguchi, 1992 and Sutter and Jurs 1997), or rhodium), also called the gate. A normal transistor electrochemical (Mari, and Barbi 1992 and operates by means of three contacts, two allow the Baltruschat, et al.1997) are still in the developmental current in (source) and out (drain), and the third acts stage and may be integrated in the next generation of as the gate contact that regulates the current through E - Noses. Such sensors can be divided into two main the transistor. In the MOSFET transistor, the gate and classes: hot (MOS, MOSFET) and cold (CP, SAW, drain contacts are shortcut, giving a diode mode BAW). The former operate at high temperatures and transistor with convenient electronics for operation, are considered to be less sensitive to moisture with characterized by an IV-curve. The applied voltage on less carryover from one measurement to another. the gate and drain contact creates an electric field, Therefore, they should offer the best ratio of drift and which influences the conductivity of the transistor. lifetime to sensitivity (Shiers, 1995). When polar compounds interact with this metal gate, the electric field, and thus the current flowing 4.1Metal oxide semiconductor sensors (MOS) through the sensor, is modified. The recorded The metal oxide coating may be either of the response corresponds to the change of voltage n-type (mainly zinc oxide, tin dioxide, titanium necessary to keep a constant preset drain current dioxide or iron (III) oxide) which responds to (Lundström et al., 1992 and 1993). The metal oxide oxidizing compounds, or of the p-type (mainly nickel semiconductor field-effect transistor (MOSFET) oxide or cobalt oxide) which responds to reducing sensors rely on a change of electrostatic potential. compounds (Mielle, 1996, Huheey, 1983 and As in the coating of MOS sensors, the gate Greenwood and Earnshaw 1988). N-type structure of a MOSFET sensor is either a thick, dense semiconductors increase the reactivity with oxidizing metal film (100–200 nm) or a thin, porous metal film molecules whereas excited p-type promotes reactions (6–20 nm). The thick, continuous metal gate responds with reducing compounds (Huheey, 1983 and almost exclusively to molecules that dissociate Greenwood and Earnshaw 1988). hydrogen on the catalytic metal surface. It is The film deposition technique further implicitly assumed that the insulator is not exposed to divides each sensor type into thin (6–1000 nm) or the ambient molecules. The dissociated hydrogen thick (10–300 µm) film MOS sensors. Film atoms diffuse within microseconds through the metal deposition includes physical or chemical vapour causing a dipole layer at the metal–insulator deposition, evaporation, or spraying for thin films, interface, leading to a potential change in the and screen printing or painting for thick films transistor. Detection of molecules such as ammonia (Demarne and Sanjines 1992). The thin film devices or carbon monoxide is not possible with such a layer offer a faster response and significantly higher since no hydrogen atoms are released (Spetz et al., sensitivities, but are much more difficult to 1992). However, it transpired that the latter manufacture in terms of reproducibility. Therefore, compounds respond well when the metal gate is commercially available MOS sensors are often based thinned. The selectivity and sensitivity of MOSFET on thick film technologies. Due to the high operating sensors may be influenced by the operating temperature (typically 200–650°C), the organic temperature (50–200°C), the composition of the volatiles transferred to the sensors are totally metal gate, and the microstructure of the catalytic combusted to carbon dioxide and water on the metal (Lundström et al, 1975 and 1993). MOSFET surface of the metal oxide, leading to a change in the sensors, like MOS sensors, have a relatively low resistance (Wünsche et al., 1995). sensitivity to moisture and are thought to be very The mechanism is based on an oxygen robust. However, high levels of manufacturing exchange between the volatile molecules and the

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G.Sujatha, N. Dhivya, K. Ayyadurai, D.Thyagarajan / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue4, July-August 2012, pp.1541-1546 expertise are necessary to achieve good quality and as chromatographic stationary phases, lipids or any reproducibility. (Schaller et al.,1998). non-volatile compounds that are chemically and thermally stable (Guilbault and Jordanel, 1988, 4.3 Conducting organic polymer sensors (CP Nieuwenhuizen and Nederlof, 1992, Holmberg , Sensors) 1997). Conducting organic polymer (CP) sensors When an alternating electrical potential is are like MOS sensors, rely on changes of resistance applied at room temperature, the crystal vibrates at a by the adsorption of gas. These sensors comprise a very stable frequency, defined by its mechanical substrate (e.g. fibre-glass or silicon), a pair of gold- properties. Upon exposure to a vapour, the coating plated electrodes, and a conducting organic polymer adsorbs certain molecules, which increases the mass such as polypyrrole, polyaniline or polythiophene as of the sensing layer and hence decreases the a sensing element. The polymer film is deposed by resonance frequency of the crystal. This change may electrochemical deposition between both electrodes be monitored and related to the volatile present previously fixed to the substrate (Mielle, 1996 and (Hodgins, 1997). The crystals may be made to vibrate Amrani et al., 1995). in a bulk acoustic wave (BAW) or in a surface As the conducting polymer is grown out of a acoustic wave (SAW) mode by selecting the solution, the deposited film contains cation sites appropriate combination of crystal cut and type of balanced by anions from the electrolyte and the electrode configuration (Wünsche et al, 1995). solvent residue (Wünsche et al., 1995 and Hodgins et BAW and SAW sensors differ in their al., 1995 and 1997).The cation sites probably consist structure. BAW are 3-dimensional waves travelling of polar ions or bipolar ions which are small regions through the crystal, while SAW are 2-dimensional of positive charge in the polymer chain providing waves that propagate along the surface of the crystal mobile holes for electron transport. at a depth of approximately one wavelength When a voltage is passed across the (Nieuwenhuizen et al, 1992 and Holmberg , 1997). electrodes, a current passes through the conducting These devices are also called „quartz crystal micro- polymer. The addition of volatile compounds to the balance‟ (QCM or QMB) because, similar to a surface alters the electron flow in the system and balance, their responses change in proportion to the therefore the resistance of the sensor (Shiers, 1995). amount of mass adsorbed. BAW sensors vibrate with The volatiles may interact at least with (i) the a frequency of 10–30 M Hz. Their thin coating (1 polymer itself, (ii) the counter ion, or (iii) the solvent µm–10 nm) is deposited by spin coating, airbrushing (Hodgins, 1997). Therefore, good selectivity in the or inkjet printing (Wünsche et al., 1995, Mielle, 1996, CP sensors may be achieved by altering one of these and Holmberg, 1997). parameters or the electrical growth of the polymer The manufacturing technique includes coating. photolithography and airbrushing, and is fully In general, these sensors show good compatible with planar integrated circuits, sensitivities, especially for polar compounds. fabrication, especially planar silicon technology. This However, their low operating temperature (< 50°C) enables SAW structures and conditioning circuits to makes them extremely sensitive to moisture (Shiers, be incorporated on the same silicon substrate, 1995). Although such sensors are resistant to resulting in robust and inexpensive SAW sensors poisoning (Zannoni, 1995), they have a lifetime of (Caliendo et al., 1992). only about 9–18 months. This short life may be due Since piezoelectric sensors may be coated to the oxidation of the polymer, or to exposure of the with an unlimited number of materials, they present sensor to different chemicals that may develop the best selectivity (Mielle, 1996 and Hodgins, 1997). contact resistances between the polymer and the However, the coating technology is not yet well electrodes. Unlike MOS sensors, the CP sensors are controlled, which induces poor batch- to-batch not yet widely marketed, and laboratory-scale reproducibility (Mielle, 1996 and Wünsche et al, manufacturing renders them expensive. The difficulty 1995). SAW sensors, though limited by the noise of producing good batch- to-batch reproducibility and caused by their high operating frequency, are more a pronounced drift of the response are their main sensitive than BAW sensors. However, both sensors disadvantages (Mielle, 1996) require a higher concentration of volatiles to elicit response levels comparable to other sensor types 4.4 Acoustic sensors (Mielle, 1996 and Hodgins, 1997). The difficulty of Piezoelectric sensors are based on a change integrating BAW and SAW sensors into an electronic of mass, which may be measured as a change in nose resides in the more complex electronics and resonance frequency. These sensors are made of tiny their high sensitivity to disturbances such as discs, usually quartz, lithium niobate (LiNbO3) or temperature and humidity fluctuations (Mielle, 1996 lithium tantalite (LiTaO3), coated with materials such and Wünsche et al., 1995).

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G.Sujatha, N. Dhivya, K. Ayyadurai, D.Thyagarajan / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue4, July-August 2012, pp.1541-1546 Table: 2 Summary of advantages of Electronic Nose Sensor Type S.No Sensor Type Advantages Disadvantage 1 Metal oxides semi- Very high sensitivity, limited High temperature operation, high conducting (MOS) sensing range and rapid response. power consumption, sulfur and weak acid poisoning. 2 Metal oxide field-effect Small sensor size and inexpensive Requires environmental control, Sensors (MOSFET) operating costs baseline drift and low sensitivity to ammonia and carbon dioxide.

3 Conducting polymer Ambient temperature operation, Sensitive to humidity and Sensors(CP) sensitive to many volatile organic temperature and sensor life is compounds, short response time, limited. inexpensive and resistive to sensor poisoning 4 Acoustic sensors: surface High sensitivity, good response time, Complex circuitry, temperature and bulk acoustic wave (SAW diverse sensor coatings, small, sensitive, specificity to analyte and BAW) inexpensive, sensitive to virtually all groups affected by polymeric- gases film sensor coating

5. CONCLUSION 6. Buck, T.M., Allen, F.G., Dalton, M. Although these sensors are sensitive and (1965), Detection of chemical species by inexpensive, they face different kinds of problems, surface effects on metals and especially drift, noise, repeatability, environmental semiconductors. In: Surface Effects In influence (temperature and humidity), poisoning and Detection; Bregmand, J.I., Dravnieks, A., non - linearity in sensor response. Among the four (Eds.), Spartan books inc.: Washington, categories discussed, the application of suitable D.C., USA; 1-27. polymer as a base material may help to overcome 7. Caliendo, C. and Verona, E. (1992) these problems. E - nose, with a nose – on – chip Surface acoustic wave (SAW) Gas sensor. which is a single computer chip containing the In: Gas Sensors, Sberveglieri, G. (Ed.), polymer sensors and processing compounds for Dordrecht: Kluwer Academic Publishers, detection of flavors will suffice the requirements of 281–306 various food industries in monitoring their products. 8. Deisingh, A.K., Stone, D.C and Thompson, M. (2004), Applications of REFERENCES electronic noses and tongues in food 1. Amrani, M.E.H., Persaud, K.C and analysis. Int. J. Food sci. Technol. 39(6), Payne. P.(1995) A high-frequency 587-604. measurements of conducting polymers - 9. Demarne, V., and Sanjinés, R. (1992), Development of a new technique for sensing Thin film semiconducting metal oxide gas volatile chemicals. Measurement Science sensors. In: Sberveglieri, G. (Ed.), Gas and Technology, 6(10), 1500–1507 sensors. Dordrecht: Kluwer Academic 2. Breer, H. (1997) Sense of smell - signal Publishers, 89–116 recognition and transductions in olfactory 10. Dickinson, T. A., White, J., Kauer, J. S. receptor neurons. In: Handbook of and Walt, D. R. A, (1996), Chemical- Biosensors and Electronic Noses: Medicine, detecting system based on a cross reactive Food,Eenvironment; Rogers, E.K, (Ed.); optical sensor array. Nature, 382, 697–700 CRC press: Boca Raton, Fl, USA, 521-532. 11. Dravnieks, A and Trotter, P.J. (1965), 3. Bartlett, P.N., Blair. N and Gardner,J. Polar vapor detector based on thermal (1993) Electronic Nose Principles, modulation of contact potential. J. Sci. Applications and Outlook. ASIC, Instrum., 42, 624-627. 15Ecolloque, Montpellier, 478–486 12. Eguchi, K. (1992) Optical gas sensors. In: 4. Baldwin, E.A., Bai, J., Plotto, A and Gas Sensors, Sberveglieri, G. (Ed.), Dea, S (2011) Electronic noses and tongues: Dordrecht: Kluwer Academic Publishers, Application for the food and pharmaceutical 307–328 industries, Sensors, 11( 1), 4744-4766; 13. Gardner, J.W.; Bartlett, P.N. (1994), A 5. Baltruschat, H., Kamphausen, I., brief history of electronic noses. Sens. Oelgeklaus, R., Rose, J., and Wahlkamp, Actuat. b: chem., 18, 211-220. M., (1997), Detection of volatile organic 14. Guilbault, G. and Jordan, J. M. (1988) solvents using potentio dynamic gas sensors. Analytical uses of piezoelectric crystal: A Analytical Chemistry, 69(4), 743–748

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