Iu2415411546

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