Research article A review of gas sensors employed in electronic nose applicationsK. Arshak E. Moore G.M. Lyons J. Harris and S. Clifford The authors K. Arshak, E. Moore, G.M. Lyons, J. Harris and S. Clifford are all based at the College of Informatics and Electronics, University of Limerick, Limerick, Ireland. Keywords Sensors, Gases Abstract This paper reviews the range of sensors used in electronic nose (e-nose) systems to date. It outlines the operating principles and fabrication methods of each sensor type as well as the applications in which the different sensors have been utilised. It also outlines the advantages and disadvantages of each sensor for application in a cost-effective low-power handheld e-nose system. Introduction The human nose has been used as an analytical tool in many industries to measure the quality of food, drinks, perfumes and also cosmetic and chemical products. It is commonly used for assessing quality through odour and this is carried out using sensory panels where a group of people fills out questionnaires on the smells associated with the substance being analysed. These sensory panels are extremely subjective as human smell assessment is affected by many factors. Individual variations occur and may be affected by physical and mental health as well as fatigue (Pearce et al., 2003). For this reason, gas chromatography and mass spectrometry have been employed to aid human panels to assess the quality of products through odour evaluation and identification and also to obtain more consistent results. However, these assistive techniques are not portable, they tend to be expensive and their performance is relatively slow (Nagle et al., 1998). The solution to the shortcomings of sensory panels and the associated analytical techniques is the electronic nose (e-nose). E-nose systems utilize an array of sensors to give a fingerprint response to a given odour, and pattern recognition software then performs odour identification and discrimination. The e-nose is a cost-effective solution to the problems associated with sensory panels and with chromatographic and mass-spectrometric techniques and can accommodate real time performance in the field when implemented in portable form. Electronic access The Emerald Research Register for this journal is available at www.emeraldinsight.com/researchregister The current issue and full text archive of this journal is available at www.emeraldinsight.com/0260-2288.htm Principle of operation of e-nose systems The e-nose attempts to emulate the mammalian nose by using an array of sensors that can simulate mammalian olfactory responses to aromas. The odour molecules are drawn into the e-nose using sampling techniques such as headspace sampling, diffusion methods, bubblers or pre-concentrators This work was conducted as part of a collaborative project between AMT Ireland, University of Limerick and University College Cork, and is funded by Enterprise Ireland under project ref. no. ATRP/ 2002/427 (Intelli-SceNT). Sensor Review Volume 24 · Number 2 · 2004 · pp. 181–198 q Emerald Group Publishing Limited · ISSN 0260-2288 DOI 10.1108/02602280410525977 181 Arshak. referred to as the response time while the time it takes the sensor to return to its baseline resistance is called the recovery time. These changes are transduced into electrical signals. 2000). Lyons. (1) Differential: the baseline xs(0) is subtracted from the sensor response xs(t) to remove any noise or drift dA present. The odour sample is drawn across the sensor array and induces a reversible physical and/or chemical change in the sensing material. such as ´ nyi. which causes changes in its output signal until the sensor reaches steady-state. The next stage in analysing the odour is sensor response manipulation with respect to the baseline. Moore. The time during which the sensor is exposed to the odorant is Figure 1 Comparison of the mammalian olfactory system and the e-nose system ¼ xs ðt Þ 2 xs ð0Þ ð 1Þ (2) Relative: the sensor response is divided by the baseline.A review of gas sensors employed in electronic nose applications Sensor Review Volume 24 · Number 2 · 2004 · 181–198 K. The three most commonly used methods as defined by Pearce et al. normalised 182 . which causes an associated change in electrical properties. (2003) are as follows. The e-nose system is designed so that the overall response pattern from the array is unique for a given odour in a family of odours to be considered by the system. Clifford (Pearce et al. The baseline manipulated response ys(t) is determined by: ys ðt Þ ¼ ðxs ðt Þ þ dA Þ 2 ðxs ð0Þ þ dA Þ E-nose sensor response to odorants The response of e-nose sensors to odorants is generally regarded as a first order time response.M. The odorant is finally flushed out of the sensor using the reference gas and the sensor returns back to its baseline as shown in Figure 2.. Harris and S. y s ðt Þ ¼ xs ðt Þð1 þ dM Þ xs ðt Þ ¼ xs ð0Þð1 þ dM Þ xs ð0Þ ð2Þ (3) Fractional: the baseline is subtracted from the response xs(t) and then divided by the baseline xs(0) from the sensor response which provides a dimensionless. The sensor is exposed to the odorant. which are preprocessed and conditioned before identification by a pattern recognition system as shown in Figure 1. This process eliminates multiplicative drift dM and a dimensionless response ys(t) is obtained. This process compensates for noise.. The first stage in odour analysis is to flush a reference gas through the sensor to obtain a baseline. Each “cell” in conductivity (Harsa the array can behave like a receptor by responding to different odours to varying degrees (Shurmer and Gardner. 2003). drift and also for inherently large or small signals (Pearce et al. E. 1992). 2003). G. J. which will be discussed later in this paper. Gopel et al. Johnson. usually calculated from baseline-manipulated data. E. J. Some aspects of these classes of sensors. the structure and layout of conductivity sensors prepared using these materials are essentially the same. adsorption or chemical reactions Conductivity Mass Optical Work function 183 . Table I Physical changes in the sensor active film and the sensor devices used to transduce them into electrical signals Physical changes Sensor devices Conductivity sensors Piezoelectric sensors Optical sensors MOSFET sensors Sensors employed in e-nose systems Gas molecules interact with solid-state sensors by absorption. Moore. 1989. The sensor device detects the physical and/or chemical changes incurred by these processes and these changes are measured as an electrical signal. intrinsically conducting polymers and metal oxides are three of the most commonly utilised classes of sensing materials in conductivity sensors. Sensitivity is the measure of the change in output of a sensor for a change in the input. A schematic of a typical conductivity sensor design is shown in Figure 3. resistance for a change in the concentration of the odorant (x) as shown in equation (4). are explored in the following sections of this review. 1997). 1996. the sensor applications and also the researchers preference. In the case of e-nose sensors. which form the electrical connections through which the relative resistance change is measured. certain manipulation techniques have been shown to be more suitable to certain sensor types and also variations in the manipulation techniques can occur in the literature.e. Arshak. Conducting polymer composite sensors Conducting polymer composites consist of conducting particles such as polypyrrole and carbon black interspersed in an insulating polymer matrix (Albert and Lewis.. in the literature several authors use different values to measure sensitivity.A review of gas sensors employed in electronic nose applications Sensor Review Volume 24 · Number 2 · 2004 · 181–198 K. These materials work on the principle that a change in some property of the material resulting from interaction with a gas/odour leads to a change in resistance in the sensor. The heater is required when metal oxides are used as the sensing material because very high temperatures are required for effective operation of metal oxide sensors. The sensing material is deposited over interdigitated or two parallel electrodes. Clifford Figure 2 E-nose sensor response to an odorant with thin or thick films of the sensor material. This is the standard definition given for the sensitivity of a sensor in several texts on sensor related topics (Fraden. Conductivity sensors Conducting polymer composites. G. However.M. Lyons. Dy S¼ ð4Þ Dx However. The most common types of changes utilised in e-nose sensor systems are shown in Table I along with the classes of sensor devices used to detect these changes. along with several examples from each class. 2000). i. The mechanisms that lead to these resistance changes are different for each material type. response ys(t) that can compensate for inherently large or small signals. however. ys ðt Þ ¼ xs ðt Þ 2 xs ð0Þ xs ð0Þ ð3Þ The choice of baseline manipulation depends on the sensor type being used. Harris and S. the sensitivity of the sensor (S ) to the odorant is the change in the sensor output parameter ( y). Moore. Different techniques have been used to apply the active material onto the substrate. typically a set of exposure cycles is quoted for each analyte. This fractional baseline manipulation is referred to as the maximum relative differential resistance change. The transport properties depend on the fractional free volume (FFV) in the polymer and on the mobility of the segments of the polymer chains. This increase in resistance is consistent with percolation theory. in some cases. both bulk and surface micromachining have also been used to produce wells on silicon substrates using patterned silicon nitride/oxide as insulation and gold electrodes as the metal contacts. initial reference gas. Arshak. as shown in Table IV. The response time depends on the rate of diffusion of the permeant into the polymer. i. 1999). The transducer device is usually a flat substrate with either two parallel electrodes or interdigitated electrodes deposited onto the substrate surface as shown in Figure 3.. However. Harris and S. which are shown in Table II. and on the effects of fillers. E. Clifford Figure 3 Typical structure of a conductivity sensor On exposure to gases.A review of gas sensors employed in electronic nose applications Sensor Review Volume 24 · Number 2 · 2004 · 181–198 K. Changes in the intrinsic conductivity of the polypyrrole particles can also occur if the odour interacts chemically with the conducting polymer backbone. J. General response times for conducting polymer composites are given in Table IV. The vapour-induced expansion of the polymer composite causes an increase in the electrical resistance of the polymer composite because the polymer expansion reduces the number of conducting pathways for charge carriers (Munoz et al. these materials change resistance and this change is due to percolation effects or more complex mechanisms in the case of polypyrrole filled composites.M. The polymer is dropped into the well using a syringe as shown in Figure 4 (Zee and Judy. 2001). plasticisers and temperature (George and Thomas. resistance changes in the polypyrrole-based composites are more difficult to predict (Albert and Lewis. 1999). 2001). milliseconds (Munoz et al. When the polymer composite sensor is exposed to a vapour. 2000). Polypyrrole-based composites have a more complex transduction mechanism because the odour molecules can cause expansion of both the insulating polymer and the polypyrrole particles. The fractional baseline manipulation response is the most common method used to record the sensor response from conducting gas sensors (Pearce et al. permeant and the crosslinking. Therefore. the concentration of the permeant and thickness of the film. analyte and final reference gas exposure times.. However. some of the vapour permeates into the polymer and causes the polymer film to expand. Therefore. The segmental mobility of the polymer chains depends on the extent of unsaturation. The diffusion rate mainly depends on the nature of the polymer. A thin film coating (40-100 nm thick) is then applied across interdigitated electrodes (typical electrode gaps were 15 mm) using dip coating (Freund and Lewis. Examples of the types of substrates and electrodes used are given in Table III along with the electrode dimensions given in the literature. Polypyrrole-based composites are usually prepared by chemically polymerizing pyrrole using phosphomolybdic acid in a solution containing the insulating polymer. 1995). DR/Rb. and. DR is the maximum change in the resistance of the sensor during exposure to the odorant and Rb is the baseline resistance before exposure. Carbon black based composites were prepared by suspending the carbon black in a solution of the insulating polymer in a suitable solvent. The overall composition of this solution is usually 80 per cent insulating polymer20 per cent carbon black by weight. 2003). Lyons. segmental mobility is significantly 184 ..e. G. Response times may vary from seconds to minutes. analytes with higher partition coefficients have higher affinity to sensors with smaller area. Sensors with constant thickness but with different surface areas have the same response when exposed to analytes with moderate partition coefficients. The response is inversely related to the vapour pressure of the analyte at the surface. The equilibrium partition coefficient is essentially the solubility coefficient of the vapour in the polymer (Vieth. reducing the sensor area increases the sensitivity of the sensor towards particular analytes. 20 N/A 50. Flattened or elongated molecules diffuse faster than spherical-shaped molecules (George and Thomas. Moore. J. Chromium Electrode dimensions (mm) N/A 10 £ 10 20 £ 10 0.. Chromium Gold Gold. Lyons.. Therefore the relationship between the partition co-efficient and the sensor geometry is an important factor in optimising polymer composite sensor response (Briglin et al. (1998a. crystallites act as large crosslinking regions in terms of the chains entering and leaving the areas around the crystallite in which diffusion occurs (Comyn. However.5 0. The presence of crystalline domains can have two different effects on the permeability of a polymer. Increased defect formation and a reduction in interlamellar links predominate and overshadow the effect of geometric impedance to such an extent that diffusivity increases despite increasing crystallinity (Vieth. Crystallites are generally impermeable to vapours at room temperature and will reduce the diffusion coefficient. 2001). (1998a) Doleman et al. The size and shape of the penetrating molecule affects the rate of uptake of the vapour by the polymer where increased size of the molecule decreases the diffusion coefficient. (1996) Doleman et al. The partial pressure. the concentration. 1991). In this case.4 Electrode thickness (nm) 50-100 50 30.1 in width 1. Clifford Table II Application techniques used to deposit carbon black based composites Reference Matthews et al. 1991). heat treatment of the composite improved the crystallinity of the matrix PE and resulted in a five-fold increase in the response of the sensor to cyclohexane vapour compared to the untreated sensor (Chen et al. (1998) Severin et al. This is due to the polymer/gas partition coefficient where low vapour pressure gas molecules have a higher tendency to inhabit the polymer and thus can be detected at much lower concentrations (Munoz et al. However. 2002). of the penetrant gas at the gas polymer interface affects the response of the sensor. G. 1999).8 in length Electrode gap (mm) 5 5 5 0. (2002) 185 .M. For example.A review of gas sensors employed in electronic nose applications Sensor Review Volume 24 · Number 2 · 2004 · 181–198 K. E. 15-30 Substrate Glass microscope slides Glass microscope slides Glass microscope slides Micromachined silicon wafer Glass microscope slide Reference Lonergan et al.e. 1985). 2002). i. Table III Electrodes and substrates used in conducting polymer composite sensors Electrode material Gold Gold Gold.86 mm 1 mm N/A Coating application technique Spray coating Spin coating Dip coating Figure 4 Bulk micromachined well as used by Zee and Judy (2001) reduced with increased numbers of double and triple bonds on the carbon-carbon backbone. Harris and S. (2000) Polymer Poly(alkylacrylate) Poly(co-vinyl-acetate) Poly(vinyl butyral) Coating thickness Thin film Thin film Thin film 0. (2002) Severin et al. b) Zee and Judy (2001) Briglin et al. in PE-grafted carbon black chemiresistors. Arshak. Low vapour pressure compounds can be detected in the low ppb range while high vapour pressure compounds need to be in the high ppm range to be detected.. J. 1999). that can be doped as semiconductors or conductors (Heeger. 2001). This is due to the fact that different polymers give different levels of response to a given odour.. Clifford Table IV General response times for conducting polymer composites Response time (s) < 2-4 60 180-240 20-200 Reference Lonergan et al. fast response and short recovery times it is essential that the sensor geometry and all the associated properties of the polymer sensing material be highly optimised. (1) The intrachain conductivity in which the conductivity along the backbone is altered. No heater is required as sensors prepared from these materials operate at room temperature. (1996) Partridge (2000) Corcoran (1993b) Doleman et al. Moore. it can be seen that for high sensitivity. polythiophene and polyaniline. Lyons. This is an important advantage with portable battery powered e-nose systems. 1998). Albert and Lewis (2000) described how the conductivity of doped polyaniline was greatly increased on interaction with ethanol. (1998) which leads to sensor drift.000 kV with sensitivities of up to 100 per cent DR/Rb per mg/ l gas depending on the filler type. (2) The intermolecular conductivity which is due to electron hopping to different chains because of analyte sorption (Charlesworth et al. 1998). Sensors prepared from these materials can operate in conditions of high relative humidity and also show highly linear responses for a wide range of gases (Munoz et al. 2000. The polymer composites show linear responses to concentration for various analytes and also good repeatability after several exposures (Severin. E. composite polymers have operating resistances of approximately 1 to 1. G. The main drawbacks of using conducting polymer composites as e-nose sensors are aging.. 2001). and also these materials are unsuitable for detecting certain gases. particle loading and vapour transport properties (Partridge et al. as shown in Figure 5. The signal conditioning circuitry required for these sensors is relatively simple as only a resistance change is being measured. High discrimination in array sensors can be easily achieved using these materials due to the wide range of polymeric materials available on the market. 186 . The doping of these materials generates charge carriers and also alters their band structure. The physical structure of the polymer also has a major influence on the conductivity (Yasufuku. Conducting polymer composites offer many advantages over other materials when utilised as gas sensors.A review of gas sensors employed in electronic nose applications Sensor Review Volume 24 · Number 2 · 2004 · 181–198 K. Dickinson et al. Intrinsically conducting polymers Intrinsically conducting polymers (ICP) have linear backbones composed of unsaturated monomers. Sensitivity to hydrophobic analytes for conducting polymer composites were found to be one to two orders of magnitude higher than those recorded for intrinsically conducting polymers (Partridge et al. 2000).. The principle of operation for ICP e-nose sensors is that the odorant is absorbed into the polymer and alters the conductivity of the polymer (Albert and Lewis. Harris and S. 2001). Yasufuku (2001) describes them as p electron conjugated polymers where the p symbol relates to the unsaturated structure of the monomer containing an unpaired carbon electron. Zee and Judy. 2000. Conducting polymer composites are also relatively inexpensive and easy to prepare. caused by the hydrogen bonds Generally. 2000). 2000. 1997). From the above discussion. i. Dickinson et al.. 2000).. Arshak. which both induce increased mobility of holes or electrons in the polymer depending on the type of doping used (Albert and Lewis. and (3) The ionic conductivity which is affected by proton tunneling induced by hydrogen bond interaction at the backbone and also by ion migration through the polymer (Albert and Lewis. as a heater would significantly increase the power consumption of the system.e. Conducting polymers such as polypyrrole. alternating double and single bonds along the backbone. Three types of conductivity are affected in intrinsically conducting polymers. are typically used for e-nose sensing. for example carbon-polymer composites are not sensitive to trimethylamine (TMA) for fish odour applications. These conducting polymers can be n-doped or p-doped depending on the doping materials used.M.. Polypyrrole gas sensors were reported to have sensitivities (mV/ppm) ranging from 0. The total charge applied during the polymerisation process determines the thickness of the resultant film. The response of ICP sensors depends on the sorption of the vapour into the sensing material. A potential is applied across the electrode that initiates the polymerisation of the polymer onto the substrate. Generally. E. Guadarrama et al. CHCl3 and NH4 (Sakurai et al. and this affects the electron density on the polymeric chains (Albert and Lewis.26 to 5. 1995). G. sensor geometries and odour molecules. 2000). (b) polypyrrole. Lyons. 30 s (Sotzing et al. (2000) described the sensor characteristics of intrinsically conducting polymers produced by pulsing the potential during polymerisation and observed a 1-50 per cent relative differential resistance change (DR/Rb) for saturated gas. ICP sensors operate at room temperature (Shurmer and Gardner... causing swelling. 2000).8 (DR/Rb for 300 ppm gas for 10 min) for gases such as CH4. Polythiophene and poly(dodecylthiophene) sensors have sensitivities from approximately 0. Electrode thickness can range from 1 to 10 mm and the electrode gap is typically 10-50 mm (Albert and Lewis. 2000).A review of gas sensors employed in electronic nose applications Sensor Review Volume 24 · Number 2 · 2004 · 181–198 K. Problems related to intrinsically conducting polymer sensors include poorly understood signal transduction mechanisms. with interdigitated electrodes. Arshak. rendering direct comparisons between sensors prepared by different researchers difficult if not impractical. J. The polymer is initially deposited onto the electrodes and then grows between them thus producing a complete thin film across the electrode arrangement as the polymerisation reactions progress. Conducting polymers show a good response to a wide range of analytes and have fast response and recovery times especially for polar compounds. difficulties in 187 . Electrochemical polymerisation is carried out using a three-electrode electrochemical cell with the electrodes on the substrate used as the working electrodes (Gardner and Bartlett. 1993a). Moore. 60 s (Pearce et al. (Freund and Lewis. while the final applied voltage determines the doping concentration. Intrinsically conducting polymers are generally deposited onto a substrate..g. 2001).. 2000). Increased discrimination when developing sensor arrays can easily be achieved with these materials as a wide range of intrinsically conducting polymers are available on the market (Albert and Lewis.M. 1992). 2002). 2003) and 180-240 s (Corcoran. Yasufuku. 2002). the reported response times for ICPs vary considerably from seconds to minutes. thereby simplifying the required system electronics. 2000).01 depending on the counter ions used in the polymers films (Fang et al. Clifford Figure 5 Chemical structures of (a) polyaniline. Partridge et al. The sorption properties of these materials essentially depend on the diffusion rate of the permeant into the polymer matrix as explained earlier for composite conducting polymers... 2000. Harris and S. Intrinsically conducting polymers have a number of advantages when used in e-nose systems. using electrochemical techniques or by chemical polymerisation. Sensors with a more simple structure can be prepared where the polymer is deposited between two conducting electrodes (Guadarrama et al. and (c) polythiophene tightening or restructuring the polymer into a more crystalline shape. These response times and sensitivities are reported for an extremely vast variety of materials prepared by various techniques.2 up to 1. e. 1995. and Cl2 as these gases remove electrons and produce holes. 1998). CO. glass. This lowers the potential barrier and allows the electrons to flow. Thick and thin film fabrication methods have been used to produce metal oxide gas sensors..e.M. demonstrated significant increases in sensitivity (Galdikas et al. (Schaller et al. thereby increasing the conductivity. 1998). The same preparative methods are used to apply the catalytic metal (Albert and Lewis. Pearce et al. J. 2000) and the sensor response can drift with time. 2003). for example. Thin film metal oxide sensors saturate quickly. Moore.. 2000. silicon or some other ceramic. the depletion region extends over the entire grain. 2001). Harris and S. i.000 nm for thin film. A heating element is printed onto the back of the substrate to provide the high temperatures required for metal oxides to operate as gas sensors. with thinner films being more sensitive to gases (Schaller et al. This produces large resistance in these areas due to the lack of carriers and the resulting potential barriers produced between the grains inhibit the carrier mobility.. Clifford resolving some types of analytes. There are two types of metal oxide sensors. the lifetime of these sensors can be quite short. The sensitivity can be improved by adding a catalytic metal to the oxide. silver or aluminium electrodes are deposited onto the substrate using the same methods. 1998).. however excessive doping can reduce sensitivity. 2003). The ratio of the grain size (D) to the electron depletion layer thickness (L). Metal oxide sensors The principle of operation of metal oxide sensors is based on the change in conductance of the oxide on interaction with a gas and the change is usually proportional to the concentration of the gas. which can reduce the sensitivity range 188 . n-type (zinc oxide. NO2.. and large variations in the properties of the sensors can occur from batch to batch (Nagle et al. Penza et al... producing charge carriers. 2000. However. typically 200-5008C. spincoating (Hamakawa et al. The effect of doping SnO2 sensors with Cu. Gold. 2001b. 2002). Film thickness ranges from 10 to 300 mm for thick film and 6-1. c).A review of gas sensors employed in electronic nose applications Sensor Review Volume 24 · Number 2 · 2004 · 181–198 K... cobalt oxide) which respond to oxidising gases (Pearce et al. titanium dioxide or iron (III) oxide) which respond to reducing gases and p-type (nickel oxide. Finally. if the sensor is introduced to a reducing gas like H2. R(g) is the reducing gas and g and s are the surface and gas. 1998). There are various electrode designs but the interdigitated structure appears to be the most common approach. P-type sensors respond to oxidising gases like O2.. platinum. typically 9-18 months. E. i. The n-type sensor operates as follows: oxygen in the air reacts with the surface of the sensor and traps any free electrons on the surface or at the grain boundaries of the oxide grains. high sensitivity to humidity (Albert and Lewis. The sensitivity of the metal oxide sensor depends on the film thickness and the temperature at which the sensor is operated.. Lyons. due to oxidation of the polymer (Schaller et al. 2002) or chemical vapour deposition (Dai. The fabrication techniques for these sensors can also be difficult and time-consuming (Nagle et al. The sensitivity of oxide gas sensors ðDR=Rb Þ=cðgasÞ. governs the sensitivity of the sensor (Behr and Fliegel. is calculated with DR ¼ R 2 Rb for oxidising gases and as DR ¼ Rb 2 R for reducing gases where Rb is the baseline resistance and R is the resistance on exposure to the odour and c(gas) is the concentration of the gas (Steffes et al. RF sputtering (Tao and Tsai. CH4. Arshak. 2003).. 1998). C2H5 or H2S the resistance drops because the gas reacts with the oxygen and releases an electron. 1998) onto a flat or tube type substrate made of alumina.e. Catalytic metals are sometimes applied on top of the oxides to improve sensitivity to certain gases. respectively (Albert and Lewis. The metal oxide films are deposited using screenprinting (Schmid et al. 1998). G. tin dioxide.. 1995). The grain size of the oxide also affects the sensitivity and selectivity to particular gases as the grain boundaries act as scattering centres for the electrons (Schaller et al. ranging below D=2L ¼ 1. Equations (1) and (2) describe the reactions occurring at the surface: 1 O2 þ e2 ! O2 ðsÞ 2 R ðgÞ þ O2 ðsÞ ! ROðgÞ þ e ð5Þ ð6Þ Where e is an electron from the oxide. 1995). They also suffer from sulphur poisoning due to irreversible binding of compounds that contain sulphur to the sensor oxide (Dickinson et al.. (Doleman et al. Moore. the sensitivity. The sensitivity of metal oxide sensors is very dependent on the operating temperature. 1992). 1993b). 1998. Harris and S.2 to 1. J. As a result. lithium niobate or quartz. 1998a. Arshak. which results in increased power consumption over sensors fabricated from materials other than metal oxides. Both types of devices work on the principle that a change in the mass of the piezoelectric sensor coating due to gas absorption results in a change in the resonant frequency on exposure to a vapour (Albert and Lewis. and relatively inexpensive to fabricate.. E. Lyons. 2001c). and an ac signal is applied across the input transducer creating an acoustic twodimensional wave that propagates along the surface of the crystal at a depth of one wavelength at operating frequencies between 100 and 400 MHz (Pearce et al. 2000). G.. for In2O3 gas sensors range from approximately 1. the sensitivity per 500 ppm CH4 ranges from 0.6 with the operating temperature varying from 350 to 4508C (Steffes et al. Surface acoustic wave sensor The SAW device is composed of a piezoelectric substrate with an input (transmitting) and output (receiving) interdigital transducer deposited on top of the substrate (Khlebarov et al..7 between 250 and 4008C (Saha et al. 2003). The general response times for tin oxide sensors. no handheld e-nose system has been fabricated utilising sensors prepared from metal oxides (Pearce et al. 2003). The sensitive membrane is placed between the transducers. The SAW device produces a surface wave that travels along the surface of the sensor while the QCM produces a wave that travels through the bulk of the sensor. the surface acoustic wave (SAW) device and the quartz crystal microbalance (QCM). The main advantages of metal oxide sensors are fast response and recovery times. have lower power consumption than thick film sensors and can be integrated directly into the measurement circuitry (Dai. Pearce et al. 2001). 1998). 2003). gases (Schaller et al. Thin film metal oxide sensors are small. which are piezoelectric in nature (Pearce et al. However.. b).. The substrates are normally prepared from ZnO. cv is the vapour concentration.M.. are 5 to 35 s and the recovery times vary from 15 to 70 s (Albert and Lewis. 2003). The response times for tin oxides tend to be very fast and can reach steady-state in less than 7 s. with gas concentrations between 0 and 400 ppm and at temperatures between 250 and 5008C. Kp is the partition coefficient. Nagle et al.. The sensitive membrane is usually polymeric or Figure 6 SAW sensor 189 . For SnO2 gas sensors doped with Al2O3. Figure 6. 1998).. DR=Rb (per 10 ppm NO2). 2000). which mainly depend on the temperature and the level of interaction between the sensor and gas (Penza et al. Corcoran gave values of 20 s for thick film metal oxide sensors and 12 s for thin film metal oxide sensors (Corcoran. 2001). 2000. 1998) and ethanol can also blind the sensor from other volatile organic compound (VOC).2 to 0.. for example. The change in frequency with sorption of a vapour is given by Df ¼ Df p cv K p =rp ð7Þ where Dfp is the change in frequency caused by the membrane.. rp is the density of the polymer membrane used (Albert and Lewis. Clifford in which the sensor can operate. they have many disadvantages due to their high operating temperatures. The mass of the gas sensitive membrane of the SAW device is changed on interaction with a compatible analyte and causes the frequency of the wave to be altered..A review of gas sensors employed in electronic nose applications Sensor Review Volume 24 · Number 2 · 2004 · 181–198 K. Piezoelectric sensors used in e-nose arrays There are two types of piezoelectric sensors used in gas sensing. Organophosphorous compounds have also been detected using these types of devices at concentrations from 10 to 100 ppm at temperatures between 25 and 508C where a sensitivity of 27 Hz/ppm at 308C (Dejous et al.. normally between 10 and 30 MHz (Schaller et al. which makes the fabrication of small sensor structures possible. which results in an increase in its mass. Moore. Sensitivities ranging from 0.. 1998).e. Polymer-coated SAW devices have quite low detection limits. 2000). 2003). E. 1997). The sensor. trichloroethylene and methoxyflurane have been detected at concentrations as low as 0. 1998). SAW devices have also been produced by utilising screen printing techniques (White and Turner. 0. This increase in mass alters the resonant frequency of the quartz crystal and this change in resonant frequency is therefore used for the detection of the vapour (Albert and Lewis.6 and 4 ppm. for example tetrachloroethylene. used an array of SAW devices with polymer coatings 20-30 nm thick to detect 16 different solvents using four different polymers. respectively (Albert and Lewis. SAW devices are produced commercially by photolithography.A review of gas sensors employed in electronic nose applications Sensor Review Volume 24 · Number 2 · 2004 · 181–198 K. however. The sensitivity is normally quoted as differential response.5 up to 12 Hz/mg/m3 have been achieved with these devices (Groves et al. 1998)... shown in Figure 7. 2000). Grate and Abraham. The sensitivity of QCMs is given by Df ð22:3 £ 1026 Þf 2 ¼ Dc A ð 8Þ Figure 7 Quartz crystal microbalance with polymer coating 190 . Generally. Lyons.. 2003). phospholipids and fatty acids deposited using Langmuir-Blodgett techniques have also been used (Albert and Lewis. Clifford liquid crystal. is composed of a quartz disc coated with the absorbing polymer layer and a set of gold electrodes evaporated onto either side of the polymer/ quartz structure.7. Groves et al. Harris and S. SAW devices suffer from poor signal to noise performance because of the high frequencies at which they operate. inkjet printing or dip coating. Coatings can be between 10 nm and 1 mm and are applied using spincoating. airbrushing. G. When an ac voltage is applied across the piezoelectric quartz crystal the material oscillates at its resonant frequency.. The sensitivity of the SAW device to a particular odour depends on the type sensitive membrane and the uptake of vapours by polymers are governed by the same factors as those for composite conducting polymer sensors as detailed earlier in this paper. dip coating and spin coating.. (Schaller et al. Pearce et al. 1986. 1995). As can be seen above. Dfp/ppm of odour (Hierlemann et al. travels through the entire bulk of the crystal. 1991) and they also offer high sensitivity and fast response times and their fabrication is compatible with current planar IC technologies (Nagle et al. The three-dimensional wave produced. where airbrush techniques are often used to deposit the thin films ( Nagle et al. Arshak.. 1998. (Schaller et al... i.M. J. Batch to batch reproducibility is difficult to achieve and the replacement of damaged sensors was also found to be problematic (Schaller et al. SAW devices can detect a broad spectrum of odours due to the wide range of gas sensitive coatings available (Carey et al. A membrane is deposited onto the surface of the crystal and this layer adsorbs gas when exposed to the vapour. Micromachining is used to fabricate the QCM devices.. 2001). 2000). However. Schaller et al.. 1998) approximately 20-30 nm thick (Groves and Zellers. Quartz crystal microbalance sensor Quartz crystal microbalance (QCM) gas sensors operate on the same principle as SAW devices.. 1998) and the circuitry required to operate them is complex and expensive (Pearce et al. 1995. 1998). 2001). The advantages of using QCMs are fast response times. 1998. The sides or tips of the optic fibres (thickness. response times of 30 s to 1 min have been reported (Carey.5 Hz shift for 1 ppm of n-heptane. (Nagle et al. 1992. can be added to the polymer to improve the response by lowering the detection limits of the sensor (Walt et al. Pearce et al. 2003)..2 Hz/mg/l (Mirmohseni and Oladegaragoze. immune to electromagnetic interference (EMI) and can operate in high radiation areas due to Bragg and other grating based optical sensors (Gopel. 1998). typically 10 s (Haug et al. Clifford where f is the fundamental frequency. Moore. 2 mm) are coated with a fluorescent dye encapsulated in a polymer matrix as shown in Figure 8. leading to increased cost.M. 1993). batch-tobatch reproducibility of QCM gas sensors and the replacement of damaged sensors are difficult (Dickinson et al. 2003). A quartz crystal microbalance gas sensor was developed and had a linear frequency shift on interaction with ethanol. These optical changes are used as the response mechanism for odour detection (Grattan and Sun. 2003). such as alumina. 2000. and the sensors have quite a short lifetime due to photobleaching (Nagle et al. 1998). 1992). ( Jin. 1987). The sensitivity depends on the type of fluorescent dye or mixture of dyes and the type of polymer used to support the dye (Nagle et al.5 ppm of ethanol and .. These compact. 2003). However. Adsorbants. changes the dye’s optical properties such as intensity change. Optical sensors used in e-nose arrays Optical fibre sensor arrays are yet another approach to odour identification in e-nose systems.. n-heptane and acetone and showed a 16 Hz shift for 2. E.... The odorant interacts with the material and causes a shift in the wavelength of the propagating light wave 191 . Gopel. 2002) demonstrating their high sensitivity to organic vapours. J. 1986). 1998). However. Walt et al. 1998). QCM devices are sensitive to a diverse range of analytes and are also very selective (Pearce et al. this problem was overcome by measuring the temporal Figure 8 A gas optical fibre sensor. lifetime change or wavelength shift in fluorescence. and poor signal to noise performance due to surface interferences and the size of the crystal (Nagle et al. 1998). 1998. QCM gas sensors have many disadvantages which include complex fabrication processes and interface circuitry. c is the concentration and A is the area of the sensitive film (Carey and Kowalski. porosity and swelling tendency (Nagle et al.5-48... As with SAW devices.A review of gas sensors employed in electronic nose applications Sensor Review Volume 24 · Number 2 · 2004 · 181–198 K. lightweight optical gas sensors can be multiplexed on a single fibre network. hydrophobicity. (Kim. However... Higher frequencies and smaller surface areas of sensitive coatings give rise to higher sensitivity (Carey. Pearce et al. (Nagle et al. less than 10 s for sampling and analysis ( Walt et al. 2001). on interaction with the vapour. 1998). G.. Optical gas sensors have very fast response times. 1998).. Polyvinylpyrrolidone modified QCMs have been used to detect various organic vapours and had sensitivities in the range of 7. Harris and S. however. spectrum change. Arshak. 1987)..000 ppm. there are several disadvantages of these types of sensors.. Polyanaline-coated optical sensors have been used to detect ammonia at concentrations as low as 1 ppm and the linear dynamic range was between 180 and 18. 1998). Lyons. Polarity alterations in the fluorescent dye. The associated electronics and software are very complex. 1. The nature of the polymer controls the response and the most important factors are polarity. 2001). The catalytic metal thickness and the operating temperatures of the MOSFETs in Figure 10 Hybrid suspended gate field effect transistor 192 . Moore. usually a catalytic metal. 2001 used three polymers (poly(ethylene-co-vinyl acetate. In the case of catalytic metals.. 1998). such as hydrogen. Harris and S. These MOSFETs. 2001).9 and 3. 1998).. which were deposited onto the FETs using a spray system. 2000). due to corresponding changes in the work functions of the metal and the oxide layers (Pearce et al. palladium (Pd) and iridium (Ir). respectively (Covington et al. Changes in the drainsource current and the gate voltage have also been used as the response mechanisms for the MOSFET gas sensors as they are also affected by changes in the work function (Albert and Lewis.. It has been observed that the change in the threshold voltage is proportional to the concentration of the analyte and is used as the response mechanism for the gas. E.. 2000. 2000).. 2003). Lyons.. The polymer thickness was between 1.. so diffusion is not necessary and therefore a wider choice of gas sensitive materials can be used (Eisele et al.. a porous gas sensitive gate material is used to facilitate diffusion of gas into the material (Eisele et al. The HSGFET is a metal air-gap insulator (MAIS) device. which Figure 9 MOSFET gas sensor with gas sensitive membrane deposited on top of SiO2 incorporate the deposition of gas sensitive catalytic metals onto the silicon dioxide gate layer (Nagle et al. Polymers have also been used as the gate material for MOSFET gas sensors (Hatfield et al. the gate material is thermally evaporated onto the gate oxide surface through a mask forming 100-400 nm thick films or 3-30 nm thin films depending on the application (Albert and Lewis. 2000). such as platinum (Pt). 2001). a hybrid suspended gate FET (HSGFET) gas sensor can also be fabricated by micromachining (Figure 10). poly(9-vinylcarbazole)) mixed with 20 per cent carbon black.A review of gas sensors employed in electronic nose applications Sensor Review Volume 24 · Number 2 · 2004 · 181–198 K. with certain gases.. The changes in the work functions occur due to the polarization of the surface and interface of the catalytic metal and oxide layer when the gas interacts with the catalytically active surface (Kalman et al.. Nagle et al. Covington et al. Therefore.. 1998).7 mm. The air-gap allows easy access to both the gate material and the insulator. This particular sensor works on the principle that the threshold voltage of the MOSFET sensor changes on interaction of the gate material. alcohols and aldehydes (Kalman et al. Apart from the standard MOSFET gas sensor architecture. poly(styrene-co-butadiene). The MOSFET sensor is a metal-insulator-semiconductor (MIS) device and its structure is shown in Figure 9 (Eisele et al. Pearce et al. Gas sensing MOSFETs are produced by standard microfabrication techniques. Clifford responses because these remain consistent despite photobleaching (Walt et al. which use polymers as the sensitive membrane are more commonly called PolFETs and can operate at room temperature.M. Metal-oxide-semiconductor field-effect transistor sensor and its use in the e-nose The metal-oxide-semiconductor field-effect transistor (MOSFET) is a transducer device used in e-nose to transform a physical/chemical change into an electrical signal. 2003).. J. the metalinsulator interface has to be accessible to the gas. Thick films have been used to detect hydrogen and hydrogen sulphide while thin films are more porous and can detect amines. In order for the physical changes in the sensor to occur. G. 2000). Arshak. 2001. UK University of Rome University of Leeds. QCM. PCA Desktop ANN www. food analysis A review of gas sensors employed in electronic nose applications Array Tech University of Manchester Institute of science and technology. consistency in foods and beverages 14 8 IR. spoilage.de/FZK2/english Electronic Sensor Technology Inc. Marconi Applied Technologies PCA Palmtop ANN. 8 MOS. PCA Laptop www.Table V Characteristics of commercial e-noses Pattern recognition/ comments www. SAW 8-28 32 QCM 6 FO – Cyrano Science Inc. MOS.airsense. Moore.hkr-sensor. medical diagnosis ANN Desktop ANN. flavour and fragrance testing Sensor type Applications No. packaging evaluation. air monitoring in textile mills. Sweden K. contamination detection. E. G. Arshak. DA. SAW 22 6-24 10 Food evaluation. of sensors Airsense analysis GmbH Alpha MOS-Multi Organoleptic Systems Applied Sensor Analysis of food types. PCA SPR Desktop PCA www. DFA. fire alarms. environmental monitoring Environmental protection. SAW GC. Clifford AromaScan PLC QCM CP Food evaluation. alcoholic beverages and perfumes Identification of purity. agriculture. PCA Desktop ANN. QCM CP 32 Environmental monitoring.com/ www. USA University of Warwick. cosmetics and perfumes. freshness. chemical quality control.estcal. environmental monitoring. bacteria identification. MOS. Quality control in food production. SAW 1 40.illumina.com Website/reference/university background Manufacturer MOS CP. Harris and S.appliedsensor. DFA. quality control of food storage. pharmaceutical product evaluation Diagnosing lung cancer. flavour and fragrance testing microbiology. J. PCA.fzk. chemical detection Illumina ANN.com Tufts University. PCA Laptop ANN.cyranosciences. analysis of dairy products. USA (Continued ) . Lyons. UK www. fresh fish.com California institute of Technology. CA. Forschungszentrum Karlsruhe Sensor Review Volume 24 · Number 2 · 2004 · 181–198 HKR-Sensorsysteme GmbH Food and beverage quality. MOS. UK ANN. CA. DC.alpha-mos. industrial process control. CP. chemical analysis. explosives and drug detection.com www. environmental analysis. MOSFET. Automotive applications Food and beverages. DA Laptop Bloodhound sensors 193 CP QCM. organic materials. degradation detection Food quality. food processing.de/ Technical University of Munich. pharmaceutical industry Life sciences. process and quality control. Germany www.M.com University of Linkopings. and petrochemical products. CP – conducting polymer.mastiff.uk/ Palmtop 194 OligoSense RST Rostock Raum-fahrt und Umweltschatz GmbH Shimadzu Co. PCA Desktop Website/reference/university background Manufacturer MOS. IR – infra red.lennartz-electronic. Bedfordshire. warning in the event of escape of hazardous substances. PCA Desktop PCA. PCA – principle component analysis.Table V Pattern recognition/ comments ANN. ANN – artificial neural network. Harris and S. CO – conductive oligomer. Lyons.com/pdf/ hazmatcad http://www. Arshak. packaging evaluation Early recognition of fires. Desktop www. leak detection. DFA – discriminant function analysis. process control Sniffs palms for personal identification Sensor type Applications No. QCM. agricultural odours and chemical analysis. QCM – quartz crystal monitor. G. workplace monitoring CP 16 www. SAW MOS 6 Diagnosing tuberculosis. food evaluation. GC – gas chromatography. Diag-Nose ANN. E. CA – cluster analysis.rst-rostock. MOSFET – metal oxide semiconductor field effect transistor.org/ Cranfield University.microsensorsystems. Microsensor Systems SAW CO MOS. DC – distance classifiers. J.co. chemical warfare (CW) agents and toxic industrial chemicals (TICs) analysis Automotive applications.be Antwerp University. Clifford Marconi technologies Mastiff Electronic Systems Ltd. bowel cancers. SAW – surface acoustic wave. of sensors Lennartz Electronic GmbH A review of gas sensors employed in electronic nose applications K. Belgium www.de/ PDF_documents University of Tubingen www.de/ www. infections in wounds and the urinary tract – 6-10 2 Chemical agent detection. QCM 16-40 Food and beverage processing.oligosense. FO – fibre optic.M. DA – discriminant analysis . Moore. England Sensor Review Volume 24 · Number 2 · 2004 · 181–198 Notes: MOS – metal oxide semiconductor. perfume oils.nose-network. M.Table VI Summary of the properties of each sensor type reviewed Fabrication ppb for HVPG ppm for LVPG. (1998) Sensor type Measurand Polymer composites Conductivity Conductivity Screenprinting. Moore. good response times. dipcoating Dipcoating 1 pg to 1 mg of vapour 1 pg mass change. airbrushing. cheap. E. 1 per cent DR/Rb/ppm.1-5 ppm 0.1ppm. chemical polymerisation 5-500 ppm Sensitive to polar analytes. integratible and reproducible Maximum response ¼ 200 mV especially for amines Notes: HVPG – high vapor pressure gas. inkjet printing. spincoating Micromachining. (1998) Suffer from photobleaching. DL # 0. Sulphides) ¼ 0. RF sputtering. suffer from baseline drift High operating temperatures. low cost sensors. (2001a) Poor signal-to-noise ratio. complex circuitry Nagle et al. Albert and Lewin (2000) and Penza et al. cheap Sensitive to temperature and humidity. 1 ng mass change Diverse range of coatings. difficult to reproduce A review of gas sensors employed in electronic nose applications K. G. dipcoating. screenprinting.8 mV/ppm for toluene.5 Hz/ppm. microfabrication Photolithography. light weight Small. J. complex interface circuitry. good response times. good batch to batch reproducibility Nagle et al. spincoating. microfabrication Electrochemical. (1998) and Kim and Choi (2002) Jin et al. CMOS 2. dipcoating. DR/Rb – relative differential resistance change . DL ¼ (amines. DL (NH3)¼ 1 ppm with polyaniline coating QCM Piezoelectricity Sensor Review Optical devices Intensity/ spectrum Screenprinting. Arshak. DL ¼ 2 ppm for octane and 1 ppm NO2 and 1 ppm H2S with polymer membranes 1. thermal evaporation. cheap. fast response times. (2001) and Nagle et al. (1998) SAW Piezoelectricity 195 Low ppb. airbrushing. (2000) Volume 24 · Number 2 · 2004 · 181–198 MOSFET Threshold voltage change Microfabrication. LVPG – low vapor pressure gas. need controlled environment Covington et al. operate at room temperature Fast response and recovery times. thermal evaporation Immune to electromagnetic interference. Harris and S. Clifford Intrinsically conducting polymers Metal oxides Conductivity Nagle et al.1-100 ppm Operate at room temperature. (2001) and Kalman et al. Gardner and Dyer (1997) and Ryan et al. diverse range of coatings Sensitive to temperature and humidity Examples of sensitivity/detection range/detection limits (DL) Advantages Disadvantages Reference Munoz et al. (1999). (1998). spincoating. Lyons. IC integratable Diverse range of coatings. cheap. high sensitivity. spray coating. restricted light sources Baseline drift. (1999) Nagle et al. suffer from sulphur poisoning. limited range of coatings Complex interface circuitry. (1986). pp. pp. pp. 54-74. (2000). Lyons. optical gas sensors and MOSFET gas sensors have been reviewed in this paper.W.. 1998. 2003). 1998. Gas sensing MOSFETs are produced by microfabrication. 1998). Conclusions Conducting polymer composite.. 1998). Vol. G. Behr. “Multicomponent analysis using an array of piezoelectric crystal sensors”. and Lewis.H. Vol. 2000). and Fliegel. Freund. Partridge. Vol. 2001). Maekawa.R. Iwata. and operating temperatures are usually between 50 and 1708C (Kalman et al. The principle of operation. Vol. 1998).1 ppm with Pt. Carey.. Ir and Pd gates and the maximum response was approximately 200 mV especially for amines (Kalman et al.. K.. Carey. B.S. 2001. Analytical chemistry. Analytical chemistry. intrinsically conducting polymer and metal oxide conductivity gas sensors. Nagle et al.. Gu et al. 97. “Electrical properties and improvement of the gas sensitivity in multiple-doped sno2”. P.. 2000). pp. MOSFET sensors have a number of advantages and disadvantages when used in e-nose arrays.R.. the electronic components of the chip have to be sealed because the sensor needs a gas inlet so it can penetrate the gate (Nagle et al.. 1998)... composition and structure of the catalytic metal (Lundstrom et al. References Albert. P. Y. J.. Analytical chemistry. 3077-84. and Yoshida. J. different sensor types depending mainly on the applications. P. 1998). 82 No. (2002). Nagle et al.S.. A. “Mechanistic studies on the interactions between poly(pyrrole) and orgaic vapors”.. advantages. 2003. and Kowalski. Response time varies from device to device and response times from milliseconds up to 300 s have been reported in the literature (Covington et al. P. M.S. 1998.. H..R.W. 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