product review
E-noses keep an eye on the future As electronic noses try to shed their bad name, more products and technologies become available. by Elizabeth Zubritsky
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here has been a flurry of recent activity in the field of electronic noses (e-noses)— “sniffing” devices that recognize volatile compounds. Familiar companies have introduced new instruments or changed names or both; newcomers have entered the field; the range of e-nose technologies has expanded; and there has been plenty of talk about new applications, especially medical diagnostics. All of this makes the technology seem mature and the possibilities endless. But this is not yet the case. As Hank Wohltjen of the e-nose company Microsensor Systems (now owned by Sawtek) says, “E-noses have developed a lot of powerful capabilities, but I’m not sure they’ve joined up effectively with the significant problems [yet].” In addition, e-noses are dogged by a bad reputation, which mainly stems from the disappointing performance of some early devices, and there is considerable debate among researchers on some fundamental technical points. This product review will discuss the technologies behind e-noses and some of the recent happenings in the field. A listing of selected commercial instruments can be found in Table 1.
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Table 1. Summary of selected electronic noses. Agilent Technologies 2850 Centerville Rd. Wilmington, DE 19808 302-633-8023 http://www.chem.agilent.com/
Alpha M.O.S. 102 Towne Centre Dr. Hillsborough, NJ 08876 800-257-4249 http://www.alpha-mos.com/
AromaScan (Osmetech) 14 Clinton Dr., Hollis, NH 03049 603-598-2922 http://www.aromascan.com/
CP1, MO, MS, QCM, SAW
BAE SYSTEMS Femtometrics Technology Center 17252 Armstrong Ave., Ste. B Irvine, CA 92614 949-833-2246 http://www.baesystems.com/
MS
CP
SAW
Bloodhound Sensors, Ltd. 175 Woodhouse Ln., Leeds LS2 3AR, United Kingdom +44-113-233-3439 http://www.bloodhound.co.uk/ bloodhound/
Cyrano Sciences 73 N. Vinedo Ave. Pasadena, CA 91107 877-744-1700 http://www.cyranosciences.com/
Electronic Sensor Technology 1077 Business Center Cir. Newbury Park, CA 91320 805-480-1994 http://www.estcal.com/
HKR Sensorsysteme GmbH Gotzinger Str. 56 D-81371 München Germany +49-89-746-0207 http://www.hkr-sensor.de/
CP
PC
SAW, OTH
MS, QCM
Lennartz Electronic GmbH2 Bismarckstrasse 136 D-72072 Tübingen Germany +49-7071-93550 http://www.lennartz-electronic.de/
Marconi Applied Technologies3 4 Westchester Plaza Elmsford, NY 10523 800-342-5338 http://www.marconitech.com/
Microsensor Systems P.O. Box 609501 Orlando, FL 32860 407-884-3300 http://www.sawtek.com/msi/msi.htm
Nordic Sensor Technologies One Exchange Place, Ste. 1000 Jersey City, NJ 07302 201-521-3193 http://www.nordicsensor.com/
MO, QCM, OTH
CP, MO, OTH
SAW
MO, OTH
Quality Sensor Systems Greenfield Business Centre, Greenfield, Flintshire CH8 7GR, United Kingdom +44-1352-718-043 http://www.q-sensors.co.uk/
SMart Nose4 Fleur-de-Lys 9, Case postale 64 CH-2074 Marin-Epagnier, Switzerland +41-32-754-3536 http://www.smartnose.com/
QCM
MS
CP = Conducting polymer MO = Metal-oxide sensor MS = Mass spectrometer OTH = Other PC = Polymer composite QCM = Quartz crystal microbalance SAW = Surface acoustic wave
1 Being phased out. 2 Distributed in the U.K. by GSG Analytical Instruments, St. James Ct., Wilderspool Causeway, Warrington, WA4 6PS, +44-1925-418-044, http://www.gsg-analytical.com/. 3 Formerly EEV Chemical Sensor Systems and Neotronics. 4 Distributed in the U.S. by Microanalytics, 2713 Sam Bass Rd., Round Rock, TX 78681, 512-218-9873.
Selecting an e-nose To choose an e-nose, a user needs to know something about the application. For in situ analyses, there are instruments of varying portability, notably the new handheld instruments. At the time this article was written, two handheld devices were on the market, and at least one more was on the way. In addition to their portability, handheld e-noses also tend to be simpler, more automated, and less expensive (~$5000 vs up to $120,000), says Steve Sunshine of Cyrano Sciences. Some vendors are even trying to make “turnkey” systems that would appeal to users who would not normally conduct chemical analyses.
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A user also must understand the complexity of an application. Generalpurpose e-noses are fine for binary decisions (e.g., does this sample conform to the standard—yes or no?) in controlled circumstances, says Julian Gardner of the University of Warwick (U.K.). Gardner is currently on the scientific advisory board of Cyrano Sciences and previously developed prototype instruments for two other companies. “In this case, you can tolerate a relatively large amount of drift and noise,” he explains. “And if you don’t even need a standard—if you are just asking, ‘Has something changed?’—then that’s even easier.” The more sophisticated instruments,
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on the other hand, draw on user-defined libraries—which are similar to libraries of IR spectra—perhaps identifying a fragrance from a roster of 10 choices. These instruments may also build libraries to track changes in the samples over time. Tony Ricco, who worked for 14 years on chemical sensors at Sandia National Laboratories and is now at ACLARA BioSciences, points out that people “tend to pick the platform that they’re accustomed to using and then see how well it works for the application at hand.” Instead of doing that, he suggests trying to understand the sensor technologies at a relatively fundamental level to
product review
determine which is most likely to respond to particular analytes. Unfortunately, there aren’t any industry-wide standards to help users evaluate e-noses (though devices for medical diagnostics will have to meet certain standards). Instead, e-nose companies release application-specific technical notes, which describe the sampling of the headspace, the running conditions, and the typical results. This approach is useful, says Gardner, but to allow direct comparisons among instruments, people are now trying to create stable synthetic standards (to test detection capabilities) and standard output databases (to test the pattern-recognition capabilities). Although these standards will be much simpler than many of the true analytes, Gardner says, “I think the electronic nose will really come of age when we have standards in place.”
E-nose technologies Strictly speaking, an e-nose uses an array of chemical sensors to detect analytes and pattern-recognition software to check the resulting chemical signature, or “fingerprint”. Resistive sensors—which can be divided into metal oxides, conducting polymers, and polymer composites—are the best known sensor technologies, but there are other choices. Although most e-noses use only one type of sensor, some companies combine various kinds to give enoses a broader applicability. Metal oxides. Metal-oxide sensors are most sensitive to gases that can act as reducing agents, including many odorless gases, such as H2, CO, and methane, and some pungent gases, such as H2S. Because of their sensitivity to H2 and methane, these sensors are good for browning or cooking processes, Gardner says. However, oxidizing species, such as ozone and oxides of nitrogen, may interfere with the signals generated by the reducing gases or may cause their own signals. That’s because the metal oxide and the oxygen in the atmosphere react to create chemisorbed oxygen sites on
the sensor surface, and these gases says, which may make them simpler may alter this equilibrium. to use. In addition, anything that can be QCM and SAW. In both surface combusted on the metal oxide can be acoustic wave (SAW) and quartz crysdetected, Gardner says. To get comtal microbalance (QCM) sensors, bustion, metal-oxide sensors run at change is measured in frequency high temperatures (usually 175–425 rather than resistance. In addition, °C). The reaction yields byproducts, both get their selectivity from a film such as CO2 and water, and induces that coats the surface, and both can charge transfer. Selectivity—and, in quantify the amount of analyte. In many cases, higher sensitivity and simple cases, QCM sensors detect the lower operating temperature—in sorption of an analyte as an increase these sensors is achieved by adding in the mass of the film, which leads to palladium or platinum as catalysts for a shift in the resonant frequency of the reactions, Ricco adds. the quartz. However, in more compliConducting polymers. The conduc- cated cases, there are other changes— tivity of these materials is a sensitive such as swelling, molecular rearrangefunction of the polymer’s structure, ment, or deformation—that are not which, in turn, can be affected by any- analyte-specific. thing that dissolves into the polymer. In SAW devices, the film can be “We’re not exactly sure how they composed of either liquid or rubbery work,” admits Gardner. “[But] we polymers, and the sorption of anadon’t believe there’s any charge trans- lytes softens the film in addition to fer.” Instead, these sensors seem to making it heavier. These changes involve complex charge carrier interalter the speed and power of an actions within the polymer chains, he acoustic wave as it travels across the explains. In practical terms, conductsensor, and the response is typically ing polymer sensors react to larger proportional to the amount of vapor polar molecules, such as ketones, alde- sorbed. hydes, and alcohols. The hydrophilic MS. Recently, vendors have also conducting polymers are also sensitive started to make so-called headspace to water and, by extension, humidity. or fingerprint MS instruments, which Polymer composites. In polymer recognize complex headspaces using composites, the swelling of the polymer in response to an anaTo select an e-nose, you need to lyte is separated from the chemiunderstand the application. cally resistive function, says Nate Lewis of the California Institute of Technology. pattern analysis. Although they do (Lewis’s polymer composite technolo- not meet the traditional definition gy has been licensed by Cyrano Sciof an e-nose, some vendors consider ences.) “We do that by using somethem e-noses. Vendors note that these thing as simple as particles of silver for instruments respond to all volatile comthe conducting phase . . . and off-the- pounds and provide structural inforshelf polymers for the swelling funcmation. And unlike traditional e-nose tion,” he explains. This way, both sensors, these systems are not affected functions can be optimized independby water or alcohol. However, MSently. Varying the type of polymer based systems cannot distinguish beprovides diversity. Composites yield tween optical isomers and are subject sensors with very linear responses, he to tuning inconsistencies.
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Sensitivity, in itself, has no value. An e-nose must have selectivity.
Optical. Although optical technology is not yet used in any commercial instruments, the company Illumina, Inc., has licensed it from David Walt of Tufts University. In his systems, the sensing elements are 6000–10,000 3-µm beads, which are dispersed among wells etched into the tip of an optical fiber. The beads are made from various polymers, all of which include a solvatochromic dye. The dye fluoresces at different wavelengths, depending on the polarity of the polymer immobilized within. This property enables the dye to report on the change in polarity once a vapor penetrates the polymer. There also may be nonlinear effects— changes in fluorescence due to the swelling of the polymer upon the sorption of vapor. In these systems, many identical beads can be made at once, so the sensors can be replaced without retraining the array, Walt says. In addition, it’s possible to look at temporal, rather than equilibrium, responses to the vapor.
Drift and humidity All e-noses are subject to drift, but the problem has received the most attention in the resistive sensor instruments. In particular, drift falls into two categories: sensor drift, which is due to the aging or degradation of individual sensors, and system drift, which encompasses all sensors. Some e-noses are designed to monitor both kinds of drift so that users know when sensors need to be replaced. In some instruments, individual sensors can be replaced, but in others, the entire array must be replaced. In all cases (except optical sensors), the e-nose must be retrained afterward. Gardner points out that some socalled drift is really a consequence of the sensor environment. For example,
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1 or 2 years, depending on the underlying chemistry and the sensor’s environment. The second reason is variations in sampling, he says. To compensate, some e-noses use autosampler systems that control the temperature, shake the samples, and perhaps, equilibrate the headspaces. Although this yields better reproducibility, these systems can be large and expensive, Ricco notes. However, Wohltjen adds that some companies have put a lot of effort into miniaturizing sampling systems recently. Selectivity is another big issue, Gardner says. “Many people focus on sensitivity, but sensitivity, in itself, has no value,” he says. “The question is: Can you discriminate the [relevant] odors?” An odor, he explains, is a complex headspace with anywhere from one to hundreds of compounds at levels varying from thousands of ppm down to
