Allman, S. L.

Boisen, Anja

Gehl, A. C.

Johansson, Alicia

Pinnaduwage, L. A.

A major research effort has been devoted over the years for the development of chemical sensors for the detection of chemical and explosive vapors. However, the deployment of such chemical sensors will require the use of multiple sensors probably tens of sensors in a sensor package to achieve selective detection. In order to keep the overall detector unit small, miniature sensors with sufficient sensitivity of detection will be needed.We report sensitive detection of dimethyl methylphosphonate DMMP, a stimulant for the nerve agents, using a miniature sensor unit based on piezoresistive microcantilevers. The sensor can detect parts-per-trillion concentrations of DMMP within 10 s exposure times. The small size of the sensor makes it ideally suited for electronic nose applications. © 2007 American Institute of Physics

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L.; Johansson, Alicia Charlotte; Boisen, AnjaPublished in:Review of Scientific InstrumentsLink to article, DOI:10.1063/1.2735841Publication date:2007Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Pinnaduwage, L. A., Gehl, A. C., Allman, S. L., Johansson, A., & Boisen, A. (2007). Miniature sensor suitable forelectronic nose applications. Review of Scientific Instruments, 78(5), 055101. DOI: 10.1063/1.2735841Miniature sensor suitable for electronic nose applicationsLal A. PinnaduwageOak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6122and Department of Physics, University of Tennessee, Knoxville, Tennessee 37996Anthony C. GehlDepartment of Physics, University of Tennessee, Knoxville, Tennessee 37996Steve L. AllmanOak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6122Alicia Johansson and Anja BoisenDepartment of Micro and Nanotechnology, Technical University of Denmark, Building 345E,2800 Kongens Lyngby, DenmarkReceived 8 January 2007; accepted 2 April 2007; published online 2 May 2007A major research effort has been devoted over the years for the development of chemical sensors forthe detection of chemical and explosive vapors. However, the deployment of such chemical sensorswill require the use of multiple sensors probably tens of sensors in a sensor package to achieveselective detection. In order to keep the overall detector unit small, miniature sensors with sufficientsensitivity of detection will be needed. We report sensitive detection of dimethyl methylphosphonateDMMP, a stimulant for the nerve agents, using a miniature sensor unit based on piezoresistivemicrocantilevers. The sensor can detect parts-per-trillion concentrations of DMMP within 10 sexposure times. The small size of the sensor makes it ideally suited for electronic noseapplications. © 2007 American Institute of Physics. DOI: 10.1063/1.2735841A wide variety of chemical sensor platforms, includingsurface acoustic wave SAW, quartz crystal microbalanceQCM, metal oxide, conductive polymer, and microcantile-ver sensors, are being explored for the detection of chemicaland explosive vapors.1,2 Most of the effort so far has beendevoted to the development of suitable chemical coatings forsensitive detection of different chemical and explosive va-pors. However, such individual coatings are not selective fora particular analyte. In order to develop an actual detectorunit that can unambiguously detect a particular vapor, selec-tive detection of such vapors is needed to be achieved. Theproposed “electronic nose”3 involves the use of multiple sen-sors with different coatings in an approach similar to thebiological olfactory system. The olfactory receptors are nothighly selective toward specific odorants; each receptor re-sponds to multiple odorants, and many receptors respond toany given odorant.4 Pattern recognition techniques, includingthose based on artificial neural networks inspired by thebiological olfactory systems, are being investigated for ana-lyzing the data from sensor arrays to identify the presence oftarget analytes in a given vapor mixture.The drive behind the electronic noses is that they areexpected to be smaller and much less expensive compared tothe established detectors based on gas chromatography, massspectrometry, or ion mobility spectrometry and are also ex-pected to be easy to use. All these features make this ap-proach attractive for a variety of applications including waron terrorism and environmental monitoring.5 In order for theelectronic nose concept to be viable, the sensors should havehigh sensitivity, detection in short times, fast recovery, andrequire long term stability of the chemical coatings used onthe sensors, just as for the biological nose.The ability to selectively detect certain analytes incomplex mixtures requires the use of multiple sensors in thesensor array. Therefore, a microelectromechanical systemMEMS-based sensor platform, such as microcantilevers, isideally suited since hundreds of individual sensors can beincorporated in a small area on the order of 1 cm2.5 How-ever, just having a miniature sensor is not enough, since thedetection circuitry can be large.Previously, we developed a nonselective handheld sensorunit that had the ability to detect low concentrations of ex-plosive vapors.6 In that sensor unit, the flow cell containing apair of microcantilevers and the detection electronics re-quired an area of roughly 100 cm2. We have recently devel-oped a sensor package that is roughly an order of magnitudesmaller. In this Note, we report the sensitive detection ofdimethyl methylphosphonate DMMP vapors using thisminiature sensor unit.The sensor package is shown in Fig. 1. The miniatureflow cell containing the microcantilever sensors is located atthe right on the circuit board in the figure. The circuit boardcontains the Wheatstone bridge circuit and all the requiredelectronics. A commercially available universal serial busUSB adapter not shown in the figure allows the sensor tobe connected to a laptop or a desktop computer using a USBcable. Therefore, a data acquisition card is not needed in thecomputer. In the future, this USB-adapter circuit may be in-corporated on the sensor board with the flow cell. It will alsobe possible to incorporate telemetry on the sensor board, thusmaking remote sensing possible.The flow cell contained a microcantilever chip with fourpiezoresistive microcantilevers fabricated with epoxy-basedphotoresist SU-8 which is highly chemically resistant andREVIEW OF SCIENTIFIC INSTRUMENTS 78, 055101 20070034-6748/2007/785/055101/4/$23.00 © 2007 American Institute of Physics78, 055101-1Downloaded 13 Aug 2009 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jspbiocompatible.7 Therefore, it is becoming widely used in mi-cromechanical and microfluidic devices, not only as a pho-toresist but also as the constitutional material of the device.Furthermore, due to the low Young’s modulus of SU-8, ahigh sensitivity can be obtained. The fabrication procedureand the characteristics of the SU-8 microcantilevers used inthis study have been discussed in detail.7 One surface of eachof the four microcantilevers on the chip was coated with a2 nm Ti adhesion layer and a 20 nm gold layer. Only two ofthe microcantilevers on a chip were used in the present study,where a 4-mercaptobenzoic acid 4-MBA self-assembledmonolayer SAM was applied to the gold surface of onemicrocantilever, with the other gold-coated cantilever usedas a reference. The selective application of the 4-MBA SAMcoating on only one microcantilever was achieved by placinga drop of the SAM solution on one gold-coated microcanti-lever by the use of a capillary tube mounted on a microma-nipulator and a stereo microscope. The relative bending ofthe 4-MBA SAM coated cantilever with respect to a gold-coated reference cantilever was monitored using a Wheat-stone bridge circuit.6,7Organophosphates are found as major components inchemical warfare agents such as tabun GA, sarin GB, andVX.8 Recently, possibly due to concern about possible ter-rorist attacks, a number of reports have appeared on the de-tection of the organophosphonate DMMP using a variety ofchemical sensor platforms: metal oxide sensors,9,10 carbonnanotube sensors,11,12 SAW sensors,13 and microcantileversensors.14,15 While some of the required features for an elec-tronic nose are satisfied according to these reports, most ofthem do not satisfy all the requirements in particular, highsensitivity at low parts-per-trillion ppt concentrations, de-tection in several seconds, and reversible within a few min-utes simultaneously.In this Note, we report fast, sensitive, and reversible de-tection of DMMP at low ppt concentrations using a self-assembled monolayer SAM coating. The stability of coat-ings used on the sensors is quite important for electronicnoses, since the repeated application of a “trained” neural netover long periods will be possible only if the sensor responseis repeatable. SAM coatings can be applied with simple pro-cedures and are extremely stable, thus providing repeatableresponses over long periods we have been using some forover three years. Thus, this sensor meets the requirementsfor use in an electronic nose. Furthermore, we used a sensorunit that is truly miniature and which can be used to accom-modate hundreds of individual sensors in the future for se-lective detection, without making the size of the sensor pack-age large.Bertilsson et al.16 has conducted a detailed investigationof the adsorption of DMMP on three SAM coatings withrespective tail groups of–OH, –CH3, and –COOH preparedby the self-assembly of alkanethiols SHCH2mX X= –OH,–CH3, –COOH on gold. The –COOH-terminated surfaceobtained with SHCH215COOH coating yielded the high-est response from DMMP, while the –CH3-terminated sur-face obtained with SHCH215CH3 coating yielded the low-est response.16The apparatus used in the present study is shown in Fig.2. A DMMP permeation tube that was estimated at0.25 ng/min at 30 °C from VICI Metronics, Inc. was usedto generate the DMMP vapor. The permeation tube was lo-cated in a permeation tube holder that was immersed in aheated/refrigerated circulator that was maintained at 30 °C.The glass beads in the permeation tube helped thermalize thecarrier gas quickly. The permeation tube holder and the glassbeads were also purchased from VICI Metronics, Inc. Flush-ing of the DMMP generator for several days was required toachieve a constant rate of delivery of DMMP from the per-meation tube, since initially there is some DMMP on theFIG. 1. Color online The sensor board with the flow cell is roughly 42 cm2 disregarding the vapor inlet and outlet tubing. A US quarter-dollarcoin is also shown for size comparison. Using a commercially availableUSB adapter, the sensor board can be connected to a personal computerPC or a laptop computer with a USB cable for data acquisition and display.FIG. 2. A schematic diagram of the apparatus. Heliumflow through the three-way valve at the bottom of thefigure provided the base line for the sensor signal. Apulse of DMMP vapor was delivered to the flow cell bysimultaneously activating both three-way valves whichare controlled by the data acquisition computer.055101-2 Pinnaduwage et al. Rev. Sci. Instrum. 78, 055101 2007Downloaded 13 Aug 2009 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jspwalls of the permeation tube due to constant permeation ofDMMP. Initially, when the generator was first attached to theapparatus, a significantly larger signal was observed due tothis extra DMMP and also possibly due to water vapor des-orbing from the walls of the permeation tube holder andother tubings in the apparatus. This signal decayed over timeand reached a plateau after 2–3 days. After that, the signallevels did not vary significantly from day to day. The datapresented in this article were taken after the generator waspurged for several days.During the experiment, two computer-controlled three-way valves were used to switch between helium and DMMPin helium delivered to the flow cell at the given flow rate.Therefore, the flow rate through the microcantilever flow cellwas maintained at the given flow rate during the whole ex-periment, and thus microcantilever bending due to change inflow was avoided.The response of the sensor to a pulse of DMMP vapor of1 min duration at different flow rates is shown in Fig. 3. Theestimated concentrations of the DMMP vapor varied from2 ppb at 25 SCCM SCCM denotes cubic centimeter perminute at STP to 0.33 ppb at 150 SCCM. Even thoughthe range of concentration levels changed by a factor of 6,the signal dropped only a factor of 3 when the flow rateincreased from 25 to 150 SCCM. Regardless of the flowrate, 0.25 ng/min is expected to be delivered to the flow cell.In order to compare with the data of Bertilsson et al.,16 acantilever on a separate chip was coated withSHCH211CH3. The response of that cantilever to a 1 minpulse of DMMP at a flow rate of 25 SCCM is also shown inFig. 3. Consistent with the results of Bertilsson et al.,16 thatresponse is much weaker compared to the response of the4-MBA coated cantilever under the same conditions. Finally,the response of a gold-coated cantilever with respect to an-other gold-coated cantilever at 25 SCCM flow rate isshown. As expected, there is no signal with this combination.Since the 4-MBA coating is quite sensitive to water va-por, it is important to verify that the observed signal is notdue to trace water vapor. A control experiment was con-ducted with the DMMP permeation tube removed from thepermeation tube holder with everything else kept the same.The “signal” due to a 1 min pulse with the blank generator ata flow rate of 25 SCCM is shown in Fig. 4. We let the “blankgenerator” purge with helium only for 1 day overnight, sothis small signal is likely to be due to leftover water vapor onthe walls of the permeation tube holder. The signal of Fig. 4for the blank generator is to be compared with the 25 SCCMdata set in Fig. 3 for the DMMP vapor stream.The rapidity with which the DMMP vapor can be de-tected at low concentrations and the relatively fast relaxationof the cantilever when the DMMP vapor is turned off can beseen in Fig. 5. When the DMMP vapor is turned on for 10 s,20 V deflection signal is observed; after the vapor streamis turned off, the cantilever relaxed back almost to the origi-nal position within 60 s. The noise level three times thestandard deviation of the background for the data of Fig. 3is 1.1 V, and thus the signal-to-noise ratio S/N is about18; i.e., the detection limit for 10 s pulses is 30 ppt.The ability to detect low concentrations of analytes intens of seconds is critically important for an electronic noseit takes less than a second for a trained neural net to analyzea data set. The achievement of sufficient selectivity with asensor array is likely to require the use of multiple individualFIG. 3. Color online The response of a 4-mercaptobenzoic acid 4-MBA-coated cantilever to a 1 min long pulse of DMMP at flow rates of 25, 50,100, and 150 SCCM in all cases, the amount of DMMP injected to thevapor stream is 0.25 ng/min. Response for a cantilever coated with theSHCH211CH3 coating at a flow rate of 25 SCCM is also shown, which isan order of magnitude smaller compared to the data for the 4-MBA chip at25 SCCM. The flat trace at the bottom was obtained when a cantilever withjust the gold coating was exposed to the same DMMP stream at 25 SCCMflow rate in all cases, the signal shown is the differential signal with respectto a gold-coated cantilever on the same cantilever chip. These data weretaken after purging the generator for several days.FIG. 4. The response of a 4-mercaptobenzoic acid 4-MBA-coated cantile-ver to a 1 min long pulse from the “blank generator” kept at 30 °C and at aflow rate of 25 SCCM. The blank generator was purged only for a day, sothis small signal is probably due to the leftover water vapor.055101-3 Miniature chemical sensor Rev. Sci. Instrum. 78, 055101 2007Downloaded 13 Aug 2009 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jspsensors in the sensor array the biological nose has thousandsof individual sensors called cilia, which are of size compa-rable to the microcantilevers. Unlike many other miniaturesensor platforms such as SAW or QCM, the microcantileversare true MEMS sensors, and it is possible to accommodatehundreds of sensors on a 1 cm2 chip. Furthermore, as weillustrate here, a simple detection circuitry can be assembledon a miniature circuit board to monitor the signals from themicrocantilever sensors. Therefore, the size of the circuitboard can be kept small. Thus, selectivity for complex mix-tures could be achieved without making the sensor packagelarge. We are currently conducting research on the selectivedetection of individual vapors in vapor mixtures using anarray of microcantilever sensors with the use of an artificialneural network for pattern recognition.Partial support for this study was provided by a Coop-erative Research and Development Agreement CRADAwith Sense Holding, Inc. ORNL is managed by UT-Battelle,LLC, for the U.S. Department of Energy under Contract No.DE-AC05-00OR22725.1 K. J. Albert, N. S. Lewis, C. L. Schauer, G. A. Solzing, S. E. Stitzel, T. P.Vaid, and D. R. Walt, Chem. Rev. Washington, D.C. 100, 2595 2000.2 J. W. Gardner and P. N. Bartlett, Electronic Noses: Principles and Appli-cations Oxford University Press, Oxford, 1999.3 K. Persaud and G. H. Dodd, Nature London 299, 352 1982.4 I. Gaillard, S. Rouquier, and D. Giorgi, Cell. Mol. Life Sci. 61, 4562004.5 L. A. Pinnaduwage, H. F. Ji, and T. Thundat, IEEE Sens. J. 5, 774 2005.6 L. A. Pinnaduwage et al., Rev. Sci. Instrum. 75, 4554 2004.7 A. Johansson, M. Calleja, P. A. Rasmussen, and A. Boisen, Sens. Actua-tors, A 123–124, 111 2005.8 Chemical Warfare Agents: Toxicity at Lower Levels CRC, Boca Raton,FL, 2000.9 A. A. Tomchenko, G. P. Harmer, and B. T. Marquis, Sens. Actuators B108, 41 2005.10 C. Yu, Q. Hao, S. Saha, and L. Shi, Appl. Phys. Lett. 86, 063101 2005.11 K. Cattanach, R. D. Kulkarni, M. Kozlov, and S. K. Manohar,Nanotechnology 17, 4123 2006.12 J. P. Novak, E. S. Snow, E. J. Houser, D. Park, J. L. Stepnowski, and R. A.McGill, Appl. Phys. Lett. 83, 4026 2003.13 C. Zimmermann, P. Mazein, D. Rebiere, C. Dejous, J. Pistre, and R. Pla-nade, IEEE Sens. J. 4, 479 2004.14 I. Voiculescu, M. E. Zaghloul, R. A. McGill, E. J. Houser, and G. K.Fedder, IEEE Sens. J. 5, 641 2005.15 Q. Zhao, Q. Zhu, W. Y. Shih, and W. Shih, Sens. Actuators B 117, 742006.16 L. Bertilsson, K. Potje-Kamloth, H. D. Liess, and B. Liedberg, Langmuir15, 1128 1999.FIG. 5. The response of a 4-mercaptobenzoic acid 4-MBA-coated cantile-ver to the periodic turning on 10 s and off 60 s of a DMMP stream of0.5 ppb concentration in helium. Since the signal-to-noise ratio is 18, thedetection limit at this fast detection rate is 30 ppt.055101-4 Pinnaduwage et al. Rev. Sci. Instrum. 78, 055101 2007Downloaded 13 Aug 2009 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp

Miniature sensor suitable for electronic nose applications

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Miniature sensor suitable for electronic nose applications

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