polymer–carbon …concentration (smac) as recorded in the training library for the jpl electronic...

714 MRS BULLETIN/OCTOBER 2004

IntroductionThe Jet Propulsion Laboratory’s electronic

nose (JPL ENose) is an autonomous, minia-ture device developed for use as an air-quality monitor in human habitats in space.1This device is under development at JPL,where it has been taken from initial work atthe California Institute of Technology2 in de-veloping polymer–carbon black compositesensors to a fully operational device capableof identifying and quantifying 20 organicand inorganic compounds that may bepresent in recycled air in spacecraft.1,3 TheJPL ENose detects the target compounds atconcentrations related to their 24 h space-craft maximum allowable concentrations(SMACs), exposure levels set by NASA.

The JPLENose first-generation device wassuccessfully demonstrated in a flight experi-

ment on space shuttle flight STS-95 in 1998;1,4

since 1998, we have focused on sensor andsensing array optimization,5 developing amolecular model of sensor response,7 andon further development of the hardware.

In this article, we focus on the polymer–carbon black composite films that composethe sensing surfaces, the approaches usedto select the polymers in the array, andwork in developing a molecular model ofsensor response. We will also discuss dataanalysis considerations and how environ-mental variations are taken into account.

Electronic Noses in SpaceElectronic noses have been proposed for

many applications within the space pro-gram, including as chemical-sensing sys-

tems for atmospheric studies on planetarysurfaces and for monitoring air quality inhuman habitats.3 Because electronic nosesare low-power devices and can be config-ured to respond to a broad range of com-pounds, they are appealing as devices to beused in planetary studies. For planetary ex-ploration, an electronic nose can be used tostudy the constituents of a planetary atmos-phere and atmospheric variations with lo-cation or time. To search for evidence of lifeon other planets, an electronic nose may beused to sample for metabolic products. Toachieve the necessary selectivity for suchanalyses, the sensing media in the array canbe chosen to distinguish compounds ofsimilar structure, including isomers andenantiomers (mirror-image isomers).6 Ap-plication of electronic noses to atmosphericstudies will require the development ofmethods to enable the electronic nose to de-convolute target vapors from an unknownbackground, either by physical or chemi-cal separation or other means. With a fullydeveloped molecular model of sensor re-sponse to analytes, it may be possible toconstruct the chemical structure of a com-pound whose response pattern has not beenpreviously recorded, much as traditionalanalytical techniques build up the chemicalstructure of an analyte from its physical orchemical characteristics.7

The immediate application of an elec-tronic nose to the space program is in real-time, continuous air-quality monitoring inhuman habitats. The ability to monitor re-cycled breathing air is important for crewhealth and safety in the living and workingquarters of a space habitat such as the In-ternational Space Station and in crew ve-hicles (e.g., the space shuttle or the new crewexploration vehicle now under develop-ment by NASA). This ability will also beimportant in future lunar or martian surfacehabitats. Today, there is no real-time air-quality monitoring in the InternationalSpace Station. Air quality is determinedanecdotally by crew members’ reports andis measured by collecting samples for returnto a terrestrial laboratory for analysis usinghigh-sensitivity analytical techniques suchas gas chromatography/mass spectrometry(GC/MS) or gas chromatography/ionmobility spectrometry (GC/IMS).

A real-time air-quality monitor would beused as an incident monitor. The crew wouldbe notified of the presence of compoundsthat are approaching dangerous levels sothat remedial action could be taken, or, in afully automated system, remediation wouldbe part of an automated environmentalmonitoring and control system.4,8,9

The specifications for an incident monitorto be used in spacecraft are that it be capableof identifying and quantifying target com-

Polymer–CarbonBlack CompositeSensors in anElectronic Nose forAir-QualityMonitoring

M.A. Ryan, A.V. Shevade, H. Zhou, and M.L. Homer

AbstractAn electronic nose that uses an array of 32 polymer–carbon black composite sensors

has been developed, trained, and tested. By selecting a variety of chemical functionalities inthe polymers used to make sensors, it is possible to construct an array capable ofidentifying and quantifying a broad range of target compounds, such as alcohols andaromatics, and distinguishing isomers and enantiomers (mirror-image isomers). A modelof the interaction between target molecules and the polymer–carbon black compositesensors is under development to aid in selecting the array members and to enableidentification of compounds with responses not stored in the analysis library.

Keywords: chemical sensors, electronic noses, polymer–carbon black compositesensors.

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Polymer–Carbon Black Composite Sensors in an Electronic Nose for Air-Quality Monitoring

MRS BULLETIN/OCTOBER 2004 715

pounds at determined levels over a fairlywide range; that it be a low-mass and low-volume device that requires low power forcontinuous operation; and that it requirelittle crew time for maintenance, calibration,and air analysis. These qualities are foundin electronic noses, and the JPL ENose hasbeen developed to meet these requirements.It can identify and quantify compounds inits target set with a dynamic range fromabout 0.01 ppm to 10,000 ppm, dependingon the compound; it lends itself to minia-turization and low-power operation; andbecause it measures deviation from a back-ground, it does not require frequent calibra-tion and maintenance.

The JPL ENose is trained to identify andquantify compounds at the 24 h SMAC to provide early warning if a compound isapproaching a dangerous concentration. In developing the JPL ENose, the 24 hSMAC was selected as the target concen-tration because environmental monitoringin crew quarters is envisioned to consist of several devices used at varying inter-vals.10 A complete picture of spacecraft airwill be generated by on-site analysis usinga more complex, high-sensitivity analyticaldevice such as GC/MS or GC/IMS with afrequency of once a day to every few days.An incident monitor such as the electronicnose is designed to monitor for changescaused by leaks, spills, or incipient firesbetween more detailed analyses.1,3,8,9 InTable I, the analytes selected as targetcompounds for the JPL ENose are listed,along with their 24 h SMACs.11 The JPLENose has been trained to identify andquantify these compounds at concentra-tions from one-third to three times the 24 h SMAC.

The JPL ENoseThe JPLENose is a complete, autonomous

instrument composed of five basic parts:� Thirty-two chemical sensors: polymer–carbon black composite films on ceramicsubstrates.� Electronics designed to operate the deviceand to acquire and store data.� A pneumatic system, including a pumpand filter system to bring air into the sens-ing chamber and to provide cleaned air forbaselining, and a flow system designed toprovide air in turbulent flow to ensure goodmixing in the sampled air.� The data analysis system, consisting of soft-ware comprising the training library de-signed to identify and quantify compounds.� The computer that controls the device op-eration, acquires and stores data, processesthe data, and provides a readout of results.

The first-generation ENose, built for the1998 flight experiment, is shown in Figure 1a.It occupies a volume of �2000 cm3, has

a mass of �1.5 kg, and uses 1.5 W averagepower (3 W peak power).1,3 The second-generation ENose (Figure 1b) has the samefunctions as the first-generation device, buthas been miniaturized to occupy �1000 cm3

with a mass of �900 g. The power require-ments of the second-generation ENose aresimilar to those of the first-generation device,even with the switch from the palmtopcomputer shown in Figure 1a to a PDA-typecomputer, but this may change if a differentcomputer is eventually chosen. The bodyand flow system of the second-generationdevice are made from a single block ofhard-anodized aluminum; this design waschosen to eliminate fittings and ensurethat there are no leaks in the flow system.

Polymer–Carbon Black CompositeSensors and the Sensing ArraySensors

The sensors used in the JPL ENose areconductometric, made from polymer–carbon black composite films (a sensor isconductometric when the transductionmethod is measurement of the dc resis-tance change in the sensor in response to atarget compound). The sensors are made by

solvent-casting polymer–carbon black com-posite films onto screen-printed Pd-Au elec-trodes on a ceramic substrate. Aphotographof one sensor substrate, consisting of eightsensors, is shown in Figure 2.1 The sensorsubstrate includes RuO2 heaters on the backside for temperature control of the sensorsduring operation. Films are made by dis-persing carbon black in a solution containingdissolved insulating polymer, then dispens-ing the solution onto the substrate. Whenthe solvent has evaporated, the films areelectronically conducting because of con-ductive pathways formed by the carbonblack in the polymer matrix.12 We are cur-rently doing work to determine the opti-mum concentration of carbon black abovethe critical amount, or percolation threshold.The carbon black concentration must besufficient to ensure formation of continuousconduction pathways without increasingthe density or thickness of the film too muchin order to detect low concentrations ofanalyte.

Responses in the sensors are read aschanges in dc resistance. A baseline resis-tance is established for each sensor, andchanges in that resistance indicate a changein the vapor constituents of the environment.The mechanism of response in polymer–carbon black composite sensing films is, atits simplest level, based on swelling in thefilm. Vapors in the environment sorb to thefilm, causing the film to swell. Swellingdisrupts conductive pathways in the filmby pushing carbon particles apart, and theelectronic resistance in the film increases.1,2,13

This increased resistance is measured andis used to construct the pattern of responseacross the sensing array (the “fingerprint”or “signature”), which is analyzed. Othermechanisms, such as ionization of analytein water trapped in the film or sorption tocarbon particles and interrupting conduc-tive pathways, may also contribute to achange in resistance in polymer–carbonblack composite sensors. It is elucidationof these mechanisms and the chemical andphysical characteristics that govern themthat are the focus of the molecular model-ing work under way at JPL.5,7

Because the polymer swelling beginsimmediately after exposure to the vapor, theresistance signals can be read in real time ornear real time. With films on the order of0.1–0.2 �m thick, the swelling (and there-fore resistance) response times to equilib-rium film swelling values range from �0.1 sto �100 s, depending on the vapor and thepolymer through which the vapor mustdiffuse.

At small swellings, the film returnsfully to its initial unswollen state after thevapor source is removed, and the film re-sistance on each array element returns to (or

Table I:Target Compounds and Their24 h Spacecraft Maximum Allowable

Concentration (SMAC) as Recorded inthe Training Library for the JPL

Electronic Nose.

Compound SMAC, 24 h(ppm)

Acetaldehyde 6Acetone 270Acetonitrile 4Ammonia 30Benzene 102-butanone 150Chlorobenzene 10Dichloromethane 35Ethanol 2000Freon 113 50Hydrazine 0.3Indole 1Methane 5300Methanol 30Methyl hydrazine 0.0022-propanol 400Tetrahydrofuran 401,1,1-trichloroethane 11Toluene 16o,p-xylenes 100

Note: The JPL ENose has been trained toidentify and quantify these compounds atconcentrations from one-third to three timesthe 24 h SMAC.



very near to) its original value. Reversibility,or the tendency of sorbed vapors to leavethe surface after the vapor source is re-moved, has been demonstrated for thepolymer composite films used in our arraysfor many thousands of vapor exposures,under a variety of ambient conditions inroom air at various relative humidities andtemperatures, for a diverse set of odorsand polymers.1,14 Such reversibility leavesthe sensor surface free to detect the targetmolecule again, increasing the sensor’slifetime.

Selecting the Sensing ArrayThe sensing array used in the JPL ENose

is a 32-element array made up of four sub-strates, each with eight sensors of the typeshown in Figure 2. Sensors made from 16polymers are cast on these substrates. Thepolymers selected as elements in the JPLENose chemical-sensing array represent awide variety of chemical functionalities,because the analyte set to be detected rep-resents a wide variety of compounds.

To obtain distinctive patterns across thearray, polymers for the second-generation

ENose were selected according to a three-step process:1

First, polymers were selected to representfive categories of chemical functionality:hydrogen-bond acidic (HBA), hydrogen-bond basic (HBB), dipolar and hydrogen-bond basic (D-HBB), moderately dipolar andweakly H-bond basic or acidic (MD-HB),and weakly dipolar with weak or nohydrogen-bond properties (WD). Responsesof these polymers to a subset of target an-alytes were measured to provide data forfurther analysis.

Second, a statistical analysis of measuredresponses of polymers to representativeanalytes (selected by functional group) wasused to determine the best distribution ofpatterns of response across the analytes.The selection of the sensor set and theircorresponding statistical weights were op-timized by maximizing distances betweengas signatures. Distance between signaturesis defined as

, (1)

where dRm(i) is the (fractional) resistancechange of the ith sensor for the mth gas, andthe summation is over N sensors used. Thegas signature is the unique pattern formedby response of the sensor array to a particu-lar analyte and is generally shown as a his-togram of the normalized magnitude ofresponse of each sensor in the array.

Third, a set of polymers was selected fromthe polymers indicated by statistical analysisas providing a high degree of definitionfrom one analyte to the next. In the finalselection, polymer films that showed thelowest-noise response and a high degree ofreproducibility of response were chosen.

�Smn �1N�Ni

�dRm�i� � dRn�i��Figure 1.The JPL ENose. (a) First-generation device, used in space shuttle flight STS-95experiment in 1998 (ruler, 10 cm); (b) second-generation device (ruler, 15 cm).

Figure 2. JPL ENose sensor substrate,a 25-mm-wide ceramic base with eightscreen-printed Pd-Au electrode sets,used in both devices in Figure 1.Polymer–carbon black composite filmsare deposited from solution on theelectrodes. Heaters have beenscreen-printed on the back side of theceramic.



The polymers used in the first-generationENose were selected based only on statis-tical analysis; chemical functionality was notconsidered. The polymers used in each de-vice and their functionality classificationsare shown in Table II.

Polymer selection depends greatly on thesuite of target compounds. If the targetcompounds represent a broad distributionof chemical functionalities, as for the JPLENose, then a broad distribution of poly-

mers will be necessary. Figure 3 showsthat the same set of polymers will distin-guish compounds of very similar structures(isomers),6 as well as compounds of verydifferent chemical structures, for instance,propanol and benzene.3 Figures 3a and 3cshow that propanol isomers can be distin-guished using 16 polymer–carbon blackcomposite sensors; Figures 3b and 3d showthat the benzene and toluene patterns aresignificantly different using the 32-sensor

array in the first-generation JPL ENose.The polymers are listed by sensor numberin Table II.

By selection of chiral polymers, it has also been shown that it is possible to differentiate enantiomers.6,15 For ex-ample, a poly(R-3-hydroxybutyrate-co-R-3-hydroxyvalerate)(77% butyrate)/carbonblack composite detector exhibited a differ-ential response to the mirror-image isomersR-2-butanol relative to S-2-butanol. Similardifferentiation was observed between enan-tiomers of �-pinene, epichlorohydrin, andmethyl-2-chloropropionate. Each enan-tiomer of a pair produced a unique relativedifferential resistance change on sensorsmade from chiral polymer–carbon blackcomposite film, whereas those same enan-tiomers produced identical responses onachiral detectors.15 This type of broadly re-sponsive differentiation to isomers andenantiomers might be useful in probing foroptically active signatures of extinct (or evenextant) extraterrestrial life as well as in moni-toring the quality of spacecraft breathing air.

Model of Sensor ResponseIn an effort to understand the chemical

and physical characteristics that influencethe analyte to sorb to the film, with thefurther goal of developing the ability topredict the response of any polymer film toany analyte based on its molecular structure,we have initiated an effort to construct amodel of sensor response.

Previous authors have shown that stud-ies of linear solvation energy relationships(LSERs) of analytes to polymers can be usedto predict the response of polymer filmsused as receptors in surface acoustic wavesensors.16,17 Similarly, such studies shouldshed light on the response of sensors basedon polymer–carbon black composites.However, carbon black particles change thephysical characteristics of the film (glass-transition temperature, rigidity, and density),and possibly the chemical characteristics(analytes can sorb or bind to the carbonsurface as well as in the film, and sorptionsites in the polymer may be occupied bycarbon), so solvation energies may not besufficient for complete description of thesensor–analyte interaction. Neither doLSERs account for analyte molecule diffu-sion within the film, film thickness, filmhydration, or interactions between analytemolecules.5,7

We have developed a model of sensingfilm–analyte interaction that takes into account the interaction energies of thepolymer, carbon black, analyte, and water.Modeling is done using Cerius2 software.To construct a polymer–carbon black com-posite model7 based on the experimentalformulations, a polymer box of appropriate

Table II: Polymers Used in First- and Second-Generation Electronic Noses andTheir Chemical Functionality Classification.

Polymer Classification Positionin Arraya

First-Generation PolymersPoly(4-vinylphenol) HBA 3, 4Methyl vinyl ether/maleic acid

50/50 copolymer HBA 29, 30Polyamide resin HBB 7, 8Poly(N-vinyl pyrrolidone) HBB 23, 24Poly(2-hydroxyethyl methacrylate) D-HBB 11, 12Vinyl alcohol/vinyl butyral, 20/80 D-HBB 13, 14Hydroxypropyl methyl cellulose, 10/30 D-HBB 31, 32Poly(2,4,6-tribromostyrene), 66% MD-HB 1, 2Cellulose triacetate MD-HB 9, 10Poly(caprolactone) MD-HB 15, 16Poly(vinyl chloride-co-vinyl acetate),

90/10 MD-HB 19, 20Poly(vinyl acetate) MD-HB 21, 22Poly(vinyl stearate) MD-HB 27, 28Poly(ethylene oxide) WD 5, 6Poly-�-methyl styrene WD 17, 18Styrene/isoprene, 14/86 ABA Block

copolymer WD 25, 26

Second-Generation PolymersPoly(4-vinylphenol-co-methyl methacrylate) HBA 1, 9Poly(ethylene-i-acrylic acid), 80/20 HBA 2, 10Poly(styrene-i-maleic acid) HBA 3, 11Poly(N-vinyl pyrrolidone) HBB 4, 12Poly(2-vinyl pyridine) HBB 5, 13Poly(4-vinyl pyridine) HBB 6, 14Vinyl alcohol/vinyl butyral copolymer D-HBB 7, 15Cyanoethyl hydroxyethyl cellulose D-HBB 8, 16Ethyl cellulose D-HBB 17, 25Poly(vinylbenzyl chloride) D-HBB 18, 26Soluble polyimide (matrimide TM 5218) MD-HB 19, 27Polyepichlorohydrin MD-HB 20, 28Poly(epichlorohydrin-co-ethylene oxide) MD-HB 21, 29Ethylene-propylene diene terpolymer WD 22, 30Polystyrene WD 23, 31Poly(ethylene oxide) WD 24, 32

a The position in the array, from 1 to 32, is listed for reference to data in Figure 3.HBA � hydrogen-bond acidic; HBB � hydrogen-bond basic; D-HBB � dipolar and hydrogen-bondbasic; MD-HB � moderately dipolar and weakly H-bond basic or acidic; WD � weakly dipolar withweak or no hydrogen-bond properties.



density is created, then carbon black mod-eled as uncharged naphthalene rings (withno hydrogen atoms) is inserted into the boxusing a random cavity search in the poly-mer matrix; naphthalene mimics thehexagonal shape of carbon black particlesin the event that shape is important in thesensor–analyte interactions. An equilibriumcomposite structure is obtained using acombination of molecular mechanics andmolecular dynamics simulation techniques.

When the polymer–carbon black com-posite is equilibrated, interaction energiesbetween one analyte molecule and the com-posite are calculated using the SORPTIONmodule in the Cerius2 software (at 300 K).The contributions to the total interactionenergy between an analyte and the com-posite are its interactions with the polymerchains, carbon black, and other compounds(such as water) that may be present. The

simulation program generates randompoints in the composite model and attemptsto insert the analyte molecules. Insertionattempts that involve the overlapping of theanalyte molecule with the compositestructure are discarded. For each composite–analyte interaction, 2–3 million simulations(analyte insertions) were run, and the av-erage energy of composite–analyte inter-actions was calculated at the end of thesimulation.

Figure 4 shows the predicted interactionenergies of a selection of target analyteswith modeled composites.7 Work is inprogress to validate the model through ex-perimental work and to describe the re-sponse mechanisms by correlating sensorresponses to molecular descriptors based onquantitative structure–activity relationship(QSAR) techniques using so-called geneticfunction approximations. As used in this

work, QSAR correlates the response (activ-ity) of the sensor to several physical andchemical properties stemming from the mo-lecular structure of both the analyte andthe sensor (molecular descriptors). Ex-amples of molecular descriptors used inthis work are the calculated energy of in-teraction between analyte and polymerand between analyte and carbon black,the glass-transition temperature of the poly-mer, and the molecular volume of the analyte.

Effect of EnvironmentalConditions on Sensors

As with most chemical sensors, local con-ditions can have a significant effect on theresponse of polymer–carbon black com-posite films used as sensors. Whereaschanges in humidity, temperature, and pres-sure tend to occur on a relatively slow timescale and so can be ignored if the device isused to monitor changes over the previousfew minutes, we have designed the JPLENose to be able to look back over longperiods (several hours) to determinewhether there is evidence of a slow buildupof target compounds. On this time scale,changes in environmental conditions canobscure small changes in sensor response,because the sensors are very sensitive tochanges in relative humidity of 1% or lessand temperature changes of �0.2�C.1,3 Todiminish the effect of local temperature vari-ations, the sensor substrates used in the JPLENose are heated to a target temperaturethat is 1–4�C above the ambient tempera-ture; target temperatures of 24�C, 28�C, 32�C,or 36�C may be used, depending on the localambient temperature. The response of thesensor to each target compound as a func-tion of temperature has been characterized.Humidity is measured independently of thepolymer–carbon black composite sensorsand taken into account in data analysisthrough empirically determined algorithms

Figure 3. Fingerprints of 60 ppm (a) i-propanol and (c) n-propanol to 16 polymers. Sensorresponse as dR/R0 is normalized to the largest response for each array fingerprint.Thefingerprints are distinct enough to distinguish the isomers of propanol. Sensor numbersrefer to polymers in the odd-numbered positions in the first-generation array (Table II). Theresponse patterns across the array of the polymer–carbon black composite sensors used inthe first-generation JPL ENose are significantly different for compounds of similar chemicalstructure, (b) benzene and (d) toluene. Sensor numbers refer to polymers in those positionsin the first-generation array (Table II).

Figure 4. Predicted target analyteinteraction energies withpolymer–carbon black composites.



that describe the effect of humidity on eachanalyte in each polymer.

To distinguish between slow changes insensor resistance caused by a slow leak of atarget compound and those caused by base-line drift, a protocol for baselining was in-cluded in the ENose. This protocol involvesswitching the air intake through a carbonfilter that removes volatile compounds toprovide cleaned air to the sensors at set in-tervals, then analyzing the virtual peakformed when the flow switches back toambient air. The peak is “virtual” because itis caused by an artificial event created bythe experimenters. If there is no change inthe magnitude or fingerprint of the virtualpeak over time, the air is considered to beunchanged and safe. If there is a change inthe magnitude or the fingerprint of thevirtual peak, the data analysis program at-tempts to identify the compound respon-sible for the change.1,3

Data AnalysisThe primary constraint in data analysis

software development was the requirementthat gas events of single or mixed gases fromthe target compounds be identified correctlyand quantified accurately. (Quantificationaccuracy was required to be only �50% ofknown concentration because toxicologicalquantities are not better known than 50%.)In addition, three conditions were foundin the data expected for air-quality monitor-ing: a large number of target compounds,some of which are of a very similar chemi-cal structure (e.g., benzene and toluene);low target concentrations with nonlinearresponses for some sensors at low concen-trations; and single gases and mixtures.

To address these considerations, we havedesigned an automated routine to identifyand quantify a gas event using an algorithmbased on Levenberg–Marquart nonlinearleast-squares fitting (LM-NLS).1,3,18 Severalother pattern recognition approaches wereconsidered before LM-NLS was selected.1,3

In the first-generation ENose, the successrate for identification and quantification was85%. This success rate takes account of falsepositives and negatives as well as incorrectidentification or quantification.1,3 Furtherwork is improving this rate.

ConclusionsThe device developed as the second-

generation ENose at JPL has been tested forits ability to respond to the compounds inTable I and is ready to operate as an au-tonomous device. Work continues in devel-oping a model of sensor–analyte interactionto optimize the selection of materials for a sensing array as well as for possible fu-ture use to identify array responses fromuntrained-for compounds, that is, com-

pounds causing array response patterns notfound in the training library.

Future application of the JPL ENose inspace habitats or planetary exploration willdepend on NASA’s needs, but eventual de-ployment of the ENose as one part of a fullyautomated environmental monitoring andcontrol system is envisioned. In this scenario,activities such as remediation of spills andleaks would be actuated through a logic sys-tem and performed robotically.

AcknowledgmentsThe work discussed here was carried out

at the Jet Propulsion Laboratory, CaliforniaInstitute of Technology, under contract withthe National Aeronautics and Space Admin-istration (NASA). Work was sponsored bythe NASA Office of Biological and PhysicalResearch, Advanced Environmental Mon-itoring and Control.

References1. M.A. Ryan, H. Zhou, M.G. Buehler, K.S.Manatt, V.S. Mowrey, S.P. Jackson, A.K. Kisor,A.V. Shevade, and M.L. Homer, IEEE Sens. J. 4(3) (2004) p. 337.2. M.C. Lonergan, E.J. Severin, B.J. Doleman,S.A. Beaber, R.H. Grubb, and N.S. Lewis, Chem.Mater. 8 (1996) p. 2298.3. M.A. Ryan and H. Zhou, in A Handbook onMachine Olfaction: Electronic Noses, edited by T.Pearce, S. Schiffman, J. Gardner, and H.T. Nagle(Wiley-VCH, Weinheim, Germany, 2002). 4. M.A. Ryan, M.L. Homer, H. Zhou, K.S. Manatt,V.S. Ryan, and S.P. Jackson, Proc. 30th Int. Conf.Environmental Systems (Society of AutomotiveEngineers, Warrendale, PA, 2000) 00ICES-259.5. M.A. Ryan, M.L. Homer, H. Zhou, K. Manatt,and A. Manfreda, Proc. 31st Int. Conf. Environ-mental Systems (Society of Automotive Engineers,Warrendale, PA, 2001) 2001-01-2308.6. M.A. Ryan and N.S. Lewis, Enantiomer 6(2001) p. 159.7. A.V. Shevade, M.A. Ryan, M.L. Homer, A.M.Manfreda, H. Zhou, and K.S. Manatt, Sens. Ac-tuators, B 93 (2003) p. 84.8. M.A. Ryan, M.L. Homer, M.G. Buehler, K.S.Manatt, F. Zee, and J. Graf, Proc. 27th Int. Conf.Environmental Systems (Society of AutomotiveEngineers, Warrendale, PA, 1997) 97-ES84.9. M.A. Ryan, M.L. Homer, M.G. Buehler, K.S.Manatt, B. Lau, D. Karmon, and S. Jackson,Proc. 28th Int. Conf. Environmental Systems (Soci-ety of Automotive Engineers, Warrendale, PA,1998) 98-1564.10. Requirements for Technology Development,Advanced Life Support/Advanced Environ-mental Monitoring and Controls, National Aero-nautics and Space Administration Web site,http://peer1.nasaprs.com/peer_review/prog/prog.html (accessed September 2004).11. Spacecraft Maximum Allowable Concentrationsfor Selected Airborne Contaminants, Vols. 1–2(National Academy Press, Washington, DC,1994–1996).12. E.K. Sichel, ed., Carbon Black–Polymer Com-posites (Marcel Dekker, New York, 1982).13. M.S. Freund and N.S. Lewis, Proc. Natl.Acad. Sci. U.S.A. 92 (1995) p. 2652.

14. E.J. Severin, B.J. Doleman, and N.S. Lewis,Anal. Chem. 72 (2000) p. 658.15. R.D. Sanner, E. Severin, B.J. Doleman, andN.S. Lewis, Anal. Chem. 70 (1998) p. 1440.16. J.W. Grate and M.H. Abraham, Sens. Actua-tors, B 3 (1991) p. 85.17. R.A. McGill, M.H. Abraham, and J.W.Grate, Chemtech 24 (1994) p. 27. 18. G. Stang, Linear Algebra and its Applications,2nd ed. (Academic Press, New York, 1980). ■■

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