774 ieee sensors journal, vol. 5, no. 4, …hj56/pdffiles/2005/moor 774.pdf774 ieee sensors journal,...

774 IEEE SENSORS JOURNAL, VOL. 5, NO. 4, AUGUST 2005

Moore’s Law in Homeland Defense: An IntegratedSensor Platform Based on Silicon Microcantilevers

Lal A. Pinnaduwage, Member, IEEE, Hai-Feng Ji, and Thomas Thundat

Abstract—An urgent need exists for the development of inex-pensive, highly selective, and extremely sensitive sensors to helpcombat terrorism. If such sensors can be made miniature, theycould be deployed in virtually any situation. Terrorists have a widevariety of potential agents and delivery means to choose from forchemical, biological, radiological, or explosive attacks. Detectingterrorist weapons has become a complex and expensive endeavor,because a multitude of sensor platforms is currently needed to de-tect the various types of threats. The ability to mass produce andcost effectively deploy a single type of sensor that can detect a widerange of threats is essential in winning the war on terrorism. Sil-icon-based microelectromechanical sensors (MEMS) represent anideal sensor platform for combating terrorism because these minia-ture sensors are inexpensive and can be deployed almost anywhere.Recently, the high sensitivity of MEMS-based microcantilever sen-sors has been demonstrated in the detection of a variety of threats.Therefore, the critical requirements for a single, miniature sensorplatform have been met and the realization of an integrated, widelydeployable MEMS sensor could be near.

Index Terms—Chemical, biological, radiological, or explosive(CBRE) detection, homeland defense, microcantilever, microelec-tromechanical sensors (MEMS) sensor, terrorism.

I. INTRODUCTION

T ERRORISTS have a huge economic advantage over law en-forcement because it is, many times, more expensive to de-

tect terrorist threats than it is to deploy terrorist threats. For ex-ample, a crude explosive device can destroy an airplane in flight.On the other hand, current explosive detection technologies de-ployed at airports are expensive and require constant operator at-tention.Achemicalorbiologicalattack,whichcanalsobecarriedout with nominal cost and effort, might even go unnoticed untilinjured people start turning up at hospitals. On the nuclear side, a“dirty bomb,” which uses radioactive material that will be spreadusing a conventional bomb, is another likely threat.

Even though sensitive detection of individual threats may becurrently possible, such techniques/sensor systems are bulky,expensive, and require time-consuming procedures. Also,

Manuscript received February 3, 2004; revised September 2, 2004. This workwas supported in part by the Bureau of Alcohol, Tobacco, and Firearms (ATF),in part by the National Safe Skies Alliance, in part by the Department of Home-land Security, in part by the Department of Energy’s NA-22 program, in part bythe Environmental Management Science Program, and in part by the Office ofBiological and Environmental Research Program. The associate editor coordi-nating the review of this paper and approving it for publication was Dr. TimothySwager.

L. A. Pinnaduwage and T. Thundat are with the Life Sciences Division, OakRidge National Laboratory, Oak Ridge, TN 37831-6122 USA, and also with theDepartment of Physics, University of Tennessee, Knoxville, TN 37996-1200USA (e-mail: [email protected]).

H.-F. Ji is with the Institute for Micromanufacturing, Louisiana Tech Univer-sity, Ruston, LA 71272 USA.

Digital Object Identifier 10.1109/JSEN.2005.845517

detection of multiple threats requires the use of a variety ofspecialized instruments based on different technologies/sensorplatforms. Therefore, a paradigm change in sensor technology isrequired for combating the war on terrorism. Ideally, homelanddefense requires a sensor system with the following features: 1) asingle-sensor platform that can detect multiple threats simulta-neously and rapidly; 2) an inexpensive, miniature, and robustsensor system that can be deployed almost anywhere; and (3)built-in telemetry for data transmission and networking. None ofthe available technologies satisfy these conditions. However, theemerging sensor technology based on MEMS has the promise ofsatisfying these conditions. Such miniature sensors could be de-ployed anywhere—airports, seaports, public buildings, strategiclocations in waterways—providing omnipresent protection.

The technical advances in a variety of fields, including com-puting, interphase chemistry, and telemetry, have maturedenough to be incorporated into MEMS sensor technology. Thus,the MEMS sensor platform is poised for such a revolution insensor technology. For example, present sensor technologycan be compared with the status of computer use in the 1960swhen any serious computing was restricted to giant computersinstalled in a handful of institutions. But the “silicon revolution,”expressedsuccinctly inMoore’sLaw[1],hasenabledwidespreadcomputer use. Currently, expensive and bulky detection systemsare sparsely deployed at strategic locations, such as airports. Ananalogous revolution in sensor technology may be possible witha sensor platform based on MEMS that will allow deploymentof intelligent, miniature sensors by the millions. Besides thewar against terrorism, such a sensor platform would be usefulin medical diagnostics, law enforcement, landmine detection,environmental monitoring, and many other applications.

Therefore, the primary issue can be stated as follows: A needexists for rapid detection of trace quantities of a wide varietyof threat agents present in complex mixtures using miniature,inexpensive sensors. Here we discuss the current status of re-search on achieving this goal with sensor arrays and point outthat microcantilever MEMS sensors provide a suitable platform.It must be noted that this paper is not intended to be a compre-hensive review paper. We will briefly review the current statusof sensor arrays and refer to selected papers on microcantilverand other sensor technologies. Our intention is to point out thepossibility of achieving a miniature sensor platform for home-land defense based on microcantilever sensors.

II. SIMULTANEOUS AND RAPID DETECTION

OF MULTIPLE ANALYTES

For just over two decades, research has been conducted ondevelopment of an “electronic nose” based on sensor arrays

1530-437X/$20.00 © 2005 IEEE

PINNADUWAGE et al.: MOORE’S LAW IN HOMELAND DEFENSE 775

[2]–[6]. In 1982, Persaud and Dodd [7] published the first paperon a modern electronic nose that attempts to mimic the olfac-tory system: They pointed out [7] that the mammalian olfactorysystem is based on broadly tuned receptor cells, and that the dis-crimination properties of the olfactory system are a property ofthe system as a whole. The olfactory receptors are not highly se-lective toward specific odorants; each receptor responds to mul-tiple odorants, and many receptors respond to any given odorant[8]. Pattern recognition methods are thought to be a dominantmode of olfactory signal processing. The electronic nose tech-nologies are based on the same concept of using broadly tunedmultiple sensors. An advantage of this approach is the ability todetect a variety of analytes simultaneously.

Several possible sensor array platforms have been studied upto now [2], [9] as candidates for an electronic nose, includingthose based on metal oxide, MOSFET, conductive polymer,fiber-optic, electrochemical, and acoustic wave sensors. Thesesensor technologies are described in detail by Gardner andBartlett [6]. A wide variety of statistical and neural networktechniques have been employed to process the data originatingfrom the sensor arrays [6].

It is not surprising that one of the first applications of elec-tronic noses based on sensor arrays has been for the evalua-tion of odors and for industrial process control [6]. Sensor ar-rays have been shown to be successful in these applications inwhich the primary interest is in qualitative analyzes that relyon changes in the sensor-array response patterns. Commercialinstruments based on sensor arrays are available for these ap-plications [6]. The commonly used sensor technologies in theseapplications are metal-oxide, conducting polymer, and acoustic[surface acoustic wave (SAW) and QCM] sensors [6]. Metaloxide sensors are bulky, but they have fairly good sensitivitiesof sub-parts-per-million (ppm) levels; conducting polymer sen-sors are small with low-power consumption, but the sensitivitiesare generally an order of magnitude lower compared with metaloxide sensors; SAWs sensors derive their vapor-detecting capa-bility from sorbent coatings and can detect the mass of the vaporadsorbed, which in some cases can be distorted by the changesof the viscoelastic properties of the coating material [8].

However, the detection of trace amounts of vapors in mix-tures has not yet been successfully achieved with sensor arrays[6], [10]. Almost all sensor array studies conducted up to nowhave used sensors with detection limits at or near 1 ppm leveland a maximum of about 12 sensors in an array [6], [10]. Betterdetection sensitivities and higher numbers of sensors per arraymay be needed to achieve trace detection in complex mixtures.A recent study [11] has concluded that increasing the number ofsensors in an array did not improve performance significantlyfor mixture analysis; a maximum of six sensors were used inthe array, and the detection limits of the SAW sensors used inthe array for the component vapors were at low ppm levels [11].Moreover, it has been noted that there are over 1000 olfactorygenes in humans and over 100 million olfactory cells in a ca-nine’s nose [2].

It is likely that MEMS sensors may provide both high sensi-tivity and the ability to use a much higher number of individualsensors in an array, thus enabling the detection of trace vaporsin complex mixtures. The high sensitivity for MEMS sensors

originates in the inherently large surface-to-volume ratio of themicroscopic objects. Thus, a MEMS sensor based on surface in-teractions for signal transduction can be expected to provide anenormous amplification in sensitivity.

The rapid development of the integrated circuit (IC) tech-nology during the past decade has initiated the fabrication ofchemical sensors on silicon or complementary metal oxidesemiconductor (CMOS) [12]–[14]. The largely two-dimen-sional integrated circuit and chemical sensor structures pro-cessed by combining lithographic, thin-film, etching, diffusive,and oxidative steps have been recently extended into the thirddimension using micromachining or MEMS technologies—acombination of special etchants, etch stops, and sacrificiallayers [12]. Therefore, MEMS technology provides an excel-lent means to meet other key criteria of chemical sensors, suchas miniaturization of the devices, low-power consumption, andbatch fabrication at low cost as well.

Currently, two CMOS-technology-based MEMS sensors arebeing studied, flexural plate wave (FPW) sensors and micro-cantilever sensors. The FPW sensor is similar in many ways tothe more common SAW and QCM sensors, and it can monitorthe mass absorbed on a coating deposited on the sensor [15],[16]. It has shown detection sensitivities of high parts-per-tril-lion (ppt) levels at the highest measured sensitivity level [15].On the other hand, the microcantilever sensors have additionaldetection modes [17], including the highly sensitive bendingmode that makes use of the high surface-to-volume ratio of theMEMS sensor element and often displays sub-ppt detection sen-sitivities as discussed in Section III.

III. MICROCANTILEVER SENSORS

In 1994, researchers observed that the microcantilevers usedin atomic force microscopy (AFM) were sensitive to externalphysical and chemical influences. Thundat et al. [18]–[21]pointed out the possible use of bending and frequency shift inmicrocantilevers for chemical sensing, and Gimzewski et al.[22]–[24] pointed out applications in thermal calorimetry. Sincethen, researchers all over the world have been reporting the useof microcantilevers for detecting various physical, chemical,biological, and radiological influences [17], [25].

Microcantilevers are miniature diving boards that are micro-machined from silicon or other materials. The length of thesecantilevers is often in the range of 100–200 m, whereas thethickness ranges from 0.3 to 1 m. (It is interesting to note thatin the human olfactory system, the site of interaction of the cellwith odorous molecules occurs at hairlike cilia, which are upto 200- m long and provide an increased surface area for odorsensing [6].) The key to the high sensitivity of the microcan-tilevers is the enormous surface-to-volume ratio, which leads toamplified surface stress, as discussed below. Fig. 1 shows a setof microcantilevers together with a human hair for comparison.

Microcantilevers have two main signal transduction methods,bending and mass-loading. In the mass-loading mode, micro-cantilevers behave just like other gravimetric sensors such asQCM, SAW, and FPW transducers: Their resonance frequenciesdecrease due to the adsorbed mass. In the “gravimetric mode,”


Fig. 1. Scanning electron microscope image of a set of cantilevers of different sizes and shapes. For comparison, a human hair is also shown.

the microcantilever detection sensitivity seems to be compa-rable with that of the other gravimetric sensors [26].

The other signal transduction method, that is, the bendingresponse, is unique to the microcantilever; for example, ifa differential surface stress is achieved by preferentially ad-sorbing target molecules to one of its broad surfaces (by usinga chemical coating on that surface), the microcantilever willbend. Since a differential surface stress is required for bending,only one broad surface should be coated for bending-modeoperation. Therefore, the mass-loading information is used onlyas a bonus. In the bending mode, microcantilever detectionsensitivity is at least an order of magnitude higher than otherminiature sensors such as SAW and QCM that are also beinginvestigated as chemical sensors. Even though it is difficultto accurately compare the detection sensitivities for differentsensors, the low- or below-ppt detection sensitivities routinelyachieved with microcantilever sensors [27]–[32] have not beenmatched by any other sensor. The closest comparison wouldbe the detection of dinitrotoluene (DNT) using SAW [33]and microcantilever [30] sensors, where both sensors usedthe same polymer coating SXFA-[poly(1-(4-hydroxy-4-tri-fluoromethyl-5,5,5-trifluoro)pent-1-enyl)methylsiloxane]. Anestimated 400-ppt detection sensitivity is achieved for 200-sexposure of DNT to the SAW sensor ([33, Fig. 10]), whereas300-ppt detection sensitivity was achieved with the microcan-tilever in a few seconds [30].

Since the microcantilever bending signal originates in surfacestress, diffusion of large amounts of vapor to a thick coating isnot necessary. Thus, even though one-monolayer-thick self-as-sembled monolayer (SAM) coatings may not be appropriate forgravimetric sensors [34], they are ideally suited for microcan-tilever sensors. Because diffusion of the analyte vapor to thebulk of the coating is avoided, the response and relaxation of amicrocantilever coated with a SAM can be fast. Fig. 2 shows theresponse of a microcantilever coated with a SAM of 4-mercap-tobenzoic acid to a vapor stream of the plastic explosive pen-taerythritol tetranitrate (PETN) at a concentration of 1.4 ppb[29]. The rapid and sensitive response of the bending signal aswell as the superiority of the bending signal compared with themass (frequency) measurement is clear.

Fig. 2. Response of a 4-mercaptobenzoic acid (4-MBA)-coated siliconcantilever to the periodic turning on (10 s) and off (60 s) of a PETN streamof 1.4-ppb concentration in ambient air. The solid curve depicts the bendingresponse, and the dots connected by dashed lines depict the resonance frequencyof the cantilever [29].

It must be emphasized that the bending of the microcantileveris not caused by the weight of the deposited material. A 40-ngmicrocantilever bends about 1 nm due to its own weight, whichis just above the noise level for a cantilever-bending signal.Therefore, the microcantilever bending caused by the weight ofthe deposited material of picogram levels is insignificant. Onthe other hand, for micron-size objects like microcantilevers,the surface-to-volume ratio is large, and the surface effects areenormously magnified. Thus, adsorption-induced surface forcescan be extremely large. The adsorption-induced force can be at-tributed to the change in surface free energy due to adsorption.Free energy density (millijoule/square meter) is the same as sur-face stress (newton/meter). This surface stress is analogous tosurface tension in a liquid. Incidentally, surface stress has theunits of a spring constant of a cantilever. Therefore, if the sur-face free-energy density change is comparable with the spring


constant of a cantilever, the cantilever will bend. When probemolecules bind to their targets, steric hindrance and electrostaticrepulsions cause the bound complexes to move apart. Becausethey are tethered at one end and because the surface area is fi-nite, they exert a force on the surface.

Another advantage of the microcantilever sensor is that itworks with ease in air and in liquid. Both resonance frequencyand bending modes can be used in liquid. Because of thesmall mass of microcantilevers, they execute thermal motion(Brownian motion) in air and liquid. Therefore, no externalexcitation technique is needed for exciting cantilevers into res-onance; the degraded quality factor in liquid can be improvedby a feedback mechanism [35], [36]. However, as in the vaporphase, the microcantilever bending signal is mainly used forsensing in liquid (see, for example, [37]–[39]).

Despite its high sensitivity, the cantilever platform offers nointrinsic chemical selectivity just like other chemical sensorssuch as SAW and QCM. One surface of the silicon microcan-tilever can be functionalized so that a given molecular specieswill be preferentially bound to that surface when it is exposedto a vapor stream. Therefore, detection sensitivity is vastly en-hanced by applying an appropriate coating on one cantilever sur-face. Such a coating can, in principle, provide selectivity as well.

Basic requirements for a sensor system are sensitivity, selec-tivity, and reversibility. Although the sensitivity and selectivityare critical, reversibility may not be necessary under some con-ditions. The lack of selectivity will lead to frequent false pos-itives (false alarms), which is as bad as false negatives due tolack of sensitivity. However, the degree of selectivity is inher-ently connected to the reversibility of detection.

Selectivity and reversibility are often competing characteris-tics of chemical sensors. The type of interaction occurring be-tween analyte molecules and the cantilever coating determinesthe adsorption and desorption characteristics. Low-energy,reversible interactions such as physisorption generally lack anacceptable degree of selectivity: The energies involved rangefrom van der Vaals interactions (energy kJ mol ) toacid-base interactions (energy kJ mol ). Furthermore,the weak interaction may lead to insufficient sorption, whichmakes sensor response weak. At the other end of the spectrum,highly selective interactions are normally covalent in nature(chemisorption) and are not reversible (binding energies are

kJ mol ) under normal conditions.Two “intermediate-range” interactions can provide limited

selectivity while being reversible. One is hydrogen bonding, andthe other is coordination chemistry. A hydrogen atom covalentlybond to an electronegative atom will have the electron cloudshifted away from it, and thus it can form a hydrogen bond withanother electronegative atom. For example, the O atoms in thecharacteristic nitro groups of explosives can participate in hy-drogen bonding. A coordination compound consists of a centralmetal atom surrounded by neutral or charged, often organic, lig-ands. In the ligand, one or more donor atoms interact with themetal ion. The selectivity now can be influenced by the choiceof the metal ions as well as by the choice of the ligand, bothfrom an electronic or steric point of view [40].

In most applications, it is desirable to have the ability toregenerate the sensor, and thus the use of “intermediate range”

interactions will be necessary, which in turn broadens the targetrange. Therefore, a single microcantilever coating may notprovide sufficient selectivity if reversible sensor operation isrequired (one exception to this is the detection of hexavalentchromium in a complex matrix using a single cantilever [38]).However, in general, it will be necessary to use an array ofmicrocantilevers with multiple coatings to obtain sufficient se-lectivity, especially if the sensor is required to monitor multiplethreats. Pattern recognition schemes need to be employed toextract the composition of the target vapor stream.

Much work on coating materials has been done over the years,especially in the development of SAW sensors [34], [41], [42].In this context, various polymer coatings have been investigated[34], [41]. These polymer coatings have been optimized pri-marily for the use in SAW and QCM sensors in which massloading is the key sensing parameter. This consideration has ledto the development of mainly polysiloxane films that allow rapiddiffusion of the analyte into the bulk of the film to provide op-timum mass loading. SAM coatings have recently gained atten-tion because of their simplicity and robustness [43], [44]. Eventhough SAM coatings may not provide high mass sensitivitydue to the lack of “volume absorption” for SAW, QCM, andsimilar sensors [34], they are ideal for microcantilever sensorsworking in the bending mode for the following reasons: 1) thecoating can be applied by a simple procedure such as soakingor touch coating [45], 2) the covalent binding of the SAM to thesensor surface provides a stable coating with direct transmissionof the stress due to analyte binding to the microcantilever, and3) because diffusion of analyte molecules to a thick coating isavoided, sensor response and relaxation is fast. Some examplesof the usage of both polymer and SAM coatings with microcan-tilever sensors will be presented below.

Over the past ten years, many breakthroughs have taken placein the area of microcantilever sensors. Advances in microma-chining made it possible to develop arrays of microcantileverbeams with required sensitivity. Many chemically selectivecoatings for chemical speciation have also been developedas discussed earlier. Receptor-ligand, antibody-antigen, orenzyme-substrate reactions have been studied for biologicaldetection [46], [47]. Advances have also been made in manyother crucial areas, such as immobilization of selective agentson cantilever surfaces and application of selective layers oncantilever arrays. Aided by such tools, physical, chemical, andbiological detection has been demonstrated using microcan-tilever sensors. The following sections of this paper discussrepresentative microcantilever-based detection results thatpertain to homeland security.

IV. SELECTED EXAMPLES OF MICROCANTILEVER STUDIES

RELEVANT TO TERRORIST THREAT DETECTION

A. Explosives

Most explosives have low-vapor pressures at ambient temper-atures. Table I shows vapor pressures of trinitrotoluene (TNT);2,4 dinitrotoluene (DNT); pentaerythritol tetranitrate (PETN);and hexahydro-1,3,5-triazine (RDX) at a range of temperatures.TNT is one of the more commonly used explosives. Even thoughDNT is not an explosive, detection of explosives can be based


TABLE IVAPOR PRESSURES (NORMALIZED TO ATMOSPHERIC PRESSURE)

OF SOME COMMON EXPLOSIVES (PARTS-PER-BILLION)

on the detection of DNT because of the following reasons. 1)Even though the main ingredient of explosives is TNT, a majorimpurity in production grade TNT is DNT. 2) DNT is a syn-thetic byproduct of TNT. 3) The saturation concentrations ofDNT in air are approximately 25 times larger than TNT (seeTable I). High explosives, such as PETN and RDX, are themost serious threats in aircraft sabotage because they can beeasily molded for concealment (the infamous “shoe bomber”had PETN hidden in his shoes), are very stable in the absenceof a detonator, and can in small amounts destroy a large airplanein flight. They are, in fact, the explosives most commonly usedfor this purpose.

We are pursuing two parallel schemes for the detection ofexplosive vapor. One is based on the commonly used “passivedetection,” where a microcantilever coated with a suitablesubstance adsorbs the explosive molecules preferentially to onesurface that leads to the bending of the cantilever. Using thisapproach, we have reported the detection of plastic explosivevapors using a 4-mercaptobenzoic acid SAM coating [29] andthe detection of dinitrotoluene using SXFA-[poly(1-(4-hy-droxy-4-trifluoromethyl-5,5,5-trifluoro)pent-1-enyl)methyl-siloxane]-polymer-coated microcantilevers [30]. Datskos et al.[51] reported the detection of TNT and DNT with microcan-tilevers with nanoporous coatings.

The second approach employs a novel “active detection”method in which explosive vapors are deposited on uncoatedpiezoresistive microcantiver surfaces, and a pulsed voltage isapplied to the microcantilever to heat it to high temperatureswhere the deposited explosive material undergoes deflagration[52], [53]. Because the deflagration event only occurs for explo-sive material [54], this method yields unambiguous detectionof explosives.

A vapor generator developed at the Idaho National Engi-neering and Environmental Laboratory (INEEL) was used togenerate the PETN, RDX, and TNT vapor streams. Flowingambient air through a reservoir containing a small amount ofthe explosive material generated the vapor stream.

Fig. 3 shows the cantilever bending and resonance frequencyvariation when a SAM-coated cantilever is exposed to PETN.As seen from Fig. 3, the bending response of the cantileverto the PETN exposure is extremely sensitive and fast. Becausethe noise level of the bending response in these experiments is

2 nm (3 standard deviation of the noise level), the detec-tion sensitivity corresponding to Fig. 3 is 14 ppt. Maximumbending of the cantilever is achieved within 20 s.

Fig. 3. Response of a 4-MBA-coated silicon cantilever to PETN vapors of1.4-ppb concentration in ambient air. The solid curve depicts the bendingresponse, and the dots depict the resonance frequency of the cantilever. Thefrequency shift due to the adsorption of PETN vapor corresponds to a massloading of �15 pg on the cantilever.

Fig. 2 shows the rapidity with which explosive vapors can bedetected and the relatively fast relaxation of the cantilever whenthe vapor stream is turned off. When the PETN stream is turnedon for 10 s, a 40-nm deflection signal is observed. When thevapor stream is turned off, the cantilever is relaxed, returningalmost to its original position within 60 s. As mentioned pre-viously, another important observation from the data shown inFig. 2 is that the resonance frequency of the cantilever does notchange significantly as a result of the small amount of PETNdeposited in 10 s. The cantilever bending is still easily detected.

It is assumed that the hydrogen bonding between the nitrogroups of the explosives molecules and the hydroxyl group of4-MBA is responsible for the easily reversible adsorption of ex-plosive vapors on the SAM-coated top surface of the cantilever[30].

In the case of “active detection” of explosive vapors, theexplosive vapor was allowed to deposit on a piezoresistivemicrocantilever, and a 10-V, 10-ms voltage pulse was appliedto it. This led to the deflagration of the explosive material,which resulted in an exothermic “bump” on the microcantileverbending signal (the bending of the cantilever was monitoredusing optical detection) [53]. In addition, the smoke plumegenerated can be clearly seen for deposited TNT mass above afew hundred picograms. Fig. 4 shows a series of images cap-tured with a high-speed camera [53]. Fig. 4(a) shows an imageof a TNT-loaded cantilever before application of a voltagepulse. Fig. 4(b)–(d) shows a sequence of frames just after theheating pulse. Fig. 4(b)–(d) clearly shows the evolution of aplume of gas from reactions occurring on the cantilever; thesmoke plume in the path of the laser light (used to monitor thecantilever bending) is illuminated, clearly displaying the smokering. Laser scattering off of the cantilever is hardly visible inFig. 4(d), whereas it is clearly visible in Fig. 4(a). This can beexplained as follows: Just before the application of the voltagepulse [Fig. 4(a)], the cantilever surfaces are covered with TNT,


Fig. 4. High-speed photos of a TNT-deposited cantilever subjected to a voltagepulse leading to deflagration (a) just before the voltage pulse and (b)–(d) asequence of frames just after the voltage pulse. The left (red) portion of thecircular plumes in (b)–(d) are illuminated by the laser light used to monitorthe cantilever bending. The amount of TNT deposited on the microcantileverwas �1 ng (calculated from the observed shift in microcantilever resonancefrequency).

which leads to higher specular reflection of the laser beamcompared with that for a TNT-free cantilever in Fig. 4(d).

The described “active detection” method for explosives, al-though not suitable as a component for a miniature sensor withmultiple-threat detection capabilities, can be used in an explo-sive-detection sensor package to provide confirmation of a pos-itive signal generated by the “passive detection” sensor arraymethod.

B. Biowarfare Agents

Although not as prevalent as explosives, the use of chem-ical/biological agents as a warfare or terrorist weapon is a seriousthreat. Several biological agents exist that can be used as warfareagents. Examples of biowarfare agents include botulinum toxin,Shiga toxin, diphtheria toxin, anthrax, and ricin. Most biologicalagents are derived from bacterium. Ricin toxin is produced from

Fig. 5. Detection of a biowarfare agent. Cantilever bending response as afunction of exposure time to ricin.

caster bean extract. Botulinum toxins are some of the most deadlysubstances known, These toxins are 100 000 times more toxicthan the nerve agent sarin, 10 000 times more toxic than VX, and1000 times more toxic than ricin [55]. The estimated lethal doseof a botulinum toxin (type A) is 1 ng/kg of body weight, and thelethal blood level of the toxin is around 20 pg/mL. The estimatedlethal dose of ricin is 3 g/kg of body weight. At present, nowidely available rapid test exists for most biological agents.

We have successfully detected the biowarfare agent ricinusing modified microcantilevers [56]. One side of the mi-crocantilever was modified with ricin antibody. When ricinwas introduced into a liquid cell housing the cantilever, thecantilever bent due to ricin-antibody interaction. Fig. 5 showsthe cantilever bending response as a function of time after theintroduction of ricin. The experiments were not done underflow conditions. The large response time is due to diffusion ofricin toward the cantilever.

A key requirement for the detection of biological species isthe ability to modify the microcantilever surface for biospecificrecognition. However, most molecular-recognition-agent-con-taining molecules are not commercially available, and thustremendous amounts of synthetic work must be done to de-velop each molecular-specific microcantilever surface. In thisregard, a general microcantilever surface-modification methodthrough layer-by-layer technology for biomolecule recognitionwas reported [57]. Weeks et al. [58] reported on the detectionof specific Salmonella enterica strains using a functionalizedmicrocantilever.

C. Chemical Warfare Agents

Basically, there are three types of chemical warfare agents:nerve agents, blister agents, and choke agents. Nerve agentsaffect the transmission of nerve impulses in the nervous system.All nerve agents belong chemically to the group of organophos-phorus compounds. Examples of nerve agents are GA (tabun),GB (sarin), and GD (soman). Blister agents burn and blisterthe skin or any other part of the body they contact. They act onthe eyes, mucous membranes, lungs, skin, and blood-formingorgans. They damage the respiratory tract when inhaled andcause vomiting and diarrhea when ingested. The blister agentsinclude sulphur mustard (HD), nitrogen mustard (HN), andLewisite (L). Choking agents are chemical agents that attack


Fig. 6. (Left) Schematic representation of the bending of a self-assembled Cu =L-cysteine bilayer-coated microcantilever upon complexation with DMMP.(Middle) Bending response as a function of time, t, for a cantilever coated with a self-assembled Cu =L-cysteine bilayer on the microcantilever after injection of10 M solutions of various electrolytes. (Right) Maximum deflection of a silicon cantilever coated with a self-assembled Cu =L-cysteine bilayer on the goldsurface as a function of the concentration of DMMP in 0.01-M tris buffer at pH = 5:0.

lung tissue, primarily causing pulmonary edema. In low con-centrations, choking agents act on the respiratory system tocause an accumulation of fluid in the lungs, which can lead todeath. In high concentrations, choking agents lead to death forthe same reason, but they might also affect the upper respiratorytract. Chemicals classified as choking agents are chloropicrin(PS), chlorine (Cl), phosgene (CG), and diphosgene (DP).

Chemical warfare agents have been used in the past. Nerveagents are among the most toxic of known substances. Thenerve agent—either as a gas, an aerosol, or a liquid—enters thebody through inhalation or through the skin. Poisoning mayalso occur if liquids or foods contaminated with nerve agentsare consumed. If a person is exposed to a high concentrationof a nerve agent (e.g., 200-mg sarin/m ), when the agent isabsorbed through the respiratory system, death may occurwithin a few minutes. When the nerve agent enters the bodythrough the skin or through consumption, death is less sudden.

A self-assembled bilayer of Cu L-cysteine on a goldsurface has recently been characterized [32] and could be usedto recognize phosphonyl groups because of the formation ofstrong P O Cu bonds. We have used a Cu L-cysteinebilayer-modified cantilever to detect nerve agents in aqueoussolution based on this mechanism. Dimethyl methyl phospho-nate (DMMP) was used as a sarin nerve gas simulant [32].The coating of the Cu L-cysteine bilayer was formed byimmersing the cantilever into a M solution of L-cysteinein tris buffer solution (pH ) for 24 h. The microcantileverwas then rinsed with tris buffer solution and immersed in

M CuSO tris buffer solution for another 24 h. WhenDMMP was introduced into the cantilever chamber, the can-tilever underwent bending due to interaction. When the DMMPwas replaced with tris buffer solution, the cantilever did notreturn to its original position. The flow rate was 4 mL/h. Fig. 6shows the cantilever bending response as a function of DMMPconcentration [32].

It has been shown that phosphonyl groups strongly bind withCu and copper complexes. Organophosphorus compoundsare unstable at high pH levels. The cantilever bending causedby exposure of DMMP is most likely the result of complexa-tion of DMMP with the Cu L-cysteine bilayer on the micro-cantilever surface through Cu O P bonds that alter thesurface stress. A cantilever deflection of almost 5 nm can bedetected even for a DMMP concentration of M. The ob-served intereference from analytes such as sodium phosphate,DL-aspartic acid, dimethylamine, 1,10-phenanthroline, aceticacid, and acetonitrile was negligible.

D. Nuclear Radiation

So far no release of radioactive materials has occurred due toterrorism. However, the threat of dirty bombs made of explo-sives and radioactive materials is still a possibility. The possi-bility of nuclear materials on the black market from other coun-tries increases the threat level and emphasizes the need for im-proved radiological sensing devices that can be massively de-ployed. Dirty bombs could be detected with an explosive-vapordetector.

We have also successfully demonstrated a micromechanicalradiation detector for alpha particles [59]. The detection wascarried out in air. In this experiment, alpha particles wereallowed to impinge on an electrically insulated metallic surfaceof 1-mm diameter. A microcantilever that is kept at a fixeddistance of a few nanometers undergoes deflection as a functionof residual charge accumulation on the surface. The staticdeflection method suffers from interference from thermal drift.To avoid thermal drift, we have used the resonance response ofa cantilever. Transients produced by continuously running forcecalibration curves on the AFM microscope accumulated theresonance transient. In an AFM, force calibration is achievedby pushing the sample (collector sphere in this case) againstthe cantilever and then withdrawing it. As it is withdrawn, the


cantilever tip does not initially break free of the surface becauseof tip-surface adhesive forces; instead, the cantilever bendsuntil its restoring force (due to its spring constant) exceeds theadhesion force. Because tip-surface attraction decreases morerapidly than the restoring force with increasing tip-surfacedistance, the cantilever rapidly accelerates away and enters theharmonic resonance mode predicted by its mass and springconstant. This transient resonance signal gradually loses itsenergy (e.g., to air or through internal friction) and comes torest. Transient oscillations were recorded on a digitizing oscil-loscope at Hz using an output signal from the deflectionmeasurement system and then analyzed using the appropriatesoftware. The force-distance curve serves as a measure ofdistance between the cantilever and the surface. Because theforce gradient plays a major role in the cantilever resonancefrequency, precise knowledge of the distance between the tipand the surface is essential for avoiding error in frequency-re-sponse measurements.

We have detected alpha particles using the shift in resonancefrequency of the microcantilever due to electrostatic force(nonuniform fields produce a change in resonance frequencydue to modification of the force constant of the cantilever as theresult of field gradients). Using this method, a single alpha par-ticle can be detected, provided the frequency is measured witha sensitivity of Hz. Frequency, however, can be detectedwith even higher sensitivity using available electronics. Thedevice can be optimized using large area collectors. We havealso used bending variation as well as variation in damping ofcantilever frequency to detect alpha particles. These techniqueshave sensitivities of the same order of magnitude as describedabove. Fig. 7 shows the resonance-frequency variation (mea-sured from transients) as a function of exposure to alphaparticles [59].

Until now, most studies on microcantilever sensors have beenconducted to illustrate that specific microcantilever coatings canbe developed to detect various species with very high sensitivity.However, to develop a miniature sensor system that can simul-taneously detect a wide variety of threats without operator as-sistance, the following basic capabilities are required.

• To selectively detect a variety of threats (or even a va-riety of chemical species), a cantilever array consistingof tens or hundreds of cantilevers may be needed.

• The signal transduction method must be simple andcompact requiring no operator assistance. It is not suf-ficient to have a MEMS sensor element if the signaltransduction cannot be miniaturized.

• Telemetry must be built into the design so that aftera threat is identified, it can be transmitted instanta-neously to a monitoring station.

We discuss the status of these capabilities in the followingsections.

V. CANTILEVER ARRAYS

Compared with other sensor technologies, relatively fewstudies have been conducted with microcantilever arrays [26],[60]–[67]. Out of these, pattern recognition algorithms were

Fig. 7. Nuclear radiation detection. Resonance frequency variation of acantilever as a function of exposure to alpha particles.

used to identify components of vapor mixtures in only twostudies [62], [63], in which simple mixtures were analyzedusing an eight-cantilever array; it is interesting to note that themicrocantilever bending signal was monitored in both studies,using optical detection.

Therefore, the feasibility of identification of components in asimple mixture has been illustrated [62], [63] with the microcan-tilever platform. Based on the lessons learned from the chemicalsensor array studies conducted up to now (for example, [2], [6],[9], [11], [34]), some or all of the following may be needed toimprove the detection capabilities of a microcantilever sensorarray so that it can detect trace amounts of multiple analytes:vapor preconcentration, careful control of experimental condi-tions to provide a “good memory” for pattern recognition, andinclusion of a wide variety of coatings to provide a sufficientsignal variation.

Detection speed is challenging, especially for chemical, bio-logical, and radiological exposures. For instance, a few secondsmay be all the time available to respond to threat-level quanti-ties of a nerve agent. Especially with the slower reaction rates ofbiological agents, detection times of seconds to minutes couldlimit the amount inhaled and simplify subsequent prophylacticaction.

Even though the enhanced sensitivity of microcantileversis due primarily to their small size (i.e., the large sur-face-to-volume ratio that greatly amplifies the bending signal),the small size also decreases the probability of target moleculesbeing captured onto the sensor surface. This loss could be com-pensated for by using a preconcentration system at the front endof the microcantilever sensor. Preconcentrators that can rapidlybring a sufficient quantity of agent into the detection volumeof the cantilever element could be essential for significantlydecreasing the detection time.

VI. PIEZORESISTIVE DETECTION OF MICROCANTILEVER

BENDING SIGNAL—A SIGNAL TRANSDUCTION METHOD

SUITABLE FOR A MINIATURE SENSOR

When making a miniature sensor, it is not enough just to havea small sensor element. Signal transduction and transmissioncapabilities also need to be incorporated into a small package.In this section, we briefly discuss a suitable signal transductionmethod for microcantilevers.


Since the advent of the AFM, several signal transductionmethods have been explored for monitoring microcantileverdeflections. These methods include optical, piezoresistive,piezoelectric, and capacitive methods [68]. In the opticalmethod, a laser diode is focused at the free end of a cantilever.The reflected light is detected with a position-sensitive detector(PSD). In the piezoresistive method, the silicon cantilever isdoped with boron or phosphorous. The electrical resistanceof the doped channel changes as a function of cantileverbending. In the piezoelectric method, cantilever bending causesa transient charge on a piezoelectric film, such as ZnO, onthe cantilever. Because the signal is transient, it is not idealfor static cantilever-bending measurements. In the capacitivemethod, the capacitance between the cantilever, which ismicromachined with a space between the cantilever and thesubstrate, is measured.

The piezoresistive microcantilever readout method was orig-inally developed by Tortonese et al. [69] for atomic force mi-croscopy. Subsequently, Boisen et al. [70], [71] developed amicrocantilever chip with four piezoresistive microcantileversspecifically for sensor applications. Such a microcantilever chiphas recently been commercialized by Cantion, Inc., Denmark.This piezoresistive microcantilever platform is ideal for the pro-posed single-sensor platform for the reasons discussed below.

We recently developed [32] a handheld sensor that makesuse of the four-microcantilever (Canti-4) chips from Cantion.Two cantilevers in the Canti-4 chips are coated with gold (a30-nm-thick gold layer on top of a 3-nm chromium adhesionlayer). In these experiments, we used only the two gold-coatedmicrocantilevers, one of which was coated with a 4-mercap-tobenzoic acid SAM [29]. We have named our first handheldsensor “SniffEx” because it is being developed for the detectionof explosive vapors. A photograph of SniffEx is shown in Fig. 8.The differential signal between the two gold-coated microcan-tilevers was obtained by using those two microcantilevers in aWheatstone bridge circuit [31]. The Canti-4 chip is located inan aluminum flow cell observed at the top of Fig. 2. A 1/16-in.stainless steel tube is connected to the input of the flow cell. In“sniffing” experiments, a small pump (shown at the top left ofFig. 8) connected to the exit port of the flow cell is used to ex-tract vapor from a sample bottle [31]. In experiments in which acalibrated explosive-vapor generator developed at INEEL [31]was used, the pump was turned off and the inlet tube was con-nected to the INEEL vapor generator, which delivered low con-centrations of explosive vapor at a flow rate of 50 standard cubiccentimeters per minute (sccm).

Fig. 9 shows the response of the SniffEx to a calibrated RDXvapor stream in nitrogen carrier gas at a flow rate of 50 sccm.Three sets of data taken on three separate days with the samecantilever chip are shown for a RDX vapor concentration of6 ppt. These show that data obtained under such controlled con-ditions have good reproducibility. The noise level (standard de-viation of the background) for the data of Fig. 9 is V,and thus, the detection limit for a single measurement is 1.8 V(3 standard deviation). This limit corresponds to RDX vaporconcentrations well below the ppt level (less than 1 part in ).It is clear that the piezoresistive microcantilever sensor has ex-tremely high sensitivity.

Fig. 8. Photograph of the SniffEx handheld sensor [31]. The flow cell islocated at the top of the figure; the small pump used to extract vapor is shownon the top left; the battery powering the pump is on the top right. The batteryto power the electronics is located at the bottom. Although normally the powercomes from the computer’s serial port, this battery will be used when thecomputer is replaced by a personal data assistant (PDA). Communication witha laptop computer is via the serial port located at the bottom of the handheldunit.

Fig. 9. Response of SniffEx to a calibrated RDX vapor stream from the INEELvapor generator [31]. The carrier gas was nitrogen at 50-sccm flow, and thegenerator temperature was 25 C, which corresponds to an RDX concentrationof 6 ppt. Three data sets obtained on three separate days are shown. A signalstrength of 1 mV corresponds to a differential surface stress of 1.25 N/m.

As shown in our measurements on the detection of the va-pors of the plastic explosives, the detection sensitivity for thepiezoresistve transduction method [31] is comparable with thatfor the optical transduction method [30], which is considered


to be the most sensitive signal transduction method for micro-cantilevers. The use of a reference cantilever for each coatedcantilever, which is inherent in the Wheatstone bridge measure-ment used with the SniffEx [31], provides common-mode rejec-tion and eliminates sensor drift caused by external effects suchas temperature and pressure variations. It is possible to massfabricate sensor chips with hundreds of pairs of such microcan-tilevers on a single chip. These features, together with the ca-pability to incorporate the all-electrical detection circuit on asingle chip, makes the piezoresistive transduction method thebest suited for an easy-to-use miniature sensor.

VII. TELEMETRY

Telemetry has also made significant advances in the pastdecade. Many digital instruments, including wireless moni-toring instruments, are in use today. But the sensors do not takefull advantage of silicon technology partly because they areanalog in nature. The development of an application-specificintegrated circuit (ASIC) with built-in telemetry (i.e., a tele-sensor) may not be available in the near future [72]. However, aminiature sensor package with telemetry that is not on a singlechip can currently be built.

Therefore, it is feasible that in a few years a cantilever-based,integrated miniature sensor (CIMS) could be used for limitedapplications in explosive-vapor detection and some chemicalvapor detection. It could take longer to extend the use of CIMSto detect a broad spectrum of chemical, biological, radiolog-ical, or explosive (CBRE) threats because of the complexitiesinvolved. As the capability to incorporate more detection capa-bilities grows, the cost of CIMS will decrease because of in-creased deployment, analogous to the technology of silicon-in-tegrated circuits that has progressed according to Moore’s Law.The fabrication of a true telesensor chip with built-in telemetrywill get us there much faster.

VIII. CONCLUSION

In the war on terrorism, the capability to detect CBREweapons in advance and the capability for the early detectionof chemical or biological agents from an attack that alreadyoccurred should be at the forefront of a long-term strategy. Thisrequires omnipresent miniature sensors that can detect multipleterrorist threats with high sensitivity and selectivity and thatcan relay the warnings instantaneously. Studies conducted onmicrocantilever sensors, together with the technological ad-vances in neural analysis and telemetry, indicate the feasibilityof a single-sensor platform based on silicon microcantilevers.Their simplicity of operation, high sensitivity of detection,miniature size, and flexibility to detect a wide variety of ter-rorist threats, together with low cost of production, make themicrocantilever-based sensors an attractive solution.

ACKNOWLEDGMENT

The authors would like to thank Dr. X. Yan, Dr. V. Boiadjiev,Dr. F. Tian, Dr. G. Muralidharan, Dr. A. Wig, Dr. P. Oden, Dr. M.Doktycz, Dr. R. Warmack, Dr. C. Britton, Dr. D. Hedden, Dr. J.

Hawk, Dr. A. Gehl, and Dr. D. Yi for their help with the exper-iments conducted in this paper.

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[72] T. L. Ferrell, C. L. Britton, W. L. Bryan, L. G. Clonts, M. S. Emery,N. Ericson, F. Merraudeau, W. Morrison, A. Passian, S. F. Smith, T.D. Threatt, G. W. Turner, and A. L. Wintenberg, “Telesensor integratedcircuits,” World J. Surg., vol. 25, no. 11, pp. 1412–1418, 2001.

Lal A. Pinnaduwage (M’97) received the Ph.D. de-gree in atomic physics from the University of Pitts-burgh, Pittsburgh, PA, in 1986.

He is a Senior Staff Scientist of the Nanoscale Sci-ence and Devices Group, Oak Ridge National Lab-oratory, Oak Ridge, TN. He is also a Research Pro-fessor with the Department of Physics, University ofTennessee, Knoxville. He research interests includeinteraction of laser and microwave fields with fastatom beams, Rydberg atoms and molecules, negativeions, electron-excited molecule interactions, gaseous

dielectrics, low-temperature plasmas, and mass spectrometry. He is currentlyworking in the areas of MEMS sensors for explosive and chemical detectionand surface physics.

Dr. Pinnaduwage is a Fellow of the American Physical Society.

Hai-Feng Ji received the Ph.D. degree from the Chi-nese Academy of Sciences, Beijing, China, in 1996.

After one year of postdoctoral study with theDepartment of Chemistry, University of Florida,Gainesville, he joined Oak Ridge National Lab-oratory, Oak Ridge, TN, for a two-and-half-yearpostdoctoral stay. He has been an Assistant Professorof chemistry since 2000 with the Department ofChemistry and the Institute for Micromanufacturing,Louisiana Tech University, Ruston. Since 1995, hehas published over 50 scientific papers in peer-re-

viewed journals with a focus on optical and micromechanical sensors. His mainresearch interests include chem/bio-microelectromechanical systems (MEMs)and nanoelectromechanical system (NEMs).

Thomas Thundat received the Ph.D. degree inphysics from the State University of New York atAlbany in 1987.

He is a Distinguished Staff Scientist and Leaderof the Nanoscale Science and Devices Group, OakRidge National Laboratory, Oak Ridge, TN. He isalso a Research Professor of physics at the Univer-sity of Tennessee, Knoxville, and a Visiting Professorat the University of Burgundy, France. He is the au-thor of over 170 publications in refereed journals, 18book chapters, 18 patents, and nine patents pending.

His research interests include novel physical, chemical, and biological detec-tion using micro- and nanomechanical sensors. His expertise includes physicsand chemistry of interfaces, solid-liquid interfaces, biophysics, scanning probes,nanoscale phenomena, and quantum confined atoms.

Dr. Thundat is the recipient of many awards, including the U.S. Department ofEnergy’s Young Scientist Award, R&D 100 Award, Discover Magazine Award,FLC Award, ASME Pioneer Award, and UT-Battelle Awards for invention, pub-lication, and research and development.

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