APPLIED PHYSICS LETTERS
VOLUME 83, NUMBER 7
18 AUGUST 2003
Sensitive detection of plastic explosives with self-assembled monolayer-coated microcantilevers L. A. Pinnaduwage,a) V. Boiadjiev, J. E. Hawk, and T. Thundat Life Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6123, and Department of Physics, University of Tennessee, Knoxville, Tennessee 37932
共Received 15 May 2003; accepted 27 June 2003兲 We report the detection of 10–30 parts-per-trillion levels of pentaerythritol tetranitrate and hexahydro-1,3,5-triazine within 20 s of exposure to a silicon microcantilever with its gold surface modified with a self-assembled monolayer of 4-mercaptobenzoic acid. These measurements correspond to a limit of detection of a few fg. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1602156兴
Of the common explosives that have been used in terrorist bombings, high explosives such as pentaerythritol tetranitrate 共PETN兲 and hexahydro-1,3,5-triazine 共RDX兲— frequently used with plastic filler—are the most serious threats in aircraft sabotage because they can be easily molded for concealment 共the infamous ‘‘shoe bomber’’ had PETN hidden in his shoes兲, are very stable in the absence of a detonator, and are able, in small amounts, to destroy a large airplane in flight. They are, in fact, the explosives most commonly used for this purpose. The vapor pressures of PETN and RDX are quite low, in the range of parts per trillion 共ppt兲 at ambient temperatures.1 Some of the techniques that have been used for the detection of explosives have been discussed in review articles.2,3 More recent and refined detection techniques are ion mobility spectroscopy 共IMS兲,4 negative-ion atmospheric pressure chemical ionization mass spectrometry,5 and laserinduced fluorescence.6 An extremely sensitive method seems to be IMS, for which limits of detection 共LOD兲 of 80 pg and 300 pg for PETN and RDX, respectively, were reported.4 However, the sampling in the reported testing was not from the vapor phase, and the explosive material was introduced by the injection of prepared solutions. The effort and technology involved in the detection of explosives are orders of magnitude more expensive than the effort and costs incurred by terrorists in their deployment. The sensors in current use are bulky and expensive and cannot be miniaturized. Only with the development of extremely sensitive and inexpensive sensors that can be mass produced can sensors be deployed in large enough numbers so that the cost of detection by law enforcement will be less than the cost of deployment by terrorists. Microelectromechanical systems 共MEMS兲 with sufficient detection capability are good candidates for such miniature detectors. We are aware of two studies using MEMS to detect vapors from explosives: The detection of dinitrotoluene 共DNT兲 using a surface acoustic wave sensor, which showed detection sensitivities in the parts-per-billion 共ppb兲 range,7 and our studies with polymer-coated cantilevers, which yielded detection levels of 100 ppt for DNT.8 The main feature distinguishing microcantilevers from a兲
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other sensors is their bending response.9 Since microcantilevers have a high surface-to-volume ratio (⬇103 ), changes in the Gibbs surface free energy induced by surface–analyte interactions lead to large surface forces. If such interactions are restricted to one surface, then the resulting differential stress leads to bending of the cantilever. Because of its smallness, the cantilever undergoes thermal vibrations, and the resonance frequency of the cantilever can also be detected by feeding the bending signal to a spectrum analyzer 共for mass loading measurements兲. The key to using microcantilevers for the selective detection of vapors is the ability to functionalize one surface of the silicon microcantilever so that a given molecular species will be preferentially bound to that surface upon exposure of the cantilever to a vapor stream. Therefore, the sensitivity of detection can be vastly enhanced by applying an appropriate coating to one cantilever surface. Another important requirement for a sensor system is fast recovery 共sensor reversibility兲, so that the sensor can be used repetitively. Significant acidity (pK a of 5–7兲 has been measured for carboxyl-terminated self-assembled monolayers 共SAMs兲 on gold substrates.10 Such acidic surfaces were expected to strongly bind basic nitrosubstituted molecules of explosive vapors via hydrogen bonding.7 SAMs of 4-mercaptobenzoic acid 共4-MBA; also known as thiosalicylic acid兲 on gold are stable11 and efficiently provide surface carboxyl groups for conjugation to enzymes, antibodies, or antigens.12 The conformational rigidity and specific surface orientation of 4-MBA aromatic SAMs prevent intermolecular hydrogen bonding and dimerization.11 In addition, earlier scanning tunneling microscopic studies of 4-MBA SAMs have found domains with a periodic row structure and no hole defects.13 All of these features of 4-MBA SAMs make them suitable for highly sensitive detection of explosives. To determine the sensitivity of 4-MBA SAMs, we used commercially available V-shaped silicon cantilevers 180 m long, 25 m wide, and 1 m thick 共Park Scientific Instruments, Inc.兲; the force constant was 0.26 N/m 共specifications of the manufacturer兲. The manufacturer had coated one side of these cantilevers with gold 共a 30 nm thick gold layer on top of a 3 nm titanium adhesion layer兲, and this side was used to reflect the laser beam for deflection measurements. Using a procedure adopted from Ref. 14, we cleaned
0003-6951/2003/83(7)/1471/3/$20.00 1471 © 2003 American Institute of Physics Downloaded 21 Dec 2004 to 134.60.13.22. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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FIG. 1. Schematic diagram of the experimental apparatus showing the PETN vapor generator. A similar generator was used for RDX measurements.
some silicon cantilevers in acetone, absolute ethanol, deionized water, and 共for only 10 s兲 piranha solution (7:3 H2 SO4 98%/H2 O2 31%). They were then rinsed with ultrapure deionized water 共three times兲 and absolute ethanol 共two times兲. The clean cantilevers were dried briefly in an oven at 80 °C in ambient air. The formation of a 4-MBA SAM on the gold surface of the cantilever was achieved by immersing the cantilever into a 6⫻10⫺3 M solution of 4-MBA 共97%, from Aldrich Chemical Company兲 in absolute ethanol for 2 days. Upon removal from the solution, the cantilever was rinsed three times with ethanol and then dried before being used in the experiments. The monolayer coating was shown to be quite stable for several months under normal operating conditions. The experimental apparatus used in the present experiments is shown in Fig. 1. The microcantilver was held in place in a vacuum-tight glass flow cell by a spring-loaded wire. A modified atomic force microscope head measured the bending response of the microcantilever. The light from a laser diode was focused at the apex of the cantilever 共the gold-coated side on which the monolayer was deposited兲. The reflected laser beam was allowed to fall on a positionsensitive detector 共PSD兲. The output from the PSD was amplified and normalized through a homemade electronics box and fed to a Stanford Research System model SR 760 fast Fourier transform spectrum analyzer 共for resonance frequency measurements兲 and a Hewlett Packard model 34970A data logger 共for bending measurements兲. This allowed simultaneous measurement of bending and resonance frequency. The flow rate of gas around the cantilever was kept constant to minimize bending due to gas flow. A vapor generator developed at Idaho National Engineering and Environmental Laboratory 共INEEL兲 was used to generate the PETN and RDX vapor streams. The vapor stream was generated by flowing ambient air through a reservoir containing PETN or RDX. The reservoir consisted of 0.1 g of PETN or RDX dissolved in acetone and deposited on glass wool contained in a stainless-steel block. The reservoir temperature was controlled via thermoelectric elements
Pinnaduwage et al.
FIG. 2. The response of a 4-MBA-coated silicon cantilever to a PETN stream of 1.4 ppb concentration in ambient air. The solid curve depicts the bending response, and the dots depict the resonance frequency of the cantilever. The frequency shift due to the adsorption of PETN vapor corresponds to a mass loading of 15 pg on the cantilever.
that cooled or heated the reservoir, generating a level of vapor saturation within the reservoir. When the explosive vapor stream was turned off, the same carrier stream was redirected to bypass the reservoir and was sent through the cantilever flow cell; i.e., the total flow rate through the cantilever flow cell was kept constant. This was done in order to avoid any potential cantilever response to the change of flow. Thus, the cantilever was always subjected to a stream of ambient air, in this case at a flow rate of 100 standard cubic centimeters per minute. Even though the data presented here were taken with the vapor generator tip sealed at the cantilever flow cell, we observed similar results even when the vapor generator tip was several millimeters below the flow cell. For the data presented in this letter, both vapor generators were operated at 50 °C. At this temperature, the vapor concentrations for PETN and RDX were 1400 ppt and 290 ppt, respectively.1 The response of a SAM-coated cantilever to a 200 s long pulse of PETN vapor is shown in Fig. 2. As seen from Fig. 2, the bending response of the cantilever to the PETN exposure is extremely sensitive and fast. Since the noise level of the bending response in these experiments is ⬇2 nm 共3⫻ standard deviation of the noise level兲, the detection sensitivity corresponding to Fig. 2 is ⬇14 ppt. Maximum bending of the cantilever is achieved within 20 s. The amount of PETN delivered by the generator in 20 s is ⬇660 pg. However, the mass of PETN exposed to the cantilever is much smaller, since the cross-sectional area of the hole in the delivery tube of the vapor generator is ⬇0.07 cm2 and the cantilever surface area is ⬇8⫻10⫺5 cm2 . Allowing for several wall bounces, it can be estimated that a few pg of PETN impinging on the cantilever in 20 s is sufficient to yield a 200 nm deflection of the cantilever. 共Actually, the number of wall bounces may be small, since we have noted that the deflection signal is not significantly reduced even if the tip of the vapor generator tube is not sealed at the cantilever flow cell but is several millimeters below the flow cell.兲 Since the minimum detection level above the noise level is a few nanometers, a low femtogram (10⫺15 g) level of LOD is implied. It should be noted that with the availability of vapor preconcentrators, the detection capabilities of a given sensor
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FIG. 4. The response of a 4-MBA-coated silicon cantilever to the periodic turning on 共10 s兲 and off 共60 s兲 of a PETN stream of 1.4 ppb concentration in ambient air. The solid curve depicts the bending response, and the dots connected by dashed lines depict the resonance frequency of the cantilever. FIG. 3. The response of a 4-MBA-coated silicon cantilever to a RDX stream of 290 ppt concentration in ambient air. The solid curve depicts the bending response, and the dots depict the resonance frequency of the cantilever. The frequency shift due to the adsorption of RDX vapor is barely discernible.
are better described by LOD 共based on the amount of explosive material兲 than by vapor concentration. The bending and frequency responses of the SAMcoated cantilever to a RDX vapor stream with a concentration of 290 ppt are shown in Fig. 3. In this case, the frequency response, and thus the mass loading, is quite small. Yet the bending response is quite clear, with a ‘‘direct’’ detection sensitivity of ⬇30 ppt. The maximum cantilever bending is achieved within ⬇25 s, and the mass of RDX delivered by the generator during this time is ⬇96 pg. Again, if we compare the cantilever cross-sectional area with that of the vapor generator, this corresponds to ⬇0.1 pg delivered to the cantilever, for a total cantilever deflection of 20 nm 共Fig. 2兲, and thus an implied LOD of a few fg. The rapidity with which explosive vapors can be detected and the relatively fast relaxation of the cantilever when the vapor stream is turned off can be seen in Fig. 4. When the PETN stream is turned on for 10 s, a 40 nm deflection signal is observed; after the vapor stream is turned off, the cantilever is relaxed back almost to the original position within 60 s. Another important observation from the data shown in Fig. 4 is that the resonance frequency of the cantilever does not change significantly as a result of the small amount of PETN deposited in 10 s. The bending of the cantilever is still quite easily detected. We suspect that the hydrogen bonding between the nitrogroups of the explosives molecules and the carboxyl group of 4-MBA is responsible for the easily reversible adsorption of explosive vapors on the SAM-coated top surface of the cantilever. This leads to strong surface stress on the top surface, making the cantilever bend downward, as shown by the data in Figs. 2– 4.
In conclusion, we have shown that 4-mercaptobenzoic acid monolayer-coated microcantilevers can directly detect PETN and RDX vapors with sensitivity in the ppt level within tens of s. These measurements indicate LOD levels of a few fg. This high sensitivity for rapid and reversible detection, together with the micrometer size of the cantilever, can lead to the development of a portable detection device for rapid and sensitive detection of explosive vapors. The authors thank Dr. Carla Miller and Tim Kaser at INEEL for their help with the INEEL vapor generators and Dr. Richard Lareau of the U.S. Department of Homeland Security for lending those vapor generators to us for the present experiments. Oak Ridge National Laboratory is operated and managed by UT-Battelle, LLC, for the U.S. Department of Energy under Contract No. DE-AC0500OR22725. 1
B. C. Dionne, D. P. Rounbehler, E. K. Achter, J. R. Hobbs, and D. H. Fine, Energe. Mater. 4, 447 共1986兲. 2 A. Fainberg, Science 255, 1531 共2001兲. 3 J. I. Steinfeld and J. Wormhoudt, Annu. Rev. Phys. Chem. 49, 203 共1998兲. 4 T. Khayamian, M. Tabrizchi, and M. T. Jafari, Talanta 59, 327 共2003兲. 5 C. S. Evans, R. Sleeman, J. Luke, and B. J. Keely, Rapid Commun. Mass Spectrom. 16, 1883 共2002兲. 6 D. Heflinger, T. Arusi-Parpar, Y. Ron, and R. Lavi, Opt. Commun. 204, 327 共2002兲. 7 E. J. Houser, T. E. Mlsna, V. K. Nguyen, R. Chung, R. L. Mowery, and R. A. McGill, Talanta 54, 469 共2002兲. 8 L. A. Pinnaduwage, T. Thundat, J. E. Hawk, D. L. Hedden, P. F. Britt, E. J. Houser, S. Stepnowski, R. A. McGill, and D. Bubb 共unpublished兲. 9 T. Thundat, R. J. Warmach, G. Y. Chen, and D. P. Allison, Appl. Phys. Lett. 64, 2894 共1994兲. 10 Z. Dai and H. X. Ju, Phys. Chem. Chem. Phys. 3, 3769 共2001兲. 11 S. E. Creager and C. M. Steiger, Langmuir 11, 1852 共1995兲. 12 S. Susmel, C. K. O’Sullivan, and G. G. Guilbault, Enzyme Microb. Technol. 27, 639 共2000兲. 13 A. H. Schafer, C. Seidel, L. F. Chi, and H. Fuchs, Adv. Mater. 共Weinheim, Ger.兲 10, 839 共1998兲. 14 A. Ulman, An Introduction to Ultrathin Organic Films 共Academic, New York, 1991兲, p. 108.
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