Surface Acoustic Wave
David S. Ballantine, Jr. Department of Chemistry Northern Illinois University DeKalb, IL 60115
Hank Wohltjen Microsensor Systems, Inc. P.O. Box 90 Fairfax, VA 22030
Interest in chemical microsensor research has increased substantially during the last decade, as indicated by the number of recent publications and presentations at professional meetings and symposia dealing with the subject (1-5). This interest has been spurred by the microsensors' small size, ruggedness, sensitivity, and low power consumption. When used in applications such as process control, clinical diagnostics, or environmental monitoring, these characteristics translate into the availability of abundant and inexpensive chemical information, with profound implications for maintaining and improving quality control in manufacturing and for improving our quality of life. Surface acoustic wave (SAW) devices are transducers based on highfrequency mechanical oscillators. They offer a simple, direct, and sensitive method for probing the chemical and physical properties of materials. SAW devices are unique in that the acoustic wave energy is constrained to the surface region of the substrate, a feature that also accounts for their versatility as chemical and physical sensors. This REPORT will introduce SAW device
technology through a brief explanation of operational principles, discuss some of the factors to be considered in the design of a SAW chemical sensor, and present an overview of the use of SAW devices both as chemical sensors and as basic research tools. Possible applications SAW device technology was born with the development of the interdigital transducer (IDT) by White and Volltmer in 1965 (6). Deposition of IDTs on the surface of piezoelectric substrates made it possible to excite a variety of elastic waves, including Rayleigh surface waves, in these materials. When first introduced, SAW devices were primarily used in rf signal processing, where they offered unique ca-
REPORT pabilities for the generation, delay, and filtering of rf signals. Because of the high sensitivity of SAW devices to the ambient environment, SAW device designers found it necessary to shield or otherwise isolate the devices to eliminate physical effects. It is somewhat ironic that this sensitivity, a potential problem in precise rf signal-processing applications, has become a boon to chemical sensor researchers (hence the adage "One man's noise is another man's signal"). Since the 1970s, chemists and engineers alike have exploited this sensitivity in developing a myriad of chemical and physical sensors.
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The use of the SAW device as a chemical sensor relies on the sensitivity of the surface wave to changes occurring in the thin surface film. Consequently, the SAW device represents a sensitive probe for characterizing thin films and for investigating interaction mechanisms between chemical vapors and coating materials. The majority of these studies rely on the mass sensitivity of the device to determine such properties as polymer/vapor sorption thermodynamics and kinetics (7); adsorption/desorption isotherms (8); and pore size, surface area, and vapor diffusivity (5). These properties are useful in characterizing materials to determine their suitability for a given application. SAW devices can also be used to study vapor/coating interaction mechanisms (9) and to rapidly identify candidate materials with appropriate solubility properties for use in gas chromatography (10). The use of SAW devices or related sensor technologies in liquid media opens the way for interesting applications. For example, studies of micromechanical properties (viscosity) and electrical (dielectric) properties can be performed. If optically transparent substrates are used, spectroelectrochemical investigations as well as investigations into electrode reaction mechanisms and biochemical reactions that occur in liquid media are feasible. SAW devices are attractive for chemical sensing because of their high mass sensitivity. The range of applicability to chemical detection problems is limited only by our ability to design, 0003-2700/89/0361 -704A/$01.50/0 © 1989 American Chemical Society
Devise for Chemical An
develop, or discover coatings for the SAW device that make it sensitive and selective for the analyte of interest. When a good coating is available, it is usually possible to detect vapors at the 10-100-ppbv concentration level, with a selectivity of 1000:1 or more over commonly encountered interferences and a dynamic range of 3-4 orders of magnitude. These features, combined with low power consumption, ruggedness, and small size, are responsible for the interest in applying SAW device technology to a broad spectrum of chemical analysis problems. Theory Both the amplitude and the velocity of the Rayleigh surface wave can be used to monitor changes in the surface medi um. For the majority of sensor applica tions, wave velocity is measured be cause of the high precision obtained when the SAW device is used in an os cillator circuit (1 part in 10 million vs. 1 part in 1000 for amplitude measure ments). Changes in the velocity of the surface wave are manifested as changes in the frequency of oscillation. In this respect, the SAW device is similar to an older technology, the quartz crystal mi crobalance (QMB). To more fully illus trate the operational principles of the SAW device and to underscore some of its unique features, let us compare it with the QMB. The QMB consists of a piezoelectric substrate (quartz) with electrodes de posited on opposite surfaces of the crystal. When a time-varying rf poten tial is applied to the electrodes, the crystal lattice undergoes particle dis
placement, the net result of which is a bulk elastic wave that propagates from one face of the crystal to the other. The wave velocity, and hence the oscillating frequency, is sensitive to changes in the mass or density of the medium in con tact with the crystal surface: Δ/ = - 2 . 3 X 10 6 F 2 AM/A
(1)
where / is the frequency shift (in Hz) associated with a given mass change, F is the fundamental operating frequen cy of the device (in MHz), AM is the change in mass on the surface of the crystal (in grams), and A is the sur face area of the device (in cm 2 ). When coated with a sorbent coating, the QMB can be used as a chemical sensor (11,12). QMB sensors exhibit a radial sensitivity function, with the mass sen sitivity at a maximum in the center of the device and decreasing toward the edges. SAW devices also require the use of a piezoelectric substrate but, unlike the QMB, the electrodes are deposited on the same side of the crystal in the form of IDTs. When a time-varying rf poten tial is applied, the crystal also under goes physical deformation, which can be confined to the surface region of the crystal and takes the form of a surface acoustic wave. In a typical delay line configuration (shown in Figure 1), one set of IDTs acts as a transmitter to launch the surface wave, and a second set of IDTs acts as a receiver to trans late the wave back into an electric sig nal. For many sensor studies, noncon ducting polymer films are employed. Equation 2 describes the response be havior for a SAW device coated with a
thin, isotropic, nonconducting film: Δ/ = (kj + k2)pfc/02 k 2 ^ / 0 2 ( W ) [(λ + μ)/(λ + 2μ)]
(2)
where ki and k 2 are material constants for the quartz substrate, VR is the Ray leigh wave velocity, h is the film thick ness, ρ is the density, μ is the shear modulus of the film material, λ is the Lamb constant, and /o is the funda mental frequency of the device (13). The first half of the equation yields the shift resulting from mass loading, whereas the second half describes the effect of changes in the elastic proper-
(a)
(b)
Figure 1. Propagation of a surface wave on a SAW delay line, (a) Top view, (b) side view.
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989 · 705 A
REPORT cific applications, but the Rayleigh SAW devices are the most popular. Sensor applications developed to date can be divided into two categories: physical sensors and chemical sensors. Some typical physical properties that can be measured using SAW devices are listed in Table I. The number of chemical sensors far exceeds the num ber of physical sensors, which is per haps a reflection of the growing interest in chemical sensors in general. For a SAW device to be effectively used as a selective chemical sensor, two criteria must be met. First, the target analyte must be trapped on or near the surface by some mechanism, usually by reaction with or sorption by a selective or semiselective coating. Second, this analyte/coating interaction must pro duce a change in the properties of the coating (i.e., density, viscosity, modu lus, or dielectric constant) and thus al ter the amplitude or velocity of the sur face acoustic wave. The sensitivity and selectivity of the SAW sensor is thus a function of the physicochemical prop erties of the coating material itself. Many sensor applications use a dual device configuration (Figure 3) consist ing of two SAW delay lines that have been constructed side by side on the
which results in smaller, cheaper de vices with lower absolute limits of detectability. Whereas the QMB exhibits a radial sensitivity function, the SAW device has uniform sensitivity over the entire active surface, which is desirable for most sensor applications. Finally, the acoustic wave in the SAW device is confined to the surface region and is always accessible, even when ruggedly packaged. Thus it can be readily sam pled or modified by surface interac tions to realize a wider range of devices than is possible with the QMB.
Horizontally polarized shear wave
Rayleigh surface wave
Sensor applications Symmetric Lamb wave
Figure 2. Illustrations of elastic waves that can be generated in piezoelectric solids.
ties of the film on the resonant fre quency. If the coating is a soft, rubbery material, the changes in frequency at tributable to modulus changes are min imal, and the second term in Equation 2 can be ignored. When values for the material constants for an ST-quartz substrate are inserted, the correspond ing surface acoustic wave equation be comes: 6
Δ/ = -1.26 Χ W flhp
(3)
Because the product of coating thick ness and density is equivalent to mass per unit area, Equation 3 is analogous to Equation 1 for the QMB. At first glance, the SAW device and the QMB appear to be opposite sides of the same coin. Both are high-frequen cy, piezoelectric devices exhibiting great sensitivity to mass/density changes on their surfaces; in fact, if a SAW device and a QMB of similar fre quency were compared directly, the QMB would appear to be the more at tractive sensor, having a greater mass sensitivity (by nearly a factor of 2) as well as lower noise levels. However, the operating frequency of the QMB is de termined in part by the thickness of the bulk crystal, so the QMB has an upper practical limit of 10-20 MHz. The SAW device is not so constrained; de vices that operate in the GHz range have been constructed. Because the sensitivity of the device increases as the square of the fundamental frequen cy, the SAW device has greater poten tial sensitivity. Also, the dimensions of the device decrease with frequency,
By careful selection of substrate and design of the oscillator circuit, a variety of elastic waves can be generated in the piezoelectric substrate. In addition to the Rayleigh surface waves already dis cussed, other types of elastic waves can be generated (Figure 2). For Lamb waves, the acoustic energy is present on both surfaces of the substrate and has both longitudinal and vertical compo nents, as does the Rayleigh wave. For the horizontally polarized shear wave, the acoustic wave is contained in planes parallel to the surface, with no vertical component. Each of these elas tic wave types has advantages for spe
Table 1.
SAW physical sensors
Property measured Temperature Pressure Force/acceleration Electric field (high voltage) Displacement Flow
Sensitivity/detectability
Reference
0.001 °C 0.1 ppm/atm 18 Hz/g 16-43 ppm/kWmm 300 Ηζ/μηι 11 Hz/sccm
14 14 14 15 14 14
_—
...
rf amp 'reference
Chemically selective coating 'difference
rf mixer
'sample
rf amp
Figure 3. Schematic of dual delay line SAW sensor configuration.
706 A · ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
REPORT tentially more sensitive to mass loading than a Rayleigh wave of the same frequency.
Cell with liquid
Output transducer
Input transducer
Piezoelectric quartz substrate
Vapor-phase sensors
ν Plate modes
Figure 4. Acoustic plate mode liquid sensor (adapted with permission from Reference 18). same substrate. One delay line can be coated with the sorptive or reactive film while the other delay line remains uncoated or inert. The frequency of the two delay lines can be compared electronically, and a frequency difference can be calculated. There are two advantages to such a configuration. First, because both delay lines are exposed to the same ambient conditions, the uncoated delay line acts as a reference to compensate for frequency deviations arising from fluctuations in temperature or pressure. Second, the frequency difference is typically on the order of kHz and can be easily sampled using inexpensive digital electronics. Chemical sensors can be classified as either liquid-phase or vapor-phase sensors, depending on the medium in which the target compound is contained.
Martin (18) have described an acoustic plate mode device (Figure 4) that is structurally similar to the conventional SAW device but can operate efficiently in liquids. Using this device, they have measured viscosities of liquids by observing the attenuation of the acoustic wave amplitude; the sensor's usefulness has already been demonstrated in applications such as corrosion monitoring, electroless film deposition, and studies of electrode reactions (19, 20). The device should also prove useful in studying other boundary layer and interface phenomena. An alternative sensor technology for use in liquid sensing is the Lamb wave device (21). Because the Lamb wave velocity in the substrate is lower than the compressional wave velocity in many liquids, there is minimal energy lost from the wave in contact with liquid. In addition, the Lamb wave is po-
Although liquid-phase sensor applications offer intriguing possibilities, the majority of SAW chemical sensors detect analytes in the vapor phase. Examples of the types of vapor sensors reported, along with the sensitivity achieved, are shown in Table II. The extensive use of the QMB as a chemical vapor sensor has been reviewed (31, 32), and the coatings/vapors used in QMB sensors could also be applied effectively to the SAW sensor. Because SAW devices are sensitive to a variety of physical effects, the sensor applications in Table II can be categorized according to the predominant transduction mechanism responsible for the response. The major categories are electric potential wave interactions (i.e., dielectric constant, conductivity) and mechanical wave interactions (i.e., mass, density, modulus). Electric potential wave interactions. Because the propagation of the surface acoustic wave results in the physical deformation/displacement of the piezoelectric crystal lattice, there is also an electric potential wave associated with the surface wave that permits probing of the dielectric and conductive properties of the surface medium. The presence of a conducting film will cause a fractional velocity change as well as a reduction in the amplitude of the surface wave. This phenomenon is only operational within a limited conductivity range, which is determined by the electromechanical coupling coefficient of the piezoelectric substrate. If the sheet conductivity of the SAW device coating is in this range and the conductivity is sensitive to the près-
Liquid-phase sensors The development of liquid-phase sensors is of interest in many research areas, including biochemistry, immunology, and electrochemistry. The use of SAW devices in liquids to measure immunochemical binding reactions has been reported (16), and this has opened up exciting possibilities for new applications because of the reported high sensitivity (ng/mL) and generic nature of the sensing method. However, the use of SAW devices as liquid sensors is controversial because of problems associated with the propagation of leaky Rayleigh waves in dense fluid media (17). Although devices using Rayleigh waves have limitations as liquid sensors, similar devices using other types of elastic waves can be used. Ricco and
Table II.
SAW chemical vapor sensors
Vapor H2 NH3 N0 2 H2S S0 2 H20 Cyclopentadiene Slyrene Vinyl acetate Organophosphorous compounds
708 A · ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
Coating
Sensitivity/ detectabiiity
Reference
Pd Pt Pb phthalocyanine W0 3 Triethanolamine None Polyimide Poly(ethylene maleate) PtCI2(ethylene)(pyridine) PtCI2(ethylene)(pyridine) Fluoropolyol
50ppm < 0.5% 2 ppm/0.5 ppm < 10 ppm 10ppb High RH 1.1kHz/%RH 200 ppm/min 5 ppm 5 ppm 0.03 ppm
24 25 22,26 27 28 23 7 29 30 5 9
REPORT ence of certain vapors, then a chemical vapor sensor can be constructed. Such a sensor was designed by Ricco and co-workers for the detection of N0 2 (22). They employed a lead phthalocyanine semiconductive film on a dual SAW device, using a LiNb03 substrate. On the reference side of the device, they added a metal film to "short out" the electric field and determine the sensor response from mass loading alone. The signal from the other delay line, which was coated only with the organic semiconductor, resulted from changes in the film conductivity when exposed to NO2. For this film, they de termined that the observed response was attributable entirely to conductiv ity effects. The humidity sensor developed by Huang (23) is another example of a SAW sensor using the electric potential wave. A dual SAW device was con structed using a nonhygroscopic piezo electric material to minimize the signal caused by adsorption of water vapor on the substrate surface. Once again, a conducting film was applied to the ref erence delay line to short out the elec tric field. The observed sensor response to high levels of humidity was attribut ed to an interaction between the elec tric field of the unshorted delay line and the highly polar water vapor mole cules. Although these sensors exhibit good sensitivity and selectivity, there are limitations precluding the use of this transduction mechanism for general vapor sensor applications. The selec tion of a substrate with the appropriate properties involves trade-offs. The LiNbU3 substrate, for example, has an electromechanical coupling constant that is compatible with the conductiv ity of the lead phthalocyanine coating. However, it exhibits a large tempera ture coefficient and can undergo fre quency shifts as large as 8800 Hz/°C. Other substrates, such as ST-quartz, are less sensitive to temperature effects but would not be as sensitive to changes in the conductivity of the semiconductor film used in this exam ple. Thus the choice of substrate auto matically limits the selection of coat ings to materials having a sheet con ductivity within the sensitive region for that substrate. This further limits the number of suitable coating materials, and hence the applications, for which this mechanism can be effectively em ployed. Mechanical wave interactions. SAW sensors relying on the physical effects of mass loading or density changes are not constrained to the same extent as those sensors that em ploy the electrical effect for transduc
tion. As a result, the majority of report ed chemical vapor sensor applications fall into this category. Some of the sen sors described in Table II use either metal films (to detect H 2 or NH 3 ) or ceramic films (to detect H2S). The oth ers use organic films, which greatly in creases the number of candidate coat ing materials from which to choose. For chemical vapor sensors using or ganic films, the question of selectivity is especially important, and two strate gies have emerged. The first takes ad vantage of recent advances in micro electronics and computer technology to apply sophisticated data reduction and analysis algorithms to sensor array data. For many vapor sensor applica tions, such as remote site monitoring or field tests, a reversible sensor response is desirable. In such cases these sensors use thin, nonconducting elastomeric films to eliminate the effects of conduc tivity and elastic modulus on the ob served responses. The sensor response can then be modeled as the dissolution or sorption of a solute vapor into a sol vent coating, a process that is con trolled to a great extent by the solubili ty properties of the coating and the va por of interest. Because such solubility interactions are not highly specific, selective re sponse to only one vapor is unlikely. However, by employing an array of sen sors with coatings of variable selectiv ity, a response pattern or fingerprint
can be obtained that is characteristic of a given vapor. Examples of vapor re sponse patterns obtained using a fourSAW sensor array are shown in Figure 5. These results demonstrate the abili ty of the sensor array to identify classes of vapors as well as to discriminate be tween individual vapors on the basis of response patterns. This approach has been used to identify target vapors in dividually (9) as well as in simple vapor mixtures (33). In addition, pattern rec ognition techniques are useful in iden tifying relationships between sensor re sponses and solubility properties (9) or coating structural functionalities (34). This can serve as a feedback process, whereby solubility and structural in formation can then be used in the selec tion or design of more selective coat ings for a particular application. The sensor array approach offers versatili ty, by using an array of sensors with varying selectivities, the same array can be used to identify a wide variety of chemical vapors. For applications in which reversibil ity of response is not a critical issue, a different strategy has evolved whereby increased selectivity is achieved through chemical means. In this ap proach, a chemical reaction occurs be tween the vapor of interest and the coating (or the reagent/coating mix ture), resulting in strong selectivity in favor of that vapor or class of vapors. Examples of such sensors include Snow
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Figure 5. Vapor response patterns obtained from a four-SAW sensor array.
710 A · ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
REPORT and Wohltjen's cyclopentadiene sensor (29) and Zellers's styrene and vinyl ace tate sensors (5,30), which are of partic ular interest because reagent is sus pended in a semiselective polymer film. Corrections are made for nonspecific sorption into the film by coating one side of a dual device with the reagentpolymer mixture and the reference side with the polymer alone. An additional advantage of these sensors is that re agents can be regenerated, whereas the cyclopentadiene-poly(ethylene maleate) system is generally irreversible. This approach has the advantage of in creased chemical specificity and can be useful when a well-known hazard exists or when the ambient environment is under partial control, such as in indus trial hygiene applications or in the monitoring of industrial processes. Future directions To date, no chemical vapor sensors have been developed that take advan tage of the sensitivity of the SAW de vice to changes in the shear elastic modulus of the coating material. Re cent investigations, however, have shown that the SAW device has poten tial for use in the characterization of polymeric materials. Parameters that
can be determined include the glass transition temperature (Tg), the melt ing temperature of crystalline poly mers (T m ), thermal expansion coeffi cients, and thermal relaxation activa tion energies (5). The SAW device pro vides a rapid, sensitive, and inexpensive alternative to existing methods. Future research will likely follow several paths. New SAW device config urations (e.g., resonators) and packag ing approaches offer potential for re ducing noise and drift. Different types of acoustic waves can be developed for chemical sensing in both liquid and va por phases. Continued vapor/coating interaction research should yield new coating formulations that will expand the range of analytes accessible to SAW device technology. Further development of pattern rec ognition algorithms should also strengthen the power of array sensors. Attempts are being made to use mod ern silicon micromachining techniques to facilitate total system miniaturiza tion of valves and pneumatic hardware often used to support the SAW sensor "chip." The successes of these efforts should make chemical sensing with SAW devices a method of choice for chemical analysis in the future.
References (1) Janata, J.; Bezegh, A. Anal. Chem. 1988,60,62 R. (2) Aucouturier, J. L.; Cauhape, J. S.; Destriaue, M.; Hagenmulle, P.; Lucat, C; Menil, F.; Portier, J.; Salardenne, J., Eds.; Proceedings of the International Meet ing on Chemical Sensors, July 1987, Bor deaux, France. (3) Transducers '87, Proceedings of the International Conference on Solid-State Sensors and Actuators-Transducers; To kyo, June 1987. (4) Proceedings of the IEEE Ultrasonic Symposium; Chicago, IL, October 1988. (5) Murray, R. W.; Heineman, W. R.; Jan ata, J.; Seitz, W. R. ACS Symposium Se ries, in press. (6) White, R. M.; Volltmer, F. W. Appl. Phys. Lett. 1965, 7, 314. (7) Brace, J. G.; Sanfelippo, T. S.; Joshi, S. G. Sens. Actuators 1988,14, 47. (8) Martin, S. J.; Ricco, A. J.; Ginley, D. S.; Zipperian, T. E. IEEE Trans. 1987, UFFC-34(2), 143. (9) Ballantine, D. S., Jr.; Rose, S. L.; Grate, J. W.; Wohltjen, H. Anal. Chem. 1986,58,3058. (10) Grate, J. W.; Snow, Α.; Ballantine, D. S., Jr.; Wohltjen, H.; Abraham, M. H.; McGill, Α.; Sasson, P. Anal. Chem. 1988, 60, 869. (11) King, W. H., Jr. Anal. Chem. 1964, 36(9), 1735. (12) Edmonds, T. E.; West, T. S. Anal. Chim. Acta 1980,117,147. (13) Wohltjen, H. Sens. Actuators 1984, 5, 307. (14) Wohltjen, H. Transducers '87, Pro-
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REPORT ceedings of the International Conference on Solid-State Sensors and ActuatorsTransducers; Tokyo, June 1987; p. 471. (15) Gatti, E.; Palma, Α.; Verona, E. Sens. Actuators 1983, 4, 45. (16) Roederer, J. E.; Bastiaans, G. J. Anal. Chem. 1983, 55, 2333. (17) Calabrese, G. S.; Wohltjen, H.; Roy, M. K. Anal. Chem. 1987,59, 833. (18) Ricco, A. J.; Martin, S. J. Appl. Phys. Lett. 1987,50(21), 1474. (19) Ricco, A. J.; Martin, S. J. Proceedings of the International Electrochemical So ciety 1988,88-12, 142. (20) Ricco, A. J.; Martin, S. J. Presented at the International Electrochemical Soci ety Meeting, Honolulu, HI, October 1987. (21) White, R. M.; Wenzel, S. W. Appl. Phys. Lett. 1988,52(20), 1653. (22) Ricco, A. J.; Martin, S. J.; Zipperian, T. E. Sens. Actuators 1985,8, 319. (23) Huang, P. H. Transducers '87, 462. (24) D'Amico, Α.; Palma, Α.; Verona, E. Sens. Actuators 1982, 3, 31. (25) D'Amico, Α.; Petri, Α.; Verardi, P.; Ve rona, E. Proc. IEEE Ultrason. Sypm.— 1987,633. (26) Venema, Α.; Niewkoop, E.; Vellekoop, M. J.; Ghijsen, W. J.; Barendz, A. W.; Nieuwenhuizen, M. S. IEEE Trans. 1987, UFFC-34(2), 148. (27) Vetelino, J. F.; Lade, R. K.; Falconer, R. S. IEEE Trans. 1987, UFFC-34(2), 156. (28) Bryant, Α.; Poirier, M.; Riley, G.; Lee, D. L.; Vetelino, J. F. Sens. Actuators 1983,4,105. (29) Snow, A. W.; Wohltjen, H. Anal. Chem. 1984,56(8), 1411. (30) Zellers, T. E.; White, R. M.; Rappo-
port, S. M.; Wenzel, S. W. Transducers '87, 459. (31) Alder, J. F.; McCallum, J. J. Analyst 1983, i 08(1291), 1169. (32) Hlavay, J.; Guilbault, G. G. Anal. Chem. 1977, 49(13), 1890. (33) Rose-Pehrrson, S. L.; Grate, J. W.; Ballantine, D. S. Anal. Chem. 1988, 60(24), 2801. (34) Carey, W. P.; Beebe, K. R.; Kowalski, B. R.; Illman, D. L.; Hirschfeld, T. Anal. Chem. 1986,58,149.
David S. Ballantine, Jr. {left) received his B.S. degree in chemistry from the College of William and Mary {1973) and his Ph.D. in analytical chemistry from the University of Maryland, Col lege Park {1983). After working for Geo-Centers, Inc., where he performed testing and evaluation on a variety of
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microsensor technologies in support of the Chemistry Division of the Naval Research Laboratory in Washington, DC, he joined the faculty of Northern Illinois University. His research focus es on the application of microsensors to basic and applied research, includ ing the use of SAW and optical wave guide devices for investigation of va por/coating interaction mechanisms, material characterization, and envi ronmental analyses. Hank Wohltjen {right) received his B.S. degrees in chemistry {1972) and in electrical engineering science {1974) from the City University of New York and his Ph.D. in chemistry from Virginia Polytechnic Institute and State University {1978). After three years as a postdoctoral fellow in the New Devices group at the IBM re search laboratory in Zurich, Switzer land, he joined the Naval Research Laboratory as a research chemist in 1981. While at NRL he initiated re search in SAW devices, organic semi conductors, and optical waveguides. In 1985 he founded Microsensor Sys tems, Inc., an organization dedicated to research and development of chemi cal microsensor instrumentation.
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