Acoustic Wave Microsensors PART II - ACS Publications - American

Acoustic TSMSAWFPWAPM Wave

Part II REPORT Acoustic wave microsensors are a versatile class of sensor with many applications. Although originally regarded as mass detectors, these sensors can also measure a variety of other physical properties at the sensor's surface. In Part I of this article, which appeared in the November 1 issue, Jay W. Grate, Stephen J. Martin, and Richard M. White discussed the types of waves and devices, operation and measurement methods, and detection via mass sensitivity. Part II will focus on sensing involving other transduction mechanisms, including sensitivity to viscoelastic thin-film properties and polymer transitions, response mechanisms of polymer-coated vapor sensors, acoustoelectric and dielectric effects, and factors influencing sensor behavior in the liquid phase. We will begin Part II with a discussion of viscoelastic thin films.

Microsensors

Jay W. Grate Molecular Science Research Center Pacific Northwest Laboratory Battelle Boulevard Richland, WA 99352

Stephen J. Martin Microsensor Research and Development Department Sandia National Laboratories Albuquerque, NM 87185

Richard M. White Berkeley Sensor and Actuator Center Department of Electrical Engineering and Computer Sciences and the Electronic Research Laboratory University of California Berkeley, CA 94720

Acoustic devices a r e u s u a l l y converted to chemical sensors by the application of a chemically selective layer (J). Polymer m a t e r i a l s h a v e been used in organic vapor sensing applications because vapor sorption in rubbery polymers is rapid and rev e r s i b l e ; p o l y m e r s form a d h e r e n t thin films; and selectivity can be tailored by varying the chemical structure. Before discussing vapor sensors in the next section, we will consider the inherent sensitivities of acoustic

devices to the physical properties of viscoelastic thin films. T h i s type of s e n s i t i v i t y w a s not noted in early models for TSM devices but was noted in early models for SAW sensors based on perturbat i o n a n a l y s i s (2, 3). These models predicted t h a t the r e s o n a n t frequency of a SAW device would be altered by both the mass and the shear m o d u l u s of a t h i n , nonconducting, isotropic film applied to its surface. E a r l y studies confirmed t h a t SAW devices w e r e c a p a b l e of d e t e c t i n g various polymer t r a n s i t i o n s (4). In recent y e a r s , a more complete picture of the effects of viscoelastic thin films on a c o u s t i c devices h a s emerged: Wave velocity and/or a m plitude changes occur in response to polymer thermal expansion, polymer relaxation processes, and film resonance effects. Viscoelastic films have both elastic and viscous properties and can therefore store and dissipate mechanical energy. When subjected to s h e a r forces, t h e s e p r o p e r t i e s a r e measured by the complex shear modu l u s G, which h a s a r e a l t e r m G', t h a t represents the storage (or elastic) modulus, and an imaginary term

ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993 · 987 A

REPORT G" that represents the loss modulus. In simple t e r m s , modulus refers to the stiffness of the material. In the context of u l t r a s o n i c devices, t h e m e a s u r e m e n t of modulus is highly f r e q u e n c y d e p e n d e n t (5). To t h e probing high-frequency waves, films of r u b b e r y polymers on u l t r a s o n i c devices appear to have moduli t h a t are characteristic of a glassy m a t e rial. (This result is a consequence of the relaxation effects described be low.) However, the state of the mate rial is unchanged by such probing. If t h e s e films w e r e probed s i m u l t a neously by a low-frequency method, the measured modulus would be that of a rubber, as expected. For refer ence, the typical modulus of a r u b bery polymer is 10 e N / m 2 , whereas that of a glassy polymer or a rubbery polymer at high frequency is usually about 10 9 N / m 2 . At room t e m p e r a t u r e and frequencies above 1 MHz, n e a r l y all r u b b e r y p o l y m e r s h a v e measured moduli typical of polymer glasses. Polymer moduli decrease with in creasing temperature as the polymer e x p a n d s . T h e s e effects a r e w e l l known in conventional b u l k - w a v e ultrasonics, where the sonic velocity through a polymer sample decreases with increasing t e m p e r a t u r e (6, 7). For reference, shear sound speed VB is directly proportional to the square root of the shear storage modulus G a n d i n v e r s e l y p r o p o r t i o n a l to t h e square root of the density p, leading to the expression

VB = (G'/pf2

(1)

Because both modulus and density decrease with t e m p e r a t u r e , the ob served decreases in sonic velocity in dicate t h a t the modulus is the domi n a n t factor i n f l u e n c i n g t h e sonic velocity. However, density and vol ume do influence sonic velocity indi r e c t l y , b e c a u s e t h e m o d u l u s is strongly volume-dependent. Increas ing volume decreases the c h a i n chain interactions, which decreases the modulus. This volume effect (via its influence on modulus) is so im portant t h a t polymer volume can be considered a fundamental influence on acoustic velocities (6). The same principles apply to poly mer thin films on planar acoustic de vices. W h e n coated to film t h i c k n e s s e s t y p i c a l l y u s e d on v a p o r sensors, polymer thermal expansion causes frequency decreases of 5 0 0 1000 Hz/°C on SAW a n d FPW de vices (8). (Recall t h a t d e c r e a s i n g o s c i l l a t o r f r e q u e n c i e s reflect d e creasing acoustic velocities.) This ef fect is quite large, and it occurs with

no m a s s per unit area change: The acoustic devices sense the decrease in modulus as the polymer volume increases. One practical consequence of this observation is t h a t efforts to reduce the temperature drifts of ultrasonic devices themselves by device engi n e e r i n g or c o m p e n s a t i o n schemes may be of limited value if the devices are to be used as chemical sensors with an applied layer whose observ able physical properties vary with temperature. For example, dual de lay-line SAW vapor sensors with a

Temperature (°C)

Figure 1. Thermal expansion effects and polymer transition processes observed in a thin film of polyvinyl acetate) on a FPW device. The frequency shifts (plotted along the left vertical axis) indicate the effect of the polymer on oscillator frequency; the inherent temperature drift of the bare device has been subtracted. The static glass transition Tg is indicated by the change in the slope of the frequency-temper ature profile. The minimum in oscillator amplitude (plotted along the right vertical axis) indicates a maximum in attenuation caused by the polymer at the dynamic glass transition 7"0.

Temperature (°C)

Figure 2. Film resonance effects observed on a quartz TSM device coated with a 15.6-μπι layer of poly(isobutylene). The point of film resonance, - 60 °C, is determined from the maximum in damping, indicated along the right vertical axis. The left vertical axis plots changes in resonance frequency, which undergoes a large increase at film resonance. (Adapted with permission from Reference 12.)

988 A · ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993

polymer layer on one delay line have been investigated (9, 10), but the uncoated reference delay line cannot compensate for the temperature drift of the polymer-coated delay line be c a u s e t h e l a t t e r is l a r g e l y d u e to polymer t h e r m a l expansion. For such a scheme to work, the reference delay line would have to be coated with the same material as the sens ing delay line, but sealed from vapor exposure. Alternatively, it m a y be better simply to control sensor tem peratures in many applications. Recent s t u d i e s h a v e shown t h a t p o l y m e r - c o a t e d F P W devices can sense a p p a r e n t changes in polymer properties occurring at both the static glass t r a n s i t i o n t e m p e r a t u r e Tg and at the so-called dynamic glass transition t e m p e r a t u r e Ta {11). Re sults for a thin film of poly(vinyl ace t a t e ) on a 5-MHz FPW device are shown in Figure 1. At Tg, a change in the slope of the frequency-tempera ture profile is observed. This change is a manifestation of the sensitivity to t h e polymer t h e r m a l e x p a n s i o n rate, which increases abruptly at T%. Relaxation processes observed at Ta involve a minimum in signal am plitude and a sigmoidal decrease in the f r e q u e n c y - t e m p e r a t u r e profile. When mechanically perturbed by the probing acoustic waves, polymer chain segments relax back to t h e i r former conditions at a r a t e t h a t is dependent on t e m p e r a t u r e and the structure of the particular polymer. At a temperature where the charac t e r i s t i c r e l a x a t i o n t i m e is m u c h longer than the period of the probing ultrasonic wave, the measured mod ulus has a value t h a t is typical of a polymer glass. At a higher tempera ture where the characteristic relax ation time is much shorter t h a n the period of the probing acoustic wave, t h e m e a s u r e d modulus is t h a t of a rubber. At i n t e r m e d i a t e t e m p e r a t u r e s where the characteristic relaxation time is comparable to the period of t h e probing waves, two effects are o b s e r v e d . Device f r e q u e n c i e s de crease because the measured modu lus undergoes a transition from val ues typical of a glass to values typical of a rubber (the dynamic glass tran sition). The amplitude is attenuated b e c a u s e e n e r g y can be efficiently coupled into t h e polymer film and dissipated when the relaxation time and wave period are similar. In the FPW device study, the results corre lated well with the properties of the test polymers as they had been de termined by standard polymer char acterization techniques such as dila-

tometry, differential scanning calorimetry (DSC), dynamic mechan ical a n a l y s i s (DMA), a n d conven tional bulk-wave ultrasonics. The quartz TSM device is also sen sitive to intrinsic polymer properties. U s i n g a network analyzer to m e a sure admittance as a function of fre quency a n d a n e q u i v a l e n t c i r c u i t model to evaluate the data, it is pos sible to extract the polymer elastic shear modulus G' and loss modulus G" (12, 13). After plotting these val ues as a function of temperature, the characteristics of a polymer relax ation process are observed for a poly(isobutylene) film. These TSM experiments and mod els also demonstrated an additional viscoelastic effect, referred to as film r e s o n a n c e , t h a t can occur in r e l a tively thick films. The film resonance effect depends on device frequency, film thickness, polymer shear modu lus, and polymer density. The effect is observed experimentally by r a m p ing t h e t e m p e r a t u r e of a polymercoated device, which c a u s e s l a r g e modulus changes in the polymer. At lower temperatures where the poly mer modulus is high and the film is rigid, the entire thickness of the film moves synchronously with the device s u r f a c e . As t h e t e m p e r a t u r e i n creases and the polymer modulus de creases, the motion at the film's u p per surface begins to lag behind the motion at the polymer-device inter face (where the polymer adheres and is forced to move synchronously with the device surface). This nonsynchronous motion induces s t r a i n in t h e film. Continuing decreases in modu lus with increasing temperature in crease the phase lag and the strain. When the phase lag reaches π/2 (i.e., the film's upper surface lags behind the motion at the polymer-device in terface by 90°), a condition of film resonance is reached. The increases in phase lag during t h i s p r o c e s s a r e a c c o m p a n i e d by changes in the particle displacement across the thickness of the film. With lossy films, t h e d i s p l a c e m e n t d e c r e a s e s as t h e d i s t a n c e from t h e polymer-device interface increases. This phenomenon can be compared with the motion in a liquid in contact with a TSM device: The molecules at the device surface follow the surface; however, this motion decays as the d i s t a n c e from t h e s u r f a c e is i n creased. The effects of film r e s o n a n c e on sensor responses are shown in Fig u r e 2 (12). At t e m p e r a t u r e s below t h a t of film r e s o n a n c e , t h e device frequency decreases with increasing

temperature as the polymer expands and the modulus decreases. As film r e s o n a n c e is a p p r o a c h e d , t h e fre quency drops more steeply. B u t a t film resonance, t h e frequency sud denly increases to values exceeding the initial frequency. In addition, the device is highly damped at film reso nance. For a given polymer, the tem perature at which film resonance oc curs decreases with increasing device frequency or w i t h i n c r e a s i n g film thickness. Although these film reso nance effects are simplest to under s t a n d on a TSM device w h e r e t h e surface motion is entirely in-plane, they also occur on SAW devices if r e l a t i v e l y t h i c k films ( e x c e e d i n g 200 nm) are used (13).

FPW

U n d e r s t a n d i n g viscoelastic film properties is also essential in the in telligent selection of polymer materi als for use on vapor sensors. When fast responses are desired, a polymer should be chosen whose T g is below the sensor's operating t e m p e r a t u r e . G r e a t e r free volume a n d p o l y m e r chain segmental motion above Tg re sult in faster vapor diffusion. To take advantage of modulus effects to max imize sensor s e n s i t i v i t i e s (see t h e next section), it is preferable t h a t the polymer's Ta be above the sensor's operating temperature. Below Ta the initial modulus of the material as it is perceived by t h e high-frequency acoustic waves is high. Finally, an understanding of film resonance ef fects is needed in order to compre hend the effects of film thickness on sensor response behavior.

Polymer-coated vapor sensors

devices can sense apparent changes in polymer properties... The s e n s i t i v i t i e s of t h e s e u l t r a sonic devices to viscoelastic proper t i e s can lead to a p p l i c a t i o n s in a number of areas. They can be used to m o n i t o r i n t r i n s i c polymer t r a n s i tions, such as the static glass transi tion a n d r e l a x a t i o n processes, and phenomena such as polymer curing or p a i n t d r y i n g (4, 11, 12, 14, 15). Properties such as shear modulus at the device frequency can also be de termined. Solid—liquid-phase transi tions can be detected, as h a s been d e m o n s t r a t e d on t h e SH-APM de vice (16). P h a s e t r a n s i t i o n s in thin liquid crystalline layers, L a n g m u i r B l o d g e t t films, a n d m u l t i b i l a y e r films have been examined with the q u a r t z TSM device (17, 18). U l t r a sonic microdevices offer the advan tages of very small sample size, trivial sample p r e p a r a t i o n , and i n s t r u m e n t a t i o n t h a t is well suited to automation and interfacing with digital electronics. On devices such as the TSM, where harmonic modes are available, one can m a k e m e a surements at various frequencies on a single device and polymer film. In addition, ultrasonic devices allow di rect measurements at high frequen cies, rather t h a n relying on extrapo l a t i o n s from low frequencies as is commonly done in DMA.

Vapor sensors based on acoustic mi crodevices invariably use some type of chemical layer to collect and con c e n t r a t e vapor molecules from t h e gas phase to the surface of t h e de vice. This method was first demon s t r a t e d by King, who u s e d q u a r t z TSM devices (19), and was extended to SAW devices by W o h l t j e n a n d Dessy (4). The same approach to va por sensing h a s been demonstrated with FPW devices (20). We will focus p r i m a r i l y on q u a r t z SAW s e n s o r s that use polymer thin films to absorb vapor reversibly. The polymer is as sumed to be a soft rubber to promote rapid vapor diffusion, and it is fur ther assumed t h a t the polymer film is quite thin so t h a t film resonance effects do not occur. The factors t h a t influence sensi tivity include the strength with which the polymer sorbs the vapor, t h e t h i c k n e s s of t h e polymer film, and the inherent sensitivities of the device to thin-film physical proper ties that are altered by vapor absorp tion. The strength with which a va por is sorbed depends only on t h e interactions between the vapor and the polymer; it is independent of the ultrasonic device. Solubility interactions such as dis persion forces, dipole-dipole interac tions, and hydrogen bonding as they apply to s e n s o r c o a t i n g m a t e r i a l s h a v e r e c e n t l y been reviewed (21). The equilibrium distribution of vapor between the gas phase and the sorb e n t p h a s e is characterized by the p a r t i t i o n coefficient Κ = Cs I C v , where C s is the concentration of the vapor in the sorbent phase and Cv is t h e vapor concentration in the gas phase.


REPORT Absorption is i l l u s t r a t e d in Fig u r e 3. The h i g h e r the Κ value, the greater the amount of vapor that will be collected a t t h e sensor's surface for a given v a p o r - p h a s e concentra tion, and hence the more sensitive the sensor. The fact t h a t polymercoated SAW sensors respond prima rily to the effects of absorbed vapor in the bulk, as opposed to surface ad sorbed vapor, has been convincingly demonstrated by comparisons of the responses of sensors of various fre quencies (8, 22). Increasing coating thickness will also increase the a m o u n t of vapor collected a t t h e surface, and hence increase sensitivity. However, the a t t e n u a t i o n of surface wave energy by rubbery polymer materials places p r a c t i c a l l i m i t a t i o n s on c o a t i n g thicknesses, especially when oscilla tor circuits are used. At some limit ing t h i c k n e s s , t h e i n s e r t i o n losses exceed the gain in an oscillator cir cuit and oscillation ceases. Polymer layer thicknesses on vapor sensors must be kept well below this limit so that the oscillator is not quenched by additional attenuation occurring as sorbed vapor softens t h e polymer. Because attenuation increases as the ultrasonic frequency increases, film thicknesses must necessarily de crease w i t h i n c r e a s i n g device fre q u e n c y . T h e s e d e c r e a s e s in film t h i c k n e s s offset i n c r e a s i n g m a s s sensitivity with frequency in vaporsensing applications using SAW de vices (see below) (22). Two questions about transduction mechanisms arise in the preparation and use of a SAW vapor sensor. First of all, what is the mechanism of the frequency decrease observed when the polymer film is applied? Second, w h a t is the m e c h a n i s m of t h e fre quency decrease when vapor is a b sorbed by t h e polymer? These pro cesses a n d t h e factors influencing t h e m a r e s u m m a r i z e d in F i g u r e 4 (23). When the polymer film is applied to t h e b a r e SAW device, t h e fre quency decrease observed is prima rily due to the m a s s of the coating. This result has been experimentally demonstrated using polymeric Langmuir-Blodgett layers of known mass per unit area (22). In theory, the fre q u e n c y shift o c c u r r i n g w h e n t h e coating is applied depends on both the mass and the modulus of the film (2). However, the modulus effect is predicted to be only about 10-15% of the mass effect if the measured mod u l u s a t t h e SAW frequency is 10 9 N / m 2 (typical of a glassy polymer or a rubbery polymer at high frequen

cy), and it should be negligible if the m e a s u r e d modulus is 10 6 N / m 2 (8, 22). Because the frequency shift is re lated to the coating mass, it provides a convenient measure of the coating thickness. "Thicknesses" of 200-300 kHz are typical for vapor sensors in fixed-gain oscillator circuits. On a 200-MHz device, 250 kHz of a poly mer of density 1 g/mL is 50 nm thick if evenly distributed. When coating t h i c k n e s s e s a r e k e p t c o n s t a n t in terms of kilohertz as device frequen cies increase, then the absolute coat ing thicknesses, in terms of nanome t e r s , d e c r e a s e w i t h t h e s q u a r e of frequency. In this case, vapor sensi t i v i t i e s a r e i n d e p e n d e n t of device frequency: The increase in the device s e n s i t i v i t y to m a s s a n d m o d u l u s changes w i t h the s q u a r e of device frequency is offset by the decrease in coating thickness, which decreases t h e a m o u n t of vapor absorbed per unit area (22). When the polymer-coated device is exposed to a vapor, the sorption of the vapor perturbs the polymer layer

Vapor Polymer thin film Sensor substrate

Figure 3. Reversible vapor absorption and the partition coefficient Κ = CJCV.

Bare SAW device Apply polymer film At /Modulus

Mass

Modulus

Figure 4. Mass and modulus effects in the coating and use of a polymercoated SAW vapor sensor. (Adapted with permission from Reference 23.)


and sensor frequencies normally de crease. These responses are in t h e correct direction for a mass loading response, and undoubtedly vapors do i n c r e a s e t h e m a s s of t h e polymer film. Therefore, it has long been as sumed t h a t such vapor sensors are measuring only the mass of the va por sorbed. The extent to which va por-induced modulus changes might contribute to these sensor responses has been quite difficult to evaluate because the moduli of the polymer film before and after vapor sorption, as they are perceived by the highfrequency waves, are unknown. However, the mass sorbed is not in dependently known either. To evaluate m a s s loading effects experimentally, an equation was de rived r e l a t i n g sensor r e s p o n s e s to partition coefficients 4/v(maSS> = ¥*CVK/P

(2)

where Admass). A4. Cv, K, and ρ are, r e s p e c t i v e l y , t h e f r e q u e n c y shift caused by the mass of the vapor, the coating thickness in kilohertz, t h e vapor concentration in the gas phase, t h e p a r t i t i o n coefficient, a n d t h e p o l y m e r m a t e r i a l d e n s i t y (8, 10). T h i s e q u a t i o n is valid for all t h e m a s s - s e n s i t i v e devices previously described. The product C^K gives Cs, the concentration of the vapor in the polymer, and hence, the mass of the vapor. Independent determinations of the partition coefficients of many v a p o r / p o l y m e r p a i r s a t 2 5 °C b y gas-liquid chromatography then provided the necessary information on the mass loading of the polymers (8, 10). The responses of polymer-coated SAW sensors at 25 °C to calibrated vapor streams were determined and c o m p a r e d w i t h t h e r e s p o n s e s ex pected from the sorbed vapor's mass. These results are shown for one poly m e r in Figure 5. Actual sensor r e s p o n s e s w e r e four to six t i m e s greater than the calculated mass loading r e s p o n s e s . Therefore, con t r a r y to p a s t a s s u m p t i o n s , m a s s loading cannot be the primary mech a n i s m by which t h e s e s e n s o r s r e spond to vapors. Sensitivity to mod ulus changes occurring when the p o l y m e r film is p e r t u r b e d by t h e sorbed vapor m u s t play a much larger role in sensor response t h a n we previously suspected. The sizes of the observed responses were within the correct range for the effect of softening a polymer whose i n i t i a l m e a s u r e d m o d u l u s is ~ 10 9 N / m 2 , as would be expected for a r u b b e r y polymer a t t h e h i g h SAW frequency. However, because the ac-

20,000 Χ

Predicted from mass loading

15,000

ι Measured

10,000 φ

5000 Η 0

NME BTL BTN DCE TOLN ISOC

Vapors

Figure 5. Comparisons of measured SAW vapor sensor responses with those predicted on the basis of mass loading. Responses of a poly(isobutylene)-coated sensor to several organic vapors. NME: nitromethane, BTL: 1 -butanol, BTN: 2-butanone, DCE: 1,2-dichloroethane, TOLN: toluene, ISOC: isooctane.

tual modulus changes were not known, the observed responses could not be compared directly with calcu lations based on the theoretical mod ulus sensitivity of the SAW device. An alternative approach, based on the interrelationships between poly mer volume, modulus, acoustic veloc ities, and oscillator frequencies, al lowed an independent estimation of s e n s o r r e s p o n s e s a t t r i b u t a b l e to modulus decreases (8). The effect of volume on sensor frequencies was calibrated from polymer thermal ex p a n s i o n e x p e r i m e n t s on SAW d e vices; these involve no mass per unit area change. From known t h e r m a l e x p a n s i o n r a t e s (typically 0.05 to 0.06%/ °C) and the measured effects of polymer thin-film expansion on SAW sensor frequencies (typically - 5 0 0 to - 1 0 0 0 Hz/°C for films 250 kHz thick), the v o l u m e effect is e s t i m a t e d to be 10,000-20,000 Hz per percent of vol ume increase. Volume increases c a u s e d by v a p o r a b s o r p t i o n (i.e., swelling) were e s t i m a t e d from t e s t vapor concentrations, p a r t i t i o n co efficients, and liquid densities of the vapors. Swelling of 0 . 3 - 3 % was typi cal for t e s t v a p o r s p r o d u c i n g r e sponses of 2000-20,000 Hz. Assuming t h a t swelling and t h e r mal expansion have similar effects on m o d u l u s a n d hence sensor fre quencies, the estimated swelling can be multiplied by the volume effect calibration to calculate the frequency decreases t h a t occur in response to swelling-induced modulus changes. These responses are four to six times g r e a t e r t h a n t h e m a s s loading r e sponses calculated by Equation 1 in P a r t I, i n good a g r e e m e n t w i t h

the experimental r e s u l t s above. Therefore, polymer-coated SAW va por sensor responses can be modeled as a sum of the effects of mass load ing a n d swelling-induced modulus c h a n g e s on s e n s o r f r e q u e n c i e s , where the latter predominates. Because vapor sorption and the re sulting volume increase also reduce polymer relaxation times, it is possi ble t h a t relaxation effects described in t h e previous section may influ ence vapor sensor responses. The ob servation of the relaxation process involves c h a n g e s in t h e perceived modulus, to which the device is sen sitive. The reduction in the sensed modulus would lead to frequency de creases t h a t would enhance the nor mal sensor response. Vapor sorption also has the poten tial to induce film resonance effects in thick films. This can cause anom alous frequency increases occurring in the opposite direction from typical responses based on mass loading and m o d u l u s d e c r e a s e s in t h i n films. Such effects have been observed in experiments monitored by the vector voltmeter method (15). These effects, previously attributed to polymer re laxation processes, are now u n d e r stood to involve film resonance (13). The combination of acoustic velocity and attenuation information in such cases can be useful in helping to dis tinguish different vapors (15).

Polymer-coated SAW sensors respond to the effects of absorbed vapor... Until recently, gas-phase sensing applications have dominated the field of acoustic wave chemical sen sors. A variety of selective materials have been investigated for the detec tion of various analytes (21, 24-29). Because reversible absorption is not 100% selective, the use of sensor ar r a y s w i t h p a t t e r n recognition h a s been investigated as a means of im proving selectivity in the identifica tion of toxic vapors and in multicomponent analysis (9, 30-34). Both selectivity a n d s e n s i t i v i t y can be enhanced through the use of preconcentrators (33, 35). Thus the

development of individual s e n s o r s leads to the development of sensor s y s t e m s in which t h e s e n s o r s a r e used as the detection elements. In addition to the sensor or sensor ar ray, a complete sensor system i n cludes a s a m p l i n g s y s t e m , s i g n a l m e a s u r e m e n t electronics, and p r e programmed signal analysis and de cision-making algorithms (33). Sen sor a r r a y s y s t e m s u s i n g p a t t e r n recognition are often described by phrases such as "smart sensor sys tems" or "electronic noses" (36, 37). Acoustoelectric and dielectric effects When an acoustic wave propagates in a piezoelectric material, it gener ates a layer of bound charge at t h e surface t h a t accompanies t h e m e chanical wave. This bound c h a r g e generates an evanescent electric field t h a t extends into an adjacent medium in contact with the surface, c a u s i n g motion of charge c a r r i e r s and dipoles in that medium. The en ergy stored and dissipated in moving t h e s e c h a r g e s a n d d i p o l e s is e x tracted from the wave and influences the wave velocity and attenuation. This acoustoelectric interaction is observed between SAWs and conduc tive thin-film overlayers, provided t h a t t h e s h e e t conductivity of t h e film is within a certain critical range a n d t h e SAW device piezoelectric m a t e r i a l h a s a sufficient e l e c t r o mechanical coupling c o n s t a n t (3840). In this regard, lithium niobate SAW devices are more sensitive to a c o u s t o e l e c t r i c effects t h a n a r e quartz SAW devices. Acoustoelectric effects caused by sheet conductivity changes are not observed when the overlay thin film is nonconducting, a s i n t y p i c a l p o l y m e r l a y e r s , or highly conductive, as in continuous metal films. Acoustoelectric effects are most pronounced when the film sheet conductivity is σ„ = V0 (e s + e 0 ), where V0 is the SAW velocity and e s and e 0 are permittivities for the sub strate and air, respectively. Weakly semiconducting lead phthalocyanine films give rise to acoustoelectric ef fects on l i t h i u m n i o b a t e SAW de vices, and this process forms the ba sis for a gas sensor (40). C h e m i s o r p t i o n of g a s e s such as N 0 2 that alter the sheet conductivity of the phthalocyanine film result in wave velocity changes caused by the acoustoelectric effect that are signif icantly greater t h a n those caused by mass loading alone. The acoustoelec tric response can be eliminated by placing a conducting metal film be tween t h e s e n s i n g p h t h a l o c y a n i n e


REPORT layer and the SAW substrate to short out the surface charge carriers. Un like mass sensitivity, the magnitude of the wave velocity change (Av / V0) caused by the acoustoelectric inter action does not depend on device fre quency. O t h e r SAW s e n s o r s w i t h semiconducting thin films include a sensor for hydrogen sulfide gas in volving t u n g s t e n trioxide on a lith ium niobate device (41) and various m e t a l l o p h t h a l o c y a n i n e s on q u a r t z SAW devices (42). The importance of the sheet con ductivity in determining the occur rence of the acoustoelectric effect can be seen in Figure 6 (43), which shows t h e effect of v a c u u m - e v a p o r a t e d nickel on SAW velocity and attenua t i o n , u s i n g a q u a r t z SAW device. (See also Reference 39.) The initial decrease in wave velocity w i t h in creasing nickel deposition is due to t h e m a s s l o a d i n g of t h e s u r f a c e . However, the change in wave veloc i t y w i t h film t h i c k n e s s b e c o m e s much steeper a t around 2 0 - 3 0 A of metal film, a n d the wave is highly a t t e n u a t e d . At t h i s t h i c k n e s s , t h e sheet conductivity of the film is in the range where acoustoelectric ef fects are significant. As the film be comes thicker, the sheet conductivity becomes much greater than the criti cal sheet conductivity for the acous toelectric interaction. The wave at tenuation diminishes, and the rate of w a v e v e l o c i t y c h a n g e w i t h film t h i c k n e s s a g a i n reflects p r i m a r i l y mass loading effects.

I n t e r a c t i o n s w i t h free dipoles a t the SAW surface can also influence wave velocity and attenuation. The evanescent electric field generated at t h e surface of t h e c r y s t a l e x t e n d s into the region above the device, de c a y i n g as exp(-ky) w h e r e k is t h e wavenumber (2π/λ) and y is the dis tance from the surface. If polar spe cies exist in the near-surface region and are free to reorient in response to the oscillating field (as opposed to being bound tightly as might occur in a film), they contribute to the electri cal energy stored and dissipated by the SAW. Corresponding wave velocity and attenuation changes occur in propor tion to the concentration of the polar species. Normally the electric field at t h e surface is not large enough to cause the interaction with dipoles to be large relative to the contribution from mass loading. However, Huang h a s developed a lithium niobate SAW sensor for use in high humidity t h a t relies on this interaction with out u s i n g a s o r p t i v e c o a t i n g (44). Water molecule concentrations near t h e surface are quite large at high humidity, and the large dipole mo ment relative to the molecular mass is ideal for detection by this mecha nism. Recently, Stone and Thompson found e v i d e n c e t h a t c a p a c i t a n c e changes in the IDTs of a SAW device may influence sensor responses (45). Sorption of polar molecules such as w a t e r or acetone between t h e IDT

Thickness (Â)

Figure 6. Acoustoelectric effect illustrated for the deposition of a nickel metal film on a SAW device. The maximum in attenuation (right vertical axis) and sigmoidal decrease in wave velocity (left vertical axis) indicate the film thickness for which the film sheet conductivity is in the range where the acoustoelectric effect is observed. (Adapted with permission from Reference 43.) 992 A · ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993

fingers increases the dielectric con s t a n t of the space between the fin gers and increases the IDT capaci t a n c e . W h e n t h e SAW device is o p e r a t e d in a n o s c i l l a t o r c i r c u i t , these vapor-induced IDT capacitance changes can lead to positive or nega tive frequency shifts, depending on the particular design of the oscillator circuit. This response, caused by per turbations of IDT capacitance, is dis tinct from the acoustoelectric inter actions between a propagating wave and dipoles or charge carriers. Base line noise and signal amplitude can also depend on IDT capacitance. Acoustoelectric effects have also been observed in liquid-phase stud ies using SH-APM devices (46, 47). The evanescent rf electrical field at the surface couples to ions in the so lution, resulting in ionic motion t h a t stores and dissipates electrical en ergy. Changes in the stored electrical energy alter the wave velocity, and dissipation leads to wave a t t e n u a t i o n . Significant c h a n g e s in wave velocity with increases in ion concen t r a t i o n s were observed experimen tally, but the attenuation was small. The r a t e at which wave velocity is influenced by ionic conductivity de pends on t h e dielectric c o n s t a n t of the solvent. A number of approaches to e l i m i n a t i n g s i g n a l s c a u s e d by variations in solution ionic conduc tivity have been suggested for appli cations where this device sensitivity would lead to interferences (46). Acoustoelectric effects have been r e p o r t e d for TSM devices (48). I n creases in the ionic conductivity of a solution in contact with the device were found to cause a decrease in the r e s o n a n t f r e q u e n c y of t h e T S M / oscillator circuit. These reports have been controversial, however, because of the presence of a metal electrode between the piezoelectric s u b s t r a t e and the solution. If common practice is observed, in which the immersed electrode is larger t h a n the one not immersed (to prevent fringing fields from e n t e r i n g t h e solution) a n d is grounded w i t h respect to t h e solu tion, then electrochemical and acous toelectric interactions cannot occur. On the other hand, if the immersed electrode is not grounded w i t h r e spect to the solution, then a parallel conduction path is created: The oscil l a t o r frequency can be "pulled" as the conduction of this path is altered by ionic concentration. In fact, this is the reason TSM devices are not to tally immersed in aqueous solutions but have only a single electrode im m e r s e d . T h u s , like t h e c h a n g e in SAW transducer capacitance by po-

lar species, this effect is probably not an acoustoelectric interaction but an external conductivity change. Liquid-phase sensing Early studies of acoustic wave microsensors focused on gas-phase ap plications because it w a s expected t h a t contact with liquids would ex cessively damp the acoustic waves. However, Konash and B a s t i a a n s r e ported in 1980 t h a t quartz TSM de vices could be u s e d in t h e liquid phase as detectors for HPLC (49). In fact, most of the devices in Table I in P a r t I can be used for liquid-phase detection applications. However, liq u i d - p h a s e operation is precluded if wave motion produces surface-nor mal particle displacements and the wave propagation velocity is greater t h a n the velocity of sound in the liq uid, as is the case with SAW devices. In this case, an angle can be found in which the direction of the compressional wave radiation from succes sive wave c r e s t s is c o h e r e n t (i.e., adds in phase), causing energy to ef ficiently "leak away" from the wave and resulting in prohibitive attenua tion. If the wave propagation velocity is less t h a n the velocity of sound in the liquid, no direction for coherent radiation exists and less wave atten uation occurs. For reference, the compressional velocity of sound in water is 1500 m/s. When the surface of a shear-mode device is placed in contact with a liq uid, the wave velocity decreases as if t h e surface were mass-loaded by a thin layer of the liquid. Figure 7 illus trates the in-plane motion at the sur face of a TSM device in contact with a liquid (50). Viscous coupling between the surface and the liquid results in a damped shear wave t h a t propagates into the liquid with a characteristic decay length. The thickness of the liq uid layer probed by a 5-MHz quartz TSM is - 0.25 μπι in water. The en trained liquid layer decreases the res onant frequency (relative to operation in a gas or vacuum phase) and damps the TSM resonance. Damping by vis cous coupling is not as great as damp ing by r a d i a t i o n of c o m p r e s s i o n a l waves and does not preclude liquidphase operation. The shift in resonance frequency on liquid loading of the TSM surface can be related to the square root of the product of the density and vis cosity of the liquid (51, 52). The sen sitivity to these properties h a s im p o r t a n t i m p l i c a t i o n s for a n a l y t e detection in liquids: E x p e r i m e n t a l c o n d i t i o n s m u s t be carefully con trolled to avoid drift and spurious re

sponses a r i s i n g from v a r i a t i o n s in t h e s e properties d u r i n g sensor ex periments. For example, isothermal conditions must be rigorously main tained because viscosity is exponen tially t e m p e r a t u r e - d e p e n d e n t . By using a network analyzer to operate TSM devices (with smooth surfaces) in the liquid phase, it is possible to distinguish responses that are caused by surface mass changes îrom responses t h a t are caused by changing liquid properties (50). T h e S H - A P M a n d F P W devices also exhibit responses related to liquid density and viscosity when operated in contact with a liquid (46, 53-

New

emphasis

on the details of molecular and physical events at interfaces... 56). In t h e s e t w o - p o r t devices t h e response arises as a change in wave velocity and attenuation, analogous to the change in resonant frequency and damping observed with one-port TSM devices. T h e s e n s i t i v i t i e s of these devices to liquid-phase properties has resulted in the development of SH-APM and FPW viscosity sensors. These s e n s o r s p e r m i t in situ monitoring and measurement of viscosity on extremely small samples of 10 |iL or less (53, 55, 56). Device operating frequency influences the measurement of liquid viscosity. When probed at low frequencies, liquids behave as purely viscous (Newtonian) fluids. At high frequencies, w h e r e t h e wave p e r i o d a p proaches the liquid relaxation time, the liquid behaves less as a n ideal N e w t o n i a n liquid a n d more like a viscoelastic fluid. In this case some liquids can be modeled as a Maxwellian fluid with a single relaxation time proportional to viscosity. The r e l a t i o n s h i p between liquid relaxation time and acoustic wave period places an upper limit on the viscosities that can be measured. The lower frequency 5-MHz F P W device can measure higher viscosities t h a n the higher frequency 158-MHz SH-APM device (53, 55). It h a s s o m e t i m e s been observed t h a t TSM devices placed in contact

with a liquid have frequency shifts greater t h a n those predicted on the basis of the liquid density and viscosity alone. A number of explanations have been proposed for these deviations, including the influences of surface roughness (51 ) and reorganization of t h e solvent at the interface such t h a t its interfacial viscosity and density are greater t h a n those oî the b u l k liquid (52, 57, 58). Surface roughness can contribute to the sensor's response because liquid can be e n t r a i n e d in t h e crevices of rough TSM surfaces. This leads to an additional frequency shift on liquid loading t h a t is proportional to the product of the density and the effective thickness of the trapped liquid. Damping is also increased by surface roughness. T h e r o u g h n e s s of c o m m e r c i a l l y available quartz crystals varies considerably, so it is worthwhile to select devices made with highly polished crystals and to evaluate their roughness with a profilometer before a t t e m p t i n g to i n t e r p r e t sensor r e sults (50). In general, surface feat u r e s must be small compared with the decay length of the damped shear wave propagating in the liquid in order for the crystal surface to be considered hydrodynamically smooth. The possibility that interfacial viscosities and slippage at the l i q u i d solid interface a t t h e TSM surface may influence TSM frequency shifts on liquid loading h a s been investigated by Thompson and co-workers (52, 57-60). It is customary to a s sume a nonslip boundary condition when deriving models for acoustic sensors in liquids, meaning t h a t the liquid molecules immediately adjacent to t h e surface move synchronously with the surface. Wetting at this interface is essential to e s t a b lishing the no-slip condition, and in one case Ricco and Martin observed

Liquid Entraînée y liquid AT-quartz Displacement Air

Figure 7. Entrainment of viscous liquid and the damped shear wave propagating from the surface of a quartz TSM device in contact with the liquid.


REPORT Table 1. Transduction mechanisms and typical applications Transduction mechanism TSM

SAW

Mass Film thickness monitor Particle detectors Chemical sensors (gas and liquid phase) Chemical sensors (gas phase) Particle detectors

FPW

Chemical sensors (gas and

SH-APM

Chemical sensors (liquid phase)

liquid phase)

Liquid density and viscosity

Viscoelastic

Acoustoelectric

Polymer transitions Film resonance Chemical sensors"

Not applicable 6

Applicable*

Polymer transitions Film resonance Vapor sensing

Electronic conductivity (gas sensors) Free dipoles (humidity sensors) Not applicable"

Not applicable''

Polymer Microliter transitions viscometer 0 Vapor sensing Microliter Gels densitometer c Polymer curing Ionic conductivity Microliter Viscosity at (liquid phase) viscometer* high frequency

a

TSM viscoelastic effects may be especially important in solution electrochemical studies where solvent and charges move in and out of polymer films with redox processes. Acoustoelectr ic effects are prevented because, in the usual device structure, a metal around plane separates the piezoelectric layer from the sensingι surface. c Sensitivities to density and viscosity are important in controlling drift and spurious responses in chemical-sensing applications. " SAW devices are not used in the liquid phase. 6

t h a t a SH-APM viscosity sensor did not respond to liquid loading until a surfactant was added to t h e liquid (55). ( I n c i d e n t a l l y , F P W viscosity sensors a p p e a r to be relatively im m u n e to the possible effects of slip because their coupling to the liquid is caused p r i m a r i l y by t h e compo nent of motion t h a t is perpendicular to the surface [73]). Thompson a n d c o - w o r k e r s h a v e observed that frequency shifts on liq uid loading can vary with the surface free energy as measured by the con t a c t a n g l e , w i t h l a r g e r frequency shifts observed for hydrophilic sur faces t h a n hydrophobic surfaces (es pecially w h e n t h e liquid is w a t e r ) (57, 59). It is possible t h a t surface free e n e r g y i n f l u e n c e s s e n s o r r e sponse to liquid loading by influenc ing how liquid is entrained by rough surfaces (57, 61). Water completely penetrates the crevices of hydrophilic surfaces, resulting in maximum liq uid trapping, but incompletely pene t r a t e s t h e crevices of hydrophobic surfaces with large contact angles, resulting in less liquid trapping. In this model, the influence of surface free energy depends on the existence of surface roughness. Using hydrodynamically smooth crystals, Martin et al. have found good a g r e e m e n t be tween frequency shifts predicted by models based only on the bulk liquid density and viscosity, regardless of

the surface free energy (61). On the other hand, deviations were observed with rough surfaces, and the amount of deviation was g r e a t e r for hydro philic surfaces t h a n for hydrophobic surfaces. A n a l y t e d e t e c t i o n in t h e liquid p h a s e u s i n g acoustic devices is a r a p i d l y e x p a n d i n g field, a n d t h e t r a n s d u c t i o n m e c h a n i s m s involved a r e still being e l u c i d a t e d a n d de b a t e d . I n t e r e s t in t h i s field s t e m s from two p r i n c i p a l a r e a s : F i r s t , a large body of literature describes the use of quartz TSM devices in liquidphase electrochemical studies, where t h e s e n s i t i v i t y to s u r f a c e m a s s changes may complement t h e elec trochemical information about redox processes at the surface (51, 62—64). Second, there is great interest in the development of biosensors (52, 6572). The sensitivity of acoustic wave devices to submonolayer coverages of small molecules indicates a consider able p o t e n t i a l to detect processes such as the binding of a n antigen to a surface-attached antibody. T h e i n t e r p r e t a t i o n of c h e m i c a l s e n s o r r e s p o n s e s in l i q u i d - p h a s e studies should consider all the fac tors discussed above, including sur face mass loading, changes in liquid density or viscosity, changes in ionic conductivity, and changes in the entrainment of liquid at the surface re l a t e d to surface r o u g h n e s s a n d / o r


surface energy. In addition, if a poly mer film is deposited on the surface, changes in its viscoelastic properties during an experiment may influence sensor responses (63). Unless all the chemical and physical processes occuring at the surface of an acoustic sensor in the liquid phase are well understood, or unless sufficient ex perimental information is available, one cannot simply assume t h a t ob served responses are solely due to mass loading. A biosensor for glucose d e t e c t i o n , for e x a m p l e , h a s b e e n shown to give responses that are sig nificantly greater t h a n those possi ble by mass-loading effects (65). The enhanced responses in this case were attributed to viscoelastic changes in the sensing layer. Summary and outlook The devices and transduction mech anisms discussed in this REPORT are all r e l a t e d to a c o u s t i c w a v e s a n d their interactions with matter at de vice surfaces. The basic principles extend from one device type to a n other and from gas- to liquid-phase sensing. For example, mass sensing is common to all these sensors and can occur in vacuum, gas, and liquid p h a s e s . Viscoelastic properties are important in the observation of the static and dynamic glass transitions in thin polymer films, the occurrence of film resonance effects, the detec tion of vapors in the gas phase with polymer-coated SAW devices, and in l i q u i d - p h a s e s e n s i n g w h e r e poly mers are present on the surface. Molecular and macromolecular r e laxation t i m e s , and t h e i r r e l a t i o n ships to acoustic wave periods, are i m p o r t a n t in t h e observation of t h e dynamic glass transition in polymer thin films and in the measurement of liquid viscosities at high frequencies. Acoustoelectric effects are important to g a s - p h a s e s e n s o r s w i t h weakly electronically conducting thin-film overlayers, gas-phase sensors for di polar species such as water, and liq u i d - p h a s e s e n s o r s in contact w i t h ionically conducting fluids. Response mechanisms and some of the applica tions of the various acoustic devices are summarized in Table I. M o s t r e v i e w s of c h e m i c a l m i crosensors group acoustic wave sen sors u n d e r m a s s s e n s o r s or piezo electric s e n s o r s . N e i t h e r of t h e s e classifications directly addresses the transduction mechanisms involved: Sensing occurs because of t h e per turbation of acoustic waves at ultra sonic frequencies. It should be evi dent t h a t the term "mass sensor" is limiting and, in many cases, inaccu-

A COMPLETE FAMILY

r a t e . M a s s d e t e c t i o n is n o t t h e only, nor always the dominant, mechan i s m by w h i c h a c o u s t i c w a v e devices function as sensors, a n d piezoelect r i c i t y is s i m p l y a m a t e r i a l p r o p e r t y t h a t is useful, b u t n o t n e c e s s a r y , for g e n e r a t i n g acoustic waves in small devices. T h e r e c o g n i t i o n of a d d i t i o n a l m e c h a n i s m s by which a n acoustic w a v e device c a n a c t a s a s e n s o r c r e a t e s m a n y n e w o p p o r t u n i t i e s for t h e d e s i g n of p h y s i c a l a n d c h e m i c a l s e n sors a n d p r o v i d e s t h e m e a n s by which sensitivities can be m u c h g r e a t e r t h a n t h o s e t h a t m i g h t be achieved by m a s s detection alone. T h e a c o u s t i c w a v e s e n s o r field h a s expanded tremendously in recent y e a r s . A t t h e r e s e a r c h e n d of t h e R & D s p e c t r u m , t h e r e is a n e w e m p h a s i s o n t h e d e t a i l s of m o l e c u l a r a n d physical events at interfaces and in t h i n films, a n d development h a s proceeded to sensor a r r a y s a n d complete m i c r o a n a l y t i c a l s y s t e m s . Select i v i t y a n d l i m i t s of d e t e c t i o n c o n t i n u e to improve, and great strides h a v e b e e n m a d e in reducing t h e size a n d p o w e r c o n s u m p t i o n of s e n s o r sytems. Many exciting applications are being explored, including elect r o n i c n o s e s for o d o r s e n s i n g , b i o s e n s i n g , s y s t e m s for s e c u r i t y a p p l i c a t i o n s ( s u c h a s t h e d e t e c t i o n of explosives, drugs, and chemical a g e n t s ) , a n d s y s t e m s for m e a s u r i n g c o n c e n t r a t i o n s of specific g a s e s a n d particulates in the stratosphere. The authors gratefully acknowledge Richard Baer, Hewlett-Packard Laboratories, for valuable suggestions on the relationships of the various acoustic devices to one another and for advance information on the STW devices. We also acknowledge Michael Thompson and David Stone, University of Toronto, for helpful discussions on interdigital capacitance, flexural rod devices, and response mechanisms in liquidphase sensing. Input on certain FPW device characteristics by Ben Costello and Stuart Wenzel, Berkeley Microinstruments, is also appreciated. Pacific Northwest Laboratory is operated for the U. S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. References (1) G r a t e , J. W.; M a r t i n , S. J.; W h i t e , R. M. Anal. Chem. 1993, 65, 940 A. (2) Wohltjen, H. Sens. Actuators 1984, 5, 307—25 (3) Tiersten, H. F.; Sinha, B. K. /. Appl. Phys. 1978, 49, 8 7 - 9 5 . (4) Wohltjen, H.; Dessy, R. E. Anal. Chem. 1979, 51, 1458-70. (5) F e r r y , J . D. Viscoelastic Properties of Polymers, 3rd éd.; John Wiley and Sons: New York, 1980. (6) Hartman, B. In Encyclopedia of Polymer Science and Engineering, 2nd éd.; Mark, H. F., Ed.; J o h n Wiley and Sons: New York, 1984; Vol. 1; pp. 131-60. (7) Massines, R.; Piche, L.; Lacabanne, C. Makromol. Chem., Macromol. Symp.

1989, 23, 121-37. (8) G r a t e , J . W.; K l u s t y , M.; McGill, R. Α.; Abraham, M. H.; Whiting, G.; Andonian-Haftvan, J. Anal. Chem, 1992, 64, 610-24. (9) Ballantine, D. S.; Rose, S. L.; Grate, J. W.; Wohltjen, H. Anal. Chem. 1986, 58, 3058-66. (10) Grate, J. W.; Snow, Α.; Ballantine, D. S.; Wohltjen, H.; A b r a h a m , M. H.; McGill, R. Α.; Sasson, P. Anal. Chem. 1988, 60, 869-75. (11) Grate, J. W.; Wenzel, S. W.; White, R. M. Anal. Chem. 1992, 64, 413-23. (12) Martin, S. J.; Frye, G. C. Proc. IEEE Ultrason. Symp. 1991, 393-98. (13) Martin, S. J.; Frye, G. C. Proceedings of the 1992 Solid State Sensor and Actuator Workshop; I E E E : New York, 1992; pp. 27-31. (14) Martin, S. J.; Ricco, A. J. Sens. Actua tors 1990, A21-A23, 712-18. (15) Martin, S. J.; Frye, G. C. Appl. Phys. Lett. 1990, 57, 1867-69. (16) Hughes, R. C ; Martin, S. J.; Frye, G. C ; Ricco, A. J. Sens. Actuators 1990, A21-A23, 693-99. (17) M u r a m a t s u , H.; K i m u r a , K. Anal. Chem. 1992, 64, 2502-07. (18) Okahata, Y.; Ebato, H. Anal. Chem. 1989, 61, 2185-88. (19) King, W. H. Anal. Chem. 1964, 36, 1735-39. (20) White, R. M.; Wicher, P. J.; Wenzel, S. W.; Zellers, E. T. IEEE Trans. Ultrason ics, Ferroelectrics, Frequency Control 1987, UFFC-34, 162-71. (21) Grate, J. W.; Abraham, M. H. Sens. Actuators Β 1991, 3, 8 5 - 1 1 1 . (22) Grate, J. W.; Klusty, M. Anal. Chem. 1991, 63, 1719-27. (23) G r a t e , J . W.; McGill, R. Α.; A b r a h a m , M. H. Proc. IEEE Ultrason. Symp. 1992 275—79 (24) Guilbault,' G. G ; Jordan, J. M. CRC Crit. Rev. Anal. Chem. 1988,19, 1-28. (25) Mierzwinski, Α.; Witkiewicz, Z. Envi ron. Pollut. 1989, 57, 181-98. (26) McCallum, J . J . Analyst (London) 1989, 114, 1173-89. (27) Nieuwenhuizen, M. S.; Venema, A. Sens. Mater. 1989, 5, 261-300. (28) Fox, C. G.; Alder, J. F. Analyst (Lon don) 1989, 114, 997-1004. (29) D'Amico, Α.; Verona, E. Sens. Actua tors 1989, 17, 55-66. (30) Carey, W. P.; Kowalski, B. R. Anal. Chem. 1986, 58, 3077-84. (31) Carey, W. P.; Beebe, K. R.; Kowal ski, B. R. Anal. Chem. 1987, 59, 1 5 2 9 34. (32) Ema, K ; Yokoyama, M.; Nakamoto, T.; Moriizumi, T. Sens. Actuators 1989, 18, 291-96. (33) Grate, J. W.; Rose-Pehrsson, S. L.; Venezky, D. L.; Klusty, M.; Wohltjen, H. Anal. Chem. 1993, 65, 1868-81. (34) Rose-Pehrsson, S. L.; Grate, J. W.; Ballantine, D. S.; J u r s , P. C. Anal. Chem. 1988, 60, 2 8 0 1 - 1 1 . (35) Kindlund, Α.; Sundgren, H.; Lundstrom, I. Sens. Actuators 1984, 6, 1-17. (36) Sensors and Sensory Systems for an Elec tronic Nose; G a r d n e r , J. W.; B a r t l e t t , P. N., Eds.; Kluwer Academic Publish ers: Dordrecht, The Netherlands, 1992. (37) Newman, A. R. Anal. Chem. 1991, 63, 585 A - 5 8 8 A. (38) Frye, G. C ; Martin, S. J. Appl. Spectrosc. Rev. 1991, 26, 73-149. (39) Ricco, A. J.; Martin, S. J. Thin Solid Films 1991, 206, 9 4 - 1 0 1 . (40) Ricco, A. J.; Martin, S. J.; Zipperian, T. E. Sens. Actuators 1985, 8, 319-333. (41) Vetelino, J. F.; Lade, R. K.; Falconer, R. S. IEEE Transactions Ultrasonics, Ferro-

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electrics, Frequency Control 1987, UFFC-34,(72) Thompson, M.; Krull, U. J. Anal. 156-61. Chem. 1991, 63, 393 A-405 A. (42) Nieuwenhuizen, M. S.; Nederlof, (73) Wenzel, Stuart W., Berkeley MicroA. J.; Barendsz, A. W. Anal. Chem. 1988, Instruments, 1993, personal communi 60, 230-35. cation. (43) Martin, S. J.; Ricco, A. J. Proc. IEEE Ultrason. Symp. 1989, 621-25. (44) Huang, P. H. Proceedings of the 4th International Conference on Solid-State Sensors and Actuators—Transducers '87; Institute of Electrical Engineers of Japan: Tokyo, 1987; pp. 462-66. (45) Stone, D. C ; Thompson, M. Anal. Chem. 1993, 65, 352-62. (46) Martin, S. J.; Ricco, A. J.; Niemczyk, T. M.; Frye, G. C. Sens. Actuators 1989, 20, 253-68. (47) Niemczyk, T. M.; Martin, S. J.; Frye, G. C; Ricco, A. J. /. Appl. Phys. 1988, 64, 5002-08. (48) Josse, F.; Haworth, D. T.; Kelkar, Jay W. Grate joined the Molecular Science U. R.; Shana, Z. A. Electron. Lett. 1990, Research Center at Pacific Northwest Lab 26, 834. oratory in 1992. Previously, he was em (49) Konash, P. L.; Bastiaans, G. J. Anal. ployed as a research chemist at the Naval Chem. 1980, 52, 1929-31. (50) Martin, S. J.; Granstaff, V. E.; Frye, Research Laboratory. Grate received his G. C. Anal. Chem. 1991, 63, 2272-81. Ph.D. in chemistry from the University of (51) Shumacher, R. Angew. Chem. Int. Ed. California, San Diego. His research inter Engl. 1990, 29, 329-438. ests include organic chemistry and surface (52) Thompson, M.; Kipling, A. L.; Dunscience, emphasizing polymers, organic can-Hewitt, W. C.; Rajakovic, L. V.; Cavic-Vlasak, B. A. Analyst (London) thin films, molecular interactions, and 1991, 116, 881-90. sorption equilibria as they apply to the de (53) Martin, Β. Α.; Wenzel, S. W.; White, velopment of chemical sensors and miR. M. Sens. Actuators 1990, A21-A23, croanalytical systems. 704-08. (54) White, R. M.; Wenzel, S. W. Appl. Phys. Lett. 1988, 52, 1653-55. (55) Ricco, A. J.; Martin, S. J. Appl. Phys. Lett. 1987, 56», 1474-76. (56) Costello, B. J.; Wenzel, S. W.; White, R. M. Proceedings Transducers '93; Insti tute of Electrical Engineers of Japan: Tokyo, 1987; pp. 704-07. (57) Yang, M.; Thompson, M.; DuncanHewitt, W. C. Langmuir 1993, 9, 80211. (58) Duncan-Hewitt, W. C ; Thompson, M.Anal. Chem. 1992, 64, 94-105. (59) Rajakovic, L. V.; Cavic-Vlasak, Β. Α.; Ghaemmaghami, V.; Kallury, K.M.R.; Kipling, A. L.; Thompson, M. Stephen J. Martin (left) is a senior mem Anal. Chem. 1991, 63, 615-21. ber of the technical staff of the Microsensor (60) Kipling, A. L.; Thompson, M. Anal. Research and Development Department Chem. 1990, 62, 1514-19. (61) Martin, S. J.; Frye, B. C ; Ricco, at Sandia National Laboratories. He re A. J.; Senturia, S. D. Anal. Chem., 1993, ceived both an M.S. degree and a Ph.D. in 65, 2910-22. electrical engineering from Purdue Uni (62) Deakin, M. R.; Buttry, D. A. Anal. versity. His work at Sandia involves the Chem. 1989, 61, 1147 A-1154 A. design, testing, and characterization of (63) Buttry, D. Α.; Ward, M. D. Chem. Rev. 1992, 92, 1355-79. acoustic wave-based sensors including (64) Ward, M. D.; Buttry, D. A. Science SAW, APM, and TSM devices. Applica 1990, 249, 1000-07; Science 1991, 251, tions for these devices include gas- and liq 1372.) uid-phase chemical detection, corrosion (65) Lasky, S. J.; Buttry, D. A. In Chemi cal Sensors and Microinstrumentation, ACS monitors, liquid viscosity and density Symposium Series 403; American measurements, fuel and lubricant degra Chemical Society: Washington, DC, dation measurements, and polymer char 1989; pp. 237-46. acterization. (66) Borman, S. Anal. Chem. 1987, 59, 1161 A-1164 A. (67) Andle, J. C; Josse, F.; Vetelino, J. F.; Richard M. White is a professor of electri McAllister, D. J. Proc. IEEE Ultrason. Symp. 1992, 287-92. cal engineering and computer sciences (68) Baer, R. L.; Flory, C. Α.; Tom-Moy, and a director at the Berkeley Sensor and M.; Solomon, D. S. Proc. IEEE Ultrason. Actuator Center at the University of Cali Symp. 1992, 293-98. fornia, Berkeley. He received his Ph.D. in (69) Gizeli, E.; Goddard, N. J.; Lowe, applied physics from Harvard University C. R.; Stevenson, A. C. Sens. Actuators Β 1992, 6, 131-37. and worked in microwave electronics be (70) Kovacs, G.; Venema, A. Appl. Phys. fore assuming his faculty post. His re Lett. 1992, 61, 639-41. search interests, centered around ultra (71) Thompson, M.; Arthur, C. L.; Dhalisonics, include thermoelastic effects, SAW wal, G. K. Anal. Chem. 1986, 58, 120609. devices, and silicon-based sensors.


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