Jay W. Grate Molecular Science Research Center Pacific Northwest Laboratory Battelle Boulevard Richland, WA 99352
coustic wave microsensors are a satile class of sensor with y applications. Although origially regarded as mass detectors, hese sensors can also measure a
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
ion via mass sensitivity. focus on sensing i
or behavior in the liquid ph e will begin Part II with a di , -
Acoustic devices are usually converted to chemical sensors by the application of a chemically selective layer (1). Polymer materials have been used in organic vapor sensing applications because vapor sorption in rubbery polymers is rapid and reversible; polymers form adherent 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 OI viscoelastic thin films. This type of sensitivity was not noted in early models for TSM devices but was noted in early models for SAW sensors based on perturbation analysis (2, 3). These models predicted that t h e resonant f r e quency of a SAW device would be altered by both the mass and the shear modulus of a thin, nonconducting, isotropic film applied to its surface. Early studies confirmed that SAW devices were capable of detecting various polymer transitions (4). I n recent years, a more complete picture of the effects of viscoelastic thin f i l m s o n a c o u s t i c devices has emerged: Wave velocity and/or amplitude changes occur in response to polymer thermal expansion, polymer relaxation processes, and film resonance effects. Viscoelastic films have both elastic a n d viscous p r o p e r t i e s a n d c a n therefore store and dissipate mechanical energy. When subjected to shear forces, these properties a r e measured by the complex shear modulus G, which has a real term G’, that represents the storage (or elastic) modulus, and an imaginary term
ANALYTICAL CHEMISTRY, VOL. 65,NO. 22, NOVEMBER 15,1993
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REPORT G that represents the loss modulus. In simple terms, modulus refers to the stiffness of the material. In the context of ultrasonic devices, t h e measurement of modulus is highly frequency dependent ( 5 ) . To t h e probing high-frequency waves, films of rubbery polymers on ultrasonic devices appear to have moduli that are characteristic of a glassy material. (This result is a consequence of the relaxation effects described below.) However, the state of the material is unchanged by such probing. If these films were probed simultaneously by a low-frequency method, the measured modulus would be that of a rubber, as expected. For reference, the typical modulus of a rubbery polymer is lo6 N/m2, whereas that of a glassy polymer or a rubbery polymer at high frequency is usually about lo9 N/m2. At room temperature and frequencies above 1 MHz, nearly all rubbery polymers have measured moduli typical of polymer glasses. Polymer moduli decrease with increasing temperature as the polymer expands. These effects a r e well known in conventional bulk-wave ultrasonics, where the sonic velocity through a polymer sample decreases with increasing temperature (6, 7). For reference, shear sound speed V, is directly proportional to the square root of the shear storage modulus G' and inversely proportional to t h e square root of the density p, leading to the expression
Vs =- (G'/p)'' Because both modulus and density decrease with temperature, the ob served decreases in sonic velocity indicate that the modulus is the domin a n t factor influencing t h e sonic velocity. However, density and volume do influence sonic velocity indirectly, because t h e modulus is strongly volume - dependent. Increas ing volume decreases t h e chainchain interactions, which decreases the modulus. This volume effect (via its influence on modulus) is so important that polymer volume can be considered a fundamental influence on acoustic velocities (6). The same principles apply to polymer thin films on planar acoustic devices. When coated to film thicknesses typically used on vapor sensors, polymer thermal expansion causes frequency decreases of 5001000 Hz/"C on SAW and FPW devices (8). (Recall t h a t decreasing oscillator frequencies reflect de creasing acoustic velocities.) This effect is quite large, and it occurs with 988 A
no mass per unit area change: The acoustic devices sense the decrease in modulus as the polymer volume increases. One practical consequence of this observation is that efforts to reduce the temperature drifts of ultrasonic devices themselves by device engineering or compensation schemes may be of limited value if the devices are to be used as chemical sensors with an applied layer whose observable physical properties vary with temperature. For example, dual delay-line SAW vapor sensors with a
Figure 1. Thermal expansion effects and polymer transition processes observed in a thin film of poly(viny1 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-temperature 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 T,.
Figure 2. Film resonance effects observed on a quartz TSM device coated with a 15.6-pm layer of poly(isobuty1ene). 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.)
ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15,1993
polymer layer on one delay line have been investigated (9, IO),but the uncoated reference delay line cannot compensate for the temperature drift of the polymer-coated delay line because t h e l a t t e r is largely due t o 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 sensing delay line, but sealed from vapor exposure. Alternatively, it may be better simply to control sensor temperatures in many applications. Recent studies have shown that polymer-coated FPW devices can sense apparent changes in polymer properties occurring a t both t h e static glass transition temperature Tgand at the so-called dynamic glass transition temperature T, ( 1 1 ) . Results for a thin film of poly(viny1 acetate) on a 5-MHz FPW device a r e shown in Figure 1.At Tg, a change in the slope of the frequency-temperature profile is observed. This change is a manifestation of the sensitivity to the polymer thermal expansion rate, which increases abruptly at Tg. Relaxation processes observed at T, involve a minimum in signal amplitude and a sigmoidal decrease in the frequency- temperature profile. When mechanically perturbed by the probing acoustic waves, polymer chain segments relax back to their former conditions at a rate that is dependent on temperature and the structure of the particular polymer. At a temperature where the characteristic relaxation t i m e is much longer than the period of the probing ultrasonic wave, the measured modulus has a value that is typical of a polymer glass. At a higher temperature where the characteristic relaxation time is much shorter than the period of the probing acoustic wave, the measured modulus is that 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 the probing waves, two effects are observed. Device frequencies de crease because the measured modulus undergoes a transition from values typical of a glass to values typical of a rubber (the dynamic glass transition). The amplitude is attenuated because energy can be efficiently coupled into the polymer film and dissipated when the relaxation time and wave period are similar. In the FPW device study, the results correlated well with the properties of the test polymers as they had been determined by standard polymer characterization techniques such as dila-
tometry, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and conventional bulk- wave ultrasonics. The quartz TSM device is also sensitive to intrinsic polymer properties. Using a network analyzer to measure admittance as a function of frequency and a n equivalent circuit model to evaluate the data, it is possible to extract the polymer elastic shear modulus G' and loss modulus G (12,13).After plotting these values as a function of temperature, the characteristics of a polymer relaxation process are observed for a poly(isobutylene) film. These TSM experiments and models also demonstrated an additional viscoelastic effect, referred to as film resonance, t h a t can occur in relatively thick films. The film resonance effect depends on device frequency, film thickness, polymer shear modulus, and polymer density. The effect is observed experimentally by ramp ing the temperature of a polymercoated device, which causes large modulus changes in the polymer. At lower temperatures where the polymer modulus is high and the film is rigid, the entire thickness of the film moves synchronously with the device surface. As t h e t e m p e r a t u r e i n creases and the polymer modulus decreases, the motion a t the film's upper surface begins to lag behind the motion at the polymer-device interface (where the polymer adheres and is forced to move synchronously with the device surface). This nonsynchronous motion induces strain in the film. Continuing decreases in modulus with increasing temperature increase the phase lag and the strain. When the phase lag reaches d2 (i.e., the film's upper surface lags behind the motion at the polymer-device interface by go"), a condition of film resonance is reached. The increases in phase lag during t h i s process a r e accompanied by changes in the particle displacement across the thickness of the film. With lossy films, t h e displacement decreases a s t h e distance 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 distance from t h e surface is i n creased. The effects of film resonance on sensor responses are shown in Figure 2 (12).At temperatures below t h a t of film resonance, the device frequency decreases with increasing
temperature as the polymer expands and the modulus decreases. As film resonance is approached, t h e frequency drops more steeply. But at film resonance, the frequency suddenly increases to values exceeding the initial frequency. In addition, the device is highly damped at film resonance. For a given polymer, the temperature at which film resonance occurs decreases with increasing device frequency or with increasing film thickness. Although these film resonance effects are simplest to understand on a TSM device where the surface motion is entirely in-plane, they also occur on SAW devices if relatively thick films (exceeding 200 nm) are used (13).
f h e sensitivities of these ultrasonic devices to viscoelastic proper ties can lead to applications in a number of areas. They can be used to monitor intrinsic polymer t r a n s i tions, such as the static glass transition and relaxation processes, and phenomena such as polymer curing or paint drying (4,11, 12, 14, 15). Properties such as shear modulus a t the device frequency can also be determined. Solid-liquid- phase transitions can be detected, as has been demonstrated on the SH-APM device (16).Phase transitions in thin liquid crystalline layers, LangmuirBlodgett films, a n d multibilayer films have been examined with the quartz TSM device (17,18).Ultrasonic microdevices offer the advantages of very small sample size, trivi a l s a m p l e p r e p a r a t i o n , an-d instrumentation that is well suited t o automation and interfacing with digital electronics. On devices such as the TSM, where harmonic m o b s are available, one can make measurements a t various frequencies on a single device and polymer film. In addition, ultrasonic devices allow di rect measurements a t high frequencies, rather than relying on extrapo-' lations from low frequencies as is commonly done in DMA.
Understanding viscoelastic film properties is also essential in the intelligent selection of polymer materials for use on vapor sensors. When fast responses are desired, a polymer should be chosen whose Tgis below the sensor's operating temperature. Greater free volume and polymer chain segmental motion above Tgresult in faster vapor difhsion. To take advantage of modulus effects to maximize sensor sensitivities (see the next section), it is preferable that the polymer's T, be above the sensor's operating temperature. Below T, the initial modulus of the material as it is perceived by the high-frequency acoustic waves is high. Finally, an understanding of film resonance effects is needed in order to comprehend the effects of film thickness on sensor response behavior. Polymer-coated vapor sensors Vapor sensors based on acoustic microdevices invariably use some type of chemical layer to collect and concentrate vapor molecules from the gas phase to the surface of the device. This method was first demonstrated by King, who used quartz TSM devices (191, and was extended t o SAW devices by Wohltjen and Dessy (4).The same approach to vapor sensing has been demonstrated with FPW devices (20).We will focus primarily on quartz SAW sensors that use polymer thin films to absorb vapor reversibly. The polymer is assumed to be a soft rubber to promote rapid vapor diffusion, and it is further assumed that the polymer film is quite thin so that film resonance effects do not occur. The factors that influence sensitivity include t h e s t r e n g t h w i t h which the polymer sorbs the vapor, the thickness of the polymer film, and the inherent sensitivities of the device to thin-film physical properties that are altered by vapor absorption. The strength with which a vapor is sorbed depends only on the interactions between the vapor and the polymer; it is independent of the ultrasonic device. Solubility interactions' such as dispersion forces, dipole-dipole interactions, and hydrogen bonding as they apply to sensor coating materials have recently been reviewed (21). The equilibrium distribution of vapor between the gas phase and the sorbent phase is characterized by the partition coefficient K = C, / C,, where C, is the concentration of the vapor in the sorbent phase and Cvis the vapor concentration in the gas phase.
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REPORT Absorption is illustrated in Figure 3. The higher the K value, the greater the amount of vapor that will be collected at the sensor’s surface for a given vapor - phase concentra tion, and hence the more sensitive the sensor. The fact that polymercoated SAW sensors respond primarily to the effects of absorbed vapor in the bulk, as opposed to surface adsorbed vapor, has been convincingly demonstrated by comparisons of the responses of sensors of various frequencies (8, 22). Increasing coating thickness will also increase the amount of vapor collected at the surface, and hence increase sensitivity. However, the attenuation of surface wave energy by rubbery polymer materials places practical limitations on coating thicknesses, especially when oscillator circuits are used. At some limiting thickness, the insertion losses exceed the gain in a n oscillator circuit 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 the polymer. Because attenuation increases as the ultrasonic frequency increases, film thicknesses must necessarily decrease with increasing device frequency. These decreases i n film thickness offset increasing mass sensitivity with frequency in vaporsensing applications using SAW devices (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, what is the mechanism of the frequency decrease when vapor is absorbed by the polymer? These processes and the factors influencing them a r e summarized in Figure 4
cy), and it should be negligible if the measured modulus is lo6 N/m2 (8, 22).
Because the frequency shift is related 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 polymer of density 1g/mL is 50 nm thick if evenly distributed. When coating thicknesses a r e kept constant i n terms of kilohertz as device frequencies increase, then the absolute coating thicknesses, in terms of nanomet e r s , decrease with t h e square of frequency. In this case, vapor sensitivities a r e independent of device frequency: The increase in the device sensitivity to m a s s a n d modulus changes with the square of device frequency is offset by the decrease in coating thickness, which decreases the amount 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
~~~~~~~~~~
M&?@ .
Figure 3. Reversible vapor absorption and the partition coefficient K = CJC,.
(23).
When the polymer film is applied to t h e bare SAW device, t h e f r e quency decrease observed is prima rily due to the mass of the coating. This result has been experimentally demonstrated using polymeric Langmuir-Blodgett layers of known mass per unit area (22). In theory, the frequency shift occurring when 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 modulus at the SAW frequency is lo9 N/m2 (typical of a glassy polymer or a rubbery polymer at high frequen990 A
Figure 4. Mass and modulus effects in the coating and use of a polymercoated SAW vapor sensor. (Adapted with permission from Reference 23.)
ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15,1993
and sensor frequencies normally de crease. These responses are in the correct direction for a mass loading response, and undoubtedly vapors do increase t h e mass of t h e polymer film. Therefore, it has long been assumed that such vapor sensors are measuring only the mass of the vapor sorbed. The extent to which vapor-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, a r e unknown. However, the mass sorbed is not independently known either. To evaluate mass loading effects experimentally, an equation was derived relating sensor responses to partition coefficients where 4v(mass), 4 s 9 C”, K,and p are, respectively, t h e frequency shift caused by the mass of the vapor, the coating thickness in kilohertz, the vapor concentration in the gas phase, t h e partition coefficient, a n d t h e polymer material density (8, IO). This equation is valid for all t h e mass - sensitive devices previously described. The product C a gives C,, the concentration of the vapor in the polymer, and hence, the mass of the vapor. Independent determinations of the partition coefficients of many vapor/polymer p a i r s a t 25 “C by gas-liquid chromatography t h e n 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 compared with t h e responses expected from the sorbed vapor’s mass. These results are shown for one polymer in Figure 5. Actual sensor responses were four to six t i m e s greater t h a n t h e calculated mass loading responses. Therefore, cont r a r y t o p a s t assumptions, m a s s loading cannot be the primary mechanism by which these sensors r e spond to vapors. Sensitivity to modulus changes occurring when t h e polymer film is perturbed by t h e sorbed vapor m u s t play a much larger role in sensor response than we previously suspected. The sizes of the observed responses were within the correct range for the effect of softening a polymer whose initial measured modulus is - lo9 N/m2, as would be expected for a rubbery polymer at the high SAW frequency. However, because the ac-
Figure 5. Comparisons of measured SAW vapor sensor responses with those predicted on the basis of mass loading. Responses of a poly(isobuty1ene)-coatedsensor to several organic vapors. NME: nitromethane, BTL: 1-butanol, BTN: 2-butanone, DCE: 1,e-dichloroethane,TOLN: toluene, ISOC: isooctane.
t u a l modulus changes were not known, the observed responses could not be compared directly with calculations based on the theoretical modulus sensitivity of the SAW device. An alternative approach, based on the interrelationships between polymer volume, modulus, acoustic velocities, and oscillator frequencies, allowed an independent estimation of sensor responses attributable to modulus decreases (8).The effect of volume on sensor frequencies was calibrated from polymer thermal expansion experiments on SAW devices; these involve no mass per unit area change. From known thermal expansion rates (typically 0.05 to 0.06%/ “C) and the measured effects of polymer thin-film expansion on SAW sensor frequencies (typically -500 to - 1000 HzPC for films 250 kHz thick), the volume effect is estimated to be 10,000-20,000 Hz per percent of volume increase. Volume increases caused by vapor absorption (i.e., swelling) were estimated from test vapor concentrations, partition coefficients, and liquid densities of the vapors. Swelling of 0.3-3% was typical for t e s t vapors producing r e sponses of 2000-20,000 Hz. Assuming that swelling and thermal expansion have similar effects on modulus and hence sensor frequencies, the estimated swelling can be multiplied by the volume effect calibration to calculate the frequency decreases that occur in response to swelling-induced modulus changes. These responses are four to six times greater than the mass loading responses calculated by Equation 1 in Part I, i n good a g r e e m e n t w i t h
t h e experimental r e s u l t s above. Therefore, polymer-coated SAW vapor sensor responses can be modeled as a sum of the effects of mass loading and swelling-induced modulus changes on sensor frequencies, where the latter predominates. Because vapor sorption and the resulting volume increase also reduce polymer relaxation times, it is possible that relaxation effects described in the previous section may influence vapor sensor responses. The observation of the relaxation process involves changes in the perceived modulus, to which the device is sensitive. The reduction in the sensed modulus would lead to frequency decreases that would enhance the normal sensor response. Vapor sorption also has the potential to induce film resonance effects in thick films. This can cause anomalous frequency increases occurring in the opposite direction from typical responses based on mass loading and modulus decreases 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 understood to involve film resonance (13). The combination of acoustic velocity and attenuation information in such cases can be useful in helping to distinguish different vapors (15).
Until recently, gas - phase sensing applications have dominated t h e field of acoustic wave chemical sensors. A variety of selective materials have been investigated for the detection of various analytes (21,24-29). Because reversible absorption is not 100% selective, the use of sensor arrays with pattern recognition has been investigated as a means of improving selectivity in the identification of toxic vapors and in multicomponent analysis (9,30-34). Both selectivity and sensitivity can be enhanced through the use of preconcentrators (33,35).Thus the
development of individual sensors leads t o the development of sensor systems in which the sensors are used as the detection elements. In addition to the sensor or sensor array, a complete sensor system includes a sampling system, signal measurement electronics, and preprogrammed signal analysis and de cision-making algorithms (33).Sensor a r r a y systems using p a t t e r n recognition are often described by phrases such as “smart sensor systems” or “electronic noses” (36,37). Acoustoelectric and dielectric effects When an acoustic wave propagates in a piezoelectric material, it generates a layer of bound charge at the surface that accompanies the mechanical wave. This bound charge generates a n evanescent electric field that extends into a n adjacent medium in contact with the surface, causing motion of charge carriers and dipoles in that medium. The energy stored and dissipated in moving these charges a n d dipoles i s extracted from the wave and influences the wave velocity and attenuation. This acoustoelectric interaction is observed between SAWS and conductive thin-film overlayers, provided t h a t t h e sheet conductivity of t h e film is within a certain critical range and the SAW device piezoelectric material h a s a sufficient electromechanical coupling constant (3840). In this regard, lithium niobate SAW devices are more sensitive to acoustoelectric 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, as i n typical polymer layers, o r highly conductive, as in continuous metal films. Acoustoelectric effects are most pronounced when the film sheet conductivity is 6,= Vo (E, + eo), where Vo is the SAW velocity and E, and eo are permittivities for the substrate and air, respectively. Weakly semiconducting lead phthalocyanine films give rise to acoustoelectric effects on lithium niobate SAW devices, and this process forms the basis for a gas sensor (40). Chemisorption of gases such a s NO, that alter the sheet conductivity of the phthalocyanine film result in wave velocity changes caused by the acoustoelectric effect that are significantly greater than those caused by mass loading alone. The acoustoelectric response can be eliminated by placing a conducting metal film between the sensing phthalocyanine
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REPORT layer and the SAW substrate to short out the surface charge carriers. Unlike mass sensitivity, the magnitude of the wave velocity change (A0 / Vo) caused by the acoustoelectric inter action does not depend on device frequency. Other SAW sensors with semiconducting thin films include a sensor for hydrogen sulfide gas involving tungsten trioxide on a lithium niobate device (41)and various metallophthalocyanines on quartz SAW devices (42). The importance of the sheet conductivity in determining the occur rence of the acoustoelectric effect can be seen in Figure 6 (43),which shows t h e effect of vacuum-evaporated nickel on SAW velocity and attenuation, using a quartz SAW device. (See also Reference 39.)The initial decrease in wave velocity with increasing nickel deposition is due to t h e mass loading of t h e surface. However, the change in wave velocity with film thickness becomes much steeper at around 20-30A of metal film, and the wave is highly attenuated. At this thickness, the sheet conductivity of the film is in the range where acoustoelectric effects are significant. As the film becomes thicker, the sheet conductivity becomes much greater than the critical sheet conductivity for the acoustoelectric interaction. The wave attenuation diminishes, and the rate of wave velocity change w i t h film thickness again reflects primarily mass loading effects.
Interactions with free dipoles at the SAW surface can also influence wave velocity and attenuation. The evanescent electric field generated at the surface of t h e crystal extends into the region above the device, decaying as exp(-Ry) where R is t h e wavenumber ( 2 d k ) and y is the distance from the surface. If polar species 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 electrical 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 the 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 that relies on this interaction without using a sorptive coating (44). Water molecule concentrations near the surface are quite large at high humidity, and the large dipole moment relative to the molecular mass is ideal for detection by this mechanism. Recently, Stone and Thompson found evidence t h a t capacitance changes in the IDTs of a SAW device may influence sensor responses (45). Sorption of polar molecules such as water or acetone between the IDT
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.)
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fingers increases the dielectric constant of the space between the fingers and increases the IDT capacitance. When t h e SAW device is operated i n a n oscillator circuit, these vapor -induced IDT capacitance changes can lead to positive or negative frequency shifts, depending on the particular design of the oscillator circuit. This response, caused by perturbations of IDT capacitance, is distinct from the acoustoelectric interactions between a propagating wave and dipoles or charge carriers. Baseline noise and signal amplitude can also depend on IDT capacitance. Acoustoelectric effects have also been observed in liquid - phase studies using SH-APM devices (46, 47). The evanescent rf electrical field at the surface couples to ions in the solution, resulting in ionic motion that stores and dissipates electrical energy. Changes in the stored electrical energy alter the wave velocity, and dissipation leads to wave attenuation. Significant changes in wave velocity with increases in ion concentrations were observed experimen tally, but the attenuation was small. The rate at which wave velocity is influenced by ionic conductivity depends on the dielectric constant of the solvent. A number of approaches to eliminating signals caused by variations in solution ionic conductivity have been suggested for applications where this device sensitivity would lead to interferences (46). Acoustoelectric effects have been reported 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 resonant frequency of t h e TSM/ oscillator circuit. These reports have been controversial, however, because of the presence of a metal electrode between the piezoelectric substrate and the solution. If common practice is observed, in which the immersed electrode is larger than the one not immersed (to prevent fringing fields from entering the solution) and is grounded with respect to the solution, then electrochemical and acoustoelectric interactions cannot occur. On the other hand, if the immersed electrode is not grounded with respect to the solution, then a parallel conduction path is created: The oscillator 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 totally immersed in aqueous solutions but have only a single electrode immersed. Thus, like t h e change 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 was expected that contact with liquids would excessively damp the acoustic waves. However, Konash and Bastiaans reported in 1980 that quartz TSM devices could be used in t h e liquid phase as detectors for HPLC (49). In fact, most of the devices in Table I in Part I can be used for liquid-phase detection applications. However, liquid-phase operation is precluded if wave motion produces surface - nor mal particle displacements and the wave propagation velocity is greater than the velocity of sound in the liquid, 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 crests is coherent (i.e., adds in phase), causing energy to efficiently “leak away” from the wave and resulting in prohibitive attenuation. If the wave propagation velocity is less than the velocity of sound in the liquid, no direction for coherent radiation exists and less wave attenuation occurs. For reference, t h e compressional velocity of sound in water is 1500 m/s. When the surface of a shear-mode device is placed in contact with a liquid, the wave velocity decreases as if the surface were mass-loaded by a thin layer of the liquid. Figure 7 illustrates the in-plane motion at the surface of a TSM device in contact with a liquid (50).Viscous coupling between the surface and the liquid results in a damped shear wave that propagates into the liquid with a characteristic decay length. The thickness of the liquid layer probed by a 5-MHz quartz TSM is - 0.25 pm in water. The entrained liquid layer decreases the resonant frequency (relative to operation in a gas or vacuum phase) and damps the TSM resonance. Damping by viscous coupling is not as great as damping by radiation of compressional 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 viscosity of the liquid (51, 52). The sensitivity to these properties has important implications for analyte detection in liquids: Experimental conditions must be carefully controlled to avoid drift and spurious re-
sponses arising from variations in these properties during sensor experiments. For example, isothermal conditions must be rigorously maintained because viscosity is exponentially temperature - dependent. By using a network analyzer to operate TSM devices (with smooth surfaces) in the liquid phase, it is possible to distinguish responses t h a t a r e caused by surface mass changes from responses that are caused by changing liquid properties (50). The SH-APM and FPW devices also exhibit responses related to liquid density and viscosity when oper ated in contact with a liquid (46, 53-
56). In these two-port devices the 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. The sensitivities of these devices to liquid - phase proper ties has resulted in the development of SH-APMand FPW viscosity sensors. These sensors permit in situ monitoring and measurement of vis cosity on extremely small samples of 10 pL 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, where the wave period a p proaches the liquid relaxation time, the liquid behaves less as a n ideal Newtonian liquid and 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 relationship between liquid relaxation time and acoustic wave period places an upper limit on the viscosities that can be measured. The lower frequency 5-MHz FPW device can measure higher viscosities than the higher frequency 158-MHz SH-APM device (53, 55). I t has sometimes been observed that TSM devices placed in contact
with a liquid have frequency shifts greater than 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 ( 5 1 ) and reorganization of the solvent a t the interface such that its interfacial viscosity and density are greater than those of the bulk liquid (52, 57, 58). Surface roughness can contribute to the sensor’s response because liquid can be entrained in the crevices of rough TSM surfaces. This leads to an additional frequency shift on liquid loading that is proportional t o the product of the density and the effective thickness of the trapped liquid. Damping is also increased by surface roughness. The roughness of commercially 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 attempting to interpret sensor results ( 5 0 ) .In general, surface features 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 a t the liquidsolid interface at the TSM surface may influence TSM frequency shifts on liquid loading has been investigated by Thompson and co-workers (52, 57-60). It is customary to assume a nonslip boundary condition when deriving models for acoustic sensors in liquids, meaning that the liquid molecules immediately adjacent t o the surface move synchronously with the surface. Wetting at this interface is essential t o establishing the no-slip condition, and in one case Ricco and Martin observed
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.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993
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memicai sensors (gas anc liquid ph
rlymer transition: Vapor sensi
GC
>hemica1 Polymer cunng Viscosity at sensors (liquid phase) high frequenc a TSM
viscoelastic effects may be especially important in solution electrochemiral cti lriipc L solvent and charges move in and out of polymer films with redox processes Acoustoelectric effects are prevented because, in the usual device structure, neIai grour plane separates the piezoelectric layer from the sensing surface. Sensitivities to density and viscosity are important in controlling drift and spurious responses rhemical-sensing applications. 4W devices are not used in the liquid phase.
that a SH-APM viscosity sensor did not respond to liquid loading until a surfactant was added to the liquid (55). (Incidentally, FPW viscosity sensors appear to be relatively immune t o the possible effects of slip because their coupling to the liquid is caused primarily by the component of motion that is perpendicular to the surface [731). Thompson and co-workers have observed that frequency shifts on liquid loading can vary with the surface free energy as measured by the contact angle, with larger frequency shifts observed for hydrophilic surfaces than hydrophobic surfaces (especially when the liquid is water) (57, 59). It is possible that surface free energy influences sensor r e sponse to liquid loading by influencing how liquid is entrained by rough surfaces (57, 61). Water completely penetrates the crevices of hydrophilic surfaces, resulting in maximum liquid trapping, but incompletely pene trates the 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 agreement between frequency shifts predicted by models based only on the bulk liquid density and viscosity, regardless of 994 A
the surface free energy (61). On the other hand, deviations were observed with rough surfaces, and the amount of deviation was greater for hydrophilic surfaces than for hydrophobic surfaces. Analyte detection in t h e liquid phase using acoustic devices is a rapidly expanding field, and t h e transduction mechanisms involved are still being elucidated and debated. Interest in this field stems from two principal areas: First, a large body of literature describes the use of quartz TSM devices in liquidphase electrochemical studies, where t h e sensitivity to surface mass changes may complement the electrochemical 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 considerable potential to detect processes such as the binding of an antigen to a surface -attached antibody. The interpretation of chemical sensor responses i n liquid-phase studies should consider all the factors discussed above, including surface mass loading, changes in liquid density or viscosity, changes in ionic conductivity, and changes in the entrainment of liquid at the surface related to surface roughness and/or
ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15,1993
surface energy. In addition, if a polymer 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 a t the surface of an acoustic sensor in the liquid phase are well understood, or unless sufficient experimental information is available, one cannot simply assume that observed responses are solely due to mass loading. A biosensor for glucose detection, for example, h a s been shown to give responses that are significantly greater than those possible 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 mechanisms discussed in this REPORT are all related to acoustic waves and their interactions with matter at device surfaces. The basic principles extend from one device type t o another 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 phases. 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 detection of vapors in the gas phase with polymer-coated SAW devices, and in liquid-phase sensing where polymers are present on the surface. Molecular and macromolecular re laxation times, and their relationships to acoustic wave periods, are important in the observation of the dynamic glass transition in polymer thin films and in the measurement of liquid viscosities at high frequencies. Acoustoelectric effects are important to gas-phase sensors with weakly electronically conducting thin-film overlayers, gas-phase sensors for dipolar species such as water, and liquid-phase sensors in contact with ionically conducting fluids. Response mechanisms and some of the applications of the various acoustic devices are summarized in Table I. Most reviews of chemical microsensors group acoustic wave sensors under mass sensors or piezoelectric sensors. Neither of these classifications directly addresses the transduction mechanisms involved: Sensing occurs because of the perturbation of acoustic waves a t ultrasonic frequencies. It should be evident that the term “mass sensor” is limiting and, in many cases, inaccu-
A C O M P L E T EFAMILY
rate. Mass detection is not the only, nor always t h e dominant, mechanism by which acoustic wave devices function as sensors, and piezoelectricity is simply a material property that is useful, but not necessary, for generating acoustic waves in small devices. The recognition of additional mechanisms by which a n acoustic wave device can act as a sensor creates many new opportunities for the design of physical and chemical sens o r s a n d provides t h e m e a n s by which sensitivities can be much greater t h a n those t h a t might be achieved by mass detection alone. The acoustic wave sensor field has expanded tremendously i n recent years. At t h e research end of t h e R&D spectrum, there is a new emphasis on t h e details of molecular and physical events at interfaces and in thin films, and development has proceeded to sensor arrays and complete microanalytical systems. Selectivity and limits of detection continue to improve, and great strides have been made in reducing the size and power consumption of sensor sytems. Many exciting applications are being explored, including electronic noses for odor sensing, biosensing, systems for security applications (such as t h e detection of explosives, drugs, a n d chemical agents), and systems for measuring concentrations of specific gases and 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 S t u a r t 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) Grate, J. W.; Martin, S. J.; White, R. M. Anal. Chem. 1993,65,94OA. (2) Wohltjen, H. Sens. Actuators 1984, 5, 307-25. (3) Tiersten, H. F.; Sinha, B. K. J. APPZ. PhyS. 1978,49, 87-95. (4) Wohltjen, H.; Dessy, R. E. Anal. Chem. 1979,51, 1458-70. ( 5 ) Ferry, J. D. Viscoelastic Properties of Polymers, 3rd ed.; John Wiley and Sons: New York, 1980. (6) Hartman, B. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Mark, H. F., Ed.; John Wiley and Sons: New York, 1984; Vol. 1; pp. 131-60. (7 j Massines, R.; Piche, L.; Lacabanne, C . Makromol. Chem., Macromol. Symp.
1989,23, 121-37. (8) G r a t e , J. W.; Klusty, M.; McGill, R. A.; 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, A.; Ballantine, D. S.; Wohltjen, H.; Abraham, M. H.; McGill, R. A.; 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; IEEE: New York, 1992; pp. 27-31. (14) Martin, S. J.; Ricco, A. J. Sens. Actuators 1990, A21423, 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, A21423, 693-99. (17) Muramatsu, H.; Kimura, 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. Ultrasonics, Ferroelectrics, Frequency Control 1987, UFFC-34, 162-71. (21) Grate, J. W.; Abraham, M. H. Sens. Actuators B 1991, 3, 85-111. (22) Grate, J. W.; Klusty, M. Anal. Chem. 1991, 63, 1719-27. (23) Grate, J. W.; McGill, R. A.; Abraham, 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, A.; Witkiewicz, Z. Environ. 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 (London) 1989,114, 997-1004. (29) D’Amico, A.; Verona, E. Sens. Actuators 1989,17,55-66. (30) Carey, W. P.; Kowalski, B. R. Anal. Chem. 1986,58,3077-84. (31) Carey, W. P.; Beebe, K. R.; Kowalski, B. R. Anal. Chem. 1987, 59, 152934. (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.; Jurs, P. C. Anal. Chem. 1988, 60, 2801-11. (35) Kindlund, A.; Sundgren, H.; Lundstrom, I. Sens. Actuators 1984, 6, 1-17. (36) Sensors and Sensory Systems for an Electronic Nose; Gardner, J. W.; Bartlett, P. N., Eds,; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992. (37) Newman, A, R. Anal. Chem. 1991, 63, 585 A-588 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 1991,206,94-101. (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, 156-61. (42) Nieuwenhuizen, M. S.; Nederlof, A. J.; Barendsz, A. W. Anal. Chem. 1988, 60, 230-35. (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. J APPl. Phys. 1988, 64, 5002-08. (48) Josse, F.; Haworth, D. T.; Kelkar, U. R.; Shana, Z. A. Electron. Lett. 1990, 26, 834. (49) Konash, P. L.; Bastiaans, G. J. Anal. Chem. 1980,52,1929-31. (50) Martin, S. J.; Granstaff, V. E.; Frye, G. C. Anal. Chem. 1991,63, 2272-81. (51) Shumacher, R. Angew. Chem. Int. Ed. Engl. 1990,29,329-438. (52) Thompson, M.; Kipling, A. L.; Duncan-Hewitt, W. C.; Rajakovic, L. V.; Cavic-Vlasak, B. A. Analyst (London) 1991,116,881-90. (53) Martin, B. A.; Wenzel, S. W.; White, R. M. Sens. Actuators 1990, A21-A23, 704-08. (54) White, R. M.; Wenzel, S. W. Appl. Phys. Lett. 1988,52, 1653-55. ( 5 5 ) Ricco, A. J.; Martin, S. J. Appl. Phys. Lett. 1987,50, 1474-76. (56) Costello, B. J.; Wenzel, S. W.; White, R. M. Proceedings Transducers '93; Institute 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, B. A.; Ghaemmaghami, V.; Kallury, K.M.R.; Kipling, A. L.; Thompson, M. Anal. Chem. 1991,63,615-21. (60) Kipling, A. L.; Thompson, M. Anal. Chem. 1990,62, 1514-19. (61) Martin, S. J.; Frye, B . C.; Ricco, A. J.; Senturia, S. D. Anal. Chem., 1993, 65, 2910-22. (62) Deakin, M. R.; Buttry, D. A. Anal. Chem. 1989, 61, 1147 A-1154 A. (63) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355-79. (64) Ward, M. D.; Buttry, D. A. Science 1990, 249, 1000-07; Science 1991, 251, 1372.) (65) Lasky, S. J.; Buttry, D. A. In Chemical Sensors and Microinstrumentation, ACS Symposium S e r i e s 403; American Chemical Society: Washington, DC, 1989; pp. 237-46. (66) Borman, S. Anal. Chem. 1987, 59, 1161 A- 1164 A. (67) Andle, J. C.; Josse, F.; Vetelino, J. F.; McAllister, D. J. Proc. IEEE Ultrason. Symp. 1992,287-92. (68) Baer, R. L.; Flory, C. A.; Tom-Moy, M.; Solomon, D. S. Proc. IEEE Ultrason. S w p . 1992,293-98. (69) Gizeli, E.; Goddard, N. J.; Lowe, C. R.; Stevenson, A. C. Sens. Actuators B 1992,6, 131-37. (70) Kovacs, G.; Venema, A. Appl. Phys. Lett. 1992,61,639-41. (71) Thompson, M.; Arthur, C. L.; Dhaliwal, G. K. Anal. Chem. 1986, 58, 120609.
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(72) Thompson, M.; Krull, U. J. Anal. Chem. 1991,63, 393 A-405 A. (73) Wenzel, Stuart W., Berkeley MicroInstruments, 1993, personal communication.
Jay W. Gratejoined the Molecular Science Research Center at Pacific Northwest Laboratory in 1992. Previously, he was employed as a research chemist at the Naval Research Laboratory. Grate received his Ph.D. in chemistry from the University of California, San Diego. His research interests include organic chemistry and suflace science, emphasizing polymers, organic thin films,molecular interactions, and so@tion equilibria as they apply to the development of chemical sensors and microanalytical systems.
Stephen J. Martin (left>is a senior member of the technical staffofthe Microsensor Research and Development Department at Sandia National Laboratories. He received both an M.S. degree and a Ph.D. in electrical engineering from Purdue University. His work at Sandia involves the design, testing, and characterization of acoustic wave-based sensors including SAW, APM, and TSM devices. Applicationsfor these devices include gas- and liquid-phase chemical detection, corrosion monitors, liquid viscosity and density measurements, fuel and lubricant degradation measurements, and Polymer characterization. Richard M. White is a professor of electrical engineering and computer sciences and a director at the Berkeley Sensor and Actuator Center at the University of California, Berkeley. He received his Ph.D. in applied physics from Harvard University and worked in microwave electronics before assuming his faculty post. His research interests, centered around ultrasonics, include thermoelastic effects,SAW devices, and silicon-basedsensors.
