Anal. Chem. 1998, 70, 5190-5197
Influence of Phase Position on the Performance of Chemical Sensors Based on SAW Device Oscillators Ju 1 rgen Reibel, Stefan Stier, Achim Voigt, and Michael Rapp*
Forschungszentrum Karlsruhe GmbH, IFIA, B341, P.O. Box 3640, D-76021 Karlsruhe, Germany
Sensor systems for the chemical analysis of organic gases based on mass-sensitive sensors, i.e., bulk acoustic wave and surface acoustic wave (SAW) sensors, are becoming more and more an interesting alternative for highly sophisticated classic instrumentation. Although the use of oscillator-operated SAW resonators as chemical sensors is widely accepted to have the best sensing properties, some important parameters of influence are not sufficient resolved yet. In general, these are effects arising from the electronic circuitry, such as the phase situation on the chemical response of these sensors. We have found that they can be almost dramatic for several types of sensitive coatings and, thus, are not negligible, especially their influence on the sensitivity. We present some exemplary results with SAW resonators, working at 433.92 MHz, coated with different polymeric films such as poly(isobutylene), poly(epichlorohydrin), or poly(dimethylsiloxane) and sampled with toluene as analyte. Depending on the thickness and homogeneity of the polymer film, serious influences of the set phase positions on the quality of the chemical response were observed, such as curve shape and signal-to-noise ratio. Also, a simulation using an equivalent circuit model of the transducers including the polymer coating is used to obtain a deeper understanding of these phenomena. Mass-sensitive sensors used in the detection of a variety of organic and inorganic gases, based on the technology of surface acoustic wave (SAW)1-5 or bulk acoustic wave (BAW)6,7 devices, are well established. These piezoelectric devices normally are coated with a chemical receptor layer and connected as the frequency-determining element of an oscillator circuit. Applied gas is absorbed and/or adsorbed by the sensitive layer, and the resultant mass increase of the coating reduces the surface or bulk wave velocity of the device. (1) Wohltjen, H. Anal. Chem. 1979, 51, 1458-1475. (2) Wohltjen, H. Sens. Actuators 1984, 5, 305-325. (3) Schickfus, M. von; Rapp, M. Acta Phys. Slovaca 1990, 40, 26-31. (4) Rapp, M.; Binz, D.; Kabbe, I.; Schickfus, M. von; Hunklinger, S.; Fuchs, H.; Schrepp, W.; Fleischmann, B. Sens. Actuators B 1991, 4, 103-108. (5) Rapp, M.; Bo¨ss, B.; Voigt, A.; Gemmeke, H.; Ache, H.-J. Fresenius J. Anal. Chem. 1995, 352, 699-704. (6) Horner, G.; Vonach, B. LaborPraxis 1995, 28. (7) Go ¨pel, W. Nachr. Chem. Technol. Lab. 1995, 43, 318-321.
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For a long time, the lack of sufficient selectivity of the applied sensitive coatings prevented the breakthrough of mass-sensitive chemical sensors in many application areas. However, since several “semiselective” sensors have been combined into arrays and coupled with subsequent pattern recognition methods, even selective analyses have been successful.8-10 Consequently, a marked increase in interest in this sensor technology has been observed in recent years, especially for the detection of organic gases. In correlation with this, a lot of work was done looking for more suitable sensor coatings, focusing on sensitivity and selectivity properties.9 Although a wide range of different kinds of specially designed chemical materials, from fullerenes11 to highly sophisticated cavity compounds,12,13 have been tested, up to now comparably simple structured polymeric substances remain the most favored coatings. Interestingly, it has been proved for some of these substances that their partition coefficients (i.e., their sensitivities) at acoustic wave sensors are much larger (up to four times) than expected by pure mass effects.14,15 Later studies of this behavior found that this effect does not apply for BAWs only but also for SAWs and can be used specifically to increase their sensitivity remarkably.16 Explanations of these observations mostly question the dominance of the mass effect but consider additional changes in other material properties, such as elasticity constants,14 conductivity, and dielectric properties, or take into account swelling effects in the polymer17 upon the absorption of vapor molecules. Unfortunately, only very few investigations are not restricted to intrinsic material properties but also take into account effects of film morphology, such as “dewetting effects”18 or “film resonance effects” as described by the model of “acoustical (8) Mu ¨ ller, R.; Lange, E. Sens. and Actuators 1986, 9, 39-48. (9) McGill, R. A.; Abraham, M. H.; Grate, J. W. Chemtech 1994, 24, 27-37. (10) Rapp, M.; Reibel, J.; Stier, S.; Voigt, A.; Bahlo, J. Proc. IEEE Freq. Control Symp. 1997, 129-132. (11) Abraham, M. H.; Du, Ch. M.; Grate, J. W.; McGill, R. A.; Shuely, W. J. J. Chem. Soc., Chem. Commun. 1993, 1863-1864. (12) Go ¨pel, W. Sens. Actuators B 1995, 24-25, 17-32. (13) Dickert, F. L.; Forth, P.; Tortschanoff, N.; Bulst, W. E.; Fischerauer, G.; Knauer, U. Proc. IEEE Freq. Control Symp. 1997, 120-123. (14) Grate, J. W.; Patrash, S. J.; Abraham, M. H. Anal. Chem. 1995, 67, 21622169. (15) Liron, Z.; Kaushansky, N.; Frishman, G.; Kaplan, D.; Greenblatt, J. Anal. Chem. 1997, 69, 2845-2854. (16) Grate, J. W.; Kaganove, S. N.; Bhetanabotla, V. R. Anal. Chem. 1998, 70, 199-203. (17) Grate, J. W.; Klusty, M.; McGill, R. A.; Abraham, M. H.; Whiting, G.; Andonian-Haftvan, J. Anal. Chem. 1992, 64, 610-624. (18) Grate, J. W.; McGill, R. A. Anal. Chem. 1995, 67, 4015-4019. 10.1021/ac9805504 CCC: $15.00
© 1998 American Chemical Society Published on Web 11/06/1998
thick and acoustical thin films”.19,20 Furthermore, we found only one author who additionally includes influences arising from the electronic circuitry which is used to operate SAW21 or BAW devices.22 Through the past years, we have developed a compact sensor array consisting of eight SAW sensor oscillators and one common SAW reference oscillator for temperature compensation, thus generating eight difference frequency signals as output. The SAW sensors were based on commercially available SAW resonators and were coated by us with different polymers after their metal housing was opened.5 When establishing a general characterization of the sensitivity properties of the individual sensors, we again and again were struck by a number of rare, but extremely strange, features of the sensor signal curves and response levels:23 (1) frequency rise instead of frequency drop upon loading with the substance to be analyzed; (2) reversal of this behavior with a different coating thickness of the same material; (3) different sensitivities of a certain sensor with different oscillator electronics; (4) strange signal curve shapes as a function of coating morphology; and (5) changes in noise level of the sensor signals during measuring and purge cycles. Especially the effect mentioned under (3) above is remarkable: frequently, when the oscillator electronics, i.e., the position within our sensor array, was changed, the same device generated not only slightly different oscillation frequencies but also different sensitivities (variations up to 20%, although the reproducibility of each measurement was definitely better than 3%) and even different signal curve shapes after exposure to a sample. After (all) other sources of error had been excluded, such as different sampling techniques and temperature conditions, respectively, the only remaining explanation for the phenomenon was differences in the oscillator itself. The only viable explanation seems to be minimal variations in impedance of the different components in the electronic circuitry, which ultimately give rise to different signal transit times in the oscillator electronics and, hence, to different phase positions of the SAW device within the oscillator. In practical applications of gas-analyzing systems based on SAW devices, the anomalies described above would have grave consequences: they prevent any reasonable calibration of the sensors, especially in view of a series production. Therefore, in the case of three coating materials used by way of example, we studied the underlying effect in greater detail. THEORY SAW Oscillators. Irrespective of the type of SAW device, the general condition for oscillation in a SAW oscillator with respect to the sum total of phase shifts of its components, including the sensing device itself, is (19) Martin, S. J.; Frye, G. C. Appl. Phys. Lett. 1990, 57, 1867-1869. (20) Martin, S. J.; Frye, G. C.; Senturia, S. D. Anal. Chem. 1994, 66, 22012219. (21) Stone, D. C.; Thompson, M. Anal. Chem. 1993, 65, 352-362. (22) Thompson, M.; Hayward, G. L. Proc. IEEE Freq. Control Symp. 1997, 114119. (23) Rapp, M.; Reibel, J.; Stahl, U.; Stier, S.; Voigt, A. Faraday Discuss. 1997, 107, 363-367; 478.
∆φtot ) ∆φa + or
τtot fo ) (τa +
∑τ )f
e o
∑∆φ
e
) 2πn
) n for n ∈1, 2, 3, ...
(1)
with ∆φtot as the total phase shift of the whole oscillator, its fractions, ∆φa of the acoustic (SAW) device and ∑∆φe as the sum total of electric components, the corresponding signal transit times, τa and Στe, and fo the oscillation frequency resulting from the condition of n ∈1, 2, 3, .... According to this, the mere finding that a different oscillation frequency is established whenever the oscillator electronics is replaced proves that the values ∑∆φe and ∑τe of the oscillator circuits actually vary within a relevant range. Only in this way can different values fo ) n/(τa + ∑τe) be generated. Hence, the conclusion that the phase position in the oscillator influences not only the oscillation frequency, but also the signal curves and signal levels measured whenever a sensor is exposed to a sample, is obvious. Influence of SAW Device Type: Resonators versus Delay Lines. Throughout our studies, SAW devices of the resonator type were used exclusively. Besides their better performance compared to the delay line type, which has already been considered in ref 24, other advantages in using SAW resonators lie in their high quality (expressed by the Q-value), their low attenuation, their small dimensions, and their low price. In principle, the quality of an SAW resonator as well as of a SAW delay line is associated with its virtual (resonators) or real (delay lines) signal transit time, τa, and hence with the phase slope at the point of resonance by way of
Q ) 2πfoτa with τa )
δφa -1 δφa ) δωo 2π δfo
(2)
In the case of resonators, a relatively large virtual signal transit time τa has principally to be taken into account because of multiple reflections which occur between the outer reflecting electrode structures. Therefore, only the virtual signal transit times are the factors decisive for the Q-value of the oscillation. In the uncoated case, resonators achieve high Q levels, as a wave group may be reflected between the reflectors, thus passing through the transit line several times. On the other hand, if the device is coated, the coating produces additional attenuation (from typically -6 up to -20 dB in some cases) and correspondingly reduces the efficiency of the reflectors. Thus, a decrease of Q and, according to (2), also of the slope of the phase curve at the resonating frequency occurs. When analyte is added, the intrinsic attenuation of the polymer changes,20 and consequently a further influence on the reflectivity of the reflectors is to be expected. As a result of the decreased virtual τa in eq 1, this ultimately will change the resonance frequency of the oscillator. All this argument in principle applies also to other SAW types, such as delay lines, which are also used for chemical sensors.1-2,20 On the other hand, and actually in the case of delay lines, the delay transit times can be influenced only by a change in surface wave velocity and not by a change of a boundary condition. (24) Mauder, A. Sens. Actuators B 1995, 26-27, 187-190.
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Figure 1. Equivalent circuit diagrams for simulating the transmission behavior of coated SAW: C, capacitor; L, inductance coil; R, resistor; TR, transformer. (a) Extended van Dyke equivalent circuit diagram. (b) Equivalent circuit diagram for simulating transmission behavior.
Hence, coated delay lines tend to retain their real τa values for the uncoated state much better than SAW resonators retain their virtual τa. This means that, in the case of coated delay lines, higher τa values must be expected in eq 1, as this is the case for resonators. Influences resulting from the oscillator circuit will come into play much less for delay lines, because, in the extreme case, even τa . ∑τe may apply also at the coated state. Consequently, influences arising from oscillator phase adjustments may be restricted mainly to SAW resonators. Van Dyke Model. The electric behavior of uncoated acoustic wave devices can be simulated by a series resonator circuit. According to the model by van Dyke, this circuit can be extended to cover also the behavior of coated BAWs.25 We transferred his model to simulate the behavior of SAW devices in the coating step (see Figure 1a). Corresponding to the van Dyke model, we assign C1, L1, and R1 to the equivalent series-connected resonator circuit of an uncoated SAW; L2, R2, and L3 to the changes in the velocity of sound and in attenuation as a result of coating of the SAW (e.g., with polymers), and Cr and Ct to the input and output capacitances of the interdigital transducers and the package. The transformer TR compensates phase shifts induced by the specific bonding conditions of the devices. EXPERIMENTAL SECTION The simulation calculations of the coated SAWs have been performed with an in-house software development. The parameters of the uncoated SAW device (C1 ) 0.2 fF, L1 ) 0.67 mH, R1 ) 145 Ω, Cr ) Ct ) 1.8 pF) were taken from the component datasheet provided by the manufacturer.26 The values of Cr, Ct, and C1 were kept constant for all simulations. Since the SAWs used are bonded to an input relative to the output phase, a response of 180° at resonance the transformer Tr was considered appropriate. The SAW devices used for these studies were the same as those used for our standard applications with the SAW sensor system: Rayleigh wave type SAW resonators from Siemens, type R2632 (AT-cut quartz, fo ) 433.92 MHz). The devices were coated with poly(isobutylene) (PIB, Aldrich), poly(epichlorohydrin) (PECH, (25) Chagnard, C.; Gilbert, P.; Watkins, A. N.; Beeler, T.; Paul, D. W. Sens. Actuators B, in press. (26) Technical data sheet of the SAW resonator, type R2632, Siemens Matsushita Components.
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Aldrich), and poly(dimethylsiloxane) (PDMS, Macherey und Nagel). The coatings were applied by spray coating (Airbrush Dispenser, Biodot) with a solution of the polymer and tetrahydrofuran (THF) as solvent. The desired coating thickness was determined by the number of spraying steps and checked by the associated frequency shift (coating frequency) in a high-frequency counter (Philips PM 6680 frequency counter). Sensors with a coating frequency below 1 MHz will be referred to below as “thincoated”, while those with a coating frequency of more than 4 MHz will be referred to as “thick-coated”. Studies of phase and frequency transmission response of SAW sensors were made in the frequency range of interest. In this case, each particular device was located in an extra sampling chamber as housing for the sensor, while the latter was directly connected with a network analyzer (Hewlett-Packard 8753C). In this way, changes in the response curves obtained by sampling have been recorded on-line. The frequency shifts of the sensors upon exposure to a gas (measuring signals) were recorded by the SAW sensor system mentioned above. However, for comfortable setting of the phase position of the oscillators, a modification of our oscillator circuits in the sensor array was necessary, adding an electrically controlled phase-shifting element, i.e., a conventional capacitance diode supplied by Siemens. Then, varying of a simple offset voltage allowed the phase position of each sensor to be set without disturbing the oscillator operation itself. The maximum voltage range of the diodes reaches 24 V, thus generating a maximum phase shift of about 60° in the range of the oscillation frequency applied. During the measurements, the phase positions of the sensors were kept constant. Between measurements they were changed by the setup described above in the range between approximately -100 and -160° for each specific oscillator. This phase range thus is of special interest and will be referred to below as the “set phase range”. Analytes used were toluene, octane, methanol, and water. As the results of these studies showed identical trends for all analytes, only the studies with toluene-nitrogen gas mixtures in the range of concentration between 1 and 55 g/m3 will be referred to below. The desired concentrations were generated by varying the mass flows of the carrier gas and saturated toluene vapor (at 20 °C) by means of a mass flow controller (Brooks Instruments MFC 5850E) and monitored by a gas chromatograph (Perkin-Elmer GC autosystem). RESULTS AND DISCUSSION Simulation of Sensor Attenuation and Phase Response. At first, the attenuation and phase curves of the uncoated SAW devices have been simulated according to the data given above. The calculated curves agreed very well with the corresponding measured transmission behavior (compare Figures 2 and 3). Afterward, a simulated coating was taken into account according to the extended van Dyke model described above. As already mentioned there, the influences of the coating are introduced by L2, L3, and R2, with L2 describing the influence of the changing viscosity on the frequency response of the SAW surface, R2 describing the influence on acoustic attenuation, and L3 describing the frequency change caused by a straightforward mass loading of the SAW. To simulate the transmission behavior of a SAW
Figure 2. Simulated SAW transmission behavior at different coating thicknesses: (a) attenuation curves, (b) phase curves. The arrows show the direction of an assumed increase of film thicknesses, respectively.
device during a coating procedure (i.e., with increasing film thickness), the van Dyke model has been simplified by combining the resistors (R1 and R2) and the inductance coils (L1, L2, L3) into R12 and L123 according to Figure 1b. Varying these two values thus simulates the behavior of the SAW under increasing mass loading. Figure 2 shows calculated amplitude and phase responses for various values of R12 and L123. Starting from the curve of the uncoated sensor (R12 ) R1, L123 ) L1), R12 and L123 were increased in steps until the curve corresponded to those of real, thick-coated sensors. It can be seen that increasing attenuation and mass loading causes the phase reserve (total phase change in the transmission range of SAW) to decrease and the minimum of the curve to move toward lower frequencies. At the same time, the resonance peak of the amplification curve is reduced. Sensor Behavior by Applying a Coating. To compare the simulation of the transmission behavior with real transmission characteristics, a SAW device was stepwise coated with PIB by spray coating. Between each coating step, the actual attenuation and phase curves were recorded by the network analyzer. The results of this experiment are shown in Figure 3. The general agreement of the measured and simulated response curves is clearly visible. From the phase and attenuation curves obtained in this way (Figures 2 and 3), the behavior of the differently coated SAWs in an oscillator (i.e., in the sensor system) can be predicted. The broadband amplifier device in our sensor system has an almost
Figure 3. Measured transmission behavior of an RS-2632 SAW resonator at different coating thicknesses during a stepwise coating procedure with PIB: (a) attenuation curves, (b) phase curves. The arrows show the direction of the increased film thicknesses, respectively. Only within the highlighted phase range would an oscillator be able to support oscillation for all these sensors with different film thicknesses.
constant phase shift in the frequency range of interest. The intersection of the phase of the amplifier with the phase curve of the SAW determines the instantaneous oscillator frequency. Figures 2 and 3 show that the usable phase range is more and more diminished when film thickness is increased. To ensure reliable oscillation in all cases, the phase of the amplifier must be shifted into the common part of the phase reserve of all these SAW sensors. This area is marked in Figure 3b, thus representing the suitable phase range of a practical SAW oscillator which should support oscillation for all possible SAW sensors. As the phase reserve tends to decrease further with increasing coating thickness, as outlined above, operating the oscillator in a stable mode becomes more and more difficult. Moreover, because of the increasingly flatter phase curve associated with thicker coatings, the influences of amplifier noise and temperature fluctuations increase. Sensor Behavior by Applying a Gas. The sections above dealt with typical phase curves for sensors with different coatings in the absence of an analyte. Therefore, these curves will be referred to below as the “unloaded curves”. As a consequence of the absorption of gas molecules onto the sensor coatings, these curves change in a characteristic way when exposed to gases, referred to below as the “loaded curves”. The differences between loaded and unloaded curves were studied by means of a network analyzer for a number of SAW sensors. Analytical Chemistry, Vol. 70, No. 24, December 15, 1998
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Figure 4. Phase responses of three typical SAW sensors without (solid lines) and with exposure to gas (dashed lines): (a) PECH, 0.28MHz coating frequency, 44.1 g/m3 of toluene; (b) PECH, 5.67-MHz coating frequency, 26.1 g/m3 of toluene; (c) PIB, 0.59-MHz coating frequency, 51.3 g/m3 of toluene.
Figure 4 shows three examples of corresponding unloaded and loaded curves for sensors bearing different coatings. The typical behavior of a thin-coated sensor is shown in Figure 4a. As a result of the exposure to gas, the phase response shifts leftward, i.e., toward lower frequencies (mass increase), while its minimum is also shifted slightly upward, i.e., toward a slightly higher phase 5194 Analytical Chemistry, Vol. 70, No. 24, December 15, 1998
value due to an increased attenuation. In the phase range between -115 and -220° (which encompasses the set phase range), the loaded curve merely seems to be shifted, relative to the unloaded curve, parallel to lower frequencies. This is an expected behavior and reflects the fact that, within a suitable phase range, the sensitivity of the corresponding sensor oscillator will not be influenced by the set phase position. However, if the coating thickness is increased (Figure 4b), the unloaded phase curve (solid) typically becomes flatter due to an increased attenuation. On sampling, the loaded phase curve (dashed) shows a remarkable upward shift of its minimum toward higher phase values besides the “classic” shift toward lower frequencies (mass increase). For thick-coated sensors, the phase curves within the phase range to be set, therefore, are obviously no longer parallel. The curves represented in Figure 4a,b are typical examples of homogeneously coated sensors and are comparable for all three coating materials. On the other hand, the spray coating parameters can be chosen so as to result in a strongly inhomogeneous mass distribution on the SAW device, e.g., due to remaining beads from droplets of the initial polymer solution. In these cases, the phase responses differ from the typical course, showing very different curve shapes. One example is presented in Figure 4c. Here, even a change in direction of the loaded curve relative to the unloaded curve can occur! In the phase range between approximately -105 and -135°, the phase curve is shifted toward higher frequencies, despite the increase in mass during loading. The curves discussed here can be used to predict sensor behavior within an oscillator circuit, e.g., within the sensor system. As SAW sensors in the oscillator circuits oscillate at a constant phase, the horizontal shift between the unloaded curve and the loaded curve in the network analyzer corresponds to the measurable frequency shift in the sensor system at the appropriate phase position and exposure to a gas. In the case of thin-coated sensors, phase-independent frequency shifting must be expected during exposure to a gas in the phase range in which both curves run parallel. On the other hand, thick-coated sensors are likely to show a strong dependence of sensitivity on the phase position of the oscillator circuit. In an inhomogeneous coating, this may even involve a change in the sign of the measured signals. Consequently, this fact may be the reason for some of the strange behaviors mentioned in the introduction and reflects the importance of considering the set phase position. Studies with the Sensor System. The three sensors discussed above by way of example were used to analyze toluene contents in nitrogen carrier gases using the sensor system (see Figure 5). In each case (each sensor), the applied toluene concentrations were identical with those used in the corresponding network analyzer measurements, and merely the phase positions of the oscillator circuit were varied within the set range (see the Experimental Section). The phase positions used in each specific case are indicated in Figure 5. Since different frequencies are generated within the sensor system, a normal response (decrease of resonant frequency) during sampling leads to an upward shift of the resulting difference frequency change (see Figures 5-8). Irrespective of the phase position set, the thin-coated sensor exhibits nearly identical curves over a phase range of 75° (Figure 5a). In contrast, the frequency shift of the thick-coated sensor
Figure 6. Zoom out of the curve shown in Figure 5c at φ ) -112°. At the beginning of the sampling (first 3 s), the frequency behaves as expected from mass loading (decrease of frequency, which is shown here in positive direction of the y-axis) but then changes its sign and shows a “strange curve shape”.
Figure 7. Sensor calibrations for toluene with the same sensors as in Figures 4a,b and 5a,b, which are linear irrespective of the phase position set. The sensitivity of the thick coated sensor depend on the phase position set.
Figure 5. Frequency shifts of the same sensors as shown in Figure 4a-c (same order) as a function of time at different phase positions. The diagram shows two successive sampling and purge cycles each.
depends very much on the phase position set (Figure 5b). In this case, the maximum measured signal drops to approximately 75% over a range of only 16° shift of phase position! A comparison of Figures 5b and 4b shows that this decrease of sensitivity correlates with a decrease of the phase position set, i.e., with an approximation toward the intersection of both curves in Figure 4b. The behavior of the sensor with an inhomogeneous coating is particularly interesting (see Figure 5c). In the phase range studied, at constant toluene concentration, the sensor exhibits a
variation in the frequency shift between +35 and -60 kHz! As predicted from Figure 4c, there are, in fact, phase-positiondependent positive and negative frequency shifts resulting from the uptake of toluene. Especially obvious are the strange curve shapes arising from this type of coating. To understand this, one should be aware that the phase curves in Figure 4c (observed by the network analyzer) represent only the initial and final states of the adsorption process. In the course of sampling (observed by the sensor system), the unloaded curve grows into the loaded curve, with first a generation of intersections and second a movement of these intersections, thus generating the “strange curve effects” mentioned in the introduction. With the sign of the actual movement of the intersections along the phase curve or/and the direction of denting at a particular phase position, the actual direction of the frequency shift response during sampling is defined. In particular, this becomes dramatic when an intersection crosses the phase position set in the oscillator during sampling or purging. An extreme example of this behavior is shown in Figure 6, a zoom-out of the curve in Figure 5c at the phase position set at φ ) -112.7°. Only in the first 3 s is a normal Analytical Chemistry, Vol. 70, No. 24, December 15, 1998
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resulting calibration with a negative slope is still a straight line (Figure 8)! Here, the phase position seems to have the dominant role to define the sensor calibration from -5.22 kHz g-1 m-3 at -140.3° to +2.00 kHz g-1 m-3 at +184.5°!
Figure 8. Sensor calibration of another sensor coated with poly(dimethylsiloxane) for toluene. Unaffected by the phase position, it again shows a linear behavior. Again, the sensitivity is a function of the phase positions set; for some settings, even negative slopes can be found!
behavior observed. After that, the intersection moves through the phase position set, causing a zero signal response although an analyte is applied. On purging, a mirrored signal course is observed if the first 3 s of the sampling mode is not considered. This means that the movement of the phase curve in both directions (purging and sampling) follows approximately the same development in time. Sensor Calibration. The effects of the electronic circuits on the sensitivities and measurement behavior of the sensors discussed above are very important in terms of sensor calibration of SAW resonators. Especially if large sensitivities are desired and, therefore, thick coated sensors have to be used, a calibration procedure definitely should include a phase adjustment step. The necessity of the latter is demonstrated in Figure 7, showing a calibration of the two PECH sensors already discussed above at a constant phase setting. The obtained calibration data can still be fitted by linear regression lines, with the slopes representing the sensitivity of the respective sensor. Of course, the sensitivity of the thin coated sensor (0.9 kHz g-1 m-3) is much smaller than that of the thick coated sensor due to the smaller amount of polymer. Also, according to Figure 5a, it cannot be increased by making phase adjustments. On the other hand, the thick coated sensorswhich is already much more sensitivescan be additionally optimized by choosing the most suitable phase position. In the example shown a variation in the set phase of only 13° led to a significant change in sensitivity (from 16.8 kHz g-1 m-3 at -110° phase position to 19.2 kHz g-1 m-3 at -97° phase position). In general, it is remarkable that a calibration versus a certain analyte turns out to remain linear, just as it is commonly known, without considering phase position effects.27 Moreover, we have found that this is also valid for all chosen phase positions within the suitable phase range of our oscillators as well as for all other sensors and coating materials used. A further example (Figure 8) with a homogeneous polysiloxane coating and again toluene as analyte shows that even in the case of negative sensor responses, which are due to a corresponding choice of phase position (comparable with those of the sensor in Figure 5c), the (27) Zellers, E. T.; Batterman, S. A.; Han, M.; Patrash, S. J. Anal. Chem. 1995, 67, 1092-1106.
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CONCLUSION We have shown that the sensor calibration of resonator-based SAW sensors strongly depends on the phase position set. Ultimately, this arises from the fact that their phase curves during the sorption process alters their shape. Several effects may be responsible for that: first, a change of the intrinsic attenuation of the sensing layer, and second, a change in the boundary conditions of the acoustic resonator, meaning a change in the reflectivity of the acoustic reflectors of a SAW resonator, which diminishes the phase slope. Unfortunately, this also means that thick coatingsswhich especially guarantee high sensitivitiesscause weaker phase slopes, making SAW resonators more susceptible to phase position changes than SAW delay lines. For the latter, the phase slope is more or less constant over a large range of phase angles. Thus, a low susceptibility for phase position effects can be assumed for any type of sensitive film (compare with the Influence of SAW Device Type section). On the other hand, all these consideration should generally be valid as well for QMBs, another type of acoustic resonator, which is also often used as a polymer-coated, oscillator-operated chemical gas sensor.6,7,12,13,16 However, comparable results with such devices have not been published to the same extent, yet. Nevertheless, we have found that SAW resonators are still better suited for our gas sensor systems compared to SAW delay lines or QMBs if one is aware of the following summarized phase position effects: (1) irrespective of the coating used and of the phase position set, all sensor calibrations remain linear over a wide concentration range; (2) the sensitivity of the sensor response and signal curve shapes can be dramatically affected by the phase position set within the SAW oscillator; and (3) the strength of these effects is a function of film thickness and homogeneity. Hence, all absolute sensor calibrations have to be questioned, by means of which an attempt is made to determine the polymer partition coefficients for certain analyte-coating pairs using oscillator-operated SAW resonators9,28 or to compare them with other methods!14-18,29 On the other hand, a relative calibration of the chemical sensor response can be made valid by making an appropriate adjustment in the phase position. As another consequence of our work, the quality of the film coating absolutely should be considered. For example, inhomogeneously coated films, at first glance, can show for some phase positions tremendous sensitivities. However, such coatings, often caused by dewetting effects, are not suitable for reliable sensor applications due to their unstable morphology. Thick but homogeneously coated sensors are preferred but must be carefully adjusted according to their phase reserve and their optimal phase point to be set. This must be done for each sensor separately (28) Grate, J. W.; Kaganove, S. N.; Bhetanabotla, V. R. Faraday Discuss. 1997, 107, 259-283. (29) Grate, J. W.; Snow, A.; Ballantine, D. S.; Wohltjen, H.; Abraham, M. H.; McGill, R. A.; Sasson, P. Anal. Chem. 1988, 60, 869-875.
since each sensor (i.e., each coating morphology and thickness) has its own phase position to achieve a maximal sensitivity, usually close to the maximal phase slope in the phase reserve. Recently, we have developed a new setup which adopts this task. Further (30) Rapp, M.; Reibel, J.; Voigt, A.; Balzer, M.; Bu ¨ low, O. Presented at the 7th International Meeting on Chemical Sensors, July 27-30, 1998, Beijing, China.
work will be done in order to simplify and automate this procedure.30 Received for review May 20, 1998. Accepted September 25, 1998. AC9805504
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