Anal. Chem. 1997, 69, 2848-2854
The Polymer-Coated SAW Sensor as a Gravimetric Sensor Zvi Liron,* Nathali Kaushansky, Gad Frishman, Doron Kaplan, and Jeremy Greenblatt
Israel Institute for Biological Research, P.O. Box 19 Ness-Ziona 70450, Israel
The absorption and desorption of chlorobenzene, odichlorobenzene, and chloroform in poly[n-butyl methacrylate] (PBMA) was studied in polymer-coated 104 MHz surface acoustic wave (SAW) sensors, and in freestanding polymer films by thermogravimetric analysis (TGA). The sorption processes were analyzed by a Fickian simulation and best-fit partition, and diffusion coefficients were derived. Good correlations were found between simulated and observed data. Partition coefficients derived from SAW response were independent of coating thickness and were found to be about two to three times bigger than those derived from the gravimetric response. In contrast, the diffusion coefficients increased linearly with coating thickness in the range 70-560 kHz. For the thickest polymer coating, SAW-derived diffusion coefficients were comparable with TGA-related diffusion coefficients. This study reconfirms the finding for other polymers that the response of SAW chemosensors is higher than that anticipated from a mass change only. The viscoelastic effect is again more pronounced than the gravimetric effect. Moreover, the similarity of diffusion coefficients obtained at higher polymer thicknesses suggests that the rate of change of the SAW viscoelastic component is similar to that of the gravimetric element. It is fair to assume that both processes originate from the same event: the absorption of the analyte in the polymer. In this view the polymer-coated SAW sensor may be regarded as an enhanced gravimetric sensor. Surface acoustic wave (SAW) devices coated with thin polymeric material are utilized as highly sensitive microsensors for the detection and monitoring of vapors and gases since 1979.1 A typical SAW device consists of two sets of interdigital transducers that have been microfabricated onto the surface of a piezoelectric crystal. When incorporated in an oscillator circuit, an acoustic Rayliegh wave is generated on the surface of the crystal. The resulting delay-line or resonator may be used as the frequencycontrolling element of a radio-frequency (rf) oscillator. Small mass changes or elastic modulus changes on the surface perturb the wave velocity and are readily monitored as a shift in oscillator frequency.2 The sensitivity of the SAW chemosensor to a specific vapor analyte is highly dependent on the choice of the polymeric overlay. Several physicochemical properties of the polymer determine the usefulness of the SAW device as a gas chemosensor. In particular, the polymer-vapor affinity, the rate of absorption and desorption (1) Wohltjen, H.; Dessy, R. E. Anal. Chem. 1979, 51, 1458-75. (2) Ballantine, D. S.; Wohltjen, H. Anal. Chem. 1989, 61, 704A-15A.
2848 Analytical Chemistry, Vol. 69, No. 14, July 15, 1997
processes, and the reversibility of the sorption process are the main factors that determine the suitability of a polymer as a sensing element in a SAW chemosensor. The transfer of the vapor analyte from the gas phase into the polymer overlay is a diffusion process characterized by the diffusion and partition coefficients. The diffusion coefficient is related to the rate of transport into the polymer, and the partition coefficient represents the chemicalpolymer affinity. Sorption of the vapor in the polymer layer involves a change of mass, and it has been claimed for many years that the SAW frequency shift is caused mainly by the absorbed or the desorbed mass.3 Thus, partition and diffusion coefficients were calculated on the basis of mass related equations.3-5 Grate et al.6 in 1992 compared SAW partition coefficients with gas-liquid chromatography (GLC) partition coefficients and found that the response of the SAW sensor exceeds the response due to purely gravimetric effects. Grate et al attributed the excess response of the polymer to swelling effects.6 Martin et al.7 have theoretically and experimentally analyzed the dynamics and responses of polymer-coated SAW devices introducing two distinct regimes of film behavior that cause different SAW responses: acoustically thin and acoustically thick films. Typically, polymer films on SAW devices behave as acoustically thin in the glassy regime and thick in the transition and rubbery regimes. For polyisobutylene (PIB)-coated SAW devices they found that the mass loading contribution accounts for only 40% of the measured frequency response for n-pentane and 67% for trichloroethylene (TCE). The remainder of the response was attributed to film plastization by the absorbed vapor molecules. A pure mass change in thin polymeric films can be measured directly by a gravimetric technique.8 A simple and convenient set-up for such measurements consists of a thermogravimetric analyzer (TGA) combined with a gas flow system that produces known concentrations of the chemical in air. However, the TGA technique is less sensitive than the SAW technique. Therefore much thicker polymeric films and higher concentrations of analyte in air have to be used in the TGA technique in order to obtain meaningful results. Bearing in mind these limitations, we tried to compare the response of the SAW chemosensors, with the pure gravimetric technique. We have followed not only the equilibrium responses, as most of the researchers do, but also (3) Grate, J. W.; Snow, D. S.; Ballantine Jr.D. S.; Wohltjen, H.; Abraham, M. H.; McGill, R. A.; Sasson, P. Anal. Chem. 1988, 60, 869-75. (4) Ballantine, D. S., Jr.; Rose L. S.; Grate, J. W.; Wohltjen, H. Anal. Chem. 1986, 58, 3058-66. (5) Bartley, D. L; Dominguez, D. D. Anal. Chem. 1990, 62, 1649-56. (6) Grate, W.; Klusty, M.; McGill, R. A.; Abraham, M. H.; Whiting, G.; AndonianHaftuan, J. Anal. Chem. 1992, 64, 610-24. (7) Martin, S. J.; Frye, G. C.; Senturia, S. D. Anal. Chem. 1994, 66, 2201-19. (8) Liron, Z.; Clewell, H. J.; McDougal, J. N. J. Pharm. Sci. 1994, 83, 692-8. S0003-2700(96)01193-6 CCC: $14.00
© 1997 American Chemical Society
the kinetics of the process, in order to gain more understanding of the fundamental processes that are involved in SAW chemosensor response. The goal of the present study was to determine to what extent are the kinetic response (diffusion) and the affinity response (partition) of the polymer-coated SAW sensor attributable to a gravimetric effect. For this purpose, we studied the absorption and desorption of a series of organic chemical vapors: chlorobenzene, o-dichlorobenzene, and chloroform, in poly(butyl methacrylate) (PBMA) films, by both SAW (where the polymer served as the sensor coating) and TGA. The contribution of the mass change to the sensor’s response is discussed on the basis of the partition and diffusion coefficients obtained for the two methods. EXPERIMENTAL SECTION Materials. Poly(butyl methacrylate) (PBMA; high molecular weight) was purchased from Aldrich. Chlorobenzene (99%) and o-dichlorobenzene (99.8%) were purchased from BDH. Chloroform (99%) and xylene (96%) were purchased from Riedel de Haen. The polymer and all other chemicals were used as received. PBMA Films for TGA. PBMA films for TGA experiments were prepared by spin-coating a solution of 30% PBMA in xylene on silicon wafers at 2000 rpm for 60 s. The spinner was a Headway EC101D. After being coated, the wafers were annealed for several hours at 70 °C, followed by overnight immersion in distilled water. The PBMA film was gently separated from the silicon wafer with thin-tipped forceps. The wet film was dried in ambient air and was kept at room temperature until used. The thickness of the PBMA film was determined by profilometry (Tencor Instruments, R step 100) at three different locations on reference polymercoated silicon wafers. The film was highly homogeneous, and its thickness was 5.4 ( 0.1 µm. For mounting in the balance pan of the thermogravimetric analyzer, films were cut into small pieces of 1-2 mg. TGA Exposure System. The thermogravimetric analyzer was a Perkin Elmer TGA-7 operating in the isothermal mode. In this mode, the TGA-7 monitors the weight change of the analyzed sample material as a function of time. Sample temperature is monitored by means of a thermocouple sensor located in close proximity to the sample holder (a titanium-made, circular pan). Changes in weight (sensitivity, (1 µg) and temperature (sensitivity, (0.01 °C) were monitored with time. The thermogravimetric analyzer was coupled to a vapor generation flow system as depicted in Figure 1. The carrier gas was clean, dry air obtained by filtering compressed air through a PALL Dominator system. A stream of the carrier air (line b in Figure 1) is saturated with vapor by passage through the liquid adsorbate contained in a thermostated bubbler. The saturated air stream is diluted with clean, dry air (line c), and the diluted air stream carrying a known exposure concentration flows through a three-way solenoid valve (SV) either to the bypass waste line (W) or into the TGA. A similar flow of clean, dry air (line a) is directed into the back of the TGA providing a constant flow to the electronic balance and the sample pan. The exposure temperature was 25 °C. In order to decrease lag of response due to experimental setup, the line between SV and the sample pan was made short as possible. The volume between SV and the pan is about 10 cm3, and the total flow rate was 100 cm3/m, giving a calculated delay of about 6 s. Mass and temperature measurements were performed every 4.4 s.
Figure 1. Schematic diagram of the TGA experimental set-up. Key: B, bubbler; TAB, thermostated air bath; O, TGA oven; NV, needle valve; SV, three-way solenoid valve; D, detector; F, charcoal filter; W, waste; M, mixing chamber; DL, dilution line.
In a typical experiment, a PBMA film weighing about 1 mg was preequilibrated in the balance pan for approximately 1-2 h under a flow of clean, dry air. This period was sufficient to reach a stable preabsorption baseline. Meanwhile, carrier air containing a predetermined concentration of the chemical was passed at a constant flow rate to the waste, through the bypass line. The weight of the sample was monitored continuously throughout the experiment. Exposure of the polymer film to the chemical vapor was started by switching on the three-way valve, thereby directing the vapor-laden flow into the TGA balance. The concentration of the chemical in the carrier air was determined by sampling periodically at two locations, before and after the TGA-7, and GLC analysis (HP 5890 gas chromatograph, HP-5 column, FID, HewlettPackard). When a steady-state absorption has been reached as shown by a plateau level of the sample weight, desorption was initiated by switching off the three-way valve to the waste line and opening the clean air desorption line (line d in Figure 1). The experiment was terminated when the initial weight of the sample was reached. In order to verify that the observed TGA weight change is entirely due to absorption in the polymer film and is not affected by interaction of the organic vapors with any part of the TGA system itself, dummy exposures to chlorbenzene were performed. In these experiments, the polymer film was replaced by a titanium piece weighing about 1-2 mg, which was required since the TGA-7 does not record weight changes referenced to zero. No response of the TGA system to a chlorobenzene step was found. It was thus evident that in the absence of an absorbent sample, the TGA system is not sensitive to chlorobenzene vapors, within the concentration range of interest in the present study. PBMA Coatings for SAW Sensor. The entire surface of the SAW sensor was spin-coated with PBMA from 2-8% solutions of the polymer in xylene, at 3000 and 5000 rpm, for 40 s. After coating, the delay lines were annealed for 10 min at 100 °C, cooled to room temperature, and inserted into the exposure set-up. Exposure experiments were started after at least 24 h, when drifts due to solvent evaporation, and possibly due to other relaxation effects, have subsided. Film thickness was monitored through the frequency shift induced by the polymer overlay. Film thickness equivalents were in the range 70-560 kHz. The actual thickness of the coatings, 4.76 × 10-2 to 3.81 × 10-1 µm, was Analytical Chemistry, Vol. 69, No. 14, July 15, 1997
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Figure 2. Schematic representation of the SAW exposure setup. SV, three-way solenoid valve.
estimated by thickness measurements on model silicon and glass substrates using ellipsometry, reflectometry, and profilometry techniques.9 SAW Module and Exposure System. SAW delay-line narrow-band filters operating at 104 MHz were similar to those described previously.9-10 The devices were mounted in standard dual-in-line microelectronic packages, and wire-bonded to the input and output pins. The oscillator electronics included an rf amplifier and a 6 dB coupler that separated the output load from the feedback circuit and minimized frequency-pulling effects. The oscillator electronics were mounted in a separate aluminum box. The SAW exposure system is shown schematically in Figure 2. A stream of saturated vapor entered a mixing chamber and was diluted with clean dry air in a predetermined ratio. The exposure cell (SAW sensors in Figure 2) was an aluminum tube (40 mm inner diameter) that included eight SAW sensors (two sets of four, centered about 25 and 40 cm from the entrance SV). The surface of each SAW delay-line element was aligned parallel to the air flow lines within the exposure cell. The exposure cell, feeding lines, and valves were placed in a thermostated chamber. The temperature, pressure, and flow rate were kept constant and were continuously monitored during the experiments. Known concentrations of the adsorbate vapor in the carrier air were obtained by bubbling the carrier through the liquid adsorbate, contained in a thermostated bubbler, at constant temperature and flow-rate, and dilution of the vapor-saturated air stream with clean, dry air as necessary. By use of four coactivated solenoid valves (SV), either dry clean air or vapor laden air were directed into the SAW sensors exposure cell. The flow rate was either 30 lpm (for high concentration experiments) or 100 lpm (for low concentration experiments), and the volume between the entrance SV and the first and second sets of SAW sensors was about 200 and 400 cm3, respectively, giving a calculated delay time of 0.10.8 s. The concentration of the chemical in the carrier air was determined by sampling and GLC analysis as described for the TGA. The PBMA-coated delay-line SAW sensors were exposed to a stepwise increasing concentration of the chemical (absorption) followed by a stepwise exposure to clean “zero” air (desorption). The exposure temperature was 25 °C. The response of the sensor was digitized and collected every 0.7 s with a personal computer and a custom-developed software as described previously.10 Data Processing. Chemical diffusivities in membranes are often evaluated from sorption data by the methods of initial rates, (9) Greenblatt, J.; Sivan, O.; Liron, Z. Presented at the 186th Electrochemical Society Meeting; Miami Beach, FL, October 9-14, 1994. (10) Liron, Z.; Greenblatt, J.; Frishman, G.; Gratziani, N.; Biran, A. Sens. Actuators, B 1993, 12, 115-22.
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Analytical Chemistry, Vol. 69, No. 14, July 15, 1997
half-life time, or the final rates of the absorption or desorption.11 However, better accuracy is achieved if the entire sorption curve is taken for analysis. This approach was adopted in the present study. The theoretical model is based on a solution of Fick’s diffusion law under transient conditions. Suppose that a polymer film, which has been conditioned to a certain concentration of a chemical, is instantaneously exposed to a different concentration. If the diffusion coefficient is constant, the amount Mt of chemical absorbed or desorbed in time t is given by the non-steady-state solution to Fick’s law, eq 1.12
Mt M∞
∞
)1-
∑
8
n)0(2n+1)
2
Π2
exp
[
]
-D(2n + 1)2Π2t 4l2
(1)
where M∞ is the amount absorbed or desorbed at t ) ∞ (equilibrium), 2l is the film thickness, and D is the diffusion coefficient of the adsorbate. The partition coefficient of the chemical vapor in the film, K, is given by eq 2:
K)
FM∞ mcv
(2)
where m is the weight of the polymer sample, F is its density, and cv is the chemical concentration in air. Equations 1 and 2 are applicable directly to the TGA data. Data from the TGA experiments were transferred to a spreadsheet program and converted to net absorbed or desorbed amounts in time. These values served as input for a simulation computer program (SCoP, Simulation Resources Inc.), by which eq 1 was solved as described earlier.7 Partition coefficients were derived from eq 2. For evaluation of the SAW data, eqs 1 and 2 may be used if it is assumed that the SAW frequency shift is proportional to mass change.6 In that case,
∆ft ∆Mt ) ∆f∞ ∆M∞
(3)
where ∆ft is the frequency shift in time t, and ∆f∞ is the frequency shift at equilibrium. The partition coefficient K can be derived from the SAW data by eq 4:
K)
( )( ) ∆f∞ F ∆fc cv
(4)
where ∆fc is the SAW frequency shift of the polymer overlay. By combining eqs 1 and 3, data from the SAW experiments were processed with the simulation computer program to yield the partition and diffusion coefficients similarly to the analysis of the TGA data. The thickness of the polymer overlay was transformed from frequency units (kHz) to length units (µm) by multiplying the overlay frequency shift by the factor 6.8 × 10-4 µm/kHz.9 RESULTS AND DISCUSSION TGA Experiments. Sorption of chlorobenzene (4900-19200 mg/m3), o-dichlorobenzene (1500-3200 mg/m3), and chloroform (11) Crank, J. The Mathematics of Diffusion; Oxford: New York, 1976; pp 2446. (12) Crank, J. The Mathematics of Diffusion; Oxford: New York, 1976; pp 47-8.
Figure 3. (a) Typical response of the PBMA film to chlorobenzene vapor in the TGA setup. (b) Simulation of the absorption phase. (c) Simulation of the desorption phase.
(92400-230900 mg/m3) in PBMA was recorded on the TGA system and was processed by the simulation technique. A typical experiment, showing absorption of chlorobenzene (cv ) 19200 mg/m3) in a PBMA film, followed by desorption in clean air at a constant temperature (24.9 (0.1 °C), is shown in Figure 3a. The desorption curve appears to be symmetrical to the adsorption curve. The data of the absorption and desorption sections in Figure 3a were processed separately. The simulated and experimental data are shown in Figure 3b (absorption) and Figure 3c (desorption). For convenience, desorption data (weight loss) are displayed as positive rather than negative numbers. Parts b and c of Figure 3 demonstrate that a very good fit was obtained between the experimental and theoretical data points, for both the absorption and desorption, as is also indicated by the serial correlation coefficients (0.98 and 0.97 for the absorption and desorption, respectively). The diffusion coefficients were found
to be 1.9 × 10-10 and 2.3 × 10-10 cm2/s, and the partition coefficients were found to be 6115 and 5795, for the absorption and the desorption processes, respectively. The similarity between the coefficients derived from absorption process and the corresponding coefficients derived from desorption process means that the two processes are symmetric and no hysteresis occurs. Partition and diffusion coefficients derived from the absorption and desorption data sections of the three compounds are given in Table 1. Both partition and diffusion coefficients were found to be insensitive to vapor concentration in the studied range. The partition coefficients indicate that o-dichlorobenzene has the highest affinity for PBMA, chlorobenzene is second, and chloroform is third, with a partition coefficient that is about thirty-fold smaller than the partition coefficient of o-dichlorobenzene. These values are in accordance with the boiling points (180.5, 132, and 61.7 °C, respectively)13 of the three analytes.14 The diffusion coefficients follow a different order, in accordance with analyte molar volume in which chloroform (VM ) 80.5 cm3/mol)13 is first, chlorobenzene (VM ) 101.8 cm3/mol) is second, and o-dichlorobenzene (VM ) 112.7 cm3/mol) is third.15 SAW Experiments. Polymer coatings on SAW sensors are limited in thickness due to attenuation of acoustic wave. Therefore we have used thinner films than those that were used by us in the TGA experiments. In addition we have studied the influence of film thickness on the SAW response. We have exposed the sensors to a wide range of analyte concentrations, although because of technical limitation we did not reach the concentration used in the TGA experiments. Seven chemosensors with PBMA coatings in the range 70560 kHz (including two different 320 kHz overlays) were exposed to a relatively low concentration (310 ( 30 mg/m3) of chlorobenzene. A typical response of a 560 kHz PBMA-coated sensor to chlorobenzene vapor at this concentration is shown in Figure 4a. Three absorption-desorption cycles are shown. Preequilibration of the sample in dry air yielded a stable base-line SAW frequency (for example, near time ) 0 before the first cycle in Figure 4a). Stepwise exposure of the sensor to chlorobenzene vapor initiated absorption, as indicated by the steep decrease of the SAW frequency. The response leveled off when equilibrium was reached. Next, exposure of the PBMA overlay to dry, clean air resulted in desorption and an increase of the SAW frequency until the base-line level was reached. The desorption curve was always symmetrical to the absorption curve. The absorption and desorption sections of each cycle were analyzed by simulation as described above. Simulated and experimental data of the first of the three cycles of Figure 4a are shown in Figures 4b (absorption) and 4c (desorption). The simulated data were found to fit the observed data well for both the absorption and desorption parts, with correlation coefficients of 0.86 and 0.85, respectively. The partition and diffusion coefficients derived from the absorption data (12 870 and 6.12 × 10-11 cm2/s, respectively) are close to those derived from the desorption data (12 830 and 5.81 × 10-11 cm2/s, respectively). To prove that SAW partition coefficient is not sensitive to vapor concentration, SAW response was tested as a function of analyte concentration in air. The ranges of concentration for chlorobenzene, odichlorobenzene, and chloroform were 60-1900, 10-350, and (13) Weast, R. W. Handbook of Chemistry and Physics, 55th ed.; CRC Press: Cleveland, OH, 1974. (14) Patrash, S. J.; Zellers, E. T. Anal. Chem. 1993, 65, 2055-66. (15) Frye, G. C.; Martin, S. J.; Ricco, A. J. Sens. Mater. 1989, 1(6), 335-57.
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Table 1. TGA Experimentsa
a
chemical vapor
concentration range, cv (mg/m3) × 10-3
partition coeff, K × 10-3
chlorobenzene dichlorobenzene chloroform
4.9-19.2 1.5-3.2 92.4-230.9
6.3 ( 0.7 32.0 ( 5.1 1.22 ( 0.04
diffusion coeff, D × 1011 (cm2/s) absorp desorp 18.0 ( 5.4 6.1 ( 4.9 3.9 ( 0.5
20.6 ( 4.8 5.5 ( 2.0 4.1 ( 0.3
n 10 7 8
At least two repeats were done at each individual concentration.
Figure 5. Typical linear correlation between SAW response and vapor concentration. Chlorobenzene absorption into 320 kHz PBMA film.
Figure 4. (a) Typical three-cycle response of the PMMA-coated (560 kHz) SAW sensor to chlorobenzene vapor. Temperature and the position of the valve to activate absorption and desorption are shown as well. (b) Simulation of the absorption phase. (c) Simulation of the desorption phase.
300-33 000 mg/m3, respectively. For all three vapors a good linear correlation was found between sensor frequency response and analyte concentration. An example of the linear correlation (r2 ) 0.99) for chlorobenzene absorption in a 320 kHz PBMA 2852 Analytical Chemistry, Vol. 69, No. 14, July 15, 1997
overlay is given in Figure 5. Introducing the value of the slope (3.7532) into eq 4 yields a partition coefficient value of 12550. The partition and diffusion coefficients obtained from exposures to representative concentrations of chlorobenzene, odichlorobenzene, and chloroform are given in Table 2. Throughout the results, the similarity between absorption-derived and desorption-derived partition and diffusion coefficients is maintained, suggesting that absorption and desorption of chlorobenzene into PBMA are symmetrical processes. Thus, the uptake of chlorobenzene by the PBMA overlay is fully reversible. With all three adsorbates, the partition coefficients of the 70 kHz film are one-third to one-half the partition coefficients for higher film thicknesses. If we temporarily ignore the results for the 70 kHz film, the partition coefficients are practically unaffected by the overlay thickness, although some variations can be noticed. The reason for this variation is not clear. A plot of the partition coefficient vs film thickness (not shown) indicate that the values for all three vapors follow a similar “sinusoidal” trend. This may be interpreted as film resonance effect as discussed by Martin et al.7 However, the fact that the exposure temperature in this study was close to the static glass transition temperature Tg(0), the relatively thin films and the small concentrations used in the present study exclude the resonance possibility. We believe that variation in K values are probably due to variations in coating application and/or morphology.16 For all the three chemicals, the diffusion coefficients were found to increase with the thickness of the polymer overlay. Again, the 70 kHz coating with its abnormally low diffusion coefficients appeared to be distinctive from the other coatings. Plots of the diffusion coefficients of the three analytes vs film thickness (Figure 6) show that diffusion coefficients increase linearly with film thickness, both for the absorption and the desorption process. This dependency is about 3-fold stronger for chloroform than for dichlorobenzene and chlorobenzene. (16) Ballantine, D. S., Jr. Anal. Chem. 1992, 64, 3059-76.
Table 2. SAW Experimentsa
chlorobenzene
dichlorobenzene
chloroform
partition coefficient, K × 10-3
diffusion coefficient, D × 1011
PBMA thickness (kHz)
absorp
desorp
absorp
desorp
70 105 138 192 320b 320 560 70 105 138 192 320b 320 560 70 105 138 192 320b 320 560
6.5 ( 0.3 14.8 ( 0.3 13.4 ( 0.4 10.7 ( 0.8 12.3 ( 0.5 16.1 ( 0.1 12.8 ( 0.1 31.3 ( 2.1 120.3 ( 8.5 106.7 ( 1.0 83.1 ( 2.4 106.2 ( 1.0 133.2 ( 1.0 110.5 ( 1.0 0.6 ( 0.2 2.3 ( 0.2 2.0 ( 0.1 1.8 ( 0.2 1.6 ( 0.2 2.3 ( 0.3 2.0 ( 0.1
6.4 ( 0.4 14.1 ( 1.0 12.1 ( 2.0 9.5 ( 0.6 12.1 ( 0.4 16.0 ( 0.2 12.8 ( 0.2 29.6 ( 2.2 112.8 ( 8.6 95.1 ( 3.9 82.5 ( 1.6 99.7 ( 4.9 129.5 ( 1.0 109.5 ( 1.0 0.6 ( 0.1 2.1 ( 0.2 2.0 ( 0.1 1.3 ( 0.1 1.6 ( 0.1 2.2 ( 0.1 2.0 ( 0.1
0.03 ( 0.01 0.7 ( 0.1 1.4 ( 0.1 1.8 ( 0.1 2.3 ( 0.1 2.8 ( 0.3 6.3 ( 0.2 0.1 ( 0.0 0.5 ( 0.1 0.8 ( 0.0 0.9 ( 0.1 1.5 ( 0.0 1.6 ( 0.2 3.4 ( 0.1 0.07 ( 0.01 1.3 ( 0.1 2.6 ( 1.6 3.7 ( 1.4 6.5 ( 0.2 8.5 ( 0.3 18.8 ( 2.1
0.03 ( 0.01 0.7 ( 0.1 1.2 ( 0.3 1.6 ( 0.0 2.4 ( 0.0 2.9 ( 0.1 5.8 ( 0.3 0.2 ( 0.0 0.3 ( 0.0 0.9 ( 0.2 0.9 ( 0.2 1.4 ( 0.0 1.6 ( 0.0 3.2 ( 0.0 0.04 ( 0.02 2.5 ( 0.5 4.0 ( 0.3 6.5 ( 0.3 7.4 ( 0.7 10.4 ( 2.2 17.7 ( 1.3
a Concentration in air: chlorobenzene 310 ( 30 mg/m3, dichlorobenzene 21 ( 2 mg/m3, chloroform 310 ( 30 mg/m3. Number of repeats was three. b Two different PBMA overlays with the same thickness were tested simultaneously in the same experiment.
Thus the SAW sensor responds not only to mass loading by the absorbed vapor but also to some other changes in the coating. This finding is in accordance with the findings of Grate et al.6 who compared SAW to a mass response based on GLC analysis, and to that of Martin et al.,7 who compared SAW to a mass response based on 5 MHz QCM response. In our opinion, TGA is a more straightforward reference technique for SAW, since the polymer mass change as well as the vapor concentration in the carrier air are monitored directly. In addition, GLC measurements represent partition coefficients at infinite dilution, while SAW and TGA measurements represent partition coefficients at finite vapor concentrations. It is interesting to note that, in spite of those differences between TGA and GLC, the results obtained by us were similar to those obtained by Grate et al. In our study KSAW/KTGA was found to be about two to three, whereas KSAW/KGLC values reported by Grate et al. are mostly between three and five. Grate et al. suggest that the principal contributor to the SAW excess response is the decreased modulus of the polymer overlay caused by a volume expansion of the polymer due to absorbed vapor. According to the notation of Grate et al., the SAW response to vapor sorption (∆fv) is given by the sum of the gravimetric effect and the modulus effect:
∆fv(sensor) ) ∆fv(gravimetric) + ∆fv(modulus) Figure 6. Effect of SAW coating thickness on analyte diffusion coefficient. (a) Absorption. (b) Desorption. Key: (9) chloroform (310 mg/m3), (0) chlorobenzene (310 mg/m3), (O) dichlorobenzene (21 mg/m3).
Sorption of a chemical vapor into a polymer film is determined by the diffusion (D) and partition (K) coefficients. A comparison between the TGA and SAW results (Tables 1 and 2) indicates that, in terms of partition coefficients, the SAW response exceeds the gravimetric response by a factor of about 2.2, 3.4, and 1.6 for chlorobenzene, o-dichlorobenzene, and chloroform, respectively.
(5)
where ∆fv(modulus) is the arithmetic difference between the two terms representing polymer modulus effects pre- and postabsorption of the vapor, respectively. In this view, the surface wave of the SAW sensor will sense the PBMA polymer (Tg(0) ) 20-27 °C) as a stiff glassy material, and a softening of the polymer by the absorbed vapor will decrease the effective modulus and induce an excess frequency shift, in addition to the mass related response. Martin et al.,7 reported that the mass loading contribution accounts for only 40% of the measured response for n-heptane and 67% for Analytical Chemistry, Vol. 69, No. 14, July 15, 1997
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TCE in PIB films. This will translate to KSAW/KQCM of 2.5 for n-heptane and 1.5 for TCE. The bigger mass loading for TCE in respect to n-heptane was explained by the fact that as the density of the absorbed species increases, there is a larger mass contribution relative to the plasticizing effect. Among the three vapors tested in the present study, chloroform has the highest density (F ) 1.5), and the finding that its KSAW/KTGA value is relatively small (1.6) is in accordance with the prediction of Martin et al.7 However, vapor density is probably not the sole factor since o-dichlorobenzene (F ) 1.3)13 and chlorobenzene (F ) 1.1) have K ratios of 3.4 and 2.2, respectively. The higher ratio for o-dichlorobenzene can be explained by an enhanced plasticizing effect due to its higher molar volume.7 The dependency of the diffusion coefficient on the film thickness is unexpected. We have seen that the SAW chemosensor response consists of a gravimetric and a viscoelastic contribution. If the two effects had different dynamic responses, and if the relative contributions of the two effects were thickness dependent, we might have seen such a dependence. However, our films were “acoustically thin”7 in all thicknesses studied. First, film thicknesses were very small in comparison with the acoustic wave length at 104 MHz. Second, the operating temperature (25 °C) was close to the static glass transition temperature (Tg(0) ) 20-27 °C), giving practically a glassy regime. And third, the low concentration of the analytes, and the linear decrease of SAW frequency with increase in vapor concentration are consistant with “acoustically thin” films. In acoustically thin films the relative contribution due to vapor absorption of the gravimetric and viscoelastic terms is constant.7 Being thickness dependent, the diffusion coefficients must be regarded as “apparent” diffusion coefficients. The dependence of the apparent diffusion coefficients on the film thickness can be explained by the presence of a thin film, either at the quartz/ polymer or at the polymer/air interface, that is more densely packed than the bulk polymer. This inner or outer “skin” can slow the diffusion process without affecting much the solubility of the analyte. Indeed, this can explain the lower diffusion coefficients for the thinner films, where the relative contribution of the “skin” effect is more pronounced and the constant “partition coefficient” for all thicknesses (except that of 70 kHz). The behavior of the 70 kHz sensor is anomalous. The extremely low diffusion coefficient (two orders of magnitude less than in the 560 kHz coating) can be probably explained by the “skin” effect. The much lower partition coefficient is harder to explain. It is interesting to note that for chlorobenzene and o-dichlorobenzene,
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partition coefficients in the 70 kHz films are practically the same as those found in the TGA experiments. In this sense, the very thin 70 kHz film coated sensor responds as a purely gravimetric sensor. In the studied polymer overlay thickness range of 70 -560 kHz (equivalent to 4.76 × 10-2 to 3.81 × 10-1 µm), the apparent diffusion coefficients increased linearly with overlay thickness, and the question arises as to what extent this function can be extrapolated. On the basis of the model described above, we would expect a function that increases with thickness and then reaches a plateau level at a certain thickness. The fact that the calculated DSAW values for the 560 kHz polymer overlay are in the range of the calculated DTGA values (lower for chlorobenzene and o-dichlorobenzene, and higher for chloroform) suggests that the apparent D values will level-off beyond an overlay thickness of 560 kHz, assuming of course that such thicknesses are still usable for SAW. In any case, when running SAW experiments with polymer overlays, the possible effect of the overlay thickness factor on the diffusion coefficients should not be ignored. The similarity of the diffusion coefficient for the thickest polymer films (where the “skin effect” is unimportant) obtained by SAW and the much thicker films used in TGA (where the “skin effect” is even less important) is very interesting. Since, the SAW response is composed of a gravimetric and a viscoelastic response, the two responses must have the same dynamic behavior, namely both responses originate from the same effect, the dissolution of the analyte in the polymer. This dissolution causes a plasticizing effect and a mass loading at the same time. No reorganization of the polymer having a different time scale was noticed. By comparing SAW and TGA closely related absorption and desorption experiments on three chemicals, we have demonstrated that, in terms of affinity, SAW-related partition coefficients are considerably larger than those derived from purely massrelated experiments such as TGA. In this sense, a prediction of the SAW response from a mass-related determination of partition coefficients will always be an underestimation. Or in other words, the SAW sensor may be regarded as an enhanced gravimetric sensor.
Received for review November 25, 1996. Accepted April 10, 1997.X AC9611935 X
Abstract published in Advance ACS Abstracts, June 1, 1997.
