Anal. Chem. 1988, 60,230-235
230
(35) Perrln, D.D. Dissociation Constants of Organic Bases in Aqueous So-
iution; Butterwodhs: London, 1965; p 90. (36) Bastow, S. H.; Bowden, F. P. Roc. R . SOC.London, A 1935. 151,
220-233. (37) Schulman, J. H.; Teorell, T. Vans. Faraday SOC. 1938, 3 4 , 1337-1 342.
RECEIVED for review March 24, 1987. Resubmitted June 22,
1987. Accepted October 15,1987. This work was supported by grants from the Science Foundation (H*F*)and by the Natural Sciences and Engineering Research Council of Canada (F.F.C.). Presented in part at the International Chemical Congress of Pacific Basin Societies, Hawaii, Dec 1984. Experimental work was performed while on study leave (F.F.C.).
Metallophthalocyanines as Chemical Interfaces on a Surface Acoustic Wave Gas Sensor for Nitrogen Dioxide Maarten S. Nieuwenhuizen,* Arnold J. Nederlof, and Anton W. Barendsz
Prins Maurits Laboratory TNO, P.O. Box 45, 2280 A A Rijswijk, The Netherlands
The response of a SAW (surface acoustlc wave) gas sensor for NO2 has been studied extenslvely by uslng NO2 and a number of other gases (CO, COP, CH,, NH,, SO2, H20, and toluene). Different metakphthaiocyanlnes (MPC; M = H2, Mg, Fey Coy NI, Cu, Pb) have been tested as a chemlcal Interface on one delay-line of a dual delay-llne oscillator. These compounds offer two sites for interaction, one at the metal ion (coordlnatlon complex formatlon) and one at the electron cloud In the periphery of the mdecule (charge transfer complex formatlon). The results are discussed In terms of general performance characteristics. At 150 *C CoPC Is preferred for Selectivity and sensitlvlty and CuPC Is preferred for response tbne. For CuPC the relation between sensltlvlty, layer thickness, and frequency (39-98 MHz) was calculated and also the effect of temperature (30-150 "C) was measured. PbPC cannot be used due to Irreversible effects. The transductlon mechanisms of these SAW chemosensors are a combination of changes In mass and in conductivity caused by several chemical and physical processes at the chemical Interface.
Surface acoustic wave (SAW) devices are attractive for chemical microsensor applications because of their small size, low cost, sensitivity, and reliability. Furthermore, SAW technology is compatible with planar integrated circuit technology. Chemosensors based on piezoelectric crystal using bulk acoustic waves (BAW) have been known for quite some time (1)and numerous examples of such sensors for various gases are known (2,3). The first gas sensor using surface acoustic waves (SAW) was reported by Wohltjen et al. (4,5 ) . They placed a SAW delay line, covered with a polymer, in the feedback loop of an amplifier and measured the oscillator frequency. Since then, Bryant et al. (6, 7) reported a SAW dual delay line and D'Amico (8)a three transducer type SAW device. The latter structures showed a better temperature stability because the second delay line is used as a reference compensating for such nonspecific effects as the variations of temperature and pressure on the substrate. The first use of a SAW resonator configuration as gas sensor was reported by Martin e t al. (9). For the adequate measurement of a specific gas in a mixture of gases the measuring delay line of the SAW device must be coated with an appropriate chemical interface. So far, many
chemical interfaces have been proposed (1-15). The interactions occurring at the interface will be responsible for the performance characteristics of the gas sensor such as selectivity, sensitivity, reversibility, and response time. Recently, a study dealing with these various aspects has been published by the authors (15). In our laboratory SAW chemosensor research is concentrating on the development of a SAW gas sensor for NO2 and CO (16-19) to be used in automotive exhaust systems, process control, or environmental monitoring. The interaction with these gases has to be sensitive, selective, reversible, and fast. Additionally, to prevent condensation of water and to reduce response times, the sensor will have to be operated at elevated temperatures, thus requiring a highly stable chemical interface. As early as 1972 it was recognized in our laboratory (20) that metal-free phthalocyanines and other polyaromatic molecules preferentially adsorb gases with high electron affinities such as NO2 and chlorine and therefore offered an interesting possibility for the detection of these gases. Phthalocyanines (PC) are p-type organic semiconductors with which the electronegative gases interact strongly resulting in a change in the electrical conductivity of these substances. Since organic semiconductors do not interact as strongly with water or oxygen as inorganic semiconductors and because organic semiconductors can easily be modified chemically to obtain tailor-made conductive properties, many papers have been published since, dealing with the development of socalled chemiresistors. For the detection of NOz or chlorine both hydrogen and metallophthalocyanines (MPCs) were investigated (21-32) as well as other compounds containing a-electron systems (20, 33-35). MPCs have also been used as sensitizers on solid electrolyte sensors (36) and as a chemical interface for piezoeledric crystal sensors (37). In previous papers (16,17) we reported on our first results using H2PC as the chemical interface on a quartz-based SAW gas sensor for NO2. Since then only Ricco et al. (14,36) reported on a single experiment using PbPC on a LiNb03-based SAW sensor. In this paper the results of a study are reported that used HzPC and a number of MPCs (M = Mg, Fe, Co, Ni, Cu, and Pb) to investigate the effect of different metal ions on important sensor performance characteristics as sensitivity, selectivity, reversibility, and response time.
EXPERIMENTAL SECTION Gases. The gases were bought from Matheson (NO2,CO, and SO,) and HoekLoos (NH3, C02, and CHI) and used
0003-2700/88/0360-0230$01.50/00 1988 American Chemical Society
ANALYTICAL CMMISTRY. VOL. BO. NO. 3. FEBRUARY 1. 1988
231
A
H m i pmthoiotyamne
Flgm 1. chsrmcal structue of
metallopMhalocyani~.
Table 1. Characteristics of the Various SAW Devicen Used in this Study: Layer Thickness ( b ) ,Frequencp of the Measuring (f,)and the Reference Delay Line (f,) as well as Drift ( D )and Noise (N) in the Frequency Differenee Signal at 150 'C sensor
code H1pC52 MgPC55 FePC59 CoPC54 NiPC62 CuPC66 CuPC65 CuPC53 CuPC70 PbPC6O
MPC h , r m f,.MHz faMHz
D,
Hzfmin
N,Hz
H,
0.14
78.38
78.45
2.6
19
Mg
0.38
78.70
1.9
25
Fe
0.26 0.26 0.36 0.29 0.43
78.67 78.71 78.71 39.15 52.46
78.50 78.45
-9.9 0.8 0.2
22
0.3
11
0.8
0.24
78.75
78.80
0.35 0.22
98.07
98.11
7 29 96
78.60
78.44
Co Ni
Cu Cu Cu Cu
Pb
78.44 78.83 39.56 52.56
0.7 -8.6 6 3
17 17
13
'Theoretical frequencies are 39.48, 52.63. 78.95. and 98.69 MHz. without further purification. Toluene and water vapors were obtained by evaporating pure liquids. The vapors and gases were diluted in a two-step dilution system with pressurized air (5% relative humidity), which was purified over a coal bed. The vapor concentrations were continuously monitored at mom temperature with a Foxborn Miran 1A CVF infrared gas analyzer at the appropriate wavelengths. It is known that NOp is in equilibrium with N2O0 The infrared analyzer has been calibrated at rmm temperature, aasuming all gas to be NO?. At room temperature considerable amounts of Np04(ca.70%) will be present. This means that at elevated temperatures the actual NO2 concentration in the sensor cell will be higher. When measurements at different temperatures me compared, this effect should be kept in mind. I t is assumed that this effect is of minor importance with respect to the discussions in this paper. SAW Devices. The configuration of the quartz-based SAW devices and dual delay-line oscillator electronics to obtain a stable and accurate frequency signal is described elsewhere (19). In Table I the characteristics of the SAW devices and the chemical interfaces used in this study are given, b e i the layer thickness, the frequencies of the measuring and reference delay line, and the maximum drift and noise during NOZ measurementa at 150 "C. Here noise is defined as the difference between maxima and minima in the signal. The phthalocyanines were bought from ECA (HJ, Fluka AG (Co, Ni, and Cu), and Pfalz and Bauer (Mg, Fe, and Ph) and were purified by vacuum sublimation a t 5 X mbar and 450-530 OC. The phthalocyanine layers were sublimated onto the measuring delay line by physical vapor deposition (PVD) with a stainlesssteel mask. The vacuum (Balzers Union MED 010 system) was mbar and the source temperature was sufficient for 0.02 pmlmin growth at a source distance of 8.5 cm. Measuring Cell. A milled brass cylindrical box contains the SAW device bonded to a ceramic plate. A silicone adhesive is used for proper heat conduction and to prevent the reflection of unwanted BAWs. The hack of the ceramic plate is provided with a thick film resistor in order to maintain the sensor a t a constant temperature (=k 0.01 "0.The cell is
npUa 2. Scanning ebcbon micrographs of the H f l and MPC layers.
purged with air (same quality as the diluting a i ) at a constant flow, 10 L/h. A t the start of a measurement the purging air stream is switched off and the diluted gas stream is drawn through. In this way a constant gas concentration was obtained within 1 min. Frequency Measurement a n d Temperature Control. A Hewlett-Packard H P 5335A frequency counter is used in combination with an HP-86B computer to sample the frequency of both delay lines every 15 s. These data as well as the temperature, time, and the calculated frequency difference are stored on a floppy disl for off-line data processing and plotting, using an H P 9121 disk drive and H P 7470A plotter. The temperature of the sensor is controlled every 2 s with a temperature sensor, an H P 3478A multimeter and the computer calculating the power to be provided by a H P 6034A power supply system to the thick film resistor. S E M Measurements. All PC layers were characterized with scanning electron microscopy using a Philips SEM 515 system at 30 keV. Layer thickness could he measured with an accuracy of 0.02 pm. In Figure 2 SEM pictures of various MPCs are shown. RESULTS AND DISCUSSION Sensitivity. As can be observed in Table I the thicknegses of the various MPC layers differ from one another. Hence, the response of the sensor should thus be related to the volume of the chemical interface. In practice, as the surface area is assumed to be constant, the response can be normalized by dividing by the layer thickness. Furthermore, i t has been observed that the reference delay line does not contribute significantly to the frequency difference signal. Therefore the response of the sensor is only dependent on the interaction of the gas with the PC layer. In formula
sensitivity = S =
response
(Hz/(ppmpm)) (2) concentration f I and foare the frequencies of the measuring and the reference delay line, in the gas-containing and the purging air stream, respectively. The sensitivity is defined as the response divided
232
ANALYTICAL CHEMISTRY, VOL. 60, NO. 3, FEBRUARY 1, 1988
%
100
I '\
I
I
I
15 8
50.
/
0 0 --
25.
01
I
I
I
I
313
353
393
633 TlKi
Flgure 4. Relative response of the H,PC52 sensor (40 O C = 100%) and the relative amount of NO, in NO, mixture as a function of tem-
perature.
LiNb0,-based SAW devices would lead to the same conclusions, when repeated on quartz-based SAW devices, viz. that only conductivity effects occur. In addition, due to the fact that PbPC is not stable at elevated temperatures, it is better to use CuPC for such experiments (vide infra). The slope of the curves as observed in Figure 3 and given in Table I11 is an indication for the affinity between NO2 and the MPCs. As NOz interacts with the electron cloud of MPCs, some correlation should exist between the affinities for NO2 and electronic structures as obtained from molecular orbital calculations (40). No correlation could be found, again pointing a t more than one transduction mechanism. Effect of Temperature. Generally, in the case of ordinary adsorption, desorption is promoted at elevated temperatures. In Figure 4 the relative response of the sensor H2PC52 vs temperature is depicted. In this figure a maximum is observed at 110 "C. This maximum could be caused by a combination of temperature effects on the affinity for NOp (changes in mass) and on the conductivity. It is reported in literature that MPC-based chemiresistors showed increasing sensitivities in the temperature range 90-150 "C (31). Also the concentration of NOz is measured a t room temperature, meaning that the relative amount of NOz in an NO, mixture will be higher at elevated temperatures leading to higher sensitivities. In Figure 4 also the relative amount of NOz in NOz/Nz04vs temperature is depicted. Table I11 and Table IV show that the position of the above-mentioned maximum differs for the various MPCs resulting in sensitivities being higher at 70 "C (FePC, CuPC) or equal (MgPC, CoPC). Effects of Frequency and Layer Thickness. Wohltjen (5)used the following formula to relate the frequency changes directly to mass changes assuming changes in mass are the only transduction mechanism:
where kl and k, are material constants for the SAW substrate, h is the thickness of the interface material, fo is the unperturbed resonant frequency of the sensor oscillator, p is the density of the interface material, V , is the unperturbed Rayleigh wave velocity, X is the Lam6 constant, and w is the modulus of the interface material. In Table I1 the R and S values of four CuPC-based sensors at different frequencies are listed. From eq 3 a linear relation is expected to exist between S and the square of the frequency. In Figure 5 this relationship is shown to be nonlinear. Another
ANALYTICAL CHEMISTRY, VOL. 60, NO. 3, FEBRUARY 1, 1988 I
Table 11. Response of CuPC Sensors for NO2 at Different Frequencies at 150 "C code
thickness, frequency, R, Hz/ m Hz ppm
CuPC66 CuPC65 CuPC53 CuPC70
39.3 52.5 78.8 98.1
0.29 0.43 0.24 0.35
8.6 19.8 19.2 38.0
S, Hz/
I
I
I
233
/I I
lOOOR/
(ppmrm)
P4
30 46 80 109
50.4 77.3 42.5 61.8
relationship was tested in which the power of the frequency is n (eq 4). It was calculated that n = 1.4 and C1 = 0.176 (correlation coefficient 0.999 86). This shows, in an empirical log S = log C1 n log fo (4)
+
R = Czhfk.4
(5) way, that the assumption of Wohltjen leading to eq 3 showing frequency changes only mass dependent might not be valid in our case. In eq 5 a linear relationship is presented which should exist between the response and the layer thickness. With the power of the frequency derived from eq 4 and the data presented in Table 11, this assumption was proven, yielding Cz = 178.5 (correlation coefficient, 0.999 70). So, in our case, when the response is related to unit volume of the chemical interface, the sensitivity is related to the frequency to the power 1.4. This implies that by using a thinner layer, which will show a faster response but lower sensitivity, the latter effect can be compensated by increasing the frequency, e.g. from 80 to 300 MHz. An increase in sensitivity by a factor 6.4 will be expected. Selectivity. In Table I n and Table IV the response is given for various gases which possibly interfere with the NOz response. These gases have been chosen as compounds present in automobile exhaust mixtures or model compounds. Interferences from other gases can be caused by two phenomena, viz. interaction with the PC molecules or "condensation physisorption" in the PC lattice. Interaction of Gases with PC Molecules. Basically, a PC molecule offers two sites for adsorption, namely the large electron cloud at the periphery of the molecule which interads with electronegative compounds (electron acceptors) forming charge transfer complexes and the positive metal ion in the center of the molecule which interacts with electropositive compounds (electron donors) forming coordination complexes. It is believed that CO, NH3, and H 2 0 form coordination compounds whereas NO2 is an example of a charge transfer
O Y 0
I
I
I
2000
4000
6000
I
BO00
I
10000
f 02
Flgure 5. Relationship between the sensltlvity (S) and the square of frequency of CuPC for different frequencies at 150 OC.
complex formation. The electron affinity (3.6 eV for NOz, and smaller than 1 eV for the other gases) is an important parameter in this respect. Percolation of Gases into the Lattice. Crystal structure determinations of PCs reveal that the critical interstitial distance between layers of PC molecules is 2.7-3.2 A (28). It depends upon the critical size of the adsorbing gas molecules in relation to the affinity of the gases toward PC to what extent physisorption will take place. It is also observed that selectivity for NOz is higher at elevated temperatures, which means that the interaction energy for the N02-MPC charge transfer complex formation is the highest. At 150 OC CoPC is the most selective PC, followed by CuPC and H2PC, while at 70 "C CuPC is most selective, followed by HzPC and FePC. Reversibility. Usually, the response curves (Figure 6) show a reversible response for NOz. In case of NH3 a negative, almost irreversible and fatiguing response was observed. The sensitivity values in Table I11 and Table IV have been calculated from the initial response. So far no clear explanation can be given for this behavior. With chemiresistors negative responses for NH3 were also observed (41) indicating a conductivity effect in the SAW response. Response Times. The response of SAW chemosensors will be rate-controlled by the physical and chemical processes taking place at the chemical interface. The transduction
Table 111. Sensitivities (Hz/(ppm*pm)) at 150 "C 100 ppm
1200 ppm
co
COZ 200 ppm
CHI 400 ppm
75 45 67 133 33 80
0 0.1 -0.1
0 0
0 0 0
NO2
HzPC52 MgPC55 FePC59 CoPC54 NiPC62 CuPC53 PbPC65
0 0
0 0.1
0 0 0
0
0
"3
200 ppm
SO2 200 ppm
HZO 8000 ppm
C7HB 200 ppm
0
0.1
0 0 0
0 -0.2 0 0 0 -0.1
0 0 0 0 0
0 0
0 -2 -10 -1 -3 -7 -31
CHI 400ppm
200ppm
200ppm
SO2
H2O 8000ppm
C7H8 200ppm
-3
0 0
-0.02 -0.02
4 2
0.01
0.02 0.07
0 0 0 0 0
0
0
0 0
0.3 0 2.7
0 0
Table IV. Sensitivities (Hz/(ppm*pm)) at 70 "C NO2 100ppm HZPC52 MgPC55 FePC59 CoPC54 CuPC53 PbPC6O
49 44 452 138 352 140
co
1200ppm
COZ 200ppm
0 0
0
1.1 0
0.1
0 0.1
0 0.1
0 0
0 0 0 0 0 0
"3
-0
-13 -5 -4 -8
0 0
234
ANALYTICAL CHEMISTRY. VOL. 80. NO. 3. EBRUARY 1. 1988
-
I
I
i
I
CUPCIN02
*.-> . -,,*. . ..
: , .
-.:,I
Table V. Rim and Fall Times for NO, and Rim Tim- for CO and NH, at 150 "C (mid NOz(on) 100ppm HzPC52 MgPC.55 FePC59 CoPC54 NiPC62 CuPC53 PbPCGO
NOz(ofO 100ppm
4
11
7 3
32 13
47
9 2
25 16 26
CO 2OOppm
200ppm
5
I8
a"
16 16
9 15 7 55
mechanism at the electrodes is a comparatively fast process (milliseconds). The processes are complex and i t is hard to establish the rate-controlling effeete. Mixing in the measuring cell, the diffusion (film diffusion, pore diffusion (physisorption)) and the chemical processes (interaction with the MPC and replenishment of NOz from N,OJ can be rate limiting. In Table V the rise times to reach 80% of the Rnal resmnse (Lao) at 150 'C are given for NO2, NH3, and CO. In addition, for NOz also the 80% fall times are given. Normally, the fall times are a factor 2-5 slower than the rise times possibly as e result of the affinity for NO?. However, in case of CoPC the rise time is extremely slow (47 min) whereas the fall time is much faster (25 min). Maybe, a relation exists between the exceptional behavior of CoPC and its good selectivity and sensitivity. The rise and fall times were found to increase with increasing concentrations. Further, no correlation could he found between the response times and the various PCs or the thickness of the layers. At 150 OC CuPC shows the fastest rise and fall times followed by FePC and HzPC. In the case of NOz the response curves indicate two mechisma. Other gases show o r d i i , first-order responsea, which are usually slower than the NOz response. The size of the gas molecules in relation to crystal structure parameters could be important. The response times are generally longer a t lower temperatures. Surprisingly, in the case of N H , response is faster at lower temperatures which can possibly be related to the irreversible and the fatiguing response.
Figure 8. Scanning electron micrographs of the PbFC layer before (left)and after (right) the measurements. In Figure 7 the relation between both the rise and fall time and temperature is given. Assuming first-order kinetics the plot In tgovs 1000/Tshould show a straight line whose slope is a measure for the activation energy. As can be seen from Figure 7 the fall times meet this assumption. With the rise times the plot indicates two mechanisms which become rate limiting in the different temperature domains. Stability. With PbPC a difference in morphology was observed between a freshly prepared layer and that m e layer after all the measurements. In Figure E both SEM pictures are shown. A reaction in a NO.-containine atmosohere at elevated temperatures is suspected to cause the destruction of the PbPC layer. This breakdown phenomenon is also illustrated by the peculiar response curve (Figure 6). Although the layer stayed intact in the case of FePC, a change in color from blue to green could be observed during the measurements. This phenomenon is also reflected by the large negative drift in the frequency signal of FePC. Upon prolonged heat treatment in NOz (50 h at 150 "C and loo0 ppm NO,) a similar breakdown phenomenon as with PbPC was observed. The negative drift of CuPC70 is not the result of thermal breakdown but is c a d by the electronic system which was not adapted to 100 MHz frequency at the time. In cnse of PbPC at 150 'C (Figure 6 ) and to a minor extent also in the case of FePC, the response for NOz has a peculiar shape. It indicates an ordinary reversible response and a much slower irreversible response (dashed lines). This codd be related to the change in morphology observed with PbPC layers during the measurements (Figure E). Jones and Walsh (42) found irreversible effects as well with PbPC layers on chemiresistors. This effect could be eliminated via inter1
ANALYTICAL CHEMISTRY, VOL. 60, NO. 3, FEBRUARY 1, 1988
mittent measuring. In our case, however, it must be concluded that PbPC cannot be used due to its irreversible behavior. Phthalocyanine layers are known to be also destructed by sublimination at temperatures higher than 150 "C. Therefore, in the future, chemically immobilized phthalocyanine derivatives will be used to obtain higher stability.
CONCLUSIONS With respect to the development of a SAW chemosensor for NOz, the following conclusions can be drawn accompanied with some final remarks: At 150 "C CoPC is preferred for selectivity and sensitivity, while CuPC is preferred for response time. As CuPC was second best in selectivity and sensitivity, this PC will be chosen for further development. The response times are still rather long for sensor applications at the moment. It is expected that thinner layers will lead to shorter response times. A corresponding decrease in sensitivity might be compensated by operating the sensor at higher frequencies. Also, higher temperatures might be applied to reduce the response times. However, this is limited due to the physical stability of the PC layers. Therefore further developments will combine the thin layer concept with increased temperature stability by chemical immobilization of CuPC derivatives at a high-frequency SAW chemosensor. It is concluded that the transduction mechanism of SAW chemosensors is a combination of changes in mass and in conductivity. Further, at the chemical interface several chemical and physical processes occur which make the sensor very complicated to describe in a model. ACKNOWLEDGMENT The authors wish to thank Dr. Venema and Mr. Vellekoop of the Department of Electrical Engineering of Delft University of Technology, the Netherlands, and Mr. Nieuwkoop of the Institute of Applied Physics TNQ-TH, Delft, The Netherlands, for their helpful discussions and amicable collaboration within our multidisciplinary project team. Mr. Duvalois of the Prins Maurits Laboratory is gratefully acknowledged for measuring and interpreting the scanning electron micrographs. Registry No. H2PC, 574-93-6; MgPC, 1661-03-6; FePC, 13216-1; COPC,3317-67-7; NPC, 1405502-8; PbPC, 15187-16-3;NOz, 10102-44-0; CO, 630-08-0; C02, 124-38-9; CHI, 74-82-8; NHB, 7664-41-7; SO2, 1446-09-5; HzO, 7732-18-5; toluene, 108-88-3.
LITERATURE CITED (1) Klng, W. H., Jr. Anal. Chem. 1984,36, 1735-1739. (2) Hlavay, J.; Qullbault, G. 0. Anal. Chem. 1977, 49, 1890-1898 and references clted. (3) Alder, J. F.; McCallum, J. J. Analyst (London) 1983, 108,1169-1189 and references cited. (4) WohltJen, H.; Dessy, R. Anal. Chem. 1970,51, 1458-1478. (5) Wohltjen, H. Sens . Actuators 1084,5 , 307-324. (6) Bryant, A.; Lee, D. L.; Vetellno, J. F. Proceedings of the IEEE Ultrasonics Symposium, Chlcago, IL, 1981, 171-174. (7) Bryant, A.; Polder, M.; Riley, 0.; Lee, D. L.; Vetellno, J. F. Sens. Actuatws 1983,4 , 105-111. (8) D'Amlco. A.; Palma, A.; Verona, E. Sens. Actuators 1982/1983,3 , 31-39.
9
235
(9) Martin, S. J.; Schweizer, K. S.; Schwa*,
S. S.; Gunshor, R. L. Proceedings of the IEEE Ultrasonics Symposium, Dallas, TX, 1984, 207-212. (10) Snow, A.; Wohltjen, H. Anal. Chem. 1984,56, 1411-1416. (11) Wohltjen, H.; Snow, A.; Ballantine, D. Proceedings of the 3rd International Conference on Sensors and Actuators, F%lladelphia, PA, 1985, 66-70. (12) Veteiino, J. F.; Lade, R.; Falconer, R. S. Proceedings of the 2nd International Meeting on Chemical Sensors, Bordeaux, 1986, 688-691. (13) DAmlco, A.; Gentilli, M.; Verardi, P.; Verona, E.; Proceedings of the 2nd International Meeting on Chemical Sensors, Bordeaux, 1986, 743-746. (14) Rlcco. A. J.; Martin, S. J.; Zlpperian, T. E. Sens. Actuators 1985, 8 , 319-333. (15) Nieuwenhuizen, M. S.; Barendsz, A. W. Sens. Actuators 1987, 11, 45-62. (16) Barendsz, A. W.; Vis, J. C.; Nieuwenhuizen, M. S.; Nieuwkoop, E.; Vellekoop, M. J.; GhlJsen. W. J.; Venema, A. Proceedings of IEEE Ultrasonics Symposium, San Francisco, CA, 1985, 586-590. (17) Nieuwenhuizen. M. S.; Barendsz, A. W.; Nieuwkoop, E.; Vellekoop, M. J.; Venema, A. Electron. Lett. 1988,22, 184-185. (18) Venema, A.; Nleuwkoop, E.; Veiiekoop, M. J.; Ghljsen, W. J.; Barendsz, A. W.; Nieuwenhuizen, M. s. I€€€ Trans. Uttrason ., Ferroelectrics Freq. Control 1087,34, 148-155. (19) Venema, A.; Nieuwkoop, E.; Vellekoop, M. J.; Barendsz, A. W.; Nieuwenhuizen, M. S. Sens. Actuators 1986, IO, 47-64. (20) Oirschot, Th. G. J. van; Leeuwen, D. van; Medema, J. J . Nectroanal. Chem. 1972,37, 373-385. (21) Helland. G.; Kohl, D. Proceedings of the 3rd International Conference on Sensors and Actuators, Philadelphla, PA, 1985, 260-263. (22) Jones, T. A.; Bott, B. Proceedings of the 3rd International Conference on Sensors and Actuators, Philadelphia, PA, 1985, 414-417. (23) Duerselen, L.; Mockert, H.; Schmeisser, D.; Goepel, W. Proceedings of the 2nd International Meeting on Chemical Sensors Bordeaux, 1986, 193- 194. (24) Jones, T. A.; Bott, B. Proceedings of the 2nd International Meeting on Chemical Sensors, Bordeaux, 1986, 167-169. (25) Blanc, J. P.; Blasquez, G.; Germain, J. P.; Larbi, A.; Maleyson, C.; Robert, H. Proceedlngs of the 2nd International Meeting on Chemical Sensors, Bordeaux, 1986, 171-174. (26) Gentry, S. J.; Waish, P. T. Proceedings of the 2nd International Meeting on Chemical Sensors, Bordeaux, 1986, 209-212. (27) Ewyk. R. L. van; Chadwick, A. V.; Wright, J. D. J . them. SOC.,Faraday Trans. 11980, 76, 2194-2205. (28) Wright, J. D.; Chadwick. A. V.; Meadows, 8.; Miasik, J. J. Mol. Cryst. Llq. Cryst. 1983,93, 315-325. (29) Jones, T. A.; Bott, 6.; Hurst, N. W.; Mann, B. Proceedings of the International Meeting on Chemical Sensors, Fukuoka. 1983, 90-94. (30) Bott, B.; Jones, T. A. Sens. Actuators 1984,5 , 43-53. (31) Jones, T. A.; Bott, B. Sens. Actuators 1986,9 , 27-37. (32) Chadwick, A. V.; Dunning, P. B. M.; Wright, J. D. Mol. Cryst. Liq. Cryst. 1086, 134, 137-153. (33) Honeybourne, C. L.; Ewen, R. J. J . fhys. Chem. Solids 1983, 44, 2 15-223. (34) Honeybourne, C. L.; Ewen, R. J. J. fhys. Chem. Solids 1983, 44, 833-838. (35) Honeybourne, C. L.; Ewen, R. J.; Hill, C. A. S. J. Chem. Soc., Feraday Trans. 11084,8 0 , 851-863. (36) Siebert, E.; Fouletier, J.; LeMoigne, J. Proceedings of the 2nd International Meeting on Chemical Sensors, Bordeaux, 1986, 281-284. (37) Mason, R.; Wlliiams, G. A.; Fielding, P. E. J . Chem. SOC.,Dalton Trans. 1979,676-683, and references cited. (38) Steinbach, F.; Schmidt, H. H. J. Catal. 1975,39, 190-196. (39) Sadaoka, Y.; Sakai, Y.,; Yamazoe, N.; Seiyama, T. Denk; Kagaku Oyob/ K q y o Butsurl Kagaku 1982,50, 457-462. (40) Schaffer, A. M.; Gouterman, M.; Davidson, E. R. Theor. Chlm. Acta 1973,30, 9-30. (41) Barger, W. R.; Wohitjen, H.;Snow, A. W.; Lint, J.; Jarvis, N. L. ACS Symp. Ser. 1988,No. 309, 155-165. (42) Jones, T. A., Walsh, P. T. (UK Health and Safety Executive), personal communication, July 1986.
RECEIVEDfor review December 22,1986. Accepted September 14, 1987. Financial support was given by the TNO Center for Microelectronics (Delft, The Netherlands). This work was presented as a poster at the Second International Meeting on Chemical Sensors, Bordeaux, 1986.
