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SAW devices-sensitivity enhancement in going from 80 MHz to 1 GHz
Sensors and Actuators B 46 (1998) 120 – 125
SAW devices-sensitivity enhancement in going from 80 MHz to 1 GHz Franz L. Dickert a,*, Peter Forth a, Wolf-Eckehard Bulst b, Gerhard Fischerauer b, Ulrich Knauer b b
a Institute of Analytical Chemistry, Vienna Uni6ersity, Wa¨hringer Straße 38, A-1090 Vienna, Austria Siemens Corporate Research and De6elopment, ZT KM 1, Otto Hahn Ring 6, D-81739 Munich, Germany
Received 26 August 1997; received in revised form 16 January 1998; accepted 19 January 1998
Abstract SAW-devices with the highest resonance frequency (1 GHz) ever reported in chemical sensor applications have been tested. The sensor response increases in a parabolic manner with the resonance frequency, as is shown, in going from 80 MHz to 1 GHz, whereas the noise level only increases approximately linearly. If a linked cyclodextrin in a height of 60 nm is used as a sensitive coating, a sensor response of 61 kHz/1000 ppm of toluene is obtained. This makes the detection of aromatic hydrocarbons, such as toluene or xylene, down to 1 ppm easily possible. © 1998 Elsevier Science S.A. All rights reserved. Keywords: SAW devices; Supramolecular chemistry; Cyclodextrins; Aromatic hydro-carbons
1. Introduction Due to their high sensitivity, SAW-devices of resonance frequencies ranging from approximately 100–600 MHz have been widely used as physical [1 – 3] and chemical sensors [4– 7], e.g. as temperature [8] or acceleration [9] sensors or gas detectors [10]. In combination with the concept of host – guest-chemistry [11], their high mass sensitivity (in the range of femto grams [12]) can lead to powerful solvent vapour detectors. The latter can be used for workplace-monitoring, pipelineleak-detection [13], filling station surveillance or similar applications. The use of analyte-adapted molecular cavities [14] as tailored sensor materials offers the advantage of analyte specific interactions. Optimized layers yield detection limits down to the ppm-range and an appreciable selectivity enhancement. Furthermore, the sensitivity might be increased by varying the resonance frequencies of the devices, due to the sensor response to mass loadings being a function of the oscillation frequency of the SAW. In this paper we have studied whether the sensitivities or the signal-to-noise ratios of * Corresponding author. Tel.: + 431 313672417; fax: +431 3196312; e-mail: [email protected] 0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00097-5
coated devices decrease or increase with rising frequencies [15]. In any case, an optimized sensitivity allows the reduction of the layer height, resulting in faster absorption--desorption processes, due to the diffusion of the analytes within the layer being hindered less. Consequently, the response times of SAW-devices could be made shorter in the course of sensitivity enhancement.
2. Experimental
2.1. De6ices Two-port dual SAW-resonators with resonance frequencies ranging from 80 MHz up to 1 GHz were used. The dual set-up renders the elimination of device effects due to temperature or humidity fluctuations possible by differential measurements between coated and uncoated devices. Since both resonators are on the same quartz substrate, deviations caused by the manufacturing process can be excluded. The piezoelectric material was ST-cut quartz, the aluminum electrodes were prepared by a photolithographical process. The innovative 1 GHz resonators were fabricated on 37.5° rot Y,Xquartz and consisted of an almost periodic arrangement
F.L. Dickert et al. / Sensors and Actuators B 46 (1998) 120–125
of 500 reflectors and 100 interdigital transducer (IDT) electrodes. These quarter-wavelength fingers were made by an aluminum metalization in a thickness of 1.8% relative to the acoustic wavelength. Without a chemically sensitive coating, the devices featured an unmatched insertion loss of 7.5 db and a 3-db bandwidth of 290 kHz, corresponding to a loaded Q of 3500. Close to the main resonance, this can be modelled by a four terminal device with two common terminals at the input and output and the ‘hot’ input and output terminals connected by a series resonant circuit (R = 120 V, L =132 mH, C =0.2fF) in parallel with a static capacitance (C0) (C0 =2.1 pF). However, to obtain more detailed information on the static characteristics, e.g. the influence of longitudinal modes in the acoustic cavity on the broadband behaviour, a fully fledged electro–acoustic analysis was run based on the P-matrix method. The layout of the devices was especially adapted to sensor applications. The resonators were designed to operate at frequencies differing by 1 MHz to suppress any injection locking between the sensor and reference oscillators. Finally, the bond pads were laid out so that no bond wires shaded the area to be coated, and the chips were mounted into a TO-8 metal package in a configuration that led to the smallest crosstalk between the resonators.
2.2. Layers The b-cyclodextrins [16] have proven their qualities as sensor materials for organic solvent vapour detection [17]. The hydrophobic hydrocarbon walls of the conelike cavity are well-suited for Van-der-Waals-interactions (Fig. 1). Bridged cyclodextrins offer improvements in sensor characteristics and long-term-stability of the formed polymers [18]. Permethylated b-cyclodextrins bridged with diiodooctane and hexafluoro benzene were taken. The hexafluorobenzene-bridged b-cyclodextrin
Fig. 1. Structure of b-cyclodextrin.
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was used to determine the signal-to-noise ratios of different resonators. Response time measurements with devices coated with varying layer heights were carried out with the octylbridged b-cyclodextrin. The permethylation reduces the hydrophilic properties of the polymer leading to minimized cross-sensitivities of the sensor to water. The synthesis of the octyl-bridged b-cyclodextrin-polymer has been described previously [19]. The hexafluorobenzene-bridged b-cyclodextrin was synthesized in analogy to [20,21]. In a solution of dimethylformamide, the ß-cyclodextrin was deprotonated with sodium hydride. After the addition of hexafluorobenzene, the reaction mixture was heated at reflux for six hours. The permethylation of the bridged cavities was performed according to a procedure described in [22].
2.3. Measurements The frequency responses of the devices were measured by a HP 8752 C network analyzer. The frequency changes at the − 3 dB-point at a higher frequency relative to the resonance maximum were monitored as a function of time. The − 3dB-point was determined in a 5× 104 Hz frequency scan (1 GHz SAW: 105 Hz) consisting of 1601 discrete points leading to a digital resolution of 31 Hz. For the LOD-measurements (limits of detection), the frequency span was decreased to obtain a better digital resolution. To increase the signal to noise ratio, a 1000 Hz bandwidth filter, a 1 or 2% smooth relative to the frequency span and average factors of 10/16/20 were applied. In addition, all resonators with the exception of the Gigahertz-SAW, were placed in an oscillator circuit with the device as a frequency-determining component to certify that the obtained results are not influenced by the network-analyzer measurement procedure. The resonance frequency of the oscillator was measured by a HP 5385 frequency counter. The resonators were placed into a thermoregulated gas flow cell and exposed to a gas stream. To obtain a gas flow containing the desired solvent vapour concentrations and relative humidity, a gas mixing apparatus was used as described elsewhere [14]. The exactness of the mixed concentrations has been confirmed by FT-IR-measurements [23]. The relative humidity was kept at 10%. To compare the sensor effects of the different resonators, all devices were coated with a layer in a height of 60 nm (permethylated b-cyclodextrin bridged with hexafluorobenzene). These SAW-resonators were exposed to a constant gas stream containing pulses of 1000 ppm toluene. Additionally, the noise traces of all uncoated devices were measured to determine the different signal to noise ratios. In another series of measurements, all resonators were coated with 40 nm of a molecular cavity (permethylated b-cyclodextrin bridged
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with hexafluorobenzene). Consequently, the lowest detectable concentration change was determined. Furthermore, the response times were measured as a function of the layer height (sensor material: permethylated b- cyclodextrin bridged with diiodooctane).
3. Results and discussion The sensor responses are square correlated to the device’s resonance frequency (Eq. (1)) Df = (k1 + k2)rlhl f 20 − k2hl f 20(4m/V 2R)[(l + m)/(l + 2m)] (1) The equation describes the sensor response Df of a SAW device coated with a thin, isotropic, nonconducting layer, where k1 and k2 are material constants for the quartz substrate, rl is the density of the layer, hl the film thickness, f0 the fundamental frequency, m the shear modulus of the film material, VR the acoustic wave velocity, and l, the Lamb constant. The first term of the equation represents the frequency shift due to mass loadings, whereas the second term describes the effect of changes in the elastic properties of the film on the resonance frequency of the device. The influence of changes of the elastic properties can be neglected for most materials. On this account, Eq. (1) can be simplified by omitting the second term and inserting the material constants: Df = − 1.26×106f 20hlrl
(2)
From Eq. (2), the frequency responses due to mass loadings rise in a parabolic manner with rising resonance frequencies can be deduced. Depending on changes of the noise level, the sensitivity and limits of detectability of the device could alter with varying resonance frequencies. The expectation, that there is a square relationship between sensor response and resonance frequency, is about to meet the experimental results depicted in Fig. 2. The frequency shifts shown in Fig. 3 demonstrate this fact, a 1 GHz device give a 5.6-fold higher frequency response than the 433 MHz-resonator. Both devices were coated homogeneously in a thickness of 60 nm of a molecular cavity (permethylated b-cyclodextrin bridged with hexafluorobenzene). As shown in Fig. 4, the noise level of a GHz-resonator, for example, is approximately twice as high as the one of a 433 MHz-SAW in case of the coated device. Measuring uncoated devices, the noise level doubles, also (Fig. 5). The noise amplitude of the uncoated GHz-device might be influenced by temperature fluctuations. At 20°C, a temperature change of 9 0.01°C (exactitude of the used thermoregulator)
Fig. 2. Sensor response Df in kHz of SAW-devices to pulses of 1000 ppm toluene as a function of the initial frequency f0; all resonators were coated with a molecular cavity (permethylated ß-cyclodextrin linked with hexafluorobenzene) in a thickness of 60 nm.
causes frequency shifts of 1.4 Hz (433-MHz-SAW) and 13 Hz (1-GHz-SAW). Therefore, some reduction of noise might be achieved by a differently cut quartz as substrate material with a smaller temperature coefficient at room temperature. Taking into account the frequency shifts (Fig. 3) and noise levels (Fig. 4) of the coated devices described above, detection limits of toluene are expected to be a few ppm. These favourable findings can further be improved by using a distinct coating technique. The sensor material which is
Fig. 3. Frequency response Df of a 1 GHz-SAW (continuous line) and a 433 MHz-SAW (dashed line) to pulses of 1000 ppm toluene, both devices coated with a molecular cavity, permethylated b-cyclodextrin linked with hexafluorobenzene, in a thickness of 60 nm, r.H. 10%.
F.L. Dickert et al. / Sensors and Actuators B 46 (1998) 120–125
Fig. 4. Noise trace of a 1 GHz-SAW (upper part) and a 433 MHz-SAW-resonator (lower part); both devices coated with 60 nm permethylated b-cyclodextrin, linked with hexafluorobenzene, r.H.10%.
necessary for a homogeneous coating of the SAW in a layer height of 40 nm was applied only to the IDT region (coating material: permethylated b-cyclodextrin bridged with hexafluorobenzene). Figs. 6 and 7 show the sensor responses to steps of 1 ppm m-xylene. In case of the 1 GHz device (Fig. 6), steps of 1 ppm can clearly be resolved, the signal to noise ratio being 90 Hz: 30 Hz meets the IUPAC-convention. In contrast to that, the 433 MHz-resonator is not able to resolve the 1 ppm steps significantly (Fig. 7), the frequency responses are only gradually visible in the noise, the signal to noise ratio is slightly better than 1:1. Therefore, the detection limits are lower in case of the 1 GHz-resonator by a factor of approximately 2 in comparison to the 433 MHz device. These LODs are lower compared to those where the sensor layers
Fig. 5. Noise trace of uncoated 1GHz-SAW-and 433 MHz-SAWdevices, r.H. 10%.
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Fig. 6. Frequency response Df of a 1GHz-SAW to pulses of 1 ppm m-xylene, coated with a molecular cavity, permethylated b-cyclodextrin linked with hexafluorobenzene, in an average thickness of 40 nm.
are applied to the whole device as described above (layer height: 60 nm). If only the IDT area is coated, a more favourable noise level results which leads to an improved signal to noise ratio, since the development of the Rayleigh waves is not disturbed by incidental inhomogeneities in the reflector area caused by the sensitive layer. The response times of the sensitive layers were studied with permethylated b-cyclodextrins, highly linked with octyl-bridges thus resulting in a rather rigid structure of the polymer. These materials have shown a high stability for chemical sensing and could be tested successfully for one year. However, some minutes are observed as t90 times, which is due to diffusion processes. Additionally, the entrance of the
Fig. 7. Frequency response Df of a 433 MHz-SAW to pulses of 1 ppm m-xylene, coated with a molecular cavity, permethylated b-cyclodextrin linked with hexafluorobenzene, in an average thickness of 40 nm, r.H. 10%.
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molecular hollows might be blocked by neighboured cavities and therefore some sterical rearrangements are necessary for analyte inclusion. These problems are solved by ultrathin layers, which yield a satisfying sensitivity with 1 GHz resonators, even in the case of monolayers. Consequently, the combination of monolayers and resonators with ultrahigh resonance frequencies is a promising strategy improving the sensitivity and the response characteristics of mass sensitive chemical sensors.
4. Conclusions An increase of the initial frequency of mass-sensitive transducers results in a higher sensitivity of the device. Consequently, the limits of detectability can be lowered by rising the oscillation frequency of the device. Additionally, high frequency devices due to their higher mass-sensitivity favour the use of thinner coatings down to monolayers, leading to shorter response times. The use of devices with higher oscillation frequency is therefore a promising strategy, only limited by the photolithographical process. Today, the smallest electrode distance feasible by photo-lithographical means is approximately half a micron. This leads to devices with frequencies up to 2.5 GHz.
References [1] M.R. Risch, Precision pressure sensor using quartz SAW resonators, Sensors and Actuators 6 (1984) 127–133. [2] E. Gatti, A. Palma, E. Verona, A surface acoustic wave voltage sensor, Sensors and Actuators 4 (1983) 45–54. [3] R. Dahint, M. Grunze, F. Josse, J.C. Andle, Probing of strong and weak electrolytes with acoustic wave fields, Sensors and Actuators B 9 (1992) 155–162. [4] J.W. Grate, S.J. Martin, R.M.White, Acoustic TSMSAWFPWAPM Wave Microsensors PartI/II, Anal. Chem. 65 (1993) 940A-948A, 987A-995A. [5] M.S. Nieuwenhuizen, A.J. Nederlof, A SAW gas sensor for carbon dioxide and water. Preliminary experiments, Sensors and Actuators B 2 (1990) 97–101. [6] M.S. Nieuwenhuizen, A.J. Nederlof, A silicon-based SAW chemical sensor for NO2 by applying a silicon nitride passivation layer, Sensors and Actuators B 9 (1992) 171–176. [7] H. Wohltjen, R.E. Dessy, Surface acoustic wave probe for chemical analysis. I. Introduction and instrument description, Anal. Chem. 51 (1979) 1458–1475. [8] L. Mingfang, L. Haiguo, SAW temperature and humidity sensor with high resolution, Sensors and Actuators B 12 (1993) 53 – 56. [9] G. Sugazaki, T. Takenaka, K. Sakata, K. Toda, Motion characteristics measurement of rotating object using surface acoustic wave oscillator, Jpn. J. Appl. Phys. 32 (1993) 4237–4240. [10] A. Mandelis, C. Christofides, Physics, Chemistry and Technology of Solid State Gas Sensor Devices, Wiley, New York, 1993.
[11] F.L. Dickert, A. Haunschild, Sensor materials for solvent vapor detection: donor – acceptor and host – guest interactions, Adv. Mater. 12 (1993) 887 – 895. [12] H. Wohltjen, D.S. Ballentine, Surface acoustic wave devices for chemical analysis, Anal. Chem. 61 (1989) 704A – 715. [13] F.L. Dickert, M. Zenkel, W.-E. Bulst, G. Fischerauer, U. Knauer, Fullerene/liquid crystal mixtures as QMB-and SAWcoatings-detection of diesel- and solvent-vapours, Fresenius J. Anal. Chem. 357 (1997) 27 – 31. [14] F.L. Dickert, U.P.A. Ba¨umler, H. Stathopulos, Mass-sensitive solvent vapor detection with Calix [4] resorcinarenes: tuning sensitivity and predicting sensor effects, Anal. Chem. 69 (1997) 1000 – 1005. [15] M. Rapp, D. Binz, I. Kabbe, M. von Schickfus, S. Hunklinger, H. Fuchs, W. Schrepp, B. Fleichmann, A new high-frequency high-sensitivity SAW device for NO2 gas detection in the subppm range, Sensors and Actuators B 4 (1991) 103 – 108. [16] I. Tabushi, Cyclodextrin catalysis as a model for enzyme action, Acc. Chem. Res. 15 (1982) 66 – 72. [17] F.L. Dickert, P.A. Bauer, The detection of halogenated hydrocarbons via host – guest chemistry — A mass-sensitive sensor study with QMB- and SAW-devices, Adv. Mater. 3 (1991) 436 – 438. [18] F.L. Dickert, T. Bruckdorfer, H. Feigl, A. Haunschild, V. Kuschow, E. Obermeier, W.E. Bulst, U. Knauer, G. Mages, Supramolecular detection of solvent vapours with QMB and SAW devices, Sensors and Actuators B 13-14 (1993) 297–301. [19] F.L. Dickert, U. Geiger, M. Keppler, M. Reif, W.E. Bulst, U. Knauer, G. Fischerauer, Supramolecular receptors for SAW and QMB devices-solvent recognition evaluated by FT-IR-spectroscopy and the BET-adsorption analysis, Sensors and Actuators B 26-27 (1995) 199 – 202. [20] J. Burdon, V.A. Damodaran, J.C. Tatlow, Polyfluoroaromatic Compounds. Part XV. The reaction of hexafluorobenzene with bifunctional nucleophiles. J. Chem.Soc. (1964)763-771. [21] G.G. Yakobson and V.M. Vlasov, Recent Synthetic Methods for Polyfluoroaromatic Compounds, Synthesis (1976) 652-672. [22] J. Szejtli, A. Lipta´k, I. Joda´l, P. Fu¨gedi, P. Na´na´siand, A. Neszme´lyi, Synthesis and 13C-NMR-spectroscopy of methylated beta-cyclodextrins, Starch 5 (1980) 165 – 169. [23] F.L. Dickert, A. Haunschild, V. Kuschow, M. Reif, H. Stathopulos, Mass-sensitive detection of solvent vapors. Mechanistic studies on host--guest sensor principles by FT-IR spectroscopy and BET-adsorption analysis, Anal. Chem. 68 (1996) 1058 – 1061.
Biographies F.L Dickert studied chemistry at the University of Erlangen/Germany and received his Ph.D in 1970 on a study of fast reactions in non-aqueous solutions. He was appointed as Professor of Physical Chemistry in 1980 and his main field of research was spectroscopic studies, especially NMR, about solvation effects concerning metal complexes and carbenium ions. Since 1990, he has focused his activities on the development of chemically sensitive coatings according to supramolecular chemistry. In 1994, he accepted a call to a chair of Analytical Chemistry at the University of Vienna.
F.L. Dickert et al. / Sensors and Actuators B 46 (1998) 120–125
P. Forth received his diploma degree in chemistry from the University of Erlangen/Germany in 1994 and his Ph.D from Vienna University in 1997. He has worked in the sensor group of F.L Dickert since 1994. W.-E. Bulst is a senior director in the Corporate Technology of Siemens in Munich/Germany and heads the
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department of surface–acoustic-wave technology and wireless sensors. U. Knauer and G. Fischauer received their Ph.D.’s from the Technical University of Munich and are responsible for projects on SAW devise fabrication and SAW sensors in extreme environments, respectively.
SAW devices-sensitivity enhancement in going from 80 MHz to 1 GHz Franz L. Dickert a,*, Peter Forth a, Wolf-Eckehard Bulst b, Gerhard Fischerauer b, Ulrich Knauer b b
a Institute of Analytical Chemistry, Vienna Uni6ersity, Wa¨hringer Straße 38, A-1090 Vienna, Austria Siemens Corporate Research and De6elopment, ZT KM 1, Otto Hahn Ring 6, D-81739 Munich, Germany
Received 26 August 1997; received in revised form 16 January 1998; accepted 19 January 1998
Abstract SAW-devices with the highest resonance frequency (1 GHz) ever reported in chemical sensor applications have been tested. The sensor response increases in a parabolic manner with the resonance frequency, as is shown, in going from 80 MHz to 1 GHz, whereas the noise level only increases approximately linearly. If a linked cyclodextrin in a height of 60 nm is used as a sensitive coating, a sensor response of 61 kHz/1000 ppm of toluene is obtained. This makes the detection of aromatic hydrocarbons, such as toluene or xylene, down to 1 ppm easily possible. © 1998 Elsevier Science S.A. All rights reserved. Keywords: SAW devices; Supramolecular chemistry; Cyclodextrins; Aromatic hydro-carbons
1. Introduction Due to their high sensitivity, SAW-devices of resonance frequencies ranging from approximately 100–600 MHz have been widely used as physical [1 – 3] and chemical sensors [4– 7], e.g. as temperature [8] or acceleration [9] sensors or gas detectors [10]. In combination with the concept of host – guest-chemistry [11], their high mass sensitivity (in the range of femto grams [12]) can lead to powerful solvent vapour detectors. The latter can be used for workplace-monitoring, pipelineleak-detection [13], filling station surveillance or similar applications. The use of analyte-adapted molecular cavities [14] as tailored sensor materials offers the advantage of analyte specific interactions. Optimized layers yield detection limits down to the ppm-range and an appreciable selectivity enhancement. Furthermore, the sensitivity might be increased by varying the resonance frequencies of the devices, due to the sensor response to mass loadings being a function of the oscillation frequency of the SAW. In this paper we have studied whether the sensitivities or the signal-to-noise ratios of * Corresponding author. Tel.: + 431 313672417; fax: +431 3196312; e-mail: [email protected] 0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00097-5
coated devices decrease or increase with rising frequencies [15]. In any case, an optimized sensitivity allows the reduction of the layer height, resulting in faster absorption--desorption processes, due to the diffusion of the analytes within the layer being hindered less. Consequently, the response times of SAW-devices could be made shorter in the course of sensitivity enhancement.
2. Experimental
2.1. De6ices Two-port dual SAW-resonators with resonance frequencies ranging from 80 MHz up to 1 GHz were used. The dual set-up renders the elimination of device effects due to temperature or humidity fluctuations possible by differential measurements between coated and uncoated devices. Since both resonators are on the same quartz substrate, deviations caused by the manufacturing process can be excluded. The piezoelectric material was ST-cut quartz, the aluminum electrodes were prepared by a photolithographical process. The innovative 1 GHz resonators were fabricated on 37.5° rot Y,Xquartz and consisted of an almost periodic arrangement
F.L. Dickert et al. / Sensors and Actuators B 46 (1998) 120–125
of 500 reflectors and 100 interdigital transducer (IDT) electrodes. These quarter-wavelength fingers were made by an aluminum metalization in a thickness of 1.8% relative to the acoustic wavelength. Without a chemically sensitive coating, the devices featured an unmatched insertion loss of 7.5 db and a 3-db bandwidth of 290 kHz, corresponding to a loaded Q of 3500. Close to the main resonance, this can be modelled by a four terminal device with two common terminals at the input and output and the ‘hot’ input and output terminals connected by a series resonant circuit (R = 120 V, L =132 mH, C =0.2fF) in parallel with a static capacitance (C0) (C0 =2.1 pF). However, to obtain more detailed information on the static characteristics, e.g. the influence of longitudinal modes in the acoustic cavity on the broadband behaviour, a fully fledged electro–acoustic analysis was run based on the P-matrix method. The layout of the devices was especially adapted to sensor applications. The resonators were designed to operate at frequencies differing by 1 MHz to suppress any injection locking between the sensor and reference oscillators. Finally, the bond pads were laid out so that no bond wires shaded the area to be coated, and the chips were mounted into a TO-8 metal package in a configuration that led to the smallest crosstalk between the resonators.
2.2. Layers The b-cyclodextrins [16] have proven their qualities as sensor materials for organic solvent vapour detection [17]. The hydrophobic hydrocarbon walls of the conelike cavity are well-suited for Van-der-Waals-interactions (Fig. 1). Bridged cyclodextrins offer improvements in sensor characteristics and long-term-stability of the formed polymers [18]. Permethylated b-cyclodextrins bridged with diiodooctane and hexafluoro benzene were taken. The hexafluorobenzene-bridged b-cyclodextrin
Fig. 1. Structure of b-cyclodextrin.
121
was used to determine the signal-to-noise ratios of different resonators. Response time measurements with devices coated with varying layer heights were carried out with the octylbridged b-cyclodextrin. The permethylation reduces the hydrophilic properties of the polymer leading to minimized cross-sensitivities of the sensor to water. The synthesis of the octyl-bridged b-cyclodextrin-polymer has been described previously [19]. The hexafluorobenzene-bridged b-cyclodextrin was synthesized in analogy to [20,21]. In a solution of dimethylformamide, the ß-cyclodextrin was deprotonated with sodium hydride. After the addition of hexafluorobenzene, the reaction mixture was heated at reflux for six hours. The permethylation of the bridged cavities was performed according to a procedure described in [22].
2.3. Measurements The frequency responses of the devices were measured by a HP 8752 C network analyzer. The frequency changes at the − 3 dB-point at a higher frequency relative to the resonance maximum were monitored as a function of time. The − 3dB-point was determined in a 5× 104 Hz frequency scan (1 GHz SAW: 105 Hz) consisting of 1601 discrete points leading to a digital resolution of 31 Hz. For the LOD-measurements (limits of detection), the frequency span was decreased to obtain a better digital resolution. To increase the signal to noise ratio, a 1000 Hz bandwidth filter, a 1 or 2% smooth relative to the frequency span and average factors of 10/16/20 were applied. In addition, all resonators with the exception of the Gigahertz-SAW, were placed in an oscillator circuit with the device as a frequency-determining component to certify that the obtained results are not influenced by the network-analyzer measurement procedure. The resonance frequency of the oscillator was measured by a HP 5385 frequency counter. The resonators were placed into a thermoregulated gas flow cell and exposed to a gas stream. To obtain a gas flow containing the desired solvent vapour concentrations and relative humidity, a gas mixing apparatus was used as described elsewhere [14]. The exactness of the mixed concentrations has been confirmed by FT-IR-measurements [23]. The relative humidity was kept at 10%. To compare the sensor effects of the different resonators, all devices were coated with a layer in a height of 60 nm (permethylated b-cyclodextrin bridged with hexafluorobenzene). These SAW-resonators were exposed to a constant gas stream containing pulses of 1000 ppm toluene. Additionally, the noise traces of all uncoated devices were measured to determine the different signal to noise ratios. In another series of measurements, all resonators were coated with 40 nm of a molecular cavity (permethylated b-cyclodextrin bridged
122
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with hexafluorobenzene). Consequently, the lowest detectable concentration change was determined. Furthermore, the response times were measured as a function of the layer height (sensor material: permethylated b- cyclodextrin bridged with diiodooctane).
3. Results and discussion The sensor responses are square correlated to the device’s resonance frequency (Eq. (1)) Df = (k1 + k2)rlhl f 20 − k2hl f 20(4m/V 2R)[(l + m)/(l + 2m)] (1) The equation describes the sensor response Df of a SAW device coated with a thin, isotropic, nonconducting layer, where k1 and k2 are material constants for the quartz substrate, rl is the density of the layer, hl the film thickness, f0 the fundamental frequency, m the shear modulus of the film material, VR the acoustic wave velocity, and l, the Lamb constant. The first term of the equation represents the frequency shift due to mass loadings, whereas the second term describes the effect of changes in the elastic properties of the film on the resonance frequency of the device. The influence of changes of the elastic properties can be neglected for most materials. On this account, Eq. (1) can be simplified by omitting the second term and inserting the material constants: Df = − 1.26×106f 20hlrl
(2)
From Eq. (2), the frequency responses due to mass loadings rise in a parabolic manner with rising resonance frequencies can be deduced. Depending on changes of the noise level, the sensitivity and limits of detectability of the device could alter with varying resonance frequencies. The expectation, that there is a square relationship between sensor response and resonance frequency, is about to meet the experimental results depicted in Fig. 2. The frequency shifts shown in Fig. 3 demonstrate this fact, a 1 GHz device give a 5.6-fold higher frequency response than the 433 MHz-resonator. Both devices were coated homogeneously in a thickness of 60 nm of a molecular cavity (permethylated b-cyclodextrin bridged with hexafluorobenzene). As shown in Fig. 4, the noise level of a GHz-resonator, for example, is approximately twice as high as the one of a 433 MHz-SAW in case of the coated device. Measuring uncoated devices, the noise level doubles, also (Fig. 5). The noise amplitude of the uncoated GHz-device might be influenced by temperature fluctuations. At 20°C, a temperature change of 9 0.01°C (exactitude of the used thermoregulator)
Fig. 2. Sensor response Df in kHz of SAW-devices to pulses of 1000 ppm toluene as a function of the initial frequency f0; all resonators were coated with a molecular cavity (permethylated ß-cyclodextrin linked with hexafluorobenzene) in a thickness of 60 nm.
causes frequency shifts of 1.4 Hz (433-MHz-SAW) and 13 Hz (1-GHz-SAW). Therefore, some reduction of noise might be achieved by a differently cut quartz as substrate material with a smaller temperature coefficient at room temperature. Taking into account the frequency shifts (Fig. 3) and noise levels (Fig. 4) of the coated devices described above, detection limits of toluene are expected to be a few ppm. These favourable findings can further be improved by using a distinct coating technique. The sensor material which is
Fig. 3. Frequency response Df of a 1 GHz-SAW (continuous line) and a 433 MHz-SAW (dashed line) to pulses of 1000 ppm toluene, both devices coated with a molecular cavity, permethylated b-cyclodextrin linked with hexafluorobenzene, in a thickness of 60 nm, r.H. 10%.
F.L. Dickert et al. / Sensors and Actuators B 46 (1998) 120–125
Fig. 4. Noise trace of a 1 GHz-SAW (upper part) and a 433 MHz-SAW-resonator (lower part); both devices coated with 60 nm permethylated b-cyclodextrin, linked with hexafluorobenzene, r.H.10%.
necessary for a homogeneous coating of the SAW in a layer height of 40 nm was applied only to the IDT region (coating material: permethylated b-cyclodextrin bridged with hexafluorobenzene). Figs. 6 and 7 show the sensor responses to steps of 1 ppm m-xylene. In case of the 1 GHz device (Fig. 6), steps of 1 ppm can clearly be resolved, the signal to noise ratio being 90 Hz: 30 Hz meets the IUPAC-convention. In contrast to that, the 433 MHz-resonator is not able to resolve the 1 ppm steps significantly (Fig. 7), the frequency responses are only gradually visible in the noise, the signal to noise ratio is slightly better than 1:1. Therefore, the detection limits are lower in case of the 1 GHz-resonator by a factor of approximately 2 in comparison to the 433 MHz device. These LODs are lower compared to those where the sensor layers
Fig. 5. Noise trace of uncoated 1GHz-SAW-and 433 MHz-SAWdevices, r.H. 10%.
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Fig. 6. Frequency response Df of a 1GHz-SAW to pulses of 1 ppm m-xylene, coated with a molecular cavity, permethylated b-cyclodextrin linked with hexafluorobenzene, in an average thickness of 40 nm.
are applied to the whole device as described above (layer height: 60 nm). If only the IDT area is coated, a more favourable noise level results which leads to an improved signal to noise ratio, since the development of the Rayleigh waves is not disturbed by incidental inhomogeneities in the reflector area caused by the sensitive layer. The response times of the sensitive layers were studied with permethylated b-cyclodextrins, highly linked with octyl-bridges thus resulting in a rather rigid structure of the polymer. These materials have shown a high stability for chemical sensing and could be tested successfully for one year. However, some minutes are observed as t90 times, which is due to diffusion processes. Additionally, the entrance of the
Fig. 7. Frequency response Df of a 433 MHz-SAW to pulses of 1 ppm m-xylene, coated with a molecular cavity, permethylated b-cyclodextrin linked with hexafluorobenzene, in an average thickness of 40 nm, r.H. 10%.
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molecular hollows might be blocked by neighboured cavities and therefore some sterical rearrangements are necessary for analyte inclusion. These problems are solved by ultrathin layers, which yield a satisfying sensitivity with 1 GHz resonators, even in the case of monolayers. Consequently, the combination of monolayers and resonators with ultrahigh resonance frequencies is a promising strategy improving the sensitivity and the response characteristics of mass sensitive chemical sensors.
4. Conclusions An increase of the initial frequency of mass-sensitive transducers results in a higher sensitivity of the device. Consequently, the limits of detectability can be lowered by rising the oscillation frequency of the device. Additionally, high frequency devices due to their higher mass-sensitivity favour the use of thinner coatings down to monolayers, leading to shorter response times. The use of devices with higher oscillation frequency is therefore a promising strategy, only limited by the photolithographical process. Today, the smallest electrode distance feasible by photo-lithographical means is approximately half a micron. This leads to devices with frequencies up to 2.5 GHz.
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Biographies F.L Dickert studied chemistry at the University of Erlangen/Germany and received his Ph.D in 1970 on a study of fast reactions in non-aqueous solutions. He was appointed as Professor of Physical Chemistry in 1980 and his main field of research was spectroscopic studies, especially NMR, about solvation effects concerning metal complexes and carbenium ions. Since 1990, he has focused his activities on the development of chemically sensitive coatings according to supramolecular chemistry. In 1994, he accepted a call to a chair of Analytical Chemistry at the University of Vienna.
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P. Forth received his diploma degree in chemistry from the University of Erlangen/Germany in 1994 and his Ph.D from Vienna University in 1997. He has worked in the sensor group of F.L Dickert since 1994. W.-E. Bulst is a senior director in the Corporate Technology of Siemens in Munich/Germany and heads the
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department of surface–acoustic-wave technology and wireless sensors. U. Knauer and G. Fischauer received their Ph.D.’s from the Technical University of Munich and are responsible for projects on SAW devise fabrication and SAW sensors in extreme environments, respectively.