Anal. Chem. 2004, 76, 3837-3840
Improvement of Surface Acoustic Wave Gas and Biosensor Response Characteristics Using a Capacitive Coupling Technique F. Bender, K. La 1 nge, A. Voigt, and M. Rapp*
Institute for Instrumental Analysis, Forschungszentrum Karlsruhe, Postfach 3640, 76021 Karlsruhe, Germany
As for many other electronic devices and circuits, electrical contact to surface acoustic wave sensors is usually made using bonding wires. This technique is known to result in reliable contact under most conditions, but it does so with several disadvantages. First, electrical contact is not reversible, impeding replacement of sensor devices. Second, bonding wires are quite delicate and should not be exposed to high gas or liquid flows. Third, the presence of bonding wires may limit the potential to miniaturize a sensor housing or flow cell. Therefore, a capacitive coupling technique was developed to replace bonding wires. This permitted redesign of flow cells and sensor arrays, resulting in flow cell volumes of 80 µL to 60 nL. As a consequence, response times were reduced to 1-2 s in gas sensing and a few minutes in liquid sensing, respectively. At the same time, sensor devices can be easily replaced, and the system is less susceptible to malfunction. Ever since its introduction in 1965,1 the interdigital transducer (IDT) has been the preferred means to launch and receive surface acoustic waves (SAW). The IDT consists of a planar electrode structure on a piezoelectric substrate. If resistance to corrosion is a major issue, the IDT may be made of gold on a chromium or titanium adhesion layer. This way, a simple and robust structure is created. However, in analytical sensing applications where the device is exposed to the environment, the weakest link in the chain is often the electrical contact to the IDT. There are different ways to make contact to an IDT. Probably the most common is wire bonding: by application of ultrasonics, a thin wire is connected to the electrode structures. For the bonding procedure, the wire must be flexible, making the connection susceptible to failure. In addition, the bonding connection is permanent, impeding replacement of the SAW device. This problem can be circumvented by attaching (and bonding) the sensor device to a replaceable holder. However, in this case, the device has to be rigidly attached to the holder by clamping or gluing. This will deteriorate the response behavior of the sensor due to mechanic stress or analyte sorption in the glue. Finally, the presence of the delicate arched wire is an obstacle in the design of a flow cell or sensor housing. * Corresponding author. E-mail:
[email protected]. (1) White, R. M.; Voltmer, F. W. Appl. Phys. Lett. 1965, 7, 314-316. 10.1021/ac035019+ CCC: $27.50 Published on Web 05/15/2004
© 2004 American Chemical Society
As an alternative, it is possible to apply some kind of mechanical pressure to make electrical contact, e.g., by using spring contact pins. However, the applied pressure may influence the signal of the sensitive SAW device, leading to drifts or sudden jumps in the output signal. In addition, the contact may degrade due to corrosion or contamination of the contacting surfaces. On the other hand, since SAW devices can be operated at ultrahigh frequencies (UHF), it is not necessary to make direct ohmic contact. Instead, the UHF signal may be coupled into the SAW device by capacitive or inductive coupling. In the case of the latter, the IDT is designed as part of a coil or antenna, which communicates with a second external coil. The feasibility of this approach has been demonstrated;2,3 however, a more compact and less sophisticated technique would still be desirable. Therefore, this work is investigating capacitive coupling as an alternative approach. For this purpose, new sensor systems based on capacitive coupling technique have been developed for sensing applications in gas and liquid phases. These systems have been optimized with regard to simple handling and quick replacement of individual SAW devices, small flow cell volumes, short response times, and robustness. Their performance is compared to that of previous sensor systems based on a wire bonding technique. Intended applications include biosensing in buffer solutions, gas analysis using a SAW sensor array with preconcentration stage, and in situ soil gas analysis with a robust, miniaturized system integrated into a soil probe. EXPERIMENTAL SECTION Materials. Parylene C dimer (di[2-chloro-p-xylylene]) was purchased from Specialty Coating Systems (Indianapolis, IN). Tetrachloroethene (purity g99.9%) was obtained from SigmaAldrich (Steinheim, Germany). Bovine serum albumin (BSA) was purchased from SERVA Feinbiochemica (Heidelberg, Germany). Phosphate buffer was based on an aqueous solution of 20 mM sodium dihydrogen phosphate; KOH was used to adjust the pH to 7.2. BSA was dissolved in phosphate buffer at a concentration of 4 mg/mL. (2) Freudenberg, J.; Schelle, S.; Beck, K.; von Schickfus, M.; Hunklinger, S. Biosens. Bioelectron. 1999, 14, 423-425. (3) Beck, K.; Kunzelmann, T.; von Schickfus, M.; Hunklinger, S. Sens. Actuators, A 1999, 76, 103-106.
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Figure 2. Cross-sectional view of the assembled sensor head (schematic). Vertical dimension expanded for clarity. (SAWR, surface acoustic wave resonator; PTFE, poly(tetrafluoroethylene).) Figure 1. Photograph of the SAW sensor microarray with three SAW chips inserted. The cover was removed. Dimensions of the board are 60 × 30 mm2.
R2632 SAW Sensor Array. This array was previously used for gas-sensing applications.4 It employs eight SAW resonators, type R2632 (Siemens/EPCOS, Mu¨nchen, Germany). The two-port quartz resonator has a frequency of operation of 433.9 MHz and was originally designed for remote control applications. It was selected for gas analysis because it is mounted in a small sensor housing (type TO39), permitting quick replacement of the device. The resonator is glued to the TO39 socket and contacted by wire bonding. For sensing applications, the housings are cut open, and the eight SAW resonators are coated with eight different polymer layers for analyte sorption. A ninth, unmodified SAW resonator is used as a reference. The devices are plugged into an electronics board containing one oscillator circuit for each device. A multiplexing technique is used to read out the devices consecutively.4 It takes 1 s to read out the eight frequency values. The array is incorporated into a common housing with integrated gas channels containing a total gas volume of ∼1 mL. A photograph of this system is shown in Figure 1 of the Supporting Information. SAW Sensor Microarray. The microarray represents a recent development for gas analysis. It employs eight SAW resonator devices designed in collaboration with I. D. Avramov, Institute of Solid State Physics, Bulgarian Academy of Sciences, Sofia, Bulgaria.5,6 The device is based on a small (5.9 × 3.9 mm2) STquartz chip and has a frequency of operation of 432.5 MHz. After deposition of the polymer coatings, the devices are mounted upside-down on the electronics board and contacted by capacitive coupling. Gas channels are milled into the electronics board, reducing the total gas volume to 80 µL.7 A photograph of the microarray is shown in Figure 1. The array is mounted inside a metal housing and combined with a temperature control system. A cross-sectional view of the entire system is shown in Figure 2. Murata Biosensor. For biosensing experiments, a single sensor device is inserted into an oscillator circuit developed inhouse. The Murata device is a 380-MHz shear-horizontal surface acoustic wave (SH-SAW) resonator based on 36°YX-LiTaO3 (type SAF380T; Murata, Kyoto, Japan). The device is contacted by wire (4) Rapp, M.; Reibel, J.; Voigt, A.; Balzer, M.; Bu ¨ low, O. Sens. Actuators, B 2000, 65, 169-172. (5) Avramov, I. D.; Rapp, M.; Voigt, A.; Stahl, U.; Dirschka, M. IEEE/EIA Int. Freq. Control Symp., Kansas City, June 7-9, 2000. (6) Avramov, I. D.; Kurosawa, S.; Rapp, M.; Krawczak, P.; Radeva, E. I. IEEE Trans. Microwave Theory Technol. 2001, 49, 827-837. (7) Bender, F.; Barie´, N.; Romoudis, G.; Voigt, A.; Rapp,M. Sens. Actuators, B 2003, 93, 135-141.
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bonding. For this device, a 50-µL flow cell was designed (Supporting Information, Figure 2, left). E062 Biosensor. For capacitive coupling, a SH-SAW resonator (type E062) was developed in cooperation with Siemens/ EPCOS.8 The resonator is based on a small (4 × 4 mm2) 36°YXLiTaO3 chip and has a frequency of operation of 428.5 MHz. For the type E062 device, two flow cells were designed. One of them has a volume of 4.8 µL (Supporting Information, Figure 2, right), the other has a volume of 60 nL (not shown). All biosensing experiments were conducted using a flow injection analysis system developed in-house. The liquid flow rate was set to 120 µL/min, independent of the flow cell volume. Vapor Deposition Polymerization. Parylene C was deposited using a commercial parylene deposition system (Labcoter 1, PDS 2010; Specialty Coating Systems). The dimer is sublimated and subsequently undergoes pyrolytic cleavage at 690 °C. The resulting monomer spontaneously polymerizes on condensation at the sensor surface in a vacuum chamber at room temperature, forming a solvent-free layer of high packing density.9 RESULTS AND DISCUSSION Design Considerations for Capacitive Coupling. To create a plate capacitor with sufficiently large capacitance, it is important to use only a thin dielectric layer. However, the layer should still be stable to ensure reproducible coupling conditions. Parylene C was used in our laboratory for years to improve polymer adhesion, protect electrode structures, and act as a waveguiding layer in SH-SAW devices.10,11 This polymer can be deposited by vapor deposition polymerization, resulting in stable, tightly packed, solvent-free films of uniform thickness. Therefore, parylene C suggests itself as a dielectric material for capacitive coupling. The usual thickness of parylene C films in gas-sensing applications is of the order of d ) 100 nm. Given a dielectric constant of ) 2.95 in the megahertz range and a size of the contacting surfaces of A ) 2.3 mm2, a capacitance of C ) 0A/d ≈ 600 pF per contact pad is calculated for this film thickness. At frequencies around f ) 430 MHz, this results in a contribution of Z ) 1/(jωC) ≈ 0.62 Ω to the total impedance (with ω ) 2πf ). Since the system impedance is usually of the order of 50 Ω, this value can be readily tolerated. In experiments, it was found that even devices using a 3.3-µm-thick parylene C film as dielectric material still work properly. (8) Rapp, M.; Bender, F.; La¨nge, K.; Kondoh, J. 202nd Meeting of the Electrochemical Society, Salt Lake City, UT, October 20-24, 2002. (9) Gorham, W. F. J. Polym. Sci. Part A-1 1966, 4, 3027-3039. (10) Barie´, N.; Rapp, M.; Sigrist, H.; Ache, H. J. Biosens. Bioelectron. 1998, 13, 855-860. (11) Barie´, N.; Stahl, U.; Bruns, M.; Rapp, M. Sens. Actuators, B 2003 (submitted).
Figure 4. Details of Figure 5 (bottom trace), demonstrating the rapid response (left) and recovery (right) of the SAW sensor microarray.
Figure 3. Frequency responses of the R2632 SAW sensor array (top) and the SAW sensor microarray (bottom) to step changes in analyte concentration. Tetrachloroethene was used as the analyte.
Another precondition for reproducible coupling is a welldefined contact pressure. This is achieved using a strip of silicone rubber, gently pressing the SAW devices onto the corresponding contact pads of the electronics board. To separate the silicone rubber from the analyte vapors present in the gas channels, a thin poly(tetrafluoroethylene) (PTFE) tape is placed between the rubber and the SAW devices; see Figure 2. In addition, the inside of the gas channels is gold-plated (Figure 1). Therefore, analyte vapors will only be absorbed in the polymer coatings intended for analyte sorption, resulting in rapid response and recovery. Response Characteristics of Gas Sensor Arrays. Figure 3 compares the SAW sensor microarray to the R2632 SAW sensor array with regard to their responses to step changes in tetrachloroethene concentration. For the R2632 SAW sensor array, large overshoots are observed, and the signal is not stable even 10 min after turning the analyte on or off. In addition, the observed frequency drifts have different slopes for different sensor devices; i.e., the pattern of the eight sensor responses is still changing minutes after a change in analyte concentration. This will render pattern recognition very difficult.12 The reason for this detrimental behavior is most likely found in the glue holding individual devices in place. The glue may absorb analyte vapor and swell, causing stress in the sensor substrate, the bonding wires, or both. Unfortunately, the problem was not solved by baking the glue prior to gas analysis experiments. The response of the SAW sensor microarray looks much better (Figure 3, lower trace). No overshoot is visible, and the response pattern of the eight sensor devices is very stable. Both response and recovery of the system are quite rapid. Indeed, the kinetics (12) Frank, M.; Hermle, T.; Ulmer, H.; Mitrovics, J.; Weimar, U.; Go¨pel, W. Sens. Actuators, B 2000, 65, 88-90.
of the response behavior of the microarray cannot be adequately assessed at the time scale used in Figure 3. Therefore, details of the lower trace of Figure 3 are shown in expanded time scale in Figure 4. For both the response and recovery of the microarray, over 96% of the frequency shift is attained within 1-2 s. To get a more accurate figure, it will be necessary to improve the time resolution of 1 s caused by the multiplexing technique. Implications for Chemical Analysis. A rapid response of the system is of particular importance when combining the gas sensor array with a preconcentration stage. In this case, analyte vapor is collected in a suitable preconcentrator (“trap”). Periodically, the trap is heated to desorb the analyte vapor, which is then aspirated by the sensor array. To maximize the preconcentration factor, a short desorption bandwidth of e3 s is desired.7,13 This necessitates response times of better than 3 s. It was found that the SAW sensor microarray harmonizes very well with a trap, permitting discrimination between closely related aromatic compounds (benzene, toluene, xylene, ethylbenzene) in sub-ppm concentrations.7 The ability of the system to analyze small sampling volumes is important for many applications. In addition, sometimes a rugged design is required. An example for a situation requiring both system characteristics is in situ soil gas analysis. For this application, a robust, miniaturized SAW sensor array is integrated into a soil probe and driven into the ground. During soil penetration, a concentration profile of soil contaminants is recorded in real time. For this purpose, the array must be very robust and shock-proof, precluding the use of delicate bonding wires. Recently, a prototype based on capacitive coupling has been designed. Testing and development of the system are in progress. Response Characteristics of Biosensors. In liquid sensing, response times are generally larger due to slow diffusion in the vicinity of a surface. However, even here capacitive coupling technique results in significant improvements in the response characteristics of the system. A previous wire-bonded device had shown several disadvantages in biosensing. In particular, the presence of the bonding wires necessitates the use of a flow cell with a minimum volume of 50 µL, resulting in slow response times. Occasionally, small air bubbles clung to the bonding wires, distorting the signal of the sensor device. In addition, the delicate bonding wires impose restrictions on the maximum liquid flow rate. (13) Lu, C.-J.; Zellers, E. T. Anal. Chem. 2001, 73, 3449-3457.
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Figure 5. Normalized frequency responses of biosensors with different flow cells to adsorption of BSA. Flow cell volumes: 50 µL (‚‚‚); 4.8 µL (s); 60 nL (- -). Arrows indicate response times for 90% frequency shift.
Recently, it was demonstrated that capacitive coupling technique permits the use of flow cells with volumes of 4.8 µL and 60 nL, respectively.14 This will result in a significant decrease in response times. To quantify this decrease, a simple biosensing experiment was conducted. The devices were inserted into an oscillator circuit and exposed to phosphate buffer. When the frequency baseline was sufficiently stable, BSA was added to the buffer solution, resulting in adsorption of BSA on the sensor surface. For the above flow cells, the response curves shown in Figure 5 were obtained. From this figure, response times (90% frequency shift) of 690 (50 µL flow cell volume), 260 (4.8 µL), and 200 s (60 nL) are determined. These results show that capacitive coupling permits a reduction in response time of 7-8 min compared to the wire-bonded device, a fact that represents an important improvement in applications such as medical diagnosis. No significant increase in attenuation due to capacitive coupling was observed. CONCLUSIONS For many applications, replaceable sensor devices are desired. In biosensing, nonreversible biochemical reactions may neces(14) La¨nge, K.; Bender, F.; Voigt, A.; Gao, H.; Rapp, M. Anal. Chem. 2003, 75, 5561-5566.
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sitate the use of disposable sensor devices. In gas sensing, when using a sensor array consisting of many individual devices, it is advantageous if a single defective device can be easily replaced to avoid malfunction of the whole array. The previous attempt to use a SAW gas sensor glued to a replaceable socket had proven unsatisfactory because of slow response times and signal overshoots, most likely due to analyte sorption in the glue. In contrast, using capacitive coupling, individual devices can be replaced quickly and easily but still give a clean response. Another major advantage of this technique is its planar nature permitting significant reduction of flow cell volumes as compared to wire bonding. In gas sensing, a flow cell volume of 80 µL for an array of eight SAW resonators was achieved, well below the value for the wire-bonded SAW sensor array (∼1 mL). As a result, response and recovery times of the system have become so short that the time resolution of data acquisition (1 s) is no longer sufficient to accurately assess the performance of the SAW sensor microarray. Therefore, only an upper limit for the response and recovery times of 2 s can be given. This distinct improvement is in part due to the absence of glue or any other absorbing material in the flow cell design, but also to the small gas volume of the flow cell. In the design of the SAW sensor microarray, the flow cell simply consists of gas channels milled into the electronics board. A single strip of silicone and PTFE tape serves both as a seal and a spring for all sensor devices. This simplifies fabrication and handling of the system and reduces the overall cost. At the same time, the resulting design is very robust, resulting in higher reliability and undistorted output signals. Similar advantages were obtained in the design of a flow cell for liquid sensing applications. ACKNOWLEDGMENT The authors thank I. D. Avramov from the Institute of Solid State Physics, Sofia, Bulgaria, for the design of the 432.5-MHz SAW resonator. This work was funded in part through research project ELMINA, grant 01SF9934, by Hermann von Helmholtz Gemeinschaft Deutscher Forschungszentren (HGF). SUPPORTING INFORMATION AVAILABLE Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review August 29, 2003. Accepted March 19, 2004. AC035019+
