Anal. Chem. 1999, 71, 3632-3636
Multichannel Quartz Crystal Microbalance Tetsu Tatsuma,*,† Yoshihito Watanabe, and Noboru Oyama*
Department of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo 184-8588, Japan Kaoru Kitakizaki and Masanori Haba
Advanced Technology Research Laboratory, Meidensha Corporation, Ohsaki, Shinagawa-ku, Tokyo 141-8565, Japan
Arrays of quartz crystal resonators are fabricated on a single quartz wafer as a multichannel quartz crystal microbalance (MQCM). Three types of four-channel array of 10-MHz resonators are prepared and tested. Mechanical oscillation of each channel is entrapped within the channel almost completely, so that the interference between the channels via the quartz crystal plate is almost negligible. A mass change on each channel is quantitatively evaluated on the basis of Sauerbrey’s law. Thus, each channel of a MQCM device can be used as an independent QCM. Influence from a longitudinal wave generated from another channel is also negligible compared to the influence from the wave from the monitored channel itself. The simultaneous oscillation of channels is also possible. The potential applicability of MQCM to the two-dimensional mapping of mass changes is demonstrated.
The quartz crystal microbalance (QCM) has been used to monitor slight mass changes on solid surfaces.1-3 Applications of the QCM include immunosensing,4,5 DNA sensing,6,7 other chemical sensing, a detector for liquid chromatography,8 and monitoring of electrochemical processes.9,10 In the present work, an array of microbalances is formed on a single plate of quartz crystal as a multichannel quartz crystal microbalance (MQCM). Three types of MQCM are prepared (Figure 1). The MQCM of type A has arrays of etched dents on both sides (Figure 1a). Types B and C have an array of dents on only one side (the other side has a flat surface or one large dent) (Figure 1b and c). All the electrodes † Present address: Department of Applied Chemistry, Faculty of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. (1) Sauerbrey, G. Z. Phys. 1959, 155, 206-212. (2) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355-1379. (3) Grate, J. W.; Martin, S. J.; White, R. M. Anal. Chem. 1993, 65, 940A948A, 987A-996A. (4) Muramatsu, H.; Dicks, J. M.; Tamiya, E.; Karube, I. Anal. Chem. 1987, 59, 2760-2763. (5) Ebersole, R. C.; Ward, M. D. J. Am. Chem. Soc. 1988, 110, 8623-8628. (6) Okahata, Y.; Matsunobu, Y.; Ijiro, K.; Mukae, M.; Murakami, A.; Makino, K. J. Am. Chem. Soc. 1992, 114, 8299-8300. (7) Yamaguchi, S.; Shimomura, T.; Tatsuma, T.; Oyama, N. Anal. Chem. 1993, 65, 1925-1927. (8) Nomura, T.; Minemura, A. Nippon Kagaku Kaishi 1980, 1621-1625. (9) Schumacher, R.; Borges, G.; Kanazawa, K. K. Surf. Sci. 1985, 163, L621L626. (10) Bruckenstein, S.; Shay, M. Electrochim. Acta 1985, 30, 1295-1300.
3632 Analytical Chemistry, Vol. 71, No. 17, September 1, 1999
Figure 1. Schematic illustrations for three types of MQCMs: (a) type A, (b) type B, and (c) type C.
are smaller than the dents for types A and B. As to type C, the electrode on the flat region is a large electrode that covers all the small dented regions. 10.1021/ac9904260 CCC: $18.00
© 1999 American Chemical Society Published on Web 07/24/1999
Figure 2. Some potential applications of the MQCM: (a) MQCM as a mass-based chemical sensor array for the multicomponent analysis, (b) series MQCM as a multichannel detector for the flow injection analysis including HPLC, (c) multifrequency QCM as a multisensitivity, multidynamic range QCM array, and (d) MQCM for the two-dimensional mass mapping of mass changes.
The MQCM has many potential applications. An array of QCMs with different receptors can serve as a sensor array for multicomponent analysis (based on mass changes) (Figure 2a), which includes (1) analysis with different selective receptors (e.g., DNA and antigen/antibody); (2) analysis with different receptors with low selectivity (e.g., ligands for metal ions; with four different ligands having different affinity toward four metal ions, it is possible to determine the concentrations of the four metal ions); and (3) analysis based on pattern recognition (e.g., taste sensor and odor sensor). In the case of a conventional QCM, several independent QCM devices must be set to a cell (a vessel for a sample solution) for multicomponent analysis; the cell assembly should be complicated. With the MQCM, however, only a single MQCM device is needed to be set to a simple cell. Additionally, one of the channels of a MQCM can be used as a reference (for a control experiment), with which the influences from the temperature, viscosity, and density of the solution can be measured and subtracted from the signals of the other channels. Although some dual QCM systems have been studied
from the same viewpoint,11-13 those were just combinations of two conventional QCM devices. Similarly, in electrochemical systems, two channels of MQCM can be used as working and counter electrodes, or as dual (or multiple) working electrodes. In these cases, measurements with one drop of an electrolyte solution (without an electrolytic cell) are also possible. The flow injection analysis is an additional application. A series MQCM (Figure 2b) of type B (or C) is suitable because the seamless, smooth surface will not disturb the solution flow (Figure 2b). Another important application is an array of QCMs with different frequencies (multifrequency QCM array) (Figure 2c). A QCM with higher resonance frequency has higher sensitivity for a mass change. However, as the frequency increases, the dynamic range decreases instead; a sensitive QCM saturates earlier as the mass increases. With conventional QCMs, therefore, sometimes the same measurements have to be repeated by using QCMs with different frequencies. On the other hand, with the multifrequency QCM array, only one measurement is necessary; one can select a channel with the optimal sensitivity and dynamic range in the course of an experiment (or even after the measurement). Such a multisensitivity, multidynamic range QCM should be useful not only for chemical sensing but also for monitoring of surface processes including electrochemical reactions. As a mass-based sensor, this type of MQCM may be modified with only one kind of receptor for the above-mentioned purpose. However, each channel may also be modified with a different receptor for a multicomponent analysis in which each channel requires different sensitivity and/or different dynamic range (e.g., analysis of DNAs with different lengths). Additionally, MQCM is potentially applicable to mapping of the two-dimensional distribution of mass changes on the solid surfaces (Figure 2d). Although lateral resolution of the device is extremely low at the present stage because each channel has a large diameter, one can improve it by making each channel smaller and thinner in the future. In our recent work, we have developed a scanning-electrode quartz crystal analysis (SEQCA) technique,14-16 which enables the qualitative mapping of mass distribution. However, since the mechanical oscillation propagates over the whole quartz plate, the quantitative mapping of mass distribution is not possible by means of SEQCA. In the present work, we designed the quartz plate so as to restrict the mechanical oscillation within the etched region. Also, MQCM needs no mechanical scanning system, which is essential for SEQCA, so that it is much less expensive. Channels of the MQCM can be oscillated (or resonated with an impedance analyzer or a network analyzer) simultaneously, if necessary (except type C). Successive oscillation (by switching each channel) is also possible. In this case, matrix-type electrodes may also be useful. (11) Bruckenstein, S.; Fensore, A.; Li, Z.; Hillman, A. R. J. Electroanal. Chem. 1994, 370, 189-195. (12) Bruckenstein, S.; Michalski, M.; Fensore, A.; Li, Z.; Hillman, A. R. Anal. Chem. 1994, 66, 1847-1852. (13) Dunham, G. C.; Benson, N. H.; Petelenz, D.; Janata, J. Anal. Chem. 1995, 67, 267-272. (14) Oyama, N.; Tatsuma, T.; Yamaguchi, S.; Tsukahara, M. Anal. Chem. 1997, 69, 1023-1029. (15) Tatsuma, T.; Yamaguchi, S.; Wakabayashi, I.; Mori, K.; Oyama, N. Faraday Discuss. 1997, 107, 53-60. (16) Tatsuma, T.; Mori, K.; Oyama, N. Anal. Sci., in press.
Analytical Chemistry, Vol. 71, No. 17, September 1, 1999
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The objective of the present paper is characterization of the three types of MQCMs and demonstration of their potential applicability to the above-mentioned applications. Especially, the absence of interference between the channels is of great importance. As mentioned above, propagation of the mechanical oscillation via the quartz plate can be responsible for the interference. As for the present MQCM, the oscillation is expected to be entrapped in the dented region, because the resonance frequency is different from that for the nondented region. EXPERIMENTAL SECTION Preparation of MQCM. Quartz crystal plates (22 × 22 mm) were processed by a photolithographic technique and chemical etching with a mixed solution of hydrofluoric acid and ammonium bifluoride as described in the literature.17 The thickness of the quartz plate is 277 µm, and the thickness of the dented region (oscillating region) is 167 µm. The diameter of the dented region is 8.0 mm. The distance between the dented regions is 1 mm. Gold electrodes (diameter, 4.5 mm; thickness, 100 nm) were prepared by sputtering on a chromium underlayer (thickness, 5 nm). The resonance frequency of each channel is 10.0-10.1 MHz. Measurements. A network analyzer (HP8753C, HewlettPackard) was used for the impedance measurement of a MQCM. Oscillation frequency of a MQCM was measured with a quartz crystal analyzer (QCA917, Seiko EG&G). Electrodeposition of silver on the gold electrode was conducted by using a potentiostat/ galvanostat (PS-07, Toho Giken) in an aqueous solution containing 1 mM AgNO3 and 0.2 M HClO4 (unless otherwise noted) at a constant current density (6.3 µA/cm2). RESULTS AND DISCUSSION Interference between Channels via Crystal. If a quartz plate has a uniform thickness, the oscillation should propagate over the plate.14-16 In this case, a mass increase of channel 1 results in frequency decreases of not only channel 1 but also all other channels. This interference between the channels disables the MQCM from the quantitative evaluation of mass changes. In the present work, we intend to restrict the oscillation within the dented region. If the separation of the oscillation is successful, massloading on a given channel will not result in frequency decreases of the other channels. To examine this interference effect, a drop of water (80 µL) was placed on a channel (channel 1, 2, 3, or 4), and changes in the impedance characteristics of channel 1 were observed. All three types of MQCMs were examined. For types B and C, a water drop was put on the flat side. Table 1 summarizes the changes in the resonance frequency FGmax and the changes in the R1 value, which is the reciprocal of the maximum value of conductance Gmax (R1 is the resistance component of the electrical equivalent circuit of the quartz crystal resonator),2 for channel 1 of type B MQCM. When the water drop was placed on channel 1, the resonance frequency FGmax decreased and the R1 value increased for channel 1. The decrease of FGmax reflects that the viscosity and density of water are higher than those of air. The increase in R1, which corresponds to the oscillation energy loss, indicates that water damps the oscillation. On the other hand, when a water drop was (17) Tanaka, M.; Ugajin, T.; Araki, N.; Oomura, Y. Jpn. J. Appl. Phys. 1997, 36, 3022-3027.
3634 Analytical Chemistry, Vol. 71, No. 17, September 1, 1999
Table 1. Changes in the FGmax and R1 Values for Channel 1 When a Drop of Water (80 µL) Was Placed on a Channel channela
∆FGmaxb (Hz)
∆R1b,c (Ω)
1 2 3 4
-3873 ( 100 32 ( 4 1(2 -40 ( 12
4267 + 113 2(1 0(1 2(3
a Channel on which a water drop was placed. b Channel 1. Mean ( standard error, n ) 5. c R1 ) 1/Gmax.
Figure 3. Changes in the oscillation frequency of channel 1 during the electrochemical deposition of silver on channel 1, 2, 3, or 4 from an aqueous solution containing 1 mM AgNO3 and 0.2 M HClO4 at a constant current density (6.3 µA/cm2). A type B MQCM is used.
placed on channel 2, 3, or 4, the changes in the FGmax and R1 values were much smaller. Similar results were obtained for types A and C as well. Although the interference was almost negligible, it was not completely absent. For instance, in the case of the results given in Table 1, the mass-loading to channel 2 and channel 4 resulted in a slight increase and decrease, respectively, of the FGmax value of channel 1. Thus, the oscillation was trapped within the channel almost completely, so that no appreciable interference between channels was observed. If the interference is needed to be less significant (
