Anal. Chem. 1997, 69, 1023-1029
Scanning Electrode Quartz Crystal Analysis Noboru Oyama,* Tetsu Tatsuma, Shuichiro Yamaguchi,† and Masanori Tsukahara
Department of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo 184, Japan
Principles and methods for scanning electrode quartz crystal analysis (SEQCA) as a technique for mapping distribution of mass and measuring interfacial rheology are described. In SEQCA, quartz crystal resonates in a region between a normal electrode and a microelectrode scanned over the opposite side of the normal electrode. The surface of the quartz crystal plate just below the scanning electrode is monitored (overscanning mode). A prototype system for the scanning electrode quartz crystal analysis is constructed. The dependence of the resonance properties on the location of the scanning electrode is studied. As a result, it was found that the qualitative mapping of the mass distribution is possible in the lateral resolution of 1 (9 MHz quartz) to 2 (5 MHz) mm. Viscoelastic properties of the load on the quartz surface are also measured qualitatively. Quartz crystal microbalance has been used to monitor minute mass changes on solid surfaces.1-3 Further characterization of solid/liquid interfacial properties,2,3 such as rheology,4-7 roughness,8 and solvophilicity,9 is also possible if one evaluates an electrical equivalent circuit of a quartz crystal resonator on the basis of impedance measurements. However, evaluation of lateral distribution of mass and so forth, including two-dimensional mapping of mass distribution, has never been attained, to the best of our knowledge. To measure a mass in a small area, we must oscillate (or resonate) quartz crystal only at a small region. To this end, we should use a microelectrode, and the microelectrode must be scanned for the mapping of mass distribution. However, an electrode of a normal quartz crystal resonator cannot be scanned. Fortunately, an electrode-separated quartz crystal resonator,10-15 whose electrode(s) is separated from the quartz plate, is known to resonate. Then, we can use a scanning microelectrode (probe) † Permanent address: R & D Center, Terumo Co., Nakai-machi, Ashigarakamigun, Kanagawa 259-01, Japan. (1) Sauerbrey, G. Z. Phys. 1959, 155, 206-212. (2) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355-1379. (3) Oyama, N.; Ohsaka, T. Prog. Polym. Sci. 1995, 20, 761-818. (4) Borjas, R.; Buttry, D. A. J. Electroanal. Chem. 1990, 280, 73-90. (5) Glidle, A.; Hillman, A. R.; Bruckenstein, S. J. Electroanal. Chem. 1991, 318, 411-420. (6) Muramatsu, H.; Kimura, K. Anal. Chem. 1992, 64, 2502-2507. (7) Tatsuma, T.; Takada, K.; Matsui, H.; Oyama, N. Macromolecules 1994, 27, 6687-6689. (8) Beck, R.; Pittermann, U.; Weil, K. G. J. Electrochem. Soc. 1992, 139, 453461. (9) Inaba, H.; Iwaku, M.; Tatsuma, T.; Oyama, N. J. Electroanal. Chem. 1995, 387, 71-77. (10) Watanabe, Y. Elektr. Nachr.-Tech. 1928, 5, 45. (11) Cady, W. G. Physics 1936, 7, 237-259. (12) Nomura, T.; Tanaka, F. Bunseki Kagaku 1990, 39, 773-777. (13) Nomura, T.; Yanagihara, T.; Mitsui, T. Anal. Chim. Acta 1991, 248, 329335.
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Figure 1. Schematic illustration (side view) of the scanning electrode quartz crystal analysis (SEQCA) in the overscanning mode (A) and the underscanning mode (B).
as one of the two electrodes of an electrode-separated quartz crystal resonator. Figure 1 illustrates our concept of scanning electrode quartz crystal microscopy (SEQCA). The microscopy operates in two different modes: an overscanning mode (Figure 1A) and an underscanning mode (Figure 1B). In the former mode, a normal-size electrode is set on the side of a quartz crystal plate opposite to the “monitored” surface, and the microelectrode is scanned over the monitored surface. If one wants to monitor electrochemical processes, another electrode may be sputtered on the monitored side of the quartz crystal. In the latter mode, an electrode is sputtered on the monitored side of the quartz crystal, and the microelectrode is scanned under the side opposite to the monitored surface. In the present work, a SEQCA system is constructed (Figure 2) and evaluated. In particular, lateral resolution of SEQCA is examined. EXPERIMENTAL SECTION Materials. Two kinds of AT-cut quartz crystal plates, the edges of which are beveled (5 MHz, 12.5 mm diameter, flat region 10 mm diameter; 9 MHz, 8 × 8 mm2, flat region 6.2 mm diameter; see Figure 3), were obtained from Hokuto Denko (Tokyo, Japan) and Epson (Suwa, Japan), respectively. A gold wire (diameter 1.4, 1.0, or 0.5 mm) was used as a scanning electrode. A black waterproof ink (Hi-Mckee, Zebra, Japan), a 1 wt % toluene solution of polystyrene, a 1-2 wt % toluene solution of silicone (PRX305, Toray Dow Corning, Tokyo, Japan), or poly(dimethylsiloxane) (14) Mo, Z.; Nie, L.; Yao, S. J. Electroanal. Chem. 1991, 316, 79-91. (15) Takada, K.; Tatsuma, T.; Oyama, N.; Nomura, T. J. Electroanal. Chem. 1994, 370, 103-107.
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Figure 2. Diagram of the SEQCA system.
mobile. The sample surface can be visually monitored in situ with a microscope and a CCD camera. Surface topographies of the mass-loaded quartz plate were measured for comparison using a surface roughness analyzer (Surfcom, Tokyo Seimitsu, Tokyo, Japan).
Figure 3. Quartz crystal plates (5 and 9 MHz) used for the SEQCA measurements.
(viscosity 10 000 cSt, Aldrich) was cast on the quartz plate to examine the lateral resolution of SEQCA. Instruments. An impedance analyzer (4194A, YokogawaHewlett-Packard, Tokyo, Japan) was used for the SEQCA measurements. A semiautomatic probe (AWP-1050HR, Wentworth Laboratory, Sandy, England) on a float desk, which is covered with a shield box, was used for SEQCA measurements. In this equipment, the probe is immobilized and the sample stage is 1024
Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
RESULTS AND DISCUSSION Distribution of Resonance Properties. Two-dimensional distribution of the fundamental resonance properties of the quartz crystal resonator was examined using a scanning electrode of 1.0 mm diameter. The scanning electrode was set ∼0.01 mm above the 5 MHz quartz crystal plate. The maximum value of conductance (Gmax) and the resonance frequency (F) which gives Gmax are plotted as functions of the location in Figure 4 (5 × 5 ) 25 points). As can be seen, the resonance frequency is higher and the Gmax value is lower at the edge than at the center of the quartz plate. Lower Gmax means that the quartz is more difficult to resonate. At around the edge, asymmetric structure of the resonator may interfere with the resonance. Since the resonance frequency is defined as the frequency giving Gmax, resolution of the resonance frequency is low if the Gmax value is so low that its S/N ratio is bad. Therefore, the SEQCA measurements should be made around the center of the quartz plate for reliable mass mapping. Distance between the Scanning Electrode and the Quartz Crystal. Dependencies of the resonance frequency and the Gmax value on the distance between the scanning electrode and the 5 MHz quartz crystal plate are shown in Figure 5 (see Figure 4C for the location of the probe). A scanning electrode of 1.0 mm diameter was used. The frequency is higher and the Gmax value is lower at a larger distance. Thus, the distance should be as short as possible for better frequency resolution. These dependencies can be explained in terms of the voltage applied across the quartz plate. Since the voltage applied between the two electrodes is constant, the voltage applied across the quartz plate is lower at a larger distance. The resonance frequency and the
Figure 5. Dependencies of the resonance frequency (A) and the Gmax value (B) on the distance between the scanning electrode (1.0 mm diameter) and the 5 MHz quartz crystal plate. See Figure 4C for the location of the scanning electrode.
Data were collected at 90 points. The resonance frequency was higher and the Gmax value was lower for the thinner electrode. Resonance frequency F of a thickness-shear mode quartz crystal plate in a dimension of x × y × z (thickness) is given by16
F)
x(
)
2 2 2 1 m Cx n Cy p Cz + + 4F x y z
(1)
(m, n, p ) 0, 1, 2, 3, ...) Figure 4. Three-dimensional distribution of the resonance frequency (F, A) and the maximum value of conductance (Gmax, B). (C) Location for (A) and (B). A scanning electrode of 1.0 mm diameter was set ∼0.01 mm above the 5 MHz quartz plate.
Gmax value are known to depend on the applied voltage,15 and the obtained relationships are quite reasonable. The capacitance between the two electrodes, which decreases with increasing distance between the electrodes, might also contribute to the dependencies. For the ideal system, the capacitance between the electrodes is not included in the serial capacitance of the electrical equivalent circuit of a quartz crystal resonator but is included in the parallel capacitance of the circuit. However, completely separate measurements of the serial and parallel capacitances are sometimes difficult in the practical system. Diameter of the Scanning Electrode. Figure 6 shows the distributions of the resonance frequency and the Gmax value for scanning electrodes with different diameters (1.0 and 0.5 mm diameter). The scanning electrode was set ∼0.01 mm above the 5 MHz quartz crystal plate and scanned along the symmetry axis.
where F (2.65 g cm-3) is the density of quartz crystal, and Cx (8.55 × 1011 dyn cm-2), Cy, and Cz (2.95 × 1011 dyn cm-2) are the elastic constants. In the fundamental mode, m ) p ) 1 and n ) 0, so that eq 1 is rewritten as
F)
x(
)
1 Cx Cz + 4F x z
(2)
Thus, the resonance frequency is independent of x when x . z. However, if x is comparable to z, resonance frequency increases with decreasing x. If the resonance frequency of a quartz plate is 5.00 MHz when x is infinity, frequency is 5.03 MHz for x ) 5 mm, 5.75 MHz for x ) 1 mm, and 7.57 MHz for x ) 0.5 mm. However, the resonance frequencies observed for the 1.0 and 0.5 mm diameter electrodes were much lower than the theoretically expected values. This must be because the dimension of the quartz plate (9.0 mm diameter) is much larger than that of the (16) Bahadur, H.; Parshad, R. Phys. Acoustics 1982, 16, 37-131.
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Figure 7. Dependence of the resonance frequency changes on the mass loaded on the quartz crystal plate (5 MHz) and its lateral distribution. A scanning electrode of 1.0 mm diameter was set ∼0.01 mm above the quartz plate.
Figure 6. Distributions of the resonance frequency (A) and the Gmax value (B) for scanning electrodes with different diameters (1.0 and 0.5 mm diameter). The distance between the scanning electrode and the 5 MHz quartz crystal plate was ∼0.01 mm. See Figure 3A for the definition of x.
electrode; the area of the resonating region may be larger than the sectional area of the probe. It has been pointed out that the resonating region is larger than the overlapping region of the two electrodes.17,18 Additionally, eqs 1 and 2 are known to become worse for smaller x values.16 These are probably responsible for the quantitative discrepancy between the theory and the experiment. Distribution of Sensitivity. Next we examined the dependence of the resonance frequency on the mass loaded on the quartz crystal and its lateral distribution. Ta2O5 was sputtered on the quartz surface for 20, 40, 60, and 80 s. Mass of the Ta2O5 was measured using a normal AT-cut, 5 MHz quartz crystal resonator with a sensitivity of -56.5 Hz cm2/µg. In the SEQCA measurement, a scanning electrode of 1.0 mm diameter was set about 0.01 mm above the quartz plate. Figure 7 shows the results. Note that this figure shows dependencies on mass of ∆F, not F. At any position, the frequency decrease was proportional to the mass of the Ta2O5 coating. Mass measurements are, therefore, possible with the present SEQCA system. The sensitivity around the edge of the quartz plate (position 4 in Figure 7, -43.5 Hz cm2/µg) was lower than those at the other positions around the center (positions 1-3, -47 Hz cm2/µg). However, the sensitivities at positions 1-3 are close to each other. This is favorable for the mapping of mass distribution. Although the mass sensitivity should depend on the resonant frequency according to Sauerbrey’s equation, the difference in the resonant frequency values at points 1 and 3 is less than 0.01% (0.5 kHz) of the resonant frequency (17) Ward, M. D.; Delawski, E. J. Anal. Chem. 1991, 63, 886-890. (18) Hillier, A. C.; Ward, M. D. Anal. Chem. 1992, 64, 2539-2554.
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values (∼5 MHz), (Figure 6). This is clearly negligible compared to the scattering of the mass sensitivity. The measured sensitivity is lower than that expected from Sauerbrey’s equation. This should be due to an undesired vibrational mode(s) other than the thickness-shear. Undesired modes can be excited when the width and length of the resonator are comparable to the thickness. The Gmax value was almost independent of the film thickness. Analysis of Rigid Films. To analyze the mass distribution by means of SEQCA, a black waterproof ink or a 1 wt % toluene solution of polystyrene was cast as spots (1-3 mm diameter) on the 5 MHz quartz surface, and the solvent was evaporated. A sputtering gold spot (1 mm diameter) was also analyzed. The scanning electrode of 1.0 mm diameter was set about 0.01 mm above the quartz plate. Data were collected at 90 points. Figures 8 and 9 depict the results for the case where the ink was cast (one and three spots, respectively). Surface topographies measured using a surface roughness analyzer were attached for comparison. Decreases in the resonance frequency were observed at any location on the quartz plate, even though the ink was cast on a limited region. This means that the quartz plate oscillates to some extent at any point, in the thickness-shear or another mode. However, the frequency decrease around the ink spot was larger than that at a bare region. Therefore, one can know where the mass is loaded from the resonance frequency distribution. The lateral resolution was about 2 mm. Similar results were obtained for the cases of polystyrene and gold. In the case where the quartz was coated with homogeneous films of Ta2O5, frequency changes were the same at most regions of the quartz plate (Figure 7). Therefore, the inhomogeneous distribution of the resonance frequency observed in the present experiments is ascribed to the deposition of the ink, polymer, or gold. Although the ink and polymer films were not very homogeneous, as shown in Figures 8C and 9C, the frequency distribution was quite similar to that of a more homogeneous gold film. This reflects the fact that the lateral resolution is not enough to see inhomogeneous feature of the ink and polymer films. Gmax increases were observed in the region where the ink or polystyrene was cast. An increase in the Gmax value was reproducible and has been observed for some different systems when a
Figure 8. Distribution of the resonance frequency change (A) and Gmax change (B) after the mass loading (an ink spot) on the 5 MHz quartz plate. A scanning electrode of 1.0 mm diameter was set ∼0.01 mm above the quartz plate. (C) Surface topography of the quartz plate. See Figure 3A for the definition of x.
resonator was coated with a rigid film.19 In the other region, Gmax decreases were observed. Although reasons for the Gmax decreases are not clear in the present stage, one can know whether the spot is rigid or not from Gmax changes. The ∆Gmax value at the edge seems higher than that at the center region, though the ∆F value did not show a clear difference between the edge and center regions. This suggests that the lateral resolution of Gmax is better than that of F, or Gmax may depend on the permittivity of the ink, because more than 50% of the probe-quartz distance is filled with the ink at the edge of the spot, while only 10% is filled at the center region. Analysis of a Viscous Film. Analysis of the viscoelastic film was also conducted. A 1-2 wt % toluene solution of silicone was cast (
