Anal. Chem. lB99, 65, 1925-1927
1925
CORRESPONDENCE
Adsorption, Immobilization, and Hybridization of DNA Studied by the Use of Quartz Crystal Oscillators Shuichiro Yamaguchi and Takeshi Shimomura Pharmaceutical Department 2, R&D Center, Terumo Company, Nakai-machi, Ashigarakami-gun, Kanagawa 259-01, Japan
Tetsu Tatsuma and Noboru Oyama. Department of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, Naka-machi, Koganei, Tokyo 184, Japan Quartz crystal oscillators have been used as microbalances, the so-called quartz crystal microbalance, because their resonant frequencies change by mass loading onto the surface of electrodes of the oscillator.' However, the resonant frequency has been revealed to be sensitive not only to mass but also to viscosity, elasticity, surface roughness, and so forth of the adhesion layer, and a rigorous measurement of mass haa been found to be very difficult. To overcome this problem, we2 and some other groups1p3are trying to characterize the surface layer of quartz crystal oscillators on the basis of the electrical equivalent circuit of the oscillator (Figure 1). As seen in the figure, the circuit consists of inductance L1, which reportedly correspondsto mass of the vibrating body (a quartz plate, electrode, and adhesion layer), capacitance C1 corresponding to mechanical elasticity of the vibrating body, resistance R1 corresponding to a loss of mechanical energy dissipated to a surrounding medium, and capacitance CO between the two electrodes on the quartz plate. These parameters can be evaluated from the conductance (G) frequency and susceptance (B)frequency spectra which can be obtained through impedance measurements. Conductance and susceptance are the real and imaginary parts of the reciprocal of impedance, respectively, and impedance equals VII, where V and I are ac voltage and current, respectively. That is, several factors affecting the resonant frequency can be separated by the impedance measurements. In the present work, this method is exploited for the study of adsorption and immobilization of DNA onto solid surfaces and of hybridization of the immobilized DNA. Although DNA sensinghas been attempted by the use of a conventionalquartz crystal microbalance modified with single-stranded DNA, which monitors the resonant frequency alone: such a microbalance is sensitive to many factors as mentioned above. Thus we employed the impedance technique for the present study. This work is the first study on the behavior of DNA on solid surfaces based on impedance measurements of quartz crystal oscillators.
EXPERIMENTAL SECTION Au-coated quartz crystal oscillators (5-MHz fundamental mode, 12-mm diameter) were prepared by successive sputtering (1) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992,92,1366-1379. (2) Okajima, T.; Sakurai, H.; Oyama, N.; Tokuda, K.; Ohsaka, T. Bull. Chem. Soc. Jpn. 1992,65,1884-1890; Electrochim. Acta 1993,38,747756. (3) Muramatau, H.; Kimura, K.; Muramatau, H.; Kimura, K. Anal. Chem. 1992,64, 2602-2607. (4) Okahata, Y.;Mataunobu, Y.;Ijiro, K.; Mukae, M.; Murakami, A.; Makiio, K. J. Am. Chem. SOC.1992,114,8299-8300. 0003-2700/93/0365-1925$04.00/0
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C0 Flgure 1. Electrical equivalent circuit for an AT-cut quartz crystal
oscillator.
of chrome (50 A) and gold (
