Anal. Chem. 1993, 65, 2177-2180
2177
Flexural Thin-Rod Acoustlc Wave Devices as Chemical Sensors Paul C. H. Li, David C. Stone, and Michael Thompson; Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 1AI
INTRODUCTION Three main types of acoustic devices are currently being employed as the architecture of chemical sensors. These are the bulk acoustic wave (BAW),surface acoustic wave (SAW), and plate mode devices. Recently, Jen et al. developed a thin-rod acoustic wave (TRAW) device based on the propagation of acousticwaves in a cylindrical rod.' A mass loading on the thin rod induces a change in the phase velocity of the acoustic wave propagated in it, thus providing a means for chemical sensing. The thin rod refers to a wire of circular cross section with a radius much smaller than the acoustic wavelength. Piezoelectric transducers, incorporated in various geometries, can generate and receive longitudinal or flexural waves with respect to the thin rod in a delay line configuration similar to that employed for the SAW device. The entire rod vibrates analogous to the plate mode device; however, the rod does not have to be fabricated from a piezoelectric material, since the propagation of acoustic waves is based entirely on mechanical processes. The torsional mode can also be investigated, but in this case the rod must be made of electrostrictive materials such as nickel. For the purposes of chemical sensing, the TRAW device offers a number of advantages over other acoustic devices. In addition to the fact that the thin-rod material does not have to possess piezoelectricproperties, the device can be fabricated in a very small geometricconfigurationand can be constructed from metal, which allows ita use as an electrode in simultaneous liquid-phase experiments. If the radius of the rod is chosen to be very small (ca. 10 pm), ita mass sensitivity, Smv, a t an operational frequency of 1MHz, is very high compared to the BAW and SAW counterparts. With respect to the flexural mode, the TRAW device offers a further advantage of the possibility of liquid operation since the phase velocity of the flexural wave is smaller than the propagation velocity of an acoustic wave in a liquid medium. This situation is similar to that for the flexural plate mode device.2 In the present paper, we concentrate on the flexural mode, and thus the acoustic device is termed the flexural TRAW (or FTRAW) device. Various geometries can be applied to excite this mode. The type employed in this work is shown in Figure 1. In the delay line configuration, when an alternating current of radio frequency is fed into the ultrasonic longitudinal transducer, an ultrasonic wave is generated with the vibration perpendicular to the transducer surface. This induces the glass horn, which is bonded to the transducer surface by phenyl salicylate, to vibrate together with it. With a 90° coupling geometry, the longitudinal wave is converted into the flexural wave, which then propagates to the other end of the delay line. The reverse process occurs a t the receiving transducer, giving rise to a signal. In view of the fact that there has been, to our knowledge, no experimental work performed to date to test the mass response of the FTRAW device, the first segment of the present work was conducted to confirm apositive-phase shift upon mass loading as predicted by theory. Second, the effect of gas flow rate, the amounts of coating, and the nature of (1)Jen, C.K.;Oliveira, J. E. B.; Yu, J. C. H.; Lai,J. D.; Busrriere, J. F.Appl. Phya. Lett. 1990,56(22). 2183-2186. (2)Wenzel, S.W.; White, R. M. ZEEE Trans. Electron Deuicea 1988, 35 (6), 736-743. 0003-2700/93/0365-2177$04.00/0
Flexural wave
v-
Gold Wire
Receiving Transducer
Generating and receivlng flexural wave In a delay configuration. horn materials on the phase shift were investigated. The effect of passing an electric current in the FTRAW device was also studied. Finally,electrochemicaldepositionof copper onto the thin rod was performed to demonstrate the operation of the device in the liquid phase. Figure 1.
THEORETICAL BACKGROUND In the low frequency limit, the lowest flexural mode Fll of a thin rod can be correctly described by the elementary theory of flexure,'
+
la-
m-a2u E&4u- 2u= 0 at2 az4 az2 where U is the transverse displacement of the F11 mode; z is the propagation distance along the rod; m = ra2p is mass per unit length of the rod; T is the tension exerted on the rod; I = ra414 is the moment of inertia of the cross section about the neutral plane of the rod; and E is Young's modulus for the material of the rod. Assuming
u = A&Ut - kZ) where A is the amplitude, k is the propagation constant, w = 2 r f , and f is the operational frequency. The phase velocity V = wlk of the F11mode can be expressed as follows: (A) For zero tension (i.e., T = 0)
v = vO = w1'2a(z) u~ 114 This is very similar to the case in Ao mode of the plate mode device.2 (B) For small tension, i.e.
p
