3F-3 Liquid Sensor Using SAW and SH-SAW on Quartz - IEEE Xplore

Liquid Sensor Using SAW and SH-SAW on Quartz Takashi Kogai and Hiromi Yatsuda Research & Development Department Japan Radio Co., Ltd. Fujimino-shi, Japan [email protected] relative permittivity and conductivity of the liquid have been studied for a long time. However, there has been a few papers for miniature one-chip biosensor using acoustic sensor devices.

Abstract— One-chip sensor system using two different acoustic waves on ST-cut quartz is presented; one is shear horizontal surface acoustic wave (SH-SAW) and the other is surface acoustic wave (SAW). On the sensor chip, there is a SH-SAW delay-line that is composed of a transmitting interdigital transducer (IDT), receiving IDT and a biochemical reaction area in between them. And there is another IDT on the sensor chip, that can excite a Rayleigh type SAW to the biochemical reaction area in the direction normal to the SH-SAW propagation direction. In order to evaluate the performance of the SH-SAW delay-line sensor, C-reactive protein antibodies with different concentrations are provided to the biochemical reaction area. The phase change in the S21 response of the SH-SAW delay-line at a fixed frequency are measured on real-time and it is confirmed that different phase changes are obtained for different antibody concentrations. On the other hand, the one-chip quartzbased liquid-phase SH-SAW delay-line sensors with pumping and agitating functions using SAW are demonstrated.

On the other hand, since a Rayleigh type SAW can be attenuated in liquid, it has been believed that the SAW cannot be utilized for liquid-phase sensor devices. However, after the phenomena that liquid on the surface of the device is dynamically moved and streamed toward the SAW propagation direction was found [6], some papers about the positioning systems of small liquid droplet using SAW [7,8] have been published. The LiNbO3 substrates were generally used for the SAW fluidic-systems because there are some SAWs that can be efficiently excited along some axes within the wafer plane. This paper presents a concept of one-chip biosensor systems using SAW and SH-SAW. The SH-SAW can be utilized for sensing and the SAW can be utilized for pumping or agitating. This work is the first step for a one-chip multifunctional biosensor or “lab-on-a-chip”.

Keywords; Sensor; SAW; SH-SAW; C-reactive protein;

I. INTRODUCTION Miniature biosensor systems have been required in some applications such as environment, food industry and medicine, since miniaturization allows to reduce the reagent volume and to shorten process time by diffusion driven reactions. Those sensor systems [1], such as a lob-on-a-chip or micro total analysis systems (uTAS), generally require a sensing function and a fluidic function on the chip.

SAW

Acoustic wave based sensors are suitable for miniaturization. Those sensors have been successfully investigated for the detection of bio-chemical compounds due to the need for real-time, rapid and direct detection where the device is in direct contact with the solution. Quartz crystal microbalances (QCMs) are one of the most popular acoustic devices in the field of bio-chemical applications and there are many papers about QCM immunosensors [2]. On the other hand, it has been known that shear horizontal surface acoustic waves (SH-SAWs) are suitable for liquid-phase sensors [3-5]. Since SH-SAW has a horizontal polarization in the direction normal to the propagation direction parallel to the substrate, the SH-SAW energy is less radiated into the liquid. Although some SH-SAWs exist on several substrates, LiTaO3 [3], La3Ga5SiO14 [4] and Quartz [5], the liquid sensing systems using SH-SAW on LiTaO3 substrate to detect the density and viscosity products,

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3200m/s SH-SAW 5100m/s

Quartz Wafer

Figure 1. SAW and SH-SAW on Quartz substrate.

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II.

SENSOR DEVICE USING SAW AND SH-SAW

SH-SAW : Sensing

A. SAW and SH-SAW on Quartz Substrate Two different acoustic waves, a Rayleigh type SAW and an SH-SAW can be efficiently excited on 37° rotated Y-cut Quartz substrate as shown in Fig. 1. The Rayleigh type SAW on 37° rotated Y-cut Quartz substrate can be effectively excited to x-direction on the surface. The elector-mechanical coupling coefficient of the SAW is about 0.16 %. It is not so big but good enough to excite SAW that can push or agitate a droplet on the substrate. On the other hand, the SH-SAW can be efficiently excited normal to the direction of the SAW propagation direction on the substrate. The SH-SAW can be suitable for liquid-phase sensors that can provide real-time, rapid and direct detection where the device is in direct contact with the solution. The SH-SAW on the Quartz substrate has a big advantage of excellent temperature stability in contrast to other SH-SAW on LiTaO3 substrate. The first order temperature coefficient is almost zero and the second order temperature coefficient is around -0.04 ppm / °C2 . On the other hand, the SH-SAW on LiTaO3 substrate has a temperature coefficient of about -35 ppm / °C .

SAW : Transportation

Reaction Area

Figure 2. One-chip sensor using SAW and SH-SAW on Quartz substrate.

Epoxy wall

B. One-chip Liquid-Phase Sensor Using SAW and SH-SAW A concept of one-chip liquid-phase sensor on Quartz substrate is shown in Fig. 2. On the sensor chip, there is a SHSAW delay-line that is composed of a transmitting interdigital transducer (IDT), receiving IDT and a biochemical reaction area in between them. And there is another IDT that excites a Rayleigh type SAW to push or agitate a droplet. When a Rayleigh type SAW is propagating under the droplet on the substrate, the SAW is attenuated and radiated a longitudinal wave into the droplet. If the SAW amplitude is high enough, the force induced by acoustic streaming is high enough to push the droplet in the SAW propagation direction. When the SAW amplitude is low, the droplet cannot be moved but the longitudinal wave that was radiated into the droplet can shake or agitate the droplet. III.

Reaction Area

Interdigital transducer (IDT) Figure 3. Configuration of SH-SAW liquid-phase sensor.

BASIC E XPERIMENT OF SH-SAW LIQUID-PHASE SENSOR ON QUARTZ

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Insertion Loss [dB]

A. SH-SAW DelayLline Sensor with Liquid Cell Surrounded with Epoxy Wall In order to evaluate the basic performance of the SH-SAW liquid-phase sensor on Quartz, we designed an SH-SAW delayline with a liquid cell as shown in Fig. 3. The transmitting and receiving SH-SAW IDTs are placed with a center-to-center distance of 6 mm between them. A liquid cell is placed between the IDTs that was surrounded with epoxy wall in order to protect the IDTs from liquid. The electrode periodicity of the IDT is a 20-micron-meter wavelength with double electrode fingers and the center frequency of the SH-SAW delay-line is around 250 MHz. The IDTs have an aperture of 2 mm and a number of finger pairs of 50.

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Figure 4. Frequency responses of SH-SAW liquid-phase sensor.

In the liquid cell, there is a reaction surface that was covered with evaporated gold film. The reaction surface was surrounded with epoxy wall with a height of 60 micron meters. The wall was realized by a photo-lithography technique using

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photosensitive epoxy film with a 60 micron meters thickness, that is a popular material for Micro Electro Mechanical Systems technology. The SH-SAW can be attenuated under the epoxy wall. The propagation loss of about 0.75 dB/wavelength at 250 MHz SH-SAW was obtained in our experiments. The photolithography technique provided a thin wall with a thickness of 40 micron meters or about two wavelengths for the 250 MHz device. Then the increase of insertion loss of SHSAW delay-line due to epoxy wall was about only 3 dB.

Personal Computer

Network Analyzer

GP-IB

B. Experimental results The frequency response of the SH-SAW delay-line sensor device is shown in Fig. 4. The dashed line shows the response without water. The insertion loss was 27 dB including the propagation loss of 3 dB at epoxy wall. The solid line shows the response with water. The increase of the insertion loss was about 12 dB due to liquid-phase sensing on the shorted layer at reaction area.

Fractional velocity change (∆V/V)

Figure 5. Measurement system.

A schematic diagram of the measurement system is shown in Fig. 5. The sensor devices are placed in the oven with a constant temperature at a 25 degree centigrade. The phase response and the insertion loss of the SH-SAW delay-line are measured using a vector network analyzer and a personal computer. When the SH-SAW is propagating at the reaction area, the SH-SAW propagation characteristics which are the phase response and insertion loss can be changed. Since the surface of the reaction area was covered with evaporated gold film, the mechanical perturbation can be efficiently detected. Then when buffer liquid with antibodies is injected into the liquid cell, the antibodies can be absorbed onto the gold surface. The velocity of the SH-SAW at the reaction area covered with gold film can be changed due to antibodies absorption. Figure 6 shows some experimental results using buffer liquid with C-reactive protein antibodies [2]. C-reactive protein antibodies with different concentrations were provided to the biochemical reaction area. The phase change in the S21 response of the SH-SAW delay-line at a fixed frequency has been monitored on real-time. The vertical axis is the fractional velocity change of the SH-SAW that was obtained from the measured S21 phase changes. It was confirmed that different velocity changes were obtained with different antibody concentrations. The insertion losses were almost the same with different concentrations. Antibody in the buffer can be adsorbed spontaneously to the gold surface in the reaction area. The different velocity changes proved that the amount of the antibody adsorbtion on the gold surface are different with different concentrations. SH-SAW velocity can be changed by surface condition with antibody adsorbtion at the reaction area. IV.

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Figure 6. Fractional SH-SAW velocity changes with different antibody concentrations.

SH-SAW Sensing

SAW Transportation

PUMPING AND AGITATING SYSTEM USING SAW

Two types of one-chip sensors using SAW and SH-SAW were demonstrated for pumping and agitating. Figure 7 shows the one-chip sensor system using a 50 MHz SAW and a 250 MHz SH-SAW in this study.

Reaction Area

A. One-chip sensor using SAW for pumping For biosensor devices, some processes may be required, which are to inject a droplet to the reaction area, to rinse the

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Figure 7. One-chip SH-SAW sensor with pumping and agitating system using SAW.

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Time [min] Figure 9. Experimental result of agitating system using SAW.

Figure 8. Experimental result of pumping system using SAW.

reaction surface and to react the immunoassay at the reaction area. After the droplet with 20 micro litters is placed at the dispense area using a dispense machine, the droplet can be transported to the reaction area by the SAW launched at the SAW IDT. The Figure 8 shows an experimental result of this process. The insertion loss and phase changes of the SH-SAW delay-line response were measured during some steps, 1)a droplet with Latex [9] with 20 micro litters was dispensed at the dispense area, 2)after 7.5 minutes, the SAW was exited for 2 minutes. As shown in Fig. 8, the droplet was transported to the reaction area by the SAW and the fractional velocity change of the SH-SAW delay-line was observed.

phase SH-SAW delay-line sensors with pumping and agitating functions using SAW were demonstrated. This work is the first step for a one-chip multifunctional biosensor or “lab-on-achip”.

B. One-chip sensor using SAW for agitating Figure 9 shows the insertion loss and phase changes of the SH-SAW delay-line response. Those were measured during some steps, 1)started the measurement, 2)after 5 minutes, a droplet with Latex with 20 micro litters was dispensed at the reaction area, 3)after 10 minutes, the SAW was exited for 2 minutes.

[2]

REFERENCES [1]

[3] [4]

After the droplet was injected into the reaction area, the SH-SAW velocity was changed slowly for the next 10 minutes. After the SAW was excited, the SH-SAW velocity was changed rapidly. It means that the reaction between the gold layer on the reaction area and Latex in liquid can be accelerated by the SAW. The SAW can be used for agitating or mixing the liquid. V.

[5]

[6] [7]

[8]

CONCLUSION

A concept of a one-chip biosensor system using SAW and SH-SAW was presented. The SH-SAW can be utilized for sensing and the SAW can be utilized for pumping or agitating. On the sensor chip, there is an SH-SAW delay-line that is composed of a transmitting IDT, receiving IDT and a biochemical reaction area in between them. And there is another IDT on the sensor chip that can excite a Rayleigh type SAW to carry a droplet or to agitate a droplet at the biochemical reaction area. The one-chip quartz-based liquid-

[9]

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