(Invited) High-Sensitive Electrochemical Detector with Micro-Stirrer Driven By Surface Acoustic Wave
Authors: Sakamoto Hiroaki , Amaya Satoshi , Saiki Tsunemasa , Takamura Eiichiro , Suye Shin-ichiro 
Published: December 2020

Micro total analysis system (μTAS), by applying microfabrication technology used in the manufacture of semiconductor integrated circuits, applies chemical processing and biochemical analysis usually conducted using test tubes in a laboratory to microscopic flow paths fabricated on a chip, and it is an ultra compact analytical system.
Advantages of μTAS are that it requires only a small amount of sample used for measurement, the space in the groove is narrow, so that the diffusion distance of molecules can be shortened, the reaction time can be shortened, automation of measurement including pretreatment, miniaturization of the equipment, disposable of the equipment, low cost, reduction of manpower and the like can be cited.
If μTAS is put into practical use, it is expected that everyone can easily and accurately inspect on site. However, as μTAS uses extremely small samples such as tens of microliters, the issues such as handling difficulty and reproducibility remain.
Turning to the electrochemical sensor, electrochemical measurement is effective as a detection method in μTAS because it has advantages such as fast response, inexpensive, easy to miniaturize, and so far, many electrochemical biosensors have been reported. Generally, in the electrochemical measurement, based on the electron transfer between the analyte on the electrode surface and the electrode, the analyte is detected from the current value. In the analysis with a sample volume of several milliliter scale, the agitator is placed in the solution cell and stirring is carried out, whereby the supply to the electrode surface of the measurement analyte is enhanced, whereby high sensitivity can be detected. However, μTAS can not place a stirrer because it uses a small amount of tens of microliters, and it is difficult to stir manually, the diffusion of the detection analyte to the electrode surface becomes rate-limiting, causing a decrease in sensor sensitivity.
Therefore, in this research, we focused on an actuator using surface acoustic wave (SAW) in order to solve the problem that sensor sensitivity decreases in electrochemical measurement of a small amount of sample. Surface acoustic waves are vibrations that concentrate and propagate on the surface of an object and can be generated by applying a high frequency potential to the IDT (Inter digital transducer) fabricated on the surface of the piezoelectric substrate. IDT can be easily fabricated by conventional photolithography technology. Since surface acoustic waves can be driven by microscale pattern electrodes fabricated by microfabrication, they are suitable for handling a very small amount of liquid droplets. Many reports have been reported as droplet flow manipulation techniques by using SAW. In the field of biotechnology, application to PCR with enhanced reaction efficiency of trace amounts of samples has been reported. Many researches have also been reported in which SAW is applied as a conversion element of a biosensor. A detection method by label free is proposed by measuring the frequency shift due to adsorption to the sensor surface.
In recent years, co-authors reported a technology to place a small amount of 5 μL droplets in the pool and drive them by SAW. They also studied the relation between the power supplied to the IDT and the angular velocity and temperature of stirring the liquid using the SAW driven micro liquid rotor.
If we could fuse this SAW technology with electrochemical measurement and stir a small amount of sample, diffusion of the target substance to the electrode surface increased, and we thought that we could solve the problem of sensitivity reduction due to diffusion control. In this research, we propose a novel measurement system which electrochemical detection is carried out by integrating 3 electrode system for electrochemical detection on SAW substrate developed by bicycle, and stirring minute amount of sample by SAW. As a result, it is possible to increase the diffusion rate of the analyte in the small amount sample to the electrode surface, aiming at constructing an electrochemical detection system with high sensitivity and good reproducibility.

Figure 1