Study On Miniaturized High-Frequency Quartz Crystal Microbalance

Quartz Crystal Microbalance (QCM) is a novel type of micro-mass sensor, which allows accurate measurements of mass, viscosity and shear modulus on nano-scale. It has been applied in many fields for its high accuracy, simple structure and low-cost. A flow-injection-based high-frequency miniaturized QCM system is proposed in this paper, which consists of a liquid injection pump, an injection valve, a flow cell and oscillating circuit. The mass-frequency sensitivity of QCM is proportional to the square of its fundamental frequency, so increasing the frequency can improve the sensitivity; On the other hand, the fundamental frequency is inversely proportional to the thickness of quartz wafer, however a thinned quartz chip with degraded mechanical strength would become fragile and difficult for assembling and further biomembrane modification. In order to overcome this problem, an inverted-mesa structure is proposed and fabricated in this work. The central region is thinned to obtain high frequency vibration, and the outer ring retains its original thickness for maintaining the necessary mechanical strength needed for chip assembly and handling. The chip is fabricated by a cheap wet etching process and characterized with the high-accuracy impedance analyzer Agilent 4294A. The test results demonstrated the success of design and fabrication for 40-50 MHz QCM chips with high quality factor higher than 20000 in air and 1000 in water respectively. As a popular analysis method, the flow injection technology has the advantages of real-time, fast-reaction and saving the sample volume. The spring needle contacting electrode based three-layer flow cell is developed in the research with simple structure and easy assembling. Most important of all, the reusability is guaranteed with this flow cell. A simple oscillating circuit is designed for driving the achieved high frequency QCM sensor. The loop gain and phase are adjustable, which enhance the operability and portability for the QCM measurement system. The output frequency and the equivalent circuit parameters of the sensor facing liquid on one side are measured and recorded with the flow-injection-based high-frequency QCM system. Under the stimulus of the oscillation circuit, the QCM chip can not only work normally in the liquid phase and the frequency stability is also high. Finally, the immunoreaction between the antigen and antibody confirms the feasibility of the on-line monitoring of the flow-injection-based QCM system.