A QCM-VNA System for Microfluidic Sensing: Leveraging Higher-Order Harmonics

This paper presents the design and development of a QCM-based VNA measurement system capable of detecting magnitude and phase responses of higher-order harmonics with a 19-Hz resolution. A fast-locking algorithm is implemented to efficiently track changes in the QCM response within a microfluidic channel. The S11 reflection measurements obtained from the VNA are used to extract the magnitude and phase characteristics of the QCM. Measurements were performed at the fifth harmonic (50 MHz) of the quartz crystal. The QCM response can be detected in 12 seconds for 500 data points using the fast sweep and locking algorithm. The measurement results are taken on glycerol in 2%, 5%, and 10% (v/v) concentrations. Limit of detection (LOD) was calculated as 2% glycerol. The baseline stability of the 5th overtone resonance peak in deionized water was calculated approximate to at 35 Hz.