Development of a Battery-Operated Handheld QCM Measurement Electronics

Abstract

There continues to be major interest in detection and discrimination of volatile organic compounds (VOCs) because these compounds are pervasive in common environments, such as buildings, factories, and in households, among others. These VOCs can be detrimental to both human health and organisms in the environment. The quartz crystal microbalance (QCM) has become a popular analytical instrument to detect various VOCs [1]. However, most QCM studies are performed in a laboratory setting with cumbersome benchtop instruments. To utilize the QCM technique in a manner that permits researchers to be outside of a laboratory environment, such as VOCs detection on-site and point-of-care testing (POCT), it is important to develop a suitably portable QCM measurement system that maintains acceptable system performance, such as frequency resolution and measurement time, which requires low-power input [2, 3]. To fulfill this demand, we propose a QCM measurement electronic system that uses a phase-locked loop (PLL) circuit as a frequency-to-voltage converter and measures an output voltage level to calculate frequency shifts of the resonator. Our proposed system does not rely on the quality of a clock frequency, which is important in typical frequency counters to improve measurement performance. Instead, a common microcontroller is used to calculate frequency shift. In this work, a portable QCM system based on this strategy was developed, and its performance successfully characterized by measuring changes in system output voltage corresponding to input frequency changes. Temperature dependence of frequency changes was also investigated from 15 to 55 °C. With a resonance frequency of 5 MHz, the system showed a frequency resolution of 0.22 Hz, which is significantly less than a common frequency counter. For validation, we employed Ionic Liquids and a Group of Uniform Materials based on Organic Salts (GUMBOS) as chemosensitive coating materials for VOCs detection [4]. Measurement results of the frequency shift caused by VOCs from the developed system were comparable to those obtained from experiments with a commercial system (QSense Analyzer, Biolin Scientific), which demonstrates the feasibility of applying this electronic strategy for development of a portable QCM measurement system.