Direct Measurement of In-Liquid Particle Masses Using Quartz Crystal Microbalance

Abstract

ABSTRACT This study proposes the utilization of Quartz Crystal Microbalance (QCM) for measuring masses of in-liquid particles. A QCM has been widely used as gas sensors, biosensors, and thin-film monitoring system because it shows high sensitivity, high resolution, and non-destructive tests in situ. However, measuring particle mass or concentration in liquid solution becomes more challenging due to complex fluidic motion in droplet, nucleation mechanism, and particle sliding effect in the evaporation process. In an effort to overcome such limitations, we decorated Carbon Nanotube (CNT) film on our sensors. A layer of CNT creates a surface roughness on QCMs. This roughness affects nucleation mechanisms of ionized droplet and particle sliding effect in the evaporation process. Ultimately it could enhance adhesion parameters for a stable operation of QCM. Additionally, CNT coated QCMs (CNT-QCMs) show a wide detectable range of over 10 μg with 40 pg Limit of Detection (LOD). CNT-QCMs also exhibit higher Quality factor (Q) values compared to bare QCMs. Such improvement in Q also reduces the power consumption and noise level of QCM integrated oscillator system. In addition, we have verified frequency response of QCM system under environmental disturbances, such as humidity and temperature. To verify humidity dependency of QCM particle mass sensor, we have experimented wet particle and dry particle generated from electrospray system with a heating system. Our results indicate that QCMs become unreliable for measuring hygroscopic particles with increasing relative humidity. In addition, our study reveals that the shift in operating temperature affects the resonance frequencies, which correspond to about 400 ng. Depending on Q, QCMs show five times deviation in the frequency response with temperature. To eliminate these environmental disturbance effects, we are planning to fabricate heater integrated QCM system and compensate environmental error from the reference QCM. We envision that our QCM sensors could have far-reaching applications for water quality monitoring system, detection of metal ion in semiconductor cleaning system, and even for airborne particle mass detection.