High Precision Liquid Salinity Sensing Using a CNT Coated Quartz Crystal Microbalance

Quartz crystal microbalance (QCM) has been used as a resonant sensing platform for chemical, biological, and mechanical events detection. Specifically, QCMs have shown great potential towards a particle sensing as the added mass induces a linear shift in resonant frequency. Although a QCM is an economic solution for the mass sensing of solid thin films, QCMs generally become unreliable for in-liquid particles analysis due to a rather complex fluidic motions and coffee ring effect of liquid droplets. Specifically, uncontrollable agglomerations of particles hinder a stable QCM operation and ultimately limit its mass sensitivity. This paper presents the integration of a layer of Carbon Nanotubes (CNTs) on a QCM for an accurate sensing of the ion concentration in liquid, or salinity. The integrated CNT layer induces a controllable nm-resolution roughness on QCMs, and such roughness affects the nucleation behavior of ionic particles and adhesion parameters, ultimately improving the particle adhesion for a stable QCM operation. CNT-QCMs exhibit a mass sensing range of up to over $10 ~\mu \text{g}$ with about 40 pg measurement resolution. Moreover, CNT-QCMs maintain higher quality factor ( ${Q}$ ) compared to the bare QCM, and such improvement in ${Q}$ could directly determine the power budget and noise performances of the QCM integrated oscillators or sensor systems. We believe our work can contribute to build an advanced sensor system for water quality monitoring and detection of liquid ion concentration in semiconductor fabrication processes.