Measurement of solvent vapor adsorption by polydimethylsiloxane using quartz crystal microbalance

A new micro-electromechanical system (MEMS) gas sensor has been developed using quartz crystal microbalance (QCM) with adsorbent coated in form of nanofibers on the QCM sensor. The nanofibers with fiber diameter typically around 200-300 nm increases the specific surface area to enhance adsorption. The QCM is made to oscillate at its natural resonance frequency. Upon exposure of the gas sensor to a given gas, the adsorbed gas onto the nanofibers adds a small mass which changes the natural frequency of the oscillation. By detecting the frequency shift due to adsorption of a given gas, the presence of the gas is detected, and by measuring the frequency shift, the amount of gas being adsorbed at a given pressure and temperature is quantified via the Sauerbrey equation [1]. A circuit has been developed to read the frequency shift due to the energy dissipation in the QCM coated with Polydimethylsiloxane (PDMS) nanofibers under the environment of several solvent vapors. The developed circuit includes two crystal oscillator circuits, two QCM's which are respectively 1MHz reference QCM and a coated QCM, RC filter and AND gates. The results of the frequency shift between the reference QCM and the coated QCM were recorded on the oscilloscope so as to investigate the relationships between the frequency shift and the amount of vapor adsorbed for different gases. Ultimately, Volatile Organic Compounds (VOCs) are the target to be monitored and a MEMS based sensor will be developed similar to the present QCM gas sensor discussed herein. This work provides the feasibility study for using nanofiber coating to enhance the adsorbent specific area and a stand-alone QCM sensor for making measurement.