Operation of an Ultrasensitive 30-MHz Quartz Crystal Microbalance in Liquids

A new ultrasensitive quartz crystal microbalance (QCM), based on chemically milled 30-MHz AT-cut shear mode crystals, for operations in gas and liquid phases, is described. In liquid media, changes in the liquid viscosity (ηl) and density (ρl) lead to changes in the resonant frequency (Δf) of the QCM. A linear relationship between Δf and (ηlPl)^1/2 was observed, in agreement with theory. Impedance analyses also showed a linear dependence of equivalent resistance on (ηlPl)^1/2. The capabilities of these resonators in electrochemical QCM (EQCM) applications are demonstrated by the electrodeposition of copper at the QCM surface. Simultaneous measurements of frequency and charge afforded sensitivity constants that were in exact agreement with theory, indicating that energy trapping of the fundamental mode is very efficient for the 30-MHz resonators. The high sensitivity of these devices portends their use in applications such as miniature viscometers, chemical and biological sensors, and for fundamental investigations of interfacial processes. These results also suggest that miniaturization and microfabrication of shear mode devices, which will require thinner quartz crystals operating at high frequency, can be accomplished with retention of performance.