OPERATION OF AN ULTRASENSITIVE 30-MHZ QUARTZ-CRYSTAL MICROBALANCE IN LIQUIDS

A new ultrasensitive quartz crystal microbalance (QCM), based on chemi cally milled 30-MHz AT-cut shear mode crystals, for operations in gas and liquid phases, is described. In liquid media, changes in the liqui d viscosity (eta(L)) and density (rho(L)) lead to changes in the reson ant frequency (DELTAf) of the QCM. A linear relationship between DELTA f and (eta(LrhoL))1/2 was observed, in agreement with theory. Impedanc e analyses also showed a linear dependence of equivalent resistance on (eta(LrhoL))1/2. The capabilities of these resonators in electrochemi cal QCM (EQCM) applications are demonstrated by the electrodeposition of copper at the QCM surface. Simultaneous measurements of frequency a nd charge afforded sensitivity constants that were in exact agreement with theory, indicating that energy trapping of the fundamental mode i s very efficient for the 30-MHz resonators. The high sensitivity of th ese devices portends their use in applications such as miniature visco meters, chemical and biological sensors, and for fundamental investiga tions of interfacial processes. These results also suggest that miniat urization and microfabrication of shear mode devices, which will requi re thinner quartz crystals operating at high frequency, can be accompl ished with retention of performance.