Scanning Electrochemical Mass Sensitivity Mapping of the Quartz Crystal Microbalance in Liquid Media

The mass sensitivity distribution of the quartz crystal microbalance, denoted S(r,θ), was determined in sltu for plano-plano and plano-convex AT-cut quartz resonators. The method Involved measuring the frequency change associated with localized mass deposits created by electrochemical deposition using a scanning microelectrode in close proximity to the QCM electroded surface. Complete mathematical descriptions of S(r,θ) were obtained from the measured data under conditions commonly encountered in QCM experiments. Our experimental results suggest that crystal contouring increases sensitivity toward the resonator center while reducing the extent of field fringing. Increasing solution viscosity reduces the sensitivity to mass changes in the center of the resonator and increases field fringing, while an increase in the rigid mass within the electroded area increases the sensitivity in the center of the resonator. Under most conditions, S(r,θ) appeared dependent upon electrode geometry and tab placement but exhibited negligible dependence upon the crystal orientation with respect to the shearing axis. The integral sensitivity, as determined by Integration of the sensitivity maps, was significantly less than that predicted by the Sauerbrey equation. These results demonstrate that detailed understanding of the sensitivity distribution is important when the QCM is used in liquid media, in the presence of nonuniform mass deposits, and under conditions favoring field fringing.