Signal amplification with multilayer arrangements on chemical quartz-crystal-resonator sensors

Viscoelastic properties of chemically sensitive coatings can enhance the ma ss sensitivity of quartz-crystal-microbalance (QCM) sensors. If analyte sor ption is accompanied by a change of the viscoelastic properties of the coat ing material, the accumulated mass cannot be calculated from the frequency shift without further information. We developed a sensor concept, which is based on a double-layer arrangement, permitting acoustic amplification and chemical sensitivity to be separated. With a proper selection of materials, the first layer realizes a constant acoustic amplification of the mass eff ect; the chemically sensitive layer acts purely gravimetrically. Major sens or design parameters are the shear modulus and the thickness of the first l ayer. From the acoustic point of view, the thickness of the chemically acti ve layer and its material properties are less critical; a glasslike, rigid coating is preferred for a stable sensor transfer function. Simultaneous me asurement of the resonant frequency of the quartz crystal and its motional resistance can be exploited to check the acoustic amplification.