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

Viscoelastic properties of chemically sensitive coatings can enhance the mass sensitivity of quartz-crystal-microbalance (QCM) sensors. If analyte sorption is accompanied by a change of the viscoelastic properties of the coating 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 effect; the chemically sensitive layer acts purely gravimetrically. Major sensor design parameters are the shear modulus and the thickness of the first layer. From the acoustic point of view, the thickness of the chemically active layer and its material properties are less critical; a glasslike, rigid coating is preferred for a stable sensor transfer function. Simultaneous measurement of the resonant frequency of the quartz crystal and its motional resistance can be exploited to check the acoustic amplification.