The quartz crystal microbalance: mass sensitivity, viscoelasticity and acoustic amplification

Quartz crystal resonators (QCR) respond to surface mass and material properties of a film coated on their surface. The acoustic load acting at the surface of the resonator is a more general parameter to describe this dependence. It can be represented by a mass factor and an acoustic factor. The quotient of resistance increase and frequency shift can be used for the determination of the acoustic factor, if the loss tangent of the coating is known. Viscoelastic properties of sensitive coatings can enhance the mass sensitivity of quartz crystal microbalance (QCM) sensors. Acoustic factor and acoustic amplification effective during chemical sensing are not the same.

We further suggest a sensor concept, which is based on a bilayer arrangement. Acoustic amplification with a viscoelastic film and chemical sensitivity is separated. With a proper selection of materials, the first layer realizes acoustic amplification while the (chemical) sensitive layer acts as a pure mass detector. Major sensor design parameters are the shear modulus and the thickness of the first layer; major challenge is the preparation of a homogeneous and uniform first film.