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

Quartz crystal resonators (QCR) respond to surface mass and material proper ties of a film coated on their surface. The acoustic load acting at the sur face of the resonator is a more general parameter to describe this dependen ce. It can be represented by a mass factor and an acoustic factor. The quot ient of resistance increase and frequency shift can be used for the determi nation of the acoustic factor, if the loss tangent of the coating is known. Viscoelastic properties of sensitive coatings can enhance the mass sensiti vity of quartz crystal microbalance (QCM) sensors. Acoustic factor and acou stic amplification effective during chemical sensing are not the same. We further suggest a sensor concept, which is based on a bilayer arrangemen t. Acoustic amplification with a viscoelastic film and chemical sensitivity is separated. With a proper selection of materials, the first layer realiz es acoustic amplification while the (chemical) sensitive layer acts as a pu re mass detector. Major sensor design parameters are the shear modulus and the thickness of the first layer; major challenge is the preparation of a h omogeneous and uniform first Nm. (C) 2000 Elsevier Science B.V. All rights reserved.