The theory of acoustic sensors application in air quality control

Abstract

Acoustic sensors based on piezoelectric resonators are widely used in the real-time monitoring of mass and in situ analysis of air composition, for example in environmental, health care and air quality control (AQC) applications, both indoor and outdoor. Recent advances in the QCM techniques include the possibility of measuring changes in resonance frequency, ∆f, and the dissipation, ∆D (so-called QCM-D sensors). Another field of application is measurements of different overtones, which can provide additional information about the system. The sensing layers in QCM devices are often polymer- or supported membrane-based, both of which are soft (viscoelastic) materials. Viscoelasticity of adsorbed layers must thus be taken into account for adequate interpretation of QCM data.This publication describes theoretical results, including analytical formulae for the resonance frequency shift and the dissipation of QCM-D, for two viscoelastic layers deposited onto resonator surface, as well as calculations for higher harmonics. In particular, the corrections in ∆f and ∆D due to the softness of the top layer, considered as a bulk, are written explicitly in terms of the layer's material parameters. The results can be useful for quantitative analysis of measured characteristics of QCM in AQC, environmental monitoring and biosensor applications.