Metallic layers, acoustic mass sensors

Compact chemical sensors can be broadly classified as being based on electronic or optical readout mechanisms [28]. The electronic sensor types would include resistive, capacitive, surface acoustic wave (SAW), electrochemical, and mass (e.g., quartz crystal microbalance (QCM) and microelectromechanical systems (MEMSs)). Chemical specificity of most sensors relies critically on the materials designed either as part of the sensor readout itself (e.g., semiconducting metal oxides, nanoparticle films, or polymers in resistive sensors) or on a chemically sensitive coating (e.g., polymers used in MEMS, QCM, and SAW sensors). This review will focus on the mechanism of sensing in conductivity based chemical sensors that contain a semiconducting thin film of a phthalocyanine or metal phthalocyanine sensing layer. 

As has been demonstrated, the fields of quartz crystal microbalance (QCM)- and surface acoustic wave (SAW)-based gas sensors are also of interest in metal film application (Miura 1991 Jakubik etal. 2003 Jakubik and Urbanczyk 2005). When the palladium or palladium-based alloy layer absorbs hydrogen, both its mass density and electrical conductivity change, and this produces a detectable change in the frequency of the SAW and resonance frequency of (JCM. Devices were able to detect hydrogen gas in a range of 1.5-4.0% concentration in air. 

The nature of acoustic waves generated in piezoelectric materials is determined by the piezoelectric material orientation as well as the metal electrodes configuration employed to generate the electric field that induces acoustic waves by converse piezoelectric effect. As gas sensors, the resonators are coated with layers which selectively absorb or adsorb analytes of interest and thereby induce a mass change that is then detected via a shift in the resonant frequency of the device (Kurosawa et al. 1990). The detection limits and the relative (5) mass sensitivities for different types of acoustic sensors are presented in Table 13.1. The comparison of various types of AW sensors is also presented in Table 13.2. Several books and reviews (Ballantine et al. 1997 Ippolito et al. 2009) provide a more detailed analysis of AW-based sensors operation. 

---