Layers acoustic mass sensors

The shear-mode acoustic wave sensor, when operated in liquids, measures mass accumulation in the form of a resonant frequency shift, and it measures viscous perturbations as shifts in both frequency and dissipation. The limits of device operation are purely rigid (elastic) or purely viscous interfaces. The addition of a purely rigid layer at the solid-liquid interface will result a frequency shift with no dissipation. The addition of a purely viscous layer will result in frequency and dissipation shifts, in opposite directions, where both of these shifts will be proportional to the square root of the liquid density-viscosity product v Pifti-... 

For liquid-phase sensing, both density and viscosity, as well as the nature of the acoustic mode, play a role in AW perturbations. For TSM and SH-APM devices, in-plane motion of the substrate surface entrains a thin layer of liquid through viscous coupling. Entraiiunent of a liquid layer by the sensor surface constitutes a mass load proportional to the product of thickness and density of the coupled liquid layer, giving rise to a velocity change. 

The key feature of all acoustic wave sensors for detecting vapors is that measurable characteristics of the acoustic wave is altered as a result of adsorption on the surface of a receptive layer or absorption into the bulk of a thin layer (Figure 3). After sorption of the vapor by a thin film on top of the acoustic resonator equilibrium conditions are established and as a consequence of the increased mass or more accurately the change in the phase velocity of the acoustic wave a signal is created. Surface coatings generally enhance the sorption of vapors with the key properties of selectivity and sensitivity while affording reversibility. Typically, rubbery polymers were used on SAW devices such as polyisobutylene or substituted polysiloxanes but also self-assembled... 

The quartz-crystal microbalance (QCM) piezoelectric sensor operating system is based on interactions between thin organic layers, coated on the surface of a quartz crystal, and analytes. The ability of a QCM sensor to selectively recognize some molecules in a pomplex mixture depends on how selective and sensitive is the coated receptor. In order to obtain selective responses the coating of the quartz must be stable and capable of specific interactions with the desired analyte. Reversibility of the responses is another essential feature which requires to resort to weak interactions, since the formation of covalent or ionic bonds would lead to irreversible saturation of the sensitive layer. On the other hand pure dispersion forces are unsuitable due to their aspecificity. Sensitivity in mass sensors depends mainly on the transduction mechanism employed. Surface acoustic wave devices (SAW) are usually at least two order of magnitude more sensitive than QCM ones with the same coating. 

Fig. 21. A surface acoustic wave dual-delay line oscillator. The sensitise layer is placed in the propagation path of one of the two SAW devices. The differenee in Ireqnency (At) between the two channels provides a dtrecl result of the mass loading and electric field effects associated w ith the sensor layer...

As the readers may see, quartz crystal resonator (QCR) sensors are out of the content of this chapter because their fundamentals are far from spectrometric aspects. These acoustic devices, especially applied in direct contact to an aqueous liquid, are commonly known as quartz crystal microbalance (QCM) [104] and used to convert a mass ora mass accumulation on the surface of the quartz crystal or, almost equivalent, the thickness or a thickness increase of a foreign layer on the crystal surface, into a frequency shift — a decrease in the ultrasonic frequency — then converted into an electrical signal. This unspecific response can be made selective, even specific, in the case of QCM immunosensors [105]. Despite non-gravimetric contributions have been attributed to the QCR response, such as the effect of single-film viscoelasticity [106], these contributions are also showed by a shift of the fixed US frequency applied to the resonator so, the spectrum of the system under study is never obtained and the methods developed with the help of these devices cannot be considered spectrometric. Recent studies on acoustic properties of living cells on the sub-second timescale have involved both a QCM and an impedance analyser thus susceptance and conductance spectra are obtained by the latter [107]. 

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. 

The conductivity of the polymer layer may also depend on the physical state of the polymer. For instance, the sorption of organic vapors (e.g., alcohol) [130, 144,151,156] or acetone [154] causes a swelling of the polymer that alters the rate of interchain electron hopping. The mass change caused by the sorption can be followed by a piezoelectric quartz-crystal microbalance (QCM) or by sitrface acoustic wave (SAW) sensors. Optical changes can also be detected, although this effect is less frequently utihzed in gas sensors. 

Kg. 1.9 Schematic diagrams of mass-sensitive gas sensors (a, b) quartz crystal microbalance (QCM) device (c) surface acoustic wave (SAW) device (d, e) microcantilever - (d) dynamic mode absorption of analyte molecules in a sensor layer leads to shift in resonance frequency, and (e) static mode the cantilever bends owing to adsorption of analyte molecules and change of surface stress at the cantilever surface (Reprinted with permission from Battison et al. (2001). Copyright 2001 Elsevier)... 

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. 

The detection of gas analytes using acoustic wave (AW) sensors can be based on changes in one or more of the physical characteristics of a thin film or layer in contact with the device surface (Ballantine et al. 1997). Some of the intrinsic film properties that can be utilized for gas detection include mass/ area, elastic stiffness (modulus), viscoelasticity, viscosity, electrical conductivity, and permittivity. Variations in any of these parameters alter the mechanical and/or electrical boundary conditions producing a measurable shift in the propagating acoustic wave phase velocity, v . Equation (13.1) illustrates the change in acoustic phase velocity, Av, as a result of external perturbations, assuming that the perturbations are small and linearly combined (Ippolito et al. 2009) ... 

SAW piezoelectric sensor has been widely reported to detect different analytes. The principle of operation of the SAW sensor is tliat mass loading on the guiding layer, results in a surface acoustic velocity change, which is detected as a frequency or phase shift of a SAW (Lee et al., 2009a, 2013). 

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