Crystal systems, acoustic mass sensors

Measurements were performed using a 24-channel acoustic-wave sensor array system shown in Fig. 19.3. The sensor system was arranged on two printed circuit boards (Fig. 19.3a). One circuit board contained sensor resonators arranged as a 6 x 4 array (Fig. 19.3b), compatible with available 24-well plates with 16-mm well diameter and positioned onto a Z-stage. Sensor resonators were 8-mm diameter polished AT-cut quartz crystals and operated at 20MHz. The crystals had 3-mm diameter Au electrode plated in the center of each face of the crystal. The mass resolution for a 20-MHz crystal was lOng. 

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. 

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]. 

Another state-of-the-art detection system contains a surface acoustic wave (SAW) device, which is based on a piezoelectric crystal whose resonant frequency is sensitive to tiny changes in its mass—it can sense a change of 10-1° g/cm2. In one use of this device as a detector it was coated with a thin film of zeolite, a silicate mineral. Zeolite has intricate passages of a very uniform size. Thus it can act as a molecular sieve, allowing only molecules of a certain size to pass through onto the detector, where their accumulation changes the mass and therefore alters the detector frequency. This sensor has been used to detect amounts of methyl alcohol (CH3OH) as low as 10 9 g. 

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. 

Quartz Crystal Microbalance (QCM) sensors detect changes in mass adsorption at an interface and may represent an alternative sensor technology for the study of biospecific interactions in real-time [78], The operating principle of these sensors is based on changes of frequency in acoustic shear waves in the substrate of the sensor. When the QCM system is used in piezoelectric detection mode, the resulting frequency will shift in direct proportion to molecular mass adsorbed at the surface of the sensor [79]. 

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