Coating, acoustic mass sensors

Acoustic Wave Sensors. Another emerging physical transduction technique involves the use of acoustic waves to detect the accumulation of species in or on a chemically sensitive film. This technique originated with the use of quartz resonators excited into thickness-shear resonance to monitor vacuum deposition of metals (11). The device is operated in an oscillator configuration. Changes in resonant frequency are simply related to the areal mass density accumulated on the crystal face. These sensors, often referred to as quartz crystal microbalances (QCMs), have been coated with chemically sensitive films to produce gas and vapor detectors (12), and have been operated in solution as Hquid-phase microbalances (13). A dual QCM that has one smooth surface and one textured surface can be used to measure both the density and viscosity of many Hquids in real time (14). 

Chemical sensors for gas molecules may, in principle, monitor physisorp-tion, chemisorption, surface defects, grain boundaries or bulk defect reactions [40]. Several chemical sensors are available mass-sensitive sensors, conducting polymers and semiconductors. Mass-sensitive sensors include quartz resonators, piezoelectric sensors or surface acoustic wave sensors [41-43]. The basis is a quartz resonator coated with a sensing membrane which works as a chemical sensor. 

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. 

Surface mass changes can result from sorptive interactions (i.e., adsorption or absorption) or chemical reactions between analyte and coating, and can be used for sensing applications in bodi liquid and gas phases. Although the absolute mass sensitivity of the uncoated sensor depends on the nature of the piezoelectric substrate, device dimensions, frequoicy of operation, and the acoustic mode that is utilized, a linear dependence is predicted in all cases. This allows a very general description of the working relationship between mass-loading and frequency shift, A/ , for AW devices to be written ... 

The relative importance of the mass-loading and viscoelastic contributions to the observed acoustic sensor response is an issue that has yet to be resolved. Capitalizing on these effects to improve chemical selectivity and detection sensitivity requires further characterization of sensor response, in terms of both velocity and attenuation changes, in addition to more accurate models describing how coating-analyte interactions affect relevant film properties. 

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. 

This paper has dealt exclusively with SAW sensors that exploit the mass sensitivity of the device to achieve chemical vapor detection. Schemes to exploit the SAW sensitivity to coating conductance changes (17) or elastic modulus changes should afford new opportunities for imaginative chemical vapor sensor designs. Finally, the field of liquid phase chemical analysis may also yield to surface acoustic wave devices that utilize plate waves and horizontally polarized shear waves to minimize acoustic losses in the liquid (18). 

In the area of chemical sensors, thin polymer films are routinely used as coatings for the semi-selective sorption of chemical vapors. One sensor technology, the surface acoustic wave (SAW) device, has demonstrated excellent sensitivity as a vapor sensor when coated with films having appropriate solubility properties (2). To date, most sensor applications have utilized the extreme mass sensitivity of the devices. In this paper, we will examine the response mechanisms of the SAW sensor and demonstrate its sensitivity to changes in the elastic properties of the coating materials. Finally, we will discuss the significance of these results in terms of current sensor applications, and the advantages of the SAW for polymeric materials characterization. 

Vapor sensor based on SAW devices were first reported in 1979, most of them rely on the mass sensitivity of the detector, in conjunction with a chemically selective coating that adsorbs the vapor of interest and result in an increased mass loading of the device [5]. In its simplest form SAW sensor consists of a delay line, which consists of an IDT at each end of an appropriate piezoelectric substrate. One IDT acts as transmitter and other acts as the receiver of acoustic energy which travels along... 

---