Sensitivity acoustic mass sensors

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. 

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). 

There are several applications of ZnO that are due to its excellent piezoelectric properties [28,164]. Examples are surface-acoustic wave (SAW) devices and piezoelectric sensors [28,165-167]. Typically, SAW devices are used as band pass filters in the tele-communications industry, primarily in mobile phones and base stations. Emerging field for SAW devices are sensors in automotive applications (torque and pressure sensors), medical applications (chemical sensors), and other industrial applications (vapor, humidity, temperature, and mass sensors). Advantages of acoustic wave sensors are low costs, ruggedness, and a high sensitivity. Some sensors can even be interrogated wirelessly, i.e., such sensors do not require a power source. 

Surface acoustic waves (SAW), which are sensitive to surface changes, are especially sensitive to mass loading and theoretically orders of magnitude more sensitive than bulk acoustic waves [43]. Adsorption of gas onto the device surface causes a perturbation in the propagation velocity of the surface acoustic wave, this effect can be used to observe very small changes in mass density of 10 g/cm (the film has to be deposited on a piezoelectric substrate). SAW device can be useful as sensors for vapour or solution species and as monitors for thin film properties such as diffusivity. They can be used for example as a mass sensor or microbalance to determine the adsorption isotherms of small thin film samples (only 0.2 cm of sample are required in the cell) [42]. 

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. 

Ogilvy J. A., Predicting mass loading sensitivity for acoustic wave sensors operating in air. Sens. Actuators B, B42, 109-117, 1997. 

TSM resonators possess lower mass sensitivity than most acoustic wave sensors that employ other mechanisms since they typically operate between only 5 and 30 MHz. Thinner devices can be used to increase frequencies that result in increased mass sensitivity, and they are more fragile and harder to manufacture and handle [5]. Recent research on this topic has been on creating high-frequency TSM resonators with piezoelectric Aims and bulk silicon micromachining techniques. 

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... 

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. 

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