Waves, acoustic mass sensors

MOS metal oxide sensor, MOSFET metal oxide semiconductor field-effect transistor, IR infrared, CP conducting polymer, QMS quartz crystal microbalance, IMS ion mobility spectrometry, BAW bulk acoustic wave, MS mass spectrometry, SAW siuface acoustic wave, REMPI-TOFMS resonance-enhanced multiphoton ionisation time-of-flight mass spectrometry... 

A piezoelectric mass sensor is a device that measures the amount of material adsorbed on its surface by the effect of the adsorbed material on the propagation of acoustic waves. Piezoelectric devices work by converting electrical energy to mechanical energy. There are a number of different piezoelectric mass sensors. Thickness shear mode sensors measure the resonant frequency of a quartz crystal. Surface acoustic wave mode sensors measure the amplitude or time delay. Flexure mode devices measure the resonant frequency of a thin Si3N4 membrane. In shear horizontal acoustic plate mode sensors, the resonant frequency of a quartz crystal is measured. 

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. 

The adsorption and desorption isotherms of an inert gas (classically N2 at 77 K) on an outgassed sample are determined as a function of the relative pressure (Prei = p/Po/ the ratio between the applied pressure and the saturation pressure. The adsorption isotherm is determined by measuring the quantity of gas adsorbed for each value of p/po by a gravimetric or a volumetric method (less accurate but simpler). A surface acoustic wave device can also be used as a mass sensor or microbalance in order to determine the adsorption isotherms of small thin films samples (only 0.2 cm of sample are required in the cell) [42,43]. 

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]. 

FIGURE 4.8 Attenuation and velocity responses arising from acoustoelectric effect for a Ni film deposited on a 97MHz ST Quartz device. The fractional velocity change has been corrected for the effect of mass loading. (Reprinted with permission from Ricco A. J. and Martin S. J., Multiple frequency surface acoustic wave devices as sensors, presented at IEEE Ultrasonics Symposium, June 4-7, 1990. Hilton Head Island, SC, USA. ( 1990 IEEE).)... 

R. Lucklum, C. Behling, and R Hauptmann, Role of mass accumulation and viscoelastic film properties forthe response of acoustic-wave based chemical sensors. Ana/. Chem., 71, 2488-2496 (1999). 

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. 

Mass sensors measure the change in mass upon interaction with the analyte. There are two main types of mass sensors quartz crystal microbalance (QCM) and surface acoustic wave (SAW). QCM measures the mass per unit area by measuring the change in frequency of a quartz crystal resonator. SAW uses a piezoelectric sensor to convert an electric signal into a mechanical wave that is then reconverted into an electric signal. Changes in amplitude, phase, frequency, or time delay between the input and output electrical signals are used to measure the concentration of the analyte. 

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). 

A new chemical sensor based on surface transverse device has been developed (99) (see Sensors). It resembles a surface acoustic wave sensor with the addition of a metal grating between the tranducer and a different crystal orientation. This sensor operates at 250 mH2 and is ideally suited to measurements of surface-attached mass under fluid immersion. By immohi1i2ing atra2ine to the surface of the sensor device, the detection of atra2ine in the range of 0.06 ppb to 10 ppm was demonstrated. 

Macrocyclic Compounds in Analytical Chemistry. Edited by Yury A. Zolotov Surface-Launched Acoustic Wave Sensors Chemical Sensing and Thin-Film Characterization. By Michael Thompson and David Stone Modern Isotope Ratio Mass Spectrometry. Edited by T. J. Platzner High Performance Capillary Electrophoresis Theory, Techniques, and Applications. Edited by Morteza G. Khaledi... 

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-... 

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...

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