Piezoelectric sensors, quartz crystal

Piezoelectric sensors Quartz crystals may be induced to resonate under electrical control. In this state they are sensitive to the mass of absorbing material. Piezoelectric sensors make use of this effect for the detection of very small amounts of inorganic or biological material, e.g. by use of surface immobilized enzymes or antibodies. 

In this entry, we focus on the discussion of the platform technology for electrochemical sensors, metal oxide semiconductive (MOS) sensors, and piezoelectric based quartz crystal microbalance (QCM) sensors. There are other types of chemical sensors, such as optical sensors, Schottky diode based sensors, calorimetric sensors, field-effect transistor (FET) based sensors, surface acoustic wave sensors, etc. Information of these specific sensors can be found elsewhere and in current journals on sensor technologies. Because of the increasing importance of microfabricated sensors, a brief discussion of microsensors is also given. 

Piezoelectric mass deposition sensor (quartz crystal microbalance)... 

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. 

A chemical microsensor can be defined as an extremely small device that detects components in gases or Hquids (52—55). Ideally, such a sensor generates a response which either varies with the nature or concentration of the material or is reversible for repeated cycles of exposure. Of the many types of microsensors that have been described (56), three are the most prominent the chemiresistor, the bulk-wave piezoelectric quartz crystal sensor, and the surface acoustic wave (saw) device (57). 

Bulk-wave piezoelectric quartz crystal sensors indirecdy measure mass changes of the coating on the surface of the sensing device. This change in mass causes changes in the resonant frequency of the device, and measurements ate based on frequency differences. 

S.J. LASKY and D.A. BUTTRY, "Sensors Based on Biomolecules Immobilized on the Piezoelectric Quartz Crystal Microbalance", ACS Symp. Ser. 403 (1989) 183. 

The history of electrochemical sensors began in the thirties of the twentieth century, when the pH-sensitive glass electrode was deployed, but no noteworthy development was carried out till the middle of that century. In 1956, Clark invented his oxygen-sensor based on a Ft electrode in 1959, the first piezoelectric mass-deposition sensor (a quartz crystal microbal-ance) was produced. In the sixties, the first biosensors (Clark and Lyons, 1962) and the first metal oxide semiconductor-based gas sensors (Taguchi, 1962) started to appear. 

Nitz, S., Kollmannsberger, H., Lachermeier, C., Horner, G. (1999) Odour assessment with piezoelectric quartz crystal sensor array, a suitable tool for quality control in food technology Adv. Food. Sci. 21 136-150. 

The scanning system is a very important part of these microscopes, and is commonly composed of a cantilever whose arms are usually made of piezoelectric quartz crystal. The electric field applied by the computer to the arms of the scanning device controls the position of the tip of the sensor to within a great spatial precision. The right variation of the electric field allows the complete scanning of the sample. 

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. 

These piezoelectric crystal oscillators are very accurate mass sensors because their resonant frequencies can be measured precisely with relatively simple electronic circuitry. For certain quartz crystals, the resonant frequency is inversely related to the crystal thickness. A crystal resonating at 5 megahertz is typically 300 micrometers thick. If material is coated or adsorbed on the crystal surface, the resonant frequency will change (decrease) in proportion to the amount of material added. The effect of adsorbed mass on the oscillator frequency varies according to the operational mode of the device. In any case, interpretation of mass via changes in frequency or amplitude assumes that the coated films are rigidly elastic and infinitesimally thin (that is, an extension of the crystal). 

There are several types of materials that exhibit the piezoelectric effect. Because it is inexpensive, and because it has a relatively strong piezoelectric coefficient, quartz is the material of choice for most piezoelectric sensor applications. It has a hexagonal crystallographic structure, with no center of symmetry. Both the magnitude of the piezoelectric coefficient and the extent of its temperature dependence are affected by the orientation of the cut of the crystal with respect to the main crystallographic axes. The most popular AT-cut is shown in Fig. 4.2. 

This is the correct name for most popular mass sensors, although they are better known as Quartz Crystal Microbalances (QCMs). A piezoelectric crystal vibrating in its resonance mode is a harmonic oscillator. For microgravimetric applications, it is necessary to develop quantitative relationships between the relative shift of the resonant frequency and the added mass. In the following derivation, the added mass is treated as added thickness of the oscillator, which makes the derivation more intuitively accessible. 

J.-M. Kim, S.M. Chang, Y. Suda and H. Muramatsu, Stability study of carbon graphite covered quartz crystal. Sensors and Actuators, A 72 (1999) pp. 140-147. Y.-C. Chao and J.-S. Shih, Adsorption study of organic molecules on fiillerene with piezoelectric crystal detection system. Anal. Chim. Acta, 374 (1998) pp. 39-46. 

Most cascade impactors do not give data in real time. The collection surfaces must be removed from the device and subjected to chemical or gravimetric analysis. However, one impactor does give data in real time. The Model PC-2 Air Particle Analyzer (California Measurements, Inc., Sierra Madre, CA) achieves a real-time measurement by using piezoelectric quartz crystal microbalance (QCM) mass sensors to electronically weigh particles at each impactor stage [62,63], The device has 10 stages and separates the aerosol into... 

A review appeared on piezoelectric quartz crystals used as detectors for phenols in air, after coating with Triton X-100 and 4-aminoantipyrine (78), or with activated carbon cloth impregnated with various compounds, such as poly(vinyl pyrrolidone). A piezoelectric sensor was proposed for determination of trace amounts of phenol and alkylphenols in air. The problems attaining selectivity of the adsoption membranes and operating conditions were addressed . An AT-cut quartz crystal, coated with a hydrophobic PVC layer and operating in the thickness shear mode, has been used to detect 4-aminophenol, after conversion to a hydrophobic indophenol dye and adsorption on the polymer layer. The mode of preparation of the PVC coating affects the sensitivity of the detector , a... 

Among piezoelectric based sensors, QCM represents a major device. A quartz crystal with a noncentro-symmetric space group will have a dipole associated with the orientation of the atoms. When under stress, the crystal exhibits a charge separation because of the displacement of its atoms. The converse effect has also been proven. It has been demonstrated that an applied alternating electric field will cause a vibration in a quartz crystal that in turn results in the generation of acoustic standing waves. Also, the crystal shows a tendency to vibrate at a characteristic resonant frequency. 

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