Piezoelectric Sensors
Although optical and electrochemical transducers are the two most popular transducers for biosensors, piezoelectric transducers have also gained some popularity in the past couple of decades. Like optical and electrochemical transducers, piezoelectric tra
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Piezoelectric Sensors
Although optical and electrochemical transducers are the two most popular transducers for biosensors, piezoelectric transducers have also gained some popularity in the past couple of decades. Like optical and electrochemical transducers, piezoelectric transducers can be used as-is as physical sensors (to sense mass), or used with bioreceptors as biosensors (to quantify biomolecules). Collectively they are called piezoelectric sensors.
11.1
Piezoelectricity
All piezoelectric sensors work on the principle of piezoelectricity. In the late nineteenth century, the Curie brothers (the younger brother, Pierre, was Marie Curie’s husband) found that an electrical voltage was generated when they compressed or stretched quartz. This is called piezoelectric effect. This effect is reversible, meaning that quartz can be lengthened or shortened when an electrical voltage is applied. Piezoelectricity refers to the material’s ability to exhibit this piezoelectric effect. Other materials can also exhibit piezoelectricity, but quartz is still the most popular and the most used material for piezoelectric sensors. Quartz (SiO2) has a unique, tetrahedron crystal structure, shown in Fig. 11.1. There are four oxygen molecules in a single tetrahedron (SiO4), where all four oxygen molecules are shared with nearby tetrahedra (thus it becomes SiO2). These tetrahedra are stacked up in a highly ordered manner to create a much larger crystal structure. Like any other materials, the distribution of polarity is not uniform throughout its crystal structure, creating dipoles. As typical quartz is monocrystal, indicating all tetrahedra are ordered in one direction, thus the dipoles are also ordered in one direction (Fig. 11.2 top). Some other materials have polycrystal structure, meaning that the directions of dipoles are not in one direction. It is possible to use this polycrystal material as J.-Y. Yoon, Introduction to Biosensors: From Electric Circuits to Immunosensors, 181 DOI 10.1007/978-1-4419-6022-1_11, # Springer Science+Business Media New York 2013
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Piezoelectric Sensors
Fig. 11.1 A quartz tetrahedron
Fig. 11.2 Dipoles are well aligned in monocrystal, but random in polycrystal. The dipoles in polycrystal can be aligned through polarization
piezoelectric sensor by applying an electrical voltage to the polycrystal to align the dipoles in one direction, called polarization (Fig. 11.2 bottom). To use the monocrystal quartz as a piezoelectric sensor, it is important to cut the crystal in a certain angle so that dipoles are aligned parallel to the electrical voltage. The most widely used angle is 35 100 , shown in Fig. 11.3, called AT-cut. Once the dipoles are aligned parallel to the electrodes, as shown in Fig. 11.4, piezoelectric effects can be observed. When a quartz crystal is compressed, the dipole itself is also compressed, creating an electrical voltage that has the same direction of the dipole. When the quartz crystal is stretched, the dipole is also stretched, creating a negative electrical voltage.
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