The word Piezoelectricity comes from Greek and means “electricity by pressure” (Piezo means pressure in Greek). This name is proposed by Hankel in 1881 to name the
phenomenon discovered a year before by the Pierre and Jacques Curie brothers [Curie and Curie,1880]. They observed that positive and negative charges appeared on several parts of the crystal surfaces when comprising the crystal in different directions, previously analysed according to its symmetry. Figure 3-1A shows a simple molecular model; it explains the generating of an electric charge as the result of a force exerted on the material. Before subjecting the material to some external stress, the gravity centres of the negative and positive charges of each molecule coincide. Therefore, the external effects of the negative and positive charges are reciprocally cancelled. As a result, an electrically neutral molecule appears. When exerting some pressure on the material, its internal reticular structure can be deformed, causing the separation of the positive and negative gravity centres of the molecules and generating little dipoles (Figure 3-1B). The facing poles inside the material are
mutually cancelled and a distribution of a linked charge appears in the material’s surfaces (Figure 3-1C). That is to say, the material is polarized. This polarization generates an electric field and can be used to transform the mechanical energy used in the material’s deformation into electrical energy.
The Curie brothers verified, the year after their discovery, the existence of the reverse process, predicted by Lippmann [1881]. That is, if one arbitrarily names direct piezoelectric effect, to the generation of an electric charge, and hence of an electric field, in certain
materials and under certain laws due to a stress, there would also exist a reverse piezoelectric effect by which the application of an electric field, under similar circumstances, would cause deformation in those materials.
3.2 Piezoelectric quartz crystal
Piezoelectric quartz crystal is a reverse piezoelectric material. A prerequisite for the occurrence of piezoelectricity in crystal is an inversion center. The quartz crystal may provide a large variety of different resonator types depending on the cut angle with respect to the crystal lattice (Figure 3-2). The cut angle determines the mode of induced mechanical
vibration. Resonators operating in the thickness shear mode, face shear mode or flexural mode can be obtained from the mother crystal with eigenfrequencies ranging from 5 × 102 to 3 × 108 Hz. AT-cut crystals, which are predominately used for quartz crystal microbalance (QCM) devices, operate in the TSM and are prepared by slicing a quartz wafer with an angle of 35¼º to the optical z-axis. AT-cut quartz crystals show a tremendous frequency stability of Δf/f ≒10-8 and a temperature coefficient which is close to zero between 0 and 50°C, rendering this particular cut the most suitable for QCM sensors. The technique determines the mass of very thin surface bound layers and simultaneously gives information about their viscoelastic properties [Fredriksson et al., 1998; Fu et al., 2003; Ebarvia et al., 2004; D'Souza et al., 2005]. This offers new opportunities to study conformational changes in layers formed on the sensor surface. Moreover, the technique takes into account water coupled to hydrated layers, in contrast to optical mass measurements, obtained from methods such as surface plasmon resonance and ellipsometry. These unique properties make piezoelectric quartz an invaluable tool for studying macromolecules at surfaces and an important
complement to existing techniques.
Many polymers, ceramics, and molecules such as water are permanently polarized: some parts of the molecule are positively charged, while other parts of the molecule are negatively charged. When an electric field is applied to these materials, these polarized molecules will align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material. Furthermore, a permanently-polarized material such as quartz or barium titanate (BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. These materials are piezoelectric, and this phenomenon is known as the piezoelectric effect. When a stress is applied to a piezoelectric material, an electric field is generated within it [Lee et al., 2004; Li et al., 2004].
Conversely, if an electric field is applied to the material it undergoes a spontaneous strain.
Crystals which acquire a charge when compressed, twisted, or distorted are said to be piezoelectric. This provides a convenient transducer effect between electrical and mechanical oscillations. Conversely, an applied electric field can cause a piezoelectric material to change dimensions. This phenomenon is known as electrostriction, or the reverse piezoelectric effect. Quartz demonstrates this property and is extremely stable (Figure 3-3).
3.3 Quartz crystal microbalance
Piezoelectric biosensor, known as quartz crystal microbalance (QCM), combines high sensitivity to mass on the surface of the quartz crystal with the high specificity of a
bioreaction. It has been extensively applied as a transducer in hybridization based on DNA biosensors for the detection of gene mutation [Tombelli et al., 2000; Su et al., 2004],
genetically modified organisms [Mannelli et al., 2003], and foodborne pathogens [Ryu et al., 2001; Mo et al., 2002; Wu et al. 2007]. In this study, we utilized QCM as sensing device [Yun et al., 1998; Zhou et al., 1999]. QCM sensors (Figure 3-4) have been widely studied and developed as detecting tools for humid, biomedical, and environmental monitoring in recent years. They are popular because of its high sensitivity , fast response, small size, low power consumption, low cost, and simplicity of use.
A piezoelectric quartz crystal resonator [Wieliczka et al., 1996; Wegener et al., 1999;
Willner et al., 1999] is a precisely cut slab from a natural or synthetic crystal of quartz. A QCM consists of a thin quartz disk with electrodes plated on it (Figure 3-4B).
Pierre and Marie Curie showed in 1880 that crystals of Rochelle salt could produce electricity when pressure is applied in certain crystallographic directions. Later they also showed the converse effect i.e. production of strain by application of electricity. These findings are the discovery of the piezoelectric effect. Piezoelectricity did not receive lot of interest in the beginning and a more detailed study of piezoelectricity is not started until 1917 when it is showed that quartz crystals could be used as transducers and receivers of ultrasound in water. In 1919 several devices of everyday interest based on the piezoelectricity of
Rochelle salt is described i.e. loudspeakers, microphones and sound pick-ups. In 1921 the first quartz crystal controlled oscillator is described. These first quartz crystal controlled oscillators are based on XT-cut crystals, which have the drawback of being very temperature sensitive. Therefore, the XT-cut crystals are nowadays used in applications where the large temperature coefficient is of little importance, such as transducers in space sonars. The dominance of the quartz crystal for all kind of frequency control applications started in 1934 when the AT-cut quartz crystal is introduced [Lasky et al., 1990; Lassalle et al., 2001; Lee et al., 2002]. The advantage with the AT-cut quartz crystal is that it has nearly zero frequency drift with temperature around room temperature. From the very beginning of using quartz crystal resonators as frequency control elements it is common to increase the frequency of the resonator by drawing pencil marks on the electrodes, or decreasing the frequency by rubbing of some electrode material with an eraser. The understanding of this mass induced
frequency shift is only known on a qualitative basis. However, in 1959 Sauerbrey published
a paper that showed that the frequency shift of a quartz crystal resonator is directly proportional to the added mass. Sauerbreys [Barraud et al., 1993; Ben-Dov et al., 1997;
Bandyopadhyay et al., 1998; Bizet et al., 1998; Abdelmaksoud et al., 2004] work is generally taken as the breakthrough and the first step towards a new quantitative tool to measure very small masses i.e. the QCM. Hence, one can describe the QCM to be an ultra-sensitive mass sensor. The heart of the QCM is the piezoelectric AT-cut quartz crystal sandwiched between a pair of electrodes. When the electrodes are connected to an oscillator and an AC voltage is applied over the electrodes the quartz crystal starts to oscillate at its resonance frequency due to the piezoelectric effect. This oscillation is generally very stable due to the high quality of the oscillation. If a rigid layer is evenly deposited on one or both of the electrodes the
resonant frequency will decrease proportionally to the mass of the adsorbed layer according to the Sauerbrey equation [Anzai et al., 1998; Bidan et al., 2000; Bizet et al., 2000; Bernhard et al., 2002; Aizawa et al., 2003]:
Where: measured frequency shift = ΔF, resonant frequency of the fundamental mode of the crystal = f0, mass change per unit area (g/cm2) = Δm, piezo-electrically active area = A, density of quartz, 2.648 g/cm3 = ρq, shear modulus of quartz, 2.947 × 1011 g/cms2 = μq
There are situations where the Sauerbrey equation does not hold, for example, when the added mass is a) not rigidly deposited on the electrode surface, b) slips on the surface or c) not deposited evenly on the electrode. Therefore, the Sauerbrey equation is only strictly
applicable to uniform, rigid, thin-film deposits. Due to this the QCM is for many years just regarded as a gas-phase mass detector [Ebato et al., 1994; Darder et al., 1999; Gomes et al., 1999]. Not until the beginning of 1980’s scientists realized that a quartz crystal can be excited to a stable oscillation when it is completely immersed in a liquid. Much of the pioneering work in liquid phase QCM measurements have been done by Kanazawa and coworkers [Kanazawa and Gordon., 1985], who showed that the change in resonant frequency of a QCM taken from air into a liquid is proportional to the square root of the liquid’s
density-viscosity product:
Where:measured frequency shift = ΔF, resonant frequency of the unloaded crystal = fu,
density of liquid in contact with the crystal = ρL, viscosity of liquid in contact with the crystal
= ηL, density of quartz, 2.648 g/cm3 = ρq, shear modulus of quartz, 2.947× 1011 g/cms2 = μq After it is found out that an excessive viscous loading would not prohibit use of the QCM in liquids and that the response of the QCM is still extremely sensitive to mass changes at the solid-liquid QCMs have been used in direct contact with liquids and/or visco-elastic films to assess changes in mass and visco-elastic properties. Even in air or vacuum, where the damping of layers has been considered to be negligible or small the QCM has been used to probe dissipative processes on the quartz crystal [Okahata et al., 1995; Okahata et al., 1998;
Okahata et al., 1999]. This is especially true for soft condensed matters such as thick polymer layers deposited on the quartz surface.
In the early days of electronic communication as a result of the limited number of quartz resonators available-frequency adjustment is accomplished by a pencil mark depositing a foreign mass layer on the crystal. In 1959 Sauerbrey showed that the shift in resonance frequency of thickness-shear-mode resonators is proportional to the deposited mass. This is the starting point for the development of a new generation of piezoelectric mass-sensitive devices.
The development of new measurement techniques represents one of the major driving forces in biotechnology that positively impacts related research areas such as polymer characterization and biochemistry and is critical to the evolution of the pharmaceutical, biotechnology, and biomaterials industries.
Since QCM is a piezoelectric [Sato et al., 1995; Sato et al., 1998], an oscillating electric field applied across the device induces an acoustic wave that propagates through the crystal and meets minimum impedance when the thickness of the device is a multiple of a half wavelength of the acoustic wave. A QCM is a shear mode device in which the acoustic wave propagates in a direction perpendicular to the crystal surface. To make this happen, the quartz crystal plate must be cut to a specific orientation with respect to the crystal axes.
These cuts belonging to the rotated Y-cut family, the AT- and BT-cuts are representative.