Resonator-based methods quartz crystal microbalance

More recently, Tohidi and coworkers (Burgass et al., 2002 Mohammadi et al., 2003) have applied a novel method for measuring gas hydrate phase equilibria (Lw-H-V), which is based on a Quartz Crystal Microbalance (QCM). Figure 6.3 shows a schematic of the QCM set up and the QCM placed in a high pressure cell. The QCM consists of a thin disk of quartz sandwiched between two electrodes. The crystal will oscillate at a particular resonant frequency when an electric current is passed across the electrodes. This frequency is a function of the properties of the crystal. Any mass (from hydrate formation) attached to the surface of the crystal disk will cause a change in the resonant frequency, and hence be detected. The pressure and temperature of the system is measured using conventional methods, namely, a pressure transducer and a thermocouple in the high pressure cell. 

The quartz crystal microbalance device (QCM)21-24 allows one to measure the change of the mass of the films. This method is based on the ability of a piezoelectric quartz crystal to oscillate at a resonance frequency determined by the mass of the crystal. For these measurements, gold is evaporated directly onto the surface of such a quartz sensor which is then exposed to the vapour or solution of the adsorbate. What makes this method very valuable is that it can be used like SPR for monitoring molecular adsorption/desorption at the surfaces in situ. 

The most effective method to measure the adsorbed mass is the quartz crystal microbalance (QCMB). This method goes back to the work of Sauerbrey. ° The apphcation of this method is based on the following equation relating the shift A/of the resonance frequency /o of a quartz crystal to the change of the mass of the crystal Am divided by A (area)... 

MIPS are also popular for the development of mass-sensitive sensors, especially quartz crystal microbalance (QCM). The sensing response is based on the linear relationship between the resonant frequency of the crystal and the mass of the detection system. In this example, MIPs were used to detect the odorant 2-methylisobomeol (MIB), created by microorganisms." The polymer with a cavity specific to MIB template was formed on the QCM surface in situ. The specific recognition depended on the hydrogen bond between the hydroxyl gronp of templates and the carboxylate group of monomers. The detection limit of this sensor was about 200 J,gL This simple and inexpensive method may have potential applications in food quality monitoring of MIB. 

Fundamental limitations of cyclic voltammetry, such as the low resolution with respect to structural information, need to be addressed by coupling of the technique with structurally richer spectroscopic or other methods of detection. For example, simultaneous application of cyclic voltammetry and various types of spectroscopy [29], diffraction techniques [30], quartz crystal microbalance [31] and plasmon resonance [32] experiments provide the detailed structural information of chemical changes that accompany an electron transfer process. Thus, a warning needs to be provided that proposals of complex mechanistic schemes based on cyclic voltammetry only should be treated with caution. Credibility requires that such mechanisms be verified by independent measurements based on in situ or ex situ techniques combined with voltammetric measurements. 

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