QCM data analysis and interpretation

Abstract

The bulk acoustic wave-thickness shear mode resonator (BAW-TSM), whose major representative is the AT-cut quartz crystal, has been introduced in Chap. 1 as a microbalance sensor (QCM). In chapter 3, the basic concepts of modeling were introduced making the use of the resonator evident as a sensor device. Figure 3.1 showed the general schema of a quartz crystal resonator with a multilayer coating, which can be reduced to that of Fig. 14.1 by modeling a 3-layer compound resonator formed by the quartz crystal in contact with a finite viscoelastic layer contacting a semi-infinite viscoelastic medium. This reduced model is appropriate for representing a large number of applications. Changes in the physical properties of the coating are transferred to the electrical admittance or impedance of the resonator, through the acoustic load impedance ZLM (see Chap. 3 and Appendix 3.A), thus allowing its use as a sensor device. Some typical applications of this acoustic wave sensor are the detection of special chemical species, the monitoring of electrochemical processes or the detection of biological molecules (see Chaps. 9, 10, 11, 12 and 13). In chapter 5, the three main steps involved in a TSM resonator sensor were introduced: 1) measurement of the appropriate experimental parameter values for a convenient representation of the resonant response of the sensor, including suitable electronic and cell interfaces (see Chap. 5); 2) extraction of the effective parameter values of the physical model used for representing the resonator sensor (2, 3 or n-layer model-see Chap. 3), and 3) final interpretation of the physical, chemical or biological phenomena responsible for the change in the effective equivalent parameters of the selected physical model. The final interpretation indicated in the third point depends on the application, but it must be coherent with the changes in the effective physicalproperties of the coating, which have been used to define the physical model of the sensor. Thus, previously to the final interpretation of the physical, biological or chemical phenomena, it is necessary the extraction of the physical properties used in the definition of the model starting from the changes in the electrical experimental parameters measured; next, an adequate interpretation of the changes in those properties must be done. This process will be called data analysis. After the data analysis, the final interpretation of the physical, chemical or biological phenomena can be accomplished starting from the changes in the effective physical properties extracted in the data analysis. In many applications, the data analysis is a complex task and, in consequence, the final interpretation is difficult as well. This chapter attempts to clarify this aspect, and for that a set of different cases (from the simplest one to the most complex) will be presented in Sects. 14.3 and 14.4. In each case the data analysis will be described. Next, in Sect. 14.5 some typical applications of the sensor will be introduced as examples for the explanation of the final interpretation of the physical, biological or chemical phenomena. Before that, in Sect. 14.2, the general extraction procedure of the effective physical properties of the coating starting from the electrical parameters measured experimentally will be described. © Springer-Verlag Berlin Heidelberg 2008.