Response Model Research And Verification Of QCM Sensors Loaded With A Viscoelastic Thin Film

Quartz Crystal Microbalance (QCM) is an ultrasensitive mass sensing device andcan be used to detect tiny mass change at the precision of nanogram(109g). It hasmany advantages, e.g. results can be shown realtime and traced online, cost isinexpensive on average, and quantitative analysis results can be given as well. It alsoholds great potential and can be widely used to high precise measurement and analysisfields, such as the detection or evaluation of agricultural pesticide residues, food safety,heavy metal pollution, air pollution, water quality, medical diagnosis and treatment, etc.and it has been a hotspot in the field of biochemical detection and analysis.Among the widely-accepted models of QCM, the Sauerbrey Model can only beapplied to the gas phase, while the Kanasawa Model to the liquid phase. The KanasawaModel multiplies the viscosity and elasticity of the liquid together as one inseparableterm. The QCM-D Model was derived based on fluid mechanics and the theory of theVoigt, in which the quation sets are conducted to explore the viscoelastic mediumadsorption phenomenon. However, the model requires certain information about thesolution, such as the density and the viscosity, which causes certain limitation inapplication. Additionally, to get the energy dissipation change in the Voigt model,instantaneous measurement method are applied, the QCM sensor is excitated tooscillation with an impulse, and then the decay time (usually in10-6seconds) isrecorded. This approach is vulnerable to the tiny perturbation, which affects theaccuracy of analysis results greatly.Based on the theory of transmission line and acoustic theory, explicit expressionsof response Models of a QCM sensor with different contacting media are derived.Relations among the frequency shift of the QCM sensor as well as the change ofdissipation factor and the characteristics of thin film adsorption are also revealed in thispaper. Characteristics of several materials are analyzed and evaluated quantitatively byexperiments as well. The innovation works in this paper mainly include:1. Based on the transmission theory and load acoustic impedance theory, the BVD(Butterworth-Van Dyke) equivalent circuit of an unloaded QCM sensor is derived. Thecorrectness of the method is verified by making comparison to the classic constitutiveequation. The Sauerbrey Model, the Kanasawa Model and the Martin Model are deduced in detail with the method, which proves the validity of these models.2. As for viscoelastic medium, current data analysis method of the QCM sensor hascertain deficiency. To solve the problem, this paper conducted response modelaccurately in a simple way. At the same time, a new method of steady statemeasurement is given based on the model. By applying the transforming relationshipbetween the frequency and the dissipation factor, the new method enables to acquire thedissipation factor accurately, from the perspective of frequency measurement. The newway improved the accuracy of the data analysis of the QCM sensor.3. Respectively from the perspective of the gas phase and the liquid phase, thispaper derived the response model of the QCM sensor with single viscoelastic thin filmabsorbed, providing certain explicit expression. The conclusion drawn in this paper isexactly same to the QCM-D Model: in gas phase, the extra mass effect is introduced bythe viscoelastic modulus of viscoelastic thin film, while in liquid phase, missing masseffect is introduced by the viscoelastic modulus of viscoelastic thin film. It is revealedby experiments that the response model conducted in this paper has certain guidingsignificance to the analysis of viscoelastic thin film characteristics. It provides ananalysis method and a scientific basis of QCM for further research in the field ofbiochemistry.4. Influencing factors of the QCM sensors caused by the properties of solutions areexplored in depth. A new model and a new detection method are presented to makeanalysis on biochemistry solutions, especially for the solution with its density andviscosity unknown. Common methods work with the output frequency and theequivalent impedance measured simultaneously, establishing equation sets to be solved.Only the information of the QCM frequency is adopted by the method introduced, andthe density and the viscosity can be seperated directly. The process of measurement isgreatly simplified and the accuracy is improved at the same time.