Carsten Behling

Role of mass accumulation and viscoelastic film properties for the response of acoustic-wave-based chemical sensors.

The sensitivity of acoustic-wave microsensors coated with a viscoelastic film to mass changes and film modulus (changes) is examined. The study analyzes the acoustic load at the interface between the acoustic device and the coating. The acoustic load carries information about surface mass and film modulus; its determination has no restrictions in film thickness. Two regimes of film behavior can be distinguished:  the gravimetric regime, where the sensor response is mainly mass sensitive, and the nongravimetric regime, where viscoelasticity gains influence on the sensor response. We develop a method, which allows the assignment of the sensor signal to a gravimetric or a nongravimetric response. The critical value can be determined from oscillator measurements. The related limits for the coating thickness are not the same for the coating procedure and mass accumulation during chemical sensing. As an example, we present results from a 10 MHz quartz crystal resonator.

Determination of complex shear modulus with thickness shear mode resonators

The electrical response of polymer-coated acoustic wave sensors depends on changes in the surface mass loading and changes in viscoelastic properties of the coating material. In this paper we consider the acoustic behaviour and the electrical response of a thickness-shear mode resonator on changes in shear parameters of the coating material at its fundamental frequency as well as its third and fifth harmonics. The changes in material properties were induced by temperature changes. Both a glassy and a rubbery polymer were investigated. The complex shear parameter and dynamic glass transition temperature were calculated from impedance measurements.

Response of quartz-crystal resonators to gas and liquid analyte exposure

Abstract The electrical response of chemical bulk acoustic wave sensors depends on changes in the surface mass loading and changes in the viscoelastic properties of the coating material. We consider here the acoustic behaviour and the electrical response of a quartz-crystal resonator to changes in surface mass and shear parameters of the coating material. Both a glassy and a rubbery poiymer have been investigated. The frequency changes of an oscillator are calculated from a full transmission-line model, an impedance approximation and Sauerbreys or Martins equation.

Gravimetric and non-gravimetric chemical quartz crystal resonators

Abstract Chemical Sensors based on quartz crystal resonators are often assumed to work as microbalance. In this paper we study the contribution from a viscoelastic coating to the sensor response. Different coating properties as well as analyte sorption, which may cause a pure mass increase or both a mass increase and a change in the viscoelastic parameters of the coating, are under investigation. Resonators in gaseous and liquid environments are studied.

Analysis of compressional-wave influence on thickness-shear-mode resonators in liquids

Abstract The operation of a thickness-shear-mode (TSM) resonator contacting a finite liquid layer has been analysed to investigate the effect of compressional-wave generation. This effect is mainly related to the non-uniform shear velocity profile across the surface of a TSM device. Hydrophone measurements show two coils of longitudinal waves. Their influence on the TSM resonator response is studied with impedance analysis, varying the spacing between resonator and reflector as well as the reflecting conditions on the top side of the liquid layer. A characteristic response with a periodicity of λ/2 is observed when the spacing of the liquid cavity or the liquid layer thickness is changed. It indicates standing longitudinal waves in the cavity. Their influence can be modelled with an additional complex impedance in the motional arm of the Butterworth-van-Dyke equivalent circuit representing an own (compressional) transmission line.

Possibilities and limitations in quantitative determination of polymer shear parameters by TSM resonators

Abstract The use of thickness-shear-mode (TSM) sensors to determine shear parameters of viscoelastic layers like polymers has been investigated. It is shown that care has to be taken in choosing the model and experimental set-up for quantitative measurements. A successful determination of shear parameters is possible but requires a knowledge of the layer thickness.

Fast three-step method for shear moduli calculation from quartz crystal resonator measurements

Quartz crystal resonator measurements can be used for polymer material characterization. The non-gravimetric regime of these resonators is exploited: the electrical response of polymer-coated quartz resonators depends on the polymer shear modulus. Previously reported methods employ an electrical admittance analysis together with difficult and time-consuming data fitting procedures to calculate the film shear modulus. This contribution presents a fast and accurate three-step method for the calculation of complex shear moduli of polymer films from quartz crystal resonator measurements. In the first step, the acoustic load impedance is calculated from the electrical admittance of the quartz crystal. The key point of this method is the application of a family of approximations for the calculation of the shear modulus from the acoustic load impedance in the second step. In the third step, the best approximation is improved further in an iterative procedure.

Error analysis of material parameter determination with quartz-crystal resonators

The application of quartz-crystal resonators for materials science is based on the dependence of acoustic-wave propagation on the film shear modulus and density. In contrast to the well-known chemical-sensor applications, the shear modulus determination of a thin film needs a complete electrical impedance (or admittance) analysis and a complex fitting procedure of the measured responses. This contribution concentrates on different error sources in the procedure to determine shear parameters.

Signal amplification with multilayer arrangements on chemical quartz-crystal-resonator sensors

Viscoelastic properties of chemically sensitive coatings can enhance the mass sensitivity of quartz-crystal-microbalance (QCM) sensors. If analyte sorption is accompanied by a change of the viscoelastic properties of the coating material, the accumulated mass cannot be calculated from the frequency shift without further information. We developed a sensor concept, which is based on a double-layer arrangement, permitting acoustic amplification and chemical sensitivity to be separated. With a proper selection of materials, the first layer realizes a constant acoustic amplification of the mass effect; the chemically sensitive layer acts purely gravimetrically. Major sensor design parameters are the shear modulus and the thickness of the first layer. From the acoustic point of view, the thickness of the chemically active layer and its material properties are less critical; a glasslike, rigid coating is preferred for a stable sensor transfer function. Simultaneous measurement of the resonant frequency of the quartz crystal and its motional resistance can be exploited to check the acoustic amplification.

Direct method for shear moduli calculation from quartz crystal resonator measurements

Quartz crystal resonator measurements can be used for polymer material characterization. The non-gravimetric behavior of these resonators, i.e. the dependence of the electrical response of polymer coated quartz resonators on the polymer shear modulus, is exploited for the determination of viscoelastic material properties of the coating. Previously reported methods applied an electrical admittance analysis together with difficult and time-consuming data fit procedures to calculate the film shear modulus. This contribution presents a new direct method for the determination of complex shear moduli of polymer films from quartz crystal resonator measurements which allows a fast calculation of the shear parameters.