Ralf Lucklum

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.

The quartz crystal microbalance: mass sensitivity, viscoelasticity and acoustic amplification

Abstract Quartz crystal resonators (QCR) respond to surface mass and material properties of a film coated on their surface. The acoustic load acting at the surface of the resonator is a more general parameter to describe this dependence. It can be represented by a mass factor and an acoustic factor. The quotient of resistance increase and frequency shift can be used for the determination of the acoustic factor, if the loss tangent of the coating is known. Viscoelastic properties of sensitive coatings can enhance the mass sensitivity of quartz crystal microbalance (QCM) sensors. Acoustic factor and acoustic amplification effective during chemical sensing are not the same. We further suggest a sensor concept, which is based on a bilayer arrangement. Acoustic amplification with a viscoelastic film and chemical sensitivity is separated. With a proper selection of materials, the first layer realizes acoustic amplification while the (chemical) sensitive layer acts as a pure mass detector. Major sensor design parameters are the shear modulus and the thickness of the first layer; major challenge is the preparation of a homogeneous and uniform first film.

SPICE model for lossy piezoceramic transducers

A transmission line equivalent circuit for piezoelectric transducers has been modified to provide modeling of lossy piezoceramic transducers. A lossy transmission line is used to model the mechanical losses. The equivalent circuit parameters are derived from analogies between electrical transmission lines and acoustic wave propagation. Implementation of the equivalent circuit model in SPICE is shown. Simulations and measurements in the time and frequency domain of a low-Q material and a multilayered ultrasonic sensor using a low-Q piezoceramic transducer are presented.

Phononic crystals for liquid sensor applications

Acoustic band gap materials, so-called phononic crystals, are introduced as a new platform for sensing material properties in small cavities. The sensor employs specific transmission windows within the band gap to determine properties of one component that builds the phononic crystal. The dependence of the frequency where transmission takes place is correlated to material properties, specifically to the sound velocity of a liquid. This value is related to several parameters of practical interest like the concentration of one component in a mixture or conversion rate in a microreactor. The capability of the concept will be demonstrated with a one-dimensional arrangement of solid plates and liquid-filled cavities and a two-dimensional periodic arrangement of liquid-filled holes in a solid matrix. The properties of 1D phononic crystals will be analysed in terms of the effective acoustic impedance and the resulting transmission behaviour and experimentally verified. The transmission properties of the 2D phononic crystal will be modelled with the layer multiple-scattering theory. Similar features which can be employed for sensing purposes will be discussed.

INTERFACE CIRCUITS FOR QUARTZ-CRYSTAL-MICROBALANCE SENSORS

The utilization of quartz-crystal-microbalance sensors in liquids yields new requirements to the applied interface circuits. In the present article, the fundamentals of the measuring principle and advantages and drawbacks of suitable interface circuits are discussed. Special requirements of oscillators as interface circuits are outlined. Possible solutions to those requirements are investigated and two recently developed oscillator circuits are presented.

New design for QCM sensors in liquids

The use of quartz-crystal microbalance (QCM) sensors in liquids for analytical purposes requires a well-designed transducer. Therefore a theoretical model is necessary to characterize the influence of liquid properties like density, viscosity, conductivity and dielectric behaviour. The high damping of the liquid loading results in a large loss of the quartz-crystal quality factor. Special electronics and a suitable sensor cell are necessary to obtain good frequency stability. The paper describes a new device based on an operational transconductance amplifier (OTA) with a very high bandwidth. Experimental results illustrate the ability of the sensor system to detect small amounts of special analytes in liquids.

Influence of viscoelasticity and interfacial slip on acoustic wave sensors

Acoustic wave devices with shear horizontal displacements, such as quartz crystal microbalances (QCM) and shear horizontally polarized surface acoustic wave devices, provide sensitive probes of changes at solid–solid and solid–liquid interfaces. Increasingly the surfaces of acoustic wave devices are being chemically or physically modified to alter surface adhesion or coated with one or more layers to amplify their response to any change of mass or material properties. In this work, we describe a model that provides a unified view of the modification in the shear motion in acoustic wave systems by multiple finite thickness loadings of viscoelastic fluids. This model encompasses QCM and other classes of acoustic wave devices based on a shear motion of the substrate surface and is also valid whether the coating film has a liquid or solid character. As a specific example, the transition of a coating from liquid to solid is modeled using a single relaxation time Maxwell model. The correspondence between parameters from this physical model and parameters from alternative acoustic impedance models is explicitly given. The characteristic changes in QCM frequency and attenuation as a function of thickness are illustrated for a single layer device as the coating is varied from liquid like to that of an amorphous solid. Results for a double layer structure are explicitly given and the extension of the physical model to multiple layers is described. An advantage of this physical approach to modeling the response of acoustic wave devices to multilayer films is that it provides a basis for considering how interfacial slip boundary conditions might be incorporated into the acoustic impedance used within circuit models of acoustic wave devices. Explicit results are derived for interfacial slip occurring at the substrate–first layer interface using a single real slip parameter, s, which has inverse dimensions of impedance. In terms of acoustic impedance, such interfacial slip acts as a single-loop negative feedback. It is suggested that these results can also be viewed as arising from a double-layer model with an infinitesimally thin slip layer which gives rise to a modified acoustic load of the second layer. Finally, the difficulties with defining appropriate slip boundary conditions between any two successive layers in a multilayer device are outlined from a physical point of view.

Transduction mechanism of acoustic-wave based chemical and biochemical sensors

Acoustic-wave-based sensors are commonly known as mass-sensitive devices. However, acoustic chemical and biochemical sensors also face so-called non-gravimetric effects, especially if they work in a liquid environment. The one-dimensional transmission-line model (TLM) is a powerful tool, which considers the influence of geometric and material properties on the sensor transduction mechanism, most importantly the influence of viscoelastic phenomena. This paper demonstrates the concept of modelling acoustic microsensors on quartz crystal resonators. Particular attention is paid to special cases which allow for simplifications or specific solutions of the TLM, like the acoustic load concept (ALC), the BVD model or the Sauerbrey equation. Deviations from the one-dimensional assumption of the TLM are suspected to significantly contribute to the acoustic sensor response in biosystems. We therefore introduce a generalization of the ALC to get access to two- or three-dimensional effects, which are up to now not considered in the TLM. As examples, signatures of interfacial phenomena or non-uniform films are discussed.

Sensor system for the detection of organic pollutants in water by thickness shear mode resonators

Abstract In this study, a sensor system is presented for the detection of organic pollutants in drinking water based on the quartz-crystal-microbalance (QCM). Hydrophobic polymers and macrocyclic calixarenes were tested as solid sensitive layers on QCM. Polymer materials with glass transition temperatures below the operating temperature supply reversible and fast sensor responses, and the microporous calixarene layers are very sensitive and selective towards analytes with high dispersion parameters.