Richard W. Cernosek

Modeling the responses of thickness-shear mode resonators under various loading conditions

We develop a general model that describes the electrical responses of thickness-shear mode resonators subject to a variety of surface conditions. The model incorporates a physically diverse set of single-component loadings, including rigid solids, viscoelastic media, and fluids (Newtonian or Maxwellian). The model allows any number of these components to be combined in any configuration. Such multiple loadings are representative of a variety of physical situations encountered in electrochemical and other liquid-phase applications, as well as gas-phase applications. In the general case, the response of the composite load is not a linear combination of the individual component responses. We discuss application of the model in a qualitative diagnostic fashion to gain insight into the nature of the interfacial structure, and in a quantitative fashion to extract appropriate physical parameters such as liquid viscosity and density and polymer shear moduli.

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.

Comparison of lumped-element and transmission-line models for thickness-shear-mode quartz resonator sensors

Both a transmission-line model and its simpler variant, a lumped-element model, can be used to predict the responses of a thickness-shear-mode quartz resonator sensor. Relative deviations in the parameters computed by the two models (shifts in resonant frequency and motional resistance) do not exceed 3% for most practical sensor configurations operating at the fundamental resonance. If the ratio of the load surface mechanical impedance to the quartz shear characteristic impedance does not exceed 0.1, the lumped-element model always predicts responses within 1% of those for the transmission-line model.

Sensitivity of the acoustic waveguide biosensor to protein binding as a function of the waveguide properties

The aim of this work is to study the effect of operating frequency, piezoelectric substrate and waveguide layer thickness on the sensitivity of the acoustic waveguide sensor during the specific binding of an antibody by a protein. Shear horizontal (SH) wave devices consisting of (a) a LiTaO3 substrate operating at 104 MHz, (b) a quartz substrate operating at 108 MHz and (c) a quartz substrate operating at 155 MHz were coated with a photoresist polymer layer in order to produce acoustic waveguide devices supporting a Love wave. The effect of the thickness of the polymer layer on the Love wave was assessed by measuring the amplitude and phase of the wave before and after coating. The sensitivity of the above three biosensors was compared during the detection of the specific binding of different concentrations of Immunoglobulin G in the range of 0.7-667 nM to a protein A modified surface. Results indicate that the thickness of the polymer guiding layer is critical for obtaining the maximum sensitivity for a given geometry but a trade-off has to be made between the theoretically determined optimum thickness for waveguiding and the device insertion loss. It was also found that increasing the frequency of operation results in a further increase in the device sensitivity to protein detection.

Measuring liquid properties with smooth- and textured-surface resonators

The response of thickness shear mode (TSM) resonators in liquids is examined. Smooth-surface devices, which viscously entrain a layer of contacting liquid, respond to the product of liquid density and viscosity. Textured-surface devices, which also trap liquid in surface features, exhibit an additional response that depends on liquid density alone. Combining smooth and textured resonators in a monolithic sensor allows simultaneous measurement of liquid density and viscosity. >

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.

Guided SH-SAW sensors for liquid-phase detection

The design and performance of guided shear horizontal surface acoustic wave (guided SH-SAW) devices on LiTaO/sub 3/ substrates are investigated for high sensitivity biochemical sensor in liquids. The device sensitivity to mass and viscoelastic loading is increased using a thin guiding layer of (cross-linked or cured) poly(methyl methacrylate) (PMMA) or cyanoethylcellulose (CEC) on the device surface. The devices have been tested in biosensing and chemical sensing experiments. Suitable design principles for these applications are discussed with regard to wave guidance, electrical passivation of the interdigital transducers (IDT) from the liquid environments, acoustic loss and sensor signal distortion. In biosensing experiments, using optimal PMMA thickness of approximately 2 /spl mu/m, mass sensitivity greater than 1500 Hz/(ng/mm/sup 2/) is demonstrated, resulting in a minimum detection limit less than 20 pg/mm/sup 2/. For chemical sensor experiments, it is found that optimal waveguide thickness must be modified to account for the chemically sensitive layer.

Design considerations for high sensitivity guided SH-SAW chemical sensor for detection in aqueous environments

Guided shear horizontal surface acoustic wave (guided SH-SAW) devices coated with a polymer waveguiding layer and/or chemically sensitive layer have been investigated for the detection of analytes in liquid environments in our previous work. Design considerations for optimizing these devices for liquid phase detection is the focus of the current work. Using dual delay line geometry on LiTaO/sub 3/, guided SH-SAW sensors are designed and analyzed. The reference line, used to correct for changes in environmental conditions such as temperature fluctuations, is coated with a waveguiding layer of poly(methylmethacrylate) (PMMA). The sensing line is coated either with a polymer that functions as both the waveguiding layer and the chemically sensitive layer (3-layer model) or with a PMMA waveguiding layer below the chemically sensitive layer (4-layer model). Experimental measurements show the 3-layer model provides higher sensitivity than the 4-layer model. Increased sensitivity when using the 4-layer model can only be achieved through rigorous selection of the guiding polymer layer and chemically sensitive layer, considering both mass loading and viscoelastic effects. Appropriate selection of the partially selective chemical layer to optimize sensitivity is also a critical design factor, particularly in sensing polar analytes in aqueous sensing applications. A methodology based on attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR) for screening the potential effectiveness of new polymer coatings for these devices has been developed and used in our work. The ATR-FTIR methodology provides an accurate determination of trends for partitioning of analytes from water into polymer coatings.

Portable acoustic wave sensor systems

A portable acoustic wave chemical sensor system that has the unique advantage of providing two independent responses, doubling the amount of information provided by the sensor, is described. These sensors utilize surface acoustic wave (SAW) devices coated with viscoelastic polymers that absorb a wide variety of volatile organic species, including chlorinated hydrocarbons (CHCs). A comparison of the relative magnitudes of these two responses, specifically the wave velocity and the wave attenuation can be used to discriminate between different isolated chemical species. This allows species identification and quantification using a single SAW sensor. Tests of this portable acoustic wave sensor (PAWS) system using polymer-coated SAW devices show rapid, reversible detection of gas phase species, rapid reestablishment of sensor baseline using an activated carbon scrubber, and discrimination of species based on a comparison of the attenuation and velocity responses.<<ETX>>

Design of a portable guided SH-SAW chemical sensor system for liquid environments

Following successful application in gas sensing, acoustic wave liquid sensors attracted considerable attention due to the need for real-time, rapid and direct detection where the device is in direct contact with the solution. More importantly, there is a need for field measurement capability with portable devices. Challenges include a physical layout of the RF circuitry to minimize parasitic and spurious noise, continuous and realtime measurements capability, and obtaining vector network analyzer (VNA) performance in a portable RF unit, especially since the sensor signal noise dictates the limit of detection (LOD). Polymer-guided shear horizontal surface acoustic wave (SH-SAW) sensor platforms operating around 105 MHz on 36deg rotated Y-cut LiTaO 3 are investigated as portable detectors in liquid environments. The described system is self-contained including RF signal source, sensor input/output signal conditioning, and sensor signal amplitude and phase measuring capabilities. Amplitude and phase signals from the sensor are differentially compared with concomitant signals available directly from the RF signal source. The two primary outputs from the system are voltages related to the detected amplitude and phase changes that are caused by the sensors response to analyte sorption by the coated device. Several devices, coated with chemically sensitive polymers, are investigated in the detection of low concentrations (10-60 ppm) of ethylbenzene and xylenes in water using the RF portable unit. The units were tested for both reproducibility and repeatability, and the results matched very well with VNA measurements