ABSTRACT
The thickness shear mode (TSM)-sensor responds to changes of mechanical properties of the material contacting the surface of the sensor. One of the material properties is the viscosity of a liquid. Abiosensor based on the TSM-resonator for the detection of endotoxin has been developed. It exploits the viscosity-density change during the reaction of endotoxin with limulus amebocyte lysate (LAL). The effect of surface properties of the sensor has been investigated to achieve better output signals. It is shown that the sensor requires a hydrophilic surface to get a better coupling between the sensor and the LAL-endotoxin solution. The TSM biosensor is able to detect an endotoxin concentration as low as 100 fg/ml by using only 50-microl standard LAL solution. The disadvantages of reusable sensors, such as the contamination from previous measurement of endotoxin and the cost of the regeneration or reclining processes of the sensor, have been eliminated by using a cost effective disposable TSM-sensor.
Subject(s)
Biosensing Techniques/instrumentation , Endotoxins/analysis , Animals , Biomechanical Phenomena , Biosensing Techniques/methods , Biosensing Techniques/standards , Endotoxins/standards , Limulus Test/instrumentation , Limulus Test/methods , Limulus Test/standards , Models, Theoretical , Quartz , ViscosityABSTRACT
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.
ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
Liquid trapped by the rough surface of a quartz resonator vibrating in thickness-shear mode (TSM) will act as a mass effect to the crystal. It has been proven that this mass effect not only depends on the liquid mass enclosed in the surface cavities, but also the liquid properties and the crystal surface features. Based on a series of experiments, this paper introduces "trapping factor" to analyze the mechanism of the liquid mass effect. Influences of different surface microstructures, including structure dimension and orientation, on the liquid mass effect have been studied on 10 MHz fundamental mode AT-cut resonators. The result indicates that the trapping factor of a chess-board structure has no advantage compared to a line-structure. For the same structure height of 0.4 microm, the mass effect of a crystal with about 3 microm distance line-structure is bigger than that of a 7.5 microm distance line-structure. With a similar surface roughness value (R (a)), the crystal with a line structured surface has a much bigger mass effect than that with a randomly rough surface.
ABSTRACT
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.
