Authors: A. Arnau, J.V. García, Y. Jiménez, V. Ferrari, M. Ferrari

Journal: Review of Scientific Instruments, vol. 79 (2008)

A new configuration of automatic capacitance compensation ACC technique based on an oscillatorlike working interface, which permits the tracking of the series resonant frequency and the monitoring of the motional resistance and the parallel capacitance of a thickness-shear mode quartz crystal microbalance sensor, is introduced. The new configuration permits an easier calibration of the system which, in principle, improves the accuracy. Experimental results are reported with 9 and 10 MHz crystals in liquids with different parallel capacitances which demonstrate the effectiveness of the capacitance compensation. Some frequency deviations from the exact series resonant frequency, measured by an impedance analyzer, are explained by the specific nonideal behavior of the circuit components. A tentative approach is proposed to solve this problem that is also common to previous ACC systems.

Authors: A. Arnau

Journal: Sensors: Special Issue: Piezolectric sensors for determination of analytes in solutions, 370-411 (2008)

 

From the first applications of AT-cut quartz crystals as sensors in solutions more than 20 years ago, the so-called quartz crystal microbalance (QCM) sensor is becoming into a good alternative analytical method in a great deal of applications such as biosensors, analysis of biomolecular interactions, study of bacterial adhesion at specific interfaces, pathogen and microorganism detection, study of polymer film-biomolecule or cell-substrate interactions, immunosensors and an extensive use in fluids and polymer characterization and electrochemical applications among others. The appropriate evaluation of this analytical method requires recognizing the different steps involved and to be conscious of their importance and limitations. The first step involved in a QCM system is the accurate and appropriate characterization of the sensor in relation to the specific application. The use of the piezoelectric sensor in contact with solutions strongly affects its behavior and appropriate electronic interfaces must be used for an adequate sensor characterization. Systems based on different principles and techniques have been implemented during the last 25 years. The interface selection for the specific application is important and its limitations must be known to be conscious of its suitability, and for avoiding the possible error propagation in the interpretation of results. This article presents a comprehensive overview of the different techniques used for AT-cut quartz crystal microbalance in insolution applications, which are based on the following principles: network or impedance analyzers, decay methods, oscillators and lock-in techniques. The electronic interfaces based on oscillators and phase-locked techniques are treated in detail, with the description of different configurations, since these techniques are the most used in applications for detection of analytes in solutions, and in those where a fast sensor response is necessary.

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Authors: A. Arnau, J.V. García, Y. Jiménez, V. Ferrari, M. Ferrari

Event: 19th International Congress on Acoustics – ICA07. Madrid, september 2-7 (2007)

 

The classical quartz crystal microbalance (QCM) is no longer only a microbalance; it has got a place as an acoustic sensor in a broad range of applications such as: fluid physical characterization, viscoelastic study of polymers, charge transfer analysis in electrochemical processes, and detection of biological components in fluid media, among other applications. In this paper, the basic operation of an AT cut quartz crystal resonator is extended to the fluid environment. In these applications the resonator is submitted to a heavy load which strongly affects the sensor response, making especially difficult the characterization of the main sensor parameters. The problem associated with the electronic interfaces for sensor characterization is introduced along with a brief reviewing and some recent improvements. After this description, an improved electronic interface is introduced in detail. The design is an interface based on a phase locked loop system which permits an accurate monitoring of the series resonant frequency and the motional resistance of the quartz crystal resonator sensor. A continuous and automatic compensation of the sensor parallel capacitance makes this possible. The report of experimental results shows the benefit of the new system, especially for heavy load QCM applications.

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Authors: C. March, J.J. Manclús, A. Arnau, Y. Jiménez, T. Sogorb, A. Montoya

Event: 7th Workshop on Biosensors and Bioanalytical μ-Techniques in Environmental and Clinical Analysis. Kusadasi, Turkey, 10-14 September (2006)

 

Immunosensors are novel analytical devices in which the extreme selectivity provided by immunological interactions is combined with the high sensitivity achieved by electronic signal transducers. In the present work, the development of a Monoclonal Antibody-based Piezoelectric Immunosensor for the analysis of pesticides (carbaryl and the chlorpyrifos and triclopyr metabolite TCP) is described, and its main analytical characteristics are presented.

Specific carbaryl and TCP haptens were conjugated to a carrier protein (BSA) and then covalently attached to gold-coated AT-cut 9 MHz  quartz crystals, previously functionalized with a self-assembled alkanethiol (thioctic acid) monolayer. The activated piezoelectric crystal was inserted in a homemade Arnite cell that allowed only one face of the crystal to be in contact with the reagents. The cell was integrated in a flow-through system and pesticide detection was subsequently accomplished by competitive immunoassay in the conjugate-coated format. The assay consisted of a previous incubation of the analyte sample with its specific monoclonal antibody, followed by the immunoreaction of the analyte-antibody mixture with the hapten derivative immobilized on the sensor surface. The frequency variations of the piezoelectric crystal working in the microgravimetric mode were measured with a commercial Research Quartz Crystal Microbalance (RQCM) and correlated with the concentration of the respective pesticide in the sample. The total time required for a complete assay cycle, including regeneration, was 20 min.

As it was expected for binding inhibition immunoassays, the frequency signals provided by the sensor were inversely proportional to the pesticide concentrations. Sigmoidal calibration curves were obtained by fitting experimental points to a four-parameter logistic equation. The immunosensor sensitivity, expressed as the analyte concentration that reduced the assay signal by 50% (I₅₀) around 25 µg/l for carbaryl and 32 µg/l for TCP. The limits of detection, calculated as the pesticide concentrations that provide 90% of the maximum signal, were around 6 and 9 µg/l, respectively. This analytical performance would allow the detection of carbaryl and TCP in fruits and vegetables at European regulatory levels. For both pesticides, the hapten-functionalized piezoelectric crystals could be reused for more than one-hundred times with only slight reductions of the maximum signal and without significant losses of sensitivity.

 

Authors: Y. Jiménez, R. Fernández, R. Torres, A. Arnau

Journal: IEEE Transactions on ultrasonics, Ferroelectrics and Frequency control. Vol.53, Iss 5, pp 1057-1072 (2006)

The problem of coating properties extraction in quartz crystal resonator (QCR) applications is one of the challenging tasks of QCR applications, not completely solved even in theoretical conditions. The present work demonstrates that the problem is determined only for a set of three parameters of the coating, which concentrate the four coating properties. The following parameters are chosen: the surface mass density, the loss tangent and the magnitude of the characteristic impedance. An algorithm is proposed that permits, in ideal conditions, an unambiguous extraction of these parameters, starting only from the admittance spectrum of a three-layer compound resonator, obtained from the one-dimensional transmission-line model (TLM) around the resonance. The algorithm introduces three additional improvements in relation to other routines: the calculation time is drastically reduced, the problem of erroneous solutions related to relative minima in typical fitting routines is eliminated, and a systematic error analysis in the extraction of the coating properties in real practice can be carried out. The use of the proposed algorithm as a tool for studying the effects of different phenomena such as slipping or surface roughness is introduced.