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.

Read more about this work here.

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.

 

Authors: Y. Jiménez, R.Torres, A. Arnau, M.Otero, E. Forzani, E.J. Calvo

Event: XVIII International Symposium on Biolectrochemistry and Bioenergetics of the Bioelectrochemical society in conjuction with 3rd spring meeting of the International Society of Electrochemistry (ISE). Coimbra, Portugal (2005)

 

An Electrochemical Quartz Crystal Microbalance is used for a continuous monitoring of the growth of the polymer poly(3,4-ethylenedioxy) thiopene tetrabutyammonium perchlorate (PEDT-TBAP), which is electro-polymerized in acetonitrile on a gold electrode of a 10 MHz quartz crystal resonator. The surface acoustic impedance of the resonator is analyzed starting from the electrical admittance continuously measured by means of a network analyzer. Changes in the acoustic impedance suggest that a mechanical resonance phenomenon occurs during the electro-deposition of the coating. In order to determine the origin of this effect, the evolution of the physical properties of the coating is analyzed. The extraction of the physical properties of the coating is carried out by means of an algorithm recently developed by the authors. Four sets of coating properties are obtained with the algorithm in four different conditions. In one of them an additional technique, ellipsometry, is used to have an alternative measurement of the coating thickness; no additional techniques are necessary in the rest of the cases. All the cases show a significant change in the viscoelastic properties of the coating during the time-interval of the suspected mechanical resonance. Changes in the surface roughness of the coating during the experiment are confirmed by scanning electron microscopy (SEM). The analysis of the surface roughness of the coating and its effect on the effective values of the coating properties, seem to indicate that the changes in the viscoelastic properties are due to the changes in the surface roughness of the coating, and that these changes are responsible for the mechanical resonance effect. This analysis is carried out by means of the same algorithm which is used, in this case, as a simulation tool.

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

Event: 5th edition of Acoustic Wave Based Sensors: Fundamentals, Concepts, New Applications. Physikzentrum Bad Honnef, Germany (2005)

 

The models that describe the surface acoustic impedance at the surface of a coated AT quartz sensor should include the effects of the roughness, both the roughness of the crystal surface and the roughness of the coating. When these effects  are not kept in mind, the values of the physical magnitudes extracted starting from the measurable characteristic parameters of the sensor, usually the complete admittance spectrum around resonance or the frequency and motional resistance shifts, can be erroneous. In this work a model that facilitates the understanding of the physical phenomena that happen when a rough surface oscillates in contact with a fluid is presented. This model characterizes the roughness like a surface of spheroids of variable magnitudes. The solution of the Navier-Stokes equation applied to this allows the obtaining of the acoustic impedance of the rough surface in contact with the liquid, and even to extend the pattern to those cases in which the rough layer has viscoelastic properties.