Noise Measurement Setup for Quartz Crystal Microbalance

Quartz crystal microbalance (QCM) is a high sensitive chemical sensor which has found widespread spectrum of applications. There are several mechanisms that are related to fluctuation phenomena. Since the aim of our research is oriented to study the sensitivity and influence of different kind of noises on sensor resolution, we modified an existing method to measure the small frequency fluctuation of QCM. The paper describes our measurement setup, in which a quartz crystal oscillator with coated active layers and a reference quartz oscillator are driven by two oscillator circuits. Each one regulates a frequency of a crystal at the minimum impedance which corresponds to the series resonance. A data-acquisition card triggers on the rise-edges of the output signal and stores these corresponding times on which the instantaneous frequency is estimated by own-written software. In comparison to other measurement setups, our approach can acquire immediate change of QCM frequency, thus, chemical processes can be even described on the basis of high-order statistics. The experiments were provided on quartz crystals with the sorption layer of polypyrrole, which is suitable for the construction of QCM humidity sensors

Measurement of water uptake in thin-film Nafion and anion alkaline exchange membranes using the quartz crystal microbalance

Water uptake, sorption mechanics and swelling characteristics of thin-film Nafion and a commercially available Tokuyama alkaline anion exchange membrane ionomer from the vapour phase is explored using a quartz crystal microbalance (QCM). The water uptake measures the number of water molecules adsorbed by the ionomer per functional group and is determined in-situ using the QCM frequency responses allowing for comparison with nanogram precision. Crystal admittance spectroscopy, along with equivalent circuit fitting, is applied to both thin films for the first time and is used to investigate the ionomer's viscoelastic changes during hydration; to elucidate the mechanisms at play during low, medium and high relative humidities

Nanoporous Quartz Crystal Microbalance: Increasing Sensitivity 1000x and Beyond

We report experimental results of the adsorption and desorption kinetics of 1-octadecanethiol onto porous gold quartz crystal microbalances (QCM). Fabricated porous gold substrates range in thickness from 75 nm to 3 µm. The porous gold was fabricated using a two-step process of sputtering and etching. First, an alloy of gold and silver was sputtered onto a flat gold QCM with the desired ratio of roughly 70 atomic percent silver and 30 atomic percent gold. Second, the sputtered silver from the alloy was removed by etching in concentrated nitric acid, leaving a gold nanoporous structure on the surface. This porous gold structure on the QCM surface greatly increases the surface area of the sensor, allowing much more thiol to be adsorbed. Experimental adsorption kinetics are compared to a Langmuir isotherm model. Thiol adsorption on porous QCM in n-Hexane is presented with concentrations ranging from 10nM (5ppb) to 3mM (900ppm). Thiol adsorption and kinetics on a nominally flat gold surface are included for comparison. Optimized sputtering conditions for porous gold substrates lead to a 1000-fold increase of thiol adsorption over flat gold QCM. It follows that the high specific area of porous gold can amplify the final sensitivity of a flat QCM by more than 3 orders of magnitude. We further present evidence that the three orders of magnitude sensitivity increase we achieve is the maximum enhancement possible when used in liquids for thiol detection. We verify our theoretical model of the QCM, describe our fabrication technique, characterize our substrates, and detail the uses of our porous gold QCM

Multichannel QCM-based system for continuous monitoring of bacterial biofilm growth

© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Quartz crystal microbalance (QCM) sensors are becoming a good alternative to analytical methods for the measurement of bacterial growth in liquid media culture. For this purpose, two essential resonance parameters allow monitoring of biofilm formation: the series resonance frequency shift and the change of the resistance at this frequency. Nevertheless, several problems arise in determining these parameters, as their relative variation is very small. This means that an accurate procedure must be implemented for the measurement of the QCM resonance parameters, including the automatic calibration of the frequency response effects of the measurement circuits and the automatic compensation of the static electrical capacitance of the QCM. In this paper, a novel multichannel system for on-line monitoring of biofilm formation based on QCM sensors is proposed. QCM resonance parameters are determined from the electrical impedance analysis by means of an auto-balanced impedance bridge. This configuration has allowed the implementation of an affordable multichannel measurement instrument. Obtained results, based on binary mixtures of water-glycerol measurements and real microorganism experiments, are in good agreement with the theoretical behaviour. These results show the great potential of this instrument to be used for monitoring microbial growth and biofilm formation.Peer ReviewedPostprint (author's final draft

Studying Soft Interfaces with Shear Waves: Principles and Applications of the Quartz Crystal Microbalance (QCM)

The response of the quartz crystal microbalance (QCM) to loading with a diverse set of samples is reviewed in a consistent frame. After a brief introduction to the advanced QCMs, the governing equation (the small-load approximation) is derived. Planar films and adsorbates are modeled with the acoustic multilayer formalism. In liquid environments, viscoelastic spectros-copy and high-frequency rheology are possible, even on layers with a thickness in the monolayer range. For particulate samples, rheology is replaced by contact mechanics. The contact stiffness can be derived. Because the stress at the contact is large, nonlinear effects are seen. Partial slip, in particular, can be studied in detail. Advanced topics include structured samples and the extension of the small-load approximation to its tensorial version

GUMBOS- and Ionic Liquid-Coated Quartz Crystal Microbalance Sensors for Detection and Molecular Weight Determination of Organic Vapors

There has been an ever-increasing demand for the development of high-performance sensing devices for detection and discrimination of volatile organic compounds (VOCs) present in different environments. Among a number of sensing devices currently available, sorption-based sensors are particularly attractive because they are simple and inexpensive, require low power, and are appropriate for fabrication of multisensor arrays. A sorption-based sensor is comprised of a chemically active coating immobilized on the surface of a physical transducer. The chemically active film interacts with analytes, and the transducer converts the binding event into an electrical signal. This dissertation is focused on a sorption-based sensor prepared by using ionic liquids (ILs) and a group of uniform materials based on organic salts (GUMBOS) as the sensing materials and the quartz crystal microbalance (QCM) as the transducer. ILs are defined as organic salts which melt below 100 °C, and similar organic salts with melting point between 25 and 250 °C are defined as GUMBOS. In this research, a series of films comprising binary blends of an IL (or GUMBOS) and polymer are deposited onto the QCM surface in order to evaluate their vapor-sensing characteristics. The QCM sensors on exposure to organic vapors displayed a change in frequency and motional resistance, and both of these parameters were simultaneously measured. Examination of the data revealed an interesting relationship between the QCM parameters and the molecular weight of the absorbed vapors. The initial findings are reported in Chapter 2 of this dissertation. Additional studies were conducted in an effort to fully understand the interesting behavior of this type of material. More elaborate studies along with the theoretical rationale for the relationship between the QCM parameters and the molecular weight of vapors are presented in Chapter 3. Another important aspect of this dissertation is the design of highly sensitive materials for vapor-sensing applications. Toward this end, two representative GUMBOS were synthesized using porphyrin and phthalocyanine derivatives. The QCM device coated with these GUMBOS exhibited a rapid response and high sensitivity toward different organic vapors. Altogether, these studies demonstrate the true potential of this type of materials for vapor-sensing applications

Applications of a fast multiple-overtone quartz crystal microbalance (QCM) in electrochemistry and beyond

Akustische Sensoren haben in der Grenzflächenanalytik und insbesondere in der Elektrochemie enorm an Bedeutung gewonnen. Der wichtigste Vertreter dieser Geräteklasse ist die Schwingquarzmikrowaage (quartz crystal microbalance, QCM). Wird eine elektrochemische Schwingquarzmikrowaage (EQCM) mit einem flüssigen Elektrolyten beladen und das Potential der Vorderelektrode variiert, ändern sich die Schwingungseigenschaften des Resonators. Die Verschiebung der Resonanzfrequenz, "Δf" , und der halben Bandbreite, "ΔΓ" , gehen sowohl auf elektrochemischen Ladungstransport an der Elektrode als auch auf die Umladung der Doppelschicht zurück. Die Einzelheiten dieser Zusammenhänge sind eine Schlüsselfrage in dieser Dissertation. Im Fall einer Elektroabscheidung ist f weitgehend proportional zur abgeschiedenen Masse, während unverändert bleibt. Eine darüberhinausgehende Modellbildung im Hinblick auf Rauigkeit-, Viskosität- und Viskoelastizität erfordert die Messung von "Δf" und "ΔΓ" auf mehreren Obertönen. Wenn intermediäre Spezies oder Submonolagen untersucht werden, liegen "Δf" und "ΔΓ" nur knapp über der Rauschgrenze. Zudem erfolgen die Änderungen in "Δf" und "ΔΓ" im Zeitraum von Millisekunden, sodass aktuelle QCMs, die auf Impedanzanalyse oder Ring-Down basieren, an ihr technisches Limit stoßen. Diese Geräte erreichen bestenfalls Zeitauflösungen im Bereich von 20 ms (Messung eines Obertons) und eine Frequenzgenauigkeit von 20 mHz (bei kleiner Datenrate). Um die Zeitauflösung zu verbessern, wird hier auf Multifrequenz Lockin Verstärkung (MLA) zurückgegriffen. Das Ausleseverfahren ist mit der Impedanzanalyse verwandt. Es wird jedoch der Frequenzsweep durch einen Kamm von bis zu 32 gleichmäßig verteilten Frequenzen ersetzt (comb drive). Dabei werden nur 6 Frequenzen benötigt, um einen einzelnen Oberton zu erfassen. Die verbleibenden Frequenzen können über weitere Obertöne verteilt werden. Die Resonanzkurven werden somit als Single-Shot erhalten. Die Zeitauflösung entspricht dem inversen Frequenzabstand im Kamm. Typische Werte sind 1 10 ms. Bei Bedarf kann die Zeitauflösung durch Ansteuern der QCM mit einer festen Frequenz (fixed-frequency drive) auf 100 µs verbessert werden. Darüber hinaus konnte die Frequenzauflösung durch Modulation des Elektrodenpotentials und Akkumulation von "Δf" und "ΔΓ" über viele Zyklen auf unter 10 mHz verbessert werden. Für die Gestalt der Potentialmodulation haben sich Stufen, lineare Rampen, und Treppen als geeignet erwiesen. Die Vorteile dieses neuen Instruments werden anhand der kapazitiven Doppelschicht-umladung, der Unterpotentialabscheidung von Kupfer in Gegenwart von Additiven sowie der reversiblen Oxidation/Reduktion der Redoxmediatoren Methylviologenchlorid (MVC) und Flavinadenindinukleotid (FAD) vorgestellt. Andere Experimente, in denen Modulation weder möglich noch notwendig ist, behandeln Trocknungsprozesse. Diese Experimente zeigen, dass auch ohne Modulation die schnelle QCM dynamische Prozesse, wie z. B. den tropfenbasierten Tintenstrahldruck oder das Elektrosprayen, zugänglich macht. Die Beispiele oben demonstrieren die entscheidende Rolle der kinetischen Information. Neben der verbesserten Empfindlichkeit ist insbesondere die Zeitauflösung wegweisend und wird in Zukunft neue Experimente ermöglichen.Acoustic sensors have achieved immense importance in interfacial analysis and especially in electrochemistry. The most important instrument in this class of devices is the quartz crystal microbalance (QCM). When an electrochemical quartz crystal microbalance (EQCM) is immersed in a liquid electrolyte and the potential of the front electrode is varied, the resonator’s resonance properties change. The shifts in resonance frequency, Δf, and half bandwidth, ΔΓ, are caused by electrochemical charge transfer at the electrode and double layer recharging. The details of this correlation are a key question in this dissertation. In the case of electrodeposition, f is largely proportional to the deposited mass, while ΔΓ remains unchanged. Modeling beyond gravimetry in terms of roughness, viscosity, and viscoelasticity requires the measurement of Δf and ΔΓ on multiple overtones. When intermediate species or submonolayers are studied, Δf and ΔΓ are only slightly above the noise level. Moreover, the changes in Δf and ΔΓ occur on a millisecond timescale, so that current QCMs based on either impedance analysis or ring-down reach their technical limit. At best, these devices achieve time resolutions in the range of 20 ms (measuring one overtone) and a frequency sensitivity of 20 mHz (at low data acquisition rate). To improve the time resolution, multifrequency lockin amplification (MLA) is employed. This interrogation scheme is related to impedance analysis but the frequency sweep is replaced by a comb of up to 32 equally spaced frequencies (comb drive). Only 6 frequencies are needed to robustly acquire one single overtone. The remaining frequencies can be distributed to further overtones. Thus, the resonance curves are obtained in a single shot. The time resolution in this mode is equal to the inverse frequency spacing in the comb. Typical values are 1 10 ms. If required, the time resolution can be improved down to 100 µs by driving the QCM with a fixed frequency (fixed-frequency drive). In addition, the frequency resolution can be improved to below 10 mHz in the liquid phase by employing modulation of the electrode potential and accumulation of Δf and ΔΓ over many cycles. Steps, linear ramps, and stairs are suitable shapes for potential modulation. The advantages of this new technique, fast modulation EQCM, are demonstrated by employing capacitive double layer recharging, underpotential deposition of copper in the presence of additives, and reversible oxidation/reduction of the redox mediators methyl viologen chloride (MVC) and flavin adenine dinucleotide (FAD). Other experiments in which modulation is neither possible nor necessary deal with drying processes. Even without modulation, these experiments show that dynamic processes, such as droplet-based inkjet printing or electrospraying, are accessible to the fast QCM. The examples above emphasize the essential role of kinetic information. In addition to the improved sensitivity, the time resolution is groundbreaking and will enable new experiments in the future