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

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

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

A modulation QCM applied to copper electrodeposition and stripping

A fast electrochemical quartz crystal microbalance with dissipation monitoring (EQCM−D) was applied to copper electrodeposition and subsequent stripping. Accumulation brings the frequency noise down to the mHz range, corresponding to 0.1 % of a monolayer. With this precision, the apparent mass transfer rate as determined from the time-derivative of the frequency shift can be directly compared to the current. Small but systematic deviations between the two can be attributed to nanoscale roughness. In the voltage range of underpotential deposition (UPD), the apparent mass transfer rate shows peaks and shoulders. The plating additive benzotriazole (BTA) leaves the magnitude of electrogravimetric signals unchanged, but shifts the UPD onset potential. The additive thiourea (TU) promotes UPD and strongly increases the bandwidth

The emergence of team resilience: A multilevel conceptual model of facilitating factors

With empirical research on team resilience on the rise, there is a need for an integrative conceptual model that delineates the essential elements of this concept and offers a heuristic for the integration of findings across studies. To address this need, we propose a multilevel model of team resilience that originates in the resources of individual team members and emerges as a team-level construct through dynamic person–situation interactions that are triggered by adverse events. In so doing, we define team resilience as an emergent outcome characterized by the trajectory of a team's functioning, following adversity exposure, as one that is largely unaffected or returns to normal levels after some degree of deterioration in functioning. This conceptual model offers a departure point for future work on team resilience and reinforces the need to incorporate inputs and process mechanisms inherent within dynamic interactions among individual members of a team. Of particular, importance is the examination of these inputs, process mechanisms and emergent states, and outcomes over time, and in the context of task demands, objectives, and adverse events. Practitioner points: Team resilience as a dynamic, multilevel phenomenon requires clarity on the individual- and team-level factors that foster its emergence within occupational and organizational settings. An understanding of the nature (e.g., timing, chronicity) of adverse events is key to studying and intervening to foster team resilience within occupational and organizational settings

Cabbage and fermented vegetables : From death rate heterogeneity in countries to candidates for mitigation strategies of severe COVID-19

Large differences in COVID-19 death rates exist between countries and between regions of the same country. Some very low death rate countries such as Eastern Asia, Central Europe, or the Balkans have a common feature of eating large quantities of fermented foods. Although biases exist when examining ecological studies, fermented vegetables or cabbage have been associated with low death rates in European countries. SARS-CoV-2 binds to its receptor, the angiotensin-converting enzyme 2 (ACE2). As a result of SARS-CoV-2 binding, ACE2 downregulation enhances the angiotensin II receptor type 1 (AT(1)R) axis associated with oxidative stress. This leads to insulin resistance as well as lung and endothelial damage, two severe outcomes of COVID-19. The nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is the most potent antioxidant in humans and can block in particular the AT(1)R axis. Cabbage contains precursors of sulforaphane, the most active natural activator of Nrf2. Fermented vegetables contain many lactobacilli, which are also potent Nrf2 activators. Three examples are: kimchi in Korea, westernized foods, and the slum paradox. It is proposed that fermented cabbage is a proof-of-concept of dietary manipulations that may enhance Nrf2-associated antioxidant effects, helpful in mitigating COVID-19 severity.Peer reviewe

Nrf2-interacting nutrients and COVID-19 : time for research to develop adaptation strategies

There are large between- and within-country variations in COVID-19 death rates. Some very low death rate settings such as Eastern Asia, Central Europe, the Balkans and Africa have a common feature of eating large quantities of fermented foods whose intake is associated with the activation of the Nrf2 (Nuclear factor (erythroid-derived 2)-like 2) anti-oxidant transcription factor. There are many Nrf2-interacting nutrients (berberine, curcumin, epigallocatechin gallate, genistein, quercetin, resveratrol, sulforaphane) that all act similarly to reduce insulin resistance, endothelial damage, lung injury and cytokine storm. They also act on the same mechanisms (mTOR: Mammalian target of rapamycin, PPAR gamma:Peroxisome proliferator-activated receptor, NF kappa B: Nuclear factor kappa B, ERK: Extracellular signal-regulated kinases and eIF2 alpha:Elongation initiation factor 2 alpha). They may as a result be important in mitigating the severity of COVID-19, acting through the endoplasmic reticulum stress or ACE-Angiotensin-II-AT(1)R axis (AT(1)R) pathway. Many Nrf2-interacting nutrients are also interacting with TRPA1 and/or TRPV1. Interestingly, geographical areas with very low COVID-19 mortality are those with the lowest prevalence of obesity (Sub-Saharan Africa and Asia). It is tempting to propose that Nrf2-interacting foods and nutrients can re-balance insulin resistance and have a significant effect on COVID-19 severity. It is therefore possible that the intake of these foods may restore an optimal natural balance for the Nrf2 pathway and may be of interest in the mitigation of COVID-19 severity

A Quartz Crystal Microbalance, Which Tracks Four Overtones in Parallel with a Time Resolution of 10 Milliseconds: Application to Inkjet Printing

A quartz crystal microbalance (QCM) is described, which simultaneously determines resonance frequency and bandwidth on four different overtones. The time resolution is 10 milliseconds. This fast, multi-overtone QCM is based on multi-frequency lockin amplification. Synchronous interrogation of overtones is needed, when the sample changes quickly and when information on the sample is to be extracted from the comparison between overtones. The application example is thermal inkjet-printing. At impact, the resonance frequencies change over a time shorter than 10 milliseconds. There is a further increase in the contact area, evidenced by an increasing common prefactor to the shifts in frequency, Δf, and half-bandwidth, ΔΓ. The ratio ΔΓ/(−Δf), which quantifies the energy dissipated per time and unit area, decreases with time. Often, there is a fast initial decrease, lasting for about 100 milliseconds, followed by a slower decrease, persisting over the entire drying time (a few seconds). Fitting the overtone dependence of Δf(n) and ΔΓ(n) with power laws, one finds power-law exponents of about 1/2, characteristic of semi-infinite Newtonian liquids. The power-law exponents corresponding to Δf(n) slightly increase with time. The decrease of ΔΓ/(−Δf) and the increase of the exponents are explained by evaporation and formation of a solid film at the resonator surface

The effects of hydrogen sulfide on platelet–leukocyte aggregation and microvascular thrombolysis

The volatile transmitter hydrogen sulfide (H2S) is known for its various functions in vascular biology. This study evaluates the effect of the H2S-donor GYY4137 (GYY) on thrombus stability and microvascular thrombolysis. Human whole blood served for all in vitro studies and was analyzed in a resting state, after stimulation with thrombin-receptor activating peptide (TRAP) and after incubation with 10 or 30 mM GYY or its vehicle DMSO following TRAP-activation, respectively. As a marker for thrombus stability, platelet–leukocyte aggregation was assessed using flow cytometry after staining of human whole blood against CD62P and CD45, respectively. Furthermore, morphology and quantity of platelet–leukocyte aggregation were studied by means of scanning electron microscopy (scanning EM). Therefore, platelets were stained for CD62P followed by immuno gold labeling. In vivo, the dorsal skinfold chamber preparation was performed for light/dye induction of thrombi in arterioles and venules using intravital fluorescence microscopy. Thrombolysis was assessed 10 and 22 h after thrombus induction and treatment with the vehicle, GYY, or recombinant tissue plasminogen activator (rtPA). Flow cytometry revealed an increase of CD62P/CD45 positive aggregates after TRAP stimulation of human whole blood, which was significantly reduced by preincubation with 30 mM GYY. Scanning EM additionally showed a reduced platelet–leukocyte aggregation and a decreased leukocyte count within the aggregates after preincubation with GYY compared to TRAP stimulation alone. Further on, morphological signs of platelet activation were found markedly reduced upon treatment with GYY. In mice, both GYY and rtPA significantly accelerated arteriolar and venular thrombolysis compared to the vehicle control. In conclusion, GYY impairs thrombus stability by reducing platelet–leukocyte aggregation and thereby facilitates endogenous thrombolysis

A quartz crystal microbalance, which tracks four overtones in parallel with a time resolution of 10 milliseconds: application to inkjet printing

A quartz crystal microbalance (QCM) is described, which simultaneously determines resonance frequency and bandwidth on four different overtones. The time resolution is 10 milliseconds. This fast, multi-overtone QCM is based on multi-frequency lockin amplification. Synchronous interrogation of overtones is needed, when the sample changes quickly and when information on the sample is to be extracted from the comparison between overtones. The application example is thermal inkjet-printing. At impact, the resonance frequencies change over a time shorter than 10 milliseconds. There is a further increase in the contact area, evidenced by an increasing common prefactor to the shifts in frequency ,∆f, and half-bandwidth, ∆Γ. The ratio ∆Γ/(−∆f), which quantifies the energy dissipated per time and unit area, decreases with time. Often, there is a fast initial decrease, lasting for about 100 milliseconds, followed by a slower decrease, persisting over the entire drying time (a few seconds). Fitting the overtone dependence of ∆f(n) and ∆Γ(n) with power laws, one finds power-law exponents of about 1/2, characteristic of semi-infinite Newtonian liquids. The power-law exponents corresponding to ∆f(n) slightly increase with time. The decrease of ∆Γ/(−∆f) and the increase of the exponents are explained by evaporation and formation of a solid film at the resonator surface