Marina V. Voinova

Viscoelastic Acoustic Response of Layered Polymer Films at Fluid-Solid Interfaces: Continuum Mechanics Approach

We have derived the general solution of a wave equation describing the dynamics of two-layer viscoelastic polymer materials of arbitrary thickness deposited on solid (quartz) surfaces in a fluid environment. Within the Voight model of viscoelastic element, we calculate the acoustic response of the system to an applied shear stress, i.e. we find the shift of the quartz generator resonance frequency and of the dissipation factor, and show that it strongly depends on the viscous loading of the adsorbed layers and on the shear storage and loss moduli of the overlayers. These results can readily be applied to quartz crystal acoustical measurements of the viscoelasticity of polymers which conserve their shape under the shear deformations and do not flow, and layered structures such as protein films adsorbed from solution onto the surface of self-assembled monolayers.

Simultaneous frequency and dissipation factor QCM measurements of biomolecular adsorption and cell adhesion

We have measured the energy dissipation of the quartz crystal microbalance (QCM), operating in the liquid phase, when mono- or multi-layers of biomolecules and biofilms form on the QCM electrode (with a time resolution of ca. 1 s). Examples are taken from protein adsorption, lipid vesicle adsorption and cell adhesion studies. Our results show that even very thin (a few nm) biofilms dissipate a significant amount of energy owing to the QCM oscillation. Various mechanisms for this energy dissipation are discussed. Three main contributions to the measured increase in energy dissipation are considered. (i) A viscoelastic porous structure (the biofilm) that is strained during oscillation, (ii) trapped liquid that moves between or in and out of the pores due to the deformation of the film and (iii) the load from the bulk liquid which increases the strain of the film. These mechanisms are, in reality, not entirely separable, rather, they constitute an effective viscoelastic load. The biofilms can therefore not be considered rigidly coupled to the QCM oscillation. It is further shown theoretically that viscoelastic layers with thicknesses comparable to the biofilms studied in this work can induce energy dissipation of the same magnitude as the measured ones.

Reversible Changes in Cell Morphology due to Cytoskeletal Rearrangements Measured in Real-Time by QCM-D

The mechanical properties and responses of cells to external stimuli (including drugs) are closely connected to important phenomena such as cell spreading, motility, activity, and potentially even differentiation. Here, reversible changes in the viscoelastic properties of surface-attached fibroblasts were induced by the cytoskeleton-perturbing agent cytochalasin D, and studied in real-time by the quartz crystal microbalance with dissipation (QCM-D) technique. QCM-D is a surface sensitive technique that measures changes in (dynamically coupled) mass and viscoelastic properties close to the sensor surface, within a distance into the cell that is usually only a fraction of its size. In this work, QCM-D was combined with light microscopy to study in situ cell attachment and spreading. Overtone-dependent changes of the QCM-D responses (frequency and dissipation shifts) were first recorded, as fibroblast cells attached to protein-coated sensors in a window equipped flow module. Then, as the cell layer had stabilised, morphological changes were induced in the cells by injecting cytochalasin D. This caused changes in the QCM-D signals that were reversible in the sense that they disappeared upon removal of cytochalasin D. These results are compared to other cell QCM-D studies. Our results stress the combination of QCM-D and light microscopy to help interpret QCM-D results obtained in cell assays and thus suggests a direction to develop the QCM-D technique as an even more useful tool for real-time cell studies.

Dynamics of viscous amphiphilic films supported by elastic solid substrates

The dynamics of amphiphilic films deposited on a solid surface is analysed for the case in which shear oscillations of the solid surface are excited. The two cases of surface and bulk shear waves are studied with the film exposed to a gas or to a liquid. By solving the corresponding dispersion equation and the wave equation while maintaining the energy balance, we are able to connect the surface density and the shear viscosity of a fluid amphiphilic overlayer with the experimentally accessible damping coefficient, phase velocity, dissipation factor, and resonant frequency shifts of shear waves.

On Mass Loading and Dissipation Measured with Acoustic Wave Sensors: A Review

We summarize current trends in the analysis of physical properties (surface mass density, viscosity, elasticity, friction, and charge) of various thin films measured with a solid-state sensor oscillating in a gaseous or liquid environment. We cover three different types of mechanically oscillating sensors: the quartz crystal microbalance with dissipation (QCM-D) monitoring, surface acoustic wave (SAW), resonators and magnetoelastic sensors (MESs). The fourth class of novel acoustic wave (AW) mass sensors, namely thin-film bulk acoustic resonators (TFBARs) on vibrating membranes is discussed in brief. The paper contains a survey of theoretical results and practical applications of the sensors and includes a comprehensive bibliography.

Steady-state electrochemical determination of lipidic nanotube diameter utilizing an artificial cell model.

By exploiting the capabilities of steady-state electrochemical measurements, we have measured the inner diameter of a lipid nanotube using Ficks first law of diffusion in conjunction with an imposed linear concentration gradient of electroactive molecules over the length of the nanotube. Ficks law has been used in this way to provide a direct relationship between the nanotube diameter and the measurable experimental parameters Deltai (change in current) and nanotube length. Catechol was used to determine the Deltai attributed to its flux out of the nanotube. Comparing the nanotube diameter as a function of nanotube length revealed that membrane elastic energy was playing an important role in determining the size of the nanotube and was different when the tube was connected to either end of two vesicles or to a vesicle on one end and a pipet tip on the other. We assume that repulsive interaction between neck regions can be used to explain the trends observed. This theoretical approach based on elastic energy considerations provides a qualitative description consistent with experimental data.

On dissipation of quartz crystal microbalance as a mechanical spectroscopy tool

We report on theoretical analysis of dissipative effects in quartz crystal resonator applications to the dynamics of complex biological fluids and soft polymer films. As a mechanical spectroscopy tool, the quartz resonator can probe the storage and loss moduli μ(ω) of a thin material sample in small amplitude oscillations where polymers exhibit linear viscoelasticity. We show how viscosity (internal friction) and slippage (interfacial friction) of the sample affect the acoustical characteristics of the quartz resonator. With respect to biosensor’s application, we present rigorous expressions for the resonant frequency and damping of the quartz crystal which allow to quantify friction effects and even distinguish between them in resonator measurements performing on various frequencies. Possible application of the results in electronic nose and electronic tongue sensors is discussed.

The theory of membrane “vitrification”

Abstract This review covers the application of the Onsager fluctuation theory and reciprocal principle to nonequilibrium membrane phase transitions (PT) with diffusion. The main features of membrane “vitrification” theory are discussed for spherical lipid membranes subjected to the fast cooling. The relevance to cryoprotection problems is discussed.

MICRO-MECHANICAL CHARGE TRANSFER MECHANISM IN SOFT COULOMB BLOCKADE NANOSTRUCTURES

Abstract Room-temperature Coulomb blockade of charge transport through nanostructures containing organic inter-links has recently been observed. A pronounced charging effect in combination with the softness of the molecular links implies that charge transfer gives rise to a significant deformation of these structures. For a simple model system containing one nanoscale metallic cluster connected by molecular links to two bulk metallic electrodes, we estimate the characteristic frequency of tunneling charge transport to be of the same order or lower than the frequency of elastic cluster vibrations determined by the elasticity of the molecular links. We show that in this case periodic oscillations of the cluster in conjunction with sequential processes of cluster charging and decharging may appear for a sufficiently large bias voltage. This new “electron shuttle” mechanism of discrete charge transfer gives rise to a current through the nanostructure, which is proportional to the cluster vibration frequency.

Shuttle electron transfer in tethered mediator biosensor

Abstract Theoretical model of mediated electron transfer processes in redox enzyme biosensor is presented. We have analyzed the charge transfer from substrate to enzyme-modified electrodes via a small mediator molecule anchored on the electrode surface by long flexible polymer. In the system considered, a mediator communicates with redox enzyme center and “shuttles” electrons to the electrode. We have shown that in this case, the output current depends on tether length and that the smaller tethers enhance the electron transfer rate.