Protein Adsorption at the Air-Water Interface by a Charge Sensing Interferometric Technique.

Protein uptake at the interface of a millimeter-sized air bubble in water is investigated by a recently developed differential interferometric technique. The technique allows the study of capillary waves with amplitudes around 10-9 m, excited at the surface of the bubble by an electric field of intensity on the order of 10 V/cm. When one studies the resonant modes of the bubble (radial and shape modes), it is possible to assess variations of interfacial properties and, in particular, of the net surface charge as a function of bulk protein concentration. Sensing the interfacial charge, the technique enables us to follow the absorption process in conditions of low concentrations, not easily assessable by other methods. We focus on bovine serum albumin (BSA) and lysozyme as representatives of typical globular proteins. To provide comprehensive insight into the novelty of the technique, we also investigated the equilibrium adsorption of sodium dodecyl sulfate (SDS) ionic surfactant for bulk concentrations at hundreds of times lower than the Critical Micelle Concentration (CMC). Results unveil how the absorption of charged molecules affects the amplitudes of the bubble resonant modes even before affecting the frequencies in a transition-like fashion. Different adsorption models are proposed and developed. They are validated against the experimental findings by comparing frequency and amplitude data. By measuring the charging rate of the bubble interface, we have followed the absorption kinetics of BSA and lysozyme recognizing a slow, energy barrier limited phenomena with characteristic times in agreement with data in the literature. The evaluation of the surface excess concentration (Γ) of BSA and SDS at equilibrium is obtained by monitoring charge uptake. At the investigated low bulk concentrations, reliable comparisons with literature data from equilibrium surface tension isotherm models are reported.

Chirality Enhancement of Porphyrin Supramolecular Assembly Driven by a Template Preorganization Effect.

Cationic polylysine promotes, under neutral conditions, the spontaneous aggregation of opposite charged ZnTPPS in water. Spectroscopic investigations evidence a different preorganization of ZnTPPS onto the polypeptide matrix depending on the chain length. Spinodal decomposition theory in confined geometry is used to model this mechanism by considering the time evolution of a homogeneous distribution of randomly adsorbed particles (porphyrins) onto a rodlike polyelectrolyte (polymer) of variable length L.

Anomalous Behavior of Ultra-Low-Amplitude Capillary Waves. A Glimpse of the Viscoelastic Properties of Interfacial Water?

We investigate, both theoretically and by a differential interferometric technique, the behavior of large-wavelength capillary waves (of the order of 10-4 m) selectively excited at the surface of drops and bubbles with typical eigenfrequencies of the order of 102 Hz. The resonance peaks of gas bubbles or hydrocarbon drops in water (radius less than 1 mm) highlight anomalously small dissipation in the region of ultralow (sub-nanometric) oscillation amplitudes, reaching a plateau at higher amplitudes. This is in sharp contrast to the usual oscillating systems, where an anomalous behavior holds at large amplitudes alone. Dissipation is strongly dependent on the excited vibrational modes and, in spite of remarkable numerical differences, water-vapor and water-hydrocarbon interfaces exhibit the same overall trend. A phenomenological model was developed, based on the assumption that water possesses a threshold viscoelasticity, above which it behaves like a regular viscous fluid. The well-known Deborah number was then estimated within the anomalous region and found to lie in the range of viscoelastic fluids. In agreement with previous studies of nanohydrodynamics (e.g., atomic force microscopy measurements with sub-nanometric tip motions), the present one lends support to the idea that every self-aggregating fluid exhibits yield stress behavior, including classical Newtonian fluids like water. The essential requirement is that the applied perturbation lie below a critical threshold, above which viscous behavior is recovered. Our differential interferometric technique seems particularly suitable for this type of studies, as it allows measurement of long-wavelength capillary waves with sub-nanometric resolution on the oscillation amplitudes.

Oscillations of Bubble Shape Cause Anomalous Surfactant Diffusion: Experiments, Theory, and Simulations.

We investigate, both theoretically and experimentally, the role played by the oscillations of the cell membrane on the capture rate of substances freely diffusing around the cell. To obtain quantitative results, we propose and build up a reproducible and tunable biomimetic experimental model system to simulate the phenomenon of an oscillation-enhanced (or depressed) capture rate (chemoreception) of a diffusant. The main advantage compared to real biological systems is that the different oscillation parameters (type of deformation, frequencies, and amplitudes) can be finely tuned. The model system that we use is an anchored gas drop submitted to a diffusive flow of charged surfactants. When the surfactant meets the surface of the bubble, it is reversibly adsorbed. Bubble oscillations of the order of a few nanometers are selectively excited, and surfactant transport is accurately measured. The surfactant concentration past the oscillating bubbles was detected by conductivity measurements. The results highlight the role of surface oscillations on the diffusant capture rate. Particularly unexpected is the onset of intense overshoots during the adsorption process. The phenomenon is particularly relevant when the bubbles are exposed to intense forced oscillations near resonance.

Atomistic Insights into Ultrafast SiGe Nanoprocessing.

Controlling ultrafast material transformations with atomic precision is essential for future nanotechnology. Pulsed laser annealing (LA), inducing extremely rapid and localized phase transitions, is a powerful way to achieve this but requires careful optimization together with the appropriate system design. We present a multiscale LA computational framework that can simulate atom-by-atom the highly out-of-equilibrium kinetics of a material as it interacts with the laser, including effects of structural disorder. By seamlessly coupling a macroscale continuum solver to a nanoscale superlattice kinetic Monte Carlo code, this method overcomes the limits of state-of-the-art continuum-based tools. We exploit it to investigate nontrivial changes in composition, morphology, and quality of laser-annealed SiGe alloys. Validations against experiments and phase-field simulations as well as advanced applications to strained, defected, nanostructured, and confined SiGe are presented, highlighting the importance of a multiscale atomistic-continuum approach. Current applicability and potential generalization routes are finally discussed.

Helical Inclusions in Phospholipid Membranes: Lipid Adaptation and Chiral Order.

The lipid bilayer is a flexible matrix that is able to adapt in response to the perturbation induced by inclusions, such as peptides and proteins. Here we use molecular dynamics simulations with a coarse-grained model to investigate the effect of a helical inclusion on a lipid bilayer in the liquid disordered phase. We show that the helical inclusion induces a collective tilt of acyl chains, with a small, yet unambiguous difference between a right- and a left-handed inclusion. This behavior is rationalized using the elastic continuum theory: The magnitude of the chiral (twist) deformation of the bilayer is determined by the interaction at the lipid/inclusion interface, and the decay length is controlled by the elastic properties of the bilayer. The lipid reorganization can thus be identified as a generic mechanism that, together with specific interactions, contributes to chiral recognition in phospholipid bilayers. An enhanced response is expected in highly ordered environments, such as rafts in biomembranes, with a potential impact on membrane-mediated interactions between inclusions.