Dynamic Assembly of Class II Hydrophobins from T. reesei at the Air-Water Interface.

Class II hydrophobins are amphiphilic proteins produced by filamentous fungi. One of their typical features is the tendency to accumulate at the interface between an aqueous phase and a hydrophobic phase, such as the air-water interface. The kinetics of the interfacial self-assembly of wild-type hydrophobins HFBI and HFBII and some of their engineered variants at the air-water interface were measured by monitoring the accumulated mass at the interface via nondestructive ellipsometry measurements. The resulting mass vs time curves revealed unusual kinetics for a monolayer formation that did not follow a typical Langmuir-type of behavior but had a rather coverage-independent rate instead. Typically, the full surface coverage was obtained at masses corresponding to a monolayer. The formation of multilayers was not observed. Atomic force microscopy revealed formation and growth of non-fusing protein clusters at the interface. The mechanism of the adsorption was studied by varying the structure or charges of the protein or the ionic strength of the subphase, revealing that the lateral interactions between the hydrophobins play a role in their interfacial assembly. Additionally, a theoretical model was introduced to identify the underlying mechanism of the unconventional adsorption kinetics.

Distribution of CoFe2O4 Nanoparticles Inside PNIPAM-Based Microgels of Different Cross-linker Distributions.

The aim of this study is to tailor the inner structure of positively charged poly-( N-isopropylacrylamid- co-allylamine) (P(NIPAM- co-AA)) microgels for a better control of the distribution of negatively charged magnetic cobaltferrite (CoFe2O4@CA) nanoparticles (MNPs) within the microgels. Therefore, two different strategies are followed for the microgel synthesis: the (one pot) batch method which leads to a higher cross-linker density in the microgel core and the feeding method which compensates different reaction kinetics of the cross-linker and the monomers. The latter one is expected to result in a homogeneous cross-linker distribution. Information about the cross-linker distribution is indirectly gained by measuring the elastic modulus via indentation experiments with an atomic force microscope. While the batch method results in a higher elastic modulus in the center of the microgel indicating a core/shell structure, the feeding method leads to a constant elastic modulus over the whole microgel. The loading with MNPs and their distribution are studied with transmission electron microscopy (TEM). The TEM images show a large difference in the MNP distribution which is correlated to the cross-linker distribution of both types of microgels. The batch method microgel has a low MNP concentration in the core. The feeding method microgel shows a much more homogeneous distribution of MNPs across the microgel. The latter one also shows a stronger charge reversal which is a hint for a higher loading of the feeding method microgel. Dynamic light scattering and electrophoretic mobility measurements demonstrate that for both types of microgels, the temperature sensitivity is preserved after loading with MNPs.

Nanomechanics and Nanorheology of Microgels at Interfaces.

The review addresses nanomechanics and nanorheology of stimuli responsive microgels adsorbed at an interface. In order to measure the mechanical properties on a local scale, an atomic force microscope is used. The tip presents an indenter with a radius of curvature of a few 10 s of nm. Static indentation experiments and dynamic studies with an excited cantilever are presented. The effect of several internal and external parameters on the mechanical properties is reviewed. The focus is on the correlation between the swelling abilities of the gels and their mechanical properties. Several results are surprising and show that the relationship is not as simple as one might expect.

Combined Cononsolvency and Temperature Effects on Adsorbed PNIPAM Microgels.

The present study addresses the multiresponsive behavior of poly(N-isopropylacrylamide) (PNIPAM) microgels adsorbed to interfaces. The microgels react to changes in temperature by shrinking in aqueous solution above their volume phase transition temperature (VPTT). Additionally, they shrink in mixtures of water and ethanol, although both individual liquids are good solvents for PNIPAM. The combination of this so-called cononsolvency effect and the temperature response of adsorbed microgels is studied by atomic force microscopy (AFM). Adsorbed microgels are of special interest because they are compressed considerably compared to those in bulk solution. It is shown that the impact of adsorption on swelling depends on the specific surface details, as well as the sample preparation. Thereby, the microgels are deposited on two different kinds of surfaces: on gold surface and on polycation (PAH) coating which show different interactions with the microgels in terms of electrostatic interaction and wettability. In addition, the microgels were deposited from different solvent mixtures. This influences the microgel structure and thereby the swelling properties. Nanorheology studies by dynamic AFM measurements lead to surprising results which are explained by the fact that not only polymer density but a subtle interaction between polymer and solvent might dominate the rheological properties. This work supports the view that preferential adsorption of ethanol at PNIPAM drives cononsolvency, while the shrinking at T > VPTT is caused by general breaking of hydrogen bonds between solvents and PNIPAM.

Poly(N-isopropylacrylamide) Microgels under Alcoholic Intoxication: When a LCST Polymer Shows Swelling with Increasing Temperature.

Poly(N-isopropylacrylamide) (PNIPAM) microgel is a smart polymer that shows a volume phase transition temperature (VPTT) at around 32 °C in aqueous solutions, above which it collapses. In this work, combining experiments and molecular simulations, it is shown that PNIPAM microgels do not always exhibit a collapsed structure above the VPTT. Instead, PNIPAM in aqueous alcohol mixtures shows a two-step conformational transition, i.e., a collapse at low temperatures (T < 32 °C) and a reswelling when T > 50 °C. The present analysis indicates that delicate microscopic interaction details, together with the bulk solution properties, play a key role in dictating the reswelling behavior. Even when PNIPAM microgels swell with increasing T, this is not a standard upper critical solution behavior.

Construction of Compact Polyelectrolyte Multilayers Inspired by Marine Mussel: Effects of Salt Concentration and pH As Observed by QCM-D and AFM.

Biomimetic multilayers based on layer-by-layer (LbL) assembly were prepared as functional films with compact structure by incorporating the mussel-inspired catechol cross-linking. Dopamine-modified poly(acrylic acid) (PAADopa) was synthesized as a polyanion to offer electrostatic interaction with the prelayer polyethylenimine (PEI) and consecutively cross-linked by zinc to generate compact multilayers with tunable physicochemical properties. In situ layer-by-layer growth and cross-linking were monitored by a quartz crystal microbalance with dissipation (QCM-D) to reveal the kinetics of the process and the influence of Dopa chemistry. Addition of Dopa enhanced the mass adsorption and led to the formation of a more compact structure. An increase of ionic strength induced an increase in mass adsorption in the Dopa-cross-linked multilayers. This is a universal approach for coating of various surfaces such as Au, SiO2, Ti, and Al2O3. Roughness observed by AFM in both wet and dry conditions was compared to confirm the compact morphology of Dopa-cross-linked multilayers. Because of the pH sensitivity of Dopa moiety, metal-chelated Dopa groups can be turned into softer structure at higher pH as revealed by reduction of Young's modulus determined by MFP-3D AFM. A deeper insight into the growth and mechanical properties of Dopa-cross-linked polyelectrolyte multilayers was addressed in the present study. This allows a better control of these systems for bioapplications.

Loading of PNIPAM Based Microgels with CoFe2O4 Nanoparticles and Their Magnetic Response in Bulk and at Surfaces.

The present paper addresses the loading of thermoresponsive poly-N-isopropylacrylamide (PNIPAM) based microgel particles with magnetic nanoparticles (MNP: CoFe2O4@PAA (PAA = poly(acrylic acid))) and their response to an external magnetic field. The MNP uptake is analyzed by transmission electron microscopy (TEM). Obviously, the charge combination of MNP and microgels plays an important role for the MNP uptake, but it does not explain the whole uptake process. The MNP uptake results in changes of size and electrophoretic mobility, which is investigated by dynamic light scattering (DLS) and a Zetasizer. The microgels loaded with MNP preserve their thermosensitivity, and they show magnetic separability and are considered as magnetic microgels. After adsorption at a surface the magnetic microgels are studied with a scanning force microscope and indentation experiments. The magnetic microgels show an elongation along the magnetic field parallel to the surface while the height of the microgels (perpendicular to the surface and to the magnetic field) is compressed. This result is in good agreement with simulations of volume change of ferrogels in a magnetic field.

Nanofluidics of thin polymer films: linking the slip boundary condition at solid-liquid interfaces to macroscopic pattern formation and microscopic interfacial properties.

If a thin liquid film is not stable, different rupture mechanisms can be observed causing characteristic film morphologies: spinodal dewetting and dewetting by nucleation of holes. This rupturing entails liquid flow and opens new possibilities to study microscopic phenomena. Here we use this process of dewetting to gain insight on the slip boundary condition at the solid-liquid interface. Having established hydrodynamic models that allow for the determination of the slip length in a dewetting experiment based on nucleation, we move on to the quantification and molecular description of slip effects in various systems. For the late stage of the dewetting process involving the Rayleigh-Plateau instability, several distinct droplet patterns can be observed. We describe the importance of slip in determining what pattern may be found. In order to control the slip length, we use polymeric liquids on different hydrophobic coatings of silicon wafers. We find that subtle changes in the coating can lead to large changes in the slip length. Thus, we gain insight into the question of how the structure of the substrate affects the slip length.