Nematic field transfer in a two-dimensional protein fibril assembly.

We perform Atomic Force Microscopy and numerical simulations of a bimodal solution containing long, semiflexible ß-lactoglobulin fibrils and short, flexible ß-lactoglobulin linear aggregates at an air-water interface. Short aggregates orient perpendicular to fibrils at very short distances and preferentially parallel at intermediate distances. At even larger distances an isotropic distribution is observed. Parallel ordering coincides with aggregate stretching: by straightening, small aggregates are able to approach the fibrils within a distance smaller than their radius of gyration. These findings contribute to the understanding of how anisotropic interactions are transferred in two-dimensional bimodal nematic fields of biopolymers at liquid interfaces.

Low-Index ZnO Crystal Plane-Specific Binding Behavior of Whole Immunoglobulin G Proteins.

Crystallographic surface-resolved examination of protein-ZnO interactions can greatly enhance the fundamental understanding of protein adsorption on these technologically important solid surfaces which, in turn, will be tremendously valuable for the emerging applications of ZnO-based biomaterials and biosensors. We examine experimentally and via computer simulations the intriguing differences in the adsorption preferences and binding behavior of whole immunoglobulin G (IgG) proteins to various, low-index ZnO crystal surfaces at the individual biomolecule level. By performing direct atomic force microscopy imaging, we determine that IgG predominantly binds to the ZnO plane of (101̅0) relative to the other three low-index planes of (0001), (0001̅), and (112̅0). This phenomenon is highly unusual, particularly considering the fact that the average binding energy of amino acids (AAs) on the ZnO (0001) facet is higher than that on the (101̅0) plane. In conjunction with combined Monte Carlo-molecular dynamics simulations, we further explain the possible origins of our unusual experimental findings with critical factors such as the specific spatial locations of strongly binding AAs in the protein and their spatial distributions on the exterior surface of the protein.

Soft repulsive interactions, particle rearrangements and size selection in the self-assembly of nanoparticles at liquid interfaces.

In the adsorption of nanoparticles at liquid interfaces, soft and short ranged repulsive effective interactions between the nanoparticles at the interface may eventually induce crowding, slow dynamics and jamming at high surface coverage. These phenomena can interfere during the adsorption process, significantly slowing down its kinetics. Here, by means of numerical simulations, we find that modifying the effective interactions, which can be achieved for example by grafting differently functionalized polymer shells on the bare nanoparticles, may qualitatively change such interplay. In particular our results suggest that, in the presence of ultrasoft particle interactions such as the ones described by a Gaussian Core Model potential, a small size polydispersity can be sufficient to decouple the adsorption kinetics from the slow dynamics that develops at the interface, due to a qualitative change from an irreversible adsorption controlled by particle rearrangements at the interface to one dominated by size selection mechanisms. These findings may be useful to achieve higher surface coverages and faster adsorption kinetics.

Conformations and effective interactions of polymer-coated nanoparticles at liquid interfaces.

We investigate conformations and effective interactions of polymer-coated nanoparticles adsorbed at a model liquid-liquid interface via molecular dynamics simulations. The polymer shells strongly deform at the interface, with the shape governed by a balance between maximizing the decrease in interfacial area between the two solvent components, minimizing unfavorable contact between polymer and solvent, and maximizing the conformational entropy of the polymers. Using potential of mean force calculations, we compute the effective interaction between the nanoparticles at the liquid-liquid interface. We find that it differs quantitatively from the bulk and is significantly affected by the length of the polymer chains and by the solvent quality. Under good solvent conditions, the effective interactions are always repulsive and soft for long chains. The repulsion range decreases as the solvent quality decreases. In particular, under poor solvent conditions, short chains may fail to induce steric repulsion, leading to a net attraction between the nanoparticles, whereas with long-enough chains the effective interaction potential may feature an additional repulsive shoulder at intermediate distances.

Assembly of nanoparticles at liquid interfaces: crowding and ordering.

Experiments with the self-assembly of nanoparticles at liquid interfaces suggest that cooperative and slow dynamical processes due to particle crowding at the interface govern the adsorption and properties of the final assembly. Here we report a numerical approach to studying nonequilibrium adsorption, which elucidates these experimental observations. The analysis of particle rearrangements shows that local ordering processes are directly related to adsorption events at high interface coverage. Interestingly, this feature and the mechanism coupling local ordering to adsorption do not seem to change qualitatively upon increasing particle size polydispersity, although the latter changes the interface microstructure and its final properties. Our results indicate how adsorption kinetics can be used for the fabrication of 2D nanocomposites with controlled microstructure.