Interfacial Properties of Film Forming Proteins and their Layer-by-Layer Assembly with Nanoparticles

Sammanfattning:  The aim of this thesis work was to investigate the interfacial properties of film forming proteins, stability of the layers created by them, and their interactions with nanoparticles. Mussel adhesive protein (Mefp-1) and hydrophobins (HFBI and HFBII) were the three main proteins used. HFBI and HFBII are relatively small globular proteins produced by filamentous fungi. They display unusual surface activity, by spontaneously forming films at many interfaces. Mefp-1 is extracted from mussel byssus and posses high ability to strongly attach to different kinds of surfaces. Fundamental understanding of their interfacial properties is of general interest because of the variety of possible applications, for example in food, biomedicine, coatings, etc. The adsorption and layer stability studies of HFBI, HFBII, and Mefp-1 at the solid/water interface were studied by means of dual polarization interferometery (DPI). It was observed that the adsorbed amount of HFBII was higher than that of HFBI and the film formed by the former protein was more compact. The adsorption kinetics of the two hydrophobins displayed some unusual features. Further, it was found that layers formed by these proteins partially desorbed when the flow was stopped, and the desorption rate for HFBII was enhanced in the presence of hydrophobins in solution.  Adsorption of Mefp-1 as a function of solution pH or degree of aggregation in the bulk was investigated. Adsorbed amount of non-aggregated Mefp-1 on silicon oxynitride increased with increasing buffer pH, but the resulting film was not more compact. Adsorption of slightly aggregated Mefp-1 resulted in higher adsorbed amount and formation of denser layer. Initial adsorption kinetics studies revealed almost no pH dependence of the non-aggregated mussel adhesive protein, but the kinetics was slowed down by presence of aggregates in the bulk solution. Moreover, the effect of ionic strength and low pH on preadsorbed Mefp-1 layer was investigated, and it was found that the protein layer exhibits higher resistance towards desorption than a synthetic polyelectrolyte with similar charge density. Finally, physical and nanomechanical properties of composite films consisting of Mefp-1 and ceria nanoparticles were examined. For fabrication of these films the layer-by-layer approach was used. Quartz crystal microbalance with dissipation (QCM-D) was utilized to measure sensed mass and visco-elastic properties of adsorbed layers. Atomic force microscopy (AFM) techniques, including peak force quantitative nanomechanical mapping (Peak Force QNM) and contact mode AFM, were used for measuring physical, mechanical and robustness properties of nanocomposite films in air. Independently on the number of layers an almost linear growth of sensed mass was observed while the total dissipation values increases after six steps. Moreover, increase in Mefp-1 concentration during deposition resulted in formation of a more rigid layer, by comparison with lower concentration. The robustness of the nanocomposite layer showed that at low pressures (about 16 MPa) plastic deformation occurs and increase in load result in abrasion of the layers at about 80 MPa.