Protein trapping of silica nanoparticles

We have observed the formation of protein-nanoparticle complexes at the air-water interfaces from three different methods of presenting the nanoparticles to proteins. The structures formed are remarkably reproducible for the three formation mechanisms. The methods of X-ray and neutron reflectivity (with isotopic contrast variation between the protein and nanoparticles) have been used to study the structures formed at the air-water interface of β-casein presented to silica nanoparticle dispersions. Whilst the silica dispersions showed no observable reflectivity, strong signals appear in the reflectivity when protein is present. Dropwise spreading of a small amount of protein at the air-silica sol interface and presentation of the silica sol to an isolated monomolecular protein film (made by the "flow trough" method (A. W. Perriman, D. J. McGillivray and J. W. White, Soft Matter, 2008, 4, 2192-2198)) gave an immediate signal. Mixing the components in solution only produces a slow response but in all cases a similar structure is formed. The different responses are interpreted in structural and stoichiometric ways.