Effect of Silica Surface Properties on the Formation of Multilayer or Sub-monolayer Protein Hard Corona: Albumin Adsorption on Pyrolytic and Colloidal SiO2 Nanoparticles

The adsorption of bovine serum albumin (BSA) on two types of silica nanoparticles (NPs), one pyrolytic (P-SiO2; namely AOX50 by Evonik) and the other colloidal (lab-made by using inverse micelles microemulsion, M-SiO2), is studied. Both materials are characterized in terms of size of primary particles (by transmission electron microscopy), amounts (by thermogravimetry) and distribution of silanols (IR spectroscopy in controlled atmosphere, augmented by H/D isotopic exchange and reaction with VOCl3, to distinguish silanols actually located at the surface of nanoparticles), water contact angle, z-potential and dispersion state in water, PBS buffer and BSA solutions in PBS (by dynamic light scattering, DLS). Proteins are found to act as dispersing agent toward the large aggregates formed by both types of NPs in PBS buffer, although monodispersion was not attained in the conditions investigated. The problem of the determination of the silica surface actually available in NPs agglomerates for protein adsorption is addressed, and a model based on the external area of the agglomerates determined by DLS is proposed, supported by the trend of z-potential in dependence on the amount of adsorbed BSA and by the UV circular dichroism spectra of adsorbed proteins. The spectra reveal the occurrence of protein-protein interactions for BSA on P-SiO2, where multilayers of irreversibly adsorbed BSA molecules (i.e. a so called protein hard corona) are proposed to be formed. Conversely, the model indicates the formation of a sub-monolayer protein hard corona on M-SiO2. The difference in protein coverage appears to be related to differences in the distribution of surface silanols, more than to differences in z-potential