Modulating Protein-Nanoparticle Interactions By pH and Electrolyte

The interaction of biological matter with nanoscale materials plays a key role in modern biotechnology. Charge-driven bridging of nanoparticles by macromolecules represents a promising route for engineering functional structures, but the strong electrostatic interactions involved when using conventional polyelectrolytes impart irreversible complexation and ill-defined structures. Here we study the combined influence of pH, electrolyte and concentration on the bridging aggregation of silica nanoparticles (diameter ~ 20 nm) with a globular protein lysozyme (Bharti et al., Langmuir 2011, Chem. Lett. 2012, Soft Matter 2014). We find that protein binding to the silica particles is determined by pH irrespective of the ionic strength. In a wide pH range lysozyme causes bridging aggregation of the silica nanoparticles. The hetero-aggregate structures formed by the silica particles with the protein were studied by small-angle X-ray scattering and the structure factor data were analyzed on the basis of a sticky hard sphere model. It is found that the electrolyte concentration determines the stickiness of particles near pH 5, where the weakly charged silica particles are bridged by strongly charged protein. An even stronger influence of the electrolyte is found near the isoelectric point of the protein (pI  11) which is attributed to the shielding of repulsive interactions between the highly charged silica particles, and to hydrophobic interactions between the bridging protein molecules.