Long-range interactions between protein-coated particles and POEGMA brush layers in a serum environment

Hydrophilic poly[oligo(ethylene glycol) methyl methacrylate] (POEGMA) brush layers with different thickness and graft densities were prepared by surface-initiated atom transfer radical polymerization (SI-ATRP) to construct a model surface to examine protein-surface interactions in a serum environment. The thickness of the POEGMA brush layers could be well controlled by the polymerization time and density of the immobilized initiators. The interactions between these brush-modified surfaces and the protein-coated polystyrene (PS) particles in newborn calf serum (NBCS) environment were then measured by total internal reflection microscopy (TIRM). In addition, protein adsorption properties onto the polymer brush surface layers were examined by atomic force microscopy (AFM). Relatively large amounts of protein adsorbed to short (4nm and 9nm-thick) POEGMA-coated surfaces or surfaces grafted with a low density of polymer chains. It was considered that shorter polymer chains or chains with low grafted density cannot fully cover the surfaces, proteins in serum could directly interact with the material surface and then deposited to form an adsorbed layer. The TIRM measurements showed that such adsorbed protein layer could mediate the interactions between the two surfaces by generating steric or bridging forces, resulting in different interaction potentials. Some particles were freely diffusing, some experienced intermittent diffusion and more than 50% of particles were irreversibly deposited to the surfaces covered by short polymer brushes. However, for longer (17 and 30nm-thick) POEGMA brush layer surfaces, material surface would be sufficiently covered by the dense coating and the first step of protein adsorption on surface was avoided. TIRM measurements showed that around 95% of the protein-coated particles could freely move in the serum and no attractive force between two surfaces was detected. The steric repulsion generated from the long POEGMA brush layer in the swollen state was long-range and strong so that the protein adsorption is very unlikely. These results concluded that the adsorbed protein layer on POEGMA surfaces plays an important role in regulating the interaction between protein-coated particles and POEGMA surfaces which are highly repellent toward protein adsorption.