Kinetics of Protein Adsorption at a Poly(Acrylic Acid) Brush Studied by Surface Plasmon Resonance Spectroscopy

The kinetics of protein adsorption at a planar poly(acrylic acid) (PAA) brush has been studied by surface plasmon resonance spectroscopy. A PAA brush has been prepared by spin-coating a gold sensor chip with a thin poly(styrene) film. Then, the diblock copolymer poly(acrylic acid)-poly(styrene) was transferred onto the film using the Langmuir–Blodgett technique generating a PAA brush with a grafting density of about 0.1 nm-2. Hen egg white lysozyme and bovine serum albumin were used as model proteins, because they carry a positive and negative net charge at the applied pH-value of 7 and interact with the negatively charged PAA brush under electrostatic attraction and repulsion, respectively. It has been found that lysozyme is strongly interacting with a PAA brush. Initial rates of adsorption are constant over time suggesting a diffusion-controlled adsorption mechanism. In contrast, a continuously decreasing adsorption rate has been observed for BSA indicating a retarding influence of already adsorbed protein molecules on the adsorption kinetics right from the beginning. For a typical protein solution concentration of 0.1 mg mL-1, the adsorbed amount of lysozyme enters a plateau region after a few seconds only, whereas in the case of BSA adsorption is slower by a factor of 100. The observed differences in the adsorption kinetics of lysozyme and BSA are discussed in the light of recent proposed models of protein adsorption at a PAA brush.