Protein binding to like-charged polyelectrolyte brushes by counterion evaporation

We report on a planar poly(acrylic acid) (PAA) brush as a new kind of materials coating showing a variable protein resistance that can be controlled by the ionic strength of the protein solution. Using neutron reflectometry it has been found that a silicon wafer covered with a PAA brush strongly binds bovine serum albumin (BSA) under electrostatic repulsion. However, when adding sodium chloride to the protein solution, the PAA brush appears essentially protein resistant, although the direct repulsive electrostatic forces between BSA and PAA are screened under this condition. This effect of salt is unique and in agreement with that found earlier for the binding of proteins to spherical PAA brush particles. From the analysis of the neutron reflectivities, the protein density profile across the interface has been determined. At low ionic strength in the solution, BSA penetrates deeply into the PAA brush. In this view, the immobilization of proteins at a poly(acrylic acid) brush may be regarded as an entrapment within a confined geometry rather than an adsorption to a two-dimensional interface. A simple mean-field argument is given that explains these experimental findings. The model predicts a large gain of free energy associated with the release of BSA counterions on transferring a BSA molecule from the solution into the PAA brush. The free energy of this counterion evaporation is entropic in nature and dominates over the electrostatic repulsion between the BSA molecule and the like-charged PAA brush.