Poly-acrylic Acid Brushes and Adsorbed Proteins

Planar polyelectrolyte brushes are prepared by Langmuir-Schaefer based grafting of perdeuterated (styrene)(49) -b-(acrylic acid)(222) block copolymer (dPS-b-PAA) to dPS pre-coated silicon supports with grafting density sigma(PAA) from 0.07 to 0.11 nm(-2). The structure of the solvent-swollen brushes, i.e. the volume fraction profile of polymer segments, phi(PAA), as a function of altitude z from the grafting plane into the liquid phase is extracted from neutron reflectivity measurements. We find that for all cases investigated phi(PAA) (z) resembles a Gaussian profile. Although very condensed, the PAA brushes can be loaded with bovine serum albumin (BSA). The integral amount of adsorbed BSA scales linearly with grafting density. We compare our z-resolved volume fraction profile phi(BSA)(z) of BSA on PAA brushes with existing literature on that system. It is found that a cross-over takes place in the adsorption scheme from ternary compressive, where proteins can approach the grafting surface only by compressing the brush, to ternary insertive, where proteins enter the brush with only local perturbation of the concentration profile, as a function of R-P/H-max, where R-P is the Stokes-Radius of the protein, and H-max is the experimentally determined maximum height of the brush.