Annealed brushes of the cationic polyelectrolyte poly(2-(dimethylamino)ethyl methacrylate), PDMAEMA, selectively bind proteins based on their net charge. These brushes can bind a highly variable amount of BSA (bovine serum albumin) by a largely electrostatic mechanism, depending on the solution pH and ionic strength and the mass of PDMAEMA grafted per unit area. In this study, surface plasmon resonance is used to measure protein binding at ionic strengths 1, 10 and 100 mM with pH ranging from 3 to 10. BSA binding is maximum at pH 7. The protein uptake at 1 mM ionic strength is higher than 10 mM from pH 4 to 6, yet comparable from pH 7 to 9. The trend is consistent with the mean-field model predictions. Much lower uptake is observed at 100 mM ionic strength across all pH values studied. At ionic strengths ≤ 10 mM, significant BSA binding was measured at approximately one unit below the isoelectric point of BSA, where the protein is net positively charged. No significant binding occurred more than one pH unit below the BSA isoelectric point. This contrasts with protein binding by anionic polyelectrolyte brushes, where others have observed significant protein binding as much as two pH units on the "wrong side" of the isoelectric point. This wrong side binding appears to be weaker for cationic brushes.

Our earlier work indicated BSA was able to penetrate deep into the brush even when the separation distance between chains was comparable to the size of BSA. Here, PDMAEMA brushes with varying grafting densities up to the maximum attainable value are prepared on silicon substrates in an attempt to identify a transition to an impenetrable brush. We will discuss how protein uptake by these high grafting density brushes differs from those with low to moderate grafting densities. The effect of BSA uptake on the brush thickness will be examined via ellipsometry and atomic force microscopy.