We consider the adsorption of bovine serum albumin (BSA) on spherical polyelectrolyte brushes (SPB). The SPB consist of a solid polystyrene core of 100 nm diameter onto which linear polyelectrolyte chains [poly(acrylic acid), (PAA)] are grafted. The adsorption of BSA is studied at a pH of 6.1 at different concentrations of added salt and buffer. We observe strong adsorption of BSA onto the SPB despite the effect that the particles as well as the dissolved BSA are charged negatively. The adsorption of BSA is strongest at low salt concentration and decreases drastically with increasing amounts of added salt. Virtually no adsorption takes place at salt concentration of 0.1 M. Moreover, the adsorbed protein can be washed out again by raising the ionic strength from low to high values. A major driving force for the adsorption is located at a lower pH within the brush at low ionic strength. Thus, the isoelectric point of the protein may be approached or even reached. In this case strong interaction between the SPB and the protein results. Moreover, the negative charge of the polyelectrolyte interacts with the patches of positive charges on the protein. In this way the protein becomes a multivalent counterion within the brush and monovalent counterions will be released. All results demonstrate that the SPB present a new class of colloidal carrier particles whose interaction with proteins can be tuned in a well-defined fashion.

We review recent experiments on the interaction of proteins with anionic polyelectrolytes in aqueous solution. Data from the literature demonstrate that proteins can form soluble complexes with linear polyelectrolytes even on the “wrong side” of the isoelectric point, that is, for pH values above the isoelectric point of the proteins under which the polyelectrolytes and the proteins are like-charged. All data published so far demonstrate that this type of adsorption becomes weaker with increasing ionic strength. A much stronger interaction is found if the polyelectrolyte chains are grafted onto solid surfaces to form polyelectrolyte brushes. Here it has been shown that spherical polyelectrolyte brushes consisting of a core of ca. 100 nm diameter and long attached polyelectrolyte chains strongly adsorb proteins at low ionic strength (“polyelectrolyte-mediated protein adsorption”; PMPA). Virtually no adsorption takes place onto the spherical polyelectrolyte brushes at high ionic strength. A critical comparison of data obtained on free polyelectrolytes and on polyelectrolyte brushes shows that both phenomena can be traced back to patches of positive charge on the surface of the proteins. Moreover, we discuss the driving force of the PMPA-process in terms of the Donnan pressure inside the brush layer. Here we find a good correlation which demonstrates that release of counterions during the process of adsorption is the main driving force.

Highly symmetric gold nanoparticle clusters with coordination numbers up to seven were produced by using coating with block copolymers. The resulting clusters were separated by density gradient centrifugation and characterized by using SEM and optical spectroscopy (see picture).