Probing multivalent host-guest interactions between modified polymer layers by direct force measurement.

The adhesion behavior between modified polysaccharide layers capable of forming host-guest complexes has been determined by direct force measurements with the atomic force microscope (AFM). Polysaccharides bearing either host or guest moieties were obtained by derivatization of chitosan with pendant β-cyclodextrin (CD) and adamantane (AD) moieties, respectively. These modified polysaccharides were covalently immobilized either to flat surfaces or to AFM-probes. The number of interacting polymer segments has been reduced significantly by covalently immobilizing chitosan to an AFM-tip with small radius and measuring the forces between the protruding polymer segments and a chitosan layer immobilized to a flat surface. By this approach, it was possible to determine the interaction between polymer layers on the level of single polymer strands. To separate contributions to the adhesion due to the formation of host-guest complexes from unspecific interactions, we performed measurements between various combinations of chitosan derivatives. With the same polymer probe of adamantane-modified chitosan, the interaction against a number of different chitosan layers has been determined, including ones that are not able to form host-guest complexes, such as unmodified chitosan or β-cyclodextrin modified chitosan, which has been blocked previously by addition of adamantane. The resulting adhesion behavior has been analyzed in terms of the total work of adhesion, the number of rupture events, and the corresponding lengths of the polymer segments as well as rupture forces. A clear difference has been found for systems where the formation of host-guest complexes is possible in comparison to the absence of specific multivalent interaction between the polysaccharide layers. In particular, the work of adhesion is increasing up to an order of magnitude upon the formation of host-guest complexes between the chitosan layers.