Probing Multivalent Host–Guest Interactions between Modified Polymer Layers by Direct Force Measurement

Abstract

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.

Recording stretching response of single polymer chains adsorbed on solid substrates

“…The atomic force microscope (AFM) has revolutionized our capabilities to image, probe, and manipulate nanoscopic objects on surfaces, especially for soft and polymeric materials. Among the different applications of this technique, which include single molecule imaging [1][2][3], measurements of forces and friction [4][5][6][7], the possibility to probe and manipulate single macromolecules seems most exciting [8][9][10][11][12][13][14][15][16].…”

“…Among these developments, the so-called single molecule force spectroscopy became a widely used tool to investigate the mechanical properties of single macromolecules, their interaction with surfaces, and chemical reaction kinetics on a single molecule level [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. The technique relies on a configuration where the substrate and the tip are bridged with a single polymer molecule, and subsequent analysis of the measured force-distance curves, normally upon retraction, but eventually also upon approach.…”

Multivalency: influence of the residence time and the retraction rate on rupture forces measured by AFM

“…Yet, even if this timescale can appear long, similar long times were reported by Kaan et al which investigated the interaction between a surface covered by chitosan bearing CD and an AFM tip coated by chitosan bearing AD groups. 29 As far as the mean adhesion force is concerned, it is a slowly varying function of the residence time for 1AD on SAM-CD and on SAM-CD(NP) and for 4AD on SAM-CD whereas it increases steadily for 1AD on PAH-CD and for 4AD on SAM-CD(NP) and on PAH-CD. In spite of this difference, the adhesion force appears to vary as log(contact time) over the domain was investigated.…”

“…By investigating the rupture between AFM tips covered by chitosan chains onto which are graed AD moieties and substrates covered by chitosan chains onto which are graed b-CDs, Kaan et al found that the rupture forces increase with the contact time over timespans of several seconds. 29 The distance at which the ruptures took place largely exceeded rupture distances found on "hard" substrates for host-guest interactions. Takemasa et al investigated the interaction between two polymers namely hydroxyethylcellulose and amylose.…”

Inclusion of picolines by a substituted binaphthyl diol host: selectivity and structure

“…6 These non-bonded interactions have also been studied with atomic force microscopy, in which the surfaces of modied chitosans were analysed. 7 A kinetic as well as thermodynamic study of a water-soluble M 4 L 6 metalorganic host allowed the determination of structure-properties relationships. 8 A Werner clathrate, Ni(NCS) 2 (4-PhPyridine) 4 has been employed recently to enclathrate selectively the isomers of xylene.…”