Force Measurement for the Interaction between Cucurbit[7]uril and Mica and Self-Assembled Monolayer in the Presence of Zn2+ Studied with Atomic Force Microscopy

Abstract

Force spectroscopy with atomic force microscopy (AFM) revealed that cucurbit[7]uril (CB[7]) strongly binds to a mica surface in the presence of cations. Indeed, Zn2+ was observed to facilitate the self-assembly of CB [7] on the mica surface, whereas monocations, such as Nat, were less effective. The progression of the process and the cation mediated self -assembled monolayer were characterized using AFM, and the observed height of the layer agrees well with the calculated CB[7] value (9.1 A). We utilized force-based AFM to further study the interaction of CB[7] with guest molecules. To this end, CB [7] was immobilized on a glass substrate, and aminomethylferrocene (am-Fc) was conjugated onto an AFM tip. The single -molecule interaction between CB[7] and am-Fc was monitored by collecting the unbinding force curves. The force histogram showed single ruptures and a unimodal distribution, and the most probable unbinding force value was 101 pN in deionized water and 86 pN in phosphate -buffered saline buffer. The results indicate that the unbinding force was larger than that of streptavidin-biotin measured under the same conditions, whereas the dissociation constant was smaller by 1 order of magnitude (0.012 s(-1) vs 0.13 s(-1)). Furthermore, a high resolution adhesion force map showed a part of the CB[7] cavities on the surface.