Host-Guest Interaction at Molecular Interfaces: Binding of Cucurbit[7]uril on Ferrocenyl Self-Assembled Monolayers on Gold

Ferrocene (Fc)-encapsulated cucurbit[7]uril (CB[7]) supramolecular host-guest complex (Fc@CB[7]) as a synthetic recognition pair has been widely adapted for coupling biomolecules and nanomaterials due to its ultrahigh binding affinity. In this paper, we have explored the binding of CB[7] on binary ferrocenylundecanethiolate/octanethiolate self-assembled monolayer on gold (FcC11S-/C8S-Au), a model system to develop our understanding of host-guest chemistry at molecular interfaces. It has been shown that upon incubation with CB[7] solution the redox behavior of FcC11S-/C8S-Au changes remarkably; i.e., a new pair of peaks appeared at more positive potential with narrowed widths. The ease of quantitation of surface bound-redox species (Fc⁺/Fc and Fc⁺@CB[7]/ Fc@CB[7]) enabled us to determine the thermodynamic formation constant of Fc@CB[7] at FcC11S-/C8S-Au (7.3 ± 1.8 × 10⁴ M^-1). With time-dependent redox responses, we were able to, for the first time, deduce both the binding and dissociation rate constants, 2.8 ± 0.3 × 10³ M^-1s^-1 and 0.08 ± 0.01 s^-1, respectively. These results showed substantial differences both thermodynamically and kinetically for the formation of host-guest inclusion complex at molecular interfaces with respect to solution-diffused, homogeneous environments.