Inclusion of bisphenols by a self-assembled monolayer of thiolated calix[6]arene on a gold surface

The molecular recognition of various kinds of bisphenols (BPs) by a self-assembled monolayer (SAM) of thiolated calix[6]arene on a gold electrode was examined using cyclic voltammetry (CV). Based on the inhibitory effect of BPs on the inclusion of hydroquinone (HQ) as a probe by the surface-confined calix[6]arene, the association constants (K_assoc) of BPs with the immobilized calix[6]arene were estimated. The K_assoc values for BPs with the SAM of thiolated hexasodium calix[6]arenehexasulfonic acid (thioSCX₆) were much smaller than those in the free calix[6]arene derivative systems reported previously. The order of the K_assoc values for BPs with thioSCX₆ was bisphenol A (BPA) > bisphenol S (BPS) > bisphenol B (BPB) > bisphenol F (BPF). The K_assoc values for BPA and BPS with thioSCX₆ were larger than that for BPB, despite the larger hydrophobicity of BPB than that of BPA and BPS. This is probably because the inclusion phenomena in this system are not simply driven by the hydrophobic interaction, but are significantly affected by several steric and structural factors with immobilization of the host. Those are (1) a decrease in flexibility of the SCX₆ cavity by the formation of SAM, (2) a decrease in inclusion ability of the SCX₆ by the presence of Au surface just beneath it, and (3) a repulsion of hydrophobic guests by the presence of sulfonate groups at the top of the SAM. Moreover, the association phenomena (adsorption and desorption processes) of a bisphenol with the SCX ₆ SAM were examined directly by using localized surface plasmon resonance spectroscopy.