Inclusion of bisphenols by a self-assembled monolayer of thiolated cyclodextrin on a gold electrode

Complexation of various kinds of bisphenols (BPs) by a self-assembled monolayer (SAM) of thiolated a-cyclodextrin (CD) on a gold electrode was examined by cyclic voltammetry (CV) using hydroquinone (HQ) as a probe. On the basis of the inhibitory effect of BP on the inclusion of HQ by the surface-confined CD, the association constants (K-assoc) of BP with the immobilized alpha-CD were estimated. For comparison, the association of BP with free alpha-CD was examined spectrophotometrically using methyl orange as a probe. The K-assoc value for BP with the SAM of 6-sulfhydryl-6-deoxy-alpha-cyclodextrin (HS-alpha-CD) was smaller than that in the free alpha-CD system. A similar tendency was obtained when 6-lipoylamido-6-deoxy-alpha-cyclodextrin (LP-alpha-CD) was used in place of HS-a-CD to introduce a spacer group between alpha-CD and the disulfide group. By the coexistence of bisphenol S (BPS) at the incubation of HS-alpha-CD or LP-alpha-CD with a bare gold electrode, furthermore, the K-assoc values for bisphenol A with the surface-confined alpha-CDs increased and became comparable to that with free alpha-CD. This is probably due to a BPS-induced fixation of the CD moiety on the electrode in a somewhat distorted shape, which is advantageous for the inclusion of bisphenols (so-called molecular imprinting). The effect of the structure of guest molecules on the association with the surface-confined alpha-CD is also discussed.