Recognition of bile acids at cyclodextrin-modified gold electrodes

Lipoylamino-β- and γ-cyclodextrin (LP-β-CD and LP-γ-CD, respectively) were adsorbed at the surface of gold electrodes by sulfur-gold bonding. The resultant electrodes exhibited quasi-reversible voltammograms for the redox reaction of Fe(CN)₆^3-/-4 in aqueous solutions, with peak-to-peak separation (ΔE_p) being 85 mV at 20 mV s^-1 as a potential sweep rate. When bile acids are added to the solution, ΔE_p values increased to 200-300 mV with increasing the concentration of bile acids. A Langmuir-type adsorption analyses satisfactorily afforded the binding constants (K_surf) of the surface-confined LP-β-CD and LP-γ-CD with the bile acids. The obtained K_surf values of LP-γ-CD are 5.0-50 times larger than the corresponding binding constants of γ-CD in homogenious aqueous solutions. Cyclic voltammetric experiments with positively, negatively, and non-charged adamantane derivatives as well as pH titration experiments revealed that the retardation of the electrode reaction of negatively charged Fe(CN) ₆^3-/4- caused by bile acids was attributable (1) to electric potential changes due to the accumulation of the negative charges at the electrode surface, and (2) to an increase in the hydrophobicity of the electrode surface due to the binding of hydrophobic bile acids to the LP-β-CD and LP-γ-CD membranes.