Strong Inclusion of Inorganic Anions into β-Cyclodextrin Immobilized to Gold Electrode

The inclusion of inorganic anions such as SO42-, NO3-, and HPO42- into the cavity of beta-cyclodextrin monolayers on Au was examined by X-ray photoelectron spectroscopy (XPS), a quartz crystal microbalance (QCM), and chronocoulometric measurements of the competitive inclusion with ferrocene. The inclusion amounts of ferrocence in 0.2 M Na2SO4, NaNO3, and Na2HPO4 solutions were less than 6% of the adsorption amount of beta-cyclodextrin on Au, resulting in the apparent inhibition of the ferrocene redox reaction. The surface association constants of these anions reached about 10 on a logarithmic scale and were much higher than those for the inclusion of common organic guest compounds. A stronger anion inclusion was also demonstrated by the QCM response corresponding to the replacement of a preincluded organic guest with sulfate upon the injection of the sulfate solution. Quantitative analysis of the XPS data suggested a 1:1 association for each of these anions per surface beta-cyclodextrin. There was no detectable inclusion for ClO4-, Cl-, and Br-.