Electrostatically-Induced Inclusion of Anions in Cyclodextrin Monolayers on Electrodes

Oxidative mercury−thiolate bond formation accounts for the assembly of densely packed monolayers of per-2,3-methylated per-6-thiolated α-, β-, and γ-cyclodextrins on the hanging mercury drop electrode. Inclusion of inorganic ions and uncharged hydrophobic guests into these monolayers was investigated by capacitance measurements. In the range of potentials where the electrode is positively charged, the interfacial capacitance depends on the type of electrolyte anions and on the applied potential. This can be explained with electrostatic double-layer forces. Whereas the smaller and less well solvated anions Cl^-, NO₃^-, and ClO₄^- are included in the cyclodextrin cavities of these monolayers, the larger and more strongly solvated anions F^-, SO₄^2-, and H₂PO₄^- are excluded. Anion inclusion constants can be obtained from the dependence of the interfacial capacitance on the anion concentration. The potential dependence of these inclusion constants shows that the nonelectrostatic contribution to the driving force for NO₃^- inclusion is negligibly small. Competitive binding of hydrophobic guest molecules decreases the interfacial capacitance. Fitting Langmuir isotherms to the plots of the interfacial capacitance as a function of adamantanol concentration yielded the binding constants 1.0 × 10⁴ and 2.6 × 10⁴ M^-1 for the β- and γ-cyclodextrin monolayers, respectively. Binding of adamantanol to α-cyclodextrin monolayers could not be observed, apparently because this guest is too large for the internal cavity of the α-cyclodextrin receptor. In contrast, 1-hexanol binds to α-cyclodextrin monolayers with the binding constant 8.9 × 10⁴ M^-1. This shows that changes in the capacitance can serve as a general signal transduction mode to monitor interactions between cyclodextrin monolayers and charged or neutral guests. Also, the extension of these types of measurements into solid electrodes and the application to other guest-selective host monolayers open the possibility of designing a novel type of electrochemical sensors for electroinactive analytes.