Host-guest interactions at self-assembled monolayers of cyclodextrins on gold

We have developed synthesis routes for the introduction of short and long dialkylsulfides onto the primary side of α-, β-, and γ-cyclodextrins. Monolayers of these cyclodextrin adsorbates were characterized by electrochemistry, wettability studies, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and atomic force microscopy (AFM). The differences in thickness and polarity of the outerface of the monolayers were measured by electrochemistry and wettability studies. On average about 70 % of the sulfide moieties were used for binding to the gold, as measured by XPS. Tof-SIMS measurements showed that the cyclodextrin adsorbates adsorb without any bond breakage. AFM measurements revealed for β-cyclodextrin monolayers a quasihexagonal lattice with a lattice constant of 20.6 Å, which matches the geometrical size of the adsorbate. The α-cyclodextrin and γ-cyclodextrin monolayers are less ordered. Interactions of the anionic guests 1-anilinonaphthalene-8-sulfonic acid (1, 8-ANS) and 2-(p-toluidinyl)naphthalene-6-sulfonic acid (2, 6-TNS) and the highly ordered monolayers of heptapodant β-cyclodextrin adsorbates were studied by surface plasmon resonance (SPR) and electrochemical impedance spectroscopy. The SPR measurements clearly showed interactions between a -cyclodextrin monolayer and 1, 8-ANS. Electrochemical impedance spectroscopy measurements gave high responses even at low guest concentrations (≤5 µM). The association constant for the binding of 1, 8-ANS (K=289 000±13 000 M-1) is considerably higher than the corresponding value in solution. (Partial) methylation of the secondary side of the β-cyclodextrin strongly decreases the binding.