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

We have developed synthesis routes for the introduction of short and long d ialkylsulfides onto the primary side of alpha-, beta-, and gamma-cyclodextr ins. Monolayers of these cyclodextrin adsorbates were characterized by elec trochemistry, 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 outerfa ce of the monolayers were measured by electrochemistry and wettability stud ies. 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 cyclode xtrin adsorbates adsorb without ally bond breakage. AFM measurements reveal ed for beta-cyclodextrin monolayers a quasihexagonal lattice with a lattice constant of 20.6 Angstrom, which matches the geometrical size of the adsor bate. The alpha-cyclodextrin and gamma-cyclodextrin monolayers are less ord ered. Interactions of the anionic guests 1-anilinonaphthalene-8-sulfonic ac id (1,8-ANS) and 2(p-toluidinyl)naphthalene-6-sulfonic acid (2,6-TNS) and t he highly ordered monolayers of heptapodant beta-cyclodextrin adsorbates we re studied by surface plasmon resonance (SPR) and electrochemical impedance spectroscopy. The SPR measurements clearly showed interactions between a b eta-cyclodextrin monolayer and 1,8-ANS. Electrochemical impedance spectrosc opy measurements gave high responses even at low guest concentrations (less than or equal to 5 mu M). The association constant for the binding of 1,8- ANS (K = 289 000 +/- 13 000 M-1) is considerably higher than the correspond ing value in solution. (Partial) methylation of the Secondary side of the b eta-cyclodextrin strongly decreases the binding.