Self-assembled monolayers of heptapodant β-cyclodextrins on gold

A route was developed for the synthesis of three different cyclodextrin adsorbates: heptakis{6-O-[3(thiomethyl)propionyl)]-2,3-di-O-methyl}-beta-cyclodextrin, heptakis{6-O-[12-(thiododecyl)dodecanoyl)]-2,3-di-O-methyl}-beta-cyclodextrin (a short and long alkyl chain sulfide cyclodextrin adsorbate, respectively), and heptakis[6-deoxy-6-(3-mercaptopropionamidyl)-2,3-di-O-methyl]-beta-cyclodextrin (a short alkyl chain thiol adsorbate). Self-assembled monolayers on gold of these three cyclodextrin adsorbates with seven sulfur moieties were fully characterized by electrochemistry, wettability studies, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The electrochemical capacitance measurements show the differences between the thicknesses of the P-cyclodextrin monolayers, and the XPS-(S-2p) measurements show the different effectivenesses of the sulfur moieties of the three monolayers in their binding to the gold surface. Sulfide-based beta-cyclodextrin monolayers use on average 4.5 of the 7 attachment points whereas the thiol-based cyclodextrin monolayer only uses 3.2 of the 7 sulfurs. These experiments show that, for adsorbates with multiple attachment points, sulfides may be more effective than thiols. TOF-SIMS measurements confirm the robust attachment of these adsorbates on gold obtained by XPS.