SUPPORTED MONOLAYERS CONTAINING PREFORMED BINDING-SITES - SYNTHESIS AND INTERFACIAL BINDING-PROPERTIES OF A THIOLATED BETA-CYCLODEXTRIN DERIVATIVE

Per-6-thio-beta-cyclodextrin (2) was prepared in two steps from beta-cyclodextrin. Receptor 2 is a beta-cyclodextrin derivative in which all the primary hydroxyl groups have been replaced by thiol groups. As such, 2 chemisorbs onto gold surfaces forming at least six S-Au bonds per receptor molecule. Although this derivatization process leads to imperfect monolayers in which a substantial fraction of the gold surface remains uncovered, the monolayer defects can be covered by treatment with a solution of ferrocene and pentanethiol. Ferrocene, an excellent substrate for beta-cyclodextrin, protects the monolayer binding sites, directing the pentanethiol molecules to seal defective sites instead of the cyclodextrin cavities. Electrodes derivatized by this procedure showed effective binding properties when immersed in aqueous solutions containing low concentrations (<60 mu M) of ferrocene. Their voltammetric response exhibited the waves anticipated for the reversible oxidation of the surface-confined (cyclodextrin-bound) ferrocene molecules. The process of interfacial ferrocene complexation was demonstrated to have the expected dynamic character by a series of competition experiments with m-toluic acid (mTA) in which the surface-confined voltammetric waves of ferrocene were gradually lost as increasing mTA concentrations were added to the solution.