A route was developed for the synthesis of three different cyclodextrin adsorbates:  heptakis{6-O-[3-(thiomethyl)propionyl)]-2,3-di-O-methyl}-β-cyclodextrin, heptakis{6-O-[12-(thiododecyl)dodecanoyl)]-2,3-di-O-methyl}-β-cyclodextrin (a short and long alkyl chain sulfide cyclodextrin adsorbate, respectively), and heptakis[6-deoxy-6-(3-mercaptopropionamidyl)-2,3-di-O-methyl]-β-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 β-cyclodextrin monolayers, and the XPS-(S2p) measurements show the different effectivenesses of the sulfur moieties of the three monolayers in their binding to the gold surface. Sulfide-based β-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.

Supramolecular host-guest interactions in aqueous environment were studied by dynamic single molecule force spectroscopy. The unbinding between ferrocene moieties immobilized on atomic force microscopy tips and -cyclodextrin receptors in highly ordered self-assembled monolayers on Au(111) was studied. The rupture force of individual ferrocene--cyclodextrin complexes was found to be 56 ( 10 pN. The value of this unbinding force was independent of the unloading rate. This was attributed to the fast (de)complexation kinetics of the host-guest complex.

We have developed synthesis routes for the introduction of short and long dialkylsulfides onto the primary side of a-, b-, and g-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 b-cyclodextrin monolayers a quasihexagonal lattice with a lattice constant of 20.6 , which matches the geometrical size of the adsorbate. The a-cyclodextrin and g-cyclodextrin monolayers are less ordered. Interactions of the anionic guests 1-anilinonaphthalene-8sulfonic acid (1,8-ANS) and 2-(p-toluidinyl)naphthalene-6-sulfonic acid (2,6-TNS) and the highly ordered monolay-ers of heptapodant b-cyclodextrin adsorbates were studied by surface plasmon resonance (SPR) and electrochemical impedance spectroscopy. The SPR measurements clearly showed interactions between a b-cyclodextrin monolayer and 1,8-ANS. Electrochemical impedance spectroscopy measurements gave high responses even at low guest concentrations ( 5 mm). The association constant for the binding of 1,8-ANS (K 289 000 AE 13 000 m À1 ) is considerably higher than the corresponding value in solution. (Partial) methylation of the secondary side of the b-cyclodextrin strongly decreases the binding.

Most applications of functionalized self-assembled monolayers (SAMs) are based on changes in electrochemical properties. Such applications require electrochemical stability. This study reveals that there is only a limited potential window from -0.8 to +0.4 VMSE where self-assembled monolayers of sulfur-containing adsorbates (thiols and alkyl sulfides) on gold are electrochemically stable. A sensitive parameter describes the quality/stability of the monolayers (viz. the charge-transfer resistance). The substitution pattern of the adsorbates influences the window. Highly ordered monolayers, like that of tetrasulfide-based cavitand (10), are more resistant to extreme potentials. The stability window from -0.9 to +0.5 VMSE for this sulfide-based monolayer is comparable to that for a decanethiol monolayer.

The nondestructive adsorption of long chain dialkyl sulfides on gold is reported. Self-assembled monolayers of decanethiol, didecyl disulfide, and didecyl sulfide were prepared and characterized by XPS, grazingangle FT-IR, wettability studies, and TOF-SIMS. The first three techniques provide evidence that wellpacked monolayers are formed. The positive SIMS spectrum of didecyl sulfide showed fragments that originate from intact dialkyl sulfide attached to gold without C-S bond cleavage. Our results underline the generally accepted nondestructive adsorption process of dialkyl sulfides and contradict the recently reported cleavage of one of the alkyl sulfur bonds of sulfides upon adsorption.

We have developed synthesis routes for the introduction of short and long dialkylsulfides onto the primary side of alpha-, beta-, and gamma-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 electro-chemistry 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 beta-cyclodextrin monolayers a quasi-hexagonal lattice with a lattice constant of 20.6 A, which matches the geometrical size of the adsorbate. The alpha-cyclodextrin and gamma-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 beta-cyclodextrin adsorbates were studied by surface plasmon resonance (SPR) and electrochemical impedance spectroscopy. The SPR measurements clearly showed interactions between a beta-cyclodextrin monolayer and 1,8-ANS. Electrochemical impedance spectroscopy measurements gave high responses even at low guest concentrations (< or = 5 microM). The association constant for the binding of 1,8-ANS (K = 289,000 +/- 13,000M-1) is considerably higher than the corresponding value in solution. (Partial) methylation of the secondary side of the beta-cyclodextrin strongly decreases the binding.

A mixed acid-ferrocene self-assembled sulfide monolayer on gold shows a pH-dependent electrochemical response by through-space communication between the receptor and ferrocene readout unit.

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