A model has been described for interpreting the binding of multivalent molecules to interface-immobilized monovalent receptors through multiple, independent interactions. It is based on the concept of effective concentration, C(eff), which has been developed before for multivalent binding in solution and which incorporates effects of lengths and flexibilities of linkers between interacting sites. The model assumes: (i). the interactions are independent, (ii). the maximum number of interactions, p(max), is known, (iii). C(eff) is estimated from (simple) molecular models. Simulations of the thermodynamics and kinetics of multivalent host-guest binding to interfaces have been discussed, and competition with a monovalent competitor in solution has been incorporated as well. The model was successfully used to describe the binding of a divalent guest to self-assembled monolayers of a cyclodextrin host. The adsorption data of more complex guest-functionalized dendrimers, for which p(max) was not known beforehand, was interpreted as well. Finally, it has been shown that the model can aid to deconvolute contributions of multivalency and cooperativity to stability enhancements observed for the adsorption of multivalent molecules to interfaces.

The divalent binding of a bis(adamantyl)-functionalized calix[4]arene (1) to an EDTA-tethered beta-cyclodextrin (CD) dimer (2) in solution (1.2 x 10(7) M(-)(1)) was 3 orders of magnitude weaker than the binding constant ( approximately 10(10) M(-)(1)) for the interaction of 1 at CD self-assembled monolayers (SAMs) on gold. This difference in binding is rationalized using a theoretical model, which interprets the divalent binding as two consecutive monovalent binding events, i.e., an intermolecular interaction followed by an intramolecular binding event, the latter of which is associated with an effective concentration term accounting for the close proximity of the two interacting species. The methodology presented in the model is applicable to divalent binding both in solution and at SAMs and indicates that the difference in observed binding constants mainly stems from a difference in effective concentration.

The rupture forces of individual host-guest complexes between beta-cyclodextrin (beta-CD) heptathioether monolayers on Au(111) and several surface-confined guests were measured in aqueous medium by single molecule force spectroscopy using an atomic force microscope. Anilyl, toluidyl, tert-butylphenyl, and adamantylthiols (0.2-1%) were immobilized in mixed monolayers with 2-mercaptoethanol on gold-coated AFM tips. For all guests and for all surface coverages, the force-displacement curves measured between the functionalized tips and monolayers of beta-CD exhibited single, as well as multiple, pull-off events. The histograms of the pull-off forces showed several maxima at equidistant forces, with force quanta characteristic for each guest of 39 +/- 15, 45 +/- 15, 89 +/- 15, and 102 +/- 15 pN, respectively. These force quanta were independent of the loading rate, indicating that, because of the fast complexation/decomplexation kinetics, the rupture forces were probed under thermodynamic equilibrium. The force values followed the same trend as the free binding energy Delta G degrees measured for model guest compounds in solution or on beta-CD monolayers, as determined by microcalorimetry and surface plasmon resonance measurements, respectively. A descriptive model was developed to correlate quantitatively the pull-off force values with the Delta G degrees of the complexes, based on the evaluation of the energy potential landscape of tip-surface interaction.

In this paper, a study on the adsorption of mixed self-assembled monolayers (SAMs) for two different combinations of thiols (Fc(CH2)6SH/HO(CH2)2SH and Fc(CH2)16SH/HO(CH2)11SH (Fc ) ferrocene)) is presented, to obtain surfaces with single isolated ferrocenylalkanethiols embedded in shorter hydroxyalkanethiols. These hydrophilic substrates with very low surface concentration of ferrocene moieties are required to perform force spectroscopy experiments on host-guest supramolecular complexes, by using SAMs of -cyclodextrin heptathioether adsorbed on gold-coated AFM tips. Several SAMs have been prepared on polycrystalline gold electrodes from 1 mM thiol solutions, changing the ferrocenylalkanethiols/ hydroxyalkanethiols ratio. The amount of electroactive component immobilized on the electrode was determined by cyclic voltammetry, and it has been related to the solution composition. The general trend is that the longer chain component is preferentially adsorbed, suggesting a thermodynamic control of the adsorption. However, relevant differences in the layer formation and composition can be observed for the two systems on the basis of a different balance of the driving forces that govern the adsorption process. Fc(CH2)16SH is adsorbed to a larger extent, compared to the Fc(CH2)6SH for the same solution composition, as shown by higher charge densities values. Furthermore for the Fc(CH2)16SH/HO(CH2)11SH system upon increase of the percentage of ferrocenylalkanethiols in solution, a shift in the redox peak position to more positive values is observed, indicating that phase segregation occurs. The differences between the two systems can be related to chain length effects from which arise favorable enthalpic contributions to the adsorption of the longer chain component.

Bottom-up nanotechnology has to start with the precise positioning of molecules. For this purpose we are developing molecular printboards, that is, self-assembled monolayers (SAMs) of molecules that have specific recognition sites, for example, molecular cavities, to which molecules can be anchored through specific and directional supramolecular interactions.[1] Such molecular printboards are prepared by the self-assembly of b-cyclodextrin (b-CD) derivatives on gold and silicon oxide surfaces. Herein we describe how to print or write, by microcontact printing (mCP) and dip-pen nanolithography (DPN), respectively, molecular patterns of guest-functionalized calixarene molecules, dendritic wedges labeled by fluorescent groups, and dendrimers on b-CDterminated printboards. The binding, as well as the desorption of the molecules, can be fine-tuned by chemical design, which allows virtually unlimited flexibility in the chemical functions that can be employed. These structures can be subsequently used to direct the adsorption of different materials, for example, fluorescent dyes.Microcontact printing has been developed by Whitesides for the preparation of patterns of molecules on bare surfaces by, for example, the transfer of thiols to gold substrates in the contact areas between a soft polymeric stamp and the substrate. [2,3] This has recently been extended by Mirkin and co-workers to writing with molecules on such surfaces by using the DPN approach.[4] Various types of molecules were deposited onto different substrates by DPN which led to arrays of, for example, DNA, [5] proteins, [6] and nanoparticles. [7] Registry capabilities have been demonstrated as well, [8] and a multipen nanoplotter able to produce parallel patterns with different ink molecules has been developed.[9]b-CD (1 a, Scheme 1) can act as a host for the binding of a variety of small, organic guest functionalities in water through hydrophobic interactions. We prepared self-assembled monolayers (SAMs) of a b-CD heptathioether adsorbate 1 b (Scheme 1) on gold as described before. [10,11] Such adsorbates form densely packed, well-ordered SAMs with equivalent

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The adsorption of a biologically important glycoprotein, mucin, and mucin-chitosan complex layer formation on negatively charged surfaces, silica and mica, have been investigated employing ellipsometry, the interferometric surface apparatus, and atomic force microscopy techniques. Particular attention has been paid to the effect of an anionic surfactant sodium, dodecyl sulfate (SDS), with respect to the stability of the adsorption layers. It has been shown that mucin adsorbs on negatively charged surfaces to form highly hydrated layers. Such mucin layers readily associate with surfactants and are easily removed from the surfaces by rinsing with solutions of SDS at concentrations > or =0.2 cmc (1 cmc SDS in 30 mM NaCl is equal to 3.3 mM). The mucin adsorption layer is negatively charged, and we show how a positively charged polyelectrolyte, chitosan, associates with the preadsorbed mucin to form mucin-chitosan complexes that resist desorption by SDS even at SDS concentrations as high as 1 cmc. Thus, a method of mucin layer protection against removal by surfactants is offered. Further, we show how mucin-chitosan multilayers can be formed.

The rupture forces of individual β-cyclodextrin (β-CD)-ferrocene host-guest complexes in an aqueous medium were measured by dynamic single molecule force spectroscopy using an atomic force microscope (AFM). The thiol-derivatized ferrocene guest was immobilized in self-assembled monolayers on goldcoated AFM tips, while the heptasulfide β-CD host was self-assembled onto atomically flat Au(111) substrates. The effects of the alkyl spacer length of the ferrocene adsorbates, the relative concentration of ferrocene in the mixed monolayer on the AFM tip, and the unloading rate on the observed molecular unbinding events were studied. Depending on the concentration of ferrocene moieties on the AFM tip, multiple or predominantly single pull-off events were observed. A statistical analysis showed that the observed rupture forces are integer multiples of one fundamental force quantum of 55 ( 10 pN, which is attributed to the rupture of a single host-guest complex. This force quantum is found to be independent of the number of interacting host-guest pairs, independent of the spacer length, and independent of the unloading rate. These results indicate that the host-guest complex rupture forces were probed under conditions of thermodynamic equilibrium.