Covalent and Supramolecular Functionalization of Self-Assembled Monolayers
The aim of the work described in this thesis was to study the reactivity of selfassembled monolayers (SAMs) on gold surfaces. The chemical reactions studied ranged from covalent coupling via nucleophilic substitution and coordination chemistry to supramolecular host-guest interactions. The focus was centered primarily on SAMs as a model system to study interfacial reactivity, to quantify the impact of packing constraints, steric effects and differences in mechanism on reaction kinetics and to illustrate what can be accomplished in micro- and nanofabrication by selfassembly. Structural characteristics of SAMs that are relevant to reactivity issues and that make them promising as a technique to micro- and nanofabrication were described, as well as the methods of patterning SAMs in the plane of the monolayer (Chapter 2). The chain length dependence of the reactivity in base-catalyzed hydrolysis of SAMs of three N-hydroxysuccinimide (NHS) ester disulfides (NHS-C2, NHS-C10, NHS-C15) having different numbers of methylene groups in the alkyl chain, was discussed in Chapter 3. The kinetics of the reaction was followed by grazing incidence reflection Fourier transform infrared (GIR-FTIR) spectroscopy and contact angle measurements. The two shorter ester disulfides (NHS-C2, NHS-C10) investigated in this study showed a pseudo first-order-kinetics, the correspondingly calculated second-order rate constants were found to be three respectively two orders of magnitude smaller than those reported for similar reactions in solution. The long chain ester (NHS-C15) showed a sigmoid kinetic behavior. The observed reactivity differences are attributed to differences in orientation, packing, and conformational order of the molecules confined in the monolayers, which vary systematically with chain length. To obtain a better understanding of sigmoid kinetics observed in Chapter 3 for the ester disulfides with long chain (NHS-C15), the temperature dependence of the hydrolysis of this ester was investigated (Chapter 4). Using ex situ contact angle measurements in conjunction with the application of the Cassie equation, the surface coverage of unreacted NHS-C15 SAMs following partial hydrolysis was determined quantitatively. For temperatures T < 40 ºC a sigmoid behavior was observed. The induction period was found to vanish at temperatures T > 40 ºC. For temperatures T > 40 ºC the reaction could be described by pseudo-first-order kinetics. The activation energy for the hydrolysis of 62 ± 5 kJ/mol was thus determined for T > 40 ºC. The differences in reactivity were ascribed to temperature dependent changes in chain tilting and defect density upon heating. In Chapter 5 the hydrolysis and aminolysis of NHS-C10 SAMs were monitored on the nanometer scale by a novel approach termed "inverted" chemical force (iCFM). In iCFM the reactants are immobilized on the tip rather than on the substrate and the chemical reactions that take place at the surface of the tip are probed by forcedisplacement measurements on an inert octadecanethiol-covered Au substrate. Data obtained by iCFM demonstrated that the aminolysis reaction is a spatially heterogeneous reaction, unlike the hydrolysis that proceeds homogeneously. The experimental data indicate that the reaction may spread from initiation or defect sites that are initially accessible for nucleophilic attack. Information about the defect density on the reactive SAMs was obtained, indicating that the initiation sites are unlikely pinholes, but may be defects in optimal head group packing. An investigation of the reactivity of a dendritic wedge containing four peripheral pyridines, immobilized via a thioether group in SAMs, with a second generation Fréchet-type dendron functionalized with a sulfur-carbon-sulfur (SCS) PdII (2) pincer moiety was reported in Chapter 6. This reaction is central to a "bottom-up" approach for the controlled construction of complex structures of nanometer dimensions starting from the molecular level. The isolated molecules and their reactivity were studied at the single molecular level by ex situ tapping mode atomic force microscopy (TMAFM). The reactivity of the dendritic wedges confined in SAMs was observed to be slowed compared to the clean, fast and quantitative reaction in solution, indicating that steric hindrance may play a crucial role in such coordination reactions at SAMs. The pattering of SAMs of ß-cyclodextrins, which possess molecular cavities as specific recognition site to anchor other molecules via specific and directional supramolecular interactions, was presented in Chapter 7. The functionalization and patterning of these "molecular printboards", with two-dimensional micro and nanostructures, having dimensions down to the sub-100 nm scale, were achieved by means of supramolecular microcontact printing (µCP) and dip-pen nanolithography (DPN). The pattering approach of the molecular printboards exploits strong multiple host-guest interactions between ß-cyclodextrin and a novel water-soluble calixarene derivative bearing two adamantyl moieties on the lower rim. Transfer of this guest by µCP and DPN onto the printboards, followed by a rinsing step, led to the highly controlled and selective formation of micrometer and sub-micrometer sized patterns. The demonstrated precisely controlled patterning of functional molecules at the nanometer scale may open new avenues for the construction of surface-based hierarchical supramolecular architectures and ultimately miniaturized functional devices. Chapter 8 reported the study of covalent surface confinement of dendrimers and the characterization of the resulting monolayer films as chemically sensitive interfaces for chemical-sensing applications. In particular, G4 poly(amidoamine) (PAMAM) dendrimers were linked to NHS-C10 SAMs via amide bond formation. The coupling reaction and the resulting assemblies were characterized by FTIR spectroscopy, CA, XPS and AFM; the obtained coverages were fitted successfully with Langmuir isotherm. Furthermore, surfaces-immobilized PAMAM dendrimers were reacted with trifluoroaetic anhydride in order to estimate the fraction of unreacted terminal amine groups of the dendrimers. The percentage was found to be 37%, for both PAMAM G4 and G5. Patterning of NHS-C10 SAMs using PAMAM dendrimers, in the micro- and nanoscale by means of µCP and DPN, was described as possible route for fabricating sub-µm scale bioarrays with high amino group density.