lnterfacial Design and Characterization of Oligo(ethylene glycol) Self-Assembled Monolayers : Templates for Biomolecular Architectures

Abstract: This thesis presents efforts to better understand and control the interfacial properties of oligo(ethylene glycol)-containing self-assembled monolayers (OEG SAMs) on gold. This has been done by means of molecular and interfacial design, followed by extensive characterization of the SAMs using a variety of surface analytical techniques. The OEG-terminated and amide group-containing alkylthiols were chosen for the study. The characterization of the OEG SAMs by contact angle goniometry, null ellipsometry, infrared reflection-absorption spectroscopy (IRAS) and X-ray photoelectron spectroscopy (XPS) revealed not only a high crystallinity and an excellent orientation of the constituent molecules in the SAMs, but also conformational differences between the investigated SAMs. It is, for example, shown that the observed all trans and helical OEG conformations depend on the oligomer chain length and/or on the lateral hydrogen bonding in the assembly. Further on, the thermal phase behavior of the OEG SAMs was investigated in ultra high vacuum by temperature programmed IRAS. This approach revealed a reversible helix-to-alltrans phase transition in the EG6 SAMs occurring at approximately 60 °C. A detailed comparative spectroscopic investigation of several analogous OEG compounds proved that the OEG phase behavior, including the unusual helix-to all trans transition, depends on the linking group between the OEG and alkylthiol chains. It was thereby demonstrated that the selection of an appropriate linking group provided means to control the OEG conformation and phase behavior via intermolecular interactions, e.g. hydrogen bonding. In order to give a more exact account of the influence of lateral hydrogen bonding on the thermal stability of the SAMs, their temperature programmed desorption was analyzed by mass spectrometry and IRAS in parallel. The results from this study clearly showed an improved thermal stability of the hydrogen bonded SAMs.Some preliminary results on the potential use of such hydrogen bonded OEG SAMs as templates for biomolecular architectures are also presented in this thesis. The amide-containing OEG compounds were chosen as a general strategy for further interfacial design and enabled the preparation of homogeneously mixed SAMs with a fixed hydrogen-bonded underlayer and a fine-tunable OEG portion. It is also shown that a spatially controlled functionalization of the SAMs can be done by incorporating compounds with terminal -COOH groups into the OEG assembly. Carboxy-derivatized molecules in OEG SAMs are expected to act as anchors for lipid bilayers, thus forming a template for supported lipid bilayer membranes. Alternatively, highly ordered SAMs can be prepared by self-assembly of extremely long compounds with the structure HSC15-amide-EG6-amide-C16 for the integration and anchoring of lipid bilayers on gold.The study thereby demonstrates a route to manipulate the interfacial properties of oligomer based SAMs on gold surfaces, by controlling the intermolecular interactions. The resulting OEG SAM interface not only enables the construction of templates for biosensors, but also novel molecular 2D and 3D architectures in general.

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