On the quality and structural characteristics of oligo(ethylene glycol) assemblies on gold: An experimental and theoretical study

This paper presents results of several years of experimental and theoretical work on a library of oligo(ethylene glycol)-containing self-assembled monolayers (OEG SAMs) on gold. The library consists of 15 different thiol compounds, which all contain alkyl and OEG portions of different length, as well as amide moieties forming a stabilizing lateral hydrogen bonding network. We have investigated the quality, conformation, orientation, defect structure and infrared (IR) signatures of these OEG SAMs prepared by spontaneous adsorption from dilute solutions. It is shown that solution concentration and incubation time are important factors to obtain high quality SAMs, in particular for those containing long OEG chains. Further on, the thiol compounds should contain a sufficiently long alkyl spacer to provide in plane van der Waals interactions strong enough to govern the formation of a densely packed alkylthiolate overlayer on the Au(1 1 1) surface. Such a highly ordered alkyl support, which is additionally stabilized by the lateral hydrogen bonds, enables us to vary the length of the terminal OEG portion from 1 to at least 12, without affecting the integrity and conformational characteristics of the supporting (alkyl) part of the SAM. Also, we discuss the importance of appropriate modeling tools to advance the understanding of IR signatures of the OEG SAMs. Finally, we demonstrate the generality of our “modular approach” by analyzing the structure of OEG SAMs formed by compounds extended with an additional terminal amide and an alkyl tail. Thus, the SAMs discussed herein provide an attractive platform for construction of advanced nanoarchitectures on surfaces, not only limited to biomaterials and fouling applications.