Local Cross-Coupling Activity of Azide-Hexa(ethylene glycol)-Terminated Self-Assembled Monolayers Investigated by Atomic Force Microscopy.

Azide-oligo(ethylene glycol)-terminated self-assembled monolayers (N-OEG-SAMs) are promising interfacial structures for surface functionalization. Its many potential applications include chemical/bio-sensing and construction of surface models owing to its cross-coupling activity that originates from the azide group and oligo(ethylene glycol) (OEG) units for non-specific adsorption resistance. However, there are only a few studies and limited information, particularly on the molecular-scale structures and local cross-coupling activities of N-OEG-SAMs, which are vital to understanding its surface properties and interfacial molecular design. In this study, molecular-scale surface structures and cross-coupling activity of azide-hexa(ethylene glycol)-terminated SAMs (N-EG-SAMs) were investigated using frequency modulation atomic force microscopy (FM-AFM) in liquid. The N-EG-SAMs were prepared on Au(111) substrates through the self-assembly of 11-azido-hexa(ethylene glycol)-undecane-1-thiol (N-EG-C-HS) molecules obtained from a liquid phase. Subnanometer-resolution surface structures were visualized in an aqueous solution using a laboratory-built FM-AFM instrument. The results show a well-ordered molecular arrangement in the N-EG-SAM and its clean surfaces originating from the adsorption resistance property of the terminal EG units. Surface functionalization by the cross-coupling reaction of copper(I)-catalyzed azide-alkyne cycloaddition was observed, indicating a structural change in the form of fluctuating structures and island-shaped structures depending on the concentration of the alkyne molecules. The FM-AFM imaging enabled to provide information on the relationship between the surface structures and cross-coupling activity. These findings provide molecular-scale information on the functionalization of the N-EG-SAMs, which is helpful for the interfacial molecular design based on alkanethiol SAMs in many applications.