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

Abstract

Azide-oligo(ethylene glycol)-terminated self-assembled monolayers (N 3 -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 3 -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 3 -EG 6 -SAMs) were investigated using frequency modulation atomic force microscopy (FM-AFM) in liquid. The N 3 -EG 6 -SAMs were prepared on Au(111) substrates through the self-assembly of 11-azido-hexa(ethylene glycol)undecane-1-thiol (N 3 -EG 6 -C 11 -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 3 -EG 6 -SAM and its clean surfaces originating from the adsorption resistance property of the terminal EG 6 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 crosscoupling activity. These findings provide molecular-scale information on the functionalization of the N 3 -EG 6 -SAMs, which is helpful for the interfacial molecular design based on alkanethiol SAMs in many applications.

Self-Assembled Monolayers of Gemini-Type Amphiphilic Hexabenzocoronenes on Gold: Contribution of Their Triethylene Glycol Side Chains to Self-Assembly Formation

“…It should be noted that an atomic force microscope (AFM), which uses force rather than tunneling current as a probe, has the potential to enable observation at the molecular scale even in environments and molecular systems that are difficult to measure with STM, but AFM technology with true atomic resolution is not commonly used and seems to be limited to specific research groups. Molecular resolution imaging of oligo EG-terminated SAMs formed on Au(111) surfaces in aqueous solution has been reported by Asakawa and Fukuma’s group using frequency modulation AFM. , They found that most of the EG chains in a hexa EG SAM are closely packed and well-ordered to present a molecularly flat surface, even in an aqueous solution.…”

“…Molecular resolution imaging of oligo EG-terminated SAMs formed on Au(111) surfaces in aqueous solution has been reported by Asakawa and Fukuma's group using frequency modulation AFM. 19,20 They found that most of the EG chains in a hexa EG SAM are closely packed and well-ordered to present a molecularly flat surface, even in an aqueous solution.…”