Molecular-scale surface structures of oligo(ethylene glycol)-terminated self-assembled monolayers investigated by frequency modulation atomic force microscopy in aqueous solution

The structure and protein resistance of oligo(ethylene glycol)-terminated self-assembled monolayers (OEG-SAMs) have been studied intensively using various techniques. However, their molecular-scale surface structures have not been well understood. In this study, we performed molecular-resolution imaging of OH-terminated SAMs (OH-SAMs) and hexa(ethylene glycol) SAMs (EG 6OH-SAMs) formed on a Au(111) surface in an aqueous solution by frequency modulation atomic force microscopy (FM-AFM). The results show that most of the ethylene glycol (EG) chains in an EG6OH-SAM are closely packed and well-ordered to present a molecularly flat surface even in an aqueous solution. In addition, we found that EG6OH-SAMs have nanoscale defects, where molecules take a disordered arrangement with their molecular axes parallel to the substrate surface. We also found that the domain size (50-200 nm) of an EG6OH-SAM is much larger than that of OH-SAMs (10-40 nm). These findings should significantly advance molecular-scale understanding about the surface structure of OEG-SAMs.