Sum frequency generation spectroscopy probing of formation of self-assembled monolayers from thiols with terminal phenylalanine ring and intrachain amide groups

Abstract

Formation of a self-assembled monolayer onto the polycrystalline gold substrate from thiols containing a terminal phenylalanine (Phe) ring and intrachain amide groups (HS-(CH₂)₇-CONH-(CH₂)₂-C₆H₅) was characterized by vibrational sum frequency generation (SFG) spectroscopy. The temporal evolution of the characteristic Phe ring stretching vibration ν(=C–H) near 3069 cm^–1 and the Amide A mode were monitored by nonlinear vibrational spectroscopy. The SFG data revealed the complete ordering of the terminal Phe rings after about 60 min of adsorption. Formation of a strong hydrogen bonding between the adjacent chains of adsorbed molecules was evident by appearance of the Amide A mode at the relative low frequency (3280–3308 cm^–1). The well-ordered and strongly hydrogen bonded SAM with the terminal Phe ring functionality is a promising platform for the analysis of interactions and the function of aromatic rings in biomolecular processes.