Adsorption and Orientation of 3,4-Dihydroxy-l-phenylalanine onto Tunable Monolayer Films

Abstract

3,4-Dihydroxy-l-phenylalanine (l-DOPA) is considered to be responsible for the mussel adhesion to a variety of surfaces. A molecular level understanding of the interactions between DOPA molecules and surfaces with different wettability and chemistry, however, posts significant challenges to control marine antifouling. Here, different self-assembled monolayers (SAMs) on gold surfaces were fabricated: (i) OH-, (ii) COOH-, and (iii) CH3-terminations. The effect of surface wettability and chemistry on the adsorption of DOPA upon the series of surfaces was investigated in situ, showing that the adsorbed mass was lower and the water content of DOPA layer was higher on hydrophilic surfaces (including OH- and COOH-terminated SAMs) than that on hydrophobic ones (including CH3-terminated SAMs and gold surface). Direct evidence regarding the DOPA orientation and the interaction between DOPA and film surfaces were obtained: on the OH-terminated surface a flexible and loose structure formed via coordinate hydrogen bonds of the hydroxyl end groups of the surface interacting with carboxyl groups of DOPA, while for the CH3-terminated surface, DOPA molecules mainly adopt a flat conformation due to the formation of hydrophobic “bonds” between the hydrophobic functional groups of alkyl chains on surface and aromatic rings of DOPA molecules. This study led a new insight into the adsorption mechanisms based on the adsorption processes and layer structures, and it proposed novel concepts for the design of antifouling and adhesive surfaces.