Thin but Strong Versus Thick but Soft: Contrasting Interfacial Water Barriers on Zwitterionic and Nonionic Antibiofouling Self-Assembled Monolayers

Both antibiofouling nonionic and zwitterionic self-assembled monolayers (SAMs) resist protein and cell adhesion similarly, despite having different chemical structures. This work elucidates their antibiofouling mechanisms by integrating surface force measurements with atomic force microscopy (AFM) and surface-enhanced infrared absorption (SEIRA) spectroscopy. The findings show that a very thin (<1 nm) water barrier near zwitterionic sulfobetaine (SB)-terminated SAMs causes strong short-range repulsion, whereas a thicker (>4 nm) water layer on nonionic hydroxy-terminated tri(ethylene glycol) (EG3-OH) SAMs results in long-range repulsion. SEIRA spectroscopy identified different hydrogen-bonding states in interfacial water: intense interactions with SB groups and weaker bonds with EG3-OH groups. These SEIRA insights clarify the nature of surface forces influenced by interfacial water. The study discusses how variations in hydration, SAM-water interfacial interactions, and the role of interfacial water molecules contribute to the antibiofouling effectiveness of these two SAM types.