A wet adhesion strategy via synergistic cation–π and hydrogen bonding interactions of antifouling zwitterions and mussel-inspired binding moieties

3,4-Dihydroxyphenylalanine (Dopa) is a mussel-inspired, unique and versatile adhesive moiety, while zwitterions are well-known for their antifouling and repellent properties. It is still unclear whether zwitterionic surfaces could effectively prevent the adhesion of Dopa units, and a zwitterion–Dopa interaction mechanism may lead to development of a novel wet adhesion strategy. Here we report single-molecule force spectroscopy (SMFS) between Dopa and zwitterionic molecules of opposite dipole orientations, with a much stronger bond dissociation energy for zwitterionic 2-methacryloyloxyethyl phosphorylcholine (MPC, ∼19.4 kBT) than for [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (DMAPS, ∼10.8 kBT). The strong MPC–Dopa interaction could be attributed to the synergy of cation–π interaction and hydrogen bonding, but the DMAPS–Dopa interaction could only be attributed to hydrogen bonding, as further demonstrated by density functional theory (DFT) computation. Based on the orientation-mediated zwitterion–Dopa binding mechanism, we design a polymer containing both MPC and Dopa moieties as a promising underwater adhesive with a measured cohesion of 7.2–14.1 mJ m−2, even stronger than reported cation–π-facilitated underwater adhesives. The elucidated molecular interaction principles shed light on designing novel zwitterion–Dopa-mediated materials with wide applications in various fields such as tissue engineering scaffolds, cell-based screening and underwater adhesives.