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, similar to 19.4 k(B)T) than for [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (DMAPS, similar to 10.8 k(B)T). The strong MPC-Dopa interaction could be attributed to the synergy of cation-pi 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-pi-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.