Fig. 3. (a) Adhesion strength of dopamine with different salts (LiCl, NaCl, and KCl) after 1 h in situ polymerization. The salt concentration is adjusted from 0 mM to 100 mM. The results suggest that the adhesion of PDA has no obvious change at different concentrations of LiCl and NaCl yet is highly sensitive on the concentration of KCl. (b) Adhesion strength between the PDA coating and various surfaces (OH-, Phe-, and N(CH3)3+-terminated surface) using an asymmetric SFA measurement configuration. The results indicate that the cation–π interaction is a major contributor in PDA adhesion, whereas π–π interactions and H-bonding are also of significance and play an auxiliary role. (c) Schematic illustration of intermolecular interactions between the PDA coating and various surfaces: OH-, Phe-, and N(CH3)3+-terminated surface. It illustrates that the cation–π interaction is much stronger than both the π–π interaction and H-bonding in PDA adhesion.
