Mussel-Inspired pH-Switched Assembly of Capsules with an Ultrathin and Robust Nanoshell

Enclosed films, also called capsules, bearing an ultrathin and robust nanoshell have sparked much interest for use in many applications, for which facile preparation methods are urgently pursued. Inspired by the pH-programmed adhesion/cohesion of mussel-secreted foot proteins, polyphenol/polyamine capsules with an ultrathin and robust nanoshell are fabricated through a pH-switched assembly on sacrificial calcium carbonate (CaCO₃) templates. Polyphenols adhere to the templates at pH 6.0 and rapidly cohere with polyamines at pH 8.0. The pH-switched assembly process is accomplished in only a few minutes where multiple instances of electrostatic interactions and chemical conjugation between polyphenols and polyamines occur. As a result, the capsules exhibit a nanoshell thickness of ∼10 nm and a superior mechanical strength of ∼1.575 GPa (elasticity modulus). Cell mimics are prepared through encasing enzymes in the lumen and present an activity recovery of ∼70% along with little activity decline during reuse. Amine or phenolic groups on the nanoshell of capsules are then applied to induce the generation of titania or silver nanoparticles, which may expand the applications of the capsules to the photo- and biorelated realms. Our study not only deepens the understanding of the adhering process of mussels but also offers a generic method toward functional materials for diverse applications.