Multifunctional Hollow–Shell Microspheres Derived from Cross-Linking of MnO₂ Nanoneedles by Zirconium-Based Coordination Polymer: Enzyme Mimicking, Micromotors, and Protein Immobilization

MnO2 microspheres were prepared with nanoneedles vertically growing on their surfaces. The MnO2 microspheres were used as templates for the growth of zirconium-based coordination polymer (Zr-CP)/metal–organic-framework, which was synthesized by coordination between Zr(IV) ions and 2-methylimidazole in methanol at room temperature. The nanoneedles were enwrapped by Zr-CP, and the bases of the MnO2 nanoneedles were cross-linked with Zr-CP. Etching of the core MnO2 produced a hollow–shell structure h-MnO2@Zr-CP. The shell, consisting of a loose outer layer and a dense inner layer, possesses oxidase- and catalase-like activities. h-MnO2@Zr-CP can work as a micromotor with H2O2 as the fuel. Hexahistidine-tagged enzymes can be selectively and strongly absorbed onto the hollow–shell microspheres with a large capacity by taking advantage of the coordinative interactions between the hexahistidine-tags and Zr-CP. d-Amino acid oxidase (DAAO) was immobilized on the hollow–shell microspheres, and the conjugate DAAO/h-MnO2@Zr-CP mimicked two-enzyme catalysis, achieving a catalysis efficiency 225% times that of free DAAO. R-ω-Transaminase (RTA) and DAAO were coimmobilized on the hollow–shell microspheres, and the conjugate RTA/DAAO/h-MnO2@Zr-CP mimicked three-enzyme catalysis. For converting (R)-1-phenylethylamine and (R)-1-aminoindan to corresponding ketones, RTA/DAAO/h-MnO2@Zr-CP achieved the conversions 99 and 95%, respectively, in contrast to corresponding conversions 48.6 and 32.3% by the mixed free enzymes RTA+DAAO. Potentially, the hollow–shell microspheres can be applied to immobilize other enzymes to establish multienzyme systems.