Polymeric template assisted synthesis of monodisperse-porous manganese oxide microspheres: a new nanozyme with oxidase-like activity allowing biomolecule determination via bimodal sensing

Abstract

“Monodisperse-porous manganese oxide (MnO_x) microspheres” that could not be obtained by current protocols were synthesized by a new template assisted method, using porous poly(methacrylic acid-co-ethylene dimethacrylate) microspheres as a template. Through the developed method, monodisperse MnO_x microspheres 3.7 μm in size, with a specific surface area of 71 m² g⁻¹, a mean pore size of 15 nm and a porous structure covering mesopores and macropores, were obtained by the calcination of polymer/manganese oxide composite microspheres at 380 °C. A transition in the porous structure from mesoporous to macroporous form and a transition in the crystal structure from monoclinic Mn₅O₈ to α-MnO₂ were observed with increasing calcination temperature. MnO_x microspheres with appropriate size and porous properties for using them as a permeable stationary phase in a continuous microcolumn were also synthesized for the first time. The monodisperse-porous MnO_x microspheres exhibited oxidase mimetic activity at neutral pH, using o-phenylenediamine (OPDA) as the substrate. The enzyme mimetic characteristics of the microspheres were also adjusted by controlling their porous properties. The decomposition of MnO_x microspheres to Mn²⁺ ions in the presence of ascorbic acid (AA) was evaluated for bimodal sensing of this agent for the first time, which could not be achieved with MnO₂ nanosheets as a frequently tried nanomaterial for biosensing applications. The particle size of the synthesized nanozyme in the micron-size range and selected substrate providing a fluorescent product allowed determination of the AA concentration via both colorimetric and fluorometric methods. The MnO_x microspheres are new materials with superiorities in biosensing, column chromatography, nanocatalysis and supercapacitor applications.