Spatial Confinement of Enzyme and Nanozyme in Silica-Based Hollow Microreactors.

Designing a strategy for encasing enzymes and nanozymes in microreactors with spatial confinement in a way to improve the selectivity and activity of nanozymes is an exciting goal. In the present work, we report a facile route to encapsulate glucose oxidase (GOx) and poly(ethylenimine) (PEI)-conjugated magnetite nanoparticles (Fe ₃ O ₄ -PEI) in the hollow interior of hybrid microreactors. The microreactors are prepared by polyallylamine hydrochloride (PAH)-mediated silica (SiO ₂ ) nanoparticle assembly on calcium carbonate (CaCO ₃ ) particles as a removable core. By tuning both shape and phase (vaterite/calcite and pure calcite) of CaCO ₃ , it allows generation of GOx and Fe ₃ O ₄ -PEI-encapsulated silica hollow microspheres (GOx-Fe ₃ O ₄ @SHS) and microcubes (GOx-Fe ₃ O ₄ @SHC). As observed, in a biomimetic cascade catalysis, the confined GOx in the microreactors is able to catalyze oxidation of glucose to gluconic acid and hydrogen peroxide (H ₂ O ₂ ), followed by the activation of H ₂ O ₂ by Fe ₃ O ₄ -PEI for the oxidation of the chromogenic substrate o -phenylenediamine (oPD) to 2,3-diaminophenazine. Comparison of the peroxidase-like activity of the encapsulated Fe ₃ O ₄ -PEI shows that the hollow microspheres (GOx-Fe ₃ O ₄ @SHS) result in activity 14 times higher than that of the hollow microcubes (GOx-Fe ₃ O ₄ @SHC), which in turn is corroborated to the differential loading capacity of GOx in microspheres and microcubes. The evaluation of kinetic parameters indicates a fivefold increase in the catalytic constant ( k _cat ) of Fe ₃ O ₄ -PEI confined in hollow microspheres (GOx-Fe ₃ O ₄ @SHS) compared to the mixture comprising free GOx and Fe ₃ O ₄ -PEI in solution. It suggests that the confined space in the microreactors allows the tandem reactions of GOx and Fe ₃ O ₄ -PEI to take place in close proximity, leading to an improved overall activity. This indeed is seen in the k _cat obtained for Fe ₃ O ₄ @SHS (GOx added externally during the assay), which is 14 times lower than that of GOx-Fe ₃ O ₄ @SHS.