Facile synthesis of magnetic hierarchical flower-like Co3O4 spheres: Mechanism, excellent tetra-enzyme mimics and their colorimetric biosensing applications.

Magnetic hierarchical flower-like Co3O4 spheres (Co3O4 nanoflowers) were facilely prepared via one-step surfactant-free and template-free wet chemical route at room temperature. The formation mechanism of Co3O4 nanoflowers was explored. The as-prepared Co3O4 nanoflowers exhibited excellent tetra-enzyme mimetic activities, including oxidase-like, peroxidase-like, catalase-like and superoxide dismutase (SOD)-like activity. The catalytic mechanism of the Co3O4 nanoflowers was studied in detail. The oxidase-like catalytic activity of Co3O4 nanoflowers was derived from the inherent oxygen vacancies of Co3O4, while the peroxidase-like catalytic activity originated from the •OH radical generated by hydrogen peroxide (H2O2). Only one specific enzyme mimics reaction with Co3O4 nanoflowers can be obtained by inhibiting specifically other nanozymes via varying pH, adding appropriate scavengers or selecting its specific substrate. Further, the steady-state kinetic and catalytic performance of the oxidase-, peroxidase- and catalase mimics of Co3O4 nanoflowers were studied. Based on the oxidase-like and peroxidase-like activities of Co3O4 nanoflowers, bifunctional colorimetric sensing platforms were constructed for the sensitive detection of acid phosphatase (ACP) with the linear range of 0.1-25 U L-1 and H2O2 with the linear range of 4-400 μM. Further, it is capable of detecting ACP in serum samples and H2O2 in water samples, respectively. Thus, the tetra-enzyme mimetic Co3O4 nanoflowers show broad prospects in the fields of biosensors, tumor therapy, environmental monitoring and biocatalysis.