Co₃O₄ nanoparticles with multi-enzyme activities and their application in immunohistochemical assay.

Co3O4 nanoparticles (Co3O4 NPs), synthesized by the coprecipitation method, showed intrinsic catalase-like, peroxidase-like, and SOD-like activity. The catalytic activity of Co3O4 NPs was much higher than analogous Fe3O4 NPs. Co3O4's mechanisms of catalytic activity were analyzed in detail using the electron spin resonance (ESR) method, which confirmed that Co3O4 NPs don't follow the classical Fenton reactions with hydrogen peroxide the way Fe3O4 NPs do. The high redox potential of Co(3+)/Co(2+) was supposed to be the leading cause of the differences in both activity and mechanism with Fe3O4. Based on the high, peroxidase-like activity, a new immunohistochemical assay was designed in which the avastin antibody was conjugated onto the surface of Co3O4 NPs. The conjugates obtained were used to detect vascular endothelial growth factor (VEGF) that was overexpressed in tumor tissue. When the experimental and control groups were stained, there were clear distinctions between them. This study showed that there are many opportunities to improve the enzyme-like activities of nanomaterials and also to improve their potential applications for biocatalysis and bioassays, especially in relatively harsh conditions.