The development of hierarchical nanostructure is demonstrated as an effective strategy to improve catalytic activity and stability of electrocatalysts. Herein, a novel leaf-like hierarchically porous heterojunction flake arrays (FAs), integrated graphitic N-doped carbon (NC) with amorphous B,N-doped carbon (BNC), has been designed and grown on flexible carbon fiber (CF) using metal-organic framework (MOF) FAs and green ionic liquids as precursors. The hierarchical heterojunction structure possesses micro- and nano-scaled pores, large surface areas, and exposed high-density catalytic active sites, which enhances electronic conductivity and offers accessible transport channels for effectively decreasing mass transport resistance. The results of discrete Fourier transform (DFT) calculations and experiments also suggest that the catalytic activity has been promoted by the heterojunction structure through narrowing the banding gap. Consequently, the resultant nanohybrid microelectrode exhibits remarkable electrochemical sensing performance towards H₂O₂ with a low detection limit of 50 nM, and a high sensitivity of 213 μA·mM^-1·cm^-2, as well as good anti-interference capability. The practical application of nanohybrid fiber microelectrode has been explored by real-time monitoring H₂O₂ released from live colon cells and surgically-resected fresh colon cancer tissue, which can provide important information for the identification of different types of cells as well as distinguish cancer cells from normal ones. We believe this research will pave the way towards the development of advanced carbon nanomaterials for application in the fields of electrochemistry, biosensing, and cancer diagnosis.