<p> An innovative organic-inorganic heterojunction based on <i>meso</i>-tetra(4-carboxyphenyl)-porphine (TCPP) and CsPbBr₃ perovskite is reported. Additionally, cobalt nitride (Co₄N) was employed as mimetic enzyme of lactic acid oxidase (LOX), thereby enabling sensitive detection of lactic acid in serum through photoelectrochemical (PEC) detection. The stability of CsPbBr₃ was significantly improved in aqueous solutions by substituting some of the long-chain ligands on its surface with TCPP, an organic semiconductor containing π-π bonds and carboxyl groups. Furthermore, the formation of a type-II heterojunction between TCPP and perovskite improved carrier separation and transport capabilities, resulting in more stable and enhanced photocurrent response. The non-enzymatic catalyst, Co₄N nanoparticles, demonstrate efficient catalytic activity toward lactic acid oxidation and enhance electron transfer to the heterojunction interface, thereby increasing the response current. The prepared PEC biosensor exhibits an effective sensing capability for lactic acid, with a working range of 0.2 mM to 10 mM and a detection limit of 0.03 mM. Furthermore, the biosensor features good selectivity and reproducibility, making it ideal for measuring lactic acid levels in human blood samples. This approach of forming heterojunctions by combining organic semiconductor materials with perovskites opens up new possibilities for constructing advanced perovskite-based photoelectric sensors.</p> Graphical abstract <p></p>