<p>Photoelectrochemical biosensor based on Bi₂S₃/Bi₂O₂CO₃ heterojunction as photoelectrically active material and lactate oxidase was constructed for sensitive detection of lactic acid in serum. Topological transformation method was utilized to prepare Bi₂S₃/Bi₂O₂CO₃ composites, which can generate electron–hole pairs triggering photoelectrochemical reactions under light illumination. The Bi₂S₃/Bi₂O₂CO₃ heterojunction effectively promoted the separation and transportation of photogenerated electrons and holes, leading to improved photoelectric conversion efficiency. Lactic acid is catalyzed by lactate oxidase to generate H₂O₂, which reacts with photogenerated holes, leading to photocurrent increase, achieving quantitative detection of lactic acid. The prepared biosensor has a wide detection range for lactic acid from 0.01 mM to 10 mM, with detection limit as low as 0.006 mM. In addition, the detection of lactic acid in serum samples further validated the potential application of the sensor.</p> Graphical abstract <p></p>