<p> A&#xa0;ultrasensitive split-type photoelectrochemical (PEC) sensor was constructed for ampicillin (AMP) detection, utilizing a metal–organic framework (MOF)-confined In₂S₃/PCN-224 Z-scheme heterojunction as the photoactive material. The prepared In₂S₃/PCN-224 was demonstrated with high charge separation efficiency and a stable PEC signal response due to the unique electron flow direction of the Z-scheme configuration. To further enhance the detection sensitivity, target-mediated in-situ ion exchange via Cd²⁺ ions was employed to modulate the photoactivity of In₂S₃/PCN-224. In the presence of AMP, the aptamers labeled with CdCO₃ were released from the DNA double-strand and then dissociated into Cd²⁺ ions after acid treatment. Ion exchange reactions will occur upon introducing the solution into the In₂S₃/PCN-224 surface. Another photoactive material may be produced on the electrode surface to amplify the original PEC signal. The resulting split-type PEC sensor exhibited an impressive linear range (0.5–200&#xa0;ng&#xa0;mL⁻¹) with a low limit of detection (LOD, 0.09&#xa0;pg&#xa0;mL⁻¹, S/N = 3). This work presents a promising strategy for the development of PEC biosensors, offering practical applications in the environmental analysis of antibiotics.</p> Graphical Abstract <p></p>