Overriding n -Type Surface Depletion via Ferroelectric Pumping for Self-Powered Cathodic Photoelectrochemical Immunoassay

The dynamics of surface carriers in conventional n -type semiconductors are generally restricted by the energy band bending at their interfaces, thereby severely limiting the extraction of photoelectrons for cathodic bioassays. Herein, we reported a nonconventional cathodic photoelectrochemical (PEC) immunoassay, utilizing an Ag-decorated Gd-doped BiFeO 3 (Ag/BGFO) Schottky junction for fundamentally overriding the n -type depletion limitations. Despite its typical n -type nature confirmed by Mott–Schottky analysis, the BGFO photoelectrode generates a robust cathodic photocurrent of 23.5 μA at 0 V, which is more than four times higher than pure BFO. This counterintuitive phenomenon is governed by the powerful spontaneous depolarization field ( E dp ) of the ferroelectric BGFO, which forcefully pumps photoelectrons to the surface, overriding the conventional depletion layer. Subsequently, the upward-bent Ag/BGFO Schottky barrier acts as a unidirectional valve, efficiently extracting these electrons into the Ag electron sinks while strictly preventing interfacial backflow. Leveraging this exceptionally electron-rich cathodic platform, a split-type PEC immunosensor was constructed for α-fetoprotein (AFP) detection. By spatially isolating the target-triggered enzymatic generation of H 2 O 2 from the photoelectric interface, the sensor thoroughly eliminates biofouling, achieving an ultralow limit of detection of 2.46 pg/mL and a remarkable 90-day storage stability (>97% retention). Furthermore, clinical validation using 50 human serum specimens demonstrated an excellent linear correlation ( R 2 = 0.9986) with the gold-standard ELISA. This work not only provides profound physical insights into ferroelectric-modulated charge dynamics but also establishes a highly robust paradigm for translational clinical diagnostics.