An ultrasensitive photoelectrochemical immunoassay based on defect-engineered Z-scheme sulfur vacancy-In₂S₃/TiO₂ heterojunctions

Overcoming the rapid recombination of electron-hole pairs and the slow transfer of charges remains the main challenge for improving the sensitivity of photoelectrochemical (PEC) immunoassays. Herein, a S-defect-engineered indium sulfide/titanium dioxide (In₂S₃/TiO₂) heterojunction-based PEC immunosensor is reported for the sensitive detection of carcinoembryonic antigen (CEA). The highly matched energy band positions and the resulting built-in electric field (BIEF) drove a Z-scheme charge transfer pathway. Concurrently, the introduced sulfur vacancies (Sv) acted as charge trapping centers, synergistically accelerating the spatial separation and transfer efficiency of photogenerated electron-hole pairs, thereby significantly enhancing the photocurrent response. For signal transduction, a sandwich-type immunoreaction system was constructed, in which alkaline phosphatase (ALP) catalyzed ascorbic acid 2-phosphate (AAP) to produce ascorbic acid (AA). The generated AA served as an efficient hole scavenger, competitively depleting the photogenerated holes involved in the electron quenching reaction. This led to a significant decrease in photocurrent intensity, realizing the quantitative detection of CEA. In the context of optimal conditions, the proposed PEC immunosensor exhibited a broad linear range from 0.01 to 40 ng mL^− 1, accompanied by an ultralow limit of detection (LOD) of 3.1 pg mL^− 1, along with satisfactory selectivity and stability. This study not only highlights the synergistic effect of defect engineering and heterojunction construction in boosting PEC performance but also provides a reliable and highly sensitive strategy for clinical biomarker detection.