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Aflatoxin B1 (AFB1) is the most toxic mycotoxin, is widely found in foods and animal feeds, and can pose a serious threat to our lives. A label-free photoelectrochemical (PEC) immunosensor was fabricated for the sensitive detection of AFB1. A Z-scheme heterojunction of gold nanoparticles (Au NPs) loaded on graphitic carbon nitride sheet and tungsten trioxide sphere composite (g-C 3 N 4 /Au/WO 3 ) acted as the highly sensitive platform. The g-C 3 N 4 /Au/WO 3 is capable, not only of immobilizing antibodies via Au NPs, but also enhancing the separation of electron-hole pairs due to its good energy band matching efficiency. The mechanism of photo-generated electron/hole transfer on g-C 3 N 4 /Au/WO 3 was explored using scavengers to eliminate active components. On this basis, an electron transfer pathway for the immunosensor was deduced. The PEC immunosensor displayed a linear concentration range from 1.0 pg mL -1 to 100 ng mL -1 and a low detection limit of 0.33 pg mL -1 (S/N = 3) for AFB1. Good reproducibility, stability, and specificity provide a solid foundation for the practical application of this immunosensor., (Copyright © 2021 Elsevier B.V. All rights reserved.)