Modulating Built-In Electric Field Strength in an In ₂ S ₃ /CdS Heterojunction via Vanadium Doping for a Photoelectrochemical Immunoassay

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Constructing a built-in electric field (BIEF) in heterojunctions is regarded as an effective strategy for improving photoelectrochemical (PEC) performance since it acts as a key factor in accelerating charge transfer and separation. Herein, a PEC immunosensor with excellent target responsiveness was presented based on vanadium-doped indium sulfide/cadmium sulfide (V–In2S3/CdS) heterojunctions with a giant BIEF and excellent peroxidase-like (POD-like) activity. The BIEF strength of the heterojunction can be effectively modulated by V doping to facilitate rapid charge transfer and separation, resulting in an enhanced photocurrent. Notably, the POD-like activity of the V–In2S3/CdS heterojunction could effectively catalyze the decomposition of hydrogen peroxide (H2O2) to produce hydroxyl radicals (•OH). To achieve a sensitive PEC immunoassay, carcinoembryonic antigen (CEA) was specifically recognized by a sandwich-type immune reaction to generate H2O2. The generated H2O2 was catalytically decomposed by the V–In2S3/CdS heterojunction to produce highly oxidative •OH radicals that could trap photoelectrons, resulting in a sharp quenching of the initial photocurrent signal. Leveraging the distinctive photocurrent variation, a highly sensitive PEC sensing platform was designed for CEA detection, exhibiting a linear detection range from 0.02 to 50 ng/mL and a low detection limit of 5.8 pg/mL. This work sets the stage for the application of the BIEF as an advanced signal modulation strategy for high-performance PEC immunoassays.