Bi₂S₃/Bi₂O₂(OH)₂ nanorods with internal electric field throughout the entire bulk phase as photoelectrochemical sensing platforms for CYFRA21-1 immunoassay

Photoelectrochemical (PEC) immunosensors are highly promising tools for monitoring biochemical molecules. Constructing high-performance heterojunctions is a general method to improve the sensitivity of PEC immunosensors. The internal electric field (IEF) formed at the heterojunction interface plays a crucial role in coordinating the separation of photogenerated carriers. Although IEF in heterojunctions shows extraordinary talents in PEC immunosensors, it is powerless to suppress the recombination of photogenerated carriers in the bulk phase. Thus, modulating the IEF of heterojunctions more than just at interfaces remains a challenge. Herein, a "signal-off" PEC immunosensor for CYFRA21-1 detection was constructed. The immunosensor was based on sulfurized bismuth oxyhydroxide compound Bi₂S₃/Bi₂O₂(OH)₂ nanorods as the platforms, N-doped hollow homogenous Mo₂C nanospheres with Ru NPs (Mo₂C:N@Ru) as the quenching label. The Bi₂O₂(OH)₂ were composed of [Bi₂O₂]²⁺ and OH^- alternating layers, which provided an IEF throughout the bulk phase. Meanwhile, the heterogeneous interface further promoted the separation of photogenerated carriers and output strong photocurrent signals by energy level matching of Bi₂O₂(OH)₂ with Bi₂S₃. Subsequently, Mo₂C:N@Ru was quantitatively introduced in the presence of CYFRA21-1 through specific recognition between antigens and antibodies. The main structure of Mo₂C:N@Ru was a homojunction formed by cubic and hexagonal phases Mo₂C, which accelerated the separation of photogenerated carriers. Moreover, doped N element and Ru NPs further enhanced the competition between Mo₂C:N@Ru and the Bi₂S₃/Bi₂O₂(OH)₂ platforms for ascorbic acid and then achieved a quenching effect. The limit of detection for CYFRA21-1 was 17 fg/mL, and linearity range was 50 fg/mL -100 ng/mL. This PEC immunosensor achieves highly sensitive detection of CYFRA21-1 and has good scalability, which provides new ideas for the development analysis and detection platforms of tumor biomarkers. Simultaneously, this strategy might provide a novel perspective for modulating the distribution of the IEF of photoactive materials applied in PEC immunosensors.