Dual-Polarity Switching Photoelectrochemical Biosensor Based on Z-Scheme Bi 2 S 3 /Bi 2 WO 6 Heterojunction for Clinical Thalassemia Screening: Cascade Signal Amplification via 3D DNA Walker and Target Recycling

The simultaneous detection of CD142 and CD41-42 mutations is critical for accurate diagnosis and precision medicine but remains challenging due to the ultralow target abundance in early stage screening. Here, we develop a dual-polarity switching photoelectrochemical biosensor based on a photocurrent polarity-switching strategy, achieving ultrasensitive dual-target detection of thalassemia genes CD142 and CD41-42 through a "cathodic-anodic-cathodic" tristate conversion. This sensor innovatively integrates a Z-scheme Bi₂S₃/Bi₂WO₆ heterojunction substrate with an Ag₂S quantum dots (anodic)/Cu₂O (cathodic) labeling system, combined with a target-triggered cascaded nucleic acid amplification strategy: target-driven toehold-mediated strand displacement reaction with exonuclease III-assisted target cycling (TSDR-EATC) and 3D DNA walker, converting background interference into characteristic signals with self-calibration capability. Experiments demonstrate that the sensor exhibits excellent linear response in the range of 0.1 fM-0.1 μM, with detection limits of 30.9 aM (CD142) and 27.8 aM (CD41-42). Clinical sample validation shows a recovery rate of 98.0-107.9% and an RSD of ≤4.8%, and the correct rate in double-blind tests exceed 93%. This work establishes a new paradigm for polarity-switchable PEC systems by integrating heterojunction engineering with nucleic acid amplification, providing a highly reliable solution for the community screening of thalassemia.