A Direct‐Contact Photocurrent‐Direction‐Switching Biosensing Platform Based on In Situ Formation of CN QDs/TiO₂ Nanodiscs and Double‐Supported 3D DNA Walking Amplification

Abstract Herein, a direct‐contact photocurrent‐direction‐switching photoelectrochemical (PEC) biosensing platform for the ultrasensitive and selective detection of soluble CD146 (sCD146) is reported for the first time via in situ formation of carbon nitride quantum dots (CN QDs)/titanium dioxide (TiO₂) nanodiscs with the double‐supported 3D DNA walking amplification. In this platform, metal organic frameworks (MOFs)‐derived porous TiO₂ nanodiscs exhibit excellent anodic photocurrent, whereas a single‐stranded auxiliary DNA (ssDNA) as biogate is absorbed onto the TiO₂ nanodiscs to block active sites. Subsequently, with the help of intermediate DNAs from target sCD146‐induced double‐supported 3D DNA walking signal amplification, the ssDNA can leave away from the surface of TiO₂ nanodiscs due to the specific hybridization with intermediate DNAs. Afterward, the successful direct contact of CN QDs on TiO₂ nanodiscs by porosity and electrostatic adsorption, leads to the effective photocurrent‐direction switching from anodic to cathodic photocurrent. Based on direct‐contact photocurrent‐direction‐switching CN QDs/TiO₂ nanodiscs system and double‐supported 3D DNA walking signal amplification, sCD146 is detected sensitively with a wide linear range (10 fg mL⁻¹ to 5 ng mL⁻¹) and a low limit of detection (2.1 fg mL⁻¹). Also, the environmentally friendly and direct‐contact photocurrent‐direction‐switching PEC biosensor has an application prospect for cancer biomarker detection.