Ultrasensitive\nPhotocathodic Biosensor Based on the\nCu<sub>2</sub>O/PTB7-Th/PDA<sup>+</sup> Composite with Enhanced Photoelectrochemical\nPerformance for the Detection of MicroRNA-375-3p

Herein, an ultrasensitive photocathodic biosensor was\nfabricated\nbased on Cu<sub>2</sub>O/PTB7-Th/PDA<sup>+</sup> photoactive materials\nwith high photocarrier separation efficiency for the detection of\nmicroRNA-375-3p. Impressively, the photocathodic signal of the Cu<sub>2</sub>O material was significantly enhanced by using PTB7-Th as\nan energy level-matching photoactive material to enhance the bulk\ncharge separation and <i>N</i>,<i>N</i>-bis (2-(trimethylammoniumiodide)\npropylene) perylene-3,4,9,10-tetracarboxydiimide (PDA<sup>+</sup>)\nas an interfacial charge transfer mediator to efficiently suppress\ncharge recombination at the photoelectrode/electrolyte interface.\nCompared with the pristine Cu<sub>2</sub>O as a photocathode, the\nobtained Cu<sub>2</sub>O/PTB7-Th/PDA<sup>+</sup> exhibited a 17 times\nhigher photocathodic signal. As a proof of concept, a PEC biosensor\nwas fabricated by using Cu<sub>2</sub>O/PTB7-Th/PDA<sup>+</sup> as\na photoactive material and a target-triggered 3D DNA walker integrated\nwith the dumbbell hybridization chain reaction (DHCR) as a signal\namplifier to achieve the sensitive detection of microRNA-375-3p with\na detection limit of 0.3 fM. This work provided a method to increase\nthe photocurrent signal and the sensitivity of PEC-sensing platforms\nfor the detection of biomarkers and disease diagnosis.