Polarity Conversion of the Ag₂S/AgInS₂ Heterojunction by Radical-Induced Positive Feedback Polydopamine Adhesion for Signal-Switchable Photoelectrochemical Biosensing

Efficient tuning of the polarity of photoactive nanomaterials is of great importance in improving the performance of photoelectrochemical (PEC) sensing platforms. Herein, polarity of the Ag₂S/AgInS₂ heterojunction is converted by radical-induced positive feedback polydopamine (PDA) adhesion, which is further employed to develop a signal-switchable PEC biosensor. In the nanocomposites, Ag₂S and AgInS₂ achieve electron-hole separation, exhibiting a strong anodic PEC response. Under the irradiation of light, the Ag₂S/AgInS₂ heterojunction is able to produce superoxide radical and hydroxyl radical intermediate species, leading to the polymerization of dopamine (DA) and the subsequent adhesion of PDA onto the Ag₂S/AgInS₂ heterojunction (Ag₂S/AgInS₂@PDA). By constructing a new electron-transfer pathway with PDA, the polarity of the Ag₂S/AgInS₂ heterojunction is converted, and the PEC response changes from anodic to cathodic photocurrents. In addition, since the photoreduction activity of PDA is stronger than that of the Ag₂S/AgInS₂ heterojunction, more superoxide radical can be produced by Ag₂S/AgInS₂@PDA once PDA is generated, thereby promoting the generation of PDA. Consequently, a positive feedback mechanism is established to enhance the polarity conversion of the Ag₂S/AgInS₂ heterojunction and amplify the responding to DA. As a result, the bioanalytical method is capable of sensitively quantifying DA in 10 orders of magnitude with an ultralow limit of detection. Moreover, the applicability of this biosensor in real samples is identified by measuring DA in fetal bovine serum and compared with a commercial ELISA method. Overall, this work offers an alternative perspective for adjusting photogenerated carriers of nanomaterials and designing high-performance PEC biosensors.