Enzyme-Induced Photocurrent Polarity Reversal in a Zr-MOF/COF Photoelectrochemical Immunosensor for Ultrasensitive Detection of Cancer Markers

Photocurrent polarity switching provides an effective strategy for improving the reliability and sensitivity of photoelectrochemical (PEC) biosensors by reducing false-positive responses. Herein, we developed a polarity-switchable PEC immunosensor for ultrasensitive detection of prostate-specific antigen (PSA) based on an in situ electron donor generation strategy using a Zr-based metal–organic framework (Zr-MOF)/thionine–cyanuric chloride covalent organic framework (Th-CC-COF) heterojunction. The modified electrode, prepared using an optimized 1:1 mass ratio of Zr-MOF to Th-CC-COF, initially exhibited a cathodic photocurrent. PSA recognition in the conventional immunosensing mode decreased the photocurrent, enabling PSA detection over a linear range from 0.10 pg/mL to 1.0 ng/mL with a detection limit of 0.023 pg/mL. To enhance the analytical performance, a sandwich immunoassay with an alkaline phosphatase-labeled secondary antibody was constructed. The enzymatic hydrolysis of ascorbic acid 2-phosphate generated ascorbic acid (AA) in situ, which acted as an electron donor and switched the photocurrent from cathodic to anodic. The resulting polarity-switchable signal-on strategy provided a wider linear range from 0.010 pg/mL to 1.0 ng/mL and a lower detection limit of 3.5 fg/mL, together with improved sensitivity compared with the cathodic sensing mode. The proposed immunosensor also showed satisfactory resistance to the tested interfering proteins, and its applicability was demonstrated by the analysis of spiked human serum samples, with results comparable to those obtained by enzyme-linked immunosorbent assay (ELISA). This study demonstrates the potential of photocurrent polarity switching as a reliable and sensitive strategy for PEC biosensing and biomolecular detection.