Exploring self-supplied photoactive layers on gold electrodes: A semiconductor-, probe-immobilization-, and label-free photoelectrochemical platform bridged by polyadenine-gold affinity.

Photoelectrochemical biosensing provides high sensitivity but often requires complex electrode modification involving semiconductors, probe immobilization, and labeling, which limits reproducibility and practicality. Here, we reveal that Au2O3 species generated during anodic oxidation of gold electrodes exhibit an intrinsic photocurrent response, which is amplified by nearly two orders of magnitude in the presence of oxygen and ascorbic acid. Unexpectedly, dithiothreitol-a common thiol compound in enzyme buffers-was found to further enhance this response. Mechanistic studies showed that dithiothreitol reduces surface Au2O3 and forms Au-S linkages, collectively decreasing interfacial electron-transfer resistance and double-layer capacitance. These interfacial effects suppress capacitive background currents and highlight the Faradaic contribution from ascorbic acid oxidation, resulting in remarkable photocurrent enhancement. Guided by these insights, we constructed a semiconductor-, probe-immobilization-, and label-free photoelectrochemical platform by coupling Escherichia coli poly(A) polymerase-catalyzed polyadenylation with intrinsic polyadenine-gold affinity, enabling sensitive quantification of microRNA-21 with a linear range of 1 fM-100 pM and a detection limit of 1.3 fM (3σ, n = 11). The practicality of this sensing platform was demonstrated by quantifying microRNA-21 in bladder urothelial carcinoma tissues, with results in good agreement with reverse transcription quantitative polymerase chain reaction. This study elucidates an effective interfacial photoelectric conversion mechanism on gold electrodes and provides an instructive strategy for constructing simplified and efficient photoelectrochemical biosensing systems.