Uncovering the Unexpected Role of DNA Molecules as Amplifiers of Photoelectrochemical Signals at Semiconductor Interfaces: Mechanistic Insights and Application in Monitoring Drug-Induced Exosomal Phenotypic Changes

The development of photoelectrochemical (PEC) biosensors with enhanced sensitivity and structural simplicity remains a key challenge in biomolecular detection. In this work, we report an unexpected and previously overlooked phenomenon in which DNA aptamers inherently act as amplifiers of PEC signals at semiconductor interfaces. Traditionally regarded solely as passive recognition elements, DNA aptamers─exemplified by the EpCAM-specific SYL3C─were found to markedly increase photocurrent when assembled on graphitic carbon nitride (g-C₃N₄)-based PEC electrodes. To further enhance interfacial charge transfer, g-C₃N₄ was covalently functionalized with 1,3,5-benzenetricarboxaldehyde (BTA), forming a donor-acceptor structured semiconductor (g-C₃N₄-BTA). Density functional theory (DFT) calculations and Mott-Schottky analysis revealed that the lowest unoccupied molecular orbital (LUMO) levels of DNA bases are positioned above the conduction band (CB) edges of both g-C₃N₄ and g-C₃N₄-BTA, enabling thermodynamically favorable injection of photoexcited electrons from DNA molecules into the semiconductor CB. This interfacial electron injection, analogous to dye-sensitized solar cells, accounts for the observed PEC signal amplification. Based on this mechanistic understanding, we developed a SYL3C/AuNPs/chitosan/g-C₃N₄-BTA-modified electrode for ultrasensitive detection of EpCAM-positive exosomes, achieving a detection limit of 988 particles mL^-1. Furthermore, the sensor demonstrated robust performance in monitoring phenotypic changes of exosomes secreted by HepG2 cells in response to chemotherapy drug treatment, highlighting its potential for functional exosome analysis in cancer research. This study not only identifies a previously unrecognized inherent property of DNA aptamers to enhance semiconductor photoactivity, but also establishes a minimalist and broadly applicable design principle for constructing high-performance PEC biosensors. The mechanistic insights presented here open new avenues for the rational design of PEC sensing interfaces and extend the utility of DNA aptamers beyond molecular recognition toward active signal amplification.