Ultrasensitive Photoelectrochemical Biosensor for microRNA-155 Based on Energy Transfer between Au Nanocages and Red Emission Carbon Dot-Assembled Nanosheets Coupled with the Duplex-Specific Nuclease Enzyme-Assisted Target Recycling Strategy.

Energy transfer (ET) is an effective tool to construct photoelectrochemical (PEC) biosensors for its high sensitivity. Since the materials to develop ET systems are limited, exploring new and universal ET systems is significant. Herein, new photoactive nanosheets (R-CDs NS) formed by self-assembling of red emission carbon dots (R-CDs) have been synthesized, which exhibit wide visible light absorption and stable photocurrent response and have an obvious sensitization effect for TiO2. Gold nanocages (AuNCs), whose absorption overlap well with the R-CDs' emission, were synthesized and served as PEC quenchers for the photosensitized system that consists of TiO2 and R-CDs. The ET between R-CDs and AuNCs can boost the recombination of photogenerated electron-hole pairs of R-CDs and results in a quenched photocurrent of this system. MicroRNA-155 was chosen as a model target. First, the nanocomposite containing R-CDs NS and AuNCs was prepared through DNA modification and hybridization. In the absence of the target, AuNCs and R-CDs were close enough for ET, with TiO2-modified FTO serving as the working electrode, and a quenched photocurrent was detected. In the presence of the target, the disintegration of the nanocomposite was induced through target hybridization and DNA hydrolyzation, leading to the separation of AuNCs and R-CDs NS, and the ET disappeared and led to a high photocurrent. With duplex-specific nuclease enzyme-assisted target recycling, the high sensitivity enabled the sensor to monitor the target in cancer cells. The sensor has a low detection limit of 71 aM. The sensing platform has high sensitivity, good selectivity, and reproducibility.