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.　©　2021　American　Chemical　Society