Microcystin-lr (MC-LR) is one of the most toxic and ubiquitous microcystins (MCs) released by cyanobacteria. Exposure to MC-LR can cause multiple organ damage and even death of the organism. Therefore, creating highly sensitive and dependable methods for detecting trace MC-LR is crucial. Herein, we developed a novel fluorescence aptasensor aided by exonuclease III (Exo III) for the highly sensitive detection of MC-LR. In the presence of MC-LR, the affinity interaction between MC-LR and aptamer A was triggered, leading to the release of blocker B. This unbound blocker can initiate Exo III-media...