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MicroRNAs (miRNAs) play important roles in cell differentiation, proliferation, and apoptosis and have been recognized as valuable biomarkers for clinical disease diagnosis. Here, we adopt for the first time zeolitic imidazolate framework-8 (ZIF-8) as a nanocarrier to efficiently deliver a nucleic acid probe to living cells and develop a novel ratiometric fluorescence strategy based on DNAzyme for miRNA-21 imaging. A Cy5-labeled 8-17 DNAzyme strand and a Cy3-labeled substrate strand containing a segment complementary to the target miRNA-21 first form a duplex probe, and fluorescence resonance energy transfer (FRET) takes place. After adsorption on the ZIF-8 surface and cellular uptake, the probe/ZIF-8 nanocomplex degrades in acidic endosome and releases duplex probes and Zn2+, and the latter can act as an effective cofactor for 8-17 DNAzyme. The intracellular miRNA-21 hybridizes with the complementary segment of the substrate strand and results in dissociation from the DNAzyme—substrate duplex probe after DNAzyme cleaves the substrate into two fragments, accompanied by the change in the FRET signal. The proposed method has been applied to image miRNA-21 expression levels in MCF-7, HeLa, and L02 cells with high contrast and reliability. The fluctuation of miRNA-21 expression level induced by miRNA-21 mimic or inhibitor can also be monitored through the obvious imaging color change. Taken together, the proposed method provides a powerful tool for cancer diagnosis and miRNA-associated biological study. [ABSTRACT FROM AUTHOR]