Visual detection of miRNAs using enzyme-free amplification reactions and ratiometric fluorescent probes

A visualized assay for miRNAs detection has been developed in this work. The presented method is based on a combination of enzyme-free amplification cascades of catalyzed hairpin assembly (CHA) and hybridization chain reaction (HCR) and fluorescence quenching of dual-emission ratiometric fluorescent probes (RF probes). MiRNAs can efficiently initiate enzyme-free amplification reactions (CHA and HCR) and produce the long nicked dsDNAs with a lot of glucose oxidases (GOD) on the surface of dynabeads bridged by the GOD-labeled hairpin DNA probes. Hydrogen peroxide (H₂O₂) is generated by oxidation of glucose catalyzed by GOD, which can quench the outer green fluorescence without affecting the internal red fluorescence of RF probes. Therefore, increased miRNA amount can result in change of the two fluorescence intensity ratios of RF probes with continuous color changes from green to red under a UV lamp, which can be easily recognized by naked eye. The proposed assay exhibits high sensitivity toward let-7a with dynamic range from 10^-13 M to 10^-8 M, and which is applied successfully to detecting let-7a in the small RNA samples.