Scheme 1. Schematic illustration of the integration of HRCA with the QD-based FRET for simultaneous detection of miR-21 and miR-221. This assay involves four steps: (1) the hybridization of miRNAs with circular templates and the subsequent HRCA reaction, (2) the hybridization of HRCA products with capture probes and reporter probes, (3) the formation of the 525QD–DNA–dye nanostructures through streptavidin–biotin binding, (4) fluorescence emission from the acceptors as a result of FRET between the 525QD and the acceptors at an excitation wavelength of 405 nm.