Fluorescence intensity of MnO2-Cy3-ssDNA-21 or MnO2-Cy5-ssDNA-155 after simultaneous incubation with 1, 5, 10, 15, 20, 25, 40, 50, 75, 90 nM miRNA-21 and miRNA-155 (a). Calibration curves of fluorescence intensity ratio I/I0 vs target miRNA concentration (b).