The FAM-labelled strand is fully complementary to the cDNA/primer, therefore is able to hybridize with the cDNA/primer sequence and separate from the BHQ1-labelled strand, leading to fluorescence emission. As the RCA reaction produces long ssDNA having numerous copies of the cDNA sequence, the RCA reaction will amplify greatly the analytical signal (Fig. ). Without RCA, the fluorescence signal caused by RBD-aptamer complexation is very weak (Fig. ). By adding the RCA step, the fluorescence signal caused by the RBDaptamer recognition becomes significantly enhanced (Fig. ). Comparison of Fig. with S7c shows clearly that the RCA is essential to achieve reliable detection of low concentration RBD (Fig. ).