In order to confirm the feasibility of the protein assays, we need to verify the complexation of aptamer with cDNA and with S protein RBD, and the competitive nature of the molecular bindings. Firstly, we verified the complexation of FAM-labelled cDNA with biotin-tagged aptamer immobilized on streptavidin-coated MBs. The FAM-cDNA has a fully complementary sequence that matches the aptamer, while the FAM-ssDNA contains three mismatched bases. We used these two sequences at three different concentrations (100 nM, 1 μM, 5 μM). As can be seen in Fig. , for all the cDNA concentrations tested, the cDNA led to lower fluorescence intensity in the supernatant than the mismatched ssDNA, confirming that the base-matched cDNA binds more effectively to the aptamer on the MBs.