The principle of analyte detection is presented in Scheme 1. In this work we used streptavidin-coated magnetic beads to immobilize biotin-cDNA/aptamer to form MBs-cDNA/aptamer. In the presence of S protein RBD, there is a competition between RBD and the cDNA to bind the aptamer. After a magnetic separation, the solution including the aptamer-S protein RBD can be removed.