Using the optimized conditions, we studied the dose-response curves of the intensity and the logarithm of RBD concentration in the range of 0.001-100 ng mL -1 , with a detection limit of 0.11 pg mL -1 (S/N=3). Using the colorimetric method, we obtained a similar calibration curve for detection and quantification of RBD. As shown in Fig. , the absorbance of the assay solution increased linearly with increasing RBD concentration, in the semi-logarithmic plot in the range of 0.001 to 100 ng mL -1 . The limit of detection of the colorimetric assay is 0.904 pg mL -1 (S/N=3). To evaluate the selectivity of the two analytical methods, we replaced RBD with a series of SARS-related proteins and carried out the protein assays. As shown in Fig. , the fluorescence and absorbance signals caused by all the other proteins are much weaker than SARS-CoV-2 RBD. Obviously, the selectivity of the present protein assays is due to the selective molecular recognition (RBD binding) provided by the aptamer. The RCA provides effective amplification of the molecular binding signal, making it possible to detect RBD at a very low concentration.