Rapid and quantitative detection of COVID-19 markers in micro-liter sized samples

Xiaotian Tan; Cory Lin; Jie Zhang; Maung Kyaw Khaing Oo; Xudong Fan

The assay mechanism and the corresponding protocol are illustrated in Figures 4(A) and S2(D), respectively. Our assay follows a single-step ELISA format. Same as in the IgG detection experiment, recombinant S1 proteins were first immobilized on the supporting surface (2 µg/mL), followed by the binding of IgG. To reduce potential sources of errors, we directly conjugated HRP molecule with purified IgG molecules (molar ratio IgG : HRP = 1 : 4, with the HRP conjugation kit from Abcam). The antibodies were then diluted to six different concentrations (1000 -1 ng/mL with 4× serial dilution) and then applied to the S1 protein-coated ELISA reactors. The immobilized IgG can be quantified after a short 6 minutes of incubation and 4 times of rinsing. Although the antibodies may not reach equilibrium by the end of the incubation, the quantity of the immobilized antibody (also the signal intensity) should still have a positive correlation with the affinity of the antibody.