General

All reagents were obtained from commercial vendors and used without any further purification. Phosphate buffer (PB, 0.1 M, pH 7.0), phosphate buffer saline (PBS, 0.01 M, pH 7.4) and carbonate buffer (0.1 M, pH 9.5) were purchased from Aladdin, Shanghai, China. Recombinant SARS-CoV-2 spike RBD-His-Avi (host cell RBD) with N- terminal His tag (Catalog # {"type":"entrez-nucleotide","attrs":{"text":"BP003052","term_id":"19494529","term_text":"BP003052"}}BP003052) and human ACE2-His-Avi (Catalog # {"type":"entrez-nucleotide","attrs":{"text":"BP003061","term_id":"19494538","term_text":"BP003061"}}BP003061) were purchased from Syd Labs, Boston, Massachusetts, USA. N-Bromosuccinimide (NBS, 0.1 M) was obtained from Sigma-Aldrich Chemie GmbH, Steinheim, Germany (Catalog # B9252). Human Serum Albumin (10%) was purchased from CSL Behring AG, Bern, switzerland (Catalog # B14200811350). PD-10 columns were from GE Healthcare, Buckinghamshire, England (catalog # 17001651). The isotope ¹²⁴I was self-produced by our laboratory using ¹²⁴Te (p, n) ¹²⁴I nuclear reaction, the activity concentration is 370 kBq/μL, and the production situation is detailed in the literature [10]. The 96-well polystyrene Stripwell^TM microplate was purchased from Corning Costar, New York, USA (Catalog # 2481); radioactive thin layer chromatography scanner (radio-TLC) was from Bioscan, Poway, California, USA (Type # AR2000); the Hypersil BDS C-18 reverse chromatographic column (5 μm, 250 mm × 4.6 mm) was from YMC Co., Ltd, kyoto, Japan. Fully automatic gamma counter was from Perkin Elmer, Waltham, Massachusetts, USA (Type # Wizard2). The radioactivity measurement activity meter was from Capintec, Ramsey, New Jersey, USA (Type # CRC-25R).