2.1. Immunoreagents, chemicals materials and samples

Recombinant SARS CoV-2 spike protein receptor-binding domain (RBD) (Cat: 40592-V05H) and recombinant human receptor angiotensin converting enzyme 2 (ACE2) (Cat:10108-H08H) were purchased from Sino Biological Inc (Beijing, China). Goat anti-human IgG and IgM and IgA antibodies (ab102416) were purchased from Abcam (Cambridge, England). Carboxyl-functionalized Europium chelate nanoparticles (EuNPs) (Catalog Number FCEU001) were obtained from Bangs Laboratories (IN, USA). Lateral Flow Nitrocellulose membranes (Vivid 120) were obtained from Pall Corporation (Washington, NY,USA). Glass fiber conjugate pads, adsorbent pads, and blood separator sample pads were purchased from Shanghai Kinbio Tech. Co., Ltd (Shanghai, China). Bovine serum albumin (BSA) was supplied by Roche (Basel, Switzerland). Mouse anti-human-RBC antibodies were obtained from Eastmo Biotech Co., Ltd (Beijing, China). 2-(N-morpholino) ethanesulfonic acid (MES), polyvinylpyrrolidone 10 (PVP10), 1-ethyl-3-(3-(dimethylamino) propyl) carbodiimide hydrochloride (EDC), and Proclin-300 were obtained from Sigma-Aldrich (St. Louis, MO, USA). N-Hydroxy-sulfosuccinimide (sulfo-NHS) was obtained from Thermo Fisher Scientific (Waltham, MA USA). Blockmaster™ PA1080 was obtained from the JSR Life Sciences (Sunnyvale, CA, USA). Tween 20 and other chemicals were purchased from Sinopharm. Amicon Ultra Centrifugal Filter Units (10 kDa) were obtained from Merck Millipore (Billerica, MA, USA). The ELISA kit was a blocking ELISA detection assay for the detection of neutralizing antibodies (cPass SARS-CoV-2 Neutralization Antibody Detection Kit, GenScript USA Inc.) (Piscataway, NJ, USA). The first WHO International Standard for anti-SARS-CoV-2 immunoglobulin, human (NIBSC code:20/136), First WHO International Reference Panel for anti-SARS-CoV-2 immunoglobulin, human (NIBSC code: 20/268), and Anti-SARS-CoV-2 Verification Panel for Serology Assay were obtained from the National Institute for Biological Standards and Control (NIBSC), UK.

The solutions were prepared as following: coating buffer (0.5% trehalose [w/v] and 10% SeaBlock [v/v] in 50 mM borate-boric acid buffer, pH 8.5); sample pad pre-treatment buffer (1.5 μg/mL mouse anti human-RBC antibody, 0.02% Tween-20 [v/v], 0.9% NaCl [w/v] in 50 mM Tris-HCl, pH8.0); conjugate buffer (8% trehalose [w/v], 1% PVP10 [w/v], 3% D-mannitol [w/v], and 0.1% BSA [w/v] in 50 mM borate-boric acid buffer, pH 8.5), activation buffer (50 mM MES, pH 5.4), coupling buffer (50 mM borate-boric acid buffer, pH 8.5), and sample diluent (0.02% Tween-20 [v/v], 0.45% NaCl [w/v], 0.3% Blockmaster™ PA1080 [w/v] in 50 mM Tris-HCl, pH 7.8).

FIC-H1W dry fluorescence immunoassay analyzer was obtained from Suzhou Helmen Precision Instruments Co., Ltd. (Suzhou, Jiangsu, China).

Serum samples were obtained from Beijing Tiantan Hospital Capital Medical University. This study was approved by the Beijing Tiantan Hospital Capital Medical University (Beijing, China) (ethics approval number: KY2021-132-02).