Label-Free Immunoassay for Sensitive and Rapid Detection of the SARS-CoV-2 Antigen Based on Functionalized Magnetic Nanobeads with Chemiluminescence and Immunoactivity

Direct detection of SARS-CoV-2 in biological specimens is often challenging due to the low abundance of viral components and lack of enough sensitivity. Herein, we developed a new type of chemiluminescent functionalized magnetic nanomaterial for sensitive detection of the SARS-CoV-2 antigen. First, HAuCl₄ was reduced by N-(aminobutyl)-N-(ethylisoluminol) (ABEI) in the presence of amino magnetic beads (MB-NH₂) to generate ABEI-AuNPs, which were directly assembled on the surface of MB-NH₂. Then, Co²⁺ was modified onto the surface to form MB@ABEI-Au/Co²⁺ (MAA/Co²⁺). MAA/Co²⁺ exhibited good chemiluminescence (CL) and magnetic properties. It was also found that it was easy for the antibody to be connected with MAA/Co²⁺. Accordingly, MAA/Co²⁺ was used as a sensing interface to construct a label-free immunoassay for rapid detection of the N protein in SARS-CoV-2. The immunoassay showed a linear range from 0.1 pg/mL to 10 ng/mL and a low detection limit of 69 fg/mL, which was superior to previously reported methods for N protein detection. It also demonstrated good selectivity by virtue of magnetic separation, which effectively removed a sample matrix after immunoreactions. It was successfully applied for the detection of the N protein in spiked human serum and saliva samples. Furthermore, the immunoassay was integrated with an automatic CL analyzer with magnetic separation to detect the N protein in patient serums and rehabilitation patient serums with satisfactory results. Thus, the CL immunoassay without a complicated labeling procedure is sensitive, selective, fast, simple, and cost-effective, which may be used to combat the COVID-19 pandemic. Finally, the CL quenching mechanism of the N protein in the immunoassay was also explored.