Label‐free optical antibody testing kit based on a self‐assembled whispering‐gallery‐mode microsphere

The appearance of antibodies in blood is a critical signal to suggest the infection. A rapid and accurate detection method for the antibody is significant to the disease diagnosis, especially for the epidemic. To this end, a highly sensitive whispering‐gallery‐mode (WGM) optical testing kit is designed and fabricated for detecting the specific immunoglobulin antibodies. The key component of the kit is a silica self‐assembled microsphere decorated with the nucleocapsid proteins (N‐proteins) of the SARS‐CoV‐2 virus. After the N‐protein antibody immunoglobulin G (N‐IgG) and immunoglobulin M (N‐IgM) solutions being injected into the kit, the WGM red‐shifts due to the antigen–antibody reaction. The wavelength displacement rates are proportional to the concentrations of these two antibodies from 1 to 100 μg/mL. A good specificity of the kit is demonstrated by the nonspecific human immunoglobulin G (H‐IgG) and immunoglobulin M (H‐IgM).