Fig. 3 

Example of COVID-19 lateral flow immunoassays (LFIA). a For antigen detection, a sample containing viral antigens is dropped on the sample pad and flows by capillary action up to the absorbent pad. The sample with viral N protein (NP) directly binds to the anti-NP Ab conjugated with nanoparticles, such as colloidal gold particles or latex nanocomposites. Then, the nanoparticle-conjugated immunocomplexes are released from the conjugation pad. The free epitope of NP is captured to the second anti-NP Ab in the test line. Unbound conjugated Abs will be recognized by immobilized anti-mouse IgG in the control line. b For Ab detection, patient serum or plasma specimens are dropped on the sample pad. The sample fluid flows through the conjugation pad which contains nanoparticle-conjugated SARS-CoV-2 spike (S) or N proteins to form antigen-Ab immunocomplexes. The immunocomplexes flow to the test line and are then captured by specific isotype immunoglobulins such as IgM and IgG. Unbound control nanoparticle-conjugated mouse IgG is captured by anti-mouse IgG at the control line