Plasmonic and quantitative lateral flow assay for grayscale imaging of neutralizing antibodies SARS-CoV-2

Lateral flow immunoassays (LFIAs) have modernized decentralized mass testing and significantly impacted the healthcare system. It provides a rapid and accessible method for serological testing, which is an important tool for screening patients with past viral phases by detecting antibodies. Here, the development of a quantitative LFIA is presented, in which the receptor-binding domain of Spike protein-specific antibody (anti-S-RBD) in serum samples from previously infected with SARS-CoV-2 patients form an immune complex with the RBD protein immobilized on gold nanoparticles. For quantitative measurements, a methodology was developed based on the plasmonic behavior of gold nanoparticles and using a digital camera that processes images of the test lines in a closed chamber using the CYMK color system. The limits of detection and quantification were 0.22 and 0.70 µg mL^–1, respectively. The proposed device has a manufacturing cost of less than US$ 1 from synthesizing the bioconjugates, assembly of the LFIA strips, and 3D printing of the cassettes. The platform could detect anti-S-RBD antibodies in human serum samples and can be particularly useful in strategies for monitoring IgG levels in the population.