The SARS-CoV-2 virus, which originated in Wuhan in late 2019, caused a global coronavirus pandemic. To assess acquired immunity to this virus, diagnostic tools are needed that can quickly, efficiently, and accurately detect antibodies against it. The SARS-CoV-2 structure is composed of four essential proteins: spike (S), envelope, membrane, and nucleocapsid. The S protein contains a short immunogenic fragment called the receptor binding domain (RBD) (SARS-CoV-2-RBD), which plays a crucial role in initiating the infection. Among the various diagnostic methods, surface plasmon resonance (SPR) immunosensors are widely used for detecting various viral and bacterial pathogens and are promising for the quantitative detection of SARS-CoV-2 antibodies. SPR immunosensors operating in a label-free direct detection format are very attractive, but often lack sufficient sensitivity. To improve sensitivity, an indirect detection format and signal amplification using nanomaterials as high-mass labels are often used. Although various nanomaterials such as carbon nanotubes, magnetic nanoparticles, or quantum dots can be applied, noble metal nanoparticles, especially gold (AuNDs), are most commonly used. Due to significantly higher mass compared to conventional molecules, nanomaterials cause a much larger change in refractive index and significantly increase the sensitivity of the immunosensor.
This research will introduce the SPR immunosensor for the quantitative analysis of antibodies against the SARS-CoV-2-RBD (anti-RBD). The immunosensor, operating in a direct detection format, was able to detect anti-RBD concentrations in the range of 0.27 – 66.60 nM, with a LOD of 0.27 nM and a LOQ of 0.47 nM. By adopting an indirect sandwich-type detection format and conjugates of biotinylated secondary antibodies against IgG and streptavidin-modified AuNDs, the immunosensor was able to detect anti-RBD antibodies in the concentration range of 0.043 – 10.66 nM, with LOD of 0.043 nM, and LOQ of 0.057 nM. It also demonstrated excellent repeatability, reproducibility, and operational stability. Finally, the immunosensor’s suitability for real sample analysis was confirmed through the quantitative determination of anti-RBD antibodies in human serum samples. This research has received funding from the Research Council of Lithuania (LMTLT), agreement No. S-MIP-22-46.