Immunoassay of SARS-CoV-2 nucleocapsid proteins using novel red emission-enhanced carbon dot-based silica spheres.

It is imminent to develop a new type of rapid COVID-19 detection method with high sensitivity. Here, we used novel red emission-enhanced carbon dot (CD)-based silica (RCS) spheres as the signals of lateral flow immunochromatography (LFI) to ultrasensitively detect novel severe acute respiratory syndrome coronavirus 2 nucleocapsid proteins (SARS-CoV-2 NPs). The red emission of CDs can be enhanced and enriched in silica spheres by a simple way. The amino ends of the N-ß-(aminoethyl)-γ-aminopropyltrimethoxy anchor carboxyl-rich CDs and enhance the red emission, while the other end is embedded in the silica carrier. Then, the composite silica spheres werecoated with 3-(triethylsilyl) propylamine to protect the CDs, promote bioconjugation and obtain RCS spheres. The optimal emission peaks of the aqueous solution and the solid state of RCS spheres were at 634 nm and 638 nm, respectively, with quantum yields (QYs) of 48.5% and 35.7%, respectively. Their red emission has a wide excitation range (from the ultraviolet region to the red region), and the best excitation wavelength is about 580 nm. Two fluorescence detection modes of the RCS-LFI technology for the SARS-CoV-2 NP assay are available: the simple mode of observation under ultraviolet light has a sensitivity of 100 pg mL-1; the advanced mode of detection under a fluorescence microscope has a sensitivity of 10 pg mL-1. This assay also exhibits the advantages of fast detection speed, high specificity, and simple operation. In addition, the feasibility of this method in actual sample detection was verified in human serum by the standard-addition method, and the results show that the method has excellent practicability. We believe that this method will be a valuable supplement for the diagnosis of COVID-19.

Development of a lateral flow immunoassay strip for rapid detection of IgG antibody against SARS-CoV-2 virus.

The ongoing worldwide SARS-CoV-2 epidemic clearly has a tremendous influence on public health. Molecular detection based on oral swabs was used for confirmation of SARS-CoV-2 infection. However, high false negative rates were reported. We describe here the development of a point-of-care (POC) serological assay for the detection of IgG antibody against SARS-CoV-2. The principle of a lateral flow immunoassay strip (LFIAs) consists of fixing SARS-CoV-2 nucleocapsid protein to the surface of the strip and coupling anti-human IgG with colloidal gold nanoparticles (Au NPs). A series of parameters of this method were optimized, including the concentration of coating antigen, BSA blocking concentration and pH value for conjugation. The entire detection process took 15-20 min with a volume of 80 µL of the analyte solution containing 10 µL of serum and 70 µL sample diluent. The performance of the established assay was evaluated using serum samples of the clinically diagnosed cases of Coronavirus Disease 2019 (COVID-19). Our results indicated that the LFIAs for SARS-CoV-2 had satisfactory stability and reproducibility. As a result, our fast and easy LFIAs could provide a preliminary test result for physicians to make the correct diagnosis of SARS-CoV-2 infections along with alternative testing methods and clinical findings, as well as seroprevalence determination, especially in low-resource countries.