Why are colored microspheres selected as markers for more and more COVID-19 antigen tests?

A lateral flow test (LFT),[1] also known as a lateral flow device (LFD), lateral flow immunochromatographic assay, or rapid test, is a simple device intended to detect the presence of a target substance in a liquid sample without the need for specialized and costly equipment. LFTs are widely used in medical diagnostics in the home, at the point of care, and in the laboratory. These tests are simple and economical and generally show results in around five to thirty minutes.[2] LFT has also been widely used in COVID-19 rapid antigen detection in many countries, which can quickly give users a result within 5–30 minutes.

Improving the sensitivity, quantitative detection and multiplex detection are the three major directions of the development of LFT, especially the sensitivity is the key factor for LFT development. However, the above important factors are all affected by immunochromatographic markers. In immunochromatographic diagnostic reagents, ideal markers should have the following advantages.

• High sensitivity, ng level, even pg and fg level
• Has no effect on the immune activity, affinity and specificity of antibodies or antigens, and does not reduce the immune response signals
• Uniformity, easy to prepare
• Suitable for stable storage, reducing inter-batch and intra-batch differences.
• Insensitivity to environmental factors
• Safety
• Easy to obtain ideal results and good repeatability

Colloidal gold is a commonly used marker in commercial LFT reagents. Colloidal gold has the advantages of convenient preparation, good stability, and easy labeling. However, its application is limited due to its single color, low sensitivity, inability to covalently cross-link, and narrow quantitative range, etc.

With the advancement of nano- and micro-microsphere technology, dyed latex particles have been widely used as immunochromatographic markers. The particle size of the dyed microspheres used in LFT is larger than that of colloidal gold, so the sensitivity of the dyed microspheres will be significantly improved. Dyed microspheres are rich in color, which is conducive to multivariate analysis and use in OTC products. Through different production processes, dyed microspheres can be coated with amino groups, carboxyl groups, sulfhydryl groups, etc. with different functional groups. Therefore, both passive physical adsorption and covalent chemical cross-linking methods can be used for the labeling of dyed microspheres. For such molecules containing multiple sulfur atoms, labeling with colored latex microspheres works well. Dyed microspheres are suitable for large-scale production, reducing batch-to-batch variation and ensuring stable product performance.