Detection of Protein Biomarkers Using Lateral Flow Tests with Photon-upconversion Nanoparticles

Rapid and sensitive detection of clinically important analytes is crucial for early disease diagnosis and initiation of treatment. Immunochemical assays are widely used to determine the presence and concentration of analytes in various complex media due to their high specificity provided by antibodies. Recently, lateral flow immunoassays (LFIAs) are becoming one of the most popular forms of immunochemical assays. LFIAs are test strips based on nitrocellulose membranes, and their most significant advantage is that there is no need for laboratory equipment to perform the assay enabling their use for point-of-care testing. Most LFIAs are based on gold nanoparticles; however, they often limit the resulting sensitivity of LFIAs. Our work focused on photon-upconversion nanoparticles (UCNPs) as a sensitive alternative label. UCNPs are lanthanide-based nanocrystals exhibiting anti-Stokes luminescence (excitation by the NIR laser and detection in the Vis region), which allows detection without optical background interference. We have developed an LFIA assay for human serum albumin (HSA) with UCNPs as a label. Using a specific anti-HSA antibody and a control anti-mouse antibody, various LFIA parameters were optimized. The immobilization of antibodies was done using acetate buffer with 1% methanol, glass membrane ST17 was used as a conjugate pad, and Tris buffer with 5% sucrose was used to stabilize the particles within the conjugate pad. The choice of a different nitrocellulose membrane had a relatively small effect on assay performance; the best results were achieved with membrane 120 HP Plus Thick. Our work has successfully demonstrated the potential of using UCNPs as sensitive labels in LFIA tests.