Lanthanide-Doped Upconversion-Linked Immunosorbent Assay for the Sensitive Detection of Carbohydrate Antigen 19-9

Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted considerable attention in detection of biological analytes and bioimaging owing to their superior optical properties, including high photochemical stability, sharp emission bandwidth, large anti-Stokes shifts, and low toxicity. In this work, we fabricated UCNP-linked immunosorbent assay (ULISA) for the sensitive detection of carbohydrate antigen 19-9 (CA19-9). The design is based on amino-functionalized SiO₂-coated Gd-doped NaYF₄:Yb³⁺,Er³⁺ upconversion nanoparticles (UCNPs@SiO₂-NH₂) as a direct background-free luminescent reporter; a secondary anti-IgG antibody (Ab₂) was conjugated to the surface of UCNPs@SiO₂-NH₂ (UCNP-Ab₂), and UCNP-Ab₂ was used for specific targeting of CA19-9. The UCNPs were well characterized by TEM, SEM, XRD, FT-IR, and UV-vis. The detection process was similar to enzyme-linked immunosorbent assay (ELISA). UCNPs were used as signal transducer to replace the color compounds for an enzyme-mediated signal amplification step. An anti-CA19-9 primary antibody (Ab₁) was fixed for capturing the CA19-9, and the fluorescence signal was obtained from the specific immunoreaction between UCNP-Ab₂ and CA19-9. Under optimum conditions, this ULISA shows sensitive detection of CA19-9 with a dynamic range of 5-2,000 U/ml. The ULISA system shows higher detection sensitivity and wider detection range compared with the traditional ELISA for CA19-9 detection. This strategy using UCNPs as signal transducer may pave a new avenue for the exploration of rare doped UCNPs in ELISA assay for clinical applications in the future.