Photon-upconverting nanoparticles (UCNPs) have become an attractive label in immunoassays because their anti-Stokes luminescence can be excited by the NIR laser and detected in the VIS region without optical background interference. Further advantages of UCNPs include good photostability, large anti-Stokes shifts, and multiple narrow emission bands that can be used for multiplexed detection. We have developed a competitive upconversion-linked immunosorbent assay (ULISA) for detection of the pharmaceutical diclofenac (DCF) in surface waters. Silica-coated UCNPs (50 nm in diameter) with carboxyl groups on the surface were synthesized and conjugated with the secondary anti-IgG antibody. The structure and monodispersity of the nanoconjugates was studied by TEM and agarose gel electrophoresis. Using a highly affine anti-DCF primary antibody, the optimized ULISA provided a detection limit of 50 pg·mL−1.Keywords: photon-upconversion nanoparticle; immunoassay; bioconjugation; antibody; diclofenac
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