Photon-upconverting nanoparticles (UCNPs) emit light of shorter wavelength under near-infrared excitation and thus avoid optical background interference. We have exploited this unique photophysical feature to establish a sensitive competitive immunoassay for the detection of the pharmaceutical micropollutant diclofenac (DCF) in water. The so-called upconversion-linked immunosorbent assay (ULISA) was critically dependent on the design of the upconversion luminescent detection label. Silica-coated UCNPs (50 nm in diameter) exposing carboxyl groups on the surface were conjugated to a secondary anti-IgG antibody. We investigated the structure and monodispersity of the nanoconjugates in detail. Using a highly affine anti-DCF primary antibody, the optimized ULISA reached a detection limit of 0.05 ng DCF per mL. This performance came close to a conventional enzyme-linked immunosorbent assay (ELISA) without the need for an enzyme-mediated signal amplification step. The ULISA was further employed for analyzing drinking and surface water samples. The results were consistent with a conventional ELISA as well as liquid chromatography-mass spectrometry (LC-MS).

CEITEC Central European Institute of Technology Masaryk University Brno 625 00 Czech Republic

Chair of Analytical Chemistry and Institute of Hydrochemistry Technical University of Munich 81377 Munich Germany

Institute of Analytical Chemistry AS CR v v i Brno 602 00 Czech Republic

Institute of Analytical Chemistry Chemo and Biosensors University of Regensburg 93040 Regensburg Germany

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