Gold nanoparticle core-europium(iii) chelate fluorophore-doped silica shell hybrid nanocomposites for the lateral flow immunoassay of human thyroid stimulating hormone with a dual signal readout

Hybrid nanocomposite particles composed of a gold core coated with a europium(iii)-chelate fluorophore-doped silica shell (AuNPs@SiO₂-Eu³⁺) have been synthesized and applied as antibody labels in lateral flow immunoassay (LFIA) devices for the determination of human thyroid stimulating hormone (hTSH). Labeling of monoclonal anti-hTSH antibodies with AuNPs@SiO₂-Eu³⁺ nanocomposites allows for both colorimetric and fluorometric observation of assay results on LFIA devices, relying on visible light absorption due to the localized surface plasmon resonance of the Au-core and the fluorescence emission of the Eu(iii)-chelate-modified shell under UV hand lamp irradiation (365 nm), respectively. The possibility for a dual signal readout provides an attractive alternative for LFIAs: instantaneous naked eye observation of the AuNP colorimetric signal as in conventional LFIAs for hypothyroidism detection, and more sensitive fluorescence detection to assess hyperthyroidism. The limits of detection (LOD) for naked eye observation of LFIA devices are 5 μIU mL^-1 and 0.1 μIU mL^-1 for the colorimetric and fluorimetric detection, respectively. Using the fluorescence detection scheme in combination with a smartphone and digital color analysis, a quantitative linear relationship between the red intensity and the logarithmic concentration of hTSH was observed (R² = 0.988) with an LOD of 0.02 μIU mL^-1. Finally, LFIA devices were effectively applied for detecting hTSH in spiked diluted human serum with recovery values between 100-116%.