Au-coated Fe3O4 core-shell nanohybrids with photothermal activity for point-of-care immunoassay for lipoprotein-associated phospholipase A2 on a digital near-infrared thermometer.

A portable photothermal immunoassay based on Au-coated magnetic Fe3O4 core-shell nanohybrids (Au-Fe3O4) was developed for point-of-care (POC) testing of lipoprotein-associated phospholipase A2 (Lp-PLA2) on a digital near-infrared (NIR) thermometer. Au-Fe3O4 photothermal materials were first synthesized through reverse micelle method, and then functionalized with polyclonal rabbit anti-human Lp-PLA2 antibody. A sandwiched immunoreaction was carried out in polyclonal mouse anti-human Lp-PLA2 antibody-coated microplate using Au-Fe3O4-labeled antibody as the detection antibody. With formation of sandwich-type immunocomplex, the captured Au-Fe3O4 on the plate converted the light into heat under an 808-nm laser irradiation (1.5 W cm-2), thereby resulting in the increasing temperature of the detection solution. The temperature variations relative to surrounding temperature was determined on a portable NIR thermometer. Several labeling protocols with gold nanoparticle, Fe3O4 nanoparticle, or Au-Fe3O4 nanohybrids were investigated for determination of Lp-PLA2 and improved analytical features were achieved with the core-shell Au-Fe3O4 nanohybrids. Under optimum conditions, Au-Fe3O4-based immunoassay exhibited good photothermal responses for the detection of Lp-PLA2 with a dynamic linear range of 0.01-100 ng mL-1 at a low detection limit of 8.6 pg mL-1. Good reproducibility and intermediate precision were less than 9.7%. Other biomarkers or proteins did not interfere with responses of this system. An acceptable accuracy was acquired for analysis of human serum sample between Au-Fe3O4-based photothermal immunoassay and commercialized human Lp-PLA2 ELISA kit.