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 Fe 3 O 4 core-shell nanohybrids (Au-Fe 3 O 4 ) was developed for point-of-care (POC) testing of lipoprotein-associated phospholipase A 2 (Lp-PLA 2 ) on a digital near-infrared (NIR) thermometer. Au-Fe 3 O 4 photothermal materials were first synthesized through reverse micelle method, and then functionalized with polyclonal rabbit anti-human Lp-PLA 2 antibody. A sandwiched immunoreaction was carried out in polyclonal mouse anti-human Lp-PLA 2 antibody-coated microplate using Au-Fe 3 O 4 -labeled antibody as the detection antibody. With formation of sandwich-type immunocomplex, the captured Au-Fe 3 O 4 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, Fe 3 O 4 nanoparticle, or Au-Fe 3 O 4 nanohybrids were investigated for determination of Lp-PLA 2 and improved analytical features were achieved with the core-shell Au-Fe 3 O 4 nanohybrids. Under optimum conditions, Au-Fe 3 O 4 -based immunoassay exhibited good photothermal responses for the detection of Lp-PLA 2 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-Fe 3 O 4 -based photothermal immunoassay and commercialized human Lp-PLA 2 ELISA kit.