Electron spin resonance spectroscopy for immunoassay using iron oxide nanoparticles as probe.

Abstract

With the help of iron oxide nanoparticles, electron spin resonance spectroscopy (ESR) was applied to immunoassay. Iron oxide nanoparticles were used as the ESR probe in order to achieve an amplification of the signal resulting from the large amount of Fe3+ ion enclosed in each nanoparticle. Rabbit IgG was used as antigen to test this method. Polyclonal antibody of rabbit IgG was used as antibody to detect the antigen. Iron oxide nanoparticle with a diameter of either 10 or 30nm was labeled to the antibody, and Fe3+ in the nanoparticle was probed for ESR signal. The sepharose beads were used as solid phase to which rabbit IgG was conjugated. The nanoparticle-labeled antibody was first added in the sample containing antigen, and the antigen-conjugated sepharose beads were then added into the sample. The nanoparticle-labeled antibody bound to the antigen on sepharose beads was separated from the sample by centrifugation and measured. We found that the detection ranges of the antigen obtained with nanoparticles of different sizes were different because the amount of antibody on nanoparticles of 10nm was about one order of magnitude higher than that on nanoparticles of 30nm. When 10nm nanoparticle was used as probe, the upper limit of detection was 40.00μgmL-1, and the analytical sensitivity was 1.81μgmL-1. When 30nm nanoparticle was used, the upper limit of detection was 3.00μgmL-1, and the sensitivity was 0.014 and 0.13μgmL-1 depending on the ratio of nanoparticle to antibody. Graphical abstract Schematic diagram of procedure and ESR spectra.