Single-core magnetic markers in rotating magnetic field based homogeneous bioassays and the law of mass action

• A rotating magnetic field based homogeneous bioassay concept was presented.
• Here, single-core iron oxide nanoparticles are applied as markers.
• The impact of the particle concentration on the bioassay results is investigated.
• The relation between particle concentration and bioassay sensitivity is nonlinear.
• This finding can be reasonably explained by the law of mass action.

In this work, we report on the effect of the magnetic nanoparticle (MNP) concentration on the quantitative detection of proteins in solution with a rotating magnetic field (RMF) based homogeneous bioassay. Here, the phase lag between 30 nm iron oxide single-core particles and the RMF is analyzed with a fluxgate-based measurement system. As a test analyte anti-human IgG is applied which binds to the protein G functionalized MNP shell and causes a change of the phase lag. The measured phase lag changes for a fixed MNP and a varying analyte concentration are modeled with logistic functions. A change of the MNP concentration results in a nonlinear shift of the logistic function with the analyte concentration. This effect results from the law of mass action. Furthermore, the bioassay results are used to determine the association constant of the binding reaction.