Towards sensitive Magnetic Particle Spectroscopy-based immunoassays with adjustable DC offset fields

Abstract

The field of homogenous magnetic immunoassays (MIAs) using magnetic nanoparticles (MNPs) as markers has been intensively studied over the past few years. The specific and sensitive measurement of their binding state is of great interest as it can be an alternative to the commonly used methods like rapid antigen tests or PCR. Several methods using various forms of Magnetic Particle Spectroscopy (MPS) are reported in literature. A common approach to achieve higher sensitivities in MPS is to use an additional magnetic DC offset field providing even harmonics or steep phase changes. However, the potential of the additional degree of freedom given by an adjustable DC offset field (ADOF) in particular is often overlooked.
Here, we present the advantages of an ADOF to find more sensitive harmonic ratios as a concentration independent metric for the binding status. In order to get a better overview of the DC field dependence for different hydrodynamic sizes, Fokker-Planck simulations for Brownian relaxation were carried out. Subsequently, the successor to our cost-effective immunoMPS with an ADOF was built, focusing on a spectrally pure magnetic excitation field and the decoupling between AC and DC paths. Initial measurements were performed on MNPs in different viscous media and compared to the simulation showing good agreement. In the next step, we are going to synthesize and measure different MNP clusters sizes before we move on to test this principle on real biological targets.