Low-Cost Magnetic Particle Spectroscopy Hardware for Low-Viral-Load Immunoassays

The use of magnetic nanoparticles (MNPs) with appropriate surface functionalization as markers in immunoassays for the detection of specific analytes, such as viruses, proteins, or antibodies, has received increased attention due to the recent pandemic. In combination with magnetic particle spectroscopy (MPS) as an evaluation method, this approach represents a promising alternative to the established testing methods. MPS measures the harmonics of the rotating MNPs, whose dynamics are the characteristics of their binding state. In this article, we discuss the development of the immunoMPS, an immunoassay-optimized as well as cost-effective MPS system, to take a big step toward higher sensitivity. To achieve this, simulations based on the Fokker-Planck equation were performed to optimize the excitation parameters for the used 80-nm-sized Bionized NanoFerrite particle system (BNF80), aiming at the detection of tiny changes in the MNP dynamics. The constructed immunoMPS only has material costs of 350 € in total, and it is mobile and self-contained and, therefore, can be used in S2+ biosafety laboratories. Despite its low price, it can detect down to 40.6-ng iron with BNF80 MNPs. The high performance of the instrument is further illustrated by our magnetic immunoassays (MIAs) with a significantly improved limit of detection (LOD) of
$3\times 10^{8}$
viruses/mL (552 fM, 33.1 amole) for the detection of mimic SARS-CoV-2. This is an improvement of about two orders of magnitude compared with previous results reported in this research topic.