Nowadays, there is an increasing demand for more accessible routine diagnostics for patients with respect to high accuracy, ease of use, and low cost. However, the quantitative and high accuracy bioassays in large hospitals and laboratories usually require trained technicians and equipment that is both bulky and expensive.In addition, the multi-step bioassays and long turnaround time could severely affect the disease surveillance and control especially in pandemics such as influenza and COVID-19. In view of this, a portable, quantitative bioassay device will be valuable in regions with scarce medical resources and help relieve burden on local healthcare systems. Herein, we introduce the MagiCoil diagnostic device, an inexpensive, portable, quantitative and rapid bioassay platform based on magnetic particle spectrometer (MPS) technique. MPS detects the dynamic magnetic responses of magnetic nanoparticles (MNPs) and uses the harmonics from oscillating MNPs as metrics for sensitive and quantitative bioassays. This device does not require trained technicians to operate and employs a fully automatic, one-step, wash-free assay with user friendly smartphone interface. Using a streptavidin-biotin binding system as a model, we show that the detection limit of the current portable device for streptavidin is 64 nM (equal to 5.12 pmole). In addition, this MPS technique is very versatile and allows for the detection of different diseases just by changing the surface modifications on MNPs. It's foreseen that this kind of portable device can transform the multi-step, laboratory-based bioassays to one-step field testing in non-clinical settings such as schools, homes, offices, etc.

In recent years, magnetic particle spectroscopy (MPS) has emerged as a new technology for immunoassay applications. In MPS, alternating magnetic fields are applied to magnetic nanoparticles (MNPs). The magnetic responses of these nanoparticles are collected and recorded by a pair of specially designed pick-up coils. These magnetic responses contain higher harmonics that are specific to the physical changes of the nanoparticles such as the binding events of target analytes to nanoparticles. This volumetric-based bioassay method analyses the response signal from the whole nanoparticle suspension, thus, allows one step and wash-free immunoassay with minimum technical requirements. In this work, we developed a handheld MPS system as a future highly sensitive, cheap, in vitro, and easy-to-use point-of-care (POC) detection kit.