Decoupling the Characteristics of Magnetic Nanoparticles for Ultrahigh Sensitivity.

Immunoassays exploiting magnetization dynamics of magnetic nanoparticles are highly promising for mix-and-measure, quantitative, and point-of-care diagnostics. However, how single-core magnetic nanoparticles can be employed to reduce particle concentration and concomitantly maximize assay sensitivity is not fully understood. Here, we design monodisperse Néel and Brownian relaxing magnetic nanocubes (MNCs) of different sizes and compositions. We provide insights into how to decouple physical properties of these MNCs to achieve ultrahigh sensitivity. We find that tricomponent-based ZnCoFeO particles, with out-of-phase to initial magnetic susceptibility χ″/χ ratio of 0.47 out of 0.50 for magnetically blocked ideal particles, show the ultrahigh magnetic sensitivity by providing a rich magnetic particle spectroscopy (MPS) harmonics spectrum despite bearing lower saturation magnetization than dicomponent ZnFeO having high saturation magnetization. The ZnCoFeO MNCs, coated with catechol-based poly(ethylene glycol) ligands, measured by our benchtop MPS show 3 orders of magnitude better particle LOD than that of commercial nanoparticles of comparable size.