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 an ultrahigh sensitivity. We find that a tri-component-based Zn0.06Co0.80Fe2.14O4 particles, with out-of-phase to initial magnetic susceptibility χ^''/χ_0 ratio of 0.47 out of nominal ratio of 0.50 for thoroughly magnetically blocked particles, show the ultrahigh magnetic sensitivity by providing rich magnetic particle spectroscopy harmonics spectrum despite bearing a lower saturation magnetization value than di-component Zn0.1Fe2.9O4 with a high value of saturation magnetization. The Zn0.06Co0.80Fe2.14O4 MNCs, coated with polyethylene glycol-based ligands, show three orders of magnitude better sensitivity than commercially available particles of comparable size.