Characterizing Physical Properties of Superparamagnetic Nanoparticles in Liquid Phase Using Brownian Relaxation

Superparamagnetic iron oxide nanoparticles (SPIONs) have been extensively used as bioimaging contrast agents, heating sources for tumor therapy, and carriers for controlled drug delivery and release to target organs and tissues. These applications require elaborate tuning of the physical and magnetic properties of the SPIONs. The authors present here a search‐coil‐based method to characterize these properties. The nonlinear magnetic response of SPIONs to alternating current magnetic fields induces harmonic signals that contain information of these nanoparticles. By analyzing the phase lag and harmonic ratios in the SPIONs, the authors can predict the saturation magnetization, the average hydrodynamic size, the dominating relaxation processes of SPIONs, and the distinction between single‐ and multicore particles. The numerical simulations reveal that the harmonic ratios are inversely proportional to saturation magnetizations and core diameters of SPIONs, and that the phase lag is dependent on the hydrodynamic volumes of SPIONs, which corroborate the experimental results. Herein, the authors stress the feasibility of using search coils as a method to characterize physical and magnetic properties of SPIONs, which may be applied as building blocks in nanoparticle characterization devices.