Estimating saturation magnetization of superparamagnetic nanoparticles in liquid phase

Superparamagnetic nanoparticles (SPMNPs), with unique physical and magnetic properties that differentiate them from their bulk magnetic materials, have been widely studied for potential applications in biomedical areas. With proper surface chemical functionalization, SPMNPs have found their applications in magnetic hyperthermia therapy, magnetic bioassays, drug delivery, magnetic manipulation, etc. These applications require elaborate tuning of the physical and magnetic properties of the SPMNPs such as saturation magnetization Ms and magnetic core size D. In this work, we present a search coil-based method to directly characterize the Ms of SPMNPs in the liquid phase. The nonlinear magnetic responses of SPMNPs under oscillating magnetic fields are exploited and the induced harmonic signals are used to analyze their Ms and D. Different combinations of Ms and D are assumed and their harmonic ratios R are summarized. Curve fitting shows that the harmonic ratio R=0.74+2.85×109∙D-4.41∙Ms-1.44, with the coefficient of determination R-square = 0.98.