Characterization of Ultrasmall Superparamagnetic Iron Oxide via Fast Field‐Cycling NMR Relaxometry at Two Temperatures

We report the nuclear magnetic resonance dispersion (NMRD) profile of superparamagnetic iron oxide nanoparticles (SPIONs) and compare the values obtained from its analysis with those from standard methods. Recorded at 1 H Larmor frequencies from 10 kHz to 600 MHz (0.23 mT to 14.1 T, static magnetic field), this profile covers water 1 H dynamics across a broad timescale encompassing all medically relevant magnetic field strengths in magnetic resonance imaging (MRI). This extensive coverage enables the extraction of accurate physicochemical parameters of the nanoparticles that dictate their efficiency (relaxivity). To support this approach, other conventional characterization methods, including TEM, DLS/zeta potential, and VSM, were employed and compared with the NMRD findings. To ensure robustness, the NMRD profiles were recorded at both 25°C and 37°C. Analysis of NMRD profiles, in conjunction with the appropriate theoretical model, successfully characterized SPIONs, yielding coherent parameters that compare favorably with those from conventional characterization methods.