Immobilization-Induced Anomalous Dynamic Magnetization Effect in Synomag Nanoparticles as Tracers for Magnetic Particle Imaging

Magnetic particle imaging (MPI) is based on the dynamic nonlinear magnetic response of magnetic nanoparticles (MNPs) used as a tracer and has gained considerable attention in medical research. An example with great potential is the tracking of MNP-labeled cells. The MPI image quality not only crucially relies on the magnetic properties of the MNPs but is at the same time impacted by the interaction of the MNPs with their physiological environment. Most often, these interactions lead to a reduction of the MPI image quality that is ascribed to the reduced dynamic response of the magnetic moments of the MNPs if they are exposed to immobilization or aggregation. Surprisingly, for the commercial iron oxide MNP system Synomag, a multicore system with a nominal hydrodynamic diameter of 30 nm and a citrate coating, the opposite behavior is observed; the dynamic magnetic response is increased after immobilization or after cellular uptake. To gain insight into this anomalous behavior, we investigated a broad range of Synomag systems with different core sizes, hydrodynamic sizes, and coatings to identify possible causes of MPI signal changes after immobilization and cellular uptake. To this end, a thorough characterization of the MNP systems is carried out using magnetic particle spectroscopy (MPS), DC magnetization (DCM), dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS). From the results, relations of two characteristic parameters describing MPS spectra as a function of magnetic moment as well as hydrodynamic diameter are derived. Furthermore, cluster SAXS analysis of immobilized MNP samples suggest that dipole–dipole interactions between closely packed particles degrade the dynamic magnetic response, whereas it leads to a signal enhancement in MPI when the cluster structures are more loosely packed. These results are of importance for the identification of tracers with superior signaling capabilities for MPI cell tracking applications.