Immobilization-Induced\nAnomalous Dynamic Magnetization\nEffect in Synomag Nanoparticles as Tracers for Magnetic Particle Imaging

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