Comparison of magnetic nanoparticle immobilization methods using MPS and MRX

Magnetic nanoparticles (MNP) are employed in many technical and clinical applications for which the knowledge of their magnetic properties is mandatory. This comprises not only the intrinsic magnetic parameters, but especially in biomedical applications the changes of the MNP behavior in a certain environment or binding state such as the immobilization of the MNP after cellular uptake, blood contact or injection into tissue. Therefore, the magnetic properties of MNP samples that have been immobilized are considered to mimic the binding of MNP to human tissue. However, the procedure and materials used for immobilization differently impact their magnetic properties. Here, we investigated common immobilization methods regarding reproducibility and variation in terms of the magnetic properties of the commercial nanoparticle system perimag® for the three available surface coatings. We considered immobilization by polyacrylamide embedding, freeze drying, gypsum crystallization, filter paper as well as cotton wool drying. We used the two magnetic measurement techniques magnetic particle spectroscopy (MPS) and magnetorelaxometry (MRX) to assess the magnetic properties and changes thereof for all immobilization methods. We found highest reproducibility and less variation in magnetic parameters for freeze dried and gypsum immobilization. A higher variation of magnetic properties was observed for evaporation-based methods filter paper and cotton wool attributed to unstructured arrangement of MNP on the fibers of the immobilization materials. The surface modification of the MNP system shows a minor impact on immobilization procedure. From our results, we deduce recommendations and practicability considerations for immobilization methods in preparation and handling of MNP reference samples.