Molecularly Restructured Artificial Co-Magnetosomes with Improved Magnetic Properties as a Tracer for Magnetic Particle Imaging

Superparamagnetic iron oxide nanoparticles (SPIONs) have found extensive applications in various biomedical fields. The interest in modifying SPIONs through doping with other transition metals has continued to gain ongoing interest due to their potential to alter magnetic properties, including magnetic anisotropy and saturation magnetization. This provides a promising avenue to customize nanoparticles for magnetic field-assisted clinical applications. However, it is important to understand how doping of transition metals in SPIONs and how the molecular structure of the SPIONs' surface postdoping affect their magnetic performance. To study these aspects, we produced oleic acid-capped SPIONs (org-SPIONs) and their 2.5% cobalt (Co)-doped version (org-Co-SPIONs) that were dispersible in an organic solvent. We also created artificial liposome-encapsulated versions of these particles (referred to as magnetosomes and Co-magnetosomes) by a facile phase transfer approach to make these particles dispersible in an aqueous medium. All particles were of similar size (similar to 12.8 nm) with a narrow size distribution, which allowed us to compare their performance for magnetic particle imaging (MPI) application. Our results demonstrate that upon doping of org-SPIONs with 2.5% Co (atomic percentage relative to the total metal content in the particles) (org-SPIONs vs org-Co-SPIONs), even though their saturation magnetization improves marginally, their MPI sensitivity and resolution deteriorate significantly. Conversely, the comparison of their liposomal formulations (magnetosomes vs Co-magnetosomes) reveals an improvement in their saturation magnetization as well as in MPI resolution and sensitivity. Interestingly, the degree of change in the saturation magnetization induced by the same aqueous phase transfer process for SPIONs vs Co-SPIONs was found to be remarkably different. In the case of SPIONs, the magnetic properties, MPI sensitivity, and MPI resolution improved by 7.1, 15.9, and 9.5%, respectively, in phase-transferred particles, whereas these parameters improved by 6.6, 83.9, and 27% for Co-SPIONs. While the influence of transition metal doping on the magnetic properties of SPIONs is regularly studied, our results provide valuable insights into the important role played by surface capping of magnetic materials in their performance.