Collectively Interacting Colloidal Magnetic Nanoparticles with Strong AC Field Response for Remote Imaging and Thermometry

Colloidal magnetic nanoparticles (MNPs) are an important class of nanomaterials being investigated for use in a host of therapeutic and diagnostic modalities such as medical imaging, remote sensing, drug delivery, and hyperthermia.¹ These applications exploit the very soft ferrimagnetic magnetic behavior found in certain materials, often ferrites, when they are confined to tens of nanometers in diameter or less. Such nanomagnets can produce a strong collective response to applied alternating current (AC) magnetic fields, while simultaneously remaining dispersed in liquid media. <br/> <br/>Recently, remote magnetic imaging of temperature has been identified as an exciting potential diagnostic application of colloidal MNPs. This thermometry measurement employs dispersed particles to construct a 3D visual representation of temperature throughout a volume. The technique, which is a variation on the magnetic particle imaging (MPI) modality, is based on the temperature-dependent response of MNPs to an applied AC magnetic field. Additionally, when applying high-frequency magnetic fields, these MNPs can also generate localized heating, thus allowing them to function as nanotheranostics. However, significant challenges remain in the development of this technology, including a need to finely engineer MNPs to increase both their magnetic thermosensitivity and magnitude of AC response.<br/> <br/>Here, we report on our development of MNPs, both as liquid dispersions and solid assemblies, specifically designed with a robust response to applied AC magnetic fields for imaging, thermometry, and temperature control. A series of colloidal nanocrystals based on ferrites were synthesized<i> via</i> highly tailorable and commercializable solution chemistry routes.² Careful selection of the synthetic reagents and precursor thermal decomposition kinetics allow for control of the resultant MNP composition (Fe, Co, Zn, V ratios) and size (5-80 nm). These MNPs were then investigated using solid- and liquid-phase AC and DC field magnetometry measurements. A home-built arbitrary-wave magnetic particle spectrometer was employed to perform relaxometry and measure magnetic AC susceptibility, allowing for a rigorous analysis of the temperature- and frequency-dependent AC response from 1 Hz to 30 MHz. The results were cross-correlated with detailed structural characterization including X-ray diffraction, light scattering, optical spectroscopy, and high-resolution electron microscopy to develop a set of structure-property relationships.<br/> <br/>Finally, we tune these parameters to optimize spatial imaging of MNP magnetic response and a robust thermosensitive AC magnetic signal using magnetic particle imaging and thermometry instrumentation specifically developed for these applications.³ We find particle size and compositional doping act as levers to optimize the AC response through the manipulation of interparticle and intraparticle magnetic spin interactions. Using these guidelines, we find the set of structural parameters that produce the most robust thermosensitive magnetic response and conduct imaging experiments whereby spatial reconstructions of colloidal MNP magnetic response are generated. The key to this improved performance, in both liquid and solid media, is the field-induced linear alignment of strongly interacting MNPs, resulting in a substantially augmented magnetic anisotropy. Collectively, these studies illuminate the complex behavior of MNPs under AC driving fields, reveal extensive correlations between nanoscale structure and magnetic response, and provide guidelines for the design of MNPs used in magnetic imaging, thermal sensing, and therapeutic applications.<br/> <br/>[1] Shasha and Krishnan<i> Adv. Mater.</i>, 33, 1904131 (2021)<br/>[2] Biacchi <i>et al</i>. <i>Int. J. Magn. Part. Imaging</i>, 6, 2 Suppl 1 (2020); Biacchi <i>et al</i>. <i>Chem. Mater.</i>, 34, 2907-<br/> 2918 (2022)<br/>[3] Bui <i>et al. J. Appl. Phys</i>., 128, 224901 (2020); Bui <i>et al.</i> <i>Appl. Phys. Lett.</i>, 120, 012407 (2022)