Lithographic Synthesis of Anisotropic Nanomagnetic Bars for In-Vivo Magnetic Particle Imaging and Spectroscopy

Magnetic Particle Imaging (MPI) is an imaging modality where a field free point is scanned over magnetic nanoparticle (MNP) tracers generating a temporally resolved spatial map of particle distribution, and velocity [1]. MPI tracers are designed to be biocompatible enabling in-vivo MPI. Additionally, by analyzing the spectral frequency response of MPI tracers as their magnetization reverses, information about their local environment can be extracted (e.g. temperature, viscosity). This approach, termed magnetic particle spectroscopy (MPS), leverages the changes in the Neel and Brownian behavior of a tracer as a function of the external parameter being measured [2].<br/>In MPS the resolution (the spectral information available) is highly dependent on the magnetic properties of the magnetic tracer. To achieve high spatial resolution, the magnetic field range over which a particle reverses its magnetization should be small. For a strong MPS signal, the particle should be physically anisotropic (to have a more pronounced Brownian rotation) and the dynamics of magnetic switching behavior should be independent of drive field frequency.<br/><br/>Guided by nanomagnetic simulations in MuMax3, we lithographically fabricated Permalloy (PY, Ni80Fe20) nanomagnetic bars of varying sizes with a length to width ratio of 5:1 and layered structure that is Ti 5nm/ PY 30nm/ Ti 5nm. Due to shape anisotropy PY structures of these dimensions are strongly uniaxial ferromagnetic with the magnetic hard axis along the length of the bars. These bars were released into solution via dissolution of a sacrificial layer. To enhance colloidal stability and enable biocompatibily, the bars were chemically coated with silica using the modified Stöber method. The silica coating also allowed the bars to be passivated with poly(ethylene glycol) via ester-carbodiimide chemistry. The resultant mixture is colloidally stable, biocompatible lithographically fabricated magnetic nanoparticles.<br/><br/>Initial MPS measurements show that due to the uniaxial ferromagnetic switching behavior demonstrated in the bars, the magnetic field range over which MPI signal is measured is a factor of 5 smaller than the current state of the art particles (Synomag®). This is directly correlated with the particle's spatial resolution as a tracer. For MPS, comparison between a large number of spectral harmonics gives information about the local environment. State of the art MPI tracers lose harmonic signal around the 15th harmonic, but anisotropic magnetic bars retain strong harmonic signal until the 24th harmonic. Additionally, harmonic strength relative to overall magnetic signal is stronger in bar particles when compared to Synomag®. Current work is focused on further optimizing the bar particles for their future applications in vivo.<br/><br/><i>References</i><br/><i>[1] Talebloo, N. et al. J. Magn. Reson. Imaging. 2020, 51: 1659-1668.<br/>[2] Wu, K. et al, ACS Appl. Nano Mater. 2020, 3: 4972–4989.</i>