Rigid-dipole magnetic nanoparticles for sub-second 3D viscosity imaging

Abstract Magnetic nanoparticles are widely explored in bio-related applications, as sensing probes, as local heat generators, and as contrast agents for imaging. The large majority of these nanoparticles are iron oxides and cubic ferrites with low to medium magnetic anisotropy, and thus a weak to medium coupling between spins and crystal lattice. Here we present stable aqueous colloidal suspensions made of a doped iron oxide (Al-doped ε-Fe₂O₃) with a strong coupling between the magnetic moment and the crystal lattice. We show that in stable aqueous suspensions, these highly anisotropic nanoparticles are cell-compatible and follow a rigid dipole model, enabling viscosity sensing, and viscosity imaging using a magnetic particle imaging (MPI) scanner. This opens new perspectives for bio-application of magnetic nanoparticles, such as 3D viscosity imaging with sub-second resolution using MPI, a tomographic technique with kHz imaging rates.