¹H Spin–Lattice Superparamagnetic Relaxation Enhancement over a Wide Range of Magnetic Fields─Theory Validation

¹H spin-lattice relaxation experiments have been performed on water solutions of 10 nm diameter Fe₃O₄ superparamagnetic nanoparticles functionalized with carboxylic acid and polyethylene glycol. The two data sets were collected over a remarkably wide frequency range from 10 kHz to 500 MHz, with varying temperature, and complemented by diffusometry to determine the translational diffusion coefficient of water molecules in the solutions. The results were used to validate the theory of superparamagnetic relaxation enhancement and to assess the extent to which the model can be used to predict and tune the relaxation effects caused by strong dipole-dipole interactions between the magnetic moments of superparamagnetic nanoparticles and ¹H nuclei. The challenging validation was successful, although some discrepancies between experiment and theory were revealed and are discussed. Furthermore, it was shown that the functionalization of superparamagnetic nanoparticles significantly affects the enhancement of ¹H spin-lattice relaxation.