1H spin–lattice superparamagnetic relaxation enhancement for very large nanoparticles – neglecting the electronic relaxation

Abstract

A theoretical model of ¹H spin–lattice superparamagnetic relaxation enhancement, specifically developed for the high anisotropy energy limit, has been tested against an extensive set of experimental data for aqueous solutions of 25 nm and 30 nm Fe₃O₄ nanoparticles coated with protein G and carboxylic acid. The model assumes negligible contribution from electron spin relaxation processes and, consequently, involves only three parameters: an effective electron spin corresponding to the nanoparticle's magnetic moment, the translational diffusion coefficient of the solvent molecules, and the distance of closest approach between the nuclear and electron magnetic moments, which is expected to be on the order of the nanoparticle radius. It has been shown that the model achieves good agreement with experimental data over a broad range of resonance frequencies (40 kHz to 40 MHz), despite the strongly constrained parameter space. Moreover, it has been demonstrated that the type of the coating considerably affects the net magnetization of the nanoparticles.