1H spin-lattice relaxation in solutions of coated superparamagnetic nanoparticles—Challenging the validity range of the low anisotropy energy model

The theoretical model for 1H spin-lattice superparamagnetic relaxation enhancement, under the assumption of low anisotropy energy, was evaluated using Fe3O4 nanoparticles (15 and 20 nm) coated with a protein G-conjugated IPG polymer and dispersed in water and water/glycerol solutions. The experimental 1H relaxation data were collected over a frequency range from 5 kHz to 40 MHz (referring to 1H resonance frequency) in the temperature range from 278 to 308 K. Distinct 1H spin-lattice relaxation maxima, as predicted by the low anisotropy energy model, were observed; however, the overall frequency dependence of the relaxation rates increasingly resembles that expected for systems with higher anisotropy energy (larger nanoparticles). A detailed comparison between the experimental data and the theoretical model predictions revealed discrepancies. The ratio between the theoretical and experimental values varies between 1.1 and 0.6, except in the case of a water solution of the 20 nm nanoparticles, for which the discrepancies are more pronounced. This effect was explained by fast electronic spin-spin relaxation. The results provide a quantitative explanation of the factors that define the applicability limits of the model of superparamagnetic relaxation enhancement derived under the assumption of low anisotropy energy and identify conditions under which its predictions remain reliable.