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Superparamagnetic nanoparticles are used as negative contrast agents in magnetic resonance imaging: owing to their large magnetic moment the water proton spins are dephased, which accelerates the nuclear magnetic relaxation of an aqueous sample containing these particles. Transverse and longitudinal relaxation times depend on several parameters of the nanoparticles such as radius and magnetization and on experimental parameters such as the static magnetic field or echo time. In this work, we introduce a new simulation methodology, using a classical formalism, allowing the simulation of the NMR signal during transverse and longitudinal relaxation induced by superparamagnetic particles in an aqueous solution, which, to our knowledge has never been done before. Nuclear magnetic relaxation dispersion profiles are obtained for a wide range of nanoparticle radii and magnetizations. The results can be classified in two regimes--the well-known motional averaging and static regimes. This generalizes previous studies focusing on transverse relaxation at high magnetic field (larger than 1 T). Simulation results correspond to analytical theories in their validity range and so far unknown dependences of the relaxation with magnetization and radii of the NMR dispersions profiles are observed, which could be used to characterize experimental samples containing large superparamagnetic particles.

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In this work, aqueous solutions of magnetite nanoparticles (NPs) are studied. Magnetite NPs are very useful in biomedicine for magnetic resonance imaging (MRI), for drug delivery therapy, and also for hyperthermia. In order to predict the NP efficiency in these applications, it is crucial to accurately characterize their size distribution and their magnetization. Magnetometry, through the dependence of NP magnetization on the magnetic induction (MB curve), can provide interesting information on these physical properties. In this work, the extraction of the NP size distribution and magnetization from experimental MB curves of aqueous solutions of magnetite NPs is discussed. The results are compared to TEM and XRD characterizations. It is shown that an expression taking into account the size distribution better fits the results than the commonly used simple Langevin function. The size distributions obtained by magnetometry seem comparable to those obtained by TEM measurements. However, a closer look at the results shows some nonnegligible discrepancies: the size distributions obtained by magnetometry vary with the temperature and are closer to the TEM ones at room temperature. Our study suggests that it could be explained by the nonnegligible anisotropy energy of the NPs at low temperature and the lack of NP Brownian rotation below the freezing point of water. This demonstrates that care must be taken when interpreting the results obtained by magnetometry of magnetite NPs: only the size and size distribution obtained at room temperature should be used.

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