No abstract

Loss processes in magnetic nanoparticles are discussed with respect to optimization of the specific loss power (SLP) for application in tumour hyperthermia. Several types of magnetic iron oxide nanoparticles representative for different preparation methods (wet chemical precipitation, grinding, bacterial synthesis, magnetic size fractionation) are the subject of a comparative study of structural and magnetic properties. Since the specific loss power useful for hyperthermia is restricted by serious limitations of the alternating field amplitude and frequency, the effects of the latter are investigated experimentally in detail. The dependence of the SLP on the mean particle size is studied over a broad size range from superparamagnetic up to multidomain particles, and guidelines for achieving large SLP under the constraints valid for the field parameters are derived. Particles with the mean size of 18 nm having a narrow size distribution proved particularly useful. In particular, very high heating power may be delivered by bacterial magnetosomes, the best sample of which showed nearly 1 kW g −1 at 410 kHz and 10 kA m −1. This value may even be exceeded by metallic magnetic particles, as indicated by measurements on cobalt particles.

Luminescent hybrid nanoparticles with a paramagnetic Gd2O3 core were applied as contrast agents for both in vivo fluorescence and magnetic resonance imaging. These hybrid particles were obtained by encapsulating Gd2O3 cores within a polysiloxane shell which carries organic fluorophores and carboxylated PEG covalently tethered to the inorganic network. Longitudinal proton relaxivities of these particles are higher than the positive contrast agents like Gd-DOTA which are commonly used for clinical magnetic resonance imaging. Moreover these particles can be followed up by fluorescence imaging. This study revealed that these particles suited for dual modality imaging freely circulate in the blood vessels without undesirable accumulation in lungs and liver.

Evaluating and understanding the performances of magnetic colloids as contrast agents for MRI requires a theory describing their magnetic interactions with water protons. The field dependence of the proton longitudinal relaxation rate (nuclear magnetic relaxation dispersion profiles) in aqueous colloidal suspensions of superparamagnetic particles is based on the so-called Curie relaxation, which essentially accounts for the high field part of the NMRD profiles (B0>0.02 T). The low-field part of the NMRD profiles can only be explained by the crystal’s internal anisotropy energy, a concept which clarifies the important difference between superpara- and paramagnetic compounds: the anisotropy energy modifies both the electronic precession frequencies and the thermodynamic probability of occupation of the crystal magnetic states. Our theory clearly explains why a low-field dispersion exists for suspensions of small size crystals, and why it does not for large crystals’ suspensions. This important effect is due to the Boltzmann factors depending on the anisotropy energy, which is itself proportional to the particle volume.