Transverse Relaxivity of Nanoparticle Contrast Agents for MRI: Different Magnetic Cores and Coatings

Four types of magnetic nanoparticles were synthesized, namely, the La 0.65 Sr 0.35 MnO 3 and La 0.80 Na 0.20 MnO 3 perovskite manganites and the Mn 0.61 Zn 0.42 Fe 1.97 O 3 and Co 0.44 Zn 0.70 Fe 1.86 O 4 spinel ferrites. Subsequent coating with silica provided typical core–shell particles with the mean shell thickness of 14–19 nm and the mean size of cores of 49–52 nm in the case of manganites and 26–33 nm in the case of ferrites. Moreover, two other surface modifications were employed for Mn–Zn ferrite nanoparticles, specifically coating with titania, leading to larger clusters with more complex morphology, and the stabilization by citrate anions. The products were characterized by powder X-ray diffraction and transmission electron microscopy, and their magnetic properties were probed by SQUID magnetometry. The main attention was devoted to the relaxometry on aqueous suspensions of particles in a magnetic field of 0.5 T. For distinct types of the prepared core–shell nanoparticles, the temperature dependence of transverse relaxivity was measured and the role of different relaxation regimes was analyzed.