Ultrasmall iron oxide nanoparticles: Magnetic and NMR relaxometric properties

Ultrasmall iron oxide (USPIO) nanoparticles, with diameter mostly less than 3 nm dispersed in an organic carrier fluid were synthesized by polyol route. The evolution of ZFC-FC magnetization curves with temperature, as well as the shift of the ac susceptibility peaks upon changing the frequency, reveal that the nanoparticles in the fluid are non-interacting and superparamagnetic with the blocking temperature T_B ∼10 K. The Mössbauer spectra analysis proposed the core/shell structure of the nanoparticles consisting of stoichiometric γ-Fe₂O₃ core and non-stoichiometric shell. The nanoparticle surface layer has a great influence on their properties which is principally manifested in significant reduction of the magnetization and in a large increase in magnetic anisotropy. Magnetic moments do not saturate in fields up to 5 T, even at the lowest measured temperature, T = 5 K. The average magnetic particle diameter is changed from 1.3 to 1.8 nm with increasing magnetic field from 0 to 5 T which is noticeably smaller than the particle sizes measured by TEM. The estimated effective magnetic anisotropy constant value, K_eff = 2 × 10⁵ J/m³, is two orders of magnitude higher than in the bulk maghemite. Measurements of the longitudinal and transverse NMR relaxivity parameters on water diluted nanoparticle dispersions at 1.5 T gave the values r₁ = 0.028 mmol⁻¹ s⁻¹, r₂ = 0.050 mmol⁻¹ s⁻¹ and their ratio r₂/r₁ = 1.8. Continuous increase of the T₁-weighted MRI signal intensity with increasing Fe concentration in the nanoparticle dispersions was observed which makes this ferrofluid to behave as a positive T₁ contrast agent.