Coating Effect on the 1 H-NMR Relaxation Properties of Iron Oxide Magnetic Nanoparticles.

We present a ¹ H Nuclear Magnetic Resonance (NMR) relaxometry experimental investigation of two series of magnetic nanoparticles, constituted of a maghemite core with a mean diameter d _TEM = 17 ± 2.5 nm and 8 ± 0.4 nm, respectively, and coated with four different negative polyelectrolytes. A full structural, morpho-dimensional and magnetic characterization was performed by means of Transmission Electron Microscopy, Atomic Force Microscopy and DC magnetometry. The magnetization curves showed that the investigated nanoparticles displayed a different approach to the saturation depending on the coatings, the less steep ones being those of the two samples coated with P(MAA- stat -MAPEG), suggesting the possibility of slightly different local magnetic disorders induced by the presence of the various polyelectrolytes on the particles' surface. For each series, ¹ H NMR relaxivities were found to depend very slightly on the surface coating. We observed a higher transverse nuclear relaxivity, r ₂ , at all investigated frequencies (10 kHz ≤ ν _L ≤ 60 MHz) for the larger diameter series, and a very different frequency behavior for the longitudinal nuclear relaxivity, r ₁ , between the two series. In particular, the first one (d _TEM = 17 nm) displayed an anomalous increase of r ₁ toward the lowest frequencies, possibly due to high magnetic anisotropy together with spin disorder effects. The other series (d _TEM = 8 nm) displayed a r ₁ vs. ν _L behavior that can be described by the Roch's heuristic model. The fitting procedure provided the distance of the minimum approach and the value of the Néel reversal time (τ ≈ 3.5 ÷ 3.9·10 ^-9 s) at room temperature, confirming the superparamagnetic nature of these compounds.