Coating Effect on the <sup>1</sup>H—NMR Relaxation Properties of Iron Oxide Magnetic Nanoparticles

oleh: Francesca Brero, Martina Basini, Matteo Avolio, Francesco Orsini, Paolo Arosio, Claudio Sangregorio, Claudia Innocenti, Andrea Guerrini, Joanna Boucard, Eléna Ishow, Marc Lecouvey, Jérome Fresnais, Lenaic Lartigue, Alessandro Lascialfari

Format: Article
Diterbitkan: MDPI AG 2020-08-01

Deskripsi

We present a <sup>1</sup>H Nuclear Magnetic Resonance (NMR) relaxometry experimental investigation of two series of magnetic nanoparticles, constituted of a maghemite core with a mean diameter d<sub>TEM</sub> = 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-<i>stat</i>-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, <sup>1</sup>H NMR relaxivities were found to depend very slightly on the surface coating. We observed a higher transverse nuclear relaxivity, r<sub>2</sub>, at all investigated frequencies (10 kHz ≤ ν<sub>L</sub> ≤ 60 MHz) for the larger diameter series, and a very different frequency behavior for the longitudinal nuclear relaxivity, r<sub>1</sub>, between the two series. In particular, the first one (d<sub>TEM</sub> = 17 nm) displayed an anomalous increase of r<sub>1</sub> toward the lowest frequencies, possibly due to high magnetic anisotropy together with spin disorder effects. The other series (d<sub>TEM</sub> = 8 nm) displayed a r<sub>1</sub> vs. ν<sub>L</sub> 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<sup>−9</sup> s) at room temperature, confirming the superparamagnetic nature of these compounds.