1H-NMR Relaxation of Ferrite Core-Shell Nanoparticles: Evaluation of the Coating Effect

Abstract

We investigated the effect of different organic coatings on the 1H-NMR relaxation properties of ultra-small iron-oxide-based magnetic nanoparticles. The first set of nanoparticles, with a magnetic core diameter ds1 = 4.4 ± 0.7 nm, was coated with polyacrylic acid (PAA) and dimercaptosuccinic acid (DMSA), while the second set, ds2 = 8.9 ± 0.9 nm, was coated with aminopropylphosphonic acid (APPA) and DMSA. At fixed core diameters but different coatings, magnetization measurements revealed a similar behavior as a function of temperature and field. On the other hand, the 1H-NMR longitudinal r1 nuclear relaxivity in the frequency range ν = 10 kHz ÷ 300 MHz displayed, for the smallest particles (diameter ds1), an intensity and a frequency behavior dependent on the kind of coating, thus indicating different electronic spin dynamics. Conversely, no differences were found in the r1 relaxivity of the biggest particles (ds2) when the coating was changed. It is concluded that, when the surface to volume ratio, i.e., the surface to bulk spins ratio, increases (smallest nanoparticles), the spin dynamics change significantly, possibly due to the contribution of surface spin dynamics/topology.

1H Spin–Lattice Relaxation Processes in Solutions of H2N–Fe3O4 Nanoparticles: Insights from NMR Relaxometry

“…This is in contrast to the extensive development of the theory of paramagnetic relaxation enhancement (PRE) describing the enhancement of 1 H relaxation in solutions of paramagnetic complexes. − The PRE theory ranges from simplified descriptions including (among other simplifications) single electron spin relaxation times even for high-spin paramagnetic complexes via several simplified concepts , to highly advanced models including a complex and frequency-dependent electron spin relaxation scenario − and going beyond the perturbation approach to relaxation processes. − The theory has been thoroughly tested, involving results from electron spin resonance to provide demanding criteria for the models . At the same time, the theoretical model of 1 H relaxation processes in solution of magnetic NPs has been proposed in a rather short series of papers. − As far as the experimental verification of the models is concerned, the studies are limited, despite very interesting experimental studies. , One should stress, at this stage, that to validate this theoretical approach, one requires relaxation data covering a broad range of magnetic fields (resonance frequencies). At high magnetic fields, the component of the electronic magnetic moment parallel to the external magnetic field reaches already its saturation (following the Brillouin function), and the electronic relaxation properties play a lesser role in the stochastic fluctuations of the magnetic dipole–dipole interactions between the magnetic moment of the nanoparticle and the nuclear magnetic moment, compared to translation diffusion of solvent molecules.…”

“…25−29 As far as the experimental verification of the models is concerned, the studies are limited, despite very interesting experimental studies. 30,31 One should stress, at this stage, that to validate this theoretical approach, one requires relaxation data covering a broad range of magnetic fields (resonance frequencies). At high magnetic fields, the component of the electronic magnetic moment parallel to the external magnetic field reaches already its saturation (following the Brillouin function), and the electronic relaxation properties play a lesser role in the stochastic fluctuations of the magnetic dipole−dipole interactions between the magnetic moment of the nanoparticle and the nuclear magnetic moment, compared to translation diffusion of solvent molecules.…”