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

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.