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

We investigated the effect of different organic coatings on the H-1-NMR relaxation properties of ultra-small iron-oxide-based magnetic nanoparticles. The first set of nanoparticles, with a magnetic core diameter d(s1) = 4.4 +/- 0.7 nm, was coated with polyacrylic acid (PAA) and dimercaptosuccinic acid (DMSA), while the second set, d(s2) = 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 H-1-NMR longitudinal r(1) nuclear relaxivity in the frequency range nu = 10 kHz divided by 300 MHz displayed, for the smallest particles (diameter d(s1)), 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 r(1) relaxivity of the biggest particles (d(s2)) 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.