NMR relaxivity of coated and non-coated size-sorted maghemite nanoparticles
The effect of polymer coating on MNR relaxometry of maghemite nanoparticles has been studied. The samples were carefully sorted by size in order to reach narrow size distribution (<0.2) with size ranging from 4.5 to 12.5 nm. Relaxation dispersion profile as well as studies at a fixed Larmor frequency, were recorded for numerous either uncoated or polymer coated samples. The NMR relaxivities r1 and r2 increase with nanoparticle diameter. We have analysed the role of polydispersity for nanoparticles with the same mean size on the dispersion curves. We have compared the role of coating on nanoparticles NMR relaxivity between bare and poly(sodium acrylate-co-maleate) coated nanoparticles. We have investigated the influence of nanoparticle size on the T1 and T2 activation energy Ea. While Ea decreases with nanoparticle diameter when determined from T1, it increases from T2 determination. The influence is more important for small particles (<9 nm) than for big particles (>9 nm). Moreover, the PAAMA coating changes the energy Ea obtained from T2: Ea becomes independent of the nanoparticle diameter. These results highlight the need of a complete characterisation of the role of the coating on the relaxation of magnetic particles.
