Coating thickness of magnetic iron oxide nanoparticles affects R2 relaxivity

Purpose: To evaluate the effect of coating thickness on the relaxivity of iron oxide nanoparticles. Materials and Methods: Monocrystalline superparamagnetic iron oxide nanoparticles (MIONs), coated with a polyethylene glycol (PEG)-modified. phospholipid micelle coating. with different PEG molecular weights, were prepared. The particle diameters were measured with dynamic light scattering (DLS) and electron microscopy (EM). The R-1 and R-2 of MIONs were measured using a bench-top nuclear magnetic resonance (NMR) relaxometer. pH was varied for some measurements. Monte Carlo simulations of proton movement in a field with nanometer-sized magnetic inhomogeneities were performed. Results: Increasing the molecular weight of the PEG portion of the micelle coating increased overall particle diameter. As coating thickness increases, the R-2 decreases and the R-1 increases. Changing pH has no effect on relaxivity. The Monte Carlo simulations suggest that the effect of coating size on R-2 relaxivity is determined by two competing factors: the physical exclusion of protons from the magnetic field and the residence time for protons within the coating zone. Conclusion: Coating thickness can significantly impact the R-2. and the R-2/R-1 ratio, of a MION contrast agent. An understanding of the relationship between coating properties and changes in relaxivity is critical for designing magnetic nanoparticle probes for molecular imaging applications using MRI.