Coating thickness of magnetic iron oxide nanoparticles affects R2 relaxivity

Abstract

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. J. Magn. Reson. Imaging 2007. © 2007 Wiley‐Liss, Inc.