PEG coating reduces NMR relaxivity of Mn 0.5 Zn 0.5 Gd 0.02 Fe 1.98 O 4 hyperthermia nanoparticles

Purpose: To investigate both T1 and T2 MR relaxation enhancement of Gd substituted Zn‐Mn ferrite magnetic nanoparticles. Both uncoated and polyethylene glycol (PEG) coated particles were used. Materials and Methods: Chemical co‐precipitation was used to synthesize particles in the form Mn 0.5 Zn 0.5 Gd 0.2 Fe 1.98 O 4 suitable for hyperthermia applications. Physical characterization of the magnetic nanoparticles included SEM, TEM, ICP, and SQUID. T1 and T2 measurements were performed at 1.5 Tesla (T). Results: The saturation magnetization was 12.86 emu/g while the particle's magnetic moment was 1.86 × 10 −19 J/T. The particle size increased due to coating, while 1/T1 and 1/T2 relaxivities (26°C) decreased from 2.5 to 0.7 and from 201.3 to 76.6 s −1 m M −1 , respectively, at a magnetic field 1.5T. Conclusion: The reduction in both 1/T1 and 1/T2 is attributed to increased distance of closest approach between the protons and the magnetic core caused by the shielding provided by the high molecular weight PEG. 1/T2 data are compared with existing theoretical models using a modified radius that takes into account both possible agglomeration of the particles and increased inter‐particle separation induced by PEG coating. J. Magn. Reson. Imaging 2011;. © 2011 Wiley Periodicals, Inc.