NMR relaxation study of water dynamics in superparamagnetic iron-oxide-loaded vesicles.

Superparamagnetic iron oxide (SPIO) nanoparticles have been introduced as contrast agents for clinical applications in magnetic resonance imaging. Recently, SPIO has been also used for tracking cells. However, NMR relaxation of water molecules behaves differently in a SPIO solution and SPIO-loaded cells. In this study, we used water-in-oil-in-water double emulsions to mimic cellular environments. The MR relaxation induced by the SPIO-loaded vesicles and SPIO solution indicates that T(2)* is sensitive to the iron concentration alone, and the behavior was very similar in both SPIO-loaded vesicles and SPIO solution. However, T(2) relaxation of water in SPIO-loaded vesicles was faster than that in a SPIO solution. In addition, the contribution of water inside and outside the vesicles was clarified by replacing H(2)O with D(2)O, and water inside the vesicles was found to cause a nonlinear iron concentration dependency. The studied dilution revealed that vesicle aggregation undergoes a structural transition upon dilution by a certain amount of water. R(2)* relaxation is sensitive to this structural change and shows an obvious nonlinear iron concentration dependency when the SPIO loading is sufficiently high. Random walk simulations demonstrated that in the assumed model, the vesicles aggregate structures causing the differences between R(2)* and R(2) relaxation of water in vesicles in the presence of SPIO particles.