Iron oxide nanocrystals clustered in oil-in-water nanoemulsions: Preparation, characterization, and transverse relaxivities

摘要

Iron oxide nanocrystals (IONCs) have attracted extensive interest due to their highmagnetization that efficiently decreases the transverse relaxation times of waterprotons. The increase in their transverse relaxivities upon crystal clustering is alsoconsidered as one of their important properties.Nanoemulsions (NEs) were developed with oil-in-water emulsions containing IONCsin an oil core stabilized by phospholipids (DSPC) and di-stearoyl-phosphatidylethanolamine-N-methoxy(polyethylene glycol)(PEG-DSPE) lipid amphiliphiels. Weprepared two different NEs: NEs containing 10 mol% and 50 mol% PEG2000-DSPE(P10 and P50), while their sizes were kept constant. Monodisperse aggregates forP10 and unevenly dispersed IONCs for P50 batches were developed.IONCs-loaded NEs were analyzed by Dynamic light scattering (DLS) to obtainthe hydrodynamic diameter, Inductively coupled plasma mass spectrometry (ICPMS)to measure iron concentration, and Transmission Electron Microscopy (TEM)to observe the IONCs in the NEs. Furthermore, transverse relaxivities, r2 weremeasured at 0.47 T, 1.5 T, 3 T and 7 T.The experimental relaxivities were interpreted based on two parameters: the numberof IONCs in each droplet and the magnetic field strengths. Experimental relaxivitiesof P10 NEs were in good agreement with the theoretical studies. Relaxivitiesincreased with increased number of IONCs in each droplet and relaxivities increasedwith increased magnetic field until they level of when magnetization saturation hasbeen attained. Experimental relaxivities of P50 were close to the prediction ofthe static dephasing regime model and much higher than the fast diffusion modelpredictions.In the current work it is evident that there is a significant difference in the experimentalrelaxivities of P10 and P50 batches. However, due to the difference ingeometry between P10 and P50, we are not able to make conclusions about theeffect of PEG density. A thorough preparation and characterization of NEs with anumerical simulations may be needed to explain the effect of PEG density.