In this study, PVP-coated iron oxide nanoparticles were synthesized by an one-step thermal decomposition method adapted to make homogeneous nanoparticles more suitable for intravenous injection. We [9
] have previously attempted to synthesize PVP-IO nanoparticles with small core size and thick coating and found that the overall size of the particles is dependent on the PVP/Fe(CO)5
reaction ratio and that the core size of the nanoparticles is dependent on the reaction time. For stable PVP-IO nanoparticles with large core size and thin coating for macrophage uptake, we found the optimal molar ratio of PVP/Fe(CO)5
to be 0.16 and the reaction time to be 5 h; in addition, more uniform PVP-IO nanoparticles were obtained after precipitation by acetone than by dialysis. The comparison of physical properties of our previously reported PVP-IO nanoparticles with small core (small core PVP-IO) [9
], PVP-IO nanoparticles with large core developed in this study (large core PVP-IO), and Feridex is summarized in . PVP-IO is highly soluble in water and various buffer solutions including saline, PBS and serum. The solubility of large core PVP-IO in PBS can be as high as 100 mg ml−1
without precipitation. The particles are also biocompatible, as no obvious cellular toxicity was observed through cell proliferation assay (data not shown).
Summary of physical properties of small core PVP-IO, large core PVP-IO, and Feridex.
Transmission electron microscopy (TEM) revealed that the average size of large core iron oxide nanoparticles is about 8–10 nm, as illustrated in . In aqueous solution, the colloidal particles have a hydrodynamic diameter of about 20–30 nm, determined by dynamic light scattering (DLS) measurement (). The small core PVP-IO previously made in our laboratory [9
] had a core size of 3–4 nm with a broad hydrodynamic size distribution of 90–150 nm. shows the x-ray diffraction (XRD) data of large core PVP-IO. The peaks appear at 30.0°, 35.3°, 43.0°, 53.3°, 56.9°, and 62.5°, which are well indexed to reflections from the (220), (311), (400), (422), (511), and (440) crystal planes of spinel ferrite, respectively. The hysteresis loop of PVP-coated iron oxide nanoparticles shown in had no coercive force, featuring superparamagnetic behavior. The large core PVP-IO nanoparticles were characterized by an augmented magnetic moment on increasing the magnetic field. The saturation magnetization of PVP-coated iron oxide nanoparticles is around 110 emu g−1
Fe, which is much higher than that of previously reported small core PVP-IO (about 35 emu g−1
] as well as Feridex (about 70 emu g−1
]. This trend matches with the report that the magnetism increases with the size and crystallization of nanocrystals [13
]. A high saturation magnetization is preferred for T2
-weighted MRI since the spin–spin relaxation process of protons in the water molecules surrounding the nanoparticles is facilitated by a large magnitude of magnetic spins in nanoparticles: large core iron oxide nanoparticles with high mass magnetization values may result in strong T2
-weighted MR signal intensity decrease as measured by MRI [13
Figure 1 Physical characterization of large core PVP-IO nanoparticles. (A) The TEM image shows monodisperse PVP-IO with core size of 8–10 nm; (B) the DLS measurement indicates that the colloidal particles in aqueous solution have a hydrodynamic diameter (more ...)
-weighted MRI was obtained with a 1.5 T MR machine (GE Excite) for the comparison of the MR contrast effect of the phantom. shows images of the synthesized large core PVP-IO colloids and Feridex in the same concentration gradient in distilled water. shows the signal intensity values converted by the image analysis tool for quantitative measurement. The results indicate that PVP-IO is slightly better than Feridex as a
negative contrast agent for MRI.
Figure 2 (A) Phantom image acquired from
-weighted MR images of Feridex and large core PVP-IO at different iron concentrations. (B) The
MR signal intensity is affected by the iron concentrations of Feridex and PVP-IO. PVP-IO is slightly better than Feridex (more ...)
-weighted spin echo images were acquired using a 3 T Siemens Tim Trio MR scanner. The measured r2
(reciprocal of T2
relaxation time) and
relaxation time) values were 174.8 and 294.3 mM−1
, respectively (). Remarkably, the large core PVP-IO show higher r2
than Feridex (r2
= 151.9 mM−1
). The higher relaxivity of PVP-coated magnetic nanoparticles is likely attributed to the high magnetic moment of PVP-IO (110 emu g−1
Fe) and effective magnetic relaxations from the proton spins around PVP-IO.
Figure 3 (A) 1/T2 and (B)
versus Fe concentration for PVP-IO (●) and Feridex (○). The relaxivity values r2 and
were obtained from the slopes of the linear fits of experimental data.
To detect inflammatory disease by MRI, it would be ideal for PVP-IO nanoparticles to possess high and persistent uptake by macrophages. To investigate this property, in vitro
cell uptake experiments were carried out using a mouse macrophage cell line RAW 264.7. The uptake of PVP-IO by macrophages was compared to that of Feridex, which is currently used clinically for MRI and is known to be taken up by macrophages due to its size. To detect the presence of iron oxide nanoparticles in cells, Prussian blue staining was carried out after 48 h incubation of Feridex and large core PVP-IO nanoparticles with macrophages at 20, 50, and 100 μ
. As shown in , the macrophage uptake of large core PVP-IO is comparable to or slightly higher than that of Feridex at all the concentrations examined, even though the overall size of PVP-IO is smaller than that of Feridex. Following the incubation,
-weighted MR images were acquired to further confirm the ability of macrophages to take up the newly synthesized large core PVP-IO. As shown in , PVP-IO exhibited somewhat more negative contrast enhancement as compared to Feridex at relatively low concentrations of Fe. Such difference was diminished when a large amount of Fe were used (e.g. 100 mg Fe ml−1
Figure 4 Mouse macrophage cells were incubated with Feridex or large core PVP-IO for 48 h at different iron concentrations (20, 50, and 100 μmol ml−1), stained by Prussian blue and then counterstained with nuclear fast red. Similar or slightly (more ...)
Figure 5 (A)
-weighted MR images and (B)
MR signal intensity of gelatin gels containing macrophages (1 × 106 cells/well) incubated with Feridex and PVP-coated iron oxide nanoparticles at the different iron concentration of 20, 50, and 100 μ (more ...)
After successful demonstration of the uptake of large core PVP-IO nanoparticle by macrophages in culture, we tested the same particles in vivo
for liver imaging. Due to the high solubility and monodispersity of the PVP-IO in aqueous buffer, we are able to perform bolus injection of PVP-IO without the presence of a filter, which is required by Feridex. Both Feridex and large core PVP-IO were able to detect a lower signal intensity in the rabbit liver parenchyma but the
effect of PVP-IO is more obvious than that of Feridex ().
-weighted MR images before, 3, 7, and 20 min after intravenous injection of Feridex (up) and PVP-coated iron oxide nanoparticles (down). Both Feridex and large core PVP-IO were able to lower signal intensity in the rabbit liver parenchyma but the
effect (more ...)