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Ultrasmall iron oxide nanoparticles for positive contrast MR imaging: structural, colloidal and magnetic properties

Ultrasmall crystalline iron oxide nanoparticles (USPIO NPs), with majority of the particles less than 3 nm in size dispersed in an organic fluid, were synthesized by polyol method. ZFCFC bifurcation and ac susceptibility peaks reveal that the nanoparticles are superparamagnetic with a blocking temperature T_B ~ 10 K. Considerably reduced measured magnetic field dependent magnetization at T > T_B, compared to the calculated dependence for noninteracting superparamagnetic NPs with a log-normal distribution of particle sizes, can be well described by modifying this distribution with field dependent average magnetic particle diameter. NMR relaxivity measurements on water diluted nanoparticle dispersions at magnetic field 1.5 T gave longitudinal and transverse relaxivity values r_1 = 0.028 mmol-1 s-1 and r_2 = 0.050 mmol-1 s-1 which resulted in the relaxivity ratio, r_2/r_1 = 1.8. The USPIO NPs dispersions have shown the properties of positive T_1 MRI contrast agents: increasing MR signal intensity in the T_1 -weighted image with increasing iron concentration in a wide concentration range. Cytotoxicity studies revealed that the USPIO NPs dispersions did not exert any significant cytotoxic activity on the B16 mouse melanoma cells at Fe concentrations of 1 mmol and below.

MRI contrasts, nanoparticles, magnetic properties

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