Highly Sensitive Mr Magnetite Nanoclusters For Cell Imaging And Initial Development Of Ultra-small Fe₃ O₄ Nanoparticles

Magnetic resonance imaging (MRI) is among the best noninvasive detection methods today in clinical applications, especially for the visualization of tissues on the anatomical and functional level. It offers a series of advantages such as excellent spatial resolution, good contrast in soft tissue, and free of exposure to radiation, etc. However, the development of MRI has been limited due to its relatively low detection sensitivity. It has been revealed that superparamagnetic iron oxide (SPION)contrast agent, with good biocompatibility, is able to significantly improve the detection sensitivity of MR imaging. Nevertheless, applied in cell imaging, especially when paucity of cells is imaged, the sensitivity of commercially available dextrin-coated SPION contrast agents is still insufficient and contrast agents with much higher sensitivity are highly desirable. Assembling individual SPION particles into compact magnetite nano-clusters (MNCs) can dramatically enhance the T2 relaxivity of the particles. Therefore, it is of great importance to develop MNCs and conduct the relevant research of the clusters for cell imaging. On the other hand, for MR molecular imaging of cancers in vivo, especially for detection of the specific surface markers on the tumor cells, the size of particles should be small enough (less than 5 nm), allowing them to get through the lesion vascular and reach the surface of the cells. Currently, the size of smallest commercial SPION contrast agent, VSOP (very small superparamagnetic iron oxide particles), is about 5 nm, with size further increased after surface modification. Therefore, it is necessary to develop a kind of SPION contrast agent with size small enough that permeation through the vessels of lesion site is allowed.In these contexts, the major contents of this dissertation are as follows:1) Synthesis of superparamagnetic MNCs: MNCs with different sizes were prepared by polios method and carefully characterized by a series of methods.2) Investation of MNCs for cell MR imaging with SHU555A (Reservist) as a reference. In this context, the cytotoxicity and the efficacy of MNC a size of 63 nm (r2: 630 s^-1mM^-1) for cell MR imaging both in vitro and in vivo were evaluated. 3) Synthesis and characterizations of ultra-small Fe₃O₄ nanoparticles. Ultra-small Fe₃O₄ nanoparticles with a size of 3 nm were prepared by polyol method. The size, magnetic property and stability in physiological condition were investigated.Results: We have synthesized four kinds of magnetite nanoclusters (MNCs) with the size of 34 nm, 60 nm, 100 nm and 166 nm, respectively. The transverse relativity of the MNCs are 490 mM^-1s^-1,630 mM^-1s^-1,420 mM^-1s^-1 and 270 mM^-1s^-1, respectively. The MNCs were well dispersed in aqueous solution.MNC with size of 63 nm was used for in vitro cell MR imaging with the commercial SHU555A (Reservist) as a control. The cluster was biocompatible. The detection limits of cells labeled with MNC and SHU555A at the iron concentration of 100μg /mL for 1 h were 3,000 and 50,000 labeled cells/mL, respectively. In order to mimic the in vivo situation, the labeled cells were implanted into mice, with detection limits of 1×10⁴ cells for MNC and 1×10⁶ for SHU555A. These results indicate that MNC with size of 63 nm has a superior sensitivity than the commonly used contrast agent (SHU555A) for cell imaging both in vitro and in vivoUltra-small Fe₃O₄ nanoparticles were synthesized by polyols method, and then modified by PEG-carboxyl's with different molecular weights. The size of the particle was about 3 nm. The Fe₃O₄ nanoparticles, before and after modification, exhibit good disperse stability in PBS buffer (pH = 7.4). The value of r₂/r₁ is 4, which is similar to the commercial SHU555C.Conclusion: The as-synthesized MNC show superior detection sensitivity than SHU555A, which is very promising for MR cell imaging. Moreover, the ultra-small Fe₃O₄ nanoparticles with a size of 3 nm have long term stability in PBS. This well-dispersed ultra-small Fe₃O_\4 nanoparticles is expected to be applied for the detection of tumor cell surface receptors for the early or personal diagnosis of cancer.