Author:

Keywords:

nanoparticles, synthetic antiferromagnetic, gold, magnetic, contrast agent, MRI, CT

Abstract:

In the last two decades, nanoparticles have become important agents in biology and medicine with applications in the sensing of biological agents and diseases, drug delivery, therapeutics and imaging. For all of these, a wide range of nanoparticles consisting of different materials and properties have been synthesized proving the great potential of these agents. However, with the aging population in our society, the burden on the healthcare system is still increasing. New and innovative treatments are therefore needed to lighten this burden. One of these is stem cell based therapy of diseases. Cells have several advantages as a therapeutic or delivery system: they are able to carry out complex functions and they are responsive to changes in the surrounding tissue of the host organism. Together with the rise of cell based therapies the need for highly sensitive non-invasive imaging tools has emerged. These are needed to locate the cells after in vivo injection and to determine the therapy efficiency. Several imaging techniques exist, each with its advantages and disadvantages but all of them require signaling agents that can be tagged to the therapeutic cells to acquire a sensitive detection in the presence of background signals such as soft tissue. PET and SPECT make use of radionuclides, ultrasound imaging uses microbubbles, optical imaging requires fluorophores, whereas X-ray CT and MRI require (respectively gold and magnetic) nanoparticles.The use of nanoparticles in the latter two imaging techniques has gained a lot of research interest leading to a publication burst in the last decade. However, the state-of-art nanoparticles still suffer from some limitations. Magnetic nanoparticles used in MRI are mostly iron oxide based and suffer from the superparamagnetic limit, thereby introducing a limited sensitivity in MRI. The proposed gold nanoparticles for use in CT are size-limited as well and induce low contrast enhancement in CT imaging. Methods have been proposed to combine magnetic materials and gold in one bimodal nanoparticle to reduce the invasiveness of both imaging methods. These often result in polydisperse particle suspensions and the introduction of a gold coating layer around the magnetic material results in decreased MRI sensitivities.To overcome these limitations, the goal of this work was to synthesize a new type of bimodal gold/magnetic nanoparticle that did not suffer from the superparamagnetic limit making it possible to synthesize highly magnetic, gold coated nanoparticles that induced high contrast generation in MRI and CT. In the first part we have shown the successful synthesis and characterization of this type of nanoparticles. Moreover, we have proven the performance of the nanoparticles as contrast agents in MRI and finally, the use of our nanoparticles as bimodal contrast agents was demonstrated in vitro and in vivo. In Chapter 2, we have shown the successful synthesis of synthetic antiferromagnetic nanoparticles (SAF-NPs) using sparse colloidal lithographytechniques. Synthesizing SAF-NPs, it is crucial to obtain control over the spacer layer thickness in between the magnetic layers. We have shown that we control the Au-spacer thickness with great precision in Ni78Fe22/Au/Ni78Fe22 and Co90Fe10/Au/Co90Fe10 particles and clear evidence of an oscillating interlayer exchange coupling was observed in these systems using AGFM. Choosing the right spacer thickness, SAF-NPs could be synthesized showing no magnetic remanence, but high susceptibilities. Moreover, this could be realized for different sizes ranging from 90 to 522 nm with high precision (only 3-5% deviation). Finally, methods were provided to functionalize the SAF-NPs, MV-NPs and HM-NCs and make a phase transfer from organic solvent to water possible. To do so, an amphiphilic phospoholipid was used that interacts with the nanoparticle surface through hydrophobic interactions and provides a hydrophilic surface group to make the particles water dispersible. Moreover, the PEG containing molecule is inert and biocompatible, which is ideal in later biological application. In Chapter 3, we demonstrated the performance of Ni80Fe20 based SAF-NPs as T2 contrast agents in MRI. The highest r2 value of 355 s−1mM-1 was observed for the smallest particles with a diameter of 90 nm. As the diameter further increased, the r2 values dropped. We adapted the theory Vuong et al. from spheres to SAF-NPs by renormalizing the SAF-NP diameter to an equivalent spherical diameter, based on the field and field gradient distributions. Herein, it was demonstrated that our experimental results follow the theory well with an expected maximum at a SAF-NP-diameter of 75 nm in the static dephasing regime followed by a decrease of r2 for larger diameters in the partial refocusing regime. These results clearly demonstrate the advantage of very uniform, highly magnetic SAF-NPs over superparamagnetic particles as the ideal r2 particle size can be reached, circumventing the superparamagnetic limit. To determine the performance of SAF-NPs in realistic conditions in vitro and in vivo experiments were conducted which were summarized in Chapter 4. The bimodal capabilities of gold SAF-NPs in vitro and in vivo in MR and CT imaging were determined using 90 nm and 222 nm SAF-NPs. In vitro, the SAF-NPs were internalized by SKOV3 cells which was confirmed through dark field and TEM imaging of the cells. Using MRI, 5,000 labeled cells could be detected and an r2 value up to 712 s−1mM−1 was measured for 222 nm SAF-NPs which outperforms any other report on bimodal gold/magnetic nanoparticles. Moreover, significant contrast enhancement in CT was observed for a minimum of 50,000 SAF-NP cells thereby confirming the bimodal capabilities of SAF-NPs. In vivo, intravenously injected SAF-NPs accumulated in the liver of a mouse and could be clearly detected in T2 or T2 weighted imaging, rendering the SAF-NPs as suitable agents for potential use in hepatic or other soft tissue imaging. In the lungs, SAF-NP labeled SKOV3 cells could clearly be detected using CT imaging which is the first proof of gold-based CT contrast enhancement in the lungs. In this work, we thus unambiguously showed the bimodal imaging capabilities of gold/magnetic SAF-NPs in vivo. In future applications, SAF-NPs can become great tools in combined imaging approaches where their outstanding performance in MRI can be used to image soft tissues and their CT contrast enhancement properties can be used for cell tracking in organs such as the lungs.