| In recent years, magnetic nanoparticles (MNPs) have attracted increasing interests due to the extensive applications in biomedical area, such as hyperthermia and controlled release or delivery of drugs. By applying gradient magnetic fields, MNPs with drugs can be delivered to the tumor site, playing a role in targeted therapy. When MNPs are used for hyperthermia, MNPs are enriched in lesion sites and producing heat due to hysteresis loss, under the influence of an external alternating magnetic field. What is more, superparamagnetic iron oxide nanoparticles can be used as a new MRI contrast agent with high relaxivity, good biocompatibility and stability. In the practical applications, the behavior of magnetic nanoparticles under magnetic field is actually dependent upon the collective magnetic property. Thus, it is very important to know the total magnetic moments quantitatively for a huge mass of colloidal magnetic nanoparticles. Magnetization which reflect the magnetized degree of magnetic materials can be obtained from the magnetic moment. By measuring the relationship the magnetization and the magnetic field, the magnetization curve can be obtained. When the magnetic field ranging from positive to negative periodically, the hysteresis loop of materials can be obtained. The hysteresis loop contains a residual magnetization of materials, such as coercivity magnetic parameters. We used the number of measuring nanoparticles as the classification criteria, and introduced magnetic force microscopy, transmission electron microscopy, superconducting quantum interference device, giant magnetoresistance sensors, magneto-optical Kerr effect method, vibrating sample magnetometer, magnetic balance, electron paramagnetic resonance instrument. Their principles, applications and developments were introduced in chapter one.The contents of this thesis are as following.we proposed a novel and convenient method for measurement of the total magnetic moments in a droplet of colloidal magnetic particles. This method was based on the kinetics of a magnetic droplet. The colloidal droplet was driven to move by a gradient magnetic field. The trajectory was monitored by CMOS and sent to computer for analysis of kinetic parameters. Then the equation between the magnetic force and the kinetic parameters can be set up according to the Newton’s Second Law. Because the magnetic force is correlative with the collective magnetization of nanoparticles, the total magnetic moments of nanoparticles can be calculated.The experimental system was shown in chapter three, Ansoft was used to simulate the distribution of magnetic field which was generated by a ’C’-shaped electromagnet. We used the Tesla to measure the field distribution and calculated field gradient using fitting of the measured data. Seen from the video, the colloidal suspension formed a spherical droplet suspending in the solvent due to the hydrophobic force. And the magnetic field was of gradient so that the droplet can be driven to move from the middle of gap toward the magnet pole.In chapter four, magnetic nanoparticles with different hydrodynamic sizes were measured by our method. It is proved that when the directions of magnetic field and the particle motion are same, the magnetic coupling between particles enhanced the magnetization. The total magnetic moments of the droplet containing bare y-Fe2O3 were much greater than those of the droplet containing DMSA-capped y-Fe2O3. Because the collective magnetic property of magnetic nanoparticles was dependent upon both the individual units and the coupling of magnetic moments between the adjacent units, it was thought that the aggregation-induced magnetic coupling enhanced the total magnetic moments of the bare y-Fe2O3 colloidal droplet. Compared different hydrodynamic Fe3O4 nanoparticles, the results measured with our method showed an obvious relationship between the aggregation state and the total magnetic moments of colloid. The near DLS sizes led to the similar measurement curves while the greatly different DLS sizes led to the distinct measurement curves. In chapter five, the magnetothermal effect was chosen to verify the point whether the maximum of total magnetic moments measured with our method can correctly evaluate the collective magnetic property for application. We chose the DMSA-capped y-Fe2O3 nanoparticles of different concentration to measure the total magnetic moments and make the experiments of magnetic hyperthermia. Based on these data, the linear correlation coefficient between the maximum of total magnetic moments and the heating temperature can be calculated, which was 0.986. The linear correlation coefficient between the saturation magnetization and the heating temperature was 0.945 and the linear correlation coefficient between the saturation magnetization and the heating temperature was 0.844. This high value demonstrated that our method can truly quantitatively reflect the magnetic property of the colloidal magnetic nanoparticles correctly. |
PDF Full Text Request