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Hyperthermia is the clinical therapy to kill malignant tissues, especially tumors, by elevating the tissue temperature up to 42 oC. Hyperthermia comprises external and internal hyperthermia. In the external hyperthermia, the heat is given by external sources, while in the internal hyperthermia the heat generators are injected into the targeted tissues. In the magnetic hyperthermia, magnetic nanoparticles are used as heat generators. Thus, magnetic hyperthermia is the internal hyperthermia. The fluid of magnetic nanoparticles in the alternating magnetic field is able to generate heat by various mechanisms such as hysteresis, relaxation, viscous, and eddy current loss, etc. It is desirable for the heat generators to have a capability to regulate the tissue temperature below 46 oC, since the normal tissue will be burnt above this temperature. This can be achieved by using magnetic nanoparticles with Curie temperature below 46 oC. Magnetic nanoparticles become paramagnetic above Curie temperature and lose their ability to generate heat by external alternating magnetic field. We can choose suitable concentration of nanoparticles or time of exposure in the external field in order to achieve the temperature around 42 C.
In this thesis, we synthesized and characterized various ferrite nanoparticles for the potential application of magnetic hyperthermia. For the use of medical application the magnetic nanoparticles should be coated with biocompatible and biodegradable materials. We coated silica on the Fe-, Co-, and Mn-ferrite nanoparticles using reverse micelle method. The coated Fe-, Co-, and Mn-ferrite nanoparticles were found to be spherical with core-shell morphology in the TEM images having uniform size distribution with average diameter of 15, 15, and 14 nm, respectively with a margin error of 0.1 nm. The coating status of silica on the surface of ferrite nanoparticles was checked by their Fourier transform infrared spectra. From the XRD measurements, the cubic spinel structure of these magnetic nanoparticles was confirmed with lattice constants of 8.4, 8.3, and 8.3Å for the Fe-, Co-, and Mn-ferrite nanoparticles, respectively. The superparamagnetic behavior with negligible coercivity was revealed in the hysteresis measurement using a vibrating sample magnetometer. Due to this small coericivity, the hysteresis loss in heating process is negligible. Thus, we can assume that all the silica-coated ferrite nanoparticles generate heat through Neel and Brownian relaxation losses. We obtained their specific absorption rates (SAR) by using an induction heating system. The highest SAR values for the silica-coated Fe-, Co-, and Mn-ferrite nanoparticles were calculated to be 140.38, 56.24, and 42.46 W/g, respectively at frequency of 260 kHz and magnetic field intensity of 5.5-kA/m which are safe for patients. In addition, the dependence of the square of the field intensity shows good agreements with the linear response theory which is given by SAR α H2. We could see the dependence of SAR on the magnetization of nanoparticles. The Fe-ferrite showed the largest SAR followed by those of Co- and Mn-ferrite nanoparticles, which is consistent with the order of the magnetization values of nanoparticles. The heating and cooling rates were obtained as a function of time from hyperthermia process. The extrapolation of the heating and cooling rates resulted in an intersection of the straight lines. The point of this intersection was interpreted as a critical or Curie temperature (Tc). The estimated Curie temperatures for the Fe-, Co-, Mn-, and Ni-ferrite nanoparticles were 525,440, 210, and 495 oC, respectively. These Curie temperatures are much higher than the desired temperature of 42 oC. To control the saturation temperature around 42 oC, we used low concentration of nanoparticles in the dispersion for magnetic hyperthermia.
The PEG-coated nickel ferrite nanoparticles were synthesized by chemical co-precipitation method. Coating of PEG was performed simultaneously along with the synthesis of nanoparticles. The coated particles were found to be cylindrical in shape with a uniform size distribution of average length and diameter of 16 and 4.5 nm, respectively with a margin error of 0.2 nm, in the TEM images. From the measurements of FTIR spectra the coating of PEG on the surface of nanoparticles was confirmed. The XRD analysis confirmed the inverse spinel structure of the nanoparticles with a lattice constant of about 8.34Å. The heating ability was checked by using an induction heater of 5 kW at frequency of 260 kHz. The measured SAR depended on the concentration of nanoparticles, and was higher than the commercial USPIOs and Feridex showing their potential application in magnetic hyperthermia.
Thesis Advisor: Prof. Ilsu Rhee