Power absorption mechanisms in high anisotropy cofe2o4 magnetic nanoparticles
- Autores: Teobaldo Enrique Torres Molina
- Directores de la Tesis: Clara Isabel Marquina García (dir. tes.), Gerardo F. Goya (dir. tes.)
- Lectura: En la Universidad de Zaragoza ( España ) en 2015
- Idioma: español
- Tribunal Calificador de la Tesis: Amílcar Labarta (presid.), Manuel Arruebo Gordo (secret.), María del Puerto Morales Herrero (voc.)
- Materias:
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- Resumen
Objectives of the work The aim of this thesis work is to study the key mechanisms involved in the power absorption taking place when magnetic nanoparticles are exposed to an alternating magnetic field. Power absorption is the basis of the magnetic hyperthermia, one of the most promising applications of magnetic nanoparticles, in the field of the biomedicine.1 One of the objectives of this thesis is to understand the role of the magnetic properties in the power absorption of magnetic nanoparticles. This work will be focused on highly anisotropic cobalt ferrite (CoFe2O4) nanoparticles.2 The work will include the synthesis of nanoparticles of different sizes and the characterization of their physicochemical properties. Once this characterization is completed, their power absorption efficiency will be determined and different theoretical models will be used to understand the physical mechanisms involved. Finally, the application of these particles in in vitro power absorption essays will be performed seeking for their future applications in magnetic hyperthermia.
STATE OF THE ART Since 1959, when the physicist and Nobel Prize laureate Richard Feynman presented "There is plenty of room at the bottom" 3 at a meeting of the American Physical Society, in which the broad possibilities of the nanoscience and nanotechnology were anticipated, numerous studies and developments have been emerged in both fields. Nanotechnology is defined as the study and manipulation of systems in which at least one of the dimensions is below 100 nanometers in size. The small size of the nano-objects results in new physical and chemical properties, which can differ notably from the bulk counterpart. Therefore, size-controlled nanomaterials would yield materials with improved optical, electronic, or magnetic properties.4 Within the biomedical research, the magnetic hyperthermia (MHT) is a new therapy aimed to provoke cell death by heating, using an alternating magnetic field, when these cells have been previously loaded with magnetic nanoparticles (MNPs)5 6. Experimentally, a large number of materials have been studied in the form of MNPs regarding their capacity as heat-generating agents. The specific power absorption (SPA) is the key parameter to measure this efficiency. 7 The mechanisms involved in heat generation with magnetic colloids containing MNPs is related to the rotation of the magnetic particle within the surrounding liquid (Brownian relaxation) and/or the rotation of the particle magnetic moment within the magnetic core (Néel relaxation). Some theories have been proposed to model these mechanisms, being the most known the Linear Response Theory LRT. 8 Recent studies by Usov and Liubimov and also by Mamiya et al.10 have led to a general understanding of these mechanisms based on the Landau stochastic movement equation.9 These models are quite general, but no analytical expressions for the general case are known. The LRT approximation, on the other hand, providing an accurate calculation of the SAR, has a limited range of application.11 All these approaches coincide in the very important role of the effective magnetic anisotropy of the nanoparticles, which for example establishes the relation between the anisotropy field of the system, Hk, and the intensity of the alternate magnetic field, H0, to be applied in a SPA measurement. In turn, the value of H0, determines the range of the validity of LRT, as showed by Carrey et al 11, by distinguishing between the viscous and magnetic modes. 11 Moreover, when working with ac magnetic fields, the role of the magnetic field frequency in the SPA value has also to be explored. Other points to take into account when trying to model the power absorption by magnetic nanoparticles are the particle size distribution and the viscosity of the solvent in which the particles are suspended for their further application. Concerning the purely experimental aspects of the SPA measurement, it is clear that for a perfect knowledge of the mechanisms governing the nanoparticle power absorption, it is necessary to carry out the experiments in the broadest possible range of both magnetic field intensity and frequency of the applied magnetic field. In this respect, there is a need in the market for an experimental facility to perform these SAR measurements easily and systematically.
Regarding the application of nanoparticles in magnetic hyperthermia, there are numerous studies on the cell-particle interaction. Cells are able to ingest the particles through a process known as endocytosis 12 and the uptake has been reported for different MNPs and cell types. A common point in all these studies is the role of the surface coating .For example some studies with nanoparticles functionalized with anionic groups have showed the high efficiency of internalization with the cellular membrane due to the average negative charges on the cell surface 13 14 15 16. Other important fact is the consensus about a lack of systematic studies on how the surface charge affects the formation of the protein corona mentioned by Tenzer 17 due at this corona have formed by more of 300 proteins 18. Finally, the phase of the cell cycle also influences the internalization of nanoparticles 19. It is also well known that for any clinical application of nanoparticles, their toxicity and influence on the cell viability after their internalization should be studied in detail.
PROJECT METHODOLOGY As was mentioned above, the average size of the particles as well as the particle size distribution, determine their physical and chemical properties. Therefore one of the objectives of this work is the synthesis of cobalt ferrite MNPs of different sizes. The nanoparticles will be synthesized by thermal decomposition of iron acetylacetonate Fe(acac)3 and cobalt acetylacetonate Co(acac)2 as precursors, in oleic acid and oleylamine, as surfactants. Different solvents such as phenyl ether, benzylether, 1-octadecene, and trioctylamine with increasing boiling temperatures, will be used in order to control the final particle size 20 21 The final product can be dispersed in hexane, and also dispersed in water, for the in vitro applications.
After the synthesis the nanoparticles morphology, crystallographic structure, chemical composition and oxidation states of their chemical constituents, as well as the organic content will be characterized. This characterization will be performed by different experimental techniques such as Transmission and Scanning Electron Microscopy (TEM) and (SEM), X-ray Diffraction (XRD), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), Ultraviolet-Visible (UV/Vis) spectrophotometry, Thermogravimetric Analysis (TGA) and X-ray Photoelectron Spectroscopy (XPS). All these analyses will be carried out on powders obtained by drying the suspensions in which the nanoparticles are re-dispersed. Dynamic Light Scattering (DLS) experiments will be performed on liquid samples, both in hexane and water, to compare the nanoparticle hydrodynamic volume in two different media.
Magnetization measurements M(T, H) and ac magnetic susceptibility measurements will be performed on a MPMS-XL SQUID Quantum Design magnetometer. The temperature dependence of the magnetization will be measured following zero-field-cooling (ZFC) and field cooling (FC) protocols .The magnetization isotherms will be measured between 5 and 400 K up to a maximum magnetic field of 3.96 MA/m. The susceptibility versus temperature will be measured applying an excitation ac field around 0.24 kA/m at different frequencies under zero external dc magnetic field. From this characterization information about the magnetic parameters such as coercive field HC, saturation magnetization MS as well as the effective anisotropy constant Keff will be obtained. Considerations about how this effective constant changes by effect of temperature will be analysed.
Other of our objectives is the study of the mechanisms involved in the power absorption of magnetic fluids consisting of Co ferrite MNPs dispersed in a solvent. To be able to perform these experiments in a systematic way, the design, optimization and assembly of the necessary experimental set-up will be carried out. As the power absorbed by the MNPs when they are in an alternating magnetic field (AMF), results in an increase of the magnetic fluid temperature, this experimental set-up is based on the measurement of the temperature increase of a magnetic fluid as a function of the frequency and intensity of the applied magnetic field. The design of the experimental set-up involves that of a magnetic field generation system that has to work in the range of RF, and the design of the temperature measurement system.
The synthesized samples will be systematically measured, in order to know the dependence of the SPA on the particle size, magnetic field and frequency. Moreover, numerical simulations based on theoretical expressions of the SPA will be carried out, to be compared with the experimental results. Besides the experiments with nanoparticles dispersed in hexane, SPA measurements will be carried out when the nanoparticles are dispersed in other solvents as toluene, chloroform and DMEM, to test the influence of the solvent on the SPA.
Finally the most suitable samples for magnetic fluid hyperthermia (i.e., those with the highest SPA) will be transferred to aqueous medium, for in vitro experiments on living cells. Their uptake in cell cultures and the intracellular distribution will be investigated by magnetization measurements on the co-cultured cells, and by visualising the cells and their cytoplasm in a Dual Beam microscope and by TEM. The nanoparticle toxicity and cell viability after particle uptake will be studied as well, by flux cytometry and Trypan Blue assays. These tests will be performed on the Neuroblastoma SH-SY5Y as cell model. Finally magnetic hyperthermia assays on these cell cultures will be also performed, to test the selected nanoparticles as heating agents for MFH.
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