Iron based magnetic nanoparticles: Synthesis using different production methods, encapsulation with silica/graphene, characterization and performance tests

Abstract

In recent years, developments in nanotechnology have accelerated in many areas, especially in energy, environmental, electronic and biomedical applications. One of the innovations that nanotechnology has brought to these application areas is magnetic nanoparticles (MNPs). MNPs are used in electronic, water or soil clean-up, catalytic and biomedical applications. Amongst them, MNPs are crucial for diagnosis and treatment of cancer. Magnetic nanoparticles are used as contrast agents in studies such as magnetic resonance imaging (MRI) in cancer diagnosis, like hyperthermia. For nanomaterials to be used in biomedical applications; particle sizes, morphology, surface properties, biocompatibility, magnetic properties, thermal and chemical stabilities are very important. Since the surface area increase with the decrease in the particle size, the surface on which drug transport and release will be increased by using magnetic nanoparticles. In addition, some Fe and Fe-based nanoparticles lose their chemical stabilities in the body fluid, unless they are surrounded by any protective layers. They could be oxidized and their magnetisation values decrease, so efficiency of imaging applications reduces. In order to prevent the decomposition of magnetic nanoparticles in body fluid and to prevent them from losing their magnetic properties, the idea of covering nanoparticles with protective layers has emerged. Materials in this form are referred to as core/shell type materials. The core with magnetic properties is coated with different inert materials, ensuring its stability in biological environments. There are studies to create a passivation layer by surrounding the metal with its own oxide or noble metal. Frequently preferred coating materials are silica and carbon-based (graphene, graphene oxide, etc.) shell materials. Many production methods have been applied in the synthesis of MNPs and their encapsulations. Fe2B as an example of magnetic nanoparticles could be synthesized using solid-state methods like mechanochemical synthesis. On the other hand magnetic nanoparticles can be encapsulated with various materials such as silica and graphene, to produce biocompatible surfaces. Silica encapsulation is generally carried out using hydrothermal methods. Although, too many methods were used for graphene encapsulation of MNPs, chemical vapor deposition is one of the most efficient ways. In this dissertation, Fe2B particles were synthesized purely from Fe2O3/B2O3/Mg powder blends using mechanochemical synthesis (MCS) followed by leaching. Milling time was examined as a process variable and after the MCS, MgO by-product was removed by MCS synthesis processes and HCl acid leaching were characterized. As a result of MCS and purification, pure Fe2B nanoparticles smaller than about 40 nm were synthesized. Additionally, encapsulation studies were carried out by treating the synthesized powders with silica. Fe2B powders embedded in silica were synthesized by Ströber method as a hydrothermal method. Magnetic Fe2B particles embedded in silica by the Ströber technique are approximately 330 nm in size and the coercivity value 168 Oe. Secondly, the MCS of pure Fe2B powders following the leaching process were carried out using Fe2O3/B/Mg powder blends. Unlike the previous case, elemental boron was used instead of B2O3 powders as a raw material. It has been a detailed study examining the variables of MCS by experimenting with different grinding times, different ball sizes and different ball-to-powder ratios (BPR). It is very unique as it is the first study in the literature to optimize the experimental pathways for the mechanochemical synthesis of Fe2B. As a result of the optimization study, the most appropriate synthesis parameters: 10/1 BPR, 1 large (14.3 mm) and 5 medium (12.4 mm) size balls and 4 hours milling time were determined. Fe2B nanoparticles synthesized under optimum conditions were obtained pure by leaching and removing MgO. These nanoparticles were around 35 nm in size. Magnetic saturation value is 87.8 emu/g and coercivity value is 229.9 Oe.Cytotoxicity tests were carried out to demonstrate the usability of these nanoparticles in biomedicine. Powders coated with polyacrylic acid (PAA) have been shown to be biocompatible for up to 72 hours as a result of studies on Vero E6 cells, HeLa and MCF7 cancer cells. In addition to particle sizes, biocompatibility and magnetic properties of nanoparticles to be used in hyperthermia applications, Specific absorption rate (SAR) values are also important. For this reason, it was measured as 9.15 W/g as a result of SAR measurements. Thirdly, Fe-nitrate (Fe(NO3)3.9H2O) and silica powders were used for precursor powder preparation. Encapsulation studies were carried out by feeding these substrates to the chemical vapor deposition (CVD) system and using methane (CH4) and hydrogen (H2) gases. Different temperatures (900-1050°C), holding times (45 min-1 h), system pressures (30 and 50 mbar) and gas flow rates (100 mL/min or 200 mL/min) were investigated as variables. According to optimization studies, optimum operating conditions for the synthesis of graphene-encapsulated nanoparticles using a Fe-nitrate-based substrate were determined as: 900°C, 60 minutes waiting time, 50 mbar pressure and 100 mL/min H2 and 100 mL/min CH4 gas flow. In accordance with the phase analyzes performed, the presence of two iron contained (FCC (Fe,C), BCC Fe), graphite/graphene and a few amount of Fe2O3 phases were detected. TEM images and elemental mapping results have clearly revealed that magnetic nanoparticles (Fe/Fe2O3) are encapsulated with multilayer graphene (number of layers ranging from 3 layers to 35 layers). Magnetic saturation and coercivity values were determined as approximately 85 emu/g and 552 Oe. These values verify that the synthesized nanoparticles have soft ferromagnetic properties and they are potential materials that can be used in biomedical applications. In this direction, toxicity analyzes of nanoparticles were made after coating with PAA, and it was shown that they are biocompatible at low incubation times below 100 μg/mL. In another case, as a follow-up of the third, is based on the effect of using different precursor materials. The precursor materials were prepared from Fe-chloride (FeCl3.6H2O) and fumed silica using a spray dryer. Variables of CVD system such as temperature, holding time, and pressure and gas flow rates were investigated, the optimum synthesis of encapsulated magnetic nanoparticles takes place at 900°C with 100 mL/min H2 and 100 mL/min CH4 gas flow for 60 minutes. When Fe-chloride-based salts are used as the precursor powders, phase analyzes have shown that the core of the nanoparticles consists of Fe/Fe3C phases. It was shown that encapsulated magnetic nanoparticles (~40 nm) have soft ferromagnetic properties by vibrating sample magnetometer (VSM) analysis (coercivity and magnetic saturation values varied between 242-344 Oe and 6.56-14.3 emu/g). In order to examine their biocompatibility, cytotoxicity and phototoxicity studies were carried out this time and it was shown that the synthesized and purified Fe/Fe3C@C nanoparticles were biocompatible. In the last two investigations Fe-sulphate (FeSO4.7H2O) and ferrocene (C10H10Fe) based precursors were prepared. Fe sulphate and ferrocene impregnated silica precursor powders were prepared using a spray dryer from ferrocene, fumed silica and ethanol contained solution and these precursor powders were fed into the CVD system. Both ferrocene reduction and graphene encapsulation on them in-situ performed during CVD studies. CVD temperatures between 850-1000°C under varied 50 or 100 mL/min flow rates of both gases. Leaching steps using HF and HCl acid solutions ensure the synthesis of pure powders free of silica and uncoated Fe and demonstrate the chemical stability of synthesized nanoparticles. According to the magnetic saturation and coercivity values obtained from VSM tests, synthesized Fe@C and Fe/Fe2O3@C nanoparticles have soft ferromagnetic properties that demonstrate potential for biomedical and environmental applications. Magnetic saturation and coercivity values of Fe/Fe2O3@C were determined as approximately 90-185 emu/g and 255-301 Oe. Also, Magnetic saturation and coercivity values of Fe @C were varied between 22-150 emu/g and 82-278 Oe. To sum up, synthesis of high quality Fe2B nanoparticles (<40 nm) was achieved via mechanochemical synthesis (MCS) at room temperature and leaching methods. After the purification with HCl acid leaching, both biocompatibility (biocompatible up to 72 h on Vero E6 cells, HeLa and MCF7 cancer cells) and SAR value measurements of synthesized nanoparticles were conducted to examine their potential as materials that can be used in biomedical applications. Another aim of presented thesis is the graphene encapsulation studies. Optimization studies on magnetic nanoparticles (Fe, Fe/Fe2O3, Fe3C) encapsulated with multilayer graphene were carried out using different raw materials (Fe-nitrate, Fe-chloride, Fe-sulphate and ferrocene) in the CVD system. Magnetic saturation and coercivity values of all synthesized core/shell nanoparticles were varied between 6.5-150 emu/g and 82-552 Oe. Both studies are novel studies that have contributed to the literature in terms of examining the biocompatibility of the encapsulated products obtained by optimizing the chemical vapor deposition system. Although the use of different raw materials affects the composition of magnetic nanoparticles, these nanoparticles were coated with graphene layers and core/shell structured materials were synthesized using optimum conditions. These nanoparticles synthesized in core/shell structures using optimized conditions, whose biocompatibility has been proven by cytotoxicity tests, are magnetic nanomaterials that are candidates for use in biomedical applications.