Sonochemical synthesis of functionalized core/shell iron oxide nanocubes for catalytic and bio-applications

Abstract

Functional magnetic nanoparticles have great importance in various fields of applications, including nanoscience, nanotechnology, environmental chemistry and biomedical applications. Although many groups have successfully described the synthesis of functional magnetic materials using various approaches and technologies, a facile, rapid, eco-friendly and economically feasible synthesis procedure that combines all the materials into a single system has remained elusive. The shape of the synthesized nanoparticles is also an important consideration when evaluating potential practical applications. Especially for biomedical applications, the nanocube surface with specified crystal plane is expected to enhance the ability to immobilize biofunctional groups because the surface to whole volume (S/V) ratio is higher than for other shapes (e.g., octahedron, dodecahedron, icosahedron and sphere shape). Therefore, integration of cube-shaped magnetic nanoparticles (MNPs) in the form of iron oxide (Fe3O4) and, thereafter, functionalization of the surface with application-specific materials would likely enhance its performance and thus becomes a promising material for various applications. Here I have developed, a novel one-pot sonochemical approach for the synthesis of size controlled high magnetization iron oxide nanocubes as a core in aqueous medium, without the use of surfactant. The surface of Fe3O4 nanocubes were subsequently functionalized with various inorganic materials, including silica (SiO2), titania (TiO2), carbon (C), silver (Ag) and gold (Au) and applied in several applications (including catalytic and bio-sensing applications). Recently, two approaches (the Stober and microemulsion methods) have emerged as the major routes for the functionalization of MNP surfaces for synthesizing of core/shell nanostructures. These methods are advantageous because they produce a uniform shell with controlled thickness, however, they require complicated environmental procedures, large amounts of surfactant and long synthesis times (in the range of 6-48 h). Here I have succeeded in developing a facile, eco-friendly and time-reducing sonochemical approach for functionalization of MNPs surfaces based on the exceptional reaction conditions generated from ultrasound of high temperatures (5000 K) and pressures (~20 MPa) with high cooling rates (~1010 K/S), thus allowing the formation of many unique materials composites. Furthermore, I have also developed a modified sol-gel method for the coating of particle surfaces using only polyvinylpyrolidone (PVP) as an amphiphilic polymer. Notably, the total reaction time for the synthesis of the core/shell nanostructures was found to be shorter than the previously reported methods. MNPs are frequently functionalized or coated with SiO2 to improve their stability, biocompatibility and functionality, giving them promise for many bio-medical applications, such as magnetic resonance imaging (MRI) contrast agents, magnetically-targeted drug delivery vehicles and agents for hyperthermia. Here I have developed a sonochemical technique as well as a modified sol-gel approach for obtaining uniform Fe3O4/SiO2 core/shell nanocubes. Furthermore, the thickness of the silica shell is easily controlled in the range of 4-18 nm by adjusting the reaction parameters. The core/shell nanocubes samples were characterized by X-ray diffractometry (XRD), transmission electron microscopy (TEM), energy dispersive spectrometer (EDS), Fourier transform infrared spectroscopy (FTIR), and vibrating sample magnetometer (VSM). The as-prepared Fe3O4/SiO2 core/shell nanocubes showed good stability in air for at least 4 month as well as against annealing condition of up to 300 °C in presence of hydrogen gas as a strong reducing agent. Furthermore, high magnetization value of 50.7 emu/g was obtained for the sample with thin silica thickness (4 nm) as a consequence of shell thickness controlled. Moreover, the biocompatibility of the core/shell nanocube was enhanced in comparison to that of pristine Fe3O4 nanocubes. In addition, the Fe3O4/SiO2 nanocubes were functionalized by Aminopropyltriethoxysilane, and then conjugated with streptavidin-Cy3 successfully as indicated by optical fluorescence microscopy. On the other hand, I used the developed sonochemical techniques for the functionalization of Fe3O4 nanocubes by TiO2 as well as Carbon. At first, Fe3O4/TiO2 nanocubes were successfully synthesized through hydrolysis and condensation of iron (II) sulfate heptahydrate and titanium isopropoxide (as precursors of Fe3O4 and TiO2, respectively) using single reaction sonochemical process. XRD, TEM, EDS and FTIR were used to characterize the crystal structure, size and morphology, elemental composition, metal-metal and metal-oxygen bonds of the core/shell nanocubes, respectively. The magnetic properties of the samples were measured by VSM at room temperature. Catalytic measurements on the samples showed an excellent efficiency for the degradation of methylene blue, and this efficiency was further promoted remarkably by addition of hydrogen peroxide (H2O2) within only 5 minutes of reaction time in the absence of ultraviolet irradiation. Even after recycling the sample for six times, the introduced catalyst was found to retain as much as 90% of initial efficiency. A possible reaction mechanism for the sonochemical deposition of TiO2 on the surface of Fe3O4 nanocubes and also for the degradation process of methylene blue by the introduced catalyst will be discussed in this thesis. However, PVP as an amphiphilic polymer was used as a surface linker in case of synthesizing the Fe3O4/C core/shell nanocubes. To functionalize Fe3O4 nanocubes with noble metals such as Ag and Au nanodots, I used the sonochemical technique and the seed mediated growth method, respectively, to obtain two different structures of Fe3O4/SiO2/Ag and Fe3O4/Au nanocubes. XRD, EDS, TEM, and FTIR analyses revealed that the Fe3O4 nanocubes were successfully functionalized using these two facile methods. The resulting Fe3O4/SiO2/Ag nanocubes showed excellent catalytic efficiency toward the reduction of p-nitraoaniline to p-phenylenediamine within very short times and a recycling efficiency of 88 % for up to 15 cycles. In addition, based on this developed sonochemical approach, I have succeeded in synthesizing different types of ferrite nanoparticles including Co-Fe2O4, NiZn-Fe2O4, and MnZn-Fe2O4. TEM results showed that different morphologies structures of spherical, cubic and mixed shapes with different particle sizes were obtained in the range of 20 to 110 nm by changing the synthesis solvent medium and compositions. I obtained high saturation magnetization value with superparamagnetic properties for the synthesized ferrite NPs samples. The obtained ferrite nanoparticles with high crystallinity, high magnetic moment, and desirable superparamagnetic properties are expected to be promising materials for various biomedical applications; and the facile aqueous approach of the sonochemical method is expected to be a promising route for the synthesis of different ferrite materials. ⓒ 2015 DGIST