Synthesis, Characterization and Applications of Magnetic Nanoparticles

Abstract

Nanostructure materials possess exceptional optoelectronic properties due to quantum effects that manifest at small sizes (1–100 nm). Nanomaterial is predominantly used as catalyst for a variety of applications that include but is not limited to water splitting, gas sensing, heavy-metal detection, biosensors, and dye degradation. Magnetic nanoparticles have been extensively utilized for several applications that include catalysis, renewable energy harvesting, solvents detoxification, heavy-metal remediation, biosensors and medical biotechnology such as drug delivery and magnetic resonance imaging. The magnetic susceptibility of these nanoparticles is the key feature, which enables them to be utilized for these applications. Among magnetic nanoparticles, iron oxide (Fe3O4, α-Fe2O3 and γ-Fe2O3) NPs represents one of the simplest types of magnetically recoverable catalysts. These NPs are robust, stable in air, amenable to functionalization, suspendable in many solvents including water or other protic benign solvents; their size, shape and crystallinity can be finely adjusted. This thesis compiles the studies carried out for the synthesis of magnetic (especially Fe3O4) nanoparticles and their application in different fields such as catalysis, sensing and electrochemistry. In the first study, a heterogeneous catalyst was synthesized for the hydrogenation of acetone to isopropyl alcohol. The magnetite’s NPs were successfully modified with tetraethylorthosilicate (TEOS) to stabilize the magnetite NPs by making core-shell and to prevent the magnetite surface from oxidation. The synthesized SiO2@Fe3O4 NPs were responsible for the efficient catalytic hydrogenation reaction regarding the 99.5% conversion of acetone to isopropyl alcohol (IPA) in the presence of microwave radiations within 60 s of reaction time. The present system is highly selective for isopropanol production and it does not show results in any byproduct. Active sites located on the surface of SiO2@Fe3O4 NPs were responsible for the quick adsorption of acetone and hydrogen on the surface of the catalyst resulting in a short time equilibrium establishment for the reaction. Moreover, the separation process was quick, simple and convenient because of magnetic separation. Henceforth, we infer that the synthesized SiO2@Fe3O4 NPs are green vi catalyst due to easily recoverable and reusable by applying a magnetic field. SiO2@Fe3O4 nanocatalyst could be recommended for hydrogenation of organic compounds with low molecular weight. The second study reports a very sensitive fluorescence sensor for the selective nanomolar detection of Ni2+ ions. The Ni2+ ions sensing is based on fluorescence quenching of the fluorophore (fluorescein) in a neutral aqueous medium. The fluorescence sensor is composed of the magnetic core and amino silica shell, functionalized with fluorescein fluorophore. The as-fabricated fluorescent nanosensor (Fe3O4@SiO2-NH2-Fluorescein) showed an enhanced fluorescence quenching response towards Ni2+ ions. The response of nanosensor was highly selective towards the target species whereas, the possible interferences from other metal cations and biological molecules were negligible. The fluorescein probe has very fast response, selective and sensitive detection limit (LOD=8.3×10-10 mol/L) towards Ni2+ ions in neutral medium with high binding constant (K) value of 3.2 x 104 L/mol for the complex formation between the sensor and Ni2+ ions. In the third study, different coated superparamagnetic Fe3O4 (magnetite) nanoparticles were synthesized, characterized and used for lysozyme (Ly) and bovine serum albumin (BSA) adsorption. SiO2, carbon nanotubes (CNTs) and graphene were used for covering the readily synthesized magnetite nanoparticles to elucidate the effect of cover layer on the protein adsorption kinetics and capacities of the nanostructure. XRD, FTIR, AFM, SEM, VSM and fluorescence measurements were used for the characterization of the samples and investigating the adsorption kinetics of Ly and BSA by these nanoparticles. The average particle size of the Fe3O4 nanoparticles are approximately found as 10 nm and VSM measurement shows that the Fe3O4 particles have superparamagnetic behavior with no hysteresis and remnant magnetization. The adsorption kinetics of proteins on nanosized material is followed via fluorescence method. All the nanostructures with different cover layers obey pseudo-first-order kinetics and SiO2 coated nanoparticles show the fastest kinetics and capabilities for Ly and BSA adsorption. In the fourth study, a simple, economic and one-pot synthetic protocol was followed to synthesize Fe3O4 nanostructures using partial oxidation co-precipitation method under aerobic conditions. After synthesis, the nanostructures were coated with tetraethyl vii orthosilicate for stabilization purpose. Later on, these nanostructures were characterized by Fourier transformed infrared spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron dispersive X-ray spectroscopy and atomic force microscopy to examine their size, shape and crystalline structure. The synthesized SiO2/Fe3O4 nanostructures with fine semi-spherical textures showed high heterogeneous catalytic activity for the reduction of 4-nitrophenol to 4- aminophenol using sodium borohydride (NaBH4) under microwave radiations. The 99.5% 4-nitrophenol reduction was achieved by using 1×10-4 g of SiO2/Fe3O4 nanocatalyst in short reaction time. Furthermore, the 4-nitrophenol reduction process utilizing SiO2/Fe3O4 nanostructures as catalyst was very economical and efficient in term of ease of synthesis, low raw materials expenditure and fast recovery/separation of the catalyst using an external magnetic field. In the 5th part of the study, magnetite (Fe3O4) nanoparticles were synthesized by simple and facile chemical co-precipitation method at ambient laboratory conditions. The synthesized Fe3O4 nanostructures were characterized for their morphology, size, crystalline structure and component analysis using Field Emission Scanning Electron Microscopy (FE-SEM), Fourier Transform Infrared spectroscopy (FT-IR), Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD) and Electron Dispersive X-Ray Spectroscopy (EDX). The catalytic properties of Fe3O4 nanoparticles were evaluated for the electrochemical oxidation of methanol. For this purpose, the magnetite NPs were coated on the surface of Indium Tin Oxide (ITO) electrode and used as a working electrode in the oxidation methanol process. The effect of potential scan rate, the concentration of methanol, the volume of electrolyte and catalyst (Fe3O4 NPs) deposition volume was studied to get high peak current densities for methanol oxidation. This study revealed that Fe3O4/ITO electrode was highly stable and selective towards methanol electrochemical oxidation in basic (KOH) media. In the 6 th study, a sensitive fluorescence sensor was synthesized for the detection of Cu2+ ions based on the fluorescence quenching of pyranine fluorophore. The sensor is composed of pyranine functionalized amino silane shell modified magnetite (Fe3O4) core nanostructures. The synthesized nanomaterials were characterized by different characterization techniques which include FTIR, XRD, VSM and TEM. The fluorescence viii response of Fe3O4@-SiO2-NH2-Pyr nanosensor towards Cu2+ ions showed an enhanced and selective fluorescence quenching. The developed nanosensor was successfully applied for the fluorescence detection and determination of trace levels of Cu2+ ions in real water samples. Pyranine functionalized magnetic nanoparticles were found to be highly selective for Cu2+ ions with no response from other interfering ions and biomolecules. The LOD value for the designed nanosensor was 6×10-9 mol/L for Cu2+ ions.