| Summary: | 博士 === 國立清華大學 === 生醫工程與環境科學系 === 100 === The Au-Fe3O4 hybrid materials, especially dumbbell-like and flower-like nanoparticles, have been demonstrated to be a potential nanocomposite for various applications because of their enhanced physicochemical properties. In this study, an effective process for the synthesis of different morphologies of Au-Fe3O4 heterostructures and other M-Fe3O4 heterostructures (M= Ag, Pt, Pd) has been developed, and the effects of different morphologies of Au-Fe3O4 heterostructures on MRI/sensing and catalysis are systematically studied. The monodisperse and size-tunable magnetic Fe3O4 and Au nanoparticles (NPs) were first synthesized and optimized. The diameters of as-synthesized Fe3O4 NPs decrease upon increasing concentrations of iron oleate complex and oleic acid/oleylamine, while the sizes of Au NPs decrease with the increase in reaction temperature. The Au-Fe3O4 heterostructures are successfully fabricated by thermal decomposition of iron oleate-complex in the presence of Au seeds through a seed-mediated growth process. Different morphologies of Au-Fe3O4 heterostructures can be easily controlled by adjusting the amount of iron oleate-complex, size of Au seeds, duration, and solvent amount. The dumbbell-like and flower-like Au-Fe3O4NPs can be synthesized using 5 nm and 10 nm Au NPs as seeds, respectively. These heterostructures show a red-shift in surface Plasmon resonance band and enhanced magnetic property. In addition, other noble metal–iron oxide nanoparticles including Ag, Pt and Pd are successfully produced using the same synthesis procedure. The structural and electronic properties of epitaxial linkage in Au-Fe3O4 heterostructures were investigated by X-ray absorption spectroscopy (XAS). After conjugation with iron oxides, the d-hole population of Au NPs increases, indicating a charge transfer from Au to Fe3O4. In addition, the increase in Fe2+ valence state was observed in Au-Fe3O4 heterostructures, which gives the strong evidence on supporting the hypothesis of the charge transfer between Au and Fe3O4. The theoretical simulation of XAS further demonstrates the presence of Au-Fe bonding in the Au-Fe3O4 heterostructures and confirms the epitaxial linkage relationship.
The dumbbell- and flower-like Au-Fe3O4 heterostructures were further used as magnetically recyclable catalysts for 4-nitrophenol and 2,4-dinitrophenol reduction. The heterostructures exhibit bifunctional properties with high magnetization and excellent catalytic activity towards nitrophenol reduction. The pseudo-first-order rate constants for nitrophenol reduction are 0.63-0.72 min-1 and 0.38-0.46 min-1 for dumbbell- and flower-like Au-Fe3O4 heterostructures, respectively. In addition, the heterostructured nanocatalysts show good separability and reusability which can be repeatedly applied for nearly complete reduction of nitrophenols for at least 6 successive cycles. The reaction mechanism for nitrophenol reduction by Au-Fe3O4 nanocatalysts is also proposed and confirmed by XPS and FTIR analyses. In addition, several environmental parameters including the initial nitrophenol concentration, pH, and temperature were optimized for the reduction of 4-trophenol. The kinetic data of nitrophenol reduction could be well-described by the Langmuir-Hinshewood model with the activation energy of 26.3 kJ mol-1, clearly indicating the nature of surface-mediated reactions. The catalytic reduction of 4-nitrophenol was also examined at various pHs and found that higher pH value retards the hydrolysis rate of borohydride, resulting in lower catalytic efficiency on nitrophenol reduction.
Different morphologies of Au-Fe3O4 heterostructures were further used as potential contrast agent for magnetic resonance imaging (MRI). Since the particle surface coated with a dense organic molecules, 8-armed PEG-Amine were chosen as surface modification agent for phase transfer and bio-functionalize. The water-dispersed Au-Fe3O4 heterostructures were then used as MRI contrast agents, and r2 values of different morphology of NPs were 142.9, 124.1, 112.9, 127.7 mM 1s-1, respectively, for Fe3O4 NPs, 5 nm Au dumbbell-like NPs, 10 nm Au dumbbell-like NPs, and 10 nm Au flower-like NPs. In addition, the Au domain in the heterostructures can serve as optical probe to sense the tau-protein via hybridization-mediated aggregation. The developed nanosensor displays a linear range (0.5-50 ng/mL) with detection limit of 3 ng/mL for tau protein detection. The results obtained in this study clearly demonstrate that the Au-Fe3O4 heterostructures are multifunctional materials which can serve as an ideal platform to apply in the fields of various heterogeneous catalytic processes and biomedical diagnosis.
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