多功能金-磁鐵礦異核結構奈米材料的製備與催化及生醫應用研究

Fabrication of multifunctional gold-magnetite heterostructures: catalytic and biomedical application

林芳新(Fang-hsin Lin)

指導教授 ： 董瑞安

國立清華大學/原子科學院/生醫工程與環境科學系/博士(2012年)

摘要

在金-磁鐵礦混和材料(hybrid materials)中，以啞鈴型奈米粒子(dumbbell-like nanoparticles)及花朵型奈米粒子(flower-like nanoparticles)為一新穎多功能奈米材料，由於其結合二種材料於單一結構中，因此可同時結合光學、催化和磁性特質而廣泛應用於各個領域。 本研究的目的主要是建立一個有效的方法來合成不同形貌之金-磁鐵礦及不同金屬-磁鐵礦異質結構奈米粒子。並探討不同型貌之金-磁鐵礦異質結構於核磁共振顯影和光學感測之能力，及其於催化還原反應之協同效應。本研究首先探討單一氧化鐵與金奈米粒子的製備，在合成步驟中，利用增加反應溫度來降低金的尺寸及增加油酸鐵複合物的濃度及油酸和油胺來降低氧化鐵奈米粒子的尺寸。同時，利用熱裂解法以合成不同形貌之磁鐵礦-金雙功能性異質結構，並探討不同油酸鐵濃度、金粒子大小、溶劑、反應時間對於顆粒形貌的影響。成功製備而得的材料進行XRD、SQUID、紫外光可見光分光光度計等以鑑定其物化特性。結果顯示，金-磁鐵礦異質結構在吸收光譜中有發現紅位移現象及磁性增強等現象。另外，本研究更將此方法推廣至不同金屬-磁鐵礦之合成，其結果顯示，此合成法可成功製備銀、鉑、鈀等金屬與磁鐵礦之異核結構。
為了解金-磁鐵礦異質結構中之界面化學，本研究利用X光吸收光譜(X-ray absorption spectroscopy，XAS)來鑑定金-磁鐵礦異質結構之細部結構與電子特性，並利用延伸X光吸收細微結構(Extended X-ray fine structure，EXAFS)來鑑定界面連接及細微結構。其結果顯示，當金奈米粒子與磁鐵礦形成異質結構後，金奈米粒子的d-hole數量會增加，證實了在合成過程中，電子會由金轉移至磁鐵礦上。此外，在金-磁鐵礦異質結構中的二價鐵增加，亦證明金與磁鐵礦間的電荷轉移。而從XAS的理論模擬結果顯示在金-磁鐵礦異質結構中具有金與鐵的鍵結，證明金與磁鐵礦於界面有連結關係。
在應用方面，首先，探討啞鈴型與花型之金-磁鐵礦異質結構於還原對-硝基苯酚及2,4-二硝基酚之應用。研究結果發現，所製備出的金-磁鐵礦異核結構具有良好的磁性與催化特性，然而，由於啞鈴形與花朵形金-磁鐵礦異核結構間的磊晶成長方式(epitaxial growth)不同，導致其在磁性與催化活性上有所不同。以啞鈴型金-磁鐵礦異核結構而言，其對硝基酚化合物擬一階反應速率常數為0.63-0.72 min-1，而對於花朵形奈米粒子而言，其擬一階反應速率常數為0.38-0.46 min-1。除此之外，所製備而得的奈米粒子具有磁性回收之特點，其回收再使用率可重覆使用達6次以上，且轉換率近乎100%。同時，本研究也利用XPS及FTIR來了解金-磁鐵礦異核結構於硝基酚化合物的還原催化反應機制。本研究更進一步探討不同環境參數對於催化還原對硝基酚之效率影響，並使用Langmuir-Hinshewood速率模式來描述動力學數據，結果顯示，異核結構於催化硝基酚為表面反應，且其活化能為26.3 kJ/mol。而在pH效應下，發現在較高pH值的情況下，會減慢氫硼化物的降解而降低催化效率。
除催化特性探討外，本研究也評估不同型貌之金-磁鐵礦異核結構於核磁共振成像(MRI)之顯影能力。首先，先利用8-arm PEG-amine進行顆粒的表面修飾，以達最佳相轉移。所得之分散奈米材料再進行核磁共振成像，其結果顯示，當異核結構中的金粒子顆粒變小，其r2值會增加，由10 nm金-磁鐵礦的112.9 mM 1s-1增加至124.1 mM 1s-1(5 nm金-磁鐵礦異質結構)。而形貌方面，以花朵型之顯影較啞鈴型好，其r2分別為127.7、112.9 mM 1s-1。除此之外，亦將金-磁鐵礦奈米粒子進行生物分子修飾，以偵測tau protein。所發展的奈米偵測器對tau protein的偵測之線性範圍為0.5-50 ng/mL及偵測極限為3 ng/mL。
由本研究的結果可知，利用油酸鐵搭配金奈米粒子於高溫熱裂解為一簡單之方法以製備金-磁鐵礦異核奈米粒子，同時，藉由調控不同反應參數可有效控制其形貌及組成。在催化應用方面，顯示金-磁鐵礦異核結構具有協同效應、再回收性且催化能力優於單一金奈米粒子，因此可作為一理想之材料，進行不同領域的異相催化，其也具有潛力應用於純化，催化，監測設備及綠色化學等領域。除此之外，在生醫應用方面，實驗結果證明金-磁鐵礦於核磁共振成像(MRI)顯影能力及光學偵測能力深受形貌影響，因此，良好的調控金-磁鐵礦異核結構之形貌以突顯其磁學及光學特性能有效的將其應用於生醫領域之雙探針成像偵測。

關鍵字

金奈米粒子 ； 磁鐵礦 ； 異核結構 ； 核磁共振 ； 還原催化

並列摘要

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.

並列關鍵字

Gold nano particle ； Magnetite ； Heterostructure ； MRI ； Catalytic reduction

參考文獻

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