金@銀核殼奈米棒之製備並作為表面增強拉曼散射基材

The Preparation of Au@Ag Core-Shell Nanorods as a Substrate for Surface-Enhanced Raman Scattering

許少勳(SHAO-HSUN HSU)

指導教授 ： 楊子萱

國立中正大學/理學院/化學暨生物化學系/碩士(2016年)

摘要

奈米銀的表面電漿子共振強度雖比奈米金高，但是在銀棒的製程上難以得到好的產率與均勻度；相對地，奈米金發展歷史悠久，至今已有不少製程能夠合成高產率且粒徑均勻的金棒，但其表面電漿子共振的強度卻相較於奈米銀弱。在幾何結構上，相較於球型角狀結構會因為電子的聚集而擁有更強的表面電漿子共振強度。因此我們想尋找一種金棒＠銀尖端的核殼結構，藉由互補的概念，既賦予此類複合型的奈米粒子既有好的表面電漿子共振強度亦能維持幾何形狀的均勻度。
我們使用無預金種法（in-situ seed growth）合成金奈米棒，藉由成核與成長在同一反應環境中，減少化學藥品的消耗，提供了簡單、快速合成金奈米棒的方法。我們藉由調整硝酸銀濃度控制金棒的長寬比。研究結果得到當硝酸銀濃度範圍控制在0.02 mM ~ 0.08 mM之間，相對應的長軸表面電漿子共振波帶及金棒長寬比皆能與硝酸銀濃度呈線性關係，其決定係數R2分別為0.9924及0.9975。
接著我們選用長寬比約莫3的金棒作為基板。由於金棒兩端曲率大，使得介面活性劑cetyltrimethyl ammonium bromide（CTAB）吸附得比側邊{110}面少，能夠讓銀離子先還原並沉積在金棒的兩端，逐漸形成Ag-Au-Ag異相結構。經由製程上不斷地改良，我們控制了成長液的pH值調整還原劑的強度，調控[Ag+]：[AA]：[Au3+] 試劑之間的比例以確保加入的銀鹽能夠完全地被還原，並克服了因金棒基材寬度不一造成曲率差異的問題。最終我們成功合成了尺寸相近，形狀均勻之金棒＠銀尖端的核殼結構。但溶液中有過多的Br-會與Ag+形成CTA-AgBr複合粒子沉澱，造成吸收光譜產生許多雜訊。故我們使用傾析法並降低CTAB/AgNO3的比例去減少吸收光譜中的雜訊干擾。
最後，我們使用R6G、4-NTP分子去測試不同形狀、組成之金銀核殼奈米粒子其拉曼增強的效果。考慮不同種類的奈米粒子會因本身的寬度而造成兩端吸附分子多寡的因素，我們將拉曼訊號強度對奈米粒子溶液濃度及粒子兩端的表面積做正規化的處理。證實拉曼增強效益會因金棒兩端奈米銀的存在而有所提升。對於4-NTP分子，拉曼訊號強度在金棒@銀尖結構對上單純金棒強度之比值為3.8倍，而在金棒@銀殼結構對單純金棒強度比值為4.4倍；對於R6G分子，拉曼訊號強度在金@銀尖結構對上單純金棒強度之比值為0.9倍，而在金@銀殼結構對單純金棒強度比值為1.0倍。由於我們假設金銀核殼粒子的濃度與原金棒濃度一致，而實際合成過程中離心會造成粒子數目的損失，故這些估計的強度比值皆為下限。

中文關鍵字:金銀核殼結構,表面增強拉曼散射

關鍵字

金銀核殼結構 ； 表面增強拉曼散射

並列摘要

Although the surface plasmon resonance of silver nanorods（Ag NRs）is stronger than that of gold nanorods, the chemical stability and geometrical uniformity of Ag NRs are relatively low. Since the sharp tips of nanoparticles accumulate more free electrons and then produce higher SPR than flat surfaces. We pursuit the development of Au NR@Ag tips core-shell structures to increase the enhancement of surface enhanced Raman scattering（SERS）.
We used in-situ seed-growth method to synthesize Au nanorods as a template for the deposition of Ag atoms. The aspect ratio and longitudinal surface plasmon resonance（LSPR）band of Au nanorods were tuned by adjusting the concentration of AgNO3 from 0.02 mM to 0.08 mM. The relation between [AgNO3] and the position of LSPR band and that with aspect ratio（AR）can be linearly fitted with the coefficient of determination R2 = 0.9924 and 0.9975, respectively.
We chosed Au NRs with AR=3 as a template, then we removed the excess cetyltrimethylammonium bromide (CTAB) in the solution by centrifugation and dispersed them in a growth solution of Ag. Since the terminal ends of Au NRs had large curvature, amount of CTAB on the ends is less than {110} facets. This results in that Ag+ preferred to deposit on the ends of Au NRs. We decreased the reducing ability of ascorbic acid by lowering pH value to between 5 to 6, making the Ag ions mildly reduced and selectively deposited on the ends of Au nanorods. We also adjusted the ratio of [Ag+]：[AA]：[Au3+] to make sure that majority Ag+ ions could be reduced on Au NRs and overcomed the curvature difference which caused by non-uniform width of Au NRs. Finally, we successfully synthesized Au@Ag core shell structure with uniform size and shape. However, the excess amount of Ag+ and Br- in the solution precipitated due to a small solubility product (Ksp = 5×10-13 M2). We decreased the ratio of CTAB to AgNO3 and conducted decantation to eliminate AgBr interference in optical spectra.
We used R6G and 4-NTP to estimate the SERS enhancement of these core-shell particles. We normalized the SERS intensity by dividing the nanoparticles concentration and the surface area of the ends of nanoparticles. For 4-NTP molecules, the ratio of SERS intensity from Au@Ag tips to that from Au NRs was 3.8；The ratio of SERS intensity for the systems of Au@Ag shell vs. Au NRs was 4.4. For R6G molecules, the ratio of SERS intensity for the systems of Au@Ag tips vs. Au NRs was 0.9 and 1.0 for the systems of Au@Ag shell vs. Au NRs. These ratios are the lowest limit.

Key words: Gold-Silver core-shell structure, Surface-enhanced Raman scattering

並列關鍵字

Gold-Silver core-shell structure ； Surface-enhanced Raman scattering

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