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

• Main Authors: HSU, SHAO-HSUN, 許少勳
• Other Authors: YANG, TZYY-SCHIUAN
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/39438111120729785999

碩士 === 國立中正大學 === 化學暨生物化學研究所 === 104 === 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