Uniformly-Assembled Metal Nanoparticles on Anodic Aluminum Oxide (AAO) Applied in Surface-Enhanced Raman Spectroscopy
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Uniformly-Assembled Metal Nanoparticles on Anodic Aluminum Oxide (AAO) Applied in Surface-Enhanced Raman Spectroscopy Zhixun Luo1, Boon H. Loo2, Jiannian Yao*,1 1
Beijing National Laboratory for Molecular Science (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China 2 Department of Chemistry, Towson University, Towson, Maryland 21252, USA ABSTRACT Colloidal Au/Ag nanoparticles can be controllably assembled on anodic aluminum oxide (AAO) surfaces; monolayer coating on the membrane on AAO with smaller pores, or a nano-net arrangement along the edges of AAO with larger pores. The supported Au and Ag nanoparticles on the AAO membranes are closely packed and exhibited localized surface plasmon resonance (LSPR). Thus AAO membrane coated with Au or Ag nanoparticles is a highly surface-enhanced Raman scattering (SERS) active substrate. High quality SERS spectra were obtained using fullerene molecules C60 & C70 as the probe molecules and the filtered Au nanoparticles as the substrate. Furthermore, new SERS systems were obtained from Au nanoparticles assembled into the pores of AAO-supported fullerene nano-tubes, and the C60/C70 nano-tube arrays loaded with Au nanoparticles. The new SERS systems made use of the contributions from AAO, the LSPR of the Au nanoparticles, and a uniform assembly of the probe molecules in the nanostructures. These approaches have also been applied to small organic molecule systems using Ag nanoparticles. Key words: AAO template; Au and Ag nanoparticles; Nanostructures; SERS; C60 and C70. INTRODUCTION Many kinds of ordering nanostructured materials with unique characteristics have been used in electronics, optics, medicine and machinery, etc. [1-5]. Template synthesis method has been recognized as one of the effective approaches in obtaining ordered nanostructures such as nanodots, nano-tubes and nano-wires [6-13]. Anodic aluminum oxide (AAO) template [1] with hexagonally ordered pores is an ideal template to be used in the synthesis of nanometer arrays [14-19]. Through anodic oxidation treatment in acid solution, high-purity aluminum foil can spontaneously form the pellumina which is a self-organizing ordered structure. Owing to the uniform aperture of template, the materials prepared also maintain the sameness in aperture and single dispersing, and importantly, it is easy to separate the nano-structures from the templates. Surface-enhanced Raman scattering (SERS) has been demonstrated to be one of the best techniques to study interfacial effects [20-26]. It has been used to study the orientation and behavior of adsorbed molecules on surfaces, and to analyze the interphase tropism, configuration and conformation of molecules. One of the key targets of the SERS technique is to optimize active substrates for ultrasensitive detection and the identification of adsorbed molecular species. We demonstrate the application of AAO templates for SERS detection by coating Au or Ag nanoparticles on the AAO surfaces, and by loading Au or Ag nanoparticles in the AAO pores [27]
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