Synthesis of Gold Nanoshells and Their Use in Sensing Applications
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Synthesis of Gold Nanoshells and Their Use in Sensing Applications Yugang Sun and Younan Xia Department of Chemistry, University of Washington, Seattle, WA 98195-1700, U.S.A. ABSTRACT A general approach involved template-engaged, galvanic replacement reactions has been developed to prepare metallic nanostructures with hollow interiors by reacting solutions of appropriate salts with metallic solid nanoparticles. The reaction between aqueous chloroauric acid and silver nanoparticles was used as a typical example to demonstrate the synthesis of gold nanoshells. The morphology, void space, and wall thickness of these hollow structures were all determined by the silver templates, which were completely converted into soluble species during the replacement reaction. The extinction peaks of these gold nanoshells were considerably redshifted as compared to solid gold colloids having approximately the same dimensions. In addition, the surface plasmon resonance of gold nanoshells exhibited a much more sensitive response toward environmental changes even when compared with solid colloids with a mean size much smaller than that of gold nanoshells. INTRODUCTION Considerable efforts have been devoted to the fabrication and engineering of metallic nanostructures due to their applications in electronics, optics, catalysis, and biosensor [1]. The properties of a metallic nanostructure are determined by its size, shape, composition, and structure [2]. In principle, any one of these parameters could be used to fine-tune the properties of this nanostructure. For example, hollow nanoparticles made of metals exhibit catalytic activities and surface plasmonic properties significantly different from their solid counterparts [3]. Palladium nanoshells with hollow interiors had been demonstrated to serve as reusable catalyst to promote the Suzuki coupling reactions with high efficiency while solid palladium nanoparticles lose their catalytic ability after the first cycle [3a]. As for optical properties, Halas et al. have shown that the surface plasmon resonance (SPR) band of gold nanoshells containing dielectric cores (such as silica beads) could be continuously tuned over the spectral regime from 600 to 1200 nm by varying their diameter and shell thickness [3b]. A general approach to the preparation of hollow metallic nanostructures is to coat the surfaces of colloidal particles (e.g., gold or silver colloids, silica beads, and polymer latexes) with thin layers of the desired material (or the precursor), followed by selective removal of the colloidal templates through calcination or wet chemical etching. For example, in preparing palladium nanoshells, mercaptopropyltrimethoxysilane modified silica spheres were used as templates to adsorb palladium acetylacetonate, which could be transformed into metallic Pd by refluxing at 250 oC. The silica templates were finally removed with HF solution to produce hollow palladium nanoshells [3a]. The metallic nanoshells prepared using this method, however, were characterized by problems such as rou
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