Letter pubs.acs.org/NanoLett
Gold Nanoframes by Nonepitaxial Growth of Au on AgI Nanocrystals for Surface-Enhanced Raman Spectroscopy Lei Zhang,† Tingzhuo Liu,† Kai Liu,† Lu Han,§ Yadong Yin,‡ and Chuanbo Gao*,† †
Center for Materials Chemistry, Frontier Institute of Science and Technology, and MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Xi’an Jiaotong University, Xi’an, Shaanxi 710054, China ‡ Department of Chemistry, University of California, Riverside, California 92521, United States § School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China S Supporting Information *
ABSTRACT: Plasmonic noble metal nanoparticles with defined interior nanogaps are of great significance to surface-enhanced Raman spectroscopy (SERS) applications owing to the presence of intraparticle hotspots. In this contribution, we discovered site-selective nonepitaxial growth of Au on nonmetallic AgI nanocrystals, and on the basis of this observation, we designed an unconventional route to synthesize monometallic Au nanoframes that possess ∼7 nm of interior nanogaps and ∼23 nm of overall size by templating of small AgI nanocrystals. Chemical bonding between Au and the iodide-rich surface of the AgI nanocrystals was proposed to play a critical role in the nonepitaxial growth of the Au nanoframes against the AgI nanocrystals. The Au nanoframes obtained from this synthesis showed superior SERS activity in detecting molecules of interest in low concentrations owing to the presence of intraparticle hotspots in additional to the interparticle ones, benchmarking against Au nanospheres. This intriguing synthesis may open up new opportunities toward a variety of noble metal/semiconductor nanoconjugates for a broad range of applications such as synergistic catalysis. KEYWORDS: gold nanoframe, nanogap, hotspot, nonepitaxial growth, surface-enhanced Raman spectroscopy
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nanostructures with controlled interior nanogaps, including nanocages20,21 and nanoframes,22−27 which are hollow nanocrystals enclosed by porous side faces and edges, respectively. These nanogaps may afford inherent hotspots for enhanced SERS activities.27−29 A few synthesis strategies have been reported to yield nanoframe structures, including dealloying of alloyed nanostructures and site-selective deposition of a metal on another metal nanocrystal. In a typical dealloying synthesis of cubic Au nanoframes, Ag nanocubes were first transformed into Au/Ag alloy nanoboxes by galvanic replacement with a Au salt, which was followed by dealloying of the nanoboxes to generate in sequence a porous structure of nanocages and eventually a nanoframe structure due to the continuously expanding pores on the side faces.22−24,27 On the other hand, a typical siteselective deposition method usually involves preferential deposition of a noble metal on the corners and edges of sacrificial metal nanocrystals, and nanoframes were obtained by subsequent etching of the nanocrystals.26,30−33 The success of both strategies relies on templating of preexsiting metal nanostructures, which usually possess relatively large feature sizes, and therefore, nanoframes were usually obtained with
lasmonic noble metal nanoparticles, Au and Ag for example, have attracted intensive interest in recent years owing to the outstanding optical properties, which render these materials a wide applicability in chemical or biomolecular sensing,1,2 clinical diagnosis,3,4 catalysis,5,6 and many other applications. In particular, these nanoparticles are capable of coupling to each other through near-field interactions, leading to a variety of interesting surface-enhanced phenomena.7−9 One important application of such plasmonic effect is the surface-enhanced Raman spectroscopy (SERS), which produces enhanced Raman signals of surface-absorbed molecules by many orders of magnitude and, therefore, enables quick and reliable sensing of molecules of interest in low abundance. The SERS sensitivity largely depends on electromagnetic “hotspots” where local electric field is extremely intense, and thus, the Raman signals from molecules at these sites are particularly strong and contribute to the main fraction of the overall intensity.8 In order to achieve maximal SERS activity, great efforts have been devoted to the creation of hotspots. In addition to interparticle nanogaps,10−12 nanostructures with inherent hotspots such as those containing interior nanogaps13−16 or sharp tips17−19 become the focus of recent studies because they may ensure a sufficient density of hotspots for sensitive detection of target molecules with high consistency by reducing the dependence on interparticle nanogaps, which could vary significantly with deposition process. In particular, Xia et al. proposed an important family of plasmonic metal © XXXX American Chemical Society
Received: February 27, 2015 Revised: June 9, 2015
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DOI: 10.1021/acs.nanolett.5b01544 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters relatively large interior nanogaps not necessarily favorable for SERS activities. Therefore, it is desirable to pursue appropriate templates that can afford nanoframes with nanogaps of a fairly small size (
