Xiujuan Wang

ZnO-nanotaper array sacrificial templated synthesis of noble-metal building-block assembled nanotube arrays as 3D SERS-substrates

This paper describes a ZnO-nanotaper array sacrificial templated synthetic approach for the fabrication of the arrays of nanotubes with tube-walls assembled by building-blocks of Ag-nanoplates, Au-nanorods, Pt-nanothorns or Pd-nanopyramids, thus possessing high-density 3D “hot spots” in sub-10-nm gaps of neighboring building blocks with nano-tips, -corners or -edges. Additionally, these hierarchical nanostructure arrays possess high surface area with rich surface chemistry, being beneficial to capturing the analyte. The Ag-nanoplateassembled nanotube arrays can be used as sensitive surface-enhanced Raman scattering (SERS) substrates with good signal uniformity and reproducibility. Using such Ag hierarchical nanostructure arrays as SERS-substrates, not only has 10−14 M rhodamine 6G been identified, but also 10−7 M polychlorinated biphenyls (PCBs, a notorious class of persistent organic pollutants) are recognized, and even two congeners of PCBs can be identified in a mixture, showing the potential applications of the materials in SERS-based rapid detection of environmental organic pollutants.

Gap-tunable Ag-nanorod arrays on alumina nanotip arrays as effective SERS substrates

Large area arrays of length-tunable alumina nanotips on the joints of hexagonally patterned conical-pores in an anodic aluminum oxide (AAO) template are achieved via a repeated process of anodizing Al foil for pore growth downwards and phosphoric acid etching for pore-widening. By top-view sputtering Ag on the alumina nanotip arrays, hexagonally patterned arrays of Ag-nanorods (Ag-NRs) on the alumina nanotips and uniformly distributed Ag-nanoparticles (Ag-NPs) on the upper rim of the inner surface of the conical-pores are obtained and they exhibit strong surface-enhanced Raman scattering (SERS) activity due to the high density of sub-10 nm gaps between the nearest neighboring Ag-NRs and between the adjacent Ag-NPs. The resultant nanostructures are tailored to attain an optimal SERS enhancement factor of ∼3.2 × 107 by tuning the Ag-sputtering duration. SERS measurements demonstrate that the as-fabricated large-scale Ag-nanostructures can serve as highly sensitive and reproducible SERS substrates. Finite element method calculation also confirms that the fabricated substrates possess excellent SERS activity. By modifying the Ag-NR arrays with mono-6-thio-β-cyclodextrin, the SERS detection limit of PCB-77 (a congener of polychlorinated biphenyls (PCBs)) reaches 10−6 M, showing potential in SERS-based rapid detection of trace PCBs, a kind of global environmental hazardous material.

Tapered Optical Fiber Probe Assembled with Plasmonic Nanostructures for Surface-Enhanced Raman Scattering Application

Optical fiber-Raman devices integrated with plasmonic nanostructures have promising potentials for in situ probing remote liquid samples and biological samples. In this system, the fiber probe is required to simultaneously demonstrate stable surface enhanced Raman scattering (SERS) signals and high sensitivity toward the target species. Here we demonstrate a generic approach to integrate presynthesized plasmonic nanostructures with tapered fiber probes that are prepared by a dipping-etching method, through reversed electrostatic attraction between the silane couple agent modified silica fiber probe and the nanostructures. Using this approach, both negatively and positively charged plasmonic nanostructures with various morphologies (such as Au nanosphere, Ag nanocube, Au nanorod, Au@Ag core-shell nanorod) can be stably assembled on the tapered silica fiber probes. Attributed to the electrostatic force between the plasmonic units and the fiber surface, the nanostructures do not disperse in liquid samples easily, making the relative standard deviation of SERS signals as low as 2% in analyte solution. Importantly, the detection sensitivity of the system can be optimized by adjusting the cone angle (from 3.6° to 22°) and the morphology of nanostructures assembled on the fiber. Thus, the nanostructures-sensitized optical fiber-Raman probes show great potentials in the applications of SERS-based environmental detection of liquid samples.

Ostwald‐Ripening‐Induced Growth of Parallel Face‐Exposed Ag Nanoplates on Micro‐Hemispheres for High SERS Activity

Ag nanoplates, as two-dimensional plasmonic nanostructures, have attracted intensive attention due to their strong shape-dependent optical properties and related applications. Here parallel face-exposed Ag nanoplates vertically grown on micro-hemisphere surfaces have been achieved by firstly electrodepositing the micro-hemispheres assembled by Ag nanoplates, whose planar surfaces are stuck together, on indium tin oxide substrates, and then Ostwald ripening the as-electrodeposited micro-hemispheres in water. The sizes of the nanoplates and the gaps between the neighboring nanoplates have been tailored by tuning the Ostwald-ripening duration, so that the SERS activity of the micro-hemispheres has been remarkably improved. The improved SERS activity can be well explained by our systematic finite-element simulation. Therefore, Ostwald ripening offers a route to the synthesis of Ag nanoplates, and the optimization of plasmon coupling and SERS activity of nanostructure-assembled systems.

In situ synthesis of pristine-graphene/Ag nanocomposites as highly sensitive SERS substrates

In this paper, we proposed a simple in situ method for the synthesis of pristine-graphene/Ag nanocomposites by chemical reduction of Ag ions in a N-methyl pyrrolidone solution in which pristine-graphene had been homogeneously dispersed. Ag nanoparticles can be directly attached on the pristine-graphene surface without any functionality. The obtained pristine-graphene/Ag nanocomposites are investigated by transmission electron microscopy, X-ray diffraction, and X-ray photoemission spectroscopy. The results show that Ag nanoparticles are homogeneously distributed on the pristine-graphene sheets with high density. We further demonstrate that this pristine-graphene/Ag nanocomposites exhibit a very strong surface-enhanced Raman scattering (SERS) effect, which can be used as ultrasensitive SERS substrates for chemical and biological analysis.