Yu Huang

Silver Nanorods Wrapped with Ultrathin Al2O3 Layers Exhibiting Excellent SERS Sensitivity and Outstanding SERS Stability.

Silver nanostructures have been considered as promising substrates for surface-enhanced Raman scattering (SERS) with extremely high sensitivity. The applications, however, are hindered by the facts that their morphology can be easily destroyed due to the low melting points (~100u2009°C) and their surfaces are readily oxidized/sulfured in air, thus losing the SERS activity. It was found that wrapping Ag nanorods with an ultrathin (~1.5u2009nm) but dense and amorphous Al2O3 layer by low-temperature atomic layer deposition (ALD) could make the nanorods robust in morphology up to 400u2009°C, and passivate completely their surfaces to stabilize the SERS activity in air, without decreasing much the SERS sensitivity. This simple strategy holds great potentials to generate highly robust and stable SERS substrates for real applications.

Enhanced surface-enhanced Raman scattering performance by folding silver nanorods

Folding straight Ag nanorods into zig-zag structures could generate corners or bends that become potential hot spots for surface-enhanced Raman scattering (SERS). Using a dynamic shadowing growth method, zig-zag silver nanorod arrays of different bending number N with a fixed total rod length are fabricated, and their SERS performance are measured and compared using the Raman probe Rhodamine 6G. The SERS intensity increases with N when Nu2009 u20094. The results suggest that folding silver nanorods into three-dimensional structures is a promising way to design highly sensitive SERS substrates.

Ag Nanorods Coated with Ultrathin TiO2 Shells as Stable and Recyclable SERS Substrates

TiO2-coated Ag nanorods (Ag@TiO2 NRs) have been fabricated as multifunctional surface-enhanced Raman scattering (SERS) substrates. Uniform TiO2 shells could sufficiently protect the internal Ag NRs against oxidation and sulfuration, thus the temporal stability of SERS substrates was markedly improved. Meanwhile, due to the synergetic effect between crystalline TiO2 and Ag, the nanocomposites could clean themselves via photocatalytic degradation of the adsorbed molecules under ultraviolet irradiation and water dilution, making the SERS substrates renewable. Such Ag@TiO2 NRs were shown to serve as outstanding SERS sensors featuring high sensitivity, superior stability and recyclability.

Hybridized plasmon modes and near-field enhancement of metallic nanoparticle-dimer on a mirror

For the attractive plasmonic structure consisting of metal nanoparticles (NPs) on a mirror, the coexistence of near-field NP-NP and NP-mirror couplings is numerically studied at normal incidence. By mapping their 3D surface charge distributions directly, we have demonstrated two different kinds of mirror-induced bonding dipole plasmon modes and confirmed the bonding hybridizations of the mirror and the NP-dimer which may offer a much stronger near-field enhancement than that of the isolated NP dimers over a broad wavelength range. Further, it is revealed that the huge near-field enhancement of these two modes exhibit different dependence on the NP-NP and NP-mirror hot spots, while both of their near-field resonance wavelengths can be tuned to the blue exponentially by increasing the NP-NP gaps or the NP-mirror separation. Our results here benifit significantly the fundamental understanding and practical applications of metallic NPs on a mirror in plasmonics.

Tunable SERS-Tags-Hidden Gold Nanorattles for Theranosis of Cancer Cells with Single Laser Beam

With the use of gold nanostructures, photothermal therapy (PTT) of cancer has great advantages compared to conventional methods, such as noninvasive targeted destruction and easily operation. Generally speaking, respective diagnosis and therapy of tumor require at least two instruments, leading to incongruence of tumor borders between diagnosis and therapy. To tackle this problem, tunable SERS-tags-hidden gold nanorattles (STHGNRs) have been designed and developed here for theranosis of cancer with single laser beam. The surface plasma resonance peak of STHGNRs can be tuned from visible region to near-infrared region by controlling the cavity size and shell thickness. The outer shells not only improve the stability of the SERS reporters but also enhance the brightness by more than two order magnitude compared to gold nanoparticles. In vitro study, immuno STHGNRs can serve as theranosis agents simultaneously for sensitive and efficient theranosis of cancer cells.

Tunable Lattice Coupling of Multipole Plasmon Modes and Near-Field Enhancement in Closely Spaced Gold Nanorod Arrays

Considering the nanogap and lattice effects, there is an attractive structure in plasmonics: closely spaced metallic nanoarrays. In this work, we demonstrate experimentally and theoretically the lattice coupling of multipole plasmon modes for closely spaced gold nanorod arrays, offering a new insight into the higher order cavity modes coupled with each other in the lattice. The resonances can be greatly tuned by changes in inter-rod gaps and nanorod heights while the influence of the nanorod diameter is relatively insignificant. Experimentally, pronounced suppressions of the reflectance are observed. Meanwhile, the near-field enhancement can be further enhanced, as demonstrated through surface enhanced Raman scattering (SERS). We then confirm the correlation between the near-field and far-field plasmonic responses, which is significantly important for maximizing the near-field enhancement at a specific excitation wavelength. This lattice coupling of multipole plasmon modes is of broad interest not only for SERS but also for other plasmonic applications, such as subwavelength imaging or metamaterials.

Rapid Detection of Polychlorinated Biphenyls at Trace Levels in Real Environmental Samples by Surface-Enhanced Raman Scattering

Detection of trace levels of persistent pollutants in the environment is difficult but significant. Organic pollutant homologues, due to their similar physical and chemical properties, are even more difficult to distinguish, especially in trace amounts. We report here a simple method to detect polychlorinated biphenyls (PCBs) in soil and distilled spirit samples by the surface-enhanced Raman scattering technique using Ag nanorod arrays as substrates. By this method, polychlorinated biphenyls can be detected to a concentration of 5 μg/g in dry soil samples within 1 minute. Furthermore, based on simulation and understanding of the Raman characteristics of PCBs, we recognized homologues of tetrachlorobiphenyl by using the surface-enhance Raman scattering method even in trace amounts in acetone solutions, and their characteristic Raman peaks still can be distinguished at a concentration of 10−6 mol/L. This study provides a fast, simple and sensitive method for the detection and recognition of organic pollutants such as polychlorinated biphenyls.

Universal Near-Field Interference Patterns of Fano Resonances in Two-Dimensional Plasmonic Crystals

AbstractWe have demonstrated directly that the physical origin of Fano resonances in two-dimensional (2D) plasmonic crystals (PCs) is a wave-interference phenomenon. This is achieved by mapping the near-field interference patterns for low and high diffraction orders via three-dimensional finite element method (FEM) calculations. We show the near-field constructive and destructive resonances center at the corresponding Rayleigh anomaly (RA) wavelength of the lattice and lead to greatly enhanced and suppressed local electric fields around the nanoparticle, respectively. These collective resonances are generated by the coupling of localized surface plasmon resonances (LSPRs) with diffraction orders perpendicular or titled to the direction of the incident polarization, but not parallel to that. Further, it is revealed that such near-field interference patterns can apply to arbitrary 2D PCs. In addition, both the near- and far-field Fano-type spectral features are found to be continuously tuned by the lattice constants, which particularly benefit applications of PCs in enhanced sensing and emission.n Graphical abstractBoth the constructive and destructive near-field interference patterns of the diffraction order (1, 1) in two-dimensional plasmonic crystals are mapped numerically

The Nanofabrication and Application of Substrates for Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) was discovered in 1974 and impacted Raman spectroscopy and surface science. Although SERS has not been developed to be an applicable detection tool so far, nanotechnology has promoted its development in recent decades. The traditional SERS substrates, such as silver electrode, metal island film, and silver colloid, cannot be applied because of their enhancement factor or stability, but newly developed substrates, such as electrochemical deposition surface, Ag porous film, and surface-confined colloids, have better sensitivity and stability. Surface enhanced Raman scattering is applied in other fields such as detection of chemical pollutant, biomolecules, DNA, bacteria, and so forth. In this paper, the development of nanofabrication and application of surface-enhanced Ramans scattering substrate are discussed.

Pinhole Effect on the Melting Behavior of Ag@Al2O3 SERS Substrates.

High-temperature surface-enhanced Raman scattering (SERS) sensing is significant for practical detections, and pinhole-containing (PC) metal@oxide structures possessing both enhanced thermal stability and superior SERS sensitivity are served as promising SERS sensors at extreme sensing conditions. Through tuning the Al2O3 precursors’ exposure time during atomic layer deposition (ALD), Al2O3 shells with different amount of pinholes were covered over Ag nanorods (Ag NRs). By virtue of these unique PC Ag@Al2O3 nanostructures, herein we provide an excellent platform to investigate the relationship between the pinhole rate of Al2O3 shells and the melting behavior, high-temperature SERS performances of these core-shell nanostructures. Pinhole effect on the melting procedures of PC Ag@Al2O3 substrates was characterized in situ via their reflectivity variations during heating, and the specific melting point was quantitatively estimated. It is found that the melting point of PC Ag@Al2O3 raised along with the decrement of pinhole rate, and substrates with less pinholes exhibited better thermal stability but sacrificed SERS efficiency. This work achieved highly reliable and precise control of the pinholes over Al2O3 shells, offering sensitive SERS substrates with intensified thermal stability and superior SERS performances at extreme sensing conditions.