Search

The progress of plasmon-based technologies relies on an understanding of the properties of the enhanced electromagnetic fields generated by the coupling nanostrucutres1–6. Plasmon-enhanced applications include advanced spectroscopies7–10, optomechanics11, optomagnetics12 and biosensing13–17. However, precise determination of plasmon field intensity distribution within a nanogap remains challenging. Here, we demonstrate a molecular ruler made from a set of viologen-based, self-assembly monolayers with which we precisely measures field distribution within a plasmon nanocavity with ~2-A spatial resolution. We observed an unusually large plasmon field intensity inhomogeneity that we attribute to the formation of a plasmonic comb in the nanocavity. As a consequence, we posit that the generally adopted continuous media approximation for molecular monolayers should be used carefully. The strength of the plasmonic field between a plasmonic particle and a Au surface can be measured at ~2-A resolution by following the Raman peaks of a suitably labelled self-assembly monolayer.

Shortly after a major renovation at the Royal Ontario Museum, it was noticed that the glass panels in many of the new display cases exhibited fogging or hazing on the surface, sometimes in ...

Surface-enhanced Raman scattering (SERS) is a powerful technique that provides fingerprint vibrational information with ultrahigh sensitivity. However, only a few metals (gold, silver and copper) yield a large SERS effect, and they must be rough at the nanoscale. Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was developed to overcome the long-standing materials and morphological limitations of SERS. It has already been applied in a variety of fields such as materials science, electrochemistry, surface science, catalysis, food safety and the life sciences. Here, the principles and applications of SHINERS are highlighted. To provide an understanding of the plasmonics involved, finite-difference time-domain (FDTD) calculations and single nanoparticle SHINERS experiments are reviewed. Next, various shell-isolated nanoparticle (SHIN) types are described. Then a number of applications are discussed. In the first application, SHINERS is used to characterize the adsorption processes of pyridine on Au(hkl) single-crystal electrode surfaces. Then, SHINERS' applicability to food inspection and cultural heritage science is demonstrated by the detection of parathion and fenthion pesticides, and Lauth's violet (thionine dye). Finally, graphene-isolated Au nanoparticles (GIANs) are shown to be effective for multimodal cell imaging, photothermal cancer therapy and photothermally-enhanced chemotherapy. SHINERS is a fast, simple and reliable method, suitable for application to many areas of science and technology. The concept of shell-isolation can also be applied to other surface-enhanced spectroscopies such as fluorescence, infrared absorption and sum frequency generation.

Electrochemical methods are combined with shell-isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS) for a comprehensive study of pyridine adsorption on Au(111), Au(100) and Au(110) single crystal electrode surfaces. The effects of crystallographic orientation, pyridine concentration, and applied potential are elucidated, and the formation of a second pyridine adlayer on Au(111) is observed spectroscopically for the first time. Electrochemical and SHINERS results correlate extremely well throughout this study, and we demonstrate the potential of EC-SHINERS for thorough characterization of processes occurring on single crystal surfaces. Our method is expected to open up many new possibilities in surface science, electrochemistry and catalysis. Analytical figures of merit are discussed.

Surface molecular information acquired in situ from a catalytic process can greatly promote the rational design of highly efficient catalysts by revealing structure-activity relationships and reaction mechanisms. Raman spectroscopy can provide this rich structural information, but normal Raman is not sensitive enough to detect trace active species adsorbed on the surface of catalysts. Here we develop a general method for in situ monitoring of heterogeneous catalytic processes through shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) satellite nanocomposites (Au-core silica-shell nanocatalyst-satellite structures), which are stable and have extremely high surface Raman sensitivity. By combining operando SHINERS with density functional theory calculations, we identify the working mechanisms for CO oxidation over PtFe and Pd nanocatalysts, which are typical low- and high-temperature catalysts, respectively. Active species, such as surface oxides, superoxide/peroxide species and Pd–C/Pt–C bonds are directly observed during the reactions. We demonstrate that in situ SHINERS can provide a deep understanding of the fundamental concepts of catalysis., Rational design of heterogeneous catalysts requires molecular understanding of catalytic processes. Here, the authors attach PtFe and Pd nanocatalysts to Raman signal-enhancing Au-silica nanoparticles, allowing them to spectroscopically observe the active species and bonds involved in CO oxidation in real time.

Surface-enhanced Raman scattering (SERS) is a powerful fingerprint vibrational spectroscopy with a single-molecule detection limit, but its applications are generally restricted to 'free-electron-like' metal substrates such as Au, Ag and Cu nanostructures. We have invented a shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) technique, using Au-core silica-shell nanoparticles (Au@SiO(2) NPs), which makes SERS universally applicable to surfaces with any composition and any morphology. This protocol describes how to prepare shell-isolated nanoparticles (SHINs) with different well-controlled core sizes (55 and 120 nm), shapes (nanospheres, nanorods and nanocubes) and shell thicknesses (1-20 nm). It then describes how to apply SHINs to Pt and Au single-crystal surfaces with different facets in an electrochemical environment, on Si wafer surfaces adsorbed with hydrogen, on ZnO nanorods, and on living bacteria and fruit. With this method, SHINs can be prepared for use in ~3 h, and each subsequent procedure for SHINERS measurement requires 1-2 h.

NSFC [20620130427]; MOST [2007DFC 40440]; National Basic Research Program of China [2009CB930703, 2007CB815303]

We very recently reported a new spectroscopic application for expanding the versatility of surface Raman called “shell-isolated nanoparticle-enhanced Raman spectroscopy” or “SHINERS”. The most important and most difficult part of the SHINERS experiment is the effective transfer of the strong electromagnetic field from a gold core through the isolating silica or alumina shell to the probed surface. For this it is essential that the chemically inert dielectric shell be ultrathin (2–5 nm) yet pinhole-free. Herein we describe experimental and theoretical aspects of our SHINERS method in more detail. We provide a protocol for the synthesis and characterization of optimized shell-isolated nanoparticles (SHINs), and we examine the advantages of SHINERS nanoparticles over bare gold nanoparticles. We also present high-quality Raman spectra obtained from gold and platinum single-crystal surfaces in an electrochemical environment by our SHINERS technique. SHINERS is a simple and cost-effective approach that expands the flexibility of surface-enhanced Raman scattering (SERS) for an unprecedented diversity of applications in materials and surface sciences.

Fairly monodisperse “dog bone” shaped gold nanorods were obtained clean and in high yield. Their average aspect ratio could be adjusted from ∼ 2 to ∼ 3 by altering the amount of silver nitrate present in the growth solution. They were coated with a uniform, ultrathin shell of silica and used to detect parathion pesticide on an orange peel by shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS).

We have rationally synthesized and optimized catalytic nanoparticles consisting of a gold core, covered by a palladium shell, onto which platinum clusters are deposited (Au@Pd@Pt NPs). The amount of Pt and Pd used is extremely small, yet they show unusually high activity for electrooxidation of formic acid. The optimized structure has only 2 atomic layers of Pd and a half-monolayer equivalent of Pt (θPt ≈ 0.5) but a further increase in the loading of Pd or Pt will actually reduce catalytic activity, inferring that a synergistic effect exists between the three different nanostructure components (sphere, shell and islands). A combined electrochemical, surface-enhanced Raman scattering (SERS) and density functional theory (DFT) study of formic acid and CO oxidation reveals that our core–shell–cluster trimetallic nanostructure has some unique electronic and morphological properties, and that it could be the first in a new family of nanocatalysts possessing unusually high chemical reactivity. Our results are immediately applicable to the design of catalysts for direct formic acid fuel cells (DFAFCs).

The electrochemically-driven desorption and readsorption of cysteine, on scratched gold surfaces placed in a highly concentrated cysteine solution, was investigated. The ordinary Raman signal from cysteine in solution was significant relative to the SERS signal from adsorbed cysteine. However, the SERS signal was isolated from the ordinary Raman background by subtracting the signal obtained with the incident laser light polarized along the scratch (ordinary Raman) from the signal obtained with it polarized across the scratch (ordinary Raman + SERS). The method described here is applicable to other electrochemical systems where signal from solution-phase species can interfere with, or even overwhelm, signal from the adsorbate. For example, it may be used in SERS studies of water or ionic liquid structure in the double-layer region.

Surface-enhanced Raman scattering (SERS) was observed for oxazine 720 molecules adsorbed on a copper film pierced by nanohole arrays. Each of the arrays had a different hole periodicity. The dependence of SERS intensity on hole periodicity was examined using two experimental geometries: forward-scattering and backscattering. Electric field intensity perpendicular to the surface of the metal was calculated for each array using the finite-difference time-domain method, and for both geometries, the array with the greatest calculated value matched the array that gave the greatest experimentally observed SERS intensity.

Polarization-dependent surface-enhanced Raman scattering (SERS) was studied for oxazine 720 molecules adsorbed on a scratched gold surface placed in situ and under electrochemical control. A quantitative method for evaluating the observed polarization dependence will be introduced. This method takes into account the polarization artifacts caused by optical elements in the light microscope used for Raman microscopy. Intensity of the SERS obtained from oxazine 720 adsorbed on scratches in gold showed a polarization dependence after correction was made for these artifacts. In contrast, intensity of the ordinary Raman signal obtained from perchlorate ions in the solution above a scratched gold surface was found to be polarization-independent. Therefore, polarization effects can be used to selectively remove solution-phase interference signals from the SERS spectrum of an adsorbed analyte. These polarization effects were found to be independent of the applied potential, meaning the methodology is applicable to electrochemical SERS studies.

We propose that aromatic nitro and amine compounds undergo photochemical reductive and oxidative coupling, respectively, to specifically produce azobenzene derivatives which exhibit characteristic Raman signals related to the azo group. A photoinduced charge transfer model is presented to explain the transformations observed in para-substituted ArNO(2) and ArNH(2) on nanostructured silver due to the surface plasmon resonance effect. Theoretical calculations show that the initial reaction takes place through excitation of an electron from the filled level of silver to the lowest unoccupied molecular orbital (LUMO) of an adsorbed ArNO(2) molecule, and from the highest occupied molecular orbital (HOMO) of an adsorbed ArNH(2) molecule to the unoccupied level of silver, during irradiation with visible light. The para-substituted ArNO(2)(-)˙ and ArNH(2)(+)˙ surface species react further to produce the azobenzene derivatives. Our results may provide a new strategy for the syntheses of aromatic azo dyes from aromatic nitro and amine compounds based on the use of nanostructured silver as a catalyst.

Au-seed Ag-growth nanoparticles of controllable diameter (50-100 nm), and having an ultrathin SiO(2) shell of controllable thickness (2-3 nm), were prepared for shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Their morphological, optical, and material properties were characterized; and their potential for use as a versatile Raman signal amplifier was investigated experimentally using pyridine as a probe molecule and theoretically by the three-dimensional finite-difference time-domain (3D-FDTD) method. We show that a SiO(2) shell as thin as 2 nm can be synthesized pinhole-free on the Ag surface of a nanoparticle, which then becomes the core. The dielectric SiO(2) shell serves to isolate the Raman-signal enhancing core and prevent it from interfering with the system under study. The SiO(2) shell also hinders oxidation of the Ag surface and nanoparticle aggregation. It significantly improves the stability and reproducibility of surface-enhanced Raman scattering (SERS) signal intensity, which is essential for SERS applications. Our 3D-FDTD simulations show that Ag-core SHINERS nanoparticles yield at least 2 orders of magnitude greater enhancement than Au-core ones when excited with green light on a smooth Ag surface, and thus add to the versatility of our SHINERS method.

We used shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) to systematically study the adsorption of pyridine on low-index Au(hkl) and Pt(hkl) single crystal electrodes. Our gold-core silica-shell nanoparticles (Au@SiO(2) NPs) boost the intensity of Raman scattering from molecules adsorbed on atomically flat surfaces. The average enhancement factor reaches 10(6) for Au(110) and 10(5) for Pt(110), which is comparable to or even greater than that obtained for bare gold NPs (a widely adopted SERS substrate). 3D-FDTD simulations reveal that this large enhancement is due to the transfer of the "hotspots" from NP-NP gaps to NP-surface gaps. We also found that the SHINERS intensity strongly depends on the surface crystallographic orientation, with differences up to a factor of 30. Periodic DFT calculations and theoretical analysis of dielectric functions indicate that this facet-dependence is predominantly governed by the dielectric property of the surface. The results presented in this work may open up new approaches for the characterization of adsorbates and reaction pathways on a wide range of smooth surfaces.

Surface-enhanced Raman scattering (SERS) is a powerful technique for detection and characterization because of its extremely high sensitivity and the rich structural information that it can offer. However, most SERS substrates are composed of Au, Ag, or Cu, and a lack of substrate generality has greatly limited the breadth of the use of SERS. Recently, we have devised a method by which SERS can be obtained from virtually any surface. Au nanoparticles are coated with ultrathin silica shells. The Au core provides Raman signal enhancement; the silica shell prevents the core from coming into direct contact with probe/analyte molecules or the surface over which these particles are spread (i.e., prevents the contamination of the chemical system under study). In the present review, we expand upon previous discussion of the enhancement mechanism; procedures for the synthesis and characterization of our nanoparticles; and applications in surface chemistry, electrochemistry, and inspection.

We present the first in situ surface Raman spectra of hydrogen on rhodium under electrochemical conditions using gold-core rhodium-shell (Au@Rh) nanoparticles for SERS or gold-core silica-shell (Au@SiO(2)) nanoparticles for SHINERS. The advantage of SHINERS lies in the versatility to study single crystal surfaces such as the H-Rh(111) system.

The conductance of single-molecule junctions may be governed by the structure of the molecule in the gap or by the way it bonds with the leads, and the information contained in a Raman spectrum is ideal for examining both. Here we demonstrate that molecule-to-surface bonding may be characterized during electron transport by 'fishing-mode' tip-enhanced Raman spectroscopy (FM-TERS). This technique allows mutually verifiable single-molecule conductance and Raman signals with single-molecule contributions to be acquired simultaneously at room temperature. Density functional theory calculations reveal that the most significant spectral change seen for a gold-4,4′-bipyridine-gold junction results from the deformation of the pyridine ring in contact with the drain electrode at high voltage, and these calculations suggest that a stronger bonding interaction between the molecule and the drain may account for the nonlinear dependence of conductance on bias voltage. FM-TERS will lead to a better understanding of electron-transport processes in molecular junctions., The conductance of single-molecule junctions is affected by the structure of the molecule and how it is bound to the electrodes, which may be examined using Raman spectroscopy. Liu et al. have developed 'fishing-mode' tip-enhanced Raman spectroscopy, which allows the simultaneous determination of conductance and Raman spectra.

A comparative study of gold nanoparticles (Au NPs) growth employing cetyltrimethylammonium bromide (CTAB) adsorbent was performed. Au nanooctahedrons transformed into slightly truncated nanocubes without centrifugation, whereas they transformed into nanocubes with centrifugation. Our results indicate that the mass transfer of Au monomers can influence the shape evolution of NPs.