Novel Ag-Cu substrates for surface-enhanced Raman scattering

Ag dendrites were deposited on rough Cu plate by a simple galvanic displacement process between Ag ion and Cu under room temperature. Surface-enhanced Raman scattering (SERS) performances have been studied using Rhodamine 6G (R6G) probe molecules on this kind of Ag-Cu substrates. The high SERS enhancements are attributed to the highly branched Ag dendritic nanostructures and Ag nanoparticles formed on the trunks, branches, and even leaves.     

A nanoforest structure for practical surface-enhanced Raman scattering substrates

A nanoforest structure for surface-enhanced Raman scattering (SERS) active substrates is fabricated and analyzed. The detailed morphology of the resulting structure can be easily controlled by modifying the process parameters such as initial gold layer thickness and etching time. The applicability of the nanoforest substrate as a label-free SERS immunosensor is demonstrated using influenza A virus subtype H1N1. Selective binding of the H1N1 surface antigen and the anti-H1 antibody is directly detected by the SERS signal differences. Simple fabrication and high throughput with strong in-plane hot-spots imply that the nanoforest structure can be a practical sensing component of a chip-based SERS sensing system.     

Silver-embedded zeolite crystals as substrates for surface-enhanced Raman scattering

This paper reports the preparation of a type of Ag-embedded zeolite crystals as surface-enhanced Raman spectroscopy (SERS) substrates by chemical reduction of Ag⁺-exchanged ZSM-5. Ag⁺ ions were loaded into the zeolite framework by ion exchange. Then the exchanged-Ag⁺ ions were reduced and metallic silver clusters formed inside the zeolite channel. The resulting Ag-embedded zeolite crystals are characterized by using a number of techniques including X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy to confirm silver formed inside the crystal channel. The fabricated Ag-embedded ZSM-5 zeolite substrates displayed strong and reproducible SERS activity for different Raman probe molecules such as Tris(2,2′-bipyridyl) ruthenium(II) chloride (RuBpy) and rhodamine 6G (R6G). Since silver embedded into the zeolite channel without changing the crystal surface property, the Ag–ZSM-5 zeolite crystal can be used to prepare different SERS-active substrate (SERS-tags), in which different probe molecules may be detected. Such Ag-embedded zeolite substrate would be useful in chemical and biological sensing and in the development of SERS-based analytical devices.     

Silver nanowire layer-by-layer films as substrates for surface-enhanced Raman scattering

In this paper, the fabrication of highly stable, surface-enhanced Raman scattering (SERS) active dendrimer/silver nanowire layer-by-layer (LBL) films is reported. Ag nanowires, approximately 100 nm in diameter, were produced in solution and transferred, using the LBL technique, onto a single fifth-generation DAB-Am dendrimer layer on a glass substrate. The Ag nanowires, and the resulting LBL films were characterized using UV-visible surface plasmon absorbance, while the LBL films were further characterized by atomic force microscopy measurements and surface-enhanced Raman and resonance Raman scattering of several analytes. The dendrimer was found to effectively immobilize the Ag nanowires with increased control over spacing and aggregation of the particles. These films are shown to be excellent substrates for SERS/SERRS measurements, demonstrating significant enhancement, and trace detection capability. Several trial analytes were tested using a variety of excitation energies, and results confirmed effective enhancement of Raman signals throughout the visible range (442-785 nm) with different molecules. Analytes were deposited onto the enhancing Ag nanowire LBL films surface using both casting and Langmuir-Blodgett monolayer transferring techniques.     

Silver clusters onto nanosized colloidal silica as novel surface-enhanced Raman scattering active substrates

A new method is proposed for obtaining surface-enhanced Raman scattering (SERS)-active substrates by photochemical reduction of silver nitrate onto colloidal silica. Transmission electron microscopy (TEM) and UV-visible absorption spectroscopy are employed to investigate the nanoscale structure of the materials. High quality SERS spectra are obtained from different organic ligands to check the efficiency of these substrates. A marked stability of the colloidal suspension is ensured by the scarce tendency of the Ag-doped silica particles to aggregate by either aging or adsorption of ligand.     

Gold/palladium and silver/palladium colloids as novel metallic substrates for surface-enhanced Raman scattering

New surface-enhanced Raman scattering (SERS) substrates, composed of gold or silver colloidal nanoparticles doped with palladium, were prepared. These novel colloids are stable and maintain a satisfactory SERS efficiency, even after long aging. The interest in doping the coinage metal nanoparticles with palladium is due to the well-known catalytic activity of this metal. Transmission electron microscopy (TEM) and ultraviolet-visible absorption spectroscopy were used to characterize the shape and size of the metal particles. It was found that these bimetallic colloidal nanoparticles have a core-shell structure, with gold or silver coated with palladium clusters.     

Silver-coated zeolite crystal films as surface-enhanced Raman scattering substrates

Silver-coated zeolite A and zeolite NaX crystal films prepared by vacuum deposition were investigated as surface-enhanced Raman scattering (SERS) substrates. The substrates were active for the enhancement of Raman scattering from uranyl ions. A detection limit of 10(-5) M for uranyl was obtained using silver-coated zeolite A films. One advantage of these zeolite-based substrates is that the negatively charged microporous framework provides the selectivity for adsorption based on static electric charges. The SERS effects of positively charged uranyl ions and neutrally charged benzoic acid were compared. For the zeolite A substrate, there was a 100-times-greater sensitivity.     

Multilayer enhanced gold film over nanostructure surface-enhanced Raman substrates

We have developed a novel class of gold multilayer, surface-enhanced Raman scattering (SERS) substrates that are capable of enhancing SERS signals by 15.3-fold over conventional gold film over nanostructure (GFON) SERS substrates, making them comparable in sensitivity to optimized silver film over nanostructure (SFON) substrates, while providing the long-term stability obtained from gold. They are fabricated by depositing 10 A thick silver oxide islands on conventional GFON substrates, followed by deposition of a second continuous gold layer. The silver oxide layer acts as a dielectric spacer between the two continuous gold films and produces significantly enhanced SERS signals, as compared to optimized single layer substrates of the same geometry or comparable substrates prepared by deposition of silver islands that are not oxidized. In addition to the enhanced sensitivity of these multilayer substrates, they also exhibit long SERS active shelf-lives (i.e., months), with no measurable degradation in SERS enhancement, and relative standard deviations in SERS enhancement of less than 5.2% across the substrate's surface.     

A practical method to fabricate gold substrates for surface-enhanced Raman spectroscopy

We describe a practical method of fabricating surface-enhanced Raman spectroscopy (SERS) substrates based on dip-coating poly-l-lysine derivatized microscope slides in a gold colloidal suspension. The use of only commercially available starting materials in this preparation is particularly advantageous, aimed at both reducing time and the inconsistency associated with surface modification of substrates. The success of colloid deposition has been demonstrated by scanning electron microscopy (SEM) and the corresponding SERS response (giving performance comparable to the corresponding traditional colloidal SERS substrates). Reproducibility was evaluated by conducting replicate measurements across six different locations on the substrate and assessing the extent of the variability (standard deviation values of spectral parameters: peak width and height), in response to either Rhodamine 6G or Isoniazid. Of particular interest is the observation of how some peaks in a given spectrum are more susceptible to data variability than others. For example, in a Rhodamine 6G SERS spectrum, spectral parameters of the peak at 775 cm(-1) were shown to have a relative standard deviation (RSD) % of <10%, while the peak at 1573 cm(-1) has a RSD of >/=10%. This observation is best explained by taking into account spectral variations that arise from the effect of a chemisorption process and the local nature of chemical enhancement mechanisms, which affects the enhancement of some spectral peaks but not others (analogous to resonant Raman phenomenon).     

Silver nanocrystal-modified silicon nanowires as substrates for surface-enhanced Raman and hyper-Raman scattering

Metal catalyzed, CVD-grown silicon nanowires decorated by chemical assembly of closely spaced Ag nanocrystals were modified with the well-known "silver mirror" reaction and investigated as substrates for surface-enhanced Raman (SERS) and hyper-Raman (SEHRS) spectroscopy. Four chromophores were examined: Rhodamine 6G, crystal violet, a cyanine dye, and a cationic donor-acceptor substituted stilbene. After soaking the substrates overnight in 10(-4) M aqueous chromophore solutions, all four chromophores gave good-quality SERS spectra in < or =60 s using <1 microW of 458-nm cw laser power, and SEHRS spectra are obtained in < or =120 s using <1 mW of mode-locked 916-nm laser power. Results from this substrate are compared with those on colloidal silver nanoparticles deposited as a film, as well as surfaces grown by the silver mirror reaction.     

Plasma reduction of silver compounds for fabrication of surface-enhanced Raman scattering substrates

Several silver compounds were reduced by low-pressure air plasma to produce porous nanostructured surfaces as surface-enhanced Raman scattering (SERS) substrates. This method is advantageous because substrates are easy to prepare and the silver metal surface is inherently clean without spectroscopic background. Silver compounds were melted into 1-2 mm slugs on quartz slides and plasma treated for different lengths of time. Silver chloride was found to be the best compound to make reproducible and stable SERS substrates. SERS activity of the substrates was tested using L-tryptophan, 4-mercaptobenzoic acid, and adenine.     

New strategy for ready application of surface-enhanced resonance Raman scattering/surface-enhanced Raman scattering to chemical analysis of organic films on dielectric substrates

Dropping of appropriately concentrated AgNO3 and NaBH4 solutions, as well as laser-ablated Ag sols, onto organic molecules results in the formation of aggregated Ag nanoparticles that can induce surface-enhanced Raman scattering (SERS) for the molecules. The addition of flocculating agents such as alkali halides can further increase the Raman signals. We demonstrate in this work that Raman spectra can be obtained even for 0.01 monolayers of R6G on Si simply by spreading silver nanoparticles and/or fabricating Ag nanoparticles and nanoaggregates at the gaps and vacant sites of R6G molecules. The application prospect of the present methodology is extremely high, not only because of its simplicity but also because of the fact that the observation of vibrational spectra is one of the most incisive methods for understanding the chemical and physical phenomena on a variety of surfaces.     

Preparation of tetrapod-like nanostructure array and used as the surface-enhanced Raman scattering substrates

Nanostructure surface composed of tetrapod-like structure with the period of about 2 microm has been made by simple nanoprinted method, the collodion was poured directly on the surface of the template and tore off after it became solidification, and then a layer of gold was sputtered on the collodion surface. The electric field distributions of the tetrapod-like structure with different size and period have been simulated using finite element method. Surface-enhanced Raman spectra mapping image gotten from the substrates illustrated the uniform enhancement property of it. The diluted aqueous solution of Rhodamine 6G (50 nM) was used to test the enhanced property of substrates for trace detection and good quality spectra has been gotten. The spectra of melamine with the concentration of 1 mg/L (7.9 microM) on the substrates were gotten to show the enhanced ability for molecule with small Raman scattering cross section.     

Self-assembly nanoparticle based tripetaloid structure arrays as surface-enhanced Raman scattering substrates

This paper reports a novel highly ordered tripetaloid structure array (TPSA) which performs very well as an active surface-enhanced Raman scattering (SERS) substrate. The TPSA is easily fabricated by anisotropic etching of a self-assembly silica-nanoparticle bilayer and a subsequent metal deposition step, with notable uniformity and reproducibility. Electromagnetic simulation indicates that the narrow inter-gaps and edge protrusions in the TPSA act as hot spots. In addition, the peak electromagnetic field intensity in the inter-gaps changes slightly and periodically as the polarization of the incident light varies from 0°?to 360°. SERS experiments show that the SERS enhancement factor (EF) of a Au-film-covered TPSA is 12 times higher than that of regular Au-film-over-nanoparticles, and not sensitive to the polarization of the incident light. The spatially averaged EF of the TPSA is as high as 5.7?×?10(6), and the local EF of its hot spots is much higher.     

Characterization of novel Ag on TiO2 films for surface-enhanced Raman scattering

Novel Ag on TiO2 films are generated by semiconductor photocatalysis and characterized by ultraviolet-visible (UV/Vis) spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM), as well as assessed for surface-enhanced Raman scattering (SERS) activity. The nature and thickness of the photodeposited Ag, and thus the degree of SERS activity, is controlled by the time of exposure of the TiO2 film to UV light. All such films exhibit the optical characteristics (lambda(max) congruent with 390 nm) of small (< 20 nm) Ag particles, although this feature becomes less prominent as the film becomes thicker. The films comprise quite large (> 40 nm) Ag islands that grow and merge with increasing levels of Ag photodeposition. Tested with a benzotriazole dye probe, the films are SERS active, exhibiting activity similar to that of 6-nm-thick vapor-deposited films. The Ag/TiO2 films exhibit a lower residual standard deviation (approximately 25%) compared with Ag vapor-deposited films (approximately 45%), which is, however, still unacceptable for quantitative work. The sample-to-sample variance could be reduced significantly (< 7%) by spinning the film during the SERS measurement. The Ag/TiO2 films are mechanically robust and resistant to removal and damage by scratching, unlike the Ag vapor-deposited films. The Ag/TiO2 films also exhibit no obvious loss of SERS activity when stored in the dark under otherwise ambient conditions. The possible extension of this simple, effective method of producing Ag films for SERS, to metals other than Ag and to semiconductors other than TiO2, is briefly discussed.     

Vapor deposition method for sensitivity studies on engineered surface-enhanced Raman scattering-active substrates

The design and optimization of a vapor-phase analyte deposition method for limit of detection (LOD) studies on engineered surface-enhanced Raman scattering (SERS)-active substrates is presented. The vapor deposition method was designed to overcome current challenges in quantitative analysis of lithographically produced SERS substrates that are relatively small (hundreds of square micrometers). A custom-built flow cell was used to deposit benzenethiol from the vapor phase onto SERS-active Ag thin films, as the control substrates, and nanoaperture arrays that were generated by electron-beam lithography. The surface coverage of benzenethiol as a function of time was monitored using the ring stretching mode 1070-cm(-1) band and the trend was fit to Langmuir adsorption kinetics. The method was deemed reliable based on agreement between the LOD determined on the control substrates and previously reported values for those substrates. Application of the new method to a 20 x 20 microm(2) nanoaperture array yielded a LOD of 4.2 +/- 0.3 amol.     

Fabrication of high performance surface enhanced Raman scattering substrates by a solid-state ionics method

Silver nanostructures were prepared by a solid-state ionics method using fast ionic conductor RbAg(4)I(5) films under a direct current electric field (DCEF). The surface morphology of the silver nanostructures grown under different constant current fields was characterized by scanning electron microscopy (SEM). Rhodamine 6G (R6G) aqueous solutions were used as probe molecules to detect the Raman enhancement performance of the silver nanostructure substrates. The effect of external electric field current intensity on the surface morphology of the silver nanostructures during the preparation was studied in detail. The enhancement effect of the silver nanostructure surface enhanced Raman scattering (SERS) substrates with different surface morphologies toward R6G was determined. We found that disordered silver nanowires (DSNW), ordered silver nanowires (OSNW), densely arranged silver nanobamboo arrays (SNBA) and compactly arranged silver nanobud clusters (SNBC) were respectively obtained when the constant current intensity was 3?μA, 5?μA, 8?μA and 12?μA under the same vacuum evaporation plating conditions. The limiting concentrations of R6G for these SERS substrates were found to be 10(-7)?mol?l(-1), 10(-13)?mol?l(-1), 10(-13)?mol?l(-1) and 10(-16)?mol?l(-1), respectively.     

Dynamic rastering surface-enhanced Raman scattering (SERS) measurements on silver nanorod substrates

Surface-enhanced Raman spectra of a thiol-modified biotin derivative on oblique-angle-deposited silver nanorod (AgNR) array substrates were measured using both static and rotating rastering methods. We find that the rotating rastering method has a strong tendency to decrease the point-to-point relative standard deviation (RSD) compared to static measurements as well as decrease the effects of cumulative excitation exposure. The AgNR substrates treated with the modified biotin typically demonstrate intra-substrate RSDs of <10%, with an average RSD of ～3% when the rastering radius r=1 mm. The quantitative studies on the relationship between rastering radius, sampling area, and rastering frequency show that only the rastering radius appears to have significant effect on the measured RSD. Our results demonstrate that under the proper measurement and sample preparation conditions, the Ag nanorod substrates are very uniform.