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.     

Silver nanoparticles deposited on porous silicon as a surface-enhanced Raman scattering (SERS) active substrate

Silver nanoparticles were deposited spontaneously from their aqueous solution on a porous silicon (PS) layer. The PS acts both as a reducing agent and as the substrate on which the nanoparticles nucleate. At higher silver ion concentrations, layers of nanoparticle aggregates were formed on the PS surface. The morphology of the metallic layers and their SERS activity were influenced by the concentrations of the silver ion solutions used for deposition. Raman measurements of rhodamine 6G (R6G) and crystal violet (CV) adsorbed on these surfaces showed remarkable enhancement of up to about 10 orders of magnitude.     

Surface-enhanced Raman scattering (SERS) and surface-enhanced resonance Raman scattering (SERRS): a review of applications

Surface-enhanced Raman scattering (SERS) and surface-enhanced resonance Raman scattering (SERRS) can provide positive identification of an analyte or an analyte mixture with high sensitivity and selectivity. Better understanding of the theory and advances in the understanding of the practice have led to the development of practical applications in which the unique advantages of SERS/SERRS have been used to provide effective solutions to difficult analytical problems. This review presents a basic theory and illustrates the way in which SERS/SERRS has been developed for practical use.     

Base effects on fabrication of silver nanoparticles embedded silica nanocomposite for surface-enhanced Raman scattering (SERS)

In this paper, we studied on the effect of organic bases in the case of ethylene glycol based fabrication of silver nanoparticles embedded silica nanocomposite (Ag SNC) without heating. Considering their chemical structures, butylamine (BA), ethanolamine (EA), triethanolamine (TEA), tributylamine (TBA), octylamine (OA) and Jeffamine 500 (JA) were used as an organic base. In addition, the effect of the concentrations of AgNO3 and organic bases on the formation of Ag SNC was also examined. In conformity with the characteristics of Ag SNC, SERS signal intensity of benzenethiol on Ag SNC was measured. As a result, the SERS signal intensity of Ag SNCs was strongly dependent on the reaction conditions. Furthermore, when reacted under the best reaction condition with concentrations of AgNO3 and OA, 3 mM and 5 mM, respectively, a large-scale production of Ag SNC was possible under the mild conditions.     

Measurement of the Raman line widths of neat benzenethiol and a self-assembled monolayer (SAM) of benzenethiol on a silver-coated surface-enhanced Raman scattering (SERS) substrate

Raman line widths of neat benzenethiol and a self-assembled monolayer (SAM) of benzenethiol on a surface-enhanced Raman scattering (SERS) substrate have been measured using a mini spectrometer with a resolution (full width at half-maximum) of 3.3 ± 0.2 cm(-1). Values of 7.3 ± 0.7, 4.6 ± 0.6, 2.4 ± 0.6, 3.2 ± 0.5, 8.8 ± 0.9, and 11.0 ± 1.1 cm(-1) have been determined for the Raman line widths of the 414, 700, 1001, 1026, 1093, and 1584 cm(-1) modes of neat benzenethiol. Values of 13.3 ± 0.7, 9.1 ± 0.7, 5.1 ± 0.6, 5.9 ± 0.6, 13.3 ± 0.5, and 8.7 ± 0.5 cm(-1) have been determined for the SERS line widths of a benzenethiol SAM on a silver-coated SERS substrate for the corresponding frequency-shifted modes at 420, 691, 1000, 1023, 1072, and 1574 cm(-1). The line widths for the SERS modes at 420, 691, 1000, 1023, and 1072 cm(-1) are about a factor of two larger than those of the corresponding Raman modes. However, the line width of the SERS mode at 1574 cm(-1) is slightly smaller than the corresponding Raman mode at 1584 cm(-1).     

Surface-enhanced Raman scattering dendritic substrates fabricated by deposition of gold and silver on silicon

This paper reports a study on the preparation of gold nanoparticles and silver dendrites on silicon substrates by immersion plating. Firstly, gold was deposited onto silicon wafer from HF aqueous solution containing HAuCl4. Then, the silicon wafer deposited gold was dipped into HF aqueous solution of AgNO3 to form silver coating gold film. Scanning electron microscopy reveals a uniform gold film consisted of gold nanoparticles and rough silver coating gold film containing uniform dendritic structures on silicon surface. By SERS (surface-enhanced Raman scattering) measurements, the fabricated gold and silver coating gold substrates activity toward SERS is assessed. The SERS spectra of crystal violet on the fabricated substrates reflect the different SERS activities on gold nanoparticles film and silver coating gold dendrites film. Compared with pure gold film on silicon, the film of silver coating gold dendrites film significantly increased the SERS intensity. As the fabrication process is very simple, cost-effective and reproducible, and the fabricated silver coating gold substrate is of excellent enhancement ability, spatial uniformity and good stability.     

Combining surface plasmon resonance (SPR) spectroscopy with surface-enhanced Raman scattering (SERS)

The simultaneous measurement of surface plasmon resonance (SPR) spectroscopy and surface-enhanced Raman scattering (SERS) on flat metallic surfaces is demonstrated on a relatively simple experimental setup based on the Kretschmann configuration. This setup requires only minor modifications to standard Raman microscopes, and we show that it can be applied successfully to the most common conditions of SPR spectroscopy, i.e., water-based solutions on gold films. Our results emphasize the peculiar properties of the Kretschmann configuration for spectroscopy in general and SERS measurements in particular, especially in terms of the asymmetry between excitation and collection requirements. The combination of simultaneous SPR-SERS spectroscopy opens up interesting prospects in analytical science to study, for example, reaction kinetics at surfaces under conditions which are already available in commercial SPR instruments.     

Dual layer and multilayer enhancements from silver film over nanostructured surface-enhanced Raman substrates

Novel dual layer and multilayer silver film over nanostructure (SFON) substrates have been developed that provide surface-enhanced Raman scattering (SERS) signal enhancements of greater than 1000% compared to conventional single layer SFON substrates. These substrates provide signal enhancement factors of 3.8 x 10(5) and greater for a variety of SERS active analytes. Substrate preparation is accomplished by vapor depositing a thick (approximately 100 nm) layer of silver on top of an underlying layer of alumina nanoparticles, followed by deposition of additional layers of silver with silver oxide layers between them. Unlike previous dual layer silver island based substrates that have been developed, these substrates do not rely on achieving an optimal morphology via deposition of silver. Instead, these substrates rely on the roughness being provided by the original under-layer, providing enhanced substrate homogeneity and more reproducible signals than either silver island substrates or colloidal substrates. In addition, the signal enhancement gives these substrates extended lifetimes compared to conventional single layer SFON substrates. Finally, this study also shows that geometric surface structure and surface roughness factors play little or no role in this enhancement process, allowing for this multilayer fabrication process to be applied to many different types of substrates achieving similar or even greater results.     

Preparation of silver nanoparticles by photo-reduction for surface-enhanced Raman scattering

A substrate for surface-enhanced Raman scattering (SERS) has been developed. Based on the surface-catalyzed reduction of Ag⁺ by citrate on the silver nanoparticles surface under light irradiation, small silver seeds on a quartz slide can be enlarged. The optical properties and characteristics of the silver films have been investigated by ultraviolet-visible spectroscopy, scan electron microscope and atomic force microscopy (AFM). The results indicate that the particle size and shape are different at different reduction time. At the first 3 h, some triangular and hexagonal nanoparticles formed; with the reduction proceeding, the shape of the silver particles became irregular and the size became larger. The silver films obtained are very suitable as SERS active substrate. The relationship between SERS intensity and the reduction time has been investigated for 1,4-bis[2-(4-pyridyl)ethenyl]-benzene molecule adsorbed on the silver film. The SERS intensity reached a maximum at 8 h reduction. The AFM measurements indicate that roughness features with an average size of 100 nm are present on the surface, which yielded the strongest SERS signal. Pyridine was used as a probe molecule to investigate the enhancement factor (EF) of the silver films. According to the formalism of Tian and co-workers, the EF of the silver films is estimated to be 3.4 × 10⁵. The silver film that can remain active for more than 50 days would seem to be suitable for various analytical applications.     

Analysis of thin film coatings on poly(ethylene terephthalate) by confocal Raman microscopy and surface-enhanced Raman scattering

The characterization of thin coatings on polymers such as poly(ethylene terephthalate) (PET) is required in order to study chemical composition and coating continuity. Two different methods of applying Raman spectroscopy for this purpose are compared in this paper. Using confocal Raman microscopy, thick coatings (> 10 microns) are relatively easily identified; however, the Raman scattering from the acrylic coatings commonly used is much weaker than that of PET and consequently, there is a background due to the substrate. Thin acrylic coatings (< 1 micron) usually cannot be detected. Surface-enhanced Raman scattering (SERS) of uncoated PET gives intense signals and if the spectra are taken from the metal-coated side, there is no evidence of the underlying Raman scattering from the bulk. Acrylic coatings do not give sufficiently strong or reproducible SERS to be reliably identified, but even thin (20 nm) coatings completely block the SERS from the substrate. Only where gaps appear in the coating is the SERS of the underlying PET seen. To detect a positive signal from the coating, SERS active labels were incorporated into the acrylic at low concentrations either as a physical mixture or as reactive co-monomers. This uniquely labels the coating and allows detection and, in principle, mapping of the coverage. Thus, for thick (> > 1 micron) coatings, normal Raman spectroscopy is an effective technique for detecting the presence of the surface coating. However, it is ineffective with thin (< 1 micron) coatings, and SERS alone only indicates where the coating is incomplete or defective. However, when a SERS label is added, spectra can be detected from very thin coatings (20 nm). The concentration of the labels is sufficiently low for the coating to remain colorless.     

Silver-doped sol-gel film as a surface-enhanced Raman scattering substrate for detection of uranyl and neptunyl ions

A surface-enhanced Raman scattering (SERS) substrate containing silver particles was prepared by an acid-catalyzed sol-gel method. Silver nitrate was first doped into the sol-gel film followed by chemical reduction of the silver ions with sodium borohydride to produce silver particles. This silver-doped sol-gel substrate exhibits strong enhancement of Raman scattering from adsorbed uranyl ions with a detection limit of 8.5 x 10(-8) M, which is comparable to existing methods of uranyl detection such as spectrophotometry, fluorometry, and a SERS method based on ligand-modified solution silver colloids. However, in the present method, no preconcentration steps, chromogens, or complexing ligands are needed. Compared with the SERS method using Ag colloidal sols, the silver-doped sol-gel film has the advantage that the silver particles trapped in the sol-gel matrix are much more stable than Ag colloids in liquid media. Furthermore, porous silica sol-gel materials are known to have affinities toward many inorganic and organic molecules. The enhanced adsorption affinities could also lead to the increased SERS sensitivity. The performance of the new silver-doped sol-gel substrate was evaluated with uranyl ions and compared to that of a SERS substrate based on silver-coated silica beads prepared by vacuum deposition. The detection limit for the silver-doped sol-gel film was 104 times lower than that for the silver-coated silica beads. The sol-gel substrate was further used to obtain, for the first time, the surface-enhanced Raman spectrum of neptunyl ions in dilute aqueous solutions.     

In-situ synchrotron X-ray scattering study of thin film growth by atomic layer deposition

We report an atomic layer deposition chamber for in-situ synchrotron X-ray scattering study of thin film growth. The chamber was designed for combined synchrotron X-ray reflectivity and two-dimensional grazing-incidence X-ray diffraction measurement to do a in-situ monitoring of ALD growth. We demonstrate ruthenium thermal ALD growth for the performance of the chamber. 10, 20, 30, 50, 70, 100, 150 and 250-cycled states are measured by X-ray scattering methods during ALD growth process. Growth rate is calculated from thickness values and the surface roughness of each state is estimated by X-ray reflectivity analysis. The crystal structure of initial growth state is observed by Grazing-incidence X-ray diffraction. These results indicate that in-situ X-ray scattering method is a promising analysis technique to investigate the initial physical morphology of ALD films.     

Fabrication of gold nanoparticle modified ITO substrate to detect beta-amyloid using surface-enhanced Raman scattering

Alzheimer's disease is a progressive neurodegenerative disorder that is characterized by the deposition of beta-amyloid (Abeta) peptide and the formation of neurofibrillary tangles in neurons. The Abeta peptide is a key molecule in the pathogenesis of Alzheimer's disease and an important marker for early diagnosis. Surface-enhanced Raman scattering (SERS) has recently been attracting keen interest in various fields such as for biosensors or immunoassays. In this study, gold nanoparticles (Au NPs) were electrochemically deposited on an indium tin oxide (ITO) substrate at different heights. Abeta antibodies were immobilized on the Au-NP-coated ITO substrate, after which the interactions between the antigen and the antibody were determined via SERS spectroscopy. The SERS responses were strongest at the Au NP array height of 91 nm, with a good linear relationship that corresponded to the change in the concentration of the antigen. This Au-NP-array-mediated SERS can be applied with a highly sensitive immunodetection biosensor.     

Synchrotron X-ray reflectivity study of high dielectric constant alumina thin films prepared by atomic layer deposition

High dielectric constant (high-k) gate dielectric alumina films were prepared with nanoscale thicknesses on p-type silicon substrates by atomic layer deposition (ALD) with alternating pulses of trimethyl aluminum, nitrogen, ozone and nitrogen, and some of them were further thermally annealed. These high-k gate dielectric films were characterized by synchrotron X-ray reflectivity (XR), and the XR data were quantitatively analyzed, providing the following structural parameters of each gate dielectric film: the surface roughness and interfacial roughness, the electron density profile, the number of layers, and the thickness of individual layers. These structural characteristics were then analyzed in detail by considering the ALD processing conditions and post-thermal annealing history.     

Reproducible surface-enhanced Raman scattering spectra of bacteria on aggregated silver nanoparticles

Surface-enhanced Raman scattering (SERS) is proven to be a powerful technique for rapid identification and discrimination of microorganisms. However, due to the heterogeneous nature of the samples, the acquisition of reproducible spectra hinders the further development of the technique. In this study, we demonstrate the influence of the experimental conditions on SERS spectra. Then, we report a simple sample preparation method coupled with a light microscope attached to a Raman spectrometer to find a proper spot on the sample to acquire reproducible SERS spectra. This method utilizes the excited surface plasmons of the aggregated silver nanoparticles to visualize the spots on the sample. The samples are prepared using the concentrated silver colloidal solutions. The collection time for one spectrum is 10 s and each spectrum is a very good representative of the other spectra acquired from the same sample. The nature of the surface charge of the silver nanoparticles influences the spectral features by determining the strength of the interactions between nanoparticles and bacteria and the aggregation properties of the nanoparticles. Although increasing the colloid concentration in the sample resulted in reproducible spectra from arbitrary points on the sample, a great variation from sample to sample prepared with the different colloidal solution concentrations is observed.     

Adsorption of a cholesteric liquid crystal polyester on silver nanoparticles studied by surface-enhanced Raman scattering and micro Raman spectroscopy

A Raman study of the adsorption of thermotropic cholesteric liquid crystal polyester PTOBDME ([C(34)H(36)O(8)](n)) on Ag surfaces is presented in this work. The affinity and adsorption mechanism of this polymer was tested on Ag metal colloids and on Ag films obtained by direct immobilization of the colloidal nanoparticles. We have first studied the structure of PTOBDME suspended in several solvents in order to identify the Raman bands used as structural markers. The adsorption of the polymer leads to a deep conformational change involving both the main chain and the aliphatic side chain. The interaction of polymers like PTOBDME with metals could be interesting in the formation of functionalized surfaces, providing them with specific physicochemical properties with possible applications in recognition phenomena, which can be easily characterized by Raman spectroscopy.     

Organic nanowire-templated fabrication of alumina nanotubes by atomic layer deposition

Alumina nanotubes were fabricated by a template method. Tris-(8-hydroxyquinoline) gallium (GaQ3) organic nanowires were used as a soft template for coating with alumina using an atomic layer deposition technique. The deposition was conducted at 25 degrees C by using trimethylaluminum and distilled water as the precursors of Al2O3. Amorphous alumina nanotubes were obtained after removing the GaQ3 by dissolving in toluene or by heat treatment at 350 degrees C. The amorphous nanotubes could be crystallized by heating at 900 degrees C for 1 h in vacuum.     

Binding of aromatic isocyanides on gold nanoparticle surfaces investigated by surface-enhanced Raman scattering

The adsorption structure and binding of phenyl isocyanide (PNC), 2,6-dimethyl phenyl isocyanide (DMPNC), and benzyl isocyanide (BZI) on gold nanoparticle surfaces have been studied by means of surface-enhanced Raman scattering (SERS). PNC, DMPNC, and BZI have been found to adsorb on gold assuming a standing geometry with respect to the surfaces. The presence of the nu(CH) band in the SERS spectra denotes a vertical orientation of the phenyl ring of PNC, DMPNC, and BZI on Au. The lack of a substantial red shift and significant band broadening of the ring breathing modes implied that a direct ring pi orbital interaction with metal substrates should be quite low. For PNC, the band ascribed to the C-NC stretching vibration was found to almost disappear after adsorption on Au. On the other hand, the C-NC band remained quite strong for DMPNC after adsorption. This result suggests a rather bent angle of C-N[triple bond]C: for the nitrogen atom of the NC binding group on the surfaces, whereas a linear angle of C-N[triple bond]C: should be more favorable on gold surfaces due to an intramolecular steric hindrance of its two methyl groups. SERS of BZI on gold nanoparticles also supports a bent angle of :C[triple bond]N-CH2 for its nitrogen atom, suggesting a preference of sp3 (or sp2) hybridization for the nitrogen atom.