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.     

Silver nanoparticles deposited on anodic aluminum oxide template using magnetron sputtering for surface-enhanced Raman scattering substrate

Low-cost and highly sensitive surface-enhanced Raman scattering (SERS) substrates have been fabricated by a simple anodizing process and a magnetron sputtering deposition. The substrates, which consist of silver nanoparticles embedded on anodic aluminum oxide (AAO) templates, are investigated by a scanning electron microscope and a confocal Raman spectroscopy. The SERS activities are demonstrated by Raman scattering from adsorbed solutions of methylene blue and pyridine on the SERS substrate surface. The most optimized SERS substrate contains the silver nanoparticles, with a size distribution of 10-30 nm, deposited on the AAO template. From a calculation, the SERS enhancement factor is as high as 8.5 × 10⁷, which suggests strong potentials for direct applications in the chemical detection and analyses.     

Novel method for preparing controllable and stable silver particle films for surface-enhanced Raman scattering spectroscopy

A new surface-enhanced Raman scattering (SERS) active substrate has been developed based on our previous study. Small silver nanoparticles on a quartz slide can be enlarged by using a mixture of commercially available reagents called Silver Enhancer and Initiator. The optical properties and characteristics of the new substrate have been investigated by ultraviolet-visible (UV-Vis) spectroscopy and atomic force microscopy (AFM). The results indicate that the small silver nanoparticles grow and some silver aggregates emerge on the quartz slide after the slide is immersed into the Silver Enhancer and Initiator Mixture (SEIM). The average diameter of the silver nanoparticles on the substrate becomes approximately double after the immersion into SEIM for 20 s. 1,4-bis[2-(4-pyridyl)ethenyl]-benzene (BPENB) was used as a Raman probe to evaluate the enhancement ability of the new silver substrate. It has been found that the SERS intensity can be increased about 10 times by using the substrate treated by SEIM compared with that without being treated by SEIM. Interestingly enough, the SERS enhancement increases with time. This may be due to the reorganization of silver nanoparticles on the quartz surface. The new substrate can remain active for more than 90 days. The adsorption mode of BPENB on the new substrate and the dependence of the BPENB configurations on the concentration of BPENB in methanol solution have also been investigated by SERS or UV-Vis spectroscopy. The SERS spectra of a self-assembled monolayer (SAM) BPENB film adsorbed on a silver substrate treated by SEIM show that BPENB molecules are chemically absorbed through the Ag-N bond. Consequently, a nearly perpendicular orientation of BPENB on the silver surface is proposed. The SERS spectra of BPENB SAMs on the new substrates fabricated from methanol solutions with different concentrations are compared. The concentration dependence of the SERS spectra reveals that the BPENB molecules are adsorbed on the silver film as monomers when the film is prepared from the solution with a lower concentration (<4 x 10(-6) M) and as aggregates when it is prepared from the solution with a higher concentration (>1 x 10(-5) M).     

Surface-enhanced Raman spectroscopy studies of surfactant adsorption to a hydrophobic interface

Surface-enhanced Raman spectroscopy is used to investigate the kinetics of adsorption of the cationic surfactant cetylpyridinium chloride (CPC) to hydrophobic surfaces from water. A hydrophobic surface, with stable and reproducible SERS activity, is produced by binding gold colloids to an amine-terminated glass slide and then modifying this surface with octadecyltrimethoxysilane. In situ SERS-detected adsorption of CPC from aqueous solution is found to follow a Frumkin isotherm. Interactions between the charged head groups could be detected in frequency shifts in the symmetric ring breathing mode, consistent with an interfacial surfactant environment similar to a CPC micelle. Rates of surfactant adsorption were determined by time-resolved SERS measurements and were found to be much slower than the diffusion-controlled limit, indicating a significant kinetic barrier to adsorption. Desorption kinetics were heterogeneous, consistent with the spectroscopic results. Alkylsilane-modified gold colloids were shown to be useful substrates for investigating amphiphile adsorption from aqueous solutions to hydrophobic surfaces, where the adsorption kinetics could also be used to determine analyte concentrations in solution.     

Controlled layer-by-layer formation of ultrathin TiO2 on silver island films via a surface sol-gel method for surface-enhanced Raman scattering measurement

A surface sol-gel process has been demonstrated to be an effective method for the surface modification of silver island films as unique SERS substrates for monitoring molecular adsorption on a dielectric titania surface. This layer-by-layer approach allows control of the thickness of the dielectric surface with a monolayer precision on silver surfaces. The enhancement of Raman scattering from adsorbed Rhodamine 6G molecules is inversely proportional to the thickness of the titania film, which is consistent with the decay of electromagnetic enhancement. Despite a reduction in the sensitivity of the film, a substantial improvement in the film was achieved as a result of the enhanced stability of this substrate compared to the silver island film without a TiO(2) coating.     

Highly sensitive SERS detection of As3+ ions in aqueous media using glutathione functionalized silver nanoparticles

A highly sensitive surface-enhanced Raman scattering (SERS) platform for the selective trace analysis of As(3+) ions was reported based on glutathione (GSH)/4-mercaptopyridine (4-MPY)-modified silver nanoparticles (AgNPs). Here, GSH conjugated on the surface of AgNPs for specifical binding with As(3+) ions in aqueous solution through As-O linkage and 4-MPY was used as a Raman reporter. When As(3+) ions were added to the system, the binding of As(3+) with GSH resulted in the aggregation of AgNPs, and excellent Raman signal of 4-MPY reporters was obtained which can reflect the concentration of As(3+) indirectly. Under optimal assay conditions, the limit of detection (LOD) was estimated to be as low as 0.76 ppb, which is lower than the WHO defined limit (10 ppb), and an excellent linear range of 4-300 ppb was obtained. The practical application had been carried out for determination of As(3+) in real water samples.     

Integrate silver colloids with silicon nanowire arrays for surface-enhanced Raman scattering

A facile method to prepare uniform and reproducible surface-enhanced Raman scattering (SERS) substrates is presented. Quasi-spherical silver colloids prepared by microwave heating and wafer-scale uniform silicon nanowire (SiNW) arrays fabricated via wet chemical etching were united together as SERS substrates. The novel SERS substrates displayed stronger Raman enhancement than conventional silver colloids as well as outstanding uniformity and reproducibility in our experiments. In addition, it was found that the cross section of SiNW arrays possessed stronger enhancement activity than the front side. The enhancement effects of two adjacent SiNWs (as a simplification of SiNW arrays) were evaluated by the finite difference time domain (FDTD) method.     

Immunoassay readout method using extrinsic Raman labels adsorbed on immunogold colloids

An immunoassay readout method based on surface-enhanced Raman scattering (SERS) is described. The method exploits the SERS-derived signal from reporter molecules that are coimmobilized with biospecific species on gold colloids. This concept is demonstrated in a dualanalyte sandwich assay, in which two different antibodies covalently bound to a solid substrate specifically capture two different antigens from an aqueous sample. The captured antigens in turn bind selectively to their corresponding detection antibodies. The detection antibodies are conjugated with gold colloids that are labeled with different Raman reporter molecules, which serve as extrinsic labels for each type of antibody. The presence of a specific antigen is established by the characteristic SERS spectrum of the reporter molecule. A near-infrared diode laser was used to excite efficiently the SERS signal while minimizing fluorescence interference. We show that, by using different labels with little spectral overlap, two different antigenic species can be detected simultaneously. The potential of this concept to function as a readout strategy for multiple analytes is briefly discussed.     

Fluorescence and Raman study of pH effect on the adsorption orientations of methyl red on silver colloids

We report the fluorescence and surface-enhanced Raman scattering (SERS) analysis of methyl red (MR) isomers (o-MR, m-MR, p-MR) on silver (Ag) colloids at different pH conditions. The changes in simultaneous fluorescence quenching around 400 nm accompanied by enhancement around 540 nm with pH are closely related to energy transfer efficiency and the interaction between the carboxylic group and the Ag surface. The change of SERS signals with pH values can be ascribed to different adsorption orientations of MR on the Ag surface, from near flat (o-MR) to near perpendicular (p-MR).     

Potential of surface-enhanced Raman spectroscopy for the rapid identification of Escherichia coli and Listeria monocytogenes cultures on silver colloidal nanoparticles

Surface-enhanced Raman (SERS) spectra of various batches of bacteria adsorbed on silver colloidal nanoparticles were collected to explore the potential of the SERS technique for rapid and routine identification of E. coli and L. monocytogenes cultures. Relative standard deviation (RSD) of SERS spectra from silver colloidal suspensions and ratios of SERS peaks from small molecules (K(3)PO(4)) were used to evaluate the reproducibility, stability, and binding effectiveness of citrate-reduced silver colloids over batch and storage processes. The results suggested consistent reproducibility of silver colloids over batch process and also stability and consistent binding effectiveness over an eight-week storage period. A variety of mixtures of E. coli/L. monocytogenes cultures with different colloidal batches revealed that, despite large variations in relative intensities and positions of SERS active bands, characteristic and unique bands at 712 and 390 cm(-1) were consistently observed and were the strongest in E. coli and L. monocytogenes cultures, respectively. Two specific bands were used to develop simple algorithms in the evaluation of binding effectiveness of silver colloids over storage and further to identify E. coli and L. monocytogenes cultures with a 100% success. A single spectrum acquisition took 5 approximately 6 min, and a minimum of 25 microL silver colloid was directly mixed with 25 microL volume of incubated bacterial culture. The short acquisition time and small volume of incubated bacterial culture make silver colloidal nanoparticle based SERS spectroscopy ideal for potential use in the routine and rapid screening of E. coli and L. monocytogenes cultures on large scales. This is the first report of the development of simple and universal algorithms for bacterial identification from the respective exclusive SERS peaks.     

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.     

High-density silver nanoparticle film with temperature-controllable interparticle spacing for a tunable surface enhanced Raman scattering substrate

The formation of high-density silver nanoparticles and a novel method to precisely control the spacing between nanoparticles by temperature are demonstrated for a tunable surface enhanced Raman scattering substrates. The high-density nanoparticle thin film is accomplished by self-assembling through the Langmuir-Blodgett (LB) technique on a water surface and transferring the particle monolayer to a temperature-responsive polymer membrane. The temperature-responsive polymer membrane allows producing a dynamic surface enhanced Raman scattering substrate. The plasmon peak of the silver nanoparticle film red shifts up to 110 nm with increasing temperature. The high-density particle film serves as an excellent substrate for surface-enhanced Raman spectroscopy (SERS), and the scattering signal enhancement factor can be dynamically tuned by the thermally activated SERS substrate. The SERS spectra of Rhodamine 6G on a high-density silver particle film at various temperatures is characterized to demonstrate the tunable plasmon coupling between high-density nanoparticles.     

Identification of organic materials in historic oil paintings using correlated extractionless surface-enhanced Raman scattering and fluorescence microscopy

A novel spectroscopic approach, correlated surface-enhanced Raman scattering (SERS) and fluorescence microscopy, is used to identify organic materials in two 18th century oil paintings. The vibrational fingerprint of analyte molecules is revealed using SERS, and corresponding fluorescence measurements provide a probe of local environment as well as an inherent capability to verify material identification. Correlated SERS and fluorescence measurements are performed directly on single pigment particles obtained from historic oil paintings with Ag colloids as the enhancing substrate. We demonstrate the first extractionless nonhydrolysis SERS study of oil paint as well as the potential of correlated SERS and fluorescence microscopy studies for the simultaneous identification of organic colorants and binding media in historic oil paintings.     

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.     

Potential-dependent surface-enhanced Raman scattering from adsorbed thiocyanate for characterizing silver surfaces with improved reproducibility

Surface-enhanced Raman scattering (SERS) spectroelectrochemistry is used to characterize electrochemically roughened and highly polished polycrystalline silver SERS-active substrates. Changes in the nitrile stretching vibrational mode of adsorbed thiocyanate are used as an in situ spectroscopic probe: the potential dependence of band position (Stark tuning), shape, and scattering intensity of this mode are measured in order to investigate differences between SERS-active sites found on smooth and roughened electrode surfaces. Results obtained from thiocyanate adsorbed onto two different types of highly polished Ag surfaces (alumina and diamond polishing) show discrete populations of SERS-active adsorption sites that remain stable over a wide potential range. This behavior stands in contrast to that observed on electrochemically roughened surfaces, where very strong Stark tuning, large vibrational bandwidths, and irreversible loss of SERS enhancement upon negative potential excursions can be attributed to a diverse population of labile SERSactive sites that exhibit strong charge-transfer interactions with the adsorbate and large chemical SERS enhancement.     

Analytical technique for label-free multi-protein detection based on Western blot and surface-enhanced Raman scattering

We have developed a new analytical procedure for label-free protein detection designated "Western SERS", consisting of protein electrophoresis, Western blot, colloidal silver staining, and surface-enhanced Raman scattering (SERS) detection. A novel method of silver staining for Western blot that uses a silver colloid, an excellent SERS-active substrate, is first proposed in the present study. During the process of silver staining, interactions between proteins and silver nanoparticles result in the emergence of SERS of proteins. In the present study, we use myoglobin (Mb) and bovine serum albumin (BSA) as model proteins. From different protein bands on a nitrocellulose (NC) membrane, we have observed surface-enhanced resonance Raman scattering (SERRS) spectra of Mb and SERS spectra of BSA. The proposed technique offers dual advantages of simplicity and high sensitivity. On one hand, after the colloidal silver staining, we can detect label-free multi-proteins directly on a NC membrane without digestion, extraction, and other pretreatments. On the other hand, the detection limit of the Western SERS is almost consistent with the detection limit of colloidal silver staining, and the SERRS detection limit of Mb is found to be 4 ng/band. This analytical method, which combines the technique of protein separation with SERS, may be a powerful protocol for label-free protein detection in proteomic research.     

A study of glutathione molecules adsorbed on silver surfaces under different chemical environments by surface-enhanced Raman scattering in combination with the heat-induced sensing method

In this study, surface-enhanced Raman scattering (SERS) in combination with a heat-induced sensing technique has been applied for investigating molecular orientations of glutathione molecules adsorbed on silver colloidal nanoparticles under different chemical environments, which has enabled us to further study how glutathione molecules are adsorbed on the silver surfaces. Factors that may affect the configurations of glutathione molecules adsorbed on the silver nanocolloids have been investigated. By observing the relative enhancement factors of SERS bands due to individual functional groups contributed from different terminals, the affinity between the different functional groups of glutathione and the silver surfaces under different conditions has been sorted and the orientations of glutathione molecules adsorbed on the silver surfaces have been investigated.     

Surface-enhanced Raman spectroscopy of self-assembled monolayers: sandwich architecture and nanoparticle shape dependence

Surface enhanced Raman scattering (SERS) spectra of 4-mercaptobenzoic acid (4-MBA) self-assembled monolayers (SAMs) on gold substrates is presented for SAMs onto which gold nanoparticles of various shapes have been electrostatically immobilized. SERS spectra of 4-MBA SAMs are enhanced in the presence of immobilized gold nanocrystals by a factor of 10(7)-10(9) relative to 4-MBA in solution. Large enhancement factors are a likely result of plasmon coupling between the nanoparticles (localized surface plasmon) and the smooth gold substrate (surface plasmon polariton), creating large localized electromagnetic fields at their interface, where 4-MBA molecules reside in this sandwich architecture. Moreover, enhancement factors depend on nanoparticle shape and vary by a factor of 10(2). This SERS geometry offers large surface enhancements for molecules adsorbed onto planar substrates and could be quite useful for determining chemical information for poor Raman scatterers from assays on 2-D substrates.     

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.     

Resonance Raman evidence of immobilization of laccase on self-assembled monolayers of thiols on Ag and Au surfaces

Resonance Raman (RR) and surface-enhanced Raman scattering (SERS) spectroscopy have been used to study immobilization of laccase on self-assembled monolayers (SAMs) of thiols containing carboxylic and amino groups, deposited on silver and gold electrodes. A new, indirect way of monitoring laccase bound to the thiol-coated Ag and Au surfaces is presented. It was demonstrated that by recording the resonance Raman spectra of the colored product of the oxidation of syringaldazine (4-hydroxy-3,5-dimethoxybenzaldehydeazine) by laccase in the presence of molecular oxygen, one may easily confirm binding as well as enzymatic activity of laccase immobilized on the SAMs modified silver and gold surfaces.