Surface enhanced Raman scattering at Ag electrodes in non-aqueous pyridine solutions

The surface enhanced Raman scattering (SERS) of interfacial pyridine and Br⁻ species at Ag electrodes from neat pyridine solutions containing tetra-n-butylammonium bromide is presented. The potential dependence of the SERS peak frequencies, bandwidths, and relative intensities is reported and interpreted in terms of pyridine reorientation in the double layer. A band at 221 cm⁻¹ is observed at certain potentials and is assigned to the AgN stretch for Ag surface pyridine species.     

Surface-enhanced Raman scattering of water adsorbed on silver electrodes

Temporal evolution of the Raman spectra of H₂O, D₂O and HDO during an oxidation-reduction cycle of a Ag electrode in aqueous 1 M KCl or KBr has been recorded with an optical multichannel analyzer. Surface enhanced Raman spectra of the adsorbed water are readily observable and are different from the Raman spectra of bulk water.     

Surface-enhanced Raman scattering of pyridine and benzene in non-aqueous electrochemical systems of alcoholic solvents

We have studied surface-enhanced Raman scattering from the following non-aqueous electrochemical systems: 0.05 M pyridine/0.1 M LiCl/methanol/Ag electrode, 0.05 M pyridine/0.1 M LiCl/ethanol/Ag electrode, and 0.05 M benzene/0.1 M LiCl/ethanol/Ag electrode. Each SERS band of pyridine is found to show two distinctive steps to reach the maximum peak intensity after the reduction potential is applied, which seems to be related to the formation of adatomic silver particle clusters. SERS spectra of benzene show three bands at 995, 986 and 993 cm⁻¹, the potential dependence of which was studied in detail to confirm chemisorption effects.     

Surface-enhanced Raman scattering measurements on silver nanoparticles covered with differently formed platinum films

Gold and silver electromagnetic nanoresonators covered by a thin layer of platinum are often used to study adsorption of various molecules on “model platinum surfaces” with surface-enhanced Raman scattering (SERS) spectroscopy. In this contribution spectra of pyridine adsorbed on films formed from core–shell Ag@Pt and Ag@Ag–Pt nanoparticles and pure Pt or Ag nanoparticles were measured using a confocal Raman microscope. The SERS spectra of pyridine adsorbed on alloy Ag@Ag–Pt nanoparticles could not be obtained as a linear combination of spectra measured on pure Ag and Pt surfaces. In other words, for silver electromagnetic nanoresonators covered by platinum there is no simple correlation between the “quality” of the deposited Pt layer and the relative intensity of SERS bands characteristic for adsorbate interacting with silver. The SERS spectra accumulated from various places of a film formed from Ag@Pt or Ag@Ag–Pt nanoclusters may differ significantly. Using Ag@Pt nanoparticles with practically negligible amount of Ag on the surface (as per the stripping measurement), it is possible to record SERS spectrum in which the contribution characteristic for pyridine adsorbed on the Ag surface is well visible. It means that, even for macroscopic samples of core–shell Ag–Pt nanoparticles, averaging of many spectra measured at various locations of the sample should be carried out to characterize reliably their properties.     

Surface-enhanced Raman scattering on colloidal nanostructures

Surface-enhanced Raman scattering combines extremely high sensitivity, due to enhanced Raman cross-sections comparable or even better than fluorescence, with the observation of vibrational spectra of adsorbed species, providing one of the most incisive analytical methods for chemical and biochemical detection and analysis. SERS spectra are observed from a molecule-nanostructure enhancing system. This symbiosis molecule-nanostructure is a fertile ground for theoretical developments and a realm of applications from single molecule detection to biomedical diagnostic and techniques for nanostructure characterization.     

Surface-enhanced Raman scattering in the ultraviolet spectral region: UV-SERS on rhodium and ruthenium electrodes

We report the first observation of surface-enhanced Raman scattering (SERS) excited with ultraviolet (UV) light from transition metal electrodes. Adsorbed pyridine and SCN- on rough rhodium (Rh) and ruthenium (Ru) electrodes, respectively, have been studied using 325 nm laser excitation. In contrast, the best enhancers in the visible and near infrared, silver and gold, do not produce UV-SERS. The experimental data of UV-SERS are in agreement with our preliminary theoretical calculation based on the electromagnetic enhancement mechanism. The enhancement factor is about 2 orders of magnitude for the Rh and Ru electrodes when they are excited at 325 nm.     

Surface-enhanced Raman scattering on mercaptopyridine-capped CdS microclusters

We obtained the high-quality Raman spectra of 4-mercaptopyridine (4-Mpy) adsorbed on CdS microclusters. The Raman signals were enhanced relative to the same molecules in solution. We compared the Raman spectra of 4-Mpy molecules adsorbed on CdS microclusters and Ag substrate. The difference of 4-Mpy molecules adsorbed on semiconductor and metal substrate was revealed. The results demonstrated that adsorbed species on semiconductor CdS can be detected by SERS spectroscopy.     

Enhanced and normal Raman scattering from pyridine adsorbed on rough and smooth silver electrodes

A multiplex spectrograph has been used to record potential difference and modulation Raman spectra of pyridine adsorbed on silver electrodes in an electrochemical cell. Spectra have been obtained from rough silver surfaces which give SERS and from surfaces where SERS has been diminished by prolonged cathodic polarisation (DSERS). Raman scattering from pyridine at smooth silver surfaces in potassium perchlorate and fluoride solutions has been distinguished from solution scatter by a potential modulation technique. The results show that the enhanced scattering caused by silver atom or cluster sites is respresentative of the surface as a whole as similar Raman spectra are obtained on smooth surfaces at a count rate as low as ?1.4 photons s^?1 (incident laser power 500 mW).Correlation of simultaneous differential capacitance data and “snapshot” SER spectra indicate that pyridine molecules in aqueous chloride ion solutions adsorb on silver in a flat π-bonded configuration at potentials markedly positive to the point of zero charge and exhibit specific reorientation at ?0.3 V and ?0.45 V (vs. SCE) to become N-bonded, perpendicular to the surface. Results also show that the adsorption behaviour of pyridine in chloride and fluoride ion solutions is largely similar.     

Surface-enhanced Raman scattering for protein detection

Proteins are essential components of organisms and they participate in every process within cells. The key characteristic of proteins that allows their diverse functions is their ability to bind other molecules specifically and tightly. With the development of proteomics, exploring high-efficiency detection methods for large-scale proteins is increasingly important. In recent years, rapid development of surface-enhanced Raman scattering (SERS)-based biosensors leads to the SERS realm of applications from chemical analysis to nanostructure characterization and biomedical applications. For proteins, early studies focused on investigating SERS spectra of individual proteins, and the successful design of nanoparticle probes has promoted great progress of SERS-based immunoassays. In this review we outline the development of SERS-based methods for proteins with particular focus on our proposed protein-mediated SERS-active substrates and their applications in label-free and Raman dye-labeled protein detection. Figure Protein-mediated SERS-active substrates for protein detection     

Surface-enhanced Raman scattering from silver-coated opals

We describe surface-enhanced Raman scattering measurements from a benzenethiol monolayer adsorbed on a silver-coated film that is, in turn, deposited on an artificial opal, where the latter is a close-packed three-dimensional dielectric lattice formed from polystyrene spheres. Data for a range of sphere sizes, silver film thicknesses, and laser excitation wavelengths are obtained. Enhancement factors can be in the range of 10(7). To partially explain these large enhancements, we have performed model finite-difference time domain simulations of the position-dependent electric fields generated at the opal surfaces for several experimentally studied laser wavelengths and sphere diameters.     

Surface-enhanced Raman scattering detection of lysophosphatidic acid

Surface-enhanced Raman scattering using silver nanoparticles was applied to detect various forms of lysophosphatidic acid (LPA) to examine its potential application as an alternative to current detection methods of LPA as biomarkers of ovarian cancer. Enhancement of the Raman modes of the molecule, especially those related to the acyl chain within the 800–1300 cm⁻¹ region, was observed. In particular, the C–C vibration mode of the gauche-bonded chain around 1100 cm⁻¹ was enhanced to allow the discrimination of two similar LPA molecules. Given the molecular selectivity of this technique, the detection of LPA using SERS may eliminate the need for partial purification of samples prior to analysis in cancer screening.     

Surface-enhanced Raman scattering on molecular self-assembly in nanoparticle-hydrogel composite

Surface-enhanced Raman scattering has been applied to study weak intermolecular interactions between small organic gelling molecules involved in the silver nanoparticle-hydrogel composite formation. Assembly and disassembly of the gelator molecules in close vicinity to embedded silver nanoparticles were followed by changes in Raman intensity of the amide II and carboxyl vibrational bands, whereas the strength of the bands related to benzene modes remained constant. This implied that the gelator molecules were strongly attached to the silver particles through the benzene units, while participating in gel structure organization by intermolecular hydrogen bonding between oxalyl amide and carboxyl groups.     

Surface-enhanced Raman scattering detection of cholinesterase inhibitors

A new sensitive surface-enhanced Raman scattering (SERS) assay for detection of cholinesterase inhibitors such as organophosphorous pesticides using silver colloidal nanoparticles was developed and optimized. Acetylcholinesterase (AChE) mediated the hydrolysis of acetylthiocholine to produce thiocholine, which interacted with the silver nanoparticles to give a specific SERS spectrum. Variation in enzyme activity due to inhibition was measured from changes in intensity of a characteristic peak (772 cm⁻¹) of the SERS spectrum that was directly correlated with the concentration of produced thiocholine. The method was demonstrated for the detection of paraoxon as reference AChE inhibitor. Limit of detection of paraoxon for 5 min incubation at 25 °C was 1.8 × 10⁻⁸ M. This assay can be utilized for the detection of trace amounts of any AChE inhibitor.     

Surface enhanced Raman scattering of pyridine on Ag electrodes formed with controlled-rate oxidation-reduction cycles

A study has been performed in which the SERS intensity of the pyridine ring breathing vibration at 1008 cm⁻¹ at Ag electrodes is found to be dependent upon both the large scale and atomic scale roughness of the electrode surface. The controlled surface morphology is generated by a double potential step ORC technique. Scanning electron microscopy of these surfaces reveals varying large scale surface morphologies that are dependent upon ORC rate. A correlation between SERS intensity and large scale surface morphology is interpreted in terms of significant contributions from electromagnetic effects in electrochemical SERS. The correlation between SERS intensity and this surface morphology is found to be independent of the presence of atomic scale roughness. The results presented here confirm and extend the results of previous investigations of the morphology of SERS-active surfaces.     

Surface-enhanced Raman spectroscopic evidence of methanol oxidation on ruthenium electrodes

Electrooxidation of methanol on Ru surfaces was investigated using in situ surface-enhanced Raman spectroscopy. Although the cyclic voltammogram did not show a significant methanol oxidation current on Ru, a Raman band at approximately 1970-1992 cm(-1) was observed from 0.4 to 0.8 V in 0.1 M HClO(4) + 1 M methanol. By comparing with the C-O stretching band (nu(CO)) of carbon monoxide (CO) adsorbed on RuO(2)(110) in the ultrahigh vacuum and on oxidized Ru electrodes, the observed spectral feature is assigned to nu(CO) of adsorbed CO (CO(ads)) on RuO(2). The formation of CO(ads) suggests that methanol oxidation does occur on Ru at room temperature, which is in contrast to the perception that Ru is not active for the reaction. The lack of significant methanol oxidation current is attributed to the competing rapid surface oxidation, which forms inactive surface oxides and therefore inhibits the methanol oxidation.     

Surface-enhanced Raman scattering from magneto-metal nanoparticle assemblies

Binary nanoparticles composed of a superparamagnetic Fe₃O₄ core and an Au nanoshell (Fe₃O₄@Au) were prepared via a simple co-precipitation method followed by seed-mediated growth process. The nanoparticles exhibited functions of both fast magnetic response and local surface plasmon resonance. The Fe₃O₄@Au nanoparticles were used as probes for surface-enhanced Raman scattering (SERS) using p-thiocresol (p-TC) as reporter molecule. With the ability of analyte capture and concentration magnetically, the Fe₃O₄@Au nanoparticles showed significant SERS properties with excellent reproducibility. Under non-optimized conditions, detection limit as low as 4.55 pM of analyte can be reached using Fe₃O₄@Au nanoparticle assemblies, which excel remarkably the cases with traditional Au nanoprobes.     

Surface-enhanced Raman scattering of 1-butanethiol in silver sol

The surface-enhanced Raman scattering (SERS) of 1-butanethiol was investigated in a silver sol. The molecule was found to be chemisorbed dissociatively on the silver surface by rupture of its SH bond. It is concluded that conformers of 1-butanethiol adsorbed selectively on the silver surface, the trans conformer around the C (1)C (2) axes being more likely adsorbed when the bulk concentration of the molecule is enough for the full monolayer coverage. The vibrational assignment of the molecule in liquid phase has also been refined by using the SERS data.     

Surface-enhanced Raman scattering of 4-aminobenzenethiol on silver nanoparticles substrate

Active surface-enhanced Raman scattering (SERS) silver nanoparticles substrate was prepared by multiple depositions of Ag nanoparticles on glass slides. The substrate is based on five depositions of Ag nanoparticles on 3-aminopropyl-trimetoxisilane (APTMS) modified glass slides, using APTMS sol–gel as linker molecules between silver layers. The SERS performance of the substrate was investigated using 4-aminobenzenethiol (4-ABT) as Raman probe molecule. The spectral analyses reveal a 4-ABT Raman signal enhancement of band intensities, which allow the detection of this compound in different solutions. The average SERS intensity decreases significantly in 4-ABT diluted solutions (from 10⁻⁴ to 10⁻⁶ mol L⁻¹), but the compound may still be detected with high signal/noise ratio. The obtained results demonstrate that the Ag nanoparticles sensor has a great potential as SERS substrate.     

Surface-enhanced Raman scattering from helical silver nanorod arrays

Helical silver nanorod arrays with different arm numbers are designed by oblique angle deposition and their surface-enhanced Raman scattering properties are characterized. Assuming that the hot spots are located at the bends between the arms, and considering the optical absorbance of different layers, the SERS behavior can be predicted qualitatively.     

Surface-enhanced Raman scattering at a flake silver surface

Supports coated with a flake silver paint show strong surface-enhanced Raman spectra for many molecules, using excitation at 1064 nm. SERS spectra thus obtained for several pyridine derivatives are illustrated, and are discussed in terms of the Brönsted and Lewis acidic properties of surface sites. The technique offers exceptional simplicity, and shows promise for analytical applications.