Surface-enhanced Raman scattering study of the adsorption of croconate violet on colloidal silver particles

Very stable silver particle suspension has been synthesized for use in surface-enhanced Raman scattering (SERS) spectroscopy with near-infrared exciting radiation. Such citrate-stabilized silver particles were obtained through a suitable control of the nucleation and growth process during the synthesis. The SERS spectra of the bis(dicyanomethylene) croconate dianion or croconate violet (CrocV) were obtained, with excitation in the near-infrared and in the visible region. The differences in the spectral patterns were correlated with a pre-resonance Raman effect of the adsorbate. The vibrational frequencies of CrocV isolated and interacting with silver surface were obtained through theoretical calculations using DFT method that together the surface selection rules allowed to perform the vibrational assignment of the SERS spectra and to infer the adsorption geometry.     

Vibrational and surface-enhanced vibrational spectra of 6-nitrochrysene

The infrared and Raman spectra of solids and thin solid films of 6-nitrochrysene, its electronic spectra, and resonance Raman scattering (RRS) obtained with UV-laser excitation at 325 nm are reported. The vibrational assignment is supported by ab initio computations at the B3LYP/6-311G(d, p) level of theory. The molecular organization in nanometric films evaporated onto smooth metal surfaces of silver and copper was probed using reflection-absorption infrared spectroscopy (RAIRS). The results of the surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA) obtained from nanometric films evaporated onto silver island films are also discussed. It was found that the molecule efficiently interacts with silver island film surfaces, and that the interaction leads to extensive photochemical reaction at the metal surface under laser illumination.     

Surface enhanced vibrational spectra of thymine

Surface-enhanced vibrational spectroscopy (SEVS) is discussed using 5-methyluracil (thymine) as a model compound. Surface-enhanced Raman scattering (SERS) and surface-enhanced infrared (SEIR) are reported and a characterization of thymine adsorbed onto silver island films is provided. The thymine SERS spectra obtained using silver colloids, silver roughened electrodes and silver island films are remarkably different due to several binding possibilities of thymine during chemical adsorption onto a silver surface. It is shown that laser induced photo dissociation may lead to further changes in the recorded spectra of the adsorbate. The surface enhanced-infrared (SEIR) spectra of thymine on silver island are reported here for the first time. The infrared spectra of thymine films were also been obtained to help the assignment of molecular vibrations in the surface enhanced spectra.     

Surface enhanced Raman scattering of surfactants adsorbed on silver mirror surfaces

The surface enhanced Raman scattering (SERS) spectra of two surfactants: Triton X-100 and n-hexadecylpyridinium bromide adsorbed on the interfaces between chemically reduced silver mirrors and water are obtained. By comparing and analyzing their bulk and SERS spectra, we have studied the adsorption configurations of the two surfactants on the silver/water interfaces.     

Surface-enhanced Raman scattering system of sample molecules in silver-modified silver film

Surface-enhanced Raman scattering (SERS) spectrum of very good quality of "silver nano-particles/sample molecules/silver film" system was reported by nesting the sample molecules to the gap of silver nano-particles and silver film, indicating that "silver nano-particles/sample molecules/silver film" is a highly SERS-active system. Not only was the number of the vibrational modes increased, but also were the frequencies of Raman bands up and down shifted. It is difficult to separate the contributions of the electromagnetic and chemical mechanisms to the great enhancement of the Raman signal. The shift by 5-30cm(-1) of the SERS bands and the change in their relative intensity compared with the ordinary Raman spectrum indicate the chemisorption of the sample molecules on the silver-modified silver surface. Furthermore, the silver nano-particles modified on the rough silver film surface play an important role in magnifying the surface local electric field near the silver surface through resonant surface plasmon excitation. From the rich information, obtained from high-quality SERS of PHBA in ternary system, we inferred that PHBA molecules in ternary system adsorb onto the metal surfaces through carboxyl at a perpendicular orientation.     

Sub-wavelength Raman spectroscopy on isolated silver islands

Surface Enhanced Raman Spectroscopy (SERS) is a valuable analytical tool for the investigation of molecules adsorbed on roughened noble metal surfaces. The shape, size, and surrounding of the metal protrusions play an important role in the Raman scattering enhancement. By combining scanning near-field optical microscopy (SNOM) with Raman spectroscopy the spatial resolution suffices for investigating isolated silver islands on SERS active substrates. We demonstrate an optical resolution below 70 nm for recording spectra on specifically prepared and fully characterized SERS substrates. For a quantitative evaluation of the SERS signal the spatial distribution of Rhodamine 6G (R6G) deposited on the SERS substrate was determined by friction force measurements. By comparing the Raman intensities of the SERS substrates with those of unmetallized support plates absolute SERS enhancement factors at specific locations on top and in the vicinity of the silver islands were determined directly.     

Fourier transform surface-enhanced Raman scattering of single-layer nucleolipid Langmuir-Blodgett films on silver island film substrates

Surface-enhanced Raman scattering (SERS) spectra of four amphiphilic nucleolipids in single-layer Langmuir-Blodgett (LB) films deposited on silver island film substrates from pure water and complementary nucleotide-containing subphase and corresponding powder normal Raman spectra were obtained. The analysis of these spectra indicates that the SERS effect is mainly caused by a charge-transfer mechanism, and only the nucleobase headgroup moieties and complementary bases combined with them through hydrogen bonds, which are directly in contact with the silver island film substrates, could be enhanced. For the amphiphilic nucleolipids with the identical nucleobase headgroups, the SERS spectra of the LB films are similar, implying that the orientations of these nucleobase moieties on the silver substrates are analogous. However, the nucleobase takes different orientations on the silver substrates before and after complementary binding. The nucleobases in the LB films deposited from pure water are nearly lying flat on the silver surface, while the complementary binding pairs transferred from the air/water interface tend to take an end-on orientation on the metal surface.     

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.     

In situ laser-induced photochemical silver substrate synthesis and sequential SERS detection in a flow cell

A new, simple, and effective approach for multianalyte sequential surface-enhanced Raman scattering (SERS) detection in a flow cell is reported. The silver substrate was prepared in situ by laser-induced photochemical synthesis. By focusing the laser on the 320 μm inner diameter glass capillary at 0.5 ml/min continuous flow of 1 mM silver nitrate and 10 mM sodium citrate mixture, a SERS active silver spot on the inner wall of the glass capillary was prepared in a few seconds. The test analytes, dacarbazine, 4-(2-pyridylazo)resorcinol (PAR) complex with Cu(II), and amoxicillin, were sequentially injected into the flow cell. Each analyte was adsorbed to the silver surface, enabling the recording of high intensity SERS spectra even at 2 s integration times, followed by desorption from the silver surface and being washed away from the capillary. Before and after each analyte passed the detection window, citrate background spectra were recorded, and thus, no “memory effects” perturbed the SERS detection. A good reproducibility of the SERS spectra obtained under flow conditions was observed. The laser-induced photochemically synthesized silver substrate enables high Raman enhancement, is characterized by fast preparation with a high success rate, and represents a valuable alternative for silver colloids as SERS substrate in flow approaches.     

An investigation of the surface enhanced Raman scattering (SERS) from a new substrate of silver-modified silver electrode by magnetron sputtering

'Pure' silver nanoparticles on silver electrode were prepared by magnetron sputtering. The silver-modified silver electrode has good stability and the silver nanoparticles on silver electrode have homogeneous size distribution. Compared with the silver colloid modified silver electrode, there were no any extraneous component ions on the electrode, for the modified silver nanoparticles are prepared by magnetron sputtering. Synchronously, we obtained much higher quality SERS spectra of adenine molecules on the silver electrode modified by magnetron sputtering (SEMMS), and the study of the adsorption behavior of adenine on the silver-modified silver electrode by surface enhanced Raman scattering (SERS) indicated that the silver-modified silver electrode was highly efficient substrates for SERS investigation. From the rich information on the SEMMS obtained from high-quality potential-dependent SERS, we may deduce the adsorption behavior of adenine and the probable SERS mechanism in the process. The probable reasons are given.     

Magnetron sputtering of silver nanowires using anodic aluminum oxide template: a new active substrate of surface enhanced Raman scattering and an investigation of its enhanced mechanism

A high quality anodic aluminum oxide (AAO) template with ordered apertures about 50-80 nm was fabricated by anodizing aluminum in electrolytes through a two-step method, and silver nanowires with diameters from 40nm to 70nm were prepared on this AAO template by magnetron sputtering. On the glass covered with silver nanowires, high quality surface enhanced Raman scattering (SERS) spectra of sudan II (C₁₈H₁₆N₂O) with enhancement factors of 10⁵ were obtained. And comparison of SERS spectra on silver nanowires with the SERS spectra of silver colloids indicates that main enhanced mode is lightning rod effect of nanorods on the Sudan II/silver nanowires system.     

Amplified light scattering and emission of silver and silver core-silica shell particles

Side versus forward light scattergrams, and fluorescence (488 nm excitation) intensity versus particle count histograms were gathered for bare, R6G-coated, and silica-R6G-coated silver particles of 150-200 nm diameter, one-by-one by flow cytometry. Fluorescence emission intensity of the composite particles monotonically increased and then reached a plateau with greater R6G concentrations, as measured by flow cytometry. Fluorescence amplification factors of up to 3.5x10(3) were estimated by reference to measurements on core-shell particles with silica instead of silver cores. Huge surface enhanced Raman scattering (SERS) intensities, at least 10(14)-fold greater than normal Raman scattering intensities, were observed with 633 nm excitation for molecules such as rhodamine 6G (R6G) on the same single particles of silver. Although routine transmission (TEM) and scanning (SEM) electron microscopies showed gross structures of the bare and coated particles, high-resolution field emission scanning electron microscopy (FE-SEM), revealed Brownian roughness describing quantum size and larger structures on the surface of primary colloidal silver particles. These silver particles were further characterized by extinction spectra and zeta potentials. Structural and light scattering observations that are reported herein were used to tentatively propose a new hierarchical model for the mechanism of SERS.     

The effects of the underpotential and overpotential deposition of lead and thallium on silver on the Raman spectra of adsorbates

It is shown that the underpotential deposition (UPD) and dissolution of monolayers of Pb and Tl onto Ag surfaces roughened in a controlled oxidation-reduction cycle produces a Ag surface which shows diminished surface enhanced Raman scattering (DSERS). Significantly enhanced Raman spectra can still be obtained from electrodes covered by complete UPD and overpotential deposited (OPD) layers of the metals. Correct choice of electrolytes for the UPD of the metal reduces the loss of enhanced Raman scattering; chloride ions, constituents of many electrolytes used in the investigation of surface enhanced Raman scattering (SERS), are shown to be especially active in causing the loss of SERS.     

The optimisation of facile substrates for surface enhanced Raman scattering through galvanic replacement of silver onto copper

A fast and cost-effective approach for the synthesis of substrates used in surface enhanced Raman scattering (SERS) has been developed using galvanic displacement. Deposition of silver onto commercially available Cu foil has resulted in the formation of multiple hierarchical structures, whose morphology show dependence on deposition time and temperature. Analysis of the surface structure by scanning electron microscopy revealed that the more complex silver structures correlated well with increased deposition time and temperature. Using Rhodamine 6G (R6G) as a model Raman probe it was also possible to relate the substrate morphology directly with subsequent SERS intensity from the R6G analyte as well as the reproducibility across a total of 15 replicate Raman maps (20 × 20 pixels) consisting of 400 spectra at a R6G concentration of 10(-4) M. The substrate with the highest reproducibility was then used to explore the limit of detection and this compared very favourably with colloidal-based SERS assessments of the same analyte.     

Surface-enhanced raman spectra of dyes and organic acids in silver solutions: chloride ion effect

We have recorded surface-enhanced Raman (SER) spectra of two different classes of compounds, cationic dyes and organic acids, and studied their chloride ion effects on the surface-enhanced Raman scattering (SERS) activities of the silver solution. For the positive charge dyes, rhodamine 6G (R6G) and 1,1'-dimethyl-2,2'-cyanine iodide (DECI), no SERS could be observed without the addition of chloride ions because of lack of the electrostatic interaction between the dye species and the silver particles in the silver solution. The chloride ions served to enlarge silver particles and to contribute the existence of the surface active sites, making the silver solution SERS active to the dye samples. Surface-enhanced resonance Raman scattering (SERRS) intensity of the dye molecules increased with the chloride ion concentration. After reaching a maximum intensity, a Cl- quenching effect on the intensity took place. For the organic acids, benzoic acid and p-aminobenzoic acid (PABA), SERS could be observed without the coexistence of chloride ions. The intensity of the Raman scattering did not vary significantly in the presence of small amount of chloride ion. At high Cl- concentration, quenching SERS intensity began to take effect.     

Surface enhanced fluorescence and Raman imaging of Langmuir-Blodgett azopolymer films

The spectroscopic properties and surface-enhanced spectra of Langmuir-Blodgett (LB) films of methacrylic homopolymer (HPDR13) are presented. It is shown that LB film displays strong fluorescence attributed to the spatial restrictions imposed by its structure. The emission is observed in conjunction with photoisomerization, a process clearly demonstrated by the formation of surface-relief gratings in the LB film [C.R. Mendon?a et al., Macromolecules 32 (1999) 1493]. Surface-enhanced Raman scattering (SERS), Surface-enhanced resonance Raman scattering (SERRS) and surface-enhanced fluorescence (SEF) were observed for LB films of HPDR13 deposited onto silver island films. SERS measurements were also carried out on a sample fabricated with one monolayer LB film deposited onto silver islands followed by one overlayer of silver (LB sandwiched between two layers of silver islands). The polymer interacts very weakly with the metal surface (physisorption), and the enhancement effect is determined by the local electric field enhancement. The strong SERS and SERRS signals were suitable for micro-Raman imaging. Line, area mapping and global images of the LB monolayer on silver island are reported. The transfer ratio in the fabrication of the LB suggests a homogeneous coating of the silver islands, thereby the chemical images show the variation of the SERS intensity due to surface enhancement.     

Raman-based detection of bacteria using silver nanoparticles conjugated with antibodies

Surface enhanced Raman scattering (SERS) has been used to detect bacteria captured by polyclonal antibodies sorbed onto protein-A-modified silver nanoparticles. The selectivity and discrimination of the technique were assured by using a specific antibody to the model bacterium, Escherichia coli. As the SERS enhancement mechanism depends upon the metal surface proximity, 8 nm was considered as the optimum distance between the bacterium and the nanoparticle surface. Spectral reproducibility was verified using Principal Components Analysis to differentiate the clusters corresponding to the biomolecules and/or bacteria sorbed onto nanoparticles. Compared to the normal Raman spectrum, the SERS technique resulted in an intensity enhancement of over 20-fold.     

Surface-enhanced Raman spectroscopy study on the structure changes of 4-mercaptopyridine adsorbed on silver substrates and silver colloids

Surface-enhanced Raman scattering (SERS) of 4-mercaptopyridine (4-mpy) adsorbed on HNO3 etched silver foil, chemically deposited silver films (silver mirror) and silver colloids were measured. The SERS study has revealed that 4-mpy was adsorbed onto the three kinds of silver surfaces by a sulfur-silver bond with the plane of pyridine ring being normal to the silver substrates. The structure of 4-mpy adsorbed on the silver surfaces depends largely on the pH values of environment. When the pH values of the environment are changed, the structure of 4-mpy adsorbed on silver surfaces can easily be altered through a protonation or deprotonation reaction occurring on the N atom of the pyridine ring, and the modified structure shows unique characters on the SERS spectrum. Owing to the remarkable enhancement ability of SERS technique and characteristic spectrum of different species, a monolayer of 4-mpy assembled on a silver mirror holds potential as a H+ sensor for highly sensitive detection of the proton concentration in an aqueous solution.     

Surface-enhanced Raman scattering of EDOT and PEDOT on silver and gold nanoparticles

Surface-enhanced Raman scattering (SERS) spectra of the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and its monomer 3,4-ethylenedioxythiophene (EDOT) on Ag and Au nanoparticles presenting different morphologies and stabilizing agents have been obtained using the excitation radiation at 633 nm. The SERS spectra of the monomer and polymer are strongly dependent both on the metal and capping agent of the substrate. SERS spectra of EDOT on Au nanospheres indicates that adsorption occurs with the thiophene ring perpendicular to the metal surface. In contrast, polymerization takes place on the silver surface of Ag nanospheres. EDOT adsorption on Ag nanoprisms with polyvinylpyrrolidone (PVP) as capping agent occurs similarly to that observed on gold. Surface-enhanced resonance Raman scattering (SERRS) spectra of PEDOT on gold nanostars that present a thick layer of PVP show no chemical interaction of PEDOT with the metal surface; however, when PEDOT is adsorbed on citrate stabilized gold nanospheres, the SERRS spectra suggest that thiophene rings are perpendicular to the surface. Oxidation of PEDOT also is observed on Ag nanospheres. The investigation of the interface between PEDOT and metal surface is crucial for the development in polymer-based optoelectronic devices since this interface plays a crucial role in their stability and performance.