Self-assembled silver nanochains for surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) integrates high levels of sensitivity with spectroscopic precision and has tremendous potential for chemical and biomolecular sensing. The key to the wider application of Raman spectroscopy using roughened metallic surfaces is the development of highly enhancing substrates for analytical purposes. Here, we demonstrate a simple strategy for self-assembling silver nanochains on glass substrates for sensitive SERS substrates. The chain length of short Ag nanochains can be controlled by adjusting the concentration of cetyltrimethylammonium bromide (CTAB) and 11-mercaptoundecanoic acid (MUA). CTAB with appropriate concentration serves as the "glue" that can link the {100} facets of two neighboring Ag nanoparticles. MUA is found to be effective in "freezing up" the aggregation of Ag short chains and preventing them from further aggregating into a long chainlike network structure. The surface plasmon bands can be tuned over an extended wavelength range by controlling the length of the nanochains. The Ag monolayer, mainly composed of four-particle nanochains, exhibited the maximum SERS enhancement factor of around 2.6 x 108, indicating that a stronger SERS enhancement can be obtained in these interstitial sites of chainlike aggregated Ag nanoparticles.     

In situ surface-enhanced Raman scattering and X-ray photoelectron spectroscopic investigation of coenzyme Q10 on silver electrode

In this study, coenzyme Q(10) (CoQ(10)) has been investigated by in situ near-infrared Fourier transform surface-enhanced Raman scattering (NIR-FT-SERS) spectroelectrochemistry and angle-resolved X-ray photoelectron spectroscopy (AR-XPS) on silver surface. The surface adsorption behavior of the coenzyme Q(10) radical intermediate could be monitored by potential-dependent SERS technique. At the applied potential lower than -0.30 V vs. SCE, the radical intermediate CoQ(10)H˙ stands perpendicularly on the silver surface with both oxygen atoms of the aromatic ring and isoprenoid side chains. When the applied potential is more positive than -0.30 V vs. SCE or at open circuit potential, the quinone ring (benzene ring) of reduced form of coenzyme Q(10) (CoQ(10)H(2)) adopts a face-on surface configuration on the surface. The responsible mechanism for the potential-dependent SERS spectra is presented. Moreover, the adsorption conformation of CoQ(10) has been further confirmed by AR-XPS at the silver surface.     

Solvent effect on surface-enhanced Raman activity of copperphthalocyanine on colloidal silver

Surface enhanced Raman spectrometry (SERS) of copperphthalocyanine (CuPc) in silver hydrosol is reported. The solvent effect on the Raman activity of the adsorbate is discussed in terms of co-adsorption and replacement kinetics of the solvent molecules at the silver surface. It is shown that the quality of the SER spectra can be improved by optimizing the solvent for the adsorbate under study.     

Improved surface-enhanced Raman scattering on optimum electrochemically roughened silver substrates

In this work, the effects of preparation conditions used in roughening silver substrates by electrochemical triangular-wave oxidation-reduction cycles (ORC) on surface-enhanced Raman scattering (SERS) were first investigated. The optimum roughening conditions for obtaining strongest SERS of Rhodamine 6G (R6G) are as follows. Ag electrodes were cycled in deoxygenated aqueous solutions containing 0.1 M NaCl from −0.3 to +0.2 V versus Ag/AgCl at 25 mV s⁻¹ for five scans. The SERS of R6G adsorbed on this optimum procedure-prepared roughened Ag substrate exhibits a higher intensity by one order of magnitude, as compared with that of R6G adsorbed on a normally roughened Ag substrate.     

Raman spectroscopy of cobalt phthalocyanine adsorbed on a silver electrode

Raman spectra of cobalt tetrasulfonated phthalocyanine adsorbed on a silver electrode in aqueous electrolytes have been recorded in situ. It is shown that the entensity of the Raman bands is directly related to the amount of charge transfered during the electrochemical activation of the silver. The strong potential dependence of distinct Raman bands is discussed with respect to the resonance properties of the adsorbate, taking into account the orientation of the molecule on the surface.     

Fabrication of flower-like silver nanoparticles for surface-enhanced Raman scattering

The flower-like silver nanoparticles have been synthesized by reducing silver nitrate (AgNO₃) with ascorbic acid (AA) as the reductant and polyvinyl pyrrolidone (PVP) as the capping agent under vigorous stirring. Such flower-like nanoparticles are aggregates of small nanoplates and nanorods. They were tested as substrates for the surface-enhanced Raman scattering (SERS), showing high sensitivity for detecting Rhodamine 6G (R6G) at a concentration as low as 10^-7 mol/L. It has been found that replacing mechanical stirring with ultrasound sonication would drastically change the particle morphology, from flower-like nanoparticles to well-dispersed smaller nanoparticles. Furthermore, when trace amounts of NaCl were added into the reagents, well-dispersed Ag nanoparticles formed even in vigorous stirring. These phenomena can be explained with the diffusion and reactant supply during nucleation and growth of Ag nanoparticles.     

Time-resolved surface-enhanced resonance Raman scattering study of the reduction process of heptylviologen monocation radical film on silver electrode surfaces

Time-resolved SERRS spectroscopy was applied to elucidate the mechanism of the reduction process of a heptylviologen monocation radical film to a neutral species on Ag electrode surfaces under various conditions. The film deposited on Ag electrodes at −0.65 V (vs. Ag/AgCl) consists of dimers, (HV^+.)₂. On application of a step potential from −0.65 to −1.2 V, the radical dimer is converted to a neutral species, HV⁰. The time-resolved spectra measured as a function of time after application of the step potential indicates that on the electrode immersed in KBr solutions (0.3 and 3 mol l⁻¹) the radical dimer is at first converted to an intermediate state, which is a surface complex of a monocation radical monomer and a Br⁻ ion (the radical monomer in a type B state), and then reduced to the neutral species. The time-resolved spectra proved also the existence of a disproportionation reaction, i.e. 2HV^+. (type B) → HV²⁺ + HV⁰. The increase in the KBr concentration (0.3 → 3 mol l⁻¹) stabilizes the intermediate surface complex, causing an appreciable decrease in the reduction rate from (HV^+.)₂ to HV⁰. The reduction process on a silver electrode in 0.3 mol l⁻¹ Na₂SO₄ consists of two reaction paths; one is a direct conversion from (HV⁺)₂ to HV⁰ and another is a path through a radical monomer, which gives SERRS features appreciably different from those of type B. The first process proceeds much faster than that on the electrode in the KBr solutions.     

Concentration-dependent surface-enhanced resonance Raman scattering of a porphyrin derivative adsorbed on colloidal silver particles

Surface-enhanced Raman spectra (SERS) of 5,10,15,20-tetrakis(1-decylpyridium-4-yl)-21H,23H-porphintetrabromide or Por 10 (H(2)Tdpyp) adsorbed on silver hydrosols are compared with the FTIR and resonance Raman spectrum (RRS) in the bulk and in solution. Comparative analysis of the RR and the FTIR spectra indicate that the molecule, in its free state, has D(2h) symmetry rather than C(2v). The SERS spectra, obtained on adsorption of this molecule on borohydride-reduced silver sol, indicate the formation of silver porphyrin. With the change in the adsorbate concentration, the SERS shows that the molecule changes its orientation on the colloidal silver surface. The appearance of longer wavelength band in the electronic absorption spectra of the sol has been attributed to the coagulation of colloidal silver particles in the sol. The long wavelength band is found to be red-shifted with the decrease in adsorbate concentration. The excitation profile study indicates that the resonance of the Raman excitation radiation with the original sol band is more important than that with the new aggregation band for the SERS activity. This indicates a large contribution of electromagnetic effect to surface enhancement.     

Low-frequency surface-enhanced Raman scattering spectroscopy at metal electrode surfaces

Low-frequency surface-enhanced Raman scattering (SERS) spectroscopy is a versatile tool for studying surface phenomena under electrochemical conditions. This spectroscopy enables us to obtain rich information on extramolecular vibrations between substrate and adsorbates, which are sensitive to atomistic surface features of the substrate. Owing to recent advancements in optical filter technology, low-frequency SERS signals are now becoming easily detectable using conventional Raman systems equipped with holographic notch filters. In addition, SERS background signals, which have been simply ignored, can provide electronic information on the metal substrate. This allows us to observe both sides of electrode–electrolyte interfaces in situ and simultaneously, which is never expected in far-infrared or terahertz absorption spectroscopy. This advanced SERS spectroscopy can help our understanding of electrochemical and electrocatalytic reactions at the molecular scale.     

Gold nanoparticles assembling on smooth silver spheres for surface-enhanced Raman spectroscopy

A simple and cost-effective chemical method was introduced to assemble gold (Au) nanoparticles on smooth silver (Ag) spheres for realizing surface-enhanced Raman scattering (SERS) enhancement by the replacement reaction between chloroauric acid and Ag spheres. In addition, the Ag-Au core-shell spheres were fabricated when a certain amount of chloroauric acid was used in the reaction solution. We found that the Ag particles decorated with small Au nanoparticles demonstrated the strongest SERS enhancement, while Ag-Au core-shell spheres showed the weakest enhancement.     

Study of the affinity of thermographic additives for silver by time-of-flight static secondary ion mass spectrometry and surface-enhanced Raman spectroscopy on silver nanoparticles

Detection of the interactions between low molecular weight organic compounds and metals in the form of sols on a nanoscale is analytically challenging. This study aims to provide experimental evidence using a combination of UV-Vis absorption spectrometry, surface-enhanced Raman spectrometry (SERS), and static secondary ion mass spectrometry (S-SIMS). The field of application is thermography where silver images are formed via heat-catalyzed reactions. Several organic compounds called tone modifiers and stabilizers are used in thermographic materials for the optimization of the image quality. With exploitation of the strengths of each of the above-mentioned methods, an affinity ranking of several tone modifiers and a stabilizer was established on the basis of competitive adsorption experiments using different model systems. Specifically, silver sols, SERS probes, and sputter-coated silver substrates were exposed to systems with one or two additives. The UV-Vis results provided insight on the aggregation of silver nanoparticles in a hydrosol, which was necessary for the interpretation of the SERS data. Both SERS and S-SIMS measurements led to a similar ranking of the relative affinity of the additives in two components, which was largely consistent with empirical knowledge derived from macroscopic behavior.     

Skeleton pseudomorphs of nanostructured silver for the surface-enhanced Raman spectroscopy

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Absorption and surface-enhanced Raman spectra of silver organosols in ethanol

Colloidal silver, dispersed in ethanol and stabilized by adsorbed polymer, shows enhanced Raman scattering of the chromophore dabsyl (N-4-dimethylaminoazobenzene-4′-sulfonyl)aspartate, DABS ASP, in the presence of excess base. The requirement of the base appears linked to the adsorption of the dabsyl aspartate by interaction with the polymer and/or promotion of growth and aggregation of the silver particles. The latter conditions are indicated by changes in the absorption spectrum of the sol upon addition of base. The enhancement is comparable to that observed earlier for DABS ASP on colloidal silver in aqueous medium.     

Concentration-dependent surface-enhanced Raman scattering of 2-benzoylpyridine adsorbed on colloidal silver particles

Surface-enhanced Raman scattering (SERS) of 2-benzoylpyridine (2-BP) adsorbed on silver hydrosols has been investigated. It has been observed that with a small change in the adsorbate concentration, the SER spectra of 2-BP show significant change in their features, indicating different orientational changes of the different part of the flexible molecule on the colloidal silver surface with adsorbate concentration. The time dependence of the SER spectra of the molecule has been explained in terms of aggregation of colloidal silver particles and co-adsorption and replacement kinetics of the adsorbed solute and solvent molecules on the silver surface. The broad long-wavelength band in the absorption spectra of the silver sol due to solute-induced coagulation of colloidal silver particles is found to be red-shifted with the increase in adsorbate concentration. The surface-enhanced Raman excitation profiles indicate that the resonance of the Raman excitation radiation with the new aggregation band contributes more to the SERS intensity than that with the original sol band.     

Surface-enhanced Raman scattering from crystal violet adsorbed on a silver electrode

SERS from crystal violet (CV) on a Ag electrode was investigated under preresonance and resonance conditions. The excitation profile of the chemisorbed species is like that of dissolved molecules but intensities are ≈ 1000 times larger. The Raman enhancement is ≈ 10⁸ and exhibits a specific potential dependence even in the absence of adsorption-desorption process. At potentials where reduction of CV occurs leuco crystal violet was detected.     

Hot spots in silver nanowire bundles for surface-enhanced Raman spectroscopy

We report a simple strategy for placing analyte molecules in hot spots between closely spaced nanowires leading to intense SERS enhancement. The results are highly reproducible from experiment to experiment likely because of the regularity of the SERS substrate, which consists of highly ordered and regular silver nanowires fabricated in porous aluminum oxide. Because the silver nanowires are sealed in the pores of PAO, this system is potentially immune to contamination until it is ready for use, at which point the alumina matrix is etched, thereby allowing the silver nanowires to collapse into bundles and form hot spots in the region of close contact between the nanowires, trapping the analyte in those junctions.