Improved surface-enhanced Raman scattering on electrochemically roughened silver substrates prepared in bielectrolyte solutions

In this work, the effect of supplemental LiClO₄ electrolytes in KCl solutions used in roughening silver substrates by electrochemical triangular-wave oxidation-reduction cycles (ORC) on surface-enhanced Raman scattering (SERS) was first investigated. To prepare SERS-active substrates by ORC procedures, electrolytes of KCl were generally employed. In contrast, LiClO₄ ones were unsuitable for producing SERS-active substrates. Encouragingly, SERS of Rhodamine 6G (R6G) adsorbed on the roughened Ag substrate prepared in an aqueous solution containing KCl and LiClO₄ electrolytes exhibits a higher intensity by one order of magnitude, as compared with that of R6G adsorbed on a roughened Ag substrate prepared in a solution only containing KCl. Further investigations indicate that the oxidation state of Cl on the roughened Ag substrate demonstrates decided effects on this improved SERS.     

Characterization of 11-mercaptoundecanoic and 3-mercaptopropionic acids adsorbed on silver by surface-enhanced Raman scattering

Functionalized n-alkanethiols such as 11-mercaptoundecanoic (MUA) and 3-mercaptopropionic (MPA) acids are likely to adsorb in silver nanoparticles (AgNPs) solely through the thiol group (-SH) or also involving the carboxylate group (−COO⁻) in their structures. The relative tendency is closely related to pH conditions, solvent or the surface potential of the metallic nanoparticles. The SERS effect (Surface Enhancement Raman Scattering) was used for improving the understanding of MUA and MPA group interaction as well as the orientation of these organic compounds adsorbed on AgNPs and the influence of Cu(II) in solution. When analyzing the MPA SERS spectrum, it was verified that the thiol moiety was preferred to adsorb on the AgNPs surface in the thiolate form, presenting both anti and gauche conformations in both acidic and basic media. MUA SERS spectrum however, indicated that solely an anti conformation for the thiol moiety adsorbed on the AgNPs surface in both acidic and basic media. Adding Cu²⁺ ion resulted in coordination to the carboxyl or carboxylate moieties was confirmed by the downshift of the band assigned to OCO stretching. The presence of Cu(II) increased the tendency of gauche conformation for MPA; the coordination of MUA to Cu(II) resulted in a more upright conformation of the carboxylic/carboxylate moieties in both acidic and basic media, respectively.     

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.     

Electromagnetic and chemical interaction between Ag nanoparticles and adsorbed rhodamine molecules in surface-enhanced Raman scattering

The critical importance of the junction between touching or closely adjacent Ag nanoparticles associated with single-molecule sensitivity (SMS) in surface-enhanced Raman scattering (SERS) was confirmed via the following observations: (1) an additional peak is observed in elastic scattering only for the SERS-active state, which originated from absorption of adsorbates, (2) local- and far-field evaluation using a finite difference time domain method could reproduce this extra peak and anticipate the significantly enhanced field even inside the adsorbates sitting at the junction through an increased coupling of the localized surface plasmons, and (3) in addition to enhanced fluorescence of adsorbed dye, an inelastic scattering peak was observed and attributed to the metal surface electron. Concerning the chemical enhancement in SERS, Cl⁻ anions activate the Ag-Cl-R6G (rhodamine) samples by inducing intrinsic electronic interaction between Ag and R6G molecules. This electronic interaction is irreversibly quenched by the addition of thiosulfate anions which dissolve Ag⁺ cations while the electromagnetic (EM) effect remains intact.     

Infrared and Raman spectroscopic study of 1,2-benzenedithiol adsorbed on silver

Adsorption of 1,2-benzenedithiol (1,2-BDT) on a silver surface has been investigated by surface-enhanced Raman (SER) and reflection-absorption infrared (RAI) spectroscopy. The molecule was adsorbed on silver very favorably by forming two Ag---S bonds after deprotonation. From the RAI spectral pattern, the benzene ring of adsorbed 1,2-BDT was presumed to be tilted by ca. 38° from the surface normal. This RAI information was used to test the validity of various proposed SER selection rules. Being frequently quoted in the literature, the presence or absence of the benzene ring CH stretching vibration in the SER spectrum seemed, in fact, to be a very useful indicator in judging the perpendicular or parallel orientation of the benzene ring with respect to the surface. However, the so-called in-plane/out-of-plane dichotomy as well as the more elaborate symmetry-based electromagnetic selection rule was found not to work in the present system.     

Cuttlebone-derived organic matrix as a scaffold for assembly of silver nanoparticles and application of the composite films in surface-enhanced Raman scattering

Biologically derived materials provide a rich variety of approaches toward new functional materials because of their fascinating structures and environment-friendly features, which is currently a topic of research interest. In this paper, we show that the cuttlebone-derived organic matrix (CDOM) is an excellent scaffold for the one-step synthesis and assembly of silver nanoparticles (AgNPs), which can be further used as substrate for surface-enhanced Raman scattering (SERS). Formation of AgNPs–CDOM composite was accomplished by the reaction of CDOM with AgNO₃ and NH₃·H₂O solution at 80 °C without using any other stabilizer and reducing agents. UV–vis spectra and TEM were utilized to characterize the AgNPs and investigate their formation process. Results demonstrate that the size and distribution of AgNPs can be partly regulated by changing incubation time; the concentration of NH₃·H₂O is critical to the formation rate of AgNPs. As a proof of principle, we show that the AgNPs–CDOM composite can be employed in trace analysis using SERS.     

Quantitative analysis of methyl green using surface-enhanced resonance Raman scattering

Surface-enhanced resonance Raman scattering (SERRS) spectra of aqueous solutions of the triphenylmethane dye methyl green have been obtained for the first time by use of citrate-reduced silver colloids and a laser excitation wavelength of 632.8 nm. Given the highly fluorescent nature of the analyte, which precluded collection of normal Raman spectra of the dye in solution and powdered state, it was highly encouraging that SERRS spectra showed no fluorescence due to quenching by the silver sol. The pH conditions for SERRS were optimised over the pH range 0.5–10 and the biggest enhancement for SERRS of this charged dye was found to be at pH 2.02, thus this condition was used for quantitative analysis. SERRS was found to be highly sensitive and enabled quantitative determination of methyl green over the range 10⁻⁹ to 10⁻⁷ mol dm⁻³. Good fits to correlation coefficients were obtained over this range using the areas under the vibrational bands at 1615 and 737 cm⁻¹. Finally, a limit of detection of 83 ppb was calculated, demonstrating the sensitivity of the technique.     

One-pot synthesis of silver colloid with body-heat for surface-enhanced Raman spectroscopy detections

In this study, a convenient method of preparing the substrate is proposed with one-pot synthesis of silver colloid under body heat, and the SERS detection uses the fresh substrate to avoid the drawback of substrates' short life of use. The synthesis of silver colloid is carried out in a 10 mL vial by using ascorbic acid as a reductant and trisodium citrate as a stabilizer. The vial is grasped with the palm of the experimenter for several minutes without shaking. The proposed method is simple, rapid, green energy and cost-effective. By adjusting the concentration of trisodium citrate, not only the particle size can be controlled from about 110 nm to 50 nm but also the homogeneity of nanoparticles can be improved. As a SERS substrate, the silver colloid has high batch reproducibility and showed good SERS activity. The relative standard deviation between different manufacturers is 5.51% when the substrate of silver colloid is used for the detection of rhodamine 6 G. Using the substrate, the lowest detection concentrations of rhodamine 6 G, crystal violet, enrofloxacin, melamine and leucomalachite green are 1.0×10~(-8), 6.1×10~(-8),1.4 × 10~(-6), 7.1 ×10~(-5) and 5.1 ×10~(-8) mol/L, respectively. Results demonstrate that the developed method has the advantage of convenience and high efficiency in the field preparation of reliable SERS substrate.     

A comparison of surface-enhanced infrared and surface-enhanced Raman spectra of pyrazine adsorbed on polycrystalline gold electrodes

The adsorption of pyrazine at a polycrystalline Au film electrode has been investigated using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), chronocoulometry, and ac impedance. Combining the SEIRA data and the thermodynamic data (the surface charge density of the electrode and the relative Gibbs surface excess of pyrazine), pyrazine was found to adsorb on the surface with a vertical end-on configuration via one N atom. The SEIRA spectra were compared with published surface-enhanced Raman (SER) spectra of pyrazine in order to clarify the reason for the breakdown of the Raman selection rule in the SER spectra. The activation of originally Raman-forbidden modes in the SER spectra is well explained by a photo-driven charge-transfer mechanism. The charge-transfer is deduced to be from filled metal states near the Fermi level to the first and second excited states of pyrazine. It is emphasized that the comparative SEIRA and SER studies are important for a better understanding of the electrochemical interface.     

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.     

Surface-enhanced Raman scattering characteristics of 4-nitrobenzenethiol adsorbed on palladium and silver thin films

Palladium is an important catalytic metal, and it is desirable to develop a surface-enhanced Raman scattering (SERS) technique to investigate the reagent and product species adsorbed on its surface. Unfortunately, Pt-group metals, e.g., Pt and Pd, have been commonly considered as non- or weak-SERS-active substrates. In this work, Ag and Pd thin films were deposited very efficiently and evenly onto the surface of glass substrates by using only corresponding metal nitrate salts (AgNO₃ and Pd(NO₃)₂) with butylamine in ethanolic solutions. In this process, pure ethanol was used for Ag deposition, while an ethanol–water (8:2) mixture was used for Pd deposition. The as-prepared Ag and Pd films exhibited SERS activity over a large area. The surface-induced photoconversion capabilities of these Ag and Pd films were then tested on 4-nitrobenzenethiol by means of SERS. It was found that at least under visible laser irradiation, the surface-catalyzed photoreaction occurs more readily on a Ag film than on a Pd film for the conversion of 4-nitrobenzenethiol to 4-aminobenzenethiol, even though Pd is known to be an important transition metal with high catalytic activity.     

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.     

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.     

Photochemical decoration of silver nanoparticles on magnetic microspheres as substrates for the detection of adenine by surface-enhanced Raman scattering

In this work, silver nanoparticles (AgNPs) decorated magnetic microspheres (MMs) are prepared as surface-enhanced Raman scattering (SERS) substrate for the analysis of adenine in aqueous solutions. To prepare these substrates, magnetic particles were first synthesized by coprecipitation of Fe(II) and Fe(III) with ammonium hydroxide. A thin layer of cross-linked polymer was formed on these magnetic particles by polymerization through suspension of magnetic particles into a solution of divinyl benzene/methyl methacrylate. The resulted polymer protected magnetic particles are round in shape with a size of 80 μm in diameter. To form AgNPs on these MMs, photochemical reduction method was employed and the factors in photochemical reduction method were studied and optimized for the preparation of highly sensitive and stable AgNPs on MMs substrates (abbreviated as AgMMs substrates). By dispersing the AgMMs in aqueous samples, cylindrical magnet was used to attract the AgMMs for SERS detections. The observed enhancement factor of AgMMs reached 7 orders in magnitude for detection of adenine with a detection limit approaching to few hundreds of nanomolar.     

X-ray and Raman study of spray pyrolysed vanadium oxide thin films

Vanadium oxide thin films were prepared by spray pyrolysis using solutions of vanadium chloride (VCl₃) with different concentrations on glass substrates heated at 200 and 250 °C. The influence of substrate temperature (T_s) and solution concentration (molarity) on structural and vibrational properties is discussed by using X-ray diffraction and Raman spectroscopy. The results revealed that at 0.05 M and T_s = 200 °C, V₄O₉ thin films are obtained. At 250 °C, V₂O₅ phases with preferential orientation are observed and the films become polycrystalline when the molarity increases.     

In-situ surface-enhanced Raman scattering and FT-Raman spectroscopy of black prints

The black inkjet and laser prints were analysed with regard to application in forensic analysis of questioned documents. The purpose of this work was to study spectral properties and compare the suitability of surface-enhanced Raman scattering (SERS) with Fourier transform Raman spectra of prints. This work aimed to find optimal surface-enhanced Raman spectroscopic approach for the future analysis of documents using statistical methods. In this work, we analysed eight prints of four laser and four inkjet devices. The samples were measured using two dispersive Raman devices; (DXR Raman microscope with excitation line 532 nm, Foram 685-2 spectrometer − 685 nm) and FT-Raman device (Bruker Spectrometer MultiRAM with excitation line 1064 nm). The silver nanoparticles (AgNPs) colloid for SERS experiment were synthesised and checked by UV–vis spectroscopy and scanning electron microscopy (SEM). The remarkable differences caused by centrifugation of silver colloid were observed just in the SEM images. The main contribution of this paper is to propose the novel approach achieving sufficient SERS signal intensity of black prints using the both, laser and inkjet printers. Moreover, this method is based on just a single metal colloid, and the analysis can be performed in-situ, i.e. directly on the printed sample surface. We consider the SERS could by highly promising and universal for applications in the forensic analysis of printed documents with the combination of statistical method when conventional methods are not effective.     

Effect of substrate on surface-enhanced Raman scattering of molecules adsorbed on immobilized silver nanoparticles

Silver nanoparticles were assembled on polyvinylpyridine (PVP) derivatized glass slides. Charge transfer between the adsorbed 4-aminothiophenol (PATP) and the immobilized silver nanoparticles was studied by surface-enhanced Raman spectroscopy with 1064 nm excitation, and compared with that of the silver nanoparticles in the colloid. It was demonstrated that the positive charges of the PVP layer could alter the charge distribution in the immobilized nanoparticles and induce the formation of the dipole in the nanoparticles, leading to less charge transfer from the metal to the adsorbed molecules. The coadsorption of chloride ions on the surface of the immobilized silver nanoparticles resulted in the redistribution of the charges in the nanoparticles and, in turn, altered the charge transfer between the adsorbed PATP molecules and the silver nanoparticles.     

Fourier-transform Raman spectroscopic study of unsaturated and saturated waxes

The application of Fourier-transform (FT)-Raman spectroscopy to the non-destructive analysis of plant biomaterials, comprising ten different waxes, is illustrated in this work. All the wax specimens studied are multicomponent systems and comparison of the individual spectra was effected. This leads to a means of distinguishing the individual wax specimen using FT-Raman spectroscopy. The samples were not pre-treated before analysis and were used directly from their respective storage collections.     

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.     

A new aptameric biosensor for cocaine based on surface-enhanced Raman scattering spectroscopy

The present study reports the proof of principle of a reagentless aptameric sensor based on surface-enhanced Raman scattering (SERS) spectroscopy with "signal-on" architecture using a model target of cocaine. This new aptameric sensor is based on the conformational change of the surface-tethered aptamer on a binding target that draws a certain Raman reporter in close proximity to the SERS substrate, thereby increasing the Raman scattering signal due to the local enhancement effect of SERS. To improve the response performance, the sensor is fabricated from a cocaine-templated mixed self-assembly of a 3'-terminal tetramethylrhodamine (TMR)-labeled DNA aptamer on a silver colloid film by means of an alkanethiol moiety at the 5' end. This immobilization strategy optimizes the orientation of the aptamer on the surface and facilitates the folding on the binding target. Under optimized assay conditions, one can determine cocaine at a concentration of 1 muM, which compares favorably with analogous aptameric sensors based on electrochemical and fluorescence techniques. The sensor can be readily regenerated by being washed with a buffer. These results suggest that the SERS-based transducer might create a new dimension for future development of aptameric sensors for sensitive determination in biochemical and biomedical studies.