An ultrasensitive method: surface-enhanced Raman scattering of Ag nanoparticles from beta-silver vanadate and copper

Ultrasensitive surface-enhanced Raman scattering signals of four typical analytes were observed on Ag nanoparticles from beta-silver vanadate and copper, even though the concentrations of these analytes were as low as 1 x 10(-16) M (Rhodamine 6G or crystal violet) and 1 x 10(-15) M (trinitrotoluene or bovine serum albumin).     

Ag nanoparticles prepared by laser photoreduction as substrates for in situ surface-enhanced Raman scattering analysis of dyes

In this work Ag nanoparticles (AgNP) with surface-enhanced Raman scattering (SERS) activity were prepared and immobilized by laser irradiation on a water/ solid interface where the aqueous phase contains the Ag+ cation and the solid surface is of hydrophilic nature (glass and cellulose). The so-prepared AgNP demonstrated a high SERS effectiveness in the detection of dispersed adsorbates such as the case of the anthraquinonic dye alizarin. The size and SERS effectiveness of AgNP increases with the irradiation time, the laser power, and the cation concentration. Laser-induced AgNP can be classified into two classes attending to the morphology: spherical and planar. The latter are formed after longer irradiation times, being more active regarding the SERS efficiency. Ag photoreduction can be employed for in situ detection of the dye alizarin, but when the dye is placed on a hydrophilic substrate. Even so, this in situ SERS technique could be attractive for analytical applications involving the in situ detection of the analyzed species, such as the case of dyes in artistical objects, textiles, foods, and surface analysis in general.     

Rapid detection of melamine with 4-mercaptopyridine-modified gold nanoparticles by surface-enhanced Raman scattering

A surface-enhanced Raman scattering (SERS) strategy based on 4-mercaptopyridine (MPY)-modified gold nanoparticles (AuNPs) was developed for the rapid and sensitive detection of melamine in milk powder. The SERS measurement of melamine strongly relied on the “hotspot” effect, in which AuNPs immediately aggregated upon the addition of melamine, leading to significantly enhanced Raman intensity of the reporter molecule MPY and a color change for the solution from red to blue-gray. The limit of detection based on a signal to noise of 3 (S/N = 3) was found to be as low as 0.1 ppb of melamine, with an excellent linearity of 0.5–100 ppb, demonstrating a higher sensitivity and a wider quantitation range than direct SERS sensing methods based on enhanced substrate. An impressive specificity for melamine detection over various common metal ions and excipients in dairy products, even at concentrations of 100-fold higher than melamine, was achieved. Good recoveries of 88.5% and 111.7% were obtained from milk samples spiked to 20 and 100 ppb levels, respectively. The proposed method is potentially applicable for the rapid in situ determination of melamine in complex matrices.     

Immunoassay using surface-enhanced Raman scattering based on aggregation of reporter-labeled immunogold nanoparticles

A one-step homogenous sensitive immunoassay using surface-enhanced Raman scattering (SERS) has been developed. This strategy is based on the aggregation of Raman reporter-labeled immunogold nanoparticles induced by the immunoreaction with corresponding antigens. The aggregation of gold nanoparticles results in a SERS signal increase of the Raman reporter. Therefore, human IgG could be directly determined by measuring the Raman signal of the reporter. The process of aggregation was investigated by transmission electron microscopy (TEM) and UV-Vis absorption spectroscopy. The effects of the temperature, time, and size of gold nanoparticles on the sensitivity of the assay were examined. Using human IgG as a model protein, a wide linear dynamic range (0.1-15 microg mL(-1)) was reached with low detection limit (0.1 microg mL(-1)) under optimized assay conditions. The successful test suggests that the application of the proposed method holds promising potential for simple, fast detection of proteins in the fields of molecular biology and clinical diagnostics.     

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.     

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.     

Rapid ultrasensitive monitoring the single-particle surface-enhanced Raman scattering (SERS) using a dark-field microspectroscopy assisted system

The observation of single-particle surface-enhanced Raman scattering(SERS) has generated considerable interest both in the nanomaterials filed and in the single-particle spectroscopy community.It is a challenge to realize rapid,facile,and high throughput SERS at single nanoparticle level.Here,without the complex experimental device and difficult experimental operations,a general single-particle SERS technique has been achieved by using dark-field-assisted surface-enhanced Raman spectroscopy(DFSERS).This advanced method provides in-situ characterization of the chemical reaction performance at single gold nanorod.     

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.     

Phospholipid membrane encapsulation of nanoparticles for surface-enhanced Raman scattering

Lipid-encapsulated surface-enhanced Raman scattering (SERS) nanoparticles, with promising applications in biomedical diagnostics, were produced. Gold nanoparticles, 60 nm in diameter, were coated with a ternary mixture of DOPC, sphingomyelin, and cholesterol. The lipid layer is versatile for engineering the chemical and optical properties of the particles. The stability of the lipid-encapsulated particles is demonstrated over a period of weeks. The versatility of the layer is demonstrated by the incorporation of three different Raman-active species using three different strategies. The lipid layer was directly observed by TEM, and the SERS spectrum of the three dye species was confirmed by Raman spectroscopy. UV-vis absorption and dynamic light scattering provide additional evidence of lipid encapsulation. The encapsulation is achieved in aqueous solution, avoiding phase transfer and possible contamination from organic solvents. Furthermore, when fluorescent dye-labeled lipids were employed in the encapsulant, the fluorescence and SERS activity of the particles were controlled by the use of dissolved ions in the preparation solution.     

Immunoassay using probe-labelling immunogold nanoparticles with silver staining enhancement via surface-enhanced Raman scattering

This paper reports a novel immunoassay based on surface-enhanced Raman scattering (SERS) and immunogold labelling with silver staining enhancement. Immunoreactions between immunogold colloids modified by a Raman-active probe molecule (e.g., 4-mercaptobenzoic acid) and antigens, which were captured by antibody-assembled chips such as silicon or quartz, were detected via SERS signals of Raman-active probe molecule. All the self-assembled steps were subjected to the measurements of ultraviolet-visible (UV-vis) spectra to monitor the formation of a sandwich structure onto a substrate. The immunoassay was performed by a sandwich structure consisting of three layers. The first layer was composed of immobilized antibody molecules of mouse polyclonal antibody against Hepatitis B virus surface antigen (PAb) on a silicon or quartz substrate. The second layer was the complementary Hepatitis B virus surface antigen (Antigen) molecules captured by PAb on the substrate. The third layer was composed of the probe-labelling immunogold nanoparticles, which were modified by mouse monoclonal antibody against Hepatitis B virus surface antigen (MAb) and 4-mercaptobenzoic acid (MBA) as the Raman-active probe on the surface of gold colloids. After silver staining enhancement, the antigen is identified by a SERS spectrum of MBA. A working curve of the intensity of a SERS signal at 1585 cm(-1) due to the [small nu](8a) aromatic ring vibration of MBA versus the concentration of analyte (Antigen) was obtained and the non-optimized detection limit for the Hepatitis B virus surface antigen was found to be as low as 0.5 [micro sign]g mL(-1).     

有序Au/Ag纳米碗阵列的简易制备及其表面增强拉曼散射性能

通过湿法化学合成基于SiO₂胶体晶体的大面积有序Au/Ag纳米碗(Au/AgNB)阵列。首先,在玻璃基板上组装3D SiO₂胶体晶体作为模板。然后,以Au纳米颗粒(AuNP)为种子,通过原位生长法在SiO₂模板上沉积一层Au纳米壳(AuNS)。再通过HCHO还原Ag⁺成Ag⁰,进一步在AuNS表面沉积Ag纳米壳,形成Ag/Au双纳米壳(Ag/AuNS)阵列。最后通过丙烯酸酯改性双向取向聚丙烯(BOPP)膜方便地获得了单层有序反转Ag/AuNB阵列。这种有序Au/AgNB阵列具有更佳的表面增强拉曼散射(SERS)活性,其SERS分析增强因子(AEF)可达2.23×10⁷。     

有序Au/Ag纳米碗阵列的简易制备及其表面增强拉曼散射性能

通过湿法化学合成基于SiO₂胶体晶体的大面积有序Au/Ag纳米碗(Au/AgNB)阵列。首先,在玻璃基板上以3D SiO₂胶体晶体作为模板。然后,在Au纳米颗粒(AuNP)种子的帮助下,通过原位生长方法在模板上沉积一层Au纳米壳(AuNS)。再通过HCHO还原Ag⁺使AuNS表面进一步沉积Ag纳米壳,形成Ag/Au双纳米壳(Ag/AuNS)阵列。通过丙烯酸酯改性双向取向聚丙烯(BOPP)方便地获得了单层有序反转Ag/AuNB阵列。这种有序Au/AgNB阵列具有更佳的表面增强拉曼散射(SERS)活性,其SERS分析增强因子(AEF)可达2.23×10⁷。     

Polarization-dependent effects in surface-enhanced Raman scattering (SERS)

A few key examples of polarization effects in surface-enhanced Raman scattering (SERS) are highlighted and discussed. It is argued that the polarization of the local field, which is felt by an analyte molecule in a location of high electromagnetic field enhancement (hot-spot), can be very different from that of the incident exciting beam. The polarization dependence of the SERS signal is, therefore, mostly dictated by the coupling of the laser to the plasmons rather than by the symmetry of the Raman tensor of the analyte. This sets serious restrictions for the interpretation of both single-molecule SERS polarization studies and for the use of circularly polarized light in techniques like surface-enhanced Raman optical activity.     

Essential nanogap effects on surface-enhanced Raman scattering signals from closely spaced gold nanoparticles

We have demonstrated the essential nanogap effects on surface-enhanced Raman scattering (SERS) signals obtained from two diagonally aligned gold nanoparticles with several nanometre separations, which were precisely fabricated on a glass substrate. This is the first proof of principle for extracting the light localization effects on SERS due to the formation of nanogaps from experimentally observed SERS signals.     

Determination of mercury(II) by surface-enhanced Raman scattering spectroscopy based on thiol-functionalized silver nanoparticles

Silver nanoparticles (Ag NPs) modified with sodium 2-mercaptoethanesulfonate (mesna) exhibit strong surface-enhanced Raman scattering (SERS). Their specific and strong interaction with heavy metal ions led to a label-free assay for Hg(II). The covalent bond formed between mercury and sulfur is stronger than the one between silver and sulfur and thus prevents the adsorption of mesna on the surface of Ag NPs. This results in a decrease of the intensity of SERS in the presence of Hg(II) ions. The Raman peak at 795?cm^?1 can be used for quantification. The effect of the concentration of mesna, the concentration of sodium chloride, incubation time and pH value on SERS were optimized. Under the optimal conditions, the intensity of SERS decreases with increasing concentration of Hg(II). The decrease is linear in the 0.01 and 2?μmol L^?1 concentration range, with a correlation coefficient (R²) of 0.996 and detection limit (S/N?=?3) is 0.0024?μmol L^?1. The method was successfully applied to the determination of the Hg(II) in spiked water samples.

Synthesis and characterization of surface-enhanced Raman scattering tags with Ag/SiO2 core-shell nanostructures using reverse micelle technology

A novel simplified method for synthesis of surface-enhanced Raman scattering tags has been reported. This synthesis method is based on reverse micelle technique using Igepal CO-520 as a surfactant and the mixed solution of silver nitrate and rhodamine dyes with isothiocyanate group as water pool followed by hydrazine hydrate reduction and TEOS polymerization leading to the formation of silica layer surrounding the silver core. Compared to the method reported in literature, the proposed methodology eliminates the necessity of vitrophilic pretreatment and makes it possible to complete all different processes including the preparation of silver nanoparticles, the conjugation of dye molecules and the formation of silica shell in the microreactor. The nanoparticle-based surface-enhanced Raman tags obtained are composed of silver core conjugated with rhodamine dyes and an encasing silica shell. Both the dyes themselves and the Ag/SiO2 core-shell nanoparticles without the encapsulation of dyes exhibit no Raman signals. However, the Ag/SiO2 core-shell nanoparticles exhibit strong Raman signals when encapsulated with these dyes. This is due to the appearance of fluorescence quenching and surface-enhanced Raman scattering effect resulting from the conjugation of dyes and silver core. The Raman tags were characterized using transmission electron microscopy (TEM), UV-visible absorption spectrometry, and Raman spectrometry.     

Effects of underpotentially deposited TI and Pb submonolayers on the surface-enhanced Raman scattering (SERS) from pyridine at Ag electrodes

SERS from pyridine on an Ag electrode is almost completely quenched by underpotential deposition of TI to a surface coverage of ≈ 3%. Removal of TI by anodic stripping does not restore the original SERS intensity. These results suggest that only a small fraction of adsorbed pyridine molecules is SERS active, and TI is specifically deposited on such a SERS active site, which then undergoes irreversible destruction. Pb appears to be non-specifically deposited on the Ag surface, but exhibits a similar irreversible quenching of SERS.     

Raman,surface-enhanced Raman scattering and DFT study of para-nitro-aniline

Raman spectra of para-nitro-aniline (pNA), a molecule with high applicability potential in molecular electronics, were recorded in solid state and in ethanol solution. Complete assignment of the experimental spectra was made by using the B3LYP/6-31G(d) theoretical results. The calculated molecular electrostatic potential shows a high negative charge localized on the nitro group of pNA and the surface-enhanced Raman scattering (SERS) spectrum of pNA adsorbed to colloidal silver particles denote the molecule interaction with the silver surface mainly through the nitro group. However, theoretical results obtained by modeling the pNA–4Ag complex also suggest the adsorption of pNA through the amino group or a flattened orientation of pNA with respect to the silver surface.     

Core (Au)-shell (Ag) structure nitrogen dots for the recognition of nitroaniline isomers by surface-enhanced Raman scattering

A facile synthesis method for NDs-Au@AgNPs SERS substrate using Au seeds prepared by nitrogen-rich quantum dots (NDs) as reducing agent and stabilizer was developed for nitroaniline isomers recognition by surface-enhanced Raman scattering.