Surface-enhanced Raman scattering of a series of n-hydroxybenzoic acids (n = P, M and O) on the silver nano-particles

Surface-enhanced Raman scattering (SERS) spectra of a series of n-hydroxybenzoic acids (n-HBA, n = P, M and O) adsorbed on the silver nano-particles were studied, respectively, in the silver colloidal solution and on the dried silver-coated filter paper. On the same substrate, the different molecules' SERS spectra were different, while on the different substrates the same molecules' SERS spectra were also different. Significant changes were found in the SERS spectra of PHBA molecules adsorbed on the two substrates, and the changes found in MHBA's spectra on two substrates were next to PHBA's, while almost no changes were found in the spectra of OHBA molecules. Moreover, it was found, on the filter paper, that the SERS spectra of the same molecules would change with the coverage density of the silver nano-particles. The analyses showed that the origins of these changes were the different adsorption behavior of molecules adsorbed on the silver nano-particles. Because in these three molecules the relative positions of the carboxyls and hydroxyls on the benzenes are different, the adsorption behaviors of these three molecules adsorbed on the silver surfaces are also different. The experimental results suggest that the surface characteristic of the substrate and the surface configuration of the adsorbate could exert a great influence on the adsorption behavior of the adsorbates on the substrates.     

Investigation of p-hydroxybenzoic acid from a new surface-enhanced Raman scattering system

In this paper, we developed a new kind of substrate, silver-coated indium tin oxide (ITO), to investigate the character of surface-enhanced Raman scattering (SERS) of p-hydroxybenzoic acid (PHBA). Homogeneous Ag-coated ITO substrate was obtained by decomposing AgNO(3) on the surface of ITO. A SERS spectrum of very good quality of "silver nanoparticles/PHBA/silver-coated ITO" was reported by adding PHBA aqueous solution and silver colloid onto the surface of silver-coated ITO repeatedly. PHBA molecules absorbed onto the surface of the silver nanoparticles through ionized carboxyl, and the PHBA molecules tended to tilt on the surface in this system. The rich information obtained from the silver nanoparticles/PHBA/silver-coated ITO system indicates that this is a highly SERS-active system. Not only was the number of the vibrational modes increased, but also were the frequencies of Raman bands shifted. The two SERS mechanisms, the "electromagnetic" and "chemical" mechanism, were mainly responsible for the experimental results. Furthermore, the silver nanoparticles modified on the silver-coated ITO surface play an important role in magnifying the surface local electric field near the silver film surface through resonant surface plasmon excitation.     

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.     

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.     

金与银电极表面甘氨酸分子吸附作用的SERS光谱研究

本文利用表面增强拉曼光谱(SERS)技术研究了甘氨酸在金与银基底表面的吸附作用特征。研究表明甘氨酸分子以COO-的不对称形式吸附于金电极表面,且NH2也是其可能的吸附位点;而在银电极表面,则主要是通过COO-的对称形式而吸附。在此基础上,进一步研究了电极电位与溶液酸碱性对吸附于粗糙化银电极表面甘氨酸分子吸附作用的影响。研究结果表明,甘氨酸分子中去质子化羧基的吸附作用受电位影响较小,而电位对-NH3+吸附作用的影响程度较大。另一方面,溶液pH值对银电极表面的甘氨酸分子吸附行为的影响也较为显著。随着溶液酸性减小羧基倾向于相对于电极表面平行吸附。这是由于随着溶液碱性增大氨基质子化程度的减小,有利于氨基在银电极表面吸附。这将改变分子的吸附构型使其更接近于电极表面。这些变化主要出现在pH值大于10的条件下。     

The investigation of a series of n-hydroxybenzoic acids (n=p, m, o) on a new surface enhanced Raman scattering active substrate

Ag nanoparticles, organized on indium tin oxide (ITO) surface, can act as a new surface enhanced Raman scattering (SERS) active substrate as well as a dry electrode. Compared with the traditional SERS-active substrate, its notable advantage is that the microcosmic changes of the film can be reflected by the SERS spectrum during the investigation of the film's electrical and other macroscopic characteristics. To illuminate the above-mentioned property of this new substrate, a series of n-hydroxybenzoic acids (n-HBA; n=p, m and o) was tested as probe molecules by SERS technique. These SERS spectra indicate that the significant changes of frequencies as well as intensities, respectively, arise from the changes of the adsorption behavior along with the proportional variation of molecules and silver nanoparticles. Excellent SERS signals prove that the silver nanoparticles-coated ITO is a highly SERS-active substrate and can efficiently reflect the microcosmic property of the film, which suggest it has promising potential of being a new technique for further application in the field of thin-film research.     

SERS of C60/C70 on gold-coated filter paper or filter film influenced by the gold thickness

SERS of C(60)/C(70) adsorbed on gold nanoparticles coated on filter paper or filter film was studied. As a new SERS substrate, dried gold-coated filter paper or filter film has a high SERS activity, whose enhancement factor can be up to about 10(5), because it avoided the influence of solvents in C(60)/C(70) solution and water in gold hydrosols. The influence of the gold thickness coated on filter paper or filter film to SERS of C(60)/C(70) adsorbed on gold nanoparticles was mainly discussed. It is indicated that the SERS effect of C(60)/C(70) was very sensitive to the distribution and aggregated characteristics of gold nanoparticles, and the SERS intensity of each mode increased at its own proportion, but it integrally tended to saturation when the thickness of colloidal gold coatings increased.     

Adsorption Behaviors of 4-Mercaptobenzoic Acid on Silver and Gold Films

The adsorption behaviors of 4-mercaptobenzoic acid on silver and gold nanoparticles were studied by surface-enhanced Raman scattering (SERS) and density functional theory. The silver and gold films by electrodeposition have the same excellent characteristics as SERS-active substrates. At the same, the SERS spectra indicate that 4-mercaptobenzoic acid molecules are adsorbed on the surfaces of gold nanoparticles through the S atom, and that the carboxyl group is far away from surface of gold nanoparticles, and that there is a certain angle between the plane of benzene ring and gold film. However, 4-mercaptobenzoic acid molecules are adsorbed on the surfaces of silver nanoparticles through the carboxyl group, and the S atom is far away from surface of silver nanoparticles, and there is also a certain angle between the plane of benzene ring and the surface of silver nanoparticles. Here it is demonstrated the calculated Raman frequencies are in good agreement with experimental values, and the calculated Raman frequencies are also helpful to infer the adsorption behaviors of 4-mercaptobenzoic acid molecules.     

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.     

对氯硝基苯吸附在银纳米粒子上的偶联反应

表面增强拉曼光谱(SERS)具有极高的检测灵敏度, 通过检测吸附分子的SERS信号, 可以获得表面吸附分子的结构以及可能发生的反应. 在拉曼激发光源的辐射下, 在碱性溶液中, 银纳米粒子表面吸附的对氯硝基苯(PCNB)的SERS光谱与其固体的常规拉曼光谱相比, 出现异常SERS谱. 通过采用密度泛函理论(DFT)计算, 对PCNB以及可能的偶联产物p,p''-二氯偶氮苯(DCAB)进行理论分析以及谱峰归属, 发现这些异常峰来自其偶联产物DCAB的偶氮C－N＝N－C基团的基频振动.     

Adsorption mechanisms of sulfathiazole on gold,silver and copper surfaces studied by SERS

Surface-enhanced Raman scattering (SERS) of sulfathiazole was studied in gold, silver and copper colloids as well as on a gold plate. SERS spectra of sulfathiazole in gold and silver colloids indicated chemisorption of molecules on the metal nanoparticles through the amide nitrogen, with the phenyl moiety orthogonally placed and the thiazole ring almost parallel positioned towards the metal surface. Although selectively enhanced phenyl bands pointed to a very similar position of the sulfathiazole molecules on the copper colloid, a chemical bonding was not implied. Unlike adsorption mechanisms and position of the molecules on the colloid metal surfaces, a sideway adsorption of sulfathiazole on the gold plate was proposed. Hereby, both, the amide nitrogen and the thiazole nitrogen were considered responsible for approaching of sulfathiazole to the gold enhancing surface.     

Surface-enhanced Raman scattering of single-walled carbon nanotubes on silver-coated and gold-coated filter paper

Surface-enhanced Raman scattering (SERS) spectra of single-walled carbon nanotubes (SWCNTs) on metal-coated filter paper are reported for the first time. Experimental results show that the metal-coated filter paper is very effective and active. The SERS spectrum not only shows that all Raman bands of SWCNTs in normal Raman scattering have been generally enhanced, but also shows many new bands, which characterize the structure of SWCNTs and the interaction between SWCNTs and silver/gold nanoparticles, arising from symmetry lowering and selection rule relaxing of SWCNTs induced by the silver/gold surface. In our case, it is difficult to separate the contributions of the electromagnetic and chemical mechanisms to the great enhancement of the Raman signal. The analysis shows that the SERS spectra of SWCNTs on the metal-coated filter paper provide convenience for probing the sample molecules with fine structures related to defects of SWCNTs, the diameter of SWCNTs, and the SERS mechanism of SWCNTs deposited on metal-coated filter paper. Moreover, this can be used as a probe technique for monitoring the synthesis quality of SWCNTs with significant higher sensitivity than other methods, which has promise of being a new technique for monitoring synthesis quality of SWCNTs.     

Solid-liquid-liquid extraction as an approach to the sensitive detection of a hydrophobic pollutant through surface-enhanced Raman spectroscopy

Surface-enhanced Raman scattering (SERS) spectra of thiram (tetramethylthiuram disulfide), a dimethyl dithiocarbamate fungicide, were recorded after the adsorption on plasmonic silver nanowires from a system of water, organic solvent and nanoparticles. As organic solvents dichloromethane and 1-octanol were involved. A method for measuring the adsorption constant of thiram as a model molecule to the silver surface by studying its partition phenomena in a binary solvent system is presented. The method is based on the extraction of a hydrophobic molecule from an organic solvent by an aqueous suspension of silver anisotropic nanoparticles. The obtained results demonstrate the effectiveness of SERS methodology for the sensitive analysis of compounds with low aqueous solubility, and a reliable SERS spectrum of thiram was obtained with excellent signal/noise ratio at low concentrations. In addition, for vibrational assignments, Density Functional Theory (DFT) was used for the simulation of the Raman and SERS spectra of thiram and its complexes with silver considering the following two models: a single silver atom and an Ag₂₀ cluster.     

Towards single-microorganism detection using surface-enhanced Raman spectroscopy

The identification and discrimination of microorganisms is important not only for clinical reasons but also for pharmaceutical clean room production and food-processing technology. Vibrational spectroscopy such as IR, Raman, and surface-enhanced Raman scattering (SERS) can provide a rapid ‘fingerprint’ on the chemical structure of molecules and is used to obtain a ‘fingerprint’ from microorganisms as well. Because of the requirement that a single bacterium cell and noble metal nanoparticles must be in close contact and the lack of a significant physical support to hold nanoparticles around the single bacterium cell, the acquisition of SERS spectra for a single bacterium using colloidal nanoparticles could be a challenging task. The feasibility of SERS for identification down to a single bacterium is investigated. A Gram-negative bacterium, Escherichia coli, is chosen as a model for the investigation. Because the adsorption of silver nanoparticles onto the bacterial cell is an exclusive way for locating nanoparticles close to the bacterium cell, the absorption characteristics of silver nanoparticles with different surface charges are investigated. It is demonstrated that the citrate-reduced colloidal silver solution generates more reproducible SERS spectra. It is found that E. coli cells aggregate upon mixing with silver colloidal solution, and this may provide an additional benefit in locating the bacterial cell under a light microscope. It is also found that a laser wavelength in the UV region could be a better choice for the study due to the shallow penetration depth. It is finally shown that it is possible to obtain SERS spectra from a single cell down to a few bacterial cells, depending on the aggregation properties of bacterial cells for identification and discrimination.     

银表面罗丹明6G的电化学表面增强拉曼光谱研究

罗丹明6G（Rhodamine 6G,R6G）是单分子表面增强拉曼光谱（SM-SERS）研究中最常用的探针分子之一,对R6G分子在表面吸附行为的研究有助于了解R6G分子和表面的相互作用. 本文应用电化学和电化学表面增强拉曼光谱技术,研究不同电位下R6G的银电极表面的吸附行为. 结果表明,随着电位负移罗丹明6G在银表面上从垂直吸附转为倾斜吸附,该变化和碱性条件下吸附于金纳米粒子上R6G的吸附构象一致. 这说明,在部分单分子实验中所发现的R6G反常光谱其来源是单个R6G分子在表面吸附取向变化. 本研究对后续详细分析SM-SERS研究中单分子SERS谱峰变化的机制有一定的参考价值.     

Investigation of chemically modified barium titanate beads as surface-enhanced Raman scattering (SERS) active substrates for the detection of benzene thiol, 1,2-benzene dithiol, and rhodamine 6G

SERS active surfaces were prepared by depositing silver films using Tollen's reaction on to barium titanate beads. The SERS activity of the resulting surfaces was probed using two thiols (benzene thiol and 1,2-benzene dithiol) and rhodamine 6G. The intensity of the SERS signal for the three analytes was investigated as a function of silver deposition time. The results indicate that the SERS intensity increased with increasing thickness of the silver film until a maximum signal intensity was achieved; additional silver deposition resulted in a decrease in the SERS intensity for all of the studied molecules. SEM measurement of the Ag coated barium titanate beads, as a function of silver deposition time, indicate that maximum SERS intensity corresponded with the formation of atomic scale islands of silver nanoparticles. Complete silver coverage of the beads resulted in a decreased SERS signal and the most intense SERS signals were observed at deposition times of 30 min for the thiols and 20 min for rhodamine 6G.     

Raman Spectra of Molecules Adsorbed on Ag Centers in Sol-Gel Matrices

Silica monoliths and submicron spheres containing silver nanoparticles have been obtained using the sol gel technology. The Ag inclusions were synthesized via the counterdiffusion method. The silver-doped matrices were immersed in solutions of an organic dye (indocyanine green) enabling the solute molecules to interact with surface of the Ag-doped silica matrices. Raman spectra of free solutions of the organic molecules under investigation, the impregnated Ag-doped matrices and the impregnated Ag-free matrices have been measured. The impregnated silica matrices which did not contain silver nanoparticles were used as a reference. These experiments have been performed in order to establish if Raman signal enhancement could be obtained by adsorption of organic molecules on the surface of Ag inclusions in the sol-gel matrices analogously to the standard surface-enhanced Raman spectroscopy (SERS) method.     

A SERS-active system based on silver nanoparticles tethered to a deposited silver film

A series of SERS-active nanostructures were produced by exposing a freshly deposited silver film (fabricated to be as free from roughness as practicable) to a solution containing a mixture of 1-decanethiol (m) and 1,9-nonanedithiol (d) of varying concentrations of m to d, then allowing colloidal silver nanoparticles to interact with the surface. Silver nanoparticles were found to bind exclusively to films which were prepared from solutions with a nonzero concentration of the dithiol implying that the nanoparticles were tethered to the silver surface by the dithiol with one of the thiolate groups bound to the nanoparticle and the other to the silver film. Intense SERS spectra were observed even from samples in which the m/d concentration ratio was so large that the adsorbed molecules in the vicinity of only approximately 8 +/- 3 nanoparticles were illuminated by the diffraction-limited focused laser beam. At such high dilution, the molecules (numbering at most approximately 330) residing in the SERS "hot spots" associated with the approximately 8 nanoparticles consisted primarily of m (although, of course, for each nanoparticle, at least one molecule in the hot spot had to be d to serve as the linker). This was corroborated by the SERS spectra. An analysis is presented, which accounts for the fact that as the concentration ratio of m/d increases, the SERS intensity associated with bands belonging to m first increases to a maximum then decreases. The nanoparticle-metal film system presented here is a simple embodiment of a more general range of SERS-active sensing platforms in which a molecular tether is used to create a SERS hot spot that (although nanosized) is large enough to accommodate analyte molecules that cannot themselves function as linkers, which are subsequently detected by SERS at the few-molecule level.     

TiO2电极表面氰基吡啶吸附的电化学SERS光谱

采用基于核壳纳米粒子的壳层隔绝纳米粒子增强拉曼(SHINERS)以及Au纳米粒子增强技术, 对比研究了4-氰基吡啶(4-CNPy)在TiO2表面的吸附行为. 结果表明, 采用2种技术所获得的光谱存在明显的差别. 利用前者得到了4-CNPy在TiO2电极上随电极电位变化的吸附方式. 在电位为0时, 分子以吡啶环上的N垂直吸附; 随电位负移, 部分分子变为倾斜吸附, 且在电位为-1.0 V时倾斜角度变大. 在正电位区间, 分子始终以吡啶环上的N垂直吸附. 而采用Au纳米粒子滴加在TiO2电极上的方式, 则得到吸附在TiO2, Au及TiO2/Au复合结构上的总光谱信息.     

银纳米粒子簇的设计、 制备和表面增强拉曼光谱

通过匹配激光光斑直径与胶体微球的尺寸, 设计制备了银纳米粒子的表面增强拉曼散射(SERS)基底, 并将其用于研究单个银纳米粒子簇的表面增强拉曼光谱. 在制备纳米粒子的过程中, 考察了等离子体刻蚀时间与银沉积厚度对“单”银纳米粒子结构与形貌的影响. 将吡啶、 巯基苯和罗丹明R6G作为SERS探针分子, 研究了其SERS效应, 通过荧光共振能量转移(FRET)机理, 实现了染料分子在单银纳米粒子簇上的SERS效应. SERS光谱测试与相关计算结果表明, 单个银纳米粒子簇的拉曼增强因子能够达到约10⁶.