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.     

Study on self-assembled monolayers of functionalized azobenzene thiols on gold: XPS,electrochemical properties,and surface-enhanced Raman spectroscopy

Self-assembled monolayers (SAMs) of 4-acetamino-4'-(4-mercaptobutoxy)azobenzene (CH3)CONH-ph-N=N-ph-O(CH2)(4)SH, abbr. aaAzoC4SH) and 4-mercaptobutoxy azobenzene (ph-N=N-ph-O(CH2)(4)SH, abbr. AzoC4SH) on a gold surface have been studied by X-ray photoelectron spectroscopy (XPS), FT Raman spectroscopy, and electrochemistry. A surface-enhanced Raman scattering (SERS)-active system with a "sandwiched" structure of Ag/R-Azo-C4S-/Au was conveniently obtained by the method of Tollen's test. The relationship between the SERS effect and the structural nature of the system indicates that the enhancement correlates to both the silver islands above and the gold substrate underneath. The redox behaviors of the self-assembly on gold electrodes showed that the SAMs of the two compounds exhibit well-behaved voltammetric responses in a Britton-Robinson buffer corresponding to the irreversible two-electron, two-proton reduction-oxidation of azobenzene. The apparent electron-transfer rate kinetics is very sluggish, and the rate constant k(app) of aaAzoC4SH/Au (1.34 x 10(-6) s(-1)) is lower than that of AzoC4SH/Au (1.63 x 10(-4) s(-1)), which may be attributed to the different spatial restriction of close-packing structures on the conformational change accompanied by electron and proton transfer in the SAMs.     

Orientation of 6-mercaptopurine SAMs at the silver electrode as studied by Raman mapping and in situ SERS

Self-assembled monolayers (SAMs) of 6-mercaptopurine (6MP) on a silver electrode in acid and alkaline media were investigated by a combination protocol of the SERS technique with Raman mapping, and it was found that the adsorption mode of 6MP SAMs changed with the pH value of the environment. Quantum calculations for the vibrational mode were performed by the BLYP/6-31G method. 6MP was adsorbed on the silver electrode with a tilted orientation via S, N1, and N7 atoms in acid medium, while the SAMs adopted head-on adsorption modes with the S atom and the N1 atom anchoring the silver surface in alkaline medium. However, 6MP SAMs turned to the same upright orientation on the electrode through the S and N7 atoms when either acid or basic solution was removed. Stability of 6MP SAMs was observed by in situ SERS spectroelectrochemical measurements. The results reveal that the desorption potentials of 6MP SAMs formed under acid and alkaline conditions from the Ag electrode were at ca. -1.3 V and -1.6 V vs SCE, respectively.     

Nanoparticle-mirror sandwich substrates for surface-enhanced Raman scattering

Sandwich surface-enhanced Raman scattering (SERS) substrates (3S) utilizing coupling between continuous metal films and plasmonic particles were fabricated using silver mirrors, electrochemically roughened films, and various sizes of silver nanoparticles. The effect of excitation wavelength and nanoparticle size on SERS spectra of poly(vinylpyridine), selected as a model compound, was studied to determine the optimum conditions for the strongest SERS signal. The Raman enhancement resulted from the plasmon coupling of silver nanoparticles to the underlying continuous film as well as the lateral plasmon coupling between the silver nanoparticles. The formation of the charge transfer complex was also observed. The 3S configuration was used to obtain SERS spectra of dipicolinic acid (DPA), a chemical signature for Bacillus anthracis.     

Surface enhanced Raman scattering based on silver dendrites substrate

A simple method of the reduction of AgNO3 by copper foil in aqueous medium was used to prepare silver dendrites, which can be used as a novel good reproducible surface enhanced Raman scattering (SERS) active substrate. The SERS spectra of 4-pyridinethiol on this novel substrate reflected the different SERS activities on the minuteness and strong Ag dendrites. The electromagnetic coupling enhancement and chemical enhancement mechanisms are used to explain the SERS effect.     

硝酸刻蚀银表面的增强拉曼光谱及其在表面化学中的应用

最近,我们摸索出的硝酸刻蚀法制备有SERS(Surface Enhanced Raman Scattering,表面增强拉曼散射)活性的银表面,有良好的热稳定性,且费用低廉,预期在研究金属表面反应、催化和金属-聚合物界面结构等方面可发挥作用。在展示了这一新方法具有极高的表面增强因子后,本文介绍用该法研究吸附质的自集合(self-assembly)和表面取向结果。     

Synthesis of a β-cyclodextrin-modified Ag film by the galvanic displacement on copper foil for SERS detection of PCBs

A mono-6-thio-β-cyclodextrin-modified silver film was synthesized via galvanic displacement on copper foil. The prepared silver films could enrich non-polar polychlorinated biphenyls (PCBs) molecules from hydrophilic phase using thiolate β-cyclodextrins (SH-β-CDs) as receptors. The components of as-prepared Ag-coated-Cu (Ag-Cu) film were confirmed by powder X-ray diffraction (XRD). Both surface-enhanced Raman spectroscopy (SERS) and energy dispersive X-ray spectroscopy (EDS) measurements gave strong evidences that the thiolated β-cyclodextrins (SH-β-CDs) had been immobilized on the surface of silver film. Compared to the substrates prepared in the absence of SH-β-CD, the surface morphology of the CD-modified Ag films was obviously changed. The interfacial enrichment and the capability of substrates to form inclusion complexes with PCBs molecules were tested by using PCB-15 (4,4'-dichlorobiphenyl) as the probe molecules via SERS technique. The measured SERS spectra could distinguish the PCB-15 molecules at micro-molar level according to the most intense CCC bending in-plane mode of PCBs. The enhancement factor (EF) of the SERS substrates for PCB-15 was 1.2×10(5), which was comparable with a number of previous reports.     

A rapid and simple chemical method for the preparation of Ag colloids for surface-enhanced Raman spectroscopy using the Ag mirror reaction

Colloidal silver (Ag) nanoparticles (AgNP) have been widely used for surface-enhanced Raman spectroscopy (SERS) applications. We report a simple, rapid and effective method to prepare AgNP colloids for SERS using the classic organic chemistry Ag mirror reaction with Tollens’ reagent. The AgNP colloid prepared with this process was characterized using SEM, and the reaction conditions further optimized using SERS measurements. It was found that Ag mirror reaction conditions that included 20 mM AgNO₃, 5 min reaction time, and 0.5 M glucose produced AgNP colloids with an average size of 319.1 nm (s.d ± 128.1). These AgNP colloids exhibited a significant SERS response when adenine was used as the reporter molecule. The usefulness of these new AgNP colloids was demonstrated by detecting the nucleotides adenosine 5′-mono-phosphate (AMP), guanosine 5′-monophosphate (GMP), cytidine 5′-monophosphate (CMP), and uridine 5′-monophosphate (UMP). A detection limit of 500 nM for AMP was achieved with the as-prepared AgNP colloid. The bacterium Mycoplasma pneumoniae was also easily detected in laboratory culture with these SERS substrates. These findings attest to the applicability of this AgNP colloid for the sensitive and specific detection of both small biomolecules and microorganisms.     

Preparation and electrochemistry of azobenzene self-assembled monolayers on gold--long range tunneling and end-group hydrogen bonding effect

A series of thiol-functionalied azobenzene derivatives (RAzoCnSH: R=H for n=3-6, abbreviated as AzoCnSH; R=CH(3)CONH for n=4, abbreviated as aaAzoC4SH) on gold electrodes were prepared and their self-assembly and electrochemical properties were studied by cyclic voltammetry. They all formed uniform and reproducible self-assembled monolayers (SAMs) on gold and showed well-behaved voltammetric responses in aqueous solution. Both the length of the alkyl chain spacer and the H-bonding of the end acetamino group had effects on the stability and the electrochemical kinetics of the SAMs, and the effect of the H-bonding was dominant. The surface coverage of the SAMs (AzoCnSH) is gradually increased with an increase of the alkyl chain spacer length, whereas the presence of the terminal acetamino group leads to a greater increase of the surface coverage. At a low scan rate, voltammetric responses corresponding to an irreversible two-electron, two-proton reduction/oxidation of the trans-azobenzene redox center were obtained in the range of +300 mV and -800 mV, which exhibited very large peak-to-peak splitting. At a high scan rate of 500 mV/s, two steps of reversible one-electron, one-proton reduction/oxidation corresponding to the cis-isomer in azobenzene-thiol SAMs (n is odd) was clearly observed between +300 and -200 mV. The apparent electron-transfer rate is decreased with increasing distance between the azobenzene redox center and the gold electrode. The existence of the end acetamino group which restricted the conformational change during the redox process also led to a decrease of the standard rate constant, and this restriction effect is more predominant than the distance effect.     

Surface-enhanced Raman scattering from ordered Ag nanocluster arrays

We have examined the effect of ordered silver nanocluster substrates on the surface-enhanced Raman spectrum of rhodamine 6G (R6G). Triangular shaped silver nanocluster arrays with order on the approximately 100 mum range were prepared using nanosphere lithography. Direct comparisons of R6G surface-enhanced Raman spectroscopy (SERS) signals between ordered nanocluster regions and amorphous Ag regions prepared under identical deposition conditions provide strong evidence of an electromagnetic field enhancement attributed to the unique nanocluster morphology. We have obtained order of magnitude enhancement factors for both 200 and 90 nm Ag nanocluster SERS substrates relative to Ag films.     

Porous GaN as a template to produce surface-enhanced Raman scattering-active surfaces

Surface-enhanced Raman spectroscopy (SERS) substrates have been prepared by depositing Au or Ag on porous GaN (PGaN). The PGaN used as the template for the metal deposition in these studies was generated by a Pt-assisted electroless etching technique. PGaN was chosen as a potential SERS template due to its nanostructured surface and high surface area, two characteristics that are important for SERS substrates. Metal films were deposited either by solution-based electroless deposition or by thermal vacuum evaporation. SERS spectra were recorded at lambda = 752.5 nm for Au films and at lambda = 514.5 nm for Ag films deposited on PGaN. The SERS signal strength across the metal coated PGaN substrates was uniform and was not plagued by "hot" or "cold" spots on the surface, a common problem with other SERS surfaces. The Ag film deposited by electroless deposition had the highest overall SERS response, with an enhancement factor (EF) relative to normal Raman spectroscopy of 10(8). A portion of the increase in EF relative to typical SERS-active substrates can be assigned to the large surface area characteristic of the PGaN-Ag structures, but some of the enhancement is intrinsic and is likely related to the specific morphology of the metal-nanopore composite structure.     

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.     

Silver nanoparticle-treated filter paper as a highly sensitive surface-enhanced Raman scattering (SERS) substrate for detection of tyrosine in aqueous solution

Highly sensitive SERS substrates based on deposition of silver nanoparticles on commercially available filter paper were prepared in this work, and used to overcome problems found in analyses of aqueous samples. To prepare silver nanoparticle- (AgNP) doped filter substrates, a silver mirror reaction was used. The procedures for substrate preparation were systematically optimized. Pretreatment of filter paper, reaction time, temperature, and concentration of reagents for silver mirror reactions were studied. The morphologies of the resulting substrates were characterized by field-emission scanning electron microscopy (FE-SEM) and correlated with the SERS signals by probing with p-nitrothiophenol (pNTP). Filter papers with different pretreatments were found to have different sizes and distributions of AgNPs. The best performance was found when filter paper was pre-treated with ammonia solution before growth of AgNPs. Based on the SEM images, the resulting AgNPs had roughly spherical shape with a high degree of uniformity. The silver-coated filter paper substrates provide much higher SERS signals compared to glass substrates and the reproducibility was improved significantly. Based on statistical analyses, the relative standard deviations for substrate-to-substrate and spot-to-spot were both were less than 8% and the enhancement factors for the substrates were, in general, higher than 107. The SERS substrates were used to selectively detect tyrosine in aqueous solution. Results indicate that filter-based SERS substrates are highly suited to detection of tyrosine. Compared to glass-based SERS substrates, 50 times more SERS signal was observed in detection of tyrosine. The linear range can be up to 100 μM with a detection limit of 625 nM (SN(-1)=3).     

银纳米粒子在云母表面的二维组装及其表面增强拉曼效应

随着纳米技术的迅速发展 ,利用共价或非共价键作用将金属纳米粒子组装到固体基片上 ,因其方法简单、重复性好而成为研究热点 .目前 ,人们已经成功地利用带有— SH[1,2 ] ,—CN,— NH2 [3 ] 等基团的单层或多层膜作为偶联剂将 Au和 Ag等金属纳米粒子固定在玻璃、石英、硅、金等固体基片上 .但在许多情况下 ,偶联剂却成为一种干扰物质 .云母是一种重要的电子工业材料 ,并具有廉价、较易获得新鲜表面等特点 ,研究金属纳米粒子在云母表面的组装和排列无疑具有重要意义 .但是 ,迄今为止 ,在表面没有偶联剂修饰的条件下 ,以云母为基底的金属纳…     

硫脲在HNO3介质中共吸附行为的喇曼光谱研究

用常规Raman谱、电化学现场表面增强喇曼散射光谱(SERS)和时间分辨SERS光谱(TRSERS)研究了硫脲(TU)在HNO₃介质中与NO-3的共吸附行为.实验结果表明,NO₃^-离子能被TU诱导共吸附在其质子化氨基上;TU以S端按σ配位键方式化学吸附在银电极表面.在较正电位区间(≥-0.6V),TU垂直吸附,电位负移(≤-0.8V),TU由垂直逐渐转变为倾斜甚至平躺吸附;在较负的电位下(≤-0.8V),TU在HNO₃介质中比在HClO₄中更稳定,甚至在-2.0V的电位下亦能检测到TU的SERS信号.     

表面增强拉曼光谱法对水中残留绿麦隆的检测

利用银镜反应原理在玻璃片上镀一层具有表面增强拉曼散射(SERS)效应的表面粗糙的银膜,通过测定农药绿麦隆在银镜表面上的表面增强拉曼光谱,探讨了绿麦隆在银镜表面的吸附取向。结果表明,在银镜表面,当绿麦隆浓度小到10-10mol L时,仍然能得到明确的光谱信息,且吸附饱和时间仅为2h,这为探索水体中微量污染物的检测方法提供了一个很好的途径。     

Raman spectroscopy probing of self-assembled monolayers inside the pores of gold nanotube membranes

Electroless deposition was used to coat porous alumina membranes with gold. This process reduced the pore diameters and provided a platform suitable for surface modifications with self assembled monolayers (SAMs). The surface enhanced Raman scattering (SERS) effect was employed in order to confirm and characterise the formation of SAMs of 3-mercaptobenzoic acid (mMBA) inside the pores of gold nanotube membranes prepared using porous alumina (PA) templates. The investigation of the coverage and reproducibility of SAMs within porous matrices is of utmost importance in the design of filtration membranes and sensing platforms. Raman spectroscopy is capable of spatially resolved techniques such as mapping which was used to characterise the distribution of mMBA assembly within the pores. Due to the highly ordered structure of porous alumina and well controlled electroless gold deposition, these gold coated membranes have the potential to develop into SERS active substrates for ultrasensitive sensing technologies.     

Surface-enhanced Raman scattering of 4,4′-bipyridine on silver by density functional theory calculations

Surface-enhanced Raman scattering (SERS) of 4,4′-bipyridine (BPy) on silver foil substrate was measured using the 488, 514.5, and 1064 nm excitation lines. Density functional theory (DFT) methods were used to calculate the structure and vibrational spectra of Ag–BPy, Ag₃–BPy and Ag₄–BPy complexes with B3LYP/6-31++G(d,p)(C,H,N)/Lanl2dz(Ag) basis set. The Raman bands of BPy were assigned on the basis of the calculation of potential energy distribution. The calculated spectra of Ag–BPy and Ag₄–BPy complexes were much closer to the experimental results of BPy adsorbed on silver surface than that of Ag₃–BPy complexes. The vibrational frequencies that are sensitive to the planar or non-planar structure of BPy and to the dihedral angle of two pyridyl rings were discussed. The DFT results showed that the angles between two pyridyl rings for Ag–BPy and Ag₄–BPy were skewed by about 38.44° and 37.1°, respectively. The energy gaps of the HOMO and LUMO from DFT were 415–912 nm for BPy–Ag complexes. The relative intensities of SERS bands changed with different excitation laser lines. Thus, a chemical enhancement mechanism should play an important role in the SERS of BPy on silver substrate.     

Silver Nano Islands Enhanced Raman Scattering on Large Area Grating Substrates Fabricated by Two Beam Laser Interference

The authors preparedlarge area surface-enhanced Raman scattering(SERS) active substrates with tunable enhancement. First the large area gratings were fabricated by scanning a photoresist with two-beam laser interference and subsequently they were coated with silver nano islands via vacuum evaporation. SERS active metal island grating substrates with four different periods(300, 400, 515 and 600 nm) and Ag nano islands uniformly coated on an area of 2.5 cm×0.5 cm were obtained. The measured SERS spectra reveal the tuning effect of the period on the Raman signals period. The highest enhancement(ca. 10⁵) for Rhodamine 6G(R6G) as probing molecule is associated with a period of 515 nm due to the perfect matching of surface plasmons and Raman excitation line. A good reproducibility of SERS signals with almost the same SERS intensity at different spots was observed on all the larger area Ag island grating substrates.     

Using a photochemical method and chitosan to prepare surface-enhanced Raman scattering–active silver nanoparticles

In this paper, we report a new strategy for the preparation of surface-enhanced Raman scattering (SERS)-active silver nanoparticles (Ag NPs), using a photochemical method and the presence of chitosan (Ch). First, Ag substrates were subjected to electrochemical oxidation/reduction cycles (ORCs) in deoxygenated aqueous solutions containing 0.1 M HNO₃ and 1 g L⁻¹ Ch (pH 6.9, adjusted by adding 1 M NaOH), resulting in Ag⁺–Ch complexes. These substrates were then irradiated with UV light at various wavelengths to yield the SERS-active Ag NPs. A stronger SERS effect was observed on the SERS-active Ag NPs prepared by using UV irradiation at 310 nm. The pH of the solution and the presence of Ch during the preparation process both affected the resulting SERS activities.