Adsorption of purpald SAMs on silver and gold electrodes: a Raman mapping study

The adsorption modes of 4‐amino‐3‐hydrazino‐5‐mercapto‐1,2,4‐trizole (purpald) self‐assembled monolayers (SAMs) formed on SERS‐active silver and gold electrodes were comparatively studied using surface‐enhanced Raman scattering (SERS), and the self‐assembling procedures were investigated by the Raman mapping technique. Purpald SAMs adopted a titled orientation with S, N2 atoms anchoring to the silver electrode and the  N7H₂ close to the surface, whereas purpald stood up on the gold electrode through S, N5 atoms and with  N8H₂ adjacent to the surface. The density functional theory (DFT) at the level of B3LYP was performed to help explain their different adsorption behaviors on the silver and gold electrodes. Copyright © 2006 John Wiley & Sons, Ltd.     

银胶和银岛膜上胸腺嘧啶的表面增强拉曼光谱对比研究

利用表面增强拉曼光谱对胸腺嘧啶在银胶和银岛膜上的吸附状态进行了对比研究.光谱分析表明,胸腺嘧啶吸附在银胶和银岛膜表面后其分子结构发生了互变异构,由原来的酮式结构变成了烯醇式结构.在银胶和银岛膜表面胸腺嘧啶主要以N(3)去质子化异构体的形式存在.胸腺嘧啶通过O(7)和N(3)倾斜地吸附在银胶颗粒表面,通过O(8)和N(3...     

Adsorption of aniline on silver mirror studied by surface‐enhanced Raman scattering spectroscopy and density functional theory calculations

The adsorption of aniline on a silver mirror was studied by surface‐enhanced Raman scattering (SERS) spectroscopy and density functional theory (DFT) calculation methods. The normal Raman and SERS spectra of pure aniline liquid and its solutions were recorded by a micro‐Raman spectrometer with excitation at 514.5 nm. Orientation of the aniline molecule adsorbed on the Ag mirror is discussed. The results indicate that pure aniline is adsorbed on the surface of the Ag mirror with a tilted orientation. The conformer with the nitrogen atom interacting with the metal surface would be dominant. DFT calculations further confirm the experimental results that charge transfer (CT) takes place from the highest occupied molecular orbital(HOMO) of aniline to the singly occupied molecular orbital (SOMO) of the silver surface. In this paper, the frontier molecular orbital theory has been successfully used to explore the interaction between the aniline molecule and the silver surface. Copyright © 2011 John Wiley & Sons, Ltd.     

二苯基乙二胺在银胶表面吸附的表面增强拉曼光谱研究

以银纳米粒子作为SERS活性基底,研究了二苯基乙二胺(DPEDA)在银表面的吸附行为,并研究了溶液的pH值和DPEDA的浓度对吸附构型的影响。对比DPEDA的Raman和SERS谱图,可以认为DPEDA是通过氮原子的孤对电子吸附在银胶表面。在酸性介质中DPEDA的氨基被质子化生成胺正离子,导致DPEDA不容易吸附到银胶表面,所以DPEDA的SERS信号强度随着溶液酸性的增加而减弱。DPEDA的浓度变化对其吸附构型中芳香环的取向影响不大,但是其C-N键与表面的角度会有一定的变化。从SERS光谱得到的信息可以加深我们对DPEDA修饰的金属催化剂的多相不对称催化机理的认识。     

Identification of species formed after pyridine adsorption on iron,cobalt, nickel and silver electrodes by SERS and theoretical calculations

The adsorption of pyridine (py) on Fe, Co, Ni and Ag electrodes was studied using surface‐enhanced Raman scattering (SERS) to gain insight into the nature of the adsorbed species. The wavenumber values and relative intensities of the SERS bands were compared to the normal Raman spectrum of the chemically prepared transition metal complexes. Raman spectra of model clusters M₄(py) (four metal atoms bonded to one py moiety) and M₄(α‐pyridil) where M = Ag, Fe, Co or Ni were calculated by density functional theory (DFT) and used to interpret the experimental SERS results. The similarity of the calculated M₄(py) spectra with the experimental SERS spectra confirm the molecular adsorption of py on the surface of the metallic electrodes. All these results exclude the formation of adsorbed α‐pyridil species, as suggested previously. Copyright © 2009 John Wiley & Sons, Ltd.     

巯基苯并咪唑在银电极表面的自组装吸附特性及表面增强拉曼光谱

采用循环伏安法处理Ag电极,得到活化的具有表面增强拉曼光谱(SERS)效应的粗糙Ag表面,进一步采用激光拉曼光谱探讨了2-巯基苯并咪唑(MBI)在其表面的自组装分子层的吸附特性。实验表明,在活性Ag表面的MBI自组装分子层能够产生理想的SERS效应,其强度随探针分子MBI浓度的增加先提高后减弱,达到一定浓度时因受其空间位阻等因素的影响,增强效应减弱。MBI在1×10^-6 mol·L^-1 浓度时增强效果最大。拉曼增强效应随着体系酸度的变化有着明显不同,在强酸性条件下的增强效应明显优于中性和碱性条件。MBI分子存在两种不同的异构体和在不同酸度下存在3种不同的存在形态,并形成动态平衡。pH <2时,MBI分子主要以硫酮式MBI⁺存在,并以巯基上的S:与活性Ag以配位方式吸附成键,其整个大π键平面垂直地吸附于Ag表面,产生相对较大的SERS信号。pH >2时,由于硫酮式和硫醇式与活性Ag的键合方式和能力不同,硫醇式上的S与Ag以S-Ag共价方式同时双键侧上的N以配位方式协同参与吸附成键,比硫酮式MBI⁺的单纯配位吸附要强,因而形成了竞争吸附,表现为SERS在pH=2~3.7之间的急剧下降。MBI硫醇式由于以S-Ag、N-Ag键的协同吸附,形成了倾斜侧卧式垂直吸附,而使拉曼增强效应相对减弱。     

表面增强拉曼光谱研究胞嘧啶在银胶上的吸附行为

为探讨胞嘧啶(Cytosine,Cy)在基底银表面的吸附特性和规律,采用表面增强拉曼散射(SERS)光谱对其吸附行为进行分析,并结合量子化学密度泛函理论(DFT)/B3LYP计算方法对Cy分子的常规拉曼光谱(NRS)及Cy与Ag团簇吸附的SERS光谱进行计算,与测定结果进行比对且对其拉曼峰进行系统指认及归属,理论计算结合实测值探讨了Cy在基底Ag上的增强效应和吸附行为。考察了Cy分子在Ag纳米粒子上的不同吸附时间、浓度、pH等条件对SERS光谱的影响及优化,发现pH影响最大,在中性和强碱性条件下的增强效应明显优于酸性。Cy分子存在2种不同的异构体和3种不同的存在形态,并随酸度变化相互转化而达动态平衡。基于Cy在不同pH时的形态分布和相应的SERS变化规律,结合DFT算得的Cy分子中的电荷分布及在银基底表面的吸附机制,详细探讨了酸碱对Cy分子的SRES光谱影响的内因和吸附机理,指出在中性和弱碱性时,是Cy中的N3和O与Ag形成配位吸附;在pH大于11时,N与Ag形成配位吸附,而O与Ag形成共价吸附。     

Visible laser–induced photoreduction of silver 4‐nitrobenzenethiolate revealed by Raman scattering spectroscopy

We have investigated the photochemical characteristics of silver 4‐nitrobenzenethiolate (Ag‐4NBT) by means of Raman spectroscopy. When Ag‐4NBT is irradiated with an argon ion laser at 514.5 nm, its Raman spectrum changes over time, resulting in the production of 4NBT‐capped silver nanoparticles. The surface‐enhanced Raman scattering (SERS) spectrum of 4NBT adsorbed on those Ag nanoparticles is subsequently converted to that of 4‐aminobenzenethiol (4ABT). These surface‐induced photoreduction characteristics were investigated by monitoring the growth of Raman peaks of 4ABT as a function of the laser exposure time. Water vapor or ambient conditions were more effective than vacuum conditions for the photoreduction of 4NBT to 4ABT. Nonetheless, the occurrence of photolysis even under vacuum conditions suggests that the benzene ring hydrogen atoms might be the H‐atom source of the nitro‐to‐amine group conversion although in ambient conditions water or solvent molecules trapped inside the Ag‐4NBT should be the primary H‐atom source and facilitate the transfer of electrons, as well as the diffusion of Ag atoms to form highly SERS‐active nanoaggregates. Copyright © 2009 John Wiley & Sons, Ltd.     

Structural and spectroscopic study of adsorption of anthracene on silver

Adsorption of anthracene on silver is investigated based on the density functional theory and the surface-enhanced Raman spectroscopy (SERS). Variations in bond and dihedral angles of the optimised geometry of anthracene indicate distortions in the hexagonal structure of the ring nearer to the silver cluster and deviations in the co-planarity of carbon atoms. Natural bond orbital analysis confirms intramolecular charge transfers from π(C–C) to π*(C–C) and π(C–C) to σ*(Ag–Ag) orbitals. Higher polarisation resulting from charge transfers on adsorption accounts for Raman enhancements of selective vibrational modes and band shifts. Surface plasmon resonance peak of silver nanoparticles after the adsorption of anthracene observed around 399 nm compares well with the theoretically simulated UV–vis spectrum derived using the time-dependent density functional theory. Theoretical and experimental SERS correlate well, confirming the process of adsorption, the tilted orientation of anthracene on the silver surface and the adsorption mechanism reported. Localisation of the electron density together with a reduced band gap after the adsorption on silver suggests its utility in the design of electro-active organic molecular devices.     

碱性银胶的表面增强拉曼效应及对牛奶中三聚氰胺的检测

在合成银胶时加入适量的氢氧化钠,得到稳定性和均一性更好的碱性银胶,研究了它对不同浓度拉曼探针分子亚甲基蓝的增强效果。相比普通银胶,浓度的变化不对其和银胶的吸附方式产生影响,根本原因是碱性银胶对亚甲基蓝分子中硫原子的优向吸附而使451 cm-1处信号始终最强。研究了银胶对同浓度不同量的亚甲基蓝分子产生的拉曼增强效应,以及该拉曼增强光谱随时间的变化关系。另外,将该碱性银胶制备成银斑点应用于掺杂三聚氰胺的牛奶样品检测,获得了三聚氰胺掺入量和拉曼信号的线性关系,该方法需要样品量仅5 μL,拉曼光谱检测时间仅需5 s,非常适合快速测定分析,利用碱性银胶对三聚氰胺在691 cm-1处的特征拉曼峰,可在3～60 mg·L-1的范围内测定三聚氰胺的掺杂量,检出限达0.28 mg·L-1。     

Improved performance of surface‐enhanced Raman scattering on silver substrates prepared in nitric acid solutions

In this work, we propose a new electrochemical method to prepare surface‐enhanced Raman scattering (SERS)‐active silver substrates in nitric acid solutions. Experimental results indicate that the SERS intensity of adsorbed Rhodamine 6G (R6G) can be significantly increased, as compared with that of R6G adsorbed on a SERS‐active Ag substrate prepared by an electrochemical method in a chloride‐containing solution, which was generally employed in the literature. Moreover, the SERS of R6G on the newly developed substrate (prepared in a nitric acid solution) still performs well at a high temperature of 250 °C. However, the enhancement capability of the SERS‐active substrate prepared in a chloride‐containing solution is seriously destroyed at temperatures higher than 150 °C. Further investigations indicate that the oxidation states of roughened Ag substrates prepared in nitric acid solutions under different experiment conditions have less influence on the corresponding SERS performances. Instead, different surface morphologies of roughened Ag substrates and different contents of nitrogen‐containing dopping ions on the roughened Ag substrates demonstrate significant effects on the corresponding SERS performances. Copyright © 2011 John Wiley & Sons, Ltd.     

Raman and surface‐enhanced Raman studies of α‐aminophosphinic inhibitors of metalloenzymes

Here, we report the nature of new di‐α‐amino (L¹–L³) and α‐amino‐α‐hydroxyphosphinic (L⁴–L⁶) acids, which are considered potential inhibitors of the aminopeptidase N, adsorbed on a colloidal silver surface by means of surface‐enhanced Raman scattering (SERS) spectroscopy. In order to reveal the adsorption mechanism of these species from their SERS spectra, Fourier‐transform Raman (FT‐RS) spectra of these nonadsorbed molecules were measured. By examining the enhancement, shift in wavenumbers, and changes in breadth of the SERS bands due to the adsorption process, we revealed that the tilted compounds interact with the colloidal silver substrate mainly through the benzene ring, amino group, and phosphinic moiety in the following way. The benzene ring of L² and L³ is ‘standing up’ on the colloidal silver surface, and the C N bond is almost vertical to it, while the tilt angle between the O PO bond and this surface is greater than 45°. On the other hand, for L¹, L⁴, and L⁵, the aromatic ring and C N bond are arranged more or less tilted, and the tilt angle between the O PO bond and the silver substrate is smaller than 45°. The elongation of the bond to the benzene ring, the L⁶ case, produces an almost horizontal orientation of the benzene ring and the O PO bond on the silver nanoparticles. For these ligands, the complement inhibition IC₅₀ tested in vitro using porcine kidney leucine aminopeptidase was correlated mainly with the behavior of the O PO and C CH N fragments on the silver surface. Copyright © 2009 John Wiley & Sons, Ltd.     

SERS imaging of silver coated nanostructured Al and Al2O3 substrates. The effect of nanostructure

The last few years have witnessed rapid development of highly ordered and reproducible surface‐enhanced Raman scattering (SERS) nanostructured substrates for their potential medical and analytical application such as biosensing and bioimaging. In this work, 5‐nm silver films deposited on nanostructured Al and Al₂O₃ templates are investigated as substrates for SERS. The chosen templates show different honeycomb nanostructures with two sets of dimension, i.e. pore diameter of ca. 25 and 50 nm and interpore distance of ca. 56 and 100 nm. The SERS imaging results reveal that the signal of the probe molecule (4‐thiazolidinone‐2‐thione) is distributed inhomogeneously on the substrate surface, and this fact is correlated with the morphology of nanostructures determined by atomic force microscopy. The variation of SERS intensity among the substrates is strongly correlated with the shape and size of potential SERS‐active sites, e.g. nanocups and nanopores. The strongest SERS response is found for the Ag/Al₂O₃ template anodized in sulfuric acid, which represents the nanopore array with the smallest dimensions (e.g. pore diameter, interpore distance etc.). Furthermore, depending on size and nanostructure shape, changes in the adsorption mechanism of the probe molecule are observed. Copyright © 2014 John Wiley & Sons, Ltd.     

In situ nucleation and growth of silver nanoparticles in membrane materials: a controllable roughened SERS substrate with high reproducibility

A controllable roughened silver surface with high surface‐enhanced Raman scattering (SERS) activity and high reproducibility has been developed in this study. This silver surface was prepared by silver nucleation in polyelectrolyte multilayers (PEMs) and silver‐enlarged growth. First, the small Ag nuclei were synthesized by NaBH₄ in situ reduction of Ag ions on a surface of PEMs. Then the small Ag nuclei formed were effectively enlarged by using a mixture of commercially available reagents named Li Silver . The optical properties and morphologies of the silver substrates have been investigated by ultraviolet–visible (UV–vis) spectroscopy and atomic force microscopy (AFM). The UV–vis and AFM results revealed that the small Ag nuclei separately appeared on the PEMs after NaBH₄ in situ reduction. The size of the enlarged Ag nanoparticles can be easily controlled with the immersing cycle in Li Silver. 4‐Mercaptopyridine (4‐MPY) and Rhodamine 6G (R6G) have been used as Raman probes to evaluate the properties of the new SERS substrates. It has been found that the enhancement factor of R6G reached ∼10⁹ after treatment in Li Silver. Reproducibility has been investigated using the SERS signal intensity at 1094 cm⁻¹ of 4‐MPY. Signals collected over multiple spots within the same substrate resulted in a relative standard deviation (RSD) of 6.38%, while an RSD of 10.33% was measured in signals collected from different substrates. Copyright © 2008 John Wiley & Sons, Ltd.     

Electrochemical and in situ SERS spectroelectrochemical investigations of 4‐methyl‐4H‐1, 2, 4‐triazole‐3‐thiol monolayers at a silver electrode

Electrochemically anticorrosive behavior of 4‐methyl‐4H‐1, 2, 4‐triazole‐3‐thiol (MTTL) self‐assembled monolayers (SAMs) on the silver electrode was studied by means of electrochemical impedance spectroscopy (EIS) and polarization measurements. The promising inhibition effect of the MTTL for silver had been affirmed. Results of surface‐enhanced Raman scattering (SERS) experiments indicated that the MTTL molecule in a tilted orientation was self‐assembled on the silver surface through S⁶ and N² atoms to form monolayers. An in situ electrochemical SERS experiment implied the changes of adsorption fashion of MTTL momolayers on the silver surface with the potential shifted to more negative direction. Copyright © 2009 John Wiley & Sons, Ltd.     

Visible light response of silver 4‐aminobenzenethiolate and silver 4‐dimethylaminobenzenethiolate probed by Raman scattering spectroscopy

Silver thiolate is a layered compound with a Raman spectrum that is known to change with time, becoming the same as the surface‐enhanced Raman scattering (SERS) spectrum of the parent thiol molecule adsorbed on Ag nanoparticles. On this basis, the Raman scattering characteristics of silver 4‐aminobenzenethiolate (Ag‐4ABT) compounds were investigated to determine whether certain peaks that are identifiable in the SERS spectrum of 4‐aminobenzenethiol (4‐ABT) but absent in its normal Raman spectrum were also apparent in the Ag salt spectrum. For comparative purposes, the Raman scattering characteristics of silver 4‐dimethylaminobenzenethiolate (Ag‐4MABT) were also examined. Raman spectra acquired while spinning the sample were typified by only a₁‐type vibrational bands of Ag‐4ABT and Ag‐4MABT, whereas in the static condition, several non‐a₁‐type bands were identified. The spectral patterns acquired in the static condition were similar to the intrinsic SERS spectra of 4‐ABT or 4‐dimethylaminobenzenethiol (4‐MABT) adsorbed on pure Ag nanoparticles. Notably, the CH₃ group vibrational bands were observable for Ag‐4MABT irrespective of the sample rotation. In addition, no decrease in intensity during irradiation with a visible laser was observed for any of the bands, suggesting that no chemical conversion actually took place in either 4‐ABT or 4‐MABT. The preponderance of evidence led to the conclusion that the non‐a₁‐type bands observable in the SERS spectra must be associated with the chemical enhancement mechanism acting on the Ag nanoparticles. The chemical enhancement effect was more profound at 514.5 nm than at 632.8 nm, and was more favorable for 4‐ABT than 4‐MABT at both wavelengths. Copyright © 2012 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering study of human serum on PVA?Ag nanofilm prepared by using electrostatic self‐assembly

Polyvinyl alcohol (PVA)‐protected silver nanoarchitecture (PVA Ag nanofilm) on the surface of the glass substrate was prepared by using electrostatic self‐assembly at a proper voltage. The two‐dimensional morphology of the PVA Ag nanofilm has been examined by scanning electron microscopy (SEM). The surface‐enhanced Raman scattering (SERS) spectra of human serum (HS) on PVA Ag nanofilms were recorded. The results show that the Raman scattering of HS can be enhanced efficiently based on these PVA Ag nanofilms. However, it also can be seen that the effect of sodium citrate (SC) acting as anticoagulant on the SERS spectrum of HS is unnegligible, which has not been discussed adequately in the previous reports. To discuss the effect of SC on the SERS spectrum of HS, we have studied the normal Raman spectra of solid SC and the SERS spectra of 1.0 × 10⁻³ mol/l aqueous solution of SC adsorbed on the PVA–Ag nanofilms. Meanwhile, Raman wavenumbers of the SC molecule were calculated by using the method of DFT‐B3LYP/6‐31G*, and the dominant assignations of the calculated wavenumbers were performed. It was found that the density functional theory (DFT) calculation of SC Raman spectrum matches well with the experimental results. With the perfect reproducibility and high SERS activity, this method will be useful in the development of HS detection methods. Copyright © 2010 John Wiley & Sons, Ltd.     

Experimental and theoretical studies of Raman spectroscopy on 4‐mercaptopyridine aqueous solution and 4‐mercaptopyridine/Ag complex system

Raman scattering and surface‐enhanced Raman scattering (SERS) have been used to study the behavior of 4‐mercaptopyridine (4‐Mpy) dissolved in water and adsorbed on silver mirrors. In order to gain the actual structure and the theoretical modes of the 4‐Mpy dissolved in water and adsorbed on the surface of silver mirror, ab initio calculation at the Hartree–Fock (HF) level and density functional theory (DFT) at Beck's three‐parameter Lee‐Yang‐Parr (B3LYP) level were performed to calculate the vibrational modes and wavenumbers. 4‐Mpy/2H₂O and 4‐Mpy/Ag complex systems were optimized, and then the corresponding Raman spectra were calculated and analyzed. Compared with the experimental results, the calculated results of 4‐Mpy and 4‐Mpy/2H₂O complex systems obtained from DFT method were more accurate. Among the results calculated with HF method, the one with three Ag atoms was economical, which took less computer time but gave equivalent results to those with more noumber of Ag atoms. Copyright © 2007 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering‐based approach for DNA detection at low concentrations via polyvinyl alcohol‐protected silver grasslike patterns

A simple method is demonstrated to detect DNA at low concentrations on the basis of surface‐enhanced Raman scattering (SERS) via polyvinyl alcohol‐protected silver grasslike patterns (PVA‐Ag GPs) grown on the surface of the common Al substrate. By the SERS measurements of sodium citrate and thymine, the PVA‐Ag GPs are shown to be an excellent SERS substrate with good activity, stability and reproducibility. With the use of the tested molecule of thymine, the enhancement factor of the PVA‐Ag GPs is up to ~1.4 × 10⁸. The PVA‐Ag GPs are also shown to be an excellent SERS substrate with good biocompatibility for DNA detection, and the detection limit is down to ~10⁻⁵ mg/g. Meanwhile, the assignations of the Raman bands and the adsorption behaviors of the DNA molecules are also analyzed. In this work, the geometry optimization and the wavenumber analysis of adenine–Ag and guanine–Ag complexes for the ground states are performed using density functional theory, B3LYP functional and the LanL2DZ basis set. The transition energies and the oscillator strengths of adenine–Ag and guanine–Ag for the lowest six singlet excited states were calculated by using the time‐dependent density functional theory method with the same functional and basis set. The results show that the charge transfer in the adenine–Ag and guanine–Ag complexes should be the chemical factor for the SERS of the DNA molecules. Lastly, this method may be employed in large‐scale preparation of substrates that have been widely applied in the Raman analysis of DNA because the fabrication process is simple and inexpensive. Copyright © 2011 John Wiley & Sons, Ltd.     

Ultrathin silver‐coated gold nanoparticles as suitable substrate for surface‐enhanced Raman scattering

Surface‐enhanced Raman scattering (SERS) is an extremely powerful tool for the analysis of the composition of bimetallic nanoparticle (BNP) surfaces because of the different adsorption schemes adopted by several molecules on different metals, such as Au and Ag. The preparation of BNPs normally implies a change in the plasmonic properties of the core metal. However, for technological applications it could be interesting to synthesize core–shell structures preserving these original plasmonic properties. In this work, we present a facile method for coating colloidal gold nanoparticles (NPs) in solution with a very thin shell of silver. The resulting bimetallic Au@Ag system maintains the optical properties of gold but shows the chemical surface affinity of silver. The effectiveness of the coating method, as well as the progressive silver enrichment of the outermost part of the Au NPs, has been monitored through the SERS spectra of several species (chloride, luteolin, thiophenol and lucigenin), which show different behaviors on gold and silver surfaces. A growth mechanism of the Ag shell is proposed on the basis of the spectroscopic and microscopic data consisting in the formation and deposit of Ag clusters on the Au NP surface. Copyright © 2009 John Wiley & Sons, Ltd.