Optical properties and ultrafast electron dynamics in gold–silver alloy and core–shell nanoparticles

Silver and gold are the two most popular metals used for many nanoparticle applications, such as surface enhanced Raman scattering or surface enhanced fluorescence, in which the local field enhancement associated with the excitation of the localized surface-plasmon–polariton resonance (SPR) is exploited. Therefore, tunability of the SPR over a wide energy range is required. For this purpose we have investigated core–shell nanoparticles composed of gold and silver with different shell thicknesses as well as the impact of alloying on these nanoparticles due to a tempering process. The nanoparticles were prepared by subsequent deposition of Au and Ag atoms or vice versa on quartz substrates followed by diffusion and nucleation. Their linear extinction spectra were measured as a function of shell thickness and annealing temperature. It turned out that different gold shell thicknesses on silver cores allow a tuning of the SPR position from 2.79 to 2.05 eV, but interestingly without a significant change on the extinction amplitude. Heating of core–shell nanoparticles up to only 540 K leads to the formation of alloy nanoparticles, accompanied by a back shift of the SPR to 2.60 eV. Calculations performed in quasi-static approximation describe the experimental results quite well and prove the structural assignments of the samples. In additional experiments, we applied the well-established persistent spectral hole burning technique to the alloy nanoparticles in order to determine the ultrafast dephasing time T ₂. We obtained a dephasing time of T ₂=(8.1±1.6) fs, in good agreement with the dephasing time of T _2,∞=8.9 fs, which is already included in the dielectric function of the bulk.     

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.     

Optical characterization of the polymer embedded alloyed bimetallic nanoparticles

A theoretical approach for the calculation of the bimetallic nanoparticles absorption spectra has been developed as an extension of the Mie theory in which nanoparticle dielectric function is found by the weighted linear combination of the dielectric functions for particles made of the corresponding pure metals. In the frame work of the theoretical model an expression for the resonance light absorption frequency were derived taking into account the interband transitions in the dielectric functions. We propose a simple method for the on-line monitoring of the bimetallic nanoparticles composition based on the measurement of the absorption peak position. Elaborated theoretical approach was used to investigate the polymer embedded Ag/Au nanoparticles which were prepared by reducing gold and silver salts (HAuCl4 and AgNO3, respectively) by ethylene glycol in presence of poly(vinyl pyrrolidone) (PVP) at room temperature. Calculated absorption spectra for the Ag/Au nanoscopic systems showed good agreement with the experimental data. Temporal evolution of the Ag/Au nanoparticles has also been investigated by this approach.     

Plasmonics properties of trimetallic Al@Al2O3@Ag@Au and Al@Al2O3@AuAg nanostructures

Bimetallic and trimetallic nanoparticles have attracted significant attention in recent times due to their enhanced electrochemical and catalytic properties compared to monometallic nanoparticles. The numerical calculations using Mie theory has been carried out for three-layered metal nanoshell dielectric–metal–metal (DMM) system consisting of a particle with a dielectric core (Al@Al₂O₃), a middle metal Ag (Au) layer and an outer metal Au (Ag) shell. The results have been interpreted using plasmon hybridization theory. We have also prepared Al@Al₂O₃@Ag@Au and Al@Al₂O₃@AgAu triple-layered core–shell or alloy nanostructure by two-step laser ablation method and compared with calculated results. The synthesis involves temporal separations of Al, Ag, and Au deposition for step-by-step formation of triple-layered core–shell structure. To form Al@Ag nanoparticles, we ablated silver for 40 min in aluminium nanoparticle colloidal solution. As aluminium oxidizes easily in water to form alumina, the resulting structure is core–shell Al@Al₂O₃. The Al@Al₂O₃ particle acts as a seed for the incoming energetic silver particles for multilayered Al@Al₂O₃@Ag nanoparticles is formed. The silver target was then replaced by gold target and ablation was carried out for different ablation time using different laser energy for generation of Al@Al₂O₃@Ag@Au core–shell or Al@Al₂O₃@AgAu alloy. The formation of core–shell and alloy nanostructure was confirmed by UV–visible spectroscopy. The absorption spectra show shift in plasmon resonance peak of silver to gold in the range 400–520 nm with increasing ablation time suggesting formation of Ag–Au alloy in the presence of alumina particles in the solution.     

Preparation of monodisperse bimetallic nanorods with gold nanorod core and silver shell and their plasmonic property and SERS efficiency

In this study, monodisperse bimetallic nanorods with gold (Au) nanorod core and silver (Ag) shell (Au@AgNRs) were synthesized through seed‐mediated growth process by reduction of AgNO₃ using Au nanorods with narrow size and shape distribution as seeds. With increasing the used amount of AgNO₃, the Ag shell thickness of their lateral facets is raised faster than that of their two tips, leading to a decrease of their aspect ratios. Four plasmon bands are observable on the extinction spectra of Au@AgNRs, which are attributed to the longitudinal dipolar plasmon mode, transverse dipolar plasmon mode, and octupolar plasmon mode of the core‐shell structured bimetallic nanorods, respectively. As their Ag shell thickness increases, their longitudinal plasmon band blue‐shifts notably with the transverse plasmon band blue‐shifting and the two octupolar plasmon bands red‐shifting slightly, due to the decrease of their aspect ratios and enhancement of Ag plasmon resonance contribution. When used as surface‐enhanced Raman scattering (SERS) substrate for probing minute amounts of 4‐mercaptobenzoic acid in aqueous solution, Au@AgNRs have much stronger SERS activity than Au nanorods, and the obtained Raman signals are highly reproducible arising from their excellent monodispersity. Their SERS activity is remarkably increased with their Ag shell thickness thanks to the enhancing surface electric field and the chemical enhancement associated with electronic ligand effect. Copyright © 2014 John Wiley & Sons, Ltd.     

利用激光烧蚀法制备表面等离子体可调谐的金核银壳层纳米颗粒胶体

利用激光烧蚀方法在水中制备了金核银壳层纳米颗粒胶体,发现这种复合胶体的等离子体振动吸收峰频率会随着激光烧蚀时间的不同而发生改变。利用等离子杂化理论定性解释了共振吸收峰可调谐的物理机制.     

An insight into the effect of composition for enhance catalytic performance of biogenic Au/Ag bimetallic nanoparticles

A comprehensive knowledge of composition‐activity property relationship of nanoparticulate materials is highly desirable for applications in various catalysis reactions. We have addressed a facile green aqueous approach for preparation of Au, Ag monometallic, Au/Ag alloy as well as core‐shell bimetallic nanoparticles. The phytochemicals present in lemon grass leaves extract were employed both as natural reducing and capping agents at room temperature. X‐ray diffraction pattern, UV‐Vis spectroscopy, and energy dispersive X‐ray studies confirmed the formation of bimetallic system. The ensuing Au core/Ag shell and Au/Ag alloy bimetallic nanoparticles were crystalline and spherical in nature with identical average diameter of ~ 18 nm as measured via transmission electron microscopy. The bimetallic systems incredibly display higher catalytic potential than their monometallic counterparts which were vividly reckoned on structural effect, lattice compression, and synergistic electronic effect.     

Study of thermal diffusivity of nanofluids with bimetallic nanoparticles with Au(core)/Ag(shell) structure

The thermal diffusivity of Au/Ag nanoparticles with core/shell structure, at different compositions (Au/Ag = 3/1, 1/1, 1/3, 1/6), was measured by using the mismatched mode of the dual-beam thermal lens (TL) technique. This study determines the effect of the bimetallic composition on the thermal diffusivity of the nanofluids. In these results we find a lineal increment of the nanofluid it thermal diffusivity when the Ag shell thickness is increased. Our results show that the nanoparticle structure is an important parameter to improve the heat transport in composites and nanofluids. These results could have importance for applications in therapies and photothermal deliberation of drugs. Complementary measurements with UV-vis spectroscopy and TEM, were used to characterize the Au(core)/Ag(shell) nanoparticles.     

A facile strategy to synthesize bimetallic Au/Ag nanocomposite film by layer-by-layer assembly technique

A facile strategy has been developed for the preparation of bimetallic gold–silver (Au–Ag) nanocomposite films by alternating absorption of poly-(ethyleneimine)–silver ions and Au onto substrates and subsequent reduction of the silver ions. The composition, micro-structure and properties of the {PEI–Ag/Au}_n nanocomposite films were characterized by ultraviolet visible spectroscopy (UV–vis), transmisson electron microscopy (TEM), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), surface enhanced Raman scattering (SERS) and cyclic voltammetry (CV). The UV–vis characteristic absorbances of {PEI–Ag/Au}_n nanocomposite thin film increase almost linear with the number of bilayers, which indicates a process of uniform assembling. Appearance of a double plasmon bands in the visible region and the lack of apparent core–shell structures in the TEM images confirm the formation of bimetallic Au–Ag nanoparticles. The result of XPS also demonstrates the existence of Ag and Au nanoparticles in the nanocomposite films. TEM and FESEM images show that these Ag and Au nanoparticles in the films possess sphere structure with the size of 20–25 nm. The resulting {PEI–Ag/Au}_n films inherit the properties from both the metal Ag and Au, which exhibits a unique performance in SERS and electrocatalytic activities to the oxidation of dopamine. As a result, the {PEI–Ag/Au}_n films are more attractive compared to {PEI–Ag/PSS}_n and {PEI/Au}_n films.     

Morphology and chemical state of PVP-protected Pt,Pt–Cu,and Pt–Ag nanoparticles prepared by alkaline polyol method

Well-dispersed, uniform monometallic Pt and bimetallic Pt–Cu, and Pt–Ag nanoparticles protected with PVP have been synthesized by a modified-protocol alkaline polyol method. The nanoparticles were characterized by various methods (TEM, XPS, and XRD) to elucidate the relationship between morphology and preparation variables. The average of monodispersed nanoparticles ranged between 4.1 and 4.9 nm. Core–shell structure was obtained in the case of bimetallic nanoparticles. The core of bimetallic nanoparticles was found to be rich in platinum, whereas the shell contained mostly copper or silver. The final structure of bimetallic nanoparticles was found to be determined by the morphology particles resulted in the first reduction step. Explanations are advanced on the light of experimental results.     

金银三层纳米管局域表面等离激元共振特性研究

相对于单一金属纳米材料,二金属复合纳米材料具有更大的潜在应用价值.基于时域有限差分方法,研究了SiO₂-Ag-Au和SiO₂-Au-Ag二金属三层纳米管的消光光谱,并对其局域表面等离激元共振(Localized Surface Plasmon Resonance,LSPR)特性进行了分析.研究发现,内核尺寸变大将导致上述两种金属纳米管LSPR峰红移;内层金属及外层金属壳层厚度增大均会导致其LSPR峰蓝移.银壳厚度变化对纳米管LSPR的调制作用大于金壳厚度变化造成的影响.上述现象可以利用等离激元杂化理论及自由电子和振荡电子变化的竞争机制进行分析.     

Laser ablation of silver and gold in liquid ammonia

Laser ablation of a silver (Ag) and/or gold (Au) target was performed in liquid ammonia (l-NH₃) at 233 K using nanosecond laser pulses of 1064, 532 and 355 nm wavelengths. An “in situ” monitoring of the ablation process by UV/vis/NIR spectroscopy has shown the evolution of the surface plasmon extinction band of silver or gold nanoparticles and thus confirmed their formation. While sols of Au nanoparticles in l-NH₃ are quite stable in air, those of Ag nanoparticles undergo oxidation to Ag(I) complexes with NH₃ ligands. On the other hand, formation of solvated electrons, namely of the (e⁻)NH₃ solvates, has not been unequivocally confirmed under the conditions of our laser ablation/nanoparticle fragmentation experiment, since only very weak vis/NIR spectral features of these solvates were observed with a low reproducibility. Reference experiments have shown that the well-known chemical production of these solvates is hindered by the presence of Ag and Au plates. Ag and Au targets can thus possibly act as electron scavengers in our ablation experiments.     

Ag核Au壳复合纳米粒子为标记溶胶免疫检测的SERS研究

以种子生长法合成Ag核Au壳复合纳米粒子,苯硫酚分子(TP)在其表面的SERS增强随Au摩尔比例的增加呈现先增强后减弱的趋势,其最大增强为相应Ag纳米粒子的10倍。将标记分子TP,羊抗小鼠抗体固定在Ag核Au壳复合纳米粒子表面形成标记免疫溶胶,其与被基底捕获抗原分子发生免疫识别,通过TP分子的SERS信号进行免疫检测。     

(Au)_核(Ag)_壳纳米微粒-火焰原子吸收光谱法测定过氧化氢

在90 ℃水浴条件下,以粒径为10 nm的纳米金做晶种,用柠檬酸三钠还原硝酸银,制备了平均粒径为30 nm的（Au）核（Ag）壳纳米微粒,用高速离心纯化除去过量的柠檬酸三钠获得了较纯的（Au）核（Ag）壳纳米微粒。在pH 3.8的HAc-NaAc缓冲溶液中,Fe2＋催化H2O2反应产生的羟基自由基可氧化（Au）核（Ag）壳纳米微粒生成银离子。离心后,离心液中的银离子可用火焰原子吸收光谱法在328.1 nm波长处测量。随着H2O2浓度增大,离心液中银离子浓度增加,其吸光度值增加。H2O2浓度在2.64～42.24 μmol·L-1范围内与上清液中银离子的原子吸收值ΔA呈良好的线性关系,回归方程为ΔA=0.014c-0.013 1, 相关系数为0.998 4,检出限为0.81 μmol·L-1 H2O2。当用于水样中H2O2的测定,获得了满意的结果。     

不同Ag壳厚度Au@Ag纳米粒子的调控制备表征及表面增强拉曼光谱的效应

采用化学还原法制备了以Au为核、包覆Ag的双金属核壳Au@Ag纳米粒子,并成功地用于表面增强拉曼光谱(SERS)分析测试。通过改变制备液中Ag/Au的量比来调控Ag壳包覆厚度。采用透射电子显微镜(TEM)和紫外-可见光谱仪(UV-Vis)对Au@Ag纳米粒子的构貌进行表征。TEM显示明显存在核壳结构,且Ag壳层随Ag/Au的量比的增加而逐渐变厚;UV-Vis表明随着Ag/Au的量比的增加,Au@Ag纳米粒子出现了Au核与Ag壳吸收峰的2个等离子体共振峰,同时伴随着Au峰的蓝移和Ag峰的红移。以双甲脒为分析物,考察了不同Ag/Au的量比时的Au@Ag纳米粒子的SERS活性。结果表明,SERS活性随Ag/Au的量比的增加先增大后减小,在6∶5时其SERS增强效应最佳,此时Ag壳厚度约为6 nm。以对巯基苯胺(4-ATP)、结晶紫(CV)和双甲脒为分析测试对象,对比了Au@Ag、Ag、Au 3种基底的SERS活性。结果表明,所制备的Au@Ag纳米粒子的SERS活性要明显优于单纯的Au、Ag纳米粒子。     

Optical properties of the Au–Ag alloy nanowire coated with an anisotropic shell

The optical properties of the Au–Ag alloy nanowire coated with a cylindrical shell with radial dielectric anisotropy are investigated based on quasistatic theory. Numerical results show that the surface plasmon resonance (SPR) peak is redshifted with increasing the component ratio of Au in the alloy, while the intensity of extinction section at the SPR increases with increasing the component ratio of Ag in the alloy. In addition, as the extent of anisotropy of the shell increases, the extinction section at SPR wavelength decreases and the SPR wavelength is redshifted, but compared with the isotropic shell consisting of comparable dielectric constant materials, the SPR has a distinct blueshift for anisotropic shell. We also find that the field enhancement can be completely concentrated inside the anisotropic shell and the electromagnetic transparency can be exhibited.     

Mono and bimetallic nanoparticles of gold,silver and palladium-catalyzed NADH oxidation-coupled reduction of Eosin-Y

Abstract  

Mono metallic (Au, Ag, Pd) and bimetallic (Au–Ag, Ag–Pd, Au–Pd) with 1:1 mol stoichiometry, nanoparticles are synthesized using one-pot, temperature controlled chemical method using cetyltrimethylammonium bromide (CTAB) as the capping agent. The particle sizes (Au = 5.6, Ag = 5.0, Pd = 6.0, Au–Ag = 9.2, Ag–Pd = 9.6, Au–Pd = 9.4 nm) are characterized by UV–Vis, HRTEM, and XRD measurements, respectively. CTAB bindings onto mono and bimetallic nanoparticles are analyzed by FTIR spectra. The catalytic activities of mono and bimetallic nanoparticles are tested on the reaction between NADH oxidation and Eosin-Y reduction. The effects of base, pH, ionic strength, nature of mono and bimetallic catalysts are studied and the reaction conditions are optimized. Bimetallic nanoparticles exhibited better catalysis than the mono metallic nanoparticles, which may be due to the electronic effects of the core to shell metal atoms.     

Bimetallic Au–Ag nanochains as SERS substrates

Au–Ag bimetallic nanochains were prepared using pulsed laser ablation in liquid medium. Synthesis was performed by ablating silver target in a gold colloidal suspension. The plasmon characteristics of the nanostructures are found to be sensitive to ablation duration. AFM and TEM images indicate that almost all nanoparticles, both Au and Ag in the suspension participate in the growth process. An attempt was also made to realize plasmon hybridization by reducing the volume of the suspension by simple evaporation. The nanochains were tested for application as SERS substrates by using crystal violet as probe molecules.     

A novel method to prepare Au nanocage@SiO_2 nanoparticle

Gold （Au） nanocage@SiO2 nanoparticles are prepared by a novel approach. The silver （Ag） nanocube@SiO2 structure is synthetized firstly. Next, the method of etching a SiO2 shell by boiling water is adopted to change the penetration rate of AuCl4- through the SiO2 shell. AuCl4- can penetrate through silica shells of different thickness values to react with the Ag nanocube core by changing the incubation time. The surface plasma resonance （SPR） peak of synthetic Au nanocage@SiO2 can be easily tuned into the near-infrared region. Besides, CdTeS quantum dots （QDs） are successfully connected to the surface of Au nanocage@SiO2, which testifies that the incubation process does not change the property of silica.     

Tuning the number of plasmon band in silver ellipsoidal nanoshell: refractive index sensing based on plasmon blending and splitting

Because of the geometric features of both rod and shell, dielectric-silver core–shell ellipsoidal nanostructure with 12–40 nm semi-major axis may bring forth four surface plasmon resonance (SPR) absorption peaks at most. Theoretical calculations based on quasi-static approximation show that there is surrounding refractive index-dependent plasmon blending and splitting in the absorption spectra, which makes the number of plasmon band of the silver ellipsoidal nanoshell is tunable. The sensitivity of the plasmon blending and splitting to the surrounding refractive index may be improved by increasing the shell thickness, aspect ratio or core refractive index. This local refractive index dependent-plasmon blending and splitting presents a new sensing picture based on tuning the number of SPR absorption peaks.