Biogenic synthesis of Ag, Au and bimetallic Au/Ag alloy nanoparticles using aqueous extract of mahogany (Swietenia mahogani JACQ.) leaves

In this paper, we have demonstrated for the first time, the superb efficiency of aqueous extract of dried leaves of mahogany (Swietenia mahogani JACQ.) in the rapid synthesis of stable monometallic Au and Ag nanoparticles and also Au/Ag bimetallic alloy nanoparticles having spectacular morphologies. Our method was clean, nontoxic and environment friendly. When exposed to aqueous mahogany leaf extract, competitive reduction of Au(III) and Ag(I) ions present simultaneously in same solution leads to the production of bimetallic Au/Ag alloy nanoparticles. UV-visible spectroscopy was used to monitor the kinetics of nanoparticles formation. UV-visible spectroscopic data and TEM images revealed the formation of bimetallic Au/Ag alloy nanoparticles. Mahogany leaf extract contains various polyhydroxy limonoids which are responsible for the reduction of Au(III) and Ag(I) ions leading to the formation and stabilization of Au and Ag nanopaticles.     

Au/Ag核一壳结构复合纳米粒子形成机制的研究

在已制备好的Au纳米粒子表面,通过化学还原的方法沉积生长Ag包覆层,通过 控制Au, Ag的比列,制备了粒度均匀且粒径可控的Au/Ag核-壳结构纳米粒子。利用 UV-vis吸收光谱和透射电子显微镜（TEM）对SAu, Ag摩尔比为1：10的复合纳米粒 子的光学性质和形态进行随时监测,直接观察了核-壳结构纳米粒子的生长过程： 一部分Ag+在Au核表面还原生长,溶液中其余Ag+还原形成银的纳米团簇向粒子表面 的继续沉积生长,壳层增厚。     

Keggin ions as UV-switchable reducing agents in the synthesis of Au core-Ag shell nanoparticles

Keggin ion-mediated synthesis of Au core-Ag shell bimetallic nanoparticles is described. Exposure of photochemically reduced aqueous (PW12O40)3- Keggin ions to AuCl4- ions leads to the formation of stable gold nanoparticles capped by the Keggin ions. The surface-bound Keggin ions may then be activated by UV irradiation and, upon exposure to Ag+ ions, reduce the metal ions to form a silver shell around the gold core. That the capping agent not only stabilizes the metal nanoparticles but also plays the role of a switchable reducing agent is a highlight of this approach with important implications in nanomaterials synthesis and catalysis.     

Structures of mixed gold-silver cluster cations (Ag(m)Au(n)+, m+n<6): ion mobility measurements and density-functional calculations

The collision cross sections of Ag(m)Au(n)+ (m+n)<6 cluster ions were determined. For bimetallic clusters, we observe a significant intracluster charge transfer leaving most of the ions positive charge on the silver atoms. The mixed trimeric ions Ag2Au+ and AgAu2+ are triangular like the pure gold and silver trimers. Most of the tetrameric clusters are rhombus shaped, with the exception of Ag3Au+, which has a Y structure with the gold atom in the center. Among the pentamers we find distorted X structures for all systems. For Ag2Au3+ we find an additional isomer which is a trigonal bipyramid. These findings are in line with predictions based on density-functional theory calculations, i.e., all these structures either represent the global minima or are within less than 0.1 eV of the predicted global minimum.     

One-pot synthesis of Ag-Au bimetallic nanoparticles with Au shell and their high catalytic activity for aerobic glucose oxidation

PVP-protected Ag(core)/Au(shell) bimetallic nanoparticles of enough small size, i.e., 1.4nm in diameter were synthesized in one-vessel using simultaneous reduction of the corresponding ions with rapid injection of NaBH(4), and characterized by HR-TEM. The Ag(core)/Au(shell) bimetallic nanoparticles show a high and durable catalytic activity for the aerobic glucose oxidation, and the catalyst can be stably kept for more than 2months under ambient conditions. The highest activity (16,890mol-glucoseh(-1)mol-metal(-1)) was observed for the bimetallic nanoparticles with Ag/Au atomic ratio of 2/8, the TOF value of which is several times higher than that of Au nanoparticles with nearly the same particle size. The higher catalytic activity of the prepared bimetallic nanoparticles than the usual Au nanoparticles can be ascribed to: (1) the small average diameter, usually less than 2.0nm, and (2) the electronic charge transfer effect from adjacent Ag atoms and protecting PVP to Au active sites. In contrast, the Ag-Au alloy nanoparticles, synthesized by dropwise addition of NaBH(4) into the starting solution and having the large mean particle size, showed a low catalytic activity.     

Observed Dramatically Improved Catalysis of Ag Shell on Au/Ag Core‐shell Nanorods is Due to Silver Impurities Released During Etching Process

Core/shell bimetallic nanoparticles are highly popular in electrocatalysis; it is argued that the core metal enhances the catalytic properties of the shell. We have investigated the electrocatalytic properties of Au/Ag core‐shell nanorods (Au/Ag NRs) where Ag shell was thinned by aging in the presence of cetyltrimethylammonium bromide. We observed excellent electrocatalysis toward hydrogen peroxide electroreduction upon decreasing the Ag shell thickness, which would, at first, appear to imply a strong synergistic effect of the Au core with the Ag shell for electrocatalysis. We show, however, that this electrocatalysis is not caused by particular Au/Ag core/shell structures but rather by the presence of residual silver impurities in the form of Ag nanoparticles (Ag NPs) formed during the preparation of the thin‐layer silver shell/gold core nanorods.     

Synthesis and characterization of Au core-Au-Ag shell nanoparticles from gold seeds: impacts of glycine concentration and pH

This paper describes the preparation of Au core-Au-Ag shell nanoparticles (NPs) in different morphologies by controlling both the pH and the glycine concentration. Using a seed-growth method, we prepared high-quality Au core-Au-Ag shell NPs from a glycine solution under alkaline conditions (pH>8.5). By controlling both the pH and the glycine concentration, we prepared dumbbell-shaped and peanut-shaped Au core-Au-Ag shell NPs readily by depositing gold and silver, reduced by ascorbate, onto the gold nanorods. We have found that the glycine concentration that is optimal for preparing high-quality Au core-Au-Ag shell NPs differs at the various values of pH. At pH<8.5, the glycine concentration is not important, but, when preparing dumbbell- and peanut-shaped Au core-Au-Ag shell NPs, it should be greater than 50 mM and greater than 20 mM at pH 9.5 and 10.5, respectively. Glycine plays a number of roles during the synthesis of the Au core-Au-Ag shell NPs by controlling the solution pH, altering the reduction potentials of gold and silver ions through forming complexes with metal ions (Au(+) and Ag(+)), minimizing the formation of Ag(2)O, AgCl, and AgBr precipitates, and stabilizing the thus-prepared NPs. At pH 9.7, we observed the changes in the morphologies of the Au core-Au-Ag shell NPs-from regular (rectangular) to peanut- and dumbbell-shaped, and finally to jewel-, diamond-, and/or sphere-shaped-that occurred during the course of a 60-min reaction. In addition, we were able to affect the shapes and sizes of the Au core-Au-Ag shell NPs by controlling the reaction time.     

Synthesis of Au, Au/Ag, Au/Pt and Au/Pd nanoparticles using the microwave-polyol method

Gold, Au/Ag, Au/Pt and Au/Pd bimetallic nanoparticles with varying mol fractions were synthesized in ethylene glycol and glycerol, using the microwave technique in the presence of a stabilizer poly(N-vinylpyrrolidone) (PVP). It was found that bimetallic colloids of Au/Ag, Au/Pd and Au/Pt form an alloy either on co-reduction of respective metal ions or on mixing individual sols.     

Synthesis of aqueous Au core-Ag shell nanoparticles using tyrosine as a pH-dependent reducing agent and assembling phase-transferred silver nanoparticles at the air-water interface

We demonstrate that the amino acid tyrosine is an excellent reducing agent under alkaline conditions and may be used to reduce Ag+ ions to synthesize stable silver nanoparticles in water. The tyrosine-reduced silver nanoparticles may be separated out as a powder that is readily redispersible in water. The silver ion reduction at high pH occurs due to ionization of the phenolic group in tyrosine that is then capable of reducing Ag+ ions and is in turn converted to a semi-quinone structure. These silver nanoparticles can easily be transferred to chloroform containing the cationic surfactant octadecylamine by an electrostatic complexation process. The now hydrophobic silver nanoparticles may be spread on the surface of water and assembled into highly ordered, linear superstructures that could be transferred as multilayers onto suitable supports by the versatile Langmuir-Blodgett technique. Further, tyrosine molecules bound to the surface of Au nanoparticles through amine groups in the amino acid may be used to selectively reduce silver ions at high pH on the surface of the Au nanoparticles, thus leading to a simple strategy for realizing phase-pure Au core-Ag shell nanostructures.     

Interfacial deposition of Ag on Au seeds leading to AucoreAgshell in organic media

A seed mediated procedure for the synthesis of hydrophobic Au(core)Ag(shell) nanoparticles in toluene is demonstrated. The reaction proceeds by way of the interfacial reduction of silver ions by 3-pentadecylphenol followed by their deposition on hydrophobized Au nanoparticles. Such a hitherto unreported interfacial seeded growth reaction leads to the formation of phase pure Au(core)Ag(shell) nanoparticles that retain the hydrophobicity of the seed particles and remain stable in toluene. Such core-shell structures are however not formed in the aqueous phase. The core-shell architecture was verified using TEM analysis and the formation process was studied by recording the UV-vis spectra of the organic phase nanoparticles as a function of time. TEM kinetics also showed gradual increase in the silver layer thickness. Conclusive evidence was however obtained on examination of the HRTEM images of the products formed. Elemental analysis using X-ray photoelectron spectroscopy of the Au(core)Ag(shell) nanostructure revealed the presence of metallic silver. Moreover changing the surface capping of the Au seed does not affect the formation of the Au(core)Ag(shell) nanostructure.     

Defining rules for the shape evolution of gold nanoparticles

The roles of silver ions and halides (chloride, bromide, and iodide) in the seed-mediated synthesis of gold nanostructures have been investigated, and their influence on the growth of 10 classes of nanoparticles that differ in shape has been determined. We systematically studied the effects that each chemical component has on the particle shape, on the rate of particle formation, and on the chemical composition of the particle surface. We demonstrate that halides can be used to (1) adjust the reduction potential of the gold ion species in solution and (2) passivate the gold nanoparticle surface, both of which control the reaction kinetics and thus enable the selective synthesis of a series of different particle shapes. We also show that silver ions can be used as an underpotential deposition agent to access a different set of particle shapes by controlling growth of the resulting gold nanoparticles through surface passivation (more so than kinetic effects). Importantly, we show that the density of silver coverage can be controlled by the amount and type of halide present in solution. This behavior arises from the decreasing stability of the underpotentially deposited silver layer in the presence of larger halides due to the relative strengths of the Ag(+)/Ag(0)-halide and Au(+)/Au(0)-halide interactions, as well as the passivation effects of the halides on the gold particle surface. We summarize this work by proposing a set of design considerations for controlling the growth and final shape of gold nanoparticles prepared by seed-mediated syntheses through the judicious use of halides and silver ions.     

Fabrication of Ag/Au bimetallic nanoparticles by UPD-redox replacement: Application in the electrochemical reduction of benzyl chloride

The bimetallic Ag/Au nanoparticles were prepared by underpotential deposition-redox replacement technique on the basis of Au nanoparticles modified glassy carbon (GC) electrode. The as-prepared Ag/Au nanoparticles were characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The Ag/Au bimetallic nanoparticles modified GC electrode with low-Ag loading exhibits much better catalytic activity for the reduction of benzyl chloride than Ag nanoparticles modified GC electrode. The result is attributed to the synergic effect between Ag and Au in the reduction process. The chronoamperometry test shows that the Ag/Au nanoparticles possess long-term performance in the electrolysis.     

Synthesis, characterization, and structure-selective extraction of 1-3-nm diameter AuAg dendrimer-encapsulated bimetallic nanoparticles

The synthesis and characterization of 1-3-nm diameter, structurally well-defined, bimetallic AuAg dendrimer-encapsulated nanoparticles (DENs) are reported. Three different bimetallic structures were examined: AuAg alloys synthesized by cocomplexation and subsequent reduction of dendrimer-encapsulated Au3+ and Ag+ and core/shell [Au](Ag) and [AuAg alloy](Ag) structures (for structured materials, brackets indicate the core metal and parentheses indicate the shell metal) synthesized by a sequential loading method. Depending on the shell metal and its oxidation state, the AuAg nanoparticles can be extracted from the dendrimer into an organic phase using different surfactants. This provides a means for analyzing the composition of the shell. UV-vis, TEM, and single-particle X-ray energy dispersive spectroscopy (EDS) were used to characterize the bimetallic DENs before and after extraction and show that the extraction step does not alter the size or composition of the bimetallic nanoparticles.     

Crystal overgrowth on gold nanorods: tuning the shape, facet, aspect ratio, and composition of the nanorods

Electrochemically prepared Au nanorods were used as seeds for the overgrowth of thin shells of gold, silver, and palladium by using a mild reducing agent, ascorbic acid, in the presence of surfactants at ambient condition. The unique crystal facets of the starting nanorods results in anisotropic crystal overgrowth. The overgrowth rates along different crystallographical directions can be further regulated by adding foreign ions or by using different metal reduction methods. This overgrowth study provides insights on how different metal ions could be reduced preferentially on different Au nanorod surfaces, so that the composition, aspect ratio, shape, and facet of the resulting nanostructures can be rationally tuned. These surfactant-stabilized bimetallic Au(core)M(shell) (M=Au, Ag, Pd) nanorod colloids might serve as better substrates in surface-enhanced Raman spectroscopy as well as exhibiting enhanced catalytic properties.     

Bimetallic (Ag)Au nanoparticles prepared by the seed growth method: two-dimensional assembling, characterization by energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, and surface enhanced raman spectroscopy, and proposed mechanism of growth

Layered core-shell bimetallic silver-gold nanoparticles were prepared by overdeposition of Au over Ag seeds by the seed-growth method using tetrachloroauric acid, with hydroxylamine hydrochloride as the reductant. The effects of pH, reduction rate, and seeding conditions on the morphology and surface plasmon extinction of the bimetallic nanoparticles were investigated. Nanoparticles prepared by a rapid reduction in the neutral ambient and assembled into two-dimensional nanoparticulate films by adsorption of 2,2'-bipyridine were characterized by energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, surface-enhanced Raman scattering spectroscopy, and transmission electron microscopy. The results are consistent with Ag core and Ag/Au-alloyed shell composition of the nanoparticles. Evidence of the presence of Ag on the surface of the nanoparticles, of enrichment of the Ag/Au alloy shell by Ag toward or at the nanoparticle surface, and of modification of the nanoparticle surface by adsorbed chlorides is also provided. Reduction of the size of the Ag seeds, alloying of Ag and Au in the shell of the nanoparticles, and modification of their surfaces by adsorbed chlorides are tentatively attributed to positive charging of the nanoparticles during the electrocatalytic overdeposition of Au over Ag seeds.     

Selective dissolution of the silver component in colloidal Au and Ag multilayers: a facile way to prepare nanoporous gold film materials

Colloidal Au/Ag multilayer films were prepared by alternate assembly of Au nanoparticles with a size of 5 +/- 1.2 nm and Ag nanoparticles with a size of 10 +/- 2.4 nm by using 1,5-pentanedithiol as cross-linker. Nanoporous gold films with a ligament size of 26.7 +/- 4.6 nm were then prepared by selective dissolution of sacrificial templates of silver particles in colloidal Au/Ag multilayers. The complete dissolution of Ag particles in colloidal Au/Ag multilayers in a mixture solution of 3.0 mM HAuCl(4) and 3 M NaCl took place at room temperature without damage of the colloidal Au film. This method to prepare nanoporous gold films was further extended to the preparation of nanoporous gold nanotubes by depositing colloidal Au/Ag film on the inner wall of anodic aluminum oxides (AAO) followed by dissolution of colloidal Ag and removal of AAO templates.     

Novel Multifunctional Graphene Sheets with Encased Au/Ag Nanoparticles for Advanced Electrochemical Analysis of Organic Compounds

This work is the first presentation of the synthesis of few‐layer graphene decorated with gold and silver nanoparticles (Gr–Au–Ag) by chemical vapor deposition over a catalytic system formed of bimetallic Au–Ag nanoclusters supported on MgO and with methane used as the source of carbon. The sheetlike morphology of the graphene nanostructures, with mean sizes in the range of hundreds of nanometers, was observed by high‐resolution electron microscopy. The distinctive feature found in all the samples was the regular rectangular or square shapes. This multi‐component organic–inorganic nanomaterial was used to modify a platinum substrate and subsequently employed for the detection of carbamazepine, an anti‐convulsion drug. UV/Vis spectroscopy revealed that a strong hypochromism occurred over time, after mixing solutions of graphene–Au–Ag with carbamazepine. This can be attributed to π–π stacking between the aromatic groups of the two compounds. Linear sweep voltammetry (LCV) provided evidence that the modified platinum substrate presented a significant electrocatalytic reaction toward the oxidation of carbamazepine. The intensity of the current was found to increase by up to 2.5 times, and the oxidation potential shifted from +1.5 to +1.35 V(Ag/AgCl) in comparison with the unmodified electrode. Electrochemical impedance spectroscopy (EIS) was further used to thoroughly assess the activity of the platinum electrode that was modified by the deposition of the Gr‐Au‐Ag composites in the presence of various concentrations of carbamazepine. The experimental EIS records were used for the generation of an equivalent electrical circuit, based on the charge‐transfer resistance (R_ct), Warburg impedance (Z_D), solution resistance (R_s), and a constant phase element (CPE) that characterizes the non‐ideal interface capacitive responses.     

Combining optical lithography with rapid microwave heating for the selective growth of Au/Ag bimetallic core/shell structures on patterned silicon wafers

We demonstrate a novel approach for the production of patterned films of nanometer-sized Au/Ag bimetallic core/shell nanoparticles (NPs) on silicon wafers. In this approach, we first self-assembled monodisperse Au NPs, through specific Au...NH(2) interactions, onto a silicon substrate whose surface had been modified with a pattern of 3-aminopropyltrimethoxysilane (APTMS) groups to form a sandwich structure having the form Au NPs/APTMS/SiO(2). These Au NPs then served as seeds for growing the Au/Ag bimetallic core/shell NPs: we reduced silver ions to Ag metal on the surface of Au seeds under rapid microwave heating in the presence of sodium citrate. Energy-dispersive X-ray analysis confirmed that the Au/Ag bimetallic core/shell NPs grew selectively on the regions of the surface of the silicon wafer that had been patterned with the Au seeds. Scanning electron microscopy images revealed that we could synthesize well-scattered, high-density (>82%) thin films of Au/Ag bimetallic core/shell NPs through the use of this novel strategy. The patterned structures that can be formed are simple to produce, easily controllable, and highly reproducible; we believe that this approach will be useful for further studies of nanodevices and their properties.     

光化学合成Au核@Pd壳复合纳米粒子及其表征

在PEG-丙酮溶液体系中, 采用紫外光辐射还原Au(Ⅲ), Pd(Ⅱ)离子混合物和以Au晶种为核、紫外光辐射还原Pd(Ⅱ)使其沉积在Au晶种表面上这两种方法, 合成了Au核@Pd壳复合纳米粒子. 通过改变Au(Ⅲ)离子或Au晶种对Pd(Ⅱ)离子的摩尔比调节复合粒子的尺寸和Pd壳厚度, 分别获得了直径范围为5.6~4.6 nm和4.6~6.2 nm的复合粒子. 利用UV-Vis吸收光谱、TEM、HR-TEM和XPS等表征手段, 证明了合成的纳米粒子为核-壳复合结构. 研究了Au@Pd纳米粒子的直径随溶液中Au(Ⅲ)/Pd(Ⅱ)摩尔比的改变而变化的规律; 对Au核向Pd壳的供电子作用以及复合粒子的光化学形成机理进行了讨论.     

A two-color-change, nanoparticle-based method for DNA detection

Herein, we describe the detailed synthesis of Ag/Au core-shell nanoparticles, the surface-functionalization of these particles with thiolated oligonucleotides, and their subsequent use as probes for DNA detection. The Ag/Au core-shell nanoparticles retain the optical properties of the silver core and are easily functionalized with thiolated oligonucleotides due to the presence of the gold shell. As such, the Ag/Au core-shell nanoparticles have optical properties different from their pure gold counterparts and provide another “color” option for target DNA-directed colorimetric detection. Size-matched Ag/Au core-shell and pure gold nanoparticles perform nearly identically in DNA detection and melting experiments, but with distinct optical signatures. Based on this observation, we report the development of a two-color-change method for the detection and simultaneous validation of single-nucleotide polymorphisms in a DNA target using Ag/Au core-shell and pure gold nanoparticle probes.