Au Ag Alloy Nanoparticles Research Articles

Mesoporous Silica‐Coated Plasmonic Nanostructures for Surface‐Enhanced Raman Scattering Detection and Photothermal Therapy

The design and fabrication of core-shell and yolk-shell nanostructures with surface plasmon resonance (SPR)-active center protected by permeable mesoporous channels can raise the new vitality into the catalysis and biological applications. Hybrid plasmonic-mesoporous silica nanocarriers consisting of Ag and Au-Ag alloy nanoparticles are fabricated through spatially confined galvanic replacement approach. The plasmonic absorption peaks can be finely controlled to the near-infrared (NIR) region (500-790 nm) that is beneficial for tissue transmittance. The mesoporous silica shell facilitates also protection of Au-Ag cores and affords the channels between the exterior and interior capsule environments, thereby endowing the multiple applications. In the present work, it is successfully demonstrated that mesoporous silica-coated Au-Ag alloy core-shell and yolk-shell nanocarriers can serve as good substrates for surface-enhanced Raman scattering (SERS) detection. The SERS signal intensities of nanocarriers are highly dependent on the SPR peaks and the contents of gold. Simultaneously, the synthesized Au-Ag@mSiO2 nanocarriers with SPR peak at ≈790 nm can be applied in NIR-sensitive SERS detection and photothermal therapy.

Antibacterial activity of Ag–Au alloy NPs and chemical sensor property of Au NPs synthesized by dextran

Gold and silver–gold alloy nanoparticles with mean diameter of 10nm and narrow size distribution were prepared by reduction of the correspondent metal precursors using aqueous dextran solution which acts as both a reducing and capping agent. The formation of nanoparticles was characterized by UV–vis spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD) and dynamic light scattering (DLS). The silver and gold nanoparticles exhibited absorption maxima at 425 and 551nm respectively; while for the bimetallic Ag–Au alloy appeared 520nm in between them. TEM images showed monodispersed particles in the range of 8–10nm. The crystallinity of the nanoparticles was assured by XRD analysis. DLS data gave particle size distribution. The dextran stabilized Au nanoparticles used as a colorimetric sensor for detection and estimation of pesticide present in water. The dextran stabilized Ag–Au alloy nanoparticles exhibited interesting antimicrobial activity against bacteria at micromolar concentrations.

Selective oxidation of methanol to methyl formate on catalysts of Au–Ag alloy nanoparticles supported on titania under UV irradiation

Au–Ag alloy nanoparticles on titania exhibit superior photocatalytic performance in selective oxidation of methanol to methyl formate.

Synthesis and plasmonic properties of monodisperse Au–Ag alloy nanoparticles of different compositions from a single-source organometallic precursor

Monodisperse Au–Ag alloy nanoparticles of different compositions are prepared through the mild decomposition of the bimetallic precursor [Au2Ag2(C6F5)4(OEt2)2]n in an organic solvent using hexadecylamine (HDA) as a stabilizing ligand. The effects of different reaction parameters on the size and composition of the nanoparticles, such as the metal : HDA ratio, the use of H2 reducing gas or the solvent (toluene, THF or mesitylene), have been studied through TEM, HRTEM, EDS, UV/Vis and 19F NMR spectroscopy. The localized surface plasmon resonance (LSPR) displayed by the spherical Au–Ag nanoparticles can be tuned as a function of the metal composition.

Synthesis of Bimetallic Nanoalloy Layer using Simultaneous Laser Ablation of Monometallic Targets

It is proposed to synthesize silver-gold alloy nanoparticles by direct pulsed laser ablation of joined Ag and Au monometallic targets. To avoid utilizing a high-vacuum chamber, this pulsed laser-assisted technique is performed in an isolated beaker containing pure helium gas at atmospheric pressure. The structure and formation mechanism of the homogeneous Ag–Au nanoalloy particles on the Si substrate surface are discussed. Here, as in other works, the formation of the Ag–Au alloy nanoparticles was verified by the appearance of a surface plasmon absorption maximum at 442 nm between the surface plasmon resonance peaks of the corresponding monometallic particles, and by results of X-ray photoelectron spectroscopy. Based on data of UV-visible spectroscopy and energy dispersive X-ray analysis, the atomic contents of Ag and Au are determined in the nanoalloy particles. Transmission electron microscopy (TEM) showed the synthesized semispherical nanoalloy particles, 5–35 nm in diameter, with a narrow particle size distribution. The related morphology, structure, and chemical composition are also investigated using atomic force microscopy, lateral force microscopy, and X-ray diffraction. The suggested approach is affordable, fast, and inexpensive.

Enhanced Raman and luminescence spectra from co-encapsulated silicon quantum dots and Au-Ag nanoalloys.

We report an approach to enhance simultaneously luminescence and SERS signals with a single excitation wavelength by co-encapsulating silicon quantum dots and Au-Ag alloy nanoparticles encoded with Raman reporter molecules inside polymeric nanoparticles. The SERS-luminescence enhancement exploits the large Stokes shift of silicon quantum dots, which allows 'room' for the display of a Raman spectrum.

Stoichiometrically controlled production of bimetallic Gold-Silver alloy colloids using micro-alga cultures

This paper reports the production of well-defined, highly stable Ag–Au alloy nanoparticles (NPs) using living cells of Chlamydomonas reinhardtii, with the composition of the bimetallic alloys being solely determined by the stoichiometric ratio in which the metal salts were added to the cultures. The NPs exhibited a single, well-defined surface plasmon resonance (SPR) band confirming that they were made of a homogeneous population of bimetallic alloys. Particle creation by the cells occurred in three stages: (1) internalization of the noble metals by the cells and their reduction resulting in the formation of the NPs; (2) entrapment of the NPs in the extracellular matrix (ECM) surrounding the cells, where they are colloidally stabilized; and (3) release of the NPs from the ECM to the culture medium. We also investigated the effect of the addition of the metals salts on cell viability and the impact on characteristics of the NPs formed. When silver was added to the cultures, cell viability was decreased and this resulted in a ∼30nm red shift on the SPR band due to changes in the surrounding environment into which the NPs were released. The same observations (in SPR and cell viability) was made when gold was added to a final concentration of 2×10−4M, but not when the concentration was equal to 10−4M, where cell viability was high and the red shift was negligible.

Atomistic modeling of the Au droplet–GaAs interface for size-selective nanowire growth

Density functional theory calculations within both the local density approximation and the generalized gradient approximation are used to study Au-catalyzed growth under near-equilibrium conditions. We discuss both the chemical equilibrium of a GaAs nanowire with an As${}_{2}$ gas atmosphere and the mechanical equilibrium between the capillary forces at the nanowire tip. For the latter goal, the interface between the gold nanoparticle and the nanowire is modeled atomically within a slab approach, and the interface energies are evaluated from the total energies of the model systems. We discuss three growth regimes, one catalyzed by an (almost) pure Au particle, an intermediate alloy-catalyzed growth regime, and a Ga-catalyzed growth regime. Using the interface energies calculated from the atomic models, as well as the surface energies of the nanoparticle and the nanowire sidewalls, we determine the optimized geometry of the nanoparticle-capped nanowire by minimizing the free energy of a continuum model. Under typical experimental conditions of 10${}^{\ensuremath{-}4}$ Pa As${}_{2}$ and 700 K, our results in the local density approximation are insensitive to the Ga concentration in the nanoparticle. In these growth conditions, the energetically most favored interface has an interface energy of around 45 meV/\AA{}${}^{2}$, and the correspondingly optimized droplet on top of a GaAs nanowire is somewhat larger than a hemisphere and forms a contact angle around 130${}^{\ensuremath{\circ}}$ for both pure Au and Au-Ga alloy nanoparticles.

Deposition of Ag and Au–Ag alloy nanoparticle films by spray pyrolysis technique with tuned plasmonic properties

Abstract Silver (Ag) and gold–silver (Au–Ag) alloy nanoparticle films have been prepared on the transparent substrates using a simple inexpensive spray pyrolysis technique. The surface plasmon resonance (SPR) properties of Ag films were investigated by varying spray solution volume. Also, the tunability of SPR properties using an electric field to the nozzle was demonstrated in this work. The reduced full width at half maximum and blue shift in the SPR band position were observed with ΔλFW ∼ 35 nm and ΔλP ∼ 30 nm at the applied voltage of 2 kV. The extinction spectra of alloy nanoparticle films show only one plasmon absorption it is concluded that mixing of gold and silver precursors leads to a homogeneous formation of alloy nanoparticles. The maximum of the SPR band tuned continuously from λmax ∼ 414–525 nm with increasing gold content. The SEM observations show the variable size and spherical structure of nanoparticle films.

Effect of organic vapors on Au, Ag, and Au–Ag alloy nanoparticle films with adsorbed 2,6-dimethylphenyl isocyanide

The physicochemical properties of metallic substrates are affected by the environment in different ways. It is generally difficult to determine these effects because the molecules in the environment interact weakly with metallic substrates. In this work, we demonstrate that even the effect of volatile organic compounds (VOCs) can be identified by utilizing the surface-enhanced Raman scattering of isocyanide molecules. The NC stretching band of 2,6-dimethylphenyl isocyanide (2,6-DMPI) adsorbed on Au, for instance, is blueshifted by 6cm−1 under an acetone flow and is redshifted by 20cm−1 under an ammonia flow. The same band of 2,6-DMPI adsorbed on Ag and Au0.5Ag0.5 alloy films is, however, redshifted equally by 8 and 13cm−1 under acetone and ammonia flows, respectively. This indicates that although the surface plasmons of Au0.5Ag0.5 alloy nanoparticles are clearly distinct from those of Ag (as well as Au) nanoparticles, both Au0.5Ag0.5 and Ag nanoparticles show a similar response to VOCs. These observations led us to conclude that the outermost parts of Au–Ag alloy nanoparticles are enriched with Ag atoms and that only the surfaces of metal nanoparticles, and not the bulk material, are affected by VOCs.

Selective oxidation of alcohols on P123-stabilized Au–Ag alloy nanoparticles in aqueous solution with molecular oxygen

A series of stable Au1−xAgx (x=0, 0.01, 0.02, 0.05, 0.10, 0.15) alloy nanoparticles with comparable particle sizes of 3–4nm have been prepared by a co-reduction method in P123 aqueous solution and used for the selective oxidation of alcohols with molecular oxygen in the presence of Na2CO3 at near ambient temperature. The results demonstrated that the addition of Ag significantly enhanced the catalytic activity, and the Au0.95Ag0.05 alloy nanocatalysts possessed the highest oxidation rate for the aerobic oxidation of benzyl alcohol. The comparisons of the product selectivities at 60% conversion of benzyl alcohol revealed that increasing reaction temperature or benzyl alcohol concentration was more favorable for producing benzaldehyde relative to benzoic acid. The aerobic oxidation of other kinds of alcohols was also examined over the Au0.95Ag0.05 alloy nanocatalysts and showed excellent catalytic activities and the product selectivities associated closely with the natures of alcohols.

Silicon/silicide grown out of nanoporous gold nanoparticles

Ordered arrays of nanoporous gold nanoparticles were fabricated via a combination of solid-state dewetting of Au/Ag bilayers on pre-patterned SiO2/Si substrates and a subsequent dealloying process. AuAg alloy nanoparticles were formed after dewetting, and a subsequent dealloying process leads to the formation of nanoporous gold nanoparticles. The dewetting was carried at 700, 800, and 900 °C, respectively. The growth of Si or silicide particles occurred after dewetting at 800 and 900 °C, and these particles remains even after the dealloying process. This growth is supposed to occur via a solid–liquid–solid (SLS) mechanism, and mainly Ag serves as catalyst. A 20 nm thick thermal SiO2 is sufficient to avoid this reaction between the Au/Ag bilayers and the Si substrate when the dewetting temperature is 700 °C.

Improvement of catalytic performance of preferential oxidation of CO in H2-rich gases on three-dimensionally ordered macro- and meso-porous Pt–Au/CeO2 catalysts

Three-dimensionally ordered macro- and meso-porous (3DOM) M/CeO2 (M=Pt, Au and Pt–Au) catalysts supported with different noble metal nanoparticles of Pt, Au and Pt–Au alloy were synthesized by thermal decomposition of cerium nitrate precursor using the close packed structures of PS colloidal crystals as templates. The obtained 3DOM M/CeO2 catalysts possess well-defined 3DOM skeletons composed of ultrafine CeO2 nanoparticles. The macroporous CeO2 skeletons contain the mesoporous walls possessing the nanopores of ∼3–4nm and the noble metal nanoparticles of Pt, Au, and Pt–Au alloy with the grain sizes of ∼5.0nm homogenously dispersed. The catalytic performance of CO preferential oxidation (PROX) in H2-rich gases on 3DOM Pt/CeO2, Au/CeO2, and Pt–Au/CeO2 catalysts were systematically studied. The superior catalytic performance with 90% CO conversion and 83% CO2 selectivity are realized on 3DOM 1wt.% Pt1Au1/CeO2 catalyst at 80°C under the weight hourly space velocity of 30,000mLg−1h−1. Moreover, the 3DOM 1wt.% Pt1Au1/CeO2 catalyst exhibits excellent catalytic stability maintaining 90% CO conversion and 56% CO2 selectivity even after the test period of 260h under the same flow rate. The three-dimensionally ordered macro- and meso-porous skeletons, the synergistic effect due to the formation of Pt–Au alloy, and the strong interaction of Pt–Au alloy nanoparticles with CeO2 supports are identified to be beneficial to the improvement of catalytic activity and stability of CO PROX. The obtained 3DOM Pt–Au/CeO2 catalysts may be potential candidates with the improved catalytic performance of CO PROX reaction in H2-rich gases for polymer electrolyte membrane fuel cells (PEMFCs) applications.

Direct Detection System for Escherichia coli Using Au–Ag Alloy Microchips

Au–Ag alloy nanoparticles (NPs) were prepared by the reduction of metal ion mixtures in aqueous sodium citrate solution using sodium borohydride (NaBH4). The resulting Au–Ag alloy NPs were analyzed by various techniques. Alloy-attached chips for the detection of microorganisms were fabricated simply by the attachment of Au–Ag alloy nanoparticles onto glass slides after silanization through self-assembled monolayers (SAMs) for the formation of activated amine (−NH2) as a terminal function group. The alloy-attached chips were investigated for their ability to bind the target Escherichia coli (E. coli) in water. E. coli was detected in water as a function of time and concentration by UV–vis spectroscopic measurements based on the interaction between the alloy-attached chip and E. coli. Field-emission scanning electron microscopy (FE-SEM) was used to directly observe the E. coli captured on the alloy chips. These studies demonstrated that E. coli in drinking water can be directly detected with Au–Ag alloy microchips without requiring any interaction between an antibody and an antigen.

Synthesis of Au–Ag alloy nanoparticles supported on silica gel via galvanic replacement reaction

Synthesis of supported Au–Ag bimetallic has attracted much attention since we found for the first time that Au and Ag had synergistic effect on CO oxidation and preferential CO oxidation in rich hydrogen. In this work, the formation of Au–Ag alloy nanoparticles supported on silica gel by galvanic replacement reaction has been investigated. We applied various characterizations including X-ray diffraction (XRD), transmission electronic microscopy (TEM), ultraviolet–visible spectroscopy (UV–vis), X-ray absorption spectroscopy (XAS) to characterize the formation process of Au–Ag alloy. Although the average particle sizes of the Au–Ag alloy nanoparticles obtained by the galvanic replacement reaction are relatively large comparing with that of loading Au first, the catalytic activity of the catalyst in preferential CO oxidation is almost the same. This result manifested that the particle size effect of Au–Ag nanoparticles was not as tremendous as that of monometallic gold. The formation of Au–Ag alloy made it less sensitive to the particle size.

P123-stabilized Au–Ag alloy nanoparticles for kinetics of aerobic oxidation of benzyl alcohol in aqueous solution

Colloidal Au–Ag alloy nanoparticles with various Ag/Au molar ratios were first prepared by a co-reduction method in P123 aqueous solution and characterized by UV–vis, TEM, and XPS. The prepared Au–Ag alloy nanoparticles were substantially stable and had comparable particle sizes (3–4nm) for aerobic oxidation kinetics of benzyl alcohol. The addition of Ag not only significantly enhanced the reaction rate but also increased the apparent activation energy (Ea) compared to that for monometallic Au nanocatalysts. The kinetics investigations indicated that on the pure Au sites, the oxidation of benzyl alcohol followed 1.5-order reaction kinetics, while on Au sites adjacent to Ag atoms, it followed 0.5-order reaction kinetics. The presence of Na2CO3 greatly improved the catalytic activity of Au–Ag nanocatalysts but decreased the selectivity to benzaldehyde.

Graphene oxide supported Au–Ag alloy nanoparticles with different shapes and their high catalytic activities

A simple method was developed to fabricate Au–Ag nanoparticle/graphene oxide nanocomposites (Au–Ag/GO) by using simultaneous redox reactions between AgNO3, HAuCl4 and GO. The Au–Ag/GO was characterized by x-ray photoelectron spectroscopy, transmission electron microscopy and energy dispersive x-ray spectroscopy. The GO nanosheets acted as the reducing agent and the support for the Au–Ag alloy nanoparticles. In addition, Au–Ag alloy nanoparticles with different shapes including core–shell-like, dendrimer-like and flower-like were obtained by simply modifying the concentration of the reactants and the reaction temperature. With no reducing or stabilizing agents added, the Au–Ag/GO nanocomposites show superior catalytic performance for the reduction of 4-nitrophenol and for the aerobic homocoupling of phenylboronic acid.

Fabrication of graphene oxide decorated with Au–Ag alloy nanoparticles and its superior catalytic performance for the reduction of 4-nitrophenol

Using graphene oxide (GO) as both a reducing agent and a support, a two-step method was used to fabricate GO decorated with Au–Ag alloy nanoparticles with an Au-rich shell. The material exhibited excellent catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol in water at room temperature and its activity was superior to that of GO decorated with Au–Ag alloy nanoparticles that was prepared by a one-step method and GO decorated with either Au or Ag nanoparticles alone.

Pt–Au/nitrogen-doped graphene nanocomposites for enhanced electrochemical activities

A facile in situ assembly strategy was developed for the fabrication of Pt–Au alloy nanoparticles (NPs) on nitrogen-doped graphene (N–G) sheets, and the as-fabricated Pt–Au/N–G nanocomposites were suitable for electrochemical applications. As characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction analysis and inductively coupled plasma-atomic emission spectroscopy techniques, Pt–Au alloy NPs with an average size of 4–5 nm were uniformly distributed on the N–G surface through intrinsic covalent bonds. The Pt–Au/N–G nanocomposites exhibited excellent electrocatalytic activity and stability towards the methanol oxidation reaction with the highest capability observed for a Pt/Au atomic ratio of 3/1. The unique electrochemical features are distinctive from those of N-free nanocomposites and commercially available Pt/C catalysts, indicative of the alloying effect of Pt–Au and their synergistic interaction with the N–G sheet, which may open up new possibilities for the preparation of N–G-based nanocomposites for other intensive applications as well.

Green synthesis of Au–Ag alloy nanoparticles using Cacumen platycladi extract

An environmentally-friendly method for the synthesis of Au–Ag alloy nanoparticles with controlled composition is proposed. The method involves the simultaneous bioreduction of HAuCl4 and AgNO3 using Cacumen Platycladi leaf extract at 90 °C. The formation of the Au–Ag alloy nanoparticles was monitored by recording the absorbance, using UV-visible light spectroscopy as a function of the reaction time and the formation process. The as-synthesized nanoparticles were characterized by X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy to verify the nature of the alloy. The Fourier transform infrared spectra show that the CC, N–H, (NH) CO, and –OH groups in the C. Platycladi extract served as a reducing agent, whereas the peptides or proteins prevented the aggregation of alloy nanoparticles. The process can be described as a purely “green technique” because no additional synthetic reagents were used as reductants or stabilizers.