Small Au Nanoparticles Research Articles

Radical scavenger for the stabilization of gold nanoparticles

In this contribution, we report the photo-catalytic synthesis of gold nanoparticles using poly(vinyl alcohol) as a radical surfactant stabilizer. Small Au nanoparticles were prepared depending on the concentration of the stabilizer. Both particle size and surface plasmon band are dependent on the concentration of poly(vinyl alcohol) and irradiation time. Transmission electron microscopic image and dynamic light scattering measurement shows that particles are spherical and monodispersed, respectively. This approach provides a simple route to fabricate gold nanoparticles that hold much promise for use in catalysis, chemical sensing, and biolabeling.

Sputter deposition onto ionic liquids: Simple and clean synthesis of highly dispersed ultrafine metal nanoparticles

Sputter deposition of gold (Au) onto ionic liquids (ILs) resulted in the formation of highly dispersed Au nanoparticles without additional chemical species, such as reducing and∕or stabilizing agents. The Au nanoparticles in 1-ethyl-3-methylimidazolium tetrafluoroborate had an average diameter (dav) of 5.5nm with a standard deviation (σ) of 0.86nm, while sputter deposition onto N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide resulted in the formation of much smaller Au nanoparticles with dav of 1.9nm and σ of 0.46nm. Prolongation of sputtering time results in a higher concentration of Au nanoparticles in ILs, but did not cause a remarkable change in their size.

Poly(vinyl pyrrolidone)-capped five-fold twinned gold particles with sizes from nanometresto micrometres

Poly(vinyl pyrrolidone)-capped multiple twinned gold (Au) particles withdecahedral shape have been synthesized by a simple and convenient solvothermalwet chemical method. In the process, hydrogen tetrachloroauric acid(HAuCl4·3H2O) was reduced by ethylene glycol (EG) to form the multiple twinned Aunanocrystals in the presence of poly(vinyl pyrrolidone) (PVP) molecules at200 °C under the extra condition of autogenous pressure. The decahedral nanoparticlestake up about 10% of the total amount and have the usual size distribution fromseveral tens to hundreds of nanometres. Some larger microsized five-twinned Auparticles with perfect decahedral shape have also been observed in the final product.Furthermore, x-ray photoelectron spectroscopy (XPS) measurements verified thatPVP molecules are adsorbed on the surface of the Au particles. Based on theexperimental results, a growth mechanism has been suggested to elucidate theformation of the small decahedral Au nanoparticles as well as their evolutioninto perfect large decahedral Au particles with the size of several micrometres.

Synthesis of gold/magnetic iron oxide composite nanoparticles for biomedical applications with good dispersibility

Composite nanoparticles consisting of gold and magnetic iron oxide were synthesized in an aqueous solution system by sonochemical and radiochemical processes. Small Au nanoparticles were immobilized on the surface of the iron oxide nanoparticles. Removal of the excess polymers in the solution leads to the good dispersibility of the composite nanoparticles. The composite nanoparticles are expected as a type of nanocarrier for biomedical applications.

From large 3D assembly to highly dispersed spherical assembly: weak and strong coordination mediated self-aggregation of Au colloids

Distinctly different 3D assemblies of ∼1.6 nm Au nanoparticles are constructed based on weak and strong coordination strategies. Reduction of KAuCl4 with NaBH4 in the presence of newly-synthesized 4-(4-phenylmethanethiol)-2,2′:6′,2′′-terpyridine (1) yields functionalized Au nanoparticles which assemble in situ into large 3D aggregates via weak coordination between alkali metal ions and terpyridine attached to separated particles. These assemblies are disassembled into individual nanoparticles via addition of DMF solvent and further reassembled into highly dispersed 3D spherical nanostructures via addition of Co2+ (strong coordination with 1). Wide and small angle XRD measurements show that the assemblies are formed from small Au nanoparticles, consistent with TEM results. It is significant that the large aggregates formed in situ can be directly transformed into nearly monodispersed 3D spherical assemblies via strong coordination (with Co2+), presenting the first example of a direct transformation of one 3D nanonetwork into another distinctly different 3D nanonetwork. The controlled assembly and disassembly processes are accompanied by distinct shifts in the surface plasmon resonance.

Synthesis and physico-chemical characterization of gold nanoparticles softly coated by AOT

Size-controlled gold nanoparticles/surfactant stable systems were prepared by the combined action of the solvated metal atom dispersion (SMAD) technique and confinement in anhydrous sodium bis(2-ethylhexyl)sulfosuccinate (AOT) micellar solution. From liquid samples, by evaporation of the organic solvent, solid gold nanoparticle–surfactant liquid crystals composites were obtained. Sample characterization was performed by X-ray diffraction (SAXS and WAXS), XPS spectroscopy and UV–vis–NIR spectroscopy. All experimental data consistently revealed the coexistence of two gold nanoparticle size populations: bigger nanoparticles (size 20–50 Å) and smaller ones (size of few Å). The two differently-sized gold nanoparticles can be separated by resuspending the gold/surfactant nanocomposite in n-heptane. This operation causes the slow selective precipitation of the bigger nanoparticles softly coated by surfactant leaving, in the surnatant, only the smaller Au nanoparticles. The latter were found to be entrapped in the core of AOT reversed micelles and stabilised by the surfactant adsorption on their surface. Such nanoparticles, as shown by SAXS data, slowly rearrange to a narrower size distribution giving a surnatant containing stable and finely size-controlled gold nanoparticles.

TEM observation of gold nanoparticles deposited on cerium oxide

The structure of Au nanoparticles supported on CeO2 using the deposition precipitation method was observed using a transmission electron microscope (TEM). The shape of Au nanoparticles and the fine structure of contact interface between the Au nanoparticles and CeO2 supports were carefully examined. It was found that there was a preferential orientation relationship between Au nanoparticles and the CeO2 support. It was also observed that small and thin Au nanoparticles disappeared during TEM observation, shrinking layer by layer down to a mono-atomic layer.

A molecular dynamics simulation of the adsorption of water molecules surrounding an Au nanoparticle

This study uses molecular dynamics simulations performed in a parallel computing environment to investigate the adsorption of water molecules surrounding Au nanoparticles of various sizes. An observation of the oxygen and hydrogen atom distributions reveals that the adsorption of the water molecules creates two shell-like formations of water in close vicinity to the Au nanoparticle surface. These shell-like formations are found to be more pronounced around smaller Au nanoparticles. The rearrangement of water molecules in this region reduces the local hydrogen bond strength to below that which is observed in the bulk region. Finally, the simulation results indicate that the absolute value of the interaction energy between the water molecules and the Au nanoparticle is reduced when the water molecules surround a nanoparticle of larger diameter. This observation implies that a stronger adsorption effect exists between smaller Au nanoparticles and water molecules. Hence, the value of the adsorption constant increases for smaller Au nanoparticles.

Triton X-100-Assisted Assembly of 5-nm Au Nanoparticles by DNA Hybridization

Abstract This report describes a Triton X-100-assisted method to assemble small Au nanoparticles by DNA hybridization. The non-specific adsorption of oligonucleotides, which is most prevalent on small Au nanoparticle surface and interferes with the hybridization of complementary sequences, is overcome by shielding the Au nanoparticle with a Triton X-100 layer. Amine-modified oligonucleotides, but not thiol-modified oligonucleotides, are able to penetrate the X-100 shell to enable the hybridization of complementary DNA sequences.

Highly dispersed gold on activated carbon fibers for low-temperature CO oxidation

Gold nanoparticles of 2–5 nm supported on woven fabrics of activated carbon fibers (ACF) were effective during CO oxidation at room temperature. To obtain a high metal dispersion, Au was deposited on ACF from aqueous solution of ethylenediamine complex [Au(en) 2]Cl 3 via ion exchange with protons of surface functional groups. The temperature-programmed decomposition method showed the presence of two main types of functional groups on the ACF surface: the first type was associated with carboxylic groups easily decomposing to CO 2 and the second one corresponded to more stable phenolic groups decomposing to CO. The concentration and the nature of surface functional groups was controlled using HNO 3 pretreatment followed by either calcination in He (300–1273 K) or by iron oxide deposition. The phenolic groups are able to attach Au 3+ ions, leading to the formation of small Au nanoparticles (<5 nm) after reduction by H 2. This was confirmed by high-resolution electron microscopy combined with X-ray energy-dispersive analysis. The catalyst with high Au dispersion demonstrated high activity in CO oxidation. The surface carboxylic groups decomposed during interaction with [Au(en) 2]Cl 3 solution and reduced Au 3+ to Au 0, resulting in the formation of bigger (>9 nm) Au agglomerates after reduction by H 2. These catalysts demonstrated lower activity as compared to the ones containing mostly small Au nanoparticles. Complete removal of surface functional groups rendered an inert support that would not interact with the Au precursor. The oxidation state of gold in the Au/ACF catalysts was controlled by X-ray photoelectron spectroscopy before and after the reduction in H 2. The high-temperature reduction in H 2 (673–773 K) was necessary to activate the catalyst, indicating that metallic gold nanoparticles are active during catalytic CO oxidation.

Ultrafast Dynamics of Gold-Based Nanocomposite Materials

Ultrafast electron dynamics are compared for small (2.5- to 4.0-nm average diameter) Au nanoparticles in two different surrounding environments. In one case, an aqueous solution contains Au nanoparticles (with average diameters of either 2.5 or 4.0 nm) embedded inside polyamidoamine (PAMAM) dendrimers. In the other case, hexanethiol-passivated (HT) Au nanoparticles, with an average diameter of 3.6 nm, are suspended in dichloromethane solution. Femtosecond two-color pump−probe spectroscopy is used to excite and probe the dynamics of the Au nanoparticles in the region of the surface plasmon resonance. The transient response is measured as a function of laser excitation fluence (J/cm2). For the metal−dendrimer nanocomposites, the transient response consists of a single-exponential decay that relaxes with a time constant of less than 1.2 ps and is due mainly to electron−phonon coupling. The relaxation time scale shows a weak dependence on excitation fluence but is essentially independent of the size of the Au...

Interactions of Small Molecules and Au Nanoparticles with Solubilized Single-Wall Carbon Nanotubes

Oxidatively end-cut single-wall carbon nanotube (SWNT) has been solubilized (sol-SWNT) in organic solvents by refluxing in aniline and purified by silica gel chromatography. NMR analysis of sol-SWNT indicates that the ratio of nanotube carbons to aniline sites is 360:1. A hydroxy-stilbene fluorophore (trans-4-nitro-4‘-hydroxy stilbene, 1) adsorbs onto sol-SWNT, but can also be demonstrably ester-coupled to the nanotube material, based on comparisons of absorbance spectra. In the combinations of 1 and sol-SWNT made by ester-coupling and adsorption, the fluorescences of hydroxy-stilbene and of sol-SWNT could be excited selectively and separately. When exciting (500 nm) the sol-SWNT in the presence of adsorbed hydroxy-stilbene, the nanotube's emission intensity decreases, but the energetics are unchanged, whereas ester-coupled hydroxy-stilbene diminishes both the sol-SWNT energy and intensity. When exciting either adsorbed (at 372 nm) or ester-coupled stilbene (at 347 nm), the stilbene emission intensities are substantially reduced, and a weak, photosensitized sol-SWNT emission could be seen. Energy transfer quenching of the stilbene by the nanotube structure produces some quantity of excited states that relax by emission. Mixed monolayer-protected Au clusters adsorb strongly onto both end-opened SWNT and sol-SWNT. Adsorption of MPCs with hydroxyl groupings in the mixed monolayer onto end-opened SWNTs occurred without change in shape of the nanotube bundles but could effect the rolling up of the more flexible sol-SWNTs. Amine-labeled MPCs were less aggressive adsorbers and the 15−20 nm nanotube bundles of sol-SWNT could be imaged, outlined with the adsorbed nanoparticles.

Hydrogenation of but-2-enal over supported Au/ZnO catalysts

The hydrogenation of but-2-enal over supported Au catalysts is discussed, together with a detailed characterisation study using X-ray diffraction, infrared spectroscopy and transmission electron microscopy. Au/ZnO catalysts are found to be selective for the formation of the unsaturated alcohol, but-2-en-1-ol rather than the saturated aldehyde, butanal. In general, the addition of thiophene is found to enhance the yield of the unsaturated alcohol. Detailed transmission electron microscopy and infrared spectroscopy studies show that thiophene modification of Au/ZnO catalysts does not affect the Au-particle size or morphology; rather, thiophene undergoes irreversible dissociative adsorption giving a surface in which the Au sites are electronically promoted by sulfur. It is observed that thiophene modification does not give any marked effect on catalyst performance for the catalysts that contain large Au-particles (10 nm) and, hence, it is considered that the sulfur promotion observed is associated with smaller Au nanoparticles. The highest but-2-en-1-ol selectivities (∽80%) are observed for 5 wt.% Au/ZnO catalysts reduced at 400°C prior to reaction. It is proposed that the origin of high selectivity is associated with large Au particles (10–20 nm in diameter) that are present in this catalyst.

Electron-phonon coupling dynamics in very small (between 2 and 8 nm diameter) Au nanoparticles

Ultrafast laser experiments were used to study electron-phonon coupling in Au nanoparticles in the 2.5 to 8 nm size range in aqueous solution. The electron-phonon coupling constants for these samples were found to be independent of the particle size. This is attributed to a weak interaction between the electron gas and the surface phonon modes in Au. Calculations were performed which show that the coupling between the hot electrons and the surface accounts for less than 10% of the total electron energy losses for these particles. Thus, bulk electron-phonon coupling dominates the relaxation of excited electrons in Au particles, for particles as small as several hundred atoms.