Synthesis of gold nanosheets at a liquid/liquid interface using an amphiphilic polyoxometallate/surfactant hybrid photocatalyst

Control of the morphology of gold nanoparticles has received considerable attention because the physical and chemical properties of gold depend significantly on its size and shape. A novel route for obtaining 2-D gold nanostructures has been developed in which chloroaurate ions (AuCl (4)(-)) are reduced at the 2-D interface between water and chloroform using an amphiphilic polyoxometallate (SiW (12)O (40)(4-))/surfactant (dimethyldioctadecylammonium; DODA) hybrid photocatalyst under UV irradiation at room temperature in air. This simple method can readily produce large single-crystalline gold nanosheets (lateral size, ca. 20 microm; thickness, ca. 150 nm).     

Three- and four-coordinate gold(I) complexes of 3,6-bis(diphenylphosphino)pyridazine: monomers,polymers, and a metallocryptand cage

The slightly yellow polymeric complexes [Au(2)Cl(2)(P(2)pz)(3)](n), 1 x 6CHCl(3), (P(2)pz is 3,6-bis(diphenylphosphino)pyridazine) and [[Au(2)(P(2)pz)(3)](PF(6))(2)](n), 2, are prepared by the stoichiometric reaction of AuCl(tht) (tht is tetrahydrothiophene) and P(2)pz in either dichloromethane or dichloromethane/methanol, respectively. Addition of 2 equiv of AuCl(tht) to a dichloromethane solution of 1 equiv of P(2)pz generates the simple (AuCl)(2)(P(2)pz) compound, 3. Compound 3 contains nearly linear P-Au-Cl units with intermolecular Au.Au separations of 3.570 A. Au(2)I(2)(P(2)pz)(3), 4, is prepared by reacting excess NaI with 2 in a dichloromethane/methanol mixture. Characterization of 1, 2, and 4 by X-ray crystallography confirms the 2:3 gold/ligand ratio of all three complexes. The coordination polymer 1 maintains a high degree of solvation in the solid-state with three chloroform adducts hydrogen-bonded to the chloride ligand on each gold atom. These chloroform molecules are sandwiched between the two-dimensional polymeric sheets of 1. The crystal structure of 4 reveals an empty, iodide-capped metallocryptand cage with the tetrahedrally distorted gold atoms and the nitrogen atoms on the pyridazine rings directed away from the center of the cavity. No metal ion encapsulation was observed for complex 4. Complex 2 forms one-dimensional arrays of [Au(2)(P(2)pz)(2)](2+) metallomacrocycles connected to each other by a third P(2)pz ligand. The electronic absorption spectra (CH(2)Cl(2)) of 1-4 show broad, nearly featureless absorption bands that tail into the visible with pi-pi bands at 296 nm and discernible shoulders at 314 nm for 2 and 334 nm for 3. Excitation into the low energy band of 2 produces only a modest emission in solution at 540 nm (lambda(ex) 468 nm) and 493 nm (lambda(ex) 403 nm). Under identical conditions, the P(2)pz ligand also emits at 540 and 493 nm.     

Composition-controlled synthesis of bimetallic gold-silver nanoparticles

This paper reports findings of an investigation of the synthesis of monolayer-capped binary gold-silver (AuAg) bimetallic nanoparticles that is aimed at understanding the control factors governing the formation of the bimetallic compositions. The synthesis of alkanethiolate-capped AuAg nanoparticles was carried out using two related synthetic protocols using aqueous sodium borohydride as a reducing agent. One involves a two-phase reduction of AuCl(4)(-), which is dissolved in organic solution, and Ag(+), which is dissolved in aqueous solution. The other protocol involves a two-phase reduction of AuCl(4)(-) and AgBr(2)(-), both of which are dissolved in the same organic solution. AuAg nanoparticles of 2-3 nm core sizes with different compositions in the range of 0-100% Au have been synthesized. The two synthetic routes were compared in terms of bimetallic composition and size properties. Our new findings have allowed us to establish the correlation between synthetic feeding of metals and metal compositions in the bimetallic nanoparticles, which have important implications to the exploration of gold-based bimetallic nanoparticles for constructing sensing and catalytic nanomaterials.     

Morphology of gold nanoparticles synthesized by filamentous cyanobacteria from gold(I)-thiosulfate and gold(III)--chloride complexes

Plectonema boryanum UTEX 485, a filamentous cyanobacterium, has been reacted with aqueous Au(S(2)O(3))(2)(3)(-) and AuCl(4)(-) solutions ( approximately 400-550 mg/L Au) at 25-100 degrees C for up to 1 month and at 200 degrees C for 1 day. The interaction of cyanobacteria with aqueous Au(S(2)O(3))(2)(3)(-) promoted the precipitation of cubic (100) gold nanoparticles (<10-25 nm) at membrane vesicles and admixed with gold sulfide within cells and encrusted on the cyanobacteria, whereas reaction with AuCl(4)(-) resulted in the precipitation of octahedral (111) gold platelets ( approximately 1-10 microm) in solutions and nanoparticles of gold (<10 nm) within bacterial cells. Functional groups imaged by negative ion TOF-SIMS on (111) faces of the octahedral platelets were predominantly Cl and CN, with smaller amounts of C(2)H and CNO.     

Platinum-catalyzed synthesis of water-soluble gold-platinum nanoparticles

The ability to control composition and size in the synthesis of bimetallic nanoparticles is important for the exploitation of the bimetallic catalytic properties. This paper reports findings of an investigation of a new approach to the synthesis of gold-platinum (AuPt) bimetallic nanoparticles in aqueous solution via one-phase reduction of AuCl(4-) and PtCl(4)(2-) using a combination of reducing and capping agents. Hydrogen served as a reducing agent for the reduction of Pt(II), whereas acrylate was used as a reducing agent for the reduction of Au(III). The latter reaction was found to be catalyzed by the formation of Pt as a result of the reduction of Pt(II). Acrylate also functioned as capping agent on the resulting nanocrystals. By controlling the feed ratios of AuCl(4-) and PtCl(4)(2-) and the relative concentrations of acrylate, an effective route for the preparation of AuPt nanoparticles with bimetallic compositions ranging from approximately 4 to 90% Au and particle sizes ranging from 2 to 8 nm has been demonstrated. The composition, size, and shell properties were characterized using transmission electron microscopy, direct current plasma-atomic emission spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. Implications of the results to the exploration of bifunctional catalysts are also briefly discussed.     

Gas-phase experiments on Au(III) photochemistry

Irradiation of AuCl(4)(-) and AuCl(2)(OH)(2)(-) in the gas-phase using ultraviolet light (220-415 nm) leads to their dissociation. Observed fragment ions for AuCl(4)(-) are AuCl(3)(-) and AuCl(2)(-) and for AuCl(2)(OH)(2)(-) are AuCl(2)(-) and AuClOH(-). All fragment channels correspond to photoreduction of the gold atom to either Au(II) or Au(I) depending on the number of neutral ligands lost. Fragment branching ratios of AuCl(4)(-) are observed to be highly energy dependent and can be explained by comparison of the experimental data to calculated threshold energies obtained using density functional theory. The main observed spectral features are attributed to ligand-to-metal charge transfer transitions. These results are discussed in the context of the molecular-level mechanisms of Au(III) photochemistry.     

Preparation of Hydrophobically Modified Poly(amidoamine) Dendrimer-Encapsulated Gold Nanoparticles in Organic Solvents

HAuCl(4) in aqueous solution was extracted to toluene or chloroform using a hydrophobically modified poly(amidoamine) dendrimer. Then, by reduction of Au(3+) ions with dimethylamineborane, gold nanoparticles in the size range of 2-4 nm were obtained in toluene or chloroform. It is suggested that gold nanoparticles are encapsulated by the dendrimer. Copyright 2000 Academic Press.     

Galvanic replacement reactions of active-metal nanoparticles

We present a systemic investigation of a galvanic replacement technique in which active-metal nanoparticles are used as sacrificial seeds. We found that different nanostructures can be controllably synthesized by varying the type of more noble-metal ions and liquid medium. Specifically, nano-heterostructures of noble metal (Ag, Au) or Cu nanocrystals on active-metal (Mg, Zn) cores were obtained by the reaction of active-metal nanoparticles with more noble-metal ions in ethanol; Ag nanocrystal arrays were produced by the reaction of active-metal nanoparticles with Ag(+) ions in water; spongy Au nanospheres were generated by the reaction of active-metal nanoparticles with AuCl(4)(-) ions in water; and SnO(2) nanoparticles were prepared when Sn(2+) were used as the oxidant ions. The key factors determining the product morphology are shown to be the reactivity of the liquid medium and the nature of the oxidant-reductant couple, whereas Mg and Zn nanoparticles played similar roles in achieving various nanostructures. When microsized Mg and Zn particles were used as seeds in similar reactions, the products were mainly noble-metal dendrites. The new approach proposed in this study expands the capability of the conventional nanoscale galvanic replacement method and provides new avenues to various structures, which are expected to have many potential applications in catalysis, optoelectronics, and biomedicine.     

Rapid room temperature synthesis of electrocatalytically active Au nanostructures

We describe a facile route for the one-pot room temperature synthesis of anisotropic Au nanostructures in aqueous solution in the absence of seeds or surfactants and their electrocatalytic activity. The Au nanostructures were synthesized using piperazine derivatives 1-(2-hydroxyethyl)piperazine and 1,4-Bis(2-hydroxyethyl)piperazine as reducing agents. The Au nanostructures were characterized by spectral, transmission electron microscopic (TEM), X-ray diffraction and electrochemical measurements. The absorption spectrum of colloidal nanoparticles displays two bands ~580 and ~930 nm, corresponding to the dipole and quadrupole plasmon resonance, respectively. TEM measurements show that the Au nanostructures have penta-twined polyhedral shape with an average size of 52 nm. X-ray and selected area electron diffraction patterns reveal the existence of face centered cubic nanocrystalline Au. The concentration of Au(III) controls the stability of the nanoparticles. The nanoparticles were immobilized on 3-D silicate network pre-assembled on a conducting support to examine their electrocatalytic activity. The nanoparticle-based electrochemical interface was characterized by spectral, voltammetric and impedance measurements. The nanoparticle shows high catalytic activity in the oxidation of NADH and reduction of oxygen. Unique inverted 'V' shape voltammogram was obtained for the oxidation of NADH at less positive potential. The nanoparticle-based interface favors two-step four-electron reduction of oxygen to water in neutral pH. Significant decrease in the overpotential for the oxidation of NADH and reduction of oxygen with respect to the polycrystalline Au electrode was observed. The electrocatalytic performance of the polyhedral nanoparticle is compared with the conventional citrate stabilized spherical nanoparticles.     

Microfluidic synthesis and catalytic application of PVP-stabilized, approximately 1 nm gold clusters

Small PVP-stabilized gold clusters were successfully prepared by the homogeneous mixing of continuous flows of aqueous AuCl 4 (-) and BH 4 (-) in a micromixer. Spectroscopic characterization revealed that microfluidic synthesis could yield monodisperse Au:PVP clusters with an average diameter of approximately 1 nm, which is smaller than clusters produced by conventional batch methods. These approximately 1 nm Au:PVP clusters exhibited higher catalytic activity for the aerobic oxidation of p-hydroxybenzyl alcohol than did Au:PVP clusters prepared by batch methods.     

Oxidation of highly unstable <4 nm diameter gold nanoparticles 850 mV negative of the bulk oxidation potential

Here we describe the oxidation of <4 nm diameter Au nanoparticles (NPs) attached to indium tin oxide-coated glass electrodes in Br(-) and Cl(-) solution. Borohydride reduction of AuCl(4)(-) in the presence of hexanethiol or trisodium citrate (15 min) led to Au NPs <4 nm in diameter. After electrochemical and ozone removal of the hexanthiolate ligands from the thiol-coated Au NPs, Au oxidation peaks appeared in the range 0-400 mV vs Ag/AgCl (1 M KCl), which is 850-450 mV negative of the bulk Au oxidation peak near 850 mV. The oxidation potential of citrate-coated Au NPs is in the 300-500 mV range and those of 4 and 12 nm diameter Au NPs in the 660-780 mV range. The large negative shift in potential agrees with theory for NPs in the 1-2 nm diameter range. The oxidation potential of Au in Cl(-) solution is positive of that in Br(-) solution, but the difference decreases dramatically as the NP size decreases, showing less dependence on the halide for smaller NPs.     

Facile synthesis of gold nanoparticles with narrow size distribution by using AuCl or AuBr as the precursor

Gold(I) halides, including AuCl and AuBr, were employed for the first time as precursors in the synthesis of Au nanoparticles. The synthesis was accomplished by dissolving Au(I) halides in chloroform in the presence of alkylamines, followed by decomposition at 60 degrees C. The relative low stability of the Au(I) halides and there derivatives eliminated the need for a reducing agent, which is usually required for Au(III)-based precursors to generate Au nanoparticles. Controlled growth of Au nanoparticles with a narrow size distribution was achieved when AuCl and oleylamine were used for the synthesis. FTIR and mass spectra revealed that a complex, [AuCl(oleylamine)], was formed through coordination between oleylamine and AuCl. Thermolysis of the complex in chloroform led to the formation of dioleylamine and Au nanoparticles. When oleylamine was replaced with octadecylamine, much larger nanoparticles were obtained due to the lower stability of [AuCl(octadecylamine)] complex relative to [AuCl(oleylamine)]. Au nanoparticles can also be prepared from AuBr through thermolysis of the [AuBr(oleylamine)] complex. Due to the oxidative etching effect caused by Br(-), the nanoparticles obtained from AuBr exhibited an aspect ratio of 1.28, in contrast to 1.0 for the particles made from AuCl. Compared to the existing methods for preparing Au nanoparticles through the reduction of Au(III) compounds, this new approach based on Au(I) halides offers great flexibility in terms of size control.     

Selective growth of Au nanoparticles on (111) facets of Cu2O microcrystals with an enhanced electrocatalytic property

The selective growth of Au nanoparticles on (111) facets of truncated octahedral and cuboctahedral Cu(2)O crystals has been achieved by exploiting the differences in the standard potential between AuCl(4)(-)/Au and Cu(2+)/Cu(2)O pairs and in surface energies between (111) and (100) planes. The density and size of Au nanoparticles can be controlled by tuning the concentration of the gold precursor. Truncated octahedral Cu(2)O-Au nanocomposites have a 10 times higher electrochemically catalytic activity toward H(2)O(2) reduction than do pure Cu(2)O crystals. The enhanced catalysis may be derived from the polarization of Au NPs at the interface, which makes Cu(2)O more active for H(2)O(2) reduction.     

Reversible synthesis of sub-10 nm spherical and icosahedral gold nanoparticles from a covalent Au(CN)2(-) precursor and recycling of cyanide to form ferric ferrocyanide for cell staining

A solution approach based on Au(CN)(2)(-) chemistry is reported for the formation of nanoparticles. The covalent character of the Au(CN)(2)(-) precursor was exploited in the formation of sub-10?nm nanospheres (≈2.4?nm) and highly monodisperse icosahedral Au nanoparticles (≈8?nm) at room temperature in a one-pot aqueous synthesis. The respective spherical and icosahedral Au morphologies can be controlled by either the absence or presence of the polymer polyvinylpyrrolidone (PVP). Using Au(CN)(2)(-) as a metal ion source, our findings suggest that the addition of citrate ions is necessary to enhance the particle formation rate as well as to generate a more homogeneous colloidal dispersion. Because of the presence of oxygen and the operation of a CN(-) etching process associated with Au(CN)(2)(-) complex formation, an interesting reversible formation-dissolution process was observed, which allowed us to repeatedly prepare spherical and icosahedral Au nanoparticles. Time-dependent TEM images and UV/Vis spectra were carefully acquired to study the reversibility of this formation-dissolution process. In view of the accompanying generation of toxic cyanide anions, we have developed a protocol to recycle cyanide in the presence of citrate ions through ferric ferrocyanide formation. After completion of particle formation, the residual solutions containing citrate ions and cyanide ions were processed to stain iron oxide nanoparticles endocytosized in cells. Additionally, the as-prepared 8?nm Au icosahedra could be isolated and grown to larger 57?nm-sized icosahedra using the seed-mediated growth approach.     

Interfacial bonding of gold nanoparticles on a H-terminated Si(100) substrate obtained by electro- and electroless deposition

Dome-shaped gold nanoparticles (with an average diameter of 10.5 nm) are grown on H-terminated Si(100) substrates by simple techniques involving electro- and electroless deposition from a 0.05 mM AuCl3 and 0.1 M NaClO4 solution. XPS depth profiling data (involving Au 4f core-level and valence band spectra) reveal for the first time the formation of gold silicide at the interface between the Au nanoparticles and Si substrate. UV-visible diffuse reflectance spectra indicate that both samples have surface plasmon resonance maxima at 558 nm, characteristic of an uniform distribution of Au nanoscale particles of sufficiently small size. Glancing-incidence XRD patterns clearly show that the deposited Au nanoparticles belong to the fcc phase, with the relative intensity of the (220) plane for Au nanoparticles obtained by electroless deposition found to be notably larger than that by electrodeposition.     

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.     

Synthesis and manipulation of high aspect ratio gold nanorods grown directly on surfaces

Here we describe the synthesis of Au nanorods directly on glass surfaces using seed-mediated deposition of Au from AuCl4- onto surface-attached 3-5 nm diameter Au nanoparticles (AuNPs) in the presence of cetyltrimethylammonium bromide (CTAB). The average length (200 nm to 1.2 microm) and aspect ratio (6-22) of the nanorods increases with increasing AuCl4- concentration. Short, low aspect ratio Au nanorods are manipulated with an atomic force microscopy (AFM) tip, while longer, high aspect ratio nanorods are bent and broken with the AFM tip.     

Gold nanoparticles grown on star-shaped block copolymer monolayers

We report on the growth of gold nanoparticles in polystyrene/poly(2-vinyl pyridine) (PS/P2VP) star-shaped block copolymer monolayers. These amphiphilic PS(n)P2VP(n) heteroarm star copolymers differ in molecular weight (149,000 and 529,000 Da) and the number of arms (9 and 28). Langmuir-Blodgett (LB) deposition was utilized to control the spatial arrangement of P2VP arms and their ability to reduce gold nanoparticles. The PS(n)P2VP(n) monolayer acted as a template for gold nanoparticle growth because of the monolayer's high micellar stability at the liquid-solid interface, uniform domain morphology, and ability to adsorb Au ions from the water subphase. UV-vis spectra and AFM and TEM images confirmed the formation of individual gold nanoparticles with an average size of 6 ± 1 nm in the P2VP-rich outer phase. This facile strategy is critical to the formation of ultrathin polymer-gold nanocomposite layers over large surface areas with confined, one-sided positioning of gold nanoparticles in an outer P2VP phase at polymer-silicon interfaces.     

An Os(II)--bisbipyridine--4-picolinic acid complex mediates the biocatalytic growth of au nanoparticles: optical detection of glucose and acetylcholine esterase inhibition

The complex Os(II)-bisbipyridine-4-picolinic acid, [Os(bpy)(2)PyCO(2)H](2+) (1), mediates the biocatalyzed growth of Au nanoparticles, Au NPs, and enables the spectroscopic assay of biocatalyzed transformations and enzyme inhibition by following the Au NP plasmon absorbance. In one system, [Os(bpy)(2)PyCO(2)H](2+) mediates the biocatalyzed oxidation of glucose and the growth of Au NPs in the presence of glucose oxidase, GOx, AuCl(4) (-), citrate and Au NP seeds. The mechanism of the Au NPs growth involves the oxidation of the [Os(bpy)(2)PyCO(2)H](2+) complex by AuCl(4) (-) to form [Os(bpy)(2)PyCO(2)H](3+) and Au(I). The [Os(bpy)(2)PyCO(2)H](3+) complex mediates the GOx biocatalyzed oxidation of glucose and the regeneration of the mediator 1. Citrate reduces Au(I) and enlarges the Au seeds by the catalytic deposition of gold on the Au NP seeds. In the second system, the enzyme acetylcholine esterase, AChE, is assayed by the catalytic growth of the Au NPs. The hydrolysis of acetylcholine (2) by AChE to choline is followed by the [Os(bpy)(2)PyCO(2)H](3+) mediated oxidation of choline to betaine and the concomitant growth of the Au NPs. The mediated growth of the Au NPs is inhibited by 1,5-bis(4-allyldimethylammonium-phenyl)pentane-3-one dibromide (3). A competitive inhibition process was demonstrated (K(M)=0.13 mM, K(I)=2.6 microM) by following the growth of the Au NPs.     

Ion-exchange properties of imidazolium-grafted SBA-15 toward AuCl4(-) anions and their conversion into supported gold nanoparticles

Imidazolium groups were successfully prepared and grafted on the surface of SBA-15 mesoporous silica. The ion-exchange properties of the functionalized porous solid (SBA-15/R(+)Cl(-)) toward AuCl(4)(-) anions were evaluated through an ion-exchange isotherm. The calculated values of the equilibrium constant (log β = 4.47) and the effective ion-exchange capacity (t(Q) = 0.79 mmol g(-1)) indicate that the AuCl(4)(-) species can be loaded and strongly retained on the functionalized surface as counterions of the imidazolium groups. Subsequently, solids containing different amounts of AuCl(4)(-) ions were submitted to a chemical reduction process with NaBH(4), converting the anionic gold species into supported gold nanoparticles. The plasmon resonance bands, the X-ray diffraction patterns, and transmission electron microscopy images of the supported gold nanoparticles before and after thermal treatment at 973 K indicate that the metal nanostructures are highly dispersed and stabilized by the host environment.