Formation Of Colloidal Gold Research Articles

Potentiostatic Electrodeposition of Au-Sn Alloys from a Non-Cyanide Bath for Soldering: Influence of Reagents Concentrations

This work presents an extensive study of a non-cyanide Au-Sn bath to obtain potentiostatically deposited Au-Sn alloys. In addition, Au-Sn bath speciation was studied through a program for chemical equilibrium simulation (CHEAQS Pro) and the electrochemical mechanisms were investigated using Cyclic Voltammetry (CV). Also, Rutherford Backscattering Spectroscopy (RBS) was used to determine the deposits characteristics and composition. Some recipes using different concentrations of ammonium citrate, SnCl2 and Na2SO3, and L-ascorbic acid presented the formation of colloidal gold during the electrodeposition processes. The incorporation of tin in the deposits obtained varied from 2 to 16 atomic % (at%).

Colloidal gold in sulphur and citrate-bearing hydrothermal fluids: An experimental study

The stability of colloidal gold (gold nanoparticles) in sulphur-bearing and citrate-bearing solutions was investigated at temperatures up to 225 °C using Synchrotron X-ray Near-edge Spectroscopy (XANES), and up to 350 °C with a visual check of colour changes. The citrate-based colloidal gold solutions, with or without colloidal silica in the solution, are only stable up to 225 °C. In contrast, the gold colloids in Na2S solutions are not stable upon heating to 150 °C, but stable up to 300 °C when 0.5–1.5 wt% of colloidal silica is present in the solution. The colloidal gold particles started to aggregate and deposit from the solution with the aggregation and growth of silica particles at 350 °C. The concentrations of gold as colloids in the solutions are up to 0.5 mmol (∼95 ppm), more than three orders of magnitude higher than gold solubility as aqueous complexes under the same condition calculated based on available thermodynamic data. These results provide the first evidence that high concentrations of colloidal gold are stable in sulphur-bearing fluids at elevated temperatures (∼300 °C). This implies that the formation of gold colloids is an effective way to concentrate gold in hydrothermal sulphur-bearing fluids to form high-grade gold ores.

Boiling-induced formation of colloidal gold in black smoker hydrothermal fluids

Gold colloids occur in black smoker fluids from the Niua South hydrothermal vent field, Lau Basin (South Pacific Ocean), confirming the long-standing hypothesis that gold may undergo colloidal transport in hydrothermal fluids. Six black smoker vents, varying in temperature from 250 °C to 325 °C, were sampled; the 325 °C vent was boiling at the time of sampling and the 250 °C fluids were diffusely venting. Native gold particles ranging from <50 nm to 2 μm were identified in 4 of the fluid samples and were also observed to precipitate on the sampler during collection from the boiling vent. Total gold concentrations (dissolved and particulate) in the fluid samples range from 1.6 to 5.4 nM in the high-temperature, focused flow vents. Although the gold concentrations in the focused flow fluids are relatively high, they are lower than potential solubilities prior to boiling and indicate that precipitation was boiling induced, with sulfide lost upon boiling to exsolution and metal sulfide formation. Gold concentrations reach 26.7 nM in the 250 °C diffuse flow sample, and abundant native gold particles were also found in the fluids and associated sulfide chimney and are interpreted to be a product of colloid accumulation and growth following initial precipitation upon boiling. These results indicate that colloid-driven precipitation as a result of boiling, the persistence of colloids after boiling, and the accumulation of colloids in diffuse flow fluids are important mechanisms for the enrichment of gold in seafloor hydrothermal systems.

Bis(diphenylphosphino)methane ligated gold cluster cations: Synthesis and gas-phase unimolecular reactivity

Electrospray ionization mass spectrometry (ESI-MS) and UV–vis spectroscopy were used to monitor the condensed phase synthesis of gold cluster cations via NaBH4 reduction of Ph3PAuCl in the presence of Ph2P-(CH2)-PPh2 (dppm, L1) a capping ligand. ESI-MS highlights that upon mixing of Ph3PAuCl and dppm a range of ligated Au(I) cations are present, including [Au2L12]2+, [Au(PPh3)2]+, [Au2L13]2+, [AuL12]+ and [Au2ClL12]+. These cationic precursors to reduction can be controlled by the ratios of the two reactants. For a ratio of 1:2 of Ph3PAuCl and dppm, [AuL12]+ is a major ion, while [Au(PPh3)2]+, which has previously been postulated to be a key precursor to gold colloid formation (J.M. Pettibone, J.W. Hudgens, Small 8 (2012) 715) is essentially absent. Upon NaBH4 reduction of a solution of this stoichiometry (Ph3PAuCl:dppm, 1:2), the following types of gold nanocluster ions are observed via ESI-MS: [Au5L13(CH(PPh2)2)]2+, [Au9L14]3+, [Au9L15]3+, [Au10L14]2+, [Au11L15]3+, [Au11L16]3+, [Au13L16]3+ and [Au14L16(CH(PPh2)2)]3+. The gas phase unimolecular chemistry of these clusters and their fragment ions were examined in a LTQ FT Hybrid Linear Ion Trap (LIT) Mass Spectrometer. Low-energy collision induced dissociation (CID) produced a range of novel gas-phase clusters of the type, [AuxL1y]z+ (x=2, 3, 6–13; y=1–6 and z=1–3) and [AuxL1y(CH(PPh2)2)]z+ (x=5, 14; y=2, 5; z=2, 3), via ligand loss and core fission fragmentation pathways. In addition fragmentation channels due to CP bond activation were also observed. Electron capture dissociation (ECD) on the Au cluster cations resulted in either one or a combination of three fragmentation channels: (i) charge reduction, (ii) charge reduction accompanied by ligand loss and/or (iii) charge reduction accompanied by CP bond activation.

The room temperature formation of gold nanoparticles from the reaction of cyclohexanone and auric acid; a transition from dendritic particles to compact shapes and nanoplates

A new straightforward method for the synthesis of gold nanoparticles from addition of cyclohexanone to aqueous solutions of auric acid at room temperature is presented. By understanding this process we have discovered a new organic chemistry transformation reaction for converting cyclic ketones to α-chloro ketones and a mechanism for the nanoparticle formation. Contrary to conventional gold nanoparticle syntheses, the reaction “self-initiates” at room temperature and forms an increasingly red solution over ≈60 minutes. By studying the gold colloid's formation using transmission electron microscopy it was observed that large dendritic (63 ± 21 nm diameter) structures made of clustered particles (6 ± 1 nm) were initially formed. These dendritic particles then compacted into an array of denser shapes that slowly increase in size until the reaction is complete. The most prominent shapes observed were spheres (43 ± 7 nm); other shapes included dodecahedra (39 ± 10 nm) triangular (≈50 nm in height) and hexagonal (≈70 nm wide) nanoplates. The solution was stable to precipitation for over 3 months. During this period the nanoplate structures substantially increased in size (triangular ≈ 250 nm, hexagonal ≈ 320 nm) whereas other structures showed no further growth. X-ray diffraction studies demonstrated that the gold nanoparticles were crystalline. The formation of the 2-chlorocyclohexanone by-product was observed in solution phase 1H & 13C NMR, gas phase chromatography and IR spectroscopy. A mechanism is presented to account for this by-product and the reduction of auric acid to gold.

Microbial synthesis of core/shell gold/palladium nanoparticles for applications in green chemistry

We report a novel biochemical method based on the sacrificial hydrogen strategy to synthesize bimetallic gold (Au)–palladium (Pd) nanoparticles (NPs) with a core/shell configuration. The ability of Escherichia coli cells supplied with H2 as electron donor to rapidly precipitate Pd(II) ions from solution is used to promote the reduction of soluble Au(III). Pre-coating cells with Pd(0) (bioPd) dramatically accelerated Au(III) reduction, with the Au(III) reduction rate being dependent upon the initial Pd loading by mass on the cells. Following Au(III) addition, the bioPd–Au(III) mixture rapidly turned purple, indicating the formation of colloidal gold. Mapping of bio-NPs by energy dispersive X-ray microanalysis suggested Au-dense core regions and peripheral Pd but only Au was detected by X-ray diffraction (XRD) analysis. However, surface analysis of cleaned NPs by cyclic voltammetry revealed large Pd surface sites, suggesting, since XRD shows no crystalline Pd component, that layers of Pd atoms surround Au NPs. Characterization of the bimetallic particles using X-ray absorption spectroscopy confirmed the existence of Au-rich core and Pd-rich shell type bimetallic biogenic NPs. These showed comparable catalytic activity to chemical counterparts with respect to the oxidation of benzyl alcohol, in air, and at a low temperature (90°C).

Time-dependent, protein-directed growth of gold nanoparticles within a single crystal of lysozyme

Gold nanoparticles are useful in biomedical applications due to their distinct optical properties and high chemical stability. Reports of the biogenic formation of gold colloids from gold complexes has also led to an increased level of interest in the biomineralization of gold. However, the mechanism responsible for biomolecule-directed gold nanoparticle formation remains unclear due to the lack of structural information about biological systems and the fast kinetics of biomimetic chemical systems in solution. Here we show that intact single crystals of lysozyme can be used to study the time-dependent, protein-directed growth of gold nanoparticles. The protein crystals slow down the growth of the gold nanoparticles, allowing detailed kinetic studies to be carried out, and permit a three-dimensional structural characterization that would be difficult to achieve in solution. Furthermore, we show that additional chemical species can be used to fine-tune the growth rate of the gold nanoparticles.

Nano-gold biosynthesis by silica-encapsulated micro-algae: a “living” bio-hybrid material

Klebsormidium flaccidum algal cells exhibiting the ability to form gold nanoparticles intra-cellularly in suspension were encapsulated within silica gels. Optical and electronic microscopy indicate that entrapped cells maintain their ability to reduce gold salts. A difference in the kinetics of gold colloid formation within silica in the absence or presence of cells could be followed by UV-visible absorption spectroscopy, confirming the bio-mediated nature of the reduction process. Study of the photosynthetic activity of the algae showed that the encapsulation process protects the cells from lethal effects arising from gold toxicity. Moreover, the first in situ imaging of entrapped cells using Raman spectroscopy allowed the investigation of the influence of the gold colloids on the photosynthetic system of the algae, in particular through modification of chlorophyll fluorescence and carotenoid signals. Such a coupling of sol–gel encapsulation and Raman imaging should allow the future development of novel photosynthesis-based cellular biosensors.

An Electrochemical Model for Gold Colloid Formation via Citrate Reduction

The mechanism of nanoparticle formation during the reduction of HAuCl4 with sodium citrate under reflux has been studied. We have found that the redox potential in the nucleating colloid remains very positive, around +0.75V vs. Ag/AgCl, until the reaction is almost complete. It then shifts suddenly to around +0.1V vs. Ag/AgCl at the end of the reaction, which is close to the final open-circuit potential in the sol. AuCl4 - continues to be reduced slowly over the course of the reaction, but is suddenly depleted as the potential shifts near the end of the reaction. The change in potential is correlated to the peptization of aggregates to form the final monodisperse gold particles. There is also a blue-shift of the surface plasmon band associated with charging of the gold particles.

Effect of a new hydrophobically modified polyampholyte on the formation of inverse microemulsions and the preparation of gold nanoparticles

A new regular polyampholyte, namely poly-( N,N-dially- N,N-dimethylammonium-alt- N-octyl-maleamic carboxylate), was synthesized by alternating free radical copolymerization. The influence of the added polymer on the range of the inverse micellar region (L 2 phase) of a SDS-based system was investigated. The phase behavior as well as conductivity measurements indicate that the polymer, which forms hydrophobic microdomains, is located more in the water core of the microemulsion droplets rather than at the interface of the surfactant film. The polyampholyte proved to be an efficient reducing and stabilizing agent for the formation of gold colloids. The process of nanoparticle formation was investigated in the absence of any other reducing agent, in water as well as in the microemulsion template phase. In both cases, nanoscalic gold particles can be synthesized, while the adsorption of the polymer on the particle surface prevents their aggregation due to electrosteric stabilization.

Speciation Analysis of Au(I) Electroplating Baths Containing Sulfite and Thiosulfate

An analysis of the speciation of gold plating solutions containing sulfite and thiosulfate as complexing agents has been performed. In the case of gold sulfite baths, it was found that the observed stability and electrochemical behavior were inconsistent with the commonly assumed stability constants of for the species. The data are, however, consistent with a much larger stability constant of . In gold plating baths containing sulfite and thiosulfate ligands, the speciation model predicts that the dominant species at neutral or basic are mixed complexes of the form and . The thiosulfate complex is only dominant under acidic conditions. It is shown that the electrodeposition characteristics reported in many studies can be interpreted in terms of the existence of such mixed ligand complexes. Finally, the effect of the stability of various gold plating solutions on colloidal gold formation has been explored. For free-standing solutions, decomposition is more likely to occur via the disproportionation of Au(I) species such as AuOH, , and rather than free aurous ions. The stability of gold baths during electrodeposition is mainly influenced by the formation of dithionite ions which reduce Au(I) complexes to form colloidal gold.

Chemical Colloids versus Biological Colloids: A Comparative Study for the Elucidation of the Mechanism of Protein Fiber Formation

Fiber formation from murine serum amyloid A1 (SAA) was compared to the linear aggregation and fiber formation of colloidal gold particles. Here we report the similarities of these processes. Upon incubation with acetic acid, SAA misfolds and adopts a new conformation, which we termed saa. saa apparently is less soluble than SAA in aqueous solution; it aggregates and forms nucleation units and then fibers. The fibers appear as a string of the nucleation units. Additionally, an external electric field promotes saa fiber formation. These properties of saa are reminiscent of colloidal gold formation from gold ions and one-dimensional aggregation of the gold colloids. Colloidal gold particles were also found to be capable of aggregating one-dimensionally under an electric field or in the presence of polylysine. These gold fibers resembled in structure that of saa fibers. In summary, protein aggregation and formation of fibers appear to follow the generalized principles derived in colloidal science for the aggregation of atoms and molecules, including polymers such as polypeptides. The analysis of colloidal gold formation and of one-dimensional aggregation provides a simple model system for the elucidation of some aspects of protein fiber formation.

Size Control and Spectroscopic Characterization of Monolayer Protected Gold Nanoparticles Obtained by Laser Ablation in Liquids

ABSTRACTIn this paper we present a study on the formation of gold colloids by laser ablation of a gold metal target in alkanes and thiol-alkane solutions. The results show a decrease of the gold particles' size up to 2 nm when thiol molecules are present in the liquid environment. In summary, we observed that laser ablation of gold targets in thiol-alkane solutions leads to the formation of stable gold clusters with size smaller than those obtained in the corresponding pure alkane. This result is a consequence of the competition between the aggregation of gold species in the plume (which allows a gold embryo to be formed and to grow) and the tendency of the dispersed thiol molecules to bond at each embryo surface stopping their growth.

Formation of isolated and clustered Au nanoparticles in the presence of polyelectrolyte molecules using a flow-through Si chip reactor

The formation of colloidal gold at room temperature was studied in a micro-reaction system. Ascorbic acid and Fe(II) were used as reducing agents. Besides citric acid, sodium metasilicate and poly(vinyl alcohol) were added as effectors for the formation of nanoparticles. The experimental set-up was based on a 2 + 1 + 1 static micro-mixer in order to test the successive addition of reaction components in a micro-continuous flow process. The residence times for a single mixing step in the chip device were chosen in the range between about 1 second and 1 minute. As a result, single particles of different sizes, simple particle aggregates, core–shell particles as well as complex aggregates and hexagonal nanocrystallites were obtained under different reaction conditions. The reaction mixtures were characterized by UV-Vis spectroscopy as well as by differential centrifugal sedimentation (DCS) and scanning electron microscopy (SEM) reflecting the variety of colloidal gold formed in the microreactor.

Chemical and Electrochemical Synthesis of Polyaniline/Gold Composites

The in situ synthesis of micrometer and nanometer gold particles in polyaniline (PANI/Au composite) using chemical or electrochemical methods, has been compared. The direct chemical synthesis of PANI/Au is initiated via the spontaneous oxidation of aniline by Gold colloid formation and subsequent reaction with PANI is monitored by in situ UV/vis spectroscopy. The emergent polymer nucleates on the gold as the PANI chain length increases, encapsulates the metal, and precipitates as its solubility limit is exceeded. Scanning electron microscopy images of these samples show relatively constant 1 μm diam gold particles. Electrochemical PANI/Au synthesis is initiated by reduction into an a priori electrochemically deposited PANI film. This method also produces a nearly uniform dispersion of Au particles but with significantly smaller 150-300 nm particles. The results indicate that electrochemical methods are more suitable for controlling particle dimensions. A minimal decrease in conductance is observed for the chemically formed PANI/Au when compared to PANI samples without the gold. A significant decrease in conductance is observed for the electrochemically formed composite. The large decrease in conductance is related to the decrease in proton doping and a greater number of oxidized units in the polymer upon electrochemical uptake and reduction of © 2004 The Electrochemical Society. All rights reserved.

Synthesis of gold colloids by laser ablation in thiol-alkane solutions

In this paper, we present a study on the formation of gold colloids by laser ablation of a gold metal target in alkanes and thiol-alkane solutions. The results show a decrease of the gold particles’ size up to 2 nm when thiol molecules are present in the liquid environment. The observation of a blue-shift of the surface plasmon resonance is discussed together with transmission electron microscopy analyses accounting the cluster size decrease and the stabilization of the obtained suspensions.

Chemical Synthesis of a Polyaniline/Gold Composite Using Tetrachloroaurate

The chemical synthesis of polyaniline (PANI) is explored using tetrachloroaurate, AuCl4-. These studies provide a simple method for the oxidation of aniline by AuCl4- and simultaneous formation of bulk quantities of a PANI/Au composite. In situ UV/vis spectroscopy indicates that the rate of formation of gold colloids and intermediate (short-chain) PANI species is rapid in comparison to that of the long-chain PANI. Longer PANI chains are produced at a slower rate, at the expense of short-chain intermediate species. The gold particles act as nucleation sites for the oxidative formation of PANI, encapsulating the metal in the form of a polymer/metal composite. Results from the elemental analysis and the FTIR spectra of the composite material are consistent with PANI produced using only ammonium persulfate as the oxidant. In addition, XPS, optical microscopy, and TEM diffraction show that the gold particles are polycrystalline with relatively constant diameter (0.8−1 μm). Finally, the PANI/Au/HBF4 conductance does not change significantly with the introduction of Au particles, relative to that of PANI/HBF4 without Au particles. These studies provide a new method for growth of PANI/metal composites where the presence of the metal in the polymer does not adversely affect the electronic structure.

Laser ablation of gold in chloroform solutions of cetyltrimethylammoniumbromide

Laser ablation of gold was studied in chloroform solutions of cetyltrimethylammoniumbromide (CTAB). Instead of the formation of gold colloids we observed the formation of cetyltrimethylammonium tetrabromo aurate and possibly cetyltrimethylammonium dibromo aurate. We show that this is due to complexation of gold ions that are formed in the initial stages of the ablation process.

The reactivity of gold atoms generated by the photolysis of gold azide complexes in acetonitrile

The photolysis of gold azide complexes in CH 3CN leads to the formation of colloidal gold and [Au I(CN) 2] −. The gold atoms which are initially formed do not only agglomerate to colloidal particles but also react with the solvent. It is suggested that CH 3CN adds oxidatively to atomic gold and [Au I(CN) 2] − is generated in subsequent reactions.

Reduction, aggregation and physicochemical properties of silver nanoparticles in propan-2-ol:cyclohexane mixtures induced by a high energy electron beam

Radiolytic reduction of silver and gold ions and subsequent formation of their aggregates have been studied in propan-2-ol:cyclohexane mixture using pulse radiolysis technique. The silver sol, produced on irradiation of Ag + solution with a train of electron pulses, has been characterized using XRD and TEM. The size of the particles has been found to be in the range of 30–50 nm. The silver sol emit light with a maximum at 340 nm on irradiation with a high energy electron beam. The intensity of emission has been found to decrease with decrease in concentration of Ag particles. Formation of colloidal gold has also been observed on irradiation of NaAuCl 4 solution in propan-2-ol:cyclohexane by train of electron pulses. The particles so formed are oxidized on exposure to air. No light emission has been observed from Au sol.