Aqueous HAuCl4 Research Articles

Electrochemical Sensor Constructed Using a Carbon Paste Electrode Modified with Mesoporous Silica Encapsulating PANI Chains Decorated with GNPs for Detection of Ascorbic Acid

In this work, an electrochemical sensor constructed using a carbon paste electrode (CPE), modified with mesoporous silica encapsulating polyaniline (PANI) decorated with gold nanoparticles (GNPs) for the detection of ascorbic acid (AA), was presented. First of all, PANI latex was synthesized by emulsion polymerization. Then, the acid-catalyzed sol–gel solutions of tetraethyl silicate (TEOS) was prepared in the presence of d-glucose, followed by encapsulation of previously prepared PANI latex into as-prepared sol-gel solution. Upon formation of silica frameworks, electroactive composite powder was obtained. Subsequently, the as-prepared composite powder was subjected to Soxhlet extraction to remove d-glucose and the final product was denoted as PM silica. On the other hand, PGM silica was obtained by immersing PM silica in 5mL of 0.10mM HAuCl4 solution for 6h, which reduced the gold nanoparticles (GNPs) onto the surface of PANI chains located inside the nanochannels of wormhole-like mesoporous silica framework. The redox reactions that mediated between EB form of PANI and aqueous HAuCl4 solution were monitored and confirmed by FTIR spectra. The appearance of GNPs inside the PGM silica was characterized by TEM, XPS, XRD, and BET analyses. PGM silica was found to reveal higher redox capability as compared to that of PM silica based on the electrochemical CV studies.It should be noted that PGM silica-modified CPE was found to exhibit high sensitivity, a relatively low limit of detection, excellent sensing repeatability and a broader linear dynamic range for AA detection than those of bare CPE or PM silica-modified CPE. Moreover, A electrochemical sensor constructed from PGM silica-modified CPE was found to reveal better selectivity for a tertiary mixture of AA/DA/UA as compared to that of PM silica-modified CPE based on a series of studies of differential pulse voltammograms. In this work, the fabricated sensor showed reliable data in detecting AA in the tested commercially available products.

Electrostatic Assembly of Functional and Macromolecular Ferricinium Chloride-Stabilized Gold Nanoparticles.

Substituted ferrocenes with various stereoelectronic effects including a ferrocene-terminated dendrimer in ether reduce aqueous HAuCl4 to gold nanoparticles (AuNPs) by interfacial electron transfer. The dependence on the stirring speed plays a crucial role, and the stereoelectronic influences on the reaction rates are dramatic. With a ferrocene-containing polymer, the reaction is conducted using an homogeneous THF/water medium, also forming AuNPs. Fully stable functional, dendritic and polymeric ferricinium chloride-stabilized AuNPs are obtained with core sizes between 13 and 35 nm, an optimal size range for potential biomedical applications. Finally the ferricinium coating of the Au nanoparticles is replaced by a more electron-rich ferricinium derivative by exergonic redox reaction with the corresponding ferrocene derivative.

Dissolution Kinetics of Oxidative Etching of Cubic and Icosahedral Platinum Nanoparticles Revealed by in Situ Liquid Transmission Electron Microscopy.

Dissolution due to atom-level etching is a major factor for the degradation of Pt-based electrocatalysts used in low-temperature polymer electrolyte membrane fuel cells. Selective surface etching is also used to precisely control shapes of nanoparticles. Dissolution kinetics of faceted metal nanoparticles in solution however is poorly understood despite considerable progress in understanding etching of two-dimensional surfaces. We report here the application of in situ liquid transmission electron microscopy for quantitative analysis of oxidative etching of cubic and icosahedral Pt nanoparticles. The experiment was carried out using a liquid flow cell containing aqueous HAuCl4 solution. The data show that oxidative etching of these faceted nanocrystals depends on the location of atoms on the surface, which evolves with time. A quantitative kinetic model was developed to account for the mass lost in electrolyte solutions over time, showing the dissolutions followed the power law relationship for Pt nanocrystals of different shapes. Dissolution coefficients of different surface sites were obtained based on the models developed in this study.

A General Approach to the Synthesis of M@Au/Ag (M = Au, Pd, and Pt) Nanorattles with Ultrathin Shells Less Than 2.5 nm Thick

This article reports a general approach to the facile synthesis of M@Au/Ag (M = Au, Pd, and Pt) nanorattles. The synthesis involves three major steps: (i) formation of M@Ag nanocubes via seed‐mediated growth, (ii) conformal deposition of ultrathin Au shells on the M@Ag nanocubes by reducing HAuCl4 with ascorbic acid under an alkaline condition, and (iii) formation of M@Au/Ag nanorattles through a galvanic replacement reaction between the Ag shells and aqueous HAuCl4. The deposition of an ultrathin Au shell prior to the galvanic replacement reaction is crucial to a successful synthesis as this shell can enhance the mechanical strength of the resultant nanorattles by preventing the Au/Ag alloy shell from fragmentation during the galvanic replacement process. The localized surface plasmon resonance peak of the nanorattles is determined by the thickness of the Au/Ag alloy shell. For Au@Au/Ag nanorattles with an edge length of 15 nm, they could be tuned to have near‐infrared absorption and excellent photothermal conversion capability for their use as transducers in the effective destruction of cancer cells through laser irradiation.

Light-Induced Polarization-Directed Growth of Optically Printed Gold Nanoparticles.

Optical printing has been proved a versatile and simple method to fabricate arbitrary arrays of colloidal nanoparticles (NPs) on substrates. Here, we show that is also a powerful tool for studying chemical reactions at the single NP level. We demonstrate that 60 nm gold NPs immobilized by optical printing can be used as seeds to obtain larger NPs by plasmon-assisted reduction of aqueous HAuCl4. The final size of each NP is simply controlled by the irradiation time. Moreover, we show conditions for which the growth occurs preferentially in the direction of light polarization, enabling the in situ anisotropic reshaping of the NPs in predetermined orientations.

A Simple Method for the Size Controlled Synthesis of Stable Oligomeric Clusters of Gold Nanoparticles under Ambient Conditions

Reducing dilute aqueous HAuCl4 with sodium thiocyanate (NaSCN) under alkaline conditions produces 2 to 3 nm diameter nanoparticles. Stable grape-like oligomeric clusters of these yellow nanoparticles of narrow size distribution are synthesized under ambient conditions via two methods. The delay-time method controls the number of subunits in the oligoclusters by varying the time between the addition of HAuCl4 to alkaline solution and the subsequent addition of reducing agent, NaSCN. The yellow oligoclusters produced range in size from ~3 to ~25 nm. This size range can be further extended by an add-on method utilizing hydroxylated gold chloride (Na+[Au(OH4-x)Clx]-) to auto-catalytically increase the number of subunits in the as-synthesized oligocluster nanoparticles, providing a total range of 3 nm to 70 nm. The crude oligocluster preparations display narrow size distributions and do not require further fractionation for most purposes. The oligoclusters formed can be concentrated >300 fold without aggregation and the crude reaction mixtures remain stable for weeks without further processing. Because these oligomeric clusters can be concentrated before derivatization they allow expensive derivatizing agents to be used economically. In addition, we present two models by which predictions of particle size can be made with great accuracy.

A Simple Method for the Size Controlled Synthesis of Stable Oligomeric Clusters of Gold Nanoparticles under Ambient Conditions.

Reducing dilute aqueous HAuCl4 with sodium thiocyanate (NaSCN) under alkaline conditions produces 2 to 3 nm diameter nanoparticles. Stable grape-like oligomeric clusters of these yellow nanoparticles of narrow size distribution are synthesized under ambient conditions via two methods. The delay-time method controls the number of subunits in the oligoclusters by varying the time between the addition of HAuCl4 to alkaline solution and the subsequent addition of reducing agent, NaSCN. The yellow oligoclusters produced range in size from ~3 to ~25 nm. This size range can be further extended by an add-on method utilizing hydroxylated gold chloride (Na+[Au(OH4-x)Clx]-) to auto-catalytically increase the number of subunits in the as-synthesized oligocluster nanoparticles, providing a total range of 3 nm to 70 nm. The crude oligocluster preparations display narrow size distributions and do not require further fractionation for most purposes. The oligoclusters formed can be concentrated >300 fold without aggregation and the crude reaction mixtures remain stable for weeks without further processing. Because these oligomeric clusters can be concentrated before derivatization they allow expensive derivatizing agents to be used economically. In addition, we present two models by which predictions of particle size can be made with great accuracy.

Ag@Au Concave Cuboctahedra: A Unique Probe for Monitoring Au-Catalyzed Reduction and Oxidation Reactions by Surface-Enhanced Raman Spectroscopy

We report a facile synthesis of Ag@Au concave cuboctahedra by titrating aqueous HAuCl4 into a suspension of Ag cuboctahedra in the presence of ascorbic acid (AA), NaOH, and poly(vinylpyrrolidone) (PVP) at room temperature. Initially, the Au atoms derived from the reduction of Au(3+) by AA are conformally deposited on the entire surface of a Ag cuboctahedron. Upon the formation of a complete Au shell, however, the subsequently formed Au atoms are preferentially deposited onto the Au{100} facets, resulting in the formation of a Ag@Au cuboctahedron with concave structures at the sites of {111} facets. The concave cuboctahedra embrace excellent SERS activity that is more than 70-fold stronger than that of the original Ag cuboctahedra at an excitation wavelength of 785 nm. The concave cuboctahedra also exhibit remarkable stability in the presence of an oxidant such as H2O2 because of the protection by a complete Au shell. These two unique attributes enable in situ SERS monitoring of the reduction of 4-nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP) by NaBH4 through a 4,4'-dimercaptoazobenzene (trans-DMAB) intermediate and the subsequent oxidation of 4-ATP back to trans-DMAB upon the introduction of H2O2.

表面修飾シリカゲルにおける重金属イオンの分配・放出過程の解析

Distribution and release processes of heavy-metal ions between spherical surface-modified silica gel microparticles and water were analyzed by microcapillary manipulation and absorption microspectroscopy. Silica gel modified with triamine (QuadraSil-TA, Q-TA) or thiol groups (QuadraSil-MP, Q-MP) was used as a surface-modified silica gel. A single microparticle was injected into an aqueous HAuCl4 or CuSO4 solution containing 0.01 M KCl (pH = 6), and the distribution rate of the metal ion from water into the microparticle was measured. The rate of release of Au(III) or Cu(II) from Q-TA into water (pH = 6) was much smaller than that of the distribution of the metal. The microparticle radius dependence of the absorbance of the metal in the microparticle and distribution of the metal by cross-sectional SEM-EDS analysis at the distribution equilibrium indicate that Au(III) and Cu(II) distribute into the microparticle interior for Q-TA, while Au(III) distributes at the outer layer in the spherical microparticle for Q-MP. The distribution processes were analyzed as the intraparticle diffusion, and the diffusion coefficient (Dp) observed in the present systems was compared with that calculated from the theoretical pore and surface diffusion model. We consider that the rate-determining step of the distribution of Au(III) in the Q-TA (pore diameter = 5.9 nm) system is the intraparticle diffusion of AuCl4−. On the other hand, the distribution of Cu(II) into Q-TA was much slower than the intraparticle diffusion.

Seeding of Au on CdSe/CdS nanoplates using Langmuir–Blodgett technique

Oleyl amine capped CdSe/CdS nanoplates formed a stable Langmuir monolayer over an aqueous HAuCl4 subphase. Au seeds grew on the edges and the flat surface of the nanoplates to form continuous Au shell-like structures on the semiconductor nanoplates.

On a role of liquid intermediates in nucleation of gold sulfide nanoparticles in aqueous media.

Previously, we found a series of fluid nanoscale intermediates preceding nucleation of gold sulfide in the reaction between aqueous HAuCl4 and sodium sulfide. Here, the effects of temperature, addition of an inert electrolyte and some other factors on characteristics of the "dense liquid" intermediates and the formation of solid nuclei were studied using UV-vis absorption spectroscopy, DLS, zeta-potential measurements, and SAXS. It was revealed, in particular, that the negatively-charged interfaces of the dense liquid species critically impede their fusion into large enough dense droplets in which nucleation can take place. The nucleation and following coagulation of poorly crystalline gold sulfide proceed not instantly but progressively as the dense liquid droplets arise, with the process being sharply accelerated by the injection of NaCl or temperature increase.

Rapid Au recovery from aqueous solution by a microorganism-mediated, surfactant-directed approach: Effect of surfactants and SERS of bio-Au

Gold recovery is of practical significance while functional Au nanostructures are of great interest in modern nanotechnology. In this work, highly and hierarchically branched Au nanowires (AuNWs) were produced to obtain bio-AuNW composites while the Au was effectively recovered from aqueous HAuCl4 solution by a microorganism-mediated, surfactant-directed (MSD) approach with Lactobacillus spp. cells (LSCs). Ternary coordination of the cell-surfactant-AA played an important role in recovering Au and producing the bio-AuNW composites in the cases of tetradecyl trimethyl ammonium bromide (TTAB), hexadecyltrimethylammonium bromide (CTAB) and octadecyl trimethyl ammonium bromide (OTAB). In contrast, Au recovery could not be achieved with hexadecyltrimethylammonium chloride (CTAC). Strong interaction between [AuCl4]− and the LSCs prior to addition of surfactant (except CTAC) and ascorbic acid (AA) was vital for the highly and hierarchically branched nanostructures. Furthermore, the bio-AuNW composites could serve as excellent surface-enhanced Raman spectroscopy (SERS) substrate. the SERS detection limits of toxic rhodamine 6G (R6G) and 4-mercaptobenzoic acid (MBA) with the bio-AuNW composites are down to 10−9 and 10−6M, respectively. This work therefore exemplifies combining recovery of Au and fabrication of functional Au nanomaterials, which was environmentally benign and application-oriented.

Stable oligomeric clusters of gold nanoparticles: preparation, size distribution, derivatization, and physical and biological properties.

Reducing dilute aqueous HAuCl4 with NaSCN under alkaline conditions produces 2–3 nm diameter yellow nanoparticles without the addition of extraneous capping agents. We here describe two very simple methods for producing highly stable oligomeric grape-like clusters (oligoclusters) of these small nanoparticles. The oligoclusters have well-controlled diameters ranging from ∼5 to ∼30 nm, depending mainly on the number of subunits in the cluster. Our first [“delay-time”] method controls the size of the oligoclusters by varying from seconds to hours the delay time between making the HAuCl4 alkaline and adding the reducing agent, NaSCN. Our second [“add-on”] method controls size by using yellow nanoparticles as seeds onto which varying amounts of gold derived from “hydroxylated gold”, Na+[Au(OH4–x)Clx]−, are added-on catalytically in the presence of NaSCN. Possible reaction mechanisms and a simple kinetic model fitting the data are discussed. The crude oligocluster preparations have narrow size distributions, and for most purposes do not require fractionation. The oligoclusters do not aggregate after ∼300-fold centrifugal-filter concentration, and at this high concentration are easily derivatized with a variety of thiol-containing reagents. This allows rare or expensive derivatizing reagents to be used economically. Unlike conventional glutathione-capped nanoparticles of comparable gold content, large oligoclusters derivatized with glutathione do not aggregate at high concentrations in phosphate-buffered saline (PBS) or in the circulation when injected into mice. Mice receiving them intravenously show no visible signs of distress. Their sizes can be made small enough to allow their excretion in the urine or large enough to prevent them from crossing capillary basement membranes. They are directly visible in electron micrographs without enhancement, and can model the biological fate of protein-like macromolecules with controlled sizes and charges. The ease of derivatizing the oligoclusters makes them potentially useful for presenting pharmacological agents to different tissues while controlling escape of the reagents from the circulation.

Speciation-controlled incipient wetness impregnation: A rational synthetic approach to prepare sub-nanosized and highly active ceria–zirconia supported gold catalysts

On the basis of calculated thermodynamic species distribution diagrams and by appropriately controlling the pH of aqueous HAuCl4 solutions, it has been possible to prepare, using a Speciation-controlled Incipient Wetness Impregnation (ScIWI) approach, Au catalysts supported on ceria–zirconia mixed oxides featuring both high gold loadings and excellent metal dispersions. This rational synthesis method is carried out at room temperature. It is both much simpler, in equipment terms, and less expensive than widely used Deposition–Precipitation (DP). Moreover, the use of ScIWI allows overcoming the severe limitations of previously assayed impregnation methods. With this procedure it is possible to prepare active catalysts in CO oxidation with high efficiency in terms of gold precursor usage, i.e. minimizing Au losses during synthesis. Therefore this, quite amenable, novel strategy for the facile preparation of highly dispersed supported gold catalysts gathers the necessary requirements for both its use at lab scale and an easy scaling-up to industrial levels.

Microorganism-mediated, CTAB-directed aggregation of Au nanostructures around Escherichia coli cells: Towards enhanced Au recovery through coordination of cell-CTAB–ascorbic acid

In this paper, a microorganism-mediated, cetyltrimethylammonium bromide (CTAB)-directed (MCD) aggregation of Au nanostructures around Escherichia coli cells (ECCs) for enhanced Au recovery, from aqueous HAuCl4 is demonstrated for the first time. Owing to the coordination of cell-CTAB–ascorbic acid (AA), one dimensional Au nanostructures grew around the ECCs, inducing the rapid aggregation of Au and the ECCs. The concentrations of CTAB and AA had insignificant effect on the precipitation if the ECC dosage and HAuCl4 concentration were matched. The Au recovery efficiency decreased with increasing CTAB concentration, while a minimum AA concentration was required for higher Au recovery efficiency. The ECC-binding Au ions were reduced to form Au nuclei over the cell surface that provided preferential nucleation sites. Meanwhile, the free ligand-substituted anions in the solution were reduced to Au atoms that served as Au source for growing the Au nanostructures around the ECCs. This work exemplifies a new avenue to enhance the efficiency of recovering Au through the engineering of Au nanostructures.

Synthesis of Au-BiVO4 nanocomposite through anodic electrodeposition followed by galvanic replacement and its application to the photocatalytic decomposition of methyl orange.

A Au-BiVO(4) nanocomposite is synthesized by a two-step strategy involving anodic electrodeposition combined with in situ galvanic replacement. First, a BiVO(4) layer is prepared by the anodic oxidation of pre-electrodeposited Bi film in a VO(4)(3-) containing electrolyte. Thus-prepared BiVO(4) film contains excess metallic Bi, which is then galvanically replaced with Au from an aqueous HAuCl(4) solution, resulting in the Au-BiVO(4) composite in the second step. Optical, photoelectrochemical and photocatalytic properties are investigated by using X-ray diffraction, energy-dispersive X-ray analysis, diffuse reflectance spectrometry, and photoelectrochemical analyses. The visible-light photocatalytic activity of the Au-BiVO(4) composite is evaluated using the decomposition of methyl orange dye and is superior to the bare BiVO(4) film counterpart.

Keggin-lysine hybrid nanostructures in the shape modulation of gold

We show here that L-lysine effectively complexes with phosphomolybdic acid (PMA) and the solution mixture when added to a 10−3 M aqueous solution of HAuCl4 after UV-irradiation for 3 h leads to the slow reduction and consequent formation of gold nanotriangles with a high degree of anisotropy. The same reaction carried out in a 12.5 kDa cutoff dialysis bag where the irradiated PMA-lysine solution was kept inside and stirred in a beaker containing aqueous HAuCl4, did not lead to the formation of gold nanotriangles. This implies that L-lysine plays the role of a shape-modulating agent and hence this study proves an improvement in the understanding of the role of such organic–inorganic hybrid structures in the synthesis and growth of anisotropic nanoparticles.

English

Many microorganisms produce intracellular metal nanoparticles. Aqueous gold, when exposed to several actinomycetal strains, become thereby, leading to the formation of gold nanoparticles. The use of microorganisms in the synthesis of nanoparticles is emerging as an eco-friendly and exciting approach gold for recovery. Streptomyces hygroscopicus was used for the biosynthesis of gold nanoparticles. UV and visible spectroscopic studies of biofilms revealed better synthesis of nanoparticles. It was observed that better biosynthesis of gold nanoparticles occurred when cell biomass treated with 10-3 and 10-4 mM HAuCl4 solution as compared to other dilutions. The pH 7.0 was found to be optimum for the biosynthesis of gold nanoparticles. The TEM study revealS that there was evidence of gold nanoparticles synthesized by S. hygroscopicus. The results demonstrate that spherical gold nanoparticles in the range of 10 to 20 nm were observed at pH value of 7.0. The actinomycetal biomass and various concentration of aqueous HAuCl4 solution were incubated, it was found that 10-4 concentration shows excellent colour of the actinomycetal biomass. Key words: Biosynthesis, optimization, gold nanoparticles, purification, characterization.

Facile Preparation and Growth Mechanism of New-Type Gold Nanoplates

When a sticky gel consisting of an aqueous HAuCl4 solution mixed with poly-vinylpyrrolidone (PVP) surfactant is kept at room temperature (about 30 °C), the HAuCl4 is reduced by the PVP, resulting in the formation of nanostructures. In this study, gold nanoplates with new shapes, which were single crystalline, several micrometers wide, and tens of nanometers thick, were mass-synthesized by adjusting the crystal growth conditions. For example, through inducing temperature decrease (10-20 °C) in the early stage of crystal growth, the product is dominated by star-like gold nanoplates, together with other new shapes such as shields, concave and convex triangles, corner snipped shapes, triple branched shapes, and shapes that are step-rich in the side plane. Based on theoretical calculations, we present the growth mechanism of these new gold nanoplates. Under certain growth conditions, the (111) plane of the gold crystal can grow not only along the direction into regular triangular or hexagonal nanoplates, but also along other directions such as and , to give new nanoplates with high-index side facets.

Preferential growth of Au on CdSe quantum dots using Langmuir–Blodgett technique

Oleyl amine capped CdSe quantum dots are allowed to form monolayer on aqueous HAuCl4 subphase and Langmuir–Blodgett technique is used judicially to grow Au tips in a directed fashion on CdSe quantum dots.