Synthesis of a stable gold hydrosol by the reduction of chloroaurate ions by the amino acid, aspartic acid

Development of reliable protocols for the synthesis of nanoparticles of well-defined sizes and good monodispersity is an important aspect of nanotechnology. In this paper, we present details of the synthesis of gold nanoparticles of good monodispersity by the reduction of aqueous chloroaurate ions by the amino acid, aspartic acid. The colloidal gold solution thus formed is extremely stable in time, indicating electrostatic stabilization via nanoparticle surface-bound amino acid molecules. This observation has been used to modulate the size of the gold nanoparticles in solution by varying the molar ratio of chloroaurate ions to aspartic acid in the reaction medium. Characterization of the aspartic acid-reduced gold nanoparticles was carried out by UV-visible spectroscopy, thermogravimetric analysis and transmission electron microscopy. The use of amino acids in the synthesis and stabilization of gold nanoparticle in water has important implications in the development of new protocols for generation of bioconjugate materials.     

Hydrophobic, organically dispersible gold nanoparticles of variable shape produced by the spontaneous reduction of aqueous chloroaurate ions by hexadecylaniline molecules

In addition to control over the size and monodispersity of nanoparticle, nanomaterial synthesis procedures are increasingly required to control their shape and assembly as well. We demonstrate in this paper synthesis of organically dispersible, hydrophobic gold nanoparticles of spherical shape and encased in triangular thin polyaniline shells by doing reaction under static conditions and assembly of these particles onto polymer nanorod/nanowire-like templates by varying the molar ratio of chloroaurate ions to hexadecylaniline and varying the solvent by the spontaneous reduction of aqueous chloroaurate ions by hexadecylaniline molecules in a biphasic reaction setup. Under stationary conditions (no stirring), a biphasic mixture of hexadecylaniline in toluene and chloroaurate ions in water leads to the electrostatic complexation of chloroaurate ions with hexadecylaniline at the liquid-liquid interface and their phase transfer into the organic phase, followed by their reduction by the hexadecylaniline molecules. By varying the conditions, the templating action of gold nanoparticles or the polyaniline nanodispersions can be tuned in the organic medium and resulting assembly.     

Lamellar multilayer hexadecylaniline-modified gold nanoparticle films deposited by the Langmuir-Blodgett technique

Organization of hexadecylaniline (HDA)-modified colloidal gold particles at the air-water interface and the formation thereafter of lamellar, multilayer films of gold nanoparticles by the Langmuir-Blodgett technique is described in this paper. Formation of HDA-capped gold nanoparticles is accomplished by a simple biphasic mixture experiment wherein the molecule hexadecylaniline present in the organic phase leads to electrostatic complexation and reduction of aqueous chloroaurate ions, capping of the gold nanoparticles thus formed and phase transfer of the now hydrophobic particles into the organic phase. Organization of gold nanoparticles at the air-water interface is followed by surface pressure—area isotherm measurements while the formation of multilayer films of the nanoparticles by the Langmuir-Blodgett technique is monitored by quartz crystal microgravimetry, UV-Vis spectroscopy, Fourier transform infrared spectroscopy and transmission electron microscopy.     

Immobilization of biogenic gold nanoparticles in thermally evaporated fatty acid and amine thin films

We have recently demonstrated the biological synthesis of gold nanoparticles by the reduction of aqueous chloroaurate ions by the fungus Fusarium oxysporum and with extract of geranium (Pelargonium graveolens) leaf. In this paper, we demonstrate the immobilization of biogenic gold nanoparticles in lipid thin films deposited by thermal evaporation. The charge on the gold nanoparticles synthesized by both the fungus and the geranium plant extract is used to facilitate their immobilization in both anionic and cationic lipid thin films. A rough estimate of the isoelectric point of the proteins capping the gold nanoparticles synthesized using the fungus could be made by pH-dependent microgravimetry studies of the immobilization process. An interesting size and shape selectivity in the immobilized gold nanoparticles is observed in the lipid thin films. The biogenic gold nanoparticle-lipid composite films were characterized using quartz crystal microgravimetry, UV-vis absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and transmission electron microscopy.     

Synthesis and capping of water-dispersed gold nanoparticles by an amino acid: bioconjugation and binding studies

We report a novel strategy for the synthesis of aqueous stable, carboxylated gold nanoparticles (GNPs) by using glutamic acid as the reducing agent. The ratio of chloroaurate ions, AuCl(-)(4) to glutamic acid was optimized in the reaction medium to obtain monodispersed GNPs. Glutamic acid reduced gold nanoparticles were characterized by UV-visible, FTIR, dynamic light scattering and transmission electron microscopy, which demonstrated high stability in aqueous solution over a period of time indicating stabilization via surface-bound amino acid. Functionalized nanoparticles were conjugated with protein molecules through electrostatic attraction between the surface-terminated negatively charged carboxylate groups (COO(-)) of glutamic acid and the positively charged amino groups (NH(+)(3)) of the protein. The conjugation efficiency of the GNP:protein conjugates was confirmed qualitatively and quantitatively through gel electrophoresis and critical flocculation concentration analysis. The interaction between functionalized GNPs with protein molecules was investigated using fluorescence spectroscopy showing the fluorescence quenching of the tryptophan residues of protein molecules after conjugation. Circular dichroism (CD) studies of the conjugates confirmed that the protein undergoes a more flexible conformational state on the boundary surface of GNPs after conjugation. There was substantial conformational transition from alpha-helix to beta-sheet structure after conjugation of protein to GNPs.     

Facile synthesis and functionalization of water-soluble gold nanoparticles for a bioprobe

In this work, we report the size tunable synthesis of water-dispersed gold nanoparticles by using octadecylamine (ODA) as the reducing agent, that electrostatically complexes with the chloroaurate ions, reduces them, and subsequently caps the gold nanoparticles. Amine-capped gold nanoparticles, thus formed, were subsequently coordinated with a secondary monolayer of an anionic surfactant, sodium bis(2-ethylhexyl)-sulfosuccinate (AOT) which helps in providing sufficient hydrophilicity to the gold nanoparticles. Functionalized gold nanoparticles were characterized by UV-vis, IR spectrophotometric, dynamic light scattering, zeta-potential and transmission electron microscopic techniques, which demonstrated high stability of gold nanoparticles in aqueous media, indicating stabilization via bilayers of ODA and AOT. The gold nanoparticles were further conjugated with a protein (bovine serum albumin) and the interaction was investigated by circular dichroism studies as well as by measuring the fluorescence quenching of the tryptophan residues of protein molecules after the binding of nanoparticles to specific sites of the protein. The binding constant and the stoichiometry values indicated that the particles with larger core size are less site-specific but show higher binding affinity with protein molecules. The use of a bio-compatible synthetic process and the stabilization of the gold nanoparticles by ODA and AOT are interesting from the point of view of making bioprobes for life science applications.     

Formation process of two-dimensional networked gold nanowires by citrate reduction of AuCl4- and the shape stabilization

Gold nanowires with a two-dimensional (2-D) network structure were formed by citrate reduction of AuCl4- with a low concentration of citrate. The structure change during the growth processes was observed by transmission electron microscopy (TEM) and the variation in concentrations of gold species in the aqueous solution was monitored by UV-vis spectra and Inductively Coupled Argon Plasma Emission Spectrophotometer (ICAP). The formation of 2-D gold nanowires was induced by the small amount of reducing agent because the preliminary gold nanoparticles formed by reduction of AuCl4- were thermodynamically unstable in the aqueous solution due to the insufficient capping of citrate. One of the key points of nanowire formation is the preferential adsorption of AuCl4- instead of citrate ions on the surface of the preliminary gold particles, which results in an attracting force between gold nanoparticles. We propose a hit-to-stick-to-fusion model, in which gold nanoparticles adhere by the attraction force and stick together, causing selective deposition of reduced gold metallic species on the concave surface of the two sticking particles, followed by fusion into nanowires. Nanowires then connect with each other, forming a network structure. The evidence obtained from TEM observation of transformation from gold nanowires on a TEM grid to large nanoparticles by hydrogen gas reduction and time-resolved measurements of gold ions suggest that gold ions not only are crucial for the growth of gold nanowires but also play an important role in stabilizing the shape of gold nanowires during the formation process. This method for synthesizing 2-D gold nanowires is simple and relatively easy application to the synthesis of other metallic nanowires such as silver or platinum is expected.     

Hollow gold and platinum nanoparticles by a transmetallation reaction in an organic solution

Transmetallation reaction between hydrophobized silver nanoparticles with hydrophobized chloroaurate and chloroplatinate ions in chloroform results in the formation of hollow gold and platinum shell nanoparticles respectively.     

Synthesis and assembly of gold nanoparticles in organized molecular films of gemini amphiphiles

Generation and assembly of gold nanostructures were investigated in the organized molecular films of a series of gemini amphiphiles. The chloroauric acid, dissolved in the aqueous subphase, was incorporated into the monolayers of the gemini amphiphiles containing ethyleneamine spacers through an interfacial assembly. The in situ formed complex monolayers were transferred onto solid substrates, and gold nanoparticles were generated in the film by a chemical or photochemical reduction. Discrete gold nanoparticles with an absorption maximum at 550 nm were generated in the films by photoirradiation, while different gold nanostructures were obtained by chemical reduction. Depending on the chemical reductant, various shape and assembly of gold nanostructures were obtained. When reduced by hydroquinone, a tree-branched assembly of the nanoparticles was obtained and the film showed a broad band centered at around 900 nm. When NaBH 4 was applied, crooked nanowires or assembly of nanoparticles were obtained, depending on concentration, and the film showed absorption at 569 or 600 nm. Furthermore, by combining the photochemical and chemical reduction methods, i.e., the chloroaurate ion-incorporated film was initially irradiated with UV light and then subjected to chemical reduction, the optical absorption of the formed gold nanostructures can be regulated.     

Gold nanosheets via reduction of aqueous chloroaurate ions by anthracene anions bound to a liquid-liquid interface

Anthracene anions bound to a liquid-liquid interface and charged by photochemically reduced Keggin ions when exposed to aqueous chloroaurate ions result in the formation of high concentration of thin, gold nanosheets at the interface.     

Free-standing nanogold membranes as scaffolds for enzyme immobilization

We demonstrate herein the formation of a free-standing gold nanoparticle membrane and its use in the immobilization of the enzyme, pepsin. The nanogold membrane is synthesized by the spontaneous reduction of aqueous chloroaurate ions at the liquid-liquid interface by the bifunctional molecule bis(2-(4-aminophenoxy)ethyl) ether (DAEE) taken in chloroform. This process results in the formation of a robust, malleable free-standing nanogold membrane consisting of gold nanoparticles embedded in a polymeric background. Recognizing that gold nanoparticles are excellent candidates for immobilization of enzymes, we have immobilized pepsin on the nanogold membrane, leading to a new class of biocatalyst. A highlight of the new pepsin-nanogold biocatalyst is the ease with which separation from the reaction medium may be achieved. The catalytic activity of pepsin in the bioconjugate was comparable to that of the free enzyme in solution. The pepsin-nanogold membrane bioconjugate material exhibited excellent biocatalytic activity over 10 successive reuse cycles as well as enhanced pH, temperature, and temporal stability.     

Rapid synthesis of Au, Ag, and bimetallic Au core-Ag shell nanoparticles using Neem (Azadirachta indica) leaf broth

We report on the use of Neem (Azadirachta indica) leaf broth in the extracellular synthesis of pure metallic silver and gold nanoparticles and bimetallic Au/Ag nanoparticles. On treatment of aqueous solutions of silver nitrate and chloroauric acid with Neem leaf extract, the rapid formation of stable silver and gold nanoparticles at high concentrations is observed to occur. The silver and gold nanoparticles are polydisperse, with a large percentage of gold particles exhibiting an interesting flat, platelike morphology. Competitive reduction of Au3+ and Ag+ ions present simultaneously in solution during exposure to Neem leaf extract leads to the synthesis of bimetallic Au core-Ag shell nanoparticles in solution. Transmission electron microscopy revealed that the silver nanoparticles are adsorbed onto the gold nanoparticles, forming a core-shell structure. The rates of reduction of the metal ions by Neem leaf extract are much faster than those observed by us in our earlier studies using microorganisms such as fungi, highlighting the possibility that nanoparticle biological synthesis methodologies will achieve rates of synthesis comparable to those of chemical methods.     

Facile synthesis of polyaniline nanofibers using chloroaurate acid as the oxidant

This work demonstrated a facile route to the synthesis of polyaniline (PANI) nanofibers by polymerization of aniline using chloroaurate acid (HAuCl(4)) as the oxidant. The reduction of AuCl(4)(-) is accompanied by oxidative polymerization of aniline, leading to uniform PANI nanofibers with a diameter of 35 +/- 5 nm and aggregated gold nanoparticles which can precipitate from the liquid phase during the reaction. The resultant PANI nanofibers and gold particles were characterized by means of different techniques, such as UV-vis, FTIR spectroscopy, and scanning and transmission electron microscopy methods. It is found that the gold aggregates are capped with polyaniline, and the conductivity of the fibers is around 0.16 S/cm.     

Formation of gold@polymer core-shell particles and gold particle clusters on a template of thermoresponsive and pH-responsive coordination triblock copolymer

Template synthesis of various morphological gold colloidal nanoparticles using a thermoresponsive and pH-responsive coordination triblock copolymer of poly(ethylene glycol)-b-poly(4-vinylpyridine)-b-poly(N-isopropylacrylamide) is studied. The template morphology of the thermoresponsive and pH-responsive coordination triblock copolymer, which can be tuned by simply changing the pH or temperature of the triblock copolymer aqueous solution, ranges from single chains to core-corona micelles and further to micellar clusters. Various morphological gold colloidal nanoparticles such as discrete gold nanoparticles, gold@polymer core-shell nanoparticles, and gold nanoparticle clusters are synthesized on the corresponding template of the triblock copolymer by first coordination with gold ions and then reduction by NaBH4. All three resultant gold colloidal nanoparticles are stable in aqueous solution, and their sizes are 2, 10, and 7 nm, respectively. The gold@polymer core-shell nanoparticles are thermoresponsive. The gold nanoparticle cluster has a novel structure, and each one holds about 40 single gold nanoparticles.     

Formation of gold nanoparticles catalyzed by platinum nanoparticles: assessment of the catalytic mechanism

The understanding of how the formation of metal nanoparticles in aqueous solutions is influenced by the presence of presynthesized nanoparticles is important for precise control over size, shape, and composition of nanoparticles. New insights into the catalytic mechanism of Pt nanoparticles are gained by studying the formation of gold nanoparticles from the reduction of AuCl(4)(-) in aqueous solution in the presence of presynthesized Pt nanoparticles as a model system. The measurement of changes of the surface plasmon resonance band of gold nanoparticles, along with TEM analysis of particle size and morphology, provided an important means for assessing the reaction kinetics. The reductive mediation of Pt-H species on the Pt nanocrystal surface is believed to play an important role in the Pt-catalyzed formation of gold nanoparticles. This important physical insight is evidenced by comparison of the rates of the Pt-catalyzed formation of gold nanoparticles in the presence and in the absence of hydrogen (H(2)), which adsorb dissociatively on a Pt nanocrystal surface forming Pt-H species. Pt-H effectively mediates the reduction of AuCl(4)(-) toward the formation of gold nanoparticles. Implications of the findings to the controllability over size, composition, and morphology of metal nanoparticles in the aqueous synthesis environment are also discussed.     

Keggin ions as UV-switchable reducing agents in the synthesis of Au core-Ag shell nanoparticles

Keggin ion-mediated synthesis of Au core-Ag shell bimetallic nanoparticles is described. Exposure of photochemically reduced aqueous (PW12O40)3- Keggin ions to AuCl4- ions leads to the formation of stable gold nanoparticles capped by the Keggin ions. The surface-bound Keggin ions may then be activated by UV irradiation and, upon exposure to Ag+ ions, reduce the metal ions to form a silver shell around the gold core. That the capping agent not only stabilizes the metal nanoparticles but also plays the role of a switchable reducing agent is a highlight of this approach with important implications in nanomaterials synthesis and catalysis.     

Deposition of gold from chloride solutions on microporous iron-carbon sorbents

The process of deposition of gold from an aqueous solution of hydrogen tetrachloroaurate(III) on microporous composites synthesized from a blend of sawdust, iron and zinc chlorides was examined. Their high sorption capacity for gold was shown, one reason of which is the interaction chloroaurate ions with iron oxide.     

Tryptophan-based peptides to synthesize gold and silver nanoparticles: a mechanistic and kinetic study

Synthetic oligopeptides with a tryptophan residue at the C-terminus have been used for the synthesis of gold and silver nanoparticles at pH 11. The tryptophan residue in the peptides is responsible for the reduction of metal ions to the respective metals, possibly through electron transfer. A mechanistic pathway has been proposed to explain the reductive properties of the tryptophan moiety of the peptide based on some spectroscopic techniques, such as UV-visible and fluorescence spectroscopy. This study reveals that some of the peptide molecules are converted to its corresponding ditryptophan, kynurenine form and some cross-linked products, all of which are highly fluorescent species. The resultant peptide-functionalized metal nanoparticles have also been characterized by UV-visible spectroscopy, transmission electron microscopy, and Fourier transform IR spectroscopy and thermogravimatric analysis.     

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.     

Stable fluorescent gold nanoparticles for detection of Cu2+ with good sensitivity and selectivity

A simple and one-pot method for the synthesis of water-soluble, red-emitting, highly fluorescent gold nanoparticles has been reported using 11-mercaptoundecanoic acid (11-MUA) as the protecting group. We found that the fluorescent gold nanoparticles could selectively detect copper ions in aqueous solution, with a limit of detection of about 87 nM.