Synthesis of L-phenylalanine stabilized gold nanoparticles and their thermal stability

Here we described the two synthesis methods of L-phenylalanine (L-phe) coated gold nanoparticles through a reduction of aqueous chloroaurate ions directly by L-phe, and also the borohydride reduction of chloroauric acid followed by capping with L-phe molecules. Phenylalanine reduced gold nanoparticles and the phenylalanine capped gold nanoparticles were evaluated by transmission electron microscope and UV-vis spectroscopy, and then their thermal stabilities were compared. We found that the phenylalanine reduced gold nanoparticles were unstable and form linearly arranged aggregates on aging, while as the phenylalanine capped gold nanoparticles were stable for months in ambient condition.     

Biological synthesis of gold and silver nanoparticles mediated by the bacteria Bacillus subtilis

Biological synthesis of gold and silver nanoparticles was carried out using the bacteria Bacillus subtilis. The reduction processes of chloroaurate and silver ions by B. subtilis were found to be different. Gold nanoparticles were synthesized both intra- and extracellularly, while silver nanoparticles were exclusively formed extracellularly. The gold nanoparticles were formed after 1 day of addition of chloroaurate ions, while the silver nanoparticles were formed after 7 days. The nanoparticles were characterized by X-ray diffraction, UV-vis spectra and transmission electron spectroscopy. X-ray diffraction revealed the formation of face-centered cubic (fcc) crystalline gold nanoparticles in the supernatant, broth solution and bacterial pellet. Silver nanoparticles also exhibited diffraction peaks corresponding to fcc metallic silver. UV-vis spectra showed surface plasmon vibrations for gold and silver nanoparticles centered at 530 and 456 nm, respectively. TEM micrographs depicted the formation of gold nanoparticles intra- and extracellularly, which had an average size of 7.6 +/- 1.8 and 7.3 +/- 2.3 nm, respectively, while silver nanoparticles were exclusively formed extracellularly, with an average size of 6.1 +/- 1.6 nm. The bacterial proteins were analyzed by sodium dodecyl sulfonate-polyacrylamide electrophoresis (SDS-PAGE) before and after the addition of metal ion solutions. We believe that proteins of a molecular weight between 25 and 66 kDa could be responsible for chloroaurate ions reduction, while the formation of silver nanoparticles can be attributed to proteins of a molecular weight between 66 and 116 kDa. We also believe that the nanoparticles were stabilized by the surface-active molecules i.e., surfactin or other biomolecules released into the solution by B. subtilis.     

A new method for the synthesis of hydrophobic gold nanotapes

Protocols for the synthesis of gold nanoparticles are increasingly focusing on controlling the morphology of the nanocrystals. We demonstrate in this article the facile, one-step synthesis of gold nanotapes that are readily dispersible in organic media. This is accomplished by the spontaneous reduction of aqueous chloroaurate ions by hexadecylaniline molecules present in chloroform at the static interface between water and chloroform. The hexadecylaniline molecules cap the gold nanotapes thus formed, rendering them hydrophobic and dispersible in a range on nonpolar and weakly polar organic solvents. Possible reasons for the growth of gold nanotapes are discussed.     

Biosynthesis of gold and silver nanoparticles using Emblica Officinalis fruit extract, their phase transfer and transmetallation in an organic solution

The design, synthesis and characterization of biologically synthesized nanomaterials have become an area of significant interest. In this paper, we report the extracellular synthesis of gold and silver nanoparticles using Emblica Officinalis (amla, Indian Gooseberry) fruit extract as the reducing agent to synthesize Ag and Au nanoparticles, their subsequent phase transfer to an organic solution and the transmetallation reaction of hydrophobized silver nanoparticles with hydrophobized chloroaurate ions. On treating aqueous silver sulfate and chloroauric acid solutions with Emblica Officinalis fruit extract, rapid reduction of the silver and chloroaurate ions is observed leading to the formation of highly stable silver and gold nanoparticles in solution. Transmission Electron Microscopy analysis of the silver and gold nanoparticles indicated that they ranged in size from 10 to 20 nm and 15 to 25 nm respectively. Ag and Au nanoparticles thus synthesized were then phase transferred into an organic solution using a cationic surfactant octadecylamine. Transmetallation reaction between hydrophobized silver nanoparticles and hydrophobized chloroaurate ions in chloroform resulted in the formation of gold nanoparticles.     

Synthesis of gold nanorods in organic media

A seed mediated approach for the synthesis of anisotropic rod shaped gold nanoparticles in organic media (toluene) is demonstrated. Pre-formed gold nanoparticles stabilized in toluene by 4-hexadecylaniline (HDA) are used as seeds. These when reacted with 1-octadecylamine (ODA) hydrophobised chloroaurate ions in toluene lead to the formation of gold nanorods. ODA or alkylamines of different chain lengths which are the chloroaurate ion phase transfer agent have been found to play a key role in the formation of the nanorods. The gold nanorods that have a five-fold symmetry evolve from multiply twinned particles and are bound at the tips by [1 11] faces and at the sides by [100] faces. The gold nanorods have been shown to grow under the shape directing effect of the alkylamines which stabilize the high energy [100] faces. The concentration of the alkylamines has been found to play a critical role in the formation of the gold nanorods. Higher concentrations of the alkylamines lead to formation of spherical particles, at times of narrow size distribution.     

Synthesis of gold nanospheres and nanotriangles by the Turkevich approach

Gold nanoparticles of triangular morphology possess interesting optical properties with potential application in medicine and infrared absorbing coatings, however, little is known about conditions that favor their growth. In this paper, we have reinvestigated a time-tested recipe for the formation of gold nanospheres by citrate reduction of aqueous gold ions under boiling conditions (Turkevich recipe). Our principle findings are that gold nanotriangle formation is kinetically controlled and is highly favored at low temperatures. Furthermore, the presence of chloride ions from the precursor chloroaurate ions plays a major role in promoting the growth of <111> oriented triangular/truncated triangular particles. The presence of bromide and iodide ions that possess the ability to replace surface-bound chloride ions inhibits triangle formation to varying degrees.     

Biological synthesis of triangular gold nanoprisms

The optoelectronic and physicochemical properties of nanoscale matter are a strong function of particle size. Nanoparticle shape also contributes significantly to modulating their electronic properties. Several shapes ranging from rods to wires to plates to teardrop structures may be obtained by chemical methods; triangular nanoparticles have been synthesized by using a seeded growth process. Here, we report the discovery that the extract from the lemongrass plant, when reacted with aqueous chloroaurate ions, yields a high percentage of thin, flat, single-crystalline gold nanotriangles. The nanotriangles seem to grow by a process involving rapid reduction, assembly and room-temperature sintering of 'liquid-like' spherical gold nanoparticles. The anisotropy in nanoparticle shape results in large near-infrared absorption by the particles, and highly anisotropic electron transport in films of the nanotriangles.     

Templated biomineralization of Au nanoplates under bovine serum albumin Langmuir monolayers

Au nanoplates were generated by spontaneous reduction of chloroaurate ions (AuCl₄⁻) under bovine serum albumin (BSA) Langmuir monolayers at room temperature. The structure of the resulting Au particulates was analyzed by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), selected-area electron diffraction (SAED), and high resolution transmission electron microscopy (HRTEM). The results showed that BSA provided dual functions for both reducing Au³⁺ ions and directing anisotropic growth of Au particles into plate-like structure as well. Amorphous Au particulates were obtained firstly in a relatively short reaction time, and then anisotropic Au nanoparticles were generated at extended reaction durations. The triangular Au nanoplates oriented along (1 1 1) basal planes were obtained via the reduction of chloroaurate ions by BSA with a relatively longer reaction duration. The present research provides a biological route to produce single-crystalline gold nanoplates with a wide variety of applications, and it also verifies that the interaction between protein/peptide and gold ions/surface may be used advantageously for green chemical synthesis of nanogold. Hopefully, this would contribute to promote genuine green biomimetic synthesis of nanomaterials with prescribed geometrical features where rationally designed multifunctional peptides are preferred.     

Design of multicomponent microgels by selective deposition of nanomaterials

In the present paper a method for the targeted deposition of different nanomaterials on aqueous microgels is described. In the first stage poly(3,4-ethylenedioxythiophene) (PEDOT) nanorods are introduced into the microgel structure by in situ oxidative polymerization. In the second stage hydrogen tetrachloroaurate is used to transform PEDOT chains to an oxidized state in the microgel structure, leading to the fixation of chloroaurate anions on the surface of the PEDOT nanorods. The reduction of chloroaurate ions induces the formation of gold nanoparticles (AuNPs) predominantly located on the PEDOT surface. Obtained microgel/PEDOT/AuNP hybrid particles with different nanoparticle loadings exhibit superior colloidal stability and temperature sensitivity. The microgel/PEDOT/AuNP hybrid microgels exhibit extraordinary catalytic activity in aqueous media.     

Influence of tyrosine on the dual electrode electrochemical detection of copper(II)-peptide complexes.

The bluret reaction makes peptides electrochemically active. This is the basis of an electrochemical method for the detection of peptides following their liquid chromatographic separation. This paper discusses the influence of tyrosine, an electroactive amino acid, on the detection of Cu(II)-peptide (bluret) complexes containing it. The dual electrode detector has an upstream anode and a downstream cathode. Tyrosine-containing peptides yield anodic signals that are approximately the sum of the tyrosine signal and the signal from the bluret complex of a nonelectroactive model peptide, phenylalanylglycyl-glycine (FGG). The cathodic signal is depressed in comparison to FGG. This is traced to the presence of an intramolecular reaction between Cu(III) and a reaction product resulting from the oxidation of the tyrosinyl residue. The rate constant for the corresponding intermolecular reaction is significant (10(6)-10(7) M-1 s-1), but in practical analytical situations, the concentrations of the reactants will be small, so the reaction will not be a major factor. Sensitivities for several bioactive peptides are reported. The dependence of the signals on the position of tyrosine in a tripeptide is also studied.     

A single-step synthesis of gold nanochains using an amino acid as a capping agent and characterization of their optical properties

We demonstrated a simple single-step synthesis of gold nanochains by reducing aqueous chloroaurate ions (AuCl(4)(-)) with sodiumborohydride (NaBH(4)) in the presence of an amino acid (glutamic acid and histidine) as a stabilizer. The structure and optical properties of gold nanochains and nanowires were characterized by transmission electron microscopy, UV-visible spectroscopy, infrared spectroscopy and femtosecond z-scan measurements. The results suggested that the gold nanochains and nanowires were formed via the nanospheres fusing into one another by an oriented attachment mechanism through dipole-dipole interactions. The z-scan measurements on Au nanowires showed a transition from saturable absorption to reverse saturable absorption as the pump intensity increased.     

Chromatographic detection using tris(2,2'-bipyridyl)ruthenium(III) as a fluorogenic electron-transfer reagent

Tris(2,2'-bipyridyl)ruthenium can be excited to fluorescence by visible light (lambda abs 454 nm, lambda em 607 nm) when in the M(II) oxidation state, but not in the M(III) state. A novel chromatographic detection method using the non-fluorescent M(III) form of the complex as a postcolumn fluorogenic reagent is demonstrated. The M(III) form is a powerful oxidizing agent (E degree = 1.27 V vs NHE, 1.05 V vs Ag/AgCl). The M(III) reagent is generated on-line from the M(II) form of the complex by a highly efficient porous carbon electrode and then reacted briefly with chromatographic effluent; the M(II) created by electron transfer from oxidation-susceptible analytes is then detected by fluorescence. The fluorescence detector can be calibrated for number of electrons transferred by injection of either M(II) or an oxidative standard such as ferrocyanide. It is hoped that this redox-based detection scheme will provide an alternative to electrochemical detection. Among the advantages are freedom from surface fouling and the potential for extremely low detection limits. The scheme was applied to detection of the peptide dynorphin A and several of its fragments. Dynorphin A contains tyrosine at the N-terminus (position 1) and tryptophan in position 15; these amino acid residues are susceptible to oxidation and peptides containing them can be detected on that basis. Flow injection testing of the model compounds Tyr-Gly-Gly-Phe-Leu and Gly-Gly-Trp-Gly indicated that tyrosine transferred approximately 1 electron to the M(III) reagent and that tryptophan transferred approximately 4 electrons. Similar results were obtained from the chromatographic runs. Dynorphin A and six dynorphin A fragments containing the N-terminal tyrosine were detected easily at 100 nM concentration (14 pmol) using laser-induced fluorescence. As expected, one fragment that did not contain tryptophan or tyrosine was not detected. A mass detection limit of 80 fmol was estimated for the tyrosine-containing fragments.     

Biocatalytically induced growth of gold nanoshells: using enzyme reaction for the controllable fabrication of nanomaterials

In the present work, the enzymatically controlled growth process of gold nanoshells (GNSs) in the presence of O2/glucose/glucose oxidase (GOx) and its chloroaurate ion electron acceptor is described. The biocatalytically stimulated growth process is one of the bio-inspired synthetic procedures directed by biological molecules which occur under ambient conditions. It is found that hydrogen peroxide (H2O2) could enlarge the gold nanoparticles (GNPs) on the surface of GNSs precursor composites, of which the preadsorbed GNPs serve as nucleation sites for further gold deposition. Here, GOx is harnessed for its unparalled level of catalytic activity and substrate specificity while H2O2 is produced as a by-product during the oxidation of D-glucose to gluconic acid by GOx. Then the bio-generated H2O2 is used as the reducing agent in the catalytic deposition process of GNSs formation. During the procedure, the localized surface plasmon resonance peaks range across hundreds of nanometers from visible to near infrared region accompanying by the resultant formation of uniform and continuous core-shell nanostructures. The corresponding optical, morphological and enzyme kinetic properties are all well investigated. The novel protocol offers a new perspective for the bio-directed synthesis method in nanotechnology.     

On-plate desalting and SALDI-MS analysis of peptides with hydrophobic silicate nanofilms on a gold substrate

We report the use of silicate nanofilms for on-plate desalting and subsequently direct laser desorption/ionization-mass spectrometric (LDI-MS) analysis of peptides. A hydrophobic octadecyltrichlorosilane (OTS) monolayer is formed on a calcinated nanofilm on a gold substrate to facilitate sample deposition and interaction with the surface that allows effective removal of MS-incompatible contaminants such as salts and surfactants by simple on-plate washing while the peptides are retained on the spot. By elimination of interferences from matrix-related ions and contaminants, sensitivity of MS analysis has been enhanced over ca. 20 times, leading to improved detection of peptides at the low-femtomolar level. A high recovery rate of the peptides is obtained by using relatively rough nanofilms, which are prepared through a modified layer-by-layer deposition/calcination process. The performance of the films has been investigated with peptide samples in the presence of high salts (NaCl and sodium acetate) and urea. Compared to matrix-assisted laser desorption/ionization analysis with CHCA matrix, LDI with on-plate desalting offers marked improvement for analysis of peptides due to low background ions and reduction of sample complexity. Additionally, selective capture of the hydrophobic components of a protein can be achieved, providing a highly useful strategy for specific peptide enrichment. LDI with on-plate desalting approach has also been successfully applied to peptide analysis from protein digests.     

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).     

Facile biosynthesis, separation and conjugation of gold nanoparticles to doxorubicin

Particle shape and size determine the physicochemical and optoelectronic properties of nanoscale materials, including optical absorption, fluorescence, and electric and magnetic moments. It is thus desirable to be able to synthesize and separate various particle shapes and sizes. Biosynthesis using microorganisms has emerged as a more ecologically friendly, simpler, and more reproducible alternative to chemical synthesis of metal and semiconductor nanoparticles, allowing the generation of rare forms such as triangles. Here we show that the plant pathogenic fungus Helminthosporum solani, when incubated with an aqueous solution of chloroaurate ions, produces a diverse mixture of extracellular gold nanocrystals in the size range from 2 to 70?nm. A plurality are polydisperse spheres, but a significant number are homogeneously sized rods, triangles, pentagons, pyramids, and stars. The particles can be separated according to their size and shape by using a sucrose density gradient in a tabletop microcentrifuge, a novel and facile approach to nanocrystal purification. Conjugation to biomolecules can be performed without further processing, as illustrated with the smallest fraction of particles which were conjugated to the anti-cancer drug doxorubicin (Dox) and taken up readily into HEK293 cells. The cytotoxicity of the conjugates was comparable to that of an equivalent concentration of Dox.     

Metal affinity capture tandem mass spectrometry for the selective detection of phosphopeptides

We report a new method called metal affinity capture that when coupled with tandem mass spectrometry (MAC-MSMS) allows for the selective detection and identification of phosphopeptides in complex mixtures. Phosphopeptides are captured as ternary complexes with Ga(III) or Fe(III) and N(alpha),N(alpha)-bis(carboxymethyl)lysine (LysNTA) in solution and electrosprayed as doubly or triply charged positive ions. The gas-phase complexes uniformly dissociate to produce a common (LysNTA + H)+ ion that is used as a specific marker in precursor ion scans. The advantages of MAC-MSMS over the current methods of phosphopeptide detection are as follows. (1) MAC-MSMS uses metal complexes that self-assemble in solution at pH <5, which is favorable for the production of positive ions by electrospray. (2) Phosphorylation at tyrosine, serine, and threonine is detected by MAC-MSMS. (3) The phosphopeptide peaks in the mass spectra are encoded with the 69Ga-71Ga isotope pattern for selective recognition in mixtures. Detection by MAC-MSMS of singly and multiply phosphorylated peptides in tryptic digests is demonstrated at low-nanomolar protein concentrations.     

Fabrication, characterization, and enzymatic activity of fungal protease--nanogold membrane bioconjugate

This study describes the synthesis of a free-standing nanogold membrane by the spontaneous reduction of aqueous chloroaurate ions by the diamine molecule DAEE at a liquid-liquid interface. The free standing nanogold membrane, provides a biocompatible surface for the immobilization of proteins. F-Protease (F-Prot) was then bound to the nanogold membrane via interaction with the gold nanoparticles leading to a new class of biocatalyst. A highlight of the new biocatalyst wherein the enzyme is bound to the nanogold membrane is the ease with which separation from the reaction medium may be achieved by simple filtration. In relation to the free enzyme in solution, the F-Prot in the bioconjugate material exhibited a slightly higher biocatalytic activity and significantly enhanced pH and temperature stability. The F-Prot nanogold membrane bioconjugate material also exhibited excellent biocatalytic activity over ten successive reuse cycles.     

Introduction of gold nanoparticles into myoglobin-Nafion film for direct electrochemistry application

An effective myoglobin-Nafion film is prepared by introducing gold nanoparticles in through a simple procedure by ion-exchange combined with electrochemical reduction. Gold nanoparticles are highly dispersed in myoglobin-Nafion film with an average size of 2.3 +/- 0.2 nm. The electrochemical behavior of myoglobin entrapped in the film has been carefully investigated with cyclic voltammetry. The results show that the introduction of gold nanoparticles into myoglobin-Nafion film makes the direct electron transfer of myoglobin efficient. A pair of well-defined redox peaks for myoglobin heme Fe(II)/Fe(III) is observed with a formal potential of -0.150 V in 0.1 M phosphate buffer (pH 7.0). The electrochemical parameters of myoglobin in the composite film are further calculated with the results of the electron-transfer rate constant (k(s)) as 0.93 s(-1) and the charge transfer coefficient (alpha) as 0.69. The experimental results also demonstrate that the immobilized myoglobin retains its electrocatalytic activity for the reduction of hydrogen peroxide and the catalytic reduction peak of myoglobin appear in a linear relationship with H2O2 concentration in the range of 10.0-235.0 microM with correlation coefficient of 0.9970. Thus fabricated Au/Mb/Nafion electrode should give a new approach for developing redox protein or enzyme-based biosensors.     

A direct and facile synthetic route for micron-scale gold prisms and fabrication of gold prism thin films on solid substrates

Flat gold prisms that have micrometer-scale edge length and nanometer-scale thickness (micro-prisms, as defined by edge length of prisms) were synthesized through a facile solution-phase synthetic method. Two effective measures were adopted in order to prepare the well-defined micro-prisms: (i) selecting poly(N-vinylpyrrolidone)(PVP) as the shape-directing agent to induce the preferential growth of small gold nanocrystals and (ii) using a weak reducing agent, ethylene glycol (EG), to slow down the reduction rate of AuCl₄^? ions. Besides, the light played an important role in inducing the formation of tiny nanoprisms in the early stages of Au(III) ion reduction. Considering that gold nanoclusters have a strong tendency to heterogeneous nucleation at solid/liquid interfaces followed by the preferential planar growth, gold prism thin films were thus fabricated on glass substrates.