Laser-induced growth and reformation of gold and silver nanoparticles

The sizes, shapes, and growth rates of gold and silver nanoparticles stabilized with polyvinylpyrrolidone in water can be controlled by using picosecond laser pulses. The nucleation of small metal clusters formed with NaBH₄ addition to produce nanoparticles takes two months with aging but 30 min with laser irradiation. Laser pulses can also induce nanoparticles to have narrow size and shape distribution or to undergo aggregation into much larger particles. The latter process is more likely found when the metal is silver or the irradiation wavelength is short. Laser-induced growth and shape transformation processes are explained in terms of BH₄ ⁻ depletion, metal fusion, and electron ejection followed by disintegration.     

Facile strategy for stability control of gold nanoparticles synthesized by aqueous reduction method

A facile strategy for controlling sizes and stabilities of gold nanoparticles synthesized by aqueous reduction method was experimentally examined and reported. When pH of the solution of HAuCl₄ and Na₃C₆H₅O₇ was controlled by introducing either NaOH or HCl with different concentration, the zeta potential of suspension of gold nanoparticles changed accordingly. With the strategy using a control of pH in a range of 5–9, the zeta potential of synthesized gold nanoparticles was regulated in a range of −60 to −40 mV, resulting in a stable red suspension of gold nanoparticles. Under a condition with pH < 5.0, gold nanoparticles could agglomerate after being kept quiescently for a day due to an adsorption of H⁺ on their surface, which in turn enhanced the attractive van der Waals interaction. On the other hand, synthesis of gold nanoparticles with pH > 9.1 would provide a lower amount of gold nanoparticles due to the formation of NaAuO₂. Based on these results, a potential mechanism of gold nanoparticle synthesis was also discussed.     

Magnetic and structural studies on CoFe2O4 nanoparticles synthesized by co-precipitation,normal micelles and reverse micelles methods

Cobalt ferrite nanoparticles were synthesized by the chemical co-precipitation, normal micelles and reverse micelles methods of iron and cobalt chlorides. X-ray diffraction analysis, Fourier Transform Infrared (FTIR) and Vibrating Sample Magnetometer were carried out at room temperature to study the structural and magnetic properties. X-ray patterns revealed the production of a broad single cubic phase with the average particle sizes of ∼12 nm, 5 nm and 8 nm for co-precipitation, normal micelles and reverse micelles methods, respectively. The FTIR measurements between 400 and 4000 cm⁻¹ confirmed the intrinsic cation vibrations of spinel structure for each one of the three methods. Moreover, the average particle sizes were lower than the single domain size (128 nm) and higher than the super-paramagnetic size (2–3 nm) at room temperature. The results revealed that the magnetic properties depend on the particle size and cation distribution, whereas the role of particle size is more significant.     

Shape and size transformation of gold nanorods (GNRs) via oxidation process: A reverse growth mechanism

The anisotropic shape transformation of gold nanorods (GNRs) with H₂O₂ was observed in the presence of “cethyl trimethylammonium bromide” (CTAB). The adequate oxidative dissolution of GNR is provided by the following autocatalytic scheme with H₂O₂: Au⁰ → Au⁺, Au⁰ + Auⁿ⁺ → 2Au³⁺, n = 1 and 3. The shape transformation of the GNRs was investigated by UV-vis spectroscopy and transmission electron microscopy (TEM). As-synthesised GNRs exhibit transverse plasmon band (TPB) at 523 nm and longitudinal plasmon band (LPB) at 731 nm. Upon H₂O₂ oxidation, the LPB showed a systematic hypsochromic (blue) shift, while TPB stays at ca. 523 nm. In addition, a new emerging peak observed at ca. 390 nm due to Au(III)-CTAB complex formation during the oxidation. TEM analysis of as-synthesised GNRs with H₂O₂ confirmed the shape transformation to spherical particles with 10 nm size in 2 h, whereas centrifuged nanorod solution showed no changes in the aspect ratio under the same condition. Au³⁺ ions produced from oxidation, complex with excess free CTAB and approach the nanorods preferentially at the end, leading to spatially directed oxidation. This work provides some information to the crystal stability and the growth mechanism of GNRs, as both growth and shortening reactions occur preferentially at the edge of single-crystalline GNRs, all directed by Br⁻ ions.     

Laser-assisted growth of gold nanoparticles: Shaping and optical characterization

We demonstrate that supported gold nanoparticles with a given well-defined shape can be produced by laser-assisted growth. For this purpose, gold nanoparticles with average radii ranging from 1.5 to 13 nm, i.e., coverage between 0.45 × 10¹⁶ and 5.6 × 10¹⁶ atoms/cm², were prepared at room temperature by self-assembly of atoms deposited on quartz and sapphire substrates. For analysis of the samples, the optical spectra of the particles were measured with p-polarized light and photon energies in the range of 1.3 to 3.1 eV. Irradiating the particles during growth with laser light of different wavelengths to stimulate surface plasmon excitation made it possible to stabilize mean axial ratios between 0.19 and 0.98. The influence of the laser fluence on the shape of the nanoparticles was also investigated and shows that the position of the surface plasmon resonance shifts to higher energies as the fluence rises. Optimum growth conditions to shape gold nanoparticles with axial ratios close to unity (spheres) with a relatively low laser fluence of 60 ± 5 mJ/cm² have also been found. The results of our experiments show that laser-assisted growth is a powerfultechnique to control the shape of nanoparticles.     

Preparation and bactericide activity of gallic acid stabilized gold nanoparticles

In this work, gold nanoparticles with three different sizes (13.7, 39.4, and 76.7 nm) were prepared using a simple aqueous method with gallic acid as the reducing and stabilizing agent, the different sizes were obtained varying some experimental parameters as the pH of the reaction and the amount of the gallic acid. The prepared nanoparticles were characterized using X-ray diffraction, transmission electron microscopy, dynamic light scattering, and UV–Vis spectroscopy. Samples were identified as elemental gold and present spherical morphology, a narrow size distribution and good stabilization according to TEM and DLS results. The antibacterial activity of this gallic acid stabilized gold nanoparticles against S. mutans (the etiologic agent of dental caries) was assessed using a microdilution method obtaining a minimum inhibitory concentration of 12.31, 12.31, and 49.25 μg/mL for 13.7, 39.4, and 76.7 nm gold nanoparticles, respectively. The antibacterial assay showed that gold nanoparticles prepared in this work present a bactericide activity by a synergistic action with gallic acid. The MIC found for this nanoparticles are much lower than those reported for mixtures of gold nanoparticles and antibiotics.     

Green synthesis of gold and silver nanoparticles using gallic acid: catalytic activity and conversion yield toward the 4-nitrophenol reduction reaction

In the present report, gallic acid was used as both a reducing and stabilizing agent to synthesize gold and silver nanoparticles. The synthesized gold and silver nanoparticles exhibited characteristic surface plasmon resonance bands at 536 and 392 nm, respectively. Nanoparticles that were approximately spherical in shape were observed in high-resolution transmission electron microscopy and atomic force microscopy images. The hydrodynamic radius was determined to be 54.4 nm for gold nanoparticles and 33.7 nm for silver nanoparticles in aqueous medium. X-ray diffraction analyses confirmed that the synthesized nanoparticles possessed a face-centered cubic structure. FT-IR spectra demonstrated that the carboxylic acid functional groups of gallic acid contributed to the electrostatic binding onto the surface of the nanoparticles. Zeta potential values of ?41.98 mV for the gold nanoparticles and ?53.47 mV for the silver nanoparticles indicated that the synthesized nanoparticles possess excellent stability. On-the-shelf stability for 4 weeks also confirmed that the synthesized nanoparticles were quite stable without significant changes in their UV–visible spectra. The synthesized nanoparticles exhibited catalytic activity toward the reduction reaction of 4-nitrophenol to 4-aminophenol in the presence of sodium borohydride. The rate constant of the silver nanoparticles was higher than that of the gold nanoparticles in the catalytic reaction. Furthermore, the conversion yield (%) of 4-nitrophenol to 4-aminophenol was determined using reversed-phase high-performance liquid chromatography with UV detection at 254 nm. The silver nanoparticles exhibited an excellent conversion yield (96.7–99.9 %), suggesting that the synthesized silver nanoparticles are highly efficient catalysts for the 4-nitrophenol reduction reaction.     

Heterogeneous nucleation and growth of silver nanoparticles on unmodified polystyrene spheres by in situ reduction

The in situ reduction growth of Ag nanoparticles (NPs) on unmodified polystyrene (PS) spheres is investigated via controlling Ag nucleation and growth rates by continuous dripping addition of reductant solution in the absence of surfactants. The sub-micro PS spheres were coated by a uniform coverage of Ag NPs with several shapes like elongated islands, spherical particles, and particle aggregates. The reaction temperature and reductant concentration are demonstrated to influence the crystal structure, distribution, and stability of the Ag NPs on the PS substrates. The heterogeneous nucleation and growth of Ag NPs on PS spheres are found to depend on the inhibition of in-solution reduction and homogeneous nucleation.     

Hydrogen-assisted fabrication of spherical gold nanoparticles through sonochemical reduction of tetrachloride gold(III) ions in water

Spherical gold nanoparticles (AuNPs) were selectively synthesized through sonochemical reduction of tetrachloride gold(III) ions ([AuCl₄]⁻) in an aqueous solution of hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl₄·4H₂O) with the aid of hydrogen (H₂) gas in the absence of any additional capping agents. On the other hand, various shaped-AuNPs such as spherical nanoparticles, triangular and hexagonal plates were formed from sonochemical reduction of [AuCl₄]⁻ in argon (Ar)-, nitrogen (N₂)- or oxygen (O₂)-purged aqueous [AuCl₄]⁻ solutions. The selective fabrication of spherical AuNPs assisted by H₂ gas is most likely attributed to the generation of hydrogen radicals (H) promoted by the reaction of H₂ introduced and hydrogen oxide radicals (OH) produced by sonolysis of water.     

Kinetics and growth mechanism of gallium arsenide crystals in gas-phase epitaxy

Conclusion In conclusion, we note that gallium arsenide itself is the material with which the physicochemical and crystallophysical fundamentals of gas-phase epitaxy are presently being developed. It is hoped that the basic principles or crystal growth in gas-phase systems discovered in gallium arsenide will prove sufficiently general to be applied to other analogous systems.The complex multistage processes occurring on a crystal surface during gas-phase crystallization require development of a more general theory of crystal growth — one which considers heterogeneous reactions and participation in the surface processes of noncrystallizing atoms and molecules. On the other hand, for construction of such a theory and its comparison to experiment information will be required not only on the composition of the gaseous phase and the growth kinetics, but also on the composition and structure of the adsorption layer and the crystallization surface, the acquisition of which in gas-phase systems is complicated in comparison to, for example, molecular-beam epitaxy systems. It is possible that these difficulties will be overcome with time. A certain part of the information on composition and structure of the surface in contact with the complex gas phase can apparently be obtained under conditions close to equilibrium. This part of the problem of gas-phase epitaxy research merges completely with problems of the characterization of the physicochemical state of semiconductor surfaces in general.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 23–37, January, 1980.     

Mono and bimetallic nanoparticles of gold,silver and palladium-catalyzed NADH oxidation-coupled reduction of Eosin-Y

Abstract  

Mono metallic (Au, Ag, Pd) and bimetallic (Au–Ag, Ag–Pd, Au–Pd) with 1:1 mol stoichiometry, nanoparticles are synthesized using one-pot, temperature controlled chemical method using cetyltrimethylammonium bromide (CTAB) as the capping agent. The particle sizes (Au = 5.6, Ag = 5.0, Pd = 6.0, Au–Ag = 9.2, Ag–Pd = 9.6, Au–Pd = 9.4 nm) are characterized by UV–Vis, HRTEM, and XRD measurements, respectively. CTAB bindings onto mono and bimetallic nanoparticles are analyzed by FTIR spectra. The catalytic activities of mono and bimetallic nanoparticles are tested on the reaction between NADH oxidation and Eosin-Y reduction. The effects of base, pH, ionic strength, nature of mono and bimetallic catalysts are studied and the reaction conditions are optimized. Bimetallic nanoparticles exhibited better catalysis than the mono metallic nanoparticles, which may be due to the electronic effects of the core to shell metal atoms.     

Microwave assisted one pot synthesis of biocompatible gold nanoparticles in Triton X-100 aqueous micellar medium using tryptophan as reducing agent

The present work reports an environmentally benign process for the synthesis of gold nanoparticles via microwave-assisted heating method, using a basic amino acid l-Tryptophan as reducing agent. The synthesis was carried out in the presence of a nonionic surfactant Triton X-100 (TX-100) aqueous micellar system. Influence of various reaction parameters such as microwave irradiation time and concentrations of precursor ion were analyzed. The effect of pH on the plasmonic properties and stability of synthesized AuNPs were studied in detail. The various size distributions of gold nanostructures obtained were characterized and analyzed by ultraviolet visible spectroscopy (UV-Vis), TEM and DLS analysis. Fourier transform infrared (FT-IR) measurements were carried out to identify the interaction sites responsible for capping and efficient stabilization by the amino acid as well as the surfactant. The synthesized gold nanoparticles were stable for more than 6 months at ambient conditions.     

Facile reduction and stabilization of ginsenoside-functionalized gold nanoparticles: optimization,characterization, and in vitro cytotoxicity studies

Gold nanoparticles (GNPs) are forecasted to provide an attractive platform in biomedicine and catalysis with their potentials of combining a variety of biophysicochemical properties into an integrated nanodevice with great therapeutic and optical functions. There are several reports of crude plant extracts mediating the conversion of metal ions into nanoparticles. However, we aimed to investigate the capability of single bioactive compounds, namely ginsenosides compound K (C-K) and Rh2, to accommodate a synergistic chemical reduction of gold salts by one-pot green chemistry. Ginsenosides C-K and Rh2 are unique triterpenoid saponins present in Panax ginseng Meyer, a perennial plant traditionally used as an oriental medicinal herbal with long history. C-K and Rh2 have demonstrated diverse pharmacological properties such as anticancer, anti-inflammation, anti-aging, and neuroprotective properties. The reduction of gold ions by these ginsenosides led to the production of nontoxic GNPs as tested in mouse macrophage (J774A.1) and human kidney epithelial (HEK-293) in vitro. The kinetics of the bioreduction and the influence of pH were examined by an ultraviolet-visible (UV-Vis) spectrophotometer. GNPs were characterized by field emission transmission electron microscopy (FE-TEM), X-ray diffraction (XRD), dynamic light scattering (DLS), and Fourier transform infrared (FTIR) spectroscopy. Ginsenoside loading efficiency of C-K-GNPs and Rh2-GNPs was determined to be approximately 62.83% and 54.91%, respectively, by thermogravimetric analysis (TGA). These results suggest that one-pot synthesis by ginsenosides C-K and Rh2 may be useful for producing ginsenoside-loaded gold nanocarriers.

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Graphical abstract

     

磁控溅射法制备金团簇纳米颗粒及性能表征

采用磁控溅射法制备金团簇纳米颗粒,用透射电镜(TEM)、X射线衍射(XRD)、紫外可见光分光光度计(UV-Vis)和X射线光电子能谱(XPS)等分析手段对其表征,研究了金团簇纳米颗粒的形貌、颗粒度、结构、光吸收性质及物质成份。研究结果表明：制备的金团簇纳米颗粒呈球形,平均粒径在10 nm左右,粒径分布均匀,无团聚、氧化现象,颗粒的结构为面心立方。在519 nm处出现团簇颗粒的表面等离子共振吸收峰,测试得到Au(4f7/2)和Au(4f5/2)电子的结合能分别为83.3 eV和86.9 eV,并且没有出现金的氧化产物。     

磁控溅射法制备金团簇纳米颗粒及性能表征

 采用磁控溅射法制备金团簇纳米颗粒,用透射电镜(TEM)、X射线衍射(XRD)、紫外可见光分光光度计(UV-Vis)和X射线光电子能谱(XPS)等分析手段对其表征,研究了金团簇纳米颗粒的形貌、颗粒度、结构、光吸收性质及物质成份。研究结果表明：制备的金团簇纳米颗粒呈球形,平均粒径在10 nm左右,粒径分布均匀,无团聚、氧化现象,颗粒的结构为面心立方。在519 nm处出现团簇颗粒的表面等离子共振吸收峰,测试得到Au(4f_7/2)和Au(4f_5/2)电子的结合能分别为83.3 eV和86.9 eV,并且没有出现金的氧化产物。     

Surface nucleation in the freezing of gold nanoparticles

We use molecular simulation to calculate the nucleation free energy barrier for the freezing of a 456 atom gold cluster over a range of temperatures. The results show that the embryo of the solid cluster grows at the vapor-surface interface for all temperatures studied and that the usual classical nucleation model, with the embryo growing in the core of the cluster, is unable to predict the shape of the free energy barrier. We use a simple partial wetting model that treats the crystal as a lens-shaped nucleus at the liquid-vapor interface and find that the line tension plays an important role in the freezing of gold nanoparticles.     

Poly(ethylene glycol) and phospholipid packing in the structure of reverse micelles

The influence of water substitution by a substance with a different polarity on the structure of phospholipid monolayer interface in water-in-oil microemulsion has been studied by the Fourier-transform pulsed-gradient spin-echo (FT PGSE)¹H nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spin-label methods. For this purpose the soybean phosphatidylcholine-based microemulsion and water soluble poly(ethylene glycol) with molecular weight 400 (PEG400) were used. Self-diffusion coefficients of all microemulsion components obtained by the FT PGSE NMR technique provided information about both the size of reverse micelles and distribution of components between different microemulsion compartments. The maximum hyperfine splitting, 2A _max, in the ESR spectra was used to characterize the degree of the phospholipid hydrocarbon chain mobility. It was shown that PEG400 alters significantly the size of the reverse micelles and the motion of the labeled segments of the lipid tails. A mechanism of PEG400 acting in solution of the phospholipid-based reverse micelles on the basis of the rough decrease of the micelle core polarity was suggested.     

A Study on the Permeability of the Triton X-100 Micelles by Fluorescence Decay Kinetics of Probe Molecules

Abstract

The third armonic (355 nm) of a pulsed Nd-YAG laser has been used to excite pyrene in micellar solutions of Triton X-100. Fluorescence quenching by nitromethane and triethylamine of the excited state of pyrene in Triton X-100 has been studied. Quenching rate constant values of 4.94 ± 0.13 × 10⁸M^?1s^?1 and 1.50 ± 0.04 × 10⁸M^?1s^?1 for deactivation by nitromethane and triethylamine, respectively, have been measured. The values obtained are discussed in terms of the interface permeability.     

Preparation of stable suspensions of gold nanoparticles in water by sonoelectrochemistry

Stable suspensions of gold nanoparticles in water were prepared with high yield by a novel one-step ultrasound assisted electrochemical process. Various strategies based on the addition of either tailor-made polymers or mixtures of commercially available polymers, in the electrochemical bath have been found successful to avoid nanoparticles aggregation commonly observed by sonoelectrochemistry. alpha-Methoxy-omega-mercapto-poly(ethylene oxide) or poly(vinyl pyrrolidone)/polyethylene oxide mixtures were able to build up a coalescence barrier around the gold nanoparticles. The results showed that the size of the gold nanoparticles could be easily tuned between 5 nm and 35 nm by simple control of the electrochemical parameters, i.e. the deposition time (T(ON)) from 10 ms to 20 ms. The properties of as-prepared gold nanoparticles were compared to the ones of gold colloids prepared by the more conventional wet nanoprecipitation method using chemical reductive agents.