X-ray absorption of gold nanoparticles with thin silica shell

Silica-coated gold (Au) nanoparticles were prepared and their morphological and X-ray absorption properties were investigated. These core-shell type nanoparticles are very stable in aqueous media and may be suitable for an X-ray contrast agent in biological systems. Transmission electron micrographs confirmed well-separated and relatively homogeneous morphology of the nanoparticles in highly concentrated colloids. Peak position for Au plasmon resonance was red-shifted with increasing shell thickness. X-ray absorption by the colloids of silica-coated Au nanoparticles was stronger than that by those of silica-coated Agl nanoparticles, a recently investigated X-ray contrast agent, at similar experimental conditions.     

Green synthesis of colloidal gold nanoparticles using latex from Hevea brasiliensis and evaluation of their in vitro cytotoxicity and genotoxicity

Latex extracted from Hevea brasiliensis tree has been used as a green alternative for preparing gold nanoparticles (Au NPs); however, no study evaluating the cytotoxic and genotoxic potential of Au NPs synthesised using H. brasiliensis has been published. The present study aimed to synthesise and characterise colloidal Au NPs using latex from H. brasiliensis and to evaluate their in vitro cytotoxicity and genotoxicity. Ideal conditions for the green synthesis of Au NPs were studied. In vitro cytotoxicity and genotoxicity of Au NPs in CHO‐K1 cells was also evaluated. Our findings indicated that the ideal synthesis conditions of pH, temperature, reduction time, and concentrations of latex and HAuCl₄ were 7.0, 85°C, 120 min, 3.3 mg/mL, and 5.0 mmol/L, respectively. LC50_{24 h} of Au NPs was 119.164 ± 5.31 μg/mL. Lowest concentration of Au NPs tested presented minimal cytotoxicity and genotoxicity. However, high concentrations of Au NPs promoted DNA damage and cell death via apoptosis. On the basis of these findings, the authors optimised the use of an aqueous solution of H. brasiliensis latex as a reducing/stabilising agent for the green synthesis of Au NPs. Low concentrations of these NPs are biocompatible in normal cell types, suggesting that these NPs may be used in biological applications.Inspec keywords: nanofabrication, biomedical materials, nanomedicine, colloids, pH, cellular biophysics, nanoparticles, gold, DNA, reduction (chemical), molecular biophysics, materials preparation, geneticsOther keywords: colloidal Au NPs, genotoxicity, green synthesis, in vitro cytotoxicity, Hevea brasiliensis, colloidal gold nanoparticle, latex concentrations, DNA damage, cell death, H. brasiliensis latex, normal cell types, Au     

One‐Pot Synthesis of Multifunctional Au@Graphene Oxide Nanocolloid Core@Shell Nanoparticles for Raman Bioimaging,Photothermal, and Photodynamic Therapy

The paper reports a facile one‐pot synthesis of core@shell nanoparticles (NPs) composed of Au core and graphene oxide nanocolloid (GON) shell. Unique properties of Au NPs and GON can be incorporated into a single nanohybrid structure to provide desirable functions for theranosis such as localized surface plasmon resonance, Raman scattering, amphiphilic surface, and photothermal conversion. Synthesis of Au@GON NPs is achieved by simple one‐pot reaction in aqueous phase utilizing GON as a reducing and stabilizing agent without any additional reducing agent. The zinc phthalocyanine, a photosensitizer, loaded Au@GON NPs show excellent multifunctional properties for combinational treatment of photothermal and photodynamic therapy in addition to Raman bioimaging with low cytotoxicity.     

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).     

Uniformly gold nanoparticles derived from P2VP-b-PCHMA block copolymer templates with different reduction methods

The micellization of poly(2-vinylpyridine)-block-poly(cyclohexyl methacrylate) (P2VP-b-PCHMA) in THF can be induced by the complexation between the P2VP blocks and HAuCl4, forming composite polymeric micelles with PCHMA being the shell and P2VP/HAuCl4 complex being the core. In order to obtain regular arrays of gold nanoparticles (Au NPs), monolayer of HAuCl4-loaded surface micelles have been produced by spin-coating the micellar solution, and Au NPs in different size have been obtained by oxygen plasma with different reduction processes. In addition, pyrole (PY) has been used as an efficient reducing agent to fabricate dispersed Au NPs within micellar structure in a short reducing time, resulting in a raspberry-like morphology of the Au-polymer composites. With the addition of annealing processes or longer reducing time (one month), different shapes of Au NPs have been observed in the cast films. Furthermore, core-shell nanostructures of gold-polypyrole (Au-PPY) have also been observed by employing vapor phase polymerization of PY onto HAuCl4-loaded polymeric solution-cast films.     

Porous Au–Ag Nanoparticles from Galvanic Replacement Applied as Single-Particle SERS Probe for Quantitative Monitoring

Plasmonic nanostructures have raised the interest of biomedical applications of surface-enhanced Raman scattering (SERS). To improve the enhancement and produce sensitive SERS probes, porous Au–Ag alloy nanoparticles (NPs) are synthesized by dealloying Au–Ag alloy NP-precursors with Au or Ag core in aqueous colloidal environment through galvanic replacement reaction. The novel designed core–shell Au–Ag alloy NP-precursors facilitate controllable synthesis of porous nanostructure, and dealloying degree during the reaction has significant effect on structural and spectral properties of dealloyed porous NPs. Narrow-dispersed dealloyed NPs are obtained using NPs of Au/Ag ratio from 10/90 to 40/60 with Au and Ag core to produce solid core@porous shell and porous nanoshells, having rough surface, hollowness, and porosity around 30–60%. The clean nanostructure from colloidal synthesis exhibits a redshifted plasmon peak up to near-infrared region, and the large accessible surface induces highly localized surface plasmon resonance and generates robust SERS activity. Thus, the porous NPs produce intensely enhanced Raman signal up to 68-fold higher than 100 nm AuNP enhancement at single-particle level, and the estimated Raman enhancement around 7800, showing the potential for highly sensitive SERS probes. The single-particle SERS probes are effectively demonstrated in quantitative monitoring of anticancer drug Doxorubicin release.     

Controlled synthesis and optical properties of Au and Au@PS nanoparticles

In this study, uniform gold (Au) nanoparticles (NPs) were prepared using seed-mediated growth method. The particle size was controlled by tuning the dosage of seed solution. Au@PS core–shell NPs were then synthesized by introducing a polystyrene (PS) shell (2–3 nm thick) around the core of Au NPs (115 nm). Evaluation of the surface plasmon (SP) optical properties indicated that wavelength of SP resonance of Au NPs increased gradually with increase in the particle size. This red shift was about 0.92 nm per 1 nm increase in particle size. The results also indicated that the zeta potential and optical properties of Au NPs could be adjusted by coating PS on the outside. Therefore, surface modifications and surface coating were effective ways to control the optical properties of Au NPs.     

Chemical transformation of Au-tipped CdS nanorods into AuS/Cd core/shell particles by electron beam irradiation

We demonstrate that electron irradiation of colloidal CdS nanorods carrying Au domains causes their evolution into AuS/Cd core/shell nanoparticles as a result of a concurrent chemical and morphological transformation. The shrinkage of the CdS nanorods and the growth of the Cd shell around the Au tips are imaged in real time, while the displacement of S atoms from the CdS nanorod to the Au domains is evidenced by high-sensitivity energy-dispersive X-ray (EDX) spectroscopy. The various nanodomains display different susceptibility to the irradiation, which results in nanoconfigurations that are very different from those obtained after thermal annealing. Such physical manipulations of colloidal nanocrystals can be exploited as a tool to access novel nanocrystal heterostructures.     

Controlling the aggregation behavior of gold nanoparticles

The aggregation of Au nanoparticles (NPs) in solution is influenced by cationic and oligocationic species. The polarization of the conduction electron oscillations in adjacent gold nanoparticles causes a new red-shifted plasmon absorbance attributed to the coupling of the plasmon absorbance of the particles. This appearance of an additional plasmon band is of particular interest to the field of SERS and has led to research works directed at the stabilization of small colloid aggregates in solution. The surface plasmon coupling can be tuned by controlling the aggregation of gold nanoparticles by the addition of some “cross-linking” agent. Here we develop a simple method to fabricate linear-chainlike aggregates of gold nanoparticles (so-called nanochains), tuning the linear optical properties in a wide wavelength range from visible to the near-infrared. The aggregation behavior and linear self-assembly mechanism of citrate-stabilized gold colloids as provoked by the addition of cetyltrimethylammonium bromide (CTAB) and 11-mercaptoundecanoic acid (MUA) are also analyzed. The line-assembly mechanism of gold nanochain is attributed to the preferential binding of CTAB molecules on a certain facet of gold NPs and the Au NP electrostatic interactions. We also found that the 11-mercaptoundecanoic acid was effective to prevent the further aggregation of CTAB-modified gold colloids.     

Synthesis of symmetrical hexagonal-shape PbO nanosheets using gold nanoparticles

Anisotropic growth of PbO with symmetrical hexagonal-shape nanosheet morphology was demonstrated for the first time via solution phase synthesis in the presence of Au nanoparticles at room temperature. Au NPs play a critical role in the formation of PbO nanosheets. No nanosheets were formed in absence of Au NPs. The effect of Au NPs appears to result from their ability to provide nucleation sites to seed anisotropic growth of the PbO nanocrystals and later the nanocrystals aggregated to form nanosheet structure. The method demonstrated here provides a facile room temperature colloidal method of producing high-quality and yield of high-symmetrical hexagonal-shape PbO nanosheets with controlled edge length.     

Stabilising agent specific synthesis of multi-branched and spherical au nanoparticles by reduction of AuCl4- by Fe2+ ions

Herein we report a new method of synthesis of Au nanoparticles (NPs) of multi-branched and spherical shapes. HAuCl4 when reduced by Fe2+ ions in aqueous solution, in the presence of sodium dodecyl sulfate (SDS), produced multi-branched Au NPs including a shape that could be termed as a "nanofan." On the other hand spherical particles were produced in the presence of Na2S and SDS or starch (instead of SDS). UV-vis, X-ray diffraction, transmission electron microscopic (TEM) studies revealed the difference in optical properties and shapes under various conditions. The essential conclusion of this report is that stabilizing agent and other reagents do have special role in controlling the shape of the NPs produced.     

Au/LaVO4 Nanocomposite: Preparation, characterization, and catalytic activity for CO oxidation

The size of the gold particles is a very important parameter to get active catalysts. This paper reports a novel colloidal deposition method to prepare Au/LaVO₄ nanocomposite catalyst with monodispersed Au colloids and uniform LaVO₄ nanoplates in nonpolar solvent. Monodispersed Au colloids with tunable size (such as 2, 5, 7, 11, 13, and 16 nm) and LaVO₄ nanocrystals with well-defi ned shapes were pre-synthesized assisted with oleic acid/amine. During the following immobilization process, the particle size and shape of Au and LaVO₄ were nearly preserved. As-prepared Au/LaVO₄ nanocomposite showed high catalytic activity for CO oxidation at room temperature. Since sizes of gold particles were well-defi ned before the immobilization process, size effect of gold particles was easy to be investigated and the results show that 5-nm Au/LaVO₄ nanocomposite has the highest activity for CO oxidation. This synthetic method can be extended further for the preparation of other composite nanomaterials.     

Self-assembly directed synthesis of poly(ortho-toluidine)-metal(gold and palladium) composite nanospheres

Poly(ortho-toluidine) (POT)-gold (Au) and palladium (Pd) composite nanospheres were successfully synthesized by the reaction of o-toluidine with the corresponding metal (Au or Pd) colloidal solution through self-assembly process in the presence of dodecylbenzenesulfonic acid (DBSA), which acts as both a dopant and surfactant, and ammonium peroxydisulfate as an oxidizing agent. The composites (POT-DBSA/Au or Pd) were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, UV-Visible (UV-Vis) spectroscopy, and electrical conductivity measurements. TEM images of the nanocomposites reveal that metal (Au or Pd) nanoparticles were well dispersed on POT spheres. TGA and XRD results show that the composites exhibit high thermal stability and are more crystalline compared with pristine POT. It was found that the electrical conductivity of the POT-DBSA/Au or Pd composites is 2 orders of magnitude higher than that of pristine polymer. Also, the POT-DBSA/Pd composite exhibits magnetic property. The formation mechanism of the POT-DBSA/Au or Pd composite nanosphere is discussed.     

Gold nanoparticles by Terminalia bellirica aqueous extract – a rapid green method

Synthesis of metal nanoparticles (NPs) using plant extracts as reducing agents is of great interest due to its ease and environmental friendly process. Reports show biogenic green synthesis reaction times in forming gold metal nanoparticles (Au NPs) varying from minutes to several hours. In this article, an instantaneous (less than 10?s) method for the green synthesis of gold NPs using aqueous extract of Terminalia bellirica as a reducing and stabilising agent has been reported. Formation of Au NPs was instantaneous and confirmed by UV-Vis spectroscopy where surface plasmon resonance band centred at 530?nm. Formation of anisotropic Au NPs was evidenced from transmission electron microscope studies. High levels of polyphenols in T. bellirica were responsible for the rapid reduction and stabilisation. The Au NPs did not display toxicity when tested by the brine shrimp (Artemia salina) assay.     

Control of Shell Morphology in p–n Heterostructured Water‐Processable Semiconductor Colloids: Toward Extremely Efficient Charge Separation

This article describes p–n heterostructured water‐borne semiconductor naonoparticles (NPs) with unique surface structures via control of shell morphology. The shell particles, comprising PC60–[6,6]‐phenyl‐C61‐butyric acid methyl ester (PC₆₁BM) composite, having n‐type semiconductor characteristics, notably influence the charge carrier behavior in the core–shell NPs. A one‐ or two‐phase methodology based on a PC60 surfactant‐water phase and PC₆₁BM n‐type semiconductor‐organic phase provides highly specific control over the shell structure of the NPs, which promote their superior charge separation ability when combined with poly‐3‐hexyl‐thiophene (P3HT). Moreover, the resulting water‐borne NP exhibits shell morphology‐dependent carrier quenching and stability, which is characterized via luminescence studies paired with structural analysis. Corresponding to the results, outstanding performances of photovoltaic cells with over 5% efficiency are achieved. The results suggest that the surrounding shell environments, such as the shell structure, and its electronic charge density, are crucial in determining the overall activity of the core–shell p–n heterostructured NPs. Thus, this work provides a new protocol in the current fields of water‐based organic semiconductor colloids.     

Synthesis of nanocomposite thin films containing Ag-Au alloy colloids for wavelength tunability

A chemical solution deposition route to synthesize silver-gold alloy colloids in thin films with diameters between 8–35 nm has been developed. Ag-Au alloy colloids were synthesized by the addition of silver ions to a polymer protected aqueous gold sol in presence of a seeding agent 'hydroxylamine hydrochloride,' followed by a heat-treatment under reducing atmosphere at temperatures ranging from 150–550°C. The resonance wavelength of Ag-Au alloy colloids exists between those of pure Ag (410 nm) and pure Au (525 nm) colloids, which can easily be controlled by selecting the molar ratio of Ag to Au. This allows the tunability of the absorption wavelength (hence the color) by using Ag-Au alloy colloids in thin films.     

Necklace-shaped Au-Ag nanoalloys: laser-assisted synthesis and nonlinear optical properties

Here in this paper, necklace-shaped Au-Ag nanoalloys (NAs) have been synthesized by a laser-based approach. A chain of Ag nanoparticles (NPs), which were joined together with Au junctions, was formed upon copper vapor laser (CVL) irradiation of a colloidal mixture of Ag and Au NPs; while the corresponding NPs were separately provided by laser ablation of gold and silver targets in deionized water by a 1064 nm Q-switched Nd:YAG laser. Dependence of the NAs development process on the CVL irradiation time in three distinct stages of as-mixed, nucleation and complete formation has been systematically studied by UV-vis optical absorption spectroscopy analysis as well as by transmission electron microscopy (TEM), which was exploited to visually confirm the NAs evolution through the process. Furthermore, the x-ray photoelectron spectroscopy (XPS) technique was accurately employed to determine the synthesized alloy content. On the other hand, using the open-and closed-aperture Z-scan technique, the nonlinear absorption (NLA) as well as nonlinear refraction (NLR) changes in Au-Ag NAs were investigated through their formation. The deduced results from the nonlinear optical properties of the colloidal NAs in the mentioned stages were interpreted considering the spectroscopic and microscopic observations. The total change of individual Au and Ag NPs saturable absorption (SA) into the reverse saturable absorption (RSA) behavior was concluded through the evolution into Au-Ag NAs.     

Electrospun polyvinyl alcohol/chitosan composite nanofibers involving Au nanoparticles and their in vitro release properties

Au nanoparticles (Au NPs) containing polyvinyl alcohol (PVA)/chitosan (CS) composite nanofibers were successfully prepared by a simple and effective method called electrospinning. Au NPs were firstly synthesized under a mild condition with CS as the reducing agent and stabilizer, followed by being mixed with PVA solution and then the resulting fibers were fabricated. The research indicated that Au NPs were indeed doped into the as-prepared fibers and the composite fibers well preserved Au NPs' unique optical characteristics. Additionally, with the adjustment of the weight ratios between PVA and CS, the diameter distribution and the morphology of the nanofibers were largely changed. In vitro drug release experiments demonstrated that the drug release rate can be conveniently controlled by changing the crosslink time.     

Growth of two-dimensional arrays of uncapped gold nanoparticles on silicon substrates

A method of preparing large area patterned 2D arrays of uncapped gold (Au) nanoparticles has been developed. The pattern has been formed using self-assembly of uncapped Au nanoparticles. The Au nanoparticles were synthesized via toluene/water two phase systems using a reducing agent and colloidal solution of Au nanoparticles was produced. These nanoparticles have been prepared without using any kind of capping agent. Analysis by TEM showed discrete Au nanoparticles of 4 nm average diameter. AFM analysis also showed similar result. The TEM studies showed that these nanoparticles formed self-assembled coherent patterns with dimensions exceeding 500 nm. Spin coating on silicon substrate by suitably adjusting the speed can self-assemble these nanoparticles to lengths exceeding 1 μm.     

Size and morphology controlled synthesis of gold nanoparticles in green solvent: Effect of reducing agents

The effect of reducing agents on the synthesis of Au(0) metallic nanoparticles (Au NPs) prepared in green solvent medium of β-d-glucose-water dispersions has been reported first. The different equivalent amounts of NaBH₄ and pH values adjusted by NaOH were tested for the reduction of Au salt (HAuCl₄·3H₂O (hydrogen tetrachloroaurate (III) trihydrate) to obtain Au NPs. The type and the amount of reducing agent and the pH of the solution affected the size and morphology of the NPs. Addition of 4 equivalents of NaBH₄ produced homogeneously dispersed 5.3 nm (σ = 0.7) diameter particles. Excess addition of NaBH₄ caused the NPs to settle down as the precipitate forming mesh or wire structure. When salt was reduced by the addition of NaOH (pH = 8.0) the particles were larger (14.2 nm) and less homogeneous (σ = 2.8). At pH = 12.2 the NPs settled at the bottom of the vial when preparation was left overnight. The wire and mesh like structures were obtained at higher pH = 12.2.