Formation and encapsulation of gold nanoparticles using a polymeric amine reducing agent

We report on the use of poly(allylamine) hydrochloride (PAH) as a reducing agent for the controlled formation of gold nanoparticles (AuNPs) in the size range of 5–50 nm. The formation of AuNPs using this polymer matrix allows for the AuNPs to be imbedded in the polymer matrix, once formed. The kinetics of AuNP formation are shown to be pseudo first-order in [HAuCl₄] at room temperature. The kinetics of AuNP formation are controlled by the ratio of reducing agent to HAuCl₄ as well as the overall concentration of the PAH and HAuCl₄. Additionally, at low PAH:HAuCl₄ mole ratios, the plasmon resonance wavelength can be controlled through the ratio of the reactants. This plamson resonance shift is shown to be related to AuNP size by means of TEM imaging data on the AuNPs.     

Optimization of accurate controlled formation of Au MNPs via non-thermal microplasma continuous glow discharge within aqueous electrochemical (GDE) method

In this work, micro plasma-induced non-equilibrium liquid chemistry was utilized to synthesize and controlled formation of gold metallic nanoparticles (Au MNPs) by governing the concentration of (HAuCl₄). These new approaches based on both plasma and liquid electrolytes contain charged species, and the interactions between the two phases represent a unique combination of physics, chemistry, and materials science. Continuous and stable DC glow discharge was done in home–made cavity to synthesize the definite sizes of (Au MNPs) by means of (3 kV) discharge voltage and (2 mA) discharge current for a period of (7 min) in aqueous solution of HAuCl₄ with four different concentrations of about 1 mM, 5 mM, 10 mM and 20 mM at room temperature. The atmospheric pressure plasma discharge between stainless steel capillary tube cathode electrode over the (HAuCl₄) solution and platinum plate as an anode dipped in solution for rapid formation of colloidal Au MNPs. Morphology aspects of the synthesized Au MNPs layer were studied by examining the (FE-SEM), HR-TEM images and X-ray difraction (XRD) pattern. Optical features of (Au MNPs) were considered via a UV–Vis beam spectrophotometer. These measurements showed that Au MNPs were organized by governing the concentration of HAuCl₄, and uniform Au MNPs with specific exclusive sizes were acquired. Grain size, specific surface area and optical stability of Au MNPs strongly be affected by the HAuCl₄ concentrations.     

Phyllanthin-assisted biosynthesis of silver and gold nanoparticles: a novel biological approach

Abstract  The anisotropic gold and spherical–quasi-spherical silver nanoparticles (NPs) were synthesized by reducing aqueous chloroauric acid (HAuCl₄) and silver nitrate (AgNO₃) solution with the extract of phyllanthin at room temperature. The rate of reduction of HAuCl₄ is greater than the AgNO₃ at constant amount of phyllanthin extract. The size and shape of the NPs can be controlled by varying the concentration of phyllanthin extract and thereby to tune their optical properties in the near-infrared region of the electromagnetic spectrum. The case of low concentration of extract with HAuCl₄ offers slow reduction rate along with the aid of electron-donating group containing extract leads to formation of hexagonal- or triangular-shaped gold NPs. Transmission electron microscopy (TEM) analysis revealed that the shape changes on the gold NPs from hexagonal to spherical particles with increasing initial concentration of phyllanthin extract. The Fourier transform infrared spectroscopy and thermogravimetric analyses reveal that the interaction between NPs and phyllanthin extract. The cyclic voltammograms of silver and gold NPs confirms the conversion of higher oxidation state to zero oxidation state. Graphical abstract  Anisotropic gold and silver nanoparticles were synthesized by a simple procedure using phyllanthin extract as reducing agent. The rate of bioreduction of AgNO₃ is lower than the HAuCl₄ at constant concentration of phyllanthin extract. The required size of the nanoparticles can be prepared by varying the concentration of phyllanthin with AgNO₃ and HAuCl₄.      

Controlled synthesis of gold nanospheres and single crystals in hydrogel

Narrow-dispersed gold nanospheres, regular single-crystal nanoplates and nanobulks were prepared, respectively, by reducing HAuCl₄ within a hydrogel system under UV irradiation. The formation of gold products with different geometric shape and size was found to depend on both the microenvironment of the gel matrix and the initial concentration of HAuCl₄. The resultant gold particles were investigated by UV–vis spectroscopy, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and energy-dispersion X-ray spectroscopy. Electronic Supplementary Material The online version of this article at (doi: ) contains supplementary material, which is available to authorised users.     

Synthesis of Au nanorods via autocatalytic growth of Au seeds formed by sonochemical reduction of Au(I): Relation between formation rate and characteristic of Au nanorods

The sonochemical formation of Au seeds and their autocatalytic growth to Au nanorods were investigated in a one-pot as a function of concentration of HAuCl₄, AgNO₃, and ascorbic acid (AA). The effects of ultrasonic power and irradiation time were also investigated. In addition, the formation rate of Au nanorods was analyzed by monitoring the extinction at 400 nm by UV–Vis spectroscopy and compared with the growth behavior of Au seeds to nanorods. Most of the reaction conditions affected the yield, size, and shape of Au nanorods formed. It was confirmed that the concentration balance between HAuCl₄ and AA was important to proceed the formation of Au seeds and nanorods effectively. The formation rate became faster with increasing AA concentration and dog-bone shaped nanorods were formed at high AA concentration. It was also confirmed a unique phenomenon that the shape of Au nanorods changed even after the completion of the reduction of Au(I) in the case of short-time ultrasonic irradiation for Au seed formation.     

Morphology control of the fabricated hydrophobic gold nanostructures in W/O microemulsion under microwave irradiation

The hydrophobic gold nanostructures with different shapes were prepared by using n-butanol situ reduction in cetyltrimethylammonium bromide (CTAB)/n-butanol/n-heptane/HAuCl₄(aq) W/O microemulsion through microwave dielectric heating. The CTAB-stabilized gold nanoparticles and nanowires with a networked structure obtained through this method were characterized and analyzed by ultraviolet visible spectroscopy (UV-vis), transmission electron microscopy (TEM) and X-ray diffraction (XRD), respectively. The morphologies of the hydrophobic gold nanocrystals were successfully controlled through the adjustment of molar ratios of CTAB/HAuCl₄ in the reverse microemulsion. The formation mechanism of anisotropic gold nanostructures was discussed, which demonstrated that CTAB played an important role in forming and stabilizing the shape of gold nanowires.     

Fabrication of a nanostructured gold-polymer composite material

A facile synthesis route is described for the preparation of a poly-(o-aminophenol)-gold nanoparticle composite material by polymerization of o-aminophenol (AP) monomer using HAuCl₄ as the oxidant. The synthesis was carried out in a methanol medium so that it could serve a dual solvent role, a solvent for both the AP and the water solution of HAuCl₄. It was found that oxidative polymerization of AP leads to the formation of poly-AP with a diameter of 50±10nm, while the reduction of AuCl₄ ^- results in the formation of gold nanoparticles (∼ 2nm). The gold nanoparticles were uniformly dispersed and highly stabilized throughout the macromolecular chain that formed a uniform metal-polymer composite material. The resultant composite material was characterized by means of different techniques, such as UV-vis, IR and Raman spectroscopy, which offered the information about the chemical structure of polymer, whereas electron microscopy images provided information regarding the morphology of the composite material and the distribution of the metal particles in the composite material.     

Experimental investigations of the formation kinetics of gold nanoparticles in polymeric media

The kinetics of UV-induced formation of gold nanoparticles in polymethylmethacrylate films doped with chloroauric acid HAuCl₄ is studied. The films are investigated by the absorption spectroscopy and smallangle X-ray scattering methods. The changing size and polydispersity of gold nanoparticles are analyzed during their formation. The growth of gold nanoparticles is determined by the properties of a polymeric matrix and the rate of diffusion fluxes of matter.     

A novel synthesis route for a gold–polymer soft composite material

We report on the synthesis of a metal–polymer composite material using an interfacial polymerization approach. The advantage of this approach is to form an intimate contact be‐tween the metal and polymer, which is an important param‐ eter for the synthesis of a nanocomposite material. It was found that polymerization of o‐phenylenediamine (PDA) us‐ing HAuCl₄ as an oxidizing agent leads to the formation of poly‐PDA with a fiber‐like morphology, while the reduction of HAuCl₄ results in the formation of well dispersed and sta‐bilized gold nanoparticles within the polymer matrix. The synthesis was carried out at the organic–aqueous interface. The resultant composite material was purely hydrophilic in nature and deposited at the aqueous fraction of the reaction medium. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Model for UV induced formation of gold nanoparticles in solid polymeric matrices

UV irradiation of polymeric PMMA films containing HAuCl₄ followed by annealing at 60-80 °C forms gold nanoparticles directly within the bulk material. The kinetics of nanoparticle formation was traced by extinction spectra of nanocomposite film changes vs annealing time. We propose that UV irradiation causes HAuCl₄ dissociation and thus provides a polymeric matrix with atomic gold. The presence of an oversaturated solid solution of atomic gold in the polymeric matrix leads to Au nanoparticle formation during annealing. This process can be understood as a phase transition of the first order. In this paper we apply several common kinetic models of the phase transition for describing Au nanoparticle formation inside the solid polymer matrix. We compare predictions of these models with the experimental data and show that these models cannot describe the process. We propose that the stabilization effect of the matrix on the growing gold nanoparticles is important. The simplest model introducing some probability for the transition from growing nanoparticle to the non-growing, stabilized form is suggested. It is shown that this model satisfactorily describes the experimentally observed evolution of the extinction spectrum of Au nanoparticles forming in a polymer matrix.     

XAS and XPS examination of the Au–S nanostructures produced via the reduction of aqueous gold(III) by sulfide ions

The S L- and Au L₃-edge X-ray absorption fine structure and X-ray photoelectron spectra of nanoscale Au–S products formed via the reduction of aqueous HAuCl₄ by sulfide ions and immobilized onto graphite have been acquired. The TEY XANES and XPS spectra implied the formation of predominant polysulfide species and metallic gold, while the transmission spectra showed Au–S bonding, the share of which increased with increasing molar Na₂S/HAuCl₄ ratio in the reaction solution. The Au–S distances derived from EXAFS analysis changes from 2.31 Å to 2.325 Å with the concentration of sodium sulfide in solution, being longer than that for cuprite-type crystalline Au₂S (2.174 Å). It has been concluded from all the evidence that the surface of gold sulfide decomposes in air and in ultra-high vacuum even before X-ray irradiation.     

Step-addition Method for Enhancing the Yield of Gold Nanoparticles in Block Copolymer Solution

Triblock copolymer poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) has been used to synthesize gold nanoparticles from hydrogen tetrachloroaureate (III) hydrate (HAuCl₄·3H₂O) salt in aqueous solution at room temperature. Measurements were performed using the triblock copolymer Pluronic P85 (EO₂₆PO₃₉EO₂₆) at a fixed concentration (1 wt%) mixed with varying HAuCl₄·3H₂O concentration in the range of 0.001 to 0.1 wt%. The surface plasmon resonance (SPR) band in UV-visible absorption spectra confirmed the formation of the gold nanoparticles. The maximum yield of the nanoparticles was found at 0.005 wt% of the salt solution. Small-angle neutron scattering (SANS) does not show any significant change in the scattering profile in these suspensions of the nanoparticles. A similar behavior was also observed in dynamic light scattering (DLS) experiments where autocorrelation function was found to be independent of the salt concentration. This can be understood since a high-block copolymer-to-gold ion ratio (r ～ 22) is required in the reduction reaction to produce gold particles. As a result, a very small fraction of the block copolymers were associated with the gold nanoparticles, and hence lead to a very low yield. Both SANS and DLS basically see the micelles of most of these block copolymers, which are not associated with nanoparticles. Based on this explanation, a step-addition method was used to enhance the yield of gold nanoparticles by manifold, where the gold salt is added in small steps to maintain higher value of r (>22), and therefore continuous formation of nanoparticles.     

Novel fabrication and catalytic application of poly(ethylenimine)-stabilized gold–silver alloy nanoparticles

Novel synthesis of amine-stabilized Au–Ag alloy nanoparticles with controlled composition has been devised using poly(ethylenimine) (PEI) as a reducing and a stabilizing agent simultaneously. The composition of Au–Ag alloy nanoparticles was readily controlled by varying the initial relative amount of HAuCl₄ and AgNO₃. Due to the presence of abundant amine functional groups in PEI, which could act as the dissolving ligand for AgCl, the precipitation problem of Ag⁺ in the presence of Cl⁻ from the gold salt was avoided. On this basis, the relatively high concentrations of HAuCl₄ and AgNO₃ salts were used for the fabrication of Au–Ag alloy nanoparticles. The PEI thus plays triple roles in this study that include the co-reducing agents for HAuCl₄ and AgNO₃, the stabilizing agents for Au–Ag alloy nanoparticles, and even the dissolving agents for AgCl. As a novel material for use in catalysis, the Au–Ag alloy nanoparticles including pure Au and Ag samples were exploited as catalysts for the reduction of 4-nitrophenol in the presence of NaBH₄. As the Au content was increased in the Au–Ag alloy nanoparticles, the rate constant of the reduction was exponentially increased from pure Ag to pure Au.     

Efficient seed-mediated method for the large-scale synthesis of Au nanorods

Seed-mediated methods are widely followed for the synthesis of Au nanorods (NRs). However, mostly dilute concentrations of the Au precursor (HAuCl₄) are used in the growth solution, which leads to a low final concentration of NRs. Attempts of increasing the concentration of NRs by simply increasing the concentration of HAuCl₄, other reagents in the growth solution and seeds lead to a faster growth kinetics which is not favourable for NR growth. Herein, we demonstrate that the increase in growth kinetics for high concentrations of reagents in growth solution can be neutralised by decreasing the pH of the solution. The synthesis of the NRs can be scaled up by using higher concentrations of reagents and adding an optimum concentration of HCl in the growth solution. The concentration of HAuCl₄ in the growth solution can be increased up to 5 mM, and 10–20 times more NRs can be synthesised for the same reaction volume compared to that of the conventional seed-mediated method. We have also noticed that a cetyltrimethylammonium bromide (CTAB)-to-HAuCl₄ molar ratio of 50 is sufficient for obtaining high yield of NRs.     

Peptide-induced patterning of gold nanoparticle thin films

In this work, the use of patterned proteins and peptides for the deposition of gold nanoparticles on several substrates with different surface chemistries is presented. The patterned biomolecule on the surface acts as a catalyst to precipitate gold nanoparticles from a precursor solution of HAuCl₄ onto the substrate. The peptide patterning on the surfaces was accomplished by physical adsorption or covalent attachment. It was shown that by using covalent attachment with a linker molecule, the influence of the surface properties from the different substrates on the biomolecule adsorption and subsequent nanoparticle deposition could be avoided. By adjusting the reaction conditions such as pH or HAuCl₄ concentration, the sizes and morphologies of deposited gold nanoparticle agglomerates could be controlled. Two biomolecules were used for this experiment, 3XFLAG peptide and bovine serum albumin (BSA). A micro-transfer molding technique was used to pattern the peptides on the substrates, in which a pre-patterned poly(dimethylsiloxane) (PDMS) mold was used to deposit a lift-off pattern of polypropylmethacrylate (PPMA) on the various substrates. The proteins were either physically adsorbed or covalently attached to the substrates, and an aqueous HAuCl₄ solution was applied on the substrates with the protein micropatterns, causing the precipitation of gold nanoparticles onto the patterns. SEM, AFM, and Electron Beam Induced Current (EBIC) were used for characterization.     

Catalytic properties of gold nanoparticles immobilized on the surfaces of nanocarriers

Gold nanoparticles are immobilized in the hydrophilic coronas of spherical micelle carriers for high catalytic activity. The micelle is formed by self-assembly of block copolymer, polystyrene-b-poly (acrylic acid), in basic aqueous solution (pH 10) and has a polystyrene core and a poly (acrylic acid) corona. The gold nanoparticles are anchored into the poly (acrylic acid) corona by in situ reduction of the mixture of HAuCl₄ and micelle with NaBH₄. The sizes of the gold nanoparticles can be adjusted by changing the content of the HAuCl₄. In the process of catalyzing p-nitrophenol to p-aminophenol, the reaction shows one-order kinetics, furthermore, the reaction rate increases with the concentration of composites as well as reaction temperature. Comparing the composites with polystyrene as core and poly (4-vinylpyridine)/Au as corona, the catalytic activity of the present composites is higher, which is ascribed to their hydrophilic corona structure.     

Size-dependent transformation from Ag templates to Au–Ag nanoshells via galvanic replacement reaction in organic medium

The transformation from Ag templates to Au?CAg nanoshells via galvanic replacement reaction with HAuCl₄ was systematically studied in an organic medium in the presence of oleylamine. Decahedral (~43?nm in size) and triangular prism (~53?nm in edge length) Ag templates transformed into equiaxed and triangular prismatic Au?CAg nanoshells, respectively. The first step involved structural and morphological changes from Ag templates to Au?CAg nanoshells with an interior cavity. In the second step, the growth of the shells continued through the deposition of Au. The shell thickness increased from ~5 to ~10?nm for the equiaxed Au?CAg nanoshells (~39-nm interior cavity) and ~5 to ~8?nm for the triangular prismatic Au?CAg nanoshells (~52-nm interior edge length). Oleylamine not only served as a surfactant but also removed AgCl precipitates and reduced HAuCl₄. For the nanoshells derived from the ~20-nm Ag decahedrons, further reaction in excess HAuCl₄ collapsed the nanoshells into Au-rich solid fragments. However, the nanoshells derived from the ~43-nm Ag decahedrons, the nanoshell structure not only persisted in excess HAuCl₄, but its shell thickness also increased. The size-dependent transformation of these nanoshells is discussed.     

General approach for fabricating nanoparticle arrays via patterned block copolymer nanoreactors

A general approach to fabricate nanoparticle arrays of different kinds of materials is demonstrated in this paper. It was found that the center-to-center distance of the nanoparticles or the nanoclusters can be controlled using patterned block copolymer nanoreactors by adding polystyrene (PS) homopolymer to poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) diblock copolymer thin film. The number of the nanoparticles formed in the P4VP nanodomains can also be adjusted by addition of polystyrene (PS) homopolymer to poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) diblock copolymer. In fabrication of Au nanoparticle arrays, HAuCl₄ precursor was directly loaded into P4VP nanodomains of the diblock copolymer thin film by using a methanol solvent, which is a good solvent for P4VP but non-solvent for PS. The Au nanoparticle arrays were then obtained by reducing HAuCl₄ with sodium citrate dihydrate, and then in situ transferred to silicon substrate by a two-step calcination method. ZnO and Fe _x O _y nanoparticle arrays were also synthesized by this approach with thermal decomposition and double decomposition reactions, respectively. Additionally, the advantage of using two-step calcination method over the air plasma method was discussed.     

Gold chloride cluster ions generated by vacuum laser ablation

In this work, a simple way for study the possibility of formation a vapor cluster species of tetrachloroauric acid (HAuCl₄), using the laser ablation in the absence of a buffer or reactive atmosphere, and without a postablation supersonic expansion on a commercial matrix assisted laser desorption/ionization time-of-flight mass spectrometer, is reported. Tetrachloroauric acid is known as precursor for the synthesis of gold nanostructures and the complex salts; therefore it is an important task to discover and quantify the species arising from HAuCl₄, in order to understand their role in the gold assisted reactions. Mass spectrum of HAuCl₄ in a reflector negative-ion mode contains the hydrated mono- and dinuclear gold clusters in the m/z range 286–436, and gold chloride clusters in the m/z range 447–795. In the first part of spectrum, m/z range 286–436, the hydrated gold cluster species of type Au _n ^? (H₂O)_m (n?=?1–2; m?=?1, 2, 5, 7, 8) and [Au_n(OH)_k]^?(H₂O)_m (n?=?1–2; k?=?1–2; m?=?1, 4–8) were found. Besides that, there are gold chloride clusters with general formula [AuH_r(HCl)₂]^?(H₂O)_m (m?=?1–5; 8–9; r?=?0–2) in this part of spectrum. In the second part of spectrum, the m/z range 447–795, only gold chloride clusters were obtained. Their general formulae can be written as [AuCl_t(HCl)_v]^?(H₂O)_m (t?=?1–4; v?=?5–8; m?=?2–4, 6–8) and [Au_n(HCl)_v]^?(H₂O)_m (n?=?1–2, v?=?4–5, m?=?1–2, 5, 7). The analysis of concentration effects on the LDI mass spectra of gold clusters reveals that the relative intensities of signals for the mono- and dinuclear Au clusters increase with decreasing the concentration of water HAuCl₄ solutions.     

Gold nanoparticles modified GC electrodes: electrochemical behaviour dependence of different neurotransmitters and molecules of biological interest on the particles size and shape

Gold colloidal nanoparticles (AuNps), synthesized by gold chloride hydrate (HAuCl₄) chemical reduction were used to realize a modified glassy carbon electrode (GCE). Different shapes and sizes were observed, varying the molar ratio of HAuCl₄ and polyvinylpyrrolidone (PVP). The electrochemical behaviour of different neurotransmitters and molecules of biological interest (dopamine, caffeic acid, catechol, uric acid, epinephrine and serotonin) were investigated by cyclic voltammetry (CV) at the AuNps modified GCE and a dependence of the electrochemical response on the size and the shape of the particles was observed. The electrochemical responses were stable during time with a generic decreasing of the peak current after 10 days ranging from 5–10% for catechol, uric acid and serotonine to 10–15% for the other analytes. A study on the electrochemical interface of modified electrodes was also carried out by means of electrochemical impedance spectroscopy (EIS).