Size and composition control of Pt-In nanoparticles prepared by seed-mediated growth using bimetallic seeds

A two-step method has been developed for precise size and composition control of bimetallic Pt-In nanoparticles. Very small (1.62 nm) PtIn seed nanoparticles with 1:1 metal ratio were prepared in the absence of capping agents followed by growth of Pt on their surface in the presence of oleyl amine as reducing and stabilizing agent. Nanoparticles with bulk compositions of Pt(4)In, Pt(3)In, and Pt(2)In could be synthesized with average diameter smaller than 3 nm. TEM, EDX, and XPS provided evidence for homogeneous growth without separate nucleation of pure platinum nanoparticles in the reaction solution. Pt(3)In nanoparticles were deposited onto SiO(2) surface by incipient wetness impregnation. Temperature-induced changes in the particle surface were monitored by in situ IR spectroscopy and CO adsorption. It was found that surface alloy composition of the particles could be tuned by using oxidizing or reducing atmospheres.     

Electrochemical determination of nitrite using a gold nanoparticles-modified glassy carbon electrode prepared by the seed-mediated growth technique.

Seed-mediated growth of gold nanoparticles on glassy carbon (GC) surfaces was developed. The field emission scanning electron microscopy (FE-SEM) and electrochemical characterization confirmed the effective attachment of gold nanoparticles on GC surface with such a wet-chemical method. The as-prepared gold nanoparticles attached glassy carbon electrode (Au/GCE) presented excellent catalytic ability toward the oxidation of nitrite. Compared with bare GCE and planar gold electrode, the Au/GCE obviously decreased the overpotential of nitrite oxidation and improved the peak current. The catalytic current was found to be linearly proportional to the nitrite concentration in the range of 1 x 10(-5) - 5 x 10(-3) M, with a detection limit of 2.4 x 10(-6) M. The Au/GCE was successfully applied to the electrochemical determination of nitrite in a real wastewater sample, showing excellent stability and anti-interference ability.     

Label-free PSA electrochemical determination by seed-mediated electrochemically-deposited gold nanoparticles on an FTO electrode

Journal of Solid State Electrochemistry - A facile and sensitive approach is introduced to precisely determine trace amounts of prostate specific antigen (PSA) by gold nanostructures deposited on...     

The crystallographic orientation of gold surfaces at the gold-aqueous solution interphases

In order to gain further understanding of the capacity-potential curves observed at the single crystal gold-aqueous solution interphases, the five faces (100), (110), (111), (311) and (210) which should have extreme electrochemical behaviour are discussed. Correlations are established between hysteresis, of C(E) curves and current peaks. A surface reconstruction exists for the three faces of lowest indices, as is observed in ultra-high vacuum (UHV) for 5d metals. The behaviour of the (311) and (210) faces is analysed. The results obtained for each face form a sequence dependent on the adsorbability of the anions. The explanation given in this paper for secondary phenomena observed on C(E) curves for gold faces, coordinates the results obtained.     

Extinction coefficients of gold nanoparticles and their dimers. Dependence of optical factor on particle size

The generalized Mie theory has been employed to calculate extinction coefficients κ for isolated gold nanoparticles of different sizes (4–80 nm) and their dimers (κ₂) at the maximum of the short-wave plasmon resonance band. It has been found that the value of κ₂ essentially depends on both interparticle distance s and particle sizes R. According to the character of variations in the κ₂(s) dependence, three ranges of the distances are distinguished, i.e., large, intermediate, and small. In the first range, the κ₂ values slightly differ from doubled κ values. Nevertheless a tendency toward an increase in κ₂ is distinctly seen as the particles approach each other, and, within some range of s values and for particles with radii R < 15 nm, κ₂ is higher than 2κ. For dimers of larger particles, κ₂ < 2κ, with its value gradually decreasing with a reduction in the s value. The behavior changes when the particle sizes are of about 50 nm. In the range of small interparticle distances, κ₂ values slightly vary with the distance between the particles somewhat decreasing or oscillating with a small amplitude about some mean value. In this range, as the sizes of the gold particles grow, the extinction coefficients of dimers increasingly deviate from the sum of the extinction coefficients of the particles composing the dimers. For 20–80 nm nanoparticles, the size-dependence plotted for the extinction efficiency of dimers in logarithmic coordinates within the range of small interparticle distances is described by a straight line, the slope of which (1.036 ± 0.039) appears to be somewhat smaller than the slope of a corresponding line for individual gold particles (1.274 ± 0.014). The ratio between κ₂ and κ predetermines the character of variations in the optical factor as depending on the particle size and the interparticle distance and governs the behavior of the sol turbidity at the stage of nanoparticle dimerization.     

Polymer size and concentration effects on the size of gold nanoparticles capped by polymeric thiols

Gold nanoparticles stabilized by thiol-terminated poly(ethylene glycol) monomethyl ethers with molecular weights ranging from 350 to 2000 have been prepared at thiol-to-gold molar ratios ranging from 3:1 to 1:8. Particle size distributions have been constructed for these particles from transmission electron microscopy images of hundreds of particles for each variation in synthetic conditions. The mean diameters of these particles range from 1.5 to 3.2 nm, with a slight increase in particle size with decreasing thiol content; these particles are smaller than those prepared using alkanethiols at similar thiol-to-gold ratios. Particles prepared under thiol-poor conditions exhibit much greater polydispersity than those prepared under thiol-rich conditions and include numerically rare large-particle outliers that contain much of the gold in the sample. The mean diameters of the gold nanoparticles decrease slightly with increasing polymer weight, especially under thiol-rich conditions. A simple model is developed to predict the trends in nanoparticle diameter that would result were the polymer's steric bulk protecting the nanoparticles from additional growth the principal factor controlling nanoparticle size in this system. This model predicts a much stronger dependence on thiol concentration than has been experimentally observed and a dependence on polymer molecular weight opposite to that experimentally observed. This suggests that the polymers' steric bulk is not the principal reason that these polymers yield smaller nanoparticles than alkanethiols at similar thiol-to-gold ratios. It is instead proposed that polar polymers may yield small nanoparticles by accelerating particle nucleation via coordination between functional groups in the polymer and atomic gold.     

Nanostructures of functionalized gold nanoparticles prepared by particle lithography with organosilanes

Periodic arrays of organosilane nanostructures were prepared with particle lithography to define sites for selective adsorption of functionalized gold nanoparticles. Essentially, the approach for nanoparticle lithography consists of procedures with two masks. First, latex mesospheres were used as a surface mask for deposition of an organosilane vapor, to produce an array of holes within a covalently bonded, organic thin film. The latex particles were readily removed with solvent rinses to expose discrete patterns of nanosized holes of uncovered substrate. The nanostructured film of organosilanes was then used as a surface mask for a second patterning step, with immersion in a solution of functionalized nanoparticles. Patterned substrates were fully submerged in a solution of surface-active gold nanoparticles coated with 3-mercaptopropyltrimethoxysilane. Regularly shaped, nanoscopic areas of bare substrate produced by removal of the latex mask provided sites to bind silanol-terminated gold nanoparticles, and the methyl-terminated areas of the organosilane film served as an effective resist, preventing nonspecific adsorption on masked areas. Characterizations with atomic force microscopy demonstrate the steps for lithography with organosilanes and functionalized nanoparticles. Patterning was accomplished for both silicon and glass substrates, to generate nanostructures with periodicities of 200-300 nm that match the diameters of the latex mesospheres of the surface masks. Nanoparticles were shown to bind selectively to uncovered, exposed areas of the substrate and did not attach to the methyl-terminal groups of the organosilane mask. Billions of well-defined nanostructures of nanoparticles can be generated using this high-throughput approach of particle lithography, with exquisite control of surface density and periodicity at the nanoscale.     

Chemometric and microscopic analyses for the size growth of monolayer-protected gold nanoparticles during their superlattice formation

Superlattices of gold nanoparticles have been produced at an air/solution interface under a highly acidic condition. The nanoparticle surface is protected by N-acetylglutathione (NAG). During the course of the superlattice formation, size growth of nanoparticles was observed: The superlattices were composed of nanoparticles of 6.6 nm in core diameter, whereas the as-prepared nanoparticles had the core diameter of 1.4 nm. The growth kinetics was pursued by the time evolution of the UV-vis absorption spectra for the sample solution. The change in the absorption spectral profiles was so small that we conducted principal-component analysis (PCA), which is known as a chemometric technique to resolve (or extract) spectra of minute chemical species submerged in the original spectra. Scanning transmission electron microscopy (STEM) corroborated the PCA results, yielding a successful explanation of the growth scheme of the NAG-protected gold nanoparticles.     

Fine-tuning size of gold nanoparticles by cooling during reverse micelle synthesis

By lowering the reaction temperature during metal ion reduction in a reverse micelle system, gold nanoparticle size can be subtly tuned from 6.6 to 2.2 nm in diameter. Under these reaction conditions, the water-to-surfactant ratio (W value) also plays an important role in controlling the particle size, enabling a wide range of products obtainable via a simple, quick, reproducible synthesis. Particle sizes were measured by HRTEM, and size trends were supported by UV-vis spectroscopy.     

Size crystallographic effects for silver and gold nanoparticles dispersed in a biopolymer matrix

Crystallographic size effects occurring during the formation of zero-valence silver and gold nanoparticles dispersed in a biopolymer polysaccharide matrix (arabinogalactan) have been studied by means of X-ray diffraction analysis. The average size of the nanoparticles has been found to increase with the increase in the metal content in the nanocomposite. Stabilization of the nanoparticles by the polymer matrix is accompanied by the decrease in the unit cell parameter of the metal correlated with the decrease in the coherent scattering length.     

Voltammetric and conductometric behavior of nanocomposites of polyaniline and gold nanoparticles prepared by layer-by-layer technique

Multilayer nanocomposites from polyaniline (PANI) and gold nanoparticles (AuNPs) were formed by layer-by-layer deposition. The formation of PANI–AuNPs multilayer structures was monitored by UV-vis absorption spectroscopy and cyclic voltammetry. Each deposited bilayer of PANI–AuNPs led to a monotonous and almost linear increase in both optical absorbance and the first current peak of PANI oxidation. The prepared multilayer nanocomposites were characterized by in situ conductivity measurements at different pH and potential and by transmission electron microscopy. Finally, chemosensitive properties of the new material based on the intrinsic affinity of gold nanoparticles were studied. Changes in the film resistance on exposure to vapors of mercury and sulfur-containing compounds were observed.     

Water-dispersible tryptophan-protected gold nanoparticles prepared by the spontaneous reduction of aqueous chloroaurate ions by the amino acid

The synthesis of water-dispersible amino-acid-protected gold nanoparticles by the spontaneous reduction of aqueous chloroaurate ions by tryptophan is described. Water-dispersible gold nanoparticles may also be obtained by the sequential synthesis of the gold nanoparticles by borohydride reduction of chloroauric acid followed by capping with tryptophan. Comparison of the proton NMR spectroscopic signatures from the tryptophan-protected gold nanoparticles obtained by the two processes indicated that the indole group in tryptophan is responsible for reduction of the aqueous chloroaurate ions. The reduction of the metal ions is accompanied by oxidative polymerization of the indole group of the tryptophan molecules and, consequently, some degree of cross-linking of the gold nanoparticles.     

Shape controlled growth of gold nanoparticles by a solution synthesis

The shape of gold nanoparticles has been successfully tuned among penta-twinned decahedrons, truncated tetrahedrons, cubes, octahedrons, hexagonal thin plates by introducing a small amount of salt into a N,N-dimethylformamide (DMF) solution containing poly(vinyl pyrrolidone) (PVP), and changing the temperature or the concentration of the gold precursor.     

Electrochemical synthesis of core–shell nanoparticles by seed-mediated selective deposition

Conventional solvothermal synthesis of core–shell nanoparticles results in them being covered with surfactant molecules for size control and stabilization, undermining their practicality as electrocatalysts. Here, we report an electrochemical method for the synthesis of core–shell nanoparticles directly on electrodes, free of surfactants. By implementation of selective electrodeposition on gold cores, 1^st-row transition metal shells were constructed with facile and precise thickness control. This type of metal-on-metal core–shell synthesis by purely electrochemical means is the first of its kind. The applicability of the nanoparticle decorated electrodes was demonstrated by alkaline oxygen evolution catalysis, during which the Au–Ni example displayed stable catalysis with low overpotential.

Core–shell nanoparticles can be synthesized by pure electrochemical methods, and the size of the core and the thickness of the shell can be precisely controlled. The nanoparticle-decorated electrodes exhibited respectable oxygen evolution catalysis.     

Shape-controlled synthesis of highly monodisperse and small size gold nanoparticles

We describe here that fine control of nanoparticle shape and size can be achieved by systematic varia-tion of experimental parameters in the seeded growth procedure in aqueous solution. Cubic and spherical gold nanoparticles are obtained respectively. In particularly, the Au cubes are highly mono-disperse in 33±2 nm diameter. The experimental methods involve the preparation of Au seed particles and the subsequent addition of an appropriate quantity of Au seed solution to the aqueous growth solutions containing desired quantities of CTAB and ascorbic acid (AA). Here, AA is a weak reducing agent and CTAB is not only a stable agent for nanoparticles but also an inductive agent for leading increase in the face of nanoparticle. Ultraviolet visible spectroscopy (UV-vis), X-ray diffraction (XRD), transmission electron microscopy (TEM) are used to characterize the nanoparticles. The results show that the different size gold nanoparticles displayed high size homogenous distribution and formed mono-membrane at the air/solid interface.     

Rapid purification and size separation of gold nanoparticles via diafiltration

Purification and size-based separation of nanoparticles remain significant challenges in the preparation of well-defined materials for fundamental studies and applications. Diafiltration shows considerable potential for the efficient and convenient purification and size separation of water-soluble nanoparticles, allowing for the removal of small-molecule impurities and for the isolation of small nanoparticles from larger nanostructures in a single process. Herein, we report studies aimed at assessing the suitability of diafiltration for (i) the purification of water-soluble thiol-stabilized 3-nm gold nanoparticles, (ii) the separation of a bimodal distribution of nanoparticles into the corresponding fractions, and (iii) the separation of a polydisperse sample into fractions of differing mean core diameter. NMR, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) measurements demonstrate that diafiltration produces nanoparticles with a much higher degree of purity than is possible by dialysis or a combination of solvent washes, chromatography, and ultracentrifugation. UV-visible spectroscopic and transmission electron microscopic (TEM) analyses show that diafiltration offers the ability to separate nanoparticles of disparate core size. These results demonstrate the applicability of diafiltration for the rapid and green preparation of high-purity gold nanoparticle samples and the size separation of heterogeneous nanoparticle samples. They also suggest the development of novel diafiltration membranes specifically suited to high-resolution nanoparticle size separation.     

Controlling the growth of particle size and size distribution of silica nanoparticles by the thin film structure

Nanostructured silicondioxide thin films were prepared by sol–gel spin coating technique. The SiO₂ films were made using a conventional mixture of tetraethoxysilane (TEOS), deionized water and ethanol with various NH₃/TEOS ratios. The nanostructured silica films were made using a mixture of the SiO₂ sol and regular SiO₂ sol to control the enlargement of the particles inside the films. The structural, morphological and optical characterizations of the as-deposited and annealed films were carried out using X-ray diffraction (XRD), atomic force microscopy, scanning electron microscopy, NKD spectrophotometer and ultraviolet–visible (UV–vis) spectroscopy. The transmittance data of the infrared spectra of the films were recorded using an FT-IR Spectrometer. The XRD studies showed that as-deposited films were amorphous and the formation of the alfa-cristobalite phase of the silica film was investigated at annealing temperature close to 1,100 °C. Optical properties of the transmittance spectra in the s and p-polarization modes were collected. Refractive indices and extinction coefficients were determined with respect to the NH₃/TEOS ratios in the compositions of the films. Optical cut-off wavelength values were investigated from the extrapolation of the absorbance spectra which was estimated from the UV–vis spectroscopy measurements. A red shift in the absorption threshold indicated that the size of silica nanoparticles was increased by an increase in the NH₃/TEOS volume ratio from 1:64 to 1:8.     

Evolution of size and shape in the colloidal crystallization of gold nanoparticles

The addition of dodecanethiol to a solution of oleylamine-stabilized gold nanoparticles in chloroform leads to aggregation of nanoparticles and formation of colloidal crystals. Based on results from dynamic light scattering and scanning electron microscopy we identify three different growth mechanisms: direct nanoparticle aggregation, cluster aggregation, and heterogeneous aggregation. These mechanisms produce amorphous, single-crystalline, polycrystalline, and core-shell type clusters. In the latter, gold nanoparticles encapsulate an impurity nucleus. All crystalline structures exhibit fcc or icosahedral packing and are terminated by (100) and (111) planes, which leads to truncated tetrahedral, octahedral, and icosahedral shapes. Importantly, most clusters in this system grow by aggregation of 60-80 nm structurally nonrigid clusters that form in the first 60 s of the experiment. The aggregation mechanism is discussed in terms of classical and other nucleation theories.     

Morphology-selective synthesis of polyhedral gold nanoparticles: what factors control the size and morphology of gold nanoparticles in a wet-chemical process

Polyhedral gold nanoparticles below 100 nm in size were fabricated by continuously delivered HAuCl(4) and PVP starting solutions into l-ascorbic acid aqueous solution in the presence of gold seeds, and under addition of sodium hydroxide (NaOH). By continuously delivered PVP and HAuCl(4) starting solutions in the presence of gold seed, the size and shape of polyhedral gold were achieved in relatively good uniformity (particle size distribution=65-95 nm). Morphological evolution was also attempted using different growth rates of crystal facets with increasing reaction temperature, and selective adsorption of PVP.