A combinatorial approach to the study of particle size effects on supported electrocatalysts: oxygen reduction on gold

A novel high-throughput technique has been developed for the investigation of the influence of supported metal particle size and the support on electrocatalytic activity. Arrays with a gradation of catalyst particle sizes are fabricated in a physical vapor deposition system that also allows selection of the support material. Simultaneous electrochemical measurements at all electrodes in the array, together with determination of the actual particle size distribution on each of the electrodes by transmission electron microscopy (TEM), then allows rapid determination of the activity as a function of catalyst center size. The procedure is illustrated using data for the reduction of oxygen on gold nanoparticles supported on both substoichiometric titanium dioxide (TiO(x)()) and carbon and the conclusions are verified using voltammetry at rotating disk electrodes. Gold centers with diameters in the range 1.4-6.3 nm were investigated and it is demonstrated that, with both supports, the catalytic activity for oxygen reduction decays rapidly for particle sizes below 3.0 nm. This may be observed as a decrease in current at constant potential or an increase in the overpotential for oxygen reduction.     

Biodistribution of colloidal gold nanoparticles after intravenous administration: effect of particle size

Purpose of the present research work was to evaluate the biological distribution of differently size gold nanoparticles (NP) up on intravenous administration in mice. Another objective was to study effect of particle size on biological distribution of gold NP to enable their diverse applications in nanotechnology. Gold NP of different particle sizes, mainly 15, 50, 100 and 200nm, were synthesized by modifying citrate ion concentration. Synthesized gold nanoparticles were characterized by SEM and their size distribution was studied by particle size analyzer. Gold NP was suspended in sodium alginate solution (0.5%, w/v) and administered to mice (1g/kg, intravenously) [n=3]. After 24h of administration of gold NP, blood was collected under light ether anesthesia, mice were sacrificed by cervical dislocation and various tissues/organs were removed. The tissues were then washed with saline, homogenized and lysed with aqua regia. The determination of gold in samples was carried out quantitatively by inductively coupled plasma mass spectrometry (ICP-MS). SEM study revealed spherical morphology of gold NP with narrow particle size distribution. Biodistribution study revealed gold NPs of all sizes were mainly accumulated in organs like liver, lung and spleen. The accumulation of gold NP in various tissues was found to be depending on particle size. 15nm gold NP revealed higher amount of gold and number of particles in all the tissues including blood, liver, lung, spleen, kidney, brain, heart, stomach. Interestingly, 15 and 50nm gold NP were able to pass blood-brain barrier as evident from gold concentration in brain. Two-hundred nanometers gold NP showed very minute presence in organs including blood, brain, stomach and pancreas. The results revealed that tissue distribution of gold nanoparticles is size-dependent with the smallest 15nm nanoparticles showing the most widespread organ distribution.     

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.     

Controllable synthesis of water-soluble gold nanoparticles and their applications in electrocatalysis and surface-enhanced Raman scattering

We report a facile method to synthesize water-soluble gold nanoparticles (AuNPs) using a biosurfactant sodium cholate as reducing reagents and protective groups in aqueous solution at ambient temperature. The diameters (13-70 nm) of uniform AuNPs can be readily adjusted by changing the initial molar ratio of sodium cholate to chloroauric acid (HAuCl(4)). Also, the alkaline condition of preparative solution is found to affect the size of as-synthesized AuNPs. This synthetic approach is one-step and "green". The obtained AuNPs exhibit a good electrocatalytic activity toward methanol oxidation. Meanwhile, the AuNPs thin films can serve as an efficient substrate for surface-enhanced Raman scattering (SERS). Furthermore, platinum nanoparticles (PtNPs) are also prepared by reducing sodium tetrachloro platinate hydrate with sodium cholate.     

Sonochemical synthesis of gold nanoparticles: effects of ultrasound frequency

The rate of sonochemical reduction of Au(III) to produce Au nanoparticles in aqueous solutions containing 1-propanol has been found to be strongly dependent upon the ultrasound frequency. The size and distribution of the Au nanoparticles produced can also be correlated with the rate of Au(III) reduction, which in turn is influenced by the applied frequency. Our results suggest that the rate of Au(III) reduction as well as the size distribution of Au particles are governed by the chemical effects of cavitation and are not significantly affected by the physical effects accompanying ultrasound-induced cavitation.     

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.     

Spherical ensembles of gold nanoparticles on silica: electrostatic and size effects

Core-shell ensembles of citrate-stabilized gold nanoparticles (20-80 nm) on submicron silica cores (330-550 nm) have been prepared by electrostatic self-assembly with shell packing densities as high as phi = 0.55.     

Influence of particle size on the binding activity of proteins adsorbed onto gold nanoparticles

We used optical extinction spectroscopy to study the structure of proteins adsorbed onto gold nanoparticles of sizes 5-60 nm and their resulting biological binding activity. For these studies, proteins differing in size and shape, with well-characterized and specific interactions-rabbit immunoglobulin G (IgG), goat anti-rabbit IgG (anti-IgG), Staphylococcal protein A, streptavidin, and biotin-were used as model systems. Protein interaction with gold nanoparticles was probed by optical extinction measurements of localized surface plasmon resonance (LSPR) of the gold nanoparticles. Binding of the ligands in solution to protein molecules already immobilized on the surface of gold causes a small but detectable shift in the LSPR peak of the gold nanoparticles. This shift can be used to probe the binding activity of the adsorbed protein. Within the context of Mie theory calculations, the thickness of the adsorbed protein layer as well as its apparent refractive index is shown to depend on the size of the gold nanoparticle. The results suggest that proteins can adopt different orientations that depend on the size of the gold nanospheres. These different orientations, in turn, can result in different levels of biological activity. For example, we find that IgG adsorbed on spheres with diameter ≥20 nm does not bind to protein A. This study illustrates the principle that the size of nanoparticles can strongly influence the binding activity of adsorbed proteins. In addition to the importance of this in cases of direct exposure of proteins to nanoparticles, the results have implications for proteins adsorbed to materials with nanometer scale surface roughness.     

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.     

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.     

Cobalt particle size effects in the Fischer-Tropsch reaction studied with carbon nanofiber supported catalysts

The influence of cobalt particle size in the range of 2.6-27 nm on the performance in Fischer-Tropsch synthesis has been investigated for the first time using well-defined catalysts based on an inert carbon nanofibers support material. X-ray absorption spectroscopy revealed that cobalt was metallic, even for small particle sizes, after the in situ reduction treatment, which is a prerequisite for catalytic operation and is difficult to achieve using traditional oxidic supports. The turnover frequency (TOF) for CO hydrogenation was independent of cobalt particle size for catalysts with sizes larger than 6 nm (1 bar) or 8 nm (35 bar), while both the selectivity and the activity changed for catalysts with smaller particles. At 35 bar, the TOF decreased from 23 x 10(-3) to 1.4 x 10(-3) s(-1), while the C5+ selectivity decreased from 85 to 51 wt % when the cobalt particle size was reduced from 16 to 2.6 nm. This demonstrates that the minimal required cobalt particle size for Fischer-Tropsch catalysis is larger (6-8 nm) than can be explained by classical structure sensitivity. Other explanations raised in the literature, such as formation of CoO or Co carbide species on small particles during catalytic testing, were not substantiated by experimental evidence from X-ray absorption spectroscopy. Interestingly, we found with EXAFS a decrease of the cobalt coordination number under reaction conditions, which points to reconstruction of the cobalt particles. It is argued that the cobalt particle size effects can be attributed to nonclassical structure sensitivity in combination with CO-induced surface reconstruction. The profound influences of particle size may be important for the design of new Fischer-Tropsch catalysts.     

Controlled functionalization of gold nanoparticles through a solid phase synthesis approach

A novel solid phase synthetic strategy was developed in this work for controlled functionalization of gold nanoparticles.     

Tailoring the particle size of monodispersed colloidal gold

Monodispersed spherical gold particles ranging in modal diameter from 80 nm to 5 μm, were prepared by reducing tetrachloroauric(III) acid with iso-ascorbic acid in aqueous solutions at 20°C. The particle size was altered by changing the pH, which affected the composition of gold(III) solute complexes. The latter controlled the redox potential of the system, essential to the formation of the initial nanosize gold dispersions. Depending on the experimental conditions, the resulting primary particles remained either stable or they aggregated to form much larger uniform spheres. The mechanisms of the precipitation of the precursors (primary) particles and of their mutual interactions to yield the final dispersions are discussed.     

In vitro permeation of gold nanoparticles through rat skin and rat intestine: effect of particle size

Purpose of the present work was to study in vitro permeation of gold nanoparticles (NPs) through isolated rat skin and intestine. Another objective was to see the effect of particle size on permeation of the gold NP. Gold NP of 15 nm, 102 nm and 198 nm were synthesized and used for study. Franz diffusion cells were used to evaluate permeation of gold NP from rat skin whereas 'intestinal sac' method was used for assessing intestinal permeation. Number density of gold NP was analyzed by UV-vis spectroscopy whereas amount of gold permeated was measured by ICP mass spectrometry. The absorption and localization of gold NP through rat skin was studied by TEM. Qualitative analysis of gold inside of the rat skin was performed by energy dispersive X-ray spectroscopy (EDS). Gold NP showed negative zeta potential. UV-vis absorption spectra of 15 nm, 102 nm and 198 nm gold NP showed lambda(max) at 520 nm, 535 nm and 577 nm, respectively. SEM study revealed spherical morphology of NP. Gold NP showed size dependent permeation through rat skin and intestine. 15 nm gold NP showed higher permeation compared to 102 nm and 198 nm gold NP. Interestingly, 102 nm and 198 nm gold NP showed lag time of 3h and 6h in case of rat skin only. As the size of the gold NP increased, permeability coefficient and diffusion coefficient was found to be decreased. The permeation of gold NP through intestine was higher than that of skin. TEM study of rat skin revealed accumulation of smaller size gold NP in deeper region of skin whereas larger particles were observed mainly in epidermis and dermis. Presence of gold inside of rat skin was confirmed by EDS. Gold NP would be an interesting carrier for transdermal as well as for oral delivery. The study demonstrated initial proof of concept of percutaneous permeation of smaller size gold particles.     

Catalytic properties of supported gold nanoparticles in organic syntheses

Gold nanoparticles exhibit unique properties due to their ability to form aggregates of atoms of diverse morphology shapes and sizes of which depend, to a considerable extent, on specific features of the nearest environment. The nature of gold nanoparticles varies in a wide range: from the particles with pronounced Lewis acidic properties to the negatively charged particles bearing a formal zero-valence charge. The most examples of new reactions catalyzed by gold nanoparticles include unsaturated compounds and strong nucleophiles (such as amines) as substrates. This short review provides a digest of the catalytic properties of gold nanoparticles. The main attention is paid to the possible role of certain forms of the metal in catalytic reactions. Of special interest are reactions in which effects of synergism of gold and other active species or second metals present in the catalyst are revealed or a size effect is established.     

Surface coverage and size effects on electrochemical oxidation of uniform gold nanoparticles

We investigated the electrooxidative dissolution of uniformly distributed Au nanoparticles (NPs) without an extra adhesion layer or capping agent. Diblock copolymer micelles were exploited to fabricate the arrays of Au NPs where not only diameter of the particles but also inter-particle spacing, and thus coverage were finely controlled. The peak potential for electrochemical oxidation shifted greater as a function of coverage of NPs than the size.     

Colloidal deposition synthesis of supported gold nanocatalysts based on Au-Fe3O4 dumbbell nanoparticles

Highly active Au catalysts with a dumbbell-like heterostructure for CO oxidation were prepared through colloidal deposition method; both activities and stabilities were investigated under different experimental conditions.     

Core size effects on the reactivity of organic substrates as monolayers on gold nanoparticles

Monolayer-protected nanoparticles (MPNs) with average core sizes of 1.7- (small), 2.2- (medium) and 4.5-nm (large) diameter have been prepared and functionalized with a variety of aryl ketone substrates, namely, 11-mercaptoundecaphenone (1), 1-(4-hexyl-phenyl)-11-mercaptoundecanone (2), 1-[4-(11-mercaptoundecyl)phenyl]hexanone (3), or 1-[4-(11-mercaptoundecyl)phenyl]undecanone (4). Upon irradiation in benzene solution, the aryl ketone-modified MPNs undergo the Norrish type II photoreaction and yield alkene- or acetophenone-modified MPNs exclusively, with no evidence for the generation of cyclobutanol. The extent of the photoreaction for the entire series of aryl ketones is dependent on the size of the MPN core. For 11-mercaptoundecaphenone, the reaction proceeds nearly to completion on the smallest MPN cores (99 +/- 1%) but occurs to a much lesser extent on medium (85 +/- 5%) and large cores (66 +/- 6%). The differences in the extents of reaction are rationalized by the decreased reactivity of substrates on terrace regions, which become increasingly larger with the core size. In lending support to this hypothesis, the edge and vertex sites of medium-sized MPNs were selectively populated with an aryl ketone probe and shown to react quantitatively, whereas selective population of the terrace sites on the same-sized MPNs results in a much lower extent of reaction. Together, these results indicate differences in reactivity of monolayer substrates on terrace versus edge/vertex sites of MPNs. The differences in reactivity with site will play a role in the design of modified MPNs for applications.     

Photochemical synthesis of gold nanoparticles in latexes

Gold nanoparticles in aqueous dispersions of 100-nm polystyrene microspheres were prepared by photochemical synthesis under exposure to monochromatic light with an excitation wavelength of 254 nm. The kinetic relationships in formation of gold nanoparticles were examined in relation to the H[AuCl₄] and polymer concentration in the photolyte.