Design and Characterization of Metal Nanoparticle Infiltrated Mesoporous Metal-Organic Frameworks.

The infiltration of palladium and platinum nanoparticles (NPs) into the mesoporous metal-organic framework (MOF) CYCU-3 through chemical vapor infiltration (CVI) and incipient wetness infiltration (IWI) processes was systematically explored as a means to design novel NP@MOF composite materials for potential hydrogen storage applications. We employed a traditional CVI process and a new ″green″ IWI process using methanol for precursor infiltration and reduction under mild conditions. Transmission electron microscopy-based direct imaging techniques combined with synchrotron-based powder diffraction (SPD), energy-dispersive X-ray spectroscopy, and physisorption analysis reveal that the resulting NP@MOF composites combine key NP and MOF properties. Room temperature hydrogen adsorption capacities of 0.95 and 0.20 mmol/g at 1 bar and 2.9 and 1.8 mmol/g at 100 bar are found for CVI and IWI samples, respectively. Hydrogen spillover and/or physisorption are proposed as the dominating adsorption mechanisms depending on the NP infiltration method. Mechanistic insights were obtained through the crystallographic means using SPD-based difference envelope density analysis, providing previously underexplored details on NP@MOF preparations. Consequently, important host-guest correlations influencing the global hydrogen adsorption properties are discussed, and they demonstrate that employing MOFs as platforms for NPs is an alternative approach to the development of versatile materials for improving current hydrogen storage technologies.

Comparative Analysis of Commercial Colloidal Silver Products.

PURPOSE: The public fear associated with the novel coronavirus (SARS-CoV-2) pandemic has triggered recently a significant proliferation of supplements touted as potential cures against bacteria and viruses. Colloidal silver has particularly benefited from this rush given its empirically and scientifically documented anti-bacterial and anti-viral actions. The lack of standards in the unregulated supplements industry remains a major roadblock in evaluating the quality and consistency of marketed products or assessing the accuracy of the information provided by manufacturers. This study is the first scientifically rigorous attempt to evaluate commercial silver colloidal products offered for sale on the internet. METHODS: Fourteen of the most popular colloidal silver products purchased from Amazon (www.amazon.com) were evaluated using state-of-the-art analytical techniques widely accepted as gold standards for investigating the properties (size, shape) and the dispersion of silver nanoparticles. RESULTS: Commercial samples were analysed using UV-Vis, FE-SEM and AAS techniques. In general, the Ag concentration was very close to those claimed by the manufacturer. The colorless product shows no absorbance in the UV-Vis analysis. The FESEM and STEM images confirmed the conclusions of the UV-Vis analysis. CONCLUSION: The results of this evaluation show clearly that 70% of the commercial products evaluated contain only ionic silver. Despite the evidence showing that silver nanoparticles are not present, eight of these products are promoted by the manufacturers as 'colloidal silver'. Considering the extensive scientific research showing major differences between silver ionic and silver nanoparticles in terms of mechanisms of action, efficacy and safety, it is clear that this misrepresentation impacts the consumers and must be addressed. This study serves as blueprint for a scientific protocol to be followed by manufacturers for characterizing their silver supplements.

Deposition of hermetic silver shells onto copper flakes.

Continuous silver shells were deposited on copper flakes using a two-stage precipitation process. A tightly packed layer of silver nanoparticles was first formed on the surface of the base metal by galvanic displacement. The size of the noble metal particles and their distribution on the substrate were controlled using complexing agents and dispersants. A continuous Ag deposit was subsequently grown by reducing slowly [Ag(NH3)2]+ ions with glucose. The final shell thickness was controlled by varying the amount of metal deposited in the second step. The electrical properties of resulting silver coated copper flakes are comparable to those measured for silver flakes of similar size and aspect ratio. By preventing the oxidation of copper cores up to 400°C, the hermetic noble metal shell dramatically extends the temperature range in which Ag/Cu flakes can successfully replace pure silver.

Preparation of concentrated stable dispersions of uniform Ag nanoparticles using resorcinol as reductant.

Uniform silver nanoparticles ranging in size from 12 to 80nm were prepared by reducing silver ammonia complex with resorcinol in presence of Arabic gum. Despite the high ionic strength of the system, the precipitation process yields stable dispersions at metal concentrations as high as 4.0wt% (0.38moldm(-3)). Stable dispersions with higher solids concentration up to 20wt% silver (1.9moldm(-3)) were obtained by subjecting the 'as precipitated' dispersions to ultrafiltration. The effects of reaction parameters (pH, concentration of reactants, metal/stabilizer ratio, and temperature) on particle size and dispersion stability are discussed and a mechanism for the reduction of silver is proposed.

Size control of noble metal clusters and metallic heterostructures through the reduction kinetics of metal precursors.

Eight precious metal salts/complexes were reduced in propylene glycol at temperatures ranging between 110 and 170 °C. We found that the reduction temperature and the size of precipitated metallic nanoparticles formed were significantly affected by the structure and reactivity of the metal precursors. The choice of noble metal precursor offers flexibility for designing, fabricating and controlling the size of metallic heterostructures with tunable properties.

Nitrilotriacetic acid: a novel reducing agent for synthesizing colloidal gold.

We report for the first time that nitrilotriacetic acid (NTA) is an effective reductant for the preparation of stable dispersions of uniform gold nanoparticles. The method described is capable of generating stable sols with a metal concentration as high as 1.5×10(-3)moldm(-3). The size of gold nanoparticles can be tuned from 10 to 160 nm by adjusting the stoichiometric excess of NTA. For a constant [Au]/[NTA] ratio the temperature affects the reduction kinetics but has little impact on the size of gold nanoparticles. The mechanisms of the reduction of Au(III) species and the formation and stabilization of gold nanoparticles are discussed.

Synthesis of selenium particles with various morphologies.

Uniform selenium spherical particles were prepared by reducing selenous acid with hydroquinone in the presence of Daxad 11G. The red colored colloidal dispersions displayed a distinct plasmon band at ~612 nm and were stable for extended time due to the negative surface potential of the particles. Structural analyses indicated that the Se spheres were aggregates of nanosize subunits crystallized in the hexagonal system. Selenium wires and rods were obtained by changing the pH and the composition of the precipitated dispersions and incubating them for extended time at moderate temperatures. The addition of a co-solvent played a major role in the re-crystallization of selenium spheres into anisotropic structures.

Precipitation of spherical and 'fiber-like' silver particles.

Silver nanoparticles with various sizes and shapes were prepared by adding silver nitrate to ascorbic acid solutions containing nitric acid and a dispersant (Daxad 11G). It is shown that the reaction environment significantly affects the nature of the interactions between silver ions and dispersant molecules. The stability of the Ag(+)/Daxad 11G intermediate species formed, which determines the reduction rate and, implicitly, the particle size and shape, is strongly affected by the dispersant/Ag(+) ratio and the concentration of nitric acid. Uniform highly dispersed Ag nanoparticles were obtained at high concentrations of HNO3, while lower acid concentrations favored the formation of 'fiber-like' metallic structures. The propensity for forming anisotropic silver structures is attributed to the planar structure and chromonic properties of dispersant molecules.

Core-shell gold/silver nanoparticles: synthesis and optical properties.

Highly dispersed gold-silver core-shell nanoparticles were synthesized in a two-step process. The stabilizer-free gold core particles with an average diameter of ~30 nm were first precipitated by rapid reduction of HAuCl(4) with l-ascorbic acid. Thin continuous silver shells of variable thickness were subsequently obtained by reducing controlled amounts of silver nitrate added in the gold sol. The plasmon band of gold gradually blue-shifted and a peak characteristic for silver eventually emerged as the amount of deposited silver increased. A strong and well-defined silver absorption band was recorded when the Ag content exceeded 60 wt.%. It is shown that the concentration of Cl(-) ions in the gold precursor solution plays a critical role in the stability of the bi-metallic sol and the structure of the deposited silver shell.

Deposition of continuous nickel shells on polymer microspheres.

Anisotropic conductive adhesives (ACAs) are widely used as interconnect materials in the manufacturing of LCD screens. To be integrated in a broader range of applications, several technical and economical issues still need to be addressed. Encapsulating the polymer particles within continuous, compact, and adhering metallic conductive shells is one of these challenges. This work describes a method for depositing nickel layers with different thickness (30-120nm) onto monosized polymer particles. The novelty of the approach consists in modifying the surface of polymer particles with linear polymeric amines. We show that by increasing amine chain length the structure and adhesion of deposited nickel shell are significantly improved. The effect of key parameters of the electroless Ni plating are discussed and illustrated.

Precipitation of silver/palladium alloy platelets from homogeneous solutions.

Dispersed silver/palladium (Ag/Pd) nanoplatelets were prepared by delivering in parallel solutions of mixed metal nitrates and L-ascorbic acid into a nitric acid solution containing Arabic gum. The shape and size of bimetallic nanoparticles varied with the silver/palladium weight ratio and the concentration of nitric acid. The optimum conditions for platelets formation were a palladium content of ~2.0 wt.% and nitric acid concentrations above 1.0 mol dm(-3). The data presented show that both parameters play a critical role in the nucleation and growth of AgPd particles. A mechanism explaining the formation of the bimetallic nanoplatelets is proposed.

Preparation of counterion stabilized concentrated silver sols.

A strategy for obtaining stable concentrated silver dispersions without dedicated stabilizing agents is presented. This approach consists of rapidly mixing aqueous solutions of silver salicylate and ascorbic acid. By using salicylate as Ag(+) counterion, it is possible to prepare stable sols with metal concentrations up to two orders of magnitude higher than with silver nitrate. The stabilizing effect of the counterion is the result of a decreased ionic strength due to salicylate protonation and its adsorption on the surface of silver. Both effects increase the range of the electrostatic repulsive forces by expanding the electrical double layer.

Precipitation of dispersed silver particles using acetone as reducing agent.

A novel environmentally friendly solution-based method for preparing dispersed silver particles is described. The simple and convenient approach consists in heating silver oxide particles dispersed in a highly alkaline water/acetone mixture. The data presented clearly show that acetone reduces completely and rapidly Ag(2)O particles to metallic silver at 60°C. A mechanism explaining the provenance of the electrons responsible for the reduction of silver is proposed.

Toxicity and developmental defects of different sizes and shape nickel nanoparticles in zebrafish.

Metallic nanoparticles such as nickel are used in catalytic sensing, and electronic applications, but health and environmental affects have not been fully investigated. While some metal nanoparticles result in toxicity, it is also important to determine whether nanoparticles of the same metal but of different size and shape changes toxicity. Three different size nickel nanoparticle (Ni NPs) of 30, 60, and 100 nm and larger particle clusters of aggregated 60 nm entities with a dendritic structure were synthesized and exposed to zebrafish embryos assessing mortality and developmental defects. Ni NPs exposure was compared to soluble nickel salts. All three 30, 60, and 100 nm Ni NPs are equal to or less toxic than soluble nickel while dendritic clusters were more toxic. With each Ni NP exposure, thinning of the intestinal epithelium first occurs around the LD10 continuing into the LD50. LD50 exposure also results in skeletal muscle fiber separation. Exposure to soluble nickel does not cause intestinal defects while skeletal muscle separation occurs at concentrations well over LD50. These results suggest that configuration of nanoparticles may affect toxicity more than size and defects from Ni NPs exposure occur by different biological mechanisms than soluble nickel.

Preparation and stabilization of monodisperse colloidal gold by reduction with aminodextran.

Stable concentrated aqueous dispersions of monodisperse gold nanoparticles were prepared using diethylaminoethyl-dextran as reductant and stabilizer. The effectiveness of dextran as reducing agent was strongly affected by the pH. In alkaline solutions, the Au(III) species were rapidly and completely reduced, yielding stable dispersions of spherical uniform gold nanoparticles. Their modal diameter could be adjusted from 18 to 40 nm by varying the pH, temperature, and the Au3+/dextran ratio. In contrast, under acidic conditions (pH approximately 4.0) the reduction was very slow, favoring the formation of large gold crystals of other shapes. A general mechanism explaining the reducing and stabilizing actions of polysaccharides in general, and of diethylaminoethyl-dextran in particular, is proposed.

A simple preparative route to highly stable dispersions of uniform silver nanoparticles.

Stable dispersions of uniform silver nanoparticles were prepared by heating silver salts in polyols in the presence of a naphthalene sulfonate/formaldehyde copolymer as dispersant. In the temperature range explored (150-190 degrees C) the modal size and the size distribution of the particles depended on the nature and concentration of the silver salt and the polyol used. Highly dispersed uniform nanoparticles with a diameter of approximately 12 nm were obtained by reducing silver salicylate in diethylene glycol at a metal concentration of 2.5 x 10(-2) mol x cm(-3). Larger silver nanoparticles (approximately 30 nm) and more concentrated dispersions (1.0 mol x cm(-3)) were prepared in ethylene glycol. In all cases the selected dispersant yielded remarkably stable silver sols and facilitated the transfer of the nanoparticles into water, while preserving the stability of the dispersions.

Computational model for the formation of uniform silver spheres by aggregation of nanosize precursors.

We present results of computational modeling of the formation of uniform spherical silver particles prepared by rapid mixing of ascorbic acid and silver-amine complex solutions in the absence of a dispersing agent. Using an accelerated integration scheme to speed up the calculation of particle size distributions in the latter stages, we find that the recently reported experimental results-some of which are summarized here-can be modeled effectively by the two-stage formation mechanism used previously to model the preparation of uniform gold spheres. We treat both the equilibrium concentration of silver atoms and the surface tension of silver precursor nanocrystals as free parameters, and find that the experimental reaction time scale is fit by a narrow region of this two-parameter space. The kinetic parameter required to quantitatively match the final particle size is found to be very close to that used previously in modeling the formation of gold particles, suggesting that similar kinetics governs the aggregation process and providing evidence that the two-stage model of burst nucleation of nanocrystalline precursors followed by their aggregation to form the final colloids can be applied to systems both with and without dispersing agents. The model also reproduced semiquantitatively the effects of solvent viscosity and temperature on the particle preparation.

Formation and structure of cubic particles of sodium magnesium fluoride (neighborite).

Uniform cubic particles of neighborite (NaMgF3) were prepared by mixing solutions of magnesium chloride and sodium fluoride, followed by aging for extended periods of time (up to 3 h). Such particles could be obtained directly either by using sodium fluoride in sufficient excess, or by first producing spherical particles of magnesium fluoride and converting them into neighborite cubes by admixing sodium fluoride. It was shown that both MgF2 and NaMgF3 particles so prepared are polycrystalline and that in both procedures to form neighborite a two stage reaction takes place. In the first stage nanosize subunits of MgF2 are formed, which are subsequently converted in the presence of excess sodium fluoride to neighborite crystallites. The latter are then reorganized into larger subunits that constitute colloidal cubes.

Zinc oxide colloids with controlled size, shape, and structure.

Highly dispersed uniform ZnO particles of different sizes and shapes were prepared by slowly adding zinc salt and sodium hydroxide solutions in parallel into aqueous solutions of Arabic gum. Except for the very early stages, the precipitated solids consisted of a well-defined zinc oxide phase. Depending on the experimental conditions, the size of the final polycrystalline particles formed by the aggregation of nanosize entities varied from 100 to 300 nm. The reaction temperature affected both the size of the nanosize precursors and their arrangement in the final particles. At ambient temperature the primary nanoparticles, approximately 10 nm in size, formed spherical aggregates, while at 600 degrees C they were much larger (44 nm) and combined to form rather uniform hexagonal ZnO prisms. The aspect ratio and the internal structure of the latter could be altered by changing the nature of the zinc salt, the addition rate, and the initial concentration of the reactants. Based on the findings of the study a two-stage mechanism for the formation of uniform polycrystalline particles with well-defined geometric shapes is proposed.