Immobilization of silver nanoparticles on silica microspheres

The silver nanoparticles (Ag NPs) have been immobilized onto silica microspheres through the adsorption and subsequent reduction of Ag⁺ ions on the surfaces of the silica microspheres. The neat silica microspheres that acted as the core materials were prepared through sol–gel processing; their surfaces were then functionalized using 3-mercaptopropyltrimethoxysilane (MPTMS). The major aims of this study were to immobilize differently sized Ag particles onto the silica microspheres and to understand the mechanism of formation of the Ag nano-coatings through the self-assembly/adsorption behavior of Ag NPs/Ag⁺ ions on the silica spheres. The obtained Ag NP/silica microsphere conglomerates were characterized by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and energy-dispersive spectroscopy (EDS). Their electromagnetic wave shielding effectiveness were also tested and studied. The average particle size of the obtained Ag NPs on the silica microsphere was found that could be controllable (from 2.9 to 51.5 nm) by adjusting the ratio of MPTMS/TEOS and the amount of AgNO₃.     

Effect of surface charge on the colloidal stability and in vitro uptake of carboxymethyl dextran-coated iron oxide nanoparticles

An easy and novel routine are reported for the preparation of metallic silver nanoparticles (AgNPs) with controlled morphology, using Na⁺–magadiite swelled with hexadecyltrimethylammonium bromide (CTA⁺–magadiite) and a layered aluminophosphate with kanemite-type structure modified with n-dodecylammonium and n-butylammonium (but,dod-AlPO-kan) as hosts. For the preparation of the metallic AgNPs (Ag⁰) in the interlamellar space, the CTA⁺–magadiite and but,dod-AlPO-kan hosts were dispersed in N,N-dimethylformamide (DMF) solution with different AgNO₃ concentrations. DMF acts as reducing agent of Ag⁺ ions leading to nanoparticles with disk-like morphology of magadiite silicate; these were characterized by TEM and UV–Vis spectroscopy. On the other hand, the AgNPs are intercalated in but,dod-AlPO-kan showing spherical-like morphology. The UV–Vis spectra of the nanocomposites based on Ag⁰ and magadiite silicate show bands at 565 nm that can be attributed to Ag⁰ nanodisks. The Ag-but,dod-AlPO-kan-based nanocomposites present a band at 422 nm attributed to the surface plasmon resonance of Ag⁰ nanospheres. The results of transmission electron microscopy agree very well with XRD and UV–Vis analysis, indicating the formation of AgNPs with different morphologies using the two kinds of lamellar materials. The magadiite host has an important role in the synthesis of Ag nanodisks, because it controls the growth of nanoparticles inside the interlayer region with disk-like morphology due the high interlayer interactions of the silicate, leading to the growth of nanoparticles in only two directions (xy plane). On the other hand, when but,dod-AlPO-kan is used a sphere-like morphology is preferred due the best accommodation of AgNPs between the layers of aluminophosphate host.     

TEM monitoring of silver nanoparticles formation on the surface of lead crystal glass

Silver nanoparticles have been formed on the surface of lead crystal glass by means of (i) ion-exchange of alkaline ions from the glass by Ag⁺ ions from a molten salts bath, and (ii) silica based sol-gel coatings containing silver. All experimental variables concerning both ion-exchange process and sol-gel coatings application were combined and studied as main parameters governing the reduction of Ag⁺ ions to Ag⁰ atoms and further aggregation to form nanosized colloids. The content of thermoreducing agents (arsenic or antimony oxides) in the lead crystal glass was essential to favour the reduction of silver ions to form nanoparticles. Optimal experimental conditions to be used for the obtaining of surface silver nanoparticles were determined. TEM was used as the principal characterisation technique for direct observation of the nanoparticles generated. The size of silver colloids varied in the 20-300 nm range for ion-exchanged samples and in the 10-80 nm range for sol-gel coated samples.     

Synergistic effect of silver seeds and organic modifiers on the morphology evolution mechanism of silver nanoparticles

Triangular, truncated triangular, quadrangular, hexagonal, and net-structured silver nanoplates as well as decahedral silver nanoparticles were manipulatively prepared starting from silver nitrate and silver seeds in the presence of poly(ethylene glycol) (PEG), poly(N-vinyl pyrrolidone) (PVP), and Tween 80 at room temperature, respectively. UV-vis spectroscopy, XRD, HRTEM, SAED, and FTIR were used to illustrate the crystal growth process and to characterize the resultant silver nanoparticles. It was found that the silver seeds and organic modifiers synergistically affected the morphology evolution of the silver nanoparticles. The co-presence of silver seeds and PEG was beneficial to the formation of triangular and truncated triangular silver nanoplates; the silver seeds and PVP favored the formation of polygonal silver nanoplates; the silver seeds and Tween 80 preferred to the formation of net-structured silver plates. The morphology evolution of the resultant silver nanoparticles was correlated with the crystallinity of the silver seeds and the adsorption ability of the organic modifiers on the crystal surfaces.     

Comparison of silver nanoparticles confined in nanoporous silica prepared by chemical synthesis and by ultra-short pulsed laser ablation in liquid

Hexagonally ordered mesoporous silica materials, MCM-41 and SBA-15, have been synthesized and loaded with Ag nanoparticles, utilizing both chemical synthesis and ultra-short pulsed laser ablation in liquid. In laser ablation, a silver target, immersed in aqueous suspension of ordered mesoporous silica SBA-15, was irradiated by ultra-short laser pulses to generate silver nanoparticles. For comparison, samples of similar silver contents were prepared either by incorporating silver into the SBA-15 during a hydrothermal synthesis or by introducing silver in MCM-41 by template ion-exchange. Samples were characterized by XRD, N₂ physisorption, TEM and UV–vis spectroscopy. All preparations contained significant amount of 5–50 nm size silver agglomerates on the outer surface of the silica particles. The laser ablation process did not cause significant destruction of the SBA-15 structure and metallic silver (Ag⁰) nanoparticles were mainly generated. It is demonstrated that by laser ablation in aqueous silica suspension smaller and more uniform metallic silver particles can be produced and loaded on the surface of the silica support than by synthesis procedures. Catalytic properties of the samples have been tested in the total oxidation of toluene. Because of its favorable Ag dispersity, the Ag/SBA-15 catalyst, generated by the laser ablation method, had better catalytic stability and, relative to its Ag load, higher activity than the conventional Ag/SBA-15 preparations.     

Synthesis and formation mechanism of Ag–Ni alloy nanoparticles at room temperature

Ag–Ni nanoparticles were prepared with a chemical reduction method in the presence of polyvinylpyrrolidone (PVP) used as a stabilizing agent. During the synthesis of Ag–Ni nanoparticles, silver nitrate was used as the Ag⁺ source while nickel sulfate hexahydrate was used as Ni²⁺ source. Mixed solutions of Ag⁺ source and Ni²⁺ source were used as the precursors and sodium borohydride was used as the reducing agent. Five ratios of Ag⁺/Ni²⁺ (9:1, 3:1, 1:1, 1:3, and 1:9) suspensions were prepared in the corresponding precursors. Ag–Ni alloy nanoparticles were obtained with this method at room temperature. Scanning electronic microscope (SEM), energy dispersive spectrum (EDS), high resolution transmission electron microscope (HRTEM) were used to characterize the morphology, composition and crystal structure of the nanoparticles. The crystal structure was also investigated with X-ray diffraction (XRD). In all five Ag/Ni ratios, two kinds of particle structures were observed that are single crystal structure and five-fold twinned structure respectively. Free energy of nanoparticles with different crystal structures were calculated at each Ag/Ni ratio. Calculated results revealed that, with identical volume, free energy of single crystal particle is lower than multi-twinned particle and the difference becomes smaller with the increase of particle size; increase of Ni content will lead the increase of free energy for both structures. Formation of different crystal structures are decided by the structure of the original nuclei at the very early stage of the reduction process.     

Influence of silver on the Sm<Superscript>3+</Superscript> luminescence in “Aerosil” silica glasses

The introduction of silver into the samarium-containing silica glasses prepared by the original solgel method leads to the formation of complex optical centers involving samarium ions and simple and/or complex silver ions. These centers are characterized by the effective sensitization of Sm³⁺ luminescence by Ag⁺, (Ag₂)⁺, and (Ag⁺)₂ ions according to the exchange mechanism for, at least, Sm³⁺-Ag⁺ centers. The formation of Sm-Ag centers is accompanied by an increase in the concentration of nonbridging oxygen ions, which prevent the reduction of silver ions by hydrogen. Silver nanoparticles formed in small amounts upon this reduction are effective quenchers of luminescence from the corresponding excited states of Sm³⁺ ions.     

A novel approach to fabrication of superparamagnetite hollow silica/magnetic composite spheres

We described a method for synthesizing hollow silica/magnetic composite spheres using sulfonic acid functionalized hollow silica spheres (SAFHSS) as templates. The Fe₃O₄ nanoparticles were deposited on or imbedded in the hollow silica shell by a precipitation reaction. The morphologies, composition and properties of the hollow composite spheres were characterized by transmission electron microscopy, Fourier transform infrared analysis, X-ray diffraction measurement and vibrating-sample magnetometry measurement. The results indicated crystal sizes and amount of the Fe₃O₄ nanoparticles on the SAFHSS. The magnetic properties of the hollow composite spheres were controlled by adjusting the proportion between Fe²⁺ and Fe³⁺ and iron ion total concentration. When appropriate loading species were added into the system, superparamagnetite hollow composite spheres were obtained. The method also could be applicable to prepare other superparamagnetite hollow silica/ferrite composite spheres.     

Growth of different morphologies of silver submicrostructures: The effect of concentrations and pH

In this letter, we demonstrated the growth of different morphologies of silver submicrostructures influenced by o-phenylenediamine (oPD) concentrations and pH values. The results indicated that concentration ratios of the oPD to Ag⁺ colloids in the solution determine the final morphology of the products. oPD is found to act as a stabilizer as well as reduction agent. Particle shape ranges from sheets to spheres, increasing with molar ratio. Meanwhile, morphologies of structures and the time consumption to form particles both depend on pH values. We could obtain plates, polyhedrons, spheres, and dendrites through adjusting pH from 1.3 to 12.3. A mechanism is proposed to interpret the controlled synthesis of silver submicrostructures.     

Growth of different morphologies of silver submicrostructures: the effect of concentrations and pH

In this letter, we demonstrated the growth of different morphologies of silver submicrostructures influenced by ortho-phenylenediamine (oPD) concentrations and pH values. The results indicated that concentration ratios of the oPD to Ag⁺ colloids in the solution determine the final morphology of the products. oPD is found to act as a stabilizer as well as reduction agent. Particle shape ranges from sheets to spheres, increasing with molar ratio. Meanwhile, morphologies of structures and the time consumption to form particles both depend on pH values. We could obtain plates, polyhedrons, spheres, and dendrites through adjusting pH from 1.3 to 12.3. A mechanism is proposed to interpret the controlled synthesis of silver submicrostructures.     

Nanostructured thin films as surface‐enhanced Raman scattering substrates

Nanoporous thin films with silver nanoparticles were synthesized with a bottom–up approach, and its potential as effective surface‐enhanced Raman scattering (SERS) substrates was demonstrated. The use of mesoporous titania films as substrates allowed to control the growth of nanoparticles on the film surface. Atomic force microscopy measurements, Ultraviolet‐visible and X‐ray diffraction analysis confirmed the photoreduction of Ag⁺ to Ag⁰ with the formation of nanoparticles with crystallite dimensions of 32 to 36 nm. The new substrates allowed the detection of two analytes (rhodamine B isothiocyanate and cytochrome c), present in solutions at very low concentrations, highlighting their potential in SERS sensing. Reproducibility, homogeneity, enhancement factor of the substrate, consistency of results and detection limits were also assessed. Copyright © 2012 John Wiley & Sons, Ltd.     

Silver aggregates and twofold-coordinated tin centers in phosphate glass: A photoluminescence study

The optical properties of silver species in various oxidation and aggregation states and of tin centers in melt-quenched phosphate glasses have been assessed by optical absorption and photoluminescence (PL) spectroscopy. Glasses containing silver and tin, or either dopant, were studied. Emission and excitation spectra along with time-resolved and temperature-dependent PL measurements were employed in elucidating the different emitting centers observed and investigating on their interactions. In regard to silver, the data suggests the presence of luminescent single Ag⁺ ions, Ag⁺-Ag⁺ and Ag⁺-Ag⁰ pairs, and nonluminescent Ag nanoparticles (NPs), where Ag⁺-Ag⁰→Ag⁺-Ag⁺ energy transfer is indicated. Tin optical centers appear as twofold-coordinated Sn centers displaying PL around 400 nm ascribed to triplet-to-singlet electronic transitions. The optically active silver centers were observed in glasses where 8 mol% of both Ag₂O and SnO, and 4 mol% of Ag₂O were added. Heat treatment (HT) of the glass with the high concentration of silver and tin leads to chemical reduction of ionic silver species resulting in a large volume fraction of silver NPs and the vanishing of silver PL features. Further characterization of such heat-treated glass by transmission electron microscopy and X-ray photoelectron spectroscopy appears consistent with silver being present mainly in nonoxidized form after HT. On the other hand, HT of the glass containing only silver results in the quenching of Ag⁺-Ag⁰ pairs emission that is ascribed to nonradiative energy transfer to Ag NPs due to the positioning of the pairs near the surface of NPs during HT. In this context, an important finding is that a faster relaxation was observed for this nanocomposite in relation to a heat-treated glass containing both silver and tin (no silver pairs) as revealed by degenerate four-wave mixing spectroscopy. Such result is attributed to Ag NP→Ag⁺-Ag⁰ plasmon resonance energy transfer. The data thus indicates that energy transfer between Ag⁺-Ag⁰ pairs and NPs is bi-directional.     

The morphology control on the preparation of silver nanotriangles

A seed-mediated growth method was commonly applied to prepare one-dimension nanomaterials. However, some associated particles were unavoidable in the formation of target nanoparticles. Herein, we reported a modified method to prepare silver nanotriangles with higher uniform shape and particle size. The size and morphology of the formed nanoparticles could be controlled by regulating reaction conditions. The results showed that cetyl trimethyl ammonium bromide (CTAB) concentration and seed concentration were related with both the morphology and the particle size. The NaOH concentration, AgNO₃ concentration, and the mole ratio of V_c/Ag⁺ mainly affected the particle size of the formed nanotriangles. The formation of silver nanotriangles may be due to the selective stacking of the new tiny nanoparticles and the oriented growth of silver seed crystals. The oxidizing action of Br⁻/O₂ existing in the CTAB system should be responsible for the final morphology of truncated triangular silver nanoplates.     

Variable Frequency Microwave Synthesis of Silver Nanoparticles

Synthesis of silver nanoparticles based on a polyol process and variable frequency microwave (VFM) was investigated. Comparing to a thermal method, the reaction by VFM radiation was much faster. The effects of silver nitrate concentration, poly(N-vinylpyrrolidone) (PVP) concentration, reaction time and reaction temperature were studied. It was found that the higher concentration of silver nitrate, longer reaction time and higher temperature increased the particle size while the higher concentration of PVP decreased the particle size.     

Use of a silver ion selective electrode to assess mechanisms responsible for biological effects of silver nanoparticles

For a detailed analysis of the biological effects of silver nanoparticles, discrimination between effects related to the nano-scale size of the particles and effects of released silver ions is required. Silver ions are either present in the initial particle dispersion or released by the nanoparticles over time. The aim of this study is to monitor the free silver ion activity {Ag⁺} in the presence of silver nanoparticles using a silver ion selective electrode. Therefore, silver in the form of silver nanoparticles, 4.2 ± 1.4 nm and 2–30 nm in size, or silver nitrate was added to cell culture media in the absence or presence of A549 cells as a model for human type II alveolar epithelial cells. The free silver ion activity measured after the addition of silver nanoparticles was determined by the initial ionic silver content. The p {Ag⁺} values indicated that the cell culture media decrease the free silver ion activity due to binding of silver ions by constituents of the media. In the presence of A549 cells, the free silver ion activity was further reduced. The morphology of A549 cells, cultivated in DME medium containing 9.1% (v/v) FBS, was affected by adding AgNO₃ at concentrations of ≥30 μM after 24 h. In comparison, silver nanoparticles up to a concentration of 200 μM Ag did not affect cellular morphology. Our experiments indicate that the effect of silver nanoparticles is mainly mediated by silver ions. An effect of silver on cellular morphology was observed at p {Ag⁺} ≤ 9.2.     

Synthesis of hollow silver spheres using poly-(styrene-methyl acrylic acid) as templates in the presence of sodium polyacrylate

Hollow silver spheres were successfully prepared by reducing AgNO₃ with ascorbic acid and using negatively charged poly-(styrene-methyl acrylic acid) (PSA) spheres as templates in the presence of sodium polyacrylate as a stabilizer. Firstly, silver cations adsorbed on the surface of PSA spheres via electrostatic attraction between the carboxyl groups and silver cations were reduced in situ by ascorbic acid. The silver nanoparticles deposited on the surface of PSA spheres served as seeds for the further growth of silver shells. After that, extra amount of AgNO₃ and ascorbic acid solutions were added to form PSA/Ag composites with thick silver shells. In order to obtain compact silver shells, the as-prepared PSA/Ag composites were heated at 150 °C for 3 h. Then hollow silver spheres were prepared by dissolving PSA templates with tetrahydrofuran.     

XAFS study of copper and silver nanoparticles in glazes of medieval middle-east lustreware (10th–13th century)

It has recently been shown that lustre decoration of medieval and Renaissance pottery consists of silver and copper nanoparticles dispersed in the glassy matrix of the ceramic glaze. Here the findings of an X-ray absorption fine structure (XAFS) study on lustred glazes of shards belonging to 10th and 13rd century pottery from the National Museum of Iran are reported. Absorption spectra in the visible range have been also measured in order to investigate the relations between colour and glaze composition. Gold colour is mainly due to Ag nanoparticles, though Ag⁺, Cu⁺ and Cu²⁺ ions can be also dispersed within the glassy matrix, with different ratios. Red colour is mainly due to Cu nanoparticles, although some Ag nanoparticles, Ag⁺ and Cu⁺ ions can be present. The achievement of metallic Cu and the absence of Cu²⁺ indicate a higher reduction of copper in red lustre. These findings are in substantial agreement with previous results on Italian Renaissance pottery. In spite of the large heterogeneity of cases, the presence of copper and silver ions in the glaze confirms that lustre formation is mediated by a copper- and silver-alkali ion exchange, followed by nucleation and growth of metal nanoparticles.     

Nano-sized silica hollow spheres: Preparation, mechanism analysis and its water retention property

In this article, synthesis of nano-sized silica hollow spheres applying positive charged polystyrene as sacrificial templates was introduced. Firstly, nano-sized polystyrene particles were synthesized by emulsifier-free emulsion polymerization under solvothermal condition. Secondly, silica hollow nanospheres were formed through a simultaneous ‘coating-etching’ process. PVP played a key role in the evolution of nano-sized hollow spheres even if the templates were positive charged and the formation mechanism was different from that of previous studies. TEM results revealed that the morphologies of nano-sized silica hollow spheres not only strongly relied on the amount of reactant, but also the sequence of adding them. TGA illustrated that the interiors of nano-sized silica hollow spheres were not completely ‘hollow’. Brunauer-Emmett-Teller (BET) analysis showed that this material had a specific area of 399 m²/g. The water retention property of the materials was also investigated.     

Evolution of the optical properties of a silver-doped phosphate glass during thermal treatment

The optical properties of a silver-doped phosphate glass have been monitored during thermal processing at several fixed temperatures by in situ optical microspectroscopy. Silver nanoparticle (NP) formation and growth processes were assessed by analysis of surface plasmon resonance spectral features. Nucleation and growth processes were distinguished, which appeared temperature and time dependent. While nucleation was favored at low temperatures, relatively high temperatures promoted NP growth by silver diffusion. Photoluminescence spectra acquired along with optical absorption data indicated a continuous reduction of Ag⁺-Ag⁰ pairs concomitant with NP precipitation, suggesting their role as nucleation centers. The work of Ag NP formation and the activation energy for silver diffusion were estimated.     

Influence of several factors on the growth of selenium nanowires induced by silver nanoparticles

This paper presents a study on the crystallization and growth mechanism of selenium nanowires induced by silver nanoparticles at ambient conditions with special reference to the effects of factors such as the shapes and size of silver nanoparticles, the induced reaction time, and the molar ratio of Ag⁰ to SeO₃²⁻ ions. The synthesis approach is conducted with no need of any stabilizers, and with no sonochemical process and/or templates. It is found that whether silver spherical particles or colloids can lead to the formation of nanowires with average diameter of 25 nm and lengths up to a few micrometers, and silver nanoplates lead to the formation of flat Se nanostructures. In particular, Au, Cu, Pt, and Pd particles cannot induce the growth of selenium nanowires in aqueous solution at room temperature. The results indicate that silver particles play a critical role in determining the growth of selenium nanowires. The lattice match between hexagonal-Se and orthorhombic- or trigonal-Ag₂Se particles is the major driving force in the growth of such nanostructures. The findings would be useful for design and construction of heterogeneous nanostructures with similar lattice parameter(s).