Immobilization of luminescent nanosilicon in a microfine polytetrafluoroethylene matrix by means of supercritical carbon dioxide

With the use of supercritical carbon dioxide (SC-CO₂), the matrix immobilization of photoluminescent silicon nanocrystals (nc-Si) in polytetrafluoroethylene microparticles (mp-PTFE) is performed, which leads to the formation of mp-PTFE/nc-Si photoluminescent nanocomposite containing ∼10³–10⁴ nc-Si particles per mp-PTFE particle (1–2 μm in size). This approach is based on the effect of polymer swelling in SC-CO₂, efficient SC-CO₂-assisted transport of nanoparticles into the internal free volume of the polymer, and contraction of the nanocomposite after the release of CO₂, an effect that prevents the subsequent agglutination of nanoparticles. Particles of nc-Si photoluminescent in the visible spectrum were synthesized from silicon suboxide powder (SiO _x , x ≈ 1) heated at various temperatures within 25–950°C and then etched in concentrated hydrofluoric acid. The hydrosilylation procedure was used to graft 1-octadecene molecules to the surface of nc-Si particles. As a result, the photoluminescence intensity of nc-Si increased substantially. According to TEM images and small angle X-ray scattering data, the maximum size of nc-Si particles did not exceed 5 nm and 7 nm, respectively, and the core of these nanoparticles consisted of crystalline silicon. The structure and spectral properties of the initial nc-Si particles and synthesized mp-PTFE/nc-Si photoluminescent nanocomposite microparticles were studied.     

Photo–induced processes in Ag and Eu β-diketonates incorporated into aerogel matrix of silicon dioxide by supercritical fluid impregnation

Samples of silicon dioxide aerogel with embedded Ag and Eu β-diketonate molecules are obtained by impregnation in supercritical carbon dioxide (SC-CO₂). The sample impregnated by Eu(tta)₃ molecules possesses photoluminescence properties. Moreover, adsorption of Eu(tta)₃ on the walls of the pores results in a strong broadening of the Stark components of its photoluminescence spectra. It is found that aerogel impregnation by AgFOD molecules followed by laser irradiation causes the formation of Ag nanoparticles in the sample volume as a result of AgFOD photolysis and subsequent diffusion self-assembly. The Ag nanoparticles assemble into filament structures due to self-organization as they focus laser radiation.     

One-step synthesis of silver nanoparticles in an aqueous solution and their antibacterial activities

A one-step simple synthesis of silver colloid nanoparticles with controllable sizes is presented in this research. In the synthesis, an amino-terminated hyperbranched polymer (HBP-NH₂) was applied as a stabilizer and a reductant. The syntheses, performed at various initial AgNO₃ concentrations (0.28–0.56 g/l) in a 2 g/l HBP-NH₂ aqueous solution, produced silver colloid nanoparticles having average sizes from 3 to 30 nm with narrow size distributions. The formation of silver colloid nanoparticles was characterized by Fourier Transform Infrared Spectrophotometry (FTIR), Dynamic Light Scattering (DLS), Transmission Electron Microscopy (TEM), UV/Visible Absorption Spectrophotometry, and X-ray Diffraction (XRD) measurements. The results indicated that both particle size and the UV absorption are strongly dependent on the initial AgNO₃ concentrations. The silver colloid nanoparticles, prepared with a 0.35 g/l AgNO₃ aqueous solution in the presences of 2 g/l HBP-NH₂, showed good antibacterial activities against Gram-negative bacteria (Escherichia coli) and Gram-positive bacteria (Staphylococcus aureus). A very low concentration of nano-silver (as low as 3.0 ug/ml Ag) also gave excellent antibacterial performance.     

The influence of low-temperature silver-ion exchange on the spectral-luminescent properties of fluorophosphate glasses doped with PbSe

Changes in the absorption and luminescence spectra of fluorophosphate glasses doped with PbSe caused by low-temperature Ag⁺–Na⁺ ion exchange are considered. It is found that the silver distribution gradient in a near-surface layer about 16 μm thick leads to two different processes of interaction between metal and semiconductor nanoparticles. PbSe molecular clusters and quantum dots more efficiently grow in deep layers with a low silver concentration. The near-surface glass layers with a high silver concentration exhibit formation of Ag metal nanoparticles, on the surface of which interaction with PbSe molecular clusters leads to the formation of Ag–Se–Pb bonds, which transform into Ag₂Se layers in the process of heat treatment. The appearance of the new phase is confirmed by X-ray diffraction.     

Corrosion processes of triangular silver nanoparticles compared to bulk silver

Excessive corrosion of silver nanoparticles is a significant impediment to their use in a variety of potential applications in the biosensing, plasmonic and antimicrobial fields. Here we examine the environmental degradation of triangular silver nanoparticles (AgNP) in laboratory air. In the early stages of corrosion, transmission electron microscopy shows that dissolution of the single-crystal, triangular, AgNP (side lengths 50–120 nm) is observed with the accompanying formation of smaller, polycrystalline Ag particles nearby. The new particles are then observed to corrode to Ag₂S and after 21 days nearly full corrosion has occurred, but some with minor Ag inclusions remaining. In contrast, a bulk Ag sheet, studied in cross section, showed an adherent corrosion layer of only around 20–50 nm in thickness after over a decade of being exposed to ambient air. The results have implications for antibacterial properties and ecotoxicology of AgNP during corrosion as the dissolution and reformation of Ag particles during corrosion will likely be accompanied by the release of Ag⁺ ions.     

The investigation of oxidized silver nanoparticles prepared by thermal evaporation and radio-frequency sputtering of metallic silver under oxygen

The investigation of oxidized silver nanoparticles by the photoemission (XPS, UPS) and HRTEM methods was performed. The nanoparticles of oxidized silver were obtained in the vacuum chamber by two methods of synthesis: thermal evaporation of silver nanoparticles followed by transferring in convective gas flow and sputtering of oxidized clusters under the action of plasma. Both methods indicated that oxygen interaction with silver nanoparticles depends strongly on its size. It was shown that the chemical bonding of oxygen species stabilized on small particles differs from the oxygen species adsorbed on bulk silver surfaces (monocrystals, foils and large particles). The low charged oxygen with molecular type of bonding stabilizes on particles of size approximately 5 nm and smaller. Increasing particle size leads to the dissociation of molecular oxygen species and the formation of strongly charged oxygen composed of oxide nanoparticles like Ag₂O or AgO type. The presence of extended defects in the microdomain large nanoparticles facilitates the formation of Ag₂O or AgO layers covering metallic nanosilver.     

Synthesis of silver nanocomposites by SCF impregnation of matrices of synthetic opal and Vycor glass by the Ag(hfac)COD precursor

Silver-containing nanocomposites were prepared by impregnating Vycor glass (a pore diameter of 4 nm) and synthesized opal matrices (an interstitial void size of 40 nm) with cyclooctadiene complex of silver hexafluoroacetylacetonate (Ag(hfac)COD), a silver precursor, dissolved in supercritical carbon dioxide and were examined by optical absorption spectroscopy, atomic force microscopy, and electron spin-resonance spectroscopy. It was demonstrated that the absorption spectra of Vycor glass and opal matrices impregnated with Ag(hfac)COD molecules and subjected to thermal treatment in air at temperatures above 50°C exhibit plasmon resonances characteristic of Ag nanoparticles at 420–430 nm. The peculiarities of the plasmon resonance band for both types of samples were attributed to the morphology of the pore space in which silver particles are formed. Paramagnetic Cu(hfac)₂ molecules (copper hexafluoroacetylacetonate) were used as a spectroscopic probe for estimating the distribution of the precursor in the pores of Vycor glass and opal matrices during supercritical fluid impregnation.     

Spectroscopic evidence of positive clusters in Ag colloids obtained by laser ablation in aqueous solutions

We have found evidence of positive cluster formation during the laser ablation process of a silver target in aqueous solutions. In particular, by employing in situ shot-by-shot UV–vis spectroscopy in the early stages of the ablation, we observed a weak and unstable absorption band around 266 nm and a more stable one around 290 nm, which could be assigned to charged clusters like Ag ₃ ²⁺ and Ag ₄ ²⁺ , respectively. Surface-enhanced Raman scattering experiments performed with a test molecule adsorbed on a silver colloid obtained in pure water were compatible with the presence of Ag ₄ ²⁺ active sites on the surface of the Ag nanoparticles.     

银/二氧化钛核壳纳米颗粒对碲化镉纳米晶的荧光增强研究

利用新合成的复合纳米结构银/二氧化钛核壳纳米颗粒,研究了金属银纳米颗粒对碲化镉纳米晶层荧光的增强情况.结果表明,这种新型复合金属纳米结构能极大地增强发光纳米晶层的荧光强度.银/二氧化钛核壳纳米颗粒是以水合肼、硝酸银和四异丙氧基钛为原材料,利用胶体化学法在水溶液中合成.透射电子显微镜图片表明这种新合成的银/二氧化钛纳米材料基本上呈球形,有较为明显的核壳结构,中间黑色的核是银纳米颗粒,外层颜色较浅部分是二氧化钛壳层.另外,包裹二氧化钛壳层后,银纳米颗粒的表面等离子吸收带从409 nm红移至430 nm,也证实了这种新型核壳纳米材料的形成.将此合成方法得到的银/二氧化钛纳米颗粒和碲化镉纳米晶用旋转涂覆方法进行直接组合后,得到了银纳米颗粒对碲化镉纳米晶荧光的明显增强,并对其增强的物理过程进行了讨论.这种能够增强荧光团发光的新型复合银纳米结构将在发光器件、荧光成像、生物探测等方面具有一定的应用价值.     

Plasmonics properties of trimetallic Al@Al2O3@Ag@Au and Al@Al2O3@AuAg nanostructures

Bimetallic and trimetallic nanoparticles have attracted significant attention in recent times due to their enhanced electrochemical and catalytic properties compared to monometallic nanoparticles. The numerical calculations using Mie theory has been carried out for three-layered metal nanoshell dielectric–metal–metal (DMM) system consisting of a particle with a dielectric core (Al@Al₂O₃), a middle metal Ag (Au) layer and an outer metal Au (Ag) shell. The results have been interpreted using plasmon hybridization theory. We have also prepared Al@Al₂O₃@Ag@Au and Al@Al₂O₃@AgAu triple-layered core–shell or alloy nanostructure by two-step laser ablation method and compared with calculated results. The synthesis involves temporal separations of Al, Ag, and Au deposition for step-by-step formation of triple-layered core–shell structure. To form Al@Ag nanoparticles, we ablated silver for 40 min in aluminium nanoparticle colloidal solution. As aluminium oxidizes easily in water to form alumina, the resulting structure is core–shell Al@Al₂O₃. The Al@Al₂O₃ particle acts as a seed for the incoming energetic silver particles for multilayered Al@Al₂O₃@Ag nanoparticles is formed. The silver target was then replaced by gold target and ablation was carried out for different ablation time using different laser energy for generation of Al@Al₂O₃@Ag@Au core–shell or Al@Al₂O₃@AgAu alloy. The formation of core–shell and alloy nanostructure was confirmed by UV–visible spectroscopy. The absorption spectra show shift in plasmon resonance peak of silver to gold in the range 400–520 nm with increasing ablation time suggesting formation of Ag–Au alloy in the presence of alumina particles in the solution.     

Silver nanoparticles synthesized by direct photoreduction of metal salts. Application in surface‐enhanced Raman spectroscopy

A simple synthesis method of silver nanoparticles and its application as an active surface‐enhanced Raman spectroscopy (SERS) colloid are presented in this work. The photoreduction of AgNO₃ in presence of sodium citrate (NaCit) was carried out by irradiation with different light sources (UV, white, blue, cyan, green, and orange) at room temperature. The evaluation of silver nanoparticles obtained as a function of irradiation time (1–24 h) and light source was followed by UV‐visible absorption spectroscopy. This light‐modification process results in a colloid with distinctive optical properties that can be related to the size and shape of the particles. The Ag colloids, as prepared, were employed as active colloids in SERS. Pyridine and caffeine were used as test molecules. Copyright © 2008 John Wiley & Sons, Ltd.     

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.     

The mechanism of metal nanoparticle formation in plants: limits on accumulation

Metal nanoparticles have many potential technological applications. Biological routes to the synthesis of these particles have been proposed including production by vascular plants, known as phytoextraction. While many studies have looked at metal uptake by plants, particularly with regard to phytoremediation and hyperaccumulation, few have distinguished between metal deposition and metal salt accumulation. This work describes the uptake of AgNO₃, Na₃Ag(S₂O₃)₂, and Ag(NH₃)₂NO₃ solutions by hydroponically grown Brassica juncea and the quantitative measurement of the conversion of these salts to silver metal nanoparticles. Using X-ray absorption near edge spectroscopy (XANES) to determine the metal speciation within the plants, combined with atomic absorption spectroscopy (AAS) for total Ag, the quantity of reduction of Ag^I to Ag⁰ is reported. Transmission electron microscopy (TEM) showed Ag particles of 2–35 nm. The factors controlling the amount of silver accumulated are revealed. It is found that there is a limit on the amount of metal nanoparticles that may be deposited, of about 0.35 wt.% Ag on a dry plant basis, and that higher levels of silver are obtained only by the concentration of metal salts within the plant, not by deposition of metal. The limit on metal nanoparticle accumulation, across a range of metals, is proposed to be controlled by the total reducing capacity of the plant for the reduction potential of the metal species and limited to reactions occurring at an electrochemical potential greater than 0 V (verses the standard hydrogen electrode).     

The irradiation influence on the properties of silver sulfide (Ag2S) colloidal nanoparticles

The aqueous solutions of different stability containing silver sulfide (Ag₂S) nanoparticles are studied. The stable, transparent, and turbid solutions have been subjected to daylight for 7 months, to ultraviolet and laser irradiation, as well as to an electron beam. Solar radiation is found to favor the Ag₂S reduction to Ag and/or the formation of Ag₂S/Ag hybrid nanoparticles in the solution. At a high amount of hybrid nanoparticles, the exciton–plasmon interaction causes asymmetry in the absorption spectra. The exposure of Ag₂S particles precipitated from the solution with the electron beam leads to the reversible growth of Ag threads. The possible exciton–plasmon interplay mechanisms in Ag₂S/Ag hybrid nanoparticles are considered. The physical mechanisms of the changing Ag₂S stoichiometry, the formation of metallic Ag and Ag₂S/Ag hybrid nanoparticles are the generation of hot carriers and the energy transfer (exciton–plasmon interaction) in a metal–semiconductor hybrid nanosystem are elucidated, as well.

     

Laser ablation of silver and gold in liquid ammonia

Laser ablation of a silver (Ag) and/or gold (Au) target was performed in liquid ammonia (l-NH₃) at 233 K using nanosecond laser pulses of 1064, 532 and 355 nm wavelengths. An “in situ” monitoring of the ablation process by UV/vis/NIR spectroscopy has shown the evolution of the surface plasmon extinction band of silver or gold nanoparticles and thus confirmed their formation. While sols of Au nanoparticles in l-NH₃ are quite stable in air, those of Ag nanoparticles undergo oxidation to Ag(I) complexes with NH₃ ligands. On the other hand, formation of solvated electrons, namely of the (e⁻)NH₃ solvates, has not been unequivocally confirmed under the conditions of our laser ablation/nanoparticle fragmentation experiment, since only very weak vis/NIR spectral features of these solvates were observed with a low reproducibility. Reference experiments have shown that the well-known chemical production of these solvates is hindered by the presence of Ag and Au plates. Ag and Au targets can thus possibly act as electron scavengers in our ablation experiments.     

Nonlinear optical properties of nanometer-size silver composite azobenzene containing polydiacetylene film

Diacetylene monomer containing p-nitrophenyl azobenzene moiety (NADA) was synthesized. Silver nanoparticles with different concentrations were adulterated in the above polymerized NADA (PNADA) films and the third-order nonlinear optical properties were investigated in detail. UV–vis spectra and transmission electron microscopy were used to confirm the formation of PNADA/Ag nanocomposite films. The silver nanoparticles (average size of 10 nm) were well dispersed in the polymer films. The value of the nonlinear refractive index n ₂ for PNADA films (8.48×10⁻¹⁵ cm²/W) was much higher than that of pure polydiacetylene films. Further, the introduction of silver nanoparticles into the PNADA polymer films led to the further enhancement of nonlinear optical properties. The maximum value of n ₂ for PNADA/Ag nanocomposite films could be 11.6×10⁻¹⁵ cm²/W. This enhancement should be ascribed to the surface plasmon resonance of silver nanoparticles.     

Effect of Ag doping on the crystallization and phase transition of TiO2 nanoparticles

TiO₂ nanoparticles doped with different Ag contents were prepared by a modified sol–gel method, using titanium tetraisopropoxide and silver nitrate as precursors and 2-propanol as solvent. Silver was incorporated into the TiO₂ matrix via decomposition of AgNO₃ during thermal treatment in different atmospheres. Effects of Ag doping on the crystallization and phase transition of the TiO₂ nanoparticles were studied using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Raman spectroscopy techniques. While air annealing incorporates silver into TiO₂ matrix in silver oxide form, annealing in nitrogen incorporates metallic silver into TiO₂. Formation of silver oxide increases the thermal stability of the TiO₂ particles. Silver oxide affects the crystallization process of TiO₂ particles and the temperature of transition form anatase to rutile. On the other hand, presence of metallic silver in the samples annealed in nitrogen atmosphere decreases the temperature of phase transition of TiO₂ nanoparticles.     

Preparation of Cu–Ag core–shell particles with their anti-oxidation and antibacterial properties

Cu–Ag core–shell particles were fabricated from Cu particles and silver sulphate with the environmental-friendly TA (tartaric acid, C₄H₆O₆) as reducing and chelating agent in an aqueous system. The influences of [TA]/[Ag] and [Ag]/[Cu] molar ratios on the formation of Ag coatings on the Cu particles were investigated. The SEM images and SEM–EDS analyses showed that [TA]/[Ag] = 0.5 and [Ag]/[Cu] ≥0.2, the Cu particles were coated with uniform Ag nanoparticles. XRD analyses revealed that for these Cu–Ag particles heated at 250 °C, the oxidation of Cu was significantly reduced. Both anti-Staphylococcus aureus (Gram-positive) and anti-Escherichia coli (Gram-negative) characteristics of this Cu–Ag composite particles showed satisfactory antibacterial ability. The characteristics of the composite Cu–Ag particles were discussed in detail.     

Silver nanoparticles generated by pulsed laser ablation in supercritical CO2 medium

Pulsed laser ablation (PLA) has been widely employed in industrial and biological applications and in other fields. The environmental conditions in which PLA is conducted are important parameters that affect both the solid particle cloud and the deposition produced by the plume. In this work, the generation of nanoparticles (NPs) has been developed by performing PLA of silver (Ag) plates in a supercritical CO₂ medium. Ag NPs were successfully generated by allowing the selective generation of clusters. Laser ablation was performed with an excitation wavelength of 532 nm under various pressures and temperatures of CO₂ medium. On the basis of the experimental result, both surface of the irradiated Ag plate and structure of Ag NPs were significantly affected by the changes in supercritical CO₂ pressure and temperature. With increasing irradiation pressure, plume deposited in the surrounding crater created by the ablation was clearly observed. In Field Emission Scanning Electron Microscopy (FE-SEM) the image of the generated Ag NPs on the silicon wafer and the morphology of Ag particles were basically a sphere-like structure. Ag particles contain NPs with large-varied diameter ranging from 5 nm to 1.2 μm. The bigger Ag NPs melted during the ablation process and then ejected smaller spherical Ag NPs, which formed nanoclusters attached on the molten Ag NPs. The smaller Ag NPs were also formed around the bigger Ag NPs. Based on the results, this new method can also be used to obtain advanced nano-structured materials.     

Formation of argentic clusters and small Ag nanoparticles in soda-lime silicate glass

The formation of argentic clusters and very small Ag nanoparticles of 0.5 to 2 nm size in commercial soda-lime glass silver-doped by Ag/Na ion exchange in a mixed nitrate melt has been studied by electron microscopy and EXAFS. Particles formation was induced already during the ion exchange procedure, or by subsequent ion irradiation with 1.5 MeV He⁺ or 3 MeV Au⁺. The presence of nanoparticles was also macroscopically revealed by their surface plasmon resonance. The structural characterization indicates that specific configurations of silver oxide-like structures, so-called argentic clusters, are involved in the initial stage of nanoparticles formation.