Evaluation of the toxicities of silver and silver sulfide nanoparticles against Gram‐positive and Gram‐negative bacteria

In this study, the endogenous lipid signalling molecules, N ‐myristoylethanolamine, were explored as a capping agent to synthesise stable silver nanoparticles (AgNPs) and Ag sulphide NPs (Ag₂ S NPs). Sulphidation of the AgNPs abolishes the surface plasmon resonance (SPR) maximum of AgNPs at 415 nm with concomitant changes in the SPR, indicating the formation of Ag₂ S NPs. Transmission electron microscopy revealed that the AgNPs and Ag₂ S NPs are spherical in shape with a size of 5–30 and 8–30 nm, respectively. AgNPs and Ag₂ S NPs exhibit antimicrobial activity against Gram‐positive and Gram‐negative bacteria. The minimum inhibitory concentrations (MIC) of 25 and 50 μM for AgNPs and Ag₂ S NPs, respectively, were determined from resazurin microtitre plate assay. At or above MIC, both AgNPs and Ag₂ S NPs decrease the cell viability through the mechanism of membrane damage and generation of excess reactive oxygen species.Inspec keywords: cellular biophysics, biomembranes, transmission electron microscopy, nanomedicine, microorganisms, molecular biophysics, antibacterial activity, nanofabrication, silver, biomedical materials, surface plasmon resonance, nanoparticles, materials preparation, silver compounds, lipid bilayersOther keywords: Gram‐negative bacteria, Gram‐positive bacteria, endogenous lipid signalling molecules, N‐myristoylethanolamine, capping agent, silver nanoparticles, Ag sulphide NPs, sulphidation, surface plasmon resonance, concomitant changes, transmission electron microscopy, minimum inhibitory concentrations, resazurin microtitre plate assay, cell viability, membrane damage, reactive oxygen species, Ag toxicities, Ag, Ag₂ S     

Preparation and photovoltaic properties of anodically grown Ag2O films

The semiconducting and photovoltaic properties of p-type Ag₂O films grown anodically on silver electrodes were studied, in view of possible applications in solar energy conversion. Films were grown in different alkaline solutions; the best results were obtained for 0.02M Ag₂SO₄ + 0.17M NH₄OH + 5.7 × 10^–3M Ba(OH)₂ saturated with Ag₂O powder, stirred mechanically at room temperature. Film thicknesses of up to 10m were thus obtained for the first time in anodically grown Ag₂O. Photovoltaic spectra taken at 300 K give a bandgap ofEg = 1.42 ± 0.04 eV. Evaporated gold on Ag₂O appears to be ohmic while aluminium and platinum are rectifying. The barrier height of Ag/Ag₂O is 0.90 ± 0.02 eV, that of Al/Ag₂O is 0.93 ± 0.02 eV, and that of platinum 0.94 ± 0.02 eV. The best cells give an open-circuit voltage,V _oc, of over 150 mV, and a short circuit current,I _sc = 100A cm^–2 under 50 mW cm^–2 illumination.     

Dissolution kinetics and diffusivity of silver in glassy layers for hybrid microelectronics

Silver and palladium/silver compositions are widely used in hybrid microelectronics, as electrodes for dielectric layers and multilayers, terminations of thick film resistors and interconnections. Interactions between Ag and the adjacent films are known to affect the microcircuit performances. The present study is aimed at collecting data on the behavior of Ag-based films in contact with glassy layers. Most experiments were performed with a glass with composition 68.2 PbO : 30.5 SiO₂ : 1.3 Al₂O₃ wt %. Two different systems were analyzed. The first system consists of thick films prepared from a paste containing glass and either 3 or 15 wt % silver particles; both fine (spherical grains, 0.5–1 m diameter) and coarse (flakes, 2–5 m, <1 m thick) Ag powders were used for these pastes. The distribution of Ag in the film was studied with X-ray diffraction, scanning electron microscopy and fluorescence analysis. The results show that Ag floats on the glassy layer. Diffraction of X-rays generated by a synchrotron radiation source allowed us to study the kinetics of silver dissolution in the glass; this phenomenon is consistent with the Avrami theory, with an apparent activation energy E _dis=0.69±0.04 eV. The second system analyzed, Ag-based terminations of glass layers fired at various peak temperatures, enabled us to obtain quantitative values for both Ag solid solubility (about 2.5 wt %) and Ag diffusion coefficients D _Ag(T ). Typical values of D _Ag(850 °C) are 30.3±11.9 10^–8 cm²/s; an apparent activation energy of the diffusion process is E _a=0.6±0.1 eV.     

Optical properties of silver sulphide thin films formed on evaporated Ag by a simple sulphurization method

Silver sulphide (Ag₂S) thin films were grown on the surface of silver films (Ag) deposited on glass substrate by using a simple chemical sulphurization method. According to X-ray diffraction analysis, the Ag₂S thin films display low intensity peaks at 34.48°, 36.56°, and 44.28°, corresponding to diffraction from (100), (112) and (103) planes of the acanthite phase (monoclinic). A model of the type Ag₂S/Ag/glass was deduced from spectroscopic ellipsometric measurements. Also, the optical constants (n, k) of the system were determined. Furthermore, the optical properties as solar selective absorber for collector applications were assessed. The optical reflectance of the Ag₂S/Ag thin film systems exhibits the expected behavior for an ideal selective absorber, showing a low reflectance in the wavelength range below 2 µm and a high reflectance for wavelengths higher than that value. An absorptance about 70% and an emittance about 3% or less were calculated for several samples.     

Correlation Between the Morphology of Ag and the Contact Resistivity of the Ag/YBa2Cu3O7?δ Thin Film Contact

The morphology of the silver films deposited and annealed on laser ablated YBa₂Cu₃O_7– thin films and the corresponding contact resistivity have been systematically investigated. A minimum contact resistivity of 6 × 10^–8 cm² was reached at 77 K by annealing Ag/YBa₂Cu₃O_7– contact at the optimum temperature. The effect of the annealing temperature on the contact resistivity was explained by considering the morphology of the silver films and the diffusion of silver into YBa₂Cu₃O_7– film, etc. The difference of the contact resistivity for Ag contact to polycrystalline, single crystal and thin film of YBa₂Cu₃O_7– were also explained.     

Microstructure and anodic polarization behavior of experimental Ag–18Cu–15Pd–12Au alloy in aqueous sulfide solution

The anodic corrosion behavior of an experimental Ag–15Pd–18Cu–12Au alloy in 0.1% Na₂S solution in relation to its microstructure was investigated using potentiodynamic and potentiostatic polarization techniques with analyses of corrosion products by X-ray diffractometry, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. The role of Pd in improvement of the corrosion resistance was also investigated. In the potential/current density curve, three distinct current peaks, at –520 mV (peak I), –425 mV (peak II) and –175 mV (peak III), were observed. The Ag-rich ₂ matrix with coarse Cu and Pd-rich lamellae was the most corrosion-susceptible region, and this region was preferentially corroded at peak I with the formation of granular deposits of Ag₂S. A small amount of Ag–Cu mixed sulfide deposited on the Cu and Pd-rich coarse particles and dissolution of Ag as AgO^– might have occurred in parallel with Ag₂S formation at peak II. Enrichment of Pd on the alloy surface occurred at peak III due to preferential dissolution of Ag and Cu. A high level of corrosion resistance was attained with the formation of a thin Pd-rich sulfide film, which enhanced the passivity of the alloy in an alkaline sulfide solution. It was found that passivity is an important phenomenon not only for base metal alloys but also for noble metal alloys to maintain high levels of resistance to corrosion and tarnishing in sulfide environments.     

Use of metastable, dissociated and charged gas species in synthesis: a low pressure analogue of the high pressure technique

Oxidation of silver using microwave-induced oxygen plasma and oxygen-ozone gas mixture was studied as a function of temperature and partial pressure. The oxide Ag₂O was formed at temperatures well above its normal decomposition temperature in oxygen plasma at a pressure of 5 Pa. The higher oxide AgO_1–x was formed in O₂₊O₃ gas mixtures at lower temperatures. The oxygen chemical potentials for the oxidation of Ag to Ag₂O, Ag₂O to AgO_1–x and AgO to Ag₂O₃ were evaluated from thermodynamic data and compared with the experimental results to obtain information on the chemical potential of oxygen in microwave plasma and gases containing ozone. The oxygen potential of the gas phase in microwave plasma operating at a pressure of 5 Pa was found to be in excess of 36 kJ/mol at 750 K. This is equivalent to a pressure of diatomic oxygen gas greater than 3 × 10⁷ Pa. In the O₂₊O₃ mixture at ambient pressure containing 5 mole percent O₃, the oxygen potential is 112 kJ/mol at 465 K. The equivalent pressure of diatomic oxygen is 4 × 10¹⁷ Pa. Thus, metastable species such as O₃ or charged species such as O^– present in plasma can be used as a powerful reagent for the syntheses of metastable oxides. Similar techniques can be used for other metastable inorganic solids such as nitrides for functional applications.     

The determination of Gibbs energy of the exchange reaction of sulphides using beta-alumina solid electrolyte

EMF measurements of the cell Na (l) |-alumina | Na₂S + A9₂S + Ag were carried out in the temperature range of 463.4 to 843.7 K. The results were represented by the equationE/V (±0.00057) = 1.75793(±0.00086) – 4.452(±0.014) × 10^–4(T) where 463.4 < T < 737.7 K. The Gibbs energy of the exchange reaction 1/2Ag₂S(s) + Na(l) 1/2Na₂S(s) + Ag(s) was determined from the EMFs. The standard Gibbs energy of formation of Na₂S in the reaction 2Na(l) + 1/2S₂(g) = Na₂S(s) was obtained from the values of EMF and in the literature as (J mol^–1 ± 1160) = –427940 + 124.92T where 463.4 < T < 737.7 K. The eutectic temperature of the Ag₂S-Na₂S system was estimated to be 738 ± 5 K.     

Enhanced photocatalytic activity of silver nanoparticles modified TiO2 thin films prepared by RF magnetron sputtering

Ag-TiO₂ nanostructured thin films with silver volume fraction of 0–40% were prepared by RF magnetron sputtering. The microstructure, surface topography, and optical properties of the films were characterized by X-ray diffractometer, transmission electron microscope, and ultraviolet–visible spectrophotometer. Photocatalytic activity of the films was evaluated by light-induced degradation of methyl orange (C₁₄H₁₄N₃NaO₃S) solution using a high pressure mercury lamp as lamp-house. The relation of photocatalytic activity and silver content was studied in detail. It is found that silver content influences microstructure of TiO₂ thin films, and silver in the films is metallic Ag (Ag⁰). Photocatalytic activity of the films increases with increasing silver content up to 5 vol.% Ag and then decreases to values significantly still bigger than that of pure TiO₂ thin films. Silver nanoparticles significantly enhance the photocatalytic activity of TiO₂ films. The better separation between electrons and holes on silver modified TiO₂ thin films surface allowed more efficiency for the oxidation and reduction reactions. The enhanced photocatalytic activity was mainly attributed to the decrease of energy gap of the films and the increase of oxygen anion radicals O₂⁻ and reactive center of surface Ti³⁺ on silver modified TiO₂ thin films surface.     

The effect of interfacial mixing on Ag/Al2O3 by N+ implantation

The Ag film/Al₂O₃ implanted with an N ion energy of 110 keV. The ion-induced interfacial mixing were examined using AES, STD and XPS. The frictional coefficient of implanted Ag/Al₂O₃ in air was determined by Drive friction precise measuring apparatus (DFPM). The influence of N⁺ implantation on the interfacial chemistry and adhesion of Ag films on Al₂O₃ substrates was examined, compounds formed by introducing a thin layer between the Ag and the Al₂O₃ at 1 × 10¹⁷ N·cm^–2. This investigation resulted in extensive interfacial grading, and new chemical bonding across the Ag/Al₂O₃ interfaces. The case for the Ag/Al₂O₃ reaction lead to the metal ceramic of 13Al₂O₃·AIN and -AgAlO₂ formation though non equilibrium processes of implantation, and friction decreased in initial cycle where ion implantation of lower vacuum lead to surface carbon film. The combination of these effects provided an adhesion increase that was approximately 3 times that obtained in unimplanted Ag/Al₂O₃ specimens.     

Reversible transformations of silver oxide and metallic silver nanoparticles inside SiO2 films

Reversible transformation of silver oxide and metallic nanoparticles inside a relatively porous silica film has been established. Annealing of Ag-doped films in oxidizing (air) atmosphere at 450 °C yielded colorless films containing AgO_x. These films were turned yellow when heated in H₂-N₂ (reducing atmosphere) due to the formation of Ag nanoparticles. This yellow coloration (due to nano Ag⁰) and bleaching (conversion of Ag⁰ → Ag⁺) are reversible. Optical and photoluminescence spectra are well consistent with this coloration and bleaching. The soaking test of the air-annealed film in Na₂S₂O₃ solution supports the presence of Ag⁺. Grazing incidence X-ray diffraction and transmission electron microscopy studies reveal the formation of Ag-oxides and Ag nanoparticles in the oxidized and reduced films, respectively.     

Ionic transport studies of the glassy silver vanadomolybdate system

Ionic conductivity of the Ag₂O-MoO₃-V₂0₅ system has been studied over a wide range of frequency, temperature and composition. A narrower glass forming region has been found in comparison to the corresponding Ag₂O-MoO₃-P₂O₅ and Ag₂O-B₂O₃-P₂O₅ systems. The highest conductivity at room temperature, _rt, = 3.21 × 10^–6–1 cm^–1 (d.c.) with an activation energy,E _act, of 0.466 eV, was observed for the glass former's ratio of unity. Further, it reached a maximum value of 2.2 × 10^–2¨-1 cm^–1 withE _act = 0.153 eV when the oxide-base glass was dissolved with Agl. D.c. conductivity, hopping rate and relaxation time in the present system have been found to be characterized by the same activation energy.     

Phase Transition of Ag-Enriched Ag2Te

The electrical and thermoelectric properties of Ag₂Te containing excess Ag (0.25 at. %) were studied. At 0.1–0.12 at. % excess Ag, Ag₂Te undergoes an electronic phase transition attributable to the formation of a new Ag sublattice.     

Electrochemical reclamation of silver from silver-plating wastewater using static cylinder electrodes and a pulsed electric field

Silver was reclaimed from silver-plating wastewater by using a pulsed electric field (PEF) combined with static cylinder electrodes (SCE). The conditions that produced the maximal silver recovery rate (RR_Ag) (99%) were as follows: average retention time of 10 min, interelectrode gap of 50 mm, solution pH of 9.0, temperature of 45 °C, initial Ag(I) concentration of 1000 mg L⁻¹, PEF pulse frequency of 1200 Hz, current density of 5.0 A m⁻² and a pulse duty cycle of 60%. Compared with the conventional direct current (DC) technology, the PEF process exhibited improvements in the silver recovery rate (RR_Ag), total energy consumption (TEC) and physical properties of the silver deposits, especially for low Ag(I) concentrations, for example, from 500 to 1000 mg L⁻¹. For an initial Ag(I) concentration of 500 mg L⁻¹, the PEF process produced an RR_Ag of up to 99%, and the TEC was 4.56 kWh (kg Ag)⁻¹. In comparison, the RR_Ag and TEC were 90% and 5.66 kWh (kg Ag)⁻¹, respectively, in the DC process. The results of SEM observation and XRD analysis indicated that the silver deposits formed by the PEF process were smaller, denser, and of a higher purity than those produced by the DC process. Therefore, the presented method was effective for reclaiming silver from silver-plating wastewater.     

Diffusional penetration of silver from electrodes into PZT ceramics

Diffusion of silver was studied in a ceramic based on lead zirconate-titanate, Pb_0.95Sr_0.05(Zr_0.53Ti_0.47)O₃+1 wt%. Nb₂O₅ (PZT), by means of a radio-tracer method. Parameters of silver diffusion and silver content in PZT after continuous diffusion saturation of the ceramic by this impurity were determined in the temperature range 500–850 °C. Concentration-depth profiles and silver content in the ceramic were obtained as a result of metal diffusion from the electrode during ceramic silvering. Our results show that silver has a high diffusivity. No evaporation of silver during metallization (T _max=750 °C) was found, but part of it ( 0.1 mg cm^–2) penetrates into the ceramic from the electrode to a depth of more than 1000 m, and the silver concentration varies from 2×10¹⁹ to 2×10¹⁸ at cm^–3. The silver concentration does not exceed 0.2 at% at diffusion saturation of the ceramic during 100–120 h over the temperature range 650–850 °C.     

Structural and electrical studies on thin films of solid electrolyte Ag6I4WO4

An electrolytic method of preparation of thin films of solid electrolyte Ag₆I₄WO₄ on a silver substrate is described. Films of different quality were obtained when the electrolysis was carried out at different temperatures and current densities. X-ray diffraction studies were carried out to confirm the compound formation. Scanning electron micrographs were taken in order to study the surface characteristics of these films. Investigations on the a.c. electrical conductivity of films prepared at different electrolysis temperatures (10 to 80° C) and two current densities (2 and 10 mA cm^–2) are also reported in the temperature range 30 to 130° C. The films deposited at 65° C gave values of room temperature conductivity and the activation energy for Ag⁺ ion conduction as 0.04 ^–1 cm^–1 and 0.132 eV, respectively.     

Infrared study of relevance of Nd2CuO4 structure to superconductivity of La-Sr-Cu-O system

A comparative study of the infrared spectra of Nd₂Cu_1–xAg_xO₄ and La_1.8Sr_0.2Cu_1–xAg_xO₄ is reported. It is shown that the appearance of an absorption peak at 680 cm^–1 is due to local distortions around the Ag impurities, and the appearance of this peak need not imply a structural transformation from Nd₂CuO₄ type of K₂NiF₄ type. It is shown that Nd₂CuO₄ structure is not important for the superconductivity of La-Sr-Cu-O system.     

Percolation threshold for electrical resistivity of Ag-nanoparticle/titania composite thin films fabricated using molecular precursor method

To find the percolation threshold for the electrical resistivity of metallic Ag-nanoparticle/titania composite thin films, Ag-NP/titania composite thin films, with different volumetric fractions of silver (0.26 ≤ φ_Ag ≤ 0.68) to titania, were fabricated on a quartz glass substrate at 600 °C using the molecular precursor method. Respective precursor solutions for Ag-nanoparticles and titania were prepared from Ag salt and a titanium complex. The resistivity of the films was of the order of 10⁻² to 10⁻⁵ Ω cm with film thicknesses in the range 100–260 nm. The percolation threshold was identified at a φ_Ag value of 0.30. The lowest electrical resistivity of 10⁻⁵ Ω cm at 25 °C was recorded for the composite with the Ag fraction, φ_Ag, of 0.55. X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), and transmission electron microscopic (TEM) evaluation of the effect of the morphology and the nanostructures of the Ag nanoparticles in the composite thin films on the electrical resistivity of the film revealed that the films consist of rutile, anatase, and metallic Ag nanoparticles homogeneously distributed in the titania matrix. It could be deduced that the electrical resistivity of the thin films formed at 600 °C was unaffected by the anatase/rutile content within the thin film, whereas the shape, size, and separation distance of the Ag nanoparticles strongly influenced the electrical resistivity of the Ag-nanoparticle/titania composite thin films.     

Microstructure Effects on the Superconducting Properties of YBa2Cu3 – x M x O7 – δ (M = Ag, Hg)

The properties of YBa₂Cu_{3 – x} Ag _x O_{7 –} and YBa₂Cu_{3 – x} Hg _x O_{7 –} (0 < x 0.5) solid solutions were studied. The solute concentration is shown to have a significant effect on the superconducting transition temperature, density, and grain size of the solid solutions. The difference in the composition dependences of the properties of the solid solutions is interpreted in terms of the crystal-chemical behavior of the Ag and Hg ions.