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.     

A Label-Free Oligonucleotide Based Thioflavin-T Fluorescent Switch for Ag<Superscript>+</Superscript> Detection with low Background Emission

A novel fluorescent Ag⁺ sensor was developed based on the label-free silver (I) specific oligonucleotide (SSO) and Thioflavine T (ThT) monomer-excimer switch. C-rich SSO which contain C-C mismatched base pairs can selectively bind to Ag⁺ ions and the formed duplexes which constructed by C-Ag⁺-C structure are thermally stabilized without largely altering the double helical structure. ThT give very weak fluorescent in bulk solution and/or in the presence of SSO. However ThT shows high fluorescence in the presence of SSO and Ag⁺ at the same time mainly because ThT excimer, which has the high quantum yield, formed and stabilized in the minor or major groove. Based on the discovery, we developed the novel Ag⁺ sensor. Under the optimum condition, the selectivity of this system for Ag⁺ over other metal ions in aqueous solution is remarkably high, and Ag⁺ can be quantified over the dynamic range of 30–450 nM, with a limit of detection of ~16 nM and a linear correlation coefficient of 0.995.     

Removal of nickel(II) ions from aqueous solutions using the natural clinoptilolite and preparation of nano-NiO on the exhausted clinoptilolite

The natural zeolite tuff (clinoptilolite) from a Serbian deposit has been studied as adsorbent for Ni(II) ions from aqueous solutions. Its sorption capacity at 298 K varies from 1.9 mg Ni g⁻¹ (for the initial solution concentration of 100 mg Ni dm⁻³) to 3.8 mg Ni g⁻¹ (for C₀ = 600 mg Ni dm⁻³) and it increases 3 times at 338 K. The sorption is best described by the Sips isotherm model. The sorption kinetics follows the pseudo-second-order model, the activation energies being 7.44, 5.86, 6.62 and 6.63 kJ mol⁻¹ for C₀ = 100, 200, 300 and 400 mg Ni dm⁻³, respectively. The sorption involves a film diffusion, an intra-particle diffusion, and a chemical cation-exchange between the Na⁺ ions of clinoptilolite and the Ni²⁺ ions. The sorption is endothermic (ΔH° being 37.9, 33.4, 30.0, 27.7 and 24.3 kJ mol⁻¹ for C₀ = 100, 200, 300, 400 and 600 mg Ni dm⁻³, respectively) and spontaneous in the 298-338 K temperature range. Thermal treatment of the Ni(II)-loaded clinoptilolite results in the formation of spherical nano-NiO particles of approx. 5 nm in diameter which are randomly dispersed in the clinoptilolite lattice.     

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.     

Synthesis of silver nanoparticles by sonogalvanic replacement on aluminium powder in sodium polyacrylate solutions

The process of the formation of silver nanoparticles (AgNPs) via the method of galvanic replacement (GR) of Ag⁺ with aluminum powder in sodium polyacrylate (NaPA) solutions in the ultrasonic (US) field has been studied. It was observed, that the yellow colloidal solutions of stabilized AgNPs with the absorption maximum at ∼ 410 nm were obtained under the application of US power by 20 W and frequency by 20 kHz in the wide range of AgNO₃ and NaPA concentrations (0.1 – 0.5 mM and 0.5 – 5.0 g/L respectively) at 25 ⁰C. It was shown, that the GR process under US field occurs without of the significant induction period. Using the UV–vis spectroscopy the kinetics of AgNPs formation has been studied and it was observed the first order kinetics with respect to Ag⁺ ions both for the nucleation and growth processes. It was found that observable rate constants of nucleation are close for the all experimental conditions but the observable rate constants of growth decreased with increasing of initial concentration of AgNO₃. Based on the obtained kinetic data it was proposed a mechanism of the formation of AgNPs consisted of the following two main stages: 1) the nucleation with the formation of primary nanoclusters (AgNCs) on aluminum surface followed by their ablation from the surface of the sacrificial metal by ultrasound into bulk of solution; 2) the transformation of AgNCs in AgNPs via growth from the Al surface and / or agglomeration of AgNCs. Using TEM it was found that the size of obtained AgNPs does not exceed of 25 nm and slightly depends on the initial concentrations of precursors. High antimicrobial activity of obtained colloidal solutions against gram-negative and gram-positive bacteria as well as against fungi was observed.     

Biasing effects on thallium and silver chlorides with dc/ac voltage

Comparison was made between crystals of thallium chloride and silver chloride on their biasing effects with dc/ac voltage. Previous reports say that, although their electrical conductivities are similar, the dominant charge carriers in the former are the Cl⁻ ions while the Ag⁺ ions in the latter. The present dc/ac study demonstrates the following: for thallium chloride, although Cl⁻ conduction may be dominant under low bias field, Tl⁺ conduction supercedes Cl⁻ conduction when the bias field is enhanced. For silver chloride, Ag⁺ conduction is overwhelming within wide temperature range, to cause easy dielectric breakdown on dc biasing. Concerning the extrinsic conductivity seen at temperatures below 60 °C (thallium chloride) or below 150 °C (silver chloride), it is ascribed to grain-boundary related electron conduction, not to grain-boundary related Tl⁺ or Ag⁺ conduction as reported earlier.     

Enhanced UV-excited luminescence of europium ions in silver/tin-doped glass

Glasses containing silver, tin and europium were prepared by the melt-quenching technique with silver nanoparticles (NPs) being embedded upon heat treatment (HT). An intensification of Eu³⁺ ions emission was observed for non-resonant excitation around 270 nm, corresponding to UV absorption in the material. Optical measurements suggest that light absorption occurs at single Ag⁺ ions and/or twofold-coordinated Sn centers followed by energy transfer to europium which results in populating the ⁵D₀ emitting state in Eu³⁺. After HT at 843 K, a quenching effect is observed on Eu³⁺ luminescence with increasing holding time in the 350–550 nm excitation range. The quenching effect shows with the presence of Ag NPs which may provide multipole radiationless pathways for excitation energy loss in europium ions.     

Deposition of silver nanoparticles on silica spheres via ultrasound irradiation

Silver-decorated silica spheres of submicrometer-sized silica spheres with a core-shell structure were obtained based on a seed-mediated growth process, where silver nanoparticles were firstly formed from reducing Ag⁺ to Ag⁰ in N,N-dimethylformamide (DMF) in the presence of poly(vinylpyrrolidone) (PVP) as protective agent under ultrasound irradiation, followed by the growth of silver shell served silver nanoparticles as nucleation sites and formaldehyde as reducer. The results revealed that the terms of PVP addition and ultrasonic surroundings had great influence on the fabrication of silver seeds.     

Spectroscopy of silver complexes in solutions of inorganic compounds

The absorption, photoluminescence, and photoexcitation spectra of a number of inorganic solid solutions with a silver ion impurity have been examined. The influence of the temperature on the spectral characteristics of haloid and oxygen-containing solutions activated with Ag⁺ ions has been investigated. The temperature dependences of the luminescence quantum yield of solid solutions with Ag⁺ impurity in the temperature range 77-150 K have been studied. It is shown that the spectra under observation are conditioned by electron transition between energetic levels of Ag⁺ ion which are deformed because of the interaction with environment.     

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.     

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.     

Preparation and mechanism investigation of highly sensitive humidity sensor based on Ag/TiO2

In this paper, a high-performance silver-doped titanium dioxide (Ag/TiO₂) humidity sensor was synthesized using a hydrothermal synthesis method for respiratory monitoring. The sensing mechanism was studied by the first principles of density functional theory (DFT). Calculations show that the doping of Ag⁺ ions increases the adsorption energy of TiO₂ to water molecules. Furthermore, the Ti–O bond in TiO₂ is broken due to the doping of Ag⁺ ions, which promotes the generation of Ti³⁺ defects. Experiments show that the doping of Ag⁺ ions can increase the hydroxide groups, Ti³⁺ defects and oxygen vacancies on the surface of TiO₂, thus effectively improving the responsivity, linearity and hysteresis of the TiO₂ humidity sensor. Compared to TiO₂, the resistance of the Ag/TiO₂ (0.5 mM) humidity sensor reaches 4.5 orders of magnitude with a high response of 39707.1, maximum hysteresis rate of 4.6%, response/recovery time of 31 s/15 s and the best linearity in a range of 11%–95% RH. In addition, the Ag/TiO₂ humidity sensor has been successfully used to detect different modes of respiration and determine the respiratory rate under different respiratory states. Significantly, this work demonstrates potential application value in human healthcare and activities monitoring.     

Impedance and Modulus studies of the solid electrolyte system 20CdI2–80[xAg2O–y(0.7V2O5–0.3B2O3)], where 1 ≤x/y ≤ 3

In the present work, an evaluation of the transport properties of super ion conducting quaternary system 20CdI₂–80[xAg₂O–y(0.7V₂O₅–0.3B₂O₃)], where 1 ≤ x/y ≤ 3, in steps of 0.25, to study the effect of changing the modifier to former ratio on the conduction phenomena has been undertaken. Electrical conductivity measurements were made using complex impedance method. The electrical conductivity and conductivity relaxation of the system were studied in the temperature range from 303 K to 333 K and in the frequency range from 100 Hz to 10 MHz. The highest conductivity at room temperature is obtained for the system with modifier to former ratio 1.75. Impedance and modulus analyses had indicated the temperature independent distribution of relaxation times and the non-Debye behavior in these materials. The co-operative motion due to strong coupling between the mobile Ag⁺ ions is assumed to give rise to non-Debye type of relaxation. The silver ionic transport number (t_Ag+) obtained by the emf technique suggested the occurrence of silver ion conduction in the CdI₂-doped Ag₂O–V₂O₅–B₂O₃ system.     

Ultrasonic assisted fabrication of silver tungstate encrusted polypyrrole nanocomposite for effective photocatalytic and electrocatalytic applications

Ultrasonication is an emerging and evergreen technique for the efficient synthesis of the catalytically active heterostructured materials. In-situ one-pot ultrasonic-assisted synthesis method was demonstrated in this work for the fabrication of silver tungstate encrusted polypyrrole nanocomposite using semi-automatic ultrasonic probe maintained at 34°C/50 kHz ultrasonic frequency and at 150 W ultrasonic power. This material retains enhanced optical, electrical, morphological properties, photocatalytic behavior in photodegradation of congo red dye, tetracycline drug and its electrochemical sensing potential for the effective determination of a broad spectrum of antibacterial drug, nitrofurazone. Optical properties were investigated using UltraViolet–Visible diffuse reflectance spectral (UV–VIS DRS) data along with Tauc’s bandgap energy calculations. The morphological properties were examined using FESEM and TEM micrographs. All the PXRD and XPS details prove the effective distribution of PPy on the surface of Ag₂WO₄ rods with the help of powerful ultrasonic assistance. PPy acted as a support for nucleation and growth of Ag₂WO₄ and an inhibitor of phase transformations. Ag₂WO₄/PPy exhibits great photocatalytic behavior while comparing with pure PPy and Ag₂WO₄ in the degradation of carcinogenic dye congo red and antibiotic drug tetracycline. In addition to that, Ag₂WO₄/PPy modified GCE exposed a widespread linear range from 0.1 to 107 µM along with a very low detection limit of 12 nM and huge sensitivity of 1.74 µA µM⁻¹cm⁻² in the electrochemical determination of nitrofurazone.     

Silver nanoparticles and silver ions stabilized in NaCl nanocrystals

This study presents a two-step synthesis of nanoparticles and the stabilization process of Ag ions in the matrix of NaCl nanocrystals. Ag⁺ ions are incorporated to NaCl with a new and attractive method that can be easily used for the different types of alkaline halides. The nanoparticles with predominant size found between 10 and 15 nm were stabilized on the surface and/or interior of NaCl nanocrystals using, in the first stages, the ionic-exchange property of zeolite A4. The optical properties of the materials were characterized through optical absorption, leading to well defined absorption bands located in the wave length values between 217–275 nm and 350–770 nm approximately, for Ag⁺ and AgNp, respectively. The antibacterial property of Ag ions and nanoparticles stabilized in NaCl was analyzed against gram-negative Escherichia Coli and Klebsiella bacteria. In order to quantify the antibacterial effect of Ag ions and nanoparticles the inhibition ratio was used as a parameter on the bacteria colonies grown in culture medium by conventional methods. Ag⁺ ions that were stabilized in NaCl nanocrystals show a mayor inhibition ratio in contact with Klebsiella bacteria, conversely Ag nanoparticles showed better results in contact with E. coli.     

Emission mechanism of radiophotoluminescence in Ag-doped phosphate glass

The objective of this study is to investigate the emission mechanism of radiophotoluminescence (RPL) in the Ag⁺-doped phosphate glass (glass dosimeter), which is now used as individual radiation dosimeter, because the emission mechanism of RPL in glass dosimeter was not fully understood. Optical properties such as optical absorption spectrum, RPL spectrum and change of RPL spectrum as a function of X-ray irradiation dose were measured for commercially available glass dosimeter. In this study, we discuss the emission mechanism of two RPL peaks at 460 nm and 560 nm, based on the fact that electrons and holes produced by X-ray irradiation are trapped at Ag⁺ ions to produce Ag⁰ and Ag²⁺ ions, respectively, when the Ag⁺-doped phosphate glass is exposed to X-ray. We would like to propose the emission mechanism of RPL peaks at 460 nm and 560 nm, concerning with Ag²⁺ and Ag⁰ ions.     

Synthesis of dendritic silver nanostructures by means of ultrasonic irradiation

In this paper, a simple and effective route for the synthesis of silver dendritic nanostructures by means of ultrasonic irradiation has been developed. Well-defined silver dendritic nanostructures were obtained by sonicating the aqueous solution of 0.04 mol/L silver nitrate with 4.0 mol/L isopropanol as reducing agent and 0.01 mol/L PEG400 as disperser for 2 h. The effects of the irradiation time, the concentration of Ag⁺ and the molar ratio of PEG to AgNO₃ on the morphology of silver nanostructures were discussed. The structures of the obtained samples were characterized by transmission electron microscopy (TEM), selected area electron diffraction (SAED) and X-ray powder diffraction (XRD), and the chemical composition of the dendrites was examined by energy-dispersive X-ray spectrum (EDS).     

A Novel Fluorescent Biosensor for Detection of Silver Ions Based on Upconversion Nanoparticles

In this paper, we report a simple and sensitive fluorescent biosensor for the quantitative analysis of silver ions (Ag⁺) by using NaYF₄:Yb³⁺, Tm³⁺ upconversion nanoparticles (UCNPs). Ag⁺ could oxidize o-phenylenediamine (OPD) to the oxidized OPD (oxOPD) directly. The fluorescence of UCNPs can be significantly quenched by oxOPD through inner filter effects (IFE). Under the optimized conditions, the Ag⁺ concentration is proportional to the changes of the fluorescence intensity of UCNPs. The proposed method shows high selectivity and Ag⁺ could be quantitatively detected in the range of 0 to 0.5 mM with a low detection limit of 33 nM for Ag⁺. The selectivity and sensitivity of the detection can also be satisfactory. More importantly, this method has potential in practical application to detect Ag⁺ in real samples without interference.     

Erratum to Formation and control of Au and Ag nanoparticles inside borate glasses using femtosecond laser and heat treatment

We report the spectroscopic properties of femtosecond laser-irradiated sodium-alumino-borate glass doped with silver and gold ions. We precipitated gold and silver nanoparticles by laser irradiation and annealing at 400°C for 30 min. The irradiation and annealing treatment produced different absorption and emission characteristics in Au³⁺ doped and Au³⁺, Ag⁺ codoped glasses, and the possible mechanisms of the observed results are discussed. The size of the nanoparticles was estimated by TEM and absorption band analysis.     

Chemical effects on x‐ray fluorescence yield of Ag+ compounds

X‐ray fluorescence measurements were carried out for silver metal and a number of silver compounds containing Ag⁺ ions such as Ag₂CO₃, Ag₂SO₄, AgNO₃, AgCl, AgBr and AgI using 59.6 keV γ‐rays, emitted from ²⁴¹Am, as the excitation source, to evaluate the value of Kβ/Kα x‐ray intensity ratio. For silver metal the value of this parameter is found to be 0.206 ± 0.003 and wide variations, 0.190 ≤ Kβ/Kα≤ 0.207, were observed for these compounds. The results are explained in terms of the charge transfer occurring between Ag⁺ and the coordinating anions. Copyright © 2005 John Wiley & Sons, Ltd.