Antibacterial activity of glutathione-coated silver nanoparticles against Gram positive and Gram negative bacteria

In the present paper, we study the mechanism of antibacterial activity of glutathione (GSH) coated silver nanoparticles (Ag NPs) on model Gram negative and Gram positive bacterial strains. Interference in bacterial cell replication is observed for both cellular strains when exposed to GSH stabilized colloidal silver in solution, and microbicidal activity was studied when GSH coated Ag NPs are (i) dispersed in colloidal suspensions or (ii) grafted on thiol-functionalized glass surfaces. The obtained results confirm that the effect of dispersed GSH capped Ag NPs (GSH Ag NPs) on Escherichia coli is more intense because it can be associated with the penetration of the colloid into the cytoplasm, with the subsequent local interaction of silver with cell components causing damages to the cells. Conversely, for Staphylococcus aureus, since the thick peptidoglycan layer of the cell wall prevents the penetration of the NPs inside the cytoplasm, the antimicrobial effect is limited and seems related to the interaction with the bacterial surfaces. Experiments on GSH Ag NPs grafted on glass allowed us to elucidate more precisely the antibacterial mechanism, showing that the action is reduced because of GSH coating and the limitation of the translational freedom of NPs.     

Green synthesis and characterization of monodispersed silver nanoparticles obtained using oak fruit bark extract and their antibacterial activity

Green synthesis of silver nanoparticles (Ag NPs) has been achieved using oak fruit bark extract as a reducing, capping and stabilizing agent. The biosynthesized Ag NPs were characterized using various techniques. UV–visible spectrum of prepared silver colloidal solution showed absorption maximum at 433 nm. X‐ray diffraction and transmission electron microscopy analysis revealed that Ag NPs have a face‐centred cubic structure being spherical in shape with an average particle size of 20–25 nm. The toxicity of the Ag NPs was tested on bacterial species such as Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa and Escherichia coli by comparison based on diameter of inhibition zone in disc diffusion tests and minimum inhibitory concentration and minimum bactericidal concentration of NPs dispersed in liquid cultures. The antimicrobial activity of Ag NPs was greater towards Gram‐positive bacteria (S. aureus and B. subtilis) compared to Gram‐negative bacteria as determined using standard Kirby–Bauer disc diffusion assay and serial dilution. Copyright © 2016 John Wiley & Sons, Ltd.     

Particle size distributions of silver nanoparticles at environmentally relevant conditions

Silver nanoparticles (Ag NPs) are becoming increasingly popular as antimicrobial agents in consumer goods with consequent risk to environmental health from discharges. Environmentally relevant fate and transport investigations are limited but essential to gain understanding towards bioavailability and toxicology. In this study, monodisperse 15 nm citrate-stabilised Ag NPs were synthesised, characterised and then fractionated by flow field-flow fractionation (FlFFF) at environmentally relevant conditions (pH 5 or 8, presence of natural organic macromolecules (NOM) and presence of sodium or calcium). At low ionic strength, Ag NPs particle size increased as pH increased from 5 to 8. However, changing the ionic strength from 10⁻³ to 10⁻² M Na increased instability of the Ag NPs, and loss of peak at pH 5 but in the presence of humic substance (HS), a reduction in NP size was seen, most likely due to a reduction in the diffuse layer. The presence of Ca²⁺ ions, at the higher ionic strengths caused complete loss of the solution Ag NPs with or without HS, most likely due to aggregation. At the lower Ca²⁺ ionic strength the Ag NPs were still unstable, but again, in the presence of HS the NPs were largely dispersed. The presence of HS improved stability of Ag NPs under these conditions by forming a surface coating resulting in both steric and charge stabilisation. This work implies that Ag NPs could have long residence times in aquatic systems in the presence of HS potentially resulting in increased bioavailability.     

Theoretical Design and Experimental Realization of Quasi Single Electron Enhancement in Plasmonic Catalysis

By a combination of theoretical and experimental design, we probed the effect of a quasi‐single electron on the surface plasmon resonance (SPR)‐mediated catalytic activities of Ag nanoparticles. Specifically, we started by theoretically investigating how the E‐field distribution around the surface of a Ag nanosphere was influenced by static electric field induced by one, two, or three extra fixed electrons embedded in graphene oxide (GO) next to the Ag nanosphere. We found that the presence of the extra electron(s) changed the E‐field distributions and led to higher electric field intensities. Then, we experimentally observed that a quasi‐single electron trapped at the interface between GO and Ag NPs in Ag NPs supported on graphene oxide (GO‐Ag NPs) led to higher catalytic activities as compared to Ag and GO‐Ag NPs without electrons trapped at the interface, representing the first observation of catalytic enhancement promoted by a quasi‐single electron.     

Cellulose/silver nanoparticles composite microspheres: eco-friendly synthesis and catalytic application

Cellulose/silver nanoparticles (Ag NPs) composites were prepared and their catalytic performance was evaluated. Porous cellulose microspheres, fabricated from NaOH/thiourea aqueous solution by a sol–gel transition processing, were served as supports for Ag NPs synthesis by an eco-friendly hydrothermal method. The regenerated cellulose microspheres were designed as reducing reagent for hydrothermal reduction and also micro-reactors for controlling growth of Ag NPs. The structure and properties of obtained composite microspheres were characterized by Optical microscopy, UV–visible spectroscopy, WXRD, SEM, TEM and TG. The results indicated that Ag NPs were integrated successfully and dispersed uniformly in the cellulose matrix. Their size (8.3–18.6?nm), size distribution (3.4–7.7?nm), and content (1.1–4.9?wt%) were tunable by tailoring of the initial concentration of AgNO₃. Moreover, the shape, integrity and thermal stability were firmly preserved for the obtained composite microspheres. The catalytic performance of the as-prepared cellulose/Ag composite microspheres was examined through a model reaction of 4-nitrophenol reduction in the presence of NaBH₄. The composites microspheres exhibited good catalytic activity, which is much high than that of hydrogel/Ag NPs composites and comparable with polymer core–shell particles loading Ag NPs.     

Colloidal systems of silver nanoparticles and high-regioregular cationic polythiophene with ionic-liquid-like pendant groups: Optical properties and SERS

We report tuning of structure dependent optical properties of colloidal systems of borate-stabilized silver nanoparticles (Ag NPs) and polythiophene-based cationic polyelectrolyte with ionic-liquid like side groups: poly{3-[6-(1-methylimidazolium-3-yl)hexyl]thiophene-2,5-diyl bromide} (PMHT-Br) towards obtaining local electromagnetic field enhancement effects. Surface-enhanced Raman scattering (SERS) studies showed that the strong electromagnetic field enhancement is related to the formation of aggregates of Ag NPs achieved at the components ratio providing the charge balance between Ag NPs and cationic polythiophene, at which Ag NPs are nearly single-polymer-layer coated, their zeta potential is close to zero and they easily form aggregates in which the mean inter-particle distance enables the occurrence of desired plasmonic effects. Fluorescence quenching is efficient only in the systems with low concentrations of PMHT-Br, in which almost all polymer chains directly interact with the Ag NPs surface.     

Sensing colorimetric approaches based on gold and silver nanoparticles aggregation: Chemical creativity behind the assay. A review

Localized surface plasmon resonance (LSPR) is one of the most remarkable features of gold nanoparticles (Au NPs) and silver nanoparticles (Ag NPs). Due to these inherent optical properties, colloidal solutions of Au and Ag NPs have high extinction coefficients and different colour in the visible region of the spectrum when they are well-spaced in comparison with when they are aggregated. Therefore, a well-designed chemical interaction between the analyte and NPs surroundings leads to a change of colour (red to blue for Au NPs and yellow to brown for Ag NPs from well-spaced to aggregated ones, respectively) allowing the visual detection of the target analyte.     

Preparation and characterization of nanostructured thin films of Au and Ag nanoparticles synthesized by ascorbic acid on modified glass surface

Immobilization of Ag and Au nanoparticles (NPs) synthesized by ascorbic acid on chemically modified glass surface has been studied. 3‐[2‐(2‐Aminoethylamino)ethylamino]propyl‐trimethoxysilane (AMPTS), N‐(2‐aminoethyl)‐3‐aminopropyltrimethoxysilan, and 3‐trimethoxysilyl‐1‐propanethiol (MSPT) were used as surface modifying agents. To improve immobilization efficiency, the ammonia solution has been used along with the silane reagents, which assisted to adsorb the metal NPs on glass surface. It was found that AMPTS and MSPT have considerable effect on deposition of Ag and AuNPs on glass substrate. The fabricated thin films were characterized by using UV‐Vis spectroscopy, atomic force microscopy, energy‐dispersive X‐ray spectroscopy and subjected to antimicrobial resistance test. The UV–Vis spectra show a distinctive plasmon resonance absorbance peak for thin films of Au and AgNPs prepared with MSPT and AMPTS, respectively. Atomic force microscopy images indicate that formation of Au and AgNPs with spherical morphology after immobilization on the glass substrate and also the dimensions of NPs on the surface appear larger than those observed in the parent colloidal solution. Energy‐dispersive X‐ray spectroscopy measurements confirmed the presence of silver and gold on the modified glass surface, and elemental composition was measured. The Au and AgNPs thin films show antibacterial activity against gram negative (Escherichia coli) and gram positive (Staphylococcus aureus) bacteria in comparison with a blank sample. Copyright © 2015 John Wiley & Sons, Ltd.     

Ternary Ag/Polyaniline/Au nanocomposites: Preparation,characterization and electrochemical properties

Ternary Ag/Polyaniline/Au nanocomposites were synthesized successfully by immobilizing of Au nanoparticles (NPs) on the surface of Ag/Polyaniline (PANI) nanocomposites. Ag/PANI nanocomposites were prepared via in situ chemical polymerization of aniline in the presence of 4-aminothiophenol (4-ATP) capped silver colloidal NPs. Then, uniform gold (Au) NPs were assembled on the surface of resulted Ag/PANI nanocomposites through electrostatic interaction to get Ag/Polyaniline/Au nanocomposites. The nanocomposites were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), ultraviolet visible spectroscopy (UV-Vis), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR). Moreover, Ag/PANI/Au nanocomposites were immobilized on the surface of a glassy carbon electrode and showed enhanced electrocatalytic activity for the reduction of H₂O₂ compared with Ag/PANI.     

Defect-mediated formation of Ag cluster-doped TiO2 nanoparticles for efficient photodegradation of pentachlorophenol

A novel strategy was designed to prepare Ag cluster-doped TiO(2) nanoparticles (Ag/TiO(2) NPs) without addition of any chemical reducing agent and/or organic additive. A defect-rich TiO(x) species was generated by laser ablation in liquid (LAL) of a Ti target. The silver ions could be reduced and deposited on the surface of TiO(2) NPs through the removal of oxygen vacancies and defects; the TiO(x) species evolved into anatase NPs in a hydrothermal treatment process. The derived Ag/TiO(2) NPs are approximately 25 nm in size, with narrow size distribution. The Ag clusters are highly dispersed inside TiO(2) and less than 3 nm in size. The doped amount can be tuned by changing the concentration of Ag(+) ions. The as-synthesized Ag/TiO(2) NPs display improved photocatalytic efficiency toward pentachlorophenol (PCP) degradation.     

Synthesis and photophysical properties of an Eu(II)-complex/PS blend: role of Ag nanoparticles in surface-enhanced luminescence

A novel Eu(II) complex with 2-ethylhexyl hydrogen 2-ethylhexyl phosphonate (EHHEHP or PC88A) was synthesized and blended with polystyrene polymer (PS). Both an independent complex and the Eu(II)/PS blend excited by near-UV light produced blue luminescence, arising from the 5d→ 4f transitions of Eu(II). Time-dependent density functional theory (TD-DFT) calculations on electronic structures of the complex molecule indicated that the absorbing and emitting center was associated with the (2)A(d(z(2))) state under the C(2) crystal field. We also synthesized silver nanoparticles (Ag NPs) with an average particle size of 4.48 nm (σ = 0.91 nm) using EHHEHP as a stabilizer. The effects of Ag NPs as a colloidal suspension and an interfacial layer on the luminescence intensity of the blend were investigated as functions of the concentration of Ag NPs and the thickness of the Ag NP layer, respectively. The critical concentration of the colloidal Ag NPs and the critical thickness of the interfacial Ag NP layer were ～355 ppm and ～0.16 μm, respectively. Under critical conditions, the Ag NPs increased the luminescence intensity by 4.4 times as a colloidal suspension in CH(2)Cl(2) and 2.2 times as an interfacial-layer state.     

The effect of negatively charged metallic nanocatalysts on their reactions with alkyl radicals

Abstract

Stored electrons on Au and Ag nanoparticles (NPs) have been found to catalyze various reduction processes initiated by ionizing irradiation or light illumination in which the NPs act as “nano-electrodes”. In the present study, we explored the effect of charging colloidal gold and silver NPs on their reaction with methyl radicals, ·CH₃. The results show that charging the colloidal metal NPs by excess of electrons, (NP)^n?, affected their reaction mechanism with methyl radicals and resulted in different product composition in comparison to the analogous reactions with uncharged particles. These results should be considered in photo(electro)catalytic processes when alkyl radicals are formed near the catalyst surface.     

Fabrication of small-sized silver NPs/graphene sheets for high-quality surface-enhanced Raman scattering

In this paper, small-sized and highly dispersed Ag nanoparticles (NPs) supported on graphene nanosheets are fabricated via a strategy for etching a copper template with Ag(+). Firstly, big-sized Cu NPs are supported on graphene, and then the small-sized and highly dispersed Ag NPs are supported on graphene by replacement reaction, mainly making use of graphene passing electrons between Cu and Ag(+). The graphene used in the experiment is prepared by in situ self-generating template and has good dispersion, excellent crystallinity and little defects. Thus, in the process of Ag/graphene synthesis, there is no any intervention of surfactant, which ensures that SERS activity sites have not been passivated. And, the little defects of graphene benefit the excellent conductivity of graphene and ensured the replacement reaction between Cu and Ag(+). The obtained material exhibits significant high-quality and distinctive SERS activity. Especially, a serial new peak of p-aminothiophenol (PATP) is observed, this is suggested two reasons: one is "surface geometry" of the PATP on small-sized Ag NPs and another is the charge-transfer between Ag and graphene.     

Close‐Packed Two‐Dimensional Silver Nanoparticle Arrays: Quadrupolar and Dipolar Surface Plasmon Resonance Coupling

Silver nanoparticles (NPs) ranging in size from 40 to 100 nm were prepared in high yield by using an improved seed‐mediated method. The homogeneous Ag NPs were used as building blocks for 2D assembled Ag NP arrays by using an oil/water interface. A close‐packed 2D array of Ag NPs was fabricated by using packing molecules (3‐mercaptopropyltrimethoxysilane) to control the interparticle spacing. The homogeneous 2D Ag NP array exhibited a strong quadrupolar cooperative plasmon mode resonance and a dipolar red‐shift relative to individual Ag NPs suspended in solution. A well‐arranged 2D Ag NP array was embedded in polydimethylsiloxane film and, with biaxial stretching to control the interparticle distance, concomitant variations of the quadrupolar and dipolar couplings were observed. As the interparticle distance increased, the intensity of the quadrupolar cooperative plasmon mode resonance decreased and dipolar coupling completely disappeared. The local electric field of the 2D Ag NP array was calculated by using finite difference time domain simulation and qualitatively showed agreement with the experimental measurements.     

Pistacia atlantica leaf extract mediated synthesis of silver nanoparticles and their antioxidant,cytotoxicity, and antibacterial effects under in vitro condition

Recently, researchers have investigated the therapeutical properties of metal nanoparticles especially silver nanoparticles in vitro and in vivo conditions. The aim of the experiment was green synthesis and chemical characterization of silver nanoparticles from aqueous extract of Pistacia atlantica leaf (Ag NPs) and evaluation of their cytotoxicity, antioxidant, and antibacterial effects under in vitro condition. Ag NPs were spherical with a size range of 40-60 nm and characterized using various analysis techniques including UV–Vis absorption spectroscopy to determine the presence of Ag NP in the solution. We studied functional groups of Pistacia atlantica extract in the reduction and capping process of Ag NP by FT-IR, crystallinity and FCC planes by XRD pattern, elemental analysis of the sample by EDS, and surface morphology, shapes, and size of Ag NPs by SEM, AFM, and TEM. Destroy initiation and termination temperatures of the Ag NPs were determined by TGA. DPPH free radical scavenging test was done to evaluate the antioxidant potentials, which indicated similar antioxidant potentials for Ag NPs and butylated hydroxytoluene. The synthesized Ag NPs had great cell viability dose-dependently and indicated this method was nontoxic. Agar diffusion tests were done to determine the antibacterial characteristic. Ag NPs revealed similar antibacterial property to the standard antibiotic. Also, Ag NPs prevented the growth of all bacteria at 1-7 μg/ml concentrations and removed them at 3-15 μg/ml concentrations. Finally, synthesized Ag NPs revealed non-cytotoxicity, antioxidant and antibacterial activities in a dose-depended manner.     

Metallogel Template Fabrication of pH‐Responsive Copolymer Nanowires Loaded with Silver Nanoparticles and Their Photocatalytic Degradation of Methylene Blue

Poly(N,N′‐methylenebisacrylamide–4‐vinylpyridine) (P(MBA‐4VP)) nanowires loaded with silver nanoparticles (Ag NPs) have been fabricated by silver metallogel template copolymerization, and subsequently, silver ions are reduced instead of the template being removed. Ag NPs with a diameter of 5–15 nm were dispersed throughout the core of P(MBA‐4VP) nanowires. The size and distribution of the formed Ag NPs could be finely controlled by reduction time. The pH sensitivity of P(MBA‐4VP) nanowires offers the possibility of Ag NP release from the nanowires under acidic conditions. The photocatalytic performance of the P(MBA‐4VP) nanowires loaded with Ag NPs was evaluated for the degradation of methylene blue (MB) under UV light irradiation. Their rate of degradation is dependent on the content and size of the Ag NPs, as well as the pH values of the MB solution. Moreover, the P(MBA‐4VP) nanowires loaded with Ag NPs exhibited high photostability, and the photocatalytic efficiency reduced by only 1.81 % after being used three times.     

Internally dispersed synthesis of uniform silver nanoparticles via in situ reduction of [Ag(NH3)2]+ along natural microfibrillar substructures of cotton fiber

Silver nanoparticles (Ag NPs) are known to have efficient antimicrobial properties, but the direct application of Ag NPs onto the surface of textiles has shown to be ineffective and raise environmental concerns because Ag NPs leach out during washing. In this study, non-leaching and stable Ag-cotton nanocomposite fiber was produced by the in situ formation of Ag NPs inside the cotton fiber. The reported method is to introduce a nanofluidic system in alkali-swollen cotton fiber. Sequential flows of [Ag(NH₃)₂]⁺ and reductant aqueous fluids into the opened microfibrillar channels yielded a self-assembly of Ag ions on the deprotonated cellulose and subsequent nucleation and particle growth on the microfibrils. Transmission electron and field emission scanning electron microscopy images showed Ag NPs evenly dispersed throughout the entire cross-section of the fiber and their fixation onto the isolated secondary cell wall, respectively. Despite the rapid reduction reaction and the absence of a stabilizing agent, the successful formation of monodispersed Ag NPs (12 ± 3 nm) was attributed to the self-controlled function of the highly organized microfibrillar substructures, which regulated the transport and mixing of reactants. Incorporation of Ag NPs into the internal structure of the cotton fiber did not significantly influence the cotton crystalline structure.     

Green Synthesis of Silver Nanoparticles Using <Emphasis Type="Italic">Commiphora caudata</Emphasis> Leaves Extract and the Study of Bactericidal Efficiency

The present study reports the synthesis of silver nanoparticles (Ag NPs) from silver nitrate solution using leaf extracts of Commiphora caudata. The formation of Ag NPs in the colloidal solution is confirmed by UV–Vis spectroscopy analysis. The identification of biomolecules is analyzed through fourier transform infrared spectroscopy. X-ray diffraction pattern shows that an average particle size of the synthesized nanoparticles are in the range of 40–24 nm. Field emission scanning electron microscopy and transmission electron microscopy confirm the formation Ag NPs in spherical shape. The photoluminescence study of the synthesized Ag NPs interprets the influence of C caudata leaf concentrations on emission behavior. Zeta potential measurement is carried out to determine the stability of synthesized Ag NPs. GC–MS analysis revealed that the C. caudata contained 11 compounds, such as Stigmasterol (24.14 %), Hexacosanoic acid, methyl ester (15.13 %) and 2-bromophenyl morpholine-4-carboxylate (11.71 %). The antibacterial activity of Ag NPs shows that these bio capped Ag NPs have higher inhibitory action for Escherichia coli, Klebsiella pheumoniea, Micrococcus flavus, Pseudomonas aeruginosa, Bacillus subtilis, Bacillus pumilus, Staphylococcus aureus.     

聚苯乙烯/银纳米粒子复合微球的合成及催化性能

首先通过乳液聚合和浓硫酸酸化制备表面富含磺酸根的磺化聚苯乙烯(PS)微球(直径532 nm),再用其静电吸附[Ag(NH_3)_2]~+离子,最后采用聚乙烯吡咯烷酮还原表面吸附的[Ag(NH_3)_2]~+离子,得到了负载银纳米粒子的PS/AgNPs复合微球.采用扫描电子显微镜、透射电子显微镜、紫外-可见光谱、红外光谱和X射线衍射表征了PS/AgNPs复合微球,并考察了其对甲基蓝(MB)的催化性能.结果表明,Ag纳米粒子高度分散在磺化PS微球表面;该PS/AgNPs复合微球对催化转化MB有较高的催化活性,并可多次重复利用.本研究在催化降解有机污染物方面有一定的实用价值.     

Colloidal behavior of aluminum oxide nanoparticles as affected by pH and natural organic matter

The colloidal behavior of aluminum oxide nanoparticles (NPs) was investigated as a function of pH and in the presence of two structurally different humic acids (HAs), Aldrich HA (AHA) and the seventh HA fraction extracted from Amherst peat soil (HA7). Dynamic light scattering (DLS) and atomic force microscopy (AFM) were employed to determine the colloidal behavior of the NPs. Influence of pH and HAs on the surface charges of the NPs was determined. zeta-Potential data clearly showed that the surface charge of the NPs decreased with increasing pH and reached the point of zero charge (ZPC) at pH 7.9. Surface charge of the NPs also decreased with the addition of HAs. The NPs tend to aggregate as the pH of the suspension approaches ZPC, where van der Waals attraction forces dominate over electrostatic repulsion. However, the NP colloidal suspension was stable in the pHs far from ZPC. Colloidal stability was strongly enhanced in the presence of HAs at the pH of ZPC or above it, but in acidic conditions NPs showed strong aggregation in the presence of HAs. AFM imaging revealed the presence of long-chain fractions in HA7, which entangled with the NPs to form large aggregates. The association of HA with the NP surface can be assumed to follow a two-step process, possibly the polar fractions of the HA7 sorbed on the NP surface followed by entanglement with the long-chain fractions. Thus, our study demonstrated that the hydrophobic nature of the HA molecules strongly influenced the aggregation of colloidal NPs, possibly through their conformational behavior in a particular solution condition. Therefore, various organic matter samples will result in different colloidal behavior of NPs, subsequently their environmental fate and transport.