Optical response of Ag-Au bimetallic nanoparticles to electron storage in aqueous medium

Composition and structure dependence of the shift in the position of the surface plasmon resonance band upon introduction of NaBH4 to aqueous solutions of gold and silver nanoparticles are presented. Silver and gold nanoalloys in different compositions were prepared by co-reduction of the corresponding salt mixtures using sodium citrate as the reducing agent. After addition of NaBH4 to the resultant nanoalloys, the maximum of their surface plasmon resonance band, ranging between that of pure silver (ca. 400 nm) and of pure gold (ca. 530 nm), is blue-shifted as a result of electron storage on the particles. The extent of this blue shift increases non-linearly with the mole fraction of silver in the nanoparticle, parallel to the trends reported previously for both the frequency and the extinction coefficient of the plasmon band shifts. Gold(core)@silver(shell) nanoparticles were prepared by sequential reduction of gold and silver, where addition of NaBH4 results in relatively large spectral shift in the plasmon resonance band when compared with the nanoalloys having a similar overall composition. The origin of the large plasmon band shift in the core-shell is related with a higher silver surface concentration on these particles. Hence, the chemical nature of the nanoparticle emerges as the dominating factor contributing to the extent of the spectral shift as a result of electron storage in bimetallic systems.     

Characterization of silver nanoparticles synthesized on titanium dioxide fine particles

Silver nanoparticles with a narrow size distribution were synthesized over the surface of two different commercial TiO(2) particles using a simple aqueous reduction method. The reducing agent used was NaBH(4); different molar ratios TiO(2):Ag were also used. The nanocomposites thus prepared were characterized using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), dynamic light scattering (DLS) and UV-visible (UV-vis) absorption spectroscopy; the antibacterial activity was assessed using the standard microdilution method, determining the minimum inhibitory concentration (MIC) according to the National Committee for Clinical Laboratory Standards. From the microscopy studies (TEM and STEM) we observed that the silver nanoparticles are homogeneously distributed over the surface of TiO(2) particles and that the TiO(2):Ag molar ratio plays an important role. We used three different TiO(2)Ag molar ratios and the size of the silver nanoparticles is 10, 20 and 80?nm, respectively. It was found that the antibacterial activity of the nanocomposites increases considerably comparing with separated silver nanoparticles and TiO(2) particles.     

Synthesis of Starch-Stabilized Silver Nanoparticles and Their Antimicrobial Activity

In this study, silver nanoparticles were prepared using silver nitrate as the metal precursor, starch as protecting agent, and sodium borohydride (NaBH₄) as a reducing agent by the chemical reduction method. The formation of the silver nanoparticles was monitored using ultraviolet-visible absorption spectroscopy, cyclic voltammetry, and particle size analyzer and characterized by transmission electron microscopy (TEM) and x-ray diffraction (XRD). Synthesis of nanoparticles were carried out by varying different parameters, such as reaction temperature, concentration of reducing agent, concentration of silver ion in feed solution, type and concentration of the stabilizing agent, and stirrer speed expressed in terms of particle size and size distribution. Dispersion destabilization of colloidal nanoparticles was detected by Turbiscan. It was observed that size of the starch stabilized silver nanoparticles were lower than 10 nm. The microbial activity of synthesized silver nanoparticles was examined by modified Kirby-Bauer disk diffusion method. Silver nanoparticles were tested for their antibacterial activity against Gram negative bacteria such as Escherichia coli, Pseudomonas aeruginosa, and Gram positive bacteria such as Staphylococcus aureus and Staphylococcus epidermidis. High bacterial activity was observed at very low concentrations of silver (below 1.39 μg/ml). The antifungal activity of silver nanoparticles has been assayed against Candida albicans.     

Preparation of silver nanoparticles having low melting temperature through a new synthetic process without solvent

This study presents a new synthetic method of silver nanoparticles using a novel polyoxyethylene maleate-based surfactant (PEOM). Unlike conventional process using large amount of a solvent to dissolve silver salts, large amount of silver salts (AgNO3) can be dissolved and stabilized by our surfactant without solvent. Silver salts can be dissociated within PEOM by the formation of charge complex between hydrophilic-COOH segments and Ag+NO3-, resulting in formation of self-assembled microstructures which acting as a nano-reactor and stabilizer. After reduction using NaBH4, uniform sized silver nanoparticles were formed in the hydrophilic domain of PEOM. Silver contents in the colloids were changed by 0.5 approximately 3 wt%. Distribution of silver nanoparticle sizes was investigated by using TEM and XRD. Melting temperature of silver nanoparticles was measured by differential scanning calorimetry, which depends upon the particle size of silver nanoparticles. The lowest melting temperature of 112 degrees was measured from 3.5 nm average-sized silver particles.     

Synthesis and anti-bacterial activity of Cu, Ag and Cu-Ag alloy nanoparticles: A green approach

Metallic and bimetallic nanoparticles of copper and silver in various proportions were prepared by microwave assisted chemical reduction in aqueous medium using the biopolymer, starch as a stabilizing agent. Ascorbic acid was used as the reducing agent. The silver and copper nanoparticles exhibited surface plasmon absorption resonance maxima (SPR) at 416 and 584 nm, respectively; while SPR for the Cu-Ag alloys appeared in between depending on the alloy composition. The SPR maxima for bimetallic nanoparticles changes linearly with increasing copper content in the alloy. Transmission electron micrograph (TEM) showed monodispersed particles in the range of 20 ± 5 nm size. Both silver and copper nanoparticles exhibited emission band at 485 and 645 nm, respectively. The starch-stabilized nanoparticles exhibited interesting antibacterial activity with both gram positive and gram negative bacteria at micromolar concentrations.     

纳米铜胶体的合成及表征

以NaBH4为还原剂,表面活性剂CTAB为稳定分散剂,通过化学反应从硫酸铜溶液制备了纳米铜胶体,并研究了稳定分散剂浓度对纳米铜胶体颗粒的氧化、大小及团聚状态的影响.结果表明:当稳定分散剂浓度足够高时,所得纳米铜胶体颗粒大小均匀、分散性好,且能有效地防止颗粒的氧化及团聚.     

Green synthesis of silver nanoparticles using hypericin-rich shoot cultures of Hypericum hookerianum and evaluation of anti-bacterial activities

The paper reports a green chemistry approach for the synthesis of silver nanoparticles (AgNPs) using hypericin-rich shoot cultures of Hypericum hookerianum as reducing agent. Normal green shoot cultures deficient in hypericin and red-pigmented shoot cultures rich in hypericin (3.01% DW) were raised in Murashige and Skoog nutrient medium containing 1.0 mg/L kinetin (KIN) and 0.2 mg/L naphthaleneacetic acid (NAA), respectively. Dried powder extracts of whole shoots were used for AgNPs formation. The effect of temperature on the formation of AgNPs is investigated. The nanoparticles obtained were characterised using UV–Vis spectroscopy, field emission scanning electron microscopy (FESEM), energy-dispersive X-ray (EDX) and X-ray diffraction (XRD) analyses. The UV–Vis spectra of AgNPs gave surface plasmon resonance (SPR) at 440 nm. The synthesised AgNPs were effective against different multidrug-resistant human pathogens such as Bacillus subtillis (Gram positive) and Pseudomonas aeruginosa (Gram negative) species. Further, the effect of hypericin concentration on anti-bacterial activity was investigated and was found to increase with increase in concentration.     

Optical study on II-VI semiconductor nanoparticles in Langmuir-Blodgett films

Cadmium sulfide (CdS) nanoparticles formed within Langmuir-Blodgett (LB) films of stearic acid and calix[8]arene were studied with different optical methods including surface plasmon resonance (SPR), ellipsometry and UV-visible absorption and fluorescence spectroscopies. For the first time, the process of formation of CdS nanoparticles within LB films was monitored in-situ with SPR. The results of ellipsometry, SPR and UV-vis absorption spectroscopy were analyzed to evaluate simultaneously the thickness, refractive index and extinction coefficient of LB films. It was shown that all three parameters increase as a result of formation of CdS nanoparticles. Photoluminescence measurements provided direct confirmation directly for previous observation with UV-vis absorption spectroscopy of the blue spectral shift caused by CdS particles formation. The observed large Stoke's shift of the luminescence band is discussed in terms of the formation of "dark excitons" in the platelet-type CdS nanoclusters. AFM study shows the formation of pseudo-two dimensional platelets of CdS with the lateral dimensions in the range of 20-30 nm.     

Synthesis and characterization of dextran-capped silver nanoparticles with enhanced antibacterial activity

Dextran-capped silver nanoparticles were synthesized by reducing silver nitrate with NaBH4 in the presence of dextran as capping agent. The characters of silver nanoparticles were investigated using UV-Vis spectrophotometer, nano-grainsize analyzer, X-ray diffraction, and transmission electron microscopy. Results showed that the silver nanoparticles capped with dextran were in uniform shape and narrow size distribution. Moreover, compared with polyvinylpyrrolidone (PVP)-capped silver nanoparticles, the dextran-capped ones possessed better stability. Antibacterial tests of these silver nanoparticles were carried out for Escherichia coli, Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa, and Klebsiella pneumoniae. Results suggested that the dextran-capped silver nanoparticles had high antibacterial activity against both Gram-positive and Gram-negative bacteria. In addition, the cytotoxicity in vitro of the dextran-capped silver nanoparticles was investigated using mouse fibrosarcoma cells (L929). The toxicity was evaluated by the changes of cell morphology and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide assay. Results indicated that these silver nanoparticles had slight effect on the survival and proliferation of L-929 cells at their minimal inhibitory concentration (MIC). After modified by dextran, the physiochemical properties of the silver nanoparticles had been improved. We anticipated that these dextran-capped silver nanoparticles could be integrated into systems for biological and pharmaceutical applications.     

Catalytic properties of silica/silver nanocomposites

The catalytic properties of silver nanoparticles supported on silica and the relation between catalytic activity of silver particles and the support (silica) size are investigated in the present article. The silver nanoparticles with 4 nm diameters were synthesized and were attached to silica spheres with sizes of 40, 78, 105 nm, respectively. The reduction of Rhodamine 6G (R6G) by NaBH4 was designed by using the SiO2/Ag core-shell nanocomposites as catalysts. The experimental results demonstrated that the catalytic activity of silica/silver nanoparticles depends on not only the concentration of catalysts (silver) but also the support silica size. Silver particles supported on small SiO2 spheres (approximately 40 nm) show high catalytic activity. Moreover, by making a comparison between the UV-vis spectra of the catalyst before and after the catalytic reaction, we found that the position of surface plasma resonance (SPR) peak of Ag nanoparticles changes little. The above results suggested that the size and morphology of silver particles were probably kept unchanged after the reduction of R6G and also implied that the catalytic activity of silver particles was hardly lost during the catalytic reaction.     

基于毛竹半纤维素的银纳米粒子的绿色合成

直接以碱溶性毛竹半纤维素为稳定剂、葡萄糖为还原剂,在水介质中绿色合成银纳米粒子,讨论了合成条件对银纳米粒子的形成和储存稳定性的影响,表征了银纳米粒子-半纤维素复合物经热处理后获得的Ag-C复合物的物理化学特性,并讨论了银纳米粒子的可能形成机理。在恒定其他反应条件下,延长反应时间会有更多银纳米粒子生成,但过度延长反应时间会使银纳米粒子发生团聚而生成大颗粒的粒子;高的葡萄糖浓度、反应温度和初始半纤维素用量会加快银纳米粒子的生成。银纳米粒子的平均粒径和粒径分布范围均随半纤维素用量的增大而减小,而银纳米粒子在4℃的储存稳定性随半纤维素用量的增大而增强。银纳米粒子-半纤维素复合物在空气气氛中300℃热处理1h后获得的Ag-C复合物中同时存在金属态的银和氧化态的银。半纤维素中呈电负性的大量自由羟基和少量羧基可能对银纳米粒子的形成起至关重要的作用。     

Synthesis of hybrid Au–ZnO nanoparticles using a one pot polyol process

In this work, we report on the synthesis of hybrid Au–ZnO nanoparticles using a one-pot chemical method that makes use of 1,3-propanediol as a solvent, a reducing agent and a stabilizing layer. The produced nanoparticles consisted of Au cores decorated with ZnO nanoparticles. The structure and morphology of the nanoparticles were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectrometry (EDX) and Raman spectroscopy. Optical extinction measurements, combined with numerical simulations, showed that the Au–ZnO nanoparticles exhibit a localized surface plasmon resonance (SPR) clearly red-shifted with respect to that of bare Au nanoparticles (AuNPs). This work contributes to the emergence of multi-functional nanomaterials with possible applications in surface plasmon resonance based biosensors, energy-conversion devices, and in water-splitting hydrogen production.     

Facile synthesis of gold nanoflowers with high surface-enhanced Raman scattering activity

Highly branched gold nanoflowers are synthesized in high yield by a simple amino-reducing method, without additional seeds or surfactant agents. We present a systematic investigation of the influence of different parameters on the size, morphology, and monodispersity of gold nanoflowers. The initial concentration of reducing agent, the solvent viscosity, and the reaction temperature play critical roles in the formation of nanoparticles. A lower concentration of reducing agent causes larger particles with sharp and dendritic tips. Moreover, with increasing solvent viscosity, the obtained particles have more and larger tips. Examination of the nanoparticles at different reaction stages with transmission electron microscopy and UV-vis spectroscopy reveals the formation of the gold nanoflowers as a classical growth process in which diffusion-controlled growth gives rise to the highly branched structures. Additionally, these gold nanoflowers have prospects for surface-enhanced Raman scattering (SERS) imaging because of their strong SERS enhancement and clean surface.     

超支化聚(胺-酯)为模板Ag纳米粒子的制备

以超支化聚(胺-酯)为模板,采用NaBH4直接还原和紫外灯光照射合成了Ag纳米粒子。超支化聚(胺-酯)对Ag纳米粒子形成起着重要的作用。TEM分析表明,Ag纳米粒子形貌为球形结构,分布均匀,粒径约为45 nm(NaBH4还原)和20 nm(紫外灯光照射)。发现在UV-Vis谱图中420 nm左右有Ag纳米粒子特征表面等离子共振吸收峰。FT-IR分析发现Ag纳米粒子和超支化聚(胺-酯)之间有较强的相互作用,超支化聚(胺-酯)对Ag纳米粒子的稳定性起保护作用。     

Synthesis and characterization of silver-carbon nanoparticles produced by high-current pulsed arc

In this paper, we report the formation of silver-carbon encapsulated metal nanoparticles (EMN's) using a high-current pulsed arc system in an argon atmosphere. The deposits were studied by Optical Extinction Spectroscopy (OES), X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM); the chemical analysis of the deposits was performed using Energy Dispersion X-ray spectroscopy (EDX). Using the total nanoparticle diameter, the bulk crystalline density of silver and an estimate amorphous carbon (a-C) density we have calculated the size of the silver nucleus and the thickness of the a-C coating as a function of the argon gas pressure. The OES spectra of the EMN's exhibited two peaks characteristic of the Surface Plasmon Resonance (SPR) of elongated/very close silver nanoparticles; a subsequent thermal annealing strongly increased the SPR peaks. The double peak SPR spectra were modeled using calculations based on the existence of silver nanoparticles in the form of prolate spheroids. The main advantage of our preparation method is that the metal nanoparticles are encapsulated in a-C from the beginning and this layer acts as an efficient chemical barrier.     

Properties of polypropylene nanocomposites containing silver nanoparticles

Silver/polypropylene (PP) nanocomposites containing silver nanoparticles smaller than 10 nm were prepared using a new synthetic method. AgNO3 crystals were dissolved into hydrophilic domain of polyoxyethylene maleate-based surfactant (PEOM), which gives self-assembly nano-structures. The AgNO3 in the nano-domains of PEOM was reduced by NaBH4 to form nanoparticles. The colloidal solutions with silver nanoparticles were diluted with ethanol and were mixed with PP pellets. Silver nanocomposites were prepared by extrusion compounding process after drying the pellets. Contents of silver nanoparticles dispersed within PP resin were changed from 100 to 1000 ppm. Formation of silver nanoparticles within PP was confirmed by UV-Vis spectroscopy and TEM. Size and distribution of dispersed silver nanoparticles were also measured by TEM. Silver/PP nanocomposites films showed not only improved thermal stability but also increased mechanical properties compared to neat PP film. Tensile properties of PP nanocomposites were largely improved compared with neat PP resin, and elongation increased also by 175% for the nanocomposites containing 1000 ppm silver nanoparticles.     

The preparation of nano-scale plate silver powders by visible light induction method

Silver can dissolve from the uncovered parts of the nanoparticles under the visible light irradiation and form extremely small spherical silver clusters. The latter can be further converted into nano-scale plate-like silver powders (PSP) in the presence of polyvinylpyrrolidone (PVP). The wavelength and intensity of visible light, the irradiation time and the weight ratio of protecting polymer to reducing agent have a significant effect on the formation and shape of PSP. The conversion process was monitored by ultraviolet-visible spectroscopy (UV-Vis). The product was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray powder diffractometer (XRD).     

纳米银/聚乙烯醇复合物的生物合成及其对6种水产病原菌的抑菌活性

纳米银作为一种新型抑菌剂有望成为传统抑菌剂的替代品,制备稳定、高效、环保的新型纳米银抑菌产品成为当今的研究热点。本研究以葡萄籽提取液为还原剂和稳定剂,聚乙烯醇(PVA)为载体,采用一步法“绿色”生物合成出一种纳米银/聚乙烯醇复合物(AgNPs/PVA)。通过紫外-可见(UV-Vis)吸收光谱、透射电镜(TEM)、X射线衍射(XRD)等手段对合成产物进行了表征。结果表明银离子被葡萄籽提取物成功还原成纳米银并附着在PVA的表面,纳米银颗粒均匀,呈现单分散状态,粒径较小,平均粒径为14 nm左右。AgNPs/PVA对鳗弧菌、溶藻弧菌、副溶血弧菌、哈维氏弧菌、灿烂弧菌及点状气单胞菌等6种典型的水产病原菌均有显著的抑菌效果。以溶藻弧菌为指示菌,AgNPs/PVA的最小抑菌浓度(MIC)为1.1 μg/mL,最小杀菌浓度(MBC)为2.2 μg/mL。AgNPs/PVA的Zeta电位为?24.1 mV,表明纳米银颗粒间有很强的排斥力,为其稳定分散提供保障,后续实验证明制备的AgNPs/PVA具有良好的稳定性和热稳定性。以上研究结果表明,AgNPs/PVA复合材料在水产养殖病害防治中具有广阔的应用前景。      

Solar photocatalytically active,engineered silver nanoparticle synthesis using aqueous extract of mesocarp of Cocos nucifera (Red Spicata Dwarf)

ABSTRACT

Silver nanoparticles synthesised using aqueous extract of Cocos nucifera (CN) mesocarp were evaluated for their photocatalytic activity under solar irradiation. The silver nanoparticles were synthesised by a green method of harnessing bioactive phytocomponents from the mesocarp of Cocos nucifera. Large-scale application of this process necessitates the manoeuvering of the process parameters for increasing the conversion of silver ions to nanoparticles. Process parameters influencing the morphological characteristics of silver nanoparticles such as precursor salt concentration and pH of the synthesis mixture were studied. The crystalline nanoparticles were characterised using UV-vis spectroscopy, XRD, FTIR, SEM and EDX analysis. CN extract and 5 mM silver nitrate solution at a ratio of 1:4 (v/v) in the synthesis mixture was found to be the optimum. Alkaline initial pH of the synthesis mixture was found to favour the synthesis of smaller sized monodispersed silver nanoparticles. Solar energy was harnessed for the photocatalytic degradation of Malachite green dye using silver nanoparticles obtained through the green synthesis method. Overall process aims at utilisation of naturally available resource for the synthesis of silver nanoparticles as well as the degradation of dyes using these nanoparticles, making it useful in the treatment of wastewater.     

Formation of silver nanoparticles in poly(perfluorosulfonic) acid membrane

The formation of silver nanoparticles by chemical reduction of Ag+-loaded Nafion-117 membrane with NaBH4 was studied using radioactivity tagged ions. The counterion-exchange method (Ag(m)+ <--> Na(s)+) was used to obtain a membrane sample with a varying proportion of Ag+ ions. The X-ray elemental mapping across the thickness of the membrane by energy-dispersive X-ray spectrometer attached to the environmental scanning electron microscope (ESEM/EDAX) indicated that Na+ and Ag+ were uniformly distributed in the membrane samples before reduction. The average size of nanoparticles formed after reduction was found to be 15 +/- 3 nm, irrespective of the concentration of silver ions present in the membrane before reduction. Energy-dispersive X-ray fluorescence (EDXRF) analyses of the membrane samples, carried out before and after reduction, indicated that the Ag concentration on the membrane surface was considerably increased after reduction. EDXRF measurements of the membrane samples, obtained from reduction carried out in a dead end cell, indicated that Ag nanoparticles were formed only on the membrane surface exposed to NaBH4 solution. Reduction carried out with NaBH4 tagged with 22Na showed that the formation of Ag nanoparticles involved exchange of Ag+ ions from ion-exchange sites in the interior of the membrane with Na+ ions, followed by reduction of Ag+ ions with BH4- ions at the surface of membrane. The study of self-diffusion of water, Na+, and Cs+ ions in the membrane loaded with Ag nanoparticles indicated that formation of Ag nanoparticles did not affect the diffusional transport properties of the membrane. The ion-exchange capacity and water uptake capacity were also not affected by the formation of Ag nanoparticles in the membrane. The spatial distribution of Ag nanoparticles across the thickness of the membrane obtained by ESEM/EDAX showed that Ag nanoparticles were confined to a few-micrometer surface layer of the membrane. Based on these observations, an attempt has been made to explain the mechanism of the formation of Ag nanoparticles in the membrane.