Microwave synthesis of silver nanofluids with polyvinylpyrrolidone (PVP) and their transport properties

Microwave synthesis has been applied to prepare stable silver nanofluids in ethanol by reduction of AgNO₃ with polyvinylpyrrolidone (PVP), used as stabilizing agent, having Ag concentrations of 1% by volume. The nanofluids were characterized by UV-vis spectroscopy, Fourier transform infrared, energy-dispersive X-ray spectroscopy, and transmission electron microscopy and systematically investigated for refractive index, electrical and thermal conductivity, and viscosity for different polymer concentrations. The size of nanoparticles was found to be in the range of 30–60 nm for two different salt-to-PVP ratios. For higher concentration of polymer in nanofluid, nanoparticles were 30 nm in size showing increase in thermal conductivity but a decrease in viscosity and refractive index, which is due to the polymer structure around nanoparticles. Thermal conductivity measurements of nanofluids show substantial increment in the thermal conductivity of nanofluid relative to the base fluid and nonlinear enhancement over the 283–323 K temperature range. Rheology of nanofluids was studied at room temperature showing effect of polymer on viscosity and confirming the Newtonian behavior of nanofluid.     

Synthesis and ligand properties towards gold and silver of the ferrocenylamidobenzimidazole ligand

The treatment of FcCOCl (Fc = (C₅H₅)Fe(C₅H₄)) with aminobenzimidazole in 1:1 or 2:1 ratio gives the ferrocenyl-amido derivatives FcCO(benzimNH₂) or (FcCO)₂(NHbenzim), respectively. The reactivity of FcCO(benzimNH₂) with silver or gold complexes has been studied. The reaction with the basic gold compounds [Au(acac)(PPh₃)] or [O(AuPPh₃)₃]ClO₄ occurs with deprotonation of the NH₂ group and coordination of one or three gold(phosphine) fragments. The treatment of this ligand with silver compounds, such as Ag(OTf) or [Ag(OTf)(PPh₃)], gives the complexes of stoichiometry [Ag(OTf)L] or [Ag(OTf)(PPh₃)L]. The ligand FcCO(benzimNH₂) and the complex [Ag(OTf){FcCO(benzimNH₂)}(PPh₃)] have been characterized by X-ray diffraction studies. DFT calculations were performed on models of this dimeric silver complex and showed that dimerization is energetically favourable, because Ag(I) achieves a four coordination environment, despite some bonds being relatively weak.     

Calculation of thermodynamic properties of gold and silver complexes of mixed composition

Computation for gold and silver complexes of mixed composition with cyanide, rhodanide, and thiosulfate anions was carried out by PM6 semiempirical method. Heats of formation in a gas phase and aqueous medium and hydration energies were determined for 224 possible structure variations. For 15 most stable structures, thermodynamic parameters of interaction with AM-2B anionite were determined: adsorption energies, energetically favorable configurations, activation energies, potential energy surfaces, etc.     

Ionic liquid-based stable nanofluids containing gold nanoparticles

A one-phase and/or two-phase method were used to prepare the stable ionic liquid-based nanofluids containing same volume fraction but different sizes or surface states of gold nanoparticles (Au NPs) and their thermal conductivities were investigated in more detail. Five significant experiment parameters, i.e. temperature, dispersion condition, particle size and surface state, and viscosity of base liquid, were evaluated to supply experimental explanations for heat transport mechanisms. The conspicuously temperature-dependent and greatly enhanced thermal conductivity under high temperatures verify that Brownian motion should be one key effect factor in the heat transport processes of ionic liquid-based gold nanofluids. While the positive influences of proper aggregation and the optimized particle size on their thermal conductivity enhancements under some specific conditions demonstrate that clustering may be another critical effect factor in heat transport processes. Moreover, the remarkable difference of the thermal conductivity enhancements of the nanofluids containing Au NPs with different surface states could be attributed to the surface state which has a strong correlation with not only Brownian motion but also clustering. Whilst the close relationship between their thermal conductivity enhancements and the viscosity of base liquid further indicate Brownian motion must occupy the leading position among various influencing factors. Finally, a promisingly synergistic effect of Brownian motion and clustering based on experimental clues and theoretical analyses was first proposed, justifying different mechanisms are sure related. The results may shed lights on comprehensive understanding of heat transport mechanisms in nanofluids.     

One-dimensional colloidal gold and silver nanostructures

One-dimensional (1-D) metallic nanoscale materials have long been of interest to many groups of scientists. Within the last 2 decades, great advances in the synthesis of metallic nanorods and nanowires have been made, with a variety of templating methods. More recently, bottom-up chemical syntheses of these materials have become increasingly reported in the literature. This Forum Article describes the synthesis, physical properties, and potential applications of 1-D metals, with an emphasis on silver and gold derived from studies in the authors' laboratories.     

Synthesis and properties of silver and gold nanocomposites in poly-1-vinyl-1,2,4-triazole matrix

Novel multifunctional hybrid nanocomposites with silver and gold nanoparticles stabilized by original polymer matrix based on poly-1-vinyl-1,2,4-triazole were synthesized and studied using UV and IR spectroscopy, X-ray diffraction analysis and transmission electron microscopy. The obtained nanocomposites comprise silver or gold nanoparticles of spherical and elliptical shape with size 3–20 nm and 1–10 nm, respectively.     

Ion-selective electrodes for gold and silver determination

Some new ion-selective electrodes for silver and gold are described. They are based on the ion-associate species formed by the cyanide, chloride or thiourea complexes of the metals, with hydrophobic anions or cations, as appropriate. The electrodes have been applied to the determination of gold and silver in various technological process solutions in industry.     

Thermophysical properties of interfacial layer in nanofluids

Although recent experiments have revealed that nanofluids have superior thermal conductivities to base fluids, the inherent physics are not fully understood. In this study, an interfacial layer, competing with Brownian motion as a corresponding mechanism, is conceptually connected with the surface-charge-induced electrical double layer. By applying colloidal science, the first explicit equations for the thickness and thermal conductivity of the layer are obtained. A fractal model including the new concept of the layer is developed. The model predictions are compared with experimental data for effects of pH, temperature, volume fraction, and primary particle size of CuO-water nanofluids.     

Synthesis, characterization and optical properties of silver and gold nanowires embedded in mesoporous MCM-41

Uniform nanowires of silver and gold inside the channels of MCM-41 were prepared by controlled reduction of their respective metal salts with sodium borohydride (NaBH₄). Presence of nanowires of silver and gold in MCM-41 were confirmed by high angle X-ray diffraction (XRD) data (peaks between 2ϑ = 30 − 60°) and transmission electron microscopy (TEM) confirmed the diameter of the nanowires. Diameter of nanowires is found to be ∼ 2.8 nm which is coincident with channel diameter of MCM-41. Optical properties of these heterostructured materials Ag-MCM-41 and Au-MCM-41 reveals the presence of surface plasmon absorption peaks of silver and gold respectively, and the shift in the absorption bands are associated to agglomeration of clusters inside the channels. Room temperature photoluminescence spectra exhibits interesting optical properties as observed for direct band gap semiconductors. Non-linear optical properties (NLO) corresponding to second harmonic generation (SHG) values were also recorded for self supported films of these heterostructured materials. Enhanced optical non-linearity was found to be arising from a corresponding increase of local field near the surface plasmon resonance. Further enhancement in SHG was found with poling due to an induction of orientation order.     

Antimicrobial and cytotoxicity properties of biosynthesized gold and silver nanoparticles using D. brittonii aqueous extract

Recently, the production of nanoparticles using biological resources has gained considerable attention due to their application for animal and human well-being. In this study, we used a green synthesis to fabricate gold and silver nanoparticles by reducing HAuCl₄ and AgNO₃ into AuNPs and AgNPs, respectively, using Dudleya brittonii (DB) extract. The physio-chemical properties of the synthesized nanoparticles were analyzed using a UV–vis spectrophotometer, FESEM, EDX, HR-TEM, AFM and FT-IR. Furthermore, the antimicrobial and cytotoxicity activities of DB-AuNPs and DB-AgNPs against livestock pathogenic bacteria and different cell lines, as well as anti-oxidant activity, were investigated. DB synthesized AuNPs and AgNPs were mostly spherical with a few triangular rods and sizes ranging of 5–25 nm and 10–40 nm, respectively. The in vitro antibacterial and antifungal studies demonstrated the DB-AuNPs and DB-AgNPs have good antibacterial activity against E. coli and other livestock pathogens, including Y. pseudotuberculosis and S. typhi. Cell studies revealed that the higher concentrations of both DB-AuNPs and DB-AgNPs (1 µg/ml to 1 mg/ml) showed potent cytotoxicity in chicken cells after 24 hrs, whereas the middle and lower concentrations of DB-AuNPs and DB-AgNPs did not show cytotoxicity in selected cell lines after 24 hrs. In addition, the DB synthesized AuNPs and AgNPs exhibited good free scavenging activity in a dose-dependent manner. Therefore, the biosynthesized nanoparticles can be utilized by the livestock industry to develop an effective source against livestock microbial infections.     

Hemoprotein bioconjugates of gold and silver nanoparticles and gold nanorods: structure-function correlations

Bioconjugates of the hemoproteins, myoglobin, and hemoglobin have been synthesized by their adsorption on spherical gold and silver nanoparticles and gold nanorods. The adsorption of hemoproteins on the nanoparticle surface was confirmed by their molecular ion signatures in matrix assisted laser desorption ionization mass spectrometry and specific Raman features of the prosthetic heme b units. High-resolution transmission electron microscopy (HRTEM) and UV-visible spectroscopy showed that the particles retain their morphology and show aggregation only in the case of silver. The binding of azide ion to the Fe(III) center of the prosthetic heme b moiety caused a red shift of the Soret band, both in the case of the bioconjugates and in free hemoproteins. This was further confirmed by the characteristic signature at 2050 cm-1 in the Fourier-transform infrared spectra, which corresponds to the asymmetric stretching of the Fe(III) bound azide. The retention of the chemical behavior of the prosthetic heme group after adsorption on the nanoparticle is interesting due to its implications in nanoparticle supported enzyme catalysis. The absence of morphology changes after the reaction of bioconjugates with azide ion observed in HRTEM studies implies the stability of nanoparticles under the reaction conditions. All these studies indicate the retention of protein structure after adsorption on the nanoparticle surface.     

Adsorption of mercury on gold and silver surfaces

In order to study the adsorption mechanism of Hg on Au and Ag substrates, thin film Au(111) and Ag(111) substrates were exposed to gaseous metallic mercury, while the mercury concentration, substrate temperature, and exposure length were varied. The resulting changes in the surface morphology of the substrates were studied with scanning tunneling microscopy (STM). The amount of adsorbed Hg required to cause saturation, i.e. a decrease in the adsorption rate was found to be dependent on the mercury concentration and substrate temperature. The observations lead to the conclusion that the adsorption includes place exchange processes and concerted adsorption of more than one Hg atom in one process. The results show that the collection efficiency of single-crystalline surfaces is a function of both mercury concentration and temperature. Therefore, results from measurements performed at different conditions using single-crystalline surfaces may not be comparable. Received: 3 February 1999 / Revised: 7 June 1999 / Accepted: 9 June 1999     

Fire-assay collection of gold and silver by copper

Gold and silver are very effectively collected in copper after fire-assay fusion at 1200 degrees . The resultant copper button is dissolved in perchloric acid and the parting solution is diluted with an equal volume of water. Both gold and silver are precipitated in the copper perchlorate medium by reduction with formic acid or hydroquinone. The two noble metals are collected, dissolved in acids, and determined by atomic-absorption spectrometry. The proposed procedure is simple, relatively rapid, and has been successfully applied to ores, concentrates, furnace products, and copper alloys. Recoveries compare favourably with those obtained by the classical lead cupellation method.     

Graphene nanofl akes and hybrid nanocomposites with gold and silver nanoparticles: optical and thermal properties

Russian Chemical Bulletin - New technologies for the preparation of graphene nanofl akes (GNF) with noble metal nanoparticles (NPs), specifi cally Au and Ag, as well as hybrid nanocomposites...     

Synthesis and anticancer properties of gold(I) and silver(I) N-heterocyclic carbene complexes

Bis(1,3-dimethylimidazol-2-ylidene)silver(I) nitrate and bis(4,5-dichloro-1,3-dimethylimidazol-2-ylidene)silver(I) nitrate were prepared by reacting the corresponding imidazolium nitrate salts with silver oxide. Bis(1,3-dimethylimidazol-2-ylidene)gold(I) nitrate and bis(4,5-dichloro-1,3-dimethylimidazol-2-ylidene)gold(I) nitrate salts were prepared via transmetallation of their silver precursors with chloro dimethylsulfide gold. The anticancer properties were determined using NCI-H460 lung cancer cells. Efficacy was established by comparison of the gold and silver compounds with cisplatin.     

Hybridization of oligonucleotide-modified silver and gold nanoparticles in aqueous dispersions and on gold films

Functionalization of silver and gold nanoparticles by 12mer-thiolated homo-oligonucleotides, SA and ST (containing only adenine or thymine, respectively), and their hybridization and dehybridization in aqueous dispersions have been described. In addition, ST and SA were self-assembled onto gold films and hybridized with their complementary pairs, unlabeled or labeled by gold and silver nanoparticles. The base pairing between DNA strands and the types of oligonucleotides (adenine or thymine) attached to the nanoparticles was detected by Polarization Modulated Fourier Transform Infrared Reflection Absorption Spectroscopy (PM-FTIRRAS).     

Cysteine-assisted growth of silver on gold nanorods

Au–Ag core–shell (Au@Ag) nanobars could be synthesized from gold nanorod (NR) seeds with cysteine additives by a two-step process of reaction temperatures. The lateral sides of gold NRs surrounded by surfactant bilayers render cysteine additives binding on both ends of the NRs, and restricted silver deposition to their lateral sides at room temperature. Further, silver deposition can take place at first on the pre-formed silver layers on the lateral sides at higher temperatures and finally resulted in the formation of Au@Ag nanobars in which gold NRs are in the corner positions of the nanobars and their longitudinal axes parallel to the longer sides of the nanobars.     

Investigation on rheological properties of carbon nanotube nanofluids

The effective viscosity of carbon nanotube nanofluids is strongly dependent on the temperature and concentration. The aggregation behaviour that carbon nanotubes exhibit in solution and the orientation variation of single carbon nanotube make rheological properties of nanofluids more complex. With the increase of shear rate, the degree of dispersion and orientation of carbon nanotubes will be improved. Based on previous studies and the fact mentioned above, a reasonable expression for viscosity of carbon nanotube nanofluids has been given, which is associated with the shear rate and aspect ratios of carbon nanotubes. The expression has been validated comparing with previous experimental data.