Biological synthesis of gold and silver nanoparticles mediated by the bacteria Bacillus subtilis

Biological synthesis of gold and silver nanoparticles was carried out using the bacteria Bacillus subtilis. The reduction processes of chloroaurate and silver ions by B. subtilis were found to be different. Gold nanoparticles were synthesized both intra- and extracellularly, while silver nanoparticles were exclusively formed extracellularly. The gold nanoparticles were formed after 1 day of addition of chloroaurate ions, while the silver nanoparticles were formed after 7 days. The nanoparticles were characterized by X-ray diffraction, UV-vis spectra and transmission electron spectroscopy. X-ray diffraction revealed the formation of face-centered cubic (fcc) crystalline gold nanoparticles in the supernatant, broth solution and bacterial pellet. Silver nanoparticles also exhibited diffraction peaks corresponding to fcc metallic silver. UV-vis spectra showed surface plasmon vibrations for gold and silver nanoparticles centered at 530 and 456 nm, respectively. TEM micrographs depicted the formation of gold nanoparticles intra- and extracellularly, which had an average size of 7.6 +/- 1.8 and 7.3 +/- 2.3 nm, respectively, while silver nanoparticles were exclusively formed extracellularly, with an average size of 6.1 +/- 1.6 nm. The bacterial proteins were analyzed by sodium dodecyl sulfonate-polyacrylamide electrophoresis (SDS-PAGE) before and after the addition of metal ion solutions. We believe that proteins of a molecular weight between 25 and 66 kDa could be responsible for chloroaurate ions reduction, while the formation of silver nanoparticles can be attributed to proteins of a molecular weight between 66 and 116 kDa. We also believe that the nanoparticles were stabilized by the surface-active molecules i.e., surfactin or other biomolecules released into the solution by B. subtilis.     

Preparation of linoleic acid-capped silver nanoparticles and their antimicrobial effect

Silver nanoparticles have been prepared through the chemical reduction of silver ions by ethanol using linoleic acid as a stabilising agent. This colloidal solution shows an absorption band in the visible range with an absorption peak at 421 nm. The peaks in the X-ray diffraction (XRD) pattern matches well with the standard values of the face-centred-cubic form of metallic silver. Transmission Electron Microscope (TEM) micrograph shows a nearly uniform distribution of the particles with an average size of 8 nm. This linoleic acid-capped silver nanoparticles show antimicrobial activity against Escherichia coli and Staphylococcus aureus.     

A green method for synthesis of silver nanodendrites

A simple, green method was developed for the synthesis of silver nanodendrites by an aqueous chemical route. This method involves the reduction of silver nitrate with absolute alcohol using polyvinyl pyrrolidone (PVP) as the surfactant. UV–vis absorption spectra, X-ray diffraction patterns (XRD) and energy dispersive spectrometry (EDS) suggest the formation of Ag nanoparticles. Transmission electron microscopy (TEM) images show the shape and sizes of the nanoparticles. During the synthesis, it was found that the morphology and size distribution of the as-prepared silver nanoparticles varied with the concentration of the precursor metal salts, reaction time and surfactant ratios. The formation mechanism of Ag nanodendrites was determined based on the investigation of the above reaction parameters. Simple methods, nontoxic chemicals and environmentally benign solvents make this synthesis ideally suited for industrial production.     

In situ synthesis of CuS nanoparticle with a distinguishable SPR peak in NIR region

In this article we report the synthesis of CuS nanoparticle with distinguishable surface plasmonic resonance (SPR) peaks in near infrared region. For this purpose in situ synthesis (one-step fabrication) has been used to prepare CuS nanoparticles in PMMA polymer matrix at room temperature. The X-ray diffraction spectrum confirms the formation of CuS nanoparticles. The transmittance spectra of nano-composite samples reveal that the samples have a good transparency. The absorption spectra show a broad absorption peak in the wavelength region from 570 to 980 nm which is a characteristic SPR peak for CuS nanoparticle. An increase of refractive index was observed for the samples containing CuS nanoparticles. The linear relationship between the refractive index and volume fraction was observed. The appearance of SPR peak in refractive index spectra was attributed to CuS nanoparticles. Shifting of absorption edge to lower photon energy has been observed for nano-composite samples. The direct energy bandgap of nano-composite samples are reduced compare to pure PMMA polymer. The plot between the direct energy band gap and refractive index reveals that the decrease in bandgap energy is associated with the increase in index of refraction. The increase of optical dielectric constant can be ascribed to the formation of CuS nanoparticles. The low band gap of CuS nanoparticles in the present work reveals their importance for applications in optoelectronic devices.     

Bare plasmonic metal nanoparticles: synthesis,characterisation and in vitro toxicity assessment on a liver carcinoma cell line

Metal nanoparticles have generated great interest due to their excellent optical and chemical properties. The widely used chemical method for synthesising nanoparticles involves capping agents for colloidal stability. However, there are scarce reports on the application of metal nanoparticles synthesised without using capping agents. Hence, there is a need to develop pristine nanoparticles devoid of capping that can be used for translational research. Here, the authors developed a facile and rapid method for synthesising bare metal nanoparticles (platinum/silver/gold) that are chemically reactive and stable for a month upon storage. They synthesised bare metal nanoparticles of sub‐15 nm and characterised using standard techniques (UV–VIS‐NIR/DLS/zeta//TEM/XRD). They assessed the safety of the synthesised nanoparticles on the liver carcinoma cell line (HepG2). Bare gold and platinum nanoparticles were non‐toxic in comparison to bare silver nanoparticles. Bare metal nanoparticles were also checked for metal detection wherein antimony, mercury and chromium were detected using bare gold and silver nanoparticles. The spectroscopic shifts of the nanoparticles when bound to metals resulted in blue and red shifting of the plasmon band, indicating the sensing of metals. These results show that bare metal nanoparticles have the potential to emerge as a promising candidate for biomedical and sensing applications.Inspec keywords: ultraviolet spectra, electrokinetic effects, liver, cellular biophysics, nanoparticles, cancer, toxicology, gold, platinum, X‐ray diffraction, silver, colloids, transmission electron microscopy, plasmonics, visible spectra, nanomedicineOther keywords: bare plasmonic metal nanoparticles, liver carcinoma cell line, capping agents, pristine nanoparticles, bare metal nanoparticles, synthesised nanoparticles, platinum nanoparticles, silver nanoparticles, XRD, TEM     

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.     

Bio-synthesis of gold and silver nanoparticles from Candida guilliermondii and their antimicrobial effect against pathogenic bacteria

In the present study we investigated the extra cellular synthesis of gold and silver nanoparticles by using the yeast Candida guilliermondii. The formation of noble metal nanoparticles was monitored by the UV-Visible spectroscopy. As prepared gold and silver nanoparticles showed distinct surface plasmon peaks at 530 nm and 425 nm respectively. Phase and morphology of the as synthesized materials were investigated by X-ray diffraction and electron microscopy techniques respectively. XRD patterns confirmed the formation of gold and silver nanoparticles with face centered cubic structures. Bio-TEM images showed the formation of near spherical, well dispersed gold and silver nanoparticles in the size range of 50-70 nm and 10-20 nm respectively. The biosynthesized nanoparticles were tested for their antimicrobial activity against five pathogenic bacterial strains. The highest efficiency for both gold and silver nanoparticles was observed against Staphylococcus aureus. A comparative study was also done to find the effect of chemically synthesized noble metal nanoparticles against the above test strains. Chemically synthesized particles had no antimicrobial activity against any of the pathogenic strains. The results obtained suggest that biosynthesized gold and silver nanoparticles can be used as effective antimicrobial agents against some of the potential harmful pathogenic microorganisms.     

Adherent amorphous hydrogenated carbon films on metals deposited by plasma enhanced chemical vapor deposition

This paper reports the findings of a study of the structural, mechanical, and tribological properties of amorphous hydrogenated carbon (a-C:H) coatings for industrial applications. These thin films have proven quite advantageous in many tribological applications, but for others, thicker films are required. In this study, in order to overcome the high residual stress and low adherence of a-C:H films on metal substrates, a thin amorphous silicon interlayer was deposited as an interface. Amorphous silicon and a-C:H films were grown by using a radio frequency plasma enhanced chemical vapor deposition system at 13.56 MHz in silane and methane atmospheres, respectively. The X-ray photoelectron spectroscopy technique was employed to analyze the chemical bonding within the interfaces. The chemical composition and atomic density of the a-C:H films were determined by ion beam analysis. The film microstructure was studied by means of Raman scattering spectroscopy. The total stress was determined through the measurement of the substrate curvature, using a profilometer, while micro-indentation experiments helped determine the films' hardness. The friction coefficient and critical load were evaluated by using a tribometer. The results showed that the use of the amorphous silicon interlayer improved the a-C:H film deposition onto metal substrates, producing good adhesion, low compressive stress, and a high degree of hardness. SiC was observed in the interface between the amorphous silicon and a-C:H films. The composition, the microstructure, the mechanical and tribological properties of the films were strongly dependent on the self-bias voltages. The tests confirmed the importance of the intensity of ion bombardment during film growth on the mechanical and tribological properties of the films.     

Synthesis and characterization of gold nanorings

In the present work, we report the formation of Au nanorings on quartz substrate by thermal evaporation of Au on quartz and subsequent annealing in certain conditions as a function of metal volume fraction and annealing temperature. Optical extinction cross-sections measurements and atomic force microscopy (AFM) studies have been performed on the as-deposited and annealed samples. No signature of nanoparticles formation is found in case of as-deposited samples, while spectra of annealed samples show a clear signature of surface plasmon resonance absorption (SPR) peaks around 580 nm, which reveals the formation of Au nanostructure. AFM images clearly show the formation of Au nanorings under certain conditions. The observed SPR frequency of the Au nanorings in our case is in agreement with the estimated frequency obtained from the formulation of Aizpurua et al. Optical extinction measurements at different incidence angles were performed, which showed splitting of SPR at angles beyond 20 degrees.     

Hydrothermal synthesis of silver nanoshells: formation and plasmon hybridization

Silver nanoshells with varying diameters are synthesized using Polyacrylamide (PAM) in a controllable way by the hydrothermal process followed by calcinations to investigate the plasmon hybridization. At relatively lower hydrothermal temperature (T_H ~ 415 K), an assembly of silver nanoparticles (NPs) along the PAM chain is formed exhibiting transverse (453 nm) and longitudinal (949 nm) surface plasmon resonance (SPR) peaks. With increasing T_H (~510 K), this assembly of NPs disrupt into segments and create a homogeneous phase of PAM–silver composites, which upon cooling form globules due to agglomeration. During calcinations at 650 K, PAM is degraded into several gases (NH₃, CO₂, and water vapor) to form nanobubbles, which are trapped inside the globule and coalescence into a single bubble as a result; the molten state containing the NPs forms a spherical shell to minimize the surface energy. The absorption spectra are de-convoluted into three peaks, which are attributed to the hybridization between anti-bonding and bonding SPR modes of individual nanoshells.     

Characterization of the optical properties of silver nanoparticle films

To understand the collective properties of nanoparticles, it is necessary to control the particle size, spacing and ordering. Here we describe the chemical synthesis of well-controlled silver nanoparticles, the wet coat preparation and the optical properties of its film. The light incidence angle and polarization dependency of the resonant spectra show distinctive surface plasmon resonance extinction peaks for isolated particles and the coupled modes of neighbouring particles. Furthermore, we discuss the thermal treatment and dielectric surrounding effects on the optical properties of silver nanoparticle film.     

Green synthesis of silver nanoparticles using aqueous solution of Ficus benghalensis leaf extract and characterization of their antibacterial activity

The present study reports an environmentally friendly and rapid method for synthesis of silver nanoparticles. Although several articles have been reported for the synthesis of silver nanoparticles from plant extract, here we have developed a green synthetic method for silver nanoparticles using Ficus benghalensis leaf extract which acts as a reducing and capping agent. It was observed that use of Ficus benghalensis leaf extract makes a fast and convenient method for the synthesis of silver nanoparticles and can reduce silver ions into silver nanoparticles within 5 min of reaction time without using any harsh conditions. Silver nanoparticles so prepared were characterized by using UV-visible spectroscopy, transmission electron microscope-energy dispersive spectra (TEM-EDS) and X-ray diffraction (XRD). Further, these nanoparticles show effective antibacterial activity toward E.coli MTCC1302 due to high surface to volume ratio.     

Hydrogen-Enhanced Catalytic Conversion of Amorphous Carbon to Graphene for Achieving Superlubricity

The solid-state conversion of amorphous carbon into graphene is extremely difficult, but it can be achieved in the friction experiments that induce macroscale superlubricity. However, the underlying conversion mechanisms remain elusive. Here, the friction experiments with Cu nanoparticles and (non-hydrogen (H) or H) a-C in vacuum, show the H-induced conversion of mechanical to chemical wear, resulting in the a-C's tribosoftening and nanofragmentating that produce hydrocarbon nanoclusters or molecules. It is such exactly hydrocarbon species that yield graphene at hydrogen-rich a-C friction interface, through reaction of them with Cu nanoparticles. In comparison, graphene isn't formed at Cu/non-H a-C friction interface. Atomistic simulations reveal the hydrogen-enhanced tribochemical decomposition of a-C and demonstrate the energetically favorable graphitization transformation of hydrocarbons on Cu substrates. The findings are of importance to achieve solid-state transformation between different carbon allotropes and provide a good strategy to synthesize other graphitic encapsulated catalysts with doped elements.     

Tuning luminescence intensity of RHO6G dye using silver nanoparticles

The photoluminescence (PL) from rhodamine (RHO6G) dye dispersed in ethanol has been studied in the presence of different amounts of citrate stabilized silver nanoparticles of size, ∼10 nm. Enhancement as well as quenching of luminescence intensity has been observed and it was found that luminescence intensity can be tuned by adding various amounts of silver nanoparticles to the RHO6G dye dispersion. The luminescence spectra of dye consist of two peaks at 440 nm and 550 nm. Peak at 440 nm shows an enhancement in intensity at all the concentrations of added silver nanoparticles with the maximum intensity for dye with 0.25 ml silver nanoparticles (82% enhancement in the luminescence intensity). PL intensity of intense peak at 550 nm of dye molecules was found to be quenched in presence of silver nanoparticles and maximum quenching was found to be 41% for the dye with 1 ml silver nanoparticles. However, for lowest concentration of silver nanoparticles viz. (0.01 ml), enhancement in intensity was observed (13% enhancement than the dye molecules). The quenching as well as enhancement in the intensity can be understood by considering the possibility of three different phenomena. It has been reported earlier that when metal nanoparticles are in close proximity to the fluorophores, quenching of luminescence occurs, whereas when metal nanoparticles are located at certain distance, enhancement in luminescence is observed. This effect has been explained by coupling of surface plasmon resonance from metal nanoparticles with fluorophore, resulting in the increase of excitation and emission rate of the fluorophore in the localized electromagnetic field. The quenching and enhancement of luminescence intensity of the dye molecules can also be explained as the transfer of electrons from dye to the silver nanoparticles and to an extent it can be attributed to the aggregation of dye molecules upon addition of silver nanoparticles.     

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.     

Surface-enhanced Raman scattering of carbon nanotubes by decoration of ZnS nanoparticles

ZnS nanoparticles anchored on the single-walled carbon nanotubes (SWNTs) were fabricated by a chemical vapor deposition (CVD) method. The CVD method shows no selectivity for growth of ZnS nanoparticles on types and defects of the SWNTs, and thus ensures the uniform decoration of all SWNTs on the substrate. ZnS nanoparticles with a diameter of 10 nm were decorated on the SWNTs surface with an interparticle distance of about 20 nm. This method provides the possibility to realize the optimal configurations of ZnS nanoparticles on SWNTs for obtaining surface-enhanced Raman spectroscopy (SERS) of SWNTs. Investigations of mechanism reveal that charge transfer (a small amount of excitation electrons) from ZnS nanoparticles to SWNTs weakly affects Raman intensity, and the coupled surface plasmon resonance (SPR) formed from plenty of excitation electrons on the surface of ZnS nanoparticles contributes to the strong surface enhancement. It would be an alternative approach for SERS after metal (normally gold or silver) nanoparticles' decoration on the SWNTs surface.     

Structural analysis of graphene oxide/silver nanocomposites: optical properties,electrochemical sensing and photocatalytic activity

In the present study, graphene oxide/silver (GO/Ag) nanocomposites were synthesized via a facile simple one pot chemical reduction method using ethylene glycol/sodium borohydrate (EG/NaBH₄) as solvent and reducing agent. GO was selected as a substrate and stabilizer to prepare GO/Ag nanocomposites. The synthesized GO/Ag nanocomposites were characterized by a series of techniques. Highly monodispersed stable crystalline silver nanoparticles having a face-centered cubic (fcc) phase were confirmed by X-ray powder diffraction (XRD) on GO signature. Scanning electron microscopy images showed that Ag nanoparticles are deposited on the GO sheet with a narrow size distribution. Transmission electron microscopy observations revealed that large numbers of Ag nanoparticles were uniformly distributed on GO sheet and well separated with an average size of 18 nm. Ultraviolet–visible (UV–Vis) spectroscopic results showed the peak of GO and surface plasmon resonance (SPR) of Ag nanoparticles. The SPR property of GO/Ag nanocomposites showed that there was an interaction between Ag nanoparticles and GO sheet. The intensities of the Raman signal of GO/Ag nanocomposites are gradually increased with attachment of Ag nanoparticles i.e. there is surface-enhanced Raman scattering activity. Electrochemical investigations indicated that the nanocomposites possessed an excellent performance for detecting towards 4-nitrophenol. An application of the obtained GO/Ag nanocomposites as a catalyst in the reduction of 4-nitrophenol to 4-aminophenol by NaBH₄ was demonstrated. The GO/Ag nanocomposites exhibited high activity and stability for the catalytic reduction of 4-nitrophenol. The prepared GO/Ag nanocomposites act as photo-catalysts.     

Real-time, in-situ, extinction spectroscopy studies on silver-nanoseed formation

In this paper, real-time extinction spectroscopy was employed to analyze in situ the fast formation process of silver seeds. The influencing factors for silver-seed formation, including the concentration of the reducer (NaBH(4)), the amount of stabilizer (citrate), and the addition procedure of NaBH(4), were evaluated. The configuration of the surface plasmon resonance (SPR) spectra of silver nanoparticles, which reflected the morphology of the produced silver seeds, was found to be distinctly affected by the NaBH(4) concentration. Homogeneous silver nanoparticles were obtained when the added NaBH(4) was lower in concentration than 2.00 mM. In contrast, higher concentrations of reducing agent (≥3.00 mM NaBH(4)) resulted in uneven silver nanoparticles. Mie theory was applied to clarify the correlation of the SPR and the size of the silver seeds. Repeated additions of a small amount of the reducing agent could increase the monodispersity and isotropy of silver seeds. Real-time extinction spectroscopy is a convenient technique that achieves the in-situ and nondestructive measurement of intermediates in the formation process of silver nanoseeds.     

Biosynthesis of silver nanoparticles using Moringa oleifera leaf extract and its application to optical limiting

The Development of biologically inspired experimental processes for the synthesis of nanoparticles is evolving into an important branch of nanotechnology. The work presented here with the biosynthesis of silver nanoparticles using Moringa oleifera leaf extract as reducing and stabilizing agent and its application in nonlinear optics. The aqueous silver ions when exposed to Moringa oleifera leaf extract are reduced resulting in silver nanoparticles demonstrating the biosynthesis. The silver nanoparticles were characterized by UV-Visible, X-ray diffraction (XRD), Fourier transform infra-red spectroscopy (FT-IR) and transmission electron microscopy (TEM) techniques. TEM analysis shows a dispersion of the nanoparticles in a range of 5-80 nm with the average around 46 nm and are crystallized in face centred cubic symmetry. To show that these biosynthesized silver nanoparticles possess very good nonlinear properties similar to those nanoparticles synthesized by chemical route, we carried out the Z-scan studies with a 6 ns, 532 nm pulsed laser. We estimated the nonlinear absorption coefficient and compare it with the literature values of the nanoparticles synthesized through chemical route. The silver nanoparticles suspended in solution exhibited reverse saturable absorption with optical limiting threshold of 100 mJ/cm2.     

Zephyranthes candida flower extract mediated green synthesis of silver nanoparticles for biological applications

The development of biologically active nanoparticles (NPs) has played a prominent role in medicinal, pharmaceuticals and bio-nanotechnology fields. Phytosynthesis is a simple, reproducible, and effective method to produce highly stable metal nanoparticles. In this present work, silver (Ag) nano particles (NPs) were produced using Zephyranthes candida (Z. candida) flower extract as a sustainable, cost-effective, and non-hazardous stabilizing agent. In the view of X-ray diffraction (XRD) analysis, the face centred cubic structure of Ag NPs was revealed. From the UV–Vis spectral analysis, the formations of Ag NPs were further confirmed through surface plasmon resonance (SPR) at the highest absorbance (λ_max) of 418 nm. FT-IR represents the spectra that reveal the presence of diverse functional groups along with their vibrational modes present in Ag NPs and Z.candida flower extract. SEM and TEM denote the formation of spherical morphology of Ag NPs. Furthermore, EDX and XPS spectra confirmed the purity of the prepared Ag NPs. Finally, the biological studies such as anti-inflammatory, anti-diabetic, anti-oxidant, anticancer confirm the bioactivity of the synthesized Zephyranthes Candida mediated Ag NPs.