A facile synthesis and characterization of Ag, Au and Pt nanoparticles using a natural hydrocolloid gum kondagogu (Cochlospermum gossypium)

An environmentally benign method for the synthesis of noble metal nanoparticles has been reported using aqueous solution of gum kondagogu (Cochlospermum gossypium). Both the synthesis, as well as stabilization of colloidal Ag, Au and Pt nanoparticles has been accomplished in an aqueous medium containing gum kondagogu. The colloidal suspensions so obtained were found to be highly stable for prolonged period, without undergoing any oxidation. SEM-EDXA, UV-vis spectroscopy, XRD, FTIR and TEM techniques were used to characterize the Ag, Au and Pt nanoparticles. FTIR analysis indicates that -OH groups present in the gum matrix were responsible for the reduction of metal cations into nanoparticles. UV-vis studies showed a distinct surface plasmon resonance at 412 and 525 nm due to the formation of Au and Ag nanoparticles, respectively, within the gum network. XRD studies indicated that the nanoparticles were crystalline in nature with face centered cubic geometry. The noble metal nanoparticles prepared in the present study appears to be homogeneous with the particle size ranging between 2 and 10 nm, as evidenced by TEM analysis. The Ag and Au nanoparticles formed were in the average size range of 5.5±2.5 nm and 7.8±2.3 nm; while Pt nanoparticles were in the size range of 2.4±0.7 nm, which were considerably smaller than Ag and Au nanoparticles. The present approach exemplifies a totally green synthesis using the plant derived natural product (gum kondagogu) for the production of noble metal nanoparticles and the process can also be extended to the synthesis of other metal oxide nanoparticles.     

Biocrystallization of silver and gold ions by inactive cell filtrate of Rhizopus stolonifer

Microbes and their cell filtrates are known to synthesize metal nanoparticles. But maintenance of aseptic conditions and irregularly shaped and sized nanoparticles are major drawbacks of the system. In this study cell filtrate from inactive biomass of Rhizopus stolonifer was used for the first time to produce near uniformly sized and shaped Ag and Au nanoparticles at room temperature. The size of Ag and Au nanoparticles were found in the range of 25–30 nm and 1–5 nm, respectively. UV–vis spectrum, TEM and XRD measurements confirmed the formation of Ag and Au nanoparticles.     

Surface-enhanced Raman studies of bradykinin using colloidal gold

In this paper, bradykinin (BK), an endogenous peptide hormone, which is involved in a number of physiological and pathophysiological processes was deposited onto the colloidal Au nanoparticles. The surface-enhanced Raman spectroscopy (SERS) was used to determine the adsorption mode of BK under different environmental conditions, including: excitation wavelengths (514.5 nm and 785.0 nm), pH of aqueous sol solutions (from pH = 3 to pH = 11), and size of the colloidal nanoparticles (10, 20, and 50 nm). The metal surface plasmon of the colloidal suspended Au nanoparticles was examined by ultraviolet-visible (UV–vis) spectroscopy. The results showed that the C-terminal part of BK plays a crucial role in the adsorption process onto the colloidal suspended Au particles. The Phe^5/8 and Arg⁹ residues of BK mainly participate in the interactions with the colloidal Au nanoparticles. At acidic pH of the solution (pH = 3), the BK COO⁻ terminal group through the both oxygen atoms strongly binds to the Au nanoparticles. The Phe⁵/Phe⁸ rings adopt tilted orientation with respect to the colloidal Au nanoparticles with diameters of 10 and 20 nm. As the particle size increases to 50 nm, the flat orientation of the Phe ring(s) with respect to the Au nanoparticles is observed.     

Competitive adsorption of toxic heavy metal contaminants by gum kondagogu (Cochlospermum gossypium): A natural hydrocolloid

Gum kondagogu (Cochlospermum gossypium), a naturally occurring tree biopolymer, is exploited as a biosorbent to remove metal ions from aqueous solutions. The removal efficiency of toxic metals by gum kondagogu was determined quantitatively in the order Cd²⁺ > Cu²⁺ > Fe²⁺ > Se²⁺ > Pb²⁺ > total Cr > Ni²⁺ > Zn²⁺ > Co²⁺ > As²⁺ at pH 5.0 ± 0.1 and temperature 25 ± 2 °C by inductively coupled plasma-mass spectrometry (ICP-MS). The biosorption (%) of various metal ions tested was found to be in the range of 97.3–16.7%, at pH 5.0. The morphological and mechanisms of interaction of toxic metal ions with gum kondagogu were assessed by scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDXA) and X-ray diffraction (XRD) spectrum. The analysis indicated that biosorption process included morphological changes, precipitation, complexation and ion exchange mechanism for the removal of metal ions by the gum. XRD analysis indicated the amorphous nature of gum kondagogu, which facilitate metal biosorption. The metal ions adsorption leads to its deposition on the gum kondagogu matrix in a crystalline state.     

Mono and bi-metallic electrocatalysts of Pt and Ag for oxygen reduction reaction synthesized by sonication

Nanoparticles of Ag, Pt and Pt–Ag were synthesized using ultrasonic irradiation with no consecutive thermal treatment to catalyze the oxygen reduction reaction. Metal nanoparticles are supported on carbon substrate. The synthesized materials were characterized by XRD, TEM, and cyclic and lineal voltammetry techniques. The kinetic formation of the metallic nanoparticles in solution was followed using UV–vis spectroscopy. The metal particles have crystalline structure and particle size with < 10 nm in size and in the form of spherical agglomerates. Ag/C exhibits lower electrochemical activity and stability for the ORR compared to Pt/C and Pt–Ag/C in acid medium. The mass and specific activity results demonstrate that the synthesized bimetallic sample exhibits 1.5 and 5 times greater electrochemical activities for the ORR compared to the commercial sample.     

Photocatalytic activity of TiO2 nanotube layers loaded with Ag and Au nanoparticles

Ag and Au nanoparticles were found to significantly enhance the photocatalytic activity of self-organized TiO₂ nanotubular structures. The catalyst systems are demonstrated to be highly efficient for the UV-light induced photocatalytic decomposition of a model organic pollutant – Acid Orange 7. The metallic nanoparticles with a diameter of ∼10 ± 2 nm (Ag) and ∼28 ± 3 nm (Au) were attached to a nanotubular TiO₂ layer that consists of individual tubes of ∼100 nm of diameter, ∼2 μm in length and approx. 15 nm of wall thickness. Both metal particle catalyst systems enhance the photocatalytic decomposition significantly more on the nanotubes support than placed on a compact TiO₂ surface.     

Sucrose ester micellar-mediated synthesis of Ag nanoparticles and the antibacterial properties

Ag nanoparticles with diameter in the range of 10–25 nm had been synthesized using a simple sucrose ester micellar-mediated method. Ag nanoparticles were formed by adding AgNO₃ solution into the sucrose ester micellar solution containing sodium hydroxide at atmospheric condition after 24 h of aging time. Trace amount of dimethyl formamide (DMF) in the sucrose ester solution served as a reducing agent while NaOH acted as a catalyst. The produced Ag nanoparticles were highly stable in the sucrose ester micellar system as there was no precipitation after 6 months of storage. The as-synthesized Ag nanoparticles were characterized using transmission electron microscope (TEM), X-ray diffractometer (XRD), dynamic light scattering (DLS) and UV–vis spectroscopy (UV–vis). Formation mechanism of Ag nanoparticles in the micellar-mediated synthesis is postulated. The antibacterial properties of the Ag nanoparticles were tested against Methicillin-resistant Staphylococcus aureus (MRSA) (Gram-positive) and Aeromonas hydrophila (Gram-negative) bacteria. This work provides a simple and “green” method for the synthesis of highly stable Ag nanoparticles in aqueous solution with promising antibacterial property.     

Ptshell–Aucore/C electrocatalyst with a controlled shell thickness and improved Pt utilization for fuel cell reactions

Nanoscale Pt_shell–Au_core/C with a controlled shell thickness was successfully synthesized based on a successive reduction strategy. With a Au core size of 4.8 nm, a complete Pt shell of thickness ∼0.6 nm was formed at Pt/Au mole ratio 1:1. The complete coverage of Au core with Pt shell was suggested by various techniques including TEM, UV–vis and cyclic voltammetry. A higher specific activity compared to conventional Pt/C was demonstrated using the probe reaction of methanol electro-oxidation, proving the improved Pt utilization with this core-shell structure.     

Gamma irradiation route to synthesis of highly re-dispersible natural polymer capped silver nanoparticles

Aqueous dispersions of highly stable, redispersible silver nanoparticles (Ag NPs) were synthesized using gamma radiolysis with gum acacia as a protecting agent. The formation of nanosized silver was confirmed by its characteristic surface plasmon absorption peak at around 405 nm in UV–vis spectra. The size of the silver nanoparticles can be tuned by controlling the radiation dose, ratio of gum acacia to silver ions and also the ionic strength of the medium. Dynamic light scattering (DLS) measurement of the as-synthesized nanoparticles indicated the size less than 3 nm at higher dose of radiation and this also corroborated the size measurement from the width of the corresponding X-ray diffraction (XRD) peak. The face centered cubic (fcc) crystallinity of the nanoparticles was evident from XRD and high resolution transmission electron microscopic (HRTEM) measurements. Fourier transform infra-red (FTIR) spectroscopic data indicate a bonding of Ag NPs with COO^? group of acacia through bridging bidentate linkage.     

Atomic force microscopy of cubic Pt nanoparticles in electrochemical environments

Real structure of cubic Pt nanoparticle has been studied at various potentials in 0.1 M NaClO₄ with the use of atomic force microscopy (AFM). Cubic Pt nanoparticles in 10 nm height are clearly imaged from 0.10 V to 1.10 V (Ag/AgCl). The height of the nanoparticle increases 1.2 ± 0.7 nm (10.4 ± 6.8%) around the onset potential of oxygen evolution (1.20 V (Ag/AgCl)). The height increase is attributed to the formation of the oxide species at the inner layers of the nanoparticle. Dissolution of the nanoparticle starts from the upper terrace, not from the edge above 1.40 V (Ag/AgCl).     

Carbon encapsulated magnetic nanoparticles produced by hydrothermal reaction

Carbon encapsulated magnetic nanoparticles(CEMNs)were synthesized by heating an aqueous glucose solution containing Fe-Au(Au coated Fe nanoparticles)nanoparticles at 160-180℃ for 2 h.This novel hydrothermal approach is not only simple but alsoprovides the surface of CEMNs with functional groups like-OH.The formation of carbon encapsulated magnetic nanoparticles wasnot favored when using pure Fe nanoparticles as cores because of the oxidation of Fe nanoparticles by H2O during the reaction and,therefore,the surfaces of the naked Fe nanoparticles had to be coated by Au shell in advance.TEM,XRD,XPS and VSMmeasurments characterized that they were uniform carbon spheres containing some embedded Fe-Au nanoparticles,with asaturation of 14.6 emu/g and the size of the typical product is$350 nm.     

Electrodeposition of monodispersed metal nanoparticles in a nafion film: Towards highly active nanocatalysts

We have explored a new and facile method for the fabrication of metal nanoparticles on the electrode surface. The approach for fabricating metal nanoparticles was carried out by two steps consisting of ion-exchange in nafion film coated on the electrode and subsequent reduction of metal ions to metallic nanoparticles by electrochemical method. The results of characterization by TEM show that metal nanoparticles were nearly monodispersed in the whole nafion film. The average diameters of Cu, Co and Ni nanoparticles were statistically measured to be 5.1 nm ± 0.2 nm, 4.6 nm ± 0.2 nm and 4.7 nm ± 0.2 nm, respectively. The amount of metal nanoparticles can be readily controlled by the amount of nafion coated on the electrode. By performing the H₂O₂ reduction at the obtained Cu nanoparticles, the high electrocatalytic activity of metal nanoparticles fabricated has been confirmed.     

Solvent effect on the synthesis of monodisperse amine-capped Au nanoparticles

A remarkable solvent effect in a single-phase synthesis of monodisperse amine-capped Au nanoparticles is demonstrated.Oleylamine-capped Au nanoparticles were prepared via the reduction of HAuCU by an amine-borane complex in the presence of oleylamine in an organic solvent.When linear or planar hydrocarbon(e.g.,n-hexane,n-octane,1-octadecylene,benzene,and toluene) was used as the solvent, high-quality monodisperse Au nanoparticles with tunable sizes were obtained.However,Au nanoparticles with poor size dispersity were obtained when tetralin,chloroform or cyclohexane was used as the solvent.The revealed solvent effect allows the controlled synthesis of monodisperse Au nanoparticles with tunable size of 3-10 nm.     

液相脉冲激光烧蚀法制备高熔点的纳米金属粒子

采用液相脉冲激光烧蚀法成功地制备了高熔点的金属Pt、Ru与Ag纳米粒子. 采用SEM、TEM、ED和紫外-可见吸收光谱表征了纳米粒子的特征. 纳米粒子的粒径基本在数个到数十个纳米的大小范围内. 发现含适量PVP(poly(vinylpyrrolidone))的水溶液有利于纳米粒子的制备, 而且还能够提高纳米粒子悬浮液的稳定性. 该制备方法较简单, 在制备高熔点的纳米金属粒子方面有着其它方法所不能比拟的优势.     

Amperometric biosensors based on deposition of gold and platinum nanoparticles on polyvinylferrocene modified electrode for xanthine detection

In this study, new xanthine biosensors, XO/Au/PVF/Pt and XO/Pt/PVF/Pt, based on electroless deposition of gold(Au) and platinum(Pt) nanoparticles on polyvinylferrocene(PVF) coated Pt electrode for detection of xanthine were presented. The amperometric responses of the enzyme electrodes were measured at the constant potential, which was due to the electrooxidation of enzymatically produced H₂O₂. Compared with XO/PVF/Pt electrode, XO/Au/PVF/Pt and XO/Pt/PVF/Pt exhibited excellent electrocatalytic activity towards the oxidation of the analyte. Effect of Au and Pt nanoparticles was investigated by monitoring the response currents at the different deposition times and the different concentrations of KAuCl₄ and PtBr₂. Under the optimal conditions, the calibration curves of XO/Au/PVF/Pt and XO/Pt/PVF/Pt were obtained over the range of 2.5 × 10^?3 to 0.56 mM and 2.0 × 10^?3 to 0.66 mM, respectively. The detection limits were 7.5 × 10^?4 mM for XO/Au/PVF/Pt and 6.0 × 10^?4 mM for XO/Pt/PVF/Pt. The effects of interferents, the operational and the storage stabilities of the biosensors and the applicabilities of the proposed biosensors to the drug samples analysis were also evaluated.     

A comparative study of morphology, reactivity and stability of synthesized silver nanoparticles using Bacillus subtilis and Catharanthus roseus (L.) G. Don

Large number of papers has been published recently on the eleventh group metallic elements such as Ag, Au and Cu. Our study was focused on biosynthesis of silver nanoparticles, their morphology, reactivity and stability. We were interested to check these properties in two different samples, S1 and S2, respectively. The biosynthesis of silver nanoparticles was achieved by reacting the samples with 1 mM concentration of silver nitrate, one involves bacteria (S1) and the other involves the plant extract (S2). Spectrophotometric analysis revealed that the particles exhibited two peaks, one at 440 nm (for S1) and the other at 390 nm (for S2). It is well known that longer wavelength corresponds to increase in particle size. Since, S1 has got a longer wavelength; it is not known, whether it forms isolated particles or agglomerates? Morphological characterization has been done by adopting the procedures of Negative staining and Wedge smear preparation methods. This hybrid method may be of interest to study agglomerated particles. Microscopic examination of the smear S1 shows predominantly triangular or hexagonal shaped agglomerated particles which were not observed in S2. Hence further characterization was done using SEM, EDAX and XRD. The S2 particles were in the range of 45–70 nm and were stable for even four months. This study indicated that particle size can be controlled from micrometer to nanometer size by varying biological reductants.     

Electrochemistry of conductive polymers 39. Contacts between conducting polymers and noble metal nanoparticles studied by current-sensing atomic force microscopy

Electrical properties of contacts formed between conducting polymers and noble metal nanoparticles have been examined using current-sensing atomic force microscopy (CS-AFM). Contacts formed between electrochemically prepared pi-conjugated polymer films such as polypyrrole (PPy), poly(3-methylthiophene) (P3MeT), as well as poly(3,4-ethylenedioxythiophene) (PEDOT) and noble metal nanoparticles including platinum (Pt), gold (Au), and silver (Ag) have been examined. The Pt nanoparticles were electrochemically deposited on a pre-coated PPy film surface by reducing a platinum precursor (PtCl62-) at a constant potential. Both current and scanning electron microscopic images of the film showed the presence of Pt islands. The Au and Ag nanoparticles were dispersed on the P3MeT and PEDOT film surfaces simply by dipping the polymer films into colloid solutions containing Au or Ag particles for specified periods (5 to approximately 10 min). The deposition of Au or Ag particles resulted from either their physical adsorption or chemical bonding between particles and the polymer surface depending on the polymer. When compared with PPy, P3MeT and PEDOT showed a stronger binding to Au or Ag nanoparticles when dipped in their colloidal solutions for the same period. This indicates that Au and Ag particles are predominantly linked with the sulfur atoms via chemical bonding. Of the two, PEDOT was more conductive at the sites where the particles are connected to the polymer. It appears that PEDOT has better aligned sulfur atoms on the surface and is strongly bonded to Au and Ag nanoparticles due to their strong affinity to gold and silver. The current-voltage curves obtained at the metal islands demonstrate that the contacts between these metal islands and polymers are ohmic.     

Green synthesis of plant-stabilized Au nanoparticles for the treatment of gastric carcinoma

In this study, gold nanoparticles (AuNPs) were green synthesized using plant extract. The obtained nanoparticles (Au NPs) were characterized by advanced physical and chemical techniques like TEM, FTIR, UV–vis, SEM, XRD and EDX. SEM image displayed the quasi-spherical shaped nanoparticles of mean diameter 20–50 nm. All the particles were of uniform shape and texture. From the XRD pattern, four distinct diffraction peaks at 38.2°, 44.2°, 64.7° and 77.4° are indexed as (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of fcc metallic gold. The in vitro cytotoxic and anti-gastric carcinoma effects of biologically synthesized Au NPs against cancer cell lines were assessed. The IC₅₀ of the Au NPs were 192, 149, 76 and 85 µg/mL against NCI-N87, MKN45, GC1401 and GC1436 gastric cancer cell lines. The anti-gastric carcinoma properties of the Au NPs could significantly remove the cancer cell lines in a time and concentration-dependent manner. So, the findings of the recent research show that biologically synthesized Au NPs might be used to cure cancer.     

Stability of Ag@SiO_2 core–shell particles in conditions of photocatalytic overall water-splitting

Core–shell nanoparticles containing plasmonic metals(Ag or Au) have been frequently reported to enhance performance of photo-electrochemical(PEC) devices. However, the stability of these particles in water-splitting conditions is usually not addressed. In this study we demonstrate that Ag@SiO_2 core–shell particles are instable in the acidic conditions in which WO_3-based PEC cells typically operate, Ag in the core being prone to oxidation, even if the SiO_2 shell has a thickness in the order of 10 nm. This is evident from in situ voltammetry studies of several anode composites. Similar to the results of the PEC experiments, the Ag@SiO_2 core–shell particles are instable in slurry-based, Pt/ZnO induced photocatalytic water-splitting. This was evidenced by in situ photodeposition of Ag nanoparticles on the Pt-loaded ZnO catalyst, observed in TEM micrographs obtained after reaction. We explain the instability of Ag@SiO_2 by OH-radical induced oxidation of Ag, yielding dissolved Ag+. Our results imply that a decrease in shell permeability for OH-radicals is necessary to obtain stable, Ag-based plasmonic entities in photo-electrochemical and photocatalytic water splitting.     

Graphene supported electrocatalysts for methanol oxidation

Graphene nanosheet was prepared by modified Hummer’s chemical method and utilized as a catalyst support of PtRu nanoparticles for the electro-oxidation of methanol. Home-made graphene nanosheet was clearly characterized by Raman spectroscopy and we applied colloidal method to synthesize with high metal content of 80 wt.% Pt–Ru catalyst, which is extensively clarified by HR-TEM and XRD analysis. 80 wt.% Pt–Ru/graphene nanosheet catalyst showed superior electrochemical activity toward methanol oxidation compared to Pt–Ru/Vulcan XC-72R. It is due to the significant increase of electrochemical active surface area for better catalyst utilization.