Effect of concentration of methanol for the control of particle size and size-dependent SERS studies.

In this paper the effect of concentration of cosolvent (methanol) for the formulation of particles size has been discussed briefly. The binary solvent system has been used which is prepared by simple mixing of two solvents. The morphology of the particles was controlled by varying the amount of cosolvent, keeping the concentration of the stabilizer and reducing agent constant. The polarity of the solvent, transport of the Au(III) ions, and coordinating ability of the solvent play vital roles for nucleus formation and the growth process, which subsequently form different size particles. The particles formed in methanol at lower composition are angular. At higher concentrations of methanol they are spherical. In all the cases the particle size increases as the concentration of the cosolvent decreases. In the methanol system, particles form multiple twined structures and the twining of the particles increase with the decrease of methanol content. The particles have been characterized by XRD analysis, UV-visible spectroscopy, and transmission electron microscopic (TEM) studies. The variable size of the particles, obtained in a water methanol system, was employed for SERS measurement. A Raman probe, Rhodamine 6G (Rh 6G), has been found to be suited for the surface modification of the gold particles and it has also been demonstrated that the larger particles show better SERS signal than the corresponding smaller ones.

Biomolecule induced nanoparticle aggregation: effect of particle size on interparticle coupling.

Gold nanoparticles of variable sizes have been prepared by reducing HAuCl(4) with trisodium citrate by Frens' method. It has been found that the gold particles under consideration produce well-ordered aggregates upon interaction with a biomolecule, glutathione in variable acidic pH condition and exhibit pronounced changes in their optical properties arising due to electromagnetic interaction in the close-packed assembly. The effect of nanoparticle size on the nature of aggregation as well as the variation in the optical response due to variable degree of interparticle coupling effects amongst the gold particles have been investigated. The optical properties of the gold aggregates have been accounted in the light of Maxwell-Garnett effective medium theory considering the changes in the filling factor in different aggregates produced by variable sizes of gold colloids. The aggregates have been characterized by UV-vis spectroscopy, FTIR, Raman, XRD and TEM studies. It has been observed that a new peak appearing at a longer wavelength intensifies and shifts further to the red from the original peak position depends on the particle size, concentration of glutathione and pH of the solution. On the basis of the first appearance of a clearly defined new peak at longer wavelength, a higher sensitivity of glutathione detection has been achieved with gold nanoparticles of larger dimension.

Light-induced hydrolysis of nitriles by photoproduced alpha-MnO2 nanorods on polystyrene beads.

A green chemistry approach has been furnished for photochemical deposition of alpha-MnO2 nanorods onto the surface of functionalized polystyrene beads through immobilization of MnO4- in alkaline condition under visible light. Then the composite material was exploited as a fruitful and novel solid-phase catalyst for the one-step and facile synthesis of amide compounds from nitriles under visible light in weakly basic medium.

Solvent effect on the electronic spectra of azine dyes under alkaline condition.

Thiazine dye, methylene blue (MB), oxazine dye, nile blue (NB), and phenazine-based dye, neutral red (NR), bear a similar basic dye skeleton with a distinctively different central heteroatom. All of them are extracted into nonpolar organic solvent from alkaline solution. The role of the heteroatom on the respective dye skeletons and redox potentials of the dyes has been examined to signature the stability of the species in organic solvent and the results have been substantiated through geometry optimization and wave function analysis at the density functional theory level. The effect of solvent polarity on the electronic absorption spectra of the three nonionic benzenoid species has been investigated with an intention to investigate the solvatochromic behavior of these compounds.

Anisotropic growth of gold clusters to gold nanocubes under UV irradiation.

A chiral reagent, 2-naphthol, has been introduced under alkaline solution as a reductant for HAuCl(4) in CTAB micelle to produce exclusively cubic gold nanoparticles under UV photoactivation. Prolonged irradiation helped the digestion of the primarily evolved spherical particles into smaller gold nanocubes, which then act as tiny cubic seeds, leading to the formation of larger nanocubes. The smaller cubes take the assistance of CTAB under alkaline condition to serve as the seed in directing the transformation of all the spherical colloids into cubic shapes under continuous irradiation via Ostwald ripening. The shape transformation of the nanoparticles has been monitored by repetitive TEM imaging and absorption spectral analysis. The FTIR analysis proves that the gold nanocubes are capped by CTAB. The XRD pattern authenticates the formation of the fcc gold nanocubes. GCMS studies in turn confirmed the presence of hydroxylation of 2-naphthol in the course of the reaction, leaving exclusively cubic gold nanoparticles at the final stage of the photoactivation reaction.

Room temperature synthesis of coinage metal (Ag, Cu) chalcogenides.

Coinage metal chalcogenides in their nanoregime have been synthesized in aqueous medium at room temperature by mixing the nanoparticles of silver or copper with selenium nanoparticles which are authenticated by UV-vis, XRD and TEM analyses.

Self-assembly of silver nanoparticles: synthesis, stabilization, optical properties, and application in surface-enhanced Raman scattering.

Silver nanoparticle aggregates were synthesized in large scale using resorcinol under alkaline condition to obtain an assembly of silver clusters. Stable dispersion of the cluster in aqueous medium has been examined out of resorcinol-capped silver nanoparticle assemblies. The UV-vis spectroscopy during the particle evolution has been studied in detail. From the high-resolution TEM (HRTEM) image and XRD pattern it was confirmed that the particles are made of pure silver only. The capping action of resorcinol has been authenticated from the FTIR spectra. UV-vis spectroscopy and TEM images reveal that the temperature, effect of vibrational energy, heat shock, and time-dependent particle evolution have unique bearing on the stability and surface properties of the clusters. The concentrations of silver nitrate, resorcinol, and NaOH have important influence on the particle evolution and its size. TEM images incite us to examine the aggregates to capitulate surface-enhanced Raman scattering (SERS) to the single molecular level using crystal violet (CV) and cresyl fast violet (CFV) as molecular probes. The SERS intensity of CV increases with increasing the size of the silver aggregate.

Exploitation of electrostatic field force for immobilization and catalytic reduction of o-nitrobenzoic acid to anthranilic acid on resin-bound silver nanocomposites.

A new solid-phase catalyst has been designed and reported here for the catalytic reduction of o-nitrobenzoic acid to anthranilic acid. Electrostatic field force helps immobilization, in turn deposition of silver nanoparticles onto solid resin surfaces and reduction of o-nitrobenzoic acid through effective catalysis. While characterization of catalyst particles has been performed by different physical methods (XRD, XPS, SEM, TEM, and EDX) in a worthwhile fashion, selective reduction of o-nitrobenzoic acid has also been achieved conveniently (approximately 95%). Different thermodynamic parameters for the reduction reaction have been presented from varied experimental conditions. Novelty of this work lies with the catalytic efficiency of nanometer size silver particles immobilized solid-phase matrix for one step synthesis of anthranilic acid over bulk silver.

Reduction of methylene blue by thiocyanate: kinetic and thermodynamic aspects.

This article reports the reduction of methylene blue (MB) by thiocyanate ions (SCN(-)) in aqueous and micellar solutions. Thiocyanate ions are found to be an effective reducing agent for the decolorization of methylene blue under ambient condition. Effects of salting-in and salting-out agents have been investigated for real-time application in the reduction process. The salting-in agent urea has been found to uniquely enhance the rate of the reduction of MB by thiocyanate ion in the presence of micelles. Again, the catalytic activity of nanoparticles in the reduction of MB has also been studied. Detailed kinetic and thermodynamic aspects have been considered to realize the interaction between methylene blue and thiocyanate. Kinetic studies revealed that the reaction is reversible and follows first-order reaction kinetics.

Layer-by-layer deposition of bimetallic nanoshells on functionalized polystyrene beads.

Bimetallic nanoshells on functionalized polystyrene beads have been fabricated through a layer-by-layer deposition technique exploiting electrostatic interaction. The synthesis has been achieved through the immobilization and successive reduction of the corresponding precursor ions. It has been shown that the thickness of the shell can be controlled by a number of cyclic depositions of respective metals onto the surface of the polystyrene beads.

Synthesis and characterization of N,N-dimethyldodecylamine-capped Aucore-Pdshell nanoparticles in toluene.

In this paper a convenient route for synthesizing Au(core)-Pd(shell) bimetallic nanoparticles in toluene has been reported as a result of co-reduction of gold(III) and palladium(II) precursors in toluene. N,N-Dimethyldodecylamine was used as a capping agent for the core-shell particles, which not only imparts stability to the organosol but also controls morphology of the evolved particles. The particles were characterized using UV-visible, transmission electron microscopy, and X-ray diffraction measurements. All results substantiate the formation of core-shell structure of the synthesized particles.

Size-selective synthesis and stabilization of gold organosol in C(n)TAC: enhanced molecular fluorescence from gold-bound fluorophores.

Gold nanoparticles of variable sizes have been synthesized in toluene employing two-phase (water-toluene) extraction of AuCl4- followed by its reduction with sodium borohydride in the presence of a series of cationic surfactants of a homologous series having the general formula C(n)TAC. The solubility features of the gold particles in the organic solvent have been accounted qualitatively by calculating the van der Waals interaction potential between the particles. The effect of thermal energy and medium dielectric constant on the stability of metal particles has been studied by measuring the surface plasmon resonance. The stabilization of surfactant-mediated gold particles as hydrosol or organosol has been elucidated by considering the double-layer interaction as a function of the dielectric constant of the solvent medium. The influence of the counterion of the phase transfer reagent and stabilizing ligand on the photochemical stability of the gold colloids has been investigated. The fluorescence probe 1-methylaminopyrene (MAP) was considered for the surface functionalization of the gold particles, and it has been found that there is an enhancement of molecular fluorescence from the gold-probe assembly.

Synthesis, characterization, and optical properties of AuSe nanoalloys.

A solution phase approach to synthesize a new metal-semiconductor nanocomposite, AuSe nanoalloy has been reported. The synthesis has been achieved through UV-photoactivation of preformed Au and Se nanoparticles in micelle. Non-ionic surfactant Triton X-100 was exploited as a micellar medium for effective fusion of gold and selenium particles under UV. Both physical and chemical studies have been performed to characterize the composition and morphology of the particles. UV-visible, TEM, SEM, XPS and AFM analyses were done for characterization purpose. The optical properties of nanocomposites have been substantiated through their interaction with a fluorescent probe, eosin in aqueous solution. The spectroscopic investigation of dye-metal-semiconductor assembly has been examined critically. It has been found that the dye experiences J and H types of aggregation on the surfaces of gold and selenium nanoparticles respectively. Again, the composition dependent change of the emission profile of the probe on different nanocomposite surfaces has been rationalized in accordance with the molecular dimerization of the dye.

Immobilization and recovery of au nanoparticles from anion exchange resin: resin-bound nanoparticle matrix as a catalyst for the reduction of 4-nitrophenol.

The immobilization of gold nanoparticles in anion exchange resin and their quantitative retrieval by means of a cationic surfactant, cetylpyridinium chloride, is studied. The resin-bound gold nanoparticles (R-Au) have been used successfully as a solid-phase catalyst for the reduction of 4-nitrophenol by sodium borohydride. At the end of the reaction, the solid matrix remains activated and separated from the product. The recycling of catalyst particles after the quantitative reduction of 4-nitrophenol and the recovery of gold nanoparticles with unaffected particle morphology from the resin-bound gold nanoparticle entity have been reported.

A new route to obtain shape-controlled gold nanoparticles from Au(III)-beta-diketonates.

Ligands with a beta-diketone skeleton have been employed for the first time as reductant to produce ligand stabilized gold nanoparticles of different shapes from aqueous HAuCl(4) solution. Evolution of stable gold nanoparticles follows first order (k approximately equal to 10(-2) min(-1)) kinetics with respect to Au(0) concentration. Growth of particles of different shapes (spherical or triangular or hexagonal) goes hand in hand under the influence of different beta-diketones, which have excellent capping and reducing properties. Chlorine insertion was observed to take place in the beta-diketone skeleton.