Showing papers by "Indian Association for the Cultivation of Science published in 2007"

Synthesis and Catalytic Application of Nanostructured Silver Dendrites

Abstract: We demonstrate a simple templateless and surfactant-free wet-chemical method of preparing silver (Ag) nanostructures with different dendritic morphologies at room temperature. This has been accomplished by aging the aqueous mixture of AgNO3 and citrate salts, carrying different cations for different time periods. Transmission electron microscopic (TEM) and X-ray diffraction (XRD) studies confirmed the formation of single-crystalline dendritic Ag nanostructures. It has been found that the nature and the concentration of citrate salt have a significant effect on the morphology of the formed Ag nanostructures. A possible formation mechanism has also been discussed on the basis of monitoring the surface plasmon resonance properties and TEM images with time during the course of formation of silver dendrites. The formed dendritic silver nanostructures showed excellent catalytic activity in the borohydride reduction of p-nitrophenol to p-aminophenol compared to the spherical silver nanoparticles.

Tryptophan-based peptides to synthesize gold and silver nanoparticles: a mechanistic and kinetic study.

Abstract: Synthetic oligopeptides with a tryptophan residue at the C-terminus have been used for the synthesis of gold and silver nanoparticles at pH 11. The tryptophan residue in the peptides is responsible for the reduction of metal ions to the respective metals, possibly through electron transfer. A mechanistic pathway has been proposed to explain the reductive properties of the tryptophan moiety of the peptide based on some spectroscopic techniques, such as UV-visible and fluorescence spectroscopy. This study reveals that some of the peptide molecules are converted to its corresponding ditryptophan, kynurenine form and some cross-linked products, all of which are highly fluorescent species. The resultant peptide-functionalized metal nanoparticles have also been characterized by UV-visible spectroscopy, transmission electron microscopy, and Fourier transform IR spectroscopy and thermogravimatric analysis.

Light-controlled gene silencing in zebrafish embryos

Abstract: Functional genomic studies in zebrafish frequently use synthetic oligonucleotides called morpholinos that block RNA splicing or translation. However, the constitutive activity of these reagents limits their experimental utility. We report here the synthesis of a photoactivatable morpholino targeting the no tail (ntl) gene. This caged reagent permits spatiotemporal gene regulation in vivo and the photochemical generation of functionally mosaic organisms.

pH-Responsive, Bolaamphiphile-Based Smart Metallo-Hydrogels as Potential Dye-Adsorbing Agents, Water Purifier, and Vitamin B12 Carrier

Abstract: A protein amino acid, phenylalanine-based bolaamphiphile 1 containing a centrally located oligomethylene group has been found to be a hydrogelator at nearly physiological pH (pH 6.5−7.2) in the presence of divalent metal salts (such as MnCl2, CoCl2, CuSO4, and NiCl2) on sonication. Other structurally related synthetic bolaamphiphiles 2−4 do not form hydrogels under similar conditions. The metallo-hydrogels have been characterized using transmission electron microscopic (TEM) studies and FT-IR studies. TEM study of these metallo-hydrogels indicates a network of nanofibrilar morphology that might be responsible for gelation. The pH sensitivity of these metallo-hydrogels can be potentially utilized for entrapment and slow release of biologically important molecules. It has been demonstrated that one of these gels entraps and slowly releases vitamin B12 molecules depending upon the pH of the medium. Moreover, these metallo-hydrogels can efficiently adsorb various toxic dyes including Crystal Violet and Naphth...

A study on the adsorption mechanism of mercury on Aspergillus versicolor biomass.

Abstract: The adsorption behavior of mercury on Aspergillus versicolor biomass (AVB) has been investigated in aqueous solution to understand the physicochemical process involved and to explore the potentiality of AVB in pollution control management. This biomass has been successfully used for reducing the mercury concentration level in the effluent of chloralkali and battery industries to a permissible limit. The results establish that 75.6 mg of mercury is adsorbed per gram of biomass. The adsorption process is found to be a function of pH of the solution, with the optimum range being pH 5.0-6.0. The process obeys the Langmuir-Freundlich isotherm model. Scanning electron microscopic analysis demonstrates a conspicuous surface morphology change of the mercury-adsorbed biomass. A nearly uniform distribution of metal ions on the mycelial surface excepting a few aggregation points is revealed by X-ray elemental mapping profiles. The results of zeta potential measurement, Fourier transform infrared (FTIR) spectroscopy, and blocking of the functional groups by chemical modification reflect the binding of mercury on the biomass occurs through electrostatic and complexation reactions. The accumulation of mercury on the cell wall associated with negligible diffusion and or transportation into cytoplasm finds support from the results of adsorption kinetics and transmission electron micrographs. Mercury adsorption on biomass also leads to elongation of cells and cytoplasmic aggregation of spheroplast/protoplasts, indicating that the cell wall acts as a permeation barrier against this toxic metal.

An improved procedure for the three-component synthesis of highly substituted pyridines using ionic liquid.

Abstract: A basic ionic liquid, [bmIm]OH, efficiently promotes a one-pot, three-component condensation of aldehydes, malononitrile, and thiophenols to produce highly substituted pyridines in high yields at room temperature. This reaction does not involve any hazardous organic solvent and toxic catalyst. The ionic liquid is recovered and recycled for subsequent reactions.

Growth kinetics of ZnO nanocrystals: a few surprises.

Abstract: Using extensive state-of-the-art experiments over a wide range of synthesis parameters, such as the temperature and concentrations of different reactants, we establish qualitatively different growth kinetics for ZnO nanocrystals compared to all growth kinetics of semiconductor nanocrystals, including ZnO, discussed so far in the literature. The growth rate is shown to be strongly dependent on the concentration of (OH)- in an intriguing nonmonotonic manner as well as on temperature and is almost invariably much slower than well-known and generally accepted growth mechanisms based on a diffusion-controlled Ostwald ripening process or that expected in the surface reaction controlled regime. We show that these qualitatively different results arise from the unexpected role played by a part of the reactants by inhibiting rather than facilitating the reaction; we explain this extraordinary result in terms of an effective passivating layer around the growing nanocrystals formed by a virtual capping shell of Na+ ions.

Mixed micelle formation and solubilization behavior toward polycyclic aromatic hydrocarbons of binary and ternary cationic-nonionic surfactant mixtures.

Abstract: Water solubility enhancements of polycyclic aromatic hydrocarbons (PAHs), viz., naphthalene, anthracene and pyrene, by micellar solutions at 25 °C using two series of surfactants, each involving two cationic and one nonionic surfactant in their single as well as equimolar binary and ternary mixed states, were measured and compared. The first series was composed of three surfactants, benzylhexadecyldimethylammonium chloride (C16BzCl), hexadecyltrimethylammonium bromide (C16Br), and polyoxyethylene(20)mono-n-hexadecyl ether (Brij-58) with a 16-carbon (C16) hydrophobic chain; the second series consisted of dodecyltrimethylammonium bromide (C12Br), dodecylethyldimethylammonium bromide (C12EBr), and polyoxyethylene(4)mono-n-dodecyl ether (Brij-30) with a 12-carbon (C12) chain. Solubilization capacity has been quantified in terms of the molar solubilization ratio, the micelle−water partition coefficient, the first stepwise association constant between solubilizate monomer and vacant micelle, and the average num...

Piezoelectric β Polymorph in Poly(vinylidene fluoride)-Functionalized Multiwalled Carbon Nanotube Nanocomposite Films

Abstract: Poly(vinylidene fluoride) (PVF2)−multiwalled carbon nanotube (MWNT) nanocomposites (PCNCs) are prepared using ester (−COOC2H5)-functionalized MWNT (F-MWNT). Due to the specific interaction of the >CO group in F-MWNT and the >CF2 group of PVF2, the dispersion of F-MWNT in PVF2 matrix is uniform. Transmission electron microscopy study reveals that the F-MWNTs in the composite are fatter than the pure F-MWNTs indicating that the PVF2 chains are wrapped on the surface of MWNTs. In the nanocomposites, the spherulitic morphology of PVF2 is lost and the fibrils are curled and smaller in length than those of pure PVF2 sample. Field emission scanning electron microscopy study indicates good dispersion of carbon nanotubes in the F-MWNT samples. The solvent-cast films have the β-polymorphic (piezoelectric) structure for F-MWNT concentration ≥1% (w/w) and have a mixture of α and β polymorphs below that concentration of F-MWNT. In the melt-cooled specimen, there occurs a mixture of α and β polymorphs and the latter is...

The Raychaudhuri equations: A brief review

Abstract: We present a brief review on the Raychaudhuri equations. Beginning with a summary of the essential features of the original article by Raychaudhuri and subsequent work of numerous authors, we move on to a discussion of the equations in the context of alternate non-Riemannian spacetimes as well as other theories of gravity, with a special mention on the equations in spacetimes with torsion (Einstein-Cartan-Sciama-Kibble theory). Finally, we give an overview of some recent applications of these equations in general relativity, quantum field theory, string theory and the theory of relativisitic membranes. We conclude with a summary and provide our own perspectives on directions of future research.

Femtosecond solvation dynamics in a neat ionic liquid and ionic liquid microemulsion: excitation wavelength dependence.

Abstract: Solvation dynamics in a neat ionic liquid, 1-pentyl-3-methyl-imidazolium tetra-flouroborate ([pmim][BF4]) and its microemulsion in Triton X-100 (TX-100)/benzene is studied using femtosecond up-conversion. In both the neat ionic liquid and the microemulsion, the solvation dynamics is found to depend on excitation wavelength (λex). The λex dependence is attributed to structural heterogeneity in neat ionic liquid (IL) and in IL microemulsion. In neat IL, the heterogeneity arises from clustering of the pentyl groups which are surrounded by a network of cation and anions. Such a nanostructural organization is predicted in many recent simulations and observed recently in an X-ray diffraction study. In an IL microemulsion, the surfactant (TX-100) molecules aggregate in form of a nonpolar peripheral shell around the polar pool of IL. The micro-environment in such an assembly varies drastically over a short distance. The dynamic solvent shift (and average solvation time) in neat IL as well as in IL microemulsions ...

White Light from Mn2+-Doped CdS Nanocrystals: A New Approach

Abstract: We report the generation of white light from a simple transition-metal-doped semiconducting nanocrystal, namely, Mn2+-doped CdS, for the first time by suitably tuning the relative surface-state emissions of the nanocrystal host and the dopant emission. White light emitted by these nanocrystals remains unchanged both in solution form as well as in the solid state and can be excited by a wide range of UV lights without disturbing the chromaticity; this desirable property arises from the intrinsic separation of the absorption energy and the emission energies due to a large stokes shift, thereby avoiding the vexing problem of self-absorption altogether.

Microwave-assisted simple and efficient ligand free copper nanoparticle catalyzed aryl-sulfur bond formation

Abstract: A new protocol for the coupling of aryl iodides with thiophenols and alkanethiols catalyzed by copper nanoparticles under ligand-free condition has been developed A variety of functionalized aryl sulfides are prepared in excellent yields under microwave irradiation for 5-7 min A plausible radical mechanism has been suggested

In-situ synthesis of soluble poly (3-hexylthiophene)/multiwalled carbon nanotube composite: Morphology, structure, and conductivity

Abstract: Soluble poly(3-hexylthiophene) (P3HT)−multiwalled carbon nanotube (MWNT) nanocomposites (PCNCs) are prepared by the in-situ oxidative polymerization of 3-hexylthiophene (3HT) in a dispersion of MWNT in CHCl3. The MWNT−P3HT nanocomposite produces blackish-brown solution in chloroform, making it an easily processable system. With increase in MWNT concentration the molecular weight of P3HT has increased, but at higher MWNT concentration (8% w/w) the molecular weight has decreased. The head−tail (H−T) regioregularity of the samples remains the same with that of pure P3HT. The TEM pictures of PCNC-1 and PCNC-8 [the number indicates percentage (w/w) of MWNT with respect to monomer in the composite] show uniform wrapping of MWNT with P3HT. The increase of outer diameter in wrapped MWNT decreases with increase in MWNT concentration, but the PCNC-2.5 exhibits phase segregation by the formation of separate spheroid-like species on the partially wrapped MWNT surface. The higher molecular weight P3HT produced in the ...

A Study of Mn2+ Doping in CdS Nanocrystals

Abstract: We report here the synthesis of water soluble Mn2+-doped CdS nanocrystals in which it has been possible for the first time to obtain a distinct Mn2+ d-related emission well-separated from the defect state emissions. By varying the reaction temperature systematically, we establish 55 °C as the optimum temperature to maximize the Mn2+ d emission, the existence of this optimum synthesis temperature being shown as the result of two opposing influences of the temperature. Most interestingly, present results establish that Mn2+ favors preferential doping of larger-sized particles even within the narrow size distribution achieved in the present synthesis, rendering the relatively smaller-sized nanocrystals depleted of Mn2+ for any given synthesis. One important aspect of the present approach is that the synthesized nanocrystals readily dissolve in water without any deleterious effect on the Mn2+ d-related emission intensity.

Intergranular Magnetoresistance in Sr 2 FeMoO 6 from a Magnetic Tunnel Barrier Mechanism across Grain Boundaries

Abstract: We present magnetization (M) and magnetoresistance (MR) data for a series of ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$ samples with independent control on antisite defect and grain-boundary densities, which reveal several unexpected features, including a novel switching-like behavior of MR with M. These, in conjunction with model calculations, establish that the MR in ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$ is dominantly controlled by a new mechanism, derived from the magnetic polarization of grain-boundary regions acting like spin valves, leading to behavior qualitatively different from that usually encountered in tunneling MR. We show that a simple and useful experimental signature for the presence of this spin-valve-type MR (SVMR) is a wider hysteresis in MR compared to that in M.

One-Dimensional Assembly of Peptide-Functionalized Gold Nanoparticles: An Approach Toward Mercury Ion Sensing

Abstract: Carboxylated peptide-functionalized gold nanoparticles (peptide−GNPs) were self-assembled into networks of one-dimensional (1D) chains in the presence of mercury ion (Hg2+) at room temperature without use of any template. Transmission electron microscopy (TEM) confirmed the formation of a 1D array of gold nanoparticle (GNP) chains and revealed that the length of the chain can be tuned by varying the Hg2+ ion concentration in the medium. Dynamic light scattering (DLS) measurements showed that the assembly of peptide−GNPs actually occurred in the medium and not during TEM specimen preparation. The assembly of peptide−GNPs resulted in a change of color of the suspension from red to purple to blue. This color change is due to the development of a new surface plasmon resonance (SPR) band at 670 nm. A mechanistic pathway is suggested for this 1D assembly on the basis of some control experiments, and we believe that the main driving force for the 1D array of GNPs is dipole−dipole interactions. The change of colo...

Highly luminescent organic : Inorganic hybrid mesoporous silicas containing tunable chemosensor inside the pore wall

Abstract: New multifunctional luminescent hybrid mesoporous silicas (LHMS) containing a tunable chemosensor diimine moiety inside the pore wall have been synthesized. XRD and TEM image analyses indicate the formation of highly ordered 2D hexagonal mesophase from a 1:3 mixture of diimine organosilica and tetraethyl orthosilicate (TEOS) assisted by the supramolecular assembly of cetyltrimethylammonium bromide (CTAB); the use of diimine organosilica precursor alone leads to disordered mesophase. 13C CP MAS, 29Si MAS NMR, and UV−visible spectroscopic data show the incorporation of bridging organic diimine in the solid mesoporous materials. N2 sorption data suggest high BET surface areas together with type IV isotherms for the LHMS samples. These hybrid mesoporous materials show very strong affinity for metal cations (Fe3+, Zn2+, etc.), which could be utilized for possible application to metal ion chemosensors. All the LHMS samples exhibit very strong photoluminescence at room temperature and strong dependence of emissi...

pH-controlled reversible assembly of peptide-functionalized gold nanoparticles.

Abstract: The assembly/disassembly process of carboxylated peptide-functionalized gold nanoparticles (peptide-GNPs) was studied within the pH interval of 2.5 to 10. The assembly process was not well controlled at pH 2.5, leading to the formation of 3D structures of GNPs, whereas at pH 4 we observed controlled assembly with the formation of only a network of 1D chains. In the pH range of 2.5 to 4, the assembly proceeded with the formation of a combination of two extremes (i.e., having both 1D and 2D nanostructures). The assembly process was reversed on changing the pH of the medium to 10. The assembly/disassembly process was monitored using UV−vis spectroscopy and finally confirmed by TEM analysis. This assembly resulted from the intermolecular H-bonding between two carboxylic acid groups of peptides bound to the two adjacent GNPs and were confirmed by FTIR spectroscopy.

Selective fluorescence zinc ion sensing and binding behavior of 4-methyl-2,6-bis(((phenylmethyl)imino)methyl)phenol: biological application.

Abstract: Zinc ion fluorescence sensing and the binding properties of 4-methyl-2,6-bis(((phenylmethyl)imino)methyl)phenol (HL) have been investigated. It displays high selectivity for Zn2+ and can be used as zinc ion-selective luminescent probe for biological application under physiological conditions. The increase in emission in the presence of Zn2+ is accounted for by the formation of hexanuclear complex [Zn6(L)2(OH)2(CH3COO)8] characterized by X-ray crystallography. An approximately 6-fold Zn2+-selective chelation-enhanced fluorescence response in HEPES buffer (pH 7.4) is attributed due to the strong coordination of Zn(II) that would impose rigidity and hence decrease the nonradiative decay of the excited state. By incubation of cultured living cells (B16F10 mouse melanoma and A375 human melanoma) with HL, intracellular Zn2+ concentration could be monitored.

ZnO Doughnuts: Controlled Synthesis, Growth Mechanism, and Optical Properties

Abstract: ZnO doughnutlike cluster particles were prepared by a simple solvothermal route using an ethylene glycol (EG)−water solvent system. It was observed that ZnO doughnuts were obtained when an equal or higher volume fraction of EG was used in the solvent. Microstructural studies revealed that these doughnut-shaped particles were clusters of small hexagonal plates arranged in a regular fashion. Because of the capping property of EG, these constituent hexagonal plates arranged in an angular oriented attachment scheme with a view to minimize their surface energies. The resulting assemblies were concave from one side and convex from the reverse side. With the increasing percentage of EG, the concentration of the capping element increased, resulting in the reduction of the size of the constituent plates, which in turn helps the formation of more and more densely packed ZnO doughnuts. Increases in temperature and pressure also favor the formation of densely packed ZnO doughnuts. These ZnO samples exhibited a strong...

Growth mechanism of nanocrystals in solution: ZnO, a case study.

Abstract: We investigate the mechanism of growth of nanocrystals from solution using the case of ZnO. Spanning a wide range of values of the parameters, such as the temperature and the reactant concentration that control the growth, our results establish a qualitative departure from the widely accepted diffusion controlled coarsening (Ostwald ripening) process quantified in terms of the Lifshitz-Slyozov-Wagner theory. Further, we show that these experimental observations can be qualitatively and quantitatively understood within a growth mechanism that is intermediate between the two well-defined limits of diffusion control and kinetic control.