A. Murugadoss

The antibacterial properties of a novel chitosan–Ag-nanoparticle composite

Escherichia coli expressing recombinant green fluorescent protein was used to test the bactericidal efficacy of a newly synthesized chitosan-Ag-nanoparticle composite. The composite was found to have significantly higher antimicrobial activity than its components at their respective concentrations. The one-pot synthesis method led to the formation of small Ag nanoparticles attached to the polymer which can be dispersed in media of pH< or =6.3. The presence of a small percentage (2.15%, w/w) of metal nanoparticles in the composite was enough to significantly enhance inactivation of E. coli as compared with unaltered chitosan. Fluorescence spectroscopy indicated that bacterial growth stopped immediately after exposure of E. coli to the composite, with release of cellular green fluorescent protein into the medium at a faster rate than with chitosan. Fluorescence confocal laser scanning and scanning electron microscopy showed attachment of the bacteria to the composite and their subsequent fragmentation. Native protein gel electrophoresis experiments indicated no effect of the composite on bacterial proteins.

Magnetically Recoverable Magnetite/Gold Catalyst Stabilized by Poly(N-vinyl-2-pyrrolidone) for Aerobic Oxidation of Alcohols

Fe3O4:PVP/Au nanocomposite synthesized via a two-step procedure was tested as a quasi-homogenous alcohol oxidation catalyst. It was found that the nanocomposite was able to carry out aerobic oxidation of alcohols in water at room temperature. Studies show rapid magnetic recoverability and reusability characteristics.

Gold/Palladium Bimetallic Alloy Nanoclusters Stabilized by Chitosan as Highly Efficient and Selective Catalysts for Homocoupling of Arylboronic Acid

Aerobic oxidative homocoupling of arylboronic acid under acidic aqueous conditions (pH 4.0) using bimetallic Au/Pd alloy nanoclusters stabilized by chitosan has been investigated. It was found that a Au0.81Pd0.19 catalyst (3.1 ± 0.8 nm) exhibited superior catalytic activities as compared to monometallic Au (2.3 ± 0.3 nm) and other series of bimetallic nanoclusters, giving the corresponding biaryls in nearly quantitative yield.

Tuning the Optical Characteristics of Poly(p-phenylenevinylene) by in Situ Au Nanoparticle Generation

A composite of 1,4-dihexyloxy-poly(p-phenylenevinylene) (DHPPV) and gold nanoparticles (AuNPs) was prepared by an efficient and simple in situ method. The formation of DHPPV-AuNP composites was confirmed through high-resolution transmission electron microscopy (TEM); UV/vis absorption spectra showed a large blue shift in the polymer absorption peak, which was greater than 60 nm in solution, accompanied by a remarkable change in the intensity, whereas the photoluminescence (PL) spectra in the solution showed only a marginal decrease in intensity in the presence of AuNPs. Solid-state UV/vis spectra of DHPPV-AuNP also showed a decrease in the intensity of the shoulder peak in the region of 400-450 nm. Interesting features were observed in the solid-state PL spectra, where the efficient energy transfer from AuNP to DHPPV results in the complete disappearance of the 585 nm peak with a dominant peak appearing at 635 nm. TEM analysis confirmed that AuNPs were embedded in the DHPPV matrix systematically, thus presenting a simple tool to assemble hybrid nanowires comprising π-conjugated organic/polymeric systems and inorganic nanoparticles with likely applications in nanosized optoelectronic devices. The optical properties of DHPPV-AuNP could be further tuned by treating the composite with octadecane thiol or sodium sulfide, resulting in a further blue shift of 65 nm.

Reduced Oxide Sites and Surface Corrugation Affecting the Reactivity, Thermal Stability, and Selectivity of Supported Au–Pd Bimetallic Clusters on SiO2/Si(100)

The morphology and surface elemental composition of Au-Pd bimetallic nanoclusters are reported to be sensitive to and affected by reduced silicon defect sites and structural corrugation on SiO2/Si(100), generated by argon ion sputtering under ultrahigh vacuum (UHV) conditions. Metastable structures of the bimetallic clusters, where Au atoms are depleted from the top surface upon annealing, are stabilized by the interaction with the reduced silica sites, as indicated from CO temperature programmed desorption (TPD) titration measurements. Acetylene conversion to ethylene and benzene has been studied as a probe reaction, revealing the modification of selectivity and reactivity enhancement in addition to improved thermal stability on substrates rich in reduced-silica sites. These observations suggest that these unique sites play an important role in anchoring thermodynamically metastable conformations of supported Au-Pd bimetallic catalysts and dictate their high-temperature activity.

Structure and Composition of Au–Cu and Pd–Cu Bimetallic Catalysts Affecting Acetylene Reactivity

Au–Cu and Pd–Cu bimetallic model catalysts were prepared on native SiO2/Si(100) substrate under ultra high vacuum (UHV) by employing buffer layer assisted growth procedure with amorphous solid water as the buffer material. The effect of the bimetallic nanoclusters (NCs) surface composition and morphology on their chemical reactivity has been studied with acetylene decomposition and conversion to ethylene and benzene as the chemical probe. It was found that among the Au–Cu NCs compositions, Au0.5Cu3 NCs revealed outstanding catalytic selectivity towards ethylene formation. These NCs were further characterized by employing TEM, XPS and HAADF-STEM coupled EDX analysis. With CO molecule as a probe, CO temperature programmed desorption has been used to investigate the distribution of gold on the top-most surface of the supported clusters. Surface segregation at high relative elemental fraction of gold leads to a decreased activity of the Au–Cu NCs towards ethylene formation. In contrast to the Au–Cu NCs, the Pd–Cu bimetallic system reveals reduced sensitivity to the relative elemental composition with respect to selectivity of the acetylene transformation toward ethylene formation. On the other hand, remarkable activity towards benzene formation has been observed at elemental composition of Cu3Pd, at comparable rates to those for ethylene formation on clean Pd NCs.

Acetanilide mediated reversible assembly and disassembly of Au nanoparticles

Herein we report the generation of Au nanoparticles (NPs) by sparingly soluble acetanilide in water. We also report the formation of linear chain-like superstructures of self-assembled Au NPs, in the presence of excess acetanilide. This was achieved in two different ways. In the first method, acetanilide was added, with increasing concentration, into aqueous HAuCl(4) to produce Au NPs as well as for the formation of assembly, which varied according to the concentration of acetanilide. The other route involved formation of spherical Au NPs at the lowest concentration of acetanilide, which was followed by the formation of assembly of various lengths upon further addition of variable amount of acetanilide. The assemblies were stable in aqueous solution for days with characteristic UV-vis absorption spectra consisting of two peaks. While the wavelength of the first peak remained the same, the position of the second peak changed to longer wavelength with increasing acetanilide concentration. Interestingly, the linear chain-like arrays could be broken into individual particles by first dilution of the solution concentration followed by treatment with ultrasonic waves. The individual Au NPs again formed linear chain-like arrays upon addition of excess acetanilide.

High yield synthesis of branched gold nanoparticles as excellent catalysts for the reduction of nitroarenes

A one-pot preparation of branched gold nanoparticles (NPs) in aqueous solution using acetanilide as both a reducing and shape-directing agent has been achieved. The structure, morphology and optical properties of the branched gold NPs can be tuned by controlling the concentration of acetanilide. In contrast to reported protocols, high quality and stable branched gold NPs can be produced exclusively without any sophisticated purification techniques. More interestingly, the gold NPs thus obtained are predominantly enriched with a uniform facet of {111}, as confirmed by HRTEM and XRD methods. When compared to reported gold NPs, the branched gold NP catalysts exhibited two to three orders of magnitude higher activity for the reduction of 4-nitrophenol into 4-aminophenol. Furthermore, the average turnover frequency of the branched gold NPs was found to be 19 molecules per min, which is significantly higher than that of spherical gold NPs and other reported branched gold NP catalyst systems.