Seamus A. Curran

Structural and Spectral Features of Selenium Nanospheres Produced by Se-Respiring Bacteria

ABSTRACT Certain anaerobic bacteria respire toxic selenium oxyanions and in doing so produce extracellular accumulations of elemental selenium [Se(0)]. We examined three physiologically and phylogenetically diverse species of selenate- and selenite-respiring bacteria, Sulfurospirillum barnesii, Bacillus selenitireducens, and Selenihalanaerobacter shriftii, for the occurrence of this phenomenon. When grown with selenium oxyanions as the electron acceptor, all of these organisms formed extracellular granules consisting of stable, uniform nanospheres (diameter, ∼300 nm) of Se(0) having monoclinic crystalline structures. Intracellular packets of Se(0) were also noted. The number of intracellular Se(0) packets could be reduced by first growing cells with nitrate as the electron acceptor and then adding selenite ions to washed suspensions of the nitrate-grown cells. This resulted in the formation of primarily extracellular Se nanospheres. After harvesting and cleansing of cellular debris, we observed large differences in the optical properties (UV-visible absorption and Raman spectra) of purified extracellular nanospheres produced in this manner by the three different bacterial species. The spectral properties in turn differed substantially from those of amorphous Se(0) formed by chemical oxidation of H2Se and of black, vitreous Se(0) formed chemically by reduction of selenite with ascorbate. The microbial synthesis of Se(0) nanospheres results in unique, complex, compacted nanostructural arrangements of Se atoms. These arrangements probably reflect a diversity of enzymes involved in the dissimilatory reduction that are subtly different in different microbes. Remarkably, these conditions cannot be achieved by current methods of chemical synthesis.

Resonance Raman and infrared spectroscopy of carbon nanotubes

We present a comparative analysis of the vibrational and structural properties of carbon nanotubes. The first-order Raman spectrum exhibits two lines at 1582 cm−1 and at 1350 cm−1. The observed ratio of the integrated intensity of these lines was found to be different as compared to polycrystalline graphite. The position and intensity of the line around 1350 cm−1 strongly depend on the energy of the exciting laser line. This dispersion effect is again different from the dispersion in nanocrystalline graphite. It is discussed in terms of a photoselective resonance process. Transmission infrared spectra of the nanotubes show one broad and asymmetric line at 1575 cm−1 and a weaker line at 868 cm−1.

Formation of tellurium nanocrystals during anaerobic growth of bacteria that use Te oxyanions as respiratory electron acceptors

ABSTRACT Certain toxic elements support the metabolism of diverse prokaryotes by serving as respiratory electron acceptors for growth. Here, we demonstrate that two anaerobes previously shown to be capable of respiring oxyanions of selenium also achieve growth by reduction of either tellurate [Te(VI)] or tellurite [Te(IV)] to elemental tellurium [Te(0)]. This reduction achieves a sizeable stable-Te-isotopic fractionation (isotopic enrichment factor [ε] = −0.4 to −1.0 per ml per atomic mass unit) and results in the formation of unique crystalline Te(0) nanoarchitectures as end products. The Te(0) crystals occur internally within but mainly externally from the cells, and each microorganism forms a distinctly different structure. Those formed by Bacillus selenitireducens initially are nanorods (∼10-nm diameter by 200-nm length), which cluster together, forming larger (∼1,000-nm) rosettes composed of numerous individual shards (∼100-nm width by 1,000-nm length). In contrast, Sulfurospirillum barnesii forms extremely small, irregularly shaped nanospheres (diameter < 50 nm) that coalesce into larger composite aggregates. Energy-dispersive X-ray spectroscopy and selected area electron diffraction indicate that both biominerals are composed entirely of Te and are crystalline, while Raman spectroscopy confirms that they are in the elemental state. These Te biominerals have specific spectral signatures (UV-visible light, Raman) that also provide clues to their internal structures. The use of microorganisms to generate Te nanomaterials may be an alternative for bench-scale syntheses. Additionally, they may also generate products with unique properties unattainable by conventional physical/chemical methods.

Evolution and evaluation of the polymer/nanotube composite

Abstract Composite structures, using MWNT and SWNT and the polymer (PmPV) exhibit properties which enhance those of the individual components. The polymer PmPV can act as an organic filter for the multiwalled system where the MWNT are indefinitely suspended in the polymer solution while the carbonaceous material falls out of solution. Raman measurements of this show a complete reduction of the amorphous line at 1350 cm-1. We see that we can alter the luminescence quantum yield of the composite, where the effects are different depending on which nanotubes are used. When we examine the SWNT/PmPV the quantum yield is increased. The MWNT composite also shows strong non-linear optical signal. The pristine polymer has an χ (3) of 10 −11 esu whereas the composite χ (3) is -10 −10 esu.

Physical doping of a conjugated polymer with carbon nanotubes

A semi-conjugated, organic polymer was mixed with carbon nanotubes to form a wholly organic composite. Composite formation from low to high nanotube concentration increases the conductivity dramatically by ten orders of magnitude, indicative of percolative behaviour. Effective mobilitys and carrier densities were calculated from the space charge regions of the current voltage characteristics for the 0% to 8% mass fractions.

Characterization of multiwalled carbon nanotubes-PMMA composites

We report, for the first time, preliminary results obtained from analysis of poly(methyl methacrylate) -multiwalled carbon nanotubes composites thin films. These films were prepared by mixing the polymer with different nanotube concentrations and were deposited by spin coating on glass substrates. The composites were characterized by Raman spectroscopy and scanning electron microscopy. The evolution of the conductivity versus the nanotube concentration was carried out in the order to determine the transport process in these materials. Such a composite is promising for use as transporting layer in multilayer diodes.

Electrical transport measurements of highly conductive carbon nanotube/poly(bisphenol A carbonate) composite

Acid-treated and pristine chemical vapor deposition grown multiwalled carbon nanotube (MWNT) and poly(bisphenol A carbonate) (PC) composites were prepared through a simple solution blending with varied nanotube weight fractions. The electrical conductivities of the composites can be described by the scaling law based on percolation theory with unprecedented high saturated ac conductivity of pristine nanotubes (σsat=1598.4 S cm−1, pc=0.19 wt %) and acid-treated nanotubes (σsat=435.4 S cm−1, pc=0.3 wt %), which correlates well with the dc behavior. We attribute the high saturated conductivities to managing the dispersions, rather than looking to have a well dispersed three-dimensional network thin film. The comparison was made between acid-treated nanotubes and pristine nanotube, both dispersed in PC at various loadings. It was found that the pristine nanotubes in PC possessed an even higher conductivity than the more evenly dispersed composites consisting of lightly acid-treated MWNT in PC.

Molecular rectifiers and transistors based on π-conjugated materials

Abstract Concepts and experimental results related to molecular rectifiers and transistors are reviewed. ‘Devices’ based on donor-δ bridge-acceptor molecules, conjugated zwitterionic molecules, macrocyclic molecules, metal clusters, fullerenes and carbon nanotubes are discussed. Current-voltage characteristics showing rectifier properties are presented, as well as experiments in which the current-voltage characteristics are modulated by a gate voltage (transistor effect).

Optical absorption and fluorescence of a multi-walled nanotube-polymer composite

We report on the optical studies of an organic composite containing an unusual phenylene vinylene copolymer and multi-walled carbon nanotubes in solution. The nanotubes appear to be held in the polymer matrix through a weak interaction between the backbone of the polymer and the lattice of the nanotube. This incorporation greatly effects the absorption and emission properties of the composite. We have also shown that it is possible to control the quantum efficiency of the system by varying the mass fraction of the nanotubes present.

Functionalization of carbon nanotubes using phenosafranin

Spectroscopic analysis and atomic force microscopy (AFM) phase imaging studies show self-assembly of phenosafranin (PSF) to multiwalled carbon nanotubes (MWNTs). The shift in absorption spectra is associated with charge transfer of valence electrons from PSF to electron accepting sites on the MWNTs. The Raman-active disorder modes are used to fingerprint PSF attachment to MWNTs via defect states. AFM phase imaging was used to obtain a molecular topographic visual confirmation of PSF attached to the MWNT.