Localized surface plasmon resonance spectroscopy of single silver triangular nanoprisms

The plasmonic properties of single silver triangular nanoprisms are investigated using dark-field optical microscopy and spectroscopy. Two distinct localized surface plasmon resonances (LSPR) are observed. These are assigned as in-plane dipolar and quadrupolar plasmon excitations using electrodynamic modeling based on the discrete dipole approximation (DDA). The dipole resonance is found to be very intense, and its peak wavelength is extremely sensitive to the height, edge length, and tip sharpness of the triangular nanoprism. In contrast, the intensity of the quadrupole resonance is much weaker relative to the dipole resonance in the single particle spectra than in the ensemble averaged spectrum. Several parameters relevant to the chemical sensing properties of these nanoprisms have been measured. The dependence of the dipole plasmon resonance on the refractive index of the external medium is found to be as high as 205 nm RIU(-1) and the plasmon line width as narrow as approximately 0.17 eV. These data lead to a sensing figure of merit (FOM), the slope of refractive index sensitivity in eV RIU(-1)/line width (eV), as high as 3.3. In addition, the LSPR shift response to alkanethiol chain length was found to be linear with a slope of 4.4 nm per CH2 unit. This is the highest short-range refractive index sensitivity yet measured for a nanoparticle.     

Change of dissolved gaseous mercury concentrations in a southern reservoir lake (Tennessee) following seasonal variation of solar radiation

A 12-month field study was conducted consecutively from June 2003 to May 2004 to quantify temporal variations of dissolved gaseous mercury (DGM) concentrations in Cane Creek Lake, a southern reservoir lake (Cookeville, TN). Diurnal changes of DGM concentrations in two periods (morning increase vs afternoon decrease with an around-noon peak) were observed, and the changes closely followed daily solar radiation variation trends. The diurnal patterns prevailed in the late spring and summer, but became vague in the late fall and winter. The monthly mean DGM concentrations peaked at 40.8 pg L(-1) in July and reached the lowest at 14.2 pg L(-1) in December and 21.9 pg L(-1) in January; this DGM concentration change closely followed the monthly mean solar radiation variation trend. The increase of the lake DGM concentration from January to July and its decrease from July to December mirror the typical daily rhythm of DGM concentration variations in the two periods. This finding supports the following hypothesis: The natural phenomenon of daily oscillation of freshwater DGM concentrations that follows diurnal solar radiation variation would manifest on a seasonal scale. High DGM concentrations were found in the spring and summer and low in the fall and winter (seasonal mean: 34.2, 37.5, 20.0, 24.4 pg L(-1), respectively). This seems to suggest an annual occurrence of two periods of the seasonal DGM level fluctuation (spring and summer high vs fall and winter low DGM levels). Linear relationships of the monthly mean DGM concentrations were found with the monthly mean global solar radiation (R2 = 0.82, P < 0.05) and UVA radiation (R2 = 0.84, P < 0.05). Linear relationships of the seasonal mean DGM concentrations were also found with the seasonal mean global solar radiation (R2 = 0.85, P = 0.08) and UVA radiation (R2 = 0.93, P < 0.05).     

Synthesis of 2,4‐ and 2,5‐Disubstituted Oxazoles via Metal‐ Catalyzed Cross‐Coupling Reactions

The rapid synthesis of 2,4‐ and 2,5‐disubstituted oxazoles via metal‐catalyzed cross‐coupling reactions is reported. The 4‐ or 5‐position of the corresponding 4‐ or 5‐halogenated 2‐butylthiooxazoles was successfully functionalized via Suzuki–Miyaura, Sonogashira and Stille cross‐coupling reactions. The 2‐position of the 2‐butylthiooxazoles obtained was further coupled to various organozinc reagents through palladium‐ or nickel‐mediated cross‐coupling reactions.     

Photodegradation of decabromodiphenyl ether adsorbed onto clay minerals, metal oxides, and sediment

The photodebromination of decabromodiphenyl ether (BDE-209) adsorbed onto six different solid matrixes was investigated in sunlight and by irradiation with 350 +/- 50 nm lamps (four lamps at 24 W each). After 14 days of lamp irradiation, BDE-209 degraded with a half-life of 36 and 44 days, respectively, on montmorillonite or kaolinite, with much slower degradation occurring when sorbed on organic carbon-rich natural sediment (t1/2 = 150 days). In late summer and fall sunlight (40.5 degrees N, elevation 600 ft), the half-lives of BDE-209 sorbed on montmorillonite and kaolinite were 261 and 408 days, respectively. Under both irradiation schemes, no significant loss of BDE-209 occurred when sorbed to aluminum hydroxide, iron oxide (ferrihydrite), or manganese dioxide (birnessite). Upon exposure to both lamp and solar light and in the presence of montmorillonite and kaolinite, numerous lesser brominated congeners (tri- to nonabromodiphenyl ethers) were produced. Nearly identical product distribution was evident on montmorillonite and kaolinite. Dark control experiments for each mineral showed no disappearance of BDE-209 or appearance of degradation products. These results suggest that photodegradation of BDE-209 on mineral aerosols during long-range atmospheric transport may be an important fate process for BDE-209 in the environment.     

A micromechanical characterization of angular bidirectional fibrous composites

A micromechanical numerical algorithm to efficiently determine the homogenized elastic properties of bidirectional fibrous composites is presented. A repeating unit cell (RUC) based on a pre-determined bidirectional fiber packing is assumed to represent the microstructure of the composite. For angular bidirectional fiber distribution, the symmetry lines define a parallelepiped unit cell, representing the periodic microstructure of an angular bidirectional fiber composite. The lines of symmetry extrude a volume to capture a three dimensional unit cell. Finite element analysis of this unit cell under six possible independent loading conditions is carried out to study and quantify the homogenized mechanical property of the cell. A volume averaging scheme is implemented to determine the average response as a function of loading in terms of stresses and strains. The individual elastic properties of the constituents’ materials, as well as, the composite can be assumed to be completely isotropic to completely anisotropic. The output of the analysis can determine this degree. The logic behind the selection of the unit cell and the implementation of the periodic boundary conditions as well as the constraints are presented. To verify this micromechanics algorithm, the results for four composites are presented. The results in this paper are mainly focused on the impact of the fiber cross angles on the stiffness properties of the composites chosen. The accuracy of the results from this micromechanics modeling procedure has been compared with the stiffness/compliance solutions from lamination theory. The methodology is to be accurate and efficient to the extent that periodicity of the composite material is maintained. In addition, the results will show the impact of fiber volume fraction on the material properties of the composite. This micromechanics tool could make a powerful viable algorithm for determination of many linear as well as nonlinear properties in continuum mechanics material characterization and analysis.     

Self-Assembly of Hybrid Peptide−DNA Nanostructures using Homotrimeric Coiled-Coil/Nucleic Acid Building Blocks

Peptides and DNA are two of the most commonly used self-assembling biological molecules for the construction of nanomaterials. However, there are only a few examples that combine these two self-assembly motifs as key structural elements in a nanostructure. We report on the synthesis of a peptide−DNA conjugate that self-assembles into a stable homotrimer based on the coiled-coil motif. The hybrid peptide−DNA trimer was then used as a novel three-way junction to link together either small DNA tile nanostructures, or to close up a triangular wireframe DNA structure. The resulting nanostructures were characterized by atomic force microscopy, and compared with a scrambled, non-assembling peptide as a control. These hybrid nanostructures enable the integration of peptide motifs and potentially bio-functionality with DNA nanostructures, and open the door to novel nano-materials that have the advantages of both molecules.     

Twisted Crystalline Organic Semiconductor Photodetectors

Optoelectronic properties of anisotropic crystals vary with direction requiring that the orientation of molecular organic semiconductor crystals is controlled in optoelectronic device active layers to achieve optimal performance. Here, a generalizable strategy to introduce periodic variations in the out-of-plane orientations of 5,11-bis(triisopropylsilylethynyl)anthradithiophene (TIPS ADT) crystals is presented. TIPS ADT crystallized from the melt in the presence of 16 wt.% polyethylene (PE) forms banded spherulites of crystalline fibrils that twist in concert about the radial growth direction. These spherulites exhibit band-dependent light absorption, photoluminescence, and Raman scattering depending on the local orientation of crystals. Mueller matrix imaging reveals strong circular extinction (CE), with TIPS ADT banded spherulites exhibiting domains of positive or negative CE signal depending on the crystal twisting sense. Furthermore, orientation-dependent enhancement in charge injection and extraction in films of twisted TIPS ADT crystals compared to films of straight crystals is visualized in local conductive atomic force microscopy maps. This enhancement leads to 3.3- and 6.2-times larger photocurrents and external quantum efficiencies, respectively, in photodetectors comprising twisted crystals than those comprising straight crystals.