January 5, 2009
Powdered bones are found in a medieval Moorish wall. If the walls of Granada could talk, they would tell about their unusual patina. C. Cardell and colleagues at the University of Granada (Spain) describe a 12-ft rampart built in the 14th century by the Moors. The rampart is covered in a patina made of CaCO3 and powdered bones. The investigators believe that medieval masons added the bone powder to the patina to make the coating more durable.
As archaeologists were excavating around the wall for a major restoration project about 2 years ago, they discovered a brick oven with a pile of black ash and bones next to it. Suspecting that the oven, ash, bones, and wall were somehow connected, the archaeologists asked Cardell to tease out the relationship between the artifacts. The patina on the wall had been analyzed by conventional X-ray diffraction, but the technique had a high detection limit and provided little insight. Cardell armed her team with micro-X-ray diffraction, scanning electron microscopy–energy-dispersive X-ray spectrometry, and IR spectroscopy, all of which have lower detection limits.
Piecing together the different analyses, the investigators concluded that the brick oven was used to burn bones down to a powder. The powder was then mixed with CaCO3 to make the patina. The investigators confirmed that the bones came from animals; and the CaCO3 was extracted from limestone typical of the kind found in mountains near Granada. (Anal. Chem. 2009, 81, ASAP Article DOI: 10.1021/ac8022444; Rajendrani Mukhopadhyay) [The contributor is a senior associate editor for Analytical Chemistry.—Ed.]
Gallium nitride and silicon form an efficient solar cell. Vertically aligned 1-D p-type nanostructures grown on n-type substrates are useful for solar cell applications because the heterojunctions make it easier to separate photogenerated hole–electron pairs. The oriented geometry also provides direct pathways for the separated carriers to move to the electrode, which leads to high carrier collection efficiency. On the basis of this design principle, C. S. Lee, S. T. Lee, and coauthors at the City University of Hong Kong and the Chinese Academy of Sciences (Shenyang) developed a heterojunction solar cell that has excellent performance.
The researchers grew a uniform array of vertically aligned magnesium-doped GaN nanorods on a silicon substrate. A solar cell made directly from the heterostructure of p-GaN nanorods on the n-Si substrate showed clear diode behavior with a rectification ratio >104:1 in the dark. Under simulated solar illumination at 100 mW/cm2, the short-circuit current density and power conversion efficiency of the cell reached 7.6 mA/cm2 and 2.73%, respectively. Light reflectance of the nanorod array was low because of the high surface areas and subwavelength sizes of the nanorods. This array may serve as an antireflection layer to reduce the light loss in the solar cell applications. (Nano Lett. 2008, 8, 4191–4195; Ben Zhong Tang)
Naphthalenebisimides are easily converted to conjugated polymers. X. Guo and M. D. Watson* at the University of Kentucky (Lexington) show that dibromonaphthalene bisimides undergo Stille coupling with a series of bis-stannylated thiophene oligomers to yield donor–acceptor conjugated copolymers that in some cases have very high molecular weights. An important structural feature is the electronically conjugated incorporation of the naphthalenebisimide (NBI) groups within the polymer backbone that produces attractive electronic properties.
The authors used established methods to prepare the NBI monomers in regioisomerically pure form in two steps. The NBIs contained aliphatic groups to ensure polymer solubility. All thiophene-based comonomers also were prepared by using standard procedures. The ligand dba is dibenzylideneacetone.
The authors considered the electron-poor NBI monomers to be ideal coupling partners for the electron-rich stannylated thiophenes. The efficient polymerization led to products with molecular weights measured at 23.6–252 kDa; however, the higher values are probably biased by aggregation in solution and may not reflect true molecular weight. All of the polymers in this study were soluble in common organic solvents, which allowed solution processing for building devices.
Optical property measurements showed a low optical energy gap for the polymer shown in the figure. The authors suggest that inter- and intramolecular charge-transfer states may contribute to this phenomenon. They also believe that the alkoxy groups on this polymer produce a large solution red shift of ~300 nm.
The optical energy gaps easily can be tuned across the visible spectrum and into the near IR by varying comonomer structure. The authors are evaluating the polymers in transistors and as the donor and acceptor components in optical devices. (Org. Lett. 2008, 10, 5333–5336; W. Jerry Patterson)
Trifluorotoluene solvent improves a palladium-mediated cyclization. As part of an effort to improve the synthesis of linezolid, an oxazolidinone antibacterial, M. Pamment and co-workers at Pfizer Global R&D (Ann Arbor, MI) studied the formation of a key oxindole intermediate. Fluoride displacement of the starting 3,4-difluoronitrobenzene with aqueous MeNH2, followed by N-acylation with ClCH2COCl, gave the oxindole precursor. Initial attempts to run the cyclization under literature-described conditions (80 °C, toluene solvent, Pd catalyst, and 1.5 equiv Et3N) met with limited success.
A solvent screen showed that the reaction worked well in DMF or trifluorotoluene (TFT). When the reaction was run in TFT, complete conversion was achieved in 5 h. The product precipitated out of the reaction mixture, which minimized decomposition of the base-sensitive oxindole. A solvent switch to i-PrOH completed the precipitation of the product; it was then washed with water to remove inorganic salts and give the oxindole in 86% yield. (Org. Process Res. Dev. 2008, 12, 884–887; Will Watson)
Phospholipid bilayer membranes deliver chloride ion. Cystic fibrosis (CF) is a disease caused by the diminished transport of chloride ion in cells. Synthetic bilayer membranes may serve as a therapy for CF by improving ion transport. B. D. Smith and co-workers at the University of Notre Dame (IN) designed membranes that contain phosphatidylcholine derivatives (1) as the transporter structures.
One of the two fatty acid chains in the phosphatidylcholines is decorated with urea-based functional groups that can bind chloride ions at the chain ends. Self-assembly of these molecules gives membranes with the hydrophobic segments forming the layer framework in the middle and the functional groups on the margins.
The authors compared three functional groups (2, 3, and 4 in the figure) that bind chloride and then transmit it through the hydrophobic bilayer in relay fashion. Similar groups on the other side “catch” the ion and release it inside the cell. Fluorescence-quenching studies showed that group 2 has the greatest interaction with chloride and group 4 the least. (J. Am. Chem. Soc. 2008, 130, 17274–17275; Sally Peng Li)
Design efficient, stable electrochromic nanotubes. P. Schmuki and co-workers at the University of Erlangen-Nuremberg (Erlangen, Germany) report a method to generate electrochromic nanotubes consisting of TiO2 doped with WO3. By electrochemically anodizing TiW alloys, they obtained well-ordered TiO2 nanotube layers (1.1–1.2 μm thick, 85–95 nm tube diam) with varying amounts (0–9 atom% W) of incorporated WO3. Adding as little as 0.2 atom% W increases the current density and lowers the applied voltage necessary for cathodic insertion.
The authors note that an increase in WO3 loading does not provide additional enhancement. Under cyclic pulse conditions (between –0.7 and +1.0 V), charge density, reversibility (ratio of anodic and cathodic charge densities), and reflectance difference increase when WO3 is added to the TiO2 nanotube array as a result of enhanced proton incorporation. The nanotubes are stable for up to 100 insertion–extraction cycles at potentials up to –0.7 V. At potentials higher than this, the nanotubes degrade as a result of disruption of the TiO2–WO3 composite array, but stability of the tubular structure improves at higher loadings of WO3.
Of extreme importance for electrochromic applications, the researchers report that color contrast (reflectance difference ~0.2) is achieved at lower negative potentials (0.6 vs 0.9 V) in the TiO2–WO3 composite layers. (J. Am. Chem. Soc. 2008, 130, 16154–16155; LaShanda Korley)
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