Phonon anharmonicity in binary chalcogenides for efficient energy harvesting

Thermoelectric (TE) materials have received much attention due to their ability to harvest waste heat energy. TE materials must exhibit a low thermal conductivity (κ) and a high power factor (PF) for efficient conversion. Both factors define the figure of merit (ZT) of the TE material, which can be increased by suppressing κ without degrading the PF. Recently, binary chalcogenides such as SnSe, GeTe, and PbTe have emerged as attractive candidates for thermoelectric energy generation at moderately high temperatures. These materials possess simple crystal structures with low κ in their pristine forms, which can be further lowered through doping and other approaches. Here, we review the recent advances in the temperature-dependent behavior of phonons and their influence on the thermal transport properties of chalcogenide-based TE materials. Because phonon anharmonicity is one of the fundamental contributing factors for low thermal conductivity in SnSe, Sb-doped GeTe, and related chalcogenides, we discuss complementary experimental approaches such as temperature-dependent Raman spectroscopy, inelastic neutron scattering, and calorimetry to measure anharmonicity. We further show how data gathered using multiple techniques helps us understand and engineer better TE materials. Finally, we discuss the rise of machine learning-aided efforts to discover, design, and synthesize TE materials of the future.

An 8-gene machine learning model improves clinical prediction of severe dengue progression

BACKGROUND

Each year 3-6 million people develop life-threatening severe dengue (SD). Clinical warning signs for SD manifest late in the disease course and are nonspecific, leading to missed cases and excess hospital burden. Better SD prognostics are urgently needed.

METHODS

We integrated 11 public datasets profiling the blood transcriptome of 365 dengue patients of all ages and from seven countries, encompassing biological, clinical, and technical heterogeneity. We performed an iterative multi-cohort analysis to identify differentially expressed genes (DEGs) between non-severe patients and SD progressors. Using only these DEGs, we trained an XGBoost machine learning model on public data to predict progression to SD. All model parameters were "locked" prior to validation in an independent, prospectively enrolled cohort of 377 dengue patients in Colombia. We measured expression of the DEGs in whole blood samples collected upon presentation, prior to SD progression. We then compared the accuracy of the locked XGBoost model and clinical warning signs in predicting SD.

RESULTS

We identified eight SD-associated DEGs in the public datasets and built an 8-gene XGBoost model that accurately predicted SD progression in the independent validation cohort with 86.4% (95% CI 68.2-100) sensitivity and 79.7% (95% CI 75.5-83.9) specificity. Given the 5.8% proportion of SD cases in this cohort, the 8-gene model had a positive and negative predictive value (PPV and NPV) of 20.9% (95% CI 16.7-25.6) and 99.0% (95% CI 97.7-100.0), respectively. Compared to clinical warning signs at presentation, which had 77.3% (95% CI 58.3-94.1) sensitivity and 39.7% (95% CI 34.7-44.9) specificity, the 8-gene model led to an 80% reduction in the number needed to predict (NNP) from 25.4 to 5.0. Importantly, the 8-gene model accurately predicted subsequent SD in the first three days post-fever onset and up to three days prior to SD progression.

CONCLUSIONS

The 8-gene XGBoost model, trained on heterogeneous public datasets, accurately predicted progression to SD in a large, independent, prospective cohort, including during the early febrile stage when SD prediction remains clinically difficult. The model has potential to be translated to a point-of-care prognostic assay to reduce dengue morbidity and mortality without overwhelming limited healthcare resources.

Influence of dopants on the impermeability of graphene

Graphene has attracted much attention as an impermeable membrane and a protective coating against oxidation. While many theoretical studies have shown that defect-free graphene is impermeable, in reality graphene inevitably has defects in the form of grain boundaries and vacancies. Here, we study the effects of N-dopants on the impermeability of few-layered graphene (FLG) grown on copper using chemical vapor deposition. The grain boundaries in FLG have minimal impact on their permeability to oxygen as they do not provide a continuous channel for gas transport due to high tortuosity. However, we experimentally show that the N-dopants in FLG display multiple configurations that create structural imperfections to selectively allow gas molecules to permeate. We used a comprehensive array of tools including Raman spectroscopy, X-ray photoelectron spectroscopy, optically stimulated electron emission measurements, and density functional theory of N-doped graphene on copper to elucidate the effects of dopant configuration on the impermeability of graphene. Our results clearly show that oxygen can permeate through graphene with non-graphitic nitrogen dopants that create pores in graphene and oxidize the underlying Cu substrate while graphitic nitrogen dopants do not show any changes compared to the pristine form. Furthermore, we observed that the work function of graphene can be tuned effectively by changing the dopant configuration.

Modulation of the Electrostatic and Quantum Capacitances of Few Layered Graphenes through Plasma Processing

It is shown that charged defect generation, through argon ion-based plasma processing, in few layer graphene, could substantially enhance the electrical capacitance for electrochemical energy storage. Detailed consideration of the constituent space charge and quantum capacitances were used to delineate a new length scale, correlated to electrically active defects contributing to the capacitance, and was found to be smaller than a structural correlation length determined through Raman spectroscopy. The study offers insights into an industrially viable method (i.e., plasma processing) for modifying and enhancing the energy density of graphene-based electrochemical capacitors.

Synthesis and characterization of gold graphene composite with dyes as model substrates for decolorization: a surfactant free laser ablation approach

A facile surfactant free laser ablation mediated synthesis (LAMS) of gold-graphene composite is reported here. The material was characterized using transmission electron microscopy, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, powdered X-ray diffraction, Raman spectroscopy, Zeta potential measurements and UV-Visible spectroscopic techniques. The as-synthesized gold-graphene composite was effectively utilized as catalyst for decolorization of 4 important textile and laser dyes. The integration of gold nanoparticles (AuNPs) with high surface area graphene has enhanced the catalytic activity of AuNPs. This enhanced activity is attributed to the synergistic interplay of pristine gold's electronic relay and π-π stacking of graphene with the dyes. This is evident when the Rhodamine B (RB) reduction rate of the composite is nearly twice faster than that of commercial citrate capped AuNPs of similar size. In case of Methylene blue (MB) the rate of reduction is 17,000 times faster than uncatalyzed reaction. This synthetic method opens door to laser ablation based fabrication of metal catalysts on graphene for improved performance without the aid of linkers and surfactants.

Direct measurement of shear properties of microfibers

As novel fibers with enhanced mechanical properties continue to be synthesized and developed, the ability to easily and accurately characterize these materials becomes increasingly important. Here we present a design for an inexpensive tabletop instrument to measure shear modulus (G) and other longitudinal shear properties of a micrometer-sized monofilament fiber sample, such as nonlinearities and hysteresis. This automated system applies twist to the sample and measures the resulting torque using a sensitive optical detector that tracks a torsion reference. The accuracy of the instrument was verified by measuring G for high purity copper and tungsten fibers, for which G is well known. Two industrially important fibers, IM7 carbon fiber and Kevlar(®) 119, were also characterized with this system and were found to have G = 16.5 ± 2.1 and 2.42 ± 0.32 GPa, respectively.

Effects of surface functional groups on the formation of nanoparticle-protein corona

Herein, we examined the dependence of protein adsorption on the nanoparticle surface in the presence of functional groups. Our UV-visible spectrophotometry, transmission electron microscopy, infrared spectroscopy, and dynamic light scattering measurements evidently suggested that the functional groups play an important role in the formation of nanoparticle-protein corona. We found that uncoated and surfactant-free silver nanoparticles derived from a laser ablation process promoted a maximum protein (bovine serum albumin) coating due to increased changes in entropy. On the other hand, bovine serum albumin displayed a relatively lower affinity for electrostatically stabilized nanoparticles due to the constrained entropy changes.

Binary III-V semiconductor core optical fiber

For the first time to the best of our knowledge a glass-clad optical fiber comprising a crystalline binary III-V semiconductor core has been fabricated. More specifically, a phosphate glass-clad fiber containing an indium antimonide (InSb) core was drawn using a molten core approach. The core was found to be highly crystalline with some oxygen and phosphorus diffusing in from the cladding glass. While optical transmission measurements were unable to be made, most likely due to free carrier absorption associated with the conductivity of the core, this work constitutes a proof-of-concept that optical fibers comprising semiconductor cores of higher crystallographic complexity than previously realized can be drawn using conventional fiber fabrication techniques. Such binary semiconductors may open the door to future fiber-based nonlinear devices.

Silicon optical fiber

Described herein are initial experimental details and properties of a silicon core, silica glass-clad optical fiber fabricated using conventional optical fiber draw methods. Such semiconductor core fibers have potential to greatly influence the fields of nonlinear fiber optics, infrared and THz power delivery. More specifically, x-ray diffraction and Raman spectroscopy showed the core to be highly crystalline silicon. The measured propagation losses were 4.3 dB/m at 2.936 microm, which likely are caused by either microcracks in the core arising from the large thermal expansion mismatch with the cladding or to SiO(2) precipitates formed from oxygen dissolved in the silicon melt. Suggestions for enhancing the performance of these semiconductor core fibers are provided. Here we show that lengths of an optical fiber containing a highly crystalline semiconducting core can be produced using scalable fiber fabrication techniques.

Superconductivity in thin films of boron-doped carbon nanotubes

Superconductivity in carbon nanotubes (CNTs) is attracting considerable attention. However, its correlation with carrier doping has not been reported. We report on the Meissner effect found in thin films consisting of assembled boron (B)-doped single-walled CNTs (B-SWNTs). We find that only B-SWNT films consisting of low boron concentration leads to evident Meissner effect with Tc=12 K and also that a highly homogeneous ensemble of the B-SWNTs is crucial. The first-principles electronic-structure study of the B-SWNTs strongly supports these results.

Electrochemical organization of monolayer protected gold nanoclusters on single-walled carbon nanotubes: significantly enhanced double layer capacitance

This paper reports a novel electrochemical route for anchoring monolayer protected gold nanoclusters (size 8 +/- 0.2 nm) on single-walled carbon nanotube bundles, resulting in the formation of hybrid materials. Monolayer protected gold nanoclusters prepared by modified Brust synthesis route were organized on SWNT bundles by cycling the potential in dichloromethane between -1 to +1 V at a scan rate of 50 mV/s. Monolayer protected nanoclusters in electrolyte solutions possess ionic space charge around them (double layer charging), making them suitable for organization on nanotube bundles, by tuning the electrostatic interactions. More significantly, analysis of the double layer capacitance of these hybrid materials shows almost ten times increase in capacitance compared to that of bare SWNT bundles. We believe that these hybrid materials are potentially useful in nanoelectronics.

A GH receptor antisense oligonucleotide inhibits hepatic GH receptor expression, IGF-I production and body weight gain in normal mice

Diabetic retinopathy and acromegaly are diseases associated with excess action of GH and its effector IGF-I, and there is a need for improved therapies. We have designed an optimised 2'-O-(2-methoxyethyl)-modified phosphorothioate oligodeoxynucleotide, ATL 227446, and demonstrated its ability to suppress GH receptor mRNA in vitro. Subcutaneous injections of ATL 227446 reduced GH receptor mRNA levels, GH binding activity and serum IGF-I levels in mice after seven days of dosing. The reduction in serum IGF-I could be sustained for over ten weeks of dosing at therapeutically relevant levels, during which there was also a significant decrease in body weight gain in antisense-treated mice relative to saline and mismatch control-treated mice. The findings indicate that administration of an antisense oligonucleotide to the GH receptor may be applicable to human diseases in which suppression of GH action provides therapeutic benefit.

Determination of the bending modulus of an individual multiwall carbon nanotube using an electric harmonic detection of resonance technique

We report a new method of measuring the amplitude and phase of oscillations of individual multiwall carbon nanotubes (MWNTs). As in many other experiments, we excite the oscillations electrostatically, but we show that we can detect the amplitude and phase of the resulting oscillation electrically. As an example, we present measurements of the fundamental and first two overtones of the diving board resonance of a MWNT at 0.339, 2.42, and 5.31 MHz in ambient conditions. The corresponding quality factors were 67, 36, and 25.

Novel electrical switching behaviour and logic in carbon nanotube Y-junctions

Carbon-nanotube-based electronics offers significant potential as a nanoscale alternative to silicon-based devices for molecular electronics technologies. Here, we show evidence for a dramatic electrical switching behaviour in a Y-junction carbon-nanotube morphology. We observe an abrupt modulation of the current from an on- to an off-state, presumably mediated by defects and the topology of the junction. The mutual interaction of the electron currents in the three branches of the Y-junction is shown to be the basis for a potentially new logic device. This is the first time that such switching and logic functionalities have been experimentally demonstrated in Y-junction nanotubes without the need for an external gate. A class of nanoelectronic architecture and functionality, which extends well beyond conventional field-effect transistor technologies, is now possible.

ZnO nanowires by pulsed laser vaporization: synthesis and properties

We report a new pulsed-laser vaporization (PLV) technique to synthesize nanowires of single-crystal ZnO having a wurtzite structure by using colloidal gold nanoparticles as seeding catalysts. The average diameter of the nanowires is approximately 13 nm, with a very narrow range of 7 to 25 nm. The nanowires are straight for the most part, with the axes parallel to the [0001] growth direction. Raman and photoluminescence spectra from the nanowires and bulk ZnO are similar except for a approximately 510 nm band in the nanowires due to oxygen vacancies. The bulk-like vibrational and electronic properties of the nanowires is due to the diameter being larger than the threshold below which quantum confinement-induced effects are expected.

Synthesis and thermoelectric power of nitrogen-doped carbon nanotubes

We have previously shown that high-purity multiwalled carbon nanotubes (pristine MWNTs) can be prepared from a mixture of xylene-ferrocene (99 at% C:1 at% Fe) inside a quartz tube reactor operating at approximately 700 degrees C. In a similar process, approximately 3 g of melamine (C3H6N6) was introduced during the growth of MWNTs to prepare nitrogen-doped nanotubes. The structural and electronic properties of nitrogen-doped MWNTs were determined by scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), and thermopower measurements. The individual nitrogen-doped nanotube exhibits a bamboo-like structure and comprises 6-16 tube walls, as evidenced by HRTEM studies. The EELS measurements yielded an average nitrogen content of approximately 5 at% in the doped tubes. The thermoelectric power data of nitrogen-doped MWNTs remained negative even after exposure to oxygen for an extended period of time, suggesting that nitrogen doping of MWNTs renders them n-type, consistent with scanning tunneling spectroscopic studies on similar nanotubes.

High-pressure Raman study of debundled single-walled carbon nanotubes

We report the pressure dependence for the radial (omega R) and tangential (omega T) band frequencies in debundled single-walled carbon nanotubes (SWNTs) derived from laser-synthesized SWNT bundles. As previously described, a chemical procedure was used to prepare debundled SWNTs from as-prepared, large SWNT bundles. The normalized pressure coefficient for omega R in the debundled sample was compared with the corresponding value in the bundled sample to quantify the strength of van der Waals interactions between tubes in these nanotube materials. Furthermore, the pressure dependences for the radial (omega R) and tangential (omega T) band frequencies in debundled tubes were also compared with corresponding dependences predicted for isolated SWNTs, obtained with generalized tight binding molecular dynamic (GTBMD) simulations described in our previous work. The results presented here collectively suggest that the van der Waals interaction is still strong in the debundled sample studied here, which contained predominantly small bundles of SWNTs rather than isolated tubes.

Raman studies of semiconducting oxide nanobelts

Crystalline nanobelts of ZnO and SnO2 were prepared from a thermal evaporation of oxide powders inside an alumina tube in the absence of catalysts. Typical dimensions of the nanobelt samples ranged from approximately 10 to 100 microns in length, 30 to 300 nm in width, and 6 to 30 nm in thickness. Room temperature Raman spectra were obtained on pressed mats of nanobelt samples and compared with the corresponding spectra of the starting oxide powders and bulk materials. Collectively, our Raman data indicated that the as-prepared nanobelt samples used in this study were oxygen deficient. Upon annealing at 900 degrees C in flowing oxygen for 1 h, the nanobelt samples exhibited Raman features that corresponded to those expected in respective bulk semiconducting oxides. The dimensions of the nanobelts were a bit too large to expect significant quantum size effects on the phonon structure similar to those observed in carbon nanotubes and short-period semiconductor superlattices.

Monitoring of indoor volatile organic compounds and polycyclic aromatic hydrocarbons arising from kerosene cooking fuel

Air samples, representing indoor environments of a kitchen in which a kerosene stove was used were collected and analysed for volatile organic compounds (VOCs) viz., n-hexane, benzene, heptane, toluene, p- and o-xylene and n-decane using a cryogenic preconcentration system and a gas chromatograph with a flame-ionisation detector. Simultaneous outdoor samples were also collected to determine indoor to outdoor (I/O) ratios for each compound. Reversed phase high performance liquid chromatography (HPLC) with ultra violet absorption detection was optimised for separation and quantification of polycyclic aromatic hydrocarbons (PAHs) in air particulate matter. Concentration of total suspended particulate matter (TSPM), benzene soluble organics and the PAHs in air samples collected in indoor environment of some tenements at Trombay, Mumbai where kerosene is used as cooking fuel are discussed in relation to the concentration of the same in outdoor environment in vicinity of the tenement. VOCs and PAHs results from samples collected in kitchens in Trombay are discussed in relation to indoor air pollution.

Monitoring of organochlorine pesticide residues in the Indian marine environment

Organochlorine pesticide residues in sediment and fish samples collected from the east and west coasts of India are presented. HCH isomers and DDT and its metabolites are the predominantly identified compounds in most of the samples. Despite the higher quantity of consumption, HCH and DDT levels in fish in India were lower than those in temperate countries suggesting a lower accumulation in tropical fish, which could be due to rapid volatilization and degradation of these insecticides in the tropical environment. The predominance of alpha- and beta-HCH reflect the use of technical grade HCH in India. The high temperature in the tropics also enhances the elimination rate of chemicals in fish, as the biological half-lives of semivolatile compounds such as DDT are short at high temperature.

Up-regulation of the peripheral-type benzodiazepine receptor expression and [(3)H]PK11195 binding in gerbil hippocampus after transient forebrain ischemia

In mammalian CNS, the peripheral-type benzodiazepine receptor (PTBR) is localized on the outer mitochondrial membrane within the astrocytes and microglia. The main function of PTBR is to transport cholesterol across the mitochondrial membrane to the site of neurosteroid biosynthesis. The present study evaluated the changes in the PTBR density, gene expression and immunoreactivity in gerbil hippocampus as a function of reperfusion time after transient forebrain ischemia. Between 3 to 7 days of reperfusion, there was a significant increase in the maximal binding site density (B(max)) of the PTBR antagonist [(3)H]PK11195 (by 94-156%; P < 0.01) and PTBR mRNA levels (by 1.8- to 2.9-fold; P < 0.01). At 7 days of reperfusion, in the hippocampal CA1 (the brain region manifesting selective neuronal death), PTBR immunoreactivity increased significantly. Increased PTBR expression after transient forebrain ischemia may lead to increased neurosteroid biosynthesis, and thus may play a role in the ischemic pathophysiology.

Effect of van der Waals interactions on the Raman modes in single walled carbon nanotubes

We have measured the Raman spectrum of individual single walled carbon nanotubes in solution and compare it to that obtained from the same starting material where the tubes are present in ordered bundles or ropes. Interestingly, the radial mode frequencies for the tubes in solution are found to be approximately 10 cm (-1) higher than those observed for tubes in a rope, in apparent contradiction to lattice dynamics predictions. We suggest that there is no such contradiction, and propose that the upshift is due rather to a decreased energy spacing of the Van Hove singularities in isolated tubes over the spacings in a rope, thereby allowing the same laser excitation to excite different diameter tubes in these two samples.

Does CDP-choline modulate phospholipase activities after transient forebrain ischemia?

Ten min forebrain ischemia/1-day reperfusion resulted in significant decreases in total phosphatidylcholine (PtdCho), phosphatidylinositol (PtdIns), and cardiolipin in gerbil hippocampus. CDP-choline restored cardiolipin levels, arachidonic acid content of PtdCho, partially but significantly restored total PtdCho, and had no effect on PtdIns. These data suggest that CDP-choline prevented the activation of phospholipase A(2) (rather than inhibiting phospholipase A(2) activity) but did not affect activities of PtdCho-phospholipases C and/or D, or phosphoinositide-phospholipase C. CDP-choline also provided significant protection for hippocampal CA(1) neurons.

Lipid alterations in transient forebrain ischemia: possible new mechanisms of CDP-choline neuroprotection

We have previously demonstrated that cytidine 5'-diphosphocholine (CDP-choline or citicoline) attenuated arachidonic acid (ArAc) release and provided significant protection for the vulnerable hippocampal CA(1) neurons of the cornu ammonis after transient forebrain ischemia of gerbil. ArAc is released by the activation of phospholipases and the alteration of phosphatidylcholine (PtdCho) synthesis. Released ArAc is metabolized by cyclooxygenases/lipoxygenases to form eicosanoids and reactive oxygen species (ROS). ROS contribute to neurotoxicity through generation of lipid peroxides and the cytotoxic byproducts 4-hydroxynonenal and acrolein. ArAc can also stimulate sphingomyelinase to produce ceramide, a potent pro-apoptotic agent. In the present study, we examined the changes and effect of CDP-choline on ceramide and phospholipids including PtdCho, phosphatidylethanolamine (PtdEtn), phosphatidylinositol (PtdIns), phosphatidylserine (PtdSer), sphingomyelin, and cardiolipin (an exclusive inner mitochondrial membrane lipid essential for electron transport) following ischemia/1-day reperfusion. Our studies indicated significant decreases in total PtdCho, PtdIns, PtdSer, sphingomyelin, and cardiolipin and loss of ArAc from PtdEtn in gerbil hippocampus after 10-min forebrain ischemia/1-day reperfusion. CDP-choline (500 mg/kg i.p. immediately after ischemia and at 3-h reperfusion) significantly restored the PtdCho, sphingomyelin, and cardiolipin levels as well as the ArAc content of PtdCho and PtdEtn but did not affect PtdIns and PtdSer. These data suggest multiple beneficial effects of CDP-choline: (1) stabilizing the cell membrane by restoring PtdCho and sphingomyelin (prominent components of outer cell membrane), (2) attenuating the release of ArAc and limiting its oxidative metabolism, and (3) restoring cardiolipin levels.

Neuroprotection by group I metabotropic glutamate receptor antagonists in forebrain ischemia of gerbil

Stimulation of group I metabotropic glutamate receptors (mGluR 1 and 5) activates G-protein coupled-phospholipase C (PLC) to release 1,2-diacylglycerol (DAG) and arachidonic acid (ArAc). To elucidate the role of group I mGluR, we tested the effects of (S)-alpha-methyl-4-carboxy-phenylglycine (MCPG, mGluR 1 and 5 antagonist), 1-aminoindan-1,5-dicarboxylic acid (AIDA, mGluR 1a specific antagonist) and 2-methyl-6-(phenylethynyl) pyridine (MPEP, mGluR 5 antagonist) on ArAc release and neuronal survival after transient forebrain ischemia in gerbils. Ischemia resulted in (a) significant release of ArAc at 1-day reperfusion and (b) significant neuronal death in the hippocampal CA1 subfield after 6-day reperfusion. MCPG and MPEP decreased ArAc release and also significantly increased neuronal survival. AIDA was less effective in decreasing ArAc release and had no effect on neuronal death. These results suggest that activation of mGluR 5 may be an important pathway in ArAc release and neuronal death after transient ischemia.

Polarized raman study of single-wall semiconducting carbon nanotubes

Polarized Raman spectra were obtained from a rope of aligned semiconducting single-wall nanotubes (SWNTs) in the vicinity of the D band and the G band. Based on group theory analysis and related theoretical predictions, the G-band profile was deconvolved into four intrinsic SWNT components with the following symmetry assignments: 1549 cm(-1) [E2(E(2g))], 1567 cm(-1) [A(A(1g))+E1(E(1g))], 1590 cm(-1) [A(A(1g))+E1(E(1g))] and 1607 cm(-1) [E2(E(2g))]. The frequency shifts of the tangential G modes from the 2D graphitelike E(2g(2)) frequency are discussed in terms of the nanotube geometry.

Molecular functionalization of carbon nanotubes and use as substrates for neuronal growth

Carbon nanotubes are strong, flexible, conduct electrical current, and can be functionalized with different molecules, properties that may be useful in basic and applied neuroscience research. We report the first application of carbon nanotube technology to neuroscience research. Methods were developed for growing embryonic rat-brain neurons on multiwalled carbon nanotubes. On unmodified nanotubes, neurons extend only one or two neurites, which exhibit very few branches. In contrast, neurons grown on nanotubes coated with the bioactive molecule 4-hydroxynonenal elaborate multiple neurites, which exhibit extensive branching. These findings establish the feasability of using nanotubes as substrates for nerve cell growth and as probes of neuronal function at the nanometer scale.

Glial glutamate transporter GLT-1 down-regulation precedes delayed neuronal death in gerbil hippocampus following transient global cerebral ischemia

Glial (GLT-1 and GLAST) and neuronal (EAAC1) high-affinity transporters mediate the sodium dependent glutamate reuptake in mammalian brain. Their dysfunction leads to neuronal damage by allowing glutamate to remain in the synaptic cleft for a longer duration. The purpose of the present study is to understand their contribution to the ischemic delayed neuronal death seen in gerbil hippocampus following transient global cerebral ischemia. The protein levels of these three transporters were studied by immunoblotting as a function of reperfusion time (6 h to 7 days) following a 10 min occlusion of bilateral common carotid arteries in gerbils. In the vulnerable hippocampus, there was a significant decrease in the protein levels of GLT-1 (by 36-46%, P < 0.05; between 1 and 3 days of reperfusion) and EAAC1 (by 42-68%, P < 0.05; between 1 and 7 days of reperfusion). Histopathological evaluation showed no neuronal loss up to 2 days of reperfusion but an extensive neuronal loss (by approximately 84%, P < 0.01) at 7 days of reperfusion in the hippocampal CA1 region. The time frame of GLT-1 dysfunction (1-3 days of reperfusion) precedes the initiation of delayed neuronal death (2-3 days of reperfusion). This suggests GLT-1 dysfunction as a contributing factor for the hippocampal neuronal death following transient global cerebral ischemia. Furthermore, decreased EAAC1 levels may contribute to GABAergic dysfunction and excitatory/inhibitory imbalance following transient global ischemia.

Elevated N1-acetylspermidine levels in gerbil and rat brains after CNS injury

The polyamine system is very sensitive to different pathological states of the brain and is perturbed after CNS injury. The main modifications are significant increases in ornithine decarboxylase activity and an increase in tissue putrescine levels. Previously we have shown that the specific polyamine oxidase (PAO) inhibitor N1,N4-bis(2,3-butadienyl)-1,4-butanediamine (MDL 72527) reduced the tissue putrescine levels, edema, and infarct volume after transient focal cerebral ischemia in spontaneously hypertensive rats and traumatic brain injury of Sprague-Dawley rats. In the present study, N1-acetyl-spermidine accumulation was greater in injured brain regions compared with sham or contralateral regions following inhibition of PAO by MDL 72527. This indicates spermidine/spermine-N1-acetyltransferase (SSAT) activation after CNS injury. The observed increase in N1-acetylspermidine levels at 1 day after CNS trauma paralleled the decrease in putrescine levels after treatment with MDL 72527. This suggests that the increased putrescine formation at 1 day after CNS injury is mediated by the SSAT/PAO pathway, consistent with increased SSAT mRNA after transient ischemia.

Neuroprotective effects of citicoline on brain edema and blood-brain barrier breakdown after traumatic brain injury

OBJECT

Cytidine 5'-diphosphocholine (CDPC), or citicoline, is a naturally occurring endogenous compound that has been reported to provide neuroprotective effects after experimental cerebral ischemia. However, in no study has such protection been shown after traumatic brain injury (TBI). In this study the authors examined the effect of CDPC on secondary injury factors, brain edema and blood-brain barrier (BBB) breakdown, after TBI.

METHODS

After anesthesia had been induced in Sprague-Dawley rats by using 1.5% halothane, an experimental TBI was created using a controlled cortical impact (CCI) device with a velocity of 3 m/second, resulting in a 2-mm deformation. Four sham-operated control animals used for brain edema and BBB breakdown studies underwent the same surgical procedure, but received no injury. Brain edema was evaluated using the wet-dry method 24 hours postinjury, and BBB breakdown was evaluated by measuring Evans blue dye (EBD) extravasation with fluorescein 6 hours after TBI. The animals received intraperitoneal injections of CDPC (50, 100, or 400 mg/kg two times after TBI [eight-10 animals in each group]) or saline (eight animals) after TBI. Traumatic brain injury induced an increase in the percentage of water content and in EBD extravasation in the injured cortex and the ipsilateral hippocampus. No significant benefit from CDPC treatment was observed at a dose of 50 mg/kg. Cytidine 5'-diphosphocholine at a dose of 100 mg/kg attenuated EBD extravasation in both regions, although it reduced brain edema only in the injured cortex. In both regions, 400 mg/ kg of CDPC significantly decreased brain edema and BBB breakdown.

CONCLUSIONS

This is the first report in which dose-dependent neuroprotective effects of CDPC have been demonstrated in the injured cortex as well as in the hippocampus, a brain region known to be vulnerable to injury, after experimental TBI. The results of this study suggest that CDPC is an effective neuroprotective agent on secondary injuries that appear following TBI.

Polarized raman study of aligned multiwalled carbon nanotubes

Polarized Raman spectra of high purity aligned arrays of multiwalled carbon nanotubes, prepared on silica substrates from the thermal decomposition of a ferrocene-xylene mixture, show a strong dependence of the graphitelike G band and the disorder-induced D band on the polarization geometry employed in the experiments. The experimental G-band intensity exhibits a minimum at straight theta(m) = 55 degrees in the VV configuration, in good agreement with theoretical predictions of a characteristic minimum at 54.7 degrees for A(1g) modes in single wall nanotubes, where straight theta(m) denotes the angle between the polarization direction and the nanotube axis.

CDP-choline: neuroprotection in transient forebrain ischemia of gerbils

CDP-choline is a rate-limiting intermediate in the biosynthesis of phosphatidylcholine (PtdCho), an important component of the neural cell membrane. The ability of CDP-choline to alter phospholipid metabolism is an important function in the treatment of ischemic injury. Exogenous treatment with CDP-choline stimulates PtdCho synthesis and prevents release of free fatty acids (FFA), especially arachidonic acid (AA), after ischemia/reperfusion. Phase III clinical trials of CDP-choline in the treatment of stroke are currently underway. Here we report the neuroprotection by CDP-choline in transient forebrain ischemia of gerbils. CDP-choline significantly attenuated the blood-brain barrier (BBB) dysfunction after ischemia with 6-hr reperfusion, and considerably reduced the increase of AA in FFA and leukotriene C(4) (LTC(4)) synthesis at 1 day. Edema was significantly elevated after 1 and 2 days, but attained maximum at 3-day reperfusion. CDP-choline substantially attenuated edema at 3 days. Ischemia resulted in 80 +/- 8% CA(1) hippocampal neuronal death after 6-day reperfusion, and CDP-choline provided 65 +/- 6% neuroprotection. CDP-choline may act by increasing PtdCho synthesis via two pathways: (1) conversion of 1, 2-diacylglycerol to PtdCho, and (2) biosynthesis of S-adenosyl-L-methionine, thus stabilizing the membrane and reducing AA release and metabolism to leukotriene C(4). This would result in decreased toxicity due to AA, leukotrienes, oxygen radicals, lipid peroxidation, and altered glutamate uptake, thus limiting BBB dysfunction, edema and providing neuroprotection.

Arachidonic acid and leukotriene C4: role in transient cerebral ischemia of gerbils

Accumulation of arachidonic acid (AA) is greatest in brain regions most sensitive to transient ischemia. Free AA released after ischemia is either: 1) reincorporated into the membrane phospholipids, or 2) oxidized during reperfusion by lipoxygenases and cyclooxygenases, producing leukotrienes (LT), prostaglandins, thromboxanes and oxygen radicals. AA, its metabolite LTC4 and lipid peroxides (generated during AA metabolism) have been implicated in the blood-brain barrier (BBB) dysfunction, edema and neuronal death after ischemia/reperfusion. This report describes the time course of AA release, LTC4 accumulation and association with the physiological outcome during transient cerebral ischemia of gerbils. Significant amount of AA was detected immediately after 10 min ischemia (0 min reperfusion) which returned to sham levels within 30 min reperfusion. A later release of AA occurred after 1 d. LTC4 levels were elevated at 0-6 h and 1 d after ischemia. Increased lipid peroxidation due to AA metabolism was observed between 2-6 h. BBB dysfunction occurred at 6 h. Significant edema developed at 1 and 2 d after ischemia and reached maximum at 3 d. Ischemia resulted in approximately 80% neuronal death in the CA1 hippocampal region. Pretreatment with a 5-lipoxygenase inhibitor, AA861 resulted in significant attenuation of LTC4 levels (Baskaya et al. 1996. J. Neurosurg. 85: 112-116) and CA1 neuronal death. Accumulation of AA and LTC4, together with highly reactive oxygen radicals and lipid peroxides, may alter membrane permeability, resulting in BBB dysfunction, edema and ultimately to neuronal death.

Studies on degradation of 14C-chlorpyrifos in the marine environment

Degradation of 14C-chlorpyrifos was studied in a marine ecosystem for 60 days and in marine sediment under moist and flooded conditions using a continuous flow system allowing a total 14C-mass balance for a period of 40 days. In the marine ecosystem, 14C-chlorpyrifos underwent rapid degradation and very little (1-2%) 14C-residues of the applied activity were detected after two months in sediments. Clams were major component of the ecosystem and played a significant role in degradation of the insecticide. In the continuous flow system chlorpyrifos did not undergo substantial mineralization. Volatilization accounted for 0.8-1% loss during first ten days of application. The amounts of extractable 14C-activity were higher in flooded sediments than in moist sediment. More bound residues were formed under moist conditions. TCP (3,5,6-trichloro-2-pyridinol) was the major degradation product formed under both moist and flooded conditions, its formation being higher in the latter conditions. These studies underline the role of clams in degradation of chlorpyrifos and lack of microbial degradation. In absence of clams, chlorpyrifos underwent abiotic degradation in marine sediment with formation of bound residues.

Contribution of polyamine oxidase to brain injury after trauma

OBJECT

The possible role of the polyamine interconversion pathway on edema formation, traumatic injury volume, and tissue polyamine levels after traumatic brain injury (TBI) was studied using an inhibitor of the interconversion pathway enzyme, polyamine oxidase.

METHODS

Experimental TBI was induced in Sprague-Dawley rats by using a controlled cortical impact device at a velocity of 3 m/second, resulting in a 2-mm deformation. Immediately after TBI was induced, 100 mg/kg of N1,N4-bis(2,3-butadienyl)-1,4-butanediamine 2HCl (MDL 72527) or saline was injected intraperitoneally. Brain water content and tissue polyamine levels were measured at 24 hours after TBI. Traumatic injury volume was evaluated using 2% cresyl violet solution 7 days after TBI occurred. The MDL 72527 treatment significantly reduced brain edema (80.4+/-0.8% compared with 81.2+/-1.2%, p < 0.05) and injury volume (30.1+/-6.6 mm3 compared with 42.7+/-13.3 mm3, p < 0.05) compared with the saline treatment. The TBI caused a significant increase in tissue putrescine levels at the traumatized site (65.5+/-26.5 nmol/g [corrected] in the cortex and 70.9+/-22.4 nmol/g [corrected] in the hippocampus) compared with the nontraumatized site (7+/-2.4 nmol/g [corrected] in the cortex and 11.4+/-6.4 nmol/g [corrected] in the hippocampus). The increase in putrescine levels in both the traumatized and nontraumatized cortex and hippocampus was reduced by a mean of 60% with MDL 72527 treatment.

CONCLUSIONS

These results demonstrate, for the first time, that the polyamine interconversion pathway has an important role in the increase of putrescine levels after TBI and that the polyamine oxidase inhibitors, blockers of the interconversion pathway, can be neuroprotective against edema formation and necrotic cavitation after TBI.

Effects of MDL 72527, a specific inhibitor of polyamine oxidase, on brain edema, ischemic injury volume, and tissue polyamine levels in rats after temporary middle cerebral artery occlusion

The possible effects of the polyamine interconversion pathway on tissue polyamine levels, brain edema formation, and ischemic injury volume were studied by using a selective irreversible inhibitor, MDL 72527, of the interconversion pathway enzyme, polyamine oxidase. In an intraluminal suture occlusion model of middle cerebral artery in spontaneously hypertensive rats, 100 mg/kg MDL 72527 changed the brain edema formation from 85.7 +/- 0.3 to 84.5 +/- 0.9% in cortex (p < 0.05) and from 79.9 +/- 1.7 to 78.4 +/- 2.0% in subcortex (difference not significant). Ischemic injury volume was reduced by 22% in the cortex (p < 0.05) and 17% in the subcortex (p < 0.05) after inhibition of polyamine oxidase by MDL 72527. There was an increase in tissue putrescine levels together with a decrease in spermine and spermidine levels at the ischemic site compared with the nonischemic site after ischemia-reperfusion injury. The increase in putrescine levels at the ischemic cortical and subcortical region was reduced by a mean of 45% with MDL 72527 treatment. These results suggest that the polyamine interconversion pathway has an important role in the postischemic increase in putrescine levels and that blocking of this pathway can be neuroprotective against neuronal cell damage after temporary focal cerebral ischemia.

Simultaneous assay of ornithine decarboxylase and polyamines after central nervous system injury in gerbil and rat

Ornithine decarboxylase (ODC) is considered the rate-limiting enzyme in polyamine biosynthesis. An increase in putrescine (a natural polyamine) synthesis after central nervous system (CNS) injury appears to be involved in blood-brain barrier dysfunction, development of vasogenic edema and neuronal death. An improved method is described to determine the ODC activity as well as polyamine levels from the same brain tissue. The polyamine results showed no significant differences from data obtained with the conventional assay. The advantages of this method are to: (1) minimize the number of animals needed for the study, and (2) eliminate any internal inconsistencies resulting from use of two independent groups of animals for ODC and polyamine measurements. Using this method, ODC activities and polyamine levels were measured in cortices and hippocampi from global transient ischemia of gerbils and traumatic brain injury (TBI) of rats.

Solution properties of single-walled carbon nanotubes

Naked metallic and semiconducting single-walled carbon nanotubes (SWNTs) were dissolved in organic solutions by derivatization with thionychloride and octadecylamine. Both ionic (charge transfer) and covalent solution-phase chemistry with concomitant modulation of the SWNT band structure were demonstrated. Solution-phase near-infrared spectroscopy was used to study the effects of chemical modifications on the band gaps of the SWNTs. Reaction of soluble SWNTs with dichlorocarbene led to functionalization of the nanotube walls.

Fluorometric assay of nitrite and nitrate in brain tissue after traumatic brain injury and cerebral ischemia

Nitric oxide synthase (NOS) is distributed within the brain, and nitric oxide (NO) is felt to be involved in the pathophysiology of deterioration after head injury and cerebral ischemia. This study determined the levels of the stable end products of NOS (NOx=nitrite+nitrate) after traumatic brain injury (TBI) and transient cerebral ischemia. A fluorometric assay using nitrate reductase and the NADPH regenerating system was used to quantitate NOx in ultrafiltered (10-kDa cutoff) cortical and hippocampal extracts after reduction of nitrate. In TBI rats, both the plasma and tissue showed a sharp increase in NOx levels 5 min after injury. Plasma NOx returned to control levels by 2 h after injury. Ipsilateral-cortex NOx levels returned to control levels approximately 6 h after injury and remained constant from 6-24 h. Contralateral-cortex returned near to control levels after 1 h. Hippocampus also followed a similar trend. In gerbils, there was a significant elevation in tissue NOx levels immediately after 10 min transient cerebral ischemia, which gradually returned to control levels over 24 h reperfusion. This striking burst of NO synthesis immediately after injury is clearly evident whether the injury is head trauma or ischemia, or whether the measurements were performed on tissue or plasma. It is unknown whether endothelial NOS, neuronal NOS, or both caused the elevation of the NO end products seen after the CNS insults.

Intraluminal suture occlusion of the middle cerebral artery in Spontaneously Hypertensive rats

In models of middle cerebral artery occlusion using intraluminal suture, the size and the distribution of ischemic injury vary considerably among laboratories. In transcranial models of cerebral ischemia, a more consistent cerebral ischemic lesion is seen in Spontaneously Hypertensive rats (SHR). In the present study, we performed intraluminal suture occlusion of the MCA in SHR and compared its reproducibility with those in Sprague-Dawley (SD) rats. Male SHR and SD rats were anesthetized with halothane and subjected to 2 h of temporary middle cerebral artery occlusion by an intraluminal suture. Comparisons of regional cerebral blood flow figures taken throughout the experiment and lesion volume figures taken at 24 h after occlusion were made between the two groups. Total lesion volume in the SHR group was 263.6+/-30.5 mm3 (mean +/- SD), significantly larger and less variable than that in the SD group (145.4+/-123.7 mm3, p < 0.02). Throughout the ischemic period there was a tendency for rCBF to be lower in the SHR group than in the SD group. In some animals of both groups, recirculation could not be produced by withdrawal of the suture from the ICA. The mortality rate was 9% in the SD group and 17% in the SHR group. Intraluminal MCA occlusion in SHRs is associated with a more consistent, reliable and reproducible volume of ischemic injury. In the studies of temporary focal cerebral ischemia, this model must be accompanied by the monitoring of regional cerebral blood flow so as to obtain more reliable results.

Effects of ifenprodil, a polyamine site NMDA receptor antagonist, on reperfusion injury after transient focal cerebral ischemia

Polyamines and N-methyl-D-aspartate (NMDA) receptors are both thought to play an important role in secondary neuronal injury after cerebral ischemia. Ifenprodil, known as a noncompetitive inhibitor of polyamine sites at the NMDA receptor, was studied after transient focal cerebral ischemia occurred. Spontaneously hypertensive male rats, each weighing between 250 and 350 g, underwent 3 hours of tandem middle cerebral artery (MCA) and common carotid artery occlusion followed by reperfusion for a period of 3 hours or 21 hours. Intravenous ifenprodil (10 microg/kg/minute) or saline infusion was started immediately after the onset of MCA occlusion and continued throughout the ischemic period. Physiological parameters including blood pressure, blood gas levels, blood glucose, hemoglobin, and rectal and temporal muscle temperatures were monitored. Six rats from each group were evaluated at 6 hours postocclusion for brain water content, an indicator of brain edema, and Evans blue dye extravasation for blood-brain barrier breakdown. Infarct volume was also measured in six rats from each group at 6 and 24 hours postocclusion. Ifenprodil treatment significantly reduced brain edema (82.5 +/- 0.4% vs. 83.5 +/- 0.4%, p < 0.05) and infarct volume (132 +/- 14 mm3 vs. 168 +/- 25 mm3, p < 0.05) compared with saline treatment, with no alterations in temporal muscle (brain) or rectal (body) temperature (35.9 +/- 0.4 degrees C vs. 36.2 +/- 0.2 degrees C; 37.7 +/- 0.4 degrees C vs. 37.6 +/- 0.6 degrees C; not significant). These results demonstrate that ifenprodil has neuroprotective properties after ischemia/reperfusion injury in the absence of hypothermia. This indicates that antagonists selective for the polyamine site of the NMDA receptors may be a viable treatment option and helps to explain some of the pathophysiological mechanisms involved in secondary injury after transient focal cerebral ischemia has occurred.

Dynamics of knowledge and attitudes about AIDS among the educated in southern India

AIDS awareness and attitudes among an educated segment of the Indian population were assessed. The study population was a total of 433 students and faculty in colleges and universities, and research & technical staff of the Public Health Service. While most knew that sexual intercourse (96%) & injection drug use (85%) could transmit HIV, and that shaking hands (95%) & mosquitoes (86%) could not, 63% did not know that breastfeeding was a mode of transmission and 71% falsely believed that they could acquire HIV by donating blood. The only variable to correlate positively with knowledge was education. Knowledge about true and false modes of transmission constituted three distinct dimensions as determined by factor analysis. An overwhelming majority (90%) harboured at least one hostile view towards persons with AIDS. Knowledge and education independently correlated with decreased hostility. There was great concern about the impact of the disease: 85% believed that AIDS is a very serious problem in India and 93% favoured increased government spending on AIDS education. These results display high levels of knowledge (with some gaps), and widespread support for increased action.

The biphasic opening of the blood-brain barrier in the cortex and hippocampus after traumatic brain injury in rats

This study examined the time course of the blood-brain barrier (BBB) opening and correlated this with brain edema formation after a lateral controlled cortical impact (CCI) brain injury in rats. Quantitative measurement of Evans blue (EB) extravasation using fluorescence was employed at 2, 4, 6 h and 1, 2, 3, 4 and 7 days after injury. Brain edema was measured by specific gravity of the tissue at corresponding time points. Two prominent EB extravasations were observed at 4-6 h and 3-day after injury in the injury-site cortex and the ipsilateral hippocampus. Brain edema became progressively more severe over time and peaked at 24 h after injury and began to decline after day 3. These results suggest that there is a biphasic opening of the BBB after CCI brain injury and the second opening of the BBB does not contribute to a further increase in edema formation.

Beneficial effects of S-adenosyl-L-methionine on blood-brain barrier breakdown and neuronal survival after transient cerebral ischemia in gerbils

We have studied the beneficial effects of S-adenosyl-L-methionine (SAM) tosylate on blood-brain barrier (BBB) breakdown and neuronal survival after transient cerebral ischemia in gerbils. BBB breakdown experiments were performed in pentobarbital anesthetized gerbils subjected to 10 min of bilateral carotid artery occlusion and 6 h of reperfusion. For BBB breakdown measurements, SAM (120 mg/kg, i.p.) was administered to gerbils just after occlusion and thereafter every hour up to 5 h. Fluorometric measurements quantified the blood-brain permeability tracer, Evans blue (EB). SAM treatment significantly reduced the BBB breakdown as indicated by reduced levels of EB fluorescence. Neuronal count experiments were conducted in gerbils subjected to transient ischemia and 7 days of reperfusion. For neuronal count experiments SAM (15-120 mg/kg) was administered at 6 and 12 h after reperfusion, and twice each day thereafter for 7 days. SAM dose dependently protected the hippocampal CA1 neurons assessed by histopathological methods. SAM has a beneficial effect on the outcome of ischemic injury by reducing the BBB breakdown and neuronal death.

Protective effects of ifenprodil on ischemic injury size, blood-brain barrier breakdown, and edema formation in focal cerebral ischemia

OBJECTIVE

Ifenprodil, a polyamine site N-methyl-D-aspartate receptor/channel antagonist, has been reported to decrease infarction volume after cerebral ischemia. However, the possible mechanisms of this protective effect have not been studied in detail. We investigated the effects of ifenprodil on ischemic injury size, blood-brain barrier (BBB) permeability, regional brain edema, and cerebral blood flow.

METHODS

Focal ischemia for 6 hours was produced by permanent occlusion of the middle cerebral artery in 15 anesthetized cats. Treatment with drug (n = 8) or vehicle (n = 7) was initiated at 5 minutes after ischemia and continued for 3 hours. Physiological variables were continuously monitored during experiments. We measured ischemic injury size, brain edema, and BBB permeability to Evans blue and determined regional cerebral blood flow by using laser doppler flowmetry.

RESULTS

Both ischemic injury size and BBB permeability were smaller in the ifenprodil-treated group, compared with the saline-treated group (P < 0.05). Ifenprodil treatment also attenuated brain edema formation in the dense ischemic region, compared with saline treatment (1.035 +/- 0.002 versus 1.028 +/- 0.002, P < 0.05). There was no significant change in cerebral blood flow with ifenprodil treatment.

CONCLUSION

Findings from this study confirm that ifenprodil treatment results in a significant decrease in the size of ischemic injury after focal ischemia. The tissue-sparing effect of ifenprodil is not related to its vasoactive properties. It is likely that its neuroprotective effects are related to its ability to antagonize N-methyl-D-aspartate receptors, which results in a decrease in brain edema and BBB permeability.