Abstract
Upon exposure to immune or inflammatory stimuli, dendritic cells (DC) migrate from peripheral tissues to lymphoid organs, where they present Ag. CC chemokines induce chemotactic and ...transendothelial migration of immature DC, in vitro. Maturation of DC by CD40L, or by LPS, IL-1, and TNF, induces down-regulation of the two main CC chemokine receptors expressed by these cells, CCR1 and CCR5, and abrogates chemotaxis to their ligands. Inhibition was rapid (<1 h) and included the unrelated agent FMLP. Concomitantly, the expression of CCR7 and the migration to its ligand EBI1 ligand chemokine (ELC)/macrophage inflammatory protein (MIP)-3β, a chemokine expressed in lymphoid organs, were strongly up-regulated, though with slower kinetics (24–48 h). Rapid inhibition of responsiveness to chemoattractants present at sites of inflammation and immune reaction may be permissive for leaving peripheral tissues. Conversely, the slower acquisition of responsiveness to ELC/MIP-3β may guide subsequent localization of DC in lymphoid organs.
Spin resonance, magnetic measurements, and structural analysis are reported for metal‐semiconductor separated SWCNTs after filling with ferrocene. Raman scattering performed after a heat treatment ...confirms partial transformation to double‐walled CNTs but results from spin resonance (FMR), X‐ray diffraction, and TEM evidence in addition the growth of ferromagnetic nanoparticles. For the metallic tubes the particles are identified as magnetite (Fe3O4) with full chemical stoichiometry. From the temperature dependence of the FMR response and from measurements of the magnetization by dc and ac SQUID the magnetite crystals are shown to undergo a Verwey transition. The transition temperature from the SQUID experiment is around 125 K as expected but considerably higher than observed from the FMR analysis. Results for semiconducting tubes are similar but magnetic particles are an order of magnitude smaller and exhibit different structures in addition to magnetite.
Electron spin resonance in the X band is reported for fully metal–semiconductor separated SWCNTs. For the experiments samples were immersed in ethanol or wrapped into a teflon foil. The response from ...the metallic tubes exhibits a strong asymmetry in the line shape whereas the asymmetry for the semiconducting (SC) tubes is comparatively small. In both cases the line widths are unusual small, of the order of 4 G. Particular attention is paid to SC nanotubes which exhibit a much stronger signal as compared to the metallic tubes. The signal intensity is nearly Curie like with a small enhancement beyond $1/T$ in the low temperature range. The $1/T$ behavior renders the ESR response unobservable at 300 K. The finite value for the asymmetry parameter in the SC tubes is assumed to originate from charges picked up during exposure to air or to ethanol.
On the diffraction pattern of C60 peapods CAMBEDOUZOU, J; PICHOT, V; ROLS, S ...
The European physical journal. B, Condensed matter physics,
11/2004, Letnik:
42, Številka:
1
Journal Article
Single-walled carbon nanotubes (SWNTs) were grown from the condensed phase by thermally post-annealing of a soot-like material, possibly containing precursors for SWNT growth. The soot-like material ...was obtained by laser ablation of Ni–Co–graphite composite targets at 550–700 °C. This initial material did not contain any SWNTs but post-annealing treatment inside an electric furnace at 1200 °C under argon flow resulted in SWNTs. The soot-like material prepared at lower temperatures (room temperature to 400 °C did not yield SWNTs even after post-annealing. These results indicate that the precursors for the growth of SWNTs can form only above a certain threshold temperature, about 550 °C. Once these precursors are present, SWNTs can grow by annealing of the metal–carbon mixture. A growth model is proposed based on these results.
We have separately probed the doping behavior of semiconducting and metallic single wall carbon nanotube (SWNT) films, by optical absorption and dc resistance measurements. Either electron acceptors ...(Br
2, I
2) or donors (K, Cs) were used as dopants with controlled stoichiometry. Disappearance of the absorption bands at 0.7, 1.2 (assigned to semiconducting SWNT) and 1.8
eV (assigned to metallic SWNT) and the concomitant decrease of resistance due to doping were attributed to electron depletion or filling in specific bands of semiconducting or metallic SWNT. This demonstrates the amphoteric doping behavior of SWNT. Semiconducting and metallic SWNT undergo charge transfer in a specific sequence: initially the transition at 0.7
eV, subsequently around 1.2
eV and finally the feature at 1.8
eV are affected depending on the concentration of the dopant. Changes in the electronic properties are discussed in terms of charge transfer mechanisms in the framework of the rigid-band model.
We demonstrated that optical absorption spectra of single wall carbon nanotubes (SWNT) changed drastically by the doping of halogens and alkali metals. From disappearance of absorption peaks ...attributable to interband optical transitions, it was established that the semiconducting phase of SWNT can be doped amphoterically. Emergence of new absorption peaks induced by heavy doping are explained by invoking the involvement of low-lying valence states (or high-lying conduction states) in the optical transition. We also found that the interband absorption peaks considerably and reversibly broadened and shifted under high pressure up to 4.1
GPa. Good correspondence of the present results to recent theoretical works suggests that this change may reflect a semiconductor–metal transition induced by intertube interactions and/or by symmetry breaking.
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