Asymptotic behavior of two Bernstein-type operators is studied in this paper. In the first case, the rate of convergence of a Bernstein operator for a bounded function f is studied at points x where f(x+) and f(x−) exist. In the second case, the rate of convergence of a Szász operator for a function f whose derivative is of bounded variation is studied at points x where f(x+) and f(x−) exist. Estimates of the rate of convergence are obtained for both cases and the estimates are the best possible for continuous points. 相似文献
A automated synthesis of symmetric integrin αvβ3-targeted radiotracer [18F]FP-PEG3-β-Glu-RGD2 was carried out by multi-step procedure on the modified PET-MF-2V-IT-I synthesizer. Firstly, the prosthetic group of 4-nitrophenyl 2-[18F]fluoropropionate ([18F]NFP) was automated synthesized by a convenient three-step, one-pot procedure. Secondly, [18F]FP-PEG3-β-Glu-RGD2 was synthesized by coupling [18F]NFP with the symmetric RGD-peptide (PEG3-β-Glu-RGD2) and purified by a solid-phase extraction cartridge. The radiochemical yields of [18F]NFP were 35 ± 5 % (n = 10, decay-corrected) based on [18F]fluoride in 80 min. [18F]FP-PEG3-β-Glu-RGD2 was obtained with yield 40 ± 10 % (n = 5, decay-corrected) from [18F]NFP within 20 min. The radiochemical purity of [18F]FP-PEG3-β-Glu-RGD2 was greater than 98 %. 相似文献
The metabonomic changes in murine RAW264.7 macrophage-like cell line induced by ultrasmall superparamagnetic particles of
iron oxides (USPIO) have been investigated, by analyzing both the cells and culture media, using high-resolution NMR in conjunction
with multivariate statistical methods. Upon treatment with USPIO, macrophage cells showed a significant decrease in the levels
of triglycerides, essential amino acids such as valine, isoleucine, and choline metabolites together with an increase of glycerophospholipids,
tyrosine, phenylalanine, lysine, glycine, and glutamate. Such cellular responses to USPIO were also detectable in compositional
changes of cell media, showing an obvious depletion of the primary nutrition molecules, such as glucose and amino acids and
the production of end-products of glycolysis, such as pyruvate, acetate, and lactate and intermediates of TCA cycle such as
succinate and citrate. At 48 h treatment, there was a differential response to incubation with USPIO in both cell metabonome
and medium components, indicating that USPIO are phagocytosed and released by macrophages. Furthermore, information on cell
membrane modification can be derived from the changes in choline-like metabolites. These results not only suggest that NMR-based
metabonomic methods have sufficient sensitivity to identify the metabolic consequences of murine RAW264.7 macrophage-like
cell line response to USPIO in vitro, but also provide useful information on the effects of USPIO on cellular metabolism. 相似文献
A simple and highly efficient method is developed for the one-step in situ preparation of carbon-encapsulated MoO2 nanocrystals (MoO2@C) with core-shell structure for high-performance lithium-ion battery anode. The synthesis is depending on the solid-state reaction of cyclopentadienylmolybdenum tricarbonyl dimer with ammonium persulfate in an autoclave at 200 °C for 30 min. The large amount of heat generated during the explosive reaction cleaves the cyclopentadiene ligands into small carbon fragments, which form carbon shell after oxidative dehydrogenation coating on the MoO2 nanocrystals, resulting in the formation of core-shell structure. The MoO2 nanocrystals have an equiaxial morphology with an ultrafine diameter of 2–8 nm, and the median size is 4.9 nm. Hundreds of MoO2 nanocrystals are encapsulated together by the worm-like carbon shell, which is amorphous and about 3–5 nm in thickness. The content of MoO2 nanocrystals in the nanocomposite is about 69.3 wt.%. The MoO2@C anode shows stable cyclability and retains a high reversible capacity of 443 mAh g?1 after 50 cycles at a current density of 3 A g?1, owing to the effective protection of carbon shell. 相似文献
Bead cellulose loaded with iron oxyhydroxide (BCF) with 47 mass% Fe content was prepared and was successfully applied to the elimination of arsenic from aqueous solutions. A clearer understanding of the arsenic removal mechanism will provide accurate prediction of the arsenic adsorptive properties of the new adsorbent. To study the mechanism of the adsorption process, we measured the extended X-ray absorption fine structure (EXAFS) spectra of arsenite and arsenate sorbed onto the adsorbent with different surface coverages. Both arsenite and arsenate were strongly and specifically adsorbed by akaganéite adsorptive centers on BCF by an inner-sphere mechanism. There was no change in oxidation state following interaction between the arsenic species and the BCF surface. The dominant complex of arsenic species adsorbed on akaganéite was bidentate binuclear corner-sharing ((2)C) between As(V) tetrahedra (or As(III) pyramids) and adjacent edge-sharing FeO(6) octahedra. On the basis of the results from EXAFS spectra, the adsorptive characteristics of arsenic, such as the effects of pH and competing anions, were satisfactorily interpreted. 相似文献
Reduced graphene oxide hollow microspheres (rGO HMS) were encapsulated with gold nanoparticles (AuNPs) by spray drying. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Raman spectroscopy were used to characterize the AuNP/rGO HMS. When placed on a glassy carbon electrode (GCE), it exhibits excellent electrochemical catalytic properties towards the oxidation of nitrite. The electrocatalytic properties were studied using various electrochemical techniques. Compared to AuNP-decorated graphene sheet based electrodes documented in the literature, the one presented here provides a larger surface area. This enhances the catalytic activity towards nitrite. The electrode, typically operated at a working potential of 0.82 V (vs. SCE), has a linear response in the 5.0 μM to 2.6 mM nitrate concentration range, and a detection limit as low as 0.5 μM (at an S/N ratio of 3).
Graphical abstract Schematic presentation of the synthesis of graphene hollow microspheres encapsulated with of gold nanoparticles (AuNP/rGO HMS) through a spray drying technique. The material was applied to the electrochemical determination of nitrite.
Crystals of RbBe2BO3F2 up to a size of 22×18×5 mm3 have been grown by the hydrothermal method. The growth conditions were optimized mainly by adjusting the mineralizers and growth temperature. The crystals linear and nonlinear optical properties, including its transmittance spectrum and second harmonic generation from 532 to 266 nm, were measured and compared with those of flux grown crystals. A maximum conversion efficiency of 20% was obtained with a 25 ps, 10 Hz laser system. The results indicate that RbBe2BO3F2 crystals grown by the hydrothermal method show similar capability for frequency conversion compared with flux grown crystals and are promising for future deep-UV harmonic generation. 相似文献