Biomedical subjects

David E McCready

Publications and source records attributed to David E McCready.

5 recordsLinked to original sources

PubMed2006

Excited carrier dynamics of alpha-Cr2O3/alpha-Fe2O3 core-shell nanostructures.

In this work alpha-Cr(2)O(3)/alpha-Fe(2)O(3) core-shell polycrystalline nanostructures were synthesized by using alpha-Cr(2)O(3) nanoparticles as seed crystals during aqueous nucleation. The formation of alpha-Fe(2)O(3) polycrystallites on alpha-Cr(2)O(3) surfaces was confirmed by X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray analysis. The excited-state relaxation dynamics of as-grown core-shell structures and "pure" alpha-Fe(2)O(3) particles of the same size were measured with femtosecond transient absorption spectroscopy. The results show the carrier lifetimes decay within a few picoseconds regardless of sample. This is likely due to fast recombination/trapping of carriers to defects and iron d-states.

Journal Article↗

PubMed2005

Upconversion luminescence from CdSe nanoparticles.

Efficient upconversion luminescence has been observed from CdSe nanoparticles ranging in size from 2.5 to 6 nm. The upconversion luminescence exhibits a near-quadratic laser power dependence. Emissions from both excitons and trap states are observed in the upconversion and photoluminescence spectra, and in the upconversion luminescence the emission from the trap states is enhanced relative to the trap-state emission in the photoluminescence. The upconversion decay lifetimes are slightly longer than the photoluminescence decay lifetimes. Time-resolved spectral measurements indicate that this is due to the involvement of long decay components from surface or trap states. Both the photoluminescence and upconversion luminescence decrease in intensity with increasing temperature due mainly to thermal quenching. All the observations indicate that trap states work as emitters rather than as intermediate states for upconversion luminescence and that two-photon absorption is the likely excitation mechanism.

Cadmium Compounds↗

PubMed2005

Synthesis and luminescence of ZnMgS:Mn2+ nanoparticles.

Efficient green emission from ZnMgS:Mn2+ nanoparticles prepared by co-doping Mg2+ and Mn2+ ions into ZnS lattices has been observed. The synthesis is carried out in aqueous solution, followed by a post-annealing process, thus showing the features of less complexity, low cost, and easy incorporation of dopants. In comparison with the emission of ZnS:Mn2+ nanoparticles, which is located generally around 590 nm, the photoluminescence of ZnMgS:Mn2+ nanoparticles is blue-shifted by 14 nm in wavelength, leading to the enhanced green emission. The X-ray diffraction, electron spin resonance, and pressure dependent photoluminescence measurements suggest that the change of the crystal field caused by Mg2+ ionic doping and the lower symmetry in the nanoparticles may account for the blue-shift of the photoluminescence. The ZnMgS:Mn2+ nanoparticles with 1% Mn2+ doping exhibit the strongest luminescence, which could potentially meet the requirements for the construction of green light emitting diodes.

Crystallization↗

PubMed2003

Observation of a distinct transition in the mode of interconversion of ring pucker conformers in non-crystalline d-ribose-2'-d from 2H NMR spin-alignment.

Internal motions of d-ribose selectively 2H-labeled at the 2' position were measured using solid state 2H NMR experiments. A sample of d-ribose-2'-d was prepared in a hydrated, non-crystalline state to eliminate effects of crystal-packing. Between temperatures of -74 and -60 degrees C the C2'-H2' bond was observed to undergo two kinds of motions which were similar to those of C2'-H2'/H2" found previously in crystalline deoxythymidine (Hiyama et al. (1989) J. Am. Chem. Soc., 111, 8609-8613): (1) Nanosecond motion of small angular displacement with an apparent activation energy of 3.6+/-0.7 kcal mol(-1), and (2) millisecond to microsecond motion of large amplitude with an apparent activation energy > or =4 kcal mol(-1). At -74 degrees C, the slow, large-amplitude motion was best characterized as a two-site jump with a correlation time on the millisecond time scale, whereas at -60 degrees C it was diffusive on the microsecond time scale. The slow, large-amplitude motions of the C2'-H2' bond are most likely from interconversions between C2'-endo and C3'-endo by way of the O4'-endo conformation, whereas the fast, small-amplitude motions are probably librations of the C2'-H2' bond within the C2'-endo and C3'-endo potential energy minima.

DNA↗

PubMedDate not supplied

Aluminum effect on dissolution and precipitation under hyperalkaline conditions: II. Solid phase transformations.

The high-level radioactive, Al-rich, concentrated alkaline and saline waste fluids stored in underground tanks have accidentally leaked into the vadose zone at the Hanford Site in Washington State. In addition to dissolution, precipitation is likely to occur when these waste fluids contact the sediments. The objective of this study was to investigate the solid phase transformations caused by dissolution and precipitation in the sediments treated with solutions similar to the waste fluids. Batch experiments at 323 K were conducted in metal- and glass-free systems under CO2 and O2 free conditions. Results from X-ray diffraction (XRD), quantitative X-ray diffraction (QXRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and energy dispersive X-ray fluorescence spectroscopy (EDXRF) indicated that significant solid phase transformations occurred in the sediments contacted with Al-rich, hyperalkaline, and saline solutions. The XRD and QXRD analyses confirmed that smectite and most likely biotite underwent dissolution. The SEM and the qualitative EDS analyses confirmed the formation of alumino-silicates in the groups of cancrinite and probably sodalite. The morphology of the alumino-silicates secondary phases changed in response to changes in the Si/Al aqueous molar ratio. The transformations in the sediments triggered by dissolution (weathering of soil minerals) and precipitation (formation of secondary phases with high specific surface area and probably high sorption capacities) may play a significant role in the immobilization and ultimate fate of radionuclides and contaminants such as Cs, Sr, and U in the Hanford vadose zone.

Accidents↗