Richard K. Hailstone

Custom cerium oxide nanoparticles protect against a free radical mediated autoimmune degenerative disease in the brain.

Cerium oxide nanoparticles are potent antioxidants, based on their ability to either donate or receive electrons as they alternate between the +3 and +4 valence states. The dual oxidation state of ceria has made it an ideal catalyst in industrial applications, and more recently, nanocerias efficacy in neutralizing biologically generated free radicals has been explored in biological applications. Here, we report the in vivo characteristics of custom-synthesized cerium oxide nanoparticles (CeNPs) in an animal model of immunological and free-radical mediated oxidative injury leading to neurodegenerative disease. The CeNPs are 2.9 nm in diameter, monodispersed and have a -23.5 mV zeta potential when stabilized with citrate/EDTA. This stabilizer coating resists being washed off in physiological salt solutions, and the CeNPs remain monodispersed for long durations in high ionic strength saline. The plasma half-life of the CeNPs is ∼4.0 h, far longer than previously described, stabilized ceria nanoparticles. When administered intravenously to mice, the CeNPs were well tolerated and taken up by the liver and spleen much less than previous nanoceria formulations. The CeNPs were also able to penetrate the brain, reduce reactive oxygen species levels, and alleviate clinical symptoms and motor deficits in mice with a murine model of multiple sclerosis. Thus, CeNPs may be useful in mitigating tissue damage arising from free radical accumulation in biological systems.

Effect of I− impurity on the efficiency of silver cluster formation on AgBr microcrystal surfaces

Computer simulation is used to study the effect of iodide impurity on silver cluster formation on AgBr microcrystals. The simulation is based on a nucleation-and-growth model of silver cluster formation in competition with recombination of electrons and holes. The efficiency of silver cluster formation is calculated as a function of microcrystal size and shown to increase with size for the impurity-free control, contrary to experimental data. This behavior is due to a partitioning of the hole between free and trapped states which favors the free side as size increases. As a result, recombination decreases and efficiency increases at large microcrystal size. Iodide impurity decreases efficiency relatively more at larger sizes because it introduces an internal recombination pathway not present in the control simulation. Because of their larger volume-to-surface-area ratios, the larger microcrystals are affected more by this additional recombination pathway than smaller microcrystals.

Sulphide centres on AgIBr (100) surfaces: characterization and energy levels

Abstract Sensitometric and spectroscopic techniques are used to characterize sensitizer centres produced by sulphur sensitization of AgIBr cubes. Sulphur sensitization primarily affects the long-wavelength sensitivity in three spectral regions: 550, 700 and 800 nm. The concentration dependence of the long-wavelength sensitivity in these spectral regions shows the first to be associated with single-sulphide centres. The other two spectral regions are attributed to multiple-sulphide species, but thiosulphate concentration-dependent activation energies for long-wavelength sensitivity precluded a more definitive assignment. A prominent 480–490 nm peak is observed in diffuse reflectance spectroscopy of these emulsions that is not observed in the long-wavelength sensitivity measurements. This peak is assigned to a product of the sulphur sensitization that is not photographically active. An energy level scheme was constructed on the basis of the activation energies and photon absorption energies. This scheme showed that all three sulphide centres are poor hole traps, but their electron trap depth increases with increasing absorption wavelength.

Is iron doping of nanoceria possible at low temperature

Iron doping of ceria (CeO2) was attempted by an aqueous reaction process in the presence of an organic stabilizer. The 2–3xa0nm diameter nanoparticles were characterized by transmission electron microscopy, X-ray diffractometry, inductively coupled plasma atomic emission spectrometry (ICP-AES), and oxygen storage capacity (OSC). ICP-AES showed that both metals were present, but only crystalline ceria was observed. The estimated amount of Fe incorporated in the ceria phase was about 2–4xa0at.%, leaving a significant amorphous iron phase postulated to be FeOOH. Heating experiments converted the amorphous phase into crystalline maghemite and hematite phases in the 59xa0at.% nominal Fe sample. OSC increased linearly with Fe concentration and was appreciably larger than any previously reported OSC values for doped ceria. These results can be explained by the high OSC inherent to the amorphous iron phase, which is converted to a crystalline hematite phase in our OSC measurement. Surprisingly, the OSC cyclability inherent to pure ceria persists in the iron-doped material.

Selenide versus sulphide centres on (100) AgIBr surfaces: characterization and energy levels

Abstract Sensitometric and spectroscopic techniques are used to characterize sensitizer centres produced by sulphur and selenium sensitization of AgIBr cubes. Sulphur sensitization primarily affects the long-wavelength sensitivity in three spectral regions—550, 700 and 800 nm. For selenium sensitization only the 550 and 800 nm spectral regions were affected, along with a weak effect at 650 nm. The concentration dependence of the long wavelength sensitivity showed the 550 nm region to be associated with single-chalconide centres. The other spectral regions are assumed to be multiple-chalconide species, but concentration-dependent activation energies for long-wavelength sensitivity precluded a more definitive assignment. A prominent 480–490 nm peak is observed in diffuse reflectance spectroscopy of these emulsions sensitized with either sulphur or selenium that is not observed in the long-wavelength sensitivity measurements. This peak is assigned to a product of the chemical sensitization that is not photographically active. An energy level scheme was constructed based on the activation energies and photon absorption energies. This scheme indicated that all chalconide centres are poor hole traps, but their electron trap depth increases with increasing absorption wavelength. The speed advantage of selenium over sulphur sensitization is suggested to be due to fewer but slightly deeper traps created by the former sensitization.

Photography: Making every photon count

Improving the sensitivity of photographic emulsions requires much chemical processing, which increases the risk of fogging. A new way of increasing emulsion sensitivity without introducing fog has the potential to improve high-speed films, which may suffer from poor image quality.

Sulphide centres on (111) AgBr surfaces: energy levels and computer simulated sensitometry

Abstract Sensitometric and spectroscopic techniques are used to characterize sensitizer centres produced by sulphur sensitization of AgBr octahedra. Activation energies for long-wavelength sensitivity were independent of thiosulphate concentration for the single-sulphide centres (550 nm), but showed a concentration dependence for the multiple-sulphide centres (700 nm). The achievement of maximum photographic speed is associated with the production of the multiple-sulphide centres. Using activation energies for long-wavelength sensitivity, with the lowest one for the multiple-sulphide centres being taken as closest to reality, an energy level scheme was constructed for the two sulphide centres. The single-sulphide centres are estimated to have an electron trap depth between 0 and 0.1 eV, whereas the multiple-sulphide centres have trap depths between 0.25 and 0.45 eV. Deconvolution of thiosulphate-induced absorption spectra suggested that only 65 per cent of the converted thiosulphtate is photographically active and that the single-sulphide centres are the predominant sulphide species. Computer simulated sensitometry based on these ideas was consistent with experimental sensitometry. Computer simulation of the sensitometry for the optimum thiosulphate concentration required a small concentration of the multiple-sulphide centres. Attempts to simulate the sensitometry of the oversensitized emulsion were less successful. Computer simulations based on the single-sulphide centres acting as hole traps were inconsistent with the experimental data.

Electronic properties of chemically produced silver clusters: electron trapping or hole removing?

Abstract Recent research by several groups has identified two types of chemically produced silver cluster. All agree that one type has the ability to capture and remove photogenerated holes. The other silver cluster category purportedly traps electrons, but its sensitometric role is controversial. An attempt is made to reconcile the different viewpoints on these silver clusters by considering the various experiments in the light of a general understanding of photoproduced silver clusters. If some of the chemically produced silver clusters trap electrons, they are not P centres as normally defined. They may be shallow electron traps with no apparent sensitometric value. Alternatively, they may be very inefficient hole-removing centres.

Sulphide centres on (111) AgBr surfaces: the effect of thiocyanate on electronic properties

Abstract Sensitometric and spectroscopic techniques are used to study the effect of sodium thiocyanate addition to sulphur-sensitized AgBr octahedra. Thiocyanate increased the speed of an oversensitized emulsion, causing it to have a speed comparable to the optimally suphur-sensitized emulsion at high irradiance and greater speed at low irradiance. Thiocyanate decreased the absorption by sulphide centres at shorter wavelengths (500–650 nm), but increased the absorption at longer wavelengths. The long-wavelength sensitivity (> 500 nm) was also increased by thiocyanate, whereas the activation energy for this response was reduced. These results are interpreted with a model that assumes that thiocyanate interacts with multiple-sulphide centres to lower their ground state energy level. This interaction reduces the degree of free-hole/trapped-electron recombination which occurs at these centres and which is responsible for the over-sensitization effect. A computer simulation based on this model gave simulated reciprocity failure data which were qualitatively consistent with the experimental trends. The effects of thiocyanate on absorption are interpreted as a change in extinction coefficient.

Luminescent Colloidal Silicon Nanocrystals Prepared by Nanoseconds Laser Fragmentation and Laser Ablation in Water

The surface states of silicon nanocrystals (Si-ncs) considerably affect quantum confinement effects and may determinate final nanocrystals properties. Colloidally dispersed Sincs offer larger freedom for surface modification compared to common plasma enhanced chemical vapor deposition or epitaxial synthesis in a solid matrix. The Si-ncs fabrication and elaboration in water by pulsed laser processing is an attractive alternative for controlling and engineering of nanocrystal surface by environmentally compatible way. We report on the possibility of direct silicon surface ablation and Si-ncs fabrication by nanosecond pulsed laser fragmentation of electrochemically etched Si micrograins and by laser ablation of crystalline silicon target immersed in de-ionized water. Two nanosecond pulsed lasers (Nd:YAG, and excimer KrF) are successfully employed to assure fragmentation and ablation in order to produce silicon nanoparticles . Contrary to the fragmentation process, which is more efficient under Nd:YAG irradiation, the laser ablation by both lasers led to the fabrication of fine and room temperature photoluminescent Si-ncs. The processing that has natural compatibility with the environment and advanced state of fabrication technologies may imply new possibilities and applications.