No description available.

Resumo: Manganita de lantânio dopada com estrôncio (La1-xSrxMnO3) tem recebido atenção da comunidade científica por apresentar propriedades elétricas e magnéticas, com funções para sensores de campo, catalisadores, eletrodos em células combustíveis, entre outras. A substituição de La3+ por Sr2+ resulta em uma transição de um estado isolante antiferromagnético para um estado metálico ferromagnético em composições de x³0,17 com valência mista Mn3+-Mn4+ responsável pelos portadores de carga. Na presença de campo magnético externo as manganitas dopadas exibem a propriedade magnetoresistência negativa gigante (GMR) e a resistividade do material varia com a temperatura e com o campo magnético aplicado. Este trabalho descreve a síntese de nanopartículas de La1-xSrxMnO3 (x=0,1; 0,2 e 0,3) pelo método da coprecipitação homogênea usando uréia como agente precipitante. Neste método, novos ligantes, tais como NH4 +, OH- e CO3 2-, podem substituir as moléculas de água nas posições de coordenação delas com os íons metálicos. Este método leva a uma nucleação homogênea prevenindo o crescimento das partículas precursoras. A dependência da magnetização DC com a temperatura para as amostras La1-xSrxMnO3 (x=0,1; 0,2 e 0,3) forneceu valores de temperatura crítica (TC) iguais a 140 K, 316 K e 357 K, respectivamente. A dependência da resistividade elétrica com a temperatura para pastilhas destes materiais apresentaram a transição metal-isolante e os valores encontrados para TC são similares aos valores de TC observados em monocristais destas manganitas. Espectroscopia infravermelho foi realizada para estudar as propriedades de transporte...(Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Lanthanum strontium-doped manganite (La1-xSrxMnO3) has attracted renewed attention at present due to the observation of interesting electrical and magnetic properties such as a giant negative magnetoresistance (GMR), catalytic, oxygen cathode reduction, field-sensing, among others. The substitution of La3+ ions by Sr2+ results in a transition from antiferromagnetic insulating state to a ferromagnetic metallic state at x ³ 0.17 with a Mn3+-Mn4+ mixed valence state responsible for the mobile charge carriers. This work describes the La1-xSrxMnO3 (0.1, 0.2 and 0.3) nanoparticle synthesis by the homogenous coprecipitation method using urea as precipitant agent. In this method, some new ligands like NH4+, OH- and CO3 2- can substitute the water molecules coordinating the metallic ions. This method allows the homogenous nucleation and prevents the particle precursors to grow. The temperature dependence of DC magnetization of the samples La1-xSrxMnO3 and critical temperature (TC) values were found to be 140 K, 316 K and 357 K, respectively for x = 0.1, 0.2 and 0.3. Temperature dependence of the electrical resistivity exhibits a metalinsulator transition at Tc in pellet samples and Tc values were found to be similar to those observed in single crystals of these manganitas. Infrared spectroscopy was performed to study spin dependent transport properties using the magnetorefractive effect (MRE), which probes the change of reflection in the infrared (IR) spectral region due to the change in electrical conductivity in the presence of a magnetic field. In the IR spectral region, the free carrier dispersion mechanism often dominates and MRE have been used to study intraband transitions of the conduction...(Complete abstract, click electronic access below) / Orientador: Miguel Jafelicci Junior / Coorientador: Carlos de Oliveira Paiva Santos / Doutor

Thesis (Ph. D.)--University of Hong Kong, 2001. / Includes bibliographical references.

Lanthanide-doped upconversion nanoparticles (UCNPs) are perceived as promising novel near-infrared (NIR) bioimaging agents characterised by high contrast and high penetration depth. However, the interactions between charged UCNPs and mammalian cells have not been thoroughly studied and the corresponding intracellular uptake pathways remain unclear. Herein, my research work involved the use of hydrothermal method and ligand exchange approach to prepare UCNP-PVP, UCNP-PEI, and UCNP-PAA. These polymer-coated UCNPs demonstrated good water dispersibility, the similar size distribution as well as similar upconversion luminescence efficiency. However, the positively charged UCNP-PEI evinced greatly enhanced cellular uptake in comparison with its neutral or negative counterparts, as revealed by cellular uptake studies. Meanwhile, it was discovered that cationic UCNP-PEI could be effectively internalized mainly through the clathrin endocytic machanism. This study is the first report on the endocytic mechanism of positively charged lanthanide-doped UCNPs. Furthermore, it allows us to control the UCNP-cell interactions by tuning surface properties. Glioblastoma multiforme (GBM) is the most common and malignant form of primary brain tumors in humans. Small molecule MRI contrast agents are used for GBM diagnosis and preoperative tumor margin delineation. However, the conventional gadolinium-based contrast agents have several disadvantages, such as a relatively low T1 relaxivity, short circulation half lives and the absence of tumor targeting efficiency. Multimodality imaging probes provide a better solution to clearly delineate the localization of glioblastoma. My research work also involved the development of multimodal nanoprobes for targeted glioblastoma imaging. Two targeted paramagnetic/fluorescence nanoprobes were designed and synthesized, UCNP-Gd-RGD and AuNP-Dy680-Gd-RGD. UCNP-Gd-RGD was prepared through PEGylation, Gd3+DOTA conjugation and RGD labeling of PEI-coated UCNP-based nanoprobe core (UCNP-NH2). It adopted the cubic NaYF4 phase, had an average size of 36 nm by TEM, and possessed a relatively intense upconversion luminescence of Er3+ and Tm3+. It also exhibited improved colloidal stability and reduced cytotoxicity compared with UCNP-NH2, and a higher T1 relaxivity than Gd3+DOTA. AuNP-Dy680-Gd-RGD was synthesized through bioconjugation of amine-modified AuNP-based nanoprobe core (AuNPPEG- NH2) by a NIR dye (Dy680), Gd3+DOTA and RGD peptide. It demonstrated a size of 3–6 nm by TEM, relatively strong NIR fluorescence centered at 708 nm, longterm physiological stability, and an enhanced T1 relaxivity compared with Gd3+DOTA. Targeting abilities of both UCNP-Gd-RGD and AuNP-Dy680-Gd-RGD towards overexpressed integrin αvβ3 receptors on U87MG cell surface was confirmed by their enhanced cellular uptake visualized by confocal microscopy imaging and quantified by ICP-MS, where their corresponding control nanoprobes were used for comparison. Furthermore, targeted imaging capabilities of UCNP-Gd-RGD and AuNP-Dy680-Gd- RGD towards subcutaneous U87MG tumors were verified by in vivo and ex vivo upconversion fluorescence imaging studies and by in vivo and ex vivo NIR fluorescence imaging and in vivo MR imaging studies, respectively. These two synthesized targeted nanoprobes, with surface-bounded cyclic RGD peptide and numerous T1 contrast enhancing molecules, are applicable in targeted MR imaging glioblastoma and delineating the tumor boundary. In addition, UCNP-Gd-RGD favors the upconversion luminescence with NIR-to-visible nature, while AuNPDy680- Gd-RGD possesses NIR-to-NIR fluorescence, and both lead to their potential applications in fluorescence-guided surgical resection of gliomas. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Thesis (Ph.D.)--University of Hong Kong, 2001. / Includes bibliographical references. Also available in print.

Thesis (Ph.D.)--University of California, Berkeley, 1963. / "UC-4 Chemistry" -t.p. "TID-4500 (24th Ed.)" -t.p. Includes bibliographical references (p. 76-78).

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Solid-state lasers are of great interest and importance in infrared sensing and imaging devices as well as in other applications. The objective of this research was to study the synthesis, crystal growth, and optical characterization of a material recently discovered for its potentially useful optical properties: lanthanum magnesium aluminate (LMA). In these experiments, samples of LMA were grown by the vertical gradient freeze (VGF) technique. This technique has most recently been used to grow high quality titanium doped sapphire crystals mainly due to the inherent stability of this system in providing a stationary heat zone during growth runs. The furnace used for this growth technique also has the advantage of requiring a reducing atmosphere which may be used, if desired, to maintain reduced valence states of the dopant ions. This thesis discusses tunable lasers, crystal growth techniques, and the results of the study on LMA.

No description available.

A eutectic in La-rich La-Cu₀.₅Ni₀.₅-Al alloys was determined by studying the melting behaviors and the microstructure observations. The microstructures of the La-Cu-Ni-Al alloys prepared by Bridgman Solidification and copper mould casting were studied by using scanning electron microscope (SEM). The results show that La₆₆[Cu₀.₅Ni₀.₅]₂₀Al₁₄ alloy is very near to a pseudo-ternary eutectic. When the cooling rate is higher than 450 K/s, fully amorphous can be formed; when the cooling rate is within 15 K/s to 450 K/s, the alloy has a microstructure of dendrite plus amorphous, when the cooling rate is within 12 K/s to 1.5 K/s, the microstructures of the alloy are dendrite plus eutectic, and when the cooling rate is lower than 0.12 K/s, the morphology of the alloy is eutectic microstructure. The off eutectic alloy has better glass forming ability, the best glass forming alloy obtained at La₆₂[Cu₀.₅Ni₀.₅>]₂₄Al₁₄ along the composition line of La₈₆₋x[Cu₀.₅Ni₀.₅]xAl₁₄. It has a potential to form bulk metallic glassy rod samples with diameter larger than 12 mm. / Singapore-MIT Alliance (SMA)