Lanthanide oxide nanotubes and nanorods: synthesis, processing, luminescence and catalytic properties

Esta tese relata estudos de síntese, caracterização da estrutura e das propriedades de fotoluminescência e aplicações de nanotubos e nanobastonetes de óxidos de lantanídeos em pontas para microscopia de força atómica, catálise heterogénea e compósitos de base polimérica. Há um interesse crescente em compreender como o confinamento quântico decorrente da redução do tamanho de partícula pode influenciar a eficiência da luminescência, a dinâmica dos estados excitados, a transferência de energia e os efeitos de termalização de nanoluminóforos. Em nanocristais dopados com lantanídeos (Ln3+), e apesar da localização dos estados 4f, ocorrem efeitos de confinamento quântico via interacção com os modos vibracionais da rede. Em particular, a termalização anómala, descrita para uma variedade de nanocristais dopados com Ln3+, tem sido atribuída à ausência de modos vibracionais de menor frequência. Este nanoconfinamento pode ter impacto na dinâmica da luminescência, bem como na transferência de energia mediada por modos vibracionais e processos de upconversion. Nesta tese, relata-se o estudo deste efeito em nanotubos de Gd2O3:Eu3+. A influência de parâmetros como a concentração de európio e as condições de calcinação também foi investigada. Algumas aplicações destes óxidos de lantanídeos também foram exploradas, nomeadamente a modificação de pontas usadas em microscopia de força atómica com nanobastonetes de Gd2O3:Eu3+, lograda através de dielectroforese, técnica que não degrada a emissão de luz (rendimento quântico 0.47). As pontas modificadas são estáveis sob condições de trabalho, podendo ser aplicadas, por exemplo, em microscopia óptica de varrimento de campo próximo (SNOM). A oxidação em fase líquida do etilbenzendo foi investigada usando como catalisador nanotubos de CeO2, em presença dos oxidantes hidroperóxido de t-butilo e H2O2, e do solvente acetonitrilo, e temperaturas entre 55 e 105 ºC. Nanobastonetes de Gd2O3:Eu3+ recobertos com sílica foram preparados pelo método sol-gel. Esta cobertura resultou num aumento, quer do rendimento quântico de emissão, de 0.51 para 0.86 (excitação a 255 nm), quer dos tempos de vida,de 1.43 para 1.80 ms (excitação a 394.4 nm). A superfície dos nanotubos cobertos com sílica foi modificada com o agente de acoplamento metacrilato de 3-(trimetoxissilil)propilo que permitiu a preparação de compósitos através da subsequente polimerização in-situ do estireno por técnicas de miniemulsão e solução. ABSTRACT: This thesis reports on the synthesis, characterisation of the structure and photoluminescence properties, and applications of nanotubes and nanorods of lanthanides oxides in atomic force microscopy tips, heterogeneous catalysis and polymer-base composites. There is a growing interest in understanding how size-dependent quantum confinement affects the photoluminescence efficiency, excited-state dynamics, energy-transfer and thermalisation phenomena in nanophosphors. For lanthanide (Ln3+)-doped nanocrystals, and despite the localisation of the 4f states, confinement effects are induced mostly via electron-phonon interactions. In particular, the anomalous thermalisation reported for a handful of Ln3+-doped nanocrystals has been rationalised by the absence of lowfrequency phonon modes. This nanoconfinement may further impact on the Ln3+ luminescence dynamics, such as phonon-assisted energy transfer or upconversion processes. Here, this effect is investigated in Gd2O3:Eu3+ nanotubes. The influence of parameters such as europium concentration and calcination procedure is also studied. Some applications of these lanthanides oxides have been explored, for instance the modification of atomic force microscopy tips with photoluminescent Gd2O3:Eu3+ nanorods, using dielectrophoresis, a technique which preserves the red emission of the nanorods (quantum yield 0.47). The modified tips are stable under working conditions and may find applications in scanning near-field optical microscopy. The liquid-phase oxidation of ethylbenzene over CeO2 nanotubes has been investigated, using tert-butyl-hydroperoxide and H2O2 as the oxidising agents, and acetonitrile as the solvent, in the range 55-105 ºC. Gd2O3:Eu3+ nanorods have been coated with silica via a sol-gel approach. The silica coating increases both, the Eu3+ absolute emission quantum yields from 0.51 to 0.86 (255 nm excitation), and decay times from 1.43 to 1.80 ms (394.4 nm excitation). The silica coating was modified with 3- (trimethoxysilyl) propyl methacrylate and, subsequently, composites have been prepared by in-situ radical polymerisation of styrene via miniemulsion and solution routes.

Silver nanoparticles flow in an aquatic trophic chain

Silver nanoparticles (AgNP) have been produced and applied in a variety of products ranging from personal care products to food package containers, clothing and medicine utilities. The antimicrobial function of AgNP makes it very useful to be applied for such purposes. Silver (Ag) is a non-essential metal for organisms, and it has been historically present in the environment at low concentrations. Those concentrations of silver increased in the last century due to the use of Ag in the photographic industry and lately are expected to increase due to the use of AgNPs in consumer products. The presence of AgNP in the aquatic environment may pose a risk for aquatic species, and the effects can vary from lethal to sublethal effects. Moreover, the contact of aquatic organisms with AgNP may not cause immediately the death of individuals but it can be accumulated inside the animals and consequently transferred within the food chain. Considering this, the objective of this work was to study the transfer of silver nanoparticles in comparison to silver ions, which was used as silver nitrate, within an aquatic food chain model. To achieve this goal, this study was divided into four steps: the toxicity assessment of AgNP and AgNO3 to aquatic test-species, the bioaccumulation assessment of AgNP and AgNO3 by Pseudokirchneriella subcapitata and Daphnia magna under different exposure scenarios, and finally the evaluation of the trophic transfer of Ag through an experimental design that included the goldfish Carassius auratus in a model trophic chain in which all the species were exposed to the worse-case scenario. We observed that the bioconcentration of Ag by P. subcapitata is mainly driven by ionic silver, and that algae cannot internalize these AgNPs, but it does internalizes dissolved Ag. Daphnia magna was exposed to AgNP and AgNO3 through different exposure routes: water, food and both water and food. The worse-case scenario for Daphnia Ag bioaccumulation was by the joint exposure of contaminated water and food, showing that Ag body burdens were higher for AgNPs than for AgNO3. Finally, by exposing C. auratus for 10 days through contaminated water and food (supplied as D. magna), with another 7 days of depuration phase, it was concluded that the 10 days of exposure were not enough for fish to reach a plateau on Ag internal concentration, and neither the 7 days of elimination were sufficient to cause total depuration of the accumulated Ag. Moreover, a higher concentration of Ag was found in the intestine of fish when compared with other organs, and the elimination rate constant of AgNP in the intestine was very low. Although a potential for trophic transfer of AgNP cannot be suggested based in the data acquired in this study, there is still a potential environmental risk for aquatic species.