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|Luminescence properties in polymer and polymer/inorganic nanocapsule
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|The main focus of this thesis was to encapsulate the hydrophobic triplet-triplet annihilation (TTA-UC) and fluorescent materials into polymeric nanocapsules and to protect the system from the influence of molecular oxygen. Different controlled strategies were used to achieve the goal of reducing the formation of singlet oxygen that quenches the properties. In Section 4.2, the shell material of the nanocapsules was varied by using different polymers with different crystallinity nature to investigate the change in upconversion properties. Poly(methyl methacrylate) (PMMA) and poly(L-lactic acid) (PLLA) nanocapsules were successfully synthesized using the solvent evaporation method to encapsulate the hydrophobic organic dye molecules (sensitizer: PdOEP and emitter: perylene). The intensive UC fluorescence of perylene with λmax = 470 nm was only observed in PMMA nanocapsules but not in PLLA ones, which may be explained by the possible reaction of PLLA with emitter molecules during the procedure of synthesis of nanocapsules. The amount of triplet energy transfer from sensitizer molecule to emitter molecule was reduced due to the lower amounts of perylene molecules within the PLLA nanocapsules. Accordingly, PMMA nanocapsules were shown to be efficient in terms of upconversion emission. Furthermore, to observe the change in upconversion, different polymeric nanocapsules were successfully synthesized via free-radical polymerization. The TTA-UC dye molecules were successfully encapsulated in polystyrene (PS), PMMA, poly(acrylonitrile) (PAN), and poly(acrylonitrile/styrene) P(AN/S). The UC fluorescence of perylene with λmax = 470 nm was only observed in PS and PMMA nanocapsules. The absence of UC emission in PAN and P(AN/S) nanocapsule was correlated with a high intensity emission at 570 nm, which could be due to the formation of molecular dimers from the emitter molecule (perylene). Overall, we conclude that using different polymeric materials in the nanocapsule shell does not influence significantly the upconversion properties. In Section 4.3, the nanocapsules containing TTA-UC dyes were successfully synthesized under protective conditions (i.e., complete darkness and argon atmosphere) by free-radical miniemulsion polymerization. Both UC fluorescence and residual sensitizer phosphorescence were strongly enhanced in the polystyrene nanocapsules synthesized under argon atmosphere and darkness when compared to the ones synthesized under ambient and daylight conditions. We demonstrate that argon and darkness play a very important role in the improvement of the UC process as a result of an increased rate of triplet-triplet transfer from the PdOEP to perylene. By using this strategy, UC emission was successfully observed upon very low excitation intensity of λ = 532 nm laser (intensity is tens of mW cm−2). In Section 4.4, the enhancement of the fluorescent process was achieved by armoring with CeO2 polystyrene nanocapsules containing a model fluorophore molecule (i.e., terrylene diimide). The photo-oxidation process of the fluorescent molecule was successfully reduced by the in-situ crystallization of the metal oxide on the surface of the nanocontainers. The presence of oxygen vacancies in the structure of cerium(IV) oxide nanoparticles on the surface of polystyrene nanocapsules are useful to enhance the fluorescence. Thus, molecular oxygen from the external environment can be trapped into vacancy sites, prohibiting its entrance into the nanocapsules, which results in the reduction of the formation of singlet oxygen within the system. The fluorescence intensity was higher in hybrid CeO2/polymer nanocapsules prepared at ambient conditions under air than in pure polymer nanocapsules carefully synthesized under oxygen-free, inert atmosphere. In Section 4.5, we compare the effectivity of using different inorganic materials (Laponite RD, hydroxyapatite and CeO2) in the armoring of polystyrene nanocapsules. Laponite RD was successfully deposited by using layer-by-layer deposition method. Controlled in-situ crystallization was used to deposit hydroxyapatite and CeO2 on the nanocapsule surface. The triplet-triplet energy transfer from the sensitizer to the emitter molecule was increased by the presence of inorganic materials on the surface and further the photodegradation process of perylene molecule was reduced in the nanocapsules. Finally in Section 4.6, we investigated the use of Pickering miniemulsions to create hybrid clay-polystyrene nanocapsules. The particles were successfully placed on the surface and results in the enhancement of UC properties. The UC enhancement can be explained by two effects: (i) the presence of clay particles on the surface of capsules, which hinders the molecular oxygen to enter into nanocapsules; and (ii) the increase in the size of capsules, which helps the free movement of sensitizer and emitter molecules within the system, resulting in an increase of the triplet-triplet energy transfer. The different strategies described in this work provide a pool of different routes to control the structure of polymer and polymer/inorganic hybrid nanocontainers to enhance their luminescent properties. A combination of various inorganic components and polymers can be used to obtain hybrid nanocontainers with a wide variety of applications.
540 Chemistry and allied sciences
|Johannes Gutenberg-Universität Mainz
|FB 09 Chemie, Pharmazie u. Geowissensch.
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