Lanthanide-doped nanoparticles exhibit upconversion fluorescence which means the emission wavelength is shorter than the excitation wavelength. Because of that they are very promising for use in biomedical imaging. The limitations of lanthanide-doped nanoparticles are low quantum yield, not sufficient chemical and colloidal stability in a biological environment, and possible toxicity. Low quantum yield can be improved with the choice of crystal matrix and dopant ions and with the optimisation of size, crystal structure, and dopant concentration. Other limitations can be reduced with additional coatings, among which one of the most promising are polymer coatings. However, they can change spectroscopic properties of nanoparticles which is why the purpose of this master thesis was spectroscopic analysis of inorganic nanoparticles with NaYF4 crystal matrix co-doped with Yb3+ and Tm3+ ions. The goal was to evaluate different coatings and their effect on upconversion fluorescence.
The measurements were done on a sensitive high-resolution spectrofluorometer. Emission spectra allowed us to evaluate the effect of size and solvent on the upconversion fluorescence and to assess the stability of nanoparticles over a longer period. The effect of wavelength and intensity of excitation light on upconversion fluorescence was also investigated. As the spectrum and intensity of fluorescence depend on the excitation conditions because of the nonlinearity of the upconversion process. For this reason, the comparison between different nanoparticles is difficult. To avoid this issue we performed time-dependent measurements of fluorescence after a short excitation pulse. Analysis of these measurements gave us lifetimes of excited states which are not dependent on the intensity of excitation light. Lifetimes allow us to assess fluorescence quenching and consequently the adequacy of different coatings.
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