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Link visio : https://meet.google.com/shu-bvrp-xrc
The defence will be in English.
Abstract: Temperature sensing with accuracy and good spectral resolution is widely demanded in research and industry, particularly in the fields of biomedicine and microelectronics. This necessity arises from the lack of suitability of the conventional thermal probes available on the market for remote measurements at scales below 10 μm. In the biological context, thermal monitoring is a useful tool because temperature changes are indicators of inflammatory areas, diseases, and tumors. Some previous studies have shown that temperature monitoring is promising not only to provide early diagnosis but also in assisting disease treatments, such as hyperthermia for cancer treatment. In this regard, luminescent nanoprobes made of inorganic materials doped with rare-earth ions have been as interesting means to measure local temperature precisely and remotely.
The thermal readout can be obtained by tracking how the ratio between two photoluminescence (PL) emission lines, hereafter referred to as the Luminescence Intensity Ratio (LIR), evolves with temperature. A calibration curve relating LIR and temperature can then be extracted from experimental data in the laboratory. Nonetheless, developing adequate luminescent nanothermometers for biological applications continues to be a major hurdle. These thermal sensors must be small in size, stable and well-dispersed in physiological solutions, have negligible toxicity, exhibit an excellent thermal response, and have intense PL emissions within the biological windows (BWs), which are the wavelength ranges in which light penetrates most deeply into biological tissues.
This work reports on the development of oxides doped with rare-earth ions aimed at nanothermometry for future biological applications. It comprised the synthesis, characterization, and analysis of thermal sensing performance using PL emissions in the BWs of Y3Al5O12 (YAG), Y2O3, and Y4Al2O9 (YAM) co-doped
with Nd3+ and Yb3+. Initially, the first two host matrices were synthesized via the modified Pechini method for co-doping engineering to determine the ideal
concentrations of Nd3+ and Yb3+ with the purpose of achieving an optimized PL emission signal of the rare-earth ions. Then, to obtain well-dispersed individual nanocrystals (NCs), YAG: Nd3+– Yb3+ and Y2O3: Nd3+-Yb3+ were synthesized by the solvothermal route and the two-step urea-based route, respectively, with conditions systematically optimized to fulfill the requirements of this thesis. The third host matrix, YAM, was studied based on the modified Pechini synthesis to investigate its thermal response when single-doped with Nd3+ and co-doped with Nd3+ and Yb3+. Lastly, a new synthesis method for YAM was explored.
The findings of this work showed that YAG: Nd3+-Yb3+ exhibited significant potential, particularly after a thin silica layer was deposited around the NCs synthesized under solvothermal conditions. This coating allowed a protected annealing at 850°C to enhance the PL emission, without causing NCs agglomeration. At physiological temperature, the resulting core-shell nanoparticles (NPs) YAG: Nd3+-Yb3+@SiO2 had a final size of 87 ± 20 nm, a relative thermal sensitivity (Sr) of 0.60%.K-1, and an outstanding thermal resolution (δT) of 0.16 K. In comparison, Y2O3: Nd3+-Yb3+ NCs of 22 ± 5 nm presented a Sr of around 0.50%.K-1, but δT ~ 0.40 K owing to a lower signal-to-noise ratio. In turn, YAM revealed a competitive thermal response when single-doped with Nd3+, presenting Sr = 0.50%.K-1 and δT = 0.26K at body temperature. On the other hand, due to the complex crystalline structure of this oxide, the insertion of both Nd3+ and Yb3+ ions hampers the thermal sensing efficiency to less than 0.40%.K-1 of Sr at physiological temperature, with δT fluctuating between 0.23 and 0.68 K across the temperature range.
Therefore, the outcomes of this work unveil a good prospective of Nd3+-Yb3+ co-doped YAG as a nanothermometer, thanks to its decreased size, good thermal sensing features, and intense PL emission within the BWs, paving the way for biological tests to verify its use in the biomedical context. Overall, the insights from this study open up new avenues for improving the synthesis and applications of the oxides in nanothermometry.
