La défense publique de la thèse de Madame Selene Acosta Morales aura lieu le 18 mai 2021 à 16h par vidéo-conférence :
Mme Carla Bittencourt
Résumé de la dissertation:
A sensor is a device developed for detecting or measuring a physical quantity such as light, sound, pressure, temperature or magnetic field strength. Sensors have an important impact in our daily life, they are used to obtain information of our surroundings and even information of our own body in real time. Currently, we use sensors to detect toxic analytes in the air, to examine the quality of food, and also to monitor our health. The development of novel sensors and the technological optimization of the conventional ones will have important positive impact in the quality of everyday life, preventing or detecting diseases in early stages, monitoring chronic diseases, and alerting about the presence of toxic compounds in outdoor and indoor air. Nanomaterials have been reported as best active materials for novel sensors. For example, in chemical sensors ⸺ chemical sensors are measurement devices that convert a chemical or physical property of a specific analyte into a measurable signal, whose magnitude is normally proportional to the concentration of the analyte ⸺ the step one of the detection is the interaction of an analyte with the surface of the sensor active layer, therefore nanomaterials having the most of atoms at their surfaces, i.e., a high surface-to-volume ratio, will have higher sensibility (larger surface interaction) than their bulk counterparts. Besides, the unique chemical, mechanical, optical and magnetic properties of nanomaterials associated to their nanoscale open up a myriad of possibilities to develop novel active materials for sensors. The aim of this PhD thesis was twofold: (1) tune nanomaterials properties using selected methods and their detailed characterization, (2) apply the designed nanomaterials as active material for sensing. Two different type of sensors were studied: nanothermomethers and chemical sensors; lanthanide based nanomaterials (Eu3+-TiO2) were evaluated as temperature sensors with nanometer spatial resolution (nanothermometers) and carbon nanotubes (CNTs) were investigated as gas sensors. In the first part of the thesis carbon nanotubes were investigated as active material for gas sensing. The intrinsically inert surface of CNTs is the main limitation for the achievement of high sensitivity in carbon nanotubes based gas sensors. For this reason, the carbon nanotubes surface was firstly functionalized with oxygen groups. The functionalization was carried out with two different techniques: low kinetic ion irradiation and plasma functionalization. Both techniques were studied and compared, it is shown that low kinetic ion irradiation is the best option for functionalization of carbon nanotubes due to its efficiency, also it is fast, clean and controllable method. Four different types of oxygen groups were grafted at the surface of carbon nanotubes: hydroxyl (C-OH), epoxy (C-O-C), carbonyl (C=O) and carboxyl (COOH). Then, the thermal stability of the oxygen groups was evaluated trough a heat treatment in a UHV chamber. Carbonyl and hydroxyl groups were the most thermal stable functionalization in the CNTs surface. Afterwards, Ox-CNTs were decorated with metal nanoparticles in order to improve their reactivity to gas molecules, Pd and Ni-Pd nanoparticles were deposited on the surface of CNTs through plasma sputtering. The characterization of this hybrid material is shown in the final part of this section. Finally, Pd-Ox-CNTs and Ni-Pd-Ox-CNTs were tested as active materials for the sensing of the following substances: H2, NO2, toluene, benzene and ethanol. In the second part of the thesis Europium-TiO2 nanomaterials are investigated as temperature sensors for biological systems. The luminescence intensity of europium (Eu3+) depends critically on temperature, this property was used to engineer a luminescent nanothermometer. First, Eu3+-TiO2 nanoparticles with three concentration of europium (1, 3 and 5 wt. %) were synthetized by the sol-gel technique. After the synthesis, the nanoparticles were characterized through several techniques: XRD, NEXAFS, TEM, SEM and XPS. Then, the dependence of luminescence intensity with temperature of Eu3+-TiO2 was investigated, two different electronic transitions were used to develop a ratiometric nanothermometer with relative sensitivity values between 1.78 and 1.41% K-1. In the final part of this section Eu3+-TiO2 were internalized in human cells (L929 fibroblast), the successful internalization was observed through fluorescence microscopy and fluorescence microspectroscopy. The luminescence of nanoparticles internalized in L929 fibroblast cells was measured when the system was heated at different temperatures. An appropriate calibration curve using the luminescence intensity of the nanoparticles was obtained. Finally, the temperature variation inside single cells was determined using the Eu3+-TiO2 nanothermometer with sensitivity of 0.5 K per 1% of luminosity change when heated.