Upconversion nanoparticles (UCNP) started to be developed at the beginning of this century, generating great expectations and interest due to their particular photophysical properties. UCNPs are nanomaterials capable of producing the upconversion (UC) phenomenon by which low-energy photons (in the near-infrared region, NIR) are transformed into high-energy photons (in the UV-vis region). The responsible species of this phenomenon are the lanthanide ions found as dopants of a transparent nanometric matrix. Therefore, in UCNPs, unlike semiconductor nanoparticles (NPs), nanometric confinement is not responsible for the optical properties but rather the interactions between the lanthanide ions in the matrix. Although, undoubtedly, the nanometer scale will affect its emissive properties. This thesis structured in four chapters. It begins with an introduction to the optical properties of UCNPs, with brief historical reviews and a detailed description of the protagonists of this unusual and interesting phenomenon: the lanthanide ions. The first chapter describes a microscopy technique developed by the group that allows characterizing the luminescence of upconversion (UC) emissions with temporal resolution. The second chapter shows how the previous technique allows measuring the lengthening of the lifetime of a functionalized organic dye on the surface of the UCNP with a strategy based on macrocyclic cucurbituril ligands. The upconversion nanohybrid (UCNH) synergistically incorporates the unique UCNP detection properties with those of the organic probe, resulting in a proof-of-concept sensor. In the third chapter, the cytotoxicity of the platform used to develop the sensor is evaluated in comparison with the starting UCNP. Finally, the fourth chapter delves into the photophysical characterization of a proof-of-concept UCNH composed of UCNP and an organic dye. In such a way that the guiding thread of the thesis is the photophysical characterization of the UCNP and the UCNH, since it is precisely the limitation in the development of these materials. Each chapter is, in turn, supported by an initial introduction with relevant bibliographic results that described the motivation that led to start the research. It also has a section on objectives, results and discussion and conclusions. Finally, the thesis is completed with the most relevant conclusions obtained in each chapter.
