Resumen de Optical rotation of luminescent microparticles for remote sensing and manipulation
The precise and non-invasive control over single particles is key for an array of physical and bio-medical applications, such as microfluidics or cell studies. In particular, the manipulation of single luminescent rare-earth-doped (RE3+) particles is of great interest due to their biocompatibility and the sensitivity of their luminescent properties to environmental conditions, such as temperature. To achieve stable three-dimensional, precise, non-contact (i.e., sterile) manipulation of individual particles, the optical trapping technique stands out. It was developed in the 1970s by Arthur Ashkin, and it is based on the radiation pressure and optical forces exerted by a focused laser beam on individual particles of small size, which experience an attractive force towards the high-intensity region created by the laser focus. The relevance and suitability of optical trapping for the study of biological systems is evidenced by the 2018 Nobel Prize in Physics to Ashkin for “his contributions to optical tweezers and their application to biological systems”. Moreover, if the trapping laser beam is circularly polarized and the trapped particle is birefringent, the transfer of spin angular momentum can generate an optical torque and induce the rotation of the particle. This rotation will be highly affected by the properties of the surrounding medium, such as temperature or viscosity, and the morphology of the particle. To explore the rotation dynamics as a sensing mechanism, in this thesis optically trapped and rotated β-NaREF4:RE3+ microparticles are presented as novel sensors to characterize a liquid medium at the microscale. By selecting the appropriate trapping wavelength and doping, the luminescence of the microparticle can be additionally excited and explored. Due to their unique luminescence mechanisms, β-NaYF4:Yb3+ and β-NaLuF4:Nd3+ microparticles are used as remote optical coolers and heaters, respectively, capable of inducing thermal changes while simultaneously monitoring the temperature and viscosity of their surrounding medium. Additionally, to decouple the thermal and mechanical measurements, a β-NaYF4:Yb3+, Er3+ microparticle is presented as a luminescent sensor capable of bias-free multiparametric sensing. The dynamics of the spinning microparticle are also affected by its volume or any change in its surface. This feature allows the development of a protein G-coated NaYF4: Yb3+, Er3+ microparticle as a biosensor capable of detecting single cells and bacteria after their adhesion to the sensor. Contrary to current biosensors, this novel method only requires a small amount of sample and a few seconds to detect. In conclusion, the incorporation of rare-earth-doped microparticles in liquid allows their use as individual remote optical sensors. Precise control over the dynamics of the microparticles is achieved via optical trapping. The analysis of their overdamped rotation dynamics is an innovative and powerful tool that makes possible not only the controlled and remote manipulation of the sensor, but also an improved characterization of the medium and fast recording of its content. This thesis aims to understand and characterize the mechanisms that occur for their application as local probes
