PhD defense: Magin Ferrer

Title

Cascading and amplified effects in fluorescence photoswitching - towards sensitive molecular dosimeters for radiation detection

Abstract

Despite regulations on their use, ionizing radiations still present risks, especially through long-term exposure to low doses, highlighting the need for sensitive dosimeters to establish a dose-response relationship in the milligray range or lower. Some photochromic terarylene derivatives exhibit a cascade effect (CE) during the cycloreversion reaction upon X-ray irradiation in chloroform solution. This leads to a large color loss with a small amount of oxidizing trigger. Based on this principle, a visual-based dosimeter with higher sensitivity is targeted. First, CE in solution was revisited to make it wavelength-selective and fatigue-resistant. Chloroform was substituted with chlorobenzene to obtain the desired features. Fluorophores were then paired with the terarylene derivatives to probe CE via amplified fluorescence photoswitching (AFPS). Polymer thin film experiments demonstrated this effect: by the photocyclization of a single photochromic unit, the quenching of multiple fluorophores by FRET was evidenced. Micro-encapsulation was then  realized to observe both CE and AFPS in a single microcapsule. Through fluorescence microscopy, we demonstrated, for the first time the presence of these effects, upon UV light trigger. Modeling revealed that both FRET and emission-reabsorption mechanisms contributed to fluorescence quenching. No response was observed, however, upon X-ray trigger. HfO2 nanoparticles were added as radiosensitizers to improve the response. However, the ligand of the nanoparticles, oleylamine, actually prevented CE from happening in solution.

PhD supervision

• Director: Keitaro Nakatani - Professor, ENS Paris-Saclay
• Co-Director: Tsuyoshi Kawai - Professor, NAIST
• Co-supervisor: Rémi Métivier - CNRS Research director, ENS Paris-Saclay