Molecular rotors as sensors of microscopic viscosity and temperature

Abstract

Microscale viscosity is a key parameter that defines the physical makeup of a system, controlling viscoelastic properties of microscopic objects. Additionally, microviscosity within a living cell controls the rate of mass transport through a cell and is hence intimately linked to the activity of a cell. Therefore, measuring viscosity on a microscale represents an important challenge within both physical and biological sciences. So far, one of the most informative and convenient ways for doing this is to use fluorescent 'molecular rotors', which are the viscosity-sensitive fluorophores. The characterisation and application of several previously unexplored molecular rotors are the main topics explored in this thesis. First, porphyrin dimers are examined and characterised as dual viscosity sensors capable of measuring viscosity using their two photophysical parameters: the ratio of two peaks in the fluorescence spectrum and the fluorescence lifetime. The dimer was thus characterised as an attractive dual viscosity sensor displaying absorption and emission in the tissue optical window. Then the porphyrin dimers are applied for imaging microviscosity in lipid monolayers and bilayers. Finally, the porphyrin dimer is used for sensing dynamic change of microviscosity in lipid monolayers and living cells undergoing oxidation by singlet oxygen. Secondly, the changes in viscosity of model lipid membranes under oxidation are further examined using the molecular rotor Bdp-C10, which fully embeds in the lipid tail region of the lipid bilayer. Changes in viscosity are measured at several different bilayer positions of the oxidant relative to the rotor: on the surface, inside the tail region and outside the bilayer in the aqueous phase. Additionally, we report striking differences in the dynamic viscosity change during Type I and Type II photosensitisations and uncover the mechanistic details of the oxidation utilising the ability of molecular rotors to provide spatially resolved information. In the last two chapters, we examine if molecular rotors are sensitive to temperature, which is an important topic that has not previously investigated. The molecular rotors demonstrate contrasting types of temperature sensitivity. Lastly, the temperature-dependence of molecular rotors was put to use by employing Kiton Red for measuring temperature in laser-heated aerosol particles and by using one of the porphyrin dimers for performing the first ever to our knowledge dual viscosity and temperature measurement on a microscopic scale.