Photoswitchable sensors: reversible ion detection using optical fibres

Abstract

In studying and diagnosing cellular systems and diseases, the ability to accurately detect and monitor the concentrations and fluctuations of metal ions is of particular importance. Fluorescent photoswitchable sensors provide a means to reversibly detect metal ions in solution. This class of sensors uses a light stimulus to chemically switch between two distinct species, one that can bind to an analyte of choice and one that cannot bind. This then provides sensors that can be turned off at will, allowing the sensor to be reset and used again at a different time point. This thesis investigates the design, synthesis and metal ion selectivity of a series of photoswitchable sensors. These sensors contain a spiropyran core unit with differing ion binding domains, such as an aza-crown ether, providing ion specificity, as well as a free carboxyl group that allows for attachment to a solid support. A discussion on the choice of this photoswitchable moiety and subsequent design and synthesis as a new metal ion sensor is presented in Chapters 2, and 3 and 4, respectively. These photoswitchable sensor molecules were then used within a microstructured optical fibre (MOF) sensing platform. Suspended core microstructured optical fibres provide a biologically suitable platform that provides a very sensitive means to sense in nanolitre volumes of sample. Covalently attaching these photoswitchable sensors to the light guiding core, via APTES silanization, provided a reversible sensing system capable of detecting picomolar concentrations of metal ions, such as Ca²⁺ in a biological sample while not contaminating the sample. The MOF not only provided a means to detect a fluorescence signal, it also allowed for repetitive on/off photocycling of the photoswitch, both in solution and attached to the surface.