Photoresponsive ion extraction/release systems (PRIONERS)

The goal of this project is the development of a universal tool for a photo-triggered extraction or release of small ionic species from a polymeric matrix, with an eventual application in cells. Initially, we will establish methods required for both irreversible and reversible extraction/release systems. Later, a successful first stage will lead to novel optical sensor concepts. In conclusion, we aim to achieve a sound foundation on which the coming years of research in this field will be built. The first part of the project will focus on the design of irreversible PRIONERS based on decomposing photoactive compounds (PACs). We will show that a complex of a charged dye and an oppositely charged PAC (i.e. a photoacid generator) will be destroyed by UV light and allow the triggered release of the dye, thereby visualizing the triggered release of charged species from a polymeric matrix. The next step is to extend the system to visible light excitation using photosensitizers. Finally, specificity for the ion-exchange will be introduced using ionophores and the ion extraction will be demonstrated using Ca2+ as a model ion. As part of this task we will establish particle casting methods for different particle sizes and properties. Employing PACs that are neither decomposed nor irreversibly converted upon illumination while still being able to attract or release protons depending on their state will convert one-way PRIONERS into dynamic, regenerative systems. As a first step to realize this task, we will establish a method for measuring the illumination-dependent pKa value of compounds and evaluate several spiropyran and azo-derivatives using a custom-made flow-cell. After identifying compounds with large pKa shift upon illumination in a polymer matrix, further optimizations and enhancements will be realized by finding the best PAC/ion-exchanger ratio, by including ionophores for selective ion-exchange properties and by analytical characterization of the photoswitching process, i.e. reproducibility, photostability and reversibility. Finally, the best performing compounds will be incorporated into particles and used for reversible extraction/release studies employing 3D optical calcium imaging methods. The return phase will focus on the development of advanced, dynamic optical ion sensors based on the results from the first stage of the project. A dynamic change of the equilibrium curve in optical ion sensors allows previously unimaginable sensor designs. As an example we will develop an optical ion sensor that can be switched on and off dynamically using light.

The aim of the project Photoresponsive Ion Extraction/Release Systems was the conduction of fundamental experiments for the development of novel tools for biologists and analytical chemists. Photoresponsive components with their ability to change the affinity towards charged substances played a crucial role in this project. Specifically, we employed dyes, which showed an increased affinity for protons under illumination with ultra violet light, while illumination with visible light had the opposite effect. In combination with lipophilic ioncarriers (ionophores), such substances allowed the development of materials for a highly controlled release or extraction of specific charged species. Despite the long history of photochromic dyes in sun-sensitive, color changing materials (sun glasses) or in research on high-density data storage materials, their change in acidity due to a structural change was hardly exploited. In this project we successfully developed materials capable of a triggered release or extraction of specific ions at a specific point of time and space. Such materials where then used for the application in switchable optical ion sensors, which were inactive when kept in dark or under visible light. Under illumination with UV light, the sensor was switched on and, after equilibration with the contacting sample solution, returned the concentration of a specific ion in form of a change in optical signal (fluorescence or absorbance). After a successful measurement, the sensor could be switched off again using visible light. An interesting application of such materials, especially in the shape of micro- and nanoparticles, lies in the investigation of biological materials. Ions play an important role in biological processes such as signal transduction. Therefore, a tool for reversibly influencing ion concentrations in situ is highly valuable. So far, the release of ionic species for this type of studies was established using photocaged compounds, i.e. ions which are bound by a photolabile complexing agent. The light triggered release is however accompanied by the irreversible destruction of the photocage which makes reversing the process impossible. Here, the novel materials introduced during this project are advantageous due to their intrinsic reversibility of the photoactivation step. The fundamental results of this project will facilitate the further development of such highly functional materials and encourage scientists and funding agencies to invest time and money in this important field of research.