Luminescent Sensors for Continuous Monitoring of Important Analytes in Ecosystems

Last decades have witness significant progress in optical sensing technology. Within this group of methods, luminescence-based sensing attracted much attention. Here an optically silent analyte interacts with the sensing material and reversibly changes its luminescent properties. Luminescent sensors benefit from highest flexibility of formats (planar sensors, microsensors, nanoparticles) and applications. A unique feature of such optical sensors (optodes) is their suitability for imaging of analyte distribution on surface (planar sensors) or in 3D (nanoparticles). Although optical sensors have been widely applied since several decades, for instance in (marine) biology, environmental monitoring etc., their even wider adoption is hindered by several factors. In order to achieve this purpose, both the sensing materials and the dedicated read-out equipment have to reach the needed robustness and fulfil the required characteristics, which may differ significantly from application to application. Furthermore, the number of analytes that can be reliably quantified by means of luminescence is currently rather limited. In this talk we will first highlight development of robust oxygen optodes for various applications (Wang and Wolfbeis 2014). These rely on dynamic quenching of luminescence of an indicator dye by molecular oxygen. Among numerous indicator dyes, platinum(II) and palladium(II) complexes with benzoporphyrins (Borisov et al. 2008) demonstrated an unmatched combination of desired photophysical properties: intense absorption in the blue and red parts of the electromagnetic spectrum, high brightness and photostability. They additionally benefit from straightforward synthesis and are now commercially available. To obtain an oxygen-sensing material, these indicator dyes are immobilized in suitable matrices, usually polymers like polystyrene. Importantly, high brightness of the indicators made it possible to minimize the thickness of the sensing layer and thus to significantly improve the response time of the resulting materials. Coating of the sensing material on a tapered glass fiber results in fast-responding sensors suitable for accessing O2 concentration multiple times in a second. These sensors are now commercially available and have been utilized in eddy covariance experiments in marine ecosystems (Merikhi et al. 2021, Berg et al. 2022). Moreover, we designed and optimized sensors for monitoring traces of oxygen in various environments. This was achieved via combination of oxygen indicators with long luminescence lifetime and highly oxygen-permeable polymeric matrices (Lehner et al. 2015). Although optical pH sensors offer a much narrower dynamic range compared to the glass electrode (typically max. 3 pH units), they are still of much interest for many applications, particularly for measurements in seawater. Numerous groups of fluorescent pH indicators have been investigated by us and other researchers over last decades (Steinegger et al. 2020). Aza-BODIPY dyes offer attractive photophysical properties (red light excitation and emission in far-red region, excellent photostability) along with the modular character that allows to easily tune the pK a value (and thus the dynamic range) of the indicator (Jokic et al. 2012, Strobl et al. 2015). Particularly, these dyes were optimized for optical measurement of pH in seawater (Staudinger et al. 2019). We showed that matrix, in which the pH indicator is imbedded, is also of highest importance for performance of the resulting sensing materials. For instance, although some of the sensors showed acceptably fast response at room temperature, they became virtually unsuitable at low temperatures (Staudinger et al. 2019). Monitoring of such important parameters like oxygen and pH is not possible without dedicated devices for the read-out of the sensing material. A submergible prototype equipped with optical feed-through, longer and battery has been developed (Staudinger et al. 2018), and utilized in proof-of-concept studies (profiling and long-term monitoring in seawater). The prototype was also successfully employed to detect potential leakage of carbon dioxide stored offshore via pH change. Further development in collaboration with an industrial partner, PyroScience GmbH (Aachen, Germany), resulted in a family of instruments and dedicated sensors for monitoring oxygen in pH in shallow water and in deep sea (up to 4000 m). The same sensing principle was utilized to sense other environmentally important analytes including ammonia (Strobl et al. 2017), carbon dioxide (Fritzsche et al. 2017) and ions like sodium (Müller et al. 2017). Together with collaboration partners we are currently working on development of compact and affordable system for robust mapping the above palette of analytes with help of planar optodes and water-dispersible particles.