Fiber optic chemical sensors for the in-situ characterization of icy moons

IntroductionFiber optic sensors [1] are suitable for the challenges posed by space missions. They are resilient to electromagnetic interference – which ensures reliable measurements in electrically noisy environments – have low power requirements, and their lightweight and flexible nature allows for highly compact architectures and high mass savings. For the last two decades, the European Space Agency (ESA) has invested in fiber optic sensors for spacecrafts, having implemented this technology in the Basic Angle Measurement (BAM) sensor on ESA’s GAIA mission and in the JUICE magnetometer (J-MAG) on ESA’s JUICE mission [2]. Still, fiber optic sensors dedicated to chemical characterization – named fiber optic chemical sensors, FOCS [3,4] – have yet to be explored in space applications. It is our belief that FOCS are worthy additions to a new generation of in-situ chemical characterization techniques [5]. Herein, we call attention to the potentialities of FOCS and showcase ongoing efforts to build space mission-dedicated prototypes.The analytical advantages of FOCS in space missions: Besides being inherently space-suitable, FOCS complement the existent suite of chemical characterization payloads. They may seamlessly interact with the environment, by probing it directly (Figure 1A) and requiring neither sample collection nor preparation. They are equally appropriate for characterizing gaseous and liquid phase environments, require small sample volumes, and can analyze multiple analytes simultaneously (resorting to different fibers connected to a shared control unit). FOCS also provide a versatile platform to different sensing strategies, previously applied to the biomedical, environmental monitoring, oil and gas and food industries [3]. Intending to argue for their applicability to the space industry, we are developing extrinsic fluorescent FOCS tailored to future space mission targets. Extrinsic FOCS use an optical fiber only to transport light to and from a responsive material, in opposition to the fiber being the sensor itself; the adopted extrinsic architecture (Figure 1) allows for higher analyte specificity. We developed off-on fluorescent responsive materials to maximize the sensitivity of our FOCS.ResultsTowards characterizing hydrocarbons in icy moons: We developed FOCS capable of detecting hydrocarbons of interest through reaction-based schemes. In an environment like Titan’s, such sensors would address both the limitations of mass spectrometry in differentiating between hydrocarbon isomers and the low specificity of infrared (IR) spectroscopy.We simulated the detection of butadiene (C4H6) on Titan’s surface. To address the slow reaction kinetics expected at Titan’s low temperatures, we focused on click reactions enabled by the Carboni-Lindsey mechanism [6]. It consists of a fast and selective reaction between a 1,2,4,5-tetrazine molecule and an olefin (butadiene, in our application), producing as its sole by-product N2, the major component of Titan’s atmosphere. Being an irreversible reaction mechanism, it accumulates signal over time, an adequate approach to low analyte concentrations [7]. We tested several tetrazine substituents to optimize the reaction kinetics and uncovered a tetrazine derivative (tz) which reacts with butadiene to form a dihidro-pyridazine molecule (dh-pyr). It produces a high emissivity product, generating an off-on fluorescence signal at 450 nm (Figure 2A), making the selected tetrazine derivative (tz) an ideal indicator of butadiene. The immobilization of tetrazine molecules in polyurethane-based membranes produces physical sensors resistant to leaching in hydrocarbon environments. Their electronic incorporation is ongoing.Towards quantifying the pH of the subsurface water oceans in icy worlds: FOCS have been applied to marine environments [8] and their pH measurements exhibited high sensitivity within the analytical range of the indicators [3]. We are developing pH FOCS to benchmark this technology against alternative strategies meant to quantify the pH of extraterrestrial aqueous bodies, such as the Enceladus alkaline subsurface ocean with an estimated pH = 8.5–10.5 and an ammonia volume mixing ratio of 0.8% (~3.6 × 10−4 M) [9], [10]. So far, the immobilization of known pH indicators in polymeric membranes demonstrated negligible leaching in aqueous solutions and fast responses to ammonia-induced pH variations (Figure 2B).Conclusions and Future WorkFOCS are a stablished sensing paradigm able to withstand challenging working conditions, in miniaturized setups with low power requirements. Also, they can be easily integrated onto existing, space-qualified, fluorescence spectrometers. Ongoing efforts to demonstrate its temperature robustness and photobleaching resistance will further demonstrate that FOCS should be considered for the new generation of scientific payloads dedicated to the chemical characterization of extraterrestrial environments, such as icy moons.AcknowledgementsThe authors acknowledge funding by Fundação para a Ciência e Tecnologia (FCT) (UIDB/00100/2020, UIDP/00100/2020, LA/P/0056/ 2020, UIDB/04565/2020, UIDP/04565/2020, LA/P/ 0140/2020, 2021.04932.BD, and2024.01442.BD). This work has the financial support of FCT for project ORIGINS (2022.05284.PTDC).References[1] Elsherif, M. et al. Adv Photonics Res 3, 2100371 (2022)[2] McKenzie, I. et al. Front Phys 9, 719441 (2021)[3] Wang, X. D. et al. Anal Chem 92, 397–430 (2020)[4] Nguyen, T. H. et al. in Optical Fibre Sensors 239–288 (John Wiley & Sons, Ltd, 2020)[5] Abrahamsson, V. et al. Front Astron Space Sci 9, 959670 (2022)[6] Oliveira, B. L. et al. Chem Soc Rev 46, 4895–4950 (2017)[7] Yang, Y. et al. Chem Rev 113, 192–270 (2013)[8] Qian, Y. et al. Sens Actuators B Chem 260, 86–105 (2018)[9] Hsu, H.-W. et al. Nature 519, 207–210 (2015)[10] Waite, J. H. et al. Nature 460, 487–490 (2009) Figure 1. Bifurcated optical fiber and spectrometer (A) used in our laboratory to demonstrate the miniaturization capabilities of FOCS. In an envisioned implementation of our sensors, an analyte-sensitive membrane (B) interacts with the environment, while its fluorescence signal is monitored through the optical fiber. Credit: Sarspec. Figure 2. (A) Response of a tetrazine derivative to butadiene through a click-chemistry reaction, producing significant variations in the absorption (top) and emission (bottom) spectra. (B) Color response of bromophenol blue immobilized in a polystyrene membrane to an ammonia solution ([NH3] = 66 mM).