1. Oxygen sensors

Our group developed a variety of oxygen sensing materials based on commercially available indicators and those synthesized in our group. They rely on the ability of molecular oxygen to efficiently quench the long-lived phosphorescence of the indicators (Fig. 1.1). These dyes have been immobilized into oxygen-permeable hydrophobic matrices. Oxygen can be precisely quantified via measuring of phosphorescence decay time or, mostly for imaging applications, the luminescence intensity. In the latter case, addition of a reference dye is essential for precise oxygen quantification. The materials include oxygen sensors based on conventional metalloporphyrins (such as complexes of meso-tetra(pentafluorophenyl)porphyrin) ^[1-5] and more advanced NIR benzoporporphyrins ^[6-9] and their analogues, ^[10,11] ultrabright materials based on cyclometallated Ir(III) complexes ^[12] or novel oxygen sensor based on Eu(III) complexes ^[13] which possess high sensitivity to oxygen and feature characteristic narrow-band emission of Eu(III) upon excitation with blue light.

Due to very high molar absorption coefficients and good luminescence quantum yields of the benzoporpyhrin complexes as well as their excellent photostability, these dyes are suitable for preparation of ultra-fast fiber optic sensors. Even a thin sensing layer coated on the tip of an optical fiber can  deliver appreciable signals and enable very fast response time (t₉₀ ~0.1 s) for measurements both in gases and in solutions (Fig. 1.2). It should be mentioned that some of the oxygen sensing materials have been commercialized in cooperation with Pyro Science GmbH (Aachen, Germany) and are now commercially available together with dedicated read-out devices optimized for various applications (www.pyro-science.com).

Trace oxygen sensors are of special interest for a number of applications in research and in industry. Recently, quantification of oxygen in “oxygen minimum zones” in oceans became very interesting since the data allow important insights of how aerobic life developed on earth milliards of years ago. The sensitivity of an oxygen sensor is guided by the combination of luminescence decay times of an indicator and the permeability of the matrix. We designed several trace oxygen sensing materials which possess different spectral properties and cover different dynamic ranges. Platinum(II) and palladium(II) meso-pentafluorophenylporphyrins were covalently bound to the surface of silica-gel beads via nucleophilic substitution of p-fluorine atoms with an amine (which was introduced via silanization). ^[1]  The beads were then dispersed in a highly oxygen-permeable silicone rubber. The sensors were found to be rather sensitive (K_SV  ~4 and 67 kPa^-1 at 25 °C) for Pt(II) and Pd(II) complexes, respectively. Evidently, the sensitivity is still limited by the decay time of Pd(II) porphyrin which is about 1 ms. In the further work, the same indicators were immobilized in highly oxygen-permeable oxidatively robust perfluorinated Hyflon AD polymer. ^[1] In combination with a dedicated read-out device LUMOS developed in our group, these sensors were able to reliably quantify concentration of dissolved oxygen in the nanomolar range (Fig. 1.3). In terms of detection limit, noise, sampling rate and easiness of sensor manufacture the trace optical sensors were superior to the STOX sensor which is the most sensitive electrochemical sensor reported (Fig. 1.3). Later the benzoporphyrin dyes also were embedded into perfluorinated polymers to enable compatibility with the read-out devices from Pyro Science. These trace sensors were applied for in-situ monitoring of oxygen in oxygen minimum zones together with collaboration partners from ERC project. ^[14]

Ultra-sensitive optical oxygen sensors were prepared by combining highly oxygen-permeable perfluorinated polymers Teflon AF and Hyflon AD with Al(III) chelates of 8-hydroxyphenalenon and the benzoannelated derivative which possess extraordinary long phosphorescence decay times (300-600 ms). ^[15] Importantly, the dyes modified with perfluoroalkyl chains had to be prepared in order to enable compatibility with the matrix material. The resulted sensing materials showed extraordinary high sensitivity and were suitable for measurements of dissolved oxygen below 1 nM (LOD – 10 pM). Importantly, soluble Teflon derivatives not only possess high oxygen permeability, but also are extremely chemically robust matrices. This is of great importance due to chemical oxygen consumption which is particularly critical at low oxygen concentrations. In fact, we noticed that highly reactive singlet oxygen produced during quenching is capable of reacting even with such fairly robust polymers as polystyrene. Despite that in Teflon AF and Hyflon AD oxygen consumption is eliminated, several remaining effects complicate the measurements. These effects were investigated in detail in our work ^[16] and are mainly cause by (i) depopulation of the ground state of the dye due to extremely long excited state lifetimes; this affects the intensity-based calibrations; (ii) triplet-triplet annihilation which becomes important due to high concentration of the excited triplet dye molecules; (iii) physical oxygen consumption resulting in minimized quenching since singlet oxygen produced can decay rather slowly in perfluorinated matrices (decay times about 1 ms). All these effect severely influence the calibration of optical sensors. Although they are particularly observable in case of ultra-trace sensors, normal range sensors operating at high light intensities (e.g. typical for microscopy) are likely also to be affected. Measuring of luminescence decay times instead of luminescence intensity, preparation of the sensors with low concentration of indicators and avoiding high light intensities are efficient strategies to overcome the above limitations. Several sensing materials based on covalently-immobilized dyes were also prepared. These materials show greater robustness compared to those based on non-covalently immobilized indicators, which is particularly visible at elevated temperatures. In fact, the migration of dye into polymeric support is completely eliminated. The sensors are based on: (i) Pt(II) complex with meso-tetra(pentafluorophenyl)porphyrin which is covalently coupled the copolymer of styrene and pentafluorostyrene using convenient thiol-based click chemistry; ^[4] (ii) benzoporphyrin dyes coupled to styrene monomers (which are subsequently co-polymerized with styrene) or covalently grafted into polystyrene-based polymer using Suzuki cross-coupling reaction of the bromo-substituted dye; ^[8] (iii) styrene-modified benzoporphyrin dyes covalently immobilized into silicone rubbers via vinyl addition reactions. ^[9] Due to high oxygen permeability of silicone rubber these materials belong to trace oxygen sensors.

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References:

[1] Borisov, S. M.; Lehner, P.; Klimant, I. Novel Optical Trace Oxygen Sensors Based on Platinum(II) and Palladium(II) Complexes with 5,10,15,20-Meso-Tetrakis-(2,3,4,5,6-Pentafluorphenyl)-Porphyrin Covalently Immobilized on Silica-Gel Particles. Analytica Chimica Acta 2011, 690 (1), 108–115. [2] Lehner, P.; Larndorfer, C.; Garcia-Robledo, E.; Larsen, M.; Borisov, S. M.; Revsbech, N.-P.; Glud, R. N.; Canfield, D. E.; Klimant, I. LUMOS - A Sensitive and Reliable Optode System for Measuring Dissolved Oxygen in the Nanomolar Range. PLoS ONE 2015, 10 (6), e0128125. [3] Mayr, T.; Borisov, S. M.; Abel, T.; Enko, B.; Waich, K.; Mistlberger, G.; Klimant, I. Light Harvesting as a Simple and Versatile Way to Enhance Brightness of Luminescent Sensors. Analytical Chemistry 2009, 81 (15), 6541–6545. [4] Koren, K.; Borisov, S. M.; Klimant, I. Stable Optical Oxygen Sensing Materials Based on Click-Coupling of Fluorinated Platinum(II) and Palladium(II) Porphyrins—A Convenient Way to Eliminate Dye Migration and Leaching. Sensors and Actuators B: Chemical 2012, 169 (0), 173–181. [5] Koren, K.; Borisov, S. M.; Saf, R.; Klimant, I. Strongly Phosphorescent Iridium(III)–Porphyrins – New Oxygen Indicators with Tuneable Photophysical Properties and Functionalities. European Journal of Inorganic Chemistry 2011, 2011 (10), 1531–1534. [6] Borisov, S. M.; Nuss, G.; Klimant, I. Red Light-Excitable Oxygen Sensing Materials Based on Platinum(II) and Palladium(II) Benzoporphyrins. Analytical Chemistry 2008, 80 (24), 9435–9442. [7] Borisov, S. M.; Papkovsky, D. B.; Ponomarev, G. V.; DeToma, A. S.; Saf, R.; Klimant, I. Photophysical Properties of the New Phosphorescent Platinum(II) and Palladium(II) Complexes of Benzoporphyrins and Chlorins. Journal of Photochemistry and Photobiology A: Chemistry 2009, 206 (1), 87–92. [8] Hutter, L. H.; Müller, B. J.; Koren, K.; Borisov, S. M.; Klimant, I. Robust Optical Oxygen Sensors Based on Polymer-Bound NIR-Emitting Platinum(II)–benzoporphyrins. J. Mater. Chem. C 2014, 2 (36), 7589–7598. [9] Müller, B. J.; Burger, T.; Borisov, S. M.; Klimant, I. High Performance Optical Trace Oxygen Sensors Based on NIR-Emitting Benzoporphyrins Covalently Coupled to Silicone Matrixes. Sensors and Actuators B: Chemical 2015, 216, 527–534. [10] Borisov, S. M.; Zenkl, G.; Klimant, I. Phosphorescent Platinum(II) and Palladium(II) Complexes with Azatetrabenzoporphyrins—New Red Laser Diode-Compatible Indicators for Optical Oxygen Sensing. ACS Applied Materials & Interfaces 2010, 2 (2), 366–374. [11] Niedermair, F.; Borisov, S. M.; Zenkl, G.; Hofmann, O. T.; Weber, H.; Saf, R.; Klimant, I. Tunable Phosphorescent NIR Oxygen Indicators Based on Mixed Benzo- and Naphthoporphyrin Complexes. Inorg. Chem. 2010, 49 (20), 9333–9342. [12] Borisov, S. M.; Klimant, I. Ultrabright Oxygen Optodes Based on Cyclometalated Iridium(III) Coumarin Complexes. Analytical Chemistry 2007, 79 (19), 7501–7509. [13] Borisov, S. M.; Fischer, R.; Saf, R.; Klimant, I. Exceptional Oxygen Sensing Properties of New Blue Light-Excitable Highly Luminescent Europium(III) and Gadolinium(III) Complexes. Adv. Funct. Mater. 2014, 24 (41), 6548–6560. [14] Larsen, M.; Lehner, P.; Borisov, S. M.; Klimant, I.; Fischer, J. P.; Stewart, F. J.; Canfield, D. E.; Glud, R. N. In Situ Quantification of Ultra-Low O2 Concentrations in Oxygen Minimum Zones: Application of Novel Optodes. Limnol. Oceanogr. Methods 2016, 14 (12), 784–800. [15] Lehner, P.; Staudinger, C.; Borisov, S. M.; Klimant, I. Ultra-Sensitive Optical Oxygen Sensors for Characterization of Nearly Anoxic Systems. Nat Commun 2014, 5. [16] Lehner, P.; Staudinger, C.; Borisov, S. M.; Regensburger, J.; Klimant, I. Intrinsic Artefacts in Optical Oxygen Sensors—How Reliable Are Our Measurements? Chem. Eur. J. 2015, 21 (10), 3978–3986.