Materials for optical oxygen sensing under high hydrostatic pressure

Optical oxygen sensors based on indicators immobilized into porous and nonporous matrix materials were investigated in regard to their sensing behaviour under high hydrostatic pressure. The sensors were subjected to step-wise pressure increase up to approximately 200 bar in multiple cycles using a custom-made chamber. The investigated materials are based on oxygen indicators (a platinum(II) benzoporphyrin dye and ruthenium(II) polypyridyl complexes) immobilized in microparticles: silica gels of different porosities, controlled pore glass, poly(phenylsilsesquioxane), crosslinked polystyrene, the metal-organic frameworks ZIF-8 and UiO-66. The microparticles are in turn dispersed in highly oxygen-permeable silicone and Hyflon AD matrices. Homogeneous films of dye doped polystyrene and polyurethane hydrogels as well as non-porous polystyrene nanospheres directly dispersed in water were used for comparison purposes. All the porous materials were found to be stable under elevated hydrostatic pressure. Although the actual oxygen concentration remained unchanged upon increasing pressure, the sensors demonstrated an apparent decrease of calculated oxygen concentration (between −0.02 and −0.45 mg O2 L-1 H2O per 100 bar), which was fully reversible. Furthermore, the kinetics of the pressure response of the sensors was determined in experiments with high temporal resolution. Spikes attributed to pressure-induced oxygen diffusion between sensor components were observed within the first seconds after pressurization/depressurization.