Improvement of optical planar oxygen sensors and application in marine environments

Abstract

First, the technical developments of sensors, camera systems and applications of the imaging techniques were reviewed. Since the first more primitive approaches, more sophisticated systems have been developed, in which both fluorescence intensity and fluorescence lifetime imaging was possible in one as well as two dimensions (planar optodes). The oxygen distributions were visualized by the use of planar sensor foils with an oxygen sensitive fluorophore layer, containing a photostable ruthenium complex, that was reversibly quenched by oxygen. The planar optodes were fabricated of organically modified sol-gel (ORMOSIL). All sensor foils were based on the use of ruthenium(II)-tris-(4,7-diphenyl-1,10-phenantrolin)-perchlorate, which was a fluorescent dye quenched dynamically by oxygen. Sensors made with different sol-gel immobilization matrices, different concentrations of precursors and indicator dye, as well as different types of scattering particles co-immobilized in the sensor foil were investigated systematically. Optimal sensor performance was obtained with dye concentrations of 2-10 mmol/kg in an immobilization matrix made of diphenyldiethoxy-silan and phenyltriethoxy-silan precursors with addition of organically coated TiO2 particles. The sensors exhibited a good mechanical stability and a high sensitivity from 0 to 100% oxygen, which remained constant over at least 36 days. The recording system was optimized for modular luminescence lifetime imaging (MOLLI). The central parts of the system were a CCD-camera with a fast electronic shutter and gated LED (light emitting diode) or Xe excitation light sources. A personal computer controlled the gating and image acquisition via a pulse delay generator. The planar optodes were used to investigate 2-dimensional O2 dynamics in burrow systems inhabited by Hediste diversicolor. Natural burrows often deviated from the classic U-shape and changed over time. This complicated quantitative assessment of the O2 dynamics of the burrow. Therefore, a stable U-shaped burrow was established surrounded by agar-solidified sediment. Visual inspection and measured ventilation patterns indicated similar behavior of polychaetes in natural and artificial systems. The volume specific O2 consumption, RP, along the primary interface in the two systems were identical. Planar optode and microelectrode measurements gave similar results along the primary sediment-water interface. The radial diffusion of O2 around a burrow was disturbed by the presence of the planar optode directly attached to the burrow wall causing an enhanced oxygen penetration depth compared to the undisturbed system. With the known geometry of the burrow system the effect could be corrected. Microbial colonization along the burrow lining increased the volume specific O2 consumption, RB, from 3.8 0.5 to 13.7 2.1 mol m-3 d-1 within two days reflecting that the burrow lining of H. diversicolor acts as a zone of intensified diagenetic activity. Extrapolated to natural densities in coastal sediments (600 m-2) the findings indicate that 88 % of the benthic O2 uptake is associated to worm burrows and that the major fraction hereof (66 %) is related to microbial activity in the burrow lining, while 22 % can be ascribed to fauna respiration.