Platinum octaetylporphyrin based planar optodes combined with an UV-LED excitation light source: An ideal tool for high-resolution O 2 imaging in O 2 depleted environments

Abstract

A planar optode system based on an O 2 quenchable luminophore platinum(II) 2,3,7,8,12,13,17,18-octaethyl-21H,23 H-porphyrin (PtOEP) and UV-light emitting diodes (LEDs) was developed and tested for high precision measurements in O 2 depleted aquatic environments. The luminescence lifetime ( τ) of the PtOEP optode changed from 85.7 to 26.7 μsec as seawater changed from anoxic to an O 2 concentration of 219 μM (100% air-saturation at S = 35.0, 23.0 °C). The dynamic range was up to 18 times higher than a parallel optode measurements performed by conventional ruthenium complexes and blue LED's (depending on the O 2 concentration). The sensitivity of the PtOEP planar optodes remained unchanged even after 19 h of continuous UV radiation at intensity comparable to that of standard measuring conditions. The prototype of a newly developed, multi-gateable CCD camera with a 14 bit A/D converter was used for imaging at a 14 bit digital resolution using a CCD-chip of 1344 × 1024 pixels. The calibrated signal correlated well to that of simultaneously obtained microelectrode measurements ( R 2 = 0.999) and the image accuracy (pixel-to-pixel variation) at anoxia and at 175 μM (80% air-saturation) was ± 0.4 and ± 7.0 μM, respectively. Pixel-to-pixel calibration of acquired images combined with image averaging and moderate pixel binning resulted in a very high precision O 2 images. Laboratory measurements in small flume aquaria documented that PtOEP based optodes combined with UV excitation was an attractive alternative to other established planar optode systems for measurements in O 2 depleted environments. The described system was used for detailed investigations along the anoxic isoline of an intertidal sediment and continuous O 2 imaging revealed previously unresolved microscale O 2 dynamics along this interface. Local O 2 depleted microniches (1.0 × 1.0 mm) appeared and reoxidized within minutes. The dynamics were induced by meiofauna activity and the observation can have important implications for early diagenesis in marine sediments.