Platinum octaetylporphyrin based planar optodes combined with an UV-LED excitation light source: An ideal tool for high-resolution O2 imaging in O2 depleted environments

A planar optode system based on an O2 quenchable luminophore platinum(II) 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin (PtOEP) and UV-light emitting diodes (LEDs) was developed and tested for high precision measurements in O2 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 O2 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 LEDʹs (depending on the O2 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 (R2 = 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 O2 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 O2 depleted environments. The described system was used for detailed investigations along the anoxic isoline of an intertidal sediment and continuous O2 imaging revealed previously unresolved microscale O2 dynamics along this interface. Local O2 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.