Time-resolved pH imaging in marine sediments with a luminescent planar optode
Limnol. Oceanogr. Methods 4:336-345 (2006) | DOI: 10.4319/lom.2006.4.336
ABSTRACT: A laboratory based optical pH sensor for 2-dimensional pH imaging at benthic interfaces is presented. The sensor consists of a single layer hydrogel matrix embedding the fluorescent pH indicator 2'L, 7'L-dihexyl-5(6)-N-octadecyl-carboxamidofluorescein ethyl ester (DHFAE) and the phosphorescent Ruthenium(II)-tris-4,7-diphenyl-1,10-phenanthroline incorporated in nanoparticles, serving as an inert reference standard. The measuring principle is based on time domain dual lifetime referencing (t-DLR). The fluorophore/phosphor couple is simultaneously excited by a green LED (λmax=530 nm) pulsed in the µs range and the sensor emission is recorded by a fast-gateable CCD camera. For each pH image, intensity images in two time windows (one during and one after the excitation phase) are taken. The ratio of these two images is proportional to the pH of the sample and not affected by the overall signal intensity. The sensor has a dynamic range suitable for marine conditions (pH 7.3-9.3) with an apparent pKa of 8.3. The sensor was long term stabile (months) when kept in darkness and had a response time <200 s when going from pH 8.3 to 7.6. Light proved to have a negative effect on the sensor performance due to photobleaching of the pH indicator, resulting in a negative drift in the signal ratio at higher pH after prolonged light exposure. The spatial resolution (83x83 µm/pixel) of the sensor was capable of resolving small scale spatial variability in the pH at a heterogeneous sediment-water interface and time series of calibrated pH images also expressed a marked temporal variability in the pH distribution across this interface. Hotspots with intensified microbial activity were observed and pH minima along burrow walls of polycheates indicated elevated diagenetic activity in these zones. Individually extracted profiles from the pH images agreed well with independently measured pH microelectrode profiles, confirming the robustness of the approach.