Oxygen imaging at the sediment‐water interface using lifetime‐based laser induced fluorescence (τLIF) of nano‐sized particles

Most applications of laser induced fluorescence (LIF) for dissolved oxygen (DO) imaging in flowing water are based on luminescence intensity measurements of a dissolved indicator. A major limitation for applying the technique in the bottom boundary layer (BBL) is the sorption of the luminescent dye to organic surfaces at the sediment. Many sediment and soil studies have used planar optodes on transparent foils as an imaging technique for observing concentration distributions across the sediment-water interface. The presence of the foil, however, is restricting the free flow and therewith the DO concentration above the sediment surface. In this study, we applied the phosphorescence lifetime LIF (tau LIF) technique in combination with nanoparticles coated with platinum complexes as DO indicators. By using a planar laser light sheet for fluorescence excitation, the three-dimensional flow field around the sampling area is not restricted. In contrast to the intensity-based LIF technique, this method extracts the lifetime of the luminescence by evaluating the exponential decrease of the phosphorescence intensity after a short excitation light pulse. The method is therefore independent of dye concentration and is not affected by spatial inhomogeneities of the excitation light intensity. We applied the technique to visualize the burrow-ventilation activity of the tube-dwelling Chironomus plumosus larvae. Sequences of the two-dimensional DO concentration distributions showed the intermittent outburst of highly-depleted plumes from the outlet of the burrows, which mixed rapidly with the ambient oxygen-rich water.