A simple, high resolution colormetric planar optode imaging approach is presented. The approach is simple and inexpensive yet versatile, and can be used to study the two-dimensional distribution and dynamics of a range of analytes. The imaging approach utilizes the inbuilt color filter of standard commercial digital single lens reflex cameras to simultaneously record different colors (red, green, and blue) of luminophore emission light using only one excitation light source. Using the ratio between the intensity of the different colors recorded in a single image analyte concentrations can be calculated. The robustness of the approach is documented by obtaining high resolution data of O 2 and pH distributions in marine sediments using easy synthesizable sensors. The sensors rely on the platinum(II)octaethylporphyrin (PtOEP) and lipophilic 8-Hydroxy-1,3,6-pyrenetrisulfonic acid trisodium (HPTS) salt derivate for O 2 and pH measurements, respectively. The brightness of both indicators is dramatically enhanced by making use of energy transfer from a donor molecule (Macrolex yellow coumarin). Furthermore, the emission from the donor serves as an internal reference for the O 2 sensor. The approach relies on semitransparent sensors, facilitating visual inspection of the sediment behind the sensors during measurements. Software for data acquisition and calibration will be available from the authors, whereas all hardware is available from a range of commercial sources. The total cost of the complete measuring system is approximately $3000 US.
The imaging of two-dimensional (2D) solute distributions with planar optodes has become an important tool in biological and medical research. The development of versatile and¯exible imaging systems, that enable both luminescence intensity and lifetime imaging, has generated various applications of planar oxygen optodes. Most of the applied optodes however, were not transparent. They either contained scattering particles in the sensing layer for signal enhancement and/or an optical insulation to separate the signal from ambient light. Since the modular luminescence lifetime imaging system (MOLLI) enables luminescence lifetime imaging, we used transparent planar oxygen optodes to investigate simultaneously the 2D distribution of oxygen and the structure that causes this distribution. This is done by either using the luminescence intensity images or different spectral illumination for structural imaging and the luminescence lifetime images for oxygen distribution imaging.As the distribution of oxygen plays a key role at different spatial scales, we present results from applications of the transparent optodes to various biological systems: (a) to a coral sand sediment sample (macrolens application: resolution of approximately 50 mm per pixel); (b) to a lichen with cyanobacteria as symbionts (endoscope application: resolution of approximately 15±62.5 mm per pixel) and (c) to a foraminifer with diatoms as symbionts (microscope application: resolution of approximately 3.8 mm per pixel). The results demonstrate the performance and some of the limits of the application of transparent optodes. Other possible ®elds of applications that are not restricted to marine environment are discussed. #
A method is presented for the measurement of depth profiles of volumetric oxygen consumption rates in permeable sediments with high spatial resolution. When combined with in situ oxygen microprofiles measured by microsensors, areal rates of aerobic respiration in sediments can be calculated. The method is useful for characterizing sediments exposed to highly dynamic advective water exchange, such as intertidal sandy sediments. The method is based on percolating the sediment in a sampling core with aerated water and monitoring oxygen in the sediment using either an oxygen microelectrode or a planar oxygen optode. The oxygen consumption rates are determined using three approaches: (1) as the initial rate of oxygen decrease measured at discrete points after the percolation is stopped, (2) from oxygen microprofiles measured sequentially after the percolation is stopped, and (3) as a derivative of steady-state oxygen microprofiles measured during a constant percolation of the sediment. The spatial resolution of a typical 3 to 4 cm profile within a measurement time of 1 to 2 h is better with planar optodes (≈0.3 mm) then with microelectrodes (2 to 5 mm), whereas the precision of oxygen consumption rate measurements at individual points is similar (0.1 to 0.5 µmol L -1 min -1 ) for both sensing methods. The method is consistent with the established methods (interfacial gradients combined with Fick's law of diffusion, benthic-chambers), when tested on the same sediment sample under identical, diffusion-controlled conditions. * E-mail: lpolerec@mpi-bremen.de AcknowledgmentsWe would like to thank Gaby Eickert, Ines Schröder, Ingrid Dohrmann, Alfred Kutsche, Georg Herz, Volker Meyer, Paul Färber, and Harald Osmers for the preparation of oxygen microsensors and the construction of various mechanical and electronic parts necessary for the experimental setup. The crew of the vessel Verandering are thanked for providing pleasant and safe conditions on board. The valuable comments of two anonymous reviewers and especially of Clare Reimers are much appreciated. This study was supported by the Bundesministerium für Bildung und Forschung (BMBF, project number 03F0284a).
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