Continuous monitoring of oxygen concentration is of great importance in many different areas of research which range from medical applications to food packaging. In the last three decades, significant progress has been made in the field of optical sensing technology and this review will highlight the one inherent to the development of oxygen indicators. The first section outlines the bioanalytical fields in which optical oxygen sensors have been applied. The second section gives the reader a comprehensive summary of the existing oxygen indicators with a critical highlight on their photophysical and sensing properties. Altogether, this review is meant to give the potential user a guide to select the most suitable oxygen indicator for the particular application of interest.

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.

A new fiber‐optic oxygen microsensor (microoptrode) based on dynamic fluorescence quenching has been developed to measure oxygen gradients in marine sediments and microbial mats. The microoptrodes are fabricated by immobilizing an oxygen‐quenchable fluorophore at the tapered tip of an optical fiber. A special optoelectronic system has been designed to measure oxygen with these microoptrodes. It is based on small and cheap optical components and can easily be miniaturized for field applications. In contrast to oxygen microelectrodes, the new oxygen microoptrodes are easy to make, do not consume oxygen, and show no stirring dependence of the signal. In addition, they show excellent long‐term stability and storage stability. Hydrogen sulfide, carbon dioxide, and other relevant chemical parameters do not interfere with the measurement. Oxygen profiles in marine sediments obtained from measurements with microoptrodes show good correlation to profiles measured with oxygen microelectrodes.

New oxygen-sensitive luminescent materials were obtained by physically immobilizing luminescent ruthenium-

A new tool ('planar optrodes') for measuring fine scale 2-dimensional O2 distributions in benthic communities is presented and discussed. The sensor consists of an 02-quenchable fluorophore cast onto a 25 X 55 mm, 175 pm thick transparent PVC sheet, which can be introduced into sediments and microbial mats. The O2 sensitive fluorescence emitted by the planar optrode was measured with a high resolution CCD camera. A 3-point calibration was performed for each pixel in the acquired images to calculate the 2-dimensional oxygen distribution with high accuracy. Calibrated images covered an area of 13 X 17 mm and the set-up allowed the 2-dimensional distribution of O2 to be resolved with a spatial resolution of 26 pm. The sensitivity of the sensor had excellent long-term stability. The planar optrode was used to record the oxygen equilibration between small gas bubbles and surrounding water with a temporal resolution of 20 S. The sensor was also used to measure the vertical and horizontal O2 distribution in an intertidal sediment. The obtained distribution correlated well with O2 concentration microprofiles measured with an O2 microelectrode. The data showed pronounced differences in the O2 penetration depths due to patchy distri.bution of h~ghly lablle organic carbon and showed differences in thickness of the diffusive boundary layer due to small topographic structures on the sediment surface Potential applications of the planar optrode are presented and discussed.