Fiber‐optic oxygen microsensors, a new tool in aquatic biology

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

An in situ instrument for planar O2 optode measurements at benthic interfaces

A new in situ instrument for two‐dimensional mapping of oxygen in coastal sediments is presented. The measuring principle is described, and potential mechanical disturbances, solute and particle smearing associated with the measurements, and calibration routines are evaluated. The first in situ measurements obtained in two different benthic communities are presented. In a shallow photosynthetic sediment (1 m of water depth), an extensive horizontal and temporal variation in the O2 distribution caused by benthic photosynthesis and irrigating fauna was resolved. Repetitive planar optode measurements performed along a transect in central Øresund, Denmark (17 m of water depth) revealed a positive correlation between the apparent O2 penetration depths (OP) measured with a lateral distance <5.0 mm, whereas OP measured with a larger horizontal distance (up to 50 m) were not correlated. Consequently, the OP varied in patches with a characteristic size of 5.0 mm. The instrument described is a powerful new tool for in situ characterization of spatiotemporal variations in O2 distributions within benthic communities. The instrument can be adapted for use at full ocean depths, e.g., on deep‐sea landers or remote operating vehicles

A novel measuring system for oxygen microoptodes based on a phase modulation technique

New fiber optic oxygen microsensors (microoptrodes) for use in aquatic environments have recently been developed as an alternative to commonly used CLark-type oxygen microelectrodes. The microoptrodes have the advantage of no oxygen consumption and no stirring sensitivity combined with a simple manufacturing process of the sensors. To avoid problems inherent to luminescence intensity measurements like photobleaching, signal dependency on the optical properties of the surrounding medium and system drifts, a novel measuring system was developed. This system uses a phase modulation method to evaluate a signal phase shift that is caused by the oxygen dependent luminescence lifetime. The measuring system is based on simple solid state technology. High reliability and low costs of the system can therefore be combined with the ability of miniaturization and low power consumption. The system consists of three units: 1) the microoptrode with the optical setup [glass fiber coupler, optical filters, lenses, light source (light emitting diode) and light detection (photon multiplier tube)], 2) the analogue signal processing unit, including a special phase detection module, and 3) the digital signal processing unit, a personal computer or a microcontroller for control of the measuring system, display and data storage. First measurements of oxygen depth profiles in sediments and biofilms at high levels of ambient light demonstrated the advantages of phase shift based O2 measurements as compared to intensity based measurements with microoptrodes

Background-free fluorescence decay time sensing and imaging of pH with highly photostable diazaoxotriangulenium dyes

Novel fluorescent diazaoxatriangulenium (DAOTA) pH indicators for lifetime-based self-referenced pH sensing are reported. The DAOTA dyes were decorated with phenolic receptor groups inducing fluorescence quenching via photoinduced electron transfer mechanism. Electron-withdrawing chlorine substituents ensure response in the most relevant pH range (apparent pK'a values ~5 and 7.5 for the p,p-dichlorophenol- and the p-chlorophenol-substituted dyes, respectively). The dyes feature long fluorescence lifetime (17-20 ns), high quantum yield (~60%) and high photostability. Planar optodes are prepared upon immobilization of the dyes into polyurethane hydrogel D4. Apart from the response in the fluorescence intensity, the optodes show pH-dependent lifetime behaviour which makes them suitable for studying 2D pH distribution with help of fluorescence lifetime imaging technique. The lifetime response is particularly pronounced for the sensors with high dye concentration (0.5-1% wt. in respect to the polymer) and is attributed to efficient homo-FRET mechanism

Highly Sensitive Poisoning-Resistant Optical Carbon Dioxide Sensors for Environmental Monitoring

A new optical carbon dioxide sensor for environmental monitoring is presented. It combines a robust and long-term stable sensing material with a compact read-out device. The sensing material relies on a NIR pH indicator immobilized into ethyl cellulose along with a quaternary ammonium base. The perfluorinated polymer Hyflon AD 60 used as a protection layer significantly enhances the long-term and mechanical stability of the sensor foils, as well as the robustness against poisoning gases, e.g. hydrogen sulfide. The sensor can be stored under ambient conditions for more than six weeks, whereas sensors covered with silicone rubber deteriorate within one week under the same conditions. The complete sensor device is applicable after a three-point (re)calibration without a preconditioning step. The carbon dioxide production and consumption of the water plant Egeria densa was measured in the laboratory. Furthermore, results of profiling carbon dioxide measurements during a research cruise on the Baltic Sea at water depths up to 225 m are presented

A New In Vivo Fluorimetric Technique To Measure Growth of Adhering Phototrophic Microorganisms

We developed a noninvasive rapid fluorimetric method for the investigation of growth of adhering (benthic) phototrophic microorganisms. The technique is based on the sensitive detection of the in vivo fluorescence of chlorophylls chlorophyll a and bacteriochlorophyll a and monitors increases in signal over time as an indicator for growth. The growth fluorimeter uses modulated excitation light of blue-light-emitting diodes and a photodiode as the detector. The light-emitting diodes are mounted geometrically in an aluminum housing for efficient and uniform illumination of the bottoms of the growth containers. The fluorimeter was characterized with respect to detection limit and dynamic range. This system is capable of resolving in vivo chlorophyll a concentrations of 0.5 (mu)g liter(sup-1) in cyanobacteria and 0.03 (mu)g liter(sup-1) in diatoms as well as in vivo bacteriochlorophyll a concentrations in phototrophic bacteria of 0.3 (mu)g liter(sup-1), which points to an extremely high sensitivity compared with that of similar available techniques. Thus, the new fluorimeter allows the determination of growth at extremely low cell densities. The instrument was used successfully to measure the growth of several adhering isolates of the filamentous cyanobacterium Microcoleus chthonoplastes from benthic microbial mats in seawater of different salinities. The data obtained demonstrate broad growth responses for all strains, which thus can be characterized as euryhaline organisms

A simple fiberoptic sensor to detect the penetration of microsensors into sediments and other biogeochemical systems

We have developed a simple and mechanically robust fiberoptic microsensor that enables optical detection of the sediment-water interface at a spatial resolution of <50 mu m The sensor measures with a tapered optical fiber the increased backscatter of near-infrared light near the sediment surface. To determine the sediment surface position independent of ambient light conditions, we developed a miniaturized opto-electronic system with an intensity-modulated laser diode (780 nm) as the light source and a photodiode as the detector. Laboratory tests of our system were done with artificial as well as with various natural sediments and biofilms. Fiberoptic microsensors for surface detection can be combined easily with both electrochemical and optical microsensors for oxygen or other reactive species