Oxygen quantification in trace amounts is essential in many fields of science and technology. Optical oxygen sensors proved invaluable tools for oxygen measurements in a broad concentration range but until now neither optical nor electrochemical oxygen sensors were able to quantify oxygen in the sub-nanomolar concentration range. Herein we present new optical oxygen sensing materials with unmatched sensitivity. They rely on the combination of ultra-long decaying (several hundred milliseconds lifetime) phosphorescent boron- and aluminium-chelates and highly oxygen-permeable and chemically stable perfluorinated polymers. The sensitivity of the new sensors is improved up to 20-fold compared to state-of-the-art analogues. The limits of detection are as low as 5 parts per billion, volume in gas phase under atmospheric pressure or 7 picomolar in solution. The sensors enable completely new applications for monitoring of oxygen in previously inaccessible concentration ranges.

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Most commercially available optical oxygen sensors target the measuring range of 300 to 2 μmol L-1. However these are not suitable for investigating the nanomolar range which is relevant for many important environmental situations. We therefore developed a miniaturized phase fluorimeter based measurement system called the LUMOS (Luminescence Measuring Oxygen Sensor). It consists of a readout device and specialized “sensing chemistry” that relies on commercially available components. The sensor material is based on palladium(II)-5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorphenyl)-porphyrin embedded in a Hyflon AD 60 polymer matrix and has a KSV of 6.25 x 10-3 ppmv-1. The applicable measurement range is from 1000 nM down to a detection limit of 0.5 nM. A second sensor material based on the platinum(II) analogue of the porphyrin is spectrally compatible with the readout device and has a measurement range of 20 μM down to 10 nM. The LUMOS device is a dedicated system optimized for a high signal to noise ratio, but in principle any phase flourimeter can be adapted to act as a readout device for the highly sensitive and robust sensing chemistry. Vise versa, the LUMOS fluorimeter can be used for read out of less sensitive optical oxygen sensors based on the same or similar indicator dyes, for example for monitoring oxygen at physiological conditions. The presented sensor system exhibits lower noise, higher resolution and higher sensitivity than the electrochemical STOX sensor previously used to measure nanomolar oxygen concentrations. Oxygen contamination in common sample containers has been investigated and microbial or enzymatic oxygen consumption at nanomolar concentrations is presented.

Herein, we present a small and versatile optode system with integrated battery and logger for monitoring of O2, pH, and pCO2 in seawater. Three sensing materials designed for seawater measurements are optimized with respect to dynamic measurement range and long‐term stability. The spectral properties of the sensing materials were tailored to be compatible with a commercially available laboratory oxygen logger that was fitted into a pressure housing. Interchangeable sensor caps with appropriate “sensing chemistry” are conveniently attached to the end of the optical fiber. This approach allows using the same instrument for multiple analytes, which offers great flexibility and minimizes hardware costs. Applications of the new optode system were demonstrated by recording depth profiles for the three parameters during a research cruise in the Baltic Sea and by measuring surface water transects of pH. The optode was furthermore used to monitor the concentration of dissolved oxygen in a seagrass meadow in the Limfjord, Denmark, and sensor packages consisting of pO2, pH, and pCO2 were deployed in the harbors of Kiel, Germany, and Southampton, England, for 6 d. The measurements revealed that the system can resolve typical patterns in seawater chemistry related to spatial heterogeneities as well as temporal changes caused by biological and tidal activity.

Conventional sensors for the quantification of O 2 availability in aquatic environments typically have limits of detection (LOD) of > 1 lmol L 21 and do not have sufficient resolution to reliably measure concentrations in strongly O 2 depleted environments. We present a novel trace optical sensor based on the palladium(II)-benzoporphyrin luminophore, immobilized in a perfluorinated matrix with high O 2 permeability. The trace sensor has a detection limit of $5 nmol L 21 with a dynamic range extending up to $2 lmol L 21. The sensor demonstrates a response time < 30 s and a small, predictable, and fully reversible response to hydrostatic pressure and temperature. The sensor showed excellent stability for continuously measurements during depth profiling in Oxygen Minimum Zones (OMZ). The novel sensor was deployed in situ using a Trace Oxygen Profiler instrument (TOP) equipped with two additional O 2 optical sensors, with higher dynamic range, allowing, when combined, measurements of O 2 concentration from $5 nmol L 21 to 1000 lmol L 21 with a single instrument. The TOP instrument was deployed in the OMZ regions of the Eastern Tropical North Pacific (ETNP) and Bay of Bengal (BoB). The measurements demonstrated that O 2 concentrations in the ETNP generally were below the LOD of the trace sensor, but that large sub-micromolar O 2 intrusions, spanning 60-80 m with maximum O 2 concentrations above 50 nmol L 21, could be observed in the OMZ core. The O 2 concentrations in the BoB were high compared to the ETNP and rarely decreased below 50 nmol L 21

Optical oxygen sensing is of broad interest in many areas of research, such as medicine, food processing, and micro- and marine biology. The operation principle of optical oxygen sensors is well established and these sensors are routinely employed in lab and field experiments. Ultratrace oxygen sensors, which enable measurements in the sub-nanomolar region (dissolved oxygen), are becoming increasingly important. Such sensors prominently exhibit phenomena that complicate calibration and measurements. However, these phenomena are not constrained to ultratrace sensors; rather, these effects are inherent to the way optical oxygen sensors work and may influence any optical oxygen measurement when certain conditions are met. This scenario is especially true for applications that deal with high-excitation light intensities, such as microscopy and microfluidic applications. Herein, we present various effects that we could observe in our studies with ultratrace oxygen sensors and discuss the reasons for their appearance, the mechanism by which they influence measurements, and how to best reduce their impact. The phenomena discussed are oxygen photoconsumption in the sensor material; depletion of the dye ground state by high-excitation photon-flux values, which can compromise both intensity and ratiometric-based measurements; triplet-triplet annihilation; and singlet-oxygen accumulation, which affects measurements at very low oxygen concentrations.

The final step of aerobic respiration is carried out by a terminal oxidase transporting electrons to oxygen (O2). Prokaryotes harbor diverse terminal oxidases that differ in phylogenetic origin, structure, biochemical function, and affinity for O2. Here we report on the expression of high-affinity (cytochrome cbb3 oxidase), low-affinity (cytochrome aa3 oxidase), and putative low-affinity (cyanide-insensitive oxidase (CIO)) terminal oxidases in the marine bacteria Idiomarina loihiensis L2-TR and Marinobacter daepoensis SW-156 upon transition to very low O2 concentrations (<200 nM), measured by RT-qPCR. In both strains, high-affinity cytochrome cbb3 oxidase showed the highest expression levels and was significantly up-regulated upon transition to low O2 concentrations. Low-affinity cytochrome aa3 oxidase showed very low transcription levels throughout the incubation. Surprisingly, however, it was also up-regulated upon transition to low O2 concentrations. In contrast, putative low-affinity CIO had much lower expression levels and markedly different regulation patterns between the two strains. These results demonstrate that exposure to low O2 concentrations regulates the gene expression of different types of terminal oxidases, but also that the type and magnitude of transcriptional response is species-dependent. Therefore, in situ transcriptome data cannot, without detailed knowledge of the transcriptional regulation of the species involved, be translated into relative respiratory activity.

This study highlights possible errors in luminescence lifetime measurements when using bright optical oxygen sensors with high excitation light intensities. An analysis of the sensor with a mathematical model shows that high light intensities will cause a depopulation of the ground state of the luminophore, which results in a non-linear behaviour of the luminescence emission light with respect to the excitation light. The effect of this non-linear behaviour on different lifetime determination methods, including phase-fluorometry, is investigated and in good agreement with the output of the model. Furthermore, the consequences of increasingly high light intensities on phase fluorometric lifetime measurements are illustrated for different oxygen sensors based on benzoporphyrin indicators. For the specific case of PdTPTBPF-based sensors an error as high as 50% is possible under high light conditions (0.25 mol m(-2) s(-1) ≈ 50 mW mm(-2)). A threshold of applied excitation light intensity is derived, thus enabling the point at which errors become significant to be estimated. Strategies to further avoid such errors are presented. The model also predicts a similar depopulation of the ground state of the quencher; however, the effect of this process was not seen in lab measurements. Possible explanations for this deviation are discussed.