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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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.

New aza-BODIPY pH indicators with spectral properties modulated solely by photoinduced electron transfer (PET) are presented.

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Long-wavelength luminescent probes and sensors become increasingly popular. They offer the advantage of lower levels of autofluorescence in most biological probes. Due to high penetration depth and low scattering of red and NIR light such probes potentially enable measurements in tissues and some of them have already reached a high level of reliability required for such applications. This review focuses on the recent progress in development and application of long-wavelength analyte-sensitive probes which can operate both reversibly and irreversibly. Photophysical properties, sensing mechanisms, advantages and limitations of individual probes are discussed.

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.

A new fluorescent sensor for Na is presented. The sensor relies on a Na selective fluoroionophore based on a bright red-emitting BODIPY chromophore. The fluorescence of the fluoroionophore is enhanced upon binding of Na-ions to the highly selective aza-crown ether receptor due to reduction of the photoinduced electron transfer (PET) quenching. Solid state sensing materials were prepared by physically embedding the fluoroionophore into water-swellable biocompatible polymer matrices (polyurethane hydrogels), thus enabling continuous measurements of aqueous samples. Despite the simple design, the sensor showed no leaching of the indicator and featured fast and reversible response. Among different polyurethane hydrogels investigated, the hydrogel D1 featuring the highest water uptake was found to be the most suitable due to the highest dynamics between "off" and "on" states. Due to little or no cross sensitivity to other ions (e.g., Mg, Ca, K) and its insensitivity to potential changes in pH, this sensor is promising for use in clinical diagnostics and for biological and marine applications. Fiber-optic sensors based on referenced read-out with a compact phase fluorimeter were prepared. To demonstrate their practical applicability, the sensors were used to determine the salinity in the seawater and brackish water of the Baltic Sea.