Ultra-sensitive optical oxygen sensors for characterization of nearly anoxic systems

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 100 ms 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 with state-of-the-art analogues. The limits of detection are as low as 5 p.p.b., volume in gas phase under atmospheric pressure or 7 pM in solution. The sensors enable completely new applications for monitoring of ​oxygen in previously inaccessible concentration ranges.

Oxygen undoubtedly belongs to the most important analytes on earth. Traditionally, most oxygen sensors were designed for the physiological range. However, numerous applications require monitoring of ​oxygen at significantly lower concentrations, for example, corrosion protection, surface treatment, the semiconductor industry and biological research. For instance, it was demonstrated that bacteria can show respiration and potentially aerobic growth far below the Pasteur point (~10 μM dissolved ​oxygen (DO)). Recently, sensors that quantify DO in concentration ranges of 100 nM and below have gained special interest as biologists explore aerobic life in areas very close to anoxic conditions. Unfortunately, few sensors are capable to resolve at such low concentration, and measurements below 0.5 nM are currently impossible.

Optical sensors proved to be indispensable tools for ​oxygen quantification that have mostly replaced the more conventional Clark electrode. Their advantages include minimal invasiveness, simplicity, suitability for miniaturization, versatility of formats (planar optodes, fibre-optic sensors, micro- and nanoparticles, paints and so on) and suitability for imaging of ​oxygen distribution on surfaces or in volume. Moreover, optical oxygen sensors are tuneable over a wide range of concentrations. Optical oxygen sensors rely on quenching of a phosphorescent indicator by the analyte. Both the nature of the indicator and the matrix (which acts as a solvent and support for the dye and as a permeation-selective barrier) are of great importance since the sensitivity of an oxygen sensor is roughly proportional to the luminescence decay time of the indicator and to the ​oxygen permeability of the matrix. State-of-the-art indicators are dominated by phosphorescent complexes with platinum group metals that possess decay times varying from microseconds to a few milliseconds. Dyes with significantly longer decay times are extremely rare and are so far limited to fullerenes(which have rather low luminescence brightness at ambient temperatures) and some phosphorescent BF₂-chelates. Both classes are not inherently compatible with highly ​oxygen-permeable matrices (for example, silicone and Teflon AF). Hence, the sensors based on these indicators and other matrices (for example, ethylcellulose and polystyrene (PS)) are not drastically more sensitive than sensors relying on more conventional dyes (for example, Pd(II) porphyrins) immobilized in highly ​oxygen-permeable polymers.

Herein, we present a new type of oxygen-sensing materials that show sensitivities well beyond state-of-the-art trace oxygen sensors. They rely on new blue light-excitable BF₂ and Al(III) chelates featuring ultra-long room temperature phosphorescence. The chelates are modified with perfluoroalkyl chains to ensure compatibility with highly ​oxygen-permeable and chemically inert perfluorinated polymers. The resulted ultra-sensitive sensors are ideally suitable for characterization of nearly anoxic systems.

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