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

Abstract

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.

Figure 1. Synthesis of the borondifluoride chelates and aluminium complexes

Figure 2. Spectral properties of the oxygen indicators

A: excitation and emission spectra of HBAN and HPhN difluoroboron chelates in polystyrene and aluminium complexes in Teflon® AF 1600; b: Temperature-dependent emission spectra of Al(HPhNPF)₃ in Hyflon® AD 60 under deoxygenated conditions acquired with a delay of 0.5 ms after excitation.

Figure 3. Oxygen sensitivity of the trace sensors at 20 °C

A: Stern-Volmer plots for BF₂HBAN and BF₂HPhN in polystyrene; b: Stern-Volmer plots for the aluminium complexes; c: photographic images of the Al(HPhNPF)₃/ Hyflon® AD 60 sensor under illumination with a 366 nm line of a UV lamp. Bluish prompt fluorescence remains constant, and the intense yellow phosphorescence slowly appears for the areas covered with anaerobic sodium sulfite solution (5% wt, containing traces of Co²⁺ as a catalyst).

Figure 4. Real-time monitoring of oxygen levels during enzymatic oxygen consumption

Three trace sensors with overlapping ranges are used to monitor oxidation of glucose catalyzed by glucose oxidase. A running average of 3 data points was used for all sensors. Whereas the reference sensors fail to resolve below 2-10 nM, the new ultra trace sensor can reliably monitor oxygen even at much lower concentrations (insert).