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

Lehner P, Staudinger C, Borisov SM, Klimant I - Nat Commun (2014)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Institute of Analytical Chemistry and Food Chemistry, Graz University of Technology, NAWI Graz, Stremayrgasse 9, 8010 Graz, Austria.

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.

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Related in: MedlinePlus

Oxygen sensitivity of the trace sensors at 20 °CA: 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).
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Figure 3: Oxygen sensitivity of the trace sensors at 20 °CA: 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).

Mentions: Due to the exceptionally long lifetimes of these chelates, resulting sensing materials are expected to have sensitivities orders of magnitude above those based on conventional sensing chemistries. Indeed, sensors composed of difluoroboron chelates in polystyrene show very high sensitivities, even though polystyrene (PS) has only moderate oxygen permeability (P ≈ 8.8×10−16 mol m−1 s−1 Pa−1).25 KSV for BF2(HPhN) and BF2(HBAN) based sensors was 60 × 10−3 ppmv−1 and 120 ×10−3 ppmv−1 respectively, (Fig. 3a) which makes them suitable for monitoring sub-nanomolar DO concentrations. The limit of detection for the BF2(HBAN)-based material, assuming a resolution of 0.5% I0, is in fact 45 ppbv (gas phase) or 60 pM (DO). Both sensing materials surpass the sensitivity of nearly all previously published sensors. Still, highly permeable matrices such as Hyflon®, Teflon® AF and poly(trimethylsilylpropyne) can potentially be used to create even more sensitive materials. Poly(trimethylsilylpropyne) has the highest known oxygen permeability (P ≈ 23000×10−16 mol m−1 s−1 Pa−1)26 but was not considered due to poor chemical stability and possession of double bonds that are prone to oxidation and thus analyte consumption.27,28 Hyflon® and Teflon® AF are amorphous, glassy, perfluorinated copolymers that combine high free volume and unmatched chemical stability. In this work we used Hyflon® AD 60 (P ≈ 170×10−16 mol m−1 s−1 Pa−1).29 and Teflon® AF 1600 (P ≈ 1200×10−16 mol m−1 s−1 Pa−1).30 Unfortunately, most conventional dyes, as well as BF2(HPhN) and BF2(HBAN), show extremely poor solubility in these matrices and aggregate readily. We failed to obtain usable sensors based on the BF2 chelates due to poor luminescence in perfluorinated polymers at any relevant concentration (0.01-1 % wt. of the dyes in respect to the polymer). Therefore, in order to render the indicators soluble in Hyflon® and Teflon® AF, HPhN and HBAN were perfluoroalkylated with perfluorooctyl iodide (Figure 1). The resulting ligands (HPhNPF and HBANPF) bear two C8F17 chains each, although in the case of HBANPF there is some impurity of tri-substituted ligand (Supplementary Fig. 2, 3). As indicated by NOESY and HMBC spectra (Supplementary Fig. 4, 5) a single isomer appears to be the predominant product for HPhNPF (Supplementary Fig. 6). The 1H NMR and COSY spectra for HBANPF (Supplementary Fig. 7, 8) showed no significant substitution in the benzannelated ring. Complete separation of the tri-substituted derivate is very challenging due to the very high similarity; however both tri- and disubstituted derivates are expected to have very similar photophysical properties und solubility. It was found that perfluoroalkylated derivates of the difluoroboron chelates have low chemical stability and dissociate in solution in presence of traces of water (Supplementary Fig. 9). Hence aluminium complexes (Al(HPhNPF)3 and Al(HBANPF)3, Figure 1) were prepared in an attempt to obtain more stable indicators. Indeed, dissociation was not observed for the aluminium chelates and they dissolve readily in fluorinated solvents (octafluorotoluene, perfluorodecalin) and also in Hyflon® and Teflon® AF. Due to the octahedral structure of the Al(III) complexes and unsymmetric substitution pattern of the ligands four sterioisomers are possible, which are inseparable because of their extreme similarity. The photophysical properties of the isomers are expected to be virtually identical. Similar to the BF2-chelates, the Al(III) complexes possess prompt fluorescence, delayed fluorescence and phosphorescence (Fig. 1). The phosphorescence decay times (250 ms for Al(HPhNPF)3 and 350 ms for Al(HBANPF)3 at 20°C in Teflon® AF 1600) are slightly shorter than those of the BF2-chelates and the Al(III) complexes possess higher molar absorption coefficients (ε465 = 23.2×103 M−1 cm−1 and ε459 = 27.1×103 M−1 cm−1 for Al(HPhNPF)3 and Al(HBANPF)3, respectively). The oxygen sensing materials based on these Al(III) complexes embedded in Hyflon® show extremely high sensitivities (Fig. 3b). In fact the KSV values are as high as 590 ×10−3 ppmv−1 (for Al(HBANPF)3, Table 1) and the sensors resolve up to 10 ppbv or 12 pM DO. They are about one order of magnitude more sensitive than the state-of-the-art sensors based on fullerenes (Table 1).14 The sensors based on Teflon® AF 1600 show a 2-fold increase in sensitivity compared to the Hyflon®-based ones. Al(HBANPF)3 in Teflon® AF 1600 is the most sensitive sensor material presented, with a KSV of 960 ×10−3 ppmv−1 and a detection limit of 5 ppbv or 7 pM. Potentially even more sensitive sensor materials based on Teflon® AF 2400 (P ≈ 3000×10−16 mol m−1 s−1 Pa−1)30 showed signs of aggregation and were therefore not further investigated. Importantly, the Stern-Volmer plots for the phosphorescence decay time are similar to those obtained for the intensity measurements (Fig. 3b and supplementary Fig. 10) so both read-out schemes (ratiometric intensity or decay time) can be used. It should be mentioned, that sensitivities are so high that obtaining reliable calibrations becomes rather challenging. Even high purity nitrogen contains oxygen impurities in the low ppm range, hence all aluminium complex-based sensor materials were calibrated using a standard addition method described in the experimental section.


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

Lehner P, Staudinger C, Borisov SM, Klimant I - Nat Commun (2014)

Oxygen sensitivity of the trace sensors at 20 °CA: 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).
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Figure 3: Oxygen sensitivity of the trace sensors at 20 °CA: 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).
Mentions: Due to the exceptionally long lifetimes of these chelates, resulting sensing materials are expected to have sensitivities orders of magnitude above those based on conventional sensing chemistries. Indeed, sensors composed of difluoroboron chelates in polystyrene show very high sensitivities, even though polystyrene (PS) has only moderate oxygen permeability (P ≈ 8.8×10−16 mol m−1 s−1 Pa−1).25 KSV for BF2(HPhN) and BF2(HBAN) based sensors was 60 × 10−3 ppmv−1 and 120 ×10−3 ppmv−1 respectively, (Fig. 3a) which makes them suitable for monitoring sub-nanomolar DO concentrations. The limit of detection for the BF2(HBAN)-based material, assuming a resolution of 0.5% I0, is in fact 45 ppbv (gas phase) or 60 pM (DO). Both sensing materials surpass the sensitivity of nearly all previously published sensors. Still, highly permeable matrices such as Hyflon®, Teflon® AF and poly(trimethylsilylpropyne) can potentially be used to create even more sensitive materials. Poly(trimethylsilylpropyne) has the highest known oxygen permeability (P ≈ 23000×10−16 mol m−1 s−1 Pa−1)26 but was not considered due to poor chemical stability and possession of double bonds that are prone to oxidation and thus analyte consumption.27,28 Hyflon® and Teflon® AF are amorphous, glassy, perfluorinated copolymers that combine high free volume and unmatched chemical stability. In this work we used Hyflon® AD 60 (P ≈ 170×10−16 mol m−1 s−1 Pa−1).29 and Teflon® AF 1600 (P ≈ 1200×10−16 mol m−1 s−1 Pa−1).30 Unfortunately, most conventional dyes, as well as BF2(HPhN) and BF2(HBAN), show extremely poor solubility in these matrices and aggregate readily. We failed to obtain usable sensors based on the BF2 chelates due to poor luminescence in perfluorinated polymers at any relevant concentration (0.01-1 % wt. of the dyes in respect to the polymer). Therefore, in order to render the indicators soluble in Hyflon® and Teflon® AF, HPhN and HBAN were perfluoroalkylated with perfluorooctyl iodide (Figure 1). The resulting ligands (HPhNPF and HBANPF) bear two C8F17 chains each, although in the case of HBANPF there is some impurity of tri-substituted ligand (Supplementary Fig. 2, 3). As indicated by NOESY and HMBC spectra (Supplementary Fig. 4, 5) a single isomer appears to be the predominant product for HPhNPF (Supplementary Fig. 6). The 1H NMR and COSY spectra for HBANPF (Supplementary Fig. 7, 8) showed no significant substitution in the benzannelated ring. Complete separation of the tri-substituted derivate is very challenging due to the very high similarity; however both tri- and disubstituted derivates are expected to have very similar photophysical properties und solubility. It was found that perfluoroalkylated derivates of the difluoroboron chelates have low chemical stability and dissociate in solution in presence of traces of water (Supplementary Fig. 9). Hence aluminium complexes (Al(HPhNPF)3 and Al(HBANPF)3, Figure 1) were prepared in an attempt to obtain more stable indicators. Indeed, dissociation was not observed for the aluminium chelates and they dissolve readily in fluorinated solvents (octafluorotoluene, perfluorodecalin) and also in Hyflon® and Teflon® AF. Due to the octahedral structure of the Al(III) complexes and unsymmetric substitution pattern of the ligands four sterioisomers are possible, which are inseparable because of their extreme similarity. The photophysical properties of the isomers are expected to be virtually identical. Similar to the BF2-chelates, the Al(III) complexes possess prompt fluorescence, delayed fluorescence and phosphorescence (Fig. 1). The phosphorescence decay times (250 ms for Al(HPhNPF)3 and 350 ms for Al(HBANPF)3 at 20°C in Teflon® AF 1600) are slightly shorter than those of the BF2-chelates and the Al(III) complexes possess higher molar absorption coefficients (ε465 = 23.2×103 M−1 cm−1 and ε459 = 27.1×103 M−1 cm−1 for Al(HPhNPF)3 and Al(HBANPF)3, respectively). The oxygen sensing materials based on these Al(III) complexes embedded in Hyflon® show extremely high sensitivities (Fig. 3b). In fact the KSV values are as high as 590 ×10−3 ppmv−1 (for Al(HBANPF)3, Table 1) and the sensors resolve up to 10 ppbv or 12 pM DO. They are about one order of magnitude more sensitive than the state-of-the-art sensors based on fullerenes (Table 1).14 The sensors based on Teflon® AF 1600 show a 2-fold increase in sensitivity compared to the Hyflon®-based ones. Al(HBANPF)3 in Teflon® AF 1600 is the most sensitive sensor material presented, with a KSV of 960 ×10−3 ppmv−1 and a detection limit of 5 ppbv or 7 pM. Potentially even more sensitive sensor materials based on Teflon® AF 2400 (P ≈ 3000×10−16 mol m−1 s−1 Pa−1)30 showed signs of aggregation and were therefore not further investigated. Importantly, the Stern-Volmer plots for the phosphorescence decay time are similar to those obtained for the intensity measurements (Fig. 3b and supplementary Fig. 10) so both read-out schemes (ratiometric intensity or decay time) can be used. It should be mentioned, that sensitivities are so high that obtaining reliable calibrations becomes rather challenging. Even high purity nitrogen contains oxygen impurities in the low ppm range, hence all aluminium complex-based sensor materials were calibrated using a standard addition method described in the experimental section.

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Institute of Analytical Chemistry and Food Chemistry, Graz University of Technology, NAWI Graz, Stremayrgasse 9, 8010 Graz, Austria.

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.

No MeSH data available.


Related in: MedlinePlus