Abstract
Optical chemical sensors are the standard for pH monitoring in small-scale bioreactors such as microtiter plates, shaking flasks or other single-use bioreactors. The dynamic pH range of the so far commercially available fluorescent pH sensors applied in small-scale bioreactors is restricted to pH monitoring around neutral pH, although many fermentation processes are performed at pH < 6 on industrial scale. Thus, two new prototype acidic fluorescence pH sensors immobilized in single-use stirred-tank bioreactors, one with excitation at 470 nm and emission at 550 nm (sensor 470/550) and the other with excitation at 505 nm and emission at 600 nm (sensor 505/600), were characterized with respect to dynamic ranges and operational stability in representative fermentation media. Best resolution and dynamic range was observed with pH sensor 505/600 in mineral medium (dynamic range of 3.9 < pH < 7.2). Applying the same pH sensors to complex medium results in a drastic reduction of resolution and dynamic ranges. Yeast extract in complex medium was found to cause background fluorescence at the sensors’ operating wavelength combinations. Optical isolation of the sensor by adding a black colored polymer layer above the sensor spot and fixing an aperture made of adhesive photoresistant foil between the fluorescence reader and the transparent bottom of the polystyrene reactors enabled full re-establishment of the sensor’s characteristics. Reliability and operational stability of sensor 505/600 was shown by online pH monitoring (4.5 < pH < 5.8) of parallel anaerobic batch fermentations of Clostridium acetobutylicum for the production of acetone, butanol and ethanol (ABE) with offline pH measurements with a standard glass electrode as reference.
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Acknowledgments
This work was funded by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology and the companies PreSens Precision Sensing GmbH, Regensburg, Germany and 2mag AG, Munich, Germany. Assistance from Michael Geitner and software support by Dirk Hebel (Technische Universität München, Garching, Germany) is gratefully acknowledged. The authors also gratefully acknowledge the support of Nils H. Janzen and Michael Schmidt by the TUM Graduate School at the Technische Universität München, Munich, Germany.
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Janzen, N.H., Schmidt, M., Krause, C. et al. Evaluation of fluorimetric pH sensors for bioprocess monitoring at low pH. Bioprocess Biosyst Eng 38, 1685–1692 (2015). https://doi.org/10.1007/s00449-015-1409-4
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DOI: https://doi.org/10.1007/s00449-015-1409-4