Novel Oxygen Optrode withstanding Autoclavation: Technical
Solutions and Performances during Implementation
H. S. Voraberger, H. Kreimaier, K. Biebernik
JOANNEUM RESEARCH, Institute of Chemical Process Development and Control,
Steyrergasse 17, 8010 Graz, Austria
Abstract
A new optical oxygen sensor withstanding sterilization is reported. The sensor is based on the
quenching of the luminescence lifetime of a dye immobilized in a suitable polymer. A
selection of suitable polymers was investigated under these harsh conditions. The matrices
polysulfone (PSU) and polyetherimide (PEI) displayed excellent characteristics for this
application. It was shown that an optimized thermal pretreatment of the polymer membranes
significantly improved the performance of the device (variation of measurement readouts was
improved from 7,1 to 0,7 vol.% at 100% oxygen). With pretreated membranes negligible
changes of the sensor readout were observed after steam sterilization (at 135°C for 60
minutes). Furthermore, a sterilization with acidic or basic solutions simulating Cleaning In
Place (CIP) showed no relevant effects. The response time of the sensor turned out to be t90 =
45 sec. for a sensor based on PSU as matrix to get from 10% O2 to an air saturated solution
(t95 = 59 sec., t99 = 115 sec.). A particular calibration model was developed, which takes the
temperature dependence of the sensor into account. The relative error due to this model was
determined to be less than 1% in the temperature range from 5C to 50C. The
implementation of this sensor in a bioreactor to measure the dissolved oxygen during
fermentation confirmed its excellent performance.
Introduction
Numerous processes, as for example yeast fermentation or the synthesis of penicillin or
brewing beer, can be controlled by continuous monitoring of the oxygen concentration in the
bioreactor. Most of those processes require sterile environments. Therefore, the demand arose
for autoclavable oxygen sensors which can remain in the fermenter during autoclavation. In
this report we describe a new optochemical oxygen sensor, which fulfils such a requirement,
and which is an attractive alternative to the presently used electrochemical sensors.
Electrodes operate on the principle of oxygen reduction at a negatively biased platinum
surface. Optical oxygen sensors in general, compared to electrodes, display numerous
advantages: they can be employed for gaseous as well as liquid analytes, no oxygen is
consumed during measurements, there is no CO2 interference, they are all solid-state devices
without the need of any electrolyte, they do not depend on the flow speed of the analyte and
they allow short response times. Furthermore they are compatible with fibre optics and are
able to withstand high pressures. The membranes can be produced easily by spin coating,
dipping, printing or casting a polymer film containing the luminescent dye onto a suitable
carrier, thus enabling the membranes to be disposable due to low production costs.
Results and Discussion
In our work the behaviour of selected polymers under the conditions of steam sterilisation was
investigated. Polysulfone and polyetherimide showed excellent characteristics for their
application as steam sterilisable membranes. Even repeated autoclavation did not cause
detectable changes.
By a thermal pre-treatment of the oxygen sensors (i. e. polymer and dye), a further
improvement of the sensor stability could be achieved. During a thermal pre-treatment,
remaining solvent and water are removed and therefore stress is reduced. This was revealed
by infrared spectroscopy. Stern-Volmer-plots of sensors pre-treated under different conditions
were established. Fig. 1 shows the improved performance of a pre-treated membrane
compared to a membrane without pre-treatment. The plots A and B were recorded before and
after autoclavation. Plot B clearly shows, that autoclavation had no significant influence on
the performance of the pre-treated sensor membrane. The variation of measurement readouts
was improved from 7,1 to 0,7 vol.% at 100% oxygen.
1.8
1.6
1.6
0/
1.8
0/
1.4
1.4
1.2
1.2
1.0
1.0
0
20
40
60
Oxygen / vol.%
A
80
100
0
B
20
40
60
80
100
Oxygen / %
Fig. 1: Stern-Volmer plots of optical oxygen sensors before (black dotted line) and after
steam sterilisation (grey line). Plot A refers to an untreated PSU film, plot B refers to a
thermally pre-treated PSU film.
air saturation / %
The measured phase shifts and the Stern-Volmer parameters are strongly dependent on the
temperature of the sensor membrane. For many applications a constant temperature cannot be
guaranteed and therefore the temperature has to be measured and to be compensated in a
calibration model. So a calibration function valid and tested for the temperature range from 5°
to 50°C was developed.
The performance of an optical oxygen
Escherichia Coli Fermentation
sensor membrane, which was inserted
100
Mettler electrode
in a bioreactor and compared to a
JR Sensor
conventional electrochemical device
80
measuring the dissolved oxygen, is
60
displayed in Fig. 2. A cultivation of
Escherichia Coil for production of
40
enzymes was monitored. It can be
20
seen, that with both sensor types
0
similar
results
were
obtained.
Therefore polymer based optical
0
2
4
6
8 10 12 14 16 18 20 22 24
time / hours
oxygen sensors are a good alternative
to electrochemical sensors.
Fig. 2: Comparison of the performances of a
conventional electrochemical oxygen electrode
and the JOANNEUM RESEARCH Optical
Oxygen Measuring Device.
Acknowledgements
We wish to thank the Institute for Biotechnology at the University of Technology, Graz, Austria, for access to
their fermenters and electrochemical O2 electrodes, Christian Konrad for his contribution to the temperature
compensation experiments and Stephanus Leibl and his co-workers at Wissenschaftlich-Technische Werkstätten
GmbH, 82362 Weilheim, Germany, for providing essential mechanical parts.