How to Choose the Right Dissolved Oxygen Sensor
As bioprocessing and biotechnology professionals know, dissolved oxygen (DO) is one of the most critical parameters for good cell culture practices. Monitoring and controlling dissolved oxygen can mean the difference between high or low cell viability and can further play an important role in complex protein production.
There are three basic varieties of dissolved oxygen sensors on the market:
• Optical Sensors
• Galvanic Cells
• Polarographic Cells
Each one of these have a role to play in scientific research. We have compared probes for dissolved oxygen measurement and the Pros and Cons of each are listed below. Hopefully you will discover which type of DO sensor technology is right for your cell culture needs.
Optical Sensors
Optical sensors are the proverbial new kid on the block for bioprocessing and biotechnology. The technology has been established since the early 2000’s but till recently has not been widely available. The sensor works by an external reader inciting fluorescence in a sensor spot located inside the cell culture vessel. The reader correlates dissolved oxygen concentration based on the quenching effect oxygen has on fluorescence.
Pros
• Optical sensors provide a high degree of accuracy from 0% to 100% DO. They are especially useful at low DO concentrations and are one of the best options for tissue culture where oxygen measurement is extremely important.
• They are non-consumptive of oxygen. Unlike Galvanic and Polarographic Cell electrodes, optical sensors do not require the utilization of oxygen for measurement.
• Limited probe maintenance is a big Pro for optical DO sensors. Because the optical reader is external and does not come in contact with cell culture media, it lasts much longer than traditional submerged probes.
• Single use, disposable sensor spots made by quality companies are pre-calibrated. The sensor spots are placed inside the vessel and are the only part of the system that comes in contact with cells. This limits constant sampling and reduces possible points of contamination.
• Optical sensors are stored dry and can last for up to six months.
Cons
• Because the use of optical dissolved oxygen technology is comparatively new, it is still not approved for all types of applications.
• Optical sensor spots can be difficult to accurately place when starting out.
• Continuous readings are currently not available. Since the optical reader needs to receive a return signal of sensor spot fluorescence oxygen quenching it cannot read without a delay of a few seconds.
Polarographic Cells
The polarographic cell type of DO sensing, commonly called amperometric or Clark electrodes, is an electrochemical sensing technology. Clark electrodes have an anode and cathode contained in an electrolyte solution filled chamber with a gas permeable membrane. Like an optical sensor, it may read 0%-100% dissolved oxygen.
Pros
• The Clark electrode has been around since 1962 and the technology is well known.
• Despite the need for regular calibration, the technology is relatively easy to use.
• Polarographic probes can be used in a variety of applications from laboratory cell culture to wastewater management.
Cons
• The probes require some time to warm up before measurements are accurate and can quickly become time consuming in laboratory settings.
• The Clark electrode consumes oxygen as part of its measurement. This is not always an issue if DO levels are high or accuracy is not a requirement, but at low oxygen levels it becomes a big issue.
• Because they consume small amounts of oxygen, it is required that the media be constantly moving to receive accurate readings.
• Clark electrodes require constant calibration at 0% and 100%. The 0% reading requires a special solution that must be replaced every few weeks.
• Continuous readings tend to have a high amount of drift because some of the oxygen is being consumed for each reading.
• If submerged in cell culture, the large electrochemical probe is somewhat cumbersome and causes disruption to the cell culture environment.
• Constant removal of cell culture media for readings increases contamination risk.
Galvanic Cells
Galvanic cells, developed in 1964, just two years after polarographic cells are similar in many ways. The main difference between the two is that Galvanic cells use different electrode materials. The anode is lead or zinc, and the cathode is gold or silver. In the presence of an electrolyte the anode is oxidized, and electrons are released.
Pros
• The Galvanic cell probe’s two dissimilar metals provide internal electric potential and users don’t need to wait for polarization to occur when the sensor is turned on. That means there is no warm-up time required.
• Galvanic cells are excellent for commercial/industrial use in the field and are the preferred option for measuring DO in streams, lakes, and rivers.
• Even though zinc or lead is consumed, the probes can last for more than a year.
• They are more stable and accurate at low DO levels than polarographic probes.
• Galvanic cells have a fast response time.
Cons
• Since oxygen is consumed during measurement, Galvanic cells require the media to be in constant movement.
• Despite lasting for a long time, the sensors require complicated maintenance every 2-8 weeks.
• Probes require daily calibration at 0% and 100% DO.
• They are less stable and accurate at low DO levels than optical sensors.
• Continuous readings tend to have a high amount of drift because some of the oxygen is being consumed for each reading.
• If submerged in cell culture, the probes are somewhat cumbersome and cause disruption to the cell culture environment.
• Constant removal of cell culture media for readings increases possible points of contamination.