Oxygen is one of the most important molecules on our earth. It is integral in the cycles of life and decay: photosynthesis in plants, respiration in animals, and most breakdown processes. Without oxygen, there would be no fire, no fermentation or biodegradation, and no oxidation and corrosion to complete the circle and enable new life.
Accurate, non-invasive measurement of oxygen is important not only for monitoring and studying these processes, but also for safety. Management of oxygen levels in fuel tanks can prevent explosions, while headspace measurements in food packaging can guarantee adequate shelf life. Monitoring oxygen can assure patient safety in point of care analysis and respiratory settings, or indicate a sterile seal on surgical instruments and drug packaging.
Optically-based oxygen sensors offer many advantages over traditional electrode oxygen sensors, including faster response and remote measurement. They do not consume any oxygen during measurement, and are chemically inert. Furthermore, advances in detection methods, miniaturization, and reduced calibration requirements make optical oxygen sensors easier than ever to use in the lab and the field.
Are you curious yet? Take a deep breath and dive into our world of oxygen sensors to see just how much we can do.
- Versatile: Able to detect molecular oxygen in gas and liquid phases across wide ranges of temperature and concentration.
- Multiple form factors: Sensors are available as traditional probes as small as 300μm, non-invasive patches, and standard cuvette sampling cells.
- Non-consuming: Oxygen is not consumed during measurement, giving true unaltered readings of your system.
- Rapid response time: Less than 1 second response time with averaging minimized.
- Low-level detection: Sensors can resolve to 0.004% oxygen at 1atm at the lowest oxygen concentrations.
Other Common Applications
- Biological environments: biofermentation processes, cell culture and growth media monitoring, sterile environments
- Biomedical & life sciences: blood oximetry, cellular analysis, tissue analysis, serum, respiration, medical devices
- Food process & storage monitoring: beverage packaging, vacuum-packaged foods, vegetable oils, wine fermentation
- Vacuum & semiconductor processes: controlled-environment glove boxes, ion deposition processes
- Aerospace: cell growth in space, O2 monitoring in confined spaces
- Process monitoring: harsh chemical environments (hexane, toluene, and mineral oil), hydrocarbons (fuels, alcohols, hydraulic fluids), packaging and headspace, pharmaceutical fermentation
- Environmental & ecological: aquaculture applications, marine organisms, natural waters (marine, surface), soils and sediments, wastewater treatment, cooling water
- Aviation: fuel storage monitoring, fuel transportation
How Can Oxygen Levels Be Measured?
A traditional electrode sensor measures oxygen in a liquid electrochemically. Oxygen diffuses into the sensor through a permeable membrane and encounters an anode and cathode. The current created when oxygen is reduced at the cathode is proportional to the oxygen levels in the liquid. The drawbacks of this method are that it consumes oxygen in the process, and it depends on the rate of oxygen diffusion into the sensor, necessitating regular stirring.
An optically based sensor (optrode), in contrast, works by coming into equilibrium with its environment. Oxygen diffuses into the sensor coating, where it alters the nature of the fluorescence of an indicator material. By looking at how the fluorescence intensity, lifetime, and phase are altered in the presence of oxygen, the oxygen level can be quickly and accurately determined without consuming any oxygen.
Optrodes have many other advantages: they have faster response, are chemically inert, offer long-term stability without the need for recalibration, and couple easily to optical fibers for remote measurements. The sensor material on which they are based can be applied to the tip of a fiber, a flat substrate, adhesive membrane, or other surface, allowing these sensors to be customized to the specific application. Optrodes can take a minimally intrusive format like an ultra-narrow probe, or can be placed directly within packaging or a cuvette for non-destructive, non-contact measurement. They can even be overcoated for deployment in harsh environments.
Our oxygen sensors measure the partial pressure of dissolved or gaseous oxygen. Because they consume no oxygen, they can be used in continuous contact with viscous samples; unlike with electrode oxygen sensors, continuous stirring is not required.
How Does an Ocean Optics Oxygen Sensor Work?
Our oxygen sensors are based on fluorescence of an indicator material that has been integrated into a matrix. The sensor matrix is then coated on a probe tip, a slide, the wall of a cuvette, or formed into an adhesive patch. We use a matrix with good thermal and mechanical stability, superior chemical compatibility, ease of production and rapid response.
We use two types of indicator materials that fluoresce – ruthenium and Pt-porphyrin complexes. Each works well for a specific range of applications like general laboratory use, high-sensitivity applications or hydrocarbon-rich sample environments. What they have in common is that both indicator materials are highly sensitive to partial pressure of oxygen.
In the presence of molecular oxygen, the fluorescence properties exhibited by these materials are altered. The most obvious change is a quenching of the fluorescence, or decrease in fluorescence intensity, as oxygen levels increase. The quenching happens because an excited indicator molecule has come into contact with an oxygen molecule, transferring its excess energy to the oxygen molecule in a non-radiative transfer (a gentle handshake of sorts). The degree of fluorescence quenching depends on the frequency of collisions, and therefore on the oxygen concentration of the sample, as well as its pressure and temperature.
Our particular oxygen sensors use a blue LED for excitation at 450 nm, transmitted to the sensor material using a fiber optic probe. Fluorescence at visible wavelengths is collected back through the same probe and routed to an avalanche photodiode for detection.
A more subtle effect of fluorescence quenching is that the average fluorescence lifetime of the indicator material decreases as oxygen concentration increases. By pulsing the excitation LED and looking at when maximum fluorescence is emitted relative to those pulses (the phase shift), the average fluorescent lifetime can be determined. This method is called phase-sensitive detection. This is the method used by all of our NeoFox systems.
Phase sensitive detection is a more accurate method than intensity-based detection, as it is insensitive to electronic and light source drift, scattering within the sample, and intensity variations due to fiber bending and ambient light. It is also unaffected by chemical degradation of the indicator material, and by refractive index changes caused by calibrating in gas prior to measurements in a liquid. Photobleaching is vastly reduced in platinum-based chemistries as compared to ruthenium-based.
Each probe has a slightly different response to oxygen concentration, so prior to use it must be calibrated using known levels of oxygen. Once the phase shift is measured, it can be related to the oxygen concentration or partial pressure using the Stern-Volmer equation. If pressure and temperature cannot be controlled and kept constant from calibration to measurement, they must also be factored into the calculations, and a multi-point calibration must be used.
It is important to note that this type of sensor requires oxygen to diffuse into the sensor material, so the sensor tip or patch must be kept in direct contact with the sample. However, since the indicator material is trapped in a matrix, it is immobilized and protected from exposure to the sample, making these sensors unusually chemically inert and robust when used with an overcoat. Notably, they have minimal response to environmental changes in pH, salinity and ionic strength.
Featured Products for Oxygen Sensing:
|NEOFOX-GT||A compact optical instrument capable of measuring fluorescence lifetime, phase, and intensity for oxygen sensing.|
|RE-BIFBORO-2||Bifurcated optical fiber with borosilicate fiber bundle has one fiber leg to transmit excitation to the patch and one fiber leg to collect the response.|
|RedEye oxygen patches||Select from among patch diameters (4 mm and 8 mm) and sensor formulations (general purpose coating, high-sensitivity coating or highly robust coating). Single patches and packs of 5 patches are available.|
|NeoFox Viewer||Windows-based software allows users to collect, manage and analyze data with NeoFox systems.|
|Calibration Service||Recommended for applications where the sample cannot be maintained at a constant temperature (±1 °C). Calibration service covers environments from 0 – 80°C and is optimized for the sensor coating formulation being used and the temperature and oxygen concentration ranges of the sample environment.|