Optical oxygen sensors, also known as luminescent dissolved oxygen (LDO) sensors or fluorescent sensors, determine dissolved oxygen concentration by measuring the quenching of luminescence in the presence of oxygen. The presence of oxygen affects both the intensity and the lifetime of the luminescence, allowing the sensor to measure either property to determine oxygen levels.
The Luminescence Process
At the heart of the optical oxygen sensor is a luminophore (a light-emitting molecule) immobilized in a sensing membrane. Here's how it works:
-
Excitation: The sensor exposes the luminophore to blue light (or light of a specific excitation wavelength).
-
Luminescence: The luminophore absorbs this light and then emits light at a longer wavelength (e.g., red light). This emission is called luminescence.
-
Quenching by Oxygen: Oxygen molecules in the surrounding environment interact with the excited luminophore molecules. This interaction causes the luminophore to return to its ground state without emitting light. This process is called "quenching." The more oxygen present, the more quenching occurs, and the weaker the luminescence signal.
-
Measurement: The sensor measures either the intensity or the lifetime of the luminescence. The intensity is simply how bright the emitted light is. The lifetime refers to the time it takes for the luminescence to decay after the excitation light is removed. Both parameters are inversely proportional to the oxygen concentration.
Intensity vs. Lifetime Measurement
Optical oxygen sensors can use two different methods to determine oxygen concentration:
-
Intensity-Based Measurement: These sensors measure the intensity of the emitted light. Higher oxygen concentrations lead to lower intensity. These sensors are simpler and generally less expensive but can be affected by variations in the light source intensity or the sensitivity of the detector.
-
Lifetime-Based Measurement: These sensors measure the time it takes for the luminescence to decay. Higher oxygen concentrations lead to a shorter decay time (shorter lifetime). Lifetime measurements are generally more robust and less susceptible to interference from changes in light source intensity or sensor fouling because they focus on a temporal characteristic rather than absolute intensity.
Advantages of Optical Oxygen Sensors
Compared to traditional electrochemical dissolved oxygen sensors (e.g., Clark cells), optical oxygen sensors offer several advantages:
- No Oxygen Consumption: Optical sensors don't consume oxygen during measurement, making them suitable for measuring oxygen in small or stagnant samples. Electrochemical sensors consume oxygen, which can alter the oxygen concentration in these situations.
- Reduced Maintenance: Optical sensors typically require less frequent calibration and maintenance than electrochemical sensors.
- Faster Response Time: Optical sensors often have a faster response time, allowing for quicker measurements and real-time monitoring.
- Less Sensitivity to Fouling: While still susceptible, optical sensors are generally less sensitive to fouling by organic matter than electrochemical sensors.
Applications
Optical oxygen sensors are used in a variety of applications, including:
- Environmental Monitoring: Measuring dissolved oxygen in rivers, lakes, and oceans to assess water quality.
- Aquaculture: Monitoring oxygen levels in fish farms and other aquaculture systems to ensure optimal conditions for aquatic life.
- Industrial Processes: Controlling oxygen levels in bioreactors, fermentation processes, and other industrial applications.
- Medical Devices: Measuring oxygen levels in blood and other bodily fluids.
In summary, optical oxygen sensors provide a reliable and convenient way to measure dissolved oxygen by exploiting the relationship between oxygen concentration and the quenching of luminescence. These sensors are increasingly replacing traditional electrochemical sensors due to their numerous advantages.
