The Optical DO Probe operates on the principle of reversible luminescence quenching of a luminophore by oxygen as it passes through the cap. The cap is coated with a luminescent compound encased in a matrix for protection. Blue light from an LED is transmitted to the cap and excites the luminophore.
A collision of an oxygen molecule with the luminophore in its electronic excited state results in energy transfer from the luminophore to oxygen. As the luminophore relaxes it emits red light. The time from when the blue light is transmitted and the red light is emitted is measured by a photodiode. The more oxygen that is present, the shorter the time it takes for the red light to be emitted.
This time is measured and correlated to the oxygen concentration. Between the flashes of blue light, a red LED is flashed onto the sensor and used as an internal reference to help validate each measurement. This process is described by the Stern-Volmer equation
τ0 / τ = 1 + Ksv[DO]
where τ0 and τ are the luminescence lifetimes in the absence and presence of oxygen, respectively, [DO] is the dissolved oxygen concentration, and Ksv is the Stern-Volmer quenching constant.
The Stern-Volmer constant (Ksv) depends directly upon the rate constant for the diffusion of oxygen, the solubility of oxygen, and the natural lifetime of the electronic excited state of the luminophore. Lifetime measurements have an advantage over intensity measurements since they are not usually affected by processes which result in loss of the complex, such as bleaching or photodegradation.
Interior schematic of the Optical DO Probe