Optical chemical sensors rely on changes in optical properties of sensor materials when they are exposed to particular chemical or physical stimuli.
- O2 fluorescence based
- pH color based
Fluorescence Quenching – Phase Fluorometry
Oxygen is sensed from the fluorescence decay rate of a trapped metal-organic fluorphore. A blue LED pulse is sent down the fiber to the tip, or to a patch with a polymer or sol-gel matrix encapsulated fluorophore. The blue light causes a red fluorescence. The red fluorescence decays over micro-seconds of time. Collision with O2 molecules causes quenching, and a faster decay rate. If the LED is modulated, the fluorescence signal is also modulated but with a phase delay that is related to the lifetime. In the absence of O2 the lifetime is long, as the partial pressure of O2 increases, the lifetime gets shorter. The relationship between lifetime and pO2 follows a Stern-Volmer relationship. Tau(0) is the lifetime in the absence of O2, Tau is the lifetime in the sample, pO2 is the partial pressure of O2 and Ksv is the sensor calibration coefficient.
Absorbance (color) change for pH
pH sensors are made from immobilized pH indicator dyes. These change color depending on the pH of the media, and these color changes are measured with a spectrometer. Typically 2 forms or colors of the dye exist and the ratio of these two forms is related to the pH.