Luminescence in solution obeys the linear Stern-Volmer relationship. Fluorescence of a molecule is quenched by specific analytes, e.g. ruthenium complexes are quenched by oxygen. When a fluorophore is immobilised within a polymer matrix a myriad of micro-environments are created. The micro-environments reflect varying diffusion co-efficients for the analyte. This leads to a non-linear relationship between the fluorescence and the quencher (analyte). This relationship is modelled in various ways, the most popular model is the two site model created by James Demas (University of Virginia).
The signal (fluorescence) to oxygen ratio is not linear, and an optode is most sensitive at low oxygen concentration, i.e. the sensitivity decreases as oxygen concentration increases. The optode sensors can however work in the whole region 0–100% oxygen saturation in water, and the calibration is done the same way as with the Clark type sensor. No oxygen is consumed and hence the sensor is stirring insensitive, but the signal will stabilize more quickly if the sensor is stirred after being put into the sample.
Major international conferences are devoted to their development e.g. Europtrode VIII Tubeingen 2006, OFS 18, Cancun 2006.