There is still a lack of commercial applications of pH sensors due to the decisive disadvantage that the signal is depending on the ionic strength of the sample. pH is defined as the negative logarithm of the activity of protons in aqueous solution, while the optical parameter (e.g. change of absorption or fluorescence) correlates with the concentration of the pH-sensitive dye. The cross-sensitivity of the calibration curve towards ionic strength can cause pH errors of up to 1.5 pH units. Since the electrolyte concentration, respectively the ionic strength, is not constant in most real...