Colorimetric ratiometry with ion optodes for spatially resolved concentration analysis

The deprotonation degree of the lipophilic pH indicator dye (chromoionophore) in ionophore-based ion optodes (so-called bulk optodes) has traditionally been measured spectrophotometrically. This makes it difficult to obtain spatially resolved concentration information, for example in the study of heterogenous systems. This article reports on a new colorimetric method that relies on a ratiometric image analysis. The acquision of image data allows one to map the deprotonation degree in two dimensions, which in turn is used to obtain the spatially-resolved ion concentration of the image. Using the detection of potassium as an example, the deprotonation degree data calculated on the basis of image analysis correlate quantitatively with those from spectrophotometry. They showed no dependence on the type of camera used in spite of their different gamma correction values and spectral sensitivities, as expected from theory. As an example, the method is successfully applied to the pixel level analysis of an ensemble of pictures acquired at different times to spatially and temporally observe potassium ion diffusion into an agarose gel containing a potassium-selective optical sensor microemulsion.