The pH-sensitive chromsionsphores brought the dream of the ion-selective me
mbrane scientist close to realization. With the help of these molecules, on
e can build pH-sensitive, ion-selective electrodes and look into the bulk o
f solvent polymeric membranes during potentiometric measurements (spectropo
tentiometry) and image concentration profiles in situ with high spatial and
temporal resolution. The combination of electrochemical and optical inform
ation helped to interpret non-idealities in the potentiometric responses, s
uppress or tailor the undesirable transport across sensor membranes, and es
timate the residual lifetime of chronically implanted sensors. These novel
opportunities provide feedback in membrane optimizations and are expected t
o lead to sensor systems with picomolar detection limits and superb selecti
vities.