Protonation of basic sites in systems comprising a chromophore and a luminescent metal center leads the way for pH sensors. Some initially proposed systems were based on pyridine derivatives but these were not stable in water. More robust sensors have been proposed in which the core is a substituted macrocycle usually bearing phosphinate, carboxylate or four amide coordinating groups. It has been observed that lanthanide luminescent probe emission increases about six-fold when decreasing the pH of the solution from six to two.