Many materials develop surface potentials when they are in contact with water solutions. With ISE sensor surfaces, these potentials change when they are contacted by certain cations or anions, due to non-faradaic phenomena. The selectivity of these sensors is mostly not as high as generally thought, certainly not for organic ionic substances. This selectivity is often adjusted by adding a molecule which has strong interactions with the analyte ion, a so-called ionophore. Despite the enormous activity in the field of macrocyclic or podand receptor molecules, good ionophores are still lacking for organic ionic molecules. The situation is better for anionic- than for cationic organic substances. Several examples of successful ligands for anionic organic molecules will be demonstrated. However, ISE selectivity for organic ions is mostly dominated by their lipophilicity (logP). This will be shown by recent examples where ISE sensors were used as a novel detection system in HPLC. The system uses a signal transformation algorithm to obtain a concentration related tracing. The analyte molecules varied from organic sulphonate and sulfate detergents and biochemicals over carboxylic acids from food industry, to pharmaceuticals. The detection limits are excellent, except for hydrophilic cationic compounds. For the latter substances, better organic ionophores and membrane components should be developed. Surprisingly, highly charged molecules such as DNA also performed very well. They yielded unexpected calibration curves (slopes) and very good detection limits. Their responses were slower than the ones of the smaller molecules. This effect was attributed to high mass transfer resistance. These data on highly charged and large biomolecules in the HPLC application receive support from recent literature data on totally different applications. This opens new exciting possibilities for the use of ISE’s in the (bio)sensor area.

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