Potentiometric Sensing of Nonsteroidal Painkillers by Acyclic Squaramide Ionophores

We report here a small library of a new type of acyclic squaramide receptors (L1-L5) as selective ionophores for the detection of ketoprofen and naproxen anions (KF^- and NS^-, respectively) in aqueous media. ¹H NMR binding studies show a high affinity of these squaramide receptors toward KF^- and NS^-, suggesting the formation of H-bonds between the two guests and the receptors through indole and -NH groups. Compounds L1-L5 have been tested as ionophores for the detection of KF^- and NS^- inside solvent PVC-based polymeric membranes. The optimal membrane compositions were established through the careful variation of the ligand/tridodecylmethylammonium chloride (TDMACl) anion-exchanger ratio. All of the tested acyclic squaramide receptors L1-L5 have high affinity toward KF^- and NS^- and anti-Hofmeister selectivity, with L4 and L5 showing the highest sensitivity and selectivity to NS^-. The utility of the developed sensors for a high precision detection of KF^- in pharmaceutical compositions with low relative errors of analysis (RSD, 0.99-1.4%) and recoveries, R%, in the range 95.1-111.8% has been demonstrated. Additionally, the chemometric approach has been involved to effectively discriminate between the structurally very similar KF^- and NS^-, and the possibility of detecting these analytes at concentrations as low as 0.07 μM with R² of 0.947 and at 0.15 μM with R² of 0.919 for NS^- and KF^-, respectively, was shown.