Extensive use of rare earth metals (REMs) in different industries has increased the concerns on the environmental fate of these elements. Accumulation of these metals in the food chain is detrimental to plants, animals, and human health. Therefore, the determination of REMs is of great importance. Potentiometric sensors with polymeric plasticized membranes containing neutral ligands are capable of precise, fast, and direct quantification of target analytes. Various ligands with diamide functions have been used for the construction of REM-sensitive sensors. The chemical structures of some of these diamides are very similar to calcium II (ETH 129) and calcium IV (ETH 5234) ionophores suggested in 80’s and successfully commercialized by Fluka Company. Thus, we have hypothesized that these commercial calcium ionophores can provide noticeable potentiometric sensitivity to REM ions. To confirm the validity of this hypothesis, PVC-plasticized sensor membranes based on ETH 129 and ETH 5234 were developed and their potentiometric performance was evaluated in acidic solutions of REM ions and compared to that of well-established neutral ligands, such as tetraoctyldiglycolamide (TODGA). Sensor membranes were synthesized using poly(vinyl chloride) as polymeric matrix, o-nitrophenyloctyl ether (NPOE) as a solvent-plasticizer, ionophore, and chlorinated cobalt dicarbollide (CCD) or fluorinated tetraphenyl borate derivatives (TFPB) as the cation exchanger. Sensors based on calcium ionophores exhibited good sensitivity across the linear concentration range of pC=3 to pC=5 M with the Nernstian slope towards most REM ions at pH 2 and with the detection limit of around 0.1-0.5 mg/l of REMs. The ETH 5234-based sensor demonstrated more remarkable sensitivity compared to the ETH 129-based sensor due to its high lipophilicity. Furthermore, the sensitivity pattern of the sensor based on ETH 5234 looked rather similar to the ligands such as TODGA. The effect of lipophilic CCD and KTFPB cation exchangers on the potentiometric response of the sensors was studied. The results revealed that the sensors containing CCD demonstrated superior sensing characteristics compared to those including TFPB.

In this study, the possibility of using commercial calcium ionophores for the determination of REM ions was presented for the first time. Reproducible and stable results and high sensing performance make them promising alternatives to the expensive ligands such as TODGA for technological monitoring purposes.