Potentiometric sensors are attractive analytical tools giving an opportunity to analyze liquids in a simple, fast, and non-destructive way with high sensitivity and reasonable accuracy. Furthermore, ion-selective electrodes (ISEs), being the most popular potentiometric sensors, are recommended for pH determination by several international standards (e.g., ISO 10523:2008(en)). Nevertheless, frequently observed non-Nernstian behavior and insufficient selectivity complicate the wider implementation of ISEs into analytical practice. Thus, a majority of existing sensors exhibit, to a different extent, a property of cross-sensitivity, being sensitive and selective not only to a target analyte, but also to other components. When chemically and physically similar analytes are simultaneously present in a sample solution (e.g., rare earth elements), it is hardly possible to develop a highly selective sensor for each individual component. A multisensor approach allows turning this problem of ISEs into an advantage [1
]. According to this method, an array of cross-sensitive and low-selective sensors demonstrating different sensitivity patterns to the components of a complex mixture is used instead of a single sensor. The application of multivariate data processing techniques to interpret a response of such multisensor arrays helps to determine and quantify several analytes at the same time. Obviously, composition and quantity of cross-sensitive sensors in a multisensor set should be adapted for a particular analytical task.
Recently, various multisensor systems have been successfully applied in different areas [2
]. This approach could be particularly useful to analyze rare earth elements (REEs) in complex media. Simultaneous determination of multiple REEs is of great interest for industrial analytical control of the spent nuclear fuel (SNF) recycling process [5
]. Currently, such high-precision spectroscopic techniques based on inductively coupled plasma (ICP) spectrometry such as ICP-MS (mass spectrometry), ICP-AAS (atomic absorption spectroscopy), and ICP-AES (atomic emission spectroscopy) are applied for REE quantification in different SNF reprocessing steps [6
]. Despite the remarkable analytical characteristics of these methods, their direct implementation into a processing line is impossible since they do not allow the continuous sampling and real-time analysis of target compounds. Moreover, these offline spectroscopic techniques require high-cost equipment, time-consuming preparation of radioactive probe, and trained staff. In contrast, chemical sensors are cost-effective and suitable for remote in-line measurements. Due to this fact, a multitude of individual ISEs have been suggested for the determination of REE cations [7
]. However, most of the proposed sensors work properly only in a pH range of 3 to 8, which is far from real industrial conditions. The selectivity of most ISEs is not enough to perform reliable measurements in complex mixtures. At the same time, multisensor systems can be successfully used for REE determination in their mixtures, as was shown in our previous work [8
A wide range of chemical molecules acting as liquid extracting agents can be used as ionophores in a polymeric matrix of potentiometric sensors due to the similar selective ion capture mechanisms [9
]. For example, N
′-tetraoctyldiamide of diglycolic acid (TODGA) [12
] and octylphenyl-N
-di-isobutylcarbamoylmethylen phosphine oxide (CMPO) [14
] were proposed for effective lanthanide and actinide cation extraction from highly acidic media. These ligands were applied as sensitive components of potentiometric membranes [16
]; it was shown that their sensing properties were similar to the extractive ones. Since the properties of some extracting agents allow their application as ionophores, it could be reasonable to study the properties of their derivatives. However, it is worth noting that the extracting behavior of a given molecule does not always result in the outstanding performance of a sensor with this component.
The extraction properties of TODGA modified by the addition of one or two methyl groups to the central carbon atoms have been recently studied [19
]. Both derivatives demonstrated a low extraction ability towards all lanthanides, but for each derivative the distribution ratio varied in a different way from light to heavy REEs [20
]. The sensors based on these molecules exhibited alternative patterns of sensitivity and selectivity in comparison with TODGA. New ligands are stable in acidic medium, which makes them attractive as potential active substances for chemical sensors.
Another interesting concept of a sensor’s development is based on the so-called pre-organization. Different individual compounds are chemically bounded into a single molecule to achieve a synergistic effect previously known for a mixture of these compounds. Chlorinated cobalt dicarbollide (CCD), used in many extraction systems for high-level radioactive waste processing, was suggested as an extracting agent for cesium [21
]. It was shown that CCD can effectively cross the phase boundary and its combination with a neutral ligand promotes extraction both through solvation and cation-exchange mechanisms [22
]. Therefore, cobalt dicarbollide (CD) ions were covalently bonded with neutral ligands, and the extracting properties of such pre-organized molecules were studied [23
]. It was shown that compounds with CD covalently bonded with diglycol amides are more efficient for the extraction of Eu (III) and Am (III) than the mixture of TODGA and CCD [25
]. Similar extracting ability improvement was observed for the covalent combination of CD and CMPO molecules [24
]. The application of neutral ligands covalently bonded with a cation-exchanger can be interesting for the development of cations’ sensors, especially due to the fact that CCD was previously used in polymeric membranes as a cation-exchanger and can be a good alternative for conventional tetraphenylborate derivatives [17
A significant synergistic effect was observed earlier for the mixtures of CCD and carbamoyl phosphine oxides. In such mixtures, the distribution coefficients of Am(III) and Eu(III) differ from the individual molecules’ characteristics by several orders of magnitude [21
]. A similar effect was reported for CCD and diglycolic acid diamides [27
In this study, we applied several modified ligands originally developed for liquid extraction as active components of chemical sensors. A number of sensors based on TODGA and its derivatives and on compounds with a CD group covalently bounded to neutral ligands (TODGA and CMPO) were prepared and their sensing properties were studied. The main purpose of this work was to expand the number of cross-sensitive sensors with variable sensitivity and selectivity patterns toward lanthanides. This would be highly useful for developing multisensor systems finely adapted for sophisticated analytical tasks, e.g., radioactive waste reprocessing analysis.
2. Materials and Methods
High-molecular weight poly(vinylchloride) (PVC), dioctyl sebacate (DOS), 2-fluorophenyl-2′-nitrophenyl ether (2F2N), and o
-nitrophenyloctyl ether (NPOE) were purchased in Fluka (Switzerland). N
′-tetraoctyldiamide of diglycolic acid (TODGA) and the structurally modified derivatives (Me-TODGA and 2Me-TODGA) were synthesized according to [19
]. Diglycolic acid (DGA) and CMPO bonded with CD (CD-DGA and CD-CMPO) were provided by Katchem (Kralupy nad Vltavou, Czech Republic) and synthesized according to [23
]. Ligand structures are shown in Figure 1
. Cesium salt of chlorinated cobalt dicarbollide (CCD) was provided by Katchem (Kralupy nad Vltavou, Czech Republic).
Polymeric sensor membranes were prepared according to the standard procedure [8
]. Corresponding quantities of membrane components were weighted, transferred to a glass beaker, and dissolved in freshly distilled tetrahydrofuran under mixing for 20 min. After the complete dissolution of all components, the mixture was transferred into a Teflon beaker with a flat bottom and was left for 24 h for solvent evaporation. The final thickness of the parent membranes was about 0.5 mm. Discs of 8 mm in diameter were chopped out from the parent membranes. Each disc was glued onto a sensor body (PVC tube). The sensors were dried in ambient conditions for 48 h. Later, they were immersed into and filled in with a 0.01 M NaCl solution for 24 h before the measurements. At least three replicated sensors were prepared for each membrane composition. Sensor membrane compositions are given in the Table 1
. Typically, there is an excess of the neutral ligand over the ion exchanger in the plasticized membrane (the corresponding ratio is about 5:1), but in the case of covalently bonded combinations of CD and ligands, the ratio was 1:3 (in the case of TODGA) and 1:2 (for CMPO) due to the molecular structure of the resulting compound. Therefore, additional membranes with a molar ratio of CCD and neutral ligands equal to that of pre-organized molecules were prepared (DGA-N, DGA-F, CMPO-N, and CMPO-F).
The response mechanism of potentiometric sensors with plasticized polymeric membranes is well known and extensively described in numerous comprehensive reviews [28
]. By default, the membrane matrix (PVC + plasticizer) is considered as neutral; however, many plasticizers may significantly affect the sensor’s sensitivity and lifetime [30
]. Several polymeric plasticizers with different dielectric constants—2F2N, NPOE, and DOS (εr
= 50.0 [30
], 24, and 3.9 [31
], respectively)—were used in this study.
Potentiometric measurements were performed in the following galvanic cell: Cu | Ag | AgCl, KClsat | sample solution | sensor membrane | 0.01 M NaCl | AgCl | Ag | Cu.
Sensor sensitivities were studied in aqueous solutions of alkaline, alkaline earth, transition, and rare earth metal ions (K, Na, Mg, Ca, Cu, Zn, Cd, Pb, La, Ce, Pr, Nd, Sm, Eu, Gd) in the concentration range of 10−7
M. Measurements of REE ions were performed at pH 2 (addition of nitric acid) to ensure the presence of triple-charged cations. The pH values were controlled by a glass pH electrode (ZIP Gomel, Belarus). REE solutions were prepared by dissolution of the metal oxides (99.9%) in nitric acid. Other calibration solutions were prepared by dissolution of the relevant salts in the water. Oxides, salts, and other chemicals used for the preparation of the calibration solutions were purchased from Vekton (St. Petersburg, Russia). Bidistilled water was used in the experiments. Sensor potentials were measured continuously every 6 s against a Ag/AgCl reference electrode (Izmeritelnaya Tekhnika LLC, Moscow, Russia) with 0.1 mV precision using a high-impedance multichannel digital mV-meter HAN-32 (Sensor Systems, LLC, St. Petersburg, Russia). The measurement time for each sample was 3 min, and the last 5 readings of the sensor during this period were averaged for data processing. Sensitivity values were calculated as a slope of the linear part of a calibration curve in the concentration range of 10−5
M. Biionic potentials (BIP) used for the selectivity estimation were calculated according to [17
] as the difference between the electromotive force (EMF) measured in a 10−3
aqueous solution and in a solution with the same concentration of another REE cation, divided by the slope value. Thermodynamically strict application of selectivity evaluation methods requires theoretical (Nernstian) sensitivity towards both the target and the interfering ion. When this condition is not fulfilled, a simplified evaluation procedure can be used—the calculation of the difference in EMF values in solutions of two ions.
The sensing properties of novel ion sensors based on different modified extracting agents have been studied. It was shown that the sensitivity and selectivity of a sensor can be changed and adapted for a particular application by modifying the structure of the ligand it comprises.
NPOE-based sensors demonstrated the best sensing characteristics and reproducible behavior in most cases.
The sensors based on TODGA and its methylated derivatives demonstrated good sensitivity to Ca2+ and Pb2+ ions, with response slopes close to theoretical values.
While the TODGA-based sensors demonstrated cross-sensitivity to a wide range of lanthanides, the increase of the methyl group number in the TODGA backbone did not result in any improvement of the sensing properties. Two additional methyl groups significantly further hindered the response to REE ions.
The covalent attachment of the cation exchanger (CD) to DGA resulted in the decreased sensitivity of such sensors towards REEs compared to the sensors based on the mixtures of these components. The selectivity of the membranes with bonded ionophores also became lower in comparison with mixed ionophores.
For CMPO, the sensitivity to REE ions was comparable for the mixed and bonded ligands, while the selectivity to REE became significantly lower for the bonded ligands.
It can be concluded that the sensors with bonded ligands are suitable for multisensor systems for simultaneous determination of several REE cations. The will be the next step of our research.