Poly(vinyl chloride) membrane cesium ion-selective electrodes based on lipophilic calix[6]arene tetraester derivatives

Talanta 53 (2000) 535 – 542 www.elsevier.com/locate/talanta

Poly(vinyl chloride) membrane cesium ion-selective electrodes based on lipophilic calix[6]arene tetraester derivatives Hyejin Oh, Eun Mi Choi, Haesang Jeong, Kye Chun Nam, Seungwon Jeon * Department of Chemistry and Institute of Basic Science, Chonnam National Uni6ersity, Kwangju 500 -757, South Korea Received 30 May 2000; received in revised form 17 July 2000; accepted 21 July 2000

Abstract New lipophilic tetraesters of calix[6]arene and calix[6]diquinone are investigated as cesium ion-selective ionophores in poly(vinyl chloride) membrane electrodes. For an ion-selective electrode based on calix[6]arene tetraester I, the linear response is 1 ×10 − 6 –1×10 − 1 M of Cs+ concentrations. The selectivity coefficients for cesium ion over alkali, alkaline earth and ammonium ions are determined. The detection limit (log aCs+ = − 6.31) and the selectivity coefficient (log k pot = −1.88) are obtained for polymeric membrane electrode containing calix[6]arene tetraester Cs+,Rb+ I. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Calix[6]arene tetraesters; Calix[6]diquinone; Ionophore; Cesium; Ion-selective electrode; Poly(vinyl chloride) membrane

1. Introduction Recently there has been increasing interest in the development of chemical sensors. Polymeric carrier-based ion-selective electrodes (ISEs) for determining the alkali metal cations such as sodium [1–4], potassium [5,6] and lithium[7] have been studied numerously. However, there has been a few study for the development of cesium ion ISE [8]. Cesium and the other alkali metal ions have been determined by spectroscopic methods such as atomic spectrometry and inductively coupled plasma mass spectrometry [9]. However, * Corresponding author. Fax: +82-62-5303389. E-mail address: [email protected] (S. Jeon).

potentiometry involving ISEs is now widely used for simple and rapid measurements of alkali metal ions, particularly in clinical and environmental assays [1–8]. Calixarenes have received considerable attention as an interesting class of ionic and molecular binding hosts [10,11]. It has been studied that various functionalized calixarenes were selectively host molecules for cations as well as anions [12–19]. The ISE dynamic response is generated by selective complexation of the target ion by neutral carrier ionophores dispersed in a poly(vinyl chloride) (PVC) matrix. Polymeric membrane ISEs provide one of the most powerful sensing methods because it is possible to select various sensory elements according to the shape and size of the target ion. Based on the recent

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advance of host-guest chemistry, polymeric membrane ISEs for inorganic metal cations have been extensively developed by the use of crown ethers and related macrocyclic hosts as well as acyclic ligands [20– 23]. Many of these ISEs exhibit excellent selectivity for metal cations as guests and are now commercially available. Recently cesium ion ISEs have received considerable attention for clinical and environmental analysis. Many macrocycles including related calixarenes have been synthesized for cesium recognition and ISEs [24– 34]. The synthesis, structures and cation-binding properties of calixarene derivatives as a new class of macrocyclic molecular receptors have been reviewed [35– 37]. In this work, PVC polymeric membrane ISEs have been prepared from lipophilic tetraesters of calix[6]arene and calix[6]diquinone (Fig. 1) as cesium selective ionophores and determined the selectivity coefficients for cesium against alkali, alkaline earth and ammonium ions. Unlikely calix[6]arene hexaester having six ester groups reported already, calix[6]arene tetraesters consist of four ester groups and two functionalized groups. It is not clear that the four ester groups of calix[6]arene tetraester can bind with target ion perfectly just as six ester groups of calix[6]arene hexaester. If four ester groups in calix[6]arene compounds are enough for the binding with target ion, the other two sites of calix[6]arene tetraester can be functionalized to various groups such as redox-switching or chromophore. ISE fablicated from calix[6]arene tetraester I gives high cesium/rubidium selectivity and the detection limit. The effect of the structure of ionophores on the potentiometric selectivity is examined.

2. Experimental

2.1. Reagents Six lipophilic tetraesters of calix[6]arene and calix[6]diquinone tested as cesium ionophores are shown in Fig. 1. They were prepared according to our literature procedure [38]. High molecular weight PVC, dioctyl sebacate (DOS), 2-nitrophenyl octyl ether (o-NPOE), potassium tetrak-

is(p-chlorophenyl)borate (KTpClPB) and tetrahydrofuran (THF), which were obtained from Fluka, were used to prepare the PVC membranes. Analytical grade chlorides of cesium, rubidium, potassium, sodium, lithium, magnesium, calcium and ammonium were used. Doubly distilled water in a quartz apparatus was used to prepare all aqueous electrolyte solutions.

2.2. Fabrication of polymeric ion-selecti6e electrodes The typical composition of PVC-based cesiumselective electrodes was 33 mg PVC, 66 mg plasticiser, 1 mg ionophore and KTpClPB (50 mol.% of ionophore if needed). Table 1 summarizes the compositions of the cesium-selective membranes employed in this study. The ionophore, plasticizer and PVC were dissolved in the appropriate volume of THF and mechanically stirred. All membrane cocktails were cast in glass rings placed on glass plates for conventional ISEs. Solvent from PVC membrane was allowed to evaporate for at least 24 h at room temperature. The thickness of the resulting membrane was about 0.3 mm.

2.3. Potentiometric measurements The electrochemical properties of the cesiumselective electrodes were investigated in the conventional configuration. Small disks were punched from the cast membranes and mounted in Philips electrode bodies (IS-561). For all electrodes, 0.1 M KCl was used as an internal filling solution. The external reference electrode was an Orion sleeve-type double-junction Ag/AgCl reference electrode (Model 90-02). The electrochemical potential was measured using home-made 16channel potentiometer coupled to a computer. The dynamic response curves were produced by adding standard solutions of cations to magnetically stirred buffer solution (0.05 M Tris–HCl, pot pH 7.2). The selectivity coefficients (K Cs +,j) were determined by the separate solution method (SSM) using 0.1 M chloride salts of the cations involved and also by the fixed interference method (FIM) using 0.01 M solutions of the interfering ions, respectively. Detection limits

H. Oh et al. / Talanta 53 (2000) 535–542

were estimated at the intersection of two linear lines, the one extrapolated from a high concentration range (10 − 4 – 10 − 1 M metal ion) and the other parallel to the x-axis drawn through the mean potential value of the lowest metal ion

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concentration used (10 − 6 M metal ion) in the plot of the potential change and the concentration of Cs+. At least three times measurement was performed, and the data were determined from the plot.

Fig. 1. Six lipophilic tetraesters of calix[6]arene and calix[6]diquinone tested as ionophores in this study.

H. Oh et al. / Talanta 53 (2000) 535–542

538

Table 1 Composition of PVC-based cesium selective membrancesa Ionophore I

II

III IV V

VI

None

a b

Number

PVC

DOS

1 2 3 4

33 33 33 33

66 66

5 6 7 8

33 33 33 33

66 66

9 10

33 33

11 12

o-NPOE

KTpClPBb

50 66 66

50

Ionophore 1 1 1 1

50

1 1 1 1

66 66

50

1 1

33 33

66 66

50

1 1

13 14 15 16

33 33 33 33

66 66

50

17 18 19 20

33 33 33 33

66 66

21 22 23 24

33 33 33 33

66 66

50 66 66

66 66

50 50

66 66

50

1 1 1 1 1 1 1 1

50 66 66

50

In wt.%. Mol.% relative to the ionophore.

3. Results and discussion Chemical modified calixarenes and calixquinones have focused increasing attention as a class of host compounds having a well-defined cavity. Extensive studies have been carried out on the selective complexation and extraction of alkali metal ions by calixarene derivatives. New lipophilic tetraesters of calix[6]arene and calix[6]diquinone are synthesized and employed as the selective ionophores for cesium ion. Calix[6]arene tetraesters have four carbonyl oxygens, which function as well-defined convergent binding sites for the metal ions that sterically fit well with the pseudocavity constructed by these oxygens, and the other two functionalized groups. In the previ-

ous studies, the calix[4]arene tetraesters and the calix[6]arene hexaesters have been shown to display selectivities for sodium and cesium ions, respectively, in the complexation, extraction, transport, liquid membrane experiments [3,28]. The calixarene structure was acted as a rigid support for the carbonyl groups that function as convergent binding sites for metal ions. Actually, calix[6]arene and calix[6]diquinone tetraesters have been shown to display selectivities for cesium ion, in the complexation, extraction, transport experiments [38]. The calix[6]arene tetraesters showed ion transport ability with efficiency decreasing in the order Cs+ \ Rb+ \ K+ \ Na+  Li+, but the calix[6]diquinone tetraesters showed ion transport ability with efficiency decreasing in the order

H. Oh et al. / Talanta 53 (2000) 535–542

Cs+ \ K+ \Rb+ \Li+ Na+. Among them, calix[6]arene tetraesters I and II are exhibited excellent extraction of alkali metal ions. In general, the extraction experiments represent a simple ion transfer system from an aqueous to an organic phase and provide a rapid method of evaluating the binding with ionophore for the target ion. However, because the polymeric membrane ISEs are a more complex system and the conditions under which the selectivity of the ISE is examined are very different, the results of extraction experiments should be considered only as a consulted data for the ISE selectivity. The potentiometric response properties of this polymeric membrane ISEs are examined for alkali metal and ammonium ions. Electrode with a membrane containing no specific selective ionophore shows negligible response to all of the alkali and alkaline earth metal ions employed. The ion exchanger KTpClPB in these experiments is known to exhibit some sensory activity for cesium ion in PVC-NPOE membrane containing no selective ionophore, but the cesium sensitivity with the addition of KTpClPB in PVC-DOS membrane containing no ionophore is largely decreased. The polymeric membrane ISEs are investigated to measure the ability of the ionophores to act as neutral carriers in the absence of the ion exchanger. Fig. 2 illustrates potentiometric response curves for alkali metal and ammonium ions, obtained in pH 7.2 (0.05 M Tris–HCl)

Fig. 2. The potentiometric response curves of electrode No. 1 based on calix[6]arene tetraester I to the concentration change of alkali metal and ammonium ions in pH 7.2 (0.05 M Tris – HCl) buffer solutions.

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buffer solutions by electrode no. 1 (PVC membrane containing DOS as a plasticizer) based on calix[6]arene tetraester I. The electrode no. 1 displays a near-Nernstian (55.7 mV per decade) response to Cs+ in CsCl solutions over the range 1× 10 − 6 –1× 10 − 1 M. The strongest response is observed for Cs+ ion with a detection limit below 10 − 6 M. Nernstian responses are also observed −4 M for Rb+, K+, and NH+ 4 , started from ca. 10 + −3 + + for Rb , and 10 M for K and NH4 . On the other hand, the response to Na+ ion is much weaker and that to Li+ ion was almost negligible. The responses to all of the alkaline earth metal ions (Mg2 + , Ca2 + , Sr2 + , Ba2 + ) were negligible. The selectivity of potentiometric responses by electrode no. 1 is in the order of Cs+ \ Rb+ \ + + 2+ , Ca2 + , Sr2 + , K+ ] NH+ 4  Na  Li  Mg 2+ Ba (no response), and its potentiometric result is identical to the extraction result which the binding efficiency of calix[6]arene tetraester I is decreasing in the order Cs+ \ Rb+ \ K+ \ Na+ pot  Li+[38]. The selectivity coefficients (K Cs +,j) depot termined are summarized in Table 2. The K Cs +,j values for the electrode with a membrane containing no specific selective ionophore are also listed in Table 2. The PVC-DOS polymeric electrode no. 1 over the PVC-NPOE polymeric electrode no. 3 or 4 based on calix[6]arene tetraester I shows the excellent selective response for cesium ion. Fig. 3 shows potentiometric response curves for Cs+ in pH 7.2 (0.05 M Tris–HCl) buffer solutions by electrodes no. 1, 2, 5, 6 shown in Table 1. As mentioned above, the PVC-DOS polymeric electrode no. 1 based on calix[6]arene tetraester I which have no para t-butyl substituents provide the lowest detection limit (log aCs+ = − 6.31) and, the higher selectivity coefficient (log k pot Cs+,Rb+ = − 1.88 by SSM, −1.81 by FIM) for Cs+ over alkali metal and ammonium ions, and slopes in excess of 50 mV per decade change in appropriate cesium concentration. The selectivity coefficient log k pot Cs+,Rb+ = − 1.88 by SSM is similar to the −1.81 obtained from FIM in electrode no. 1. However, the PVCDOS polymeric electrode no. 5 based on the calixarene tetraester II which have para t-butyl substituents give the lower selectivity coefficient + (log k pot Cs+,Rb+ = − 0.49). The preference for Cs

H. Oh et al. / Talanta 53 (2000) 535–542

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Table 2 Electrochemical properties of PVC-based cesium selective membranes Ionophore

Number

Slope (mV per decade)

Detection limit (log aCs+)

pot log k Cs +, j

j= Rb+

j= NH+ j= K+ 4

j=Na+ j= Li+

I

1 2 3 4

55.7 50.0 41.4 73.7

−6.31 −3.32 −4.31 −4.31

−1.88 −0.41 −1.02 −1.72

−3.01 −0.61 −1.78 −1.83

−2.23 −0.54 −1.66 −1.99

−3.25 −1.00 −2.29 −3.35

−5.12 −1.35 −2.36 −3.99

II

5 6 7 8

59.5 50.0 41.9 53.0

−3.62 −3.31 −2.67 −3.88

−0.49 −0.41 −0.18 −0.31

−1.18 −0.51 −0.65 −0.89

−0.52 −0.46 −0.41 −0.53

−1.42 −0.88 −0.80 −1.74

−2.35 −1.22 −1.21 −2.61

III

9 10

55.1 49.5

−5.14 −4.37

−1.28 −0.61

−2.05 −1.00

−2.74 −0.98

−2.99 −1.84

−3.58 −2.43

IV

11 12

54.1 51.5

−4.83 −3.25

−0.35 −0.33

−1.28 −0.28

−0.41 −0.48

−1.98 −0.96

−3.13 −1.14

V

13 14 15 16

60.2 60.7 34.1 57.3

−4.09 −3.60 −3.48 −4.14

−0.35 −0.54 −0.46 −0.53

−1.83 −0.96 −0.91 −1.72

−1.10 −0.66 −0.71 −2.46

−2.31 −1.62 −1.09 −2.67

−3.01 −2.35 −1.46 −3.04

VI

17 18 19 20

57.4 54.2 43.0 54.8

−4.01 −4.50 −3.41 −4.16

−0.83 −0.46 −0.37 −0.68

−1.56 −1.06 −0.68 −1.71

−1.71 −0.73 −0.44 −1.09

−1.83 −1.72 −1.03 −2.31

−2.74 −2.33 −1.64 −3.06

None

21 22 23 24

53.7 46.0 8.6 58.0

−2.85 −2.78 −2.59 −3.89

0.20 −0.05 −0.09 −0.61

−0.36 −0.03 0.08 −1.15

0.06 0.03 −0.07 −0.95

−0.86 −0.30 −0.24 −2.30

−1.40 −0.56 −0.24 −2.76

can be reasonably explained by an excellent geometric fit with the pseudocavity constructed by the four carbonyl oxygens in calix[6]arene tetraester I which have no para t-butyl substituents rather than calixarene tetraester II which have para t-butyl substituents. The selectivity coefficient (log k pot Cs+,Rb+=−1.88) of ISE based on calix[6]arene tetraester I is very similar to log k pot Cs+,Rb+=−1.85 of ISE based on calix[6]arene hexaester [28] and log k pot Cs+,Rb+=−1.8 of optode membrane based on 1,3-calix[4]bisnaphthyl-crown-6 [32]. This result indicates that the four ester groups of calix[6]arene tetraester can bind with cesium ion perfectly, and the other two sites of calix[6]arene tetraester can be functionalized to give redox-switching or chromophore

compounds such as calix[6]diquinone tetraesters studied in this work. According to the results in Table 2, the cesium selectivity and sensitivity of calix[6]arene and calix[6]diquinone tetraesters in PVC-DOS polymeric electrodes are usually lowered with the addition of KTpClPB as an additive reagent. However, the cesium selectivity and sensitivity of calix[6]arene and calix[6]diquinone tetraesters in PVC-NPOE polymeric electrodes are usually increased with the addition of KTpClPB. These results may attribute to the difference of lipophilicity of plasticizers DOS (10.1) and NPOE (5.9). Meanwhile the cesium selectivity and sensitivity of calix[6]arene tetraester III are lower than those of calix[6]arene tetraester I when they are compared in any electrodes. The cesium selectivity

H. Oh et al. / Talanta 53 (2000) 535–542

and sensitivity of calix[6]diquinone tetraester V having redox-swtching group are also lower than those of calix[6]arene tetraester I. Examination of the selectivity data for the electrodes employed in this study indicates that the ionophore structure is the major factor determining the selectivity, and the other factor determining it is the kind of plasticizer used in PVC membrane electrodes. The addition of the additive to PVC membranes containing ionophores greatly changed the selectivity of all guests employed, and the selectivity depends on the kind of polymeric electrodes. All the electrodes responded rapidly to changes in cesium concentration with time constants of the order of a few seconds. The rate of response was only limited by the speed of stirring and the injection technique.

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electrode based on calix[6]arene tetraester I shows the good detection limit and the excellent selectivity coefficient in pH 7.2 (0.05 M Tris–HCl) buffer solutions, and the linear response in Cs+ concentrations of 1× 10 − 6 –1× 10 − 1 M. The cesium pot selectivities (K Cs +,j) are dependent upon ionophore as well as plasticizer used in membrane electrodes.

Acknowledgements This work was supported by the Ministry of Education of Korea (BK21 project) and Ministry of Science and Technology as a part of the Nuclear R&D Program.

References 4. Conclusion PVC-polymeric cesium ISEs based on lipophilic tetraesters of calix[6]arene and calix[6]diquinone have been fabricated and exhibit high selectivity against alkali metal, earth metal and ammonium ions. In general the polymeric electrodes combined with the calix[6]arene tetraesters which have no para t-butyl substituents had superior characteristics to those combined with those which have para t-butyl substituents. Among them, PVC polymeric

Fig. 3. The potentiometric response curves of electrodes (a) No. 1, (b) No. 2, (c) No. 5, and (d) No. 6 to the concentration change of cesium ion in pH 7.2 (0.05 M Tris–HCl) buffer solutions.

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