Cesium selective electrodes based on novel double flexible spacers bridged biscalix[4]arenes

Analytica Chimica Acta 447 (2001) 41–46

Cesium selective electrodes based on novel double flexible spacers bridged biscalix[4]arenes Langxing Chen a , Hongfang Ju a,c , Xianshun Zeng b , Xiwen He a,∗ , Zhengzhi Zhang b b

a Department of Chemistry, Nankai University, Tianjin 300071, China Key State Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China c Department of Chemistry, Chengshu College, Jiangshu 215500, China

Received 16 March 2001; received in revised form 11 July 2001; accepted 19 July 2001

Abstract Two novel double flexible spacers brideged biscalix[4]arenes 25,25 ,27,27 -bis(1,3-dioxypropane)-bis(5,11,17,23-p-terttetrabutylcalix[4]arene-26,28-diol) (1) and 25,25 ,27,27 -bis(1,4-dioxybutane)-bis(5,11,17,23-p-tert-tetrabutylcalix[4]arene26,28-diol) (2) have been evaluated as cesium ion-selective ionophore in polymeric membrane electrodes. The electrodes all give the good Nernstian response of 51 mV per decade for Cs+ in the concentration range from 1 × 10−1 to 1 × 10−5 M and good selectivity. The potentiometric selectivity of Cs+ -ISEs based on 1 and 2 for cesium ion over other alkali metal ions, alkaline-earth metal ions and NH4 + have been assessed. Effects of membrane composition and pH on the response of Cs+ -ISEs based on 1 and 2 were also discussed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Bridged biscalix[4]arenes; Cesium; Ion-selective electrode

1. Introduction Potentiometric ionophore-based membrane sensors are well-established analytical tools routinely used for the on-line detection of cations in the biological and environmental areas. A great effort has been made in the synthesis and characterization of these ionophores in order to improve selectivity and detection limits. Many cyclic and acyclic ionophores with defined cavities having ion-complexing properties, have been used extensively in construction of PVC-based ion-selective electrodes for alkali metal cations, such as Na+ , and K+ [1,2]. However, in comparison with Na+ and K+ , there are only a limited Cs+ -ISEs based on crown

∗ Corresponding author. Fax: +86-22-23504853. E-mail address: [email protected] (X. He).

ethers or natural ionophore valinomycin which have been reported [3–6]. Calixarene compounds have been widely regarded as an important class of macrocyclic host molecule for a couple of decades because of their structural and electronic feature which allow a three-dimensional (3D) control of metal ion complexation, resulting in highly selective and efficient binding properties for specific metal ions [7–9]. The ability of the calixarene compounds to act as carriers has been widely exploited for the construction of ion-selective electrodes for alkali metal and alkaline-earth metal ions [2,10,11]. The Cs+ -ISE based on calix[6]arene hexaester showed high selectivity for Cs+ relative to Na+ [12,13]. Removal of the para-t-butyl substituents significantly improves the electrode characteristics by calix[6]arene tetraesters as Cs+ -selective ionophore [14]. Also, some calixarene-crown ether derivatives

0003-2670/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 0 1 ) 0 1 3 0 0 - 9

42

L. Chen et al. / Analytica Chimica Acta 447 (2001) 41–46

have been incorporated in PVC membrane for Cs+ [15–18]. Recently, the design of double calixarenes as such enforced cavities coupled in “head-to-head” fashion have been reported [19–23]. Most of the double calix[4]arenes with enforced cavities lack the ability to accommodate potential guest molecules due to the absence of suitable binding sites. Meanwhile, double calix[4]arenes coupled in “tail-to-tail” fashion in which link the two calix[4]arene moieties are usually rigid have also been documented [24–28]. Some of these calix[4]arenes exhibit a sodium selectivity in the evaluation experiment. In recent years, we have synthesized some calixarene derivatives containing S or N atoms as suitable neutral ionophores in Ag+ -ISEs [29–32]. In this study, we report the ion-selective characteristics of the double calix[4]arene with enforced cavities in “tail to tail” fashion. The two new types of double flexible bridged biscalix[4]arene 1 and 2 connected in short spacers exhibited a good cesium selectivity and sensitivity.

Fig. 1. The structure of ionophores 1 and 2 used in cesium ion-selective electrodes in this work.

2.2. Electrode preparation 2. Experimental 2.1. Chemicals 25,25 ,27,27 -bis(1,3-dioxypropane)-bis(5,11,17,23p-tert-tetrabutylcalix[4]arene-26,28-diol) (1) and 25, 25 ,27,27 -bis(1,4-dioxybutane)-bis(5,11,17,23-p-terttetrabutylcalix[4]arene-26,28-diol) (2) (Fig. 1) were synthesized by the reaction of p-tert-butylcalix[4]arene with preorganized 25,27-bis(3-bromoproxyl) calix[4]arene-26,27-diol and 25,27-bis(3-bromobutoxyl) calix[4]arene-26,27-diol in the presence of K2 CO3 and KI according to literature [33]. Poly(vinyl chloride) (PVC) with high relative molecular mass, potassium tetrakis(4-chlorophenylborate) (KTpClPB) were obtained from Fluka (Buchs, Switzerland). Dibutyl phthalate (DBP), tetrahydrofuran (THF) (freshly distilled), and metal chlorides of lithium, sodium, potassium, cesium, ammonium, calcium and magnesium were of analytical grade and were supplied by Shanghai Chemical Reagent Corporation (Shanghai, China). The standard stock solutions and working solution of metal chlorides were prepared in redistilled deionized water.

The general procedure for the preparation of the polymeric membrane was as follows. The ionophore (1 wt.%), plasticizer (65–66 wt.%), PVC (33 wt.%) and 35 mol% KTpClPB relative to the ionophore content were mixed and dissolved in 3 ml THF. The resulting PVC–THF syrup was poured into a glass mould and THF was allowed to evaporate off at room temperature over 24 h. A flexible, transparent membrane with a thickness of 0.2–0.4 mm was obtained. The discs of 6 mm diameter were cut using a cork borer, then pasted onto the PVC tip clipped to the end of the electrode body which consisted of an Ag–AgCl wire immersed in an internal solution of 0.01 M cesium chloride. The PVC membrane electrodes were pre-conditioned by immersion in the 0.01 M cesium chloride for at least 12 h prior to use for determination of the electrode characteristics except selectivity coefficients. The potentiometric selectivity coefficients for several cations relative to Cs+ were determined by the separate solution method developed by Bakker [34,35] and calculated from Eq. (1). The solution of 0.01 M NaCl served as the internal filling solution and the electrodes were conditioned in 0.01 M NaCl solution for at least 12 h before

L. Chen et al. / Analytica Chimica Acta 447 (2001) 41–46

43

measurement. pot log Kij


 Ej − Ei Zi = + 1− log ai 2.303RT/Zi F Zj

(1)

pot

where logKCs/M is the selectivity coefficient, i the primary ion (Cs+ ), j the interfering ion, E the measured potential (mV), R the gas constant, T the thermodynamic temperature in K, F the Faraday constant, Z the charge of the ion, and a is the activity calculated from activity coefficients estimated by the Debye–Hükel equation [36]. For each membrane composition, two electrodes were prepared. 2.3. emf measurements All membrane electrode potential measurements were performed at constant temperature (25 ± 0.5◦ C). The representative electrochemical cell for the emf measurement was as follows: Ag, AgCl | int. soln. (0.01 M CsCl | PVC membrane | sample | salt bridge (1 M KNO3 ) | 3 M KCl | Hg2 Cl2 , Hg. The potential readings of the electrodes were measured in stirred solutions via a PXD-12 meter (Jiangshu Electroanalytical Instruments Corp., China). As reference electrode, the saturated calomel (Shanghai Dian Guang Device Factory, China) was used. The performance of the electrodes was examined by measuring the emf of the primary ion solutions in the concentration range of 10−7 to 10−1 M.

3. Results and discussion The potential responses of Cs+ -ISEs based on ionophore 1 and 2 with 35 mol% of the lipophilic additive KTpClPB and DBP as the membrane plasticizer were investigated. The potential response of the electrodes based on 1 and 2 for different metal ions are shown in Fig. 2. The electrodes exhibited a marked selectivity for cesium ion over other alkali metal (monovalent) cations and alkaline-earth metal (divalent) cations. A Nernstian response (>50 mV per decade) was obtained for Cs+ -ISEs based on 1 and 2 in the range of 1×10−1 to 1×10−5 M CsCl solutions. The response slopes and detection limits obtained for membranes with two ionophores were determined according to analytical commission rules [37]. The

Fig. 2. Cations response of Cs+ -ISEs based on ionophores 1 and 2: (a) ionophore 1; (b) ionophore 2. (1) Li+ ; (2) Na+ ; (3) K+ ; (4) Cs+ ; (5) NH4 + ; (6) Ca2+ ; (7) Mg2+ . Membrane compositions: ionophore/DBP/35 mol% KTpClPB.

electrode properties are summarized in Table 1. As can be seen from Fig. 2 and Table 1, the sensitivities of the response for alkali metal ions decrease in the following general order Cs+ > K+ > Na+ > Li+ . It is also found that the response of the ionophore 2 to cesium ions with butyl group as bridged spacer is slightly larger than that of 1 with proxy group. This may be mainly due to the larger enforced cavity of 2 in comparison with 1. The effect of the KTpClPB content in the membrane was also examined. It is well known that the

44

L. Chen et al. / Analytica Chimica Acta 447 (2001) 41–46

Table 1 Performance characteristics of electrodes based on ionophores 1 and 2 Na+

K+

Cs+

NH4 +

Ca2+

Mg2+

Slope (mV per decade) 1 40.2 2 40.5

47.3 48.1

50.2 49.3

51.8 52.7

45.0 45.1

23.5 21.0

18.8 17.8

Linear range (mol/l) 1 10−4 to 10−1 2 10−4 to 10−1

10−4 to 10−1 10−4 to 10−1

10−5 to 10−1 10−5 to 10−1

10−5 to 10−1 10−5 to 10−1

10−5 to 10−1 10−5 to 10−1

10−5 to 10−2 10−5 to 10−2

10−5 to 10−1.6 10−5 to 10−1.6

Detection limits (mol/l) 1 4.0 × 10−5 2 3.2 × 10−5

2.5 × 10−5 2.5 × 10−5

3.2 × 10−6 1.3 × 10−6

2.0 × 10−6 1.2 × 10−6

6.3 × 10−6 6.3 × 10−6

7.9 × 10−6 7.9 × 10−6

6.3 × 10−6 7.9 × 10−6

Ionophore

Li+

presence of lipophilic anion in cation-selective membrane electrodes can significantly influence the performance characteristics of membrane sensors [38–40]. As the content of KTpClPB increases, the Cs+ selectivity over Na+ and K+ decreases. As shown in Fig. 3, the Cs+ /K+ and Cs+ /Na+ selectivities of electrode based on 1 above 70 mol% of KTpClPB relative to ionophore content are decreased largely. The electrode without KTpClPB suffered from potential drift and slow response times mainly due to high mem-

Fig. 3. Effect of the concentration of KTpClPB in the membrane upon the Cs+ selectivity for 1-ISE with DBP as the plasticizer.

brane resistance. An increased content of KTpClPB in the membrane increases the linear range of electrode response in the higher concentration range of Cs+ ions. The Cs+ -ISEs without KTpClPB showed a linear response within the concentration 10−5 to 10−2 , and the response resembled an S-curve. The linear response range of a membrane with over 35 mol% KTpClPB was lower 1 log (activity) unit than that of the membrane without KTpClPB. When the concentration of KTpClPB is over 35 mol% the membrane is so lipophilic that sample anions are exclude from the membrane, which prevents the electrode from being selective for Cs+ . With this composition of 35 mol% KTpClPB, further experiments were carried out. The values of selectivity coefficients are listed in Fig. 4. Selectivity coefficient values for Cs+ versus the alkaline-earth metal ions are much smaller than that towards alkali metal ions due to the difference in charge density and ion radius. While most interfering ions are K+ , NH4 + , their size is close to that of cesium ion. The cesium ion with a radius of 1.96 Å is a good fit for the cavity in molecules 1 and 2, much better than that of the smaller cations, such as Li+ (0.68Å), Na+ (1.02 Å), Mg2+ (0.72 Å), Ca2+ (0.99 Å), K+ (1.33 Å), NH4 + (1.59 Å). The selectivity of 2 toward Cs+ over alkali metal ions and ammonium ions is better than that of 1, which indicated that the cavity of 2 is better match with Cs+ than that of 1. Fig. 5 is shown the correlation between the values of selectivity coefficient and the ionic radius. This indicated that the same membrane composition, the Cs+ selectivity is mainly influenced by the relative size of biscalix[4]arene’s

L. Chen et al. / Analytica Chimica Acta 447 (2001) 41–46

pot

Fig. 4. Selectivity coefficients expressed as log KCs/M of Cs+ -ISEs based on ionophore 1 and 2. Membrane compositions: ionophore/DBP/35 mol% KTpClPB.

enforced cavity to the cation’s diameter and the related charged density. The influence of the pH on the response of Cs+ -ISEs based on ligand 1 and 2 with DBP as

Fig. 5. Cesium selectivity against alkali metal cations vs. ionic radius for ionophore 1 and 2. Membrane compositions: ionophore/DBP/35 mol% KTpClPB.

45

Fig. 6. Effect of pH of the test solution on the potential response of 2-ISE. Membrane compositions: ionophore/DBP/35 mol% KTpClPB.

plasticizer was studied at concentrations of 1 × 10−3 and 1 × 10−2 M CsCl solution, where the pH was adjusted with dilute HNO3 and NaOH solutions. The pH range of 2-DBP was illustrated in Fig. 6. As can be seen, the emf remained constant in range pH 2–12. The electrode based on 1 showed similar result. The response time (t90% ) was measured by injection of 2 ml 0.1 M CsCl solution into 20 ml 1 × 10−3 M CsCl to produce a 1 × 10−3 to 1 × 10−2 M change in concentration and the time required to achieve a 90% of the equilibrium mV values was fast (t90% < 8 s). The electrode lifetime was studied by periodically recalibrating the cesium response in standard CsCl solution, which was kept in a 10−3 M CsCl solution. The electrode based on 1 and 2 had been repeatedly calibrated seven times during a period of 1 month, no change in the performance of the electrode was observed. In all instances, there was a rapid response (t90% < 8 s) and good linearity with a constant slope 51 ± 2 mV per decade. We come to the conclusion that biscalix[4]arene 1 and 2 are very good cesium-selective ionophores showing good discrimination against alkali, alkalineearth metal cations with a useful detection range between 10−5 and 10−1 M.

46

L. Chen et al. / Analytica Chimica Acta 447 (2001) 41–46

Acknowledgements This work was supported by the National Natural Science Foundation of China (Project Grant no. 29975014). References [1] Y. Umezawa, CRC Handbook of Ion-Selective Electrodes, CRC Press, Boca Raton, FL, 1990. [2] P. Bühlmann, E. Pretsch, E. Bakker, Chem. Rev. 98 (1998) 1593. [3] B. Rieckemann, F. Umland, Fresenius J. Anal. Chem. 323 (1986) 241. [4] K. Kimura, T. Tamura, T. Shono, J. Electroanal. Chem. 105 (1979) 335. [5] K.W. Fung, K.H. Wong, J. Electroanal. Chem. 111 (1980) 359. [6] R.F. Cosgrove, A.E. Beezer, Anal. Chim. Acta 105 (1979) 77. [7] C.D. Gutsche, in: F.J. Stoddard (Ed.), Calixarenes: Monographs in Supramolecular Chemistry, Royal Society of Chemistry, Cambridge, UK, 1989. [8] J. Vicens, V. Böhmer, (Eds.), Calixarenes: A Versatile Class of Macrocyclic Compounds, Kluwer Academic Publishers, Dordrecht, 1991. [9] V. Böhmer, Angew. Chem. Int. Ed. Engl. 34 (1995) 713. [10] D. Diamond, M.A. Mckervey, Chem. Soc. Rev. 25 (1996) 15. [11] R. Ludwig, Fresenius J. Anal. Chem. 367 (2000) 103. [12] A. Cadogen, D. Diamond, M.R. Smyth, G. Svehla, M.A. Mckervey, E.M. Seward, S.J. Harris, Analyst 115 (1990) 1207. [13] K. Cunningham, G. Svehla, S.J. Harris, M.A. Mckervey, Analyst 111 (1993) 341. [14] C. Bocchi, M. Careri, A. Casnati, G. Mori, Anal. Chem. 67 (1995) 4234. [15] H.J. Oh, E.M. Choi, H.S. Jeong, K.C. Nam, S.W. Jeon, Talanta 53 (2000) 535. [16] J.S. Kim, A. Ohki, R. Ueki, T. Ishizuka, T. Shimotashiro, S. Maeda, Talanta 48 (1999) 705. [17] C. Pérez-Jiménez, L. Escriche, J. Casabó, Anal. Chim. Acta 371 (1998) 155.

[18] J.S. Kim, I.H. Suh, J.K. Kim, M.H. Cho, J. Chem. Soc., Perkin Trans. 1 15 (1998) 2307. [19] S. Kanamathareddy, C.D. Gutsche, J. Org. Chem. 60 (1995) 6070. [20] A. Siepen, A. Zett, F. Vögtle, Liebigs. Ann. 5 (1996) 757. [21] A. Arduini, G. Manfredi, A.R. Pochini, R. Ungaro, J. Chem. Soc., Chem. Commun. 13 (1991) 936. [22] K. Araki, K. Sisido, K. Hisaichi, S. Shinkai, Tetrahedron Lett. 34 (1993) 8297. [23] V. Böhmer, H. Goldman, W. Vogt, J. Vicens, Z. Asfari, Tetrahedron Lett. 30 (1989) 1391. [24] P. Schmitt, P.D. Beer, M.G.B. Drew, P.D. Sheen, Tetrahedron Lett. 39 (1998) 6383. [25] Z.L. Zhong, Y.Y. Chen, X.R. Lu, Tetrahedron Lett. 36 (1995) 6735. [26] J.S. Li, Z.L. Zhong, Y.Y. Chen, X.R. Lu, Tetrahedron Lett. 39 (1998) 6507. [27] F. Ohseto, S. Shinkai, Chem. Lett. 12 (1993) 2045. [28] J.D. van Loon, D. Kraft, M.J.K. Ankoné, W. Verboom, S. Harkema, W. Vogt, V. Böhmer, D.N. Reinhoudt, J. Org. Chem. 55 (1990) 5176. [29] L.X. Chen, X.S. Zeng, X.W. He, Z.Z. Zhang, Fresenius J. Anal. Chem. 617 (2000) 535. [30] L.X. Chen, X.W. He, B.T. Zhao, Y. Liu, Anal. Chim. Acta 417 (2000) 51. [31] L.X. Chen, X.S. Zeng, H.F. Ju, X.W. He, Z.Z. Zhang, Microchem. J. 65 (2000) 129. [32] X.S. Zeng, L.H. Weng, L.X. Chen, X.B. Leng, Z.Z. Zhang, X.W. He, Tetrahedron Lett. 41 (2000) 4917. [33] X.S. Zeng, L.H. Weng, L.X. Chen, H.F. Ju, X.B. Leng, X.W. He, Z.Z. Zhang, Chin. J. Chem. 19 (2001) 493. [34] E. Bakker, Anal. Chem. 69 (1997) 1061. [35] E. Bakker, E. Pretsch, P. Bühlmann, Anal. Chem. 72 (2000) 1127. [36] P.C. Meier, Anal. Chim. Acta 136 (1982) 363. [37] Commission on Analytical Nomenclature, Pure Appl. Chem. 48 (1975) 129. [38] D. Ammann, W.E. Morf, P. Anker, P.C. Meier, E. Pretsch, W. Simon, Ion-Selective Electrode Rev. 5 (1983) 3. [39] E. Lindner, E. Gráf, Z. Niegreisz, K. Tóth, E. Pungor, R.P. Buck, Anal. Chem. 60 (1988) 295. [40] T. Rosatzin, E. Bakker, K. Suzuki, W. Simon, Anal. Chim. Acta 280 (1993) 197.