Tripodal Ionophore with Sulfate Recognition Properties for Anion

Anal. Chem. 2000, 72, 5295-5299

Tripodal Ionophore with Sulfate Recognition Properties for Anion-Selective Electrodes Maria J. Berrocal, Aurelio Cruz, Ibrahim H. A. Badr,† and Leonidas G. Bachas*

Department of Chemistry and Center of Membrane Sciences, University of Kentucky, Lexington, Kentucky 40506-0055

Ionophore topology has a profound effect on the behavior of ion-selective electrodes. This is demonstrated with a new class of ionophores that incorporates aminochromenone moieties linked through urea spacers to different scaffolds that preorganize the ionophore binding cleft into tripodal topologies. Tris(2-aminoethylamine) and cis1,3,5-tris(aminomethyl)cyclohexane were employed as the scaffolds. The two differ in their rigidity and in the size of ionophore cavity that they create. The electrodes based on the ionophore that incorporates the tris(2aminoethylamine) scaffold show anti-Hofmeister behavior with an improved selectivity for sulfate. In contrast, the ionophore with the cis-1,3,5-tris(aminomethyl)cyclohexane scaffold exhibits a more Hofmeister-like response. The design and synthesis of new ionophores for anions has been one of the goals in the development of membrane-based ionselective electrodes (ISEs) and optodes. 1,2 Some of the first anionselective ionophores used in ISEs were ion exchangers, such as tridodecylmethylammonium chloride (TDMAC). The major drawback of this type of ionophore is that the selectivity toward different anions correlates with their relative lipophilicity, following the Hofmeister series: organic anions > ClO4- > SCN- > I- ∼ salicylate- > NO3- > Br- > Cl- > HCO3- > SO42-, HPO42-. As a consequence of this pattern, a strong interference of hydrophobic anions is observed when this type of sensor is used in the determination of more hydrophilic anions that appear at the end of the Hofmeister series. This is especially critical in many practical applications including clinical analysis of hydrophilic blood/serum anions (e.g., chloride) in the presence of hydrophobic anions such as thiocyanate, bromide, and salicylate.3 Several strategies can be followed in the design of ionophores with optimal selectivity toward a particular anion. The ionophore may contain a variety of functionalities, which must be organized to complement the size and shape of the anion. For instance, multiquaternary ammonium salts,4,5 cyclic polyamines,6 and guani* Corresponding author: (e-mail) [email protected]; (phone) (859) 2576350; (fax) (859) 323-1069. † On leave from the Department of Chemistry, Faculty of Science, Ain-Shams University, Cairo, Egypt. (1) Bu ¨ hlmann, P.; Pretsch, E.; Bakker, E. Chem. Rev. 1998, 98, 1593-1687. (2) Antonisse, M. M. G.; Reinhoudt, D. N. Electroanalysis 1999, 11, 10351048. (3) Oesch, U.; Ammann, D. A.; Simon, W. Clin. Chem. 1986, 32, 1448-1459. (4) Wotring, V. J.; Johnson, D. M.; Bachas, L. G. Anal. Chim. Acta 1997, 338, 209-214. (5) Li, Z.; Liu, G.; Duan, L.; Shen, G.; Yu, R. Anal. Chim. Acta 1999, 382, 165170. 10.1021/ac000241p CCC: $19.00 Published on Web 09/28/2000

© 2000 American Chemical Society

dinium-based receptors7-9 are among successful ionophores that contain charged groups in strategic positions or frameworks. Several metallomacrocyclic ligands, such as lipophilic derivatives of vitamin B12 and metalloporphyrins, have been introduced as ionophores in ISEs as well.10 Organotin derivatives11 and other organometallic compounds such as mercuracarborands12 take advantage of the Lewis acid properties of the metal centers to bind anions. Several ionophores with hydrogen-bonding character have been reported recently in ISEs. A cytosine and a cytidine derivative have been used in nucleotide-selective electrodes.13 Also, urea and thiourea have been proposed as functionalities for the development of anion receptors.14-17 For instance, a bis(thiourea) derivative, R,R′-bis(N′-phenylthioureylene)-m-xylene, was used as an ionophore in a sulfate-selective electrode.18 A similar compound with a xanthene spacer linking the two thiourea units displayed chloride selectivity.19 The topology of the ionophore is of importance in determining the overall ionophore-ion interaction. For example, the donor groups in valinomycin and crown ether compounds exhibit a circular topology (Figure 1A), which creates a cavity that allows the ionophore to engulf the cation it is selective for. The selectivity of such ionophores can be controlled by modifying the size of the ring and by introducing heteroatoms, such as nitrogen, sulfur, and phosphorus in the ring chain.20 The synthesis of cyclic (6) Carey, C. M.; Riggan, W. B. Anal. Chem. 1994, 66, 3587-3591. (7) Hutchins, R. S.; Molina, P.; Alajarı´n, M.; Vidal, A.; Bachas, L. G. Anal. Chem. 1994, 66, 3188-3192. (8) Hutchins, R. S.; Baansal, P.; Molina, P.; Alajarı´n, M.; Vidal, A.; Bachas, L. G. Anal. Chem. 1997, 69, 1273-1278. (9) Fibbioli, M.; Berger, M.; Schmidtchen, F. P.; Pretsch, E. Anal. Chem. 2000, 72, 156-160. (10) Hutchins, R. S.; Bachas, L. G. In Biofunctional Membranes; Butterfield, D. A., Ed.; Plenum Press: New York, 1996; pp 35-44. (11) Liu, D.; Chen, W. C.; Yang, R. H.; Shen, G. L.; Yu, R. Q. Anal. Chim. Acta 1997, 338, 209-214. (12) Badr, I. H. A.; Diaz, M.; Hawthorne, M. F.; Bachas, L. G. Anal. Chem. 1999, 71, 1371-1377. (13) Bu ¨ hlmann, P.; Amemiya, S.; Nishizawa, S.; Xiao, K. P.; Umezawa, Y. J. Inclusion Phenom. Mol. 1998, 32, 151-163. (14) Ball, J. C.; Hutchins, R. S.; Raposo, C.; Mora´n, J. R.; Alajarı´n, M.; Molina, P.; Bachas, L. G. ACS Symp. Ser. 1998, No. 690, 248-256. (15) Nishizawa, S.; Bu ¨hlmann, P.; Iwao, M.; Umezawa, Y. Tetrahedron Lett. 1995, 36, 6483-6486. (16) Tobe, Y.; Sasaki, S.; Mizuno, M.; Naemura, K. Chem. Lett. 1998, 8, 835836. (17) Ameniya, S.; Bu ¨ hlmann, P.; Umezawa, Y.; Jagessar. R. C.; Burns, D. H. Anal. Chem. 1999, 71, 1049-1054. (18) Nishizawa, S.; Bu ¨ hlmann, P.; Xiao, P. K.; Umezawa, Y. Anal. Chim. Acta 1998, 358, 35-44. (19) Xiao, P. K.; Bu ¨ hlmann, P.; Nishizawa, S.; Amemiya, S.; Umezawa, Y. Anal. Chem. 1997, 69, 1038-1044.

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Figure 1. Examples of ionophore topologies: (A) circular, (B) acyclic, and (C) tripodal.

ionophores for anions is much more challenging due to their larger size. There are only a few examples of anion-selective cyclic ionophores, such as polyammonium macrocycles and mercuracarborand derivatives.6,12 On the other hand acyclic anion-selective ionophores (Figure 1B) are more readily available synthetically. Tripodands (Figure 1C) constitute a special class of acyclic ionophores, which consist of multiarmed ligands with each arm bearing functional groups that can coordinate with the targeted ion.21 We hypothesized that tripodal ionophores, which are between cyclic and acyclic ionophores with regard to preorganization, should be able to complex anions more effectively compared to analogous acyclic ionophores. Therefore, in this article, we explore a new class of ionophores for anions that are based on a tripodal topology. Although examples of tripodal anion receptors exist in the literature,22-25 to our knowledge, this is the first time that a tripodal receptor molecule has been employed as the ionophore in anion-selective electrodes. EXPERIMENTAL SECTION Reagents. Bis(2-ethylhexyl) sebacate (DOS), 2-nitrophenyl octyl ether (o-NPOE), 2-fluorophenyl 2-nitrophenyl ether (FPNPE), poly(vinyl chloride) (PVC), tridodecylmethylammonium chloride, and tetrahydrofuran (THF) were purchased from Fluka (Ronkonkoma, NY). N-2-Hydroxyethylpiperazine-N′-2-ethanesulfonic acid (HEPES), 2-(N-morpholino)ethanesulfonic acid (MES), and tris(hydroxymethyl)aminomethane (Tris) were obtained from Research Organics (Cleveland, OH). 1-Chloronaphthalene and the sodium salts of nitrate and thiocyanate were obtained from Aldrich (Milwaukee, WI). The sodium salts of acetate, sulfate, chloride, and perchlorate were acquired from J. T. Baker (Phillipsburg, NJ). Sodium bromide was purchased from Mallinckrodt (St. Louis, (20) Tsukube, H. In Crown Ethers and Analogous Compounds; Hiraoka, M., Ed.; Elsevier: Amsterdam, The Netherlands, 1992; pp 100-197. (21) Tu ¨ mmler, B.; Maass, G.; Vo ¨gtle, F.; Sieger, H.; Heimann, U.; Weber, E. J. Am. Chem. Soc. 1979, 101, 2588-2598. (22) Valiyaveetitil, S.; Engsbersen, J. F. J.; Verboom, W.; Reinhoudt, D. N. Angew. Chem., Int. Ed. Engl. 1993, 32, 900-901. (23) Metzger, A.; Lynch, V. M.; Anslyn, E. V. Angew. Chem., Int. Ed. Engl. 1997, 36, 862-865. (24) Sato, K.; Arai, S.; Yamagishi, T. Tetrahedron Lett. 1999, 40, 5219-5222. (25) Kavallieratos, K.; Bertao, C. M.; Crabtree, R. H. J. Org. Chem. 1999, 64, 1675-1683.

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MO), and sodium salicylate, sodium phosphate, and sodium borate from Sigma (St. Louis, MO). All the aqueous solutions were prepared with deionized distilled water (Milli-Q water purification system). Details on the synthesis of the different ionophores can be found in refs 26-28. The synthesis of ionophore 1 is analogous to the one described in ref 26, with the exception of the starting material: a 6-bromochromene derivative was used in that reference, whereas for ionophore 1, a 6-tert-butylchromene was used instead. Apparatus. Membrane potentials were monitored with an inhouse custom-built six-channel high-impedance amplifier with unity gain, coupled to an analog-to-digital converter (G. W. Instruments; Somerville, MA) connected to a Macintosh computer using SuperScope v. 1.2 (G. W. Instruments) software. The reference electrode was an Orion Ag/AgCl double-junction electrode. Membranes. To prepare the membranes, 2 mg of ionophore (corresponding to 1 wt %), different amounts of the lipophilic salt TDMAC (20-80 mol % relative to the ionophore), and plasticizer and PVC, in a 2:1 ratio, were dissolved in 2 mL of THF. This cocktail was poured in a 22-mm-diameter glass ring, and the membranes were formed after evaporation of the solvent at room temperature. Smaller disks were cut, placed at the tip of a Philips IS-561 electrode body (Glasblaserei Mo¨ller, Zurich, Switzerland). The assembled electrode was then conditioned overnight in a 1.0 × 10-2 M solution of the primary ion. Measurement Procedure. Potentiometric measurements were obtained by using the following cell assembly:

Ag/AgCl | KCl (saturated) || 0.0100 M HEPES, pH 7.0 || sample | membrane | 0.0100 M Na2SO4, 0.0100 M KCl, 0.0100 M HEPES, pH 7.0 | Ag/AgCl The change in the potential of the cell, ∆E, was recorded for every addition of aliquots of standard solutions to 50.0 mL of buffer. The selectivity coefficients were determined by the matched potential method.29 Due to the sensitivity of this particular method to the experimental conditions, all experimental conditions were kept as close as possible to those of earlier reported electrodes in order to be able to compare values from different ionophores in the literature. The buffer that was used throughout the experiments was 0.0100 M HEPES-NaOH, pH 7.0. This buffer system provided calibration plots for sulfate with lower detection limits and slopes that were closer to theoretical. Other buffers that were evaluated included the following: 0.0100 M Tris-HCl, pH 7.0, 0.0100 M MES-NaOH, pH 6.5, 0.0100 M phosphate buffer, pH 7.0, 0.0100 M acetic-acetate, pH 4.75, and 0.0100 M borate, pH 8.0. RESULTS AND DISCUSSION The design of synthetic receptors for anions of tetrahedral geometry has been a challenging task. The most successful (26) Raposo, C.; Crego, M.; Mussons, M. L.; Caballero, M. C.; Mora´n, J. R. Tetrahedron Lett. 1994, 35, 3409-3410. (27) Raposo, C.; Almaraz, M.; Martı´n, M.; Weinrich, V.; Mussons, M. L.; Alca´zar, V.; Caballero, M. C.; Mora´n, J. R. Chem. Lett. 1995, 759-760. (28) Raposo, C.; Pe´rez, N.; Almaraz, M.; Mussons, M. L.; Caballero, M. C.; Mora´n, J. R. Tetrahedron Lett. 1995, 36, 3255-3258. (29) Umezawa, Y.; Umezawa, K.; Sato, H. Pure Appl. Chem. 1995, 67, 507-518.

Figure 2. Structures of the ionophores used in this study.

strategies employ polytopic receptors as anion recognition elements3-6,9,13-15,22 that are typically biologically inspired. For example, structural information suggests that the guanidinium group of arginine residues is commonly found in the binding site of several oxoanion binding proteins, a fact that was exploited in the design of guanidinium-based ionophores for this class of anions.7-9 With the goal of identifying possible candidates to be used as sulfate-selective ionophores, we examined the only available X-ray structure of a sulfate-binding protein complexed with sulfate.30 This structure reveals that the anion is tightly held by seven hydrogen bonds, five of which are donated by mainchain amide NH groups, but also that there are no positively charged groups near the sulfate anion. The ionophores described in this article, share similar structural features (Figure 2). Ionophores 2 and 3, in which the aminochromenone moieties are linked through urea spacers to different scaffolds that preorganize the ionophore binding cleft into tripodal topology, mimic the way by which sulfate is complexed in the sulfate-binding protein. In this three-dimensional cleft, the sulfate anion would be complexed by the hydrogen bonds formed through the amide NH groups of the ionophore. When implemented as ionophore in an ISE membrane in combination with o-NPOE and different amounts of TDMAC, the bis(aminochromenone) 1 showed preference for the more hydrophobic anions (Figure 3). Although the selectivity pattern is mostly Hofmeister-like, the electrode demonstrated some selectivity toward salicylate (cf. responses to salicylate and perchlorate). This is consistent with the predicted favorable interaction of the carboxylate group of this anion with the ionophore from CPK models.26 The main difference between the tripodands 2 and 3 is the use of two different scaffolds. In ionophore 2, the scaffold is tris(2-aminoethylamine), whereas in ionophore 3, a cis-1,3,5- tris(aminomethyl)cyclohexane scaffold is used. The scaffold in 3 is more rigid than tris(2-aminoethylamine) and provides a slightly larger cavity.27 Both facts can affect greatly the selectivity of ionophores.20 Indeed, ionophore 2 demonstrates selectivity to the hydrophilic anion sulfate, while ionophore 3 exhibits almost no response to hydrophilic anions, but shows, in general, larger responses toward more hydrophobic ones (i.e., Hofmeister-like responses). These results clearly indicate the effect of the scaffold (30) Pflugrath, J. W.; Quiocho, F. A. J. Mol. Biol. 1988, 200, 163-180.

Figure 3. Potentiometric responses of an o-NPOE-plasticized membrane containing 1 wt % ionophore 1 and 50 mol % TDMAC (relative to the ionophore) in 0.0100 M HEPES-NaOH pH 7.0: (1) sulfate, (2) chloride, (3) bromide, (4) nitrate, (5) iodide, (6) thiocyanate, (7) perchlorate, and (8) salicylate.

on the ionophore selectivity. By being more flexible, ionophore 2 can adapt more easily to the size and shape of the anion than ionophore 3. It is now well-established, both theoretically and experimentally, that the addition of lipophilic ionic species to plasticized PVC membranes, containing neutral or charged ionophores, is beneficial for the performance of the resulting ISEs.31 Effects of the addition of lipophilic additives include the decrease of the counterion interference (Donnan exclusion failure) and improvement of the selectivity of the sensor.31,32 TDMAC was incorporated into the membrane, in an amount that corresponds to 20, 50, and 80 mol % with respect to the ionophore. The addition of the tetraalkylammonium salt provides lipophilic cationic sites within the membrane. Membranes containing 80 mol % TDMAC were chosen for the rest of the study since smaller amounts of TDMAC yielded electrodes with sub-Nernstian slopes; membranes with 80 mol % TDMAC also showed improved selectivity toward sulfate with respect to monovalent interferences. The improvement in the selectivity coefficients is consistent with prior studies and suggests that the ionophore acts as a neutral carrier in the membrane environment.32 One possible explanation for the change in the slopes of the calibration plots as a function of lipophilic salt content could be that the concentration of sulfate within the membrane is not well buffered for the lower concentrations of TDMAC. As predicted by theory, the concentration of the main ion has to be kept constant within the membrane in order to obtain a Nernstian response.33 Similar improvement in slopes has also been observed with a different class of ionophore ([12]-mercuracarborand-4).34 (31) Schaller, U.; Bakker, E.; Spichiger, U. E.; Pretsch, E. Anal. Chem. 1994, 66, 391-398. (32) Bakker, E.; Bu ¨ hlmann, P.; Pretsch, E. Chem. Rev. 1997, 97, 3083-3132. (33) Cobben, P. L. H. M.; Egberink, R. J. M.; Bomer, J. G.; Bergveld, P.; Reinhoudt, D. N. J. Electroanal. Chem. 1994 368, 193-208. (34) Badr, I. H. A.; Bachas, L. G. Unpublished data.

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Table 1. Response Characteristics and Selectivity Coefficients of Electrodes Based on Ionophore 2 with Different Amounts of Lipophilic Additivea mole ratio of ionophore and TDMACb

detn limit (M) linear range (M) slope (mV/dec) pot KSO 224 ,HPO4 pot KSO42-,Clpot KSO 24 ,Br pot KSO 2-,NO 4 3 pot KSO 24 ,salicylate pot KSO 24 ,ClO4 aStandard

20 mol %

50 mol %

80 mol %

6 wt % TDMAC18

2((0.8) × 10-4 10-3-10-1 -15.8 -2.5 -0.1 +0.5 +1.3 +2.4 +3.6

3((0.5) × 10-5 10-4-10-1 -21.2 -1.1 -0.3 -0.6 +0.5 +1.5 +2.8

3((0.1) × 10-6 10-5-10-1 -28.9 -0.9 -1.4 -0.4 +0.4 +1.6 +2.6

2.5((0.1) × 10-5 2.5 × 10-4-10-2 -26.3 -1.1 +0.8 +2.2 +2.9 +5.0

deviations (n ) 3) are given in parentheses. b Membrane composition: 1 wt% 2, ∼66 wt % o-NPOE, ∼33 wt% PVC, and TDMAC.

Figure 4. Comparison of the calibration plots for sulfate obtained with ionophore 2 and 1-chloronaphthalene (squares), FPNPE (diamonds), or o-NPOE (triangles) as plasticizer.

One of the main roles of a plasticizer is to facilitate the dissolution of the ionophore in the polymeric matrix. When a plasticizer with low dielectric constant like DOS (r ) 3.88) was used to prepare the membranes, none of the tested ionophores was completely soluble, and the resulting electrodes showed subNernstian slopes. To improve the solubility of the ionophore, and to study the influence of the plasticizer on the electrode response, more polar plasticizers were evaluated, such as o-NPOE (r ) 24) and FPNPE (r ) 50). In addition, since the interaction between ionophore and anion is through hydrogen bonds, 1-chloronaphthalene was examined as a plasticizer. 1-Chloronaphthalene has a low hydrogen-bonding character, in contrast to o-NPOE and FPNPE, which have a moderate hydrogen-bonding character. However, the slopes obtained when 1-chloronaphthalene was used as the plasticizer were sub-Nernstian for all the tested ionophores. The lower solvation ability of this plasticizer could be the cause of the observed behavior. A comparison of calibration plots for sulfate obtained with ionophore 2 and the different plasticizers is shown in Figure 4. The response of ionophore 2 to sulfate is remarkable (Figure 5), with slope values very close to the theoretical value of -29.6 5298 Analytical Chemistry, Vol. 72, No. 21, November 1, 2000

Figure 5. Potentiometric responses of an o-NPOE-plasticized membrane containing 1 wt % ionophore 2 and 80 mol % TDMAC (relative to the ionophore) in 0.0100 M HEPES-NaOH pH 7.0: (1) chloride, (2) phosphate, (3) sulfate, (4) nitrate, (5) bromide, (6) salicylate, (7) thiocyanate, and (8) perchlorate.

mV/decade and detection limits of 3 × 10-6 M (o-NPOE as plasticizer). The plasticizers o-NPOE and FPNPE showed similar responses for most of the tested anions (see Table 1 for a comparison of responses to different anions). The response time, defined as the time between addition of analyte to the sample solution and the time when less than 0.1 mV/min change in potential has been reached, was