Anal. Chem. 1990, 62, 1506-1510
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(8) Freemau, T. M.: S e k , W. R. Anal. Chem. 1981, 53, 98-102. (9) Durliat, H.; Barrau, M. B.; Comtat, M. Bioekfrmbem. Bioenefg.
(11) Bird, C. L.; Kuhn, A. T. Chem. SOC.Rev. 1981, 10, 49-83. (12) Comyn, J. Polymer Permeabiky; Elsevier: New York, 1985.
1088. 19 413-423
Polymeric Membrane Anion-Selective Electrodes Based on Diquaternary Ammonium Salts Vanessa J. Wotring, David M. Johnson,l and Leonidas G . Bachas*
Department of Chemistry and Center of Membrane Sciences, University of Kentucky, Lexington, Kentucky 40506-0055
Hydrophoblc dlquaternary ammonlum salts were used as Ionophores to develop poiymerlc membrane ion-selectlve electrodes. These cOmpOunds had two quaternary ammonium groups separated by elther an ethylene or a propylene group. When the two posltlvely charged nitrogens were separated by a chaln of two carbon atoms, the resuiting electrodes exhibited an anion selectlvtty pattern that devlated from the l y selective for Hofmelster serles. These electrodes were w iodide, showed Nernstlan response, and were stable over at least 1 month. When the length of the chain that separated the two quaternary nttrogens was Increased to three carbon atoms, the selectlvlty of the prepared electrodes was that predicted by the Hofmeister series. This Is the same selectivity pattern obtalned with prevlously reported electrodes based on monoquaternary ammonium derlvatlves. The reported data suggest that the distance separating the positively charged nitrogens in dlquats Is crucial In obtaining electrodes with unique selectlvlty properties.
between the ion and the ionophore. Thus far, most of the electrodes that have shown a unique anion selectivity have been based on organometallic ionophores. Specifically, an electrode with high selectivity for iodide has been reported recently using a lipophilic vitamin BI2derivative ( 5 ) . Lipophilic cobyrinates have also been used as ionophores in nitrite-selective electrodes (6-9).Further, Sn(IV), Mn(III), and Co(II1) metalloporphyrins have been employed successfully in the development of ISEs for salicylate and thiocyanate (10-14). Another electrode that takes advantage of an interaction between an organometallic compound and an anion is the hydrogen phosphate electrode reported by Glazier and Arnold (15, 16). In all of the above electrodes, there is an exchange of a coordinated anion a t the metal center of the ionophore with anions in the sample solution. One exception to this is the carbonate-selective electrode, which is based on the formation of an adduct between carbonate and a trifluoroacetophenone derivative in the membrane (17). It has been reported that electrodes prepared with a N,-
N,N’,”-tetradecyl-N,N’-dimethylhexamethylenediammonium INTRODUCTION Ion-selective electrodes (ISEs) based on ionophore-impregnated polymer membranes are commonly employed in a variety of analyses. In that respect, several electrodes with high selectivity for specific cations have been reported and are available commercially (for examples, see review articles 1-4). However, only a limited number of anion-selective electrodes have been developed. In anion-selective electrodes, it is well established that there is a correlation between the observed ionic selectivity and the lipophilicity of anions. The selectivity pattern that is controlled by the relative lipophilicity of the anions is known as the Hofmeister series, according to which “nonselective” electrodes respond in decreasing order to the following anions: lipophilic anions > perchlorate > thiocyanate iodide > nitrate > bromide > chloride > fluoride. One class of ionophores that exhibits this type of response is quaternary ammonium salts. Two quaternary ammonium salts that have been used as ionophores in ISEs are tridodecylmethylammonium chloride (TDMAC)and Aliquat 336 (mixture with the major component being trioctylmethylammonium chloride) ( 3 ) . To develop electrodes with selectivity patterns that deviate from the Hofmeister series, a specific interaction must occur
-
Permanent address: Life Science Division, Ferrum College, Ferrum, VA 24008. 0003-2700/90/0362-1506$02.50/0
salt, which has two quaternary ammonium groups separated by a chain of six carbon atoms, demonstrate Hofmeister selectivity (18). This implies that the two positively charged nitrogens act independently of each other. In this paper, we describe the development and evaluation of polymer membrane based anion-selective electrodes using diquaternary ammonium salts (diquats) as ionophores. The ionophores used in this study (Figure 1)have the two positively charged atoms closer together and separated by a chain of two or three carbon atoms. The closer proximity between the two charged nitrogens induces spatial recognition properties to these ionophores. Indeed, ionophores 1 and 2 give electrodes that are highly selective for iodide over a variety of anions, while ionophores 3 and 4 respond according to the Hofmeister series. The response is Nernstian for the preferred anion and the electrodes are stable for at least 1 month.
EXPERIMENTAL SECTION Reagents. N,N,Nr,”-Tetramethyldiaminomethane, N,N,N‘,N‘-tetramethylethylenediamine, N,N,Nr,N’-tetramethylpropylenediamine, 1,4-diazabicyclo[2.2.2]octane(Dabco),quinuclidine, 1-iodooctadecane, 1-iodododecane, and dimethylformamide were purchased from Aldrich (Milwaukee, WI). Poly(viny1 chloride) (PVC) was obtained from Polyscienes, Inc. (Warrington, PA), and o-nitrophenyl octyl ether (NPOE), bis(1-butylpentyl)adipate (BBPA), bis(2-ethylhexyl) sebacate (DOS), and tris(2ethylhexyl) phosphate (TOP) were from Fluka (Ronkonkoma, NY). 2-(N-Morpholino)ethanesulfonic acid (MES), sodium acetate, sodium salicylate,and all inorganic salts were purchased Q 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 14, JULY 15, 1990 CH3
H3C
\*
+I
R-N-(CH&--N-R
\CH3 1 CH3
+I
R-N-
I
H3C
H3C
+I
\+
\
I
R-N-(CH2)3-N-R CH3 H3C
2
-3
H3C
CH3
\+
CH2-N-R
\ CH3 H3CI
4
CH3
9
1507
4
I
R--+N-R
I
-120
CH3
s
Figure 1. Compounds 1-3 are structures of diquaternary ammonium ionophores used in this study. Compound 4 is an octadecyl derivative of quinuclidine, and 6 is a dimethyidioctadecylammonium salt. The synthesis of 5 (dioctadecyl derivative of tetramethyidiaminomethane) and the counterion is was unsuccessful. In all cases, R is iodide.
from Sigma Chemical Co. (St. Louis, MO) or Fisher Scientific Co. (Cincinnati, OH). Tetrahydrofuran (THF) was also obtained from Fisher Scientific Co. The buffers and salt solutions were prepared by using deionized (Milli-QWater Purification System, Millipore Corp., Bedford, MA) distilled water. Apparatus. Voltages were monitored with a Fisher Accumet (Model 810 or Model 825 MP) digital pH/mV meter. The potential was recorded by using a Fisher Recordall (Series 5000) strip-chart recorder. The electrode cell was maintained at constant temperature by using a Fisher Isotemp constant temperature circulator (Model 80) set at 27 O C . A Fisher saturated calomel electrode was used as the reference. Preparation of Ionophores. Ionophores 1-4 were prepared by following the method employed by Tabushi et al. for the synthesis of diquaternary salts (19). The appropriate diamine (0.50 g) and 10.0 g of iodoalkane were heated in 25 mL of dimethylformamide for 2 days at 70 "C. The reaction mixture was then added to 100 mL of benzene to precipitate the diquaternary salt. The precipitate was filtered to remove excess iodoalkane and washed with hot water to remove any monoquaternary salt present. The diquaternary salt was recrystallized by using dimethylformamide and dried. Ionophore 6 was prepared as described above, except the mixture was extracted with water instead of adding it into benzene. The organic layer was eliminated in a rotaevaporator (Buchi, RE 111) to yield the quaternary ammonium salt, which was recrystallized as described above. Membranes and Cell Assembly. The membranes were prepared by using 33 mg of PVC, 66 mg of plasticizer, 1 mg of ionophore, and THF as the solvent. First, the ionophore was dissolved in the plasticizer. Then, the PVC and THF were added and allowed to mix for half an hour. This mixture was transferred to a casting mold with a diameter of 1.5 cm, and the solvent was allowed to evaporate at room temperature Overnight (20). Smaller disks were cut from the resulting membrane and mounted in a Philips electrode body (IS-561, Glasblaserei Moller, Zurich). All potentiometric measurements were made with the following electrochemical cell: SCEllKCl (saturated)llsample solutionlmembranell.00 X M sodium salicylatelAg-AgC1 M NaC1, 1.00 X Since the ionophores do not show any significant response to chloride,sodium salicylate was added to the internal fding solution to yield a stable internal solution-membrane interface. Evaluation of Response and Selectivity. The potentiometric response of the membranes was determined by adding different volumes of a standard solution to 10.00 mL of a buffered sample solution (0.0500 M acetateHC1, pH 4.0 or 0.0500 M MES-NaOH, pH 5.5). The response of the electrodes was followed by using a pH/mV meter and recorded on a strip-chart recorder. Before use, the membranes were conditioned in 5 X lo4 M NaI for 3 days (ionophores 1, 2, and 3) or in 1 X M NaI overnight (ionophores 4 and 6). When not in use, the electrodes were stored M NaI, respectively. The conditioning in 5 X lo4 M or 1 X step and the storage of these electrodes were important in
-160
6
-6
-5
-4 log [anion]
-3
1
-2
Figure 2. Anion selectivity of an electrode based on ionophore 1. A€ corresponds to the change in potential from the base line. The
electrode was exposed to the sodium salts of iodide (l),perchlorate (3),bromide (4), salicylate (5), chloride (6), nibate (7), and the potassium salt of thiocyanate (2). Solutions of these salts were added to a 0.0500 M acetate-HCI, pH 4.0 buffer. maintaining electrodes that had reproducible slopes and starting potentials. Selectivity coefficients were determined by using the fixed interference method (21). In the experiment where the pH 4.0 buffer was used, the acetate ion concentration in the buffer (Le., 7.45 x low3M) was considered as the fixed interferent. Automatic Potentiometric Titration of Silver. Twenty milliliters of a solution of 6.00 X lo4 M AgN03 in MES-NaOH buffer was titrated with 1.00 X 10-3 M NaI at a flow rate of 1.05 mL/min. The free iodide concentration was monitored by using an electrode based on ionophore 1. The response was followed on a strip-charge recorder. The end point was determined by taking the first derivative of the resulting titration curve.
RESULTS AND DISCUSSION The specific interaction between organometallic compounds and anions has been used in several occasions for the development of ISEs. In one such case, it has been demonstrated that an iodide-selective electrode could be prepared by using a lipophilic vitamin BIz derivative as the ionophore ( 5 ) . Further, it has been postulated that in this electrode the selectivity toward iodide may be a result of a simultaneous interaction of this anion with both the positively charged Co(II1) metal center and the protonated nitrogen of the imidazole ring of the ionophore (5). Since in diquaternary ammonium salts the distance between the two positively charged nitrogens is controlled by the length of the carbon chain that separates the two charges, it appears that some of these compounds may interact selectively with anions. In view of the potential usefulness of such diquaternary ammonium salts as ionophores in ion-selective electrodes, several PVC-based liquid membranes were prepared. T o prepare a functional membrane for use in ISEs, it is usually desirable to select a plasticizer in which the ionophore is soluble. For this reason several plasticizers were studied. All ionophores were more soluble in NPOE as compared to BBPA, DOS, or TOP. Thus, in the rest of the studies NPOE was used as the plasticizer. Membrane electrodes based on ionophore 1gave a selectivity pattern that deviates from the Hofmeister series. Specifically, these electrodes were highly selective for iodide over the other anions tested (Figure 2). In these graphs, hE is the difference between the steady-state potential and the starting potential (i.e., potential of cell before any addition of anions). The response toward iodide was Nernstian and linear to a t least 1 X lo4 M with the IUPAC-defined limit of detection (21) being 4.2 X M. Perchlorate, thiocyanate, and bromide were the main interferents. It is interesting to note that these electrodes do not respond to chloride or nitrate. As indicated in the figure, there is actually a slight positive slope due to dilution of the acetate buffer upon addition of
1508
ANALYTICAL CHEMISTRY, VOL. 62, NO. 14, JULY 15, 1990
- 8
-1-
-120
't,
i
-160 -6
-5
-4
-3
-2
log [anion]
Figure 3. Anion selectivity of an electrode based on ionophore 2 in 0.0500 M acetate-HCI, pH 4.0. For a key to the numbers associated with each calibration curve see the legend in Figure 2.
the standard solution of these anions. However, there could be some response toward chloride or nitrate in a different buffer environment, but this response should be less than that of acetate. Other anions that were tested, but not shown on the graph, are iodate, dihydrogenphosphate, lactate, and sulfite. The electrodes showed no response toward these ions in a MES-NaOH pH 5.5 buffer. The observed selectivity pattern is comparable to the one reported by Tabushi et al. (19). In their work, compound 1 was used to transport purines and pyrimidines across a hydrophobic liquid membrane. Several anions were used to strip these compounds from 1, the more effective being perchlorate, thiocyanate, and bromide. In addition, chloride and dihydrogen phosphate were unable to liberate adenosine diphosphate from 1. Unfortunately, Tabushi et al. did not test iodide in their transport studies. Several storage conditions for these electrodes were evaluated. It was found that the best results were obtained when the electrodes were kept at room temperature and conditioned in 5 x M NaI between experiments. Under these conditions and with data from five experiments, the standard deviation of the starting potential was found to be less than 6 mV. The slopes of different electrodes ranged from 55 to 59 mV/decade. However, for the same electrode the slopes were reproducible to f l mV/decade. The response time of these electrodes toward iodide was on the order of 1 min; response time is defined as the length of time between when the concentration of the ion of interest is changed and the first instant at which the potential of the cell becomes equal to its steady-state value within 1mV (21). It was also observed that the response was pH-dependent. Above pH 7.0, there was a strong interference from hydroxide. For this reason the test solutions were buffered either a t pH 4.0 or 5.5. An electrode prepared by using the didodecyl derivative of 1,4-diazabicyclo[2.2.2]octanegave the same selectivity pattern as electrodes based on ionophore 1. These electrodes lasted for only 1 week as compared to a t least 1 month for electrodes based on 1. It is postulated that because the didodecyl derivative (i.e., R is C12H25) is less lipophilic than I, it can leach from the membrane at a faster rate than 1. These data suggest that the length of the two lipophilic carbon chains does not affect the selectivity pattern but dictates the lifetime of the electrode. For this reason, the rest of the ionophores used in this study were prepared by using iodooctadecane. Membrane electrodes based on ionophore 2 had a selectivity pattern that also deviated from the Hofmeister series. Compared to 1, the positively charged nitrogens in 2 are also separated by a two-carbon chain, but this ionophore lacks the two extra bridging carbon chains, and thus its structure is not as rigid as 1. However, electrodes prepared by using 2 were also selective for iodide over the other anions tested (Figure 3). The response toward iodide was Nernstian and linear to
Ionophore 1
Ionophore 2
Ionophore 3
Ionophore 4
Ionophore 6
Figwe 4. Comparison of electrodes prepared with ionophores 1-4 and 6 with respect to the selectivity coefficient, kPorx,acetats In this figure Sali- refers to the salicylate anion.
at least 1 X lo4 M with the IUPAC-defined limit of detection being 9.1 x M. Perchlorate, thiocyanate, and salicylate were the main interferents. Bromide, which is not shown in the figure, is also an interferent. The response curve for this ion is approximately at the same position as the other interferents (perchlorate, thiocyanate, and salicylate), but in many cases the starting potential and response characteristics of the electrode would shift after being in contact with bromide. These electrodes did not respond toward chloride or nitrate in an acetate buffering system. Because of the similarity in the selectivity patterns of the electrodes prepared with 1 and 2, it appears that the rigidity of the ionophore is not as important in determining the observed selectivity. Storage conditions for these electrodes were the same as for electrodes based on ionophore 1. The slopes for iodide ranged from 53 to 60 mV/decade with different electrodes, but the response within the same electrode was reproducible to f2 mV/decade. The response time of these electrodes was also on the order of 1 min. With data from five experiments, the standard deviation of the starting potential was found to be less than 5 mV. Membrane electrodes based on ionophore 3 were not selective for iodide. In fact, the response pattern for these electrodes followed the Hofmeister series (Figure 4). In this compound, the positively charged nitrogens are separated by a propylene group that holds the nitrogens farther apart than in ionophores 1 and 2. This suggest that the distance between the positively charged nitrogens is crucial in determining the relative selectivity of the ionophore. One possible explanation for the observed selectivity for iodide in the case of ionophores 1 and 2 is the size correlation between the anion and the distance separating the two nitrogen centers. A distance of 3.9 (22) and 3.6 8, (23) separates the nitrogens in ionophores 1 and 2, respectively. These distances refer to the neutral molecules (i.e., the tertiary amines used as the starting materials), and thus the actual numbers should be slightly larger due to repulsion between the positive sites. From these data, it appears that iodide (diameter of 4.1 A (24))may interact simultaneously with both positive nitrogens of ionophores 1 and 2. Depending on the conformation of 3, the distance separating the two nitrogens ranges from 3.1 to 5.1 A. The lipophilicity of the ions is also playing a role in determining the overall selectivity of these electrodes, since some response is observed for anions that are in the beginning of the Hofmeister series. Like ionophore 1, ionophore 4 has a bicyclic structure; however, it contains only one quaternary nitrogen. This ionophore was studied to further support the proposed mechanism for the interaction between iodide and 1. The response of electrodes based on this ionophore followed the Hofmeister series (Figure 4). In that respect, this ionophore had the same response pattern as electrodes prepared with straight chain quaternary ammonium salts, such as trido-
ANALYTICAL CHEMISTRY, VOL. 62, NO. 14, JULY 15, 1990
decylmethylammonium chloride (3). Since there was no observed selectivity for iodide, it may be concluded that both nitrogens in 1 are somehow involved in the selective interaction with iodide. This reinforces the conclusion that the distance between the positively charged nitrogens influences the selectivity of the diquaternary ionophores. The synthesis of compound 5 was unsuccessful. The reaction of tetramethyldiaminomethanewith iodooctadecane gives two molecules of dimethyloctadecylammonium iodide and formaldehyde (25). Since there is excess iodooctadecane present, the quaternary amine 6 is formed. Carbon NMR verifies that the carbon between the two nitrogens is not present (only one resonance line was observed above 60 ppm from +NCH3). One report of the synthesis of 5 has appeared in the literature, but the reported proton NMR data failed to include the protons on the carbon between the two nitrogens (26). These protons are difficult to identify, because they may be in the same region (3.5-3.8 ppm) as the +NCH3 and +N-CH,C protons. Elemental analysis of this compound was assigned as C41H88N2Br2 (26), but it may be possible that the compound was dimethyloctadecylammonium bromide (C20H44NBr). In the case reported, the reaction was not performed in the presence of excess iodooctadecane, and therefore it is unlikely that dimethyldioctadecylammonium iodide ((&HmNBr) would have been synthesized. A report of a similar compound, hexamethylmethylenediamine iodide (prepared from tetramethyldiaminomethane and excess iodomethane) does include complete proton NMR data and consistent elemental analysis (25), which indicate formation of the diquat. This method was followed to make 5, but in our case no reaction took place, possibly because of the different iodoalkane used. Membrane electrodes based on dimethyldioctadecylammonium iodide (ionophore 6) had a selectivity pattern that followed the Hofmeister series. If ionophore 5 could have been synthesized, the distance between the two positively charged nitrogens would have been 2.4 A. On the basis of the results obtained with the other ionophores, one might expect electrodes based on 5 to be selective for fluoride (diameter 2.4 A (25)). The observed selectivity pattern is another proof that compound 6 was prepared rather than 5. While this research was in progress, two reports on anionselective electrodes based on a lipophilic macrocyclic polyamine appeared in the literature (27, 28). Specifically, 15hexadecyl-l,4,7,10,13-pentaazacyclohexadecane was incorporated into a PVC liquid membrane. A t the pH used for these experiments, the ionophore has a 3+ charge. It was demonstrated that the protonated nitrogens in this ionophore interact with nucleotide and dicarboxylate anions in solution. Specifically, by using ions such as malonate, succinate, and adipate, which have different distances between the terminal carboxyl groups, a potentiometric response was observed that was indicative of a relatively small but selective interaction between the ionophore and the anion. Higher selectivity may have been observed if the ionophore contained only two, appropriately positioned, positively charged nitrogens. In any event, these data support our experimental observation that the distance between the positively charged atoms affects the relative selectivity of the ionophore. Polymer membrane iodide-selective electrodes may find applications in a variety of fields. To demonstrate one such application, the iodide electrode based on ionophore 1 was used in an automatic potentiometric titration to indirectly determine the concentration of silver ions in a solution. A quantity of 1.20 X mol of silver ions was titrated with NaI. As iodide was added to the solution, AgI precipitated. Although at the end point an inflection point in the titration curve (plotted as the change in potential vs the volume of NaI
1500
added) was quite visible, the first derivative of the titration curve was taken to increase accuracy and precision. The amount of silver ions was found to be 1.19 X 10" f 4.1 X lo-' mol (average f standard deviation, for three determinations). The average percent error was 0.8%. The relatively low values for the standard deviation and percent error suggested that the precipitation of AgI did not harm the response of the electrode toward iodide. Indeed, this electrode provided identical response toward iodide before and after the titrations. In conclusion, it has been shown that electrodes based on two structurally different ionophores, which have two positively charged nitrogens separated by a chain of two carbon atoms, are highly selective for iodide over other ions tested. However, Hofmeister selectivity was observed for an ionophore in which the two positively charged nitrogens were separated by a propylene group. This suggests that both positively charged nitrogens of 1 and 2 are interacting simultaneously with iodide to give electrodes that deviate from the Hofmeister series. Finally, it appears that not only the distance between the two quaternary nitrogens but also the lipophilicity of the anion plays a role in determining the relative selectivity of these electrodes, since some response is observed for the anions at the beginning of the Hofmeister series. It should be noted that this is one of the first examples of using principles of host-guest recognition chemistry in the rational development of anion-selective electrodes. Furthermore, the results of this study suggest that this approach could be successful in developing new anion-selective electrodes with unique selectivity properties.
ACKNOWLEDGMENT We thank Yigang Fu for supplying literature searches of crystal structures of the tertiary amines used in this study. LITERATURE CITED Mayerhoff, M. E.; Opdycke, W. N. A&. Clln. Chem. 1988, 2 5 , 1-47. Oesch, U.; Ammann, D.; Simon, W. Clln. Chem. 1988, 3 2 , 1448-1459. Yu, R. Ion-Sel. Rev. 1988, 8 , 153-172. Thomas, J. D. R. Anal. Chim. Acta 1986, 180, 269-297. Daunert, S.; Bachas, L. G. Anal. Chem. 1989, 61, 499-503. Stepinek. R.; Krautler, B.; Schubess, P.; Lindemann, B.; Ammann, D.; Simon, W. Anal. Chim. Acta 1988, 182. 83-90. Schulthess, P.; Arnmann, D.; Krautler, B.;Caderas, C.; Stepinek, R.; Simon, W. Anal. Chem. 1985, 5 7 , 1397-1401. Schulthess, P.; Amman, D.; Simon, W.; Caderas, C.; Stepinek, R.; Krautler, B. Helv. Chlm. Acta 1984, 6 7 , 1026-1032. Daunert, S.; Witkowski, A.; Bachas, L. G. Prog. Clin. 8/01.Res. 1989, 292, 215-225. Chaniotakis, N. A.: Park, S. B.; Meyerhoff, M. E. Anal. Chem. 1989, 61, 566-570. Chaniotakis, N. A.; Chasser, A. M.; Meyerhoff, M. E.; Groves, J. T. Anal. Chem. 1988, 6 0 , 188-191. Ammann, D.; Huser, M.; Krautler, B.;Rusterholtz, B.; Schulthess, P.; Llndemann, B.; Halder, E.; Simon, W. Helv. Chim. Acta 1988, 6 9 , 849-854. Hodini, A.; Jyo, A. Chem. Lett. 1988, 993-996. Hodini, A.; Jyo, A. Anal. Chem. 1989, 6 1 , 1169-1171. Glazier, S. A.; Arnold, M. A. Anal. Chem. 1988, 60, 2540-2542. Glazier, S. A.; Arnold, M. A. Anal. Lett. 1988, 2 2 , 1075-1088. Meyerhoff, M. E.; Pretsch, E.; Helti, D. H.; Simon, W. Anal. Chem. 1986, 5 9 , 144-150. Nikol'skil, B. P.; Materova, E. A,; Timofeev, S. V.; Arkhangel'skil, L. K. Elektrokhimiya 1978, 14, 61-71. Tabushi, 1.; Kobuko, Y.; Imuta, J. I . J. Am. Chem. SOC.1981, 103, 6152-6 157. Moody, G. J.; Thomas, J. D. R. I n Ion-Selective Electrodes Methodology; Covington, A. K., Ed.; CRC Press: Boca Raton, FL, 1980; pp 111-130. Commission on Analytical Nomenclature, Pure Appl. Chem . 1975, 48, 129-132. Hon, P. K.; Mak, C. W. J. Crystallogr. Spectrosc. Res. 1987, 17, A. I.Q - - A.Z-I-. Pasquali, M.; Floriani, C.: Gaetani-Manfredotti, A. Inorg. Chem. 1981, 2 0 , 3382-3387. Douglas, B. E.; McDaniel, D. H.: Alexander, J. J. Concepts and Models of Inorganic Chemistry; Wiley: New York, 1983; p 221. Bohrne, H.; Dahne, M.; Lehners, W.; Riier. E. Lieb/gs Ann. Chem. 1969, 723, 34-40. Fujii, Y.; Pacey, G. E. Anal. Chim. Acta 1987, 200, 181-189. Kataoka, M.; Naganawa, R.: Odashima, K.; Umezawa, Y.; Kimura, E.; Koike. T. Anal. Lett. 1989, 22, 1089-1105.
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(26) Umezawa, Y.; Kataoka, M.; Takami, W.; Kirnura. E.; Koike, T.; Nada, H. Anal. Chem. 1988, 60. 2392-2396.
RECEIVEDfor review December 12,1989. Accepted April 12, 1990. Acknowledgment is made to the National Science
Foundation (R11-86-10671) and to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for support of this research. D.M.J., a visiting scholar, received support from the UK Faculty Scholars Program with funding from the Mable Pew Myrin Charitable Trust.
Polymeric Membrane Sodium-Selective Electrodes Based on Lipophilic Calix[ 4larene Derivatives Keiichi Kimura,* T s u t o m u Miura, Mitsunori Matsuo, a n d Toshiyuki Shono Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565, J a p a n
Three klnds of llpophillc callx[4]aryl ester and amlde derivatives (1-3) have been designed for Na+-selectlve polymeric membrane electrodes. The callxarene Ionophores have thus proved to be excellent Na' neutral carriers. Furthermore, selectlon of the electrode membrane condltlons such as membrane solvents and llpophlllc salts allowed high Na' selectlvltles agalnst K', the selectlvlty coefflclent for Na' wRh respect to K' belng 3.8 X lo-' In the electrode based on caHN4bryl decyl ester 1. The cdxarenebased Na'-selectlve electrode was successfully applled to Na' assay In human blood sera.
Considerable efforts have been directed toward the development of neutral carrier-type Na+-selective electrodes as alternatives of Na+-selective glass electrodes, because glass electrodes have some difficulties in clinical uses; i.e. special treatment is required at regular intervals to prevent electrode deterioration. Simon et al. have designed acyclic polyetheramide derivatives as neutral carriers for Na+-selective electrodes ( I ) and applied them to Na+ microelectrodes for intracellular studies (2). We have synthesized bis(l2-crown-4) derivatives as Na+ ionophores, taking advantage of high Na+ selectivity based on the bis(crown ether) effect (3),and fabricated Na+-selective electrodes of practical use ( 4 ) . Cyclic oligomers of tert-butylphenol-formaldehyde condensates, what we call calixarenes, when in the cone conformation, are apt to form inclusion compounds with small organic molecules being bound in the cavities. Incorporation of carbonyl groups such as ester, amide, and ketone linkages into the phenolic oxygen atoms of calixarenes provides them with ionophoric properties. In that case, a metal cation is in the cavity of the calixarene, interacting with the carbonyl and phenolic oxygen atoms. The ionophoric calixarene derivative are, therefore, candidates for neutral carriers of ion-selective electrodes. Diamond et al. and we independently applied calix[4]arenes carrying carbonyl groups to Na+-selective electrodes (5, 6). We have chosen lipophilic calix[l]arene derivatives incorporating alkoxycarbonyl and alkylcarbamoyl substituents, 1-3, as the Na+ neutral carriers. In this pub-
0003-2700/90/0362-1510$02.50/0
lication, we describe the electrode properties and membrane optimization of the Na+-selective polymeric membrane electrodes based on the lipophilic calix[4]arene derivatives. As an application of the calixarene-based electrodes, Na+ assay in human blood sera is also mentioned. EXPERIMENTAL SECTION Synthesis of Calixarene Neutral Carriers. The parent calixarene, tert-butylcalix[4]arene,was prepared according to the Gutsche method (7). Decyl a-bromoacetate and cyclohexyl abromoacetate were obtained by esterification of bromoacetic acid with decyl and cyclohexyl alcohols in benzene in the presence of H2S04. Treating chloroacetyl chloride with di-n-butylamine and triethylamine (HCl accepter) in benzene afforded Nfl-di-n-butyl-a-chloroacetamide. Calix[4]aryldecyl ester 1 was synthesized by the following reaction of tert-butylcalix[4]arene with decyl bromoacetate. Under a nitrogen atmosphere, tert-butylcalix[4]arene (2.03 mmol) was dissolved in 150 mL of a dry mixed solvent (1:1) of N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) by heating. To the solution, which had been cooled to room temperature, was added NaH (37.5 mmol) and decyl bromoacetate (23.1 mmol). The mixture was then refluxed for 3 h under a nitrogen atmosphere. After the reaction, the solvent was evaporated off and then water was added to the residue. The mixture was extracted with chloroform and the organic layer was dried over MgS04 The chloroform and the excess of decyl bromoacetate were removed under reduced pressure. The residue was subjected to silica gel column chromatography (benzene), which afforded pure product 1. Calix[4]aryl dibutylamide, 2, and calix[4]aryl cyclohexyl ester, 3, were obtained by similar reactions using Nfl-dibutylchloroacetamide and cyclohexyl bromoacetate, respectively. Silica gel chromatography using hexane/ethyl acetate gave the pure products. 1 (yield, 30%): colorless oil; 'H NMR (CDC1,) 6 0.87 (12 H, t, CH3CHJ, 1.05 (36 H,S, (CH,)&), 1.26 (64 H,9, CH,(CH2)&, 3.18 and 4.84 (8 H, d, PhCHtPh), 4.10 (8 H, t, OCHZCHZ), 4.77 (8 H, s, OCH2CO),6.77 (8 H, s, phenyl H). Anal. Calcd for CgZHl4O1$ C, 76.62; H, 10.06. Found: C, 76.54; H, 10.37. 2 (yield, 41%): white crystal; mp 208.5-209.5 O C ; 'H NMR (CDCl,) 6 0.90 (24 H, t, CH3CHz),1.08 (36 H, s, (CH,),C), 1.2-1.6 (32 H, m, CH3(CZf2)2),3.16 and 5.24 (8 H, d, PhCHZPh), 3.27 (16 H, t, CHZN), 5.02 (8 H, s, OCH2CO),6.81 (8 H, s, phenyl, H). Anal. Calcd for C, 76.09; H, 10.03; N, 4.23. Found: C, 75.77; H, C84H13208N4: 10.14; N, 4.13. 3 (yield, 31%): white crystal (from ethanol); mp 182-183 "C; 'H NMR (CDCl3) b 1.08 (36 H, 9, (CH,)&), 1.2-2.0 (40 H, m, CHZ(CH2CH2),CH2CO),3.13 and 4.86 (8 H, d, PhCH,Ph), 4.80 (12 H, s, OCH,CO,CH), 6.77 (8 H, s, phenyl H). Anal. Calcd for C76H104012: C, 75.46; H, 8.67. Found: C, 75.22; H, 8.70. Other Chemicals. Poly(viny1chloride) (PVC) with an average polymerization degree of 1100 was purified by reprecipitation from THF in methanol. The plasticizers or membrane solvents, 0nitrophenyl octyl ether (B), o-nitrophenyl phenyl ether (NPPE) 0 1990 American Chemical Society
