Anal. Chem. 1995, 67,2401 -2404
Practical Applicability of Silicone Rubber Membrane Sodium=SelectiveElectrode Based on Oligosiloxane-Modified Calix[4]arene Neutral Carrier Yutaka Tsujimura, Masaaki Yokoyama, and Keiichi Kimura* Chemical Process Engineering, Faculty of Engineering, Osaka University, Yamadaska, Suita, Osaka 565, Japan
Silicone rubber-basedNa+-sensingmembranes, which do not require any special plasticizer and are therefore quite resistant to membrane deterioration, can be practically applied to Na+-selectiveelectrodes, as far as the neutral carrier is highly dispersible in silicone rubber. The silicone rubber membrane Na+-selectiveelectrodes based on an oligosiloxane-mo-ed ~aW41areneneutral carrier are reliable for sodium assays in human body fluids such as blood sera and urine. ltvo other Na+ neutral carriers that are excellent ones for plasticized poly(viny1 chloride) membranes did not give good results for silicone rubber membranes of the ion-selective electrodes, due to their poor dispersibility in silicone rubber. Typical potentiometric ion sensors, ion-selective electrodes enjoy widespread use as convenient, handy, iondetermining tools. Specifically, various neutral carrier-type ion-selective electrodes have been developed due to their high ion selectivities. An excellent membrane material for the neutral carrier-type ion electrodes is plasticized poly(viny1 chloride) (WC). Plasticized W C ion-sensing membranes, however, still have some disadvantage; Le., they need high volume percents of special plasticizer, which are easy to exude from the membranes to aqueous phases and therefore result in membrane deterioration. There is also some work claiming that neutral carrier-type W C membrane ionselective electrodes may give some negative errors in cation assays in biological systems, as described later. An alternative membrane material for ion-selective electrodes is cross-linked poly(dimethylsiloxane),silicone rubber, which was first applied to a heterogeneous precipitate-based membrane for silver-selective electrodes.' In general, any special plasticizer is not required in silicone rubber-based ion-sensing membranes. Some biocompatibility can be also expected on using silicone rubber membrane ion-selective electrodes in biological systems. However, silicone rubber-based ion-sensing membranes possess high electrical resistivity, which often causes unstable emf readings. This problem is serious especially in ion-sensing membranes containing electrically neutral ion exchanger, neutral carriers, although valinomycin, a typical K+ ionophore, seems to be applicable to silicone rubber-based K+-selective (1) Pungor, E.; Havas, J.; T6th, IC;MandArasz, G. French Patent 1,402,343,1965. (2) Pick, J.; Toth, IC; Pungor. E. Anal. Chim. Acta 1973,64, 477-480. Riess, C.; Ammann, D.; Magyar, B.; Asper, R.; Simon, W. (3) Jenny, H.-B.; Mikrochim. Acta 1980,2, 309-315. (4) Mostert, L. A; Anker, P.; Jenny, H.-B.; Oesch, U.; Morf, W. E.; Ammann, D.;Simon, W. Mikrochim. Acta 1985,1, 33-38.
0003-2700~95/0367-2401$9.00/0 0 1995 American Chemical Society
We have successfully applied silicone rubber membranes to neutral carrier-type ion-sensitivefield-effect transistors (ISFFCTs), taking advantage of the high adhesion of silicone rubber to the inorganic FIT gate surface. It was found during the study that highly dispersible neutral carriers can afford silicone rubber-based ion-sensing membranes with relatively low electrical resisti~ity.~~~ It occurred to us that highly dispersible calix[4larene neutral carriers can be employed as silicone rubber-based ion-sensing membranes for Na+-selective electrodes. Here we describe the practical applicability of silicone rubber membrane Na+-selective electrodes based on oligosiloxane-modified calix[41arene neutral carrier 1, which is highly dispersible in the polymer matrix.
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Comparisons are also made with silicone rubber membrane Na+selective electrodes based on a popular calixarene neutral carrier 2 and our previous Na+ neutral carrier, a bis(l2-crown-4) derivative 3. EXPERIMENTAL SECTION
Materials. Calii[4]arene triethyl siloxanylundecyl ester 1* and calix[4]arenetetraethyl ester 2 were prepared according to a procedure described in the literature. Bis(l2crown-4-methyl) 2-dodecyl-2-methylmalonate(3) was employed as received from Dojindo. Unless otherwise noted, the onecomponent room temperaturevulcanizing silicone rubber employed was Shin-Etsu Silicone KE47T, where cross-linking and thereby hardening of the polysiloxane was achieved by alcohol-evolving condensation of its silanol and alkoxysilane. W C with an average (5) LeBlanc, 0. H.; Grubb, W. T. Anal. Chem. 1976,48, 1658-1660. (6) Crowe, W.; Mayevsky, A; Mela, L. Am. J. Physiol. 1977,233, C56-C60. (7) Kimura, K; Matsuba, T.; Tsujimura, Y.; Yokoyama, M. Anal. Chem. 1992, 64, 2508-2511. (8) Tsujimura, Y.; Yokoyama, M.; Kimura, K Electroanalysis 1993,5. 803807. (9) Chang, S:K; Cho, 1.1. Chem. SOC.,Perkin Trans. 1 1986,211-214.
Analytical Chemistty, Vol. 67,No. 14,July 15, 1995 2401
polymerization degree of 1020was purified by reprecipitation from THF in methanol. 2-Ethylhexyl sebacate @OS) was purified by vacuum distillation. Alkali and alkaline-earth metal chlorides and ammonium chloride were of analytical reagent grade. Water was deionized. Control blood sera (Wako, control serums I and II) and control urine @io-Rad, quantitative urine control 1) were purchased. The concentrations for Na+ and K* are as follows; 145 mmol dm-3 Na+ and 4.1 mmol dm-3 K+for serum I, 124 mmol dm-3 Na+ and 6.5 mmol dm-3 K+for serum 11, and 59 mmol dm-3 Na' and 21 mmol dm-3 K+ for urine 1. Dilution of the samples (10- and 100-fold) was performed with deionized water. Electrode Fabrication. The general procedure for casting ion-sensing membranes is as follows: RTV silicone rubber (90 mg) and a neutral carrier (10 mg) were dissolved in chloroform (0.6 cm3). The whole solution was poured into a Teflon-made Petri dish with an inner diameter of 17 mm. Gradual evaporation of the chloroform and hardening at ambient temperature for 2 days afforded an elastic, semitransparent membrane with a thickness of 0.1-0.2 mm. Similarly, PVC membranes were prepared by casting from a solution consisting of PVC (50 mg), DOS (100 mg), neutral carrier (10 mg), and tetrahydrofuran (2 cm3) on a flat 20-mm inner diameter Petri dish. A 7-mm diameter disk was cut from the membrane with a cork borer and was then incorporated into an electrode body (Philips IS561 type). The mol dm-3 NaCl aqueous internal filling solution was 1 x solution. Conditioning of the electrodes was made by soaking in the NaCl solution overnight. Measurements. Potential measurements were made at room temperature using a pH/mV meter (Toko, TP-lOOO). The external reference electrode was a double-junction-type Ag/AgCl electrode with 3 mol dm-3 KC1 internal solution and 1mol dm-3 CH3C02Li external solution. The electrochemical cell was Ag-AgCl/l x mol dm-3 NaCl//silicone rubber membrane//sample solution/l mol dm-3 CH3COzLi/3 mol dm-3 KCl/AgCl-Ag. The measuring cation activities were changed by injection of highconcentration solutions to the testing solutions while stirring with a magnetic stir bar. The emf readings were made after the potential reached a constant value. The activity coefficients ( y ) were calculated according to the Debye-Huckel equation, log y = -0.511(I)1/2/(l PI2), using the values of ionic strength (I). Selectivity coefficients for Na' with respect to other cations were determined by a mixed-solution method (FIM). The background cation concentrations were 1 x 10-1 mol dm-3 for K+ and Hf, 5 x 10-1 mol dm-3 for Li+,Ca2', and Mg2+, and 1 mol dm-3 for NHd-. The sodium assay was carried out by a calibration plot method and a standard addition method. For the standard addition method, the volumes for the sample and adding solution (2 mol dm-3 Nai) were 5 and 1 cm3, respectively. The slopes were determined in advance by a five-time addition of the standard solution. The membrane electrical resistivity was measured by an ac impedance method, using a U-type cell and a frequency response analyzer (Solartron, 1253) equipped with a current a m p u e r (Keithley, 427). In the measurement cell, an ion-sensing membrane with 1-cm2 area separated two aqueous phases with an identical Na+ concentration, in which a pair of spiral Ag-AgC1 electrodes of about 1-cm2 area were placed. The frequency range and applied ac voltage were 3000-0.1 Hz and 1 V, respectively. The electrical resistivity was calculated by Cole-Cole plots.
+
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Analytical Chemistry, Vol. 67, No. 14, July 15, 1995
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RESULTS AND DISCUSSION
Characterization of Silicone Rubber Membranes Based on Neutral Carriers. Oligosiloxane-modified calix[.lIarene neutral carrier 1 was chosen as a neutral carrier that is highly dispersible in silicone rubber. The high dispersibility can be attributed to the oligosiloxane moiety incorporation and the low molecular cohesion derived from its structural ~nsymmetrization.~ For comparison, calix[4larene tetraethyl ester 2 and bis(l2crown-4) 3 15-li were also used as a popular calix[4larene neutral carrier and a famous crown-ether-based Na+ neutral carrier, respectively. Membrane impedance for the silicone rubber membranes containing 1-3 was first measured in various concentrations of Na+ aqueous solutions by a conventional ac impedance method (Figure 1). The silicone rubber membrane of oligosiloxanemodified calixarene 1 possesses very low electrical resistivity as compared with the membrane containing no neutral carrier. This indicates that neutral carrier 1is highly dispersible in the polymer matrix and is capable of exchanging Na' effectively. In contrast, the electrical resistivity for the membrane of caliiarene tetraethyl ester 2 is quite high and even higher than that for the membrane without any neutral carrier. This suggests that the dispersibility of 2 is quite low in silicone rubber. Microscopic observation of the 2 membrane also showed that some crystal phase of 2 was in the membrane. The poor dispersion of the neutral carrier in silicone rubber caused signiticant membrane heterogeneity, thus resulting in increased whole electrical resistivity of the membranes due to the frequent occurrence of interfacial resistance between the immiscible phases. This is almost the case with the silicone rubber membrane of bis(crown ether) neutral carrier 3. The (10) Diamond, D.; Svehla. G.; s w a r d , E. M.; McKervey, M. A. Anal. Chim. Acta 1988, 204, 223-231. (11) Cadogan, A. M.: Diamond, D.; Smyth, M. R.; Deasy, M.; McKervey, M. A; Hams, S. J. Analyst (London) 1989, 114, 1551-1554. (12) Kimura, K; Miura, T.; Matsuo, M.; Shono. T. Anal. Chem. 1990,62,15101513. (13) Sasaki. T.; Harada, T.; Deng, G.; Kawabata, H.; Kawahara, Y.; Shinkai, S. J. Inclusion Phenom. 1992, 14, 285-302. (14) Careri, M.; Casnati, A; A. Guarinoni; Mangia, A,: Mori, G.: Pochini, A,; Ungaro, R Anal. Chem. 1993, 65, 3156-3160. (15) Shono. T.; Okahara, M.; Ikeda, I.; Kimura, K; Tamura, H. J. Electroanal. Chem. Inte$acial Electrochem. 1982, 132, 99-105. (16) Shibata, Y.; Mamizumi, T.; Miyagi, H. Nippon Kagaku Kaishi 1992,961967. (17) Moody, G. J.; Saad. B. B.; Thomas, J. D. R. Anal. Proc. 1989,26, 8-11.
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heterogeneity of the 3 membrane, however, seems to be much more serious than that of the 2 membrane, the former membrane possessing extremely high electrical resistivity, which is close to the high detection limit for our impedance measurement system. Electrode Response. Figure 2 shows a typical potential response for a silicone rubber membrane Na+-selectiveelectrode based on calixarene neutral carrier 1,together with those for the other neutral carriers, 2 and 3. The silicone rubber membrane of 1 a€forded high electrode sensitivity with a Nemstian slope in the wide Na+ activity range of 3 x 10-5-6 x lo-’ mol dm-3. On the other hand, both the silicone rubber membrane electrodes based on 2 and 3 exhibited only poor sensitivity to Na+ activity changes and even anion response in the high NaCl concentration range. Comparisons were made between membranes of silicone rubber and plasticized PVC for Na+-selectiveelectrodes based on the neutral carriers. In any of the Na+-selectiveelectrodes with WC/DOS membranes of 1-3,the potential response profile was quite similar to that for the Na+-selectiveelectrodes with silicone rubber membranes of 1 in Figure 2. This means that 1-3 are all good neutral carriers for the plasticized W C Na+-sensing membranes. It is obvious that the poor dispersibility of neutral carriers 2 and 3 in silicone rubber and therefore the high electrical resistivity of their silicone rubber membranes lead to the low sensitivity for their Na+-selective electrodes shown in Figure 2. A distinct difference in response time was found among Na+selective electrodes with silicone rubber membranes of 1-3. Time-response profiles for the three silicone rubber membrane electrodes are summarized in Figure 3. A fast response was attained in the electrodes with silicone rubber membranes of 1, the response time, tw, being about 2 s. The electrode response was, however, sluggish in the 2/silicone rubber membrane system. The 3 membrane system exhibited a longer response time than the 2 membrane system. The response time in both the silicone rubber membrane systems of 2 and 3 was in the order of minutes. High dispersibility of oligosiloxane-modified calixarene neutral carrier 1 again contributes to the fast electrode response for its silicone rubber membrane Na+-selective electrodes, possibly due to its effective cation exchange. The selectivities for silicone rubber membrane Na+-selective electrodes based on 1 are equal or even slightly superior to those for the electrodes with the corresponding plasticized PVC membranes (Figure 4). The selectivitycoefficient for Na+ with respect
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Time I min Figure 3. Time course changes of potential response for silicone rubber membrane Na+-selective electrodes based on neutral carriers 1-3 on changing Na+ concentration from 1 x mol to 3 x dm-3.
Figure 4. Selectivity comparison between Na+-selectiveelectrodes with silicone rubber and plasticized PVC membranes of neutral carriers 1.
to K+ in the silicone rubber membrane electrode of 1 was 3 x 10-3. Applicability to Sodium Assay in Human Fluids. Some deviation of the found values from the corresponding actual values has been so far observed for the cation assay in biological systems by using alkali-metal ion-selectiveelectrodes with plasticized WC membranes. For instance, W C membrane K+-selectiveelectrodes based on valinomycin give negative errors for the urine potassium assay under undiluted condition^.^ Serious negative deviation occurs in serum sodium assay by using Na+ ISFETs with plasticized W C membranes of a calixarene neutral carrier.1s (18)Tsujimura,Y.;Yokoyama, M.; Kimura, K Sens. Actuaton 1994,822,195199.
Analytical Chemistry, Vol. 67,No. 14, July 15, 1995
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Figure 5. Na+ calibration plots and Na+ concentrations in sera obtained with silicone rubber membrane Na+-selective electrodes based on calixarene 1 by a calibration plot method: (0) Na+ calibration plots; (A)found values.
Figure 6. Nat calibration plots and Na+ concentrations in urine obtained with silicone rubber membrane Na+-selective electrodes based on calixarene 1 by calibration plot method: (0)Na' calibration plots; (A)found values.
Attempts were made to apply the silicone rubber membrane Natselective electrodes based on oligosiloxane-modified calixarene neutral carrier 1to sodium assay in human body fluids for their applicability check. Na+ concentrationsin human blood sera was first determined with silicone rubber membrane Na+-selective electrodes based on 1, using the Na+ calibration plots (Figure 5). Undiluted, l(lfo1d diluted, and lOOfold diluted control sera samples (serum I) were employed here as the serum sample. The results show that the found values for Nat concentration in both of the samples are in good agreement with the Na' calibration plots. Little potential shift was observed from the calibration even in the undiluted serum samples. Sodium assay in two undiluted serum samples containing 145 and 124 mmol dm-3 Nat (serums I and II) was also carried out with silicone rubber membrane Na+selective electrodes based on 1by a standard addition method. Five-time analysis for each of the serum samples afforded a NaT concentration average value of 146.6 mmol dm-3 with only 0.4% relative standard deviation for serum I and a value of 125.7 mmol dm-3 with only 0.8%of the standard deviation for serum 11. These results again support the reliability of silicone rubber membrane Na+-selectiveelectrodes based on 1 in the serum sodium assay. The Na+-selectiveelectrodes also allowed good results in the sodium assay of human urine by a calibration plot method, as demonstrated in Figure 6. In the serum sodium assay by the electrode with a DOS/WC/l membrane, on the other hand, a significant negative potential shift was found in the high-Na+ activity range. Also, in the silicone rubber membrane Na+selective electrodes based on 2 and 3,serious potential drift was
observed in the sodium assay, mainly due to their high membrane resistivities. Thus, the silicone rubber membrane Na+-selective electrodes based on 1 are quite reliable in a sodium assay in human body fluids. In conclusion, highly dispersible calixl4larene neutral carrier can afford practically applicable silicone rubber membrane Na+selective electrodes that have several advantages over their corresponding plasticized W C membrane electrodes due to the effective cation-exchanging ability and relatively low electrical resistivity of the resulting ion-sensing membranes. To the best of our knowledge, the present Na+ electrode is the first example for neutral carrier-type silicone rubber membrane Na+-selective electrodes that are applicable to sodium assays in human body fluids. Enhancement in dispersibility of neutral carriers by molecular design would allow various silicone rubber membrane ion-selective electrodes to be of great practical use.
2404 Analytical Cbemjsty, Vol. 67,No. 14, July 15, 1995
ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, and Culture. Y.T. also acknowledges financial support by a Grant-in-Aid for Encouragement of Young Scientists. Received for review November 28, 1994. Accepted April 19, 1995.@ AC9411561 @
Abstract published in Adounce ACS Abstructs, June 1, 1995.
