Anal. Chem. 1997, 69, 1919-1924
Donnan Exclusion Failure of Neutral Ionophore-Based Ion-Selective Electrodes Studied by Optical Second-Harmonic Generation Setsuko Yajima,† Koji Tohda, Philippe Bu 1 hlmann, and Yoshio Umezawa*
Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
Optical second harmonic generation (SHG) at the liquidliquid interfaces of ionophore-free and neutral ionophoreincorporated membranes has been measured in the presence and absence of ionic sites in order to better understand the influence of ionic sites on the potentiometric responses of liquid membrane ion-selective electrodes. Quick and lasting EMF and SHG responses were observed upon increasing the primary ion activity in aqueous solutions in contact with membranes with ionic sites. For membranes without ionic sites, transient EMF responses due to extraction of primary ion salts occurred. They were accompanied by very similar transient changes in the SHG intensity. No EMF and SHG responses to the primary ion activity were, however, observed for site-free membranes in equilibrium with the aqueous phase. The extent of charge separation at the interfaces of such membranes, as observed by SHG from primary ions or primary ion complexes, does not depend on the primary ion activity in the aqueous phase. This explains why membranes free of ionic sites do not exhibit EMF responses and confirms that ionic sites are a necessity for counterion-independent primary ion responses of potentiometric sensors based on neutral carriers. The response of ion-selective electrodes (ISEs) based on liquid or solvent polymeric membranes containing an ionophore or ion carrier (i.e., a lipophilic complexing agent that reversibly binds ions) strongly depends on the amount of ionic sites trapped in the membrane phase. Deliberately added ionic sites are nowadays routinely used to reduce the interference of lipophilic counterions (for cation-selective electrodes, “anionic effect”)1 and the electric membrane resistance, control the potentiometric selectivity, and shorten response times.2-8 A smaller concentration of ionic sites regularly originates from impurities of membrane components, such as the membrane plasticizer or the polymer matrix (e.g., poly(vinyl chloride) (PVC)) or from membrane supports.9-13 The † Present address: Department of Chemistry, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan. (1) Boles, J. H.; Buck, R. P. Anal. Chem. 1973, 45, 2057-2062. (2) Morf, W. E.; Kahr, G.; Simon, W. Anal. Lett. 1974, 7, 9-22. (3) Morf, W. E.; Simon, W. Helv. Chim. Acta 1986, 69, 1120-1131. (4) Morf, W. E. The Principles of Ion-Selective Electrodes and of Membrane Transport; Elsevier: New York, 1981. (5) Eugster, R.; Gehrig, P. M.; Morf, W. E.; Spichiger, U. E.; Simon, W. Anal. Chem. 1991, 63, 2285-2289. (6) Schaller, U.; Bakker, E.; Spichiger, U. E.; Pretsch, E. Anal. Chem. 1994, 66, 391-398. (7) Bakker, E.; Na¨gele, M.; Schaller, U.; Pretsch, E. Electroanalysis 1995, 7, 817-822. (8) Bakker, E. Electroanalysis 1997, 9, 1.
S0003-2700(96)01315-7 CCC: $14.00
© 1997 American Chemical Society
presence of such ionic sites, whether introduced deliberately or not, has been commonly used to explain the permselectivity of neutral carrier ISE membranes, which is a prerequisite for primary ion-selective EMF responses. Only very little is, however, known on the properties of membranes free of ionic sites because with very few exceptions all investigators in the field have so far used membranes containing at least ionic impurities. A better understanding of the potentiometric properties of membranes free of ionic sites seemed however necessary to fully assess the significance of ionic sites for ISEs and ion-selective field effect transistors (ISFETs). Only recently, we have shown experimentally for a selection of neutral ionophores and carefully purified, typical PVC plasticizers that in absence of ionic sites Nernstian EMF responses could not be obtained.14 For plasticizers of low polarity, no EMF responses were observed at all. Transient responses due to salt extraction even with the highly hydrophilic counterion chloride were observed in the case of the more polar nitrobenzene. Lasting primary ion-dependent charge separation at the liquid-liquid interfaces of ISEs, resulting in a stable EMF response, seemed therefore only possible in the presence of ionic sites confined to the membrane phase. Because membranes free of impurity sites have a very large bulk resistance, we worked with field effect transistors directly covered with polymer-free liquid membranes.14,15 The quick response of such membranes to tetraphenylphosphonium chloride and subsequent establishment of a steady potential as well as solvent extraction experiments confirmed the suitability of our experimental approach. To further corroborate these potentiometric results, we used optical second harmonic generation (SHG), which is the conversion of two photons of frequency ω to a single photon of frequency 2ω. The particular aptness of optical SHG for the characterization of liquid-liquid interfaces is based on the fact that it only occurs in noncentrosymmetric media and therefore discriminates very selectively between surface species and species in the adjacent bulk media.16-18 We have previously shown the first application of optical SHG for studying the surface of ionophore-incorporated (9) Perry, M.; Lo ¨bel, E.; Bloch, R. J. Membr. Sci. 1976, 1, 223-235. (10) Horvai, G.; Graf, E.; Toth, K.; Pungor, E.; Buck, R. P. Anal. Chem. 1986, 58, 2735-2740. (11) van den Berg, A.; van der Wal, P. D.; Skowronska-Ptasinka, M.; Sudho¨lter, E. J. R.; Reinhoudt, D. N.; Bergveld, P. Anal. Chem. 1987, 59, 2827-2829. (12) Iglehart, M. L.; Buck, R. P.; Horvai, G.; Pungor, E. Anal. Chem. 1988, 60, 1018-1022. (13) Na¨gele, M.; Pretsch, E. Mikrochim. Acta 1995, 121, 269-279. (14) Bu ¨ hlmann, P.; Yajima, S.; Tohda, K.; Umezawa, Y. Electrochim. Acta 1995, 40, 3021-3027. (15) Masadome, T.; Wakida, S.; Kawabata, Y.; Imato, T.; Ishibashi, N. Anal. Sci. 1992, 8, 89-91.
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PVC liquid membranes.19,20 It was found that, just as the EMF, the observed SHG intensity also increased with increasing primary ion concentration, showing that the number of SHG-active species was quantitatively correlated to the charge separation and, therefore, the potential drop across the interface between membrane and aqueous sample solution. To determine the influence of ionic sites on the charge separation at the membrane interface, we have measured in this study SHG with ionophore-free and ionophore-incorporated liquid membranes in the absence and presence of ionic sites. The dependence of the SHG intensity on the activity of the primary ion in the aqueous solution is presented and compared to the corresponding EMF. EXPERIMENTAL SECTIONS Reagents. KSCN, NaSCN, KCl, LiOAc (Wako Pure Chemical Co., Osaka, Japan), potassium tetrakis(p-chlorophenyl) borate (KTpClPB; Dojindo Laboratories, Kumamoto, Japan), and bis[(benzo15-crown-5)-4-methyl] pimelate (bis(benzo-15-crown-5); potassium ionophore, Dojindo Laboratories) were all of analytical grade and used without further purification. Tridodecylmethylammonium thiocyanate (TDDMA-SCN) was prepared from tridodecylmethylammonium chloride (TDDMA-Cl; purity 98%, Aldrich Chemical Co., Milwaukee, WI) as follows: 100 mg of TDDMA-Cl was dissolved in 40 mL of benzene (analytical reagent grade, Wako Pure Chemicals) and shaken three times with 50 mL of aqueous 0.1 M NaSCN, leading to the exchange of chloride for thiocyanate ions. The organic phase was then washed three times with water and the solvent evaporated. The chloride concentration in the organic phase after the ion exchange was not distinguishable from the background level, as determined by ion chromatography. The organic solvents used in the present study were selected based not only on their polarity but also on their density, because the experimental setup for the SHG measurements allowed changing of the primary ion activity in the sample solution more easily when the organic solvent had a higher density than the aqueous solution. Nitrobenzene (analytical reagent grade, Wako Pure Chemicals) was used without further purification, and 1,2dichloroethane (spectroscopy grade, Nacalai Tesque Inc., Kyoto, Japan) was passed twice through a column of alumina (activated, 200 mesh, column chromatography grade, Wako Pure Chemicals). Preparation of Sample Solutions. Organic phases with ionic sites were obtained by dissolving the ion exchanger, TDDMASCN, in the organic solvents in a ratio of 1 wt %. Organic phases with neutral ionophore contained 1 wt % bis(15-crown-5). If appropriate, K-TpClPB in a mole ratio of 50% to the ionophore was added. Deionized and charcoal-treated water (specific resistance >18.2 MΩ cm) was prepared by a Milli-Q Type I reagent grade water system (Millipore Corp., Bedford, MA). The aqueous and organic phases used for the SHG measurements were equilibrated before use by shaking in a separatory funnel. EMF Measurements. Measurements of the EMF were carried out with cells of the following type:15 (16) Grubb, S. G.; Kim, M. W.; Rasing, T.; Shen, Y. R. Langmuir 1988, 4, 452454. (17) Higgins, D. A.; Corn, R. M. J. Phys. Chem. 1993, 97, 489-493. (18) Corn, R. M.; Higgins, D. A. Chem. Rev. 1994, 94, 107-125. (19) Yoshiyagawa, S.; Tohda, K.; Umezawa, Y.; Hashimoto, S.; Kawasaki, M. Anal. Sci. 1993, 9, 715-718. (20) Tohda, K.; Umezawa, Y.; Yoshiyagawa, S.; Hashimoto, S.; Kawasaki, M. Anal. Chem. 1995, 67, 570-577.
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Ag | AgCl | 3 M KCl || 1 M LiOCOCH3 || sample solution | liquid membrane | FET A Si3N4-based field effect transistor (FET; Shindengen Co. Ltd., Saitama, Japan) was inserted into a tapered glass capillary (inner diameter at the tip ∼0.4 mm) whose surface had been modified with octadecyltrichlorosilane. The capillary was then filled with the liquid membrane (height of ∼1 cm), the capillary force preventing the membrane from flowing out at the bottom. A double-junction type Ag/AgCl electrode (Denki Kagaku Keiki Co., Tokyo, Japan), containing a 3 M KCl solution in the inner and a 1 M LiOCOCH3 solution in the outer compartment, was used as reference electrode. The EMF was measured with an ISFET/ mV meter (Shindengen) at room temperature. The source-drain voltage and current were kept at 4.0 V and 150 µA, respectively, and the electrochemical cell was shielded by a Faraday cage. The liquid membranes were immersed in 50 mL of water for at least 12 h, resulting in EMF drifts of ∼2 mV/h. Small increments of a highly concentrated primary ion solution (10-2, 10-1, 100, 3 × 100 M) was then added to the sample solution, giving 10-5-100 M primary ion solutions. No electrolytes other than the primary ion salt were added. The sample solutions were stirred throughout the experiments by use of a Teflon-coated magnetic stirring bar. The measured EMF values were corrected for changes in the liquid-junction potential by applying the Henderson equation,21 and concentrations in the aqueous solution were converted to activities by use of activity coefficients. SHG Measurements. The SHG measurements were made with the s-polarized 1064 nm output of a Q-switched Nd:YAG laser system (Quanta-Ray, Model DCR130, Spectra-Physics, Mountain View, CA) with a 10 ns pulse width and 10 Hz repetition rate. The laser light was focused by a quartz lens (focal length, 10 cm) and passed an infrared transmittance filter (85IR, Sigma Koki, Saitama, Japan). The beam was first reflected by a mirror and then totally reflected by the liquid-liquid interface. The SHG measurement cell was made of Pyrex glass and was roundbottomed to allow irradiation from various incident angles. The typical energy density of the incident laser light was 50 mJ/cm2, as measured by an optical power meter (Model 407, Scientec, Boulder, CO). The second harmonic was detected with a photomultiplier (H1161, Hamamatsu Photonics, Hamamatsu, Japan) after passing an interference filter (532 nm, 10 nm of fwhm). The output of the SHG signal was averaged with a gated integrator (SR250, Stanford Research System, Sunnyvale, CA). All experiments were carried out at room temperature. Changes in the primary ion activity in the sample solution were made by adding without stirring small increments of a concentrated primary ion solution (10-2, 10-1, 100, or 3 × 100 M) to the sample solution, giving 10-4-100 M primary ion solutions for the ion-exchanger system and 10-5-100 M solutions for the neutral ionophore system. No electrolytes other than the primary ion salt were added. RESULTS AND DISCUSSION Membranes without Ionophore. Ion-exchanger electrodes based on a salt of a primary ion and a lipophilic counterion, as for (21) Meier, P. C.; Ammann, D.; Morf, W. E.; Simon, W. In Medical and Biomedical Applications of Electrochemical Devices; Koryta, J., Ed.; Wiley: New York, 1980; pp 13. (22) Umezawa, Y. Handbook of Ion-Selective Electrodes: Selectivity Coefficients; CRC Press: Boca Raton, FL, 1990.
Figure 1. KSCN activity dependence of the EMF response of 1,2dichloroethane containing (2) no or (O) 1 wt % TDDMA-SCN and (b) nitrobenzene membranes containing no TDDMA-SCN.
example realized in many electrodes developed for drugs and fairly lipophilic inorganic anions,22 present the simplest type of ISEs, the corresponding membranes containing no ionophore and specific interactions between primary ion and their counterions being absent.4,23-26 In the following, we compare the EMF and SHG responses of such ISEs to responses of electrodes with membranes free of ionic sites. As in our previous potentiometric investigation with typical membrane plasticizers,14 we decided to work with membranes of low and high polarity. Due to further experimental considerations (cf. Experimental Section), our choice of the membrane solvents fell on 1,2-dichloroethane (dielectric constant, ) 10.4) and nitrobenzene ( ) 34.8). As the ion exchanger, TDDMA-SCN was used. (a) 1,2-Dichloroethane Membranes with Ionic Sites. We have previously shown20 that changes in the charge separation across the phase boundary of ionophore-based ion-selective electrode membranes are accompanied by changes in the SHG intensity. The same is expected for ion exchanger electrodes too. Figure 1 (O) shows the EMF responses of a 1,2-dichloroethane membrane containing tridodecylmethylammonium ions as ionic sites. A Nernstian response is obtained in the activity range between 10-4 and 10-1 M KSCN. The corresponding SHG intensities decreased steadily with the KSCN activity in the aqueous solution, attained a minimum at ∼10-2 M KSCN and then strongly increased again (Figure 2A, O). To interpret the SHG response, it is necessary to know that the square root of the SHG signal intensity, (I(2ω))1/2, is proportional to the surface concentration of SHG-active species, N, the molecular second-order nonlinear electric polarizability, R(2), and the coordinate transformation, 〈T〉, connecting the laboratory and molecular axes, where the broken brackets denote an average over all molecular orientations. Because K+ ions, the water molecules, and the solvent molecules (1,2-dichloroethane) are not significantly SHG-active at 1064 nm and the same is also expected for the tridodecylmethylammonium cations, the only species with SHG activity at 1064 nm is oriented SCN-.17,20 The minimum in (23) Buck, R. P.; Stover, F. S.; Mathis, D. E. J. Electroanal. Chem. 1979, 100, 63-70. (24) Vanysek, P.; Buck, R. P. J. Electroanal. Chem. 1991, 297, 19-35. (25) Melroy, O. R.; Buck, R. P. J. Electroanal. Chem. 1983, 143, 23-36. (26) Girault, H. H. J.; Schiffrin, D. J. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15; pp 1-141.
Figure 2. KSCN activity dependence of the SHG response of (A) 1,2-dichloroethane and (B) nitrobenzene membranes containing (b) no or (O) 1 wt % TDDMA-SCN.
the plot of the SHG intensity vs KSCN activity (Figure 2) can, therefore, be explained by the disappearance of oriented SHGactive SCN- anions at the interface at this KSCN activity. For membranes with an ion exchanger,25 a surface excess of positive charges on the organic side of the membrane interface is expected at low SCN- activities, where SCN- ions are transferred from the organic into the aqueous phase. Due to the requirement of electroneutrality, an equal surface excess of negative charges must occur on the aqueous side of the interface. The SHG-active species providing excess negative surface charge on the aqueous side must be SCN- because the aqueous solution contains no electrolyte other than KSCN. When the activity of KSCN in the aqueous phase is increased, fewer SCN- ions leave the membrane, leading finally to the disappearance of the charge separation of TDDMA+ and oriented SHG-active SCN- across the liquid-liquid interface. Upon further increasing the KSCN activity, more SCNions enter the organic phase. This results in a surface excess of negative charges on the organic side of the phase boundary. An analogous effect has been used to explain maxima in the interfacial tension at very similar liquid-liquid interfaces.26 (b) 1,2-Dichloroethane Membranes Free of Ionic Sites. In contrast to membranes with TDDMA-SCN, no significant equilibrium EMF and SHG responses to KSCN were observed with 1,2-dichloroethane membranes containing no ion exchanger (Figure 1, 2, and Figure 2A, b). This suggests that, at equilibrium, the extent of charge separation of K+ and oriented SHGactive SCN- across the membrane interface is very low and does not depend on the concentration of KSCN in the aqueous phase. This agrees with the theoretical expectation that partitioning of a single salt between two immiscible solvents gives a potential drop Analytical Chemistry, Vol. 69, No. 10, May 15, 1997
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Figure 5. Time course of SHG responses of nitrobenzene membranes (b) without and (O) with 1 wt % TDDMA-SCN to a concentration step from 0 to 10-2 M KSCN. Figure 3. Time course of the (O) EMF and (b) SHG responses of 1,2-dichloroethane containing no TDDMA-SCN upon a concentration step from 0 to 10-2 M KSCN.
Figure 4. Time course of the EMF response to a concentration step from 0 to 10-2 M KSCN of nitrobenzene membranes containing (b) no or (O) 1 wt % TDDMA-SCN.
across the interface of the two solvents that does not depend on the salt activity.24,26 The EMF response to KSCN of a water-saturated 1,2-dichloroethane membrane containing no ion exchanger was, however, time-dependent (Figure 3, O), suggesting a transient charge separation due to extraction of KSCN into the organic solvent. A very small transient decrease of the EMF (8 mV) upon addition of KSCN to electrolyte-free water to give a 0.01 M solution was observed. The measured potential passed through a minimum after ∼15 min and then increased again for several hours, with a derivative considerably larger than the background drift of the FET. Also, the intensity of the second harmonic as generated at the 1,2-dichloroethane/water interface in a different measurement cell but under otherwise identical conditions was transient (Figure 3, b). It increased immediately upon addition of KSCN to the aqueous phase but then decreased to background level within 40 min. In contrast, very different EMF responses were obtained for the membranes with ionic sites: a strong and fast anionic response was observed. It was followed by a drift that is not distinguishable from the background drift of the FET. The SHG response was also very quick, and no substantial changes in the intensity were observed. The transient SHG and EMF responses 1922 Analytical Chemistry, Vol. 69, No. 10, May 15, 1997
for membranes without ionic sites thus confirm the occurrence of time-dependent charge separation across the phase boundary due to transfer of K+ as well as SCN- into the organic phase. (c) Nitrobenzene Membranes with and without Ionic Sites. When we previously investigated the potentiometric properties of carefully purified plasticizers of low polarity, no EMF responses were observed, whereas for a more polar solvent (nitrobenzene), transient EMF responses were obtained.14,27 Because of this large difference, we were also interested in the combined SHG and EMF responses of more polar ion-exchange membranes. As previously, we used nitrobenzene for this purpose. When allowing for complete equilibration with aqueous KSCN, no KSCN dependence of the EMF was obtained for nitrobenzene membranes without ionic sites (Figure 1, b). Again, this suggests that the extent of charge separation across the interface of membranes without ionic sites does not change with the KSCN activity. This interpretation is confirmed by the intensity of the second harmonic, which showed no dependence on the KSCN activity upon equilibration of the two liquid phases (Figure 2B, b). On the other hand, incorporation of TDDMA-SCN into nitrobenzene led to an SHG response (Figure 2B, O) closely resembling that for TDDMA-SCN containing 1,2-dichloroethane membranes (Figure 2A), showing once more the correspondence between the EMF and the SHG response. As in the 1,2-dichloroethane case, transient EMF and SHG responses to KSCN were observed for the nitrobenzene membranes without ionic sites. This suggests that here too not only SCN- but also K+ ions are transferred into the nitrobenzene phase (Figure 4 and Figure 5). Salt extraction into the bulk of the organic phase, in analogy to similar observations previously reported for neutral ionophore-incorporated liquid membranes without ionic sites,14 was indeed independently confirmed by atomic absorption spectrometry. Figure 6 shows the concentration of K+ in nitrobenzene equilibrated at room temperature with a 10-2 M aqueous solution of KSCN as a function of equilibration time. The presence of the ion exchanger TDDMA-SCN efficiently suppresses KSCN extraction into the organic phase, but in its absence, a substantial amount of KSCN enters the nitrobenzene phase. The trends of the EMF and the SHG responses are therefore very similar in spite of the different polarities of the plasticizers. Ionophore-Incorporated Membranes. We previously found that the SHG responses to primary cations of ISEs based on (27) Bu ¨ hlmann, P.; Yajima, S.; Tohda, K.; Umezawa, K.; Nishizawa, S.; Umezawa, Y. Electroanalysis 1995, 7, 811-816.
Figure 6. Time course of the potassium concentration in nitrobenzene containing (b) no or (O) 1 wt % TDDMA-SCN upon equilibration with 10-2 M KSCN, as determined by AAS.
Figure 8. KCl activity dependence of the (A) EMF and (B) SHG responses of nitrobenzene membranes containing 1 wt % bis(benzo15-crown-5) and (b) no ionic sites or (O) K-TpClPB in a mole ratio of 50% to the ionophore.
Figure 7. KCl activity dependence of the (A) EMF and (B) SHG responses of 1,2-dichloroethane membranes containing 1 wt % bis(benzo-15-crown-5) and (b) no ionic sites or (O) K-TpClPB in a mole ratio of 50% to the ionophore.
several crown ether ionophores could be correlated to the number of primary ion complexes at the phase boundary, which contributed to the membrane potential.19,20 We have now incorporated the K+ ionophore bis[(benzo-15-crown-5)-4-methyl] pimelate into 1,2-dichloroethane and nitrobenzene membranes and determined EMF and SHG responses to KCl in the presence and absence of ionic sites. Figure 7A (O) shows the EMF responses of a 1,2-dichloroethane membrane containing anionic sites (K-TpClPB). A Nernstian response was obtained. An SHG response to KCl was observed at activities of the latter above 10-2 M (Figure 7B, O). These results can be interpreted in the same way as for ionophore-
incorporated PVC liquid membranes, for which we have shown19,20 that the concentration of oriented cation complexes at the liquidliquid interface can explain both the observed SHG signal and EMF response. The present SHG responses thus suggest primary ion concentration-dependent charge separation at the interface of the 1,2-dichloroethane membranes incorporated with ionic sites. For 1,2-dichloroethane membranes without anionic sites, on the other hand, negligible EMF responses were obtained (Figure 7A, b). The intensity of the SHG signal did not depend on the KCl activity (Figure 7B, b). This suggests that there is no change in the extent of charge separation across the membrane phase boundary upon increasing the primary ion activity in the case of the membranes without ionic sites. The EMF and SHG responses for nitrobenzene as solvent were qualitatively identical to those for 1,2-dichloroethane (Figure 8B). The only difference between the results for the two solvents was that the SHG response to KCl of nitrobenzene membranes with ionic sites was more sensitive than for 1,2-dichloroethane membranes with ionic sites, where an increase in the SHG intensity was observed at KCl activities above 10-4 M KCl. Use of ionophore-incorporated membranes thus leads to the same conclusions as described above for the ionophore-free membranes. Here too, the SHG measurements suggest that a permanent, primary ion-dependent charge separation at the liquid-liquid interface, and therefore a potentiometric response, is only possible when the membrane contains ionic sites. CONCLUSIONS Salt extraction into membranes with and without neutral ionophore is efficiently suppressed in the presence of ionic sites. Analytical Chemistry, Vol. 69, No. 10, May 15, 1997
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Changes in the primary ion concentration of the aqueous sample solutions result in Nernstian EMF responses. The corresponding changes in the extent of charge separation at the sample/ membrane interface are manifested by changes in the concentration of oriented SHG-active species. For membranes without ionic sites, on the other hand, either transient EMF responses due to extraction of primary ion salts occur or no dependence of the EMF on the primary ion activity is observed. The EMF at equilibrium is not primary ion dependent because the concentration of SHGactive primary ions or primary ion complexes at the interface is very low and independent of the primary ion activity, and therefore, the extent of charge separation at the membrane/ solution interface remains constant. The SHG responses further corroborate that the control of the total concentration of impurity and added ionic sites in the membranes of neutral carrier ISEs and ISFETs cannot only be used to improve the potentiometric performance in many aspects (for example, reduction of counter-
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ion effect and electric membrane resistance, control of potentiometric selectivities),2-8 as is well-known, but that counterionindependent EMF responses of neutral carrier-based ISEs and ISFETs are indeed only possible in the presence of such sites. ACKNOWLEDGMENT The authors are indebted to Prof. Mitsuo Tasumi for lending us the laser equipment. We thank Prof. Masao Sugawara for valuable discussions. This work was supported by grants from the Ministry of Education, Science and Culture, Japan, and the Nissan Science Foundation. Received for review December 31, 1996. Accepted March 7, 1997.X AC961315T X
Abstract published in Advance ACS Abstracts, April 15, 1997.
