Method of Achieving Desired Potentiometric Responses of

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Anal. Chem. 2008, 80, 3921–3924

Method of Achieving Desired Potentiometric Responses of Polyacrylate-Based Ion-Selective Membranes Agata Michalska,* Krystyna Pyrzyn ´ ska, and Krzysztof Maksymiuk Department of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland We introduce a simple procedure allowing preparation of cation-selective electrodes with poly(n-butyl acrylate)based membranes containing different proportions of primary and interfering ions introduced already at the membrane preparation step, by using two different liphophilic salts of the same anion. With this approach the time required to achieve saturation of polyacrylate membranes with primary ions can be significantly shortened. Moreover, depending on the ratio of the primary and interfering ions introduced to the membrane cocktail, different potentiometric responses are obtained ranging from typical (with micromolar detection limit), through lower detection limits to super-Nernstian ones. In recent years considerable progress in understanding the mechanisms of ion-selective electrodes responses in low activities has been achieved.1 The requirements for desirable responses are usually known; however, these are often not easy to achieve from a practical, experimental point of view. Ion-selective electrodes incorporating neutral ionophores, regardless of membrane material applied, contain also a liphophilic salt-ion-exchanger. A deviation from this general recipe is sometimes needed for applications requiring membranes of lower resistance; then another liphophilic salt of nontransferable cation and anion is included in the membrane composition.2–4 Nevertheless, for potentiometric applications cation-selective electrodes are nowadays most often prepared using one of the two commercially available tetrakis[3,5-bis(trifluoromethyl)phenyl]borates (sodium or potassium). Alternatively, potassium tetrakis(4-chlorophenyl)borate is included in cation-selective membranes. According to a general recipe, the ion-exchanger of choice is introduced to the membrane in an amount equal to ca. 60 mol. % of applied ionophore.1 For optimal potentiometric responses, freshly prepared membranes require conditioningsequilibration with solutionsleading, among other things, to achievement of adequate saturation with water and uptake of primary cations. The latter becomes an especially important issue if sensors of improved detection limits are to be prepared. Thus, conditioning solutions of relatively high * Corresponding author. E-mail: [email protected]. Fax: +48 22 8225996. (1) Bakker, E.; Pretsch, E. Trends Anal. Chem 2005, 24, 199–207. (2) Jadhav, S.; Meir, A. J.; Bakker, E. Electroanalysis 2000, 12, 1251–1257. (3) Long, R.; Bakker, E. Electroanalysis 2003, 15, 1261–1269. (4) Shvarev, A.; Bakker, E. Anal. Chem. 2003, 75, 4541–4550. 10.1021/ac8000622 CCC: $40.75  2008 American Chemical Society Published on Web 04/02/2008

concentrations of primary ions, e.g. 10-3 M, were in recent years often replaced by more diluted ones or by solutions of constant and low activity of primary ions and high activity of interfering ones, e.g. by ions buffers.5,6 Using these conditioning media results in membranes containing both primary and interfering ions and ultimately in more favorable analytical parameters of sensors.1,5,7 The time required to achieve optimal (for anticipated purposes) contents of primary/ interferent ions in the ion-selective phase of the same thickness and contents of ionophore/ion exchanger, using the same pretreatment media, depends on ions’ diffusion rate in the membrane. Therefore, this time is usually longer for membranes using photopolymerized poly(n-butyl acrylate) matrix compared to those based on poly(vinyl chloride) characterized with ions’ diffusion coefficients close to 10-11 and 10-8 cm2/s, respectively.7,8 On the other hand, once the desired contents of primary/interfering ions in the poly(n-butyl acrylate)-based membrane are achieved, due to the same reason it is more difficult to alter it in the course of the sensor routine application. Slower ion diffusion within the membrane is also beneficial for sensor responsessfor the same ionophore and liphophic salt content, for polyacrylate-based membranes lower detection limits are obtained.9 Achieving proper contents of primary and interfering ions in the membrane can take even a couple of days, being in some cases impractical and difficult to achieve for some applications. Therefore, in this report we propose an alternative approach leading to potentiometric sensors of tailored responses. It is based on introducing primary and interferent cations to the membrane cocktail, and ultimately to the poly(n-butyl acrylate) membrane, using two different liphophilic salts of the same anion, to make up together liphophilic anion contents equal to (traditional) 60 mol % of applied ionophore. As both primary and interfering cations (both being transferable ions) are present in the membrane obtained from such a cocktail at the desired level, the conditioning time usually required for introduction of primary ions to the membrane can be significantly shortened and the procedure simplified. Moreover, the same simple, pretreatment step is (5) Sokalski, T.; Ceresa, A.; Fibbioli, M.; Zwickl, T.; Bakker, E.; Pretsch, E. Anal. Chem. 1999, 71, 1210–1214. (6) Chumbimuni-Torres, K. Y.; Rubinova, N.; Radu, A.; Kubota, L. T.; Bakker, E. Anal. Chem. 2006, 78, 1318–1322. (7) Michalska, A.; Wojciechowski, M.; Wagner, B.; Bulska, E.; Maksymiuk, K. Anal. Chem. 2006, 76, 5584–5589. (8) Heng, L. Y.; Toth, K.; Hall, E. A. H. Talanta 2004, 63, 73–87. (9) Michalska, A. J.; Appaih-Kusi, Ch.; Heng, L. Y.; Walkiewicz, S.; Hall, E. A. H. Anal. Chem. 2004, 76, 2031–2039.

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required to obtain sensors of different characteristics. To our knowledge, this relatively simple approach has not been reported earlier. As model systems potassium and calcium-selective membranes based on photopolymerized poly(n-butyl acrylate) membrane in an all-solid-state arrangement were chosen. For potassium sensor commercially available sodium and potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borates were used. Calcium-selective sensors were obtained using sodium and calcium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate salts; the latter was obtained using standard ion-exchange procedure. EXPERIMENTAL SECTION Apparatus. In the potentiometric experiments a multichannel data acquisition setup and software, Lawson Laboratories, Inc. (3217 Phoenixville Pike, Malvern, PA 19355), was used. The pump systems 700 Dosino and 711 Liquino (Metrohm, Herisau, Switzerland) were used to obtain sequential dilutions of calibrating solution. The double junction silver/silver chloride reference electrode with 1 M lithium acetate in the outer sleeve (Möller Glasbläserei, Zürich, Switzerland) was used. The recorded potential values were corrected for the liquid junction potential calculated according to the Henderson approximation. The mean ion activities were calculated according to Debye– Hückel theory.10 All experiments were performed at ambient temperature (23 °C). Reagents. Calcium-selective ionophore [N,N-dicyclohexylN′,N′-dioctadecyl-3-oxapentanediamide (ETH 5234)] and potassium-selective ionophore – valinomycin, potassium and sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (KTFPB or NaTFPB, respectively)] were from Fluka AG (Buchs, Switzerland), and 1,6-hexanedioldiacrylate (HDDA), 2,2-dimethoxy-2-diphenylacetophenone (DMPP) and n-butyl acrylate were from Aldrich (Germany). Doubly distilled and freshly deionized water (resistance 18.2 MΩ cm, Milli-Qplus, Millipore, Austria) was used throughout this work. All salts used were of analytical grade and were obtained from POCh (Gliwice, Poland). The cartridge with ion-exchange resin Sulfonic 7090-09 for SPE (500 mg, 3 mL) obtained from J.T. Baker was used. Electrodes. Glassy carbon (GC) electrodes of area 0.07 cm2 were used. The substrate electrodes were polished with Al2O3, 0.3 µm and rinsed well in water. Ion-Selective Membranes. Potassium-Selective Membranes. Potassium composite membrane cocktails contained (by weight) 1.0% of tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anions, 2% valinomycin, 0.2% HDDA, 1.4% DMPP and n-butyl acrylate; the ratio ion-exchanger to ionophore was equal to 60 mol % in each case. Membrane cocktails contained tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anions added either as potassium salt or as sodium salt; alternatively compositions of both KTFPB and NaTFPB in weight ratio 1:2 or 2:1 were applied. Calcium-Selective Membranes. Calcium composite membrane cocktails contained (by weight) 1.6% ETH 5234, 0.2% HDDA, 1.4% DMPP, n-butyl acrylate and 1.0% of tetrakis[3,5-bis(trifluorometh(10) Meier, P. C. Anal. Chim. Acta 1982, 136, 363–368.

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yl)phenyl]borate anions, applied either as sodium salt or as calcium salt, alternatively both sodium and calcium salts of tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anions (with sodium to calcium salt weight ratio 3:1). The ratio ion-exchanger to ionophore was equal to 60 mol % in each case. Calcium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate was prepared using silica cation-exchange resin with a typical ionexchange procedure. The as obtained cation-exchange resin was equilibrated with 0.1 M Ca(NO3)2 for 12 h, well washed with water, and then washed with a water-tetrahydrofuran (THF) mixture (1:1 by volume). NaTFPB was introduced onto a column after dissolving in a water-THF mixture (1:1 by volume); the filtrate collected was dried and recrystallized from a water-THF mixture (1:1 by volume) and then from THF. The yield of the procedure was equal to 76% (w/w). Although quantitative analysis of a water-THF solution of the obtained product was not the aim, a qualitative test using the ICP-MS method and apparatus described earlier7 confirmed lack of sodium and the presence of calcium in the sample. All-Solid-State Sensors. As a solid contact, poly(3,4-dioctyloxythiophene) (PDOT) was used.11 PDOT as obtained from synthesis, i.e. in neutral, semiconducting, ion-free form, was dissolved in chloroform to achieve a concentration of 2 mg/mL. 20 µL of PDOT solution was applied on the surface of a GC electrode, placed in an upside-down position and left to dry in a laboratory atmosphere, yielding a PDOT layer. 10 µL of the membrane cocktail was applied on the top of a GC electrode previously coated with PDOT, placed in upside-down position. Photopolymerization was carried out using a UV lamp (360 nm) for 5 min under argon. The obtained sensors were conditioned before measurements for 3 h (potassium sensors) or 5 h (calcium sensors) in 10-3 M chloride solutions of primary ions. Between measurements electrodes were stored dry. RESULTS AND DISCUSSION Theoretical considerations (e.g., refs 12 and 13) point out that, in ion transport within the membrane phase of potentiometric sensors characterized with improved detection limits, both primary and interfering ions take part. Also results of elemental analysis of ion-selective membranes show that both primary and interfering ions are present in the membrane phase.7,14,15. Insufficient presence of primary cations in the membrane prevails as so-called super-Nernstian behavior (e.g., ref 5); on the other hand, the membrane equilibrated well with primary ions (characterized with micromolar detection limts) is virtually free from interfering ions.7 The membranes of sensors characterized with low detection limits naturally fall between the two above cases. The usual approach to achieve optimal saturation of an ionselective membrane is to apply conditioning. Poly(vinyl chloride)(11) Michalska, A.; Skompska, M.; Mieczkowski, J.; Zagórska, M.; Maksymiuk, K. Electroanalysis 2006, 18, 763–771. (12) Sokalski, T.; Zwickl, T.; Bakker, E.; Pretsch, E. Anal. Chem. 1999, 71 (763), 1204–1209. (13) Sokalski, T.; Lingenfelter, P.; Lewenstam, A. J. Phys. Chem. B 2003, 107, 2443–2452. (14) Michalska, A.; Wojciechowski, M.; Bulska, E.; Maksymiuk, K. Electrochem. Commun. 2008, 10, 61–65. (15) Rzewuska, A.; Wojciechowski, M.; Bulska, E.; Hall, E. A. H.; Maksymiuk, K.; Michalska, A. Anal. Chem. 2008, 80, 321–327.

Figure 1. Open circuit potentiometric responses of potassiumselective electrodes prepared using KTFPB and/or NaTFPB. Symbols denote membranes containing tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anions introduced in the form of (b) NaTFPB, mixture of both potassium and sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate salts in w/w ratio (4) 1:2 or (O) 2:1 or (3) KTFPB only. For easy comparison all curves were shifted to give equal potential at log a K+ ) -4.

Figure 2. Open circuit potentiometric responses of calcium-selective electrodes prepared using as liphophilic salt in the membrane tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anions introduced in the form of (4) NaTFPB, (O) calcium salt of tetrakis[3,5-bis(trifluoromethyl)phenyl]borate or (b) mixture of sodium and calcium (in w/w ratio 3:1) salts of tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anions. For easy comparison all curves were shifted to give equal potential at log a Ca2+ ) -3.

based membranes are typically characterized with ion diffusion coefficients, D, on the order of 10-8 cm2/s.7 Assuming that there is no depletion of the sample boundary region due to uptake of primary ions to the membrane and assuming typical membrane thickness, L, in the range of 150 µm, the time, t, required to achieve full membrane saturation can be estimated. From a simple relation t ) L2/D, the calculated time is in the range of a few hours. However, for the same conditions in the case of poly(nbutyl acrylate)-based membranes, characterized with diffusion coefficient on the order of 10-11 cm2/s,8 the time required to achieve full saturation of ion-selective membrane is in the range of a few months. This coarse estimation is indicative of severe differences between pretreatment protocol requirements for different membrane materials. Moreover, a few months long conditioning time required for full saturation of polyacrylate-based membranes is unacceptable from the industrial sensor production point of view, pointing to the need for simpler (quicker) procedures. A remedy for the above problem can be usage of liphophilic salt of primary cation in the course of membrane preparation. Additional benefits arise from the possibility of tailoring of potentiometric response patterns according to needs that can be achieved when different liphophilic salts, one of primary ion and another one of interfering ion, are used to prepare the membrane. Moreover, due to low value of ion diffusion coefficients within the membrane, achieved ion contents are expected to be relatively stable in the course of sensor lifetime. Figure 1 presents responses recorded for potassium-selective sensors obtained using different proportions of KTFPB to NaTFPB in the membrane. All E vs log a dependencies were linear within the KCl activities range from 10-3 to 10-6 M with the slope close to 54 mV/dec. For activities lower than 10-6 M, despite the same pretreatment applied, different responses were obtained. The electrode containing only potassium salt (KTFPB) in the membrane cocktail was characterized with a detection limit equal to 10-7.2 M. Despite relatively short conditioning (3 h), potentiometric responses with typical detection limits were obtained.9 Introducing sodium ions to the membrane composition has resulted in a slightly extended linear response range. The

detection limits obtained for the sensors containing both KTFPB and NaTFPB in a weight ratio 2:1 and in a ratio 1:2 were equal to 10-7.7 M and 10-8.3 M, respectively. However, as expected for the electrode with membrane containing only NaTFPB, the potential change corresponding to KCl activities shift from 10-6 to 10-7 was close to 100 mV, pointing to super-Nernstian behavior. This indicates insufficient membrane saturation with potassium ions during the pretreatment step (although this time was sufficient for other potassium sensors tested in parallel). The above-described results support the thesis that introducing one liphophilic salt or mixture of tetrakis[3,5-bis(trifluoromethyl)phenyl]borate salts to the membrane composition (keeping the usual ratio ion-exchanger anions to ionophore equal to 60 mol %) allows control of responses of potassium sensors within low activity ranges. The “within day” stability of potentials recorded for tested K-ISEs characterized with a low detection limit was on the order of ±2 mV for higher activities and ±10 mV for lower activities tested. The above-described concept was studied further with Caselective electrodes, with in-house prepared calcium liphophilic salt. Figure 2 presents responses recorded for calcium-selective electrodes with different salts of tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anions present in the membrane cocktail, all pretreated in the same way. The electrode with the membrane containing (only) NaTFPB was, as expected, characterized with linear Nernstian responses within a CaCl2 activity range from 10-2 to 10-5 M, followed by a super-Nernstian response range for lower activities. On the other hand, for the sensor with membrane containing only calcium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, despite short conditioning applied, linear responses (slope 26.1 ± 0.5 mV/dec, R2 ) 0.999) were obtained for the above given activity range, and the detection limit was equal to 10-5.6 M. The characteristic recorded shows clearly that introduction of calcium ions to the membrane, achieved in the course of membrane preparation, instead of months long conditioning, yields a response Analytical Chemistry, Vol. 80, No. 10, May 15, 2008

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typical for classical ion-selective electrode with membrane saturated with primary ions. For the electrode with membrane containing a mixture of sodium and calcium salts of tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anions linear responses with slope equal to 24.1 ± 0.5 mV/dec, R2 ) 0.997 were obtained for activities ranging from 10-2 to 10-10 M CaCl2, showing that mixing ion-exchangers of different cations in the membrane cocktail allows simple tailoring of the detection limit of ion-selective electrodes. The logarithms of selectivity coefficients obtained for this sensor (separate solution method, mean value for activity ranges from 10-2 to 10-4 using experimental slope; in parentheses are given values obtained in a parallel experiment for the sensor containing only NaTFPB in the membrane phase) were equal to -8.1 ± 0.1 (-9.2 ± 0.1) for Mg2+, -4.2 ± 0.4 (-5.7 ± 0.3) for Na+, -4.6 ± 0.4 (-5.7 ± 0.4) for K+ and -2.9 ± 0.2 (-3.8 ± 0.3) for H+. As expected, in all cases logarithms of selectivity coefficients obtained for the sensor with the membrane containing both sodium and calcium salts of tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anions were comparable, within the limit of experimental error, with values reported earlier for poly(vinyl chloride)-based membrane electrode characterized with low detection limit.5 Similarly, the values obtained for the sensor with super-Nernstian E vs log a dependence type (containing only NaTFPB in the membrane cocktail) were comparable within the limit of experimental error with those reported earlier for a poly(vinyl chloride)-based counterpart.5 The stability of responses of a Ca-selective sensor of membrane prepared using a mixture of sodium and calcium tetrakis[3,5bis(trifluoromethyl)phenyl]borates was tested by repeating calibration within the activity range from 10-3 to 10-9 M CaCl2 (ten times) over 10 days; the SD of potential values recorded was irrespectively of solution activity close to ±9 mV, and the variation of recorded potentials was random, i.e., no systematic change of recorded potentials was observed.

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Similarly as for other potentiometric sensors working in broad activity ranges, both response time and potential stability at different concentrations were changing while decreasing sample activity.5 For Ca-selective sensor with the membrane prepared using a mixture of sodium and calcium tetrakis[3,5-bis(trifluoromethyl)phenyl]borates in 10-3 M CaCl2 solution short-time (i.e., during measurement) potential stability was in the range of 0.1 mV/min, whereas for 10-7 M CaCl2 it was close to 1.5 mV/min. Response time in given activities changes from immediate at 10-3 M CaCl2 to about 20 s for 10-7 M. CONCLUSIONS The alternative for long-term conditioning required to achieve desired saturation of polyacrylate-based ion-selective membranes, by introducing both primary and interfering cations to the membrane already at cocktail preparation step, was shown. As a result, different potentiometric response patterns, depending on the proportion of two cations, were obtained. On the other hand, if only primary cations are introduced to the poly(n-butyl acrylate) ion-selective membrane phase, the conditioning time required to full saturation of the membrane can be shortened from months to a few hours. ACKNOWLEDGMENT Dr. Marcin Wojciechowski’s help in the ICP-MS test of prepared calcium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate is kindly acknowledged. The authors are grateful to Professor M. Zagórska and Dr. M. Skompska for samples of poly(3,4-dioctyloxythiophene) and to Professor J. Mieczkowski for the synthesis of 3,4-dioctyloxythiophene. Received for review January 10, 2008. Accepted March 4, 2008. AC8000622