Factors Affecting the Potentiometric Response of All-Solid-State

The selectivity of the constructed electrode is better than that of the conventional calcium-selective electrode with internal filling solution of 10-...
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Anal. Chem. 2004, 76, 6410-6418

Factors Affecting the Potentiometric Response of All-Solid-State Solvent Polymeric Membrane Calcium-Selective Electrode for Low-Level Measurements Anna Konopka,† Tomasz Sokalski,‡ Agata Michalska,† Andrzej Lewenstam,‡ and Magdalena Maj-Zurawska*,†

Faculty of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland, and Process Chemistry Centre, c/o Centre for Process Analytical Chemistry and Sensor Technology ‘ProSens’, A° bo Akademi University, Biskopsgatan 8, 20500 Turku, Finland

An all-solid-state calcium-selective electrode was constructed with poly(pyrrole) solid-contact doped with calcium complexing ligand Tiron. The potentiometric response of this sensor can have a linear range down to 10-9 M with a slope close to Nernstian and detection limit equal to 10-9.6. The effects of pH and the activity of the interfering ion in the conditioning solution on the potentiometric behavior of the constructed sensors were examined. Potential stability, reproducibility, and impedance studies were performed. The selectivity of the constructed electrode is better than that of the conventional calcium-selective electrode with internal filling solution of 10-2 M CaCl2 and comparable to that of the best liquid-contact electrodes. Ion-selective electrodes (ISEs) are currently finding a widespread use for direct determination of ions in whole blood, serum, urine, and other biological samples as well as in environmental analysis.1-5 The observed detection limit (DL) usually lies in micromolar range for both conventional electrodes with an internal filling solution (IFS) and the so-called all-solid-state or coatedwire arrangements. It was shown that the presence of primary ion buffers in the sample solution strongly improves the detection limit of ISEs with solid-state and plastic membranes.6-8 This indicated, based upon the steady-state diffusion layer model interpretation, that the detection limit is the property not only of * To whom correspondence should be addressed. E-mail: mmajzur@ chem.uw.edu.pl. † Warsaw University. ‡ A° bo Akademi University. (1) Bakker, E.; Pretsch, E. Anal. Chem. 2002, 72, 420A-426A. (2) Bakker, E.; Pretsch, E. Trends Anal. Chem. 2001, 20, 11-19. (3) Bakker, E. Anal. Chem. 2004, 76, 3285-3298. (4) Bakker, E.; Diamond, D.; Lewenstam, A.; Pretsch, E. Anal. Chim. Acta 1999, 393, 11-18. (5) Lewenstam, A.; Maj-Zurawska, M.; Hulanicki, A. Electroanalysis 1991, 3, 727-734. (6) Hulanicki, A.; Sokalski, T.; Lewenstam, A. Microchim. Acta 1988, III, 119129. (7) Sokalski, T.; Maj-Zurawska, M.; Hulanicki, A. Microchim. Acta I 1991, I, 285-291. (8) Hulanicki, A.; Lewenstam, A. Talanta 1976, 23, 661-665.

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the ion-selective membrane but of the whole system. It was determined that such a high DL is a result of the difference between the primary ion concentrations in the diffusion layer at the outer interface of the electrode and sample solution.9,10 In the case of solvent polymeric membranes, which are of interest in this paper, the diffusion layer is enriched in the primary ions due to ion transport across the membrane, which is caused by the cation and anion coextraction processes from the inner solution, ion exchange of primary ions by interfering ions in the phase boundary layer, or both.9,11-15 For the conventional ISEs with a solvent polymeric membrane and IFS, this problem was solved by using an IFS with a metal buffer and a correspondingly low activity of the primary ion coupled with high activity of the interfering ion. Consequently, the flux of primary ions from the membrane to the sample was decreased, and the detection limit shifted down to the picomolar level.11 In this way, new possibilities for practical analytical applications of ISEs have arisen.1 Model calculations predicting the influence of many parameters on the potentiometric behavior were published.9,10 The effects of the composition of the IFS,14,16,17 the concentration of the ionophore in the membrane, the thickness of the diffusion layer,16,17 the presence of the interfering ions in the sample,17 and the influence of the type of plasticizer used in the membrane18 were examined. A lower detection limit in the range of 10-7-10-12 M has been reached for Pb2+, Ca2+, Cd2+, Ag+, NH4+, K+, ClO4-, and I- as (9) Sokalski, T.; Zwickl T.; Bakker E.; Pretsch E. Anal. Chem. 1999, 71, 12041209. (10) Zwickl, T.; Sokalski, T.; Pretsch, E. Electroanalysis 1999, 11, 673-680. (11) Sokalski, T.; Ceresa, A.; Zwickl, T.; Pretsch, E. J. Am. Chem. Soc. 1997, 119, 11347-11348. (12) Maj-Zurawska, M.; Erne, D.; Ammann, D.; Simon, W. Helv. Chim. Acta 1982, 65, 55-62. (13) Mathison, S.; Bakker, E. Anal. Chem. 1998, 70, 303-309. (14) Sokalski, T.; Ceresa, A.; Fibbioli, M.; Zwickl, T.; Bakker, E.; Prestch, E. Anal. Chem. 1999, 71, 1210-1214. (15) Gyurcsanyi, R., E.; Pergel, E.; Nagy, R.; Kapui, I.; Lan, B., T., T.; Toth, K.; Bitter, I.; Lindner, E. Anal. Chem. 2001, 73, 2104-2111. (16) Sokalski, T.; Bedlechowicz, I.; Maj-Zurawska, M.; Hulanicki, A. Fresenius’ J. Anal. Chem. 2001, 370, 367-370. (17) Ceresa, A.; Sokalski, T.; Pretsch, E. J. Electroanal. Chem. 2001, 501, 7076. (18) Bedlechowicz, I.; Maj-Zurawska, M.; Sokalski, T.; Hulanicki, A. J. Electroanal. Chem. 2002, 537, 111-118. 10.1021/ac0492158 CCC: $27.50

© 2004 American Chemical Society Published on Web 10/01/2004

shown in a recent review.1 Alternatively, another way to improve the detection limit of ISEs is to apply a current to compensate ions fluxes across the ion-selective membrane.19-22 Recently it was shown that it is also possible to improve the detection limit of Ca2+-selective ISFET-based sensors.23 All-solid-state ion-selective electrodes are of special interest in the field of potentiometric sensors because they are free of some limitations imposed by the presense of the IFS such as the need to work in a vertical position or evaporation of the inner solution. They are maintenance-free and easy for miniaturization. In all-solid-state ion-selective electrodes, a conducting polymer can be placed between the electronically conducting substrate and an ionically conducting solvent polymeric membrane.24 Conducting polymers such as poly(pyrrole), poly(thiophene), poly(3,4-ethylenedioxythiophene), and poly(aniline) possess both ionic and electronic conductivity and function well as an ion-to-electron transducting layer.25 This construction improves the potential stability of the sensors compared to that of coated-wire electrodes (CWE),26 where the plastic membrane is placed directly on the electronic substrate and the interfacial potential is not well determined. It was recently reported that the potential drift for CWEs can also be caused by the formation of a thin aqueous layer between the electronic substrate and the poly(vinyl chloride) (PVC) membrane.27 All-solid-state sensors based on conducting polymers demonstrate a potential stability equivalent to that of conventional ion-selective electrodes with an IFS24,28-31 and display comparable linear potentiometric response ranges and selectivity coefficient values.24,28-30 Recent studies point out that it is possible to induce cationic sensitivity of conducting polymer films by doping them with complexing ligands.32 Conducting polymer layers with adsorbed ethylenediaminetetraacetic acid (EDTA) were sufficient to induce a super-Nernstian behavior, although the lifetime was very short.33 In this work, a calcium-selective electrode with a poly(pyrrole) mediating layer doped with the calcium complexing ligand Tiron is examined to determine its suitability as a chemical sensor with a lowered detection limit. (19) Lindner, E.; Gyurcsanyi, R., E.; Buck, R., P. Electroanalysis 1999, 11, 695702. (20) Pergel, E.; Gyurcsanyi, R., E.; Toth, K.; Lindner, E. Anal. Chem. 2001, 73, 4249-4253. (21) Morf, W., E.; Badertscher, M.; Zwickl, T.; de Rooij, N., F.; Pretsch, E. J. Electroanal. Chem. 2002, 526, 19-28. (22) Michalska, A.; Dumanska, A.; Maksymiuk, K. Anal. Chem. 2003, 75, 49644974. (23) Bratov, A.; Abramova, N.; Dominguez, C. Talanta 2004, 62, 91-96. (24) Cadogan, A.; Gao, Z.; Lewenstam, A.; Ivaska, A.; Diamond, D. Anal. Chem. 1992, 64, 2469-2501. (25) Bobacka, J.; Ivaska, A.; Lewenstam, A. Electroanalysis 2003, 15 (5-6), 366374. (26) Catrall, R., W.; Hamilton, I., C. Ion-Selective Electrode Rev. 1984, 6, 125172. (27) Fibbioli, M.; Morf, W., E.; Badertscher, M.; de Rooij, N., F.; Pretsch, E. Electroanalysis 2000, 12, 1286-1292. (28) Hulanicki, A.; Michalska, A. Electroanalysis 1995, 7, 692-693. (29) Michalska, A.; Hulanicki, A.; Lewenstam, A. Analyst 1994, 119, 2417-2420. (30) Michalska, A.; Hulanicki, A.; Lewenstam, A. Microchem. J. 1997, 57, 5964. (31) Bobacka, J. Anal. Chem. 1999, 71, 4932-4937. (32) Migdalski, J.; Blaz, T.; Lewenstam, A. Anal. Chim. Acta 1996, 322, 141149. (33) Michalska, A.; Konopka, A.; Maj-Zurawska, M. Anal. Chem. 2003, 75, 141144.

Figure 1. Deposition curve for PPy(Tiron) layer electrodeposited at a constant potential of +650 mV vs Ag/AgCl/KCl(1 M) reference electrode from an aqueous solution of 0.46 M monomer and 0.5 M Tiron.

EXPERIMENTAL SECTION Reagents. High molecular weight PVC, bis(2-ethylhexyl) sebacate (DOS), calcium ionophore (ETH 1001) [(-)-(R,R)-N,N′bis[11-(ethoxycarbonyl)undecyl]-N,N′-4,5-tetramethyl-3,6-dioxaoctane-diamide], and potassium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (KTFPB) were from Fluka (Buchs, Switzerland). Pyrrole (Sigma-Aldrich, Steinheim, Germany) was doubly distilled and stored under argon atmosphere at temperature of -20 °C, protected from light. 1,2-Dihydroxybenzene-3,5-disulfonic disodium salt (Tiron) was from Sigma-Aldrich. Tetrahydrofuran (THF) (Merck, Darmstadt, Germany) was freshly distilled. All salts were of analytical grade (POCH, Gliwice, Poland). Doubly distilled and freshly deionized water (resistance 18.2 MΩ cm, Milli-Q Plus, Millipore) was used to prepare all solutions. The stock solutions (0.1 M) of metal chlorides were obtained by weighing the appropriate salts and dissolving them in water. Ion-Selective Membranes. The composition of the calciumselective membrane was as follows (m/m), 32.1% PVC, 0.6% KTFPB, 66.0% DOS, and 1.3% ETH 1001; 200 mg of membrane components mixture was dissolved in 2 mL of THF to give the membrane cocktail. An average amount of 25 µL was cast onto glassy carbon coated with the polymer film, which resulted in a membrane thickness of ∼130 µm, as measured by using a micrometercaliper. Ion-Selective Electrodes. The glassy carbon working electrodes (diameter 3 mm) were equipped with a screwed cup with the opening (diameter 3.5 mm) located on the top of the carbon surface. This arragement has prevented the ion-selective membrane from peeling off from the solid-contact phase. The cup was added to conducting polymer film after electrochemical polymerization just before casting of the ion-selective membrane cocktail. Before the electrochemical polymerization of pyrrole, the working electrodes were polished with sand paper (Grid 1000) and rinsed with water. Poly(pyrrole) films doped with Tiron (PPy(Tiron)) were electrodeposited potentiostatically, under constant potential +0.65 V versus Ag/AgCl/KCl(1 M) reference electrode, passing the charge 1.4 C/cm2 from the aqueous solution containing 0.46 M PPy and 0.5 M Tiron. The deposition curve for PPy(Tiron) film is presented in Figure 1. All the obtained PPy films were smooth and have good mechanical properties. Just before membrane casting, the electrodes were rinsed with water and THF. The calcium-selective membrane cocktail was Analytical Chemistry, Vol. 76, No. 21, November 1, 2004

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Table 1. Composition of Conditioning Solutions series

Ca2+ concn (M)

Na+ concn (M)

pHa

A B C

10-5 10-5 10-5

10-1 10-2 10-3

7, 8, 9, 10, 11, 12, 13 7, 8, 9, 10, 11, 12 11

a In every series, conditioning solutions differ in pH values, e.g., A7 means 10-5 M Ca2+, 10-1 M Na+, pH 7.

pipetted into the cup opening on the glassy carbon electrode coated with polymer film, when the electrode was placed in an upside-down position. After overnight evaporation of THF, the obtained all-solid-state calcium-selective electrodes were conditioned for at least 2 days before potentiometric measurements. The compositions of the conditioning solution are presented in Table 1. The conventional calcium-selective electrodes with IFS containing only 10-2 M CaCl2 and calcium-selective electrodes with an IFS containing a low Ca2+ and high Na+ activity were tested in parallel. The composition of the IFS of the latter electrodes was as follows: 10-3 M CaCl2, 0.05 M Na2EDTA, adjusted with 1 M HCl to pH 3.81, resulting in pCa ) 4.2 and pNa ) 1.1. For these electrodes, the aforementioned membrane cocktail was poured into a glass ring (diameter 28 mm) placed on a glass plate and the solvent was allowed to evaporate. Next, circular pieces of the plastic membrane were cut out with a cork borer and affixed inside Philips IS 561 electrode bodies. The conventional Ca ISE was conditioned in 10-2 M CaCl2, and the Ca ISEs with lowered calcium ion activity were conditioned in the same conditioning solutions as the electrodes with PPy(Tiron) mediating layer (Table 1). Calcium-selective coated-wire electrodes were also prepared. Before casting, the glassy carbon electrodes were polished with Al2O3 (0,3 µm) and rinsed with water and THF. The membrane cocktail was directly pipetted onto the glassy carbon. After overnight evaporation of THF, the obtained calcium-selective coated-wire electrodes were conditioned in the same conditioning solutions as desribed for the electrodes with PPy(Tiron) solid contact (Table 1). Potentiometric Measurements. The potentiometric measurements were performed using a 16-channel data acquisition setup and software (Lawson Labs, Inc., 3217 Phoenixville Pike, Malvern, PA 19355). A pH-meter LPH430T (Tacussel, France) was used to measure the pH of solutions. A double-junction Ag/AgCl/ KCl(3 M) reference electrode (Mo¨ller, Zu¨rich, Switzerland) with 0.1 M KCl solution in the outer sleeve was used. Sequential dilutions of stock solutions were performed using the 700 Dosino and 711 Liquino pump systems (Metrohm, Herisau, Switzerland). The electrode potential was measured during 20 min at each dillution in stirred solutions. Stable potential readings were taken (potential drift smaller than 0.2 mV/min). All recorded potential values were corrected for the liquid-junction potential, calculated according to the Henderson approximation. The single ion activities were calculated according to the Debye-Hu¨ckel theory.34 Electrochemical Experiments. In the electrochemical experiments, a galvanostat-potentiostat CH Instruments model 660A (34) Meier, P., C. Anal. Chim. Acta 1982, 136, 363-368.

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Figure 2. Comparison of potentiometric response of the tested allsolid-state calcium-selective electrode with the conventional calciumselective electrode: ([) PPy(Tiron)/Ca2+-membrane electrode conditioned in solution B12 (Table 1); (2) a conventional calcium-selective electrode with IFS of 10-2 M CaCl2.

Figure 3. Effect of composition of conditioning solution on the response of Ca2+ electrode with PPy(Tiron) solid contact; conditioning solutions: ([) C11, (9) B12, (2) B11, (0) B10, (/) B9, (b) B8, (]) B7, and (4) from A7 to A13 (Table 1).

(Austin, TX) and an Autolab general purpose electrochemical system (PGSTAT12, AUT71682, Eco Chemie, B. V., The Netherlands) were used. Electrochemical polymerization of pyrrole was performed in a conventional, three-electrode cell. The working electrode was a glassy carbon disk (area 0.07 cm2) (Detektor, Poland); the reference electrode was Ag/AgCl/KCl(1 M) gel electrode (Detektor). Pt wire was used as auxiliary electrode. EDAX Experiments. The elemental energy-dispersive analysis of X-rays (EDAX) experiments were performed with a ThermoNoran model Vantage at 10 keV. For EDAX, the films were electrodeposited on platinum sheets in the same way as described above for the glassy carbon substrate. The ion-selective membrane cocktail was subsequently cast onto the polymer layer. After overnight evaporation of THF, the Pt sheets coated with polymer and PVC film were conditioned for at least two weeks in the same solutions as described for the tested electrodes (Table 1). Before EDAX examination, the PVC films were removed. EIS Measurements. Electrochemical impedance spectroscopy (EIS) studies were performed using an Autolab general

Figure 4. EDAX spectra for PPy(Tiron) films (a) after electropolymerization and (b) after 2 weeks of conditioning in solution containing calcium and sodium ions (solution B12, Table 1).

purpose electrochemical system and an Autolab frequency response analyzer system (AUT20.FRA2-Autolab, Eco Chemie, B. V.) in an one-compartment, three-electrode cell where the tested electrode was connected as the working electrode. A glassy carbon rod was used as the auxiliary electrode, and the reference electrode was an Ag/AgCl/KCl(3 M) electrode (Metrohm). The impedance spectra were recorded in the frequency range 100 kHz-10 mHz by using a sinusoidal excitation signal (single sine) with an excitation amplitude of 100 mV to enhance the signal/ noise ratio. The measurements were performed in 0.1 M CaCl2 at room temperature. RESULTS AND DISCUSSION The goal of this work was to construct an all-solid-state solvent polymeric calcium-selective electrode for low-level calcium concentration measurements. As was mentioned before, this was achieved upon recognition for the ion-selective electrodes, by using an IFS characterized by low activity of the primary ion and high activity of the interfering ion.11 In this work, the internal filling solution was replaced by a conducting polymer layer (PPy) doped with Tiron to maintain and stabilize the low calcium activity at the inner side of the ion-selective membrane. In this way, the flux of the primary ion from the internal solution toward the sample was eliminated, and an unbiased detection limit and selectivity coefficients were obtained. To achieve lowered detection limits, a high activity of an interfering ion is required in the IFS to facilitate ion exchange between the primary and the interfering ion at the inner side of plastic membrane.9,11 In this work, the

interfering ions at the inner side of the plastic membrane are hydrogen or sodium ions introduced to the PPy(Tiron) layer during electrodeposition or conditioning via mechanisms discussed in detail by numerous authors.35-38 Potentiometric Responses. The representative calibration curve obtained for electrodes with PPy(Tiron) solid contact and the calibration curve for conventional ISEs with an IFS containing 10-2 M primary ion are presented in Figure 2. The tested electrode with PPy(Tiron) mediating layer was conditioned in solution containing Ca2+ in a concentration of 10-5 M and Na+ in a concentration of 10-2 M at pH 12 (solution B12, Table 1). The characteristic property of this sensor is a linear response within the Ca2+ activities from 10-4 to 10-9 with a 29.8 mV/decade slope (SD ) 0.2 mV/decade, n ) 6, R2 ) 0.999). The DL, calculated as the calcium activity at which the measured potential starts to deviate by (RT/2F) ln 2 from the Nernstian value,9 of this electrode is equal to 10-9.6. The calcium activity at the internal side of the membrane is maintained low and stable due to Tiron, which forms complexes with Ca2+ ions (log βCaL ) 5.8, log βCaHL ) 14.9 in aqueous solution39). According to this and due to the presence of the interfering ions in the poly(pyrrole) structure, the ion fluxes across the ion-selective membrane were reduced (35) Krivan, E.; Visy, C.; Kankare, J. J. Phys. Chem. B 2003, 107, 1302-1308. (36) Michalska, A., Maksymiuk, K. Electrochim. Acta 1999, 44, 2125-2129. (37) Migdalski, J. Chem. Anal. (Warsaw) 2002, 47, 595-611. (38) Migdalski, J., Blaz, T., Lewenstam, A. Chem. Anal. (Warsaw) 2002, 47, 371-384. (39) Inczedy, J. Analytical Applications of Complex Equilibria; Horwood: Chichester, 1976.

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Figure 5. Scheme of the ion-exchange processes, which can occur at the interfaces of electrode with PPy(Tiron) solid contact during the conditioning and calibration process.

and detection limit was shifted to low values. For conventional ISEs, the typical response was a linear range extending to 10-410-6 M Ca2+ (slope 28.1 mV/decade, SD ) 0.9 mV/decade, n ) 3, R2 ) 0.999) and a DL equal to 10-6,7. We have observed a strong influence from the composition of the conditioning solution on the shape of the potentiometric response of the calcium electrodes with a PPy(Tiron) solid contact, Figure 3. EDAX measurements performed just after electrochemical polymerization without any conditioning process (Figure 4a) showed that there are neither calcium nor sodium ions and confirmed that Tiron ions were present in the PPy structure. The presence of sulfur atoms proves the incorporation of Tiron into polymer structure during electropolymerization. It is well known 6414

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that, in order to obtain a Nernstian response for an ISE, the ionselective membrane must be conditioned in a solution containing the primary ion. On the other hand, as was mentioned above, to lower the detection limit of an ISE, the interfering ions must be present in the internal side of the ion-selective membrane. In the case of all-solid-state electrodes, the way to introduce the interfering ions into the solid-contact polymer layer is by conditioning the electrodes in a solution containing both the primary and the interfering ions. Therefore, we have examined the potentiometric response of the constructed electrodes with PPy(Tiron) solid contact after conditioning in solutions containing different concentrations of interfering ions at varying pH values (Table 1). Different pH values were used to influence the complex equilibria.

The presence of both calcium and sodium ions in the PPy structure after conditioning was confirmed by EDAX performed after two weeks of conditioning in solution of Ca2+ and Na+ (solution B12, Table 1), Figure 4b. During calibration in CaCl2 solutions performed after conditioning, the following phenomena were observed: a super-Nernstian response for electrodes conditioned in solution containing Na+ ions at a concentration of 10-1 M (solutions A7-A13, Table 1), a linear Nernstian response to 10-9 M for electrodes conditioned in solution containing Na+ ions at a concentration of 10-2 M (solutions B7-B12, Table 1), and a linear Nernstian response to 10-5 M for electrodes conditioned in solution containing Na+ ions at a concentration of 10-3 M (solution C11, Table 1), Figure 3. To explain this potentiometric behavior, two phases must be taken into account, the solid-contact polymer phase and the plastic membrane phase, in which a certain equilibrium state is established for all their components when the electrode is in contact with the solutions containing primary and interfering ions, as shown schematically in Figure 5. The lipophilic salt and ionophore present in the membrane are responsible for binding the ions. ETH 1001 is an ionophore, which is selective to calcium ions. However, after conditioning in a concentrated solution of sodium ions (e.g., 10-1 M Na+), a certain part of ETH 1001 in the membrane forms complexes with sodium ions. The amount of sodium in the membrane may be estimated from selectivity coefficient values of ETH 1001. During calibration in CaCl2 solutions, the ion exchange between calcium ions from the diffusion layer and sodium ions weakly bound to ionophore in the membrane phase takes place. That is why the super-Nernstian response of the electrode with PPy(Tiron) solid contact was observed after conditioning in solutions containing 10-1 M Na+ and 10-5 M Ca2+ (solutions A7-A13, Table 1), Figure 3. The same process occurs at the poly(pyrrole)/ion-selective membrane interface, Figure 5. If sodium concentration in the conditioning solution was lowered to 10-2 M (solutions B7-B12, Table 1), then the linear range of the electrode was extended to 10-9 M. Under these conditions, almost all of the ETH 1001 molecules in the membrane are bound to calcium and then Tiron and interfering ions present in the poly(pyrrole) phase are responsible for extending the linear part of calibration curve. Due to the calciumcomplexing properties of Tiron, a low and stable free-calcium ion activity in the polymer phase is maintained, and due to the presence of interfering ions, the ion-exchange reaction between calcium and interfering ions is possible, which is required to achieve lowered detection limit. To confirm this explanation, after conditioning for 24 h in solutions (A) 10-5 M CaCl2, 10-1 M NaCl and (B) 10-5 M CaCl2, 10-2 M NaCl, the potential of electrodes with PPy(Tiron) solid contacts was recorded in 10-7 M CaCl2, Figure 6. For comparison, the same experiment was performed for calcium-selective coated-wire electrodes and electrodes with IFSs with lowered calcium activity (linear response to 10-10 M),17 Figure 6. For all of these electrodes, after conditioning in solution containing 10-1 M NaCl, the recorded potential in 10-7 M CaCl2 increased with time by ∼160 mV. Replacement of sodium ions by calcium in complexes with ETH 1001 in the membrane is responsible for this effect. The potential achieved a constant value for the electrode with IFS (after ∼2 h); in the the case of the electrode with PPy(Tiron) solid contact and for coated-wire electrode, the potential stabilized after ∼4 h. For all types of

Figure 6. Time-dependent potential response in 10-7 M CaCl2 for the tested electrodes after conditioning in solutions with different concentrations of interfering ion (sodium ion); (A) 10-5 M CaCl2, 10-1 M NaCl; (B) 10-5 M CaCl2, 10-2 M NaCl.

electrodes conditioned in solution containing 10-2 M NaCl, the recorded potential in 10-7 M CaCl2 stabilized within a maximum of 15 min. This shows that the extension of the linear range for the electrodes with PPy(Tiron) solid contact conditioned in 10-2 M NaCl and 10-5 M CaCl2 originates from the properties of the polymer mediating layer. The concentration of sodium ions in the internal side of the plastic membrane, introduced by conditioning in solution containing 10-3 M Na+ and 10-5 M Ca2+ (solution C11, Table 1) is not enough to provide the extended range of potentiometric response, Figure 3. In aqueous solution, the calcium complexation by Tiron is pH-dependent (log βHL ) 12.7, log βH2L ) 20.439). Some influence of the pH of the conditioning solution on the shape of the calibration curve for the electrode with PPy(Tiron) mediating layer has been observed, Figure 3. Lower potentials in diluted calibrants were observed for higher pH of the conditioning solution. Analytical Chemistry, Vol. 76, No. 21, November 1, 2004

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Figure 7. Impedance spectra for Ca2+ electrodes: (a) GC/PPy(Tiron)/Ca2+ membrane, (b) GC/Ca2+ membrane (CWE), and (c) conventional Ca2+ selective electrode with IFS.

Impedance Responses. The impedance measurements were made for three types of electrodes: (a) the all-solid-state electrodeglassy carbon/PPy(Tiron)/Ca2+ membrane, (b) a coated-wire electrode-glassy carbon/Ca2+ membrane, and (c) a conventional calcium-selective electrode with a 10-2 M CaCl2 IFS. The spectra were recorded in 10-1 M CaCl2 at a constant potential polarization of +200 mV, at which PPy is in its oxidation state. The observed spectra are presented in Figure 7. All spectra show a highfrequency semicircle, which is due to bulk resistance of the ionselective PVC membrane, and the diameter of this semicircle is equal to the bulk resistance of the membrane.40 Placing the conducting PPy film between Glassy carbon (GC) and PVC (40) Horvai, G.; Graf, E.; Toth, K.; Pungor, E.; Buck, R., P. Anal. Chem. 1986, 58, 2735-2740.

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membrane decreases the resistance of the membrane from 7.2 (Figure 7b) to 2.7 MΩ (Figure 7a). This effect was earlier reported and explained by Cadogan et al.24 According to these authors, by placing an electronically and ionically conducting PPy layer between the electronically conducting GC and ionically conducting PVC ion-selective membrane, the charge transfer across the interfaces is facilitated; i.e., the PPy layer acts as an ion-to-electron transducer. The resistance of the conventional ion-selective membrane is also quite high, 6.7 MΩ (Figure 7c). The lowfrequency part of the impedance spectrum for CWE shows the semicircle arising from a small capacitance in parallel with a large charge-transfer resistance at the “blocked” interface GC/PVC membrane31 (Figure 7b). The PPy layer between GC and PVC membrane increases low-frequency capacitance and decreases

Table 2. Standard Deviation of Ca2+ Concentration Measurements at Different Levels, n ) 7 Ca2+ concn in the sample (M)

standard deviation (M)

10-4 10-5 10-6 10-7 10-8 10-9 10-10

(0.10 × 10-4 (0.08 × 10-5 (0.10 × 10-6 (0.14 × 10-7 (0.43 × 10-8 (0.48 × 10-9 (0.89 × 10-10

Figure 8. Potential stability during response of Ca2+ electrode with PPy(Tiron) solid contact with an extended linear range (conditioned in solution B12, Table 1) and super-Nernstian characteristic (conditioned in solution A13, Table 1).

Figure 10. Potentiometric responses of Ca2+ electrode with PPy(Tiron) solid contact recorded in metal chloride solutions.

Figure 9. Repeatability of potential of Ca2+ electrode with PPy(Tiron) solid contact within 6 weeks, error bars SD, n ) 7.

charge-transfer resistance at the interface GC/PVC membrane, which is characteristic for “unblocked” interfaces (Figure 7a). For conventional ISEs, the capacitance arises from the redox reaction of the inner reference electrode connected with ion transport in the internal filling solution31 (Figure 7c). Potential Stability. Time-dependent responses of the all-solidstate electrodes with the extended linear range (conditioned in solution B12, Table 1) and super-Nernstian characteristic (conditioned in solution A13, Table 1) are presented in Figure 8. For both electrodes, the response was fast and the potential stabilized within under 1 min in the 10-4-10-7 M sample solutions. In