Hyphenated FT-IR-Attenuated Total Reflection and Electrochemical

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Hyphenated FT-IR-Attenuated Total Reflection and Electrochemical Impedance Spectroscopy Technique to Study the Water Uptake and Potential Stability of Polymeric Solid-Contact Ion-Selective Electrodes Tom Lindfors,*,†,‡ Lajos H€ofler,§,^ Gyula Jagerszki,z and Robert E. Gyurcsanyi§,z †

Laboratory of Analytical Chemistry, Process Chemistry Centre, Åbo Akademi University, Biskopsgatan 8, FI-20500 Åbo, Finland Academy of Finland, Helsinki, Finland § Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, H-1111 Budapest, Szt. Gellert ter 4, Hungary ^ Department of Chemistry, The University of Michigan, 930 N. University, Ann Arbor, Michigan 48109-1055, United States z Research Group of Technical Analytical Chemistry of the Hungarian Academy of Sciences, H-1111 Budapest, Szt. Gellert ter 4, Hungary ‡

bS Supporting Information ABSTRACT: A new hyphenated method utilizing FT-IR-attenuated total reflection (ATR) and electrochemical impedance spectroscopy (EIS) is presented to correlate the water uptake with concomitant potential and impedance changes of polymeric coated-wire electrodes (CWEs) and solid-contact ion-selective electrodes (SCISEs). The Ca2þ-selective silicone rubber (RTV 3140) based SCISEs with poly(3-octylthiophene) (POT) as the solid-contact (SC) showed good correlation between a very low water content at the Pt-coated ZnSe substrate/SC interface and a superior potential stability. This is due to the hydrophobicity of both RTV 3140 and POT and the approximately 2 orders of magnitude lower water diffusion coefficients in POT compared to RTV 3140. Practically no potential drift could be observed during 24 h when unconditioned CaSCISEs were contacted with 103 M CaCl2, in contrast to the Ca2þ-selective CWEs with considerably higher water uptake and potential drift. The CaSCISEs had a fast Nernstian response with a detection limit of 8  109 M Ca2þ and a good reproducibility and stability of the standard potential, which indicates that the CaSCISEs does not require any conditioning prior to use.

I

on-selective electrodes (ISEs) with the detection limit (LOD) in the nano- and picomolar range have gained attention since 1997 when it was reported that a detection limit of 5  1012 M could be reached with a conventional Pb2þ-selective ISE by optimizing the composition of the inner solution, i.e., buffering the primary ion concentration to a constant low level.1 This was done to prevent the transmembrane flux2,3 of primary (analyte) ions from the inner solution through the plasticized poly(vinyl chloride) (PVC) based ion-selective membrane (ISM) phase into the sample solution. At low primary ion concentrations in the sample, the ISE responds to the elevated primary ion concentration established by the transmembrane flux at its surface and not to the real bulk analyte concentration. Several methods have been proposed to impede the transmembrane flux of primary ions through the ISM in order to improve the LOD of conventional ISEs. These methods are usually based on an applied chemical gradient1,4 or electrical current59 or on establishing enhanced mass transport in the solution.10 Alternatively, the ion fluxes can be also suppressed by simply using ISMs with low ion diffusion rates such as different r 2011 American Chemical Society

types of poly(acrylate) (PA) membranes,11 which have approximately 3 orders of magnitude lower ion diffusion rates than plasticized PVC. Thus, the most attractive approach to eliminate the leakage of primary ions into the sample solution is to combine such low diffusivity ISMs with solid-contact ISEs (SCISEs) that eliminate the conventional liquid contact acting as a reservoir sustaining the outflow of primary ions.1214 The optimization of SCISEs for ultratrace measurements reduces, therefore, solely to a proper conditioning of the ISMs.15 In the SCISEs concept, most often an electrically conducting polymer (CP) acts as an ion-to-electron transducer layer between the electronically conducting substrate and the ISM. However, the leaching of primary ions cannot either completely be eliminated with the SCISE construction if an aqueous layer is formed below the ISM due to its water uptake.1620 If a water layer is formed beneath the ISM, both the coated-wire electrode (CWE) and the SCISE Received: March 10, 2011 Accepted: May 5, 2011 Published: May 05, 2011 4902

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Analytical Chemistry are converted into conventional liquid contact ISEs, however, with a small inner solution volume. This may result in unstable and drifting electrode potentials because of composition changes that may occur in the aqueous layer due to transmembrane fluxes (diffusion) of primary and interfering ions, water as well as neutral species (e.g., CO2, NH3, and small organic molecules).21 The presence of an aqueous layer is traceable using an ingenious experimental method, the so-called potentiometric aqueous layer test.22,23 This test, however, does not provide mechanistic information about the water uptake process. Therefore, the water uptake of ISMs received recently accentuated attention in this context although the water uptake of conventional ISMs (symmetrical setup) was extensively studied already in the 1990s.1618 The aqueous layer formation of a plasticized PVC based ISM applied in the CWE setup (asymmetrical setup) was studied with neutron reflectometry (NR), electrochemical impedance spectroscopy (EIS), and secondary ion mass spectrometry (SIMS).24 It was concluded that a 100 ( 10 Å water layer was formed between the silica substrate and the ISM. It was later shown that the formation of a water layer or scattered islands (pools of water) could be eliminated using a poly(methyl methacrylate)/poly(decyl methacylate) (PMMA/PDMA) ISM matrix in combination with poly(3-octylthiophene) (POT) as the solid-contact (SC) layer (asymmetrical setup).25,26 Alternatively, we have introduced FT-IR spectroscopy in the attenuated total reflection (ATR) mode as a powerful tool to investigate the water uptake of plasticized PVC, PA, and silicone rubber (SR) based ISMs in the coated-wire (CWE) configuration.19,20 The advantage of the FT-IR-ATR technique is that it is possible to distinguish between monomeric nonhydrogen bonded water, hydrogen-bonded dimers, and clustered and bulk water.27 The FT-IR-ATR measurements showed that the PA based membranes had the highest equilibrium water uptake of the different membrane types in deionized water whereas it was lowest for the SR based ISMs,20 although the diffusion coefficients for water in the SR membranes were almost similar to plasticized PVC membranes. However, until now, there have been no attempts to correlate simultaneously in real time the water content of polymeric ISMs with changes in their potential response and bulk impedance, which should be the ultimate goal of the water uptake studies, as it is of primary relevance for the practical use of SCISEs. This work presents a new hyphenated method for measuring simultaneously the water uptake, impedance spectra, and potential stability of hydrophobic SR and POT based SCISEs. It combines the FT-IR-ATR and EIS techniques and makes it possible to follow in situ how the water uptake of the ISMs influences the potential response and bulk resistance of initially unconditioned ISMs until they reached their equilibrium water content. ISMs based on SR were chosen for this study due to their low equilibrium water content20 and the clear need for alternative ISM matrices with a low water uptake.28 This paper focuses on the correlation between the presence of water at the electrode substrate/ISM (CWE) and the substrate/SC (SCISE) interfaces and the potential stability of SR based Ca2þ-selective CWEs and SCISEs (CaCWEs and CaSCISEs).

’ EXPERIMENTAL SECTION Chemicals. Room temperature vulcanizing silicone rubber (RTV 3140) was obtained from Dow Corning. Potassium tetrakis[3,5bis(trifluoromethyl)phenyl]borate (KTFPB), calcium ionophore

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I (ETH 1001), and tetrahydrofuran (THF) of Selectophore grade were received from Fluka. The regioregular poly(3-octylthiophene-2,5-diyl) (Mn = 34 000) was purchased from SigmaAldrich. Electrode Preparation. The CaCWEs, CaSCISEs, and neat poly(3-octylthiophene) POT films for the simultaneous FT-IRATR and EIS measurements, which were performed with the same electrode setup, were prepared as follows: (a) CaCWE: The Ca2þ-selective ISMs with thicknesses of ca. 250300 μm were deposited by drop casting (2  75 μL) on Pt-sputtered ZnSe crystals. The membrane solution dissolved in tetrahydrofuran (THF) had a dry weight of 20.0 wt % and consisted of 98.1 wt % silicone rubber (RTV 3140), 1.0 wt % calcium ionophore I (ETH 1001), and 0.9 wt % (5 mmol/kg) KTFPB.29 The ISMs were allowed to dry for ca. 72 h prior to the FT-IRATR measurements. The ISM thicknesses were determined with a micrometer. See Supporting Information for a more detailed experimental description. (b) CaSCISE: The ZnSe crystals were pretreated according to the same procedure as for the CaCWEs. However, 50 μL of regioregular POT (10 mg/mL in chloroform) was applied by drop casting on the Pt-sputtered ZnSe substrate and allowed to dry overnight before applying the upper Ca2þselective ISM by drop casting on top of the POT layer. (c) POT: The POT layer was deposited on the sputtered ZnSe crystals by drop casting 50 μL of the POT solution on the crystal and allowing the formed films to dry overnight. The CaCWEs and CaSCISEs for the potentiometric measurements were prepared according to the following procedures: (a) CaCWE: ISMs with the same composition as in the FT-IR-ATR measurements and with a thickness of usually ca. 280300 μm were deposited on glassy carbon (GC) electrodes with PEEK bodies by applying 45 μL of the membrane solution on top of the GC/PEEK electrodes (outer diameter: 6 mm). The ISMs were allowed to dry for ca. 72 h prior to the potentiometric measurements. (b) CaSCISE: 2  0.6 μL of the POT solution described above was applied as the SC layer on the GC substrate and was allowed to dry for 4 h before applying 45 μL of the Ca2þ-selective ISM cocktail on top of the POT layer. The ISM covered the entire surface of the GC/PEEK electrode. FT-IR-ATR Measurements. The FT-IR-ATR setup has been described in detail previously.19 Before starting the FT-IR-ATR measurements, the sample compartment was purged with dry air for 30 min. The background spectrum as well as the first FT-IR spectrum in the measurement sequence was measured without electrolyte. After measuring the first spectrum, the cell was quickly filled with 103 M CaCl2 (see also Supporting Information). Impedance Measurements. Impedance spectra within the frequency range of 100 kHz10 mHz were measured in 103 M CaCl2 every hour during 24 h with an Autolab potentiostat PGSTAT20 equipped with an impedance module (FRA). All impedance measurements were done at the open circuit potential with an amplitude of (ΔE) of either 10 mV (POT) or 100 mV (CaCWE and CaSCISE). Additionally, the impedance was measured only at 100 kHz (ΔE = 0.2 mV) every 15 min during the entire measurement time to register the open circuit potential. A Ag/AgCl wire was used as a quasireference electrode in the impedance measurements. The impedance and the FT-IR-ATR measurements were started simultaneously, and the results of these two measurements could, therefore, be compared. Potentiometric Measurements. Selectivity coefficients. The selectivity coefficients of Ca2þ against Naþ, Kþ, Hþ, and Mg2þ of the CaCWEs and CaSCISEs were measured for previously 4903

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Figure 1. FT-IR-ATR spectra reflecting the water uptake (24 h) of a 7 μm thick POT membrane contacted with 103 M CaCl2.

unconditioned electrodes in 0.1 M chloride solutions prepared with deionized water. The measured selectivity coefficients were, therefore, not biased by the primary ions leaching from the ISM phase. It must be stressed that there is a difference between the classical and unbiased selectivity only if the logarithmic values of the selectivity coefficients determined with the classical method are lower than 4, which is not the case for the SR based ISEs presented in this paper. (See the Results and Discussion section.). The electrodes were, thus, not calibrated in the interfering ion solutions. A double junction Ag/AgCl electrode filled with 3.0 M KCl and 1.0 M KCl in the inner and outer compartments, respectively, was used as the reference electrode (RE). Potentiometric calibrations. The CaCWEs and CaSCISEs were calibrated from 104 M to 3  1012 M CaCl2 with an automatic diluting system (Metrohm 700 Dosino and 711 Liquino controller); see also Supporting Information. The CaCWEs and CaSCISEs were first kept overnight in a 105 M CaCl2 solution and were then conditioned prior to the potentiometric calibrations for 3 h in a 109 M CaCl2 solution containing 105 M NaCl as the background electrolyte, which was used to suppress the leaching of primary ions from the ISM and consequently to lower the LOD. This solution was changed to a fresh one every hour. Aqueous layer test. The potentiometric aqueous layer test22 was done by measuring the electrode potentials of the CaCWEs and CaSCISEs in the following solutions: (1) 0.1 M CaCl2, (2) 0.1 M NaCl (interfering ion: Naþ), and (3) 0.1 M CaCl2. The potentials were measured for 24 h in each solution using the same RE as in the selectivity measurements. All potentiometric measurements were done in stirred solutions with a 16-channel high impedance (1015 Ω) voltmeter (Lawson Lab Inc., Malvern, PA).

’ RESULTS AND DISCUSSION Water Uptake, Potential Stability, and Impedance Spectra Measured Simultaneously. The water uptake of the POT solid-

contact layer and the CaCWE was measured separately to gain deeper insight in the water uptake of the CaSCISE. Figure 1 shows the FT-IR-ATR spectra measured for 24 h during the water uptake of a 7 μm thick POT membrane in contact with a 103 M CaCl2 solution. The penetration depth of the IR beam is 0.50.6 μm in the wavenumber region of 30003700 cm1.30 The IR beam is, therefore, sensing water only at the ZnSe/ membrane interface for all electrode types studied. The spectra

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Figure 2. Potential stability of the (b) POT (membrane thickness: 7 μm), (O) CaCWE (248 μm), and (9) CaSCISE (267 μm) membranes measured in 103 M CaCl2.

show the presence of water at the ZnSe/POT interface already after 10 min, indicating that water penetrates quickly through the thin POT membrane to the Pt-sputtered ZnSe substrate. The water content at the ZnSe/POT interface increased rather rapidly during the first 6 h of the measurement, reaching then a saturation level after ca. 18 h after which the water content increased only to a minor extent. Mathematical simulations of the integrated band area of the strong OH-stretching band (2960 3750 cm1) showed that the water uptake of the POT membrane was best described with a model consisting of three water diffusion coefficients: D1 = (4.7 ( 3.6)  1010 cm2/s, D2 = (5.1 ( 3.6)  1011 cm2/s, and D3 = (4.7 ( 3.4)  1012 cm2/s (Figure S-1, Supporting Information). These were interpreted to be related to monomeric and dimeric water (D1), clustered water (D2), and bulk water (D3). The UVvis transmission spectra of the POT film was also measured for 24 h in direct contact with 103 M CaCl2 to obtain information about possible changes occurring in the oxidation state of the neat POT film during the water uptake. The spectra (not shown here) are characteristic of the neutral nonconducting form of POT with absorption peaks at ∼445, 560, and 603 nm31,32 and showed no changes in the oxidation state during 24 h. On the other hand, the FT-IR spectrum measured with the external reflection technique (θ = 60) of the POT film surface, which had not been contacted with an aqueous electrolyte solution, showed bands of the neutral form of POT at 1454,3335 1464,31,34 and 1510 31,33,34 cm1 but also strong infrared active vibrational (IRAV) doping induced bands at 995,34 1147,34,36 and 13043436 cm1 originating from the electrically conducting form of POT and, thus, confirming the presence of electrically conducting segments within the POT matrix. However, most probably only a minor amount of these segments are present in the POT matrix due to its low electrical conductivity, i.e., low doping level, reported by the manufacturer (1  106 S/cm). The vibrational bands of the neutral and the electrically conducting form of POT observed in the wavenumber region of 7001600 cm1 in the FT-IR spectra are discussed in the Supporting Information section (Figure S-2). The open circuit potential of the POT film was measured in 103 M CaCl2 to obtain additional information about the potential stability of the SC layer (i.e., the stability of the oxidation state) when it is exposed to direct contact with an 4904

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Figure 3. Impedance spectra of (a) POT (membrane thickness: 7 μm), (b) CaCWE (248 μm), and (c) CaSCISE (267 μm) measured at t = (b) 0.1, (O) 1.9, (2) 6.8, and (Δ) 23.3 h; f = 39.8 kHz10 mHz, ΔE = 100 mV (POT: 10 mV). The insets show (a) the high frequency part of the POT spectra and the low frequency part of the (b) CaCWE and (c) CaSCISE spectra.

aqueous electrolyte solution. As shown in Figure 2, the open circuit potential of the neat POT film decreased from ca. 230 mV to 50 mV during the water uptake measurements, indicating that the rather nonconducting POT film was further reduced to the neutral form in direct contact with water (Figure 3). However, depending on the hydrophobicity of the outer ISM used in the SCISEs, the amount of water diffusing to the underlying SC may be very low. The potential stability of the POT solid-contact layer cannot, therefore, be predicted from the potential stability curve of POT shown in Figure 2, but it still gives valuable information about the stability of the oxidation state of POT. It was previously reported that a doped poly(thiophene) film can be reduced to the neutral state even by washing with water, and it is, therefore, expected that the same can also happen with POT.37 This assumption is supported by the impedance measurements which were done simultaneously with the FT-IR-ATR measurements (Figure 3a). It is qualitatively seen from the high frequency part of the impedance spectra of POT that the charge transfer resistance at the Pt-sputtered ZnSe/POT interface increased in magnitude with the time, reflecting that the electrical conductivity of the POT matrix probably decreased to some extent during the water uptake. (See the inset of Figure 3a.) The low frequency part of the impedance spectra can be related to an interfacial impedance originating from the POT/solution interface.38 It was found that the low frequency part of the impedance spectra of POT measured after a contact time of 5 min with 103 M CaCl2 could be described by a slightly suppressed semicircle with the (interfacial) resistance of 25.8 MΩ and a time constant (τ) of 15 s. However, the resistance and time constant increased to 228 MΩ and 123 s, respectively, after a contact time of 6.8 h but decreased then slightly with increasing time. The increase of the interfacial impedance indicates that the neat POT solidcontact layer will be partly reduced when the SC is in direct contact with water. The FT-IR-ATR spectra measured during the water uptake of a CaCWE membrane in 103 M CaCl2 are shown in Figure 4a. A distinct water band was observed in the spectra already after 30 min increasing in intensity throughout the measurement. Most of the water uptake took place during the first 4 h of the measurement, which is in good correlation with the initial drift of the open circuit potential of the CaCWE (Figure 2) that was measured simultaneously with the water uptake of the SR based ISM. The potential of the CaCWE showed an initial potential

drift of 29 mV during the first 4 h of the measurement but only a very small continuous drift of 6 mV during the rest of the water uptake measurement. The water uptake of the CaCWE membrane was best described with a mathematical model consisting of three diffusion coefficients: D1 = (7.5 ( 1.4)  108 cm2/s; D2 = (1.1 ( 0.3)  108 cm2/s, and D3 = (3.4 ( 1.8)  1010 cm2/s (Figure S-1, Supporting Information), which are all approximately 2 orders of magnitude higher than the corresponding values of the water diffusion coefficients in POT, showing that water diffuses quicker through the SR based membrane than POT. In accordance with the POT film, the diffusion coefficients were related to monomeric and dimeric water (D1), clustered water (D2), and bulk water (D3). The impedance measurements showed that the water uptake has only a minor influence on the bulk properties of the CaCWE membrane (Figure 3b). The impedance spectra in Figure 3b revealed only minor time-dependent changes in the bulk resistance of the CaCWE membranes from 35.5 MΩ to 31.9 MΩ during the water uptake process. The dielectric constant calculated from the bulk semicircle showed that the water uptake influenced the dielectric constant of the SR based CaCWE membrane only to a minor extent and changed its value from εr = 2.0 (t = 0.08 h) to εr = 2.2 (t = 23.3 h). The calculated dielectric constants are in good accordance with the dielectric constant of 2.52 for neat RTV 3140 reported by Dow Corning. It was verified with mathematical modeling that the second suppressed semicircle observed at low frequencies in Figure 3b (inset) was connected to the exchange of Kþ (originating from KTFPB) to Ca2þ in the membrane phase during the water uptake measurements in 1 mM CaCl2 for 24 h. According to the mathematical model, the second semicircle is due to the lower diffusivity of Ca2þionophore complex compared to the diffusivity of Kþ and/or the Kþionophore complex in the SR matrix. The ion-exchange of Kþ to Ca2þ results in a growing layer with lower diffusivity on the sample side of the SR membrane that causes a higher resistance in that region. This phenomenon can be observed in the impedance spectrum as a second semicircle. In accordance with the model, the second semicircle was absent in a separate experiment (not shown here) where the impedance spectra of the CaCWE was measured for 24 h in 103 M KCl, further proving that the second suppressed semicircle is related to the ion-exchange of Kþ to Ca2þ. Impedance fitting showed that the resistance associated with 4905

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Figure 4. FT-IR-ATR spectra measured in 103 M CaCl2 during the water uptake (24 h) of the SR based (a) CaCWE and (b) the CaSCISE membranes with the thicknesses of 248 and 267 μm, respectively.

the second semicircle in Figure 3b increased from 3.1 MΩ at t = 0.08 h to 11.8 MΩ at t = 23.3 h. This can be explained by the process of Ca2þ replacing an increasing amount of Kþ in the membrane; thus, the region with higher resistance is growing in time. The small low frequency “tail” in the impedance spectra at the lowest frequencies originates from the blocked charged transfer at the substrate/ISM interface of the CaCWE.39,40 The FT-IR-ATR spectra in Figure 4b measured during the water uptake of the CaSCISE membrane show clearly that POT prevented almost all of the water from reaching the ZnSe/POT interface, in comparison with the much higher water content detected at the ZnSe/ISM interface for the CaCWE membranes without the SC layer of POT (Figure 4a). A signal amplification of 32 was applied in all FT-IR-ATR measurements, and the water content at the ZnSe/POT interface for the CaSCISE membrane is, therefore, very low. The low water content at this interface is due to the approximately 2 orders of magnitude lower water diffusion coefficients in POT compared to SR (RTV 3140). It was also recently reported that pinholes in the POT layer could cause water pooling at the substrate/POT interface in a PMMA/ PDMA based SCISE25 and the pinhole formation cannot either be completely excluded for the SR based SCISEs. The very low water content at the ZnSe/POT interface in the CaSCISE membranes correlated well with a superior potential stability of the CaSCISEs measured simultaneously with the water uptake in 103 M CaCl2, showing the importance of preventing water from penetrating through the ISM and SC to the electrode substrate (Figure 2). The potential of the CaSCISE drifted only 4 mV during the first 4 h of the measurement but remained completely stable after that. The first potential value of the POT, CaCWE and CaSCISE membranes shown in Figure 2 was measured with initially unconditioned electrodes after a contact time of 1 min in 103 M CaCl2. The superior potential stability of the CaSCISE clearly demonstrates that hydrophobic ISMs and SCs should preferably be used in the manufacturing of SCISEs. The very stable potential of the CaSCISE indicates also that the oxidation state of the POT film beneath the SR based ISM was probably not influenced to any greater extent by the water uptake, in contrast to when the entire POT film was exposed to water. As with the CaCWE, the water uptake had only a minor influence on the bulk resistance and dielectric constant of the SR based ISM (Figure 3c). The bulk resistance changed from 63.1 MΩ to 52.5 MΩ, which corresponds to dielectric constants of 1.9 and 2.3, respectively. The POT solid-contact layer was slightly dissolved in

the outer ISM during the drop casting procedure, but it does not seem to have any bigger influence on the dielectric constant. The resistance of the second suppressed low frequency process related to the ionexchange of Kþ to Ca2þ changed from 4.1 MΩ (t = 0.08 h) to 20.0 MΩ (t = 23.3 h). The absence of the low frequency “tail” in the impedance spectra of the CaSCISE, which was observed for the CaCWE, confirms that the charge transfer is facilitated at the substrate/POT interface, contributing to the superior potential stability of the CaSCISE. Potentiometric Calibrations and Selectivity Measurements. The potential traces and the calibration graphs of the CaCWEs and CaSCISEs (GC/PEEK bodies) were recorded in 3  1012104 M CaCl2 solutions by calibrating the ISEs from high to low concentration (Figure 5). Both electrode types had slightly sub-Nernstian slopes of 27.1 ( 0.7 mV/decade (CaSCISE) and 25.6 ( 0.7 mV/decade (CaCWE) and a LOD of 8  109 M (Table 1). The selectivity coefficients are given in Table 1, and the selectivity coefficients of Ca2þ over Naþ and Kþ are in rather good agreement with those reported earlier for SR membranes with the same composition as in this work.29 Both the CaCWEs and the CaSCISEs had very fast potential responses to the concentration changes of Ca2þ, and the potentials of the CaSCISEs stabilized immediately after the concentration change whereas the potentials of the CaCWEs showed a continuous slow drift and had a higher noise (inset of Figure 5a). The superior response stability of the CaSCISEs can be related to the very low water content at the GC/POT interface, and the slow potential drift of the CaCWE can be related to much higher water content at the blocked GC/ISM interface. The superior potential stability is also in good accordance with the very good reproducibility of the standard potential (E0) of the CaSCISEs compared to the CaCWEs (Table 1). The potentials of three identical CaSCISEs and CaCWEs were 224.2 ( 6.7 mV and 370.6 ( 27.6 mV in 103 M CaCl2 (Table 1), respectively, but the standard deviation of the CaSCISEs could be decreased to only 0.63.5 mV (n = 3) when completely new GC/PEEK electrodes were used for the electrode preparation. The CaCWEs and the CaSCISEs were calibrated 5 times during 120 h, and almost no drift in the standard potential of the CaSCISEs could be observed (potentials measured in 104 M CaCl2) whereas the standard potential of the CaCWE showed a considerable drift of >100 mV (Figure S-3, Supporting Information). This strongly indicates that the CaSCISEs with POT as the SC does not require any 4906

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Figure 5. (a) Potential traces of (1) the CaCWE and (2) the CaSCISE measured in CaCl2 solutions for 3 min at each concentration. Enlargements of the potential traces measured in the beginning of the calibrations are shown in the inset. (b) The calibration plots of the (9) CaCWE and (0) CaSCISE in the CaCl2 solutions. The calibration plot of the CaSCISE has been shifted to higher potentials to have the same potential as the CaCWE in 104 M CaCl2. The thickness of the CaISM was 290 μm for both electrode types.

Table 1. Response Characteristics of the SR Based CaCWEs and the CaSCISEs with POT as the SC (n = 3)

CaCWE

slope (mV/dec)a potential reproducibility (mV)b limit of detection, LOD (M)

CaSCISE

25.6 ( 0.7

27.1 ( 0.7

370.5 ( 27.6

224.2 ( 6.7

8  109

8  109

selectivity coefficients (log Kpot Ca,j) þ

a

j = Na

2.5

2.9

j = Kþ

2.6

2.9

j = Hþ

2.9

3.0

j = Mg2þ

3.6

3.8

107  2  105 M CaCl2. b Determined in 104 M CaCl2.

Figure 6. (a) Aqueous layer test in 0.1 M primary ion (Ca2þ) and interfering ion (Naþ) solutions: The SR based (a) CaCWE and (b) CaSCISE and (c) the plasticized PVC based CaCWE. The thicknesses of the SR based CaISMs were 290 μm, whereas the plasticized PVC based CaISM membrane had a thickness of 225 μm (curve c).

conditioning prior to use, which could be especially beneficial for single-use devices. However, this must still be studied in more detail. Potentiometric Aqueous Layer Test. The result of the potentiometric aqueous layer test22 showed no aqueous layer

formation either for the CaSCISE or the CaCWE (Figure 6). Completely stable, driftless potentials were obtained with the CaSCISEs as well as almost driftless potentials for the CaCWEs. The minor potential drift can possibly be related to the presence of small amounts of water at the ZnSe/ISM interface, which was confirmed by the FT-IR-ATR measurements. On the other hand, a response pattern typical for the aqueous layer formation was observed for a plasticized poly(vinyl chloride) (PVC) based ISM consisting of 32.92 wt % PVC, 65.83 wt % bis(2-ethylhexyl)sebacate (DOS), 0.8 wt % calcium ionophore IV, and 0.45 wt % KTFPB. However, the potentiometric aqueous layer test may not be completely reliable for the SR membranes due to the lower diffusion coefficients of the ionionophore complex in the SR matrix compared to plasticized PVC. The time required for the transmembrane flux of ions through the SR membrane will, therefore, be much longer than for plasticized PVC. This would probably require much longer measurement times than those shown in Figure 6.

’ CONCLUSION A new method of measuring simultaneously FT-IR-ATR and impedance spectra provides a deeper understanding of the processes taking place in ISMs or CPs when they are exposed to an aqueous electrolyte solution. The FT-IR-ATR measurements verified that a superior potential stability of initially unconditioned CaSCISEs correlated with very low water content at the ZnSe substrate. Practically no potential drift could be observed in 0.1 M CaCl2 during 24 h which indicates that no conditioning is required prior to use for the SR based CaSCISEs having a 0.3 ppb LOD. The low water content at the ZnSe/POT interface is due to the hydrophobicity of both the SR based ISM and the POT solidcontact layer. Their water uptake was best described with a model consisting of three diffusion coefficients with 2 orders of magnitude lower diffusion coefficients for POT than for the Ca2þselective outer SR based ISM, which explains why POT prevents water from reaching the ZnSe/POT interface. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: Tom.Lindfors@abo.fi. Fax: þ358-2-2154479.

’ ACKNOWLEDGMENT Dr. Fredrik Sundfors and Dr. Pia Damlin are gratefully acknowledged for technical assistance. This work is part of the activities of the Åbo Akademi Process Chemistry Centre within the Finnish Centre of Excellence Program (Academy of Finland) and connected to the scientific program of the development of quality-oriented and harmonized RþDþI strategy at BME. T.L. acknowledges the Academy of Finland and the Hungarian Academy of Sciences for financial support. The financial support of the Hungarian Scientific  MOP-4.2.1/B-09/1/KMR-2010Fund (OTKA NF 69262) and TA 0002 is also gratefully acknowledged. ’ REFERENCES (1) Sokalski, T.; Ceresa, A.; Zwickl, T.; Pretsch, E. J. Am. Chem. Soc. 1997, 119, 11347–11348. (2) Mathison, S.; Bakker, E. Anal. Chem. 1998, 70, 303–309. (3) 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. (4) Ceresa, A.; Sokalski, T.; Pretsch, E. J. Electroanal. Chem. 2001, 501, 70–76. (5) Pergel, E.; Gyurcsanyi, R. E.; Toth, K.; Lindner, E. Anal. Chem. 2001, 73, 4249–4253. (6) Lindner, E.; Gyurcsanyi, R. E.; Buck, R. P. Electroanalysis 1999, 11, 695–702. (7) H€ofler, L.; Bedlechowicz, I.; Vigassy, T.; Gyurcsanyi, R. E.; Bakker, E.; Pretsch, E. Anal. Chem. 2009, 81, 3592–3599. (8) Bedlechowicz, I.; Sokalski, T.; Lewenstam, A.; Maj-Zurawska, M. Sens. Actuators, B: Chem. 2005, B108, 836–839. (9) Peshkova, M. A.; Sokalski, T.; Mikhelson, K. N.; Lewenstam, A. Anal. Chem. 2008, 80, 9181–9187. (10) Vigassy, T.; Gyurcsanyi, R. E.; Pretsch, E. Electroanalysis 2003, 15, 375–382. (11) Heng, L. Y.; Hall, E. A. H. Anal. Chim. Acta 1996, 324, 47–56. (12) Nikolskii, B. P.; Materova, E. A. Ion-Selective Electrode Rev. 1985, 7, 3–39. (13) Cadogan, A.; Gao, Z.; Lewenstam, A.; Ivaska, A.; Diamond, D. Anal. Chem. 1992, 64, 2496–2501. (14) Cattrall, R. W.; Freiser, H. Anal. Chem. 1971, 43, 1905–1906. (15) Sutter, J.; Lindner, E.; Gyurcsanyi, R. E.; Pretsch, E. Anal. Bioanal. Chem. 2004, 380, 7–14. (16) Li, Z.; Li, X. Z.; Rothmaier, M.; Harrison, D. J. Anal. Chem. 1996, 68, 1726–1734. (17) Li, Z.; Li, X. Z.; Petrovic, S.; Harrison, D. J. Anal. Chem. 1996, 68, 1717–1725. (18) Chan, A. D. C.; Li, X. Z.; Harrison, D. J. Anal. Chem. 1992, 64, 2512–2517. (19) Lindfors, T.; Sundfors, F.; H€ofler, L.; Gyurcsanyi, R. E. Electroanalysis 2009, 21, 1914–1922. (20) Sundfors, F.; Lindfors, T.; H€ofler, L.; Gyurcsanyi, R. E. Anal. Chem. 2009, 81, 5925–5934. (21) Lindner, E.; Gyurcsanyi, E. R. J. Solid State Electrochem. 2009, 13, 51–68. (22) Fibbioli, M.; Morf, W. E.; Badertscher, M.; de Rooij, N. F.; Pretsch, E. Electroanalysis 2000, 12, 1286–1292. (23) Fibbioli, M.; Bandyopadhyay, K.; Liu, S. G.; Echegoyen, L.; Enger, O.; Diederich, F.; Gingery, D.; B€uhlmann, P.; Persson, H.; Suter, U. W.; Pretsch, E. Chem. Mater. 2002, 14, 1721–1729. (24) De Marco, R.; Veder, J.-P.; Clarke, G.; Nelson, A.; Prince, K.; Pretsch, E.; Bakker, E. Phys. Chem. Chem. Phys. 2008, 10, 73–76.

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dx.doi.org/10.1021/ac200597b |Anal. Chem. 2011, 83, 4902–4908