Direct Evidence of Ionic Fluxes Across Ion-Selective Membranes: A

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Anal. Chem. 2001, 73, 2104-2111

Direct Evidence of Ionic Fluxes Across Ion-Selective Membranes: A Scanning Electrochemical Microscopic and Potentiometric Study Robert E. Gyurcsa´nyi,†,‡ E Ä va Pergel,§ Rena´ta Nagy,§ Imre Kapui,† Bui Thi Thu Lan,† Kla´ra To´th,§ | Istva´n Bitter, and Erno 1 Lindner*,‡

Research Group of Technical Analytical Chemistry of the Hungarian Academy of Sciences, Institute of General and Analytical Chemistry, Budapest University of Technology and Economics, 1111 Budapest, Szt. Gelle´ rt te´ r 4, Hungary, Institute of General and Analytical Chemistry, Budapest University of Technology and Economics, 1111 Budapest, Szt. Gelle´ rt te´ r 4, Hungary, Institute of Organic Chemical Technology, Budapest University of Technology and Economics, 1111 Budapest, Mu¨egyetem rkp. 3, Hungary, and Joint Graduate Program in Biomedical Engineering, The University of Memphis and University of Tennessee Health Science Center, Herff College of Engineering, Memphis, Tennessee 38152-6582

Scanning electrochemical microscopy (SECM) supplemented with potentiometric measurements was used to follow the time-dependent buildup of a steady-state diffusion layer at the aqueous-phase boundary of lead ionselective electrodes (ISEs). Differential pulse voltammetry is adapted to SECM for probing the local concentration profiles at the sample side of solvent polymeric membranes. Major factors affecting the membrane transportrelated surface concentrations were identified from SECM data and the potentiometric transients obtained under different experimental conditions (inner filling solution composition, membrane thickness, surface pretreatment). The amperometrically determined surface concentrations correlated well with the lower detection limits of the lead ion-selective electrodes. Ion-selective electrodes (ISEs) are chemical sensors for the selective determination of ions in complex matrixes. The largest area of routine application of ISEs is clinical chemistry.1 Electrodes are available for the most important clinically relevant ions (Na+, K+, Cl-, Ca2+, H+), and ion activities are routinely determined with an ion-selective potentiometry technique in biological fluids such as whole blood, serum, plasma, and urine.2-4 ISEs are also utilized in many other fields, including physiology, process control, and environmental analysis. † Research Group of Technical Analytical Chemistry of the Hungarian Academy of Sciences, Institute of General and Analytical Chemistry, Budapest University of Technology and Economics. ‡ The University of Memphis and University of Tennessee Health Science Center, Herff College of Engineering. § Institute of General and Analytical Chemistry, Budapest University of Technology and Economics. | Institute of Organic Chemical Technology, Budapest University of Technology and Economics. (1) Bakker, E.; Diamond, D.; Lewenstam, A.; Pretsch, E. Anal. Chim. Acta 1999, 393, 1-18. (2) Lindner, E.; Buck, R. P. Anal. Chem. 2000, 72, 336A-345A. (3) Meyerhoff, M. E. Clin. Chem. 1990, 36, 1567-1572. (4) Burritt, M. F. Clin. Chem. 1990, 36, 1562-1566.

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The detection limit (DL) is one of the most important analytical characteristics of ISEs.5 It was accepted for decades that the detection limits of ISEs are in the micromolar range. This view persisted despite of reports on nano- or picomolar DLs in metal ion buffer solutions6-8 or with optode membranes based on the same ion-exchange equilibrium as the corresponding potentiometric membranes.9 Only recently has it been fully realized that in certain concentration ranges minor ionic fluxes across ionselective membranes have an important role in determining the potentiometric response. The transport of ions is due to the nonperfect permselectivites and ion selectivities of the membranes. At micromolar and submicromolar sample activities, even minor ion transport significantly alters the surface concentrations; e.g., leaching ions perturb ionic concentrations in the immediate vicinity of the sensing surface. Since surface concentrations dominate the phase boundary potentials, these altered concentrations determine the electrode calibration slopes and the lower detection limits.10 Only a few papers discussed the importance of altered surface concentrations in interpreting nonidealities in the potentiometric responses. Those papers dealt mostly with precipitate-based sensors and pointed to the sensing membrane as a possible source of contamination. Pungor and To´th11 have shown that the solubility products of the precipitates constituting the ion-selective membranes determine the detection limits of precipitate-based electrodes. Later the stoichiometry of the precipitates12 and the (5) Buck, R. P.; Lindner, E. Pure Appl. Chem. 1995, 66, 2528-2536. (6) Amman, D.; Pretsch, E.; Simon, W.; Lindner, E.; Bezegh, A.; Pungor, E. Anal. Chim. Acta 1985, 171, 119-129. (7) Ruzicka, J. R.; Hansen, E. H.; Tjell, J. C. Anal. Chim. Acta 1973, 67, 155178. (8) Sokalski, T.; Maj-Zurawska, M.; Hulanicki, A. Microchim. Acta 1991, I, 285291. (9) Bakker, E.; Willer, M.; Pretsch, E. Anal. Chim. Acta 1993, 282, 265-271. (10) Bakker, E.; Bu ¨ hlmann, P.; Pretsh, E. Chem. Rev. 1997, 97, 3083-3132. (11) Pungor, E.; To´th, K. Analyst 1970, 95, 625-648. (12) Morf, W. E.; Kahr, G.; Simon, W. Anal. Chem. 1974, 46, 1538-1543. 10.1021/ac000922k CCC: $20.00

© 2001 American Chemical Society Published on Web 04/03/2001

adsorption/desorption processes13 were identified as factors influencing ion activities in the sample solution in the close proximity of the membrane and thus the detection limit.14,15 By determining the ion-exchange capacities of solvent polymeric membranes, it became clear that ionic impurities can be released from the IS membranes.16 The release of potassium ions from a potassium-selective membrane was determined by atomic absorption spectrometry,17 and the salt coextraction was related to the attainable detection limit.18 The dramatic improvement in the DL of ionophore-based solvent polymeric membranes by Sokalski and Pretsch19 initiated the current interest in membrane transport phenomena for restoring the nominal sample concentrations in the close proximity of the sensing membrane.20-23 Applying a concentration gradient across the membrane, and directing the transport from the sample toward inner filling solution, eliminated the undesirable leaching of primary ions from the membrane into the sample. Soon it was realized that galvanostatically controlled current can also be used to improve the detection limit. Lindner and co-workers22 showed that the best results are expected when the current-controlled membrane transport is combined with high linear flow rate, in the sample solution. In this way, the mass transport is controlled from both the membrane and the solution sides of the interface, and surface concentrations equal to the solution bulk can be achieved. Despite all the recent interest in the DL of ISEs, until now there are only indirect proofs for the existence of a diffusion layer generated by the release of ions on the surface of solvent polymeric membranes. Anticipated processes contributing to the buildup of a steady-state diffusion layer are ion exchange, coextraction, ad- and desorption, dissolution, and diffusion across the sensing membrane. In this paper, scanning electrochemical microscopy (SECM)24-26 and a novel potentiometric technique are used to track the buildup of the diffusion layer in time. In addition, SECM was applied for tracing primary ion concentration profiles in the solution layers adjacent to the IS membrane. SECM is an established method for monitoring concentration profiles in the close proximity of various targets utilizing voltam(13) Lindner, E.; To´th, K.; Pungor, E. Anal. Chem. 1982, 54, 202-207. (14) Harsa´nyi, E. G.; To´th, K.; Pungor, E.; Umezawa, Y.; Fujiwara, S. Talanta 1984, 31, 579-584. (15) Morf, W. E. The principles of ion-selective electrodes and of membrane transport; Akade´miai Kiado´: Budapest, 1981. (16) Lindner, E.; Graf, E.; Niegreisz, Zs.; To´th, K.; Pungor, E.; Buck, R. P. Anal. Chem. 1988, 60, 295-301. (17) Bu ¨ hlmann, P.; Yajima, S.; Tohda, K.; Umezawa, K.; Nishizawa, S.; Umezawa, Y. Electroanalysis 1995, 7, 811-816. (18) Mathison, S.; Bakker, E. Anal. Chem. 1998, 70, 303-309. (19) Sokalski, T.; Ceresa, A.; Zwickl, T.; Pretsh, E. J. Am. Chem. Soc. 1997, 119, 11347-11348. (20) Sokalski, T.; Zwickl, T.; Bakker, E.; Pretsch, E. Anal. Chem. 1999, 71, 12041209. (21) Sokalski, T.; Ceresa, A.; Fibbioli, M.; Zwickl, T.; Bakker, E.; Pretsch, E. Anal. Chem. 1999, 71, 1210-1214. (22) Lindner, E.; Gyurcsa´nyi, E. R.; Buck, R. P. Electroanalysis 1999, 11, 695702. (23) Morf, W. E.; Badertscher, M.; Zwickl, T.; de Rooij, N. F.; Pretsch, E. J. Phys. Chem. B 1999, 103, 11346-11356. (24) Engstrom, R. C.; Meany, T.; Tople, R.; Wightman, R. M. Anal. Chem. 1987, 59, 2005-2010. (25) Bard, A. J.; Fan, F.-R. F.; Pierce, D. T.; Unwin, P. R.; Wipf, D. O.; Zhou, F. Science 1991, 254, 68-74. (26) Bard, A. J.; Fan, F.-R. F.; Mirkin, M. V. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1993; Vol. 18.

metric27,28 or potentiometric probes.29,30 Mapping the diffusion layer of voltammetric electrodes constituted one of the earliest applications of SECM;24 improved techniques have been reported very recently.31 SECM has been a valuable technique for studying membranes and polymer films.32-34 It was utilized to monitor the ion transport through porous synthetic and biological membranes including mica,35 skin,36 dentine,37,38 and laryngeal cartilage.39 The permeation of weak acids,40 ammonium ions,41 and ferrocene42 through planar lipid bilayer membranes has also been successfully tracked. Despite the multitude of SECM applications, studies of ion-selective membranes are limited to the mapping of pH changes, caused by cyanide corrosion, in the close vicinity of silver iodide pellets.43,44 To monitor leaching primary ions from a solvent polymeric ion-selective membrane, minor concentration alterations (µM) should be measured. Thus, a sensitive, sequential differential pulse voltammetric (DPV) measurement mode was adapted to the SECM methodology. A lead-selective membrane was selected as model because of the sensitivity of differential pulse voltammetry for lead determinations. EXPERIMENTAL SECTION Reagents and Materials. Poly(vinyl chloride) (PVC), 2-nitrophenyl octyl ether (o-NPOE), bis(2-ethylhexyl) sebacate (DOS), sodium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (NaTFPB), and tetrahydrofuran (THF) were from Fluka AG (CH-8071, Buchs, Switzerland). A novel lead ionophore, N,N,N′,N′-tetracyclohexyl2,2′-[2,2′-sulfenediilbis(5-chlorophenoxy)] diacetamide (BME 3806) was synthesized and provided by the Department of Organic Chemical Technology (Budapest University Technology and Economics).45 All the reagents were of the highest analytical grade available (Fluka, Merck (Darmstadt, Germany), Reanal (Budapest, Hungary)) and were used without further purification. The (27) Bard, A. J.; Fan, F. R. F.; Kwak, J.; Lev, O. Anal. Chem. 1989, 61, 132138. (28) Engstrom, R. C.; Pharr, C. M. Anal. Chem. 1989, 61, 1099A-1100A, 1102A, 1104A. (29) Wei, C.; Bard, A. J.; Nagy, G.; Toth, K. Anal. Chem. 1995, 67, 1346-1356. (30) Denuault, G.; Frank, M. H. T.; Peter, L. M. Faraday Discuss. 1992, 94, 23-35. (31) Amatore, C.; Szunerits, S.; Thourin, L.; Warkocz, J.-S. Electrochem. Comm. 2000, 2, 353-358. (32) Lee, C.; Bard, A. J. Anal. Chem. 1990, 62, 1906-13. (33) Frank, M. H. T.; Denuault, G. J. Electroanal. Chem. 1993, 354, 331-339. (34) Kapui, I.; Gyurcsa´nyi, E. R.; Nagy, G.; To´th, K.; Arca, M.; Arca, E. J. Phys. Chem. B 1998, 102, 9934-9939. (35) Scott, E. R.; White, H. S.; Phipps, J. B. J. Membr. Sci. 1991, 58, 71-87. (36) Scott, E. R.; White, H. S.; Phipps, J. B. Solid State Ionics 1992, 53-56, 176-83. (37) Macpherson, J. V.; Beeston, M. A.; Unwin, P. R. J. Chem. Soc., Faraday Trans. 1995, 91, 1407-1410. (38) Nugues, S.; Denuault, G. J. Electroanal. Chem. 1996, 408, 125-140. (39) Macpherson, J. V.; O′Hare, D.; Unwin, P. R.; Winlove, C. P. Biophys. J. 1997, 73, 2771-2781. (40) Antonenko, Y. N.; Denisov, G. A.; Pohl, P. Biophys. J. 1993, 64, 17011710. (41) Antonenko, Y. N.; Pohl, P.; Denisov, G. A. Biophys. J. 1997, 72, 21872195. (42) Yamada, H.; Matsue, T.; Uchida, I. Biochem. Biophys. Res. Commun. 1991, 180, 1330-1334. (43) Horrocks, B. R.; Mirkin, M. V.; Pierce, D. T.; Bard, A. J.; Nagy, G.; Toth, K. Anal. Chem. 1993, 65, 1213-1224. (44) To´th, K.; Nagy, G.; Horrocks, B. R.; Bard, A. J. Anal. Chim. Acta 1993, 282, 239-246. (45) Lan, B. T. T.; Horvath, M.; To´th, K.; Gru ¨ n, A.; Bitter, I. Anal. Chim. Acta, submitted.

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aqueous solutions were prepared with deionized (DI) water doubly distilled from quartz. The platinum and gold wires used for voltammetric microelectrode preparation were obtained from Goodfellow Cambridge Ltd. (Cambridge, England). Ion-Selective Membranes and Electrodes. Membranes contained 1.0 wt % lead ionophore BME 3806, 0.63 mol % NaTFPB, 62.7 wt % o-NPOE or DOS, and 35.4 wt % PVC. The membrane components (total, ∼200 mg) were dissolved in 2 mL of THF and poured into a glass ring (i.d. 39 or 28 mm) fixed on a glass plate.46 After evaporation of THF membranes of ∼ 80- or 200-µm thickness were obtained, respectively. Six-millimeter-diameter disks were punched from the membranes and glued to the end of a PVC tube (6-mm outer and 3-mm inner diameter) with a mixture of THF and PVC. The inner compartments of the electrodes were filled with 1 mM or 0.5 M Pb(NO3)2 solutions or with a lead ion buffer solution of 5 × 10-2 M Na2EDTA and 1 mM Pb(NO3)2, pH 4.5, to set the free Pb2+ ion concentration to 10-12 M. For pH adjustment glacial acetic acid was used. The inner reference electrode (SCE) was connected to the internal filling solution (IFS) through a salt bridge containing 0.1 M lithium acetate (LiOAc) in 3% agar-agar gel. Before measurements, the ISEs were conditioned in 1 mM Pb(NO3)2, unless it was indicated otherwise. EMF Measurements. All potentiometric measurements were performed at room temperature versus Ag/AgCl reference electrode (Radelkis, Budapest, Hungary) with 0.1 M LiOAc as salt bridge electrolyte. The potentiometric data were recorded with an OP 208/1 model (Radelkis) or a 16-channel pH meter (Lawson Labs, Inc., Malvern, PA) interfaced to personal computers. Before measurements, the electrodes were washed thoroughly with distilled water and then immersed into 100 mL of a 10-2 M solution of Mg(OAc)2 pH 4.5. The calibration curves were recorded between 10-8 and 10-2 M Pb2+ by adding small increments of Pb(NO3)2 stock solutions to the background buffer solution. The effect of flow rate on the detection limit was evaluated using the exponential dilution method47 in a wall-jet cell arrangement. The flow rate was varied between 2.5 and 9.5 mL/min. These values correspond to 5.3 and 20.2 cm/s or 6.8 and 25.8 cm/s linear flow rate perpendicular or parallel to the electrode surface, respectively.48 The flow rate was checked before and after each dilution experiment. The activity coefficients were calculated using the extended Debye-Hu¨ckel equation, and the measured potentials were corrected for the liquid junction potential according to the Henderson formalism.49 The IUPAC recommendation was applied to determine the detection limits.5 Potentiometric and SECM Monitoring of Ionic Breakthrough across a Primary Ion-Free Membrane. For these experiments, membranes that were never in contact with lead ions were used. The membranes were glued to the end of a Tygon (PVC) tube and were fully equilibrated with the background buffer solution (10-2 M solution of Mg(OAc)2 pH 4.5). The conditioning solution in the inner compartment was replaced by 500 µL of 1 mM NaCl and the data acquisition was started. Constant chloride (46) Craggs, A.; Moody, G. J.; Thomas, J. D. R. J. Chem. Educ. 1974, 51, 541544. (47) Horvai, G.; To´th, K.; Pungor, E. Anal. Chim. Acta 1976, 82, 45-54. (48) Lindner, E.; To´th, K.; Pungor, E. Dynamic characteristics of ion-selective electrodes; CRC Press: Boca Raton, FL, 1988. (49) Meier, P. C.; Ammann, D.; Morf, W. E.; Simon, W. In Medical and Biological Applications of Electrochemical Devices; Koryta, J., Ed.; Wiley: Chichester, New York, Brisbane, Toronto, 1980; pp 13

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concentration is necessary for stable potential of the inner reference electrode. At t ) 0, the inner solution was spiked with 500 µL of 1 M or 2 mM lead nitrate solution in 1 mM NaCl. During potentiometric measurements, the membrane potential change, due to lead ion transport across the membrane, was recorded as a function of time. In the SECM studies, at t ) 0 the inner filling solution compartment of the electrode was filled with 0.5 M or 1 mM Pb(NO3)2 and the appearance of lead ions on the opposite side of the membrane was monitored in real time with a mercury film microelectrode. Preparation of Mercury Film Microelectrodes. SECM microelectrodes were prepared by sealing platinum and gold wires (125- and 25-µm diameter) into soft glass capillaries and polishing off the sealed end. The microelectrodes were transformed into SECM tips by conically sharpening the glass shielding around the metal disk as was described earlier.26 The preparation of the mercury film on platinum followed a slightly modified recipe of Wehmeyer and Wightman.50 The surface of the platinum electrode was activated by cycling the electrode potential between -200 and +1500 mV at 50 mV/s polarization rate in 0.05 M H2SO4 for 3 min.51 Then the potential of the electrode was held at +200 mV vs SCE in the same solution for 4 min. The mercury film was electrochemically deposited, holding the potential of the Pt electrode at 0 mV vs SCE in a deoxygenated 1 M KNO3 solution containing 5.7 mM Hg(I)NO3 and 0.5% HNO3 for 15 or 120 min (15 min for the 25-µm and 120 min for 125-µm-diameter platinum disk electrodes). The electrodes used for SECM measurements had a coherent mercury deposit in the shape of a spherical segment with a thickness of ∼10 µm as determined by optical microscopy. The mercury film was renewed by removing the old mercury layer in concentrated HNO3 and repeating all steps of the preparation procedure. The simplicity of preparation made the amalgamated gold electrode an attractive candidate as SECM probe. Unfortunately, with the amalgamated gold electrode, we could not achieve the same performance as with the platinum-based mercury film electrodes. The slope of the calibration curve was continuously changing and the DL was not adequate (5 × 10-6 M). Instrumentation. The electrochemical pretreatment of the platinum electrode and the electrochemical deposition of the mercury film were performed with a model 174A potentiostat (EG&G, PAR). The SECM measurements were carried out with a home-built SECM apparatus described in ref 52. For the electrochemical detection, a Pgstat 10 bipotentiostat (Ecochemie, Utrecht, The Netherlands) equipped with a current preamplifier module for low-current measurement was used. The arrangement of the electrochemical cell used in the SECM is shown in Figure 1. RESULTS AND DISCUSSION SECM Probe for Imaging Pb2+ Concentration Profiles. Analytical Characteristics of Mercury Film Microelectrodes. Tracing lead ion concentration profiles in the close proximity of ISE membranes requires a microscopic probe that works reliably below the DL of the ISEs. As voltammetric techniques offer a very (50) Wehmeyer, K. R.; Wightman, R. M. Anal. Chem. 1985, 57, 1989-1993. (51) Nyholm, L.; Bjorefors, F. Anal. Chim. Acta 1996, 327, 211-222. (52) Kapui, I.; Csa´ny, B.; Nagy, G.; To´th, K. Magy. Ke´ m. Foly. 1998, 5, 195207.

Figure 1. Schematic design of the SECM cell.

sensitive method for measuring lead ion concentrations; a mercury film microelectrode was selected as a SECM probe. In our studies, repeated scan DPV with a platinum-based mercury film electrode was used to determine the kinetics of primary ion leaching and to map the aqueous diffusion layer. The use of repeated scans instead of constant-potential amperometry, as is common in SECM studies, had several advantages. The perturbation of the probed concentration profiles is less likely during short DPV scans, separated by adequate resting periods, compared to the continuous electrolysis in amperometry. The use of the intermittent DPV mode allows the use of larger SECM probe electrodes (with more stable response). In the time frame of a single DPV scan, a ∼50-µm-thick diffusion layer evolves in the vicinity of the 125-µm platinum-based mercury film electrode used in these studies. However, the perturbation caused by the probe is compensated with long, approximately 5-10-min “resting” periods (see examples later) and large approaching steps (larger than 20 µm). Finally, the selectivity of the DPV method over constant-potential amperometry was essential to minimize the interference related to fluctuations in the residual oxygen concentration of the sample. By selecting 10-mV pulse amplitude, the Pb2+ and the first oxygen reduction peak could be separated. In summary, a DL of 5 × 10-7 M Pb2+ could be routinely reached; however, loss of sensitivity and detection limit of the electrodes under continuous use was a general experimental fact. Typical voltammetric recordings and the corresponding calibration curve are shown in Figure 2. Repeated calibration of the electrode demonstrated unchanged voltammetric response over at least 2 days (stored in DI water). After 2 days, the mercury films had to be renewed because losses of mercury were visible by optical microscopy. The slight shift in the peak potentials toward more negative values with increasing Pb2+ concentration is due to the amalgamation process (the free energy of the amalgamation is added to the standard redox potential). The distance from the target to the electrode could not be estimated by the commonly used SECM technique. The approaching curves based on dissolved oxygen were distorted as oxygen readily permeates the ISE membrane. Introducing other redox mediators in the solution would represent a contamination source for the electrochemical cell. Thus, the following semiquantitative approach was applied. The tip positioning was followed

Figure 2. DPV curves recorded in Pb(NO3)2 solutions (background: 10 mM Mg(OAc)2, pH 4.5) with a 125-µm-diameter mercury film working electrode. Pulse time, 50 ms; pulse amplitude, 10 mV. The inset shows the relevant calibration curve.

laterally with a magnification device providing ∼5-µm resolution. The tip was advanced very slowly toward the membrane using the stepper motor until the solution gap between the membrane and the mercury film electrode disappeared. The free movement in the lateral direction was checked to ensure that they were not in contact with each other. At this point, the residual distance between the tip (the mercury film) and the membrane was assumed to be within 5 µm (“d ) 0”) from the target surface. We considered that further approach of the tip was not safe because of the mechanical fragility of the mercury film and the nonideal geometry of the system. Once the position of the target membrane was fixed, the target electrode was retracted to a well-defined distance. For mapping the diffusion layer, the direction of the movement was reversed and the electrode was driven back the same distance. As the sensor tip approached the ISE surface in the vertical (z) direction, its movement was stopped at certain time intervals and a DPV curve was recorded. The procedure was repeated in a preset timebased scheme until the microelectrode tip “made contact” with the target. Tracking the Kinetics of Membrane Transport with SECM and Potentiometry. The aqueous diffusion layer developed in the proximity of an ion-selective membrane at steady state assumes balanced fluxes at the membrane interface (i.e., equal fluxes in each phase). In our experiments, the release of ions from the membrane is compensated by the transport of ions toward the solution bulk according to eq 1,18,20 where, C′I, and CIsample are

(C′I - CIsample)

Diaq δiaq

DILmem

) ([IL]′′ - [IL]′)

δm

(1)

surface and bulk concentrations of primary ions in the aqueous solution; [IL]′ and [IL]′′ are the ionophore-primary ion complex mem concentrations at the two sides of the membrane; Daq i and DIL are diffusion coefficients of the primary ion and the ionophoreprimary ion complex in the aqueous solution and in the membrane, respectively; δaq is the diffusion layer thickness in the aqueous solution; and δm is the membrane thickness. Analytical Chemistry, Vol. 73, No. 9, May 1, 2001

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Figure 3. The buildup of a steady-state diffusion layer. Currenttime profiles recorded at ∼5-µm distance from the membrane of a lead-selective electrode with a 125-µm SECM probe. At t ) 0, the inner compartment of the ISE is filled with (1) 0.5 M Pb(NO3)2 or (2) 10-3 M Pb(NO3)2. Background electrolyte: 10-2 M Mg(OAc)2, δm ) 80 µm.

In unstirred solutions, δiaq ≈ δm, there is a 3 orders of magnitude difference between the membrane and solution diffusion coefficients in eq 1. To satisfy the equation, the concentration gradient term on the left side (solution transport) must be 3 orders of magnitude smaller compared to the similar term on the right side (membrane transport). In other words, the large difference in the diffusion coefficients in the two phases is compensated by a similarly large but reversed difference in the concentration gradients. Recently, several papers were published on evolving concentration profiles within the bulk of ion-selective membranes during anion and cation interference.53,54 In this work, the concentration profiles in the steady-state diffusion layer were tracked and the measured surface concentrations correlated with the DL of ISEs. To characterize the ionophore-mediated and/or coextractioninduced primary ion fluxes across ion-selective solvent polymeric membranes, we were interested in the time-dependent buildup of the steady-state diffusion layer. The ion-selective membrane was approached with the SECM measuring tip at ∼5 µm as is shown in Figure 1. At t ) 0, the inner compartment of the electrode was filled with 10-3 or 0.5 M Pb(NO3)2, respectively. The surface concentration of lead ions was intermittently measured by recording DVP curves. Figure 3 shows the peak current of DPV scans as a function of time. After the breakthrough, the lead ion concentration on the surface gradually increases until steady state is attained. At steady state, the superficial Pb2+ concentration for the 0.5 M and 1 mM Pb(NO3)2 IFSs, corresponds to 7.9 and 1.7 µM, respectively. Apparently, the breakthrough of lead ions occurs in shorter time (1.8 vs 6.7 h) when the inner filling solution is more concentrated. The breakthrough time (1.8 h) in combination with the membrane thickness (80 µm) offers a simple way to calculate an estimated diffusion coefficient of the (53) Schneider, B.; Zwickl, T.; Federer, B.; Pretsch, E.; Lindner, E. Anal. Chem. 1996, 68, 4342-4350. (54) Lindner, E.; Zwickl, T.; Bakker, E.; Lan, B. T. T.; To´th, K.; Pretsch, E. Anal. Chem. 1998, 70, 1176-1181.

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Figure 4. Potential-time transients of a lead ion-selective electrode, which have not been in contact with lead ions before the experiment, as an effect of induced lead ion transport from the IFS toward the sample solution (pH 4.5, 10-2 M Mg(OAC)2, δm ) 80 µm) (A). At t ) 0, the lead ion concentration of the IFS was fixed at (1) 0.5 M and (2) 1 mM with lead nitrate solutions. The evaluation of the breakthrough times (B) and the effect of stirring on the potential of a leadselective ISE with 0.5 M Pb2+ in the inner filling solution at the end of a 24-h experiment (C) are included.

ionophore-lead ion complex in the membrane by the EinsteinSmoluchowski formula. The calculated diffusion coefficients (∼5 × 10-9 cm2/s) are somewhat smaller compared to the published values (∼2 × 10-8 cm2/s 55,56). The experimental conditions explain the discrepancy. As the SECM probe has a limited DL (∼1 × 10-7 M), the breakthrough times should be considerably overestimated, resulting in smaller diffusion coefficients. The large difference in the breakthrough times determined with different internal filling solutions can be explained similarly. The much smaller flux across the membrane with the more dilute IFS needs more time to establish a measurable concentration in the superficial layer. One of our reviewers suggested monitoring of the membrane potential in identical experiments to prevent overestimation of the breakthrough time. It was expected that the breakthrough of lead ions would dramatically shift the electrode potential toward more positive potential values. The shape of the potential transients for lead-selective electrodes with 1 mM and 0.5 M lead nitrate IFSs and membrane thickness of ∼80 µM is shown in Figure 4 A. A large potential drop is observed at the addition of lead to the inner compartment. Subsequent to this potential change, the measured potential increased gradually. The change in the slope of the curve, at the expected breakthrough time, was blurred. This behavior is most likely related to concomitant, gradual change of Pb2+ concentration in the ion-selective membrane and at both sides of (55) Pendley, B. D.; Lindner, E. Anal. Chem. 1999, 71, 3673-3676. (56) Iglehart, M. L.; Buck, R. P.; Pungor, E. Anal. Chem. 1988, 60, 290-295.

the membrane solution interfaces. Concentration changes at the two sides of the membrane or two sides of the membrane/solution interface have opposite effects on the membrane potential. Since in these experiments the lead ion concentration is continuously changing within the membrane, the measured potential cannot be solely correlated with the lead concentration in the aqueous diffusion layer. However, the breakthrough times obtained with this potentiometric method are significantly smaller than those determined by SECM measurements, and the calculated diffusion coefficients agree well with the previously published values (D ) (2.58 ( 1.12) × 10-8 cm2/s, N ) 10). The relative high standard deviation is related to the difficulty in evaluating the vague breakthrough times (see Figure 4B) and variations of the membrane thickness. The diffusion coefficients obtained for 0.5 M and 1 mM Pb2+ IFSs were the same at the 95% confidence level. Mapping of the Aqueous Diffusion Layer of Lead-Selective Membrane Electrodes. For the direct imaging and quantification of the diffusion layer generated by a lead-selective electrode, the SECM cell was filled with the background electrolyte. Mg(OAc)2 solution was selected because of the superior selectivity of the pot lead ionophore over magnesium ions (-log KPb,Mg ) 5.2). The pH of the solution was adjusted to pH 4.5 with acetic acid. At this pH, the formation of carbonate complex is negligible, precipitation of lead could be avoided, and the hydrogen ions do not interfere with the lead ion measurement. The local lead concentrations were monitored with SECM in the aqueous diffusion layer (deaerated solution) at steady state as a function of the lead concentrations in the IFS, the membrane thickness, polarity, and surface treatment. The membranes used in these studies were fully equilibrated in 10-3 M Pb(NO3)2 solution for a minimum of 1 day. Before the SECM measurements, the electrode membranes were thoroughly rinsed with distilled water or soaked in stirred distilled water for 1 h. The results of these studies are summarized in Figure 5 and Table. 1. The finite dimension of the tip limits the accuracy of the measured concentration values. First, due to the curved shape mercury film, the SECM tip probes a surface-weighted average concentration of the diffusion layer generated by the ISE. Thus, correlating the measured current with the lead ion concentration, using the calibration curves determined in homogeneous solution, a systematic, negative error is introduced. However, the SECM electrode placed in proximity of the membrane surface can impede the free diffusion of leaching lead ions. The latter may contribute to a positive error in the calculated surface concentration. Figure 5 shows that the lead ion concentration increases in all of the studied experimental conditions as the SECM tip approaches the surface of the lead-selective membrane. These increases are more pronounced for ISEs with more concentrated IFS (Figure 5A), thinner membranes (Figure 5B), and polar membrane plasticizer (Table 1). As expected from the kinetic experiments (Figures 3 and 4), the steady-state concentration profile recorded with 0.5 M Pb(NO3)2 as IFS is running above the concentration profile measured with 10-3 M Pb(NO3)2 inner filling solution (Figure 5A). The surface concentrations with an 80-µm-thick membrane were determined as 3 × 10-5 and 6 × 10-6 M, respectively. Increasing the membrane thickness from 80 to 200 µm decreases the ionic flux across the membrane, which

Figure 5. Concentration profiles in the close proximity of a leadselective liquid membrane. The SECM approaching curves were recorded in “sequential DPV” mode. Working electrode, 125-µmdiameter mercury film electrode; background electrolyte, 10 mM Mg(OAc)2. (A) δm ) 80 µm; IFS, (1) 10-3 M Pb(NO3)2 and (2) 0.5 M Pb(NO3)2. (B) (1) δm ) 200 µm and (2) δm ) 80 µm; IFS, 0.5 M Pb(NO3)2. Table 1. Summary of the Superficial Lead Ion Concentrations Measured with DPV-SECM for Different Electrode Membranes, Inner Filling Solutions, and Conditioninga

membrane BME 3806 o-NPOE 80 µm

inner electrolyte (Pb(NO3)2) concn (M) 10-3 0.5

BME 3806 o-NPOE 200 µm BME 3806 DOS 80 µm BME 3806 DOS 200 µm

0.5 0.5 0.5

conditioning

surface concn of Pb2+ (µM)

A B A B A B A B A

5.6 2.0 30 8.1 15 6.5 18 7.0 9.8

a Before the SECM measurement, the ISEs were rinsed with DI water (A) or soaked for 1 h in DI water under continuous stirring (B). The measurements were performed in 10 mM Mg(OAc)2 pH 4.5 solution.

contributes to a drop in the surface concentration as is shown in Figure 5B. Long-time conditioning is beneficial in minimizing the undesirable contamination of the surface layers by the sensing membrane (Table 1). The lead ion concentrations determined in close proximity of the long-time-conditioned membranes were 2.0 µM for 1 mM Pb2+ and 8.1 µM for 0.5 M Pb2+ IFSs. These values are in very good agreement with the steady-state superficial Analytical Chemistry, Vol. 73, No. 9, May 1, 2001

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concentration values, determined during breakthrough experiments, for membranes that have not been in contact with lead ion solutions before the measurement, 1.7 and 7.9 µM, respectively. The surface lead concentrations, summarized in Table 1, can be utilized to calculate the ionic flux across the aqueous diffusion layer and to estimate the diffusion coefficient of the ionophore-ion complex in the membrane. Inserting the experimentally determined 3 × 10-5 M surface lead concentration (C′I) in combination with the thickness of the steady-state aqueous diffusion layer, as shown in Figure 5 (∼200 µm), into the left side of eq 1 gives a flux of 15 pmol/cm2‚s. For this calculation, an aqueous diffusion coefficient of Diaq ) 1.0 × 10-5 cm2/s is used and the contribution of the sample bulk concentration is neglected (CIsample ) 0). At steady state, the flux across the membrane is equal to the flux across the aqueous diffusion layer as is given by eq 1. To calculate the diffusion coefficient of the ionophore-lead ion complex, a linear concentration profile is assumed in the membrane with [IL]′ ≈ 0 at the sample solution and [IL]′′ ≈ 10-2 M on the IFS side. This gives a calculated diffusion coefficient DILmem ∼1.2 × 10-8 cm2/s for the 80 µm and 1.8 × 10-8 cm2/s for the 200-µm-thick o-NPOE plasticized membrane. These values are in the range of the published diffusion coefficient data ((1-5) × 10-8 cm2/s53,55-58). Since the steady-state concentration values were used for their calculation, the calculated values are not influenced by the sensitivity of the SECM probe. In all SECM measurements, the lead concentration is actually measured in a stream of lead ions (15 pmol/cm2‚s) constantly flowing from the membrane perpendicularly to the SECM probe. Under these conditions, the perturbation related to the consumption of Pb2+ ions by the probe is assumed to be negligible (calculated from the total charge during one DPV scan). In addition, the “active” surface area (in contact with the IFS) of the ISE is ∼500 times larger than the area of the 125-µm SECM tip. Figure 4C shows the stirring sensitivity of the steady-state potentiometric signal. It proves clearly that the aqueous diffusion layer reestablishes in a relatively short time even after extensive and rigorous stirring. The perturbation in the diffusion layer by the voltammetric SECM probe is negligible compared to intensive stirring. Based on the above, the depletion-related perturbation of the SECM probe is expected to fade away between the repeated scans. Effect of Convection on the Detection Limit of ISEs. The presented data clearly demonstrate that surface lead concentration often strongly deviates from the bulk values. The deviations depend on the experimental conditions and the membrane parameters. The potentiometrically determined DLs of the leadselective electrodes, used as targets in the SECM studies, correlated well with the surface concentrations determined with DPV in the close proximity of these sensor membranes. According to eq 1, the flux of primary ions away or to the electrode surface can be controlled also from the sample solution side. Intensive mixing works against the buildup of an extensive diffusion layer. In Figure 6A it is demonstrated that the unfavorable effect of primary ion leaching from the polymeric membrane could be partially offset22 when the calibrations are performed in intensively stirred solutions or in flow-through systems at high flow rate. (57) Armstrong, R. D.; Horvai, G. Electrochim. Acta 1990, 35, 1-17. (58) Nahir, T. M.; Buck, R. P. J. Phys. Chem. 1993, 97, 12363-12372.

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Figure 6. Effect of flow rate and IFS composition on the calibration curves of lead-selective electrodes. (A) Calibration curves of a leadselective electrodes with 0.5 M Pb(NO3)2 (1, 2) and 10-3 M Pb(NO3)2 (3, 4) IFSs, in stirred (2, 4) and unstirred (1, 3) solutions δm ) 80 µm; plasticizer, o-NPOE; background electrolyte, 10 mM Mg(OAc)2, pH 4.5. (B) Effect of flow rate on the calibration curve of the lead-selective electrode ((1) 2.5, (2) 6.0, and (3) 9.5 mL/min). IFS, 5 × 10-2 M EDTA-Na2, 1 mM Pb(NO3)2, δm ) 200 µm; plasticizer, o-NPOE; background electrolyte, 10-4 M Mg(OAc)2, pH 4.5.

The reduction of the diffusion layer thickness is similarly advantageous when the ionic fluxes are directed toward the inner filling solution.59,60 This is the approach of Sokalski et al. to eliminate the disadvantageous effect of undesirable leaching on the DL.19 If the ion flux in the membrane is too high, it depletes primary ions in the adjacent solution layer and an apparent superNernstian response is recorded. To control or adjust the ion transport in the membrane for an extended Nernstian response is difficult. However, an increased mass transport in the sample solution can counterbalance the ion uptake rate by the membrane. Figure 6B shows the potential response of a lead-selective electrode at different flow rates when Pb2+ ion transport is guided toward the inner filling solution. The thickness of the Nernstian diffusion layer could be estimated by assuming ideal geometry of the wall-jet setup,48

δN ) 3L1/2vpar-1/2ν1/6D1/3

(2)

where vpar is the linear flow rate parallel to the electrode surface,

L is the coordinate in the direction of flow (0-0.3 cm), ν is viscosity (∼10-6 m2/s), and D is the diffusion coefficient in the aqueous solution layer. The calculated mean δN values were 28, 18, and 14 µm for 2.5, 6, and 9.5 mL/min flow rates, respectively. The results clearly demonstrate that by enhancing the flow rate in the solution the diffusion layer thickness can be very effectively reduced. Thus, the increased flux of primary ions could correct the apparent super-Nernstian response of the electrode. CONCLUSIONS SECM and direct potentiometry were used to track the breakthrough of lead ions from the inner filling solution across lead-selective membranes. Both the potential of the target ISE and the current recorded by a mercury film electrode placed on the opposite (sample) side of the membrane were used to detect independently the appearance of these ions. SECM proved to be a valuable tool for tracking the ion transport-related buildup of a steady-state diffusion layer at the surface of ionophore-based (59) Kapui, I.; Pergel, E.; Lan, B. T. T.; To´th, K. In Symposium on Electrochemical and Biosensors, Book of Abstracts 78: Ma´trafu ¨ red, Hungary, 1998. (60) Zwickl, T.; Sokalski, T.; Pretsch, E. E. Electroanalysis 1999, 11, 673-680.

solvent polymeric membranes. The conjunction of DPV and SECM provided the first direct evidence for the time-dependent release of lead ions from lead-selective solvent polymeric membranes. By mapping the concentration profiles in the close proximity of ionophore-based liquid membranes, the major factors affecting the surface concentrations and the experimentally attainable detection limit of ISEs could be unambiguously identified. Although the experimentally determined surface concentration values cannot be considered fully quantitative, the surface concentrations and ISE detection limits correlate well with each other. ACKNOWLEDGMENT The financial support of the Hungarian Scientific Research Fund (OTKA T 016583 and T 022507) and the FRG grant 2-22307 of the University of Memphis are gratefully acknowledged.

Received for review August 4, 2000. Accepted February 6, 2001. AC000922K

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