Picomolar Detection Limits with Current-Polarized Pb2+ Ion

The steady-state Na+ ion flux (in our example) enforced by the positive current ..... Ives, D. J. G.; Yanz, G. J. Reference Electrodes; Academic Press...
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Anal. Chem. 2001, 73, 4249-4253

Picomolar Detection Limits with Current-Polarized Pb2+ Ion-Selective Membranes E Ä va Pergel,† Robert E. Gyurcsa´nyi,‡,§ Kla´ra To´th,† and Erno 1 Lindner,*,§

Institute of General and Analytical Chemistry, Budapest University of Technology and Economics, 1111 Budapest, Szt. Gelle´ rt te´ r 4, Hungary, Research Group of Technical Analytical Chemistry of the Hungarian Academy of Sciences, 1111 Budapest, Szt. Gelle´ rt te´ r 4, Hungary, and Joint Graduate Program in Biomedical Engineering, The University of Memphis and University of Tennessee Health Science Center, Herff College of Engineering, 330 Engineering Technology, Memphis, Tennessee, 38152-6582

Minor ion fluxes across ion-selective membranes bias submicromolar activity measurements with conventional ion-selective electrodes. When ion fluxes are balanced, the lower limit of detection is expected to be dramatically improved. As proof of principle, the flux of lead ions across an ETH 5435 ionophore-based lead-selective membrane was gradually compensated by applying a few nanoamperes of galvanostatic current. When the opposite ion fluxes were matched, and the undesirable leaching of primary ions was eliminated, Nernstian response down to 3 × 10-12 M was achieved. Until recently, it was accepted that the analytical range of solvent polymeric ion-selective electrodes (ISEs) is limited by anion and cation interference.1 At high concentrations, indeed, the loss of permselectivity or salt coextraction levels the potential response. However, the nonidealities in the potentiometric responses of ISEs at low concentrations cannot be solely related to the lack of ion selectivity. In solutions containing “only” primary ions, for example, it is difficult to explain deviations from the Nernstian response based on nonideal selectivities. Recently, it was shown that minor ion fluxes in to and out of the ion-selective membrane can contribute to apparent super- and sub-Nernstian response slopes and impair the analytical measurement.2,3 In the presence of ion fluxes, the surface concentrations deviate from the concentrations in the sample bulk and dominate the phase boundary potential. Primary ion outflow from the membrane increases the primary ion activity in the close proximity of the membrane surface and taints the experimentally determined detection limit (DL) and selectivity coefficients. A schematic example is shown in Figure 1A. Indeed, Mathison and Bakker4 showed that the DL of valinomycin-based potassium-selective membranes is a function of the inner filling solution (IFS) †

Budapest University of Technology and Economics. Research Group of Technical Analytical Chemistry of the Hungarian Academy of Sciences. § Herff College of Engineering. (1) Bakker, E.; Bu ¨ hlmann, P.; Pretsch, E. Chem. Rev. 1997, 97, 3083-3132. (2) Sokalski, T.; Zwickl, T.; Bakker, E.; Pretsch, E. Anal. Chem. 1999, 71, 12041209. (3) Sokalski, T.; Ceresa, A.; Fibbioli, M.; Zwickl, T.; Bakker, E.; Pretsch, E. Anal. Chem. 1999, 71, 1210-1214. (4) Mathison, S.; Bakker, E. Anal. Chem. 1998, 70, 303-309. ‡

10.1021/ac010094a CCC: $20.00 Published on Web 07/27/2001

© 2001 American Chemical Society

Figure 1. Elimination of primary ion leaching from the inner filling solution across the membrane, into the sample by negative current. Hypothetical concentration profiles in a lead-selective membrane before (A) and after (B) applied negative current. The arrows indicate the direction of net ion fluxes.

concentration. More concentrated IFS generates larger ion flux from inside out and the DL appears at higher concentrations. The increased surface concentrations and the kinetics of the transport across plasticized PVC membranes were determined with scanning electrochemical microscopy (SECM).5 In their pioneering work, Sokalski and Pretsch6 eliminated the undesired leaching of primary ions from the membrane into the sample solution. Using an ion buffer solution on the backside of the membrane, they established extremely low free primary ion activity in the IFS and reversed the direction of ion transport across the membrane. In this way, the analytical range of lead and calcium ion-selective sensors was extended into the subnanomolar concentration range.3,6 The prerequisite for this impressive improvement was the existence of ionophores with extremely (5) Gyurcsa´nyi, R. E.; Pergel, EÄ .; Nagy, R.; Kapui, I.; Lan, B. T. T.; To´th, K.; Bitter, I.; Lindner, E. Anal. Chem 2001, 73, 2104-2111. (6) Sokalski, T.; Ceresa, A.; Zwickl, T.; Pretsch, E. J. Am. Chem. Soc. 1997, 119, 11347-11348.

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Figure 2. Elimination of primary ion transport from the sample solution across the membrane, into the inner filling solution by positive current. Hypothetical concentration profiles in a lead-selective membrane before (A) and after (B) applied positive current. The arrows indicate the direction of net ion fluxes.

small selectivity coefficients. As soon the primary ion leaching was eliminated, these intrinsic selectivity coefficients could be determined with simple separate solution method.7 However, the approach of Sokalski and Pretsch occasionally lead to apparent super-Nernstian response.2,3,8,9 At low sample activities, the primary ion transport toward IFS depletes the superficial solution layer in primary ions and decreases the surface primary ion activity below the solution bulk value. A schematic example is shown in Figure 2A. The influence of key parameters on the shape of the calibration curves and the lower detection limit were summarized recently.9 The disturbing effect of ion leaching or uptake by the sensor membrane can be decreased in flowing solutions10,11 because the flowing solution prevents the buildup of an extensive diffusion layer in the proximity of the membrane.5 Passing nanoampere-level direct current across the membrane is also an appealing possibility to prevent minor ion fluxes completely.11 In this work, we will demonstrate that reproducible, Nernstian response down to 3 × 10-12 M can be achieved without apparent super-Nernstian response when there is no net primary ion flux across the membrane; i.e., the ion fluxes are balanced by external current. EXPERIMENTAL SECTION Reagents. For all experiments, deionized water (MilliQ Millipore, Bedford MA; 18 MΩ) and chemicals of puriss p.a. grade (7) Bakker, E.; Pretsch, E.; Bu ¨ hlmann, P. Anal. Chem. 2000, 1127-1133. (8) Morf, W. E.; Badertscher, M.; Zwickl, T.; de Rooij, N. F.; Pretsch, E. J. Phys. Chem. B 1999, 103, 11346-11356. (9) Ceresa, A.; Sokalski, T.; Pretsch, E. J. Electroanal. Chem. 2001, 501, 7076. (10) To´th, K.; Fucsko´, J.; Lindner, E.; Fehe´r, Z.; Pungor, E. Anal. Chim. Acta 1986, 179, 359-370.

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were used. Poly(vinyl chloride) (PVC), bis(2-ethylhexyl) sebacate (DOS), sodium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (NaTFPB), tetrahydrofuran (THF), and the lead-selective ionophore (lead ionophore III, ETH 5435) were Selectophore from Fluka. Membranes and Electrodes. The membranes were prepared in two different thickness (200 and 400 µm) according to the method of Craggs et al.12 with 33% PVC and 65% plasticizer. The double-thick membranes were prepared by fusing two layers of membrane with a small amount of PVC/plasticizer/THF mixture. The membranes were cast with 10 mM/membrane ionophore and 3 mM/membrane added site (30 mol %) concentration. The membranes were glued at the end of a Tygon tube with 3-mm outer diameter and 1.4-mm inner diameter. A homemade Ag/ AgCl electrode served as inner reference electrode.13 As inner filling solution, 10-3 M PbCl2 and 10-2 M EDTA + 10-3 M Pb(NO3)2 + 2 M NaCl were used. The pH of the inner filling solution (pH 3.0) was not adjusted because in the presence of swamping excess of sodium (CNa > CEDTA > CPb) the free lead ion concentration becomes pH independent.14 The calculated free lead ion concentration in the EDTA containing IFS was as 9.2 × 10-18 M. A double-junction Ag/AgCl reference electrode RH44/2-SD/1 (Mo¨ller Glassbla¨serei, Zurich, Switzerland) with sleeve junction served as outer reference electrode. One molar lithium acetate solution was used as salt bridge electrolyte. The EMF values were always corrected for changes in the liquid junction potential according to the Henderson formalism.15 To evaluate the slopes of the calibration curves, the primary ion activities were calculated with the help of the extended Debye-Hu¨ckel equation.16 Electrochemical Cells and Procedure. The calibrations were made in flowing solutions using the exponential dilution method17 in combination with a wall-jet cell. The cell design was published in our previous paper.11 The exponential dilution experiments were carried out with the help of a Minipuls3 (Gilson Inc., Middleton, WI) variable-speed peristaltic pump using 1.65mm-i.d. Tygon tubing (Ismatec SA, Glattbrugg-Zu¨rich, Switzerland). The flow rate was checked before and after each dilution experiment. Flow rates of ∼1.6 mL/min were common. For the galvanostatic current control, a Princeton Applied Research model 283 potentiostat/galvanostat (Oak Ridge, TN) was used. RESULTS AND DISCUSSION Ion-exchange and coextraction-induced concentration gradients18,19 and related ion fluxes can deteriorate the Nernstian response of ion-selective electrodes.2,3 In this work, we report our approach to restore the ideal membrane behavior with enforced (11) Lindner, E.; Gyurcsa´nyi, R. E.; Buck, R. P. Electroanalysis 1999, 11, 695702. (12) Craggs, A.; Moody, G. J.; Thomas, J. D. R. J. Chem. Educ. 1974, 51, 541. (13) Ives, D. J. G.; Yanz, G. J. Reference Electrodes; Academic Press: New York, 1961. (14) Perrin, D. D. Buffers for pH and metal ion control; Chapman Hall: London, 1974. (15) Meier, P. C.; Ammann, D.; Morf, W. E.; Simon, W. In Medical and Biological Applications of Electrochemical Devices; Koryta, J., Ed.; John Wiley & Sons: Chichester, 1980; pp 13-90. (16) Meier, P. C. Anal. Chim. Acta 1982, 136, 363-368. (17) Horvai, G.; To´th, K.; Pungor, E. Anal. Chim. Acta 1976, 82, 45. (18) Lindner, E.; Buck, R. P. Anal. Chem. 2000, 72, 336A-345A. (19) Schneider, B.; Zwickl, T.; Federer, B.; Pretsch, E.; Lindner, E. Anal. Chem. 1996, 68, 4342-4350.

galvanostatic current. The externally applied current can drive ions across the membrane.20,21 The current is assigned with negative sign when the galvanostatic cation transport is directed toward the IFS, as is shown in Figure 1B. The current is marked with a positive sign when cations are driven from the IFS into the sample (Figure 2B). It is assumed that the applied current does not disturb the Nernstian equilibrium at the membrane/solution interface.22, 23 When the concentration of the primary ion in the inner filling solution is much higher than in the sample solution and the experimentally determined DL is biased by ions leaching from the membrane, the application of negative current11 is the electrochemical alternative of the method of Sokalski et al.6 Positive current drives cations from the membrane into the sample. It generally deteriorates the sensor response at low concentrations because it increases the primary ion concentration on the sample side of the membrane. However, positive current is expected to be useful in eliminating excessive primary ion fluxes toward the IFS, responsible for the “super-Nernstian” response of certain electrodes at low concentration ranges. The beneficial effect of positive current can be understood on the basis of Figure 2B. To have well-defined surface conditions, the lead-selective membranes used in this study were conditioned for at least 2 days in 10-2 M NaCl solution before use. Next, the electrodes were calibrated in NaCl solutions. This simple test informs about possible primary ion contamination in the proximity of the membrane surface due to leaching, desorption, etc. The importance of testing the response in pure interfering ion solutions is discussed in a recent paper of Bakker et al.7 The long-term conditioning in NaCl effectively removes the Pb2+ ions and other contamination from the surface layers of the membrane. After this extensive conditioning in NaCl, the leadselective electrode gives a close to theoretical calibration curve in NaCl solutions with excellent reproducibility and a DL of 4.6 × 10-5 M. A follow-up calibration in Pb(NO3)2 solutions is also very promising even though 10-3 M Pb(NO3)2 was used as IFS. A slope of 30.6 mV/pPb and DL of 1.4 × 10-11 M could be achieved in the first run (not shown). However, in repeated calibration experiments in Pb(NO3)2 solutions, the response slopes decreased, and simultaneously, the DL was shifting to higher concentrations as the membrane surface became “contaminated” with leaching Pb2+ ions from the IFS (Figure 1A) or the sample solution. This gradual contamination of the membrane surface can be avoided with an infinitesimal flux of primary ions toward the IFS. It can be achieved by a concentration gradient slanted toward the IFS (Figure 2A) or by negative current (Figure 1B). However, when primary ion flux toward the IFS is set too large, it depletes Pb2+ ions from the surface and the response becomes “superNernstian” as discussed earlier. Figure 3. shows three sets of calibration curves with a leadselective membrane electrode after 2 days of conditioning in 10-2 (20) Buck, R. P.; Nahir, T. M.; Cosofret, V. V.; Lindner, E.; Erdoˆsy, M. Anal. Proc. 1994, 31, 301-319. (21) Thoma, A. P.; Viviani-Naurer, A.; Arvanitis, S.; Morf, W. E.; Simon, W. Anal. Chem. 1977, 49, 1567-1572. (22) Armstrong, R. D.; Covington, A. K.; Evans, G. P.; Handyside, T. Electrochim. Acta 1984, 29, 1127-1131. (23) Cammann, K.; Rechnitz, G. A. In Ion-selective electrodes; Pungor, E., Ed.; Akade´miai Kiado´: Budapest, 1985; pp 35-55.

Figure 3. Calibration curves of a 200-µm-thick lead ion-selective electrode in NaCl and Pb(NO3)2 solutions using the exponential dilution method. IFS: 10-2 M EDTA + 10-3 M Pb(NO3)2 + 2 M NaCl. A 10-4 M, pH 4.7 Mg(OAc)2 solution was used as diluting electrolyte. The curves were recorded at zero current polarization; however, before recording Pb-2 curve +0.5 nA polarization was used for 12 min in 10-2 M NaCl solution to restore optimal membrane and surface conditions. The Pb-3 calibration curve was measured immediately after the exponential dilution experiment providing the Pb-2 curve.

M NaCl. The free lead ion activity in the IFS was set to 9.2 × 10-18 M by its composition (10-2 M EDTA + 10-3 M Pb(NO3)2 + 2 M NaCl, pH 3.0).6 The calibration curves were recorded by the exponential dilution method using 10-4 M, pH 4.7 Mg(OAc)2 as diluting electrolyte. The calibration curve recorded in NaCl solutions has a close to theoretical slope (59.8 mV/pNa) and a DL ) 6.2 × 10-5 M. However, the calibration curves in Pb(NO3)2 solutions have a “super-Nernstian” section between 10-7 and 10-8 M (curve Pb-1). It is in agreement with the results published by Pretsch and co-workers.2,3,24,25 Positive current is expected to tip the established lead ion concentration gradient in the membrane back to horizontal and restore depleted surface layers, if any (Figure 2B). The calibration curve labeled Pb-2 was recorded after a short positive current “treatment” (0.5 nA for 12 min). It has a close to theoretical slope in the whole range (25.5 mV/pPb) and a detection limit of 3 × 10-11 M. No “super-Nernstian” section appeared on the curve. In repeated calibration runs, without further current treatment, the DL shifted toward higher concentration values and the calibration curve slowly approached the calibration curve recorded before the current treatment (e.g., curve Pb-3). It was shown that the total voltage across the membrane26 can be expressed as three terms: iR∞ and two interfacial potential differences (EM):27

E ) iR∞ + E1 - E2 ) R∞ + EM

(1)

s m EM ) S log(as1am 2 /a2a1 )

(2)

where iR∞ is the ohmic potential drop in the membrane (i is the applied current and R∞ is the high-frequency resistance), E1 and (24) Qin, W.; Zwickl, T.; Pretsch, E. Anal. Chem. 2000, 72, 3236-3240. (25) Zwickl, T.; Sokalski, T.; Pretsch, E. Electroanalysis 1999, 11, 673-680. (26) Iglehart, M. L.; Buck, R. P.; Horvai, G.; Pungor, E. Anal. Chem. 1988, 60, 1018. (27) Morf, W. E. The Principles of Ion-Selective Electrodes and of Membrane Transport; Elsevier: New York, 1981.

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Figure 4. Calibration curves of a 400-µm-thick lead ion-selective electrode in NaCl and Pb(NO3)2 solutions using the exponential dilution method. A 10-4 M, pH 4.7 Mg(OAc)2 solution was used as diluting electrolyte and 10-2 M EDTA + 10-3 M Pb(NO3)2 + 2 M NaCl as IFS. The curves were recorded with 0, +1, and + 2 nA current polarization.

E2 are the phase boundary potentials at the two sides of the membrane, S is the Nernstian response slope (59.16 mV/zi log a at 25 °C) zI is the charge number of the primary ion, and as and am are solution and membrane activities on the two sides of the membrane, respectively. A temporary current treatment induces a shift in the offset voltage because the enforced galvanostatic current changes three of the four activities in the logarithmic term. The ion activities in the inner filling solution (as2) are fixed by the buffer. The slow drift after the current is switched off is attributed to relaxation processes in the membrane as the activities approach their i ) 0 steady-state value. To minimize drift in practical applications, e.g., continuous monitoring, the parameters of the current treatment has to be optimized (time, current density, repetition frequency, etc.). To have well-controlled, steady-state conditions, we tried to use continuous positive current polarization in combination with a “negative” concentration gradient. In our approach, the concentration gradient induced ion fluxes toward the IFS (“negative” gradient) are carefully balanced by ion fluxes prompted by positive, galvanostaticly controlled current in the opposite direction (Figure 2B). The opposite fluxes keep the membrane in constant, steady-state condition. The concentration gradient slanted toward the IFS prevents the detrimental primary ion leaching to the sample. The applied positive current offsets the excessive ion transport from the sample to the IFS and eliminates the superNernstian response. The steady-state Na+ ion flux (in our example) enforced by the positive current (Figure 2B) keeps the membrane surface contamination free. To find the optimal current setting, we calibrated our sensor at gradually increasing positive currents until the disappearance of the super-Nernstian section in the calibration curve. The three calibration curves shown in Figure 4. were recorded at 0, +1, and +2 nA polarization current. With the increasing positive current, the super-Nernstian section of the curves gradually vanishes and the linear section is extended to ∼3 × 10-11 M. The detection limit was determined as (3.0 ( 0.3) × 10-12 M in four consecutive calibrations. This value is far below the DL achieved after the temporary positive current treatment (shown in Figure 3) and the 4252 Analytical Chemistry, Vol. 73, No. 17, September 1, 2001

action limit set by the U.S. Environmental Protection Agency,28,29 which makes the current polarized lead sensor attractive for environmental applications. We assume that at +2 nA external current there is no net primary ion flux across the membrane and the primary ion concentration profile in the membrane is horizontal. The shifts in the offset voltage experienced at zero current after a temporary positive current treatment (Figure 3) or during continuous polarization (Figure 4) are similar. It suggests that the changes in the surface activities in eq 1 are determining the magnitude of this shift and the iR∞ potential drop in the membrane only modifies its value. Curves NaCl(1-3) are calibration curves in NaCl solutions under zero current. The three curves run above each other and have slopes of ∼57.4 mV/pNa and DL of ∼3 × 10-5 M. To evaluate the Na+ selectivity of the lead-selective membrane under positive current polarization, the NaCl calibration curve was also recorded with +2 nA external current (curve NaCl(4) in Figure 4). As can be seen, this Na+ calibration curve is practically the same as the one measured at zero current, only the absolute potential values are shifted to more positive potentials. The calibration slopes remained the same, and the shift in the DL toward higher concentrations (from ∼3 × 10-5 to ∼5 × 105 M) is hardly significant. However, it should be noted that the shift in the offset voltage is similar to the shift experienced when the current-polarized calibration were made in Pb2+ solutions. In addition, similar to the zero current calibrations, the potential values measured in the vicinity of the detection limit during NaCl and Pb(NO3)2 calibrations are very close to each other (compare curves NaCl(1-3) and Pb + 0 nA, or NaCl(4) and Pb + 2 nA, respectively). The close to perfect agreement between the detection limits determined from the Na+ calibration curves, recorded at 0 and +2 nA current, is an unambiguous proof that Na + ions are the dominating species in the charge transport during positive current polarization. The enforced current does not contaminate the sensing surface with Pb2+ ions! The Na+ ion transport is accompanied by increased sodium concentrations at the sensing surface (Figure 2B), which explains the small shift in the DL toward higher concentrations. However, this increased detection limit in NaCl solutions (∼5 × 10-5 M) is still very small; it is equivalent to a 5 × 10-14 M DL in Pb2+ solutions, based on the Nikolsky equation and a selectivity pot coefficient for sodium of -log KPb,Na ) 4.7.6 The potential difference between the Pb2+ and Na+ calibration curves (a measure of the experimentally determined selectivity coefficient) at +2 nA is somewhat smaller than the value determined at zero current. The decreased potential difference is translated to a change in the selectivity coefficient from -log pot pot KPb,Na ) 4.4 to -log KPb,Na ) 3.7. The small apparent loss in the sodium selectivity is understandable because due to the applied positive current the Na+ ion concentration is somewhat larger at the phase boundary compared to its solution bulk value as is shown in Figure 2B. (28) Lead and Copper Rule: Minor Revisions, Envitonmental Protection Agency, 1999, EPA 815-F-899-010, 1999. (29) Bakker, E.; Pretsch, E. Trends Anal. Chem. 2001, 20, 11-19.

CONCLUSION To improve the DL and approach the inherent selectivity coefficients of ionophore-based ion-selective electrodes, the technique of Sokalski et al. is combined with a galvanostatically applied positive current. The current is used to eliminate minor primary ion fluxes across the membrane generated by concentration differences between the sample and the inner filling solution. These fluxes often impair the potentiometric measurement at submicromolar concentrations. The calibration curve is used as a feedback to set the optimal current. When the opposite ionic fluxes were “matched”, a Nernstian response with good reproduc-

ibility and a DL of 3 × 10-12 M was achieved with an ETH 5435 ionophore-based lead-selective electrode. ACKNOWLEDGMENT The financial support of the U.S. Hungarian Joint Fund (JF 568) and the FRG grant 2-22307 to the University of Memphis is gratefully acknowledged. Received for review January 22, 2001. Accepted June 26, 2001. AC010094A

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