Thick Membrane, Solid Contact Ion Selective Electrode for the

A new approach for decreasing the lower detection limit of a lead ion selective electrode (ISE) is presented. The ISE is designed using nonfunctionali...
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Anal. Chem. 2005, 77, 1780-1784

Thick Membrane, Solid Contact Ion Selective Electrode for the Detection of Lead at Picomolar Levels Maria Fouskaki and Nikolas A. Chaniotakis*

Laboratory of Analytical Chemistry, University of Crete, Iraklion 71 409 Crete, Greece

A new approach for decreasing the lower detection limit of a lead ion selective electrode (ISE) is presented. The ISE is designed using nonfunctionalized porous glassy carbon loaded with ionophore/plasticizer/additive cocktail. This material acts both as the support for the liquid polymeric membrane and as the signal transducer of the ISE. The high purity of the glassy carbon, together with its high conductivity, allows for the development of a thick, low-resistance composite membrane. This sensor element enables the continuous measurement of lead down to picomolar levels, with very small detection limit deterioration due to the lead ion transport within the bulk of the thick membrane. Ion selective electrodes (ISEs) are chemical sensors for the selective determination of ions in complex matrices and have found widespread application in many fields, including clinical chemistry1 and environmental analysis. One of the most important analytical characteristics of ISEs is the lower detection limit (LDL).2 Until recently, it was accepted that the LDL of ISEs lay in the micromolar range.3 However, much lower LDLs have been obtained recently using ISEs in carefully buffered solutions,4,5,6 as well with optodes.7,8 During the last years it became clear that the observed low LDL was not an intrinsic property of ISEs but a consequence of the experimental setup. In fact, research in the field of polymer membrane-based ISEs has proved that transmembrane diffusion of electrolyte extracted from the inner side of the membrane results in concentration gradients within the membrane bulk and in nonzero membrane internal diffusion potential.9 This process * Corresponding author. Tel +30 2810 393 618. Fax +30 2810 393 601. E-mail: [email protected]. (1) Bakker, E.; Diamond, D.; Lewenstam, A.; Pretsch, E. Anal. Chim. Acta 1999, 393, 11-18. (2) Buck, R. P.; Lindner, E. Pure Appl. Chem. 1995, 66, 2528-2536. (3) Buhlmann, P.; Pretsch, E.; Bakker, E. Chem. Rev. 1998, 98, 1593-1688. (4) Schefer, U.; Ammann, D.; Pretsch, E.; Oesch, U.; Simon, W. Anal. Chem. 1986, 58, 2282-2285. (5) Ammann, D.; Pretsch, E.; Simon, W.; Lindner, E.; Bezegh, A.; Pungor, E. Anal. Chim. Acta 1985, 171, 119-129. (6) Sokalski, T.; Maj-Zurawska, M.; Hulanicki, A. Microchim. Acta 1991, I, 285291. (7) Lerchi, M.; Bakker, E.; Rusterholz, B.; Simon, W. Anal. Chem. 1992, 64, 1534-1540. (8) Bakker, E.; Willer, M.; Pretsch, E. Anal. Chim. Acta 1993, 282, 265-271. (9) Sokalski, T.; Lingenfelter, P.; Lewenstam, A. J. Phys. Chem. B 2003, 107, 2443-2452.

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is so slow that it can be ignored if the analyte activity in the test solution is high or if the analysis time is short. However, when ISEs operate in their LDL levels, ion transport through the membrane plays an important role in the measured potential.10 Based on the fact that reducing or eliminating the ion fluxes through the membrane is the key parameter in improving the LDL of ISEs, several methodologies aimed at dealing with this issue have appeared in the literature. The first effort was based on the precise control of the primary ion activity in the inner filling solution of the electrode.11 This was achieved by the use of a complexing agent such as EDTA and a rather high concentration of an interfering ion or, alternatively, by the use of an ion-exchange resin.12 However, in sample solutions with a wide range of primary ion activities, the precise control of ion fluxes through the membrane is a very difficult experimental task, because the optimum ratio of sample to reference solution analyte activities cannot always be easily achieved. In another method, the control of the activity of the primary ion in the inner membrane interface was based on the use of a conductive polymer solid contact ISE, doped with complexing agents.13 In all the above cases, there is a bulk membrane transport that results in nontheoretical sensitivity, nonreproducible results, and the inability of real sample application. A more elegant method to control ion fluxes through the membrane is to work under zero current conditions. In this method, an external current is applied to the sensor element at a level that counteracts the current generated due to carrier-induced ion transport through the membrane. Under optimized conditions, this method eliminates the ion transport through the membrane and imposes the ideal concentration profiles within the membrane phase.14 The current control method can be a very powerful tool, but it requires sophisticated instrumentation and complicated experimental procedure, since the membrane ion current depends on the membrane potential generated. This means that when the ISE potential increases, the current that passes through the membrane increases (V ) IR), and thus the compensation current must also increase. (10) Bakker, E.; Pretsch, E. Anal. Chem. 2002, 420A-421A. (11) Sokalski, T.; Ceresa, A.; Zwickl, T.; Pretsch, E. J. Am. Chem. Soc. 1997, 119, 11347-11348. (12) Qin, W.; Zwickl, T.; Pretsch, E. Anal. Chem. 2000, 72, 3236-3240. (13) Michalska, A.; Konopka, A.; Maj-Zurawska, M. Anal. Chem. 2003, 75 (1), 141-144. (14) Lindner, E.; Gyurcsanyi, R. E.; Buck, R. P. Electroanalysis 1999, 11, 695702. 10.1021/ac049013b CCC: $30.25

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Alternatively, the effect of membrane contamination by ion transport can be decreased by using flow analysis manifolds.14 This method requires that the carrier solution is capable of continuously extracting the primary ions from the membrane phase. This in turn decreases the potential buildup at the outer diffusion layer of the sensing membrane, and thus, the LDL decreases. In this work, a new approach for decreasing the LDL of ISEs is presented. In particular, the ISE membrane has been designed based on two basic principles: (a) increased thickness, so that there is essentially no inner reference element contamination from the primary ion from the test solution, and (b) the elimination of the internal reference solution with the use of a solid contact system. It has already been reported that increasing the membrane thickness, the polymer content, or both results in lower LDL of ISEs, since an increase in the diffusion coefficients in the membrane phase occurs and the concentration gradients reduce.15 However, the steady state is reached more slowly when the ISE membrane is of large thickness, high polymer content, or both.16 In our case, the membrane thickness can be as large as the experimental parameters allow, since there is no resistance limitation.17,18 This is achieved with the use of a porous conductive glassy carbon loaded with the ionophore-doped plasticizer, which in conjunction with a conventional liquid polymeric membrane is the basis for the design of the composite membrane employed. EXPERIMENTAL SECTION Reagents. Poly(vinyl chloride) (PVC), 2-nitrophenyl octyl ether (o-NPOE), potassium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (KTFPB), and the lead ionophore III N,N,N′,Ν′tetradodecyl-3,6-dioxaoctandithioamide (ETH 5435) were from Fluka. Ethylenediaminetetraacetic acid disodium salt dihydrate used for Pb2+ activity buffering was purchased from Fluka, while nitric acid was from Merck. All other chemicals were of puriss p.a. grade. All aqueous solutions were prepared using Nanopure water (∼18 ΜΩ, EASY-pure model D7033, Barnstead). Electrode Construction. The solid contact electrode was based on a previously reported electrode design.17,18 A rod (length 4.1 mm, diameter 3.8 mm) of microporous vitreous carbon (MAST carbon Ltd.) (Figure 1) was used as the matrix for thick membrane preparation as described below. Analysis of the carbon proves that this material is free from any oxygen groups, while its BET surface area is 926 m2 g-1.19 First, a solution containing 99% w/w plasticizer o-NPOE, 1% w/w lead ionophore III, and borate additive at a mole ratio relative to ionophore of 25% is prepared. Borate concentration was chosen based on optimization results with membranes containing 0, 25, or 50 mol %. The plasticizer o-NPOE was selected due to its high dielectric constant, which aids the partition of the divalent lead ions into lipophilic polymeric membranes.20 This solution was used (15) Ceresa, A.; Sokalski, T.; Pretsch, E. J. Electroanal. Chem. 2001, 501 7076. (16) Fu, B.; Bakker, E.; Yun, J. H.; Yang, V. C.; Meyerhoff, M. E. Anal. Chem. 1994, 66, 2250-2259. (17) Vamvakaki, M.; Chaniotakis, N. A. Anal. Chim. Acta 1996, 320, 53-61. (18) Andredakis, G. E.; Moschou, E. A.; Matthaiou, K.; Froudakis, G. E.; Chaniotakis, N. A. Anal. Chim. Acta 2001, 439, 273-280. (19) Personal communication with MAST Carbon Ltd. Co.

Figure 1. SEM image of the porous glassy carbon used for construction of the thick membrane solid contact ISE.

Figure 2. Schematic representation of the thick membrane, solid contact ISE.

as the soaking solution, into which the glassy carbon was placed, under vacuum, for 30 min, in order for all the pores to be filled with this non-PVC-containing membrane cocktail. Subsequently, the carbon was inserted into a holder made out of PVC with a hole of the same size as the carbon. The PVC holder is equipped with a Ag/AgCl reference element, which makes direct contact with the carbon rod. A small 0.5-mm grove on top of the carbon allows for the space required to place the PVC-based membrane with same composition as the soaking solution, as the final step. The liquid polymeric membrane used contained 66% w/w plasticizer o-NPOE, 33% w/w PVC, and 1% w/w lead ionophore III while the mole ratio of borate to ionophore was 25%. Figure 2 shows the schematic representation of the resulting sensor. Calibration curves of the sensor were performed from lower to higher activities either using standard Pb2+ solutions prepared as described below or by performing standard additions of Pb(NO3)2 in aqueous solutions adjusted to pH 3.8 using HNO3. Selectivity measurements were obtained using 10-5 M solutions (20) Oesch, U.; Xu, A.; Brzozka, Z.; Suter, G.; Simon, W. Chimia 1986, 40.

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Table 1. Lead Ion Activities and the Corresponding pH Values of the Standard Solutions Used for the Calibration Curves of the Solid Contact Pb2+ Selective Electrode log [Pb2+] (mol/L)

pH

-6 -7 -8 -9 -10 -11

2.44 2.81 3.23 3.70 4.19 4.69

of chloride or nitrate salts of different cations (adjusted to pH 3.8 with HNO3). pH response experiments were performed in EDTAbuffered solutions by adding HNO3 or NaOH. Instrumentation. Potentiometric measurements were performed with a Xenon CI-317 eight-channel electrometer versus a Ag/AgCl double-junction reference electrode (Orion). The data were collected using a personal computer with software written in basic. Standard Pb2+ Solutions. The activities of free Pb2+ in the standard solutions were determined by the titration of metal ion (Pb) with a solution of ligand (EDTA) to form a 1:1 complex.21,22 All calculations were done using Mathematica 3.0 software. All standard Pb2+ solutions were of 10-5 M total lead concentration and 2 × 10-5 M EDTA. The free Pb2+ activities were fixed by adjusting pH with HNO3. Table 1 shows the Pb2+ activities and the corresponding pH values of the standard solutions used for the calibration curves of the solid contact lead selective electrode. FIA Measurements. The flow injection analysis (FIA) system used was a laboratory-built system consisting of an Orion syringe pump model 362 for buffer delivery at a flow rate of 0.5 mL/min and an Upchurch six-port injection valve, model V540, for sample injections. The injection loop volume was 200 µL, while all tubing was of polyether ether ketone (Upchurch). The buffer used in the FIA measurements was a standard Pb2+ solution where the Pb2+ activity was fixed at 10-15 M, pH 4.7. RESULTS AND DISCUSSION The design of an ISE with detection limits that reach the ionophore capabilities is not a trivial matter. The main problem arises from the fact that the potentiometric measurements must always be obtained under equilibrium conditions, to have a stable and reproducible analytical system. The sensor element in the ISEs is the area in which a wide range of physicochemical processes take place during the measurement. The response mechanism of these sensors is usually not very clear, since it is based on ion separation due to two distinct processes generating the Volta potential and Galvani potential.23,24 The Volta potential is purely electrostatic and exists on the outer membrane solution interface. On the other hand, the Galvani (21) Harris, D. C. Quantitative Chemical Analysis, 4th ed.; Freeman: New York, 1995. (22) Ammann, D.; Bu ¨ hrer, T.; Schefer, U.; Mu ¨ ller, M.; Simon, W. Pflu ¨ gers Arch 1987, 409, 223-228. (23) Schmickler, W. Interfacial Electrochemistry; Oxford University Press: New York 1996. (24) Antropov, L. I. Theoretical Electrochemistry; Mir Publishers: Moscow 1972.

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potential evolves in the ionophore-doped inner membrane phase and has to do with the chemical energy difference of the primary ion between the organic membrane phase and the aqueous test solution. Differentiating these two potential generating processes is usually not possible, mainly because the inner potential cannot be measured, as such. In potentiometry, the sum of all potentials through the membrane is measured, since as mentioned above, one cannot separate the Volta potential from the Galvani potential. In other words, the measured potential is the difference of Volta and Galvani potentials of the membrane system. This means that ion transport through the membrane will affect the test solution interface as soon as there is an equilibrium established at that point, but it will affect the inner membrane interface potential only after the ions have traveled through the membrane and have reached the inner reference point. Increasing the thickness of the membrane increases, accordingly, the time requires for inner reference element contamination. The use of thick membranes is usually hampered by the high impedance introduced. The ISE system in this paper was designed to take advantage of this idea, providing a sensor with improved detection limit, without the need for cumbersome experimental setups. The membrane designed in this work can be as thick as the experimental parameters allow, since there is not resistance limitation due to the highly conductive nature of the porous glassy carbon material used as the membrane matrix. This carbon material was produced under experimental conditions that resulted in an ultraclean, minimum functionalized surface.19 The ionophore chosen for this pilot study was the wellstudied lead ionophore III. The use of this ionophore would allow us to compare the results with those already published in the literature.11,25 Figure 2 shows the schematic representation of the sensor. The dimensions of the carbon used for the design of the sensor presented here were chosen based on the existence in these dimensions of the special tool (high-pressure/high-temperature mold) required to make the porous glassy carbon. Porous glassy carbon is a very hard and brittle material; thus, machining larger pieces to desired dimensions is not possible. As described in the Experimental Section, the design proposed results in a completed sensor that has a sensing element that is ∼20 times thicker than the normal PVC membrane. The diffusion coefficients of ions in the membrane phase is on the order of 10-8 cm2 s-1;10,26 so some days are required for an ion to diffuse through 4.3-mm sensor element thickness and reach the inner reference element. In addition, the low resistivity of the glassy carbon (F ) 0.0028 Ω cm) results in a very low overall membrane resistance, which is basically controlled by the thickness of the PVC-based membrane and is on the order of a few kiloohms. The design characteristics of this sensor suggest that it can be used for several days without any significant increase in the detection limit due to the bulk membrane ion transport. The potentiometric measurement of the Pb2+ activity is a complicated matter, mainly due to the formation of complexes with hydroxide ions and other organic and inorganic ligands. To have a controlled and well-known value of the Pb2+ activity, metal or pH buffers should be used in the test solution. When lead (25) Pergel, E.; Gyurscsanyi, R. E.; Toth, K.; Lindner, E. Anal. Chem. 2001, 73, 4249-4253. (26) Sokalski, T.; Zwickl, T.; Bakker, E.; Pretsch, E. Anal. Chem. 1999, 71, 12041209.

Figure 4. Calibration curve of the solid contact lead ion selective electrode obtained by adding aliquots of Pb(NO3)2 in aqueous solutions adjusted to pH 3.8 with HNO3 (slope ) 27.5 mV/decade, LDL ) 5 × 10-8 M).

Figure 3. (A) Consecutive calibration curves of the solid contact lead ion selective electrode using standard Pb2+ solutions. (O) First calibration curve (super Nernstian response for [Pb2+] > 10-9M), (0) second calibration curve (theoretical response, slope, 29.2 mV/ decade, LDL < 10 pM). (B) Chart recording of consecutive calibrations of the solid contact lead ion selective electrode using standard Pb2+ solutions; (solid line) first calibration and (dotted line) second calibration.

activity in the test solution is controlled bellow 1 µm using EDTA at a specific pH, the amounts of EDTA required are of extremely small difference; thus, the accuracy of the method is not sufficient for analytical purposes. For this reason, solutions with fixed EDTA and lead concentrations were used. The lead ion activity was adjusted by changing the pH of the solution, as shown in Table 1.21 Since the pH of the standard solutions changes, this method can be used only if the sensor does not suffer from hydrogen ion interference. The pH response of the sensor was first evaluated within the pH range required to adjust the activities of lead to be used. The potentiometric response for pH values from 4.26 to 2.45 is only 10 mV, indicating that the sensor has very limited pH interference at this range, and thus, lead measurement can be performed with a small error introduced by the pH differences of the standard solutions. Using the solutions from Table 1, the calibration curves and the corresponding chart recordings of the sensor to lead ion activity in a static mode are shown in Figure 3. In the first calibration curve (Figure 3A, lower trace), a super Nernstian response is observed for activities of free Pb2+ over 10-9 M, which is in accordance with previous literature.12,25 This phenomenon has been attributed to the extraction of the lead ions into the membrane phase, which in turn results in a buildup of an excess of Galvani potential. To determine the effect of the lead ion partitioning within the membrane phase, a second calibration

curve (Figure 3A, upper trace) was obtained immediately after the first calibration curve without any specific treatment of the electrode except for rinsing with Nanopure water. As can be seen from the second calibration curve, the sensor exhibits a theoretical sensitivity to lead ion activity (29.2 mV/decade, r2 ) 0.995) within the same range of response. The detection limit of the lead sensor based on this calibration curve is at least 10 pM. The response time of the sensor calculated from the chart recording shown in Figure 3B ranges from 20 to 30 s, depending on the free lead ion concentration, while the initial potential was slightly shifted to higher values mainly due to the lead ion uptake in the membrane, which can be reversed if the membrane is washed for a longer period of time (30-50 min). To validate these results, a second method was used to obtain a calibration curve. This time, the calibration curve shown in Figure 4 was obtained by adding aliquots of Pb(NO3)2 in aqueous solutions adjusted to pH 3.8 with HNO3. As can be seen from Figure 4, the sensitivity of the sensor is 27.5 mV/decade within the lead ion activity range of 10-7-10-4 M, while the LDL is calculated to be 5 × 10-8 M. Even though this value is higher than that obtained when the EDTA-buffered solutions were used, it is still considerably lower than that previously reported using conventional ISE setups.8 The lower detection limit obtained when EDTA-buffered standard solutions are used can be attributed to the fact that EDTA continuously extracts the primary ions from the outer membrane interface decreasing the potential buildup, which in turn results in lower LDL. Under these experimental conditions, no interference from alkali and alkaline earth metal ions was observed, while significant interference can arise in the presence of cadmium or copper ions as expected from the reported ionophore selectivities.7 For this reason, care must be taken so that these cations are absent for low-level lead detection during real sample analysis. The signal stability and reproducibility of the sensor was examined using a FIA experimental setup in order to have more reproducible experimental conditions. For this, the potential of the sensor was monitored, while successive injections of solutions with different lead ion activities were introduced into the flow stream. Figure 5 shows the chart recording of the response of the sensor to solutions with different Pb2+ activities. In order for Analytical Chemistry, Vol. 77, No. 6, March 15, 2005

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the same lead ion activity is excellent, with a CV of less than 4% for four measurements. The narrow linear range of response under FIA conditions compared to that obtained in static mode is attributed to the fact that no optimization of the FIA parameters were performed. From these results it is clear that the sensor presented in this work can measure lead ion activity in the absence of cadmium and copper ions at activity levels as low as 10 pM.

Figure 5. Chart recording of solid contact lead ion selective electrode calibration in a FIA system, using standard Pb2+ solutions. Baseline was obtained with a standard Pb2+ solution ([Pb2+] ) 10-15 M, pH 4.7) delivered at a flow rate of 0.5 mL/min. The numbers given correspond to the following free Pb2+ activities (in mol/L): (1) 1.2 × 10-11, (2) 1.2 × 10-10, and (3) 1.0 × 10-9.

the system to be at equilibrium, the baseline is obtained using a solution with very low lead ion activity, as described in the Experimental Section. This way, the differential response of the sensor to the lead ion activities of the injected standard solutions is recorded. Injections of solutions containing three different lead ion activities (1.2 × 10-11, 1.2 × 10-10, and 1.0 × 10-9 M) were repeated four times to study the reproducibility, baseline return, and stability of the system. As shown from the chart recording, the stability of the baseline under these conditions is very good. Additionally, the signal reproducibility between the injections of

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CONCLUSIONS In this paper, we introduce a new method for the development of an ISE that can be used for the measurement of ions at much lower detection limits compared to the existing sensors. The elimination of the internal reference solution together with the use of a low-resistance, thick composite membrane has allowed for the development of a lead ISE with detection limit below 10 pM. This lead ISE based on the ionophore ETH-5435, the o-NPOE as the plasticizer, and KTFPB as the additive shows excellent response characteristics. The sensitivity of the sensor in the region between 10 pM and 10 µM is close to theoretical, while the response time, stability, and reproducibility are excellent, as shown from FIA experiments. The use of this sensor setup is expected to find future application in the design of other ISEs for routine low-level measurements in batch or FIA mode.

Received for review July 6, 2004. Accepted December 28, 2004. AC049013B