Anal. Chem. 1999, 71, 3657-3664
Voltammetric and Amperometric Transduction for Solvent Polymeric Membrane Ion Sensors Smita Jadhav and Eric Bakker*
Department of Chemistry, Auburn University, Auburn, Alabama 36849
This paper describes basic response features of solvent polymeric membrane ion sensors with voltammetric and amperometric transduction. The model systems used here contain no ionophore for simplicity reasons. Reasonable simplifications of the theory are introduced that allow one to understand the response mechanism in view of a practical application of these sensors. It is shown that ionsensing membranes preferentially contain no ion-exchanger properties in order to function optimally in a voltammetric mode. As with the systems studied by Kihara, both liquid-polymer interfaces of the membrane are preferably polarizable. Specifically, they contain the highly lipophilic electrolyte tetradodecylammonium tetrakis(4-chlorophenyl)borate (ETH 500) in the membrane to improve lifetime, increase the magnitude of the potential window, and prohibit exchange reactions with sample ions. An ohmic behavior that is associated with an assisted electrolyte-transfer process is observed only above a threshold potential which can be quantitatively predicted by theory. The threshold potential depends on the nature and activity of sample anions and cations in the sample and inner filling solution of the membrane electrode. Within the experimental conditions discussed in this paper, these sensors seem to measure sample ion activities, not concentrations, since the rate-limiting step is the diffusion of extracted ions away from the interface into the membrane bulk. Similarly, no effect of sample stirring on the measured current is observed. This contrasts to work done on liquid-liquid electrolyte-transfer reactions, where large diffusion coefficients in the organic phase often lead to substantial sample depletion effects. The detection of anions and cations with the same membrane is demonstrated in a cyclic voltammetric mode. Direct continuous detection of one type of anion is accomplished by pulsed amperometry to ensure a rapid, repetitive renewal of the membrane composition between measurements.
the ion selectivity of such membranes has often been grossly underestimated, especially for cation-selective systems.3,4 A number of important advances have recently been established, such as the design of potentiometric ion sensors with improved detection limits 4 and/or higher sensitivity than predicted from the Nernst equation.5,6 It is obvious that the high binding selectivity of the numerous available ionophores can be adapted to other applications. In the past decade, much work has been devoted to developing and characterizing optical sensors based on the same ionophores and same membrane materials of the corresponding potentiometric systems (for reviews, see refs 1, 2, and 7). Most of these optical sensors function on the basis of an ionophore-mediated, competitive extraction of two ions from the sample into a thin solvent polymeric film coated onto a solid substrate. A fluorescence or absorbance change of a chromophore-containing molecule within the sensing film is monitored. Often, a variety of components are embedded in such films, with each component having a distinct function in the ion extraction properties and/or optical response characteristics of the sensing film.8 The differences between optical and potentiometric sensors based on the same selective components have been discussed.1,9 In both cases, the sensor is an essentially passive device where the signal is monitored on the basis of a spontaneous equilibrium or steady-state process. In potentiometry, the measured electromotive force is, ideally, directly related to sample activity of one particular ion. With optical sensors, the response is instead often directly related to ion activity ratios or products. In principle, the recorded spectrum adds an additional dimension to the measurement and could in some cases be used for multicomponent analysis. In many cases, however, the optical response stems from a molecule that binds to a reference ion such as H+, and the uptake of ions other than the target analyte does not lead to changes in the absorbance or fluorescence spectrum. Most of these sensors are, in analogy to their potentiometric counterparts, so-called one-dimensional sensors as well.
Potentiometric ion sensors based on solvent polymeric membranes are versatile analytical tools that are well established for a number of routine applications. They may exhibit an extremely high sensing selectivity owing to the binding selectivity of an embedded lipophilic ionophore.1,2 Recent work has shown that
(3) Bakker, E. Anal. Chem. 1997, 69, 1061. (4) Sokalski, T.; Ceresa, A.; Zwickl, T.; Pretsch, E. J. Am Chem. Soc. 1997, 119, 11347. (5) Fu, B.; Bakker, E.; Yun, J. H.; Yang, V. C.; Meyerhoff, M. E. Anal. Chem. 1994, 66, 2250. (6) Amemiya, S.; Bu ¨ hlmann, P.; Umezawa, Y. Anal. Chem. 1998, 70, 445. (7) Fiber Optic Chemical Sensors and Biosensors; Wolfbeis, O. S., Ed.; CRC Press: Boca Raton, 1991; Vols. 1 and 2. (8) Shortreed, M.; Bakker, E.; Kopelman, R. Anal. Chem. 1996, 68, 2656. (9) Bakker, E. Anal. Chim. Acta 1997, 350, 329.
(1) Bakker, E.; Bu ¨ hlmann, P.; Pretsch, E. Chem. Rev. 1997, 97, 3083. (2) Bu ¨ hlmann, P.; Pretsch, E.; Bakker, E. Chem. Rev. 1998, 98, 1593. 10.1021/ac990195x CCC: $18.00 Published on Web 07/16/1999
© 1999 American Chemical Society
Analytical Chemistry, Vol. 71, No. 17, September 1, 1999 3657
In view of the challenging design of highly selective ionophores, especially for anion- and heavy-metal-sensing applications, it would be valuable to extend the dimensionality of solvent polymeric membrane sensors. In classical electrochemistry with metal electrodes, this is accomplished with voltammetric techniques where the applied potential is an additional degree of freedom in the measurement. The same transition may be done with ionophore-based ion sensors. Voltammetry at the interface of two immiscible electrolyte solutions (ITIES) is now a wellstudied field.10-12 While most of the works published in that area have been fundamental in nature,13 a number of highly interesting analytical applications have been published by Koryta 14-16 and more recently by Girault 17-19 and others.20,21 In view of the historic development of potentiometric ionophore-based sensors, it is obvious that their voltammetric counterparts must make a transition from studies on well-defined organic solvents to polymeric materials that can be engineered into practical sensing arrays. Indeed, a number of studies focused on gelified organic solvents such as nitrobenzene solidified with a few percent dissolved PVC.22-25 Interestingly, papers on the voltammetric application of the solvent polymeric membranes used in potentiometric sensors have now just started to emerge.18,21,26,27 Since ion mobilities within plasticized polymeric materials are substantially smaller than in pure organic solvents, the response behavior may be different. Some research involving such polymeric membranes has focused on the development of a solid contact internal redox element in view of a voltammetric application.26,28 However, an aqueous internal contact may be more easily understood theoretically and is often preferred with potentiometric sensors as well. This paper aims at establishing basic response features of voltammetric solvent polymeric membrane ion sensors that have two polarizable interfaces. Theoretical and practical results are here obtained for ionophore-free membranes as important model systems. Most work done on ITIES focused on the ion- or electrontransfer processes at one single interface. Classically, a reference electrode and a counter electrode were each immersed into the organic phase as well. For a gelified or polymeric membrane, most researchers chose to avoid this concept for practical reasons and immersed the internal electrodes in an aqueous solution in contact with the membrane backside. Concentration polarizations at the
internal interface were avoided by using a common ion in both phases. In many cases, the membrane contains tetrabutylammonium tetrakis(4-chlorophenyl)borate and an optional ionophore and is in contact at the backside by an aqueous solution containing tetrabutylammonium chloride.17,18,24,29 The voltammetric waves obtained with such membranes can be largely attributed to singleion transfer characteristics at the sample side. For ionophore-free membranes, this seems to be a viable system since tetrabutylammonium ions are preferred over simple cations such as potassium or sodium by as much as 10 orders of magnitude.30 With voltammetric ion-sensing membranes containing highly selective ionophores, however, the stabilized extracted cations may now expel the tetrabutylammonium ions much more readily from the membrane. Girault himself has estimated the logarithmic formation constant of the valinomycin-potassium complex in PVCNPOE membranes to be ∼16,24 which would indicate a complete displacement of tetrabutylammonium ions at that interface with moderate sample potassium concentrations. This process will depend on the sample composition and likely lead to unstable sensor behavior. Reliable sensing membranes must therefore contain a highly lipophilic electrolyte such as tetradodecylammonium tetrakis(4-chlorophenyl)borate. In this case, however, the internal interface will be polarizable as well. Kihara and co-workers have studied such liquid as well as lipid bilayer membrane systems extensively.31-33 They found that the obtained potential windows are about twice as large as traditionally observed, which can be regarded as an added benefit from an practical standpoint. His group has, however, not studied the behavior of plasticized PVC, and his systems usually contained rather water soluble electrolytes such as the tetraphenylborate salt of crystal violet, which was shown to transfer into the sample phase at moderately high potentials.33
(10) Beattie, P. D.; Willington, R. G.; Girault, H. H. J. Electroanal. Chem. 1995, 396, 317. (11) Valent, O.; Koryta, J.; Panoch, M. J. Electroanal. Chem. 1987, 226, 21. (12) Girault, H. H. Electrochim. Acta 1987, 32, 383. (13) Vanysek, P.; Buck, R. P. J. Electroanal. Chem. 1984, 163, 1. (14) Koryta, J.; Kozkov, Y. N.; Skalicky, M. J. Electroanal. Chem. 1987, 234, 335. (15) Sabela, A.; Koryta, J.; Valent, O. J. Electroanal. Chem. 1986, 204, 267. (16) Vanysek, P.; Ruth, W.; Koryta, J. J. Electroanal. Chem. 1983, 148, 117. (17) Lee, H. J.; Beriet, C.; Girault, H. H. Anal. Sci. 1998, 14, 71. (18) Lee, H. J.; Girault, H. H. Anal. Chem. 1998, 70, 4280. (19) Beattie, P. D.; Infelta, P. P.; Girault, H. H. Anal. Chem. 1994, 66, 52. (20) Sawada, S.; Torii, H.; Osakai, T.; Kimoto, T. Anal. Chem. 1998, 70, 4286. (21) Horvath, V.; Horvai, G.; Pungor, E. Mikrochim. Acta 1990, I, 217. (22) Marecek, V.; Colombini, M. P. J. Electroanal. Chem. 1988, 241, 133. (23) Marecek, V.; Janchenova, H.; Brezina, M. Anal. Chim. Acta 1991, 244, 15. (24) Lee, H. J.; Beriet, C.; Girault, H. H. J. Electroanal. Chem. 1998, 453, 211. (25) Ji, H.; Wang, E. Analyst 1988, 113, 1541. (26) Cammann, K.; Ahlers, B.; Henn, D.; Dumschat, C.; Shul’ga, A. A. Sens. Actuators, B 1996, 35, 26. (27) Horvath, V.; Horvai, G. Anal. Chim. Acta 1993, 273, 145. (28) Sun, L.; Li, S.; Jung, S. O.; Valenta, J.; Weber, S. G. Using Sensor Principles in Extraction and Microextraction. Pittsburgh Conference, New Orleans, LA, 1998; Abstr. 999.
(29) Silva, F.; Sousa, M. J.; Pereira, C. M. Electrochim. Acta 1997, 42, 3095. (30) Scholer, R.; Simon, W. Helv. Chim. Acta 1972, 55, 1801. (31) Shirai, O.; Yoshida, Y.; Matsui, M.; Maeda, K.; Kihara, S. Bull. Chem. Soc. Jpn. 1996, 69, 3151. (32) Shirai, O.; Kihara, S.; Yoshida, Y.; Matsui, M. J. Electroanal. Chem. 1995, 389, 61. (33) Shirai, O.; Kihara, S.; Suzuki, M.; Ogura, K.; Matsui, M. Anal. Sci. 1991, 7, 607. (34) Homolka, D.; Hung, L. Q.; Hofmanova, A.; Khalil, M. W.; Koryta, J.; Marecek, V.; Samec, Z.; Sen, S. K.; Vanysek, P.; Weber, J.; Brezina, M.; Janda, M.; Stibor, I. Anal. Chem. 1980, 52, 1606. (35) Girault, H. H. J.; Schiffrin, D. J. J. Electroanal. Chem. 1985, 195, 213. (36) Girault, H. H. J.; Schiffrin, D. J. J. Electroanal. Chem. 1984, 170, 127. (37) Stewart, A. A.; Campbell, J. A.; Girault, H. H.; Edddowes, M. Ber. BunsenGes. Phys. Chem. 1990, 94, 83. (38) Kakiuchi, T.; Senda, M. J. Electroanal. Chem. Interfacial Electrochem. 1991, 300, 431. (39) Senda, M.; Kakiuchi, T.; Osakai, T. Electrochim. Acta 1991, 36, 253. (40) Matsuda, H.; Yamada, Y.; Kanamori, K.; Kudo, Y.; Takeda, Y. Bull. Chem. Soc. Jpn. 1991, 64, 1497. (41) Samec, Z.; Papoff, P. Anal. Chem. 1990, 62, 1010. (42) Iglehart, M. L.; Buck, R. P.; Horvai, G.; Pungor, E. Anal. Chem. 1988, 60, 1018. (43) Sandifer, J. R.; Iglehart, M. L.; Buck, R. P. Anal. Chem. 1989, 61, 1, 1624.
3658 Analytical Chemistry, Vol. 71, No. 17, September 1, 1999
THEORETICAL SECTION This section outlines important voltammetric and amperometric response features of solvent polymeric membranes. The general theory of the voltammetric response at the interface of two immiscible electrolyte solutions (ITIES) has been established mainly by Koryta,14,34 Girault,12,35-37 Kakiuchi,38,39 Matsuda,40 Samec,41 and Buck.13,42,43
Diffusion coefficients in solvent polymeric membranes are on the order of 10-8 cm2 s-1, whereas classical organic solvents used in work involving ITIES show diffusion coefficients that are ∼1000 times larger. Many researchers, including Kihara and Girault, have observed and repeatedly stated that the diffusion coefficients are drastically reduced in gelified liquid membranes and plasticized polymeric membranes.23,24,31 Perhaps since most traditional response treatments were done for the electrochemistry at ITIES, however, a simplified response theory was not yet given for cases where sample depletion can be neglected. The treatment proposed in this paper is targeted toward practical sensor design and the understanding of its general response behavior. If an external potential is imposed upon an ion-selective membrane, current can flow through the cell by ion transfer from and to the membrane phase and the aqueous solutions in contact with it. The overall potential drop can be divided between two phase boundary potential drops, EPB and EPB′, and the so-called iR drop across the membrane bulk as a result of the high bulk resistance of the polymeric membrane material:
EM ) EPB - EPB′ + iRmem
(1)
where the prime (′) indicates the phase boundary at the membrane-inner electrolyte side. It is well established that ion-transfer reactions are ordinarily by orders of magnitude faster than diffusion processes within the Nernst diffusion layers involved. Therefore, the phase boundary potential for the transfer of cations IzI+ and anions AzA- can in most cases be described by the following relationship:
EPB )
RT kIaI(aq) RT kAaA(aq) )ln + ln z zIF [I I ] zAF [AzA-] pb
(2)
pb
where aion and [Ion]pb are the activity and concentration of the ion in the aqueous- and organic-phase boundaries, respectively, kion is a direct function of the free energy of transfer of the ion, and R, T, and F are the gas constant, the absolute temperature, and the Faraday constant, respectively (note that R is different from the membrane bulk resistance Rmem in eq 1). Any assisted ion transfer will invariably lead to concentration polarizations at the phase boundary relative to the aqueous and membrane bulk compositions. Since the interfacial ion-transfer reaction itself is rapid, the observed current will be limited by mass transport in at least one of the two adjacent diffusion layers. In a onedimensional system, therefore, the current can be described for each diffusion layer as follows:
i ) AFzIDI(δcI/δx)t,x)0
(3)
where A is the exposed membrane area, DI is the diffusion coefficient, and (δcI/δx)t,x)0 is the concentration gradient of the transferring ion at the interface (x ) 0) and time t. In a steadystate situation, it is customary to simplify eq 3 by assuming a linear concentration gradient within a Nernst diffusion layer of thickness δ as follows:
i ) AFzIDI(∆cI/δ)
(4)
Figure 1. Schematic presentation of relevant ion-transfer mechanisms that are expected for voltammetric membranes (A) with and (B) without ion-exchange properties: I+, sample cation; A-, sample anion; R- and R+, lipophilic anionic and cationic sites. Membranes of type B may in principle be used to determine sample cations or anions by sign reversal of the applied potential.
This relationship is valid for each of the four diffusion layers observed with ion-selective membranes. It can be expected that most of the applied potential will contribute to the iR drop since membrane resistances are known to be on the order of 1 MΩ. Expected currents with applied voltages on the order of 1 V are therefore, according to Ohm’s law, ∼1 µA. This information can be used to estimate which of the separate diffusion layers is mass transport limiting. With a diffusion coefficient in the aqueous phase DI(aq) ) 10-5 cm2 s-1, an electrode area A ) 0.1 cm2, an ionic charge zI ) 1, and a Nernst diffusion layer thickness δ ) 3 × 10-3 cm, the ionic concentration difference between phase boundary and sample bulk is predicted to be ∆cI ) 3 × 10-8 mol cm-3 or 3 × 10-5 M. Evidently, for typical samples with concentrations on the order of 10-4 M or higher, depletion in the aqueous-phase boundary is often not significant. It can therefore be concluded that mass transport from the phase boundary into the bulk of the membrane phase is in most cases rate limiting. The same eq 4 yields for DI(org) ) 10-8 cm2 s-1,44 A ) 0.1 cm2, zI ) 1, an estimated δ ) 10-2 cm, and i ) 1 µA a concentration difference between phase boundary and membrane bulk ∆cI ) 1 × 10-4 mol cm-3 or 1 × 10-1 M. Evidently, the much smaller diffusion coefficient in the polymeric membranes effectively accumulates the extracted ions in the phase-boundary region of the membrane phase. In a classical amperometric experiment, the diffusion layer thickness in the organic phase will continuously increase until δ matches the membrane thickness. In practice, this situation would only be observed after a number of hours. According to eq 4, therefore, the current will continuously decrease as δ increases with time. Voltammetric Response of Solvent Polymeric Membranes Containing Lipophilic Ion-Exchanger Sites. Ion-selective electrodes must have ion-exchange properties to function properly in a potentiometric mode (see Figure 1A). While many electrode membranes give Nernstian responses without purposely added (44) Schneider, B.; Zwickl, T.; Federer, B.; E. Pretsch; Lindner, E. Anal. Chem. 1996, 68, 4342.
Analytical Chemistry, Vol. 71, No. 17, September 1, 1999
3659
ionic sites, Horvai et al.45 and van den Berg and co-workers46 showed that ionic impurities present in the membrane are responsible for this behavior. A rigorous purification of the membrane materials yielded extremely small electrode slopes, even with the ionophore valinomycin embedded in these membranes.47 This requirement can be explained with eq 2. In potentiometry, under zero current conditions, an apparent Nernstian slope is only observed if the ionic concentration in the organicphase boundary remains approximately constant as the sample composition is altered.48 Under these circumstances, the phase boundary potential equation reduces to the Nernst equation. In voltammetry or amperometry, however, these criteria might be different. For an ionophore-free cation-responsive membrane containing cation-exchanger sites RT- (see Figure 1A), the concentration of cations in the bulk membrane is approximately given by the electroneutrality condition, [IzI+] ) RT-/zI. As noted above, the observed current is in most cases limited by mass transport from the organic phase boundary to the membrane bulk. Equation 4 can therefore be rewritten as +
+
[IzI ]pb - [IzI ] ) i ) AFzIDI(org) δ +
+
[IzI ]′ - [IzI ]′pb AFzIDI(org) (5) δ′ The applied membrane potential can, in accordance to eqs 1 and 2, be written as +
zI RT kIaI(aq)[I ]pb′ ln + EM ) + iRmem zIF [IzI ] k a (aq)′
(6)
pb I I
A current is observed by a net transport of cations IzI+ from one aqueous phase to the other across the membrane. Therefore, the concentration polarization at the membrane-inner electrolyte interface has the opposite sign than at the sample side, as noted in eq 5. Inserting these current-concentration relationships into eq 6 yields
EM )
[ (
aI(aq) RT RT iδ′ ln zIF aI(aq)′ zI DI(org)zIAF
(
)]
-
)
RT RT iδ ln + + iRmem (7) zIF zI DI(org)zIAF It is notable that eq 7 is still a function of the ion activities in the sample and inner electrolyte. If concentration polarizations in both aqueous Nernst diffusion layers are expected to be negligibly small, the phase boundary activities correspond to the bulk sample activities. The response of such voltammetric membrane systems is therefore related to that of ion-selective electrodes measured (45) Horvai, G.; Buck, R. P.; Graf, E.; Pungor, E.; Toth, K. Anal. Chem. 1986, 58, 2735. (46) Van den Berg, A.; Reinhoudt, D. N.; Skowronska-Ptasinska, M.; Sudholter, E.; Van der Wal, P. D. Anal. Chem. 1987, 59, 2826. (47) Bu ¨ hlmann, P.; Yajima, S.; Tohda, K.; Umezawa, K.; Nishizawa, S.; Umezawa, Y. Electroanalysis 1995, 7, 811. (48) Bakker, E.; Xu, A.; Pretsch, E. Anal. Chim. Acta 1994, 295, 253.
3660 Analytical Chemistry, Vol. 71, No. 17, September 1, 1999
Figure 2. (A) Calculated current-potential relationship for a solvent polymeric membrane containing cation exchanger sites R- as a function of different sample activities according to eq 7. The resulting cyclic voltammogram reflects the large iR drop across the membrane and is analytically not as useful. Parameters chosen: activity of inner electrolyte aI(aq)′ ) 1 mM; charge of ion zI )1; diffusion coefficient in membrane phase DI(org) ) 10-8 cm2 s-1; surface area of electrode A ) 0.3 cm2; membrane resistance Rmem ) 1 MΩ; diffusion layer thickness in membrane δ ) δ′ ) 10-2 cm; concentration of ion exchanger in membrane RT ) 0.005 M; sample electrolyte activity aI(aq) ) 0.1, 0.01, and 0.001 M (top to bottom trace). (B) Calculated current-potential relationship for a solvent polymeric membrane without ion-exchange properties as a function of different sample activities according to eq 11. The current increases according to Ohm’s law only above a threshold potential that can be estimated according to eq 13. All parameters are otherwise identical to (A).
under zero-current conditions. Figure 2A shows some predicted current-potential curves based on eq 7. The response should in most cases be dictated by the large iR drop across the membrane. If only small concentration perturbations in the two organic-phase boundaries are observed relative to the large [IzI+], eq 7 readily reduces to
EM ≈
RT aI(aq) ln + iRmem zIF aI(aq)′
(8)
As shown in Figure 2A, the expected response follows the Nernst equation with a large superimposed iR term. For analytical purposes, therefore, it seems that voltammetric systems operating according to eq 8 yield basically the same information as their
potentiometric counterparts. This mode may find special applications but does not generally seem to offer any distinct advantages over zero-current potentiometry. These conclusions are corroborated by the fact that virtually all reports involving amperometric and voltammetric transduction at ITIES have made use of organic solvent without added lipophilic ion exchangers. Voltammetric Response of Solvent Polymeric Membranes without Ion-Exchanger Properties. With membranes lacking ion-exchanger properties, no transferable anions or cations are intrinsically present in the organic phase (see Figure 1B). If an applied potential forces cations to transfer from the sample phase to the organic phase, for example, anions AzA- must transfer from the inner electrolyte into the backside of the membrane.31 This concept is illustrated in Figure 1B. The applied potential according to eq 1 can now be rewritten as
EM ) -
RT kAaA(aq) RT kIaI(aq)′ ln ln + + iRmem (9) + zAF [AzA-] zIF [IzI ]′ pb
pb
As above, it is expected that sample depletion at both phase boundaries is negligible. The current is therefore again limited by mass transport from the phase boundary to the bulk of the membrane phase. For both phase boundaries, eq 4 is therefore modified to -
+
[AzA ]pb [IzI ]′pb i ) -AFzADA(org) ) AFzIDI(org) (10) δ δ′ Inserting eq 10 into 9 therefore gives
EM )
RT ln[(kAaA(aq))1/zA(kIaI(aq)′)1/zI] + F
RT ln[(AFzADA(org)/iδ′)1/zA((AFzIDI(org)/iδ))1/zI] + F iRmem (11) Figure 2B shows typical calculated response curves for the case where the concentration of extracted sample ions is zero prior to the experiment. This corresponds to a forward voltammetric scan. Evidently, the calculated current response assumes a linear dependence on the potential only after a certain threshold potential. As the current increases, the iR term becomes significant while the second term of eq 11 becomes increasingly small. Equation 11 therefore reduces at high currents to
EM ≈
RT ln[(kAaA(aq))1/zA(kIaI(aq)′)1/zI] + iRmem F
(12)
The approximate threshold potential is found by extrapolating eq 12 to zero current:
EM(threshold) ≈
RT ln[(kAaA(aq))1/zA(kIaI(aq)′)1/zI] (13) F
Evidently, this threshold potential is dictated by the composition and nature of the sample as well as the inner electrolyte. Again, this information is closely related to that obtained under
zero-current conditions with a solvent polymeric membrane containing lipophilic ion-exchanger sites. However, this particular system seems capable of determining sample cations and anions, since a simple sign reversal of the applied potential will yield the observation of the complementary ion-extraction process, with cations being extracted from the sample and anions from the inner electrolyte. In addition, the reversal of the applied potential in a cyclic voltammetric or pulsed amperometric experiment will lead to phase transfer of the extracted membrane ions back into their aqueous phases. This process will yield kinetic information about the extracted ions, since the current-time response will depend on δ, DI(org), and the accumulated ion concentration. Finally, such a technique may be used in an accumulation and stripping mode analogous to stripping voltammetry,17 thereby yielding lower detection limits than observed with traditional voltammetry and giving the potential for convenient multianalyte detection. In these respects, voltammetric detection may certainly expand the possible uses of solvent polymeric membranes. The accumulation and concomitant stripping of sample ions may also be accomplished by intermittent holding of the potential sufficiently above and below the threshold potential as defined in eq 13. Such an amperometric detection mode should serve two main purposes: (1) it will repeatedly lead to an effective renewal of the membrane composition, thereby eliminating current drifts that are expected to occur due to increase of δ and δ′ as a function of time (see eq 10); (2) it keeps the diffusion layer thicknesses in the membrane phase small for short pulsing intervals. This increases sensitivity according to eq 10 since smaller δ values give larger currents. EXPERIMENTAL SECTION Reagents. The salts, acids, and the membrane components tetradodecylammonium tetrakis(4-chlorophenyl)borate (TDDATpClPB), sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB), bis(2-ethylhexyl) sebacate (DOS), o-nitrophenyl octyl ether (NPOE), high-molecular-weight poly(vinyl chloride) (PVC), and tetrahydrofuran (THF) were purchased in puriss. or Selectophore quality from Fluka Chemical Corp. (Milwaukee, WI). Aqueous solutions were prepared by dissolving the appropriate salts in Nanopure purified distilled water. Membrane Preparation. Ion-selective electrode membranes (∼200 µm thick) contained PVC and NPOE (1:2 by weight) and 1.0 wt % of the inert salt TDDA-TpClPB. The membrane was prepared by solvent casting, with THF as solvent, in complete analogy to preparations of traditional ion-selective electrode membranes.49 Experimental Setup. The membranes were mounted in a Philips body electrode (IS-561, Glasbla¨serei Mo¨ller, Zu¨rich, Switzerland) typically used for potentiometry, with a 0.1 M NaCl inner filling solution. The electrodes were conditioned overnight in a solution identical to the inner filling solution. The measurements were performed at laboratory ambient temperature (21 °C) in a three-electrode cell system, where the internal redox element of the ion-selective electrode acted as the working electrode, and the external reference electrode and counter electrode were immersed in the sample. A 1 M LiOAc solution was used as the bridge electrolyte of the reference electrode. Cyclic voltammetric and amperometric experiments were performed with a AFRDE5 (49) Bakker, E. J. Electrochem. Soc. 1996, 143, L83.
Analytical Chemistry, Vol. 71, No. 17, September 1, 1999
3661
Pine potentiostat (Pine Instruments, Grove City, PA), and the data recorded with a LabView 3.1 program (National Instruments, Austin, TX) on a Macintosh computer equipped with a 12-bit data acquisition card (PCI-1200, National Instruments). The voltage signals were low-pass filtered before reaching the A/D board. Pulsed amperometric experiments were software controlled with the same setup. Calibration curves were produced by adding aliquots of a 1 M NaClO4 solution to 50 mL of distilled water. Reproducibility experiments involved batch replacement of the respective samples. RESULTS AND DISCUSSION Basic Response Principles. It is well established that the faradaic current associated with voltammetric sensors on liquidliquid interfaces is associated with an ion-transfer process.34 Hence, more extreme potentials lead to the uptake of more hydrophilic ions. If the response is mass transport limited, a direct relationship between sample concentration and observed current is established. Most work on the voltammetry at ITIES used a fourelectrode potentiostat, with a reference and a counter electrode each immersed into the organic and aqueous solution to be measured, although two-electrode setups were often found to be adequate. The resulting currents are associated with interfacial ion-transfer processes. In this work, a configuration was chosen that purposely mimics that of a potentiometric setup. Potentials were applied with a regular three-electrode potentiostat, with the working electrode immersed in the inner filling solution and the reference and counter electrodes in the sample. This was done in view of a realistic real-world application of such sensors, where they would likely be used within an array of other sensing elements that may or may not function potentiometrically. Potentiometric sensors based on solvent polymeric membranes must have ion-exchanger properties to function properly. Only if the concentration of extracted sample ions is comparatively invariant relative to the sample concentration is a Nernstian electrode slope observed.50 With voltammetric sensors, spontaneous uptake of ions is not desired. In fact, the recorded cyclic voltammogram of a PVC-DOS membrane containing the sodium salt of the lipophilic anionic additive NaTFPB, in contact on either side with 0.1 M NaCl, showed a simple ohmic behavior with slope of 14 µA V-1 (data not shown), suggesting a bulk resistance Rmem ) 6.9 × 105 Ω. This is predicted theoretically on the basis of eq 8 (see Figure 2). At large ion-exchanger concentrations in the membrane, no substantial ion depletion in the sample or accumulation in the membrane occurs as the applied potential is increased. Most of the applied potential drop is associated with the iR drop within the membrane, thereby leading to the ohmic behavior of the cell. According to eq 8, such a mode should still yield ion currents that depend on the sample composition since the Nernst equation partially dictates the response, but the information is no more interesting than that obtained with traditional potentiometric sensors. Analogous results were reported by Horvath and Horvai on PVC membranes without polarizable interfaces.27 In further work, voltammetric membranes had no substantial ion-exchange properties and contained the lipophilic salt tetra(50) Bakker, E.; Na¨gele, M.; Schaller, U.; Pretsch, E. Electroanalysis 1995, 7, 817.
3662 Analytical Chemistry, Vol. 71, No. 17, September 1, 1999
Figure 3. Cyclic voltammogram of a PVC-NPOE membrane containing the lipophilic salt tetradodecylammonium tetrakis(4-chlorophenyl)borate, in contacted with 0.1 M NaCl on both sides. Scan rate, 10 mV s-1. At positive potentials, the extraction and release of sample anions occurs, while at negative potentials, the extraction and release of sample cations is observed (see text). The so-called threshold potential is estimated by extrapolating the ohmic portions of the plot to zero current (see eq 13).
dodecylammonium tetrakis(4-chlorophenyl)borate to reduce the bulk membrane resistance. This electrolyte is far more lipophilic than what is ordinarily used in the field of ion-transfer voltammetry. The more polar plasticizer NPOE (mem ) 14)51 was preferred. As stated in the Theoretical Section, four separate diffusion layers are principally encountered with such a setup. It was estimated that, with moderately concentrated samples, depletion in the aqueous phase is in most cases negligible. This suggests that the two diffusion layers in the membrane phase, adjacent to both phase boundaries, are rate limiting. Figure 3 shows the voltammetric response of an NPOE-PVC membrane to 0.1 M NaCl in the sample and inner filling solution. Since the working electrode was immersed in the inner filling solution, positive applied potentials must lead to a net flux of negative charge from the counter electrode to the working electrode. As Figure 3 shows, the current starts to increase around 450 mV and assumes an ohmic behavior similar to the data shown in Figure 2. According to theory, this process is accompanied with the extraction of chloride anions at the sample side and sodium ions at the inner filling solution side. The threshold potential (see eq 13) where this process occurs should directly correspond to the activities of these two ions in both solutions and to the overall coextraction constant, which is a direct function of the Gibbs’ free energy of transfer of the electrolyte. Figure 3 indicates that the threshold potential occurs at ∼655 mV on the forward scan and -733 mV on the backward scan. Correcting for potential asymmetries in the cell, this translates to an applied membrane potential of about (694 mV, which would, according to eq 13, correspond to a coextraction constant kNakCl ≈ 10-9.6. This is a reasonable number for solvent polymeric membranes.1 As the potential is reversed, the current decreases again according to Ohm’s law. The current (51) Armstrong, R. D.; Horvai, G. Electrochim. Acta 1990, 35, 1.
Figure 4. Cyclic voltammograms of a PVC-NPOE membrane containing the lipophilic salt tetradodecylammonium tetrakis(4-chlorophenyl)borate, with 0.1 M NaCl as inner electrolyte and different 1 mM sample electrolytes. Scan rate, 5 mV s-1.
on the backward scan is smaller than on the forward scan since the diffusion layer thickness within the membrane continuously increases, which leads to smaller currents (see eq 10). As the threshold potential is reached, the current reverses sign and a peak shape is observed. This peak is interpreted as the back extraction process of the ions now contained in the membrane. As the potential is further reduced, the observed current eventually again assumes an ohmic behavior below -500 mV. This is interpreted as the uptake of sample cations and corresponding chloride anions from the inner filling solution. In a fully symmetric experiment, this process should occur at the same potential magnitude as in the forward scan. However, a number of potential contributions are not symmetrical in this experiment since different electrode constructions were used on either side of the membrane, which explains the observed shift. Again, the magnitude of current gradually decreases as the scan direction is reversed, and a current peak with opposite sign is observed around the threshold potential for the cation extraction process. This experiment indicates that anions and cations can in principle be measured voltammetrically within the same scan. This is one demonstration of the principle multianalyte detection capability of such voltammetric sensors. Figure 4 shows observed cyclic voltammograms of different sample electrolytes, all measured at millimolar concentrations. As predicted by theory (eq 13), the threshold potentials continuously shift to higher values as less lipophilic anions are present in the sample, with the Hofmeister sequence of ClO4- > NO3- > Br- > Cl- > OAc-. This compares well to data obtained from potentiometric selectivity determinations with ionophore-free anion-selective electrodes,52 which would predict a potential shift by +187, +226, +310, and +392 mV for the voltammetric uptake of the ions NO3-, Br-, Cl-, and OAc- relative to ClO4-. Figure 4 also shows the cyclic voltammetric behavior of 10-3 M KClO4 in the sample. As (52) Schaller, U.; Bakker, E.; Spichiger, U. E.; Pretsch, E. Anal. Chem. 1994, 66, 391.
expected, the current response behavior at positive potentials, where anion extraction from the sample occurs, is nearly identical to the corresponding NaClO4 experiment. At negative potentials, where sample cations are transferred into the membrane, the threshold potential occurs earlier and larger currents are observed as a result. Again, literature values from potentiometric experiments predict a threshold potential shift of about -60 mV on the basis of eq 13,52 which is in good agreement with the data presented here. The limiting current of classical voltammetric or amperometric experiments on metal electrodes is ideally dictated by limiting mass transport from the bulk sample to the electrode surface. A similar situation is typically observed with voltammetry at liquidliquid interfaces since the high diffusion coefficients within the organic phase lead to rapid diffusion of ions away from the interface into the bulk of the phase. Under the conditions presented here, however, it seems that mass transport in the solvent polymeric membrane is rate limiting. This has a number of interesting implications. If depletion on the sample side is negligible, the phase boundary concentration of extracted sample ions will be a direct function of the sample activity and the applied potential (see eq 9). Higher sample activities dictate a higher activity of extracted ions at the interface. These ions continuously diffuse from the phase boundary into the membrane bulk where ion concentrations are lower. The observed current is given by this diffusion rate, since additional ions must transfer into the membrane phase to make up for the ions diffusing away from the interface. This process conveniently explains why the observed current on a forward voltammetric scan is still directly dependent on the sample composition. It must be pointed out that this seems to be one of very few voltammetric systems that apparently measure ion activities, not concentrations. Consequently, no effect of sample stirring on the current was observed experimentally (data not shown). A decrease in scan rates from 100 to 5 mV s-1 gradually enlarged the stripping peaks, evidently since different times were allowed to uptake ions from the sample during the forward voltammetric scan, and shifted these peaks by up to 100 mV to more extreme potentials, again since more time was allowed for the stripping process to occur (data not shown). Forward scans showed somewhat larger currents with faster scan rates, which can be attributed to smaller Nernst diffusion layer thicknesses under these conditions. Direct Continuous Ion Detection Modes: Cyclic Voltammetry and Pulsed Amperometry. It is evident from the above discussion that solvent polymeric membranes under potentiostat control cannot be used in complete analogy to classical amperometric sensors. With metal electrodes, the diffusion layer thickness is kept constant by sample stirring or by introduction of a diffusion barrier between electrode and sample. With the membranes discussed here, the diffusion layer is located within the membrane. A classical amperometric measurement would invariably lead to a continuous increase of the diffusion layer thickness until δ matches the membrane thickness. With typical 40-200µm-thick membranes, however, response times would approach many hours. It seems therefore more practical to use these membranes in a voltammetric mode where the diffusion layer is repeatedly renewed. One example is the cyclic voltammetric data shown above in Figures 3 and 4. Since no effect of sample stirring Analytical Chemistry, Vol. 71, No. 17, September 1, 1999
3663
Figure 5. Demonstration of the pulsed amperometric detection principle for eliminating current drifts with 1 mM NaClO4 as sample (for other parameters, see Figure 4). The potential is repeatedly stepped between two values that initiate uptake and release of sample ions into and from the membrane. The current gradually decreases within a pulse owing to a continuous increase of the diffusion layer thicknesses in the membrane. The current is reproducible from pulse to pulse.
is observed here, cyclic voltammograms have more quantitative utility than ordinary metal electrode systems. One drawback, however, is that δ assumes large values in a slow-scanning experiment (see above). Higher currents should be observed with smaller δ values, which could be achieved by rapid stepping of the potential between two discrete values, each associated with the assisted ion uptake and stripping to and from the membrane. The two applied potentials should be sufficiently above and below the threshold potential according to eq 13. This principle is demonstrated in Figure 5 where the potential is successively stepped between +1.0 and +0.1 V in 1-s intervals for a 10-3 M NaClO4 sample. According to the cyclic voltammogram for NaClO4 in Figure 4, these two potentials should lead to currents in opposite direction, as desired. The data in Figure 5 show that the current continuously decreases within one potential pulse, which is mostly due to a continuously increasing δ within the membrane. The sign of the current reverses upon application of 0.1 V, and the same general behavior is observed. Upon application of the next 1-V pulse, the current returns to the previous value, showing that the membrane composition has been successfully renewed during the preceding step. The reproducibility of the pulsed amperometric response was tested by immersing the electrode repeatedly in 10-3 and 10-4 M NaClO4 solutions. Figure 6 shows the mean current during the last 100 ms of each +1.0-V pulse of 1-s duration as a function of time. Evidently, electrode drift was virtually eliminated and relative standard deviations were found to be (0.2 and (0.4% for 10-3 and 10-4 M NaClO4 solutions, respectively. Pulsed amperometric detection has also been proposed as a preferred detection mode in other recent works,18,20 further demonstrating the usefulness of the technique.
3664 Analytical Chemistry, Vol. 71, No. 17, September 1, 1999
Figure 6. Reproducibility of the pulsed amperometric mode shown in Figure 5 for two different NaClO4 solutions (for other parameters, see Figure 4). The current is sampled for the last 100 ms of each pulse and plotted as a function of time. Drift is virtually eliminated (see text).
CONCLUSIONS Voltammetry on solvent polymeric membranes is a feasible technique with considerable practical promise as a transduction principle for these types of sensors. The response mechanism is, perhaps surprisingly, quite strongly related to their potentiometric counterparts. Apparently, these sensors ordinarily still measure ion activities, not concentrations as with traditional voltammetric systems. However, membranes without ion-exchanger properties seem most useful for this detection mode. This makes the simultaneous detection of sample anions and cations possible, since the sign of the applied potential dictates the charge sign of the sample ion that is extracted into the membrane. Naturally, in a practical system the membrane selectivity must be adequate to make use of this interesting detection possibility. Classical amperometric detection seems not feasible with this technique since the rate of diffusion away from the interface in the direction of the membrane bulk continuously decreases as time progresses, leading to drifts in the observed currents. Interestingly, cyclic voltammetry seems more useful as a quantitative tool here than with classical electrochemical detectors since mass transport within the sample phase is often not limiting the observed current. A pulsed amperometric approach is however a direct replacement for classical amperometry, since current drifts can be eliminated by careful adjustment of the experimental parameters. ACKNOWLEDGMENT The authors thank the Petroleum Research Fund (administered by the American Chemical Society) and the National Institutes of Health (Grant R01-GM58589) for financial support, Shigeru Amemiya for helpful discussions, and Erno Pretsch for careful reading of the manuscript.
Received for review February 17, 1999. Accepted June 10, 1999. AC990195X
