amperometric detection of alkali metal ions on micro-fabricated composite polymer membranes

Journal of Electroanalytical Chemistry 453 (1998) 211–219

Amperometric detection of alkali metal ions on micro-fabricatedcomposite polymer membranes

Hye Jin Lee, Carine Beriet, Hubert H. Girault *

Laboratoire d’Electrochimie, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland

Received 3 March 1998; received in revised form 20 April 1998

Abstract

Transfer reactions of alkali metal ions, either direct or facilitated by the ionophores dibenzo-18-crown-6 and valinomycin, havebeen studied at the water�2-nitrophenyloctylether-polyvinyl chloride gel micro-interface. The polarised micro-hole array interfacebetween water and the composite polymer membrane is used as a selective transducer for the amperometric detection of alkalimetal ions. The formal Gibbs transfer energies of ion transfer from the aqueous phase to the 2-nitrophenyloctylether-polyvinylchloride gel phase are evaluated from voltammetric measurements. The association constants are calculated to determine theselectivity imparted by the ionophores such as dibenzo-18-crown-6 and valinomycin on transferring the alkali metal ion at the gelmicro-interface. © 1998 Elsevier Science S.A. All rights reserved.

Keywords: Alkali metal ion transfer; Polymer membrane; Amperometric sensor; Dibenzo-18-crown-6; Valinomycin

1. Introduction

Amperometric detection of non redox ionic species ispossible by measuring the current associated with iontransfer reactions across a polarised liquid�liquid inter-face such as the water�1,2-dichloroethane interface [1,2].To transpose this approach to the design of a trans-ducer is difficult for two main reasons. First, the me-chanical instability of a liquid�liquid interface is a majorproblem which can be circumvented by gelification ofone or two of the phases. Most publications related toion transfer reactions at a liquid�gel interface haveshown that although the diffusion coefficient of the ionsin the gel is much reduced, ion transfer reactions couldstill be used for amperometric purposes [3–22].

The second difficulty associated with ion transferreactions is the resistive nature of the organic phase.This inherent disadvantage can be reduced by usingmicro-interfaces. Different approaches to support a mi-

cro liquid�liquid interface have been proposed and in-clude the use of micropipettes [23,24] or micro-holes inthin polymer films [25–28]. Enzyme sensors using microliquid�liquid interface arrays have been developed forthe assay of urea and creatinine, where the transducerrelies on the amperometric detection of ammonium[29,30].

Recently, we have developed micro-fabricated com-posite polymer membranes which combine the advan-tages of using gelified organic phases andmicro-interfaces [31]. We have shown that reproduciblecomposite polymer membranes could be fabricated forthe amperometric detection of choline using either di-rect ion transfer reactions or stripping ion transferreactions [32].

Due to the fact that alkali metal cations are veryhydrophilic and usually limit the potential window atthe ITIES, the introduction of ionophores in the or-ganic phase is required to translate the transfer of thesecations within the potential window. This approach waspioneered by Koryta [33] who studied the transfer ofalkali metal ions facilitated by ionophores such as

* Corresponding author. Tel.: +41 21 6933151; fax: +41 216933667; e-mail: [email protected]

0022-0728/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.PII S0022-0728(98)00171-5

H.J. Lee et al. / Journal of Electroanalytical Chemistry 453 (1998) 211–219212

dibenzo-18-crown-6 and valinomycin. The main effectof the ionophore is to lower the Gibbs energy oftransfer of the cation.

Extensive studies have been carried out on a largenumber of neutral or synthetic macromolecules able toform a complex with alkali and alkaline earth metalions at the ITIES. Some examples of facilitated iontransfer reactions include the study of alkali metal ionfacilitated transfer by hydrophilic crown ethers [34], theinvestigations of potassium ion facilitated transfer bymonoaza-18-crown-6 [35] and valinomycin [36] and thefacilitated ammonium ion transfer by the syntheticcrown ether, dibenzo-18-crown-6. The study of facili-tated ion transfer by dibenzo-18-crown-6 using sup-ported tip micro-pipettes has been reported [26,37]. Thefacilitated lithium ion transfers by ETH1810 [38] anddibenzyl-14-crown-4 [39] have also been studied bycyclic voltammetry.

Mechanisms for these facilitated ion transfer reac-tions have been published covering a wide range ofionophores. More recently, Matsuda et al. [40] haveproposed a theoretical treatment for reversible iontransfer reactions facilitated by neutral ligands, and thiswork was recently extended by Beattie et al. [41] and byReymond et al. [42,43].

In this paper, a novel amperometric detectionmethod for facilitated sodium, ammonium and potas-sium transfer by dibenzo-18-crown-6 and valinomycinbased on a polymer composite membrane is demon-strated. The performance of the novel membrane fea-turing a micro-interface array is considered. Theassociation constants are evaluated for alkali metal ioncomplexation. A thermodynamic study of ion transferat micro-gel interfaces by electrochemical methods isalso presented.

2. Experimental

2.1. Chemicals

The aqueous and organic phase solvents are de-ionised water (Milli-Q, Millipore, CH) and 2-nitro-phenyloctylether (NPOE) (Fluka, CH) respectively.Tetramethylammonium chloride (TMACl), tetraethyl-ammonium chloride (TEACl), tetrapropylammoniumchloride (TPrACl), tetrabutylammonium chloride(TBACl), dibenzo-18-crown-6 (DB18C6) and valino-mycin are supplied by Fluka (CH). Ammonium chlo-ride (NH4Cl), lithium chloride (LiCl), sodium chloride(NaCl) and potassium chloride (KCl) are also suppliedby Fluka (CH). High molecular weight polyvinylchlo-ride is supplied by Sigma (CH). Bis(triphenylphospho-ranylidene)ammonium tetrakis(4-chlorophenyl)borate(BTPPATPBCl) is prepared by metathesis of BTPPACland potassium tetrakis(4-chlorophenyl)borate (KTP-

BCl) and tetrabutylammonium tetrakis(4-chlorophenyl)borate (TBATPBCl) is prepared by metathesis ofTBACl and KTPBCl. Tetraphenylarsonium tetra-phenylborate (TPAsTPB) is prepared by metathesis oftetraphenylarsonium chloride (Fluka, CH) and sodiumtetraphenylborate (Fluka) [44]. All other chemicals usedare analytical grade or better.

2.2. Micro-machined composite polymer membrane

The membrane consists of two polymer layers; asupporting film of polyethylene terephthalate (12 mmthick, Melinex type ‘S’ from ICI Films, UK) with amicro-hole array of 66 holes (11×6) which is etched bya UV Excimer laser and acts as a supporting film for agelified NPOE phase with polyvinyl chloride (PVC).The process of laser micro-machining of the polyester isperformed as described elsewhere [31]. The entrancediameter is 22 mm and the exit 13 mm due to the effectof the anisotropic etching [45]. The polyvinyl chloridelayer is prepared by dissolving PVC (2.8% m/m) in asolution of TBATPBCl (10 mM) or BTPPATPBCl (10mM) or TPAsTPB (1 mM) in NPOE either including orexcluding ligands, at a temperature of approximately120°C. The dual layer membranes are produced asdescribed elsewhere [31].

2.3. Electrochemical measurements

Electrochemical measurements are performed in atwo electrode mode and the cell featuring the micro-hole array supported between water and NPOE-PVCgel has been described in a previous report [31]. Cyclicvoltammetry is performed using a computer controlledpotentiostat (Sycopel Scientific, UK) or a Tacussel Pol150 workstation (Radiometer, France) the data ofwhich is acquired using Windows-driven software. Herethe Galvani potential difference across the water andPVC-NPOE gel phase, Do

wf, is given by:

Dowf=fwater−foil

The sweep rate in the cyclic voltammetry work is 10mV s−1 (unless otherwise specified). All the aboveexperiments are carried out at a room temperature of2392°C.

3. Results and discussion

3.1. Gibbs energy of transfer of ions

The TATB assumption which is usually used forevaluating the Gibbs energy of transfer of ionic speciesis based on the fact that the standard Gibbs transferenergies of TPAs+ cation (DG tr,TPAs+

0,w�o ) and TPB− an-ion (DG tr,TPB−

0,w�o ) can be assumed to be equal [46]. Thus,

H.J. Lee et al. / Journal of Electroanalytical Chemistry 453 (1998) 211–219 213

Fig. 1. A steady-state cyclic voltammogram for the transfer of TPAs+ and TPB− ions. x, y and z in the Cell 1 are 0. (···) represents the zeropoint of the potential scale. Sweep rate=10 mV s−1.

simply the centre of symmetry of the current-potentialcurve obtained with TPAsTPB as the base organicelectrolyte and LiCl as an aqueous base electrolyteusing Cell 1 (x, y and z=0) can be considered as thezero of the TATB potential scale (Fig. 1), since in thiscase the potential window is limited by the respective-transfers of TPAs+ on the left and TPB− on the right.To obtain this point graphically we need to estimate thesteady-state current for TPB− transfer which is difficultto measure directly. Using the approximate ratio of thesteady-state currents, it is also possible to extract theratio of the diffusion coefficients of these two ions. Themagnitude of these currents should be directly propor-tional to the ionic size assuming the Stokes-Einsteinequation (D=kT/6pha with D, the diffusion coeffi-cient, a, the hydrodynamic radius of the ion and, thesolvent viscosity). Our estimation of the steady statecurrent for TPB− as shown in Fig. 1 yields a ratio ofthe diffusion coefficients (DTPB−/DTPAs+) equal to1.290.1 and that of the ionic radii to be approximately0.8390.1 which is in relatively good agreement withthe ratio of the atomic radii of the central arsoniumand boron atoms [47].

Having defined the origin of the TATB potentialscale for Cell 1, it is possible to obtain from thevoltammetric data the formal Gibbs transfer energyvalues in this scale. In this way, a value of −33.2 kJmol−1 is obtained for both TPAs+ and TPB−. Theformal Gibbs energy of transfer is defined from theformal ion transfer potential, Do

wf i0%, by:

DowG tr

0%, w�o= −ZiFDowf i

0% (1)

Fig. 2. A steady-state cyclic voltammogram for TPrA+ ion transferin the TATB potential scale. x, y and z in Cell 1 are 0, 0 and 0.05.Sweep rate=10 mV s−1.

Fig. 2 shows a cyclic voltammogram for TPrA+ iontransfer at the polarised water�2.8% PVC-NPOE gelinterface using Cell 1 (x, y=0 and z=0.05). Themeasured values for the half-wave potentials and for-mal transfer potentials of TPrA+, TEA+, TMA+,TPB− and TPAs+ together with the formal Gibbstransfer energies are listed in Table 1.

Considering the asymmetry of the diffusion fields inthe micro-interface array used, the relationship betweenthe formal transfer potential and the experimental halfwave potential is derived for inlaid micro-discs andwritten by [48]:

Dowf1/2=Do

wf0%+RTzF

ln�Dwgo

Dogw

�+

RTzF

ln�4d

pr+1

�(2)

where d and r are the thickness and the radius of themicrohole, respectively. As a first approximation, we


Fig. 3. Cyclic voltammograms of alkali metal ion transfer into thePVC-NPOE gel membrane in the TATB potential scale. x and y inCell 2 are 0 and 5, respectively. Sweep rate=10 mV s−1.

Table 1The measured values for the half-wave potentials and formal transferpotentials of TPrA+, TEA+, TMA+, TPB− and TPAs+ togetherwith the formal Gibbs transfer energies

Dowf0%/mVbDo

wf1/2/ DowG tr

0%, w�gel/ DowG tr

0, w�o/kJIonmol−1dmVa kJ mol−1c

−11.991.5−34915 −8.7TPrA+ −123915TEA+ 36915 3.491.5 2.6(2.6*)125915

125915214915 11.591.5 10.7TMA+

−33.291.5 −31.5(344915TPB− 255915

−30.3*)TPAs+ −344915 −33.291.5 −30.3*−255915

50.091.5Na+ —607915 51891544.691.5 —462915551915NH4+

442915531915 42.691.5 —K+

a Half wave potentials.b Formal transfer potentials in the TATB potential scale using Cell 1or Cell 2, respectively.c The formal Gibbs energy of transfer in the TATB scale across thewater�2.8% PVC-NPOE gel interface.d The standard Gibbs energy of transfer taken from the voltammetryand solubility* measurements in pure NPOE solvent carried out bySamec et al. [49].

assume that the activity coefficients in water and theorganic gel are equal. The ratio of the diffusion coeffi-cients between water and the organic gel can, in asecond approximation, be taken to be equal to that ofthe viscosity between the pure solvents namely waterand NPOE (138 kg m−1 s−1) [49]. This approximationis reasonable based on the work of Armstrong et al.who measured the conductivity of the NPOE-PVC gelas a function of the PVC composition. From their data,the difference of the half-wave potential for an iontransfer in pure NPOE and PVC-NPOE gel was esti-mated to be approximately 5 mV [50]. The validity ofthe assumption is corroborated by the fact that thevalues obtained in this experiment are in a relativelygood agreement with those evaluated in pure NPOEsolvent from the solubility and voltammetry measure-ments carried out by Samec et al. [49] (Table 1).

For the study of the formal Gibbs transfer energiesof K+, NH4

+ and Na+ ions, it is difficult to measuredirectly the experimental half-wave potential becausethe cation itself is a potential limiting species. Cyclicvoltammograms of the transfer of these metal ionsacross the PVC-NPOE gel using Cell 2 are presented inFig. 3.

To circumvent this difficulty we choose to measurethe half wave transfer potential of potassium first, andto refer all the others to this value by comparison of thetransfer potential for a constant current value. Fig. 4shows a linear sweep voltammogram of K+ ion trans-fer using Cell 2 with IR compensation. The logarithmicplot of log(Iss−I/I) versus potential gives a straightline with a slope of 71 mV which probably indicatesthat the ohmic drop is not fully compensated (seeinsert). In this present analysis, the migration effect due

to no supporting electrolyte in the aqueous phase isneglected so that we may consider that our data areoverestimated by 20 mV (S. Wilke, private communica-tion). Nevertheless, from the forward wave of thevoltammogram, the half wave potential of potassium isestimated as 531915 mV.

Using TMA+ as an internal reference, we measurethe half-wave potential and formal transfer potentialvalues of alkali metal cations (Do

wf1/2, M+) in the TATBpotential scale. The data and the respective formalGibbs transfer energies are listed in Table 1.

3.2. Facilitated potassium, sodium and ammonium iontransfer by DB18C6

To lower the standard Gibbs transfer energy of eachmetal ion, an ionophore is introduced into the gelphase. DB18C6 which has a large KD (highly insolublein water) is selected due to its ability to form stablecomplexes with alkali and alkaline earth metal ions[26,51–53].

An important quantity for the complexation proper-ties of the ionophore is the ratio of the radius of thecation to the cavity size of the ionophore. It is knownthat alkali metal ions form 1:1 stoichiometry (metal:ligand) complexes with DB18C6. The system for the


Fig. 4. (a) Extended cyclic voltammogram for K+ ion transfer in the TATB potential scale using Cell 2 (x=5, y=0). Sweep rate=5 mV s−1.(b) logarithmic plot of log(Iss−I/I) versus potential for K+ ion transfer. (- - -) Represents the regression line.

study of facilitated potassium, sodium and ammoniumion transfer by DB18C6 is presented in Cell 3. Cyclicvoltammograms obtained for the transfer by interfacialcomplexation of the sodium ion by DB18C6 at thewater�2.8% PVC-NPOE gel interface are shown in Fig.5 (x=0.1–10 for Na+ in Cell 3).

At liquid�liquid interfaces, in the case of an excess ofthe ligand, the observed limiting current should beproportional to the concentration of the metal ionspecies, whereas for an excess concentration of themetal, the limiting current should be proportional tothe concentration of the ligand. However, for the wa-ter�PVC-NPOE gel system, the current is always limitedby the diffusion of the ligand in the gel phase even fora concentration ratio of metal: ligand down to 1:500.This is essentially due to the much lower diffusioncoefficient of DB18C6 in the gel phase compared tothat of the cations in the liquid phase.

If we consider that the ion transfer reaction is limitedby the mass transport of the ligand, the geometry of theinlaid micro-hole filled with the PVC-NPOE gel [31]should be considered as a 12 mm depth recessed micro-hole interface as shown in Fig. 6. It allows us to use thebehaviour of the recessed solid micro-electrode as a

Fig. 5. Steady-state voltammograms of Na+ ion transfer into thePVC-NPOE gel membrane facilitated by 10 mM DB18C6 in theTATB potential scale using Cell 3 (x= (i) 0.1, (ii) 1, (iii) 10). Sweeprate=10 mV s−1.

model. In this case, at short times the facilitated iontransfer reaction is mainly controlled by the linear fluxof the ligand in the micro-hole which explains the slightpeak-shape of the voltammogram (Fig. 5). At longertimes, the spherical diffusion flux dominates the iontransfer so a steady-state current is reached. Neverthe-less the ion transfer can be regarded as reversible orquasi-reversible. From this steady-state current it ispossible to estimate the diffusion coefficient of DB18C6


Fig. 6. Simplified diagram of the ion flux in the micro-hole array.

Fig. 7. Plot of the half wave potential of (i) Na+ �DB18C6 complexversus, (ii) NH4

+ �DB18C6 complex versus and (iii) K+ �DB18C6complex versus. (- - -) Represents the regression line.

in the gel using the equation formerly derived for therecessed solid micro-electrode [54].

I0�w=4mzFDocor� pr

pr+4L�

(3)

where L is the distance between the recessed micro-in-terface and the surface of the support PET film. Thecentre to centre distance chosen for the array can beconsidered as large enough to neglect the shieldingeffect as the diffusion coefficients are very low [55].The calculated diffusion coefficient of DB18C6 in thePVC-NPOE gel is found to be approximately 2.7×10−11 m2 s−1. This low value confirms that even inthe presence of an excess concentration of ionophore,the current is limited by the mass transfer of theligand.

For a facilitated ion transfer reaction, the associa-tion constant can be calculated from the expressionderived for the reversible half wave potential for theion transfer reaction in the case of excess metal sys-tems [52]. Here we assume that the expression validfor a micro-disc can be used:

Dowf1/2=Do

wf0%+RTF

ln� DL

DLM

�−

RTzF

ln(b1ocM+) (4)

where b1o is the association constant (normally ex-

pressed as logb1o), cM+ is the metal ion concentration,

and DL and DLM+ are the diffusion coefficients of theionophore and ionophore complex respectively. As-suming that DL= [52] and taking the formal potentialof metal ions from Table 1, log b1

o can be obtainedfrom Eq. (4).

Fig. 7 shows the concentration dependence of thehalf-wave potential for Na+, NH4

+ and K+ iontransfer facilitated by DB18C6 using Cell 3 and therelevant wave information is shown in Table 2. Thehalf-wave potentials of each metal ion are evaluatedin accordance with the TATB assumption in the

form,

Dowf1/2=Do

wf1/2, M(exp)+

− [Dowf1/2, TMA(exp)

+ −DowfTMA+

0% ] (5)

where DowfTMA+

0% is 125 mV. The slopes of the regres-sion lines for Na+, NH4

+ and K+ are −0.0645,−0.0562 and −0.0562 V M−1, respectively. The calcu-lated association constants (log b1

0) of alkali metalcations are found to be Na+ (11.3), NH4

+ (9.4) andK+ (10.6). These values indicate that DB18C6 has ahigher complexing strength for sodium in PVC-NPOEgel than for potassium and ammonium. The trend inthe association constants and the values themselves atthe water�PVC-NPOE interface can be regarded as rea-sonable with regard to the values Na+ (10.4), NH4

+

(8.6) and K+ (9.9) at the water�1,2 DCE interfacereported by several authors [56,57].

3.3. Facilitated potassium, sodium and ammonium iontransfer by 6alinomycin

A macro-cyclic depsipeptide, valinomycin has beenused as an ionophore forming stable complexes withalkali metal ions depending on the ionic radius. Thesystem for the study of facilitated potassium, sodiumand ammonium ion transfer by valinomycin is pre-sented in Cell 4. Steady-state voltammograms ob-tained for the facilitated potassium ion transfer byvalinomycin are shown in Fig. 8 using Cell 4 with 10mM BTPPATPBCl (x varies from 0.05 to 5).


The facilitated ion transfer by valinomycin is alsolimited by the diffusion of the ligand over the concen-tration range studied. The calculated diffusion coeffi-cient of valinomycin in the PVC-NPOE gel using Eq.(3) is found to be 2.4×10−11 m2 s−1. This diffusioncoefficient of valinomycin is slightly lower than that ofDB18C6 which results in a more peak-shaped voltam-mogram and also confirms that the micro-hole interfaceis recessed. Fig. 9 shows the concentration dependenciesof the half-wave potentials for Na+, NH4

+ and K+ iontransfer facilitated by valinomycin using Cell 4 and therelevant wave information is shown in Table 3. Theslopes of the regression lines for Na+, NH4

+ and K+

are −0.0563, −0.0569 and −0.0613 V M−1,respectively.

In the case of potassium and ammonium ion transfer,it is difficult to measure the half wave potential accu-rately using TBA+ as the organic electrolyte cationbecause these cations transfer at more negative poten-tials than sodium. Therefore, the association constantsof ammonium and potassium are calculated from thecyclic voltammograms recorded with BTPPA+ as theorganic electrolyte cation which extends the potentialwindow towards more negative potentials. In the caseof sodium, the obtained values of the half wave poten-tial referenced versus the transfer of the TMA+ ionover the concentration range studied, are almost identi-cal with those obtained when either TBA+ orBTPPA+ are used as the organic phase supportingelectrolyte cation (Table 3).

Using the values from Table 3 and Eq. (4), thecalculated association constants (log b1

o) of alkali metalcations are found to be Na+ (12.5), NH4

+ (14.6) andK+ (16.0). These values indicate that valinomycin has a

higher complexing strength for potassium in PVC-NPOE gel than for ammonium. Furthermore, valino-mycin imparts a relatively smaller degree of complexingstrength towards the sodium ion compared to potas-sium and ammonium. These values can be consideredas reasonable in comparison to the values of 14.8 and13.1 for potassium and ammonium facilitated ion trans-fer at the water�1,2 DCE reported by Osborne et al.[52]. The trend of the selectivity in this experiment isalso reasonably similar to that obtained from potentio-metric measurements using ion selective electrodesmade of 30% PVC and 70% NPOE with valinomycin asa potassium selective ionophore. Here the selectivitycoefficients at the ISE membrane for potassium versusammonium (log KKNH4

) and potassium versus sodium(log KKNa) are reported as approximately, −1 and−3.7, respectively [58].

Fig. 8. Steady-state voltammograms of K+ ion transfer into themembrane facilitated by 10mM valinomycin in the TATB potentialscale using Cell 4. BTPPA+ is used as the organic electrolyte cation.x in the Cell 4 is (i) 0.05, (ii) 0.5, (iii) 1, (iv) 5. Sweep rate=10 mVs−1.

Table 2Half wave potentials of sodium, ammonium and potassium ion versus TMA+ and each metal ion concentration for the Na+, NH4

+ and K+ iontransfer facilitated by 10 mM DB18C6 as in Cell 3

cNa+/M Dowf1/2, Na+ versus TMA+/V cNH4

+/M Dowf1/2, NH4

+ versus TMA+/V cK+/M Dowf1/2, K+ versus TMA+/V

0.000050.0980.0001 0.0660.0000 50.1540.0080.001 0.00050.029 0.1020.0005

−0.037 0.005 0.041 0.005 −0.0530.01−0.095 0.05 −0.013 0.05 −0.1010.1


Table 3Half wave potentials of sodium, ammonium and potassium ion versus TMA+ and each metal ion concentration for the Na+, NH4

+ and K+ iontransfer facilitated by 10 mM valinomycin as in Cell 4

Dowf1/2, NH4

+ vs TMA+/V cK+/McNa+ Dow f1/2, Na+ vs TMA+/V cNH4

+/M Dowf1/2, K+ vs TMA+/M

−0. 137 −0.2610.000050.000050.00005 0.03 1(0.025)0.0005 −0.3110.0001 0.014 0.0001 −0.167

−0.3480.001−0.1 950.0050.0005 −0.01 6(−0.045)0.05 −0.3800.001 −0.041 0.001 −0.2140.01 −0.4010.005 −0.078 (−0.086) 0.005 −0.25 4

−0.2680.01 −0.108 0.01 0.050.1 −0.156−0.316−0.128(−0.134) 0.05

0.5 (−0.169)

The values in parentheses are obtained using 10 mM TBATPBCl as an organic electrolyte.

Fig. 9. Plot of the half wave potential of (i) and (i)* Na+ �valinomycincomplexes versus, using BTPPA+ and TBA+ as the organic elec-trolyte cation, respectively, (ii) NH4

+ �valinomycin complex versus and(iii) K+ �valinomycin complex versus using BTPPA+ as the organicelectrolyte cation. The (- - -) are obtained by linear regression.

also wish to acknowledge the financial support given bythe Fonds National pour la Recherche Scientifique Suisseunder grant no. FN 5002-045020. Laboratoire d’Elec-trochimie is part of the European Training and MobilityNetwork on ‘Organisation, Dynamics and Reactivity atElectrified Liquid�Liquid Interfaces’ (ODRELLI).

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4. Conclusions

Electrochemical data characterising the facilitated andnon-facilitated transfer of metal ion species at the wa-ter�PVC-NPOE gel interface are obtained. Dibenzo-18-crown-6 and valinomycin give well-defined facilitatedalkali metal cation transfer responses. DB18C6 is slightlymore selective towards sodium, whereas valinomycin isslightly more selective towards potassium. The PVC-NPOE gel including valinomycin can be used effectivelyas either a potassium or ammonium selective electrodein a matrix with excess sodium. The experimental resultsshow that the micro-interface polymer membranes withthe ionophores studied can be used as a transducer forthe amperometric sensing of alkali metal ions.

Acknowledgements

The authors wish to thank Dr Stefan Wilke for helpfuldiscussions and providing unpublished data. The authors


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