Potentiometric Responses of IonSelective Membranes Containing Mixtures of Two Different Ionophores

Feature Article

Potentiometric Responses of Ion-Selective Membranes ContainingMixtures of Two Different IonophoresMiklos Erdosy,a Vasile V. Cosofret,a Richard P. Buck,b Kla¬ra To¬th*c

a Instrumentation Laboratory, Inc., 101 Hartwell Ave., Lexington, MA 02421b Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, N.C. 27599c Department of General and Analytical Chemistry, Budapest University of Technology and Economics, Budapest, Hungary,H-1111; [email protected]

Received: December 21, 2001Final version: April 29, 2002

AbstractResearch related to optical sensors (optodes) directed the attention towards plasticized polymeric membranescontaining more than one ionophore at the same time. Studying the potentiometric behavior of such membranesystems helps to characterize the effect of cross-contamination in multi-electrode systems used for biomedicalapplications. In this article, a theoretical model is introduced, which describes the potentiometric behavior ofmembranes doped with two different ion carriers. The theory is compared to those results obtained with a modelsystem containing both K� and H�-ionophores in the membrane phase. The study has also been extended to ion-selective membranes containing two other ion-selective carriers in different ratios. These results give guidelines tooptimize the physical arrangement of potentiometric multi-electrode systems.

Keywords: Ion-selective membrane, Mixed ionophores, Cross-contamination, Selectivity, Complex stability constant

Dedicated to Professor Gary Christian on the Occasion of His 65th Birthday

1. Introduction

The potentiometric responses of plasticized membranesincorporating mixtures of ionophores appear to be arelatively new research topic in the view of the excitingfield of optical devices (optodes). These two-ionophore-containing systems were developed at ETH inZ¸rich by theresearch group of the late Professor Simon [1 ± 6].Membranes, incorporating mixture of ionophores were

studied by Bakker and Pretsch [7, 8] in order to determinethe stability constants of ion-carrier complexes. Theirmethod was based on the theory that the hydrogen ionselectivity of a neutral hydrogen ion carrier and a secondionophore mixture containing membrane depends on thecomplex stability constant of the second ionophore as well.The H� ion selectivity coefficient ratio of the neutralhydrogen selective ionophore-containing membrane andthe H� ionophore and a second ionophore mixturecontaining membrane is proportional to the complexformation constant of the non-hydrogen ion selectiveionophore. The limitation of this method is that thedetermined stability constants depend on the neutralhydrogen ionophore and its linear range. Still the stabilityconstant values were in good correlation with thoseobtained by optical methods.In a recent study, Bakker and his co-workers [9, 10]

described a more accurate method for determination ofstability constants of different ionophores. This method,using so-called sandwich structured membranes was devel-

opedbasedon the theory ofMikhelson [11] andMokrov andStefanova [12]. In this article, their results are comparedwith our findings on the selectivity of mixed ionophoresystems.A practical situation, relevant to the two-ionophore

systems is the cross-contamination of membrane compo-nents from different neighboring ion-selective membranes.This problem arises in two different cases:1) When a sensor array is formed by placing individual

membrane cocktails on a poly(vinylchloride) (PVC) sup-port, and the diffusion of plasticizer across the unplasticized(or slightly plasticized) PVC carries ionophores intoneighboring membranes; or similarly, when a single,contiguous membrane sheet, typically PVC, is externallydoped with two or more different ionophore-plasticizercocktails to form an array detector, and the diffusion of theplasticizer in the unplasticized membranes carries theionophores from one membrane to the other one [13].This case was a common problem for manufacturers ofpoint-of-care blood gas analyzers that incorporate PVCbased sensor cartridges.2) When an array of different ion sensors is interconnect-

ed only by the sample, typically blood, in the channel of alinear flow sensor array, or by natural convection when aflexible array is implanted into the live tissue for longerperiod of time [14]. During in vivo experiments, a mixingprocess of the ionophores may occur. Flexible, implantablesensor arrays [15 ± 17] and point-of-care blood gas analyzerscan introduce these types of cross-contamination processes

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Electroanalysis 2002, 14, No. 19±20 ¹ 2002 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim 1040-0397/02/1910-1433 $ 17.50+.50/0

due to the close placing of the different sensors on a specificelectrode platform.In potentiometry, another case of sensors containing two

ionophores is the unsuccessful experimental attempt to mixionophores for the same ionic species, typically H�, in aneffort to extend the pH response ranges. In spite ofexpectations, it was found that the most stable complex-forming ionophore in greater concentration dominated theresponse [18, 19]. Related experiments used an aminatedPVC polymer with a proton carrier [20].Aswe are going to discuss in detail, we have found that the

cross-contamination can lead to sub-Nernstian potentio-metric responses. In some special cases, the slope is half ofthe theoretical one. These non-Nernstian responses are notnew in the field of ion selective sensors. Suzuki and co-workers [21, 22] reported twice Nernstian slopes for ISEscontaining polyether antibiotics with carboxyl groupsduring calibration by Ca2� or Mg2�. Umezawa and co-workers [23, 24] developed a theoretical model in order todescribe twice-Nernstian responses of such membranescontaining acidic ionophores and anionic sites. The range ofthe super-Nernstian response was found to be dependent onthe pH of the bathing electrolyte [24].In this article, we describe the response of solvent

polymeric membranes doped with two different iono-phores using a new theoretical model. Based on exper-imental data, we suggest a method to optimize the arrange-ment of multi-sensor arrays to reduce the disadvantageouseffects of cross-contamination.

2. Theory

2.1. Theoretical Description of the Mixed IonophoreSystems

The conventional Teorell-Meyer-Sievers theory [25, 26] ofthe net membrane potential difference splits the overallpotential difference into three components: two interfacialand one bulk diffusion-migration potentials. Each interfa-cial potential difference is calculated by equating theelectrochemical potentials of each exchanging chargedspecies in the membrane and in the bathing solution. Thenetmembrane potential difference is independent of the siteconcentration in themembranewhen the sites are uniformlydistributed. Thus, for single ionophore based membranebathed in a salt of a single permeable species, H� or K�, theobserved membrane potential difference is independent ofsite concentration and independent of ion pairing with site.As long as the conductivity of themembrane is not too small,reliable potentiometric measurements can be made evenwhen ion pairing is significant, especially in low dielectricconstant membranes. The internal potential difference (i. e.,bulk diffusion-migration potential) is zero for a singlepermeating species (or even for two species when they havethe same mobility).Accordingly, the expected potentiometric responses of

single ionophore-containing membranes arise from perme-

ability of a single species. For pH sensors, a Nernstianresponse is observed over a verywide (3 ± 12) pH range untilDonnan failure occurs at high acidities and the alkali ionresponse limits the proton response at high basicities.Similarly, a Nernstian response is expected for K� ion overa wide (pK�� 1 ± 6) concentration range until interferenceeffects appear at high concentrations of lipophilic anions.However, when a membrane for pH also contains an ioncarrier for K�, then in a two-ion containing solutions as pHincreases (H� ion concentration decreases), the responsebecomes constant and independent of pH corresponding tothe constant K� background electrolyte concentration. Thebackground interference is expected to increase withincreasing activity of K� in solution, but also with increasingpotassium carrier concentration in the membrane or byincreasing the ionophore complex stability constant.Aswe are going to prove in the light of the theory derived,

a new effect is also expected to occur in the potentiometricresponsewhen the two ionophores of comparable formationconstants reach nearly equal concentrations in the mem-brane phase. At this point, the response of either speciesdevelops a sub-Nernstian slope, specifically half of thenormal Nernstian slope. This reduced slope is caused by thefact that in such a case the membrane activities of bathingions increase as the square root of bathing activities, insteadof reaching a constant value, independent of bathingactivity, as required for a Nernstian response.It is assumed in the theory of neutral carrier transport,

that in themembrane bulk there are always asmany free andcarrier complexed counter-ions as there are sites. This is aresult of membrane electroneutrality [27].In the present study, a potassium and proton ionophore-

containing system is described although the results are notspecific to this ionophore pair. In the discussion, as a firstapproximation, ion-pairing and activity coefficients areignored. While we understand the importance of ion-pairing, we realized that by taking it into account wouldcomplicate the theory far too much. The future work willconsider to extend the theory by taking into account ion-pairing effect also. Further on, we assume arbitrary concen-trations of two ionophores (forming 1 :1 adducts), and weconsider a total mobile site concentration equal to the sumof the concentrations of the ion/ionophore complexes plusthe concentrations of the remaining free ions.Considering the above-mentioned assumptions, six defin-

ing equations can be established based onmaterial balancesand the following three equilibria (a, b, c):

(Ks�)� (Hm

�) � (Km�)� (Hs

�) (a)

(Km�)� (com1) � (Kcom1

�) (b)

(Hm�)� (com2) � (Hcom2

�) (c)

Q�Kiex

K�s

� ��

�

� �� K�m

� �H�

m�� 1�

1434 M. Erdosy et al.

Electroanalysis 2002, 14, No. 19±20

(Hm�)� (Hcom2

�)� (Km�)� (Kcom1

�)� S� (2)

Kcom�1� �

K�m

� � � com1� ��KKc1 �3�

Hcom�2� �

H�m

� � � com2� ��KHc2 �4�

(Kcom1�)� (com1)�C1 (5)

(Hcom2�)� (com1)�C1 (6)

where subscript m refers to the membrane species, andsubscript s is for solution species.K� refers to the potassiumandH� to the proton concentration. The site concentrationisS, and theneutral carrier ionophores are com1 and com2. InEquation 1,Kiex refers to the equilibriumconstant of the ion-exchange reaction between the solution and the membranephase. Formation constants of the charged complexes areKKc1 and KHc2 corresponding to potassium carrier 1 andproton carrier 2. C1 and C2 are the total concentrations ofthe respective ionophores. Q is a convenient runningvariable that is the ratio of the ionic species in themembrane phase.The relationship betweenC1,C2 and S can be represented

in the following way:

C1�C2� S or C1�C2� S or C1�C2� S (7)

The following different cases have importance frompractical point of view:Case 1: C1�C2� S. In this case either one of the

ionophore concentrations or neither of them exceeds thesite concentration, S. A special subcase can occur whendifferent sensors are nearby in an array. When theionophores leak, then there is the possibility that C2��C1

or C1��C2. In this case, C1 �S or C2�S. In our experi-ments, we are exploring the question, how do traces offoreign ionophore cause interference in the detection of theprincipal species of the sensor.Case 2: C1�C2�S. This is a particularly simple case to

solve for the speciation profiles. It is further simplified whenC1�C2�S/2.Case 3: C1�C2� S.This is not a useful case from practical

point of view since in this case the interference is controlledby an ion exchange mechanism and not by the ionophoreinduced selective responsemechanism (i. e., by the selectivecomplex formation with the ionophore).The general theory of ion-carrier interactions at zero

current (potentiometry) and ion-carrier transport (chro-noamperometry and impedance studies) system-responsesfall into two categories: one is the more selective (e.g.,logKpot

X,Y��1.5) complex formation or less selective ion-exchange responses, and the other is the more selective orless selective transport processes. The significant change in

selectivity occurs when the carrier concentration exceedsthe site concentration or conversely. At high total carrierconcentrations, typically twice the site concentration, allions exist as cation-carrier complexes within themembrane.In this case, the membrane is selective for the ion that formsthe most stable complex. At low carrier concentrations, themembrane contains some ion-carrier complexes, but mustalso contain free ions, especially foreign ions from thesolution, to maintain electroneutrality in the membranephase. In such a case, different ions may enter into themembrane phase and the selectivity is determined by the ionexchange properties of the membrane.The experience from the studies of mixed ion carriers in

optodes also shows that total carrier concentrations abouttwice the site concentration are satisfactory for permittingchanges in both the detecting and the indicating carrier-complexes. In normal operation, the detecting carriervalinomycin, for example, in an optode for K� is in excessof the indicating carrier, a coloredproton carrier.Changes inunknown bathing activities of Ks

� cause changes in theconcentration ofKcom1

� complex within themembrane. Atthe same time, required by electroneutrality, the concen-tration of proton-carrier complex must change also. Anincrease in Ks

� activity at constant Hs� activity produces a

decrease in colored proton-carrier complex that can bemeasured optically and interpreted in terms of bathing Ks

�

activities.

2.2. Solving the Mixed Ionophore Case

The speciation problem for the model of a potassiumionophore and a proton ionophore is solvable from amastercubic equation. A convenient running bathing electrolyteconcentration variable, Q, is introduced (see Equation 1).From the Equation 1,Hm

� is expressed as:

Hm��Km

� / Q (8)

and then from Equation 3, com1 is expressed as:

com1�Kcom1� / (Km

�KKc1) (9)

Expressing com2 from Equation 4 and eliminating Hm� by

Km�/Q using Equation 8:

com2�Q* Hcom2� / (Km

�KHc2) (10)

Combining Equations 5 and 3 and eliminating all thevariables except Km

�, it gives:

Kcom�1 �

C1

1� 1K�

mKKc1

�11�

Similarly, combining Equations 6 and 4 and eliminating allthe variables except Km

�, we can expressHcom2�:

1435Potentiometric Responses of Ion-Selective Membranes


Hcom�2 �

C2

1� QK�

m�KHc1

�12�

Combining Equations 9, 10, 11, 12 with Equation 2, thefollowing equation can describe Km

�:

K�m�

S

1� 1Q

� C1KKc1

KKc1K�m � 1

� C2KHc2

KHc2K�m �Q

�13�

In this equation, the only variable isKm�. Re-arranging this

equation, the result forKm� is the following cubic equation:

A3 (Km�)3�A2 (Km

�)2�A1 (Km�)�A0� 0 (14)

where

A3�1�QQ

KKc1KHc2 �15�

A2� 1�Q� � � KKc1 �KHc2

Q

� �� C1 � C2 � S� �KKc1KHc2 �16�

A1�C1QKKc1 � C2KHc2 � S QKKc1 �KHc2� � � 1�Q� � (17)

A0��Q* S (18)

Equation 14 is a general description of the two-ionophoremembrane problem. It does not restrict the relationshipbetween C1, C2 and S. In order to simplify the results,considering the earlier mentioned three practical cases, wefocus on the following one:

C1�C2� S (19)

Using this assumption, the following simplified expressionscan be written for A0*, A1*, A2* and A3*:

A3*�A3 (20)

A2*� 1�Q� � KKc1 �KHc2

Q

� �

A1*�� C1KHc2 � C2QKKc1 � 1�Q� �

A0*�A0 (23)

Equation 14 can be solved for Km� using the Cardano

mathematical formula [28]. The potential related toHs� and

Ks� can be described with the following equations:

PotK� (RT/F) ln(Ks�/Km

�) (24)

PotH� (RT/F) ln(Hs�/Hm

�) (25)

The calculated results appear as species concentrations inthe membrane. The concentrations include the simple ionsand the carrier-complex ions as a function of the naturalvariable Q defined in Equation 1. Equation 1 relatesmembrane bulk concentrations to solution concentrationsusing the unknown ion exchange constant, Kiex. Finally, therunning variable is simply proportional toKs

�/Hs�, the ratio

of ion concentrations in the buffers. For simplicity, byassuming the ion exchange constant is unity, the interior freeion concentrations in themembranewill have aunity ratio atthe bathing solution unity ratio. This means that we can plotthe calculated species concentrations in the bulk of themembrane as a function of Ks

�/Hs� and that species plot

becomes almost symmetrical for most species on (Ks�/Hs

�)� 1 (Fig. 1a and 1b).When both Km

� and Kcom1� are constant in the

membrane phase (simultaneously Hm� and Hcom2

� arevariable), the response to Ks

� is Nernstian. Similarly, whenHm

� and Hcom2� are constant in the membrane phase

(simultaneously Km� and Kcom2

� are variable), theresponse to Hs

� is Nernstian as well. These conditions canbe met when the formation constants are different, and thisis true formost practical situations. However, when they areequal, it is not possible to find a range of bathing electrolytecompositions that allow the free ion and complex concen-trations for either ions to become constant. This means thatthe free ion and complex concentrations change as thebathing electrolyte composition changes. As a result, thesensor response does not follow the Nernstian slope.Figure 1a and b show the concentrations of the species in

the bulk of the membrane computed for three differentvalues of the formation constant for the potassium-ioncomplex, called KHc1�101, 103, and 105, using a single valueof the formation constant for the proton-carrier complex:KKc2� 105. Both ionophores have equal concentrations inthe membrane phase. As Figure 1b indicates, the potassiumion ± ionophore complex (Kcom1

�) concentration is almostconstant at any potassium-ion/proton concentration ratio.Simultaneously,Hcom2

� concentration is almost constant athigher bathing electrolyte proton concentrations, especiallywhen KHc2 is similar to KKc1., When the two stabilityconstants become equal, as it is expected, the membranefree ion concentrations become linearly dependent from thelogarithmof the ion concentration ratio (Q). In this case, theslope of the theoretical potential response is half of theNernstian slope.The computer generated potential response curves, as a

function of pH, at constant K� ion concentration for threedifferent combinations of the formation constants, areshown in Figure 2a, where C1 and C2 are equal (50% ofthe total site concentration). On Figure 2b, we show thepotential curves when the amount of the potassiumionophore is only 10% of the total site concentration andthe hydrogen ionophore is 90%. In this case, the slope isclose to the Nernstian value at high proton concentrationsfor all the formation constants. The half Nernstian slope isreduced to a small concentration region. This shows that wecan expect to measure half Nernstian slope only at equal



carrier concentrations, while the ion-carrier formationconstants should be close value to each other.

3. Experimental

3.1. Reagents

For all experiments, deionized water (Barnstead, NanopureII) and chemicals of analytical-reagent gradewere used. Themembrane components were supplied by FlukaAG (Buchs,Switzerland): tridodecylamine (TDDA; #95292); hydrogenionophore IV (Hy-IV; ETH 1778; #95296), chromoiono-phore I (ETH 5294; #27086); valinomycin (Val; #94675);calcium ionophore I (Ca-I, ETH 1001, #21192); sodiumionophore VI (Na-VI; #71739); o-nitrophenyl octyl ether(o-NPOE, #73741); potassium tetrakis (4-chlorophenyl)borate (KTpClPB, #60591). The material used for themembrane matrix was high molecular weight PVC (HMW-

PVC, #81392). The chemicals used for preparing thesolutions were purchased from Aldrich.

3.2. Polymeric Membrane and Electrode Preparation

The membranes were prepared according to the classicalprocedure [29]. Their compositions in regard to ionophore/salt additive ratio are listed in Tables 1a and b.Conventional high molecular weight PVC-based mem-

branes were used in this study. Their typical compositionswere 32 wt% PVC and 65.0 wt% plasticizer. The ionophoreand potassium tetrakis (p-chloro)phenyl borate concentra-tions of the membranes are listed in Tables 1a and 1b.All studiedmembranes were incorporated into Philips IS-

560 membrane electrode bodies (Mˆller Glasblaserei,Z¸rich, Switzerland) and tested for their electroanalyticalproperties.

Fig. 1. a) Calculated concentrations of different species as a function of (Ks�/Hs

�). (�–±�) Km�; (–±) Hm

�; KKc1� 105 for all curves;solid line: KHc2�101; dotted line: KHc2�103; dashed line: KHc2�105. b) Calculated concentrations of different species as a function of (Ks

�

/Hs�). (�–±�) Kcomp�; (–±) Hcomp�; KKc1� 105 for all curves; solid line: KHc2�101; dotted line: KHc2�103; dashed line: KHc2�105



3.3. Calibrating and Inner Filling Solutions

For pH and K� ionophore based electrodes a weak citrate ±borate buffer solution (6.0 mM LiOH�0.66 mM citric acid�1.14 mM boric acid) was used for calibrating the sensorsand as an inner electrolyte. The K� ion content of thesolutionswas varied from10�1 ± 10�5 M (the compositions oftheir inner filling solutions are mentioned in Tables 1a and1b.) The final pH of the buffer solutions were adjusted witheither 0.1 M NaOH or 0.1 M HCl using an Orion combina-tion glass pH electrode (Model 90-32) connected to anOrion pH/mV meter (Model 730A). The K� ion content ofthe solutions was varied from 10�1 ± 10�5 M.Additionally, electrodes based on two different iono-

phores selective to cations other than H� were calibrated ina solution containing a mixture of the two different cations.

The solutions were made in de-ionized water by usingcorresponding dry chloride salts. The ion concentrationswere varied from 10�1 ± 10�5 M for each cation componentindependently (e.g. Na� and K� or Na� and Ca2�).

3.4. Emf Measurements

Emfmeasurementswere carried out at room temperature inan air-conditioned laboratory at 24.0� 0.5 �C with an OrionpH/mVmeter (Model 730A) connected to an Orion Model605 electrode switchbox that allowed us to test up to sixelectrodes at the same time. As a reference electrode, anOrion Model 90-02 Ag/AgCl double junction was usedthroughout. The solution in the outer compartment of thereference electrode consisted of 1.0 M lithium acetate.The different pH and K� ionophore-containing mem-

brane electrodes were preconditioned by soaking them in a10�2 M KCl, pH 7.0 citrate buffer solution for at leastovernight. The calibrations were performed by measuringthe emf values in citrate buffer solutions at constant KClbackground concentrations. The pH of these calibratingsolutions were varied at different desired pH values byadding small increments of either 0.1 M NaOH or 0.1 MHCl. The emf values were taken when the readings werestable within �0.2 mV/min. The other two-ionophore-containing electrodes were preconditioned in a solutioncontaining both ions at 10�3M concentrations. The emfvalues weremeasured by keeping one of the ion componentconstant, while the other one was varied from 10�1 ± 10�5 M,using series of solutions.The Nicolsky-Eisenmann selectivity coefficients (KI,J

pot)were determined by the separate solution method, using10�1 M concentration unbuffered solutions of the chloridesalts for the different cations [30].

3.5. Calculation of the Theoretical Curves

MathCad 5.0 software (Mathsoft) was used to plot thetheoretical curves. The software was running under Win-dows 98 operating system.

4. Results and Discussion

4.1. Model System

The idealmodel for verifying the theory is a system that usestwo different types of ionophores (e.g., for two differentbathing ions), with relatively close values of complexstability constants when using the same plasticizer. In thatcase the ion exchange constant will be near unity, soQ valuewill be near unity as well, when the membrane transitionfrom excess ionic species 1 to excess species 2 occurs.It is a requirement for each ionophore to be highly

selective (log KpotX,Y � about �4) for its primary ion.

Otherwise, the membrane will cease ion-exchange at high

Fig. 2. a) Calculated potential vs. hydrogen ion concentration(H�) curves at constant potassium ion concentration (Ks

�� 10�3

M). C2� 0.5S; C1� 0.5S; KHc2� 105; (–±) KKc1� 101; ( ¥¥ ¥ ¥ ¥ ¥ ) KKc1

� 103; (- - - -) KKc1� 105. b) Calculated potential vs. hydrogen ionconcentration (H�) curves at constant potassium ion concentra-tion (Ks

�� 10�3 M). C2� 0.9S; C1� 0.1S; KHc2� 105; (–±) KKc1�101; ( ¥ ¥ ¥ ¥ ¥ ¥ ) KKc1� 103; (- - - -) KKc1� 105.



bathing solution concentrations since both ionophores willcoordinate the same ion. The model system is plasticizedPVCmembranes containing potassium and proton selectiveionophores: valinomycin (Val) and hydrogen ionophore IV(ETH 1778; Hy-IV). Val forms a relatively strong andselective complex with potassium ion. The ETH 1778hydrogen ionophore is very selective (log Kpot

H,Y �� 8) toH� ion, like the other proton carriers, and it shows goodlinearity profile in the pH1 ± 7 range. In our experiments, westudied the membrane behavior using o-nitrophenyl octylether (o-NPOE) as a high dielectric constant solvent.When the Val and Hy-IV concentration ratio was varied

systematically from 100% Val to 100% ETH 1778(Table 1a), the potentiometric response to pH improvedfrom near zero slope to close to Nernstian slope. Theoptimal pH response was best illustrated for low constantionic backgrounds of KCl, when o-NPOE was used as theplasticizer. Figure 3 is an example of pH responses of fivedifferent carrier composition ratios at 10�4 M KCl back-ground. The slope was close to half of the Nernstian at the50%composition of both carriers. It is important tomentionthat the calibration plot remained close to linear for the pH2 ± 7 range while the slope was reduced to 50% of the

Table 1. Compositions of polymeric membranes used for the mixed-ionophore studies. All polymeric membrane compositions areaccurate within �2% error. The internal filling solution was pH 4 citrate-borate buffer with 10�3 M KCl background for all themembranes. All polymeric membranes contained o-NPOE as plasticizer.

a)Membrane Ionophore 1 (Val ) Ionophore 2 (Hy-IV) (� ionoph.)/

(sites)mmol/100 mgmembrane

mol% oftotal ionophore

mmol/100 mgmembrane


[%]

pH/K-1 2.2 100 0 0 116pH/K-2 1.84 85 0.15 15 118pH/K-3 1.32 50 0.89 50 120pH/K-4 0.39 16 0.69 84 110pH/K-5 0 0 0.97 100 130

b)Membrane Ionophore 1 Ionophore 2 Negative sites

mmol/100 mgInternalfilling

mmol/100 mgmembrane

mol% of totalionophore

mmol/100 mgmembrane


membrane electrolyte

Na-VI ValNa-1 5.16 100 0 0 4.10 10�3 M NaClNa/K-2 5.16 97 0.17 3 4.10 10�3 M NaClNa/K-3 5.16 89 0.64 11 4.10 10�3 M NaClNa/K-4 5.16 78 1.44 22 4.10 10�3 M NaCl

Ca-I ValCa-1 2.93 100 0 0 2.05 10�2 M CaCl2Ca/K-2 2.93 95.5 0.14 4.5 2.05 10�2 M CaCl2Ca/K-3 2.93 89 0.35 11 2.05 10�2 M CaCl2Ca/K-4 2.93 71 0.81 29 2.05 10�2 M CaCl2

Val Ca-IK-1 2.15 100 0 0 1.40 10�3 M KClK/Ca-2 2.15 96 0.08 4 1.40 10�3 M KClK/Ca-3 2.15 90 0.24 10 1.40 10�3 M KClK/Ca-4 2.15 75 0.71 25 1.40 10�3 M KCl

Fig. 3. Potentiometric response of ISEs based on containingvalinomycin and hydrogen ionophore IV (Hy-IV) in differentratios. The plasticizer is o-NPOE in all the membranes. Thecalibrating solution contains 10�4 M KCl background electrolyte.S100%�51.0 mV/pH; S84%� 50.1 mV/pH; S50%� 25.2 mV/pH; S15%

� 1 mV/pH (based on linear fit for pH 2 ± 6).



Nernstian one. Based on the data, the calibration plotsshowed similar trend to those curves suggested by theory(see Fig. 2a).

4.2. Other Two-Ionophore-Containing Membranes

In order to extend thepractical aspects of this study, a coupleof other ionophore combinations were compared. The goalwas to optimize the arrangement of the sensors in a multisensor array. Our preliminary data showed that certain ionselective membranes are very sensitive to cross-contami-nation while others are not. A systematic study withdifferent two-ionophore based membrane systems has

been conducted. While the results cannot be directlycorrelated to the theory, their investigation proves thesignificance of cross-contamination. This part of our studywas very important from practical point of view because ithelped to determine the best arrangement for the electrodesin a multi-sensor system. It gave guidelines for placingsensors in a multi-sensor array in order to avoid thedeterioration of potentiometric performance due to cross-contamination.The studied systems contained Ca2�, Na� and K�

ionophores in different combinations. These systems wereslightly different from the Val ± Hy-IV containing combi-nations. These membranes had constant concentrations ofthe main ion carrier and the salt additive. The mainionophore concentration was in excess of the site concen-tration. The second ionophore content of the membranephase was gradually increased in order to see the effect ofcontamination on the sensor behavior. This concentrationschemewas similar to thatwhich canbeobserved for sensorsin multi-sensor arrays.The effect of cross-contamination depends on the binding

constants of various ionophores towards their primary ionand various interfering ions. The overall excess of theprimary carrier over the negative sites does not guaranteethe proper selectivity of the membrane when a secondcarrier is extracted into the membrane phase. Table 2summarizes the measured slope values in the significantlyhigh 10�1 ± 10�2 M or 10�2 ± 10�3 M concentration ranges ofthe primary ion, using membranes with different secondionophore contents (see Table 1b for the compositions ofthe membranes). Sensors containing 5.16 mmol/100 mgmembrane Na-VI carrier showed significant interference

Table 2. Slope values of different ion-selective membranes calibrated in 10�1 ± 10�2 M or 10�2 ± 10�3 M solutions of the primary cationsand containing interfering cations in various concentrations

Membrane Slope in 10�1 ± 10�2M NaCl concentration range at different KCl background concentrations (cKCl)

10�2M 10�3M 10�4M 10�5M

Na-1 56.9 56.0 55.4 56.0Na/K-2 13.1 35.4 50.8 53.6Na/K-3 0 10.3 35.8 49.7Na/K-4 �1.1 3.3 28.5 47.5

Slope in 10�1 ± 10�2M CaCl2 concentration range at different KCl background concentrations (cKCl)

10�2M 10�3M 10�4M 10�5MCa-1 29.4 29.4 28.9 28.1Ca/K-2 20.7 28.7 28.1 29.6Ca/K-3 12.3 27.3 27.3 28.2Ca/K-4 6.6 25.0 27.1 28.7

Slope in 10�2 ± 10�3M KCl concentration range at different CaCl2 background concentrations (cCaCl2)

10�1M 10�2M 10�3M 10�4MK-1 54.0 56.0 57.3 57.3K/Ca-2 53.2 55.6 57.3 57.4K/Ca-3 52.1 55.3 57.3 57.4K/Ca-4 49.6 54.3 56.7 57.9

Table 3. Selectivity coefficients (log KpotX,Y) of different ion-

selective electrodes based on membranes listed in Table 1b.

Membrane log KpotX,Y [a]

Na� K� Ca2� H�

Na-1 0 �1.7 �3.2 �2.8Na/K-2 0 0.9 �2.2 �1.8Na/K-3 0 1.9 �1.4 �1.6Na/K-4 0 2.0 �1.4 �1.6Ca-1 �3.8 �3.8 0 �4.0Ca/K-2 �3.8 0.6 0 �4.2Ca/K-3 �3.8 1.6 0 �4.2Ca/K-4 �3.6 2.2 0 �3.9K-1 �3.5 0 �3.9 �4.0K/Ca-2 �3.5 0 �3.7 �3.9K/Ca-3 �3.4 0 �3.3 �3.8K/Ca-4 �3.5 0 �3.0 �3.7

[a] Determined by separate solution method at 10�1 M concentrations.



at 10�4 M KCl concentration even at 3 mol% Val content(Na/K-2 membrane). A 9% slope decrease was observed inthese conditions (Ca/K-3). Calcium-selective membrane(2.93 mmol/100 g membrane ETH 1001) showed much lessinterference in the presence of valinomycin even at higherconcentrations (see Table 2). Calcium selective membranewith 11 mol% potassium ionophore contamination at 10�3

M KCl background electrolyte showed about 7% slopedecrease. It is important tomention that potassium selectivemembranes contaminated with ETH 1001 calcium ioncarrier showed only negligible interference. A 25% calciumionophore content (K/Ca-4) reduced the slope for thepotassium response by less than 2% at 10�3 M CaCl2background electrolyte concentration. The slope decreasewas just 5%, even at 10�2 M CaCl2 level, and the slopedecrease was about 12% at 10�1 M CaCl2 backgroundconcentration.Table 4 summarizes the stability constant values of the

ionophores used in this study. It is interesting to compare theobservations about cross-contamination with the ratios ofthe stability constant values. Valinomycin forms 103 timesmore stable complex with potassium ion than the Na-VIionophorewithNa�. This data is in good agreement with thesignificant loss of potassium ion selectivity in case ofmembranes containing Na-VI as primary and valinomycinas the secondary ionophore (Na/Kmembranes). The loss ofselectivity is supported by the fact that the K� ion selectivitycoefficient (log Kpot

K,Y) of the Na-VI ionophore is about�1.7 (Table 3) and the Na� ion selectivity of Val is about�4.5. Thismeans thatNa-VI is not very selective for sodiumby itself and therefore the valinomycin contaminationsignificantly decreases the potassium selectivity.More complex issue is the effect of cross-contamination in

case of the Ca-I ionophore and Val containing membranes.The Ca-I ionophore has a very high formation constantvalue (log �� 24.5) but the Val contamination still signifi-cantly reduces the calcium ion selectivity over potassium ionselectivity. Oppositely, as the data indicate, the potassiumselective membrane (log �� 11.63) does not lose selectivityas the Ca-I ionophore contamination increases. Bothionophores show about the same selectivity coefficient(log Kpot

X,Y) for each others primary ion (Table 3). The factthat Val forms 1 :1 ion ± ionophore complex while Ca-Iforms 1 :2 complex may partially explain the unexpected

experimental results, we would still expect opposite situa-tion based on the formation constants of the ionophores.This points to an important conclusion: the theory and theformation constants can provide valuable data for the betterunderstanding of membranes containing ionophore mix-tures, but the complex effect of cross-contamination canonly be fully determined by experiments with variousmembrane compositions.The calculated Nicolsky-Eisenmann selectivity coeffi-

cients (KpotI,J), measured by using the separate solution

method (Table 3), confirmed the above described interfer-ence results. The selectivity decreased not only toward theprimary ion of the second ionophore, but toward other ionsas well, due to the limited selectivity of both carriers. Thiscan significantly effect the results of whole blood measure-ment in blood gas analyzer systems. As the data in Table 3indicates for the Na-VI ± Val containing membrane,increased hydrogen and calcium-ion interference could beobserved at the same time. The Val ± Ca-I containingmembrane showed less but similar effect, while the Ca-I ±Val system showed negligible decrease in selectivity towardsother cations.Based on similar studies, it is possible to optimize the

arrangement of sensors in a multi-sensor array. It candetermine which ionophores have the most significantimpact on the neighboring sensors considering the physio-logical ranges for the different ions. It is advisable, forexample, not to place sodium and potassium sensors side-by-sidewhile the potassiumand calcium sensor can be safelyplaced next to each other in a sensor array platform.

5. Conclusions

The theoretical model that describes the behavior ofmembrane systems containing two different ionophoreshelps us to better understand the behavior of ion-selectivemembranes, especially in multi-sensor-systems. The meas-ured potential responses support the data predicted by thetheory. The results also help to improve the use-life/shelf-life time ofmulti-sensor arrays. Based on these experiments,it is possible to keep those sensors further away from eachother that can develop more serious effect of the cross-contamination. This means that during extended wetstorage (e.g., 12 months) or blood exposure (e.g., 60 daysor longer) the cross-contamination of the membranecomponents will not deteriorate significantly the perform-ance of the sensors. We could implement these data tooptimize the sensor sequence in multi electrode arrayarrangements used for in vivomeasurements and for wholeblood analysis with blood gas analyzers.

6. Acknowledgements

This work was supported in part by NSF EngineeringResearch Center, Grant CDR-8622201, NIH GrantR01HL-49818, The Hungarian Ministry of Education,

Table 4. Formation constants of the ionophores used in presentstudy based on the literature data.

Ionophore/plasticizer Complexstochiometry

Formation constantlog � com

[12] [8]

Val/DOS n� 1 10.10 9.32Val/o-NPOE n� 1 11.63 ±Na-VI/DOS n� 1 6.55 ±Na-VI/o-NPOE n� 1 9.19 ±Ca-I/DOS n� 2 19.7 19.7Ca-I/o-NPOE n� 2 24.54 ±



Grant FKFP 0572/1999 as well as by InstrumentationLaboratory, Co.

7. References

[1] W. E. Morf, K. Seiler, P. R. Sorenson, W. Simon, in Ion-Selective Electrodes (Ed: E. Pungor), Vol. 5, AkademiaiKiado, Budapest 1989, pp. 141 ± 152.

[2] W. E. Morf, K. Seiler, B. Lehmann, C. Behringer, S. S. Tan, K.Hartman, P. R. Sorenson, W. Simon, in Ion-Selective Electro-des, (Ed: E. Pungor), Vol. 5, Akademiai Kiado, Budapest1989, pp. 115 ± 124.

[3] E. Bakker, W. Simon, Anal. Chem. 1992, 6, 1805.[4] K. Seiler W. Simon, Anal. Chim. Acta 1992, 266, 73.[5] E. Bakker, M. Lerchi, T. Rosatzin, B. Rusterholz, W. Simon,

Anal. Chim. Acta 1993, 278, 221.[6] E. Bakker, M. Willer, M. Lerchi, K. Seiler, E. Pretsch, Anal.

Chem. 1994, 66, 516.[7] E. Bakker, E. Pretsch, J. Electrochem. Soc. 1997, 144, L125.[8] E. Bakker, E. Pretsch, Anal. Chem. 1998, 70, 295.[9] Y. Mi, E. Bakker, Anal. Chem. 1999, 71, 5279.[10] Y. Qin, Y. Mi, E. Bakker, Anal. Chim. Acta 2000, 421, 207.[11] K. N. Mikhelson, Sens. Actuators B 1994, 18 ± 19, 31.[12] S. B. Mokrov, O. K. Stefanova, Elektrokhimiya 1985, 21, 540.[13] K. Bezegh, A. Bezegh, J. Janata, U. Oesch, A. Xu , W. Simon,

Anal.Chem. 1987, 59, 2846.[14] E. Lindner, V. V. Cosofret, S. Ufer, R. P. Kusy, R. P. Buck,

R. B. Ash, H. T. Nagle, J. Chemical Soc., Faraday Trans. 1993,89, 361.

[15] V. V. Cosofret, M. Erdosy, T. A. Johnson, R. P. Buck, R. B.Ash, M. R. Neuman, Anal. Chem. 1995, 67, 1647.

[16] R. P. Buck, V. V. Cosofret, E. Lindner, S. Ufer, M. B.Madaras, T. A. Johnson, R. B. Ash, M. R. Neuman, Electro-analysis 1995, 9, 846.

[17] E. Lindner, V. V. Cosofret, S. Ufer, T. A. Johnson, R. B. Ash,H. T. Nagle, M. R. Neuman, R. P. Buck, Fresenius J. of Anal.Chem. 1993, 346, 584.

[18] E. Bakker, A. Xu, E. Pretsch, Anal. Chim. Acta 1994, 295,253.

[19] S. C. Ma, N. A. Chaniotakis, M. E. Meyerhoff, Anal. Chem.1988, 60, 2293.

[20] E. Lindner, V. V. Cosofret, R. P. Kusy, R. P. Buck, T.Rosatzin, U. Schaller, W. Simon, J. Jeney, K. Toth, E.Pungor, Talanta 1993, 40, 957.

[21] K. Suzuki, K. Tohda, H. Aruga, M. Matsuzone, H. Inoue, T.Shirai, Anal. Chem. 1988, 60, 1714.

[22] K. Suzuki, K. Tohda, Trends Anal. Chem. 1993, 12, 287.[23] S. Amemiya, P. B¸hlmann, K. Tohda, Y. Umezawa, Anal.

Chim. Acta 1997, 341, 129.[24] S. Amemiya, P. B¸hlmann, Y. Umezawa, Anal. Chem. 1998,

70, 445.[25] T. Teorell, Proc. Soc. Exp. Bio. Med. 1935, 33, 282.[26] K. H. Meyer, J. F. Sievers, Helv. Chim. Acta 1936, 19, 649.[27] R. P. Buck, T. M. Nahir, V. V. Cosofret, E. Lindner, M.

Erdosy, Anal. Proceedings 1994, 31, 301.[28] E. T. Whittaker, G. Robinson, in The Calculus of Observa-

tions: A Treatise on Numerical Mathematics, 4th ed. NewYork, Dover 1967, pp. 124 ± 126.

[29] G. J. Moody, R. B. Oke, J. D. R. Thomas; Analyst 1970, 95,910.

[30] E. Pungor, K. Toth, A. Hrabeczy-Pall, Pure Appl. Chem.1979, 51, 1913.



Potentiometric Responses of IonSelective Membranes Containing Mixtures of Two Different Ionophores

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