Composite Polyacrylate-Poly(3,4-ethylenedioxythiophene) Membranes for ImprovedAll-Solid-State Ion-Selective SensorsAnna Rzewuska,† Marcin Wojciechowski,† Ewa Bulska,† Elizabeth A. H. Hall,‡Krzysztof Maksymiuk,† and Agata Michalska*,†

Department of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland, and Institute of Biotechnology, Universityof Cambridge, Tennis Court Road, Cambridge, CB2 1QT, UK

A novel type of self-plasticizing polyacrylate-based mem-brane was developed for all-solid-state ion-selective po-tentiometric electrodes. The membrane composition con-tains a conducting polymer (CP): poly(3,4-ethylene-dioxythiophene) end capped with methacrylate groups,chemically grafted with the membrane during the photo-polymerization step. This composition results in ion-selective membranes with the following advantages: lowerelectrical resistance compared to the CP-free membrane,facile ion-to-electron transduction between the membraneand the electrode support, controlled low activity ofanalyte ions, and high concentration of interferent ions(incorporated with the CP) within the membrane, poten-tially resulting in improved analytical parameters. Ca2+-and K+-selective membranes were chosen as modelsystems to study the effect of pretreatment and CP contenton the potentiometric sensor’s characteristics. For Ca2+

sensors, reproducible and stable Nernstian characteristicswere obtained within the range from 0.1 to 10-9 M CaCl2,

without a time-consuming preconditioning step. For K+-selective sensors, the influence on Nernstian responserange was observed for varying KCl concentrations in theconditioning solution, with the lowest detection limitfound close to 10-8 M KCl. Mass spectrometry coupledwith laser ablation studies of the membranes revealed thatin this case the detection limit is not related to primaryion content in the membrane contacting a sample solu-tion, but is affected by interfering ion concentration closeto the membrane surface.

Ion-selective electrodes are established electroanalytical toolsallowing determination of many analytes, including clinically orenvironmentally important ions, in concentrations reaching tracelevels.1 An interesting modification, leading to sensors that couldbe potentially easier to manufacture and handle, is replacementof the internal solution with a conducting polymer ion-to-electrontransducer, thereby creating all-solid-state sensors2,3 (ASS-CP-

ISEs). A conducting polymer layer included in the sensorconstruction has been reported to contribute to significant increasein stability of recorded potentials2,3 compared to other internalsolution free arrangements.

ASS-CP-ISEs are usually produced in at least two steps, withinitial deposition of the conducting polymer transducer layer,which is next covered by an ion-selective membrane, most oftenplasticized poly(vinyl chloride) (PVC).2,3 From a practical pointof view, simplicity in sensor construction is desirable. Theapproach used for poly(vinyl chloride)-based membranes takesadvantage of solubility of some of conducting polymers in solventsthat can be added to the plasticized poly(vinyl chloride)-based ion-selective membrane formulation.4-9 Unfortunately, solubility canbe achieved only for some of the available conducting polymers:undoped (semiconductor form) poly(3-octylthiophene),4 proto-nated (conducting) form of polyaniline (PANI) and its deriva-tives,4-6,9 or doped poly(pyrrole).5,8 Calcium or lithium sensorsobtained with polyaniline or poly(3-octylthiophene) included inthe membrane up to a few weight percent (single piece elec-trodes4,6) have resulted in a performance that is slightly worsethan for traditional internal solution containing ion-selectiveelectrodes; e.g., detection limits achieved were close to 10-4 M.4,6

Slightly lower detection limits were also obtained for planarminiature potassium sensors containing PANI in the ion-selectivemembrane phase.7,8

On the other hand, plasticized PVC is sometimes not anoptimal choice of membrane material, since leakage of plasticizerfrom the membrane compromises the performance, especially iflong-term usage of the sensor is required.10,11 Self-plasticizingpolyacrylate membranes seem to be an interesting alternative, e.g.,

* To whom correspondence should be addressed. E-mail: agatam@chem.uw.edu.pl.

† Warsaw University.‡ University of Cambridge.

(1) Bakker, E.; Pretsch, E. Trends Anal. Chem. 2005, 24, 199-207.(2) Bobacka, J. Electroanalysis 2006, 18, 7-18.

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Chem. 1995, 67, 3819-3823.(5) Lindfors, T.; Bobacka, J.; Lewenstam, A.; Ivaska, A. Analyst 1996, 121,

1823-1827.(6) Lindfors, T.; Sjoberg, P.; Bobacka, J.; Lewenstam, A.; Ivaska, A. Anal. Chim.

Acta 1999, 385, 163-173.(7) Zachara, J. E.; Toczylowska, R.; Pokrop, R.; Zagorska, M.; Dybko, A.;

Wroblewski, W. Sens. Actuators, B 2004, 101, 207-212.(8) Toczylowska, R.; Pokrop, R.; Dybko, A.; Wroblewski, W. Anal. Chim. Acta

2005, 540, 167-172.(9) Grekovich, A. L.; Markuzina, N. N.; Mikhelson, K. N.; Bochenska, M.;

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Anal. Chem. 2008, 80, 321-327

10.1021/ac070866o CCC: $40.75 © 2008 American Chemical Society Analytical Chemistry, Vol. 80, No. 1, January 1, 2008 321Published on Web 12/07/2007

refs 12-20, especially as sensors with these membrane materialshave shown some improved analytical parameters compared toPVC (e.g., refs 16 and 17). A polyacrylate matrix also enablesgrafting of ionophore13,14,21 or ion-exchanger20 within the mem-brane phase. However, polyacrylate membranes usually have highresistivity (e.g., refs 16 and 19), which can cause noisy, unstablepotentiometric readings. It has been shown (e.g., ref 19) thatadding a plasticizer to the polyacrylate membrane formulationhelps to decrease resistivity, but obviously reduces the advantageof using self-plasticizing media. To date, all the proposed ASS-CP-ISEs seem to have required rigorous conditioning protocols(e.g., refs 17, 22-27) to achieve detection limits and selectivitiesthat are comparable with the best internal solution electrodes28,29

(regardless of the ion-selective membrane matrix used). Alterna-tive engineering of the transducer composition30 or even galvano-static polarization31 has been tried to overcome this, but simplifyingor eliminating such procedures would clearly be beneficial.

An alternative remedy for this problem is explored in the workreported herein, whereby a conducting polymer is combined withthe polyacrylate to form a composite membrane. In contrast tosingle piece electrodes4,6 (obtained by mixing of the conductingpolymer with ion-selective membrane components), this approachexplores for the first time chemical grafting of the conductingpolymer within the poly(n-butyl acrylate) matrix containing theusual additives (ionophore and ion-exchanger) during a singlephotopolymerization step. The conducting polymer explored inthis context was poly(3,4-ethylenedioxythiophene) end cappedwith methacrylate groups, used in the form of a suspensionstabilized by p-toluenesulfonate anions. Thus, the electricalresistivity of ion-selective membranes was decreased and ad-ditional benefits arise, since together with the conducting polymersuspension, p-toluenesulfonate countercations (sodium or potas-sium ions) are also introduced to the membrane, at the polym-

erization step. The sensors obtained are expected to be charac-terized by high stability of the potential readings, low detectionlimits, and high selectivities.

EXPERIMENTAL SECTIONApparatus. In the potentiometric experiments, a multichannel

data acquisition setup and software, Lawson Labs. Inc. (3217Phoenixville Pike, Malvern, PA 19355) was used. In otherelectrochemical measurements (electrochemical impedance spec-troscopy (EIS), chronopotentiometry), galvanostat-potentiostatCH-Instruments model 660A (Austin, TX), and a conventionalthree-electrode cell, with platinum sheet as a counter electrode,was used. The pump systems 700 Dosino and 711 Liquino(Metrohm, Herisau, Switzerland) were used to obtain sequentialdilutions of calibrating solution.

The double junction silver/silver chloride reference electrodewith 1 M lithium acetate in the outer sleeve (Moller Glasblaserei,Zurich, Switzerland) was used. The recorded potential values werecorrected for the liquid junction potential calculated according tothe Henderson approximation.

An inductively coupled plasma mass spectrometer ELAN 9000(Perkin-Elmer) (LA-ICPMS) equipped with the laser ablationsystem LSX-200+ (CETAC) was used.32 The applied laser energywas 3.2 mJ, repetition rate was 5 Hz, and spot size was 100 µm.The changes in distribution of elements (23Na, 39K, 44Ca, 32S) withinthe ion-selective membrane were followed. A comparative measureof their amounts in different membranes was achieved; the fullquantitative analysis of the membrane components was not theaim. Each LA-ICPMS analysis was performed in duplicate (for atleast two different points on the ISM surface).

Reagents. Calcium-selective ionophore N,N-dicyclohexyl-N′,N′dioctadecyl-3-oxapentanediamide (ETH 5234), potassium-selective ionophore valinomycin, and sodium tetrakis[3,5-bis-(trifluoromethyl)phenyl]borate (NaTFPB) were from Fluka AG(Buchs). 1,6-Hexanedioldiacrylate (HDDA), 2,2-dimethoxy-2-diphenylacetophenone (DMPP), and n-butyl acrylate were fromAldrich.

Poly(3,4-ethylenedioxythiophene) (PEDOT) tetramethacrylateend-capped solution (mean molar mass ∼6000 g/mol) in ni-tromethane was obtained from Aldrich (Germany). This solutionwas concentrated prior use by slow, mass change controlled,evaporation of nitromethane at room temperature. The concen-trated conducting polymer (CP) composition obtained was usedin the membrane preparation.

Doubly distilled and freshly deionized water (resistance 18.2MΩ cm, Milli-Qplus, Millipore) was used throughout this work.All salts used were of analytical grade and were obtained fromPOCh (Gliwice, Poland).

Electrodes. Glassy carbon (GC) electrodes of area 0.07 cm2

were used. The substrate electrodes were polished with Al2O3,0.3 µm, and rinsed well in water.

Ion-Selective Membrane. Calcium composite membranecocktails contained the following (by weight): 1.1% NaTFPB, 1.4%ETH 5234, 0.2% HDDA, 1.4% DMPP, 6.4 or 8.2% CP, and n-butylacrylate. Potassium composite membrane cocktails contained thefollowing (by weight): 1.3% NaTFPB, 1.7% valinomycin, 0.2%

(12) Heng, L. Y.; Hall, E. A. H. Anal. Chim. Acta 1996, 324, 47-56.(13) Heng, L. Y.; Hall, E. A. H. Anal. Chem. 2000, 72, 42-51.(14) Malinowska, E.; Gawart, Ł; Parzuchowski, P.; Rokicki, G.; Brzozka, Z. Anal.

Chim. Acta 2000, 421, 93-101.(15) Heng, L. Y.; Toth, K.; Hall, E. A. H. Talanta 2004, 63, 73-87.(16) Michalska, A. J.; Appaih - Kusi, C.; Heng, L. Y.; Walkiewicz, S.; Hall, E. A.

H. Anal. Chem. 2004, 76, 2031-2039.(17) Chumbimuni-Torres, K. Y.; Rubinova, N.; Radu, A.; Kubota, L. T.; Bakker,

E. Anal. Chem. 2006, 78, 1318-1322.(18) Grygolowicz-Pawlak, E.; Wygladacz, K.; Sek, S.; Bilewicz, R.; Brzozka Z.;

Malinowska, E. Sens. Actuators, B 2005, 111-112, 310-316.(19) Wygladacz, K.; Durnas, M.; Parzuchowski, P.; Brzozka Z.; Malinowska, E.

Sens. Actuators, B 2003, 95, 366-372.(20) Qin, Y.; Bakker, E. Anal. Chem. 2003, 75, 6002-6010.(21) Qin, Y.; Peper, S.; Radu, A.; Ceresa, A.; Bakker, E. Anal. Chem. 2003, 75,

3038-3045.(22) Michalska, A.; Dumanska, J.; Maksymiuk, K. Anal. Chem. 2003, 75, 4964-

4974.(23) Sutter, J.; Radu, A.; Peper, S.; Bakker, E.; Pretsch, E. Anal. Chim. Acta

2004, 523, 53-59.(24) Sutter, J.; Lindner, E.; Gyurcsanyi, R.; Pretsch, E. Anal. Bioanal. Chem.

2004, 380, 7-14.(25) Michalska, A.; Maksymiuk, K. J. Electroanal. Chem. 2005, 576, 339-352.(26) Michalska, A.; Konopka, A.; Maj-Zurawska, M. Anal. Chem. 2003, 75, 141-

144.(27) Konopka, A.; Sokalski, T.; Michalska, A.; Lewenstam, A.; Maj-Zurawska,

M. Anal. Chem. 2004, 76, 6410-6418.(28) Bakker, E.; Pretsch, E. Anal. Chem. 2002, 74, 420A-426A.(29) Sokalski, T.; Ceresa, A.; Zwickl, T.; Pretsch, E. J. Am. Chem. Soc. 1997,

119, 11347-11348.(30) Michalska, A.; Skompska, M.; Mieczkowski, J.; Zagorska, M.; Maksymiuk,

K. Electroanalysis 2006, 18, 763-771.(31) Michalska, A. Electroanalysis 2005, 17, 400-407.

(32) Michalska, A.; Wojciechowski, M.; Wagner, B.; Bulska, E.; Maksymiuk, K.Anal. Chem. 2006, 78, 5584-5589.

322 Analytical Chemistry, Vol. 80, No. 1, January 1, 2008

HDDA cross-linker, 1.4% DMPP, 6.4% CP, and n-butyl acrylate.For both membranes, the ratio of numbers of acrylate groupsoriginating from CP and n-butyl acrylate is below 1%.

Membranes for electrochemical experiments contained 0.2%HDDA, 1.4% DMPP, and n-butyl acrylate (blank polyacrylatemembrane); 0.2% HDDA, 1.4% DMPP, CP, n-butyl acrylate or 1.1%NaTFPB, 1.4% ETH 5234, 0.2% HDDA, 1.4% DMPP, and n-butylacrylate (CP-free membrane used to obtain coated disk elec-trodes).

A 10-µL sample of the composite membrane cocktail wasapplied on the top of a glassy carbon electrode placed in up-sidedown position. Photopolymerization was carried out using a UVlamp (360 nm) for 5 min under argon.

The same membrane cocktails were used to prepare samplesfor (LA-ICPMS) analysis; 5 µL of the membrane cocktail solutionwas pipetted on an acetate transparency and photopolymerizedas above.

Conditioning of the Composite Membranes. Before EIS andchronopotentiometric measurements, the membranes were con-ditioned for 20 h in 0.1 M KCl (potassium sensors) or 0.1 M CaCl2

(calcium sensors).For potentiometric measurements, calcium-selective sensors

were conditioned for 3 days in 10-3 M CaCl2 and then kept in-between measurements in EDTA buffer of constant and low Ca2+

ion activity (0.05 M Na2EDTA, 3.5 × 10-2 M CaCl2, 0.146 M NaCl,adjusted with NaOH to pH 10.3, calculated aCa2+ ) 2.3 × 10-9

M, calculated aNa+ ) 0.175 M), alternatively the sensors wereconditioned in the above buffer only, without pretreatment.Potassium sensors were conditioned and were kept in-betweenmeasurements in 1 or 10-3 M KCl.

Samples for LA-ICPMS analysis were conditioned in 10-3 MCaCl2 (calcium sensors) or 1 or 10-3 M KCl (potassium sensors)for 20 h.

RESULTS AND DISCUSSIONResistance and Capacitance of Composite Membranes.

Poly(n-butyl acrylate) membranes are typically characterized witha relatively high resistance, determined from EIS data correspond-ing to a phase angle close to zero. In this instance, for the blankpoly(n-butyl acrylate) membrane (free from conducting polymer,ion-exchanger or ionophore addition), the frequency range cor-responding to a purely resistive response was 0.1 Hz ÷ 10 Hz,and the recorded resistance was close to 1010 Ω. However,introducing CP into the membrane led to a substantial decreasein the resistance. For example, adding ∼2% CP to the membraneresults in a resistance close to 107 Ω (determined in the frequencyrange from 0.1 to 100 Hz), whereas further increase in CP contentin the membrane to ∼6% reduces the resistance to a limiting valueof ∼106 Ω. This value does not change further for higher amountsof CP in the poly(n-butyl acrylate) phase. These resistance valuesare also largely comparable with those reported previously forclassical plasticized PVC ion-selective membranes (e.g., ref 33).

The chronopotentiometric curves obtained for compositecalcium-selective membranes containing poly(n-butyl acrylate),ionophore, ion-exchanger, and 6.4 or 8.2% w/w CP are presentedin Figure 1. The resistance estimated in this experiment, from

potential jump upon current direction change, is slightly higher(compared to EIS data) and is close to 107 and 106.9 Ω for themembrane containing 6.4 and 8.2% w/w CP, respectively. Foranodic and cathodic polarization, potentials recorded were (ir-respective of amount within the studied range) only slightlyaffected by 10-8 A galvanostatic polarization for both compositionss

potential drift reaching a few millivolts for 60 s. Therefore,according to previous reports,34 in the presence of CP within themembrane, significantly higher potential stability than in the caseof coated disk arrangement is suggested for both cases. Thecapacitance (determined from the linear potential versus timedependence34) is then estimated to be above 10-3 F/cm2, muchhigher than a typical double layer capacitance of GC electrode(∼10-6 F/cm2). This confirms the transducing properties of theCP and points to the absence of a blocked interface at themembrane/electrode substrate. The stabilizing effect on electrodepotential can be explained by the amount of redox-active polymerpresent in the membrane, resulting from photopolymerization of10 µL of cocktail. It is in the range of 10-3 mmol, i.e., comparable

(33) Pawlowski, P.; Michalska, A.; Maksymiuk, K. Electroanalysis 2006, 18,1339-1346. (34) Bobacka, J. Anal. Chem. 1999, 71, 4932-4937.

Figure 1. Chronopotentiograms (applied current (10-8 A) for (A)composite membrane calcium-selective sensors containing (1) 6.4and (2) 8.2% conducting polymer and (B) for calcium-selectivemembrane, free from CP additive, recorded in 0.1 M CaCl2 followingconditioning in 10-3 M CaCl2 (note the potential scale difference forplots A and B).

Analytical Chemistry, Vol. 80, No. 1, January 1, 2008 323

with a typical amount of electropolymerized PEDOT in a film, usedfor solid-state contact sensors, where the potential stabilizing effectis well documented.34

In a chronopotentiometric experiment conducted for compositepotassium-selective membranes containing 6.4% CP (results notshown) in 0.1 M KCl, the recorded potentials were also practicallyunaffected by the applied current (single mV drift observed),regardless of whether the sensor was conditioned in 1 or 10-3 MKCl. The resistance of the composite potassium-selective mem-branes, estimated from this experiment was close to 107.4 Ω.

In contrast, in a parallel experiment conducted for the ion-selective membrane, free from CP additive, coated directly on aGC support (coated disk arrangement), a pronounced potentialdrift was observed, Figure 1. Using the procedure proposed byBobacka34 for the calcium-selective sensor, the estimated capaci-tance of this arrangement is close to 10-6 F/cm2, correspondingto the typical value for a noncoated GC electrode.

Calcium-Selective Composite Membranes. Calcium-selec-tive composite membranes were used as model systems to studythe effect of the CP content. For sensors containing 6.4% CP inthe membrane, linear Nernstian responses were obtained withinthe CaCl2 activity range from 10-9 to 0.1 M; the reproducibility ofpotentials obtained within a one-week period (n ) 10) is presentedin Figure 2. These sensors were characterized by a wide linearresponse range and high stability of the measured potentialvalues: the slope of the line presented in Figure 2A, within theactivity range from 10-9 to 0.1 M, is 24.9 ( 0.5 mV/decade (R2 )0.997). Although the sensor was repeatedly tested both in high-and low-activity solutions, the potentials recorded for activities ofg10-7 M were reproducible within a few millivolts. With loweringactivity, within the range from 10-7 to 10-9 M, increasingfluctuations in potential values were observed. However, even inthis activity range, the variability obtained within 1 day did notexceed a few millivolts. It is particularly noteworthy that thesehighly reproducible, low detection limit responses of the calcium-selective electrodes were obtained even without applying time-consuming, complicated pretreatment procedures.17,27,35

Also noteworthy is that, for Ca sensors prepared without CPin the membrane phase (i.e., coated disk arrangement), precon-ditioned in the same way, super-Nerstian responses were obtainedjust after pretreatment. This supports the thesis of the beneficialrole of conducting polymer and its components when included inthe membrane phase.

In line with the considerations above, when the activity rangewas restricted to 10-3-10-9 M CaCl2, a linear responses wereobtained with slope 31.1 ( 1.0 mV/decade, R2 ) 0.992 with SDreaching 7 mV for an activity range from 10-7 to 10-9 M and closeto 3 mV for higher activities for data obtained over two weeks,Figure 2B.

Increasing the CP content in the composite membrane to 8.2%w/w had little effect on both slope and reproducibility of thepotential values recorded for calcium ion activities higher than10-6 M. Within the activity range from 10-6 to 0.1 M, slopes closeto Nernstian were obtained (for this range, the slope of the linepresented in Figure 2C is 29.9 ( 0.8 mV/decade, R2 ) 0.997).For activities lower than 10-6 M, a greater variation in the recorded

potentials was observed compared with the 6.4% CP preparation.In all cases, the overall potential change in the range from 10-6

(35) Konopka, A.; Sokalski, T.; Lewenstam, A.; Maj-Zurawska, M. Electroanalysis2006, 18, 2232-2242.

Figure 2. Reproducibility of potentiometric responses of calcium-selective composite membrane electrodes containing (A) 6.4% CPin the membrane within one week, error bars SD, n ) 10, (B) 6.4%CP in the membrane for activities ranging from 10-3 to 10-9 M withintwo weeks, error bars SD, n ) 9, and (C) 8.2% CP in the membranewithin one week, error bars SD, n ) 10. Before measurementselectrodes were conditioned in 10-3 M CaCl2 for 3 days, in-betweenmeasurements, electrodes were kept in EDTA buffer of constant andlow activity of Ca2 ions.

324 Analytical Chemistry, Vol. 80, No. 1, January 1, 2008

to 10-9 M was greater than Nernstian; however, the magnitudeof the super-Nernstian potential change and its onset on theactivity scale was variable. Furthermore, no trend could beelucidated from the observed changes, but the variability recordedfor calcium activities lower than 10-6 M is reflected in the higherSD presented in Figure 2C. The variability of potentials recordedwithin the range from 10-7 to 10-9 M was close to 30 mV; i.e.,close to a decade equivalent change in concentration.

It is clear, therefore, that comparison of potentiometricresponses from composite membranes containing 6.4 and 8.2%w/w CP, pretreated in the same way and tested in parallel, leadsto the conclusion that CP present in the composition significantlyaffects the properties of the resulting phase. To the authors’ bestknowledge, the addition of a CP component to the membrane hasnot been previously characterized by improved detection limits.In this instance, we attribute the effect to the ion-exchangeproperties of CP doping anions to the presence of sodium/potassium cations in the CP composition. The dispersion of poly-(3,4-ethylenedioxythiophene) methacrylate end capped in ni-tromethane is stabilized by the presence of p-toluenesulfonateanions. The ICPMS analysis of the conducting polymer suspensionhas revealed the presence of a high amount of sodium andpotassium (p-toluenesulfonate counterions) and the presence ofsulfur. The estimated sodium content is close to 1 M, andpotassium to 0.15 M. The presence of sulfur, sodium, andpotassium (and the presence of calcium in the conditionedcomposite membrane) was also confirmed in an LA-ICPMSexperiment. Therefore, both sodium and potassium are introducedto the poly(acrylate) composite ion-selective membrane togetherwith the CP suspension. This implies that for the first time ion-selective membranes with a high loading of interferent ions canbe obtained by polymerization, without necessitating pretreatmentprocedures.

The p-toluenesulfonate ions affect the free calcium ion activitywithin the membrane and in this way help to prevent unwantedleakage of Ca2+ ions from the membrane in a low-activity range.On the other hand, sodium and potassium ions introduced to themembrane together with CP composition result in high interferingions loading of the composite membrane, which is also an essentialprerequisite to obtain improved sensor responses (e.g., ref 1).Therefore, the CP composition applied in this work not only hasresulted in enhancement of charge transfer between the mem-brane and the support but also has significantly contributed toimprovement of analytical parameters of the sensor.

As seen in Figure 2, changing the CP content in the compositemembrane leads to change in the sensor characteristics frombroad range linear Nernstian response to super-Nernstian in alimited low-activity range. Interestingly, this change affects thestability of the potential values recorded in these low-activitysolutions, without influence on the reproducibility of potentialsvalues recorded for high activities of Ca2+ solutions. At highactivities, leakage of primary ions from the membrane or depletionof the boundary region due to incorporation of ions into themembrane is not significant compared with the high activity ofprimary ions in solution. In this range, the stability of thepotentiometric response is determined mainly by charge-transferprocesses at the membrane/substrate interface, which can bestudied by means of chronopotentiometry.34 As shown earlier,

currents in the range of nanoamperes, to tens of hundreds ofnanoamperes, do not affect ion fluxes at the ion-selective mem-brane/solution interface for solution activities higher than 10-3

M.22,33,34 On the other hand, at activities lower than 10-5-10-6 M,the ion fluxes at the membrane/solution interface give rise tosignificant potential changes, especially when super-Nernstianslopes are observed. The magnitude and direction of primary ionflux can be affected by many factors related to the membraneitself, internal solution if used, sample, etc.,36 and higher fluctua-tions in potential are observed even for well-defined chargetransfer at the back side.37 Thus, composite membranes containinga higher amount of CP and higher amount of p-toluenesulfonate,and sodium/potassium ions, with a tendency to incorporatecalcium ions into the membrane phase (as proven by a super-Nerstian slope at low activities37), are characterized with apparentlylower stability.

Table 1 presents selectivity coefficients determined for com-posite calcium membrane sensors. Notwithstanding the ion-exchange properties introduced with the CP preparation, similarvalues were obtained regardless of the amount of CP introducedto the membrane. Log KCa,Mg

pot and log KCa,Bapot were comparable

with values reported earlier for tailored internal solution electrodeswith PVC membranes containing the same ionophore.37 However,composite membrane selectivity toward Na+, K+, and Li+ ions waslowered compared to the tailored internal solution electrode withPVC membranes, but still, selectivity coefficients were comparablewith those obtained for electrodes with this type of membraneand “conventional” (10-2 M CaCl2) internal solution.37 The valuesof the selectivity coefficients for these monovalent cations are notunexpected taking into account that high amounts of sodium andpotassium are introduced to the membrane with the CP composi-tion. Nevertheless, contrary to the “conventional” calcium-selectiveelectrodes,37 the sensors with composite membranes gave slopesrecorded in the presence of interfering ions that were close toNernstian, with the exception of Mg2+ interference.

Introduction of a higher amount of CP to the ion-selectivemembrane can sometimes result in increased redox sensitivityof the membrane.4 Therefore, the effect of a redox couple(Fe(CN)6

3-/4-) present in the sample was studied with oxidized

(36) Ceresa, A.; Sokalski, T.; Pretsch, E. J. Electroanal. Chem. 2001, 501, 70-76.

(37) Sokalski. T.; Ceresa, A.; Fibioli, M.; Zwickl, T.; Bakker, E.; Pretsch, E. Anal.Chem. 1999, 71, 1210-1214.

Table 1. Mean Potentiometric Selectivity Coefficients,log KCa,J

pot ( SD and Slope (mV/decade) Obtained withinthe Activity Range from 10-2 to 10-4 mol/dm3,Separate Solution Method, for Ca-Selective Electrodeswith Composite Membranes Following Conditioning inEDTA Buffer

J log KCa,Jpot ( SD

6.4 % PEDOT 8.2 % PEDOT lit. data37

Li+ -3.2 ( 0.1 (51.2) -3.2 ( 0.3 (53.2) -4.9 ( 0.6K+ -3.0 ( 0.1 (52.6) -3.0 ( 0.2 (54.1) -5.6 ( 0.8Na+ -3.1 ( 0.2 (53.7) -3.0 ( 0.3 (55.3) -6.4 ( 0.3Mg2+ -7.6 ( 0.2 (19.1) -7.7 ( 0.1 (24.5) -8.6 ( 0.3Ba2+ -2.9 ( 0.2 (30.6) -2.5 ( 0.3 (33.2) -3.1 ( 0.1

Analytical Chemistry, Vol. 80, No. 1, January 1, 2008 325

to reduced form concentration ratios varying from 1/10 to 10/1,spiked with KCl to achieve constant K+ concentration equal to 1M. It was found for the composite membrane, containing either6.4 or 8.2% w/w CP, that the recorded potentials were practicallyindependent of logarithm of [Fe(CN)6

3-/Fe(CN)64-] (slope of the

dependence <3 mV/decade). This effect proves that the com-posite membranes are quite resistive toward solution redoxpotential changes. Low redox sensitivity of the tested membranecan result from a low amount of PEDOT (compared to n-butylacrylate units) and can be also related to accumulation of the CPat the membrane/supporting electrode interface (e.g., sedimenta-tion just before polymerization is started), which in fact can bealso beneficial for ion-to-electron transduction.

Using the procedure proposed recently32 and following changesin 44Ca signal intensities, the diffusion coefficient for Ca2+ ions inthe composite membrane containing 8.2% w/w CP was estimatedto be close to (4 ( 1.5) × 10-9 cm2/s, following 20-h conditioningin 10-3 M CaCl2. This value is ∼1 order of magnitude lower thanreported for PVC-based membranes,32 but still ∼100 times highercompared to typical poly(acrylate)-based ones.15 A lower value ofdiffusion coefficient, compared to that found for PVC-basedmembranes, can be advantageous, as it results in lower trans-membrane fluxes of ions between the sample solution and themembrane. Thus, super-Nernstian effects should be less exposedand the potentiometric characteristic for low-activity range morestable than for the PVC-based counterparts.

Potassium-Selective Composite Membranes. Potassium-selective composite membranes were used as a model system tostudy the effect of primary ion concentration in the conditioningsolution on the sensor’s response (Figure 3). The electrodesconditioned in 1 M KCl were characterized with a linear depen-dence of potential on the logarithm of potassium ion activity, foractivities higher than 10-5 M (slope of dependence presented inFigure 3 is equal to 54.6 ( 4.2 mV/decade, R2 ) 0.989); for loweractivity, potentials recorded were independent of KCl activity.Changing the conditioning solution to 10-3 M KCl gave sensors

with linear responses within the range from 10-8 to 0.1 M KCl(the slope of the dependence presented in Figure 3 is equal to57.1 ( 1.5 mV/decade, R2 ) 0.996). The reproducibility of thepotential recorded was in the range of a few millivolts for KClactivities higher than 10-6 M and ∼30 mV for lower activities.

To correlate the membrane composition with the observedpronounced difference in linear range of the tested potassiumsensors, LA-ICPMS experiments were performed. The resultingprofiles of elements within the membranes conditioned in 10-3

or 1 M KCl are shown in Figure 4; as can be seen, regardless ofthe applied conditioning solution, maximum potassium signalintensities were close to the membrane interface that had beenin contact with the solution and decreased moving deeper intothe membrane. The maximum sodium signal intensities, on theother hand, regardless of conditioning solution applied, werelocated in a deeper region of the membrane, closer to the middleof the membrane, Figure 4. This result is not unexpected takinginto account that sodium ions were introduced into the membranecocktail with the CP prior to the polymerization step, not throughconditioning. It should be stressed that the maximum for thesulfur signal intensities (results not shown) were also observedat a distance from the membrane supporting the idea of possibleaccumulation of CP close to the GC electrode surface. Interest-ingly, however, there was a pronounced difference in sodiumsignal intensity close to the membrane surface depending onapplied conditioning solution, Figure 4. In the case of lower

Figure 3. Reproducibility of potentiometric responses of potassium-selective composite membrane electrodes containing 6.4% CP in themembrane within three weeks, error bars SD, n ) 15 conditionedbefore (for 20 h) and in-between measurements in (b) 10-3 and (O)1 M KCl. For easy comparison, experimental lines were shifted togive the same value at 10-3 M KCl.

Figure 4. Intensity of the measured signal as a function of laserablation penetration depth obtained for tested composite potassium-selective membranes conditioned for 20 h in (A) 10-3 or (B) 1 M KCl.Red lines correspond to potassium signal and black lines to sodiumsignal.

326 Analytical Chemistry, Vol. 80, No. 1, January 1, 2008

concentration of KCl solution (10-3 M), sodium signals near thesurface were relatively high, reaching ∼60% of its maximumintensity recorded for this membrane. On the other hand, for themembrane that was in contact with higher KCl concentration (1M KCl), sodium signal intensities at ∼10% of membrane thicknessfrom the surface were significantly smaller compared to themaximum value.

These data clearly confirm that the detection limit of thecomposite, potassium-selective membrane not only is related toprimary ion content in the membrane or at the membrane surfacethat is in contact with solution but is affected by interfering ionspresent close to the membrane surface. To achieve a lowerdetection limit, at least comparable amounts of primary andinterfering ions are required close to the membrane/solutioninterface.

CONCLUSIONSPotentiometric ion sensors with polyacrylate-based membranes

containing chemically grafted conducting polymer, poly(3,4-ethylenedioxythiophene). represent a kind of ion-selective elec-trode that is potentially useful for practical applications. Advan-tages of such sensors result from a simple one-step preparationmethod (photopolymerization), stable and reproducible responsesin a wide concentration range, not requiring pretreatment orconditioning steps. Potential stability results from the presenceof the conducting polymer, assuring ion-to-electron signal trans-

duction, while conditioning procedure is simplified because anadequate amount of interferent ion is introduced with the conduct-ing polymer during photopolymerization.

The proposed approach resulted in ion-selective self-plasticizedmembranes with the following benefits: (i) decreased resistance,(ii) facile ion-to-electron transduction between the membrane andelectrode support, stabilizing the potentiometric responses, (iii)ability to ensure a constant and low activity of analyte cations dueto cation-exchanging properties of doping p-toluenesulfonateanions, and (iv) presence of high activity of interfering cations(sodium and potassium) in the membrane. The results can besupported by the LA-ICPMS examination of the distribution of Kand Na in the subsurface domain of the membrane and highlightthe role of primary and interferent ion in influencing the limits ofdetection.

ACKNOWLEDGMENTFinancial support of the research project 3 T09A 017 27 in

the years 2004-2007, from KBN, Poland, is gratefully acknowl-edged.

Received for review April 27, 2007. Accepted August 21,2007.

AC070866O

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