Journal of Electroanalytical Chemistry 633 (2009) 246–252

Contents lists available at ScienceDirect

Journal of Electroanalytical Chemistry

journal homepage: www.elsevier .com/locate / je lechem

Poly(3,4-ethylenedioxythiophene) (PEDOT) doped with carbon nanotubesas ion-to-electron transducer in polymer membrane-based potassiumion-selective electrodes

Zekra Mousavi, Johan Bobacka, Andrzej Lewenstam, Ari Ivaska *

Åbo Akademi University, Process Chemistry Centre, c/o Laboratory of Analytical Chemistry, Biskopsgatan 8, FI-20500 Åbo-Turku, Finland

a r t i c l e i n f o

Article history:Received 7 April 2009Received in revised form 8 June 2009Accepted 11 June 2009Available online 13 June 2009

Keywords:All-solid-state ion-selective electrodePotassium ion sensorConducting polymersPEDOTMulti-walled carbon nanotubes

0022-0728/$ - see front matter � 2009 Published bydoi:10.1016/j.jelechem.2009.06.005

* Corresponding author. Tel.: +358 2 215 4420; faxE-mail address: ari.ivaska@abo.fi (A. Ivaska).

a b s t r a c t

Negatively charged multi-walled carbon nanotubes (MWCNTs) were used as dopants in the electrochem-ical synthesis of the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT). The resultingelectroactive film, PEDOT(CNT), was used as ion-to-electron transducer (solid contact) in potassiumion-selective electrodes (K+-ISEs) based on plasticized PVC membrane containing valinomycin as neutralionophore. Potentiometric measurements were carried out to study the analytical performance ofsolid-contact K+-ISEs, the influence of dissolved O2 and CO2 on the potential of the electrodes, and theformation of the interfacial aqueous film. The prepared electrodes were also characterized by cyclic vol-tammetry (CV), electrochemical impedance spectroscopy (EIS), and chronopotentiometry. The experi-mental results showed that PEDOT(CNT) has the capability to function as solid contact in fabrication ofK+-ISEs. These electrodes, based on PEDOT(CNT) as ion-to-electron transducer, showed high sensitivityand selectivity to K+ ion which can be related to the plasticized PVC-based ion-selective membrane con-taining valinomycin. The stability of the electrode potential, however, was found to depend on the con-ducting substrate used for deposition of the PEDOT(CNT) film. Results from the CV and EIS revealed thatthe PEDOT(CNT) contact exhibits high redox capacitance that is favorable for a solid contact.

� 2009 Published by Elsevier B.V.

1. Introduction

Potentiometric ion sensors, or ion-selective electrodes (ISEs),are the most frequently used chemical sensors in analytical appli-cations in medical, environmental, and industrial analysis [1–4]. Asan analytical device, ISE offers many advantages such as selectivity,fast response, simple analytical procedure, and relatively low cost.The necessity of robust, maintenance-free, and reliable ISEs led tothe invention of the so called coated-wire electrode (CWE) which isa polymeric membrane-based all-solid-state ISE [5,6]. However,the configuration of CWE involves some drawbacks such as driftin the electrode potential due to the high charge-transfer resis-tance and low double-layer capacitance at the interface betweenthe electrically conducting substrate and the ionically conductingion-selective membrane (ISM) [7,8]. To solve this problem, solid-contact ISEs were fabricated by using a layer with mixed electronicand ionic conductivity to act as ion-to-electron transducer be-tween the ISM and the solid substrate [9]. The excellent and uniqueproperties of the electrically conducting polymers (ECPs) makethem attractive materials that can function as ion-to-electrontransducers in solid-contact ISEs [10–12]. Poly(3,4-ethylenedioxy-

Elsevier B.V.

: +358 2 215 4479.

thiophene) (PEDOT), is one of the most interesting conductingpolymers that has been used for this purpose [8,13,14].

On the other hand, carbon nanotubes (CNTs) with remarkablestructural, electrical, mechanical, and thermal properties have gen-erated a considerable interest since being discovered by Iijima in1991 [15]. It has been shown that incorporating CNTs into a poly-mer matrix enhances the mechanical and electrical properties ofthe original polymer [16–18]. Another important application ofCNTs was incorporating them in electrochemical transducers inthe fabrication of biosensing devices [19,20]. The ability of CNTsto function as gas, pressure, temperature, and mass sensors hasbeen demonstrated [21]. Carbon nanotubes have also been usedas the ion-to-electron transducing layer in ISEs [22,23]. The proce-dure used to prepare these electrodes, however, was tedious andtime consuming. Also other carbon-based materials with high sur-face area have successfully been used for the same purpose [24,25].

The aim of this study was to combine the properties of conduct-ing polymers and carbon nanotubes to prepare a composite thatcan be used as ion-to-electron transducer in poly (vinyl chloride)(PVC)-based ion-selective electrodes. The conducting polymerpoly(3,4-ethylenedioxythiophene) (PEDOT) doped with negativelycharged multi-walled carbon nanotubes (MWCNT) was electro-chemically synthesized to prepare the composite material PED-OT(CNT) that was coated with a PVC-based K+-selective

Z. Mousavi et al. / Journal of Electroanalytical Chemistry 633 (2009) 246–252 247

membrane containing valinomycin as neutral carrier. The electro-chemical properties of PEDOT(CNT) film and the analytical perfor-mance of the K+-ISEs based on this film as ion-to-electrontransducer were studied using different techniques.

2. Experimental

2.1. Chemicals

The monomer 3,4-ethylenedioxythiophene (EDOT; >97%) wasobtained from Bayer AG. Multi-walled carbon nanotubes(MWCNTs) with 10–30 nm diameter and purity >90%, were pur-chased from Sun Nanotech Co. Ltd. (Jiangxi, China). The carbonnanotubes (CNTs) were first refluxed in 3 M nitric acid for 48 h inorder to purify them and to create the negatively charged groups(mainly carboxylic groups) on them. The number of defects andfunctional groups, which affect the electrical and structural proper-ties of carbon nanotubes, depends on the type and concentration ofthe acid, and refluxing time used in the pretreatment procedure. Inaddition, the treatment of MWCNTs produces functional groups ontheir outer sidewalls while preserving the structure of the innersidewalls, whereas similar treatment procedure may damage thestructure of single-walled carbon nanotubes (SWCNTs) and changeits electrical properties. The acid treated CNTs were filteredthrough a 0.45 lm pore size nylon membrane and washed repeat-edly with deionized water until pH of the solution was neutral.Finally, the purified CNTs were dried in vacuum oven at 60 �C for8 h. Potassium ionophore I (valinomycin), potassium tetrakis(4-chlorophenyl)borate (KTpClPB), bis(2-ethylhexyl)sebacate (DOS),poly(vinyl chloride) (PVC) of high molecular weight, and tetrahy-drofuran (THF, >99.5%) were selectophore reagents obtained fromFluka. All other chemicals were analytical-reagent grade. Distilledand deionized water (ELGA, 18.2 MX cm) was used to preparethe aqueous solutions.

2.2. Electrochemical synthesis of PEDOT and fabrication of all-solid-state K+-ISEs

PEDOT was deposited on the working electrode by galvanostaticelectrochemical polymerization using an Autolab General PurposeElectrochemical System (AUT30.FRA2-Autolab, Eco Chemie, B.V.,The Netherlands) connected to a conventional one-compartmenttwo-electrode electrochemical cell. The working electrodes usedin this study were glassy carbon (GC) disk electrodes with 3 mmdiameter, screen-printed carbon electrodes (SPCEs) with 4 mmdiameter, and SPCEs modified with carboxyl functionalized mul-ti-walled carbon nanotubes (SPCE-MWCNT). The SPCE and SPCE-MWCNT electrodes were purchased from DROPSENS, Spain. TheGC working electrodes were polished with 0.3 lm alumina slurry,rinsed with water, and cleaned ultrasonically before they wereused. A GC rod was used as the auxiliary electrode. No referenceelectrode was used during the galvanostatic electropolymerizationprocedure. The polymerization solution, containing 0.01 M EDOTand 0.2 wt% of negatively charged CNTs dissolved in distilledwater, was sonicated for ca. 1 h in order to get a stable suspension.The solution was then purged with N2 gas for at least 15 min andN2 inlet was kept above the solution during the polymerizationprocedure. To produce a polymerization charge of about 10 mC, aconstant current of 0.014 mA was applied for 714 s. The resultingPEDOT doped with carbon nanotubes will hereafter be called PED-OT(CNT). As the only anions present in the polymerizationsolution, the negatively charged CNTs will act as charge compensa-tors to the positively charged backbone of PEDOT resulting fromthe galvanostatic polymerization of EDOT. However, the presenceof CNTs in PEDOT(CNT) film was still verified by scanning electronmicroscopy (SEM) and RAMAN measurements [26] (results not

shown). For comparison purpose, PEDOT doped with chloride, i.e.PEDOT(Cl), was also prepared from a deaerated aqueous solutioncontaining 0.01 M EDOT and 0.1 M potassium chloride and usingthe same conditions as used in preparation of PEDOT(CNT). Afterpolymerization, the electrodes were rinsed with distilled waterand conditioned in 0.1 M KCl solution for at least 1 day before fur-ther use.

To prepare all-solid-state potassium ion-selective electrodes(K+-ISEs), 70 lL potassium ion-selective membrane (K+-ISM) cock-tail was applied on GC/PEDOT(CNT) or GC/PEDOT(Cl) electrodes.The resulting electrodes are denoted as GC/PEDOT(CNT)/K+-ISMand GC/PEDOT(Cl)/K+-ISM, respectively. When using SPCE orSPCE-MWCNT substrates, 40 lL K+-ISM cocktail was used. Theion-selective membrane was applied on bare GC and SPCE-MWCNTsubstrates to prepare the coated disk electrodes denoted asGC/K+-ISM and SPCE-MWCNT/K+-ISM, respectively. Compositionof the ISM in% (w/w) was: 1.19 valinomycin, 0.31 KTpClPB, 66.4DOS, and 32.1 PVC. These components were dissolved in THF(dry fraction of the membrane cocktail is ca. 18%). The resultingK+-ISEs were conditioned in 0.01 M KCl solution before furthermeasurements and also between the measurements.

2.3. Potentiometric measurements

A homemade multi-channel mV-meter connected to a PC fordata acquisition was used to carry out the potentiometricmeasurements. The reference electrode was Ag/AgCl/3 M KCl, orAg/AgCl/3 M KCl with a salt bridge containing 1 M lithium acetate(LiAc). The potentiometric sensitivity of the K+-ISEs was studied byrecording calibration curves for the electrodes in KCl solutionswith different concentrations. The extended Deby-Hückel equation[27] was used to convert the concentrations to activities. Correc-tion for the liquid-junction potential was applied using the Hen-derson equation [28]. The selectivity coefficients of the electrodesto potassium ion in the presence of several cations were deter-mined by using the separate solution method (SSM) [29]. Theselectivity measurement was carried out in 0.01 M solutions ofthe studied cations (chloride salt).

Sensitivity of the GC/PEDOT(CNT) electrodes to O2 and CO2

gases was studied in 0.1 M KCl solution before and after depositingthe K+-ISM. The solution was purged with O2 or CO2 for 1 h whilethe potential was measured. The solution was purged with N2

gas for 1 h before and after saturating the solution with O2 orCO2 gas.

Test for formation of an aqueous-layer between the ISM and theunderneath substrate was carried out on the GC/PEDOT(CNT)/K+-ISM, and GC/K+-ISM electrodes in which 70 lL K+-ISM cocktailwas applied on the GC/PEDOT(CNT) and GC substrates, respec-tively. The potential of the electrodes was first measured in0.1 M KCl solution for 90 min, then in 0.1 M NaCl solution for120 min, and finally in 0.1 M KCl for 240 min.

2.4. Cyclic voltammetry (CV) and electrochemical impedancespectroscopy (EIS)

The cyclic voltammetric measurements were carried out indeaerated 0.1 M KCl solution. Three cycles were recorded at bareGC, GC/PEDOT(Cl), and GC/PEDOT(CNT) electrodes between +0.5and �0.5 V with the scan rate of 0.1 V/s. The Autolab General Pur-pose Electrochemical System connected to a conventional one-compartment three-electrode electrochemical cell was used inrecording the cyclic voltammograms. The electrodes mentionedabove were used as the working electrode.

Electrochemical impedance spectroscopy (EIS) measurementswere performed in deaerated 0.1 M KCl solution by using an Auto-lab Frequency Response Analyzer System (AUT30.FRA2-Autolab,

248 Z. Mousavi et al. / Journal of Electroanalytical Chemistry 633 (2009) 246–252

Eco Chemie, B.V.) connected to a conventional one-compartmentthree-electrode electrochemical cell. The impedance spectra wererecorded at the open-circuit potential of GC/PEDOT(CNT) andGC/PEDOT(Cl) electrodes before and after applying the ISM onthem. In these measurements the sinusoidal excitation signal withexcitation amplitude of 10 mV was used in the frequency range100 kHz–10 mHz. A GC rod was used as the auxiliary electrodeand Ag/AgCl/3 M KCl as the reference electrode in both CV and EIS.

2.5. Chronopotentiometry

Chronopotentiometric measurements were carried out onGC/PEDOT(CNT)/K+-ISM, GC/PEDOT(Cl)/K+-ISM, and GC/K+-ISMelectrodes in 0.1 M KCl solution by applying a constant current of+1 nA for 60 s followed by �1 nA current for another 60 s whilethe potential of the electrodes was measured as a function of time[8]. The measurements were performed using the same Autolabinstrument, reference and auxiliary electrodes as used in cyclicvoltammetric measurement.

All the measurements were performed at room temperature(23 ± 2 �C).

3. Results and discussion

3.1. Sensitivity and selectivity of all-solid-state K+-ISEs

The GC/PEDOT(CNT)/K+-ISM, GC/PEDOT(Cl)/K+-ISM, and GC/K+-ISMelectrodes were prepared as described in Section 2.2. Afterconditioning in 0.01 M KCl for 2 days, the electrodes were cali-brated in 10�1–10�7 M and then back to 10�1 M KCl solutions.The reference electrode was Ag/AgCl/3 M KCl with a salt bridgecontaining 1 M lithium acetate. As can be seen in Fig. 1a, the differ-ent types of K+-ISEs give linear response in the activity range10�1–10�6 M of K+ ion. The GC/PEDOT(CNT)/K+-ISM electrodeshows the least hysteresis among the studied electrodes. Theslopes of the linear part of the calibration curves are 57.7, 57.5,and 56.8 mV/decade for GC/PEDOT(CNT)/K+-ISM, GC/PEDOT(Cl)/K+-ISM, and GC/K+-ISM, respectively. After being conditioned in0.01 M KCl solution for three weeks, the same electrodes werecalibrated again in 10�1–10�6 M and then back to 10�1 M KCl solu-tions. Results from this calibration experiment shown in Fig. 1bindicate that the potential values of the GC/PEDOT(CNT)/K+-ISMelectrode were shifted to more positive values by ca. 52 mV. Theelectrode, however, still gives linear response in the activity range10�1–10�6 M with an almost Nernstian slope; 56.5 mV/decade.Compared to the calibration curve in Fig. 1a, a slight deteriorationcan be seen in the calibration plot of GC/PEDOT(Cl)/K+-ISM in the

-7 -6 -5 -4 -3 -2 -1 0-250

-200

-150

-100

-50

0

50

100

150

200

E, m

V

log a K +

a

Fig. 1. Calibration plots for GC/PEDOT(CNT)/K+-ISM (j), GC/PEDOT(Cl)/K+-ISM (d), and2 days conditioning in 0.01 M KCl, and (b) in 10�1–10�6 then back to 10�1 M KCl solutio

lowest studied concentration of potassium ion (10�6 M). TheGC/K+-ISM electrode shows increased hysteresis and significantpotential drift that are typical for a coated disk electrode (CDE).

PEDOT(CNT) was also electrochemically deposited on the SPCEand SPCE-MWCNT electrodes. The resulting SPCE/PEDOT(CNT)and SPCE-MWCNT/PEDOT(CNT) electrodes were first conditionedin 0.1 M KCl and after drying, K+-ISM cocktail was added on topof the polymer to prepare the SPCE/PEDOT(CNT)/K+-ISM andSPCE-MWCNT/PEDOT(CNT)/K+-ISM electrodes. After conditioningin 0.01 M KCl for 2 days, the electrodes were calibrated in10�1–10�6 M KCl solutions. The Ag/AgCl/3 M KCl referenceelectrode was used in this case. The calibration plots of theseelectrodes are shown in Fig. 2.

Results from Fig. 2 show that the potentiometric responses ofthe SPCE/PEDOT(CNT)/K+-ISM and the SPCE-MWCNT/PED-OT(CNT)/K+-ISM electrodes are similar and both electrodes givelinear response in the activity range 10�1–10�6 M of K+ ion. Theslopes of the calibration curves are 56.9 and 57.1 mV/decade andthe standard potential (E0) values are 393 and 399 mV forSPCE/PEDOT(CNT)/K+-ISM and SPCE-MWCNT/PEDOT(CNT)/K+-ISM,respectively. Calibration of the SPCE-MWCNT/PEDOT(CNT)/K+-ISM electrode was repeated after ca. 2 and 10 weeks conditioningin 0.01 M KCl solution. The calibration curves obtained in this case(not shown) revealed that even after such long conditioning times,the electrode still responded linearly to K+ in the same activityrange (10�1–10�6 M). The slopes calculated from the linear rangeof the calibration plots were 56.6 and 56.9 mV/decade, and E0 val-ues were 396 and 386 mV after 2 and 10 weeks, respectively.

The SPCE-MWCNT/PEDOT(CNT)/K+-ISM, SPCE-MWCNT/PEDOT(Cl)/K+-ISM, and SPCE-MWCNT/K+-ISM electrodes were preparedas described before. The calibration of these electrodes in10�1–10�6 M KCl solutions was carried out after 2 weeks condi-tioning in 0.01 M KCl solution (Fig. 3). The reference electrode usedwas Ag/AgCl/3 M KCl with a salt bridge containing 1 M lithiumacetate.

As can be seen in Fig. 3, the SPCE-MWCNT/PEDOT(CNT)/K+-ISMelectrode shows linear response in the range 10�1–10�6 M of K+

ions with a slope of 55.2 mV/decade. Both the SPCE-MWCNT/PED-OT(Cl)/K+-ISM, and SPCE-MWCNT/K+-ISM electrodes respond line-arly to K+ in the activity range 10�1–10�5 M.

The potentiometric selectivity coefficients (as logKKþ ;j) of theGC/PEDOT(CNT)/K+-ISM, GC/PEDOT(Cl)/K+-ISM, and GC/K+-ISMelectrodes determined by using the SSM in the presence of someinterfering cations are shown in Fig. 4. These results indicate thatdifferent types of K+-ISEs exhibit high selectivity towards K+ incomparison to the interfering cations, except for ammonium ionwhich is the most notable interfering cation among the tested ions

-7 -6 -5 -4 -3 -2 -1 0-150

-100

-50

0

50

100

150

200

250

300

log a K+

E, m

V

b

GC/K+-ISM (h) electrodes in (a) 10�1–10�7 then back to 10�1 M KCl solutions afterns after being conditioned for 3 weeks.

-6 -5 -4 -3 -2 -1 0

50

100

150

200

250

300

350

Slope= 56.9 mV/decade

log a K+

E, m

V

-6 -5 -4 -3 -2 -1 0

50

100

150

200

250

300

350

Slope= 57.1 mV/decade

log a K+

E, m

V

a b

Fig. 2. Calibration plots in 10�1–10�6 M KCl solutions for (a) SPCE/PEDOT(CNT)/K+-ISM and (b) SPCE-MWCNT/PEDOT(CNT)/K+-ISM electrodes.

-7

-6

-5

-4

-3

-2

-1

0

interfering cation, j

log

KK

+ ,j

GC/PEDOT(CNT)/K-ISM

GC/PEDOT(Cl)/K-ISM

GC/K-ISM

Li+

Mg2+

NH4+

Na+

H+Ca2+

Fig. 4. Selectivity coefficient (log KKþ ;j) of the GC/PEDOT(CNT)/K+-ISM, GC/PEDOT(Cl)/K+-ISM, and GC/K+-ISM electrodes determined by the SSM.

-1 0 1 2 3 4 5 60

50

100

150

200

250

300

350

Time, h

E, m

V

N2

CO2N

2O

2N

2

Fig. 5. Sensitivity to O2 and CO2 in 0.1 M KCl solution for the GC/PEDOT(CNT) (solidline) and GC/PEDOT(CNT)/K+-ISM (dotted line) electrodes.

-6 -5 -4 -3 -2 -1-50

0

50

100

150

200

250

300

350

log a K+

E, m

V

Fig. 3. Calibration plots for SPCE-MWCNT/PEDOT(CNT)/K+-ISM (j), SPCE-MWCNT/PEDOT(Cl)/K+-ISM (d), and SPCE-MWCNT/K+-ISM (h) electrodes in 10�1–10�6 MKCl solutions after 2 weeks conditioning in 0.01 M KCl solution.

Z. Mousavi et al. / Journal of Electroanalytical Chemistry 633 (2009) 246–252 249

ðlog KKþ ;NHþ4¼ �1:7� 0:05Þ. As shown in Fig. 4, the selectivity of the

electrodes is not influenced by the type of the solid contact used, in

agreement with the earlier results [8]. This confirms that the selec-tivity is solely determined by the ion-selective membrane and notby the solid contact.

3.2. Sensitivity to O2 and CO2

Sensitivity of the electroactive film PEDOT(CNT) to O2 and CO2

gases was studied before and after applying the ISM. Potential ofthe GC/PEDOT(CNT) and GC/PEDOT(CNT)/K+-ISM electrodes wasmeasured in 0.1 M KCl. While measuring the potential of the elec-trodes, the solution was first purged with N2 for 1 h, then with O2

for 1 h. After O2, the solution was again purged with N2 for 1 h,then for 1 h with CO2 and finally 1 h with N2 (Fig. 5).

As shown in Fig. 5, the GC/PEDOT(CNT) electrode with andwithout K+-ISM is only slightly sensitive to O2. On the other hand,GC/PEDOT(CNT) shows relatively high sensitivity to CO2 (pHchanging). This is obviously due to the carboxylic groups presenton the functionalized carbon nanotubes in the PEDOT(CNT) com-posite, because PEDOT doped with poly(styrene sulfonate) showsmuch less sensitivity to CO2 [30]. The GC/PEDOT(CNT)/K+-ISM,however, does not show any sensitivity to CO2.

3.3. Aqueous-layer test

The probable formation of an aqueous-layer between theISM and the underneath substrate was examined for the

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-150

-100

-50

0

50

100

150

E, V

I, μA

Fig. 7. Cyclic voltammograms (3rd cycle) for bare GC (solid line), GC/PEDOT(Cl)(dotted line), and GC/PEDOT(CNT) (dashed line) recorded in 0.1 M KCl at a scan rateof 0.1 Vs�1.

250 Z. Mousavi et al. / Journal of Electroanalytical Chemistry 633 (2009) 246–252

GC/PEDOT(CNT)/K+-ISM and GC/K+-ISM electrodes that were incontact with the conditioning solution for 3 weeks and 4 days,respectively. The experiment was carried out by first measuringthe potential of the electrodes in 0.1 M KCl for 90 min and thenin the solution containing the interfering ion Na+, i.e. 0.1 M NaCl,for 120 min. Finally, the potential was measured in 0.1 M KCl for240 min (Fig. 6).

According to Fibbioli et al. [31], the presence of an aqueous-layer causes a positive potential drift when changing the samplesolution from the primary ion to a solution with an interferingion. In addition, when changing the sample solution from the inter-fering ion to the primary ion solution a negative potential driftindicates the existence of an aqueous layer. As can be seen inFig. 6, the coated disk electrode (GC/K+-ISM) shows slightly posi-tive potential drift in the interfering ion (Na+) solution and a clearnegative potential drift when changing the sample solution fromNaCl to KCl. On the contrary, the GC/PEDOT(CNT)/K+-ISM electrodeshows rather stable potential upon similar sample changing de-spite being in contact with the conditioning solution for longertime, indicating that the PEDOT(CNT) film effectively reduces for-mation of a water-layer between the ISM and the underlying layer.

3.4. CV and EIS

The cyclic voltammograms (3rd cycle) recorded at bare GC,GC/PEDOT(Cl), and GC/PEDOT(CNT) electrodes in 0.1 M KClsolution are shown in Fig. 7.

In the scale shown in Fig. 7, a negligible current is observed atbare GC electrode in the potential range �0.5 to 0.5 V. Whereas,distinct capacitive current is obtained at both GC/PEDOT(CNT)and GC/PEDOT(Cl) electrodes. In this potential range, however,the capacitive current obtained at GC/PEDOT(CNT) electrode isca. 2–3 times (depending on Edc) higher than the one obtained atGC/PEDOT(Cl) electrode. The higher capacitive current observedat PEDOT(CNT), which is due to the presence of CNTs in the film,means higher redox capacitance in comparison to PEDOT(Cl) as so-lid contact. Having a solid contact with a high redox capacitance isone of the stability conditions required for an all-solid-state poten-tiometric ion sensor [32]. The relationship between the potentialstability of solid-contact K+-ISEs and the capacitance of conductingpolymer as solid-contact material was extensively studied byBobacka [8].

The impedance spectra shown in Fig. 8a are recorded in 0.1 MKCl solution for the GC/PEDOT(CNT) and GC/PEDOT(Cl) electrodes

0 60 120 180 240 300 360 420 480-50

0

50

100

150

200

250

0.1 M KCl0.1 M KCl

Time, min

E, m

V

0.1 M NaCl

Fig. 6. Aqueous-layer test for the GC/PEDOT(CNT)/K+-ISM (solid line) and GC/K+-ISM (dotted line) electrodes.

without the ISM. The shape of the impedance spectra is typicalfor PEDOT films in an aqueous electrolyte [33]. As can be seen fromthe low-frequency part of the impedance spectra of the GC/PED-OT(CNT) and GC/PEDOT(Cl) electrodes, the imaginary part ofimpedance is ca. 3 times lower for the PEDOT(CNT) film whencompared to the PEDOT(Cl) film. This low-frequency region is re-lated to the bulk redox capacitance of PEDOT(CNT) and PEDOT(Cl).The redox capacitance was estimated using the equationCLF = 1/(2pfZ0 0), where f is the lowest frequency used to recordthe spectra (0.01 Hz), and Z0 0 is the imaginary part of the imped-ance at this frequency. The calculated CLF was ca. 721 and 250 lFfor PEDOT(CNT) and PEDOT(Cl), respectively. These results are ingood agreement with the results obtained by CV where thePEDOT(CNT) membrane shows higher redox capacitance thanthe PEDOT(Cl) membrane. The calculated redox capacitance ofPEDOT(CNT) film is even higher than the redox capacitance of PEDOTdoped with PSS which was prepared using similar conditions [8].

The impedance spectra for the GC/PEDOT(CNT)/K+-ISM andGC/PEDOT(Cl)/K+-ISM electrodes were also recorded in 0.1 M KClsolution (Fig. 8b). As can be seen in this figure, the impedance spec-tra of the GC/PEDOT(CNT)/K+-ISM and GC/PEDOT(Cl)/K+-ISM elec-trodes are almost identical. The high-frequency semicircleobtained in this case is due to the PVC-based ion-selective mem-brane which, in both electrodes, is supposed to be of the samethickness. The absence (negligible) of a low-frequency semicircleshows that the ion-to-electron transduction occurs properly bothfor PEDOT(CNT) and PEDOT(Cl) [8].

3.5. Chronopotentiometry

The potential stability of all-solid-state K+-ISEs was studiedusing the constant-current chronopotentiometric measurementsuggested by Bobacka [8]. Constant current of ±1 nA was appliedon the K+-ISEs while the potential of the electrodes was measuredin 0.1 M KCl solution. Fig. 9 shows the chronopotentiograms re-corded for the GC/PEDOT(CNT)/K+-ISM, GC/PEDOT(Cl)/K+-ISM,and GC/K+-ISM electrodes.

The chronopotentiograms in Fig. 9 indicate that all the K+-ISMelectrodes show a potential jump upon the change in the appliedcurrent from +1 to �1 nA. The potential jump in the GC/PED-OT(CNT)/K+-ISM response was used to calculate the total resistanceof the electrode. The calculated resistance in this case was 8.1 MXwhich is in good agreement with the resistance estimated from theEIS measurement (Fig. 8b). The potential drift of the electrodes was

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

7

8

9

10

-Z'',

MΩ

Z', MΩ0 10 20 30 40 50 60 70

0

10

20

30

40

50

60

70

Z', kΩ

-Z'',

kΩ

a b

Fig. 8. Impedance spectra of (a) the GC/PEDOT(Cl) (h), and GC/PEDOT(CNT) electrodes (j) and (b) the GC/PEDOT(Cl)/K+-ISM (s), and GC/PEDOT(CNT)/K+-ISM (d) electrodesrecorded in 0.1 M KCl solution. The spectra were recorded at the open-circuit potential, and DEac = 10 mV. The frequency range was 100 kHz–10 mHz.

0 20 40 60 80 100 120100120140160180200220240260280300320340

current = -1nA

E, m

V

Time, s

current = +1nA

Fig. 9. Chronopotentiograms for the GC/K+-ISM (dashed line), GC/PEDOT(Cl)/K+-ISM (solid line), and GC/PEDOT(CNT)/K+-ISM (dotted line) recorded in 0.1 M KCl.The applied current is +1 nA for 60 s and �1 nA for 60 s.

Z. Mousavi et al. / Journal of Electroanalytical Chemistry 633 (2009) 246–252 251

calculated from the slope of the curves at longer time (DE/Dt). Thecalculated potential drift values were 12, 17.6, and 1867 lV/s forthe GC/PEDOT(CNT)/K+-ISM, GC/PEDOT(Cl)/K+-ISM, and GC/K+-ISM electrodes, respectively. The low-frequency capacitance (CLF)of the solid-contacts PEDOT(CNT) and PEDOT(Cl) in the GC/PED-OT(CNT)/K+-ISM and GC/PEDOT(Cl)/K+-ISM electrodes were calcu-lated using the equation DE/Dt = i/CLF. The estimated CLF was 83and 57 lF for PEDOT(CNT) and PEDOT(Cl), respectively. Coverageof the PEDOT(CNT) and PEDOT(Cl) solid-contacts with the ISM isthe reason for the lower capacitance values calculated here in com-parison to those calculated in the EIS measurements (Section 3.4).However, the capacitance estimated for the PEDOT(CNT) solid con-tact is higher than that of PEDOT(Cl) which is in qualitative agree-ment with the results obtained both by CV and EIS. This shows thatthe low-frequency capacitance (CLF) of PEDOT is strongly depen-dent on the contacting medium. The 0.1 M KCl solution offers moreefficient ion transport to/from PEDOT compared with a plasticizedPVC membrane. The same effect was observed earlier for PEDOTdoped with poly(styrene sulfonate) [8].

4. Conclusions

Poly(3,4-ethylenedioxythiophene) (PEDOT) was electrochemi-cally synthesized using negatively charged multi-walled carbon

nanotubes as doping ions. The resulting PEDOT(CNT) compositewas used as the solid contact in all-solid-state potassium ion-selec-tive electrode (K+-ISE) by applying a PVC-based potassium ion-selective membrane (K+-ISM) on the composite. Results from thepotentiometric measurements show that K+-ISEs based on PED-OT(CNT) as the ion-to-electron transducer are sensitive to K+ inthe concentration range 10�1–10�6 M with close to Nernstianslopes. The electrodes show high selectivity to K+ in the presenceof some interfering cations. Interestingly, not only the conductingpolymer but also the conducting substrate used for the depositionof PEDOT(CNT) seems to affect the long-term potential stability ofthe electrode. The standard potential of the GC/PEDOT(CNT)/K+-ISM electrode, with glassy carbon as the electrically conductingsubstrate, drifted by ca. 52 mV when keeping the electrode condi-tioned in KCl solution for 3 weeks. However, for SPCE-MWCNT/PEDOT(CNT)/K+-ISM electrode, in which the PEDOT(CNT) filmwas deposited on carbon electrodes modified with multi-walledcarbon nanotubes (SPCE-MWCNT), the standard potential and theanalytical performance were more stable. The change in the stan-dard potential of the electrode conditioned in 0.01 M KCl solutionfor 2 and 10 weeks was ca. 3 and 13 mV, respectively. The linearrange of the calibration curves (10�1–10�6 M) and the calculatedslopes in this range, however, were rather stable for all the elec-trodes based on PEDOT(CNT) as the solid contact. Based on thesefindings, the SPCE-MWCNT/PEDOT(CNT)/K+-ISM electrode is con-sidered to be the best electrode compared with the other elec-trodes studied in this work. This may partially be due to thecharge-transfer properties of CNTs present both in the substrateand in the PEDOT(CNT) composite. The results from cyclic voltam-metric and electrochemical impedance spectroscopic measure-ments reveal that PEDOT(CNT) film exhibits higher redoxcapacitance than PEDOT(Cl) and PEDOT(PSS) [8]. This can be attrib-uted to the presence of CNTs in the PEDOT(CNT) film. No aqueouslayer was detected at the interface between PEDOT(CNT) and theion-selective membrane. These results show that PEDOT(CNT) issuitable as ion-to-electron transducer in solid-contact ISEs.

Acknowledgments

The authors thank Prof. Carita Kvarnström for the RAMAN mea-surement. Financial support from the Åbo Akademi UniversityFoundation is gratefully acknowledged. This work is part of theactivities of the Åbo Akademi Process Chemistry Centre withinthe Finnish Centre of Excellence Program (2000–2011) by theAcademy of Finland.

252 Z. Mousavi et al. / Journal of Electroanalytical Chemistry 633 (2009) 246–252

References

[1] E. Bakker, D. Diamond, A. Lewenstam, E. Pretsch, Anal. Chim. Acta 393 (1999)11–18.

[2] V.K. Gupta, S. Chandra, R. Mangla, Electrochim. Acta 47 (2002) 1579–1586.[3] E.G. Kulapina, O.V. Barinova, Pharm. Chem. J. 31 (1997) 667–672.[4] F. Jalali, M.J. Rajabi, G. Bahrami, M. Shamsipur, Anal. Sci. 21 (2005) 1533–1535.[5] R.W. Cattrall, H. Freiser, Anal. Chem. 43 (1971) 1905–1906.[6] R.W. Cattrall, I.C. Hamilton, Ion-Sel. Electrode Rev. 6 (1984) 125–172.[7] R.P. Buck, in: H. Freiser (Ed.), Ion Selective Electrodes in Analytical Chemistry,

vol. 1, Plenum Press, New York, 1980, p. 58.[8] J. Bobacka, Anal. Chem. 71 (1999) 4932–4937.[9] A. Cadogan, Z. Gao, A. Lewenstam, A. Ivaska, D. Diamond, Anal. Chem. 64

(1992) 2496–2501.[10] T. Alaviuhkola, J. Bobacka, M. Nissinen, K. Rissanen, A. Ivaska, J. Pursiainen,

Chem. Eur. J. 11 (2005) 2071–2080.[11] J. Bobacka, A. Ivaska, A. Lewenstam, Electroanalysis 15 (2003) 366–374.[12] J. Bobacka, Electroanalysis 18 (2006) 7–18.[13] M. Vázquez, J. Bobacka, A. Ivaska, A. Lewenstam, Talanta 62 (2004) 57–63.[14] M. Vázquez, P. Danielsson, J. Bobacka, A. Lewenstam, A. Ivaska, Sens. Actuators

B 97 (2004) 182–189.[15] S. Iijima, Nature 354 (1991) 56–58.[16] L.S. Schadler, S.C. Giannaris, P.M. Ajayan, Appl. Phys. Lett. 73 (1998) 3842–

3844.

[17] K. Lota, V. Khomenko, E. Frackowiak, J. Phys. Chem. Solids 65 (2004) 295–301.

[18] T.-M. Wu, Y.-W. Lin, Polymer 47 (2006) 3576–3582.[19] G.A. Rivas, M.D. Rubianes, M.L. Pedano, N.F. Ferreyra, G.L. Luque, M.C.

Rodríguez, S.A. Miscoria, Electroanalysis 19 (2007) 823–831.[20] J. Wang, Analyst 130 (2005) 421–426.[21] L. Dai, P. Soundarrajan, T. Kim, Pure Appl. Chem. 74 (2002) 1753–1772.[22] G.A. Crespo, S. Macho, F.X. Rius, Anal. Chem. 80 (2008) 1316–1322.[23] G.A. Crespo, S. Macho, J. Bobacka, F.X. Rius, Anal. Chem. 81 (2009) 676–681.[24] C.-Z. Lai, M.A. Fierke, A. Stein, P. Bühlmann, Anal. Chem. 79 (2007) 4621–4626.[25] M. Fouskaki, N. Chaniotakis, Analyst 133 (2008) 1072–1075.[26] D. Wei, C. Kvarnström, T. Lindfors, A. Ivaska, Electrochem. Commun. 9 (2007)

206–210.[27] J. Koryta, J. Dvorak, L. Kavan, Principles of Electrochemistry, second ed., Wiley,

Chichester, 1993 (p. 38).[28] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and

Applications, second ed., John Wiley & Sons Inc., New York, 2001 (p. 72).[29] R.P. Buck, E. Lindner, Pure Appl. Chem. 66 (1994) 2527–2536.[30] M. Vázquez, J. Bobacka, A. Ivaska, A. Lewenstam, Sens. Actuators B 82 (2002)

7–13.[31] M. Fibbioli, W.E. Morf, M. Badertscher, N.F. de Rooij, E. Pretsch, Electroanalysis

12 (2000) 1286–1292.[32] B.P. Nikolskii, E.A. Materova, Ion-Sel. Electrode Rev. 7 (1985) 3–39.[33] J. Bobacka, A. Lewenstam, A. Ivaska, J. Electroanal. Chem. 489 (2000) 17–27.