Journal of

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Journal of Electroanalytical Chemistry 601 (2007) 29–38

ElectroanalyticalChemistry

Impedance spectroscopic investigations of ITO modifiedby new Azo-calix[4]arene immobilised into

electroconducting polymer (MEHPPV)

A. Rouis a,b,*, R. Mlika a, J. Davenas b, H. Ben Ouada a, I. Bonnamour c, N. Jaffrezic d

a Laboratoire de Physique et Chimie des Interfaces, Faculte des Sciences de Monastir, Avenue de l’environnement, 5000 Monastir, Tunisiab Ingenierie des Materiaux Polymeres: LMPB, 43 boulevard du 11 Novembre 1918, Universite Claude Bernard – Lyon 1, 69622 Villeurbanne, France

c Laboratoire d’Application de la Chimie a l’environnement (LACE), 43 Boulevard du 11 Novembre 1918,

Universite Claude Bernard – Lyon 1, 69622 Villeurbanne, Franced CEGELY, Ecole Centrale de Lyon, 36 rue Guy de Collongue, 69134 Ecully, France

Received 18 April 2006; received in revised form 2 October 2006; accepted 11 October 2006Available online 22 November 2006

Abstract

A sensing material based on calix[4]arene molecules (Azo-C[4]) dispersed in a conjugated polymer (MEHPPV) is deposited by spincoating onto a transparent conducting substrate (ITO) to fabricate new chemical sensors. The electrochemical properties of the sensorare investigated by impedance spectroscopy. Transfer and ion diffusion processes of metal cations (Cu2+, Eu3+) in the Azo-C[4]-MEHPPV membranes are studied. The calixarene molecule behaviour into the polymer membranes is characterised by impedance spec-troscopy showing the cation complexation by both, Azo-calix[4]arene and MEHPPV, components of the membrane. The differences inselectivity are found to be represented by variations in the bulk resistance of the membrane (Rm) and in the charge transfer resistance(Rtc) of the interface. The impedance behaviour of the Azo-C[4]-MEHPPV electrodes is modelled by an equivalent electrical circuit. Opti-cal excitation tests of these membranes show that the sensing activity of the calixarene molecules entrapped in the polymer matrix ispreserved.� 2006 Elsevier B.V. All rights reserved.

Keywords: Indium thin oxide; Azo-C[4]-MEHPPV membrane; Impedance spectroscopy; Equivalent electrical circuit; Optical excitation

1. Introduction

The modification of electrode surfaces with organicmolecular films attracted a growing interest in variousfields during the last decades. Macrocyclic compounds rep-resent a well-known family of organic molecular materialsexhibiting many potential applications in the domain ofchemical sensors [1,2]. Calixarenes have in particular beenat the origin of extensive studies for recognition and supra-

0022-0728/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jelechem.2006.10.032

* Corresponding author. Address: Laboratoire de Physique et Chimiedes Interfaces, Faculte des Sciences de Monastir, Avenue de l’environn-ement, 5000 Monastir, Tunisia. Tel.: +216 73 500 274; fax: +216 73 500278.

E-mail address: rouisahlem2@yahoo.fr (A. Rouis).

molecular chemistry [3,4]. Several authors have shown thatcalix[n]arenes have good extraction properties due to theirability to recognise ions and organic molecules accordingto a key–lock interaction mechanism [5–7].

Recently, the inclusion of cage molecules into electro-conducting polymers, ECPs, has opened new perspectivesof application for the construction of ionic sensors. Sincethe last three decades, a number of ion-selective electrodesinvolving polymeric membranes have been investigated.Therefore, Shinkai et al. [8] reported that calix[6]arene-p-hexasulfonate bonded to the soluble poly (ethylene imine)has a high affinity for the uranyl ion. Yilmaz’group [9,10]has synthesised different polymeric calixarenes and hasinvestigated their ionophoric properties for some metal cat-ions. In 1997, Bidan and Niel [11] studied the affinity of

Fig. 1. Chemical structures of (a) MEHPPV and (b) Azo-calix[4]arene.

30 A. Rouis et al. / Journal of Electroanalytical Chemistry 601 (2007) 29–38

electrodes modified by sulfonated calixarenes immobilisedinto a polypyrrole film for trimethyl(ferrocenylmethyl)-ammonium cation (FcTMA+). In 2002, Mahajan et al.[12] used ISEs based on Schiff-base-p-tert-butylca-lix[4]arene derivatives for the selective detection of silver.

These studies have confirmed that the ability of recogni-tion and complexation of calixarenes is preserved upontheir immobilization in polymer matrices. Electrodes mod-ified by conducting polymers doped with calixarenesappear then as good candidates for the detection of targetspecies.

Electrochemical studies involving various micro-sensorsas ion-sensitive-electrode, ion-sensitive field-effect transis-tors (ISFET) and electrolyte–insulator–semiconductor(EIS) have been developed for analysis using many typesof substrates (Si/SiO2 [13–15], Si/SiO2/Si3N4 [16], platinum[17], gold [18]. . .). However, few investigations wereperformed using ITO (indium thin oxide) substrates inmicro-sensor applications [19,20], whatever extensivelyused in opto-electronic and electronic devices like organiclight emitting diodes (OLED) [21], photovoltaic diodes[22] and field-effect-transistor (FET) [23]. These oxidesare well known for their unique electrical and optical prop-erties, i.e. high electrical conductivity and high opticaltransmittance in the visible region [24].

Impedance spectroscopy is commonly used to character-ise the electrical response of a material as a function of fre-quency. In the present paper, we report sensor impedancestudies of composite membranes formed by new Azo-calix[4]arene esters [25] incorporated into an electro-conducting polymer (MEHPPV) on ITO electrode. Thephenomena occurring at the electrode/solution interfaceduring the complexation process have been investigatedby electrochemical impedance spectroscopy [26]. Finally,complementary information could be obtained about thecomplexation mechanism by the optical excitation with ablue laser diode (kexci � 430 nm) of functionalized ITOsubstrates.

2. Experimental

2.1. Materials

The sensing molecules used in this work, a 5,17-bis(4-nitrophenylazo)-26,28-dihydroxy-25,27-di(ethoxycarbonyl-methoxy)-calix[4]arene (Azo-C[4]), have been synthesisedand purified by the LACE (Laboratoire d’Application dela Chimie a l’Environnement) at ‘‘Claude Bernard Univer-sity, Lyon’’ according to the previously reported synthesisroute [25]. The new Azo-calix[4]arene derivative has beencharacterised by the presence of chromophores at theupper rim and an ester group at the lower rim.

Calixarene molecules are incorporated into, poly[2-meth-oxy-5-(2 0-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), which is purchased from Aldrich, by mixing the twocomponents in a common solvent. The optimum weightratio of the membranes having electroactive material

(Azo-C[4]) with MEHPPV was 3:7 (w/w). The chemicalstructures of the molecules are shown in Fig. 1.

2.2. Electrode elaboration

ITO substrates (ITO-thickness 100 nm, sheet resistance20 X/cm�2) are purchased from Merck Display Technolo-gies and are cut into 1 · 1 cm square slides. Prior to thinlayer deposition, the ITO substrates are successivelycleaned for 20 min in acetone and isopropyl alcohol in anultrasonic bath and finally dried by a nitrogen gas flow.The cleaned ITO substrates are functionalized, respec-tively, by MEHPPV polymeric matrix, which is chosen asa reference and MEHPPV/calixarene composite. Filmsare performed by the spin-coating technique from CHCl3solution at a controlled speed of 2300 rpm. Followingdeposition, films are dried at 80 �C for 30–60 min.

2.3. Impedance spectroscopy and electrical circuit model

Impedance measurements were done in the frequencyrange from 100 kHz to 0.05 kHz with the help of an imped-ance analyser ‘‘Voltalab 40’’ controlled by a computer. Alldata were obtained at AC amplitude of 10 mV. An aqueoussolution of 0.1 M of ammonium acetate (CH3COO�,NHþ4 Þ at pH 7 was used as the electrolyte in the electro-chemical cell with three electrodes: the working electrode(ITO), a platinum counter electrode and a saturated

0 20 40 60 80 1000

20

40

60

80

100

-Zi (

MΩ

cm

-2)

Zr (MΩ cm-2)

Fig. 2. Electrochemical conditions of ITO/MEHPPV-Azo-calix[4]arenemembranes versus polarization: (h) �200 mV/SCE, (s) �400 mV/SCE,(m) �600 mV/SCE.

0.0 0.5 1.0 1.5 2.00

200

400

600

800Z

r (K

Ω c

m-2

)

ω-1/2(s-1/2)

Fig. 3. Determination of coverage rate of ITO electrode: (j) bare ITOelectrode, (s) ITO/MEHPPV-Azo-C[4] electrode.

A. Rouis et al. / Journal of Electroanalytical Chemistry 601 (2007) 29–38 31

calomel reference electrode (SCE). The top half of ITOelectrode was used for electrical contact. Therefore, thebottom half was used as an electroactive electrode area(0.125 cm2). All measurements were carried out at roomtemperature. The analysis of the impedance data was doneby using the Z-View2 software. With its help the equivalentelectric circuits were obtained. The parameters of theequivalent circuit were optimised to ensure the best fit ofthe experimental data over the entire frequency range using[27].

3. Results and discussion

3.1. Adhesion test

The adhesion of Azo-calix[4]arene derivative films onITO electrodes is usually poor, inducing an early degrada-tion of the device performance. So, it is necessary to inves-tigate the factors controlling adhesion over a broad rangeof experimental conditions. In fact, the ITO cleaning pro-cedure, the film drying duration and the electrolyte solu-tion concentration have little influence on the adhesion.Our prospect is to use a conducting polymer for theimmobilization and electrical contact of the calixarenesmolecules on ITO substrates. In literature, a number ofstudies were performed using the electropolymerization ofconducting polymers (cf. polypyrrole: PPy, polyaniline:PANI) [28,29] incorporating guest molecules to produceISEs. We report the fabrication of chemical sensors real-ized involving Azo-calix[4]arene derivative ionophoresentrapped in a polymeric layer deposited on ITO substrates(cf. PVK (poly(N-vinylcarbazole)), PMPP (a derived ofPPV) and MEHPPV). Only, in the presence of theMEHPPV layer that the stability of the electrodes increasesgreatly, whereas the accessibility of the calixarene complex-ing sites remains preserved.

3.2. Polarization optimisation

The first step for the impedance studies is the determina-tion of the polarization domain to be investigated. Undernegative overvoltages (�0.2, �0.4 and �0.6 V) versusSCE, an obvious decrease of the Warburg straight lineand a decrease of the half-circle diameter are observed(Fig. 2). This latter reflects the effect of the association ofthe charge-transfer resistance (Rtc) and the Helmoltz dou-ble layer interfacial capacitor (Cdl). Under �600 mV over-voltage polarization, a dark spot covering the active area ofthe work electrode is observed, indicating a chemical degra-dation of the ITO electrode. So, the potential polarizationis optimised at �600 mV along this work.

3.3. Coverage rate determination of the ITO electrode

In Fig. 3, the impedance real part of bare and function-alized ITO electrodes has been plotted as a function of theinverse of the square root of the sinusoidal excitation pul-

sation. The extrapolation of the linear zone to the high fre-quencies provides the sum of the charge-transfer resistance(Rtc), the membrane resistance (Rm) and the electrolyticsolution resistance (Rs). The latter resistance has been gen-erally small as compared to the first one. A coverage of83% is obtained using Eq. (1) [30,31]. This value can beexplained by the formation of a compact film:

H¼ 1� ½Rtc ðbare electrodeÞ=Rtc ðfunctionalized electrodeÞ�ð1Þ

3.4. Equivalent circuit modelling

The complex plane plot of impedance is formed by twotypical regions as seen in Fig. 4. The first region consists ofa small semicircle at high frequencies followed by a straightline with a slope slightly below p/4 (cf. inset) and thesecond region of a bigger semicircle expanding over the

Z"

-500000

0 250000 500000

-250000

0

Z'

0 25000 50000 75000

-75000

-50000

-25000

0

Z'

Z"

Fig. 4. (a) Nyquist plot of Azo-C[4]-MEHPPV membrane, (b) modulusversus frequency (continuous line: experimental data, dash line: fittedspectrum).

(c)

0 100 200 300 400 500 600 700 8000

100

200

300

400

500

600

700

800

0 10 20 30 40 50 600

10

20

30

40

50

60

-Zi (K

Ωcm

-2)

Zr (KΩcm-2)

(b)-Z

i (K

Ω c

m-2)

Zr (KΩ cm-2)

p[Cu2+]= 10

p[Cu2+]= 9.58

p[Cu2+]= 9.06

p[Cu2+]= 8.64

p[Cu2+]= 8.08

p[Cu2+]= 7.65

(a)

32 A. Rouis et al. / Journal of Electroanalytical Chemistry 601 (2007) 29–38

middle and low-frequency range. Only one semicircle couldbe observed for the ITO uncovered electrode. The firstsemicircle is ascribed to the bulk of the membrane, there-fore the second semicircle occurring in the low-frequencyrange is related to the ion charge transfer at the ITO/aque-ous solution interface. The line inclined at 45� observed inthe medium-frequency range is attributed to the Warburgimpedance (W) [32,33]. As equivalent circuit a serial asso-ciation of three components is proposed, as shown inFig. 5.

In this circuit, CPE1 and CPE2 are two constant phaseelements that take into account the interfacial irregularities(porosity, roughness, geometry) [34]. It can be described byEq. (2) [35]:

Z ¼ 1

AðjxÞn ð2Þ

400

600

800

0 5 10 15 20 250

5

10

15

20

25

-Zi (K

Ω c

m-2

)

Zr (KΩcm-2)

(d)

i (K

Ω c

m-2)

p[Cu2+]= 10 p[Cu2+]= 9.58 p[Cu2+]= 9.18 p[Cu2+]= 8.78 p[Cu2+]= 8.5 p[Cu2+]= 8.3 p[Cu2+]= 7.92 p[Cu2+]= 7.55 p[Cu2+]= 7.06 p[Cu2+]= 6.17 p[Cu2+]= 5.73

where A is a frequency independent term and the exponentn (0 < n < 1) determines the phase angle. Perfectly smoothelectrodes are described by n = 1 and CPE acts then like acapacitor with (A) being its capacitance.

The finite length Warburg diffusion impedance Zw iscontrolled by the ionic species diffusion within the film.

Rtc

W

CPE1

Rs

Rm

CPE2

Zw

Fig. 5. Equivalent circuit used to fit the impedance. Rm: bulk resistanceof the membrane, Zw: finite length Warburg diffusion impedance, CPE1and CPE2: constant phase elements, Rtc: charge transfer resistance, Rs:resistance of the solution and ITO glass.

The mathematical form of the impedance is given by Eq.(3) [35]:

ZD ¼ RD

coth½ðjxsDÞ12�

ðjxsDÞ12

ð3Þ

where x is the angular frequency, RD is the diffusionresistance and sD denotes the time constant for diffusionof species along the distance l with the diffusion coefficientD [36,37]:

sD � l2=D ð4ÞThe parameters of the electrical circuit shown in Fig. 5

can be calculated from a fit of the equivalent circuit tothe experimental impedance data. The typical impedancespectra in Fig. 4 shows the experimental data together withtheir theoretical fits for ITO/Azo-C[4]-MEHPPV/electro-lyte system.

0 200 400 600 8000

200

-Z

Zr (KΩ cm-2)

Fig. 6. Complex plane electrochemical impedance spectra for differentCu2+ concentrations: (curves a and c) represents, respectively, the wholespectra of MEHPPV and Azo-C[4]-MEHPPV membranes. The inset(curves b and d) shows, respectively, the high frequency part of MEHPPVand Azo-C[4]-MEHPPV membranes.

A. Rouis et al. / Journal of Electroanalytical Chemistry 601 (2007) 29–38 33

3.5. Detection process

3.5.1. Cu2+ sensitivity

The electrochemical impedance of the modified ITOelectrode is studied to get insight on the role of the modi-fied Azo-calix[4]arene in the selective response of the elec-trode for copper ions.

Fig. 6(a) and (c) shows that the impedance behaviours ofMEHPPV and Azo-C[4]-MEHPPV membranes are depen-dent on the p[Cu2+] added to the electrolyte solution. Aslightly depressed semicircle in the complex-plane imped-ance plots is observed for the two electrode types athigh frequencies. It has been found that the diameter ofthe Nyquist plots decreases with increasing Cu2+

concentration.The circuit parameters of ITO/MEHPPV/electrolyte

and ITO/Azo-C[4]-MEHPPV/electrolyte structuresdeduced from the fit to the previous equivalent circuit arepresented in Table 1. A decrease of the diffusion time con-stants (sD) calculated from a finite-length diffusion imped-ance by means of the software Z-View2 (Fig. 6(b) andTable 1) is noted for increasing Cu2+ concentration in theMEHPPV matrix. This variation can be explained by thediffusion process of Cu2+ cations into the polymer matrix.sD time constants are increased when incorporating Azo-calix[4]arene molecules inside the polymer matrix. Indeed,calixarene molecules can improve the porosity of polymericelectrodes but a lower dependence on the concentration of

Table 1Electrochemical parameters of MEHPPV and Azo-C[4]-MEHPPV membrane

Membrane p[Cu2+] Rm (kX) CEP

MEHPPV 10 39.6 ± 0.5 82.09.58 36.7 ± 0.5 98.89.06 34.1 ± 0.5 107.88.64 32.4 ± 0.5 139.68.08 30.9 ± 0.4 164.17.65 30.4 ± 0.4 173.07.08 30.6 ± 0.4 192.2

Azo-C[4]-MEHPPV 10 12.1 ± 1.4 63.39.58 11.6 ± 1.6 60.79.18 11.2 ± 1.3 61.98.78 11.0 ± 1.3 61.98.5 10.6 ± 1.2 62.68.3 10.3 ± 1.2 62.4

MEHPPV under light excitation 10 10.2 ± 0.2 102.59.70 9.7 ± 0.2 105.58.74 9.1 ± 0.2 100.57.93 9.0 ± 0.2 123.46.70 8.5 ± 0.2 114.76.22 8.4 ± 0.2 113.8

Azo-C[4]-MEHPPV under light excitation 10 8.5 ± 0.1 46.99.52 8.3 ± 0.1 46.49.06 8.2 ± 0.1 46.68.72 8.1 ± 0.1 46.18.48 7.7 ± 0.1 45.08.27 7.6 ± 0.1 44.77.91 7.1 ± 0.1 45.8

analyte is observed on the diffusion time constant (Fig. 6(d)and Table 1). The diffusion pathway limited by the calixa-rene inside the doped membrane can be at the origin of thissmaller concentration dependence.

As illustrated in Fig. 7, the membrane containing Azo-C[4] molecules is obviously less resistive than the polymermembrane. We note, on one hand, that the bulk resistanceRm decreases when [Cu2+] increases and becomes constant.Moreover, in Fig. 7(a), bulk resistance plot versus p[Cu2+]decreases carefully in the Azo-C[4]-MEHPPV membrane.In terms of membrane conductivity (c), we can deduce thatthe Azo-C[4]-MEHPPV membrane is more conductivethan MEHPPV membrane.

On the other hand, the charge transfer resistance Rtc

also decreases with the increase of the detected ion concen-tration [Cu2+] for the two types of interfaces. The Rtc slopeis higher for the Azo-C[4]-MEHPPV/ITO interface thanthe MEHPPV/ITO interface (Fig. 7(b)) on account ofthe Cu2+ complexation by Azo-calix[4]arene molecules.Comparisons of Rm and Rtc slope values are illustrated inTable 2. Therefore, the incorporation of Azo-calix[4]arenemolecules into a polymer matrix limits the diffusion andimproves the charge transfer across the compositemembrane.

3.5.2. Eu3+ sensitivityLikewise a comparable study is carried out on ITO/

MEHPPV and ITO/Azo-C[4]-MEHPPV sensors versus

s versus Cu2+ concentration

1(nF) sD (s) Rtc (kX) CPE2 (lF) v2 (10�4)

± 2.1 0.23 ± 0.03 635.0 ± 4.7 1.42 ± 0.01 4.6± 2.7 0.22 ± 0.03 628.3 ± 4.7 1.46 ± 0.01 4.9± 3.1 0.18 ± 0.02 613.3 ± 4.5 1.47 ± 0.01 5.4± 4.3 0.16 ± 0.02 624.0 ± 4.6 1.46 ± 0.01 6.0± 4.9 0.10 ± 0.01 592.6 ± 4.2 1.51 ± 0.01 6.1± 5.5 0.10 ± 0.02 585.1 ± 4.5 1.53 ± 0.01 6.6± 5.9 0.06 ± 0.01 550.5 ± 5.0 1.57 ± 0.01 7.0

± 1.8 0.52 ± 0.02 693.1 ± 6.9 1.38 ± 0.18 2.0± 1.8 0.53 ± 0.02 692.8 ± 6.9 1.39 ± 0.19 2.1± 1.6 0.51 ± 0.02 662.7 ± 7.2 1.38 ± 0.18 1.5± 1.6 0.52 ± 0.02 650.4 ± 5.8 1.46 ± 0.18 1.6± 1.7 0.52 ± 0.02 642.3 ± 5.7 1.50 ± 0.18 1.7± 1.6 0.49 ± 0.01 606.8 ± 5.4 1.55 ± 0.19 1.5

± 5.2 0.23 ± 0.02 245.2 ± 1.9 3.71 ± 0.69 3.6± 5.5 0.24 ± 0.02 243.8 ± 1.9 3.76 ± 0.52 3.7± 5.4 0.23 ± 0.01 251.7 ± 1.7 3.74 ± 0.45 3.5± 7.0 0.399 ± 0.005 251.7 ± 1.7 3.76 ± 0.49 3.6± 6.5 0.371 ± 0.005 247.1 ± 1.7 3.83 ± 0.51 3.5± 6.4 0.370 ± 0.005 248.7 ± 1.7 3.88 ± 0.52 3.4

± 2.2 0.24 ± 0.26 205.0 ± 4.7 2.540 ± 0.68 4.2± 2.2 0.27 ± 0.27 129.6 ± 2.8 2.675 ± 0.74 4.2± 2.3 0.25 ± 0.30 185.7 ± 4.2 2.670 ± 0.79 4.8± 2.2 0.28 ± 0.31 176.8 ± 4.2 2.768 ± 0.84 4.3± 2.3 0.29 ± 0.42 169.1 ± 4.5 2.791 ± 0.94 4.3± 2.3 0.30 ± 0.46 156.8 ± 4.8 2.890 ± 1.12 4.7± 2.4 0.31 ± 0.54 154.1 ± 4.7 2.921 ± 1.15 4.3

6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5

8

12

16Rm1 (ITO /Azo-C[4]-M EH PPV )

p [Cu 2+]

Rm

1 (K

Ω c

m-2

)

32

36

40Rm2 (ITO /M EH PP V)

Rm

2 (KΩ

cm-2)

6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5

540

560

580

600

620

640

660

680

700

Rtc

(1.

2)(K

Ω c

m-2)

p [Cu 2+]

R(tc)1 (ITO /A zo -C [4]-M EH PP V)

R(tc)2 (ITO /M EH PP V)

Fig. 7. (a) Rm plots versus p[Cu2+] of MEHPPV and Azo-C[4]-MEHPPVmembranes, (b) Rtc plots versus p[Cu2+] of MEHPPV and Azo-C[4]-MEHPPV membranes.

Table 2Comparisons of Rm and Rtc slope values of ITO/MEHPPV and ITO/Azo-C[4]-MEHPPV membranes, with and without excitation, for the detectionof Cu2+ cations

Membranes Rm (X) Rtc (X)

ITO/MEHPPV (without light excitation) 566.2 3347.7ITO/Azo-C[4]-MEHPPV (without light excitation) 111.6 6908.1ITO/MEHPPV (with light excitation) 88.3 �0ITO/Azo-C[4]-MEHPPV (with light excitation) 78 3079.6

Electroactive electrode area was 0.125 cm2.

0 200 400 600 8000

200

400

600

800

0 200 400 600 8000

100

200

300

400

500

600

700

800

900 (a)

-Zi (

KΩ

cm-2

)

Zr (KΩcm-2)

p[Eu3+]=10p[Eu3+]=9.3p[Eu3+]=8.68p[Eu3+]=8.22p[Eu3+]=7.66p[Eu3+]=6.92p[Eu3+]=6.29p[Eu3+]=5.68p[Eu3+]=5.1

p[Eu3+]= 9

p[Eu3+]= 8

p[Eu3+]= 7.52

p[Eu3+]= 6.88

p[Eu3+]= 6.48

p[Eu3+]= 5.63

p[Eu3+]= 4.91

-Zi(

ΚΩ

cm

-2)

Zr (KΩcm-2)

(b)

Fig. 8. Complex plane electrochemical impedance spectra for differentconcentrations of Eu3+, (a and b) represents, respectively, the wholespectra of MEHPPV and Azo-C[4]-MEHPPV membranes.

34 A. Rouis et al. / Journal of Electroanalytical Chemistry 601 (2007) 29–38

additions of Eu3+ cations. The whole impedance spectra ofMEHPPV and Azo-C[4]-MEHPPV membranes are repre-sented in Fig. 8. Complex plane plots show a slight shiftin impedance values (Fig. 8(a)) for ITO/MEHPPV struc-ture in presence of Eu3+ cations in electrolytic solution.Therefore, a significant variation is observed for MEHPPVmembrane doped with calixarene in the same conditions(Fig. 8(b)).

The fitted parameters of the equivalent circuit are pre-sented in Table 3. Diffusion time sD for ITO/Azo-C[4]-MEHPPV sensor is higher than corresponding value forITO/MEHPPV sensor, as seen in Table 1. It can be influ-enced by the porosity and the thickness of the compositemembrane [19]. After addition of Eu3+ cations in electro-

lytic solution, sD parameter decreases and becomes con-stant. The diffusion time constant is so limited.

The Rm and Rtc versus the detected ion concentration ofEu3+ for the two types of membranes are shown in Fig. 9.We observe that the bulk resistance (Rm) of the polymermatrix is not affected by the addition of Eu3+ cations.Therefore, the bulk resistance of MEHPPV membranedoped with Azo-C[4] molecules responds slowly versusp[Eu3+] (Fig. 9(a)).

On another hand, the Rtc fitted values decrease withincreasing detected Eu3+ concentrations for the two typesof interfaces. The Rtc slope was higher for the calixarenedoped polymer membrane than for the undoped one onaccount of the complexation of Eu3+ cations (Fig. 9(b)).Different results of Rm and Rtc slopes are illustrated inTable 4. Therefore, the incorporation of Azo-calix[4]arenemolecules into the ECP matrix improves the charge trans-fer across the membrane.

3.6. Effect of a light excitation

The light absorption of a semiconductor is controlled byits optical band gap (Eg). Creation of electron hole pairsgenerally result from the absorption of photons with ener-gies larger than the band gap. Our samples have beenilluminated with a blue laser diode (kexci � 430 nm,Eexci � 2.8 eV). The ITO and the solution are transparentto the excitation beam due to their large band gap, whereasfree carriers are created upon the transition of electrons inthe excited state in the MEHPPV polymer (Eg = 2.4 eV)and Azo-calixarene molecules (Eg = 2.34 eV) upon theexcitation with 2.8 eV photons. The intrinsic conductivitycan then be increased for both semiconducting componentsby the light induced charge carriers. Complementary infor-mation about the detection mechanism are expected fromthe perturbation introduced by this excitation on the con-ductivity and trapping of ionic species.

Table 3Electrochemical parameters of MEHPPV and Azo-C[4]-MEHPPV membranes versus Eu3+ concentration

Membrane p[Eu3+] Rm (kX) CPE1 (nF) sD (s) Rtc (kX) CPE2 (lF) v2 (10�4)

MEHPPV 10 11.3 ± 6.4 1577 ± 31 0.0044 ± 0.0006 715.0 ± 9.2 0.97 ± 0.02 5.38.68 11.6 ± 6.7 1558 ± 29 0.0037 ± 0.0005 707.3 ± 6.3 0.99 ± 0.01 4.77.66 11.5 ± 6.9 1561 ± 30 0.0032 ± 0.0003 684.2 ± 5.4 1.02 ± 0.01 4.86.92 10.9 ± 6.2 1558 ± 30 0.0030 ± 0.0004 662.9 ± 5.1 1.03 ± 0.01 4.56.29 10.7 ± 6.5 1540 ± 27 0.0029 ± 0.0004 657.7 ± 4.8 1.05 ± 0.01 4.1

Azo-C[4]-MEHPPV 9 31.8 ± 1.0 1386 ± 49 1.24 ± 0.12 724.1 ± 29.6 0.77 ± 0.02 2.68 26.9 ± 0.6 1271 ± 40 1.21 ± 0.06 669.3 ± 22.7 0.85 ± 0.02 2.67.52 23.3 ± 0.5 1212 ± 38 1.00 ± 0.04 571.6 ± 18.2 0.97 ± 0.02 2.36.88 16.7 ± 0.7 942 ± 65 0.93 ± 0.09 512.1 ± 29.1 1.18 ± 0.06 276.48 14.9 ± 0.7 892 ± 64 0.93 ± 0.08 483.2 ± 26.5 1.26 ± 0.06 275.63 14.8 ± 0.6 865 ± 61 0.92 ± 0.07 433.4 ± 24.2 1.38 ± 0.07 28

MEHPPV under light excitation 9.7 16.5 ± 0.1 23.8 ± 0.7 0.75 ± 0.06 304.2 ± 4.8 2.74 ± 0.07 3.38.85 16.0 ± 0.1 25.1 ± 0.8 0.76 ± 0.05 301.4 ± 4.8 2.80 ± 0.07 3.28.27 15.7 ± 0.1 25.5 ± 0.8 0.78 ± 0.05 294.8 ± 4.7 2.88 ± 0.07 3.37.67 15.6 ± 0.1 27.1 ± 0.8 0.72 ± 0.04 288.0 ± 4.0 2.97 ± 0.07 3.66.86 15.3 ± 0.1 27.3 ± 0.9 0.77 ± 0.05 290.1 ± 4.3 3.01 ± 0.07 3.3

Azo-C[4]-MEHPPV under light excitation 10 11.3 ± 1.1 16.8 ± 1.3 0.41 ± 0.02 697.0 ± 22.3 0.53 ± 0.03 2.59.58 9.9 ± 1.5 17.0 ± 1.4 0.58 ± 0.09 690.6 ± 31.0 0.55 ± 0.03 3.89.18 9.2 ± 1.0 13.7 ± 1.0 0.54 ± 0.07 603.6 ± 21.1 0.63 ± 0.02 2.58.78 8.2 ± 1.0 12.9 ± 1.0 0.83 ± 0.20 592.0 ± 19.5 0.71 ± 0.02 2.88.25 7.3 ± 0.9 12.5 ± 0.9 1.00 ± 0.25 544.4 ± 14.7 0.78 ± 0.01 2.77.59 6.9 ± 0.8 12.4 ± 0.7 1.53 ± 0.45 524.3 ± 11.5 0.88 ± 0.01 2.46.84 4.9 ± 0.6 10.9 ± 0.5 1.53 ± 0.58 512.9 ± 8.2 0.92 ± 0.01 1.4

5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

10

15

20

25

30

35

Rm

2 (KΩ

cm-2)

Rm1 (ITO /Azo-C[4]-M EH PPV )

p [Eu 3+]

Rm

1 (K

Ω c

m-2

)

10

15

20

25

30

35Rm2 (ITO /M EH PPV )

5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0400

450

500

550

600

650

700

750

800

p [Eu3+]

Rtc

(1.

2)(K

Ω c

m-2)

R(tc)1 (ITO/Azo-C[4]-M EHPP V)

R(tc)2 (ITO/MEHPPV)

Fig. 9. (a) Rm plots versus p[Eu3+] of MEHPPV and Azo-C[4]-MEHPPVmembranes, (b) Rtc plots versus p[Eu3+] of MEHPPV and Azo-C[4]-MEHPPV membranes.

Table 4Comparisons of Rm and Rtc slope values of ITO/MEHPPV and ITO/Azo-C[4]-MEHPPV membranes with and without excitation, for the detectionof Eu3+ cations

Membranes Rm (kX) Rtc (kX)

ITO/MEHPPV (without light excitation) �0 2716.9ITO/Azo-C[4]-MEHPPV (without light excitation) 845.1 12260.6ITO/MEHPPV (with light excitation) 67.2 1079ITO/Azo-C[4]-MEHPPV (with light excitation) 232.3 9637.1

Electroactive electrode area was 0.125 cm2.

A. Rouis et al. / Journal of Electroanalytical Chemistry 601 (2007) 29–38 35

3.6.1. Cu2+ sensitivity

Previous experimental conditions are conserved for thesample elaboration. The electrochemical impedance mea-surements shown in Section 3.5 have been carried underthe illumination of the front face of the functionalizedITO sample as work electrode.

Fig. 10(a) and (b) shows Nyquist impedance plots ofITO/MEHPPV and ITO/Azo-C[4]-MEHPPV samples,respectively, under blue light excitation. The effect of calix-arene molecules is evidenced by the variation of the imped-ance plot upon Cu2+addition. In fact, Nyquist spectra ofdoped polymer membrane decreases when increasing thecopper concentration. In contrast to the previous behav-iour and to the MEHPPV membrane behaviour withoutexcitation, no changes of the impedance spectrum uponCu2+ addition is observed for the undoped membraneunder illumination. All spectra are modelled by the equiv-alent circuit in Fig. 5 and the fitted parameters are pre-sented in Table 1. The diffusion time values (sD) now

0 50 100 150 200 250 3000

50

100

150

200

250

300

0 50 100 150 200 250 3000

50

100

150

200

250

300

-Zi (K

Ωcm

-2)

Zr (KΩcm-2)

p[Cu2+]= 9.7p[Cu2+]= 8.74

p[Cu2+]= 7.93p[Cu2+]= 6.7

p[Cu2+]= 6.22

p[Cu2+]= 5.33

(a)

-Zi (K

Ω c

m-2)

Zr (KΩ cm-2)

p[Cu2+]= 10

p[Cu2+]= 9.52

p[Cu2+]= 9.04

p[Cu2+]= 8.72

p[Cu2+]= 8.48

p[Cu2+]= 8.27

p[Cu2+]= 7.91

p[Cu2+]= 7.55

p[Cu2+]= 7.05

p[Cu2+]= 6.16

p[Cu2+]= 5.17

(b)

Fig. 10. Complex plane electrochemical impedance spectra for differentconcentrations of Cu2+ under blue light illumination, (a and c) represents,respectively, the whole spectra of MEHPPV and Azo-C[4]-MEHPPVmembranes. The inset (curves b and d) represents, respectively, the highfrequency part of MEHPPV and Azo-C[4]-MEHPPV membranes.

6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.56.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

10.5

p [Cu2+]

Rm

(1.2

)(K

Ω c

m-2)

Rm1 (ITO/Azo-C[4]-MEHPPV)

Rm2 (ITO/MEHPPV)

6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5

150

200

250

300

Rtc

(1.

2)(K

Ω c

m-2

)

p [Cu2+]

R(tc)1 (ITO/Azo-C[4]-MEHPPV)

R(tc)2 (ITO/MEHPPV)

Fig. 11. (a) Rm plots versus p[Cu2+] of MEHPPV and Azo-C[4]-MEHPPVmembranes under blue light excitation, (b) Rtc plots versus p[Cu2+]of MEHPPV and Azo-C[4]-MEHPPV membranes under blue lightexcitation.

36 A. Rouis et al. / Journal of Electroanalytical Chemistry 601 (2007) 29–38

increase slowly, upon the addition of Cu2+, in the two cases(MEHPPV and Azo-C[4]-MEHPPV membranes).

The evolution of fitted values of Rm and Rtc versus thedetected ion concentration of copper(II) for ITO/MEHPPV and ITO/Azo-C[4]-MEHPPV under light exci-tation is represented in Fig. 11(a) and (b). We notice, inFig. 11(a), a decrease of Rm slopes for both types of illumi-nated samples in comparison with their corresponding val-ues without excitation as a consequence of the free carrierscreated under light excitation. Therefore, the slopes ofcurves illustrated in Table 2 are similar indicating thatthe introduction of Azo-calix[4]arene molecules has noeffect on the bulk conductivity which is controlled underillumination by the polymer.

In the previous part, we have demonstrated that theintroduction of calixarene molecules into polymeric matriximproves the charge transfer and the same behaviour isobserved under illumination. The variation of (Rtc) values,represented in Fig. 11(b), of MEHPPV and dopedMEHPPV membranes upon addition of Cu2+ areimproved by the presence of calixarene molecules in thepolymeric matrix. The charge transfer resistance is loweredunder illumination in comparison with the correspondingRtc value of the nonexcited samples. In fact, curve (b)shows an effect of charge neutralization, lowering iontransfers into the polymer membrane. The Rtc slope ofdoped MEHPPV membrane under illumination is approx-imately the difference of Rtc slopes of MEHPPV and dopedMEHPPV membranes without excitation. It can beconcluded that MEHPPV polymer shows the main imped-imetric response which is not influenced by the presence ofAzo-calixarenes under light excitation. The complexationof copper cations by the Azo-calixarene molecules can thenbe isolated under light excitation from the polymer contri-bution which is then cancelled (Table 2). In fact, the chargetransfer in the MEHPPV matrix is limited by diffusion [38].

3.6.2. Eu3+ sensitivity

Likewise, we studied the ITO/MEHPPV and ITO/Azo-C[4]-MEHPPV sensors versus additions of Eu3+ cationsunder illumination. The impedance spectra are presentedin Fig. 12(a) and (b) and the resulting parameters of theequivalent circuit in Table 3. The evolution of the fitted val-ues of Rm and Rtc versus the detected ion concentration ofEu3+ under illumination is given in Table 4 for the twotypes of membranes.

Fig. 12 shows a decrease of the impedance upon theaddition of Eu3+ cations resulting from the complexationof europium by calixarenes into the MEHPPV membrane.The bulk resistance (Rm) slope of Azo-C[4]-MEHPPVmembrane (Fig. 13(a)) decreases under illumination asalready observed upon Cu2+ complexation. Thus, the Rtc

slope of MEHPPV membranes involving Eu3+ complexedcalixarenes is smaller than the corresponding value of thesample without excitation. The Rtc slope value of dopedMEHPPV membrane under illumination is approximatelythe difference of Rtc slope values of MEHPPV and dopedMEHPPV membranes without excitation. This observationconfirms the behaviour observed upon Cu2+ complexation.Slope values of Rm and Rtc are illustrated in Table 4. We

0 200 400 600 800 1000 12000

200

400

600

800

1000

1200

0 100 200 300 4000

100

200

300

400

-Zi (

KΩ

cm

-2)

Zr (KΩ cm-2)

p[Eu3+]=9.7p[Eu3+]=8.85

p[Eu3+]=8.27

p[Eu3+]=7.67

p[Eu3+]=7.21

(a)

(b)

-Zi (K

Ω c

m-2)

Zr (KΩ cm-2)

p[Eu3+]= 10p[Eu3+]= 9.58p[Eu3+]= 9.19p[Eu3+]= 8.79p[Eu3+]= 8.25p[Eu3+]= 7.59p[Eu3+]= 6.84p[Eu3+]= 6.26p[Eu3+]= 5.76p[Eu3+]= 5.25

Fig. 12. Complex plane electrochemical impedance spectra for differentconcentrations of Eu3+, (a and b) represents, respectively, the wholespectra of MEHPPV and Azo-C[4]-MEHPPV membranes under opticalexcitation.

6 8 10

4

6

8

10

12 R m1 (ITO/Azo-C[4]-MEHPPV)

Rm

2 (KΩ

cm-2)

p [Eu3+]

Rm

1 (K

Ω c

m-2

)

14

16

18

20

22R m2 (ITO/MEHPPV)

6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5

450

500

550

600

650

700

R(tc)2 (K

Ω cm

-2)

R(tc)1 (ITO/Azo-C[4]-MEHPPV)

p [Eu3+]

R(t

c)1 (

KΩ

cm

-2)

250

300

350

400

450

500R(tc)2 (ITO/MEHPPV)

Fig. 13. (a) Rm plots versus p[Eu3+] of MEHPPV and Azo-C[4]-MEHPPVmembranes under blue light excitation, (b) Rtc plots versus p[Eu3+] ofMEHPPV and Azo-C[4]-MEHPPV membranes under blue lightexcitation.

A. Rouis et al. / Journal of Electroanalytical Chemistry 601 (2007) 29–38 37

can conclude that the illumination of doped samples has noeffect on the charge transfer into the composite membrane.

The effect of polymer matrix (MEHPPV) is neutralized andAzo-calixarene molecules responded normally under lightexcitation.

4. Conclusion

We have shown that the immobilization of Azo-calix[4]-arenes into an electroconducting polymer layer on ITOsubstrate is a solution to overcome the poor adhesion ofthese molecules to the ITO surface preserving at the sametime their recognition and complexing properties. Theselective properties of the modified ITO electrode havebeen studied by electrochemical impedance spectroscopy(EIS) of MEHPPV and Azo-C[4]-MEHPPV films depos-ited by spin coating. Charge transfer and diffusion of cop-per and europium cations across these nanocompositemembrane have been characterised and modelled by anequivalent electrical circuit. Selectivity towards Cu2+ andEu3+ is found to be reflected by the dependency of the bulkresistance of the membrane (Rm) and the charge transfer(Rtc) at the membrane/ITO interface upon the additionof cations into the electrolyte solution. The comparativestudy of MEHPPV and Azo-C[4]-MEHPPV membraneshas shown that the conjugated polymer and the host mol-ecules are both active in complexing metallic cations.Under blue light excitation, the affinity versus cations ofthe polymer matrix (MEHPPV) is neutralized by the gener-ated charge carriers and the calixarene sensitivity versuscation concentration can be isolated. The attractive poten-tialities of such nanocomposite membranes in complexingcations will be exploited to develop chemical sensors exhib-iting simultaneously an electrical and optical response. Thestudy of the optical detection is reported in previous papers[39,40] and will be developed for the realization of optodes.

Acknowledgement

Mrs. Nabiha BCHIR is gratefully acknowledged for herattribution in the correction of the English.

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