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Electrochemical properties of gold electrodes functionalized by new pseudo-polyrotaxanes of polyaniline and chemically modified β-cyclodextrin inclusion complex
Synthetic Metals 162 (2012) 186–192
Contents lists available at SciVerse ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Electrochemical properties of gold electrodes functionalized by new pseudo-polyrotaxanes of polyaniline and chemically modified ˇ-cyclodextrin inclusion complex R. Mlika a,∗ , S. Hbaieb b , R. Ben Chaabene a , Y. Chevalier c , R. Kalfat b , H. Ben Ouada a a
Laboratoire de Physique et Chimie des Interfaces, Faculté des Sciences de Monastir, Université de Monastir, 5019 Monastir, Tunisia Laboratoire de Méthodes et Techniques d’Analyses, Institut National de Recherche et d’Analyse Physico-chimique, 2020 Sidi Thabet, Tunisia c Laboratoire d’Automatique et de Génie des Procédés (LAGEP), CNRS UMR 5007, Université Claude Bernard Lyon 1, F-69622 Villeurbanne, France b
a r t i c l e
i n f o
Article history: Received 25 May 2011 Received in revised form 26 November 2011 Accepted 28 November 2011 Available online 20 December 2011 Keywords: Pseudo-polyrotaxanes Thermal evaporation Gold electrodes Electrochemical impedance spectroscopy (EIS)
a b s t r a c t This work explored the development of impedimetric sensors based on conducting polyaniline (PANI) and PANI-ˇ-CDpNH2 thin films, formed by the complex inclusion of polyaniline with the modified ˇcyclodextrin with amino groups. Thin films about 100 nm were deposited by thermal evaporation on gold electrodes. These thin films were characterized by the wettability technique for hydrophobicity analysis. The electrical properties of these polymer films have been studied by electrochemical impedance spectroscopy (EIS) technique. This study has revealed, according to an equivalent circuit fitting the experimental parameters, that PANI-ˇ-CDpNH2 film is more conductive than PANI one. This result was explained by the PANI conformation changes from coiled and disorderly chain to rodlike one coated in the cavities of ˇ-CD. Finally we have studied the electrochemical response of these electrodes towards Pb2+ cations in the aim to probe the contribution of PANI inclusion with ˇ-CDpNH2 molecules to the enhancement of the sensitivity and recognition properties of these later. We have found that PANI-ˇ-CDpNH2 film exhibits a good sensitivity towards the Pb2+ cations. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Conducting polymers have been attracted great attention in the last two decades with regard to scientific study and technological applications. Previous works have discussed the possibility of use of these polymers as potential materials for sensors devices [1–3]. These conducting polymer based sensors have the advantage to exhibit improved response properties and are sensitive to small perturbations due to their electrical conductivity or charge transport properties [4]. Among these conducting polymers, polyaniline has emerged as a promising candidate for chemical sensors [5,6] due to its chemical stability, good electrical, optical and electrochemical properties [7]. Recently, the association of conduction polymer with macrocyclic compounds has opened new perspectives in the electrochemistry conductive polymers domain. Cyclodextrin (CD) is one of these macrocyclic compounds, and was considered as an ideal
∗ Corresponding author. E-mail address: [email protected] (R. Mlika). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.11.029
candidate for this pursuit due to its well defined cavity, large enough to serve as an anchoring site for polymer molecule [8]. Lee et al. have investigated the electrochemical properties of polymer films of gold electrodes modified with thiolated ˇcyclodextrin [9]. Yang et al. have developed an electrochemical sensor for cinchonine (CNN) using ˇ-cyclodextrin modified poly (N-acetylaniline) [10]. Subramanian et al. have studied the influence of the ˇcyclodextrin as an encapsule and an inclusion complex dopant on conducting polyaniline [11]. Kozlowski et al. have used modified cyclodextrin polymers as selective ion carriers for Pb2+ separation across plasticized thin films [12]. In 2010, the electrochemical deposition of polyaniline in the presence of ˇ-cyclodextrin (ˇ-CD) or sulfated ˇ-cyclodextrin (ˇSCD) has been reported by Rokovic´ et al. [13]. The morphology and electrochemical properties of the resulting layer have been studied. In this work, we report on the development of impedimetric sensors based on new pseudo-polyrotaxanes film formed by the inclusion of polyaniline with modified ˇ-CDpNH2 molecules. The association of conducting polymer with ˇ-cyclodextrin molecules permits to enclose a new compound presenting two essential
R. Mlika et al. / Synthetic Metals 162 (2012) 186–192
187
(NH2 .HCl)7
(NH2 .HCl)7
HCl 1M
NH2 +
NH2.HCl
(OH)7
(OH)7
(OH)
(OH)
(NH2 .HCl)7
+.
NH
NH _ Cl
(NH4)2 S2O8
+.
NH
NH _ Cl
n
x
Scheme 1. Schematic diagram of pseudo-polyrotaxanes. Synthesis route.
aspects, at the same time, conduction and recognition. As result, this association can lead to better sensors performance. These pseudo-polyrotaxanes were deposited by thermal evaporation on gold electrodes and then characterized by the wettability technique for hydrophobicity analysis. After, these electrodes were analyzed by electrochemical impedance spectroscopy (EIS) technique to study the electrical properties of the deposited films at various dc potential. The EIS technique was applied also to study the recognition properties of these pseudo-polyrotaxanes towards Pb2+ cations. Finally, the impedance behavior of these electrodes was modeled by an equivalent circuit to investigate the processes taking place at the different interfaces.
2.3. Elaboration of thin films by evaporation The immobilization of the receptor on the gold surface was done by evaporation technique under a vacuum of 132 × 10−11 bar. This technique permits the measurement of the deposited thin film thickness by using a vibrating piezoelectric quartz crystal microbalance. The polyrotaxanes powder was introduced into a cylindrical oven heated by an electrical filament. The evaporation temperature was maintained at about 150 ◦ C [16] with an evaporation rate of 0.5 nm/s. The measured deposited film thickness was about 100 nm carried out using dektak-3010 surface profilometer. 2.4. Contact angle measurements
2. Experimental 2.1. Pseudo-polyrotaxanes The pseudo-polyrotaxanes (Scheme 1) were synthesized by chemical polymerization in the Laboratoire de Méthodes et Techniques d’Analyses, Institut National de Recherche et d’Analyze Physico-chimique, Tunisia. The synthesis route, the FTIR and RMN results are discussed in our previous work [14].
Contact angle measurements were performed with a model contact instrument (Digidrop) form GBX (Romans, France) [17] in order to verify the non degradation of the deposited thin film after thermal evaporation. These measurements were performed by applying 4 l of deionized water on the polyrotaxanes thin film surface. The image of the water droplet behavior on the surface was then acquired with a digital camera and analyzed. 2.5. Impedance spectroscopy measurements
2.2. Gold electrodes substrates Gold substrates are purchased from Centre de Recherche LAAS (France) and cut into 1 cm × 1 cm square solids. The layer gold thickness is about 50 nm. In electrochemical experiments, the quality of the active surface will affect the measurements. Also, the current peak in cyclic voltammetry and the frequency response during electrochemical impedance spectroscopy will be dependent on the surface composition of the gold. For this reason, the gold substrate must be cleaned in piranha solution for about 5 min in order to activate the surface [15] and then rinsed with ultra pure water.
Electrochemical impedance spectroscopy (EIS) technique is an effective tool for the qualitative and quantitative characterization of electrochemical processes occurring in the conducting polymer films [18], and is able to study the interfacial properties of modified electrode. In the present work, impedance spectroscopy measurements were carried out at room temperature by using an impedance analyzer “Autolab PGSTAT” supported by Frequency Response Analysis System software (FRA2) controlled by computer. A sinusoidal excitation signal with amplitude of 10 mV was applied and the impedance spectra were recorded in the frequency range [0.05–100] kHz.
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Fig. 1. The contact angle () value with water as a liquid probe, before and after membrane deposition.
For the electrochemical measurements, a three electrode system was used: a working electrode (gold), a platinum counter electrode and a saturated calomel reference electrode (SCE). An aqueous ammonium acetate (CH3 COO− , NH4 + ) solution of 0.1 M, with pH = 7, was used as electrolyte.
Scheme 2. Schematic representation of pseudo-polyrotaxanes orientation at the gold surface.
Table 2 Comparison of contact angle measurements of PANI, PANI-ˇ-CD and PANI-ˇCDpNH2 thin films on gold and quartz substrate. Film
(◦ ) (Gold)
(◦ ) (Quartz)
PANI PANI-ˇ-CD PANI-ˇ-CDpNH2
80 93 74
81 94 73
3. Results and discussions
3.2. Electrochemical impedance spectroscopy (EIS) measurements
3.1. Hydrophobicity study
3.2.1. Polarization optimization Generally, the first step for the impedance studies is the determination of the polarization domain which depends on the thin film type. For our study, different negative potentials (E = −0.7, −0.8, −0.9 and −1 V) were applied in a large frequency range. Under E = −1 V over voltage versus SCE, an obvious decrease of the Warburg straight line and the half circle diameter was observed for the gold electrodes based on PANI and PANI-ˇ-CDpNH2 thin films (Figs. 3 and 4). This decrease can be explained by the reduction of the charge transfer resistance as a function the dc potential as shown in Fig. 5. This reduction renders possible the observation of the kinetics of cations at the thin film/solution interface.
In order to check the orientation of the pseudo-polyrotaxanes at the gold surface [19], a hydrophobicity study was carried out before and after thin film deposition, with water as a liquid probe. We have found that the contact angle value () increases from 70◦ , for the gold substrate, to 80◦ and 93◦ for the PANI and PANI-ˇcyclodextrin thin films, respectively (Fig. 1 and Table 1). The high value determined in the case of the PANI-ˇ-cyclodextrin thin film, proved the hydrophobic character due to the presence of the inner cavity of the ˇ-cyclodextrin molecule [20]. As a consequence, we can presume that the pseudo-polyrotaxanes were turned up at the gold surface (Scheme 2). The contact angle value ( = 74◦ ), found in the case of the PANIˇ-CDpNH2 thin film, shows the hydrophilic character of this film due the presence of the hydrophilic grafted NH2 groups [21]. On the other hand, we ran this wettability technique to verify the non-degradation of pseudo-polyrotaxanes after evaporation process. For this reason, we have deposited, during the polymerization route, PANI, PANI-ˇ-CD and PANI-ˇ-CDpNH2 thin films on quartz substrate and we have determined the contact angle values (Table 2), which were found to be very close to those determined for the evaporated thin films case (Fig. 2).
Table 1 Contact angle measurements of gold bare electrode, PANI, PANI -(-CD and PANI-ˇCDpNH2 thin films. Electrode
(◦ )
Gold Gold/PANI Gold/PANI-ˇ-CD Gold/PANI-ˇ-CDpNH2
70 80 93 74
Fig. 2. Comparison of the contact angle () value, with water as a liquid probe, of Pani and pseudo-polyrotaxanes membranes deposited on quartz and gold substrates.
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189
Fig. 3. Optimization of the polarization potential of the gold electrode based on evaporated PANI thin film.
Fig. 6. Nyquist and Bode plots of the electrode based on PANI-ˇ-CDpNH2 thin film.
3.2.2. Equivalent circuit modeling Fig. 6 reveals that phase plot presents two phase angle maxima. Consequently, the Nyquist plot is analyzed as a combination of two closely interacting semicircles, which can indicate the presence of other components in the equivalent electrical circuit model [22]. The larger one is attributed to the thin film bulk at high frequency range, which could be modeled to RC component. This later is formed by an electrical dipole based on parallel (Rf , CPEf ), where: • Rf is the total resistance of the thin film; • CPEf is the capacitance generated by the imbalance within the film.
Fig. 4. Optimization of the polarization potential of the gold electrode based on evaporated PANI-ˇ-CDpNH2 thin film.
The smaller second semicircle, corresponding to the lowfrequency range, is related to the gold/electrolyte interface and could be modeled to another RC formed by an electrical dipole based on parallel composed by a charge transfer resistance (Rct ) and Constant Phase Element (CPE1) (Fig. 7). The CPE impedance is given by the following relation [23]: Z=
1 Q (jω)
n
(1)
where: - Q is a frequency independent term giving information about the homogeneity, roughness and surface porosity;
Fig. 5. The variation of the charge transfer resistance as a function the dc potential for the PANI and PANI-ˇ-CDpNH2 thin films.
Fig. 7. Equivalent circuit used to fit the impedance spectra, Rf : bulk resistance of the thin film, CPEf and CPEdl : Constant Phase Elements, Rct : charge transfer resistance, Re : resistance of the electrolyte.
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Scheme 3. Schematic diagram of polyaniline coiled chain (a) and polymer chain coated in the into the ˇ-CD cavities (b).
leads to an increase of the film conductivity. This increase is due to the PANI chain conformation changes from coiled and disorderly chain to rodlike one coated in the cavities of ˇ-CDpNH2 [24,25] (Scheme 3). Comparing the charge transfer resistance (Rct ) value of PANI and PANI-ˇ-CDpNH2 films, it is apparent that Rct (PANI-ˇCDpNH2 ) < Rct (PANI). This result indicates that the charge transfer process is better demonstrated after the inclusion of the PANI in the cavities of modified ˇ-CDpNH2 , which can be explained by the PANI chain stretching process [26]. Also, we can remark that the capacitive element CPEf value of the PANI film is larger than the PANI-ˇ-CDpNH2 one due to a larger permittivity (εr ) of the PANI film [27].
Fig. 8. Comparison of the Nyquist diagrams of bare gold electrode, PANI and PANIˇ-CDpNH2 thin films at E = −1 V/SCE potential of polarization.
- n is a correction factor (0 < n < 1). 3.2.3. Electrical properties of PANI and PANI-ˇ-CD thin films Fig. 8 shows the Nyquist plots, at the same polarization potential (E = −1 V), of the PANI and PANI-ˇ-CDpNH2 thin films on gold electrode. We can deduce that the impedance increases after evaporation process, which can be attributed to the deposited thin film. The fit of the electrochemical impedance spectra was done, using the FRA2 software, to the equivalent circuit shown in Fig. 7. We have determined the Rf and CPEf electrical parameters values for PANI and PANI-ˇ-CDpNH2 films (Table 3). From this table, we can note that the Rf resistance value for the PANI thin film is larger than the PANI-ˇ-CDpNH2 one, which reveals a lower resistivity since Rf is inversely proportional to the film conductivity according to the following relation [17]: Rf =
d S
(2)
where d is the film thickness and S its active area. So we can conclude that the PANI-ˇ-CDpNH2 film presents a higher conductivity than PANI film, which can be explained by the fact that the inclusion of the PANI in the cavities of ˇ-CDpNH2
3.2.4. Sensitivity of PANI and PANI-ˇ-CDpNH2 thin films towards Pb2+ cations The sensing properties of benzylated ˇ-CD films towards Pb2+ in aqueous solution for sensor application have been tested in our previously work [28] and have shown a good sensitivity towards these cations. For this reason, we have to study the sensing properties of PANI and PANI-ˇ-CDpNH2 thin film towards these same cations. The cations kinetics at the thin film/solution interface was investigated by impedance spectroscopy for the two thin films towards Pb2+ cations (Figs. 9 and 10). From these figures, we can remark that there is an enhancement of the impedance with the Pb2+ concentration. This increase is far more pronounced for the PANI-ˇ-CDpNH2 thin film. The circuit parameters of the PANI and PANI-ˇ-CDpNH2 thin films, deduced from the fit, to the previous equivalent circuit were presented in Tables 4 and 5. Fig. 11 shows the variation of the bulk resistance Rf as function of Pb2+ concentration cologarithm (p[Pb2+ ]). We observe that Rf for the PANI thin film is not affected by the addition of Pb2+ cations which can be explained by the hundreds of some kind of disorder in PANI chain. Furthermore, Rf for thin PANI-ˇ-CDpNH2 film increases versus [Pb2+ ] concentration indicating a high activity and mobility of these cations within the film due to the presence of ˇ-CDpNH2 molecules enable to encage these cations (Scheme 4). The charge transfer resistance Rct dependence on cologarithm of [Pb2+ ] cation concentration is shown in Fig. 12. We can note that the Rct of PANI-ˇ-CDpNH2 film is not affected by the addition of the Pb2+ cations. However, a significant increase of Rct was observed for
Table 3 Electrical fit parameters values of gold bare electrode, PANI-ˇ-CDpNH2 and PANI thin films. CPE
Gold Gold/PANI Gold/PANI-ˇ-CDpNH2
Rct (k)
Q (F m−2 S−(1−n) )
n
6.95 5.33 17.4
0.81 0.82 0.78
2.57 6.00 3.60
Rf (k)
7.22 3.71
CPEf Q (F m−2 S−(1−n) )
n
6.75 5.20
0.92 0.95
R. Mlika et al. / Synthetic Metals 162 (2012) 186–192
Fig. 9. Nyquist diagrams for different Pb2+ concentrations: of the gold electrode based on PANI thin film.
191
Fig. 11. Variation of the bulk resistance (Rf ) vs. p[Pb2+ ] of PANI and PANI-ˇ-CDpNH2 thin films.
Scheme 4. Synoptic schematic diagram of Pb2+ cations recognition by pseudopolyrotaxanes.
Fig. 10. Nyquist diagrams for different Pb2+ concentrations: of the gold electrode based on PANI-ˇ-CDpNH2 thin film. Table 4 Fitting data of gold electrode based on PANI thin film for different concentrations of Pb2+ cations. The active surface (S) was about 0.3 cm2 . p[Pb2+ ] 7.69 6.7 5.5 4.5 3.7 2.7 1.7 205 219 224 238 240 244 241 Re () 3.23 3.41 3.30 3.00 3.09 3.30 3.57 Rf (k) CPEf Q(F m−2 S−(1−n) ) 3.6 3.4 3.6 3.6 3.6 3.4 3.4 0.9 0.89 0.88 0.94 0.93 0.98 0.94 n 1.1 1.36 1.69 1.78 2.00 2.20 2.20 Rct (k) CPE −2 −(1−n) ) 3.9 3.6 4.4 5.1 6.5 6.7 7 Q (F m S 0.75 0.67 0.64 0.6 0.63 0.63 n 0.7 2 (×10−3 ) 7.1 7.5 4.1 5.3 7.2 6.3 8.1
the PANI film with increasing Pb2+ concentration. This result suggests that the electron transfer to the interface electrode solution is slowed down due to the spatial hindrance of Pb2+ cations. Fig. 13 shows the variation of thin film Constant Phase Element (CPEf ) as function of cologarithm of Pb2+ concentration. As shown, for the two PANI-ˇ-CDpNH2 and PANI thin films, the CPEf value is not affected by the increase of the Pb2+ concentration.
Table 5 Fitting data of gold electrode based on PANI-ˇ-CD pNH2 thin film for different concentrations of Pb2+ cations. The active surface (S) was about 0.3 cm2 . p[Pb2+ ] 7.69 6.7 5.5 4.5 3.7 2.7 1.7 Re () 200 207 206 195 213 202 200 4.51 5.22 5.61 6.08 6.41 7.48 8.00 Rf (k) CPEf Q (F m−2 S−(1−n) ) 2.6 2.6 2.6 2.7 2.7 2.4 2.4 0.89 0.88 0.87 0.85 0.86 0.87 0.88 n 1.00 1.10 1.28 1.30 1.30 1.20 1.20 Rct (k) CPE 1.00 1.31 1.20 1.17 1.11 1.19 2.00 Q (F m−2 S−(1−n) ) 0.8 0.87 0.88 0.85 0.84 0.88 0.86 n 8.1 6.5 7.3 7.2 4.3 8.2 7.1 2 (×10−3 )
Fig. 12. Variation of the charge transfer resistance (Rct ) vs. p[Pb2+ ] of PANI and PANIˇ-CDpNH2 thin films.
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by rodlike chain and a cavity diameter of about 7.8 A˚ which is large enough to serve as an anchoring site for cations. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] Fig. 13. Variation of the Constant Phase Element (CPEf ) vs. p[Pb2+ ] of PANI and PANIˇ-CDpNH2 thin films.
[15] [16]
4. Conclusion In this work, PANI and PANI-ˇ-CDpNH2 thin films have been deposited by thermal evaporation process on gold surface. The hydrophobicity study, by wettability technique, has proved the hydrophilic character of the PANI-ˇ-CDpNH2 film due the presence of the hydrophilic grafted NH2 groups. The impedance spectroscopy measurements have revealed an increase of the conductivity of the PANI-ˇ-CDpNH2 thin film due to the PANI conformation changes from coiled and disorderly chain to rodlike one coated in the cavities of ˇ-CD. The sensitivity towards Pb2+ cations was investigated by impedance spectroscopy for the two thin films. We have found that the recognition has improved for PANI-ˇ-CDpNH2 thin film formed
[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
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Contents lists available at SciVerse ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Electrochemical properties of gold electrodes functionalized by new pseudo-polyrotaxanes of polyaniline and chemically modified ˇ-cyclodextrin inclusion complex R. Mlika a,∗ , S. Hbaieb b , R. Ben Chaabene a , Y. Chevalier c , R. Kalfat b , H. Ben Ouada a a
Laboratoire de Physique et Chimie des Interfaces, Faculté des Sciences de Monastir, Université de Monastir, 5019 Monastir, Tunisia Laboratoire de Méthodes et Techniques d’Analyses, Institut National de Recherche et d’Analyse Physico-chimique, 2020 Sidi Thabet, Tunisia c Laboratoire d’Automatique et de Génie des Procédés (LAGEP), CNRS UMR 5007, Université Claude Bernard Lyon 1, F-69622 Villeurbanne, France b
a r t i c l e
i n f o
Article history: Received 25 May 2011 Received in revised form 26 November 2011 Accepted 28 November 2011 Available online 20 December 2011 Keywords: Pseudo-polyrotaxanes Thermal evaporation Gold electrodes Electrochemical impedance spectroscopy (EIS)
a b s t r a c t This work explored the development of impedimetric sensors based on conducting polyaniline (PANI) and PANI-ˇ-CDpNH2 thin films, formed by the complex inclusion of polyaniline with the modified ˇcyclodextrin with amino groups. Thin films about 100 nm were deposited by thermal evaporation on gold electrodes. These thin films were characterized by the wettability technique for hydrophobicity analysis. The electrical properties of these polymer films have been studied by electrochemical impedance spectroscopy (EIS) technique. This study has revealed, according to an equivalent circuit fitting the experimental parameters, that PANI-ˇ-CDpNH2 film is more conductive than PANI one. This result was explained by the PANI conformation changes from coiled and disorderly chain to rodlike one coated in the cavities of ˇ-CD. Finally we have studied the electrochemical response of these electrodes towards Pb2+ cations in the aim to probe the contribution of PANI inclusion with ˇ-CDpNH2 molecules to the enhancement of the sensitivity and recognition properties of these later. We have found that PANI-ˇ-CDpNH2 film exhibits a good sensitivity towards the Pb2+ cations. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Conducting polymers have been attracted great attention in the last two decades with regard to scientific study and technological applications. Previous works have discussed the possibility of use of these polymers as potential materials for sensors devices [1–3]. These conducting polymer based sensors have the advantage to exhibit improved response properties and are sensitive to small perturbations due to their electrical conductivity or charge transport properties [4]. Among these conducting polymers, polyaniline has emerged as a promising candidate for chemical sensors [5,6] due to its chemical stability, good electrical, optical and electrochemical properties [7]. Recently, the association of conduction polymer with macrocyclic compounds has opened new perspectives in the electrochemistry conductive polymers domain. Cyclodextrin (CD) is one of these macrocyclic compounds, and was considered as an ideal
∗ Corresponding author. E-mail address: [email protected] (R. Mlika). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.11.029
candidate for this pursuit due to its well defined cavity, large enough to serve as an anchoring site for polymer molecule [8]. Lee et al. have investigated the electrochemical properties of polymer films of gold electrodes modified with thiolated ˇcyclodextrin [9]. Yang et al. have developed an electrochemical sensor for cinchonine (CNN) using ˇ-cyclodextrin modified poly (N-acetylaniline) [10]. Subramanian et al. have studied the influence of the ˇcyclodextrin as an encapsule and an inclusion complex dopant on conducting polyaniline [11]. Kozlowski et al. have used modified cyclodextrin polymers as selective ion carriers for Pb2+ separation across plasticized thin films [12]. In 2010, the electrochemical deposition of polyaniline in the presence of ˇ-cyclodextrin (ˇ-CD) or sulfated ˇ-cyclodextrin (ˇSCD) has been reported by Rokovic´ et al. [13]. The morphology and electrochemical properties of the resulting layer have been studied. In this work, we report on the development of impedimetric sensors based on new pseudo-polyrotaxanes film formed by the inclusion of polyaniline with modified ˇ-CDpNH2 molecules. The association of conducting polymer with ˇ-cyclodextrin molecules permits to enclose a new compound presenting two essential
R. Mlika et al. / Synthetic Metals 162 (2012) 186–192
187
(NH2 .HCl)7
(NH2 .HCl)7
HCl 1M
NH2 +
NH2.HCl
(OH)7
(OH)7
(OH)
(OH)
(NH2 .HCl)7
+.
NH
NH _ Cl
(NH4)2 S2O8
+.
NH
NH _ Cl
n
x
Scheme 1. Schematic diagram of pseudo-polyrotaxanes. Synthesis route.
aspects, at the same time, conduction and recognition. As result, this association can lead to better sensors performance. These pseudo-polyrotaxanes were deposited by thermal evaporation on gold electrodes and then characterized by the wettability technique for hydrophobicity analysis. After, these electrodes were analyzed by electrochemical impedance spectroscopy (EIS) technique to study the electrical properties of the deposited films at various dc potential. The EIS technique was applied also to study the recognition properties of these pseudo-polyrotaxanes towards Pb2+ cations. Finally, the impedance behavior of these electrodes was modeled by an equivalent circuit to investigate the processes taking place at the different interfaces.
2.3. Elaboration of thin films by evaporation The immobilization of the receptor on the gold surface was done by evaporation technique under a vacuum of 132 × 10−11 bar. This technique permits the measurement of the deposited thin film thickness by using a vibrating piezoelectric quartz crystal microbalance. The polyrotaxanes powder was introduced into a cylindrical oven heated by an electrical filament. The evaporation temperature was maintained at about 150 ◦ C [16] with an evaporation rate of 0.5 nm/s. The measured deposited film thickness was about 100 nm carried out using dektak-3010 surface profilometer. 2.4. Contact angle measurements
2. Experimental 2.1. Pseudo-polyrotaxanes The pseudo-polyrotaxanes (Scheme 1) were synthesized by chemical polymerization in the Laboratoire de Méthodes et Techniques d’Analyses, Institut National de Recherche et d’Analyze Physico-chimique, Tunisia. The synthesis route, the FTIR and RMN results are discussed in our previous work [14].
Contact angle measurements were performed with a model contact instrument (Digidrop) form GBX (Romans, France) [17] in order to verify the non degradation of the deposited thin film after thermal evaporation. These measurements were performed by applying 4 l of deionized water on the polyrotaxanes thin film surface. The image of the water droplet behavior on the surface was then acquired with a digital camera and analyzed. 2.5. Impedance spectroscopy measurements
2.2. Gold electrodes substrates Gold substrates are purchased from Centre de Recherche LAAS (France) and cut into 1 cm × 1 cm square solids. The layer gold thickness is about 50 nm. In electrochemical experiments, the quality of the active surface will affect the measurements. Also, the current peak in cyclic voltammetry and the frequency response during electrochemical impedance spectroscopy will be dependent on the surface composition of the gold. For this reason, the gold substrate must be cleaned in piranha solution for about 5 min in order to activate the surface [15] and then rinsed with ultra pure water.
Electrochemical impedance spectroscopy (EIS) technique is an effective tool for the qualitative and quantitative characterization of electrochemical processes occurring in the conducting polymer films [18], and is able to study the interfacial properties of modified electrode. In the present work, impedance spectroscopy measurements were carried out at room temperature by using an impedance analyzer “Autolab PGSTAT” supported by Frequency Response Analysis System software (FRA2) controlled by computer. A sinusoidal excitation signal with amplitude of 10 mV was applied and the impedance spectra were recorded in the frequency range [0.05–100] kHz.
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Fig. 1. The contact angle () value with water as a liquid probe, before and after membrane deposition.
For the electrochemical measurements, a three electrode system was used: a working electrode (gold), a platinum counter electrode and a saturated calomel reference electrode (SCE). An aqueous ammonium acetate (CH3 COO− , NH4 + ) solution of 0.1 M, with pH = 7, was used as electrolyte.
Scheme 2. Schematic representation of pseudo-polyrotaxanes orientation at the gold surface.
Table 2 Comparison of contact angle measurements of PANI, PANI-ˇ-CD and PANI-ˇCDpNH2 thin films on gold and quartz substrate. Film
(◦ ) (Gold)
(◦ ) (Quartz)
PANI PANI-ˇ-CD PANI-ˇ-CDpNH2
80 93 74
81 94 73
3. Results and discussions
3.2. Electrochemical impedance spectroscopy (EIS) measurements
3.1. Hydrophobicity study
3.2.1. Polarization optimization Generally, the first step for the impedance studies is the determination of the polarization domain which depends on the thin film type. For our study, different negative potentials (E = −0.7, −0.8, −0.9 and −1 V) were applied in a large frequency range. Under E = −1 V over voltage versus SCE, an obvious decrease of the Warburg straight line and the half circle diameter was observed for the gold electrodes based on PANI and PANI-ˇ-CDpNH2 thin films (Figs. 3 and 4). This decrease can be explained by the reduction of the charge transfer resistance as a function the dc potential as shown in Fig. 5. This reduction renders possible the observation of the kinetics of cations at the thin film/solution interface.
In order to check the orientation of the pseudo-polyrotaxanes at the gold surface [19], a hydrophobicity study was carried out before and after thin film deposition, with water as a liquid probe. We have found that the contact angle value () increases from 70◦ , for the gold substrate, to 80◦ and 93◦ for the PANI and PANI-ˇcyclodextrin thin films, respectively (Fig. 1 and Table 1). The high value determined in the case of the PANI-ˇ-cyclodextrin thin film, proved the hydrophobic character due to the presence of the inner cavity of the ˇ-cyclodextrin molecule [20]. As a consequence, we can presume that the pseudo-polyrotaxanes were turned up at the gold surface (Scheme 2). The contact angle value ( = 74◦ ), found in the case of the PANIˇ-CDpNH2 thin film, shows the hydrophilic character of this film due the presence of the hydrophilic grafted NH2 groups [21]. On the other hand, we ran this wettability technique to verify the non-degradation of pseudo-polyrotaxanes after evaporation process. For this reason, we have deposited, during the polymerization route, PANI, PANI-ˇ-CD and PANI-ˇ-CDpNH2 thin films on quartz substrate and we have determined the contact angle values (Table 2), which were found to be very close to those determined for the evaporated thin films case (Fig. 2).
Table 1 Contact angle measurements of gold bare electrode, PANI, PANI -(-CD and PANI-ˇCDpNH2 thin films. Electrode
(◦ )
Gold Gold/PANI Gold/PANI-ˇ-CD Gold/PANI-ˇ-CDpNH2
70 80 93 74
Fig. 2. Comparison of the contact angle () value, with water as a liquid probe, of Pani and pseudo-polyrotaxanes membranes deposited on quartz and gold substrates.
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Fig. 3. Optimization of the polarization potential of the gold electrode based on evaporated PANI thin film.
Fig. 6. Nyquist and Bode plots of the electrode based on PANI-ˇ-CDpNH2 thin film.
3.2.2. Equivalent circuit modeling Fig. 6 reveals that phase plot presents two phase angle maxima. Consequently, the Nyquist plot is analyzed as a combination of two closely interacting semicircles, which can indicate the presence of other components in the equivalent electrical circuit model [22]. The larger one is attributed to the thin film bulk at high frequency range, which could be modeled to RC component. This later is formed by an electrical dipole based on parallel (Rf , CPEf ), where: • Rf is the total resistance of the thin film; • CPEf is the capacitance generated by the imbalance within the film.
Fig. 4. Optimization of the polarization potential of the gold electrode based on evaporated PANI-ˇ-CDpNH2 thin film.
The smaller second semicircle, corresponding to the lowfrequency range, is related to the gold/electrolyte interface and could be modeled to another RC formed by an electrical dipole based on parallel composed by a charge transfer resistance (Rct ) and Constant Phase Element (CPE1) (Fig. 7). The CPE impedance is given by the following relation [23]: Z=
1 Q (jω)
n
(1)
where: - Q is a frequency independent term giving information about the homogeneity, roughness and surface porosity;
Fig. 5. The variation of the charge transfer resistance as a function the dc potential for the PANI and PANI-ˇ-CDpNH2 thin films.
Fig. 7. Equivalent circuit used to fit the impedance spectra, Rf : bulk resistance of the thin film, CPEf and CPEdl : Constant Phase Elements, Rct : charge transfer resistance, Re : resistance of the electrolyte.
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Scheme 3. Schematic diagram of polyaniline coiled chain (a) and polymer chain coated in the into the ˇ-CD cavities (b).
leads to an increase of the film conductivity. This increase is due to the PANI chain conformation changes from coiled and disorderly chain to rodlike one coated in the cavities of ˇ-CDpNH2 [24,25] (Scheme 3). Comparing the charge transfer resistance (Rct ) value of PANI and PANI-ˇ-CDpNH2 films, it is apparent that Rct (PANI-ˇCDpNH2 ) < Rct (PANI). This result indicates that the charge transfer process is better demonstrated after the inclusion of the PANI in the cavities of modified ˇ-CDpNH2 , which can be explained by the PANI chain stretching process [26]. Also, we can remark that the capacitive element CPEf value of the PANI film is larger than the PANI-ˇ-CDpNH2 one due to a larger permittivity (εr ) of the PANI film [27].
Fig. 8. Comparison of the Nyquist diagrams of bare gold electrode, PANI and PANIˇ-CDpNH2 thin films at E = −1 V/SCE potential of polarization.
- n is a correction factor (0 < n < 1). 3.2.3. Electrical properties of PANI and PANI-ˇ-CD thin films Fig. 8 shows the Nyquist plots, at the same polarization potential (E = −1 V), of the PANI and PANI-ˇ-CDpNH2 thin films on gold electrode. We can deduce that the impedance increases after evaporation process, which can be attributed to the deposited thin film. The fit of the electrochemical impedance spectra was done, using the FRA2 software, to the equivalent circuit shown in Fig. 7. We have determined the Rf and CPEf electrical parameters values for PANI and PANI-ˇ-CDpNH2 films (Table 3). From this table, we can note that the Rf resistance value for the PANI thin film is larger than the PANI-ˇ-CDpNH2 one, which reveals a lower resistivity since Rf is inversely proportional to the film conductivity according to the following relation [17]: Rf =
d S
(2)
where d is the film thickness and S its active area. So we can conclude that the PANI-ˇ-CDpNH2 film presents a higher conductivity than PANI film, which can be explained by the fact that the inclusion of the PANI in the cavities of ˇ-CDpNH2
3.2.4. Sensitivity of PANI and PANI-ˇ-CDpNH2 thin films towards Pb2+ cations The sensing properties of benzylated ˇ-CD films towards Pb2+ in aqueous solution for sensor application have been tested in our previously work [28] and have shown a good sensitivity towards these cations. For this reason, we have to study the sensing properties of PANI and PANI-ˇ-CDpNH2 thin film towards these same cations. The cations kinetics at the thin film/solution interface was investigated by impedance spectroscopy for the two thin films towards Pb2+ cations (Figs. 9 and 10). From these figures, we can remark that there is an enhancement of the impedance with the Pb2+ concentration. This increase is far more pronounced for the PANI-ˇ-CDpNH2 thin film. The circuit parameters of the PANI and PANI-ˇ-CDpNH2 thin films, deduced from the fit, to the previous equivalent circuit were presented in Tables 4 and 5. Fig. 11 shows the variation of the bulk resistance Rf as function of Pb2+ concentration cologarithm (p[Pb2+ ]). We observe that Rf for the PANI thin film is not affected by the addition of Pb2+ cations which can be explained by the hundreds of some kind of disorder in PANI chain. Furthermore, Rf for thin PANI-ˇ-CDpNH2 film increases versus [Pb2+ ] concentration indicating a high activity and mobility of these cations within the film due to the presence of ˇ-CDpNH2 molecules enable to encage these cations (Scheme 4). The charge transfer resistance Rct dependence on cologarithm of [Pb2+ ] cation concentration is shown in Fig. 12. We can note that the Rct of PANI-ˇ-CDpNH2 film is not affected by the addition of the Pb2+ cations. However, a significant increase of Rct was observed for
Table 3 Electrical fit parameters values of gold bare electrode, PANI-ˇ-CDpNH2 and PANI thin films. CPE
Gold Gold/PANI Gold/PANI-ˇ-CDpNH2
Rct (k)
Q (F m−2 S−(1−n) )
n
6.95 5.33 17.4
0.81 0.82 0.78
2.57 6.00 3.60
Rf (k)
7.22 3.71
CPEf Q (F m−2 S−(1−n) )
n
6.75 5.20
0.92 0.95
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Fig. 9. Nyquist diagrams for different Pb2+ concentrations: of the gold electrode based on PANI thin film.
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Fig. 11. Variation of the bulk resistance (Rf ) vs. p[Pb2+ ] of PANI and PANI-ˇ-CDpNH2 thin films.
Scheme 4. Synoptic schematic diagram of Pb2+ cations recognition by pseudopolyrotaxanes.
Fig. 10. Nyquist diagrams for different Pb2+ concentrations: of the gold electrode based on PANI-ˇ-CDpNH2 thin film. Table 4 Fitting data of gold electrode based on PANI thin film for different concentrations of Pb2+ cations. The active surface (S) was about 0.3 cm2 . p[Pb2+ ] 7.69 6.7 5.5 4.5 3.7 2.7 1.7 205 219 224 238 240 244 241 Re () 3.23 3.41 3.30 3.00 3.09 3.30 3.57 Rf (k) CPEf Q(F m−2 S−(1−n) ) 3.6 3.4 3.6 3.6 3.6 3.4 3.4 0.9 0.89 0.88 0.94 0.93 0.98 0.94 n 1.1 1.36 1.69 1.78 2.00 2.20 2.20 Rct (k) CPE −2 −(1−n) ) 3.9 3.6 4.4 5.1 6.5 6.7 7 Q (F m S 0.75 0.67 0.64 0.6 0.63 0.63 n 0.7 2 (×10−3 ) 7.1 7.5 4.1 5.3 7.2 6.3 8.1
the PANI film with increasing Pb2+ concentration. This result suggests that the electron transfer to the interface electrode solution is slowed down due to the spatial hindrance of Pb2+ cations. Fig. 13 shows the variation of thin film Constant Phase Element (CPEf ) as function of cologarithm of Pb2+ concentration. As shown, for the two PANI-ˇ-CDpNH2 and PANI thin films, the CPEf value is not affected by the increase of the Pb2+ concentration.
Table 5 Fitting data of gold electrode based on PANI-ˇ-CD pNH2 thin film for different concentrations of Pb2+ cations. The active surface (S) was about 0.3 cm2 . p[Pb2+ ] 7.69 6.7 5.5 4.5 3.7 2.7 1.7 Re () 200 207 206 195 213 202 200 4.51 5.22 5.61 6.08 6.41 7.48 8.00 Rf (k) CPEf Q (F m−2 S−(1−n) ) 2.6 2.6 2.6 2.7 2.7 2.4 2.4 0.89 0.88 0.87 0.85 0.86 0.87 0.88 n 1.00 1.10 1.28 1.30 1.30 1.20 1.20 Rct (k) CPE 1.00 1.31 1.20 1.17 1.11 1.19 2.00 Q (F m−2 S−(1−n) ) 0.8 0.87 0.88 0.85 0.84 0.88 0.86 n 8.1 6.5 7.3 7.2 4.3 8.2 7.1 2 (×10−3 )
Fig. 12. Variation of the charge transfer resistance (Rct ) vs. p[Pb2+ ] of PANI and PANIˇ-CDpNH2 thin films.
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by rodlike chain and a cavity diameter of about 7.8 A˚ which is large enough to serve as an anchoring site for cations. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] Fig. 13. Variation of the Constant Phase Element (CPEf ) vs. p[Pb2+ ] of PANI and PANIˇ-CDpNH2 thin films.
[15] [16]
4. Conclusion In this work, PANI and PANI-ˇ-CDpNH2 thin films have been deposited by thermal evaporation process on gold surface. The hydrophobicity study, by wettability technique, has proved the hydrophilic character of the PANI-ˇ-CDpNH2 film due the presence of the hydrophilic grafted NH2 groups. The impedance spectroscopy measurements have revealed an increase of the conductivity of the PANI-ˇ-CDpNH2 thin film due to the PANI conformation changes from coiled and disorderly chain to rodlike one coated in the cavities of ˇ-CD. The sensitivity towards Pb2+ cations was investigated by impedance spectroscopy for the two thin films. We have found that the recognition has improved for PANI-ˇ-CDpNH2 thin film formed
[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
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