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Effect of electrolyte solvent on the morphology of polypyrrole films: Application to the use of polypyrrole in pH sensors
Synthetic Metals 158 (2008) 453–461
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Effect of electrolyte solvent on the morphology of polypyrrole films: Application to the use of polypyrrole in pH sensors ´ Stephanie Carquigny, Olivier Segut, Boris Lakard ∗ , F. Lallemand, Patrick Fievet Institut UTINAM, Universit´e de Franche-Comt´e, Bˆ atiment Prop´edeutique, 16 route de Gray, Besanc¸on Cedex 25030, France
a r t i c l e
i n f o
Article history: Received 28 September 2007 Received in revised form 1 March 2008 Accepted 12 March 2008 Available online 24 April 2008 Keywords: Polypyrrole Electrochemical oxidation Polymer-modified electrodes Scanning electron microscopy Atomic force microscopy pH sensors
a b s t r a c t Electrochemical and morphological characteristics of polypyrrole (PPy) films electro-deposited from three different electrolyte solutions (acetonitrile, water and acetonitrile + water) have been investigated using atomic force microscopy and scanning electron microscopy. Experimental parameters including the electrolyte and the deposition time were shown to affect the morphologies of polypyrrole films. After characterization of the polypyrrole film morphologies, these polymer films were successfully tested as sensitive layers in pH sensors since the pH sensor responses were fast, linear and sensitive to pH changes. More, these responses of the pH sensors were dependent on the experimental conditions of the electro-deposition (thickness and solvent). © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since polyacetylene was shown to have high electrical conductivities when properly doped [1,2], -conjugated polymers have been studied extensively from both fundamental and practical points of view because these classes of polymers showed possibilities of a wide range of applications [3–5]. For example, conducting polymers are regarded as potential materials in field effect transistors [6–8], light-emitting diodes [9], solar cells [10–12], electrochromic devices [13,14], electronic circuits [15,16], elastic textile composites of high electrical conductivity [17], supercapacitors for energy storage and secondary batteries [18], protection of metals [19,20], ion exchange membranes that respond to external stimulations [21,22], sensors and biosensors [23–29]. Among all the conducting polymers, polypyrrole (PPy) is one of the most studied for its ease of preparation, high electronic conductivity and good stability in air and aqueous media. Electrochemical polymerization of pyrrole is the main method in the preparation of polypyrrole films. The polymerization parameters, such as solvent [30–33], temperature [34], potential or current density [35], concentration of monomer [36], supporting electrolyte [37] and the nature of working electrode [38], have strong effect on the polymerization processes of pyrrole.
∗ Corresponding author. Tel.: +33 3 63 08 25 78; fax: +33 3 81 66 62 88. E-mail address: [email protected] (B. Lakard). 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.03.010
The polypyrrole films can be prepared from organic or aqueous solutions. For example, it is now well known that water has a marked effect on the electropolymerization of pyrrole in acetonitrile [39–43]. But, there has been much speculation about this ‘water effect’. Initially, it was suggested that water might reduce the solubility of oligomers and thus, leads to a faster deposition on the electrode [43]. Some authors proposed that water, owing to its higher dielectric constant (80 compared to 37 of acetonitrile), reduces the Coulombic repulsion between the radical cations and consequently, facilitates the radical–radical coupling [44–48]. Zotti et al. [49] proposed that the reaction-released protons should protonate pyrrole and its oligomers and that such protonated species in front of the electrode should prevent further electro-oxidation. Water then scavenges protons because of its stronger basicity compared with pyrrole. Then, by showing chemical polymerization, of pyrrole via protonation in HClO4 -acidified acetonitrile electrolytic solution, Otero and Rodriguez [50] proposed the coexistence of an electrochemically generated conducting polymer and a chemically produced non-conducting polymer. They further assumed that the addition of water to acetonitrile solution prevented the protoncatalyzed generation of an insulating film, since water has a greater proton-accepting ability than pyrrole. Thus, many polymerization parameters are strongly modified by this ‘water effect’ as it was proved by Warren and Anderson [51]. Indeed, this latter study proved that the addition of 1 wt.% (ca. 0.5 mol dm−3 ) to a solution of pyrrole in acetonitrile increases the polymerization rate, improves the film adherence, and enhances the conductivity of the resulting polymer. Thus, one order differ-
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ence of the magnitude of conductivity was observed between the polypyrrole films prepared in aqueous solutions and those formed in acetonitrile solutions [51]. Diaz and Hall investigated the effect of the addition of water into acetonitrile solution on the conductivity of polypyrrole [52]. They found that the conductivity and the mechanical properties of polypyrrole films greatly depended on the water concentration since the addition of water into acetonitrile resulted in a decrease of the conductivity and mechanical properties. In this work, we have studied this ‘water effect’ (by using either water, or acetonitrile, or water + acetonitrile as solvent) on the morphology and the thickness of the polypyrrole films since the morphology and the thickness are two other important parameters, that can lead to a strong modification of the polymer film properties. The morphology of the electrosynthesized polymers were studied using atomic force microscopy (AFM) and scanning electron microscopy (SEM), and the thickness was estimated using electrochemistry. Then, the different polypyrrole films were tested as sensitive layers in pH sensors in an attempt to rely their polymerization conditions to their pH sensitive properties. The responses of these pH sensors based on polypyrrole films were obtained, and it appeared that the pH responses were dependent on the experimental conditions of the electro-deposition.
Fig. 1. Schematic drawing of the pH sensor.
2.4. Atomic force microscopy
2. Materials and methods
Examinations of polymer topographies and morphologies were also performed by an atomic force microscope (Atomic Force Microscopy PicoSPM from Molecular Imaging, USA), in contact mode with gold-coated Si3 N4 tip (200-m long triangular cantilever and 0.1 N/m force constant).
2.1. Electrochemistry
2.5. Fabrication of pH sensors
All cyclic voltammetry experiments were carried out and recorded with an Autolab potentiostat galvanostat, model PGSTAT 20 (Ecochemie, The Netherlands), controlled by a PC computer via the GPES software interface. The electrochemical cell consisted in a classical three-electrode setup with a saturated calomel electrode (SCE), XR100 model from radiometer analytical as the reference electrode, a platinum disk as the counter-electrode and a platinum surface (2.4 mm × 2.4 mm) deposited on a SiO2 wafer as the working electrode. All electrochemical experiments were carried out at room temperature (293 K). Pyrrole was from Sigma–Aldrich and used as received. The electrolyte solution contained 10−1 M pyrrole and 10−1 M LiClO4 in different electrolytes: water, acetonitrile, or water (1%, v/v) in acetonitrile.
The pH sensors were fabricated using microsystem technologies, and in particular using lift-off process that consists in a photolithography followed by a sputtering of platinum on a SiO2 wafer. The first step of the photolithography process consisted in drawing the required pattern (see Fig. 1) with commercial mask design software Cadence. Then a Cr/Glass mask, on which the shape of the pattern has been drawn, was made with an electromask optical pattern generator. The process started with a 100-oriented standard 3 in. silicon wafer, which was thermally wet-oxidized, at 1200 ◦ C in water vapor, in order to produce a 1.3-m thickness SiO2 layer. Next, a 1.4-m thickness layer of negative photoresist (AZ 5214, from Clariant), suitable for lift-off, was deposited by spin coating. Then, the wafer was first exposed with the mask to a 36mJ/cm2 UV radiation flux delivered by an EVG 620 apparatus, and then without any mask to a 210-mJ/cm2 UV radiation flux. Thus, the pattern was transferred to the resist, which was then developed, using AZ 726 developer, to dissolve the resist where the metal was deposited. Then, a magnetron sputtering (Alcatel SCM 441 apparatus) was used to coat microsystems with titanium (30 nm, used to improve platinum layer), then platinum (150 nm). The fabrication parameters for Pt and Ti films were the following ones: base pressure: 4.6 × 10−7 mbar, pressure (Ar) during sputtering: 5 × 10−3 mbar, power: 150 W, target material purity: 99.99%, film thickness: 150 nm for Pt films and 30 nm for Ti films. The remaining resist layer was then dissolved using acetone. After the pH sensors have been fabricated, the pattern and the dimensions are controlled using an optical microscope. More details about the microsystem fabrication can be found in a previous paper [29].
2.2. X-ray photoelectron spectroscopy The polymer surface was characterized by X-ray photoelectron spectroscopy (XPS, SSX-100 spectrometer). XPS was used to control the elemental composition and to determine the oxidation state of elements. All recorded spectra were recorded at a 35◦ take-off angle relative to the substrate with a spectrometer using the monochromatized Al K␣ radiation (1486.6 eV). The binding energies of the core-levels were calibrated against the C 1s binding energy set at 285.0 eV, an energy characteristic of alkyl moieties. The peaks were analyzed using mixed Gaussian–Lorentzian curves (80% of Gaussian character). 2.3. Scanning electron microscopy
2.6. pH responses Examinations of polymer morphologies were performed using a high-resolution scanning electron microscope. Once synthesized and dried, polymer samples were examined in a LEO microscope (scanning electron microscopy LEO stereoscan 440, manufactured ¨ by Zeiss–Leica, Koln, Germany) with a electron beam energy of 15 keV.
The responses of pH sensors were measured in buffered solutions versus the SCE. The buffered solutions were from Prolabo, France and were stored at +4 ◦ C. Electrode potential in the different buffered solutions were measured using a pH-meter PHN130T model (Tacussel Electronics, France), used as a high impedance volt-
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meter (>10 M). Consequently, each potentiometric measurement was performed with an accuracy of ±1 mV. 3. Results and discussion 3.1. Electrochemical studies of polypyrrole in various solvents 3.1.1. Electrochemical synthesis of polypyrrole Electrochemical synthesis of polypyrrole was performed by cyclic voltammetry, from solutions of pyrrole in various solvents, on the platinum square located in the middle of the fabricated microsystems. To test the influence of the polypyrrole film thicknesses on the morphology and responses to pH, polymer films of different thicknesses were synthesized by varying the number of scans used for the electropolymerization (from 1 to 30 scans). Thus, the polymerization of 0.1 M pyrrole was performed in aqueous solution (Fig. 2a), anhydrous acetonitrile solution (Fig. 2b), and acetonitrile + water (1%, v/v) solution (Fig. 2c), charged with 0.1 M LiClO4 , at a platinum electrode using a potential sweep rate of 0.1 V s−1 between −0.3 and +1.5 V SCE−1 . For aqueous solutions of pyrrole (Fig. 2a), the first scan showed the oxidation peak of pyrrole at +1.4 V SCE−1 . All following scans showed the oxidation peak of polypyrrole at +0.4 V SCE−1 and the reduction peak of polypyrrole at about +0.1 V SCE−1 . The oxidation peak current during the first scan was equal to 2.23 × 10−3 A, and this current appeared to decrease after the first scan. For acetonitrile and acetonitrile + water solutions containing pyrrole, only the oxidation and reduction peaks of polypyrrole were observed at +0.4 V SCE−1 and +0.1 V SCE−1 , respectively. On the contrary, the oxidation peak obtained at +1.4 V SCE−1 in aqueous solutions did not appear in Fig. 2b and c. However, the presence or the lack of this peak is known to be dependent on the solvent used, and it was previously shown by Zhou and Heinze [53] that the oxidation of pyrrole in water-free solvents leads only to the oxidation and reduction peaks of polypyrrole and not to an oxidation peak of pyrrole during the first scan at +1.4 V SCE−1 . Concerning the oxidation peak currents during the first scan, a value of 2.23 × 10−3 A was obtained
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when acetonitrile + water was used as solvent (this value is similar to the one obtained in aqueous solutions of pyrrole), and a value of 1.50 × 10−3 A was obtained when acetonitrile was used as solvent. Consequently, the presence of water, even a small amount of water, led to an important increase of the current and in particular of the oxidation current peak. This result is not surprising since it was already shown that the addition of water in acetonitrile solutions, used as solvent for the oxidation of pyrrole, leads to an important increase of the conductivity [51,52]. 3.1.2. Characterization of the polypyrrole film The polymer surface was characterized by XPS. XPS was used to control the elemental composition of the polymer films. Thus, Fig. 3 shows the XPS spectrum of polymer films obtained from the oxidation of an acetonitrile solution containing pyrrole and LiClO4 (the spectra obtained for other solvents are almost identical). The XPS analyses confirm the presence of polypyrrole, incorporating ClO4 − doping agents, on the platinum surfaces. Indeed, XPS spectra of polymer samples reveal the presence of C, N, O, Cl and Pt for all polymers. Thus, C 1s signal (Fig. 3a) can be fitted by six different carbon species at 284.0, 284.8, 286.1, 287.8 and 289.8 eV. The two components at the lowest binding energy relevant to  and ␣ carbon atoms, respectively, revealed the first interesting finding. In fact, the comparison of these two carbon atoms areas showed that, following overoxidation, the  carbons in the film were less abundant than the ␣ ones. This indicates, that the  positions were the ones involved in the polymer functionalization. The third peak at 286.1 eV is attributed to carbons of the polymer C N or C–N+ , the fourth one at 287.8 eV to C N+ carbons and the peak much weaker at 289.8 eV to carbonyl C O groups. The appearance of a C O component may be associated with the overoxidation of polypyrrole at the -carbon site in the pyrrole rings. The N 1s spectra (Fig. 3b) indicate the presence of four peaks in the case of polypyrrole. It contains a main signal at 399.6 eV which is characteristic of pyrrolylium nitrogens (–NH-structure) and a high BE tail (BE = 400.4 and 402.0 eV) attributable to the pos-
Fig. 2. Oxidation of a solution of pyrrole (0.1 M pyrrole, 0.1 M LiClO4 , 0.1 V s−1 , −0.3 to +1.5 V SCE−1 , 5 scans) at a platinum electrode in (a) water, (b) acetonitrile and (c) acetonitrile + water (1%, v/v).
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Fig. 3. Survey-scan XPS (a) of polypyrrole film grown during five cycles in acetonitrile, pyrrole and LiClO4 , (b) C 1s (b) N 1s and (c) Cl 2p XPS spectra of the same polypyrrole film.
itively charged nitrogen (–NH + (polaron) and NH + (bipolaron). The spectra also show a small contribution at 397.0 eV that we associate with N-structure. Fig. 3c represents the Cl 2p core-level XPS spectrum at 207.5 eV binding energy due to the perchlorate anions present in the film as a doping agent. Consequently, these XPS spectra confirm that polypyrrole films incorporating ClO4 − doping agents are obtained from the oxidation of pyrrole in various solvents. 3.1.3. Polypyrrole film thickness To obtain polypyrrole films of different thickness, we varied the number of scans used for the electropolymerization. To determine the thickness of each electrosynthesized polymer film, we analyzed the electrical charge obtained for each cyclic voltammetry. The required charge density to grow a film with an average thickness of 10 m (4 C cm−2 ) was calculated assuming a polypyrrole density of 1.5 g cm−3 [54,55], and an electron loss z of 2.25 [43,56]. Thus, Table 1 presents the mean film thickness of polypyrrole (x), estimated from the electrical charge (q), associated with pyrrole oxidation by application of Faraday’s Law and assuming 100% current efficiency for polypyrrole formation: x = qM/AzF, where M is the molar mass of the polymer, F is the Faraday constant,
is the density of the polymer and z is the number of electrons involved. The nominal density of the polypyrrole films () was taken as 1.5 g cm−3 [54,55]. Table 1 indicates that polypyrrole film thicknesses increased linearly with the number of cycles. More, the films synthesized in acetonitrile were thinner that those obtained in water or mixture of water and acetonitrile. On the contrary, no marked difference was observed between polypyrrole films obtained with water or water + acetonitrile. These observations are coherent with the electrochemical study since the currents observed in the cyclic voltammograms were less important in acetonitrile that those obtained in water or mixture of water and acetonitrile. This is probably due to the low conductivity of pyrrole + acetonitrile solutions compared to the conductivity of pyrrole aqueous solutions. 3.2. Surface morphology of polypyrrole films Since we want to rely the surface morphology of polypyrrole films to the electrodeposition conditions and since the knowledge of the polymer morphology is of great interest to understand the behavior of the polymer films as pH sensitive sensors, we used scanning electron microscopy and atomic force microscopy to study
Table 1 Thicknesses (x) of polypyrrole films produced with different number of scans and different electrolytes Number of scans
0.1 M pyrrole + 0.1 M LiClO4 in acetonitrile (m)
0.1 M pyrrole + 0.1 M LiClO4 in acetonitrile + water (1%, v/v) (m)
0.1 M pyrrole + 0.1 M LiClO4 in water (m)
1 scan 3 scans 5 scans 10 scans 15 scans 30 scans
x = 0.4 x = 1.2 x = 2.0 x = 3.8 x = 5.5 x = 11.3
x = 0.6 x = 1.9 x = 3.3 x = 6.6 x = 9.3 x = 20.3
x = 0.7 x = 2.1 x = 3.0 x = 5.8 x = 7.4 x = 21.8
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Fig. 4. Scanning electron microscopy images of polypyrrole films grown during five cycles in (a) acetonitrile, (b) water and (c) acetonitrile + water.
polymer films synthesized in different electrolytes and at different thicknesses. 3.2.1. Scanning electron microscopy Fig. 4 shows the scanning electron microscopic images of polypyrrole films, grown during five scans from water, acetonitrile and water + acetonitrile. These images confirmed the estimated polymer film thicknesses since polypyrrole films grown in acetonitrile covered only a small part of the surface when polypyrrole films grown in water or water + acetonitrile covered all the platinum surface even if their morphology are slightly different. Fig. 5 shows the scanning electron microscopy images of polypyrrole films, grown from a mixture of water + acetonitrile at different thicknesses since they were grown during 1, 3, 5, 10, 15 and 30 cycles. The growth of the polymer film on the surface can be observed in these images since the surface is progressively covered by all the polymer film. In general, nodular surface structures were observed for all polypyrrole films. The thick polypyrrole films showed larger aggregates, mountains and valleys when compared with the thin films as shown in Fig. 5. However, the thinnest films showed less uniform surfaces usually with little difference in nodule diameter. In general, the roughness and pore size of polypyrrole films increased with film thickness. Similar results were reported by others [57–60]. 3.2.2. Atomic force microscopy Fig. 6 shows the atomic force microscopic images of polypyrrole films, grown during five scans from water, acetonitrile and water + acetonitrile. The roughness of the different polymer films was deduced from these images. Thus, the roughness of polypyrrole synthesized in acetonitrile was estimated to 0.46 m, when the roughnesses of polypyrrole synthesized in water and water + acetonitrile were estimated to 2.8–2.1 m, respectively, indicating once again that these latter polymer films are thicker than those grown in acetonitrile. Fig. 7 shows the atomic force microscopy images of polypyrrole films, grown from a mixture of water + acetonitrile at different thicknesses since they were grown during 1, 3, 5, 10, 15 and 30 cycles. These contact-atomic force microscopy images indicate that
the surface was progressively covered by all the polymer film. For the thinnest polypyrrole films nodular surface structures were observed, these structures could correspond to the anchoring of molecular strands on the platinum surface. On the contrary, for the thickest polypyrrole films, platinum electrodes seemed to be completely covered by polypyrrole, the formation of aggregates could be observed, and the structure of these films seemed to be more amorphous than the one observed for the thinnest polypyrrole films. The diameter of the polymer nodules or aggregates of polypyrrole films varied in sizes from less than 1 (Fig. 7a) to 5 m (Fig. 7f). It can also be noticed that the roughness and pore size of polypyrrole films increased with film thickness. Indeed, the roughness varied from 300 nm (Fig. 7a) to 2.5 m (Fig. 7f). 3.2.3. Influence of polypyrrole morphology It is likely that the variations in surface morphology and film structures could result in the changes in the potentiometric behavior for polypyrrole films of different thicknesses. As shown in this work and as proposed by Yang et al. [61], thin polypyrrole films comprised mainly ordered molecular strands anchored directly on the electrode surface, while thick films were of amorphous structures. The diffusion and exchange of anions and cations in this ordered thin films should be very fast and the entrapment of the ions should be small, so the potentiometric response was controlled mainly by the surface reaction involving protonation and deprotonation of the polypyrrole film. On the other hand, the threedimensional growth of the polypyrrole film to form the thick film resulted in the formation of porous amorphous structures [62]. Diffusion of ions through the thick polypyrrole film became slower or required longer time. Therefore, thick polypyrrole films showed a diffusion-controlled potentiometric behavior. 3.3. Use of polypyrrole as pH sensitive film We examined the potentiometric responses of the sensors coated by polypyrrole as a function of the pH changes since polypyrrole films contained an important proportion of –NH groups, that can be protonated or deprotonated as a function of pH changes. In all cases the sensors were immersed into different buffered solu-
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Fig. 5. Scanning electron microscopy images of polypyrrole films grown in acetonitrile + water during (a) 1, (b) 3, (c) 5, (d) 10, (e) 15 and (f) 30 cycles.
Fig. 6. Atomic force microscopy images of polypyrrole films grown during five cycles in (a) acetonitrile, (b) water and (c) acetonitrile + water.
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Fig. 7. Atomic force microscopy images of polypyrrole films grown in acetonitrile + water during (a) 1, (b) 3, (c) 5, (d) 10, (e) 15 and (f) 30 cycles.
tions and the potential equilibrium response time was less than 1 min. Thus, the potentials were measured as a function of the pH to obtain E = f(pH) linear curves. Then, the slopes of these linear curves were calculated and gathered in Table 2. In this table, the measured potentials were given as a function of the pH. The linear correlation coefficient of the linear regression E = f(pH) were also calculated and given in Table 2. As proved by the correlation coefficients, all the sensors showed a linear behaviour from pH 4 to pH 10.
Table 2 shows that pH response slopes were strongly dependent on the polymer film thicknesses. Indeed, polypyrrole films obtained after five scans (noted polypyrrole5) appeared to be the most sensitive to pH changes since they exhibited the most important slopes, in particular polypyrrole5 grown in water + acetonitrile (Fig. 8). This can be explained by the fact that the thinnest films did not covered the whole surface and so are not as efficient as polypyrrole5 films. More, the thickest films are maybe too thick, so the entrapment of
Table 2 Evolution of the potentiometric responses E for the different polymer films modified sensors Number of scans
0.1 M pyrrole + 0.1 M LiClO4 in acetonitrile
0.1 M pyrrole + 0.1 M LiClO4 in acetonitrile + water (1% v/v)
0.1 M pyrrole + 0.1 M LiClO4 in water
1 scan
s = 45.0 R = 0.96
s = 54.3 R = 0.99
s = 48.8 R = 0.99
3 scans
s = 50.8 R = 0.95
s = 56.5 R = 0.96
s = 45.8 R = 0.97
5 scans
s = 57.7 R = 0.98
s = 59.5 R = 0.99
s = 55.9 I = 0.97
10 scans
s = 50.5 R = 0.98
s = 52.0 R = 0.95
s = 54.8 R = 0.92
15 scans
s = 55.2 R = 0.92
s = 50.1 R = 0.94
s = 51.8 R = 0.97
30 scans
s = 50.3 R = 0.89
s = 43.9 R = 0.96
s = 46.2 R = 0.97
s = slope of E/SCE (mV) = f(pH) (mV/pH unit), R = linear correlation coefficient.
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Table 3 Influence of the ageing on the potentiometric responses to pH changes of the sensors Days
0.1 M pyrrole + 0.1 M LiClO4 in acetonitrile
0.1 M pyrrole + 0.1 M LiClO4 in acetonitrile + water (1%, v/v)
0.1 M pyrrole + 0.1 M LiClO4 in water
1
s = 57.7 R = 0.98
s = 59.4 R = 0.99
s = 55.9 R = 0.97
10
s = 39. R = 0.99
s = 43. R = 0.99
s = 38. R = 0.98
20
s = 38. R = 0.99
s = 40. R = 0.99
s = 38. R = 0.99
30
s = 36. R = 0.99
s = 40. R = 0.99
s = 39. R = 0.99
s = slope (mV/pH unit), R = linear correlation coefficient.
estimated by electrochemistry, scanning electron microscopy and atomic force microscopy. It was shown that polypyrrole films grown from acetonitrile solutions led to thin films, containing small nodules. On the contrary, polypyrrole films grown from water and water + acetonitrile solutions led to thicker films because of the low conductivity of pyrrole + acetonitrile solutions compared to the conductivity of pyrrole aqueous solutions. More, these later films consisted in aggregates when the time of electro-deposition is sufficient. The differences of thickness and morphologies led to differences of behavior when using polypyrrole films as pH sensitive layers. Thus, polypyrrole films grown from water + acetonitrile during five scans appeared to be the best sensors since they were the most sensitive to pH changes. More, the responses of all sensors were fast and linear to pH changes.
References Fig. 8. Potentiometric responses to pH changes of the sensors coated with polypyrrole grown in water + acetonitrile during five scans.
the protons in the polymer film should be too important to favour the diffusion and exchange of protons between the films and the solution. Comparing the results obtained with the different solvents, mixtures of water and acetonitrile seemed to lead to the most efficient polypyrrole films since they showed the best sensitivities to pH changes and since their potentiometric responses exhibited a very good linearity to pH changes. However, the films grown either from acetonitrile or from water led to good results, in terms of sensitivity and linearity, too. The chemical responses to pH changes of the polypyrrole5 sensors grown in acetonitrile, water or acetonitrile + water, the ones that exhibited the best sensitivities according to Table 2, were studied during a period of 30 days to test the stability in time of these sensors. During all this period of test, the potentiometric responses to pH changes remained linear from pH 4 to pH 10 (Table 3). However, the sensitivity of these pH sensors decreased strongly after the first use, before remaining constant during the rest of the period of test (with a sensitivity of about −40 mV pH−1 ). Consequently, the sensors electrochemically coated with polypyrrole were very efficient pH sensors during their first use since their response was linear and fast, and since their sensitivity was important. After the first use, the sensors remained linear and fast but their sensitivity is less important. 4. Conclusions Polypyrrole films were electro-synthesized from three different electrolyte solutions (acetonitrile, water and acetonitrile + water). The electro-deposited film thicknesses and morphologies were
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Effect of electrolyte solvent on the morphology of polypyrrole films: Application to the use of polypyrrole in pH sensors ´ Stephanie Carquigny, Olivier Segut, Boris Lakard ∗ , F. Lallemand, Patrick Fievet Institut UTINAM, Universit´e de Franche-Comt´e, Bˆ atiment Prop´edeutique, 16 route de Gray, Besanc¸on Cedex 25030, France
a r t i c l e
i n f o
Article history: Received 28 September 2007 Received in revised form 1 March 2008 Accepted 12 March 2008 Available online 24 April 2008 Keywords: Polypyrrole Electrochemical oxidation Polymer-modified electrodes Scanning electron microscopy Atomic force microscopy pH sensors
a b s t r a c t Electrochemical and morphological characteristics of polypyrrole (PPy) films electro-deposited from three different electrolyte solutions (acetonitrile, water and acetonitrile + water) have been investigated using atomic force microscopy and scanning electron microscopy. Experimental parameters including the electrolyte and the deposition time were shown to affect the morphologies of polypyrrole films. After characterization of the polypyrrole film morphologies, these polymer films were successfully tested as sensitive layers in pH sensors since the pH sensor responses were fast, linear and sensitive to pH changes. More, these responses of the pH sensors were dependent on the experimental conditions of the electro-deposition (thickness and solvent). © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since polyacetylene was shown to have high electrical conductivities when properly doped [1,2], -conjugated polymers have been studied extensively from both fundamental and practical points of view because these classes of polymers showed possibilities of a wide range of applications [3–5]. For example, conducting polymers are regarded as potential materials in field effect transistors [6–8], light-emitting diodes [9], solar cells [10–12], electrochromic devices [13,14], electronic circuits [15,16], elastic textile composites of high electrical conductivity [17], supercapacitors for energy storage and secondary batteries [18], protection of metals [19,20], ion exchange membranes that respond to external stimulations [21,22], sensors and biosensors [23–29]. Among all the conducting polymers, polypyrrole (PPy) is one of the most studied for its ease of preparation, high electronic conductivity and good stability in air and aqueous media. Electrochemical polymerization of pyrrole is the main method in the preparation of polypyrrole films. The polymerization parameters, such as solvent [30–33], temperature [34], potential or current density [35], concentration of monomer [36], supporting electrolyte [37] and the nature of working electrode [38], have strong effect on the polymerization processes of pyrrole.
∗ Corresponding author. Tel.: +33 3 63 08 25 78; fax: +33 3 81 66 62 88. E-mail address: [email protected] (B. Lakard). 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.03.010
The polypyrrole films can be prepared from organic or aqueous solutions. For example, it is now well known that water has a marked effect on the electropolymerization of pyrrole in acetonitrile [39–43]. But, there has been much speculation about this ‘water effect’. Initially, it was suggested that water might reduce the solubility of oligomers and thus, leads to a faster deposition on the electrode [43]. Some authors proposed that water, owing to its higher dielectric constant (80 compared to 37 of acetonitrile), reduces the Coulombic repulsion between the radical cations and consequently, facilitates the radical–radical coupling [44–48]. Zotti et al. [49] proposed that the reaction-released protons should protonate pyrrole and its oligomers and that such protonated species in front of the electrode should prevent further electro-oxidation. Water then scavenges protons because of its stronger basicity compared with pyrrole. Then, by showing chemical polymerization, of pyrrole via protonation in HClO4 -acidified acetonitrile electrolytic solution, Otero and Rodriguez [50] proposed the coexistence of an electrochemically generated conducting polymer and a chemically produced non-conducting polymer. They further assumed that the addition of water to acetonitrile solution prevented the protoncatalyzed generation of an insulating film, since water has a greater proton-accepting ability than pyrrole. Thus, many polymerization parameters are strongly modified by this ‘water effect’ as it was proved by Warren and Anderson [51]. Indeed, this latter study proved that the addition of 1 wt.% (ca. 0.5 mol dm−3 ) to a solution of pyrrole in acetonitrile increases the polymerization rate, improves the film adherence, and enhances the conductivity of the resulting polymer. Thus, one order differ-
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ence of the magnitude of conductivity was observed between the polypyrrole films prepared in aqueous solutions and those formed in acetonitrile solutions [51]. Diaz and Hall investigated the effect of the addition of water into acetonitrile solution on the conductivity of polypyrrole [52]. They found that the conductivity and the mechanical properties of polypyrrole films greatly depended on the water concentration since the addition of water into acetonitrile resulted in a decrease of the conductivity and mechanical properties. In this work, we have studied this ‘water effect’ (by using either water, or acetonitrile, or water + acetonitrile as solvent) on the morphology and the thickness of the polypyrrole films since the morphology and the thickness are two other important parameters, that can lead to a strong modification of the polymer film properties. The morphology of the electrosynthesized polymers were studied using atomic force microscopy (AFM) and scanning electron microscopy (SEM), and the thickness was estimated using electrochemistry. Then, the different polypyrrole films were tested as sensitive layers in pH sensors in an attempt to rely their polymerization conditions to their pH sensitive properties. The responses of these pH sensors based on polypyrrole films were obtained, and it appeared that the pH responses were dependent on the experimental conditions of the electro-deposition.
Fig. 1. Schematic drawing of the pH sensor.
2.4. Atomic force microscopy
2. Materials and methods
Examinations of polymer topographies and morphologies were also performed by an atomic force microscope (Atomic Force Microscopy PicoSPM from Molecular Imaging, USA), in contact mode with gold-coated Si3 N4 tip (200-m long triangular cantilever and 0.1 N/m force constant).
2.1. Electrochemistry
2.5. Fabrication of pH sensors
All cyclic voltammetry experiments were carried out and recorded with an Autolab potentiostat galvanostat, model PGSTAT 20 (Ecochemie, The Netherlands), controlled by a PC computer via the GPES software interface. The electrochemical cell consisted in a classical three-electrode setup with a saturated calomel electrode (SCE), XR100 model from radiometer analytical as the reference electrode, a platinum disk as the counter-electrode and a platinum surface (2.4 mm × 2.4 mm) deposited on a SiO2 wafer as the working electrode. All electrochemical experiments were carried out at room temperature (293 K). Pyrrole was from Sigma–Aldrich and used as received. The electrolyte solution contained 10−1 M pyrrole and 10−1 M LiClO4 in different electrolytes: water, acetonitrile, or water (1%, v/v) in acetonitrile.
The pH sensors were fabricated using microsystem technologies, and in particular using lift-off process that consists in a photolithography followed by a sputtering of platinum on a SiO2 wafer. The first step of the photolithography process consisted in drawing the required pattern (see Fig. 1) with commercial mask design software Cadence. Then a Cr/Glass mask, on which the shape of the pattern has been drawn, was made with an electromask optical pattern generator. The process started with a 100-oriented standard 3 in. silicon wafer, which was thermally wet-oxidized, at 1200 ◦ C in water vapor, in order to produce a 1.3-m thickness SiO2 layer. Next, a 1.4-m thickness layer of negative photoresist (AZ 5214, from Clariant), suitable for lift-off, was deposited by spin coating. Then, the wafer was first exposed with the mask to a 36mJ/cm2 UV radiation flux delivered by an EVG 620 apparatus, and then without any mask to a 210-mJ/cm2 UV radiation flux. Thus, the pattern was transferred to the resist, which was then developed, using AZ 726 developer, to dissolve the resist where the metal was deposited. Then, a magnetron sputtering (Alcatel SCM 441 apparatus) was used to coat microsystems with titanium (30 nm, used to improve platinum layer), then platinum (150 nm). The fabrication parameters for Pt and Ti films were the following ones: base pressure: 4.6 × 10−7 mbar, pressure (Ar) during sputtering: 5 × 10−3 mbar, power: 150 W, target material purity: 99.99%, film thickness: 150 nm for Pt films and 30 nm for Ti films. The remaining resist layer was then dissolved using acetone. After the pH sensors have been fabricated, the pattern and the dimensions are controlled using an optical microscope. More details about the microsystem fabrication can be found in a previous paper [29].
2.2. X-ray photoelectron spectroscopy The polymer surface was characterized by X-ray photoelectron spectroscopy (XPS, SSX-100 spectrometer). XPS was used to control the elemental composition and to determine the oxidation state of elements. All recorded spectra were recorded at a 35◦ take-off angle relative to the substrate with a spectrometer using the monochromatized Al K␣ radiation (1486.6 eV). The binding energies of the core-levels were calibrated against the C 1s binding energy set at 285.0 eV, an energy characteristic of alkyl moieties. The peaks were analyzed using mixed Gaussian–Lorentzian curves (80% of Gaussian character). 2.3. Scanning electron microscopy
2.6. pH responses Examinations of polymer morphologies were performed using a high-resolution scanning electron microscope. Once synthesized and dried, polymer samples were examined in a LEO microscope (scanning electron microscopy LEO stereoscan 440, manufactured ¨ by Zeiss–Leica, Koln, Germany) with a electron beam energy of 15 keV.
The responses of pH sensors were measured in buffered solutions versus the SCE. The buffered solutions were from Prolabo, France and were stored at +4 ◦ C. Electrode potential in the different buffered solutions were measured using a pH-meter PHN130T model (Tacussel Electronics, France), used as a high impedance volt-
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meter (>10 M). Consequently, each potentiometric measurement was performed with an accuracy of ±1 mV. 3. Results and discussion 3.1. Electrochemical studies of polypyrrole in various solvents 3.1.1. Electrochemical synthesis of polypyrrole Electrochemical synthesis of polypyrrole was performed by cyclic voltammetry, from solutions of pyrrole in various solvents, on the platinum square located in the middle of the fabricated microsystems. To test the influence of the polypyrrole film thicknesses on the morphology and responses to pH, polymer films of different thicknesses were synthesized by varying the number of scans used for the electropolymerization (from 1 to 30 scans). Thus, the polymerization of 0.1 M pyrrole was performed in aqueous solution (Fig. 2a), anhydrous acetonitrile solution (Fig. 2b), and acetonitrile + water (1%, v/v) solution (Fig. 2c), charged with 0.1 M LiClO4 , at a platinum electrode using a potential sweep rate of 0.1 V s−1 between −0.3 and +1.5 V SCE−1 . For aqueous solutions of pyrrole (Fig. 2a), the first scan showed the oxidation peak of pyrrole at +1.4 V SCE−1 . All following scans showed the oxidation peak of polypyrrole at +0.4 V SCE−1 and the reduction peak of polypyrrole at about +0.1 V SCE−1 . The oxidation peak current during the first scan was equal to 2.23 × 10−3 A, and this current appeared to decrease after the first scan. For acetonitrile and acetonitrile + water solutions containing pyrrole, only the oxidation and reduction peaks of polypyrrole were observed at +0.4 V SCE−1 and +0.1 V SCE−1 , respectively. On the contrary, the oxidation peak obtained at +1.4 V SCE−1 in aqueous solutions did not appear in Fig. 2b and c. However, the presence or the lack of this peak is known to be dependent on the solvent used, and it was previously shown by Zhou and Heinze [53] that the oxidation of pyrrole in water-free solvents leads only to the oxidation and reduction peaks of polypyrrole and not to an oxidation peak of pyrrole during the first scan at +1.4 V SCE−1 . Concerning the oxidation peak currents during the first scan, a value of 2.23 × 10−3 A was obtained
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when acetonitrile + water was used as solvent (this value is similar to the one obtained in aqueous solutions of pyrrole), and a value of 1.50 × 10−3 A was obtained when acetonitrile was used as solvent. Consequently, the presence of water, even a small amount of water, led to an important increase of the current and in particular of the oxidation current peak. This result is not surprising since it was already shown that the addition of water in acetonitrile solutions, used as solvent for the oxidation of pyrrole, leads to an important increase of the conductivity [51,52]. 3.1.2. Characterization of the polypyrrole film The polymer surface was characterized by XPS. XPS was used to control the elemental composition of the polymer films. Thus, Fig. 3 shows the XPS spectrum of polymer films obtained from the oxidation of an acetonitrile solution containing pyrrole and LiClO4 (the spectra obtained for other solvents are almost identical). The XPS analyses confirm the presence of polypyrrole, incorporating ClO4 − doping agents, on the platinum surfaces. Indeed, XPS spectra of polymer samples reveal the presence of C, N, O, Cl and Pt for all polymers. Thus, C 1s signal (Fig. 3a) can be fitted by six different carbon species at 284.0, 284.8, 286.1, 287.8 and 289.8 eV. The two components at the lowest binding energy relevant to  and ␣ carbon atoms, respectively, revealed the first interesting finding. In fact, the comparison of these two carbon atoms areas showed that, following overoxidation, the  carbons in the film were less abundant than the ␣ ones. This indicates, that the  positions were the ones involved in the polymer functionalization. The third peak at 286.1 eV is attributed to carbons of the polymer C N or C–N+ , the fourth one at 287.8 eV to C N+ carbons and the peak much weaker at 289.8 eV to carbonyl C O groups. The appearance of a C O component may be associated with the overoxidation of polypyrrole at the -carbon site in the pyrrole rings. The N 1s spectra (Fig. 3b) indicate the presence of four peaks in the case of polypyrrole. It contains a main signal at 399.6 eV which is characteristic of pyrrolylium nitrogens (–NH-structure) and a high BE tail (BE = 400.4 and 402.0 eV) attributable to the pos-
Fig. 2. Oxidation of a solution of pyrrole (0.1 M pyrrole, 0.1 M LiClO4 , 0.1 V s−1 , −0.3 to +1.5 V SCE−1 , 5 scans) at a platinum electrode in (a) water, (b) acetonitrile and (c) acetonitrile + water (1%, v/v).
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Fig. 3. Survey-scan XPS (a) of polypyrrole film grown during five cycles in acetonitrile, pyrrole and LiClO4 , (b) C 1s (b) N 1s and (c) Cl 2p XPS spectra of the same polypyrrole film.
itively charged nitrogen (–NH + (polaron) and NH + (bipolaron). The spectra also show a small contribution at 397.0 eV that we associate with N-structure. Fig. 3c represents the Cl 2p core-level XPS spectrum at 207.5 eV binding energy due to the perchlorate anions present in the film as a doping agent. Consequently, these XPS spectra confirm that polypyrrole films incorporating ClO4 − doping agents are obtained from the oxidation of pyrrole in various solvents. 3.1.3. Polypyrrole film thickness To obtain polypyrrole films of different thickness, we varied the number of scans used for the electropolymerization. To determine the thickness of each electrosynthesized polymer film, we analyzed the electrical charge obtained for each cyclic voltammetry. The required charge density to grow a film with an average thickness of 10 m (4 C cm−2 ) was calculated assuming a polypyrrole density of 1.5 g cm−3 [54,55], and an electron loss z of 2.25 [43,56]. Thus, Table 1 presents the mean film thickness of polypyrrole (x), estimated from the electrical charge (q), associated with pyrrole oxidation by application of Faraday’s Law and assuming 100% current efficiency for polypyrrole formation: x = qM/AzF, where M is the molar mass of the polymer, F is the Faraday constant,
is the density of the polymer and z is the number of electrons involved. The nominal density of the polypyrrole films () was taken as 1.5 g cm−3 [54,55]. Table 1 indicates that polypyrrole film thicknesses increased linearly with the number of cycles. More, the films synthesized in acetonitrile were thinner that those obtained in water or mixture of water and acetonitrile. On the contrary, no marked difference was observed between polypyrrole films obtained with water or water + acetonitrile. These observations are coherent with the electrochemical study since the currents observed in the cyclic voltammograms were less important in acetonitrile that those obtained in water or mixture of water and acetonitrile. This is probably due to the low conductivity of pyrrole + acetonitrile solutions compared to the conductivity of pyrrole aqueous solutions. 3.2. Surface morphology of polypyrrole films Since we want to rely the surface morphology of polypyrrole films to the electrodeposition conditions and since the knowledge of the polymer morphology is of great interest to understand the behavior of the polymer films as pH sensitive sensors, we used scanning electron microscopy and atomic force microscopy to study
Table 1 Thicknesses (x) of polypyrrole films produced with different number of scans and different electrolytes Number of scans
0.1 M pyrrole + 0.1 M LiClO4 in acetonitrile (m)
0.1 M pyrrole + 0.1 M LiClO4 in acetonitrile + water (1%, v/v) (m)
0.1 M pyrrole + 0.1 M LiClO4 in water (m)
1 scan 3 scans 5 scans 10 scans 15 scans 30 scans
x = 0.4 x = 1.2 x = 2.0 x = 3.8 x = 5.5 x = 11.3
x = 0.6 x = 1.9 x = 3.3 x = 6.6 x = 9.3 x = 20.3
x = 0.7 x = 2.1 x = 3.0 x = 5.8 x = 7.4 x = 21.8
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Fig. 4. Scanning electron microscopy images of polypyrrole films grown during five cycles in (a) acetonitrile, (b) water and (c) acetonitrile + water.
polymer films synthesized in different electrolytes and at different thicknesses. 3.2.1. Scanning electron microscopy Fig. 4 shows the scanning electron microscopic images of polypyrrole films, grown during five scans from water, acetonitrile and water + acetonitrile. These images confirmed the estimated polymer film thicknesses since polypyrrole films grown in acetonitrile covered only a small part of the surface when polypyrrole films grown in water or water + acetonitrile covered all the platinum surface even if their morphology are slightly different. Fig. 5 shows the scanning electron microscopy images of polypyrrole films, grown from a mixture of water + acetonitrile at different thicknesses since they were grown during 1, 3, 5, 10, 15 and 30 cycles. The growth of the polymer film on the surface can be observed in these images since the surface is progressively covered by all the polymer film. In general, nodular surface structures were observed for all polypyrrole films. The thick polypyrrole films showed larger aggregates, mountains and valleys when compared with the thin films as shown in Fig. 5. However, the thinnest films showed less uniform surfaces usually with little difference in nodule diameter. In general, the roughness and pore size of polypyrrole films increased with film thickness. Similar results were reported by others [57–60]. 3.2.2. Atomic force microscopy Fig. 6 shows the atomic force microscopic images of polypyrrole films, grown during five scans from water, acetonitrile and water + acetonitrile. The roughness of the different polymer films was deduced from these images. Thus, the roughness of polypyrrole synthesized in acetonitrile was estimated to 0.46 m, when the roughnesses of polypyrrole synthesized in water and water + acetonitrile were estimated to 2.8–2.1 m, respectively, indicating once again that these latter polymer films are thicker than those grown in acetonitrile. Fig. 7 shows the atomic force microscopy images of polypyrrole films, grown from a mixture of water + acetonitrile at different thicknesses since they were grown during 1, 3, 5, 10, 15 and 30 cycles. These contact-atomic force microscopy images indicate that
the surface was progressively covered by all the polymer film. For the thinnest polypyrrole films nodular surface structures were observed, these structures could correspond to the anchoring of molecular strands on the platinum surface. On the contrary, for the thickest polypyrrole films, platinum electrodes seemed to be completely covered by polypyrrole, the formation of aggregates could be observed, and the structure of these films seemed to be more amorphous than the one observed for the thinnest polypyrrole films. The diameter of the polymer nodules or aggregates of polypyrrole films varied in sizes from less than 1 (Fig. 7a) to 5 m (Fig. 7f). It can also be noticed that the roughness and pore size of polypyrrole films increased with film thickness. Indeed, the roughness varied from 300 nm (Fig. 7a) to 2.5 m (Fig. 7f). 3.2.3. Influence of polypyrrole morphology It is likely that the variations in surface morphology and film structures could result in the changes in the potentiometric behavior for polypyrrole films of different thicknesses. As shown in this work and as proposed by Yang et al. [61], thin polypyrrole films comprised mainly ordered molecular strands anchored directly on the electrode surface, while thick films were of amorphous structures. The diffusion and exchange of anions and cations in this ordered thin films should be very fast and the entrapment of the ions should be small, so the potentiometric response was controlled mainly by the surface reaction involving protonation and deprotonation of the polypyrrole film. On the other hand, the threedimensional growth of the polypyrrole film to form the thick film resulted in the formation of porous amorphous structures [62]. Diffusion of ions through the thick polypyrrole film became slower or required longer time. Therefore, thick polypyrrole films showed a diffusion-controlled potentiometric behavior. 3.3. Use of polypyrrole as pH sensitive film We examined the potentiometric responses of the sensors coated by polypyrrole as a function of the pH changes since polypyrrole films contained an important proportion of –NH groups, that can be protonated or deprotonated as a function of pH changes. In all cases the sensors were immersed into different buffered solu-
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Fig. 5. Scanning electron microscopy images of polypyrrole films grown in acetonitrile + water during (a) 1, (b) 3, (c) 5, (d) 10, (e) 15 and (f) 30 cycles.
Fig. 6. Atomic force microscopy images of polypyrrole films grown during five cycles in (a) acetonitrile, (b) water and (c) acetonitrile + water.
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Fig. 7. Atomic force microscopy images of polypyrrole films grown in acetonitrile + water during (a) 1, (b) 3, (c) 5, (d) 10, (e) 15 and (f) 30 cycles.
tions and the potential equilibrium response time was less than 1 min. Thus, the potentials were measured as a function of the pH to obtain E = f(pH) linear curves. Then, the slopes of these linear curves were calculated and gathered in Table 2. In this table, the measured potentials were given as a function of the pH. The linear correlation coefficient of the linear regression E = f(pH) were also calculated and given in Table 2. As proved by the correlation coefficients, all the sensors showed a linear behaviour from pH 4 to pH 10.
Table 2 shows that pH response slopes were strongly dependent on the polymer film thicknesses. Indeed, polypyrrole films obtained after five scans (noted polypyrrole5) appeared to be the most sensitive to pH changes since they exhibited the most important slopes, in particular polypyrrole5 grown in water + acetonitrile (Fig. 8). This can be explained by the fact that the thinnest films did not covered the whole surface and so are not as efficient as polypyrrole5 films. More, the thickest films are maybe too thick, so the entrapment of
Table 2 Evolution of the potentiometric responses E for the different polymer films modified sensors Number of scans
0.1 M pyrrole + 0.1 M LiClO4 in acetonitrile
0.1 M pyrrole + 0.1 M LiClO4 in acetonitrile + water (1% v/v)
0.1 M pyrrole + 0.1 M LiClO4 in water
1 scan
s = 45.0 R = 0.96
s = 54.3 R = 0.99
s = 48.8 R = 0.99
3 scans
s = 50.8 R = 0.95
s = 56.5 R = 0.96
s = 45.8 R = 0.97
5 scans
s = 57.7 R = 0.98
s = 59.5 R = 0.99
s = 55.9 I = 0.97
10 scans
s = 50.5 R = 0.98
s = 52.0 R = 0.95
s = 54.8 R = 0.92
15 scans
s = 55.2 R = 0.92
s = 50.1 R = 0.94
s = 51.8 R = 0.97
30 scans
s = 50.3 R = 0.89
s = 43.9 R = 0.96
s = 46.2 R = 0.97
s = slope of E/SCE (mV) = f(pH) (mV/pH unit), R = linear correlation coefficient.
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Table 3 Influence of the ageing on the potentiometric responses to pH changes of the sensors Days
0.1 M pyrrole + 0.1 M LiClO4 in acetonitrile
0.1 M pyrrole + 0.1 M LiClO4 in acetonitrile + water (1%, v/v)
0.1 M pyrrole + 0.1 M LiClO4 in water
1
s = 57.7 R = 0.98
s = 59.4 R = 0.99
s = 55.9 R = 0.97
10
s = 39. R = 0.99
s = 43. R = 0.99
s = 38. R = 0.98
20
s = 38. R = 0.99
s = 40. R = 0.99
s = 38. R = 0.99
30
s = 36. R = 0.99
s = 40. R = 0.99
s = 39. R = 0.99
s = slope (mV/pH unit), R = linear correlation coefficient.
estimated by electrochemistry, scanning electron microscopy and atomic force microscopy. It was shown that polypyrrole films grown from acetonitrile solutions led to thin films, containing small nodules. On the contrary, polypyrrole films grown from water and water + acetonitrile solutions led to thicker films because of the low conductivity of pyrrole + acetonitrile solutions compared to the conductivity of pyrrole aqueous solutions. More, these later films consisted in aggregates when the time of electro-deposition is sufficient. The differences of thickness and morphologies led to differences of behavior when using polypyrrole films as pH sensitive layers. Thus, polypyrrole films grown from water + acetonitrile during five scans appeared to be the best sensors since they were the most sensitive to pH changes. More, the responses of all sensors were fast and linear to pH changes.
References Fig. 8. Potentiometric responses to pH changes of the sensors coated with polypyrrole grown in water + acetonitrile during five scans.
the protons in the polymer film should be too important to favour the diffusion and exchange of protons between the films and the solution. Comparing the results obtained with the different solvents, mixtures of water and acetonitrile seemed to lead to the most efficient polypyrrole films since they showed the best sensitivities to pH changes and since their potentiometric responses exhibited a very good linearity to pH changes. However, the films grown either from acetonitrile or from water led to good results, in terms of sensitivity and linearity, too. The chemical responses to pH changes of the polypyrrole5 sensors grown in acetonitrile, water or acetonitrile + water, the ones that exhibited the best sensitivities according to Table 2, were studied during a period of 30 days to test the stability in time of these sensors. During all this period of test, the potentiometric responses to pH changes remained linear from pH 4 to pH 10 (Table 3). However, the sensitivity of these pH sensors decreased strongly after the first use, before remaining constant during the rest of the period of test (with a sensitivity of about −40 mV pH−1 ). Consequently, the sensors electrochemically coated with polypyrrole were very efficient pH sensors during their first use since their response was linear and fast, and since their sensitivity was important. After the first use, the sensors remained linear and fast but their sensitivity is less important. 4. Conclusions Polypyrrole films were electro-synthesized from three different electrolyte solutions (acetonitrile, water and acetonitrile + water). The electro-deposited film thicknesses and morphologies were
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