Talanta 78 (2009) 199–206 Contents lists available at ScienceDirect Talanta journal homepage: www.elsevier.com/locate/talanta Ammonia gas sensor based on electrosynthesized polypyrrole films Stéphanie Carquigny a,b , Jean-Baptiste Sanchez a , Franck Berger a , Boris Lakard b,∗ , Fabrice Lallemand b a LCPR-AC, UMR CEA E4, Université de Franche-Comté, Bâtiment Propédeutique, 16 route de Gray, 25030 Besanc¸ on Cedex, France b Institut UTINAM, UMR CNRS 6213, Université de Franche-Comté, Bâtiment Propédeutique, 16 route de Gray, 25030 Besanc¸ on Cedex, France article info Article history: Received 28 July 2008 Received in revised form 24 October 2008 Accepted 31 October 2008 Available online 11 November 2008 Keywords: Gas sensor Ammonia Polypyrrole Electrochemistry abstract In this work, design and fabrication of micro-gas-sensors, polymerization and deposition of poly(pyrrole) thin films as sensitive layer for the micro-gas-sensors by electrochemical processing, and characterization of the polymer films by FTIR, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), are reported. The change in conductance of thin polymer layers is used as a sensor signal. The behaviours, including sensitivity, reproducibility and reversibility, to various ammonia gas concentrations ranging from 8 ppm to 1000 ppm are investigated. The influence of the temperature on the electrical response of the sensors is also studied. The experimental results show that these ammonia gas sensors are efficient since they are sensitive to ammonia, reversible and reproducible at room temperature. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Polypyrrole (PPy) has attracted considerable attention because of the possibility to use redox reactions for transforming it into states of strongly differing electrical conductivity [1] and because PPy has a good stability in air and aqueous media. PPy and other conductive polymers have, therefore, also been classified as organic metals. There exists a wide range of applications to use organic metals, such as: cell culture substrates [2,3], field effect transistors [4–6], light-emitting diodes [7], solar cells [8–10], electrochromic devices [11,12], electronic circuits [13,14], elastic textile composites [15], supercapacitors for energy storage and secondary batteries [16], protection of metals [17,18], ion exchange membranes that respond to external stimulations [19,20], sensors and biosensors [21–27]. More, in recent years, attention has also been given to the use of conducting polymers as active layers in chemical gas sen- sors, and it has been proved that adsorbed gas molecules (ammonia, NO 2 ,CO 2 ) and organic vapors (alcohols, ethers, halocarbons) cause a change of electrical conductivity in the polymer matrix of organic metals [28–33]. In comparison with most of thecommercially avail- able sensors, based usually on metal oxides and operating at high temperatures, the sensors made of conducting polymers have many improved characteristics. They have high sensitivities and short response time; especially, these feathers are ensured at room tem- perature. ∗ Corresponding author. Tel.: +33 3 63 08 25 78. E-mail address: boris.lakard@univ-fcomte.fr (B. Lakard). Thus, in this paper, an original ammonia gas sensor based on micropatterned microelectrodes functionalized by electropolymer- ization of polypyrrole films is studied. Electrochemical deposition has been chosen since it is the most convenient method to deposit conducting polymer films [34–36]. Indeed, the thickness of the film can be controlled by the total charge passed through the elec- trochemical cell during the film growing process. More, such a deposition also allows the preparation of films at a well-defined redox potential in the presence of a given counter-ion, which then also defines the level and characteristics of the doping reaction [37]. Thus, electropolymerization is used in this study to fabricate a gas sensor consisting in PPy films deposited on microstructured electrode arrays and also across the insulating gap separating the microstructured electrodes of the sensor. Indeed, if the insulat- ing gap between the neighboring electrodes is close enough (a few micrometers), the growing film can cover the insulated gap and connect electrodes [38,39]. This is important in fabricating chemiresistors for gas sensing. A microstructured interdigitated electrode array was chosen since it represents the most suit- able geometry to serve as a transducer in chemical gas sensors, based on conductivity changes. Following their deposition, theelec- tropolymerized polypyrrole layers are characterized by infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Then,polymerfilms are tested as ammonia gas sensors. In particular, their response, in terms of conductance changes when exposed to different ammonia concen- tration, was studied. The reproducibility and the reversibility of the signal exhibited by the PPy films to ammonia exposure but also the influence of temperature on this response are also studied. 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2008.10.056 200 S. Carquigny et al. / Talanta 78 (2009) 199–206 Fig. 1. (a) Experimental set-up used for the analysis of NH 3 vapors and (b) schematic drawing of the ammonia gas sensor. 2. Experimental 2.1. Fabrication of gas sensors The gas sensors were fabricated using microsystem technolo- gies, and in particular using lift-off process that consists in a photolithography followed by a sputtering of platinum on a SiO 2 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  silicon wafer, which was thermally wet-oxidized, at 1200 ◦ C in water vapor, in order to produce a 1.3 ␮m thickness SiO 2 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 36- mJ/cm 2 UV radiation flux delivered by an EVG 620 apparatus, and then without any mask to a 210-mJ/cm 2 UV radiation flux. Thus, the pattern was transferred to the resist, which was then devel- oped, 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 fab- rication 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 gas sensors have been fabricated, thepattern and the dimensions are controlled using an optical microscope. More details about the microsystem fabrication can be found in a previous paper [27]. A microstruc- tured interdigitated electrode array was chosen since it represents the most suitable geometry to serve as a transducer in chemical gas sensors, based on conductivity changes. Thewidth and length of the 100 bands (50 bands on each microelectrode array) were 10 0 ␮m and 9996 ␮m, respectively (Fig. 1). The width of the gap between the two microelectrode array was 4 ␮m to allow the coating of the gap by the polypyrrole film. 2.2. Electrochemistry Pyrrole (Py) and LiClO 4 were obtained from Sigma–Aldrich (ana- lytical grade). Pyrrole was use d at the concentration of 0.05 M in an aqueous solution of 0.1 M LiClO 4 . The electrochemical appara- tus was a classical three-electrode set-up using a Tacussel PGZ301, from Radiometer, potentiostat–galvanostat. The microsystem was used as working electrode. The reference electrode was a saturated calomel electrode (SCE) and the counter-electrode was a platinum wire. All electrochemical experiments were carried out at room temperature (293 K). Cyclic voltammetry experiments were car- ried out with a sweep rate of 100 mV s −1 between −0.3 V/SCE and +1.5 V/SCE. Each solution was purged by ultrahigh purity argon. Chronoamperometry experiments were carried out at a potential of +1.3 V/SCE. 2.3. Characterization of the polymer films 2.3.1. XPS 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 monochro- matized 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% ofGaussian character). 2.3.2. SEM Examinations of polymer morphologies were performed using a high-resolution scanning electron microscope. Once synthesize d and dried, polymer samples were examined in a LEO microscope (SEM LEO stereoscan 440, manufactured by Zeiss–Leica, Köln, Ger- many) with an electron beam energy of 15 keV. 2.3.3. IRTF-ATR All spectra were recorded using a Shimadzu spectrometer (IR-Prestige 21) in ATR reflexion mode. The specific accessory used for these analyses is the ATR Miracle Diamond/KRS5 which allowed us to record spectra between 4000 cm −1 and 700 cm −1 . Resolution was fixed at 4 cm −1 and 60 scans were realized to acquire each spectrum. All samples were constituted with PPy powder. S. Carquigny et al. / Talanta 78 (2009) 199–206 201 Fig. 2. Electrochemical synthesis of polypyrrole films by oxidation of an aqueous solution of pyrrole and LiClO 4 by cyclic voltammetry (a) or chronoamperometry (b). 2.4. Gas measurements An initial ammonia concentration equal to 1000 ppm in nitro- gen was used for the experiment. All the studies were carried out in nitrogen atmosphere. The sensor’s electrical responses were obtained by monitoring the variations in the sensor’s instantaneous conductance versus acquisition time, for a constant temperature of the sensitive layer. The conductance measurements lasted about 3 h for each acquisition. The protocol used for the conductance experiments was the same for each sensor. Each sensor’s electri- cal response was obtained under a constant gas flow (N 2 or NH 3 diluted in N 2 )rateof50mLmin −1 . Specially designed equipment was developed for this study. Mass flowmeters were used to obtain different NH 3 concentrations. This experimental set-up allowed PPy-based gas sensors to be exposed to the different ammonia concentrations. The effect of gases on the sensor’s electrical properties was recorded using a basic divisor voltage bridge (Fig. 1). With these experimental conditions, the relationship between the variation of the sensor’s conductance and the variation of the voltage U R is defined as: G C = 1 R((E/U R ) − 1) Any decrease (or increase) of the sensor’s conductance was recorded as a decrease (or increase) of the electrical signal. Each new sensor was exposed to a constant nitrogen flow for 12 h before conducing each experiment. This process allowed for the desorption of pollutant chemical compounds adsorbed onto the sensitive layer during the storage. 3. Results and discussion 3.1. Electrochemical synthesis of polymer films Electrochemical synthesis of polypyrrole was performed by cyclic voltammetry, from an aqueous solution containing 0.1 M pyr- role and 0.1 MLiClO 4 , on the platinum microelectrodes of the sensor using a potential sweep rate of 0.1 V s −1 between −0.3 V/SCE and +1.5 V/SCE (Fig. 2a). For this aqueous solution of pyrrole, the first scan showed the oxidation of pyrrole at +1.25 V/SCE. Following scans showed the oxidation peak of polypyrrole at +0.5 V/SCE and the reduction peak of polypyrrole at about +0.2V/SCE. Moreover, the polymer film is a conductive one since the current remains constant during all the potential scans. PPy films can also be formed at a constant potential. As in the case of polymer formation by potential scans, the films are homo- geneous and very adherent to the substrate. From Fig. 2a, we chose to carry out the potentiostatic depositions at +1.3 V/SCE. Fig. 2b shows the I–t curves obtained for a fixed potential of +1.3 V/SCE and for a deposition time of 60 s. This chronoamperometric curve shows that, after an increase corresponding to the formation of pyrrole cation radicals, the current decreases following a linear rela- tionship with t −1/2 . This behaviour indicates a diffusion-controlled process. This current response is due to the nucleation and growth of the polymer. At longer times, after the nucleation transient, the chronoamperometry shows a constant I–t response for PPy. 3.2. XPS characterization of the polypyrrole films Fig. 3a shows the XPS of the films obtained from the oxidation of an aqueous solution containing pyrrole and LiClO 4 since this tech- nique is widely used to control the elemental composition of a solid film. The XPS analyses confirm the presence of PPy, incorporating ClO 4 − doping agents, on the platinum surfaces. Indeed, XPS spec- tra of polymer samples reveal the presence of C, N, O, Cl, Pt for all polymers. Thus, C 1s signal (Fig. 3b) can be fitted by five differ- ent 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 NorC 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 PPy at the ␤ carbon site in the pyrrole rings. The N 1s spectra (Fig. 3c) indicate the presence of four peaks in the case of PPy. It contains a main signal at 399.6eV which is characteristic of pyrrolylium nitro- gens ( NH-structure) and a high BE tail (BE = 400.4 and 402.0 eV) attributable to the positively 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. 3d 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 ClO 4 − doping agents are obtained from the oxidation of pyrrole in various solvents. 202 S. Carquigny et al. / Talanta 78 (2009) 199–206 Fig. 3. Survey-scan XPS of polypyrrole films electrosynthesized by oxidation of an aqueous solution of pyrrole and LiClO 4 (a). (b) C 1s (c) N 1s (d) Cl 2p XPS spectra of the same polypyrrole film. 3.3. Morphological characterization Scanning electron microscopy was used to determine the sur- face morphology of the polypyrrole films on the microstructured electrode arrays but also to check that PPy films were deposited across the insulating gap on the microstructured electrode arrays. Thus, Fig. 4 shows that the whole surface of the platinum micro- electrodes is coated by a homogeneous and very compact film of polypyrrole composed of many nodules (1–2 ␮m long). The mean Fig. 4. SEM image of PPy film grown on the sensor surface. film thickness of this polypyrrole film (x) was estimated to 2.25 ␮m 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 ()wastakenas1.5gcm −3 and an electron loss z of 2.25 was considered. More, Fig. 4 shows that nodules of PPy are also present in the insulating gap between the microelectrodes indicating that the growing film covers the insulated gap and connect microelectrodes. This point is important since the PPy layer must connect each pair of interdigitated electrodes in order to obtain the sensitive layer of the gas sensor. 3.4. Evaluation of the sensor’s electrical signal under NH 3 flow Firstly, the sensor was exposed to a NH 3 flow at a concentration equal to 500 ppm with a temperature of the sensitive layer near to room temperature. The signal’s electrical variation was recorded versus time. Fig. 5a represents the evolution of the PPy’s conduc- tance in presence of ammonia and nitrogen. The curve shows the evolution of the electrical signal when the sensor is first stabilised under N 2 flow (until 300 s.), second exposed to NH 3 flow (from 300 s to 6000 s) and then to nitrogen flow (from 600 s to 14,000 s). This acquisition protocol was used for all the experiments described in this paper. In presence of pollutant in the gas chamber (NH 3 ), we clearly notice a decrease in sensor’s sensitive layer conductance. Ammo- nia reacts with the PPy and induces a modification of the sensor’s sensitive layer electrical properties. After ammonia exposition, the S. Carquigny et al. / Talanta 78 (2009) 199–206 203 Fig. 5. Sensor’s electrical response under NH 3 (500 ppm) and N 2 flow. sensor is submitted to a nitrogen flow (after 6000 s). Fig. 5a indi- cates an increase of the sensor’s conductance. This modification of conductance can be attributed to the desorption of ammonia from sensitive layer. Among to this electrical variation under NH 3 flow, it is possible to obtain one supplementary information. Looking at the beginning of the exposition to pollutant flow, the conductance variation is linear with time. In this way, the calculation of the slope value gives us information about the sensitivity of the gas sensor. For a concentration of ammonia equal to 500 ppm the value of the slope equals to 63.20 nS s −1 . In order to evaluate a possible reproductibility of the gas sensor under ammonia flow at room temperature, we studied two succes- sive electrical responses of the same gas sensor under pollutant. The purpose was to compare thesensor’s conductance between two successive acquisitions. In Fig. 5b is represented the first electrical response obtained under a constant NH 3 flow and nitrogen flow. The second curve shows the successive response under NH 3 flow and N 2 flow after a nitrogen flow exposition of the sensitive layer during 12 h at room temperature. As shown in Fig. 5b, we notice a superposition of the signal under ammonia flow during the first minutes of acquisition. If we consider that the sensor’s electrical response is measured by referring to experimental point obtained at the beginning of the exposition of the sensor, one can say that the electrical signal is reproductible with the same sensor at room tem- perature. The values of the slope are nearly the same (63.20 nS s −1 and 65.42 nS s −1 ). By comparing the two acquisitions (Fig. 5b curves 1 and 2), when the sensor is rinsed with nitrogen flow, the second electrical response is slightly shifted. This phenomenon is probably due to a chemical modification of thesensitive surface after a first detection. This point will be confirmed with infrared analysis. Various ammonia concentrations from 8 ppm to 1000 ppm were tested using the same sensor. Fig. 6a shows some of the electri- cal responses obtained for various ammonia concentrations. Before each pollutant expositions, the gas sensor is stabilised by nitrogen flow during 12 h at room temperature. Fig. 6a shows a decrease of the instantaneous conductance for each NH 3 concentrations. This decrease depends on the concen- trations of the pollutant in the gas chamber. In particular, if we determine the value of theslope of the electrical responses obtained for each ammonia concentrations, we can plot the variation of the slope versus ammonia concentrations. Fig. 6b represents the evolution of the slope versus the ammonia concentration. Above concentrations of 500 ppm, we noticed a smooth plate which was due to the saturation of the sensitive layer. For lower concentra- tions, there is a linear relationship between the value of the slope and the ammonia’s concentration. 3.5. Influence of the sensitive layer’s temperature on the sensor’s electrical signal under NH 3 flow Chemiresistors, based on metallic oxides, generally works at high temperatures (about 450 ◦ C) in order to optimize the elec- trical response. In order to evaluate the impact of temperatures on the electrical signal of the PPy-based gas sensor, the sensing layer was heated at temperatures ranging from 25 ◦ Cto100 ◦ C(Table 1). A concentration of ammonia equals to 500 ppm was used for this experiment. Curves show the evolution of the electrical signal when Fig. 6. Sensor’s electrical responses to various ammonia concentrations (a). Slope of the gas sensor’s electrical response vs. NH 3 concentrations (b). 204 S. Carquigny et al. / Talanta 78 (2009) 199–206 Fig. 7. Infrared spectra of polypyrrole powder before and after an exposition to ammonia flow for 1 h. Table 1 Values of the slope under ammonia flow at different temperatures. PPy’s temperature ( ◦ C) Slope (nS s −1 ) 25 116.72 50 72.29 75 74.04 100 78.57 the sensor is first stabilised under N 2 flow (until 400 s), second exposed to NH 3 flow (from 400 s to 1200 s) and then to nitrogen flow (from 1200 s to 3500 s). Looking at Table 1 which represents the values of the slope for each temperature, we understand that an increase of the PPy’s sen- sitive layer temperature decreases the sensitivity of the gas sensor. The best sensitivity was obtained at room temperature. Conse- quently, in term of power consumption, the PPy-based gas sensor show very interesting detection properties compared to resistive sensors which have higher working temperatures (300–500 ◦ C). 3.6. Interaction mechanism In order to understand the interaction mechanism between ammonia and the gas sensor’s sensitive surface we proceed to an infrared characterization of PPy powder before and after being exposed to NH 3 flow. First we characterized the PPy powder unexposed to NH 3 vapors. The spectrum represented in Fig. 7a shows the presence of broad absorption bands characteristics of polypyrrole material. These absorption bands corresponds to: N H stretching in sec- ondary amine (3600 cm −1 ), N H deformation in secondary amine (1600 cm −1 ), C C aromatic stretching (1400 cm −1 )etC N stretch- ing (1050cm −1 ). Then, in order to understand the mechanism of ammonia adsorption onto PPy surfaces, polypyrrole samples were exposed during 1 h to NH 3 1000 ppm flow. The spectra obtained for this sample, compared to PPy under N 2 , is represented in Fig. 7b. Com- pared to the PPy powder spectrum (Fig. 7a), two broad absorption bands appeared when PPy was exposed to NH 3 flow. The first one at 3260 cm −1 may be attributed to the stretching vibration of N H binding in NH 3 +◦ radical group. The second one seems to be superposed to the C C aromatic stretching band centered at 1400 cm −1 . This latter band may be attributed to N H bend- ing vibration. These results confirmed that the interaction between NH 3 and PPy-induced chemical modifications of the sensitive layer. These infrared analyses explained the shift observed between two successive NH 3 detections using PPy-based gas sensors. According to the infrared results, we propose an interaction mechanism for the adsorption of ammonia onto polypyrrole thin films. The different stages of ammonia adsorption onto the PPy layer which is considered as a p-type semi-conducting material (positive hole conduction) are the following ones: S. Carquigny et al. / Talanta 78 (2009) 199–206 205 The first step of this mechanism is the lost of an electron by the doublet of nitrogen of some nitrogens of the polymer backbone. This electron transfer between ammonia molecule and the poly- mer’s positive hole induces a diminution of the sensitive positive charge density which leads to a decrease in the conductance layer. After adsorption of NH 3 , the polymer becomes less conducting. In this mechanism, it is proposed that ammonia is adsorbed onto PPy surface forming NH 3 + ◦ radical groups according to infra-red spec- tra. This mechanism is completing the various works realised on the ammonia detection studies using PPy-based gas sensors [40,41]. 3.7. Comparison with other works Before this study, other authors used conducting polymer films to develop gas sensors. These polymer films were obtained using different techniques. The most often used technique was the chemical deposition by dip-coating [42–45], and the oth- ers were: spin-coating from soluble conducting polymers [46,47], thermal evaporation by heating and deposition of the conduct- ing polymer on a substrate [48], vapor deposition polymerization [49], drop-coating of a dried polymer solution [50,51], UV- photopolymerization [52], deposition of Langmuir–Blodgett film [53] and electrochemical deposition [54,55]. We decide to use this latter technique since the thickness of the film can be controlled by the total charge passed through the electrochemical cell dur- ing film growing process. Moreover, the film can be deposited on patterned microelectrode arrays [38]. However, if the insulat- ing gap between the neighboring electrodes is close enough, the growing film can cover the insulated gap and connect electrodes [39]. Amongst the various polymer films, polypyrrole is one of the most studied andinteresting in particularthanks to its high conduc- tivity. Consequently, many papers have already used this polymer as active layer of gas sensors. Thus, PPy obtained by chemical oxi- dation was used for the detection of CO [56],CO 2 [57], xylene [58], alcohols [43,59,60] or acetone [61]. PPy obtained by vapor deposition polymerization was also used for the detection of methanol, ethanol, CCl 4 and benzene [62]. PPy obtained by UV- photopolymerization was used for the detection of sevoflurane [52]. It can also be noticed that the surfaces coated by PPy in all these studies were either gold microelectrode arrays deposited on aluminia substrates [43,52,56,58,60] or ITO substrates [57,59,62]. Polypyrrole deposited by electrochemical way was also incorpo- rated in gas sensors for the detection of ethanol gas [54], benzene, xylene and toluene [55]. Other studies focused on ammonia gas sensors using polypyr- role as sensitive layer. Thus, an ammonia gas sensor based on Langmuir–Blodgett PPy film was developed but its lower detectable limit was of 100 ppm of NH 3 in N 2 [53]. Bai et al. have electrochemi- cally co-polymerized polypyrrole and sulfonated polyaniline on an ITO substrate to obtain an ammonia sensor but it was efficient only for ammonia concentration higher than 20 ppm [63]. Another study from Brie et al. presents an ammonia sensor using electrosynthe- sized PPy film, with various doping agents, but this study is limited to concentrations higher than 10 ppm [32]. Thus, only a study by Guernion et al. presented an ammonia sensor giving a response below 10 ppm but in this study PPy is chemically oxidized on a poly(etheretherketone) surface [64]. Concerning ammonia sensors using polymer films deposited on microelectrode arrays, an inter- esting work was carrie d out by Lin et al. [65]. In this work an electrosynthesized copolymer PPy–poly(vinyl alcohol) was used and was efficient for ammonia gas concentrations ranging from 50 ppm to 150 ppm. Consequently, the results obtained in our study are competitive with all these results since the ammonia gas sen- sors developed in this paper showed a detectable limit of 8ppm of NH 3 in N 2 . More, the best responses were obtained at room temperature and were reproductible. 4. Conclusion The aim of this work was to validate the use of polypyrrole-based gas sensor for the detection of ammonia at concentrations lower than 10 ppm. From this study we first electrosynthesized PPy films doped with small anions ClO − 4 on metallic electrodes to develop a chemical resistor gas sensor. A homogeneous polymer deposited film with a thickness close to the micrometer was obtained. The various tests conducted under ammonia flow showed an interest- ing sensitivity (lower than 10 ppm) and a good reproductibility. 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