DSpace at VNU: A conductive polypyrrole based ammonium ion selective electrode

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A CONDUCTIVE POLYPYRROLE BASED AMMONIUM ION SELECTIVE ELECTRODE DO PHUC QUAN, CHU XUAN QUANG, LE THE DUAN and PHAM HUNG VIET∗ Centre of Environmental Chemistry, Vietnam National University, Hanoi, Vietnam (∗ author for correspondence, e-mail: cec@fpt.vn) Abstract In view of the development of miniaturized sensor arrays, a solid-contact ammonium ion selective electrode has been investigated A conductive polypyrrole film was electrochemically deposited on a glassy carbon surface and used as an internal solid contact layer between the sensing membrane and solid electrode surface A systematic evaluation of the important parameters affecting the electromotive force (emf) response is presented The performances of this solid-contact sensor were verified using a batch-mode measurement setup and a wall-jet flow cell system The designed sensor exhibited excellent selectivity for the primary ion and a linear response over the pNH+ range 1–5 with a slope of 56.3 mV decade−1 The sensor has a fast response and is relatively robustness, and was also used to determine ammonium concentrations in natural waters, with promising results Keywords: ammonium, conductive polypyrrole, ion selective electrode, solid-contact sensor Introduction The presence of ammonium ions in environmental samples can indicate the extent of pollution and the eutrophication of natural water (Nigel, 1994; Gerald et al., 1999), and over recent years, the growing importance of controlling the levels of environmental pollutants has increased interest in the development of novel sensors for the detection of these ions (Erkang et al., 1997; Magalhaes et al., 1997; Deviteri and Diamond, 1994; Peter et al., 1997) Ion selective electrodes (ISES) offer a simple and useful method for the direct detection of inorganic ammonium ions They offer great advantages, which include, speed and ease of preparation, simplified procedures, relatively fast response, reasonable selectivity, and a wide linear dynamic range, at a relatively low cost Furthermore, ISE developments offer the possibility of sensor miniaturization, based on solid-state ion sensors, which use solvent polymeric membranes that allow the sensing liquid membrane to be cast on the solid electrode surfaces and eliminates the need for an internal solution (Henry, 1987; Lemke and Cammann, 1989) However, instability is a problem frequently encountered with solid-state ion selective electrodes It is generally agreed that this is caused by the lack of a stable internal reference potential at the boundary between the sensing membrane and the inner reference element Attempts have been made to overcome this problem, notably by the development of solid contact membrane sensors, in which, the transfer Environmental Monitoring and Assessment 70: 153–165, 2001 © 2001 Kluwer Academic Publishers Printed in the Netherlands 154 DO PHUC QUAN ET AL from ionic to electronic conductivity is provided by a solid contact layer having a mixed ionic and electronic conductivity between the inner reference element and the sensing membrane Using this approach, a photo-cured ammonium electrode (Peter et al., 1997), and an ammonium-ISFET sensor (Cecilia and Jordi, 1997) based on a graphite-epoxy composite used as the conductive layer between the ISFET surface and the PVC membrane, have been fabricated Recently, the use of conducting electroactive polymers, such as those incorporating polypyrrole, polythiophene or polyaniline as the active components of amperometric sensors have attracted much intention These are based on the system’s ability to incorporate and expel ionic species during the switching of the polymer from the oxidized to the reduced state (Erkang and Anhua, 1991; Omowunmi and Wallace, 1994; Barisci et al., 1997) Further application of conducting polymers in potentiometric sensors involve the incorporation of an organic solvent soluble ionophore in the polymer, which then allows the fabrication of wholly solid-state potentiometric sensors (Pia et al., 1999) In this study, it was found that adding the polypyrrole film by electropolymerization, as solid contact layer, significantly improves the potentiometric stability of solid-state potentiometric sensors, which is believed to be the result of a better defined interfacial potential between the sensing membrane and the solid electrode contact We report upon the determination of ammonium levels in a range of natural waters using such a solid-contact ammonium ion selective electrode Materials and Methods 2.1 M ATERIALS All reagents used were of analytical reagent grade Standards and buffer solutions were prepared with Milli-Q water Pyrrole purchased from Fluka was redistilled under vacuum prior to use and covered with aluminium foil in the refrigerator to prevent UV degradation Nonactin and monactin were kindly donated by Dr Beat Muller of EAWAG, Switzerland 2-nitrophenyl-octyl ether (2-NPOE), bis(1-butylpentyl)adipate (BBPA), high molecular weight polyvinyl chloride (PVC), potassium tetrakis(4chlorophenyl)borate (KT4ClPB) and tetrahydrofuran (THF) were obtained from Fluka (Buchs, Switzerland) 2.2 E LECTRODE PREPARATION 2.2.1 Preparation of a Conventional Internal Electrolyte Electrode The membrane components in Table I (200 mg in total) were dissolved in mL of fresh the distilled THF This solution was placed in a glass ring of 24 mm i.d resting on a glass plate After solvent evaporation overnight, the resulting membrane was peeled off the glass mould and discs of mm i.d were cut out, and mounted A CONDUCTIVE POLYPYRROLE BASED AMMONIUM ION SELECTIVE ELECTRODE 155 TABLE I Composition of the ammonium solvent polymeric membrane mixtures prepared in this study Membrane composition Membranes AM1 AM2 AM3 AM4 (w./w %) Nonactin/monactin 2-Nitrophenyl-octyl ether Dioctyl sebacate Bis(1-butylpentyl) adipate Potassium tetrakis(4-chlorophenyl) borate in a 70% molar to the inophore High molecular weight PVC 67 – – – – 67 – – – – 67 – – – 67 30 30 30 30 in Philips IS 561 electrode bodies (Eindhoven, The Netherlands) for electromotive force (EMF) measurements in batch-mode setup A 10 mM solution of NH4Cl was used as the internal filling solution 2.2.2 Fabrication of the Solid-Contact Ion Selective Electrode Polypyrrole (PPy) synthesis and characterization were performed in a conventional three electrode system comprising, the mm i.d glassy carbon disc (GC) working electrode (6.1204.110 GC, Metrohm, Switzerland), a platinum wire gauze auxiliary electrode and an Ag/AgCl (3 M NaCl) reference electrode, against which all potentials were measured A laboratory-made microgalvanostat was used for the electropolymerization Chronopotentiograms were recorded using this instrument with data acquisition system support Cyclic voltammetric measurements of conductive polymer were performed using a PC-controlled system for Voltammetry (Model 757 VA Computrace, Metrohm, Switzerland) Before polymerization, the surface of the glassy carbon working electrode was polished on a polishing cloth with alumina slurry (0.05 µm) and then cleaned with double distilled water and finally in a water-filled ultrasonic bath for 30 sec The polypyrrole film was prepared by anodic galvanostatic electropolymerization of the pyrrole monomer from aqueous solution (0.5 M) onto the electrode surface The counterion solution used for polymerization contained M of chloride Solution was deoxygenated with nitrogen for to remove any trace of oxygen from the solution, prior to the polymer synthesis A current density of mA cm−2 for 150 sec was used to achieve electropolymerisation 156 DO PHUC QUAN ET AL Scheme The polymeric membrane-coated modified electrode was a solution of a mixture of ammonium membrane AM4 solution (30 µL) was applied directly on the top of the polypyrrole film, then dried for hr under a gently nitrogen atmosphere, and conditioned overnight in 10 mM of the selected primary ion solution prior to any measurement 2.3 P OTENTIOMETRIC MEASUREMENTS Batch-mode potentiometric measurements were made while stirring at a constant rate and with the electrodes immersed to the same depth in the solution The calibration curve was obtained by a standard addition method, involving the additions of 10−6 to 10−1 M of the primary ion The potentials were measured against a double junction Ag/AgCl reference electrode (Orion 90-02-00) using a 692 pH/Ions meter (Metrohm, Switzerland), the accuracy of the potential measurements were +0.1 mV Flow injection potentiometric measurement was performed with a simple flow manifold including a four channel peristaltic pump (Ismatec - Switzerland), a low pressure six way injection valve (5020 Rheodyne) with 100 µL sample loop and a wall-jet flow cell (Metrohm, Switzerland) The carrier eluent in the flow injection experiments contained mM of sodium acetate and mM of sodium chloride including µM of ammonium, to provide base line stability All measurements were carried out at room temperature Results and Discussion 3.1 C YCLIC VOLTAMMETRY MEASUREMENTS A polypyrrole film was deposited on electrode surface by oxidation of monomer A CONDUCTIVE POLYPYRROLE BASED AMMONIUM ION SELECTIVE ELECTRODE 157 Figure Cyclic voltammetry of a PPy/Cl electrode in 0.1 M of different cation solutions (a) NH+ 4; (b) K+ ; (c) Na+ ; (d) Li+ at scan rate 100 mV sec−1 from an aqueous solution containing the appropriate counterion as supporting electrolyte Chronopotentiograms were recorded during film growth Relatively constant potential was observed throughout the polymerization, indicating the formation of a conductive polymer layer Since the polypyrrole film plays the role of the solid contact layer in the ion selective electrode, the investigation of the incorporation process of ions of interest into the polymer film is of great importance to the development process Counterion injection and release to accompany the redox cycling of electropolymerized polypyrrole films in electrolytes is well known Recent studies (Lien et al., 1991; John and Wallace, 1993) of the doping-dedoping process occurring at polypyrrole have reported that, at least in some instances, two distinct processes occur, as shown in Scheme 1, one involving anion transport and the other cation transport The degree to which each of these processes occur depends upon the nature of the polymer materials used and the mobilities of the corresponding anion and cation in solution and through the polymer In the present study, cyclic voltammograms recorded after the electropolymerization of the polypyrrole indicated that polymer PPy/Cl was conductive in the 158 DO PHUC QUAN ET AL Figure Cyclic voltammetry of a PPy/Cl electrode in 0.1 M of different cation solutions (a) NH+ 4; (b) Ba2+ ; (c) Ca2+ ; (d) Mg2+ at scan rate 100 mV sec−1 counterion solution In order to investigate the incorporation of cation into the polypyrrole film, cyclic voltammogram measurements of the PPy/Cl electrode in solutions of different cations were taken The most striking feature of these voltammograms was the fact that the second reduction wave which occurs at a more negative potential was present in the univalent cation solutions (Figure 1) Figure also indicates that the second reduction response moved in the positive direction with a decrease in the solvated cation size according to Li+ > Na+ > K+ > NH+ The absence of a second reduction response in the divalent cation solutions (Figure 2) suggests that polymer was not reduced to the extent that occurred in the univalent cation solutions This was confirmed by the fact that the subsequent oxidation peaks were smaller in solutions containing the divalent cations 3.2 P OTENTIOMETRIC RESPONSE CHARACTERISTICS OF ELECTRODES Principle electrode characteristics, such as a Nerntian slope, dynamic linear range, detection limit and selectivity, depend on the composition of the ion selective membrane One of the first tasks in a development of this type involves the determ- A CONDUCTIVE POLYPYRROLE BASED AMMONIUM ION SELECTIVE ELECTRODE 159 TABLE II Determination of ammonium in natural water samples using the designed solid-contact ammonium selective electrode as compared with the ammonia gas electrode Real samples were prefiltered through filter membrane of 0.45 µm pore size Sample place Solid-contact NH+ selective liquid membrane electrode NH3 gas electrode (Orion Inc., U.S.A.) −1 (SRD) Ammonium (NH+ ) in groundwater samples, mg L Phap Van Ha Dinh Yen Phu Tuong Mai 13.9 10.2 4.7 6.9 (0.88%) (0.87%) (0.79%) (0.82%) 13.4 9.5 4.4 6.4 (0.95%) (0.92%) (0.86%) (0.89%) −1 (SRD) Ammonium (NH+ ) in river and ponds water samples, mg L Thanh Tri fish pond Thanh Tri fish pond Kim Nguu River To Lich River Lu River 3.8 5.1 5.5 5.2 5.0 (1.45%) (1.46%) (1.53%) (1.51%) (1.48%) 3.2 4.6 5.0 4.8 4.3 (1.49%) (1.52%) (1.62%) (1.58%) (1.53%) ination of the best composition for the preparation of a solid-contact ammonium ion selective electrode The optimal composition of the ion selective membrane for the ammonium was determined in several experiments by varying the nature and the percentage of various liquid membrane plasticizers, results are summarized in Table II Three ammonium membrane electrodes employing three different plasticizers were prepared The electrode responses observed for the AM1, AM2 and AM3 ISEs were both Nernstian in character over range between 0.01 and 100 mM in pure ammonium chloride solutions However, the AM2 and AM3 ISE gave superior response characteristics with a Nernstian slope of 57.6 and 58.3 mV dec−1 , respectively Furthermore, this study also indicated that anion interference at concentrations >100 mM, for the liquid membrane – based ISE, are more evident for a polar plasticizer, such as NPOE The severity of the anion interference is also dependent on the lipophilicity of the anion, that is, the greater the lipophilicity of the anion, the greater the penetration of the anion into the liquid membrane, and consequently, the greater the interference exerted upon the cation selective membrane at higher concentrations Therefore, in order to reduce anion interference at higher concentrations, a lipophilic salt, such as potassium tetrakis(4-chlorophenyl) borate, 160 DO PHUC QUAN ET AL Figure Charge transfer process occurring in (A) a conventional internal electrolyte selective electrode; and (B) in a solid-contact conducting polymer based electrode was added to the membrane In the following section, we describe a solid-contact sensor that was fabricated with the optimal AM4 mixture composition Figure compares the charge transfer process occurring in a conventional internal electrolyte selective electrode with that in a solid-contact conducting polymer based electrode Comparisons of the charge transfer between the sensing membrane and inner element of both ammonium electrodes showed similar trends The potential response of the solid-contact electrode was very stable in the measured ammonium concentration range Calibration curves for a conventional internal electrolyte electrode and the solid-contact electrode in ammonium solutions, are shown in Figure This result demonstrates that the response was linear over the investigated range of 10−5 to 10−1 M L−1 of ammonium with a slope of 56.3 mV dec−1 Response time was also investigated, because it is a very important factor in terms of the practical use of solid-contact sensors The time taken for the both electrodes to attain 90% of the steady-state response was typically a few seconds, suggesting that this electrode is ideal for flow injection measurement A CONDUCTIVE POLYPYRROLE BASED AMMONIUM ION SELECTIVE ELECTRODE 161 Figure Comparison of ammonium ion concentration calibration curves for the internal electrolyte ammonium ISE and the solid-contact ammonium electrode in mM NaCl, to adjust the ionic strength The selectivity of solvent polymeric membrane electrodes in the presence of interfering cations were determined by the separateg solution method (SSM) (Morf, 1981) and calculated using Equation (1) pot log Ki,j = zi (Ej − E) + 1− S zj lg , (1) where Ki,j is the selectivity coefficient, i is the primary ion (NH+ ), j is the interfering ion, E is the measured potential (mV), S is the Nernstian slope factor (mV/decade), z is the electrical charge of the ion, and a is the activity calculated from the activity coefficients The selectivity coefficient values obtained by the separate solution method are given in Figure and are compared to the selectivity coefficients reported for ammonium electrodes in previous studies (Thomas et al., 1988) Comparison of the selectivity coefficients of interfering ions showed that the potassium ion is the most notable interference ion of the ions tested The solid-contact ammonium ion selective electrode AM4SCS was mounted in the FIA system described in the Experimental Section Typical FIA signals obpot 162 DO PHUC QUAN ET AL pot Figure Selectivity coefficients, logKNH ,j for the ammonium selective electrodes made during this work, compared to a previously reported ammonium ISE Results were obtained by the separate solution methods in Tris buffered solutions of 0.1 M chloride salts at pH 7.1 AM (1–4): four conventional internal electrolyte ammonium selective electrodes; ISE∗ : ammonium ion selective microelectrode (Thomas et al., 1988); and AM4SCS: solid-contact ammonium ion selective electrode tained for series injection of different ammonium solutions are shown in Figure 6, which demonstrates the high reproducibility of the observed peak heights The potentiometric response was linear over the range investigated i.e., between 10−5 and 10−1 M L−1 of ammonium 3.3 WATER ANALYSIS Finally the analytical performance of the solid-contact ammonium ion selective electrode was tested in terms of the determination of ammonium in natural water The samples of natural water used were representative complexes of real matrices containing high concentrations of inorganic and organic substrates Moreover, the concentration of ammonium in these samples represented an important quality level determining factor In order to verify the analytical results obtained using this A CONDUCTIVE POLYPYRROLE BASED AMMONIUM ION SELECTIVE ELECTRODE 163 Figure Potentiometric responses of solid-contact ammonium selective electrode in an FIA system Flow injection conditions were as reported in the Experimental Section electrode, the same samples were analyzed using an ammonia gas sensor, and the results obtained using the two techniques were found to be close agreement These results demonstrate clearly the benefits of the features of the ammonium solidcontact electrode in a flow injection system for the determination of ammonium levels in real complex matrices Conclusions The solid-contact ion selective electrode, based on a conductive polypyrrole membrane, showed good sensitivity in the linear pNH+ range 1–5 with a Nernstian slope of 56.3 mV dec−1 This sensor exhibited a fast response (5 sec) and relatively high robustness in both batch-mode measurements and flow injection systems, using a wall-jet flow cell It is hoped that this study will contribute to the design and construction of an ammonium selective solid-contact electrode based 164 DO PHUC QUAN ET AL on integrated planar sensor technology Further investigations to improve the electrode, involving lowering the detection limit and lengthening the life-time of the solid-contact sensor will be conducted in our laboratory The use of this conceptual approach for development of solid-contact selective sensors for other relevant environmental ions, such as nitrite and potassium remains challenging work Acknowledgements This work was supported by the Vietnam National University, Hanoi (Grant QGTD.99.02) and partly supported by the SDC (Swiss Agency for Development and Cooperation, Switzerland) in framework of the co-operation Project ESTNV between CEC, VNU Hanoi and EAWAG, Switzerland in the field of Environmental Chemistry Do Phuc Quan express his deep gratitude to Prof Dr Gordon G Wallace, Director of the Intelligent Polymer Research Institute (University of Wollongong, Australia) for the effective support during his former academic stay in the Gordon G Wallace’s Laboratory and for many challenging discussions The authors also express their thankfulness to thank Dr Beat Müller (EAWAG, Switzerland) for his kind donation of the nonactin/monactin ionophore References Barisci, J N., Wallace, G G and Clarke, A.: 1997, ‘Amperometric detection of electroinactive anions using conducting polymer electrodes subsequent to chromatographic separation’, Electroanalysis 9(6), 461 Beck, F and Oberst, M.: 1987, ‘Electrodeposition and cycling of polypyrrole’, Markromol Chem Macromol Symp 8, 97–125 Buhrer, T., Gehrig, P and Simon, W.: 1988, ‘Neutral-carrier-based ion-selective microelectrodes design and application A review’, Anal Sci 4, December Jimenez, C and Bartoli, J.: 1997, ‘Development of an ion-sensitive 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A CONDUCTIVE POLYPYRROLE BASED AMMONIUM ION SELECTIVE ELECTRODE 159 TABLE II Determination of ammonium in natural water samples using the designed solid-contact ammonium selective electrode as... L−1 of ammonium 3.3 WATER ANALYSIS Finally the analytical performance of the solid-contact ammonium ion selective electrode was tested in terms of the determination of ammonium in natural water... water The samples of natural water used were representative complexes of real matrices containing high concentrations of inorganic and organic substrates Moreover, the concentration of ammonium