Overoxidized polypyrrole film directed single-walled carbon nanotubes immobilization on glassy carbon electrode and its sensing applications

Biosensors and Bioelectronics 22 (2007) 3120–3125

Overoxidized polypyrrole film directed single-walled carbon nanotubes immobilization on glassy carbon electrode and its sensing applications Yongxin Li a,b,∗ , Po Wang a , Lun Wang a , Xiangqin Lin b,∗∗ a

Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China b Department of Chemistry, University of Science and Technology of China, Hefei 230026, PR China Received 28 July 2006; received in revised form 27 January 2007; accepted 6 February 2007 Available online 11 February 2007

Abstract In this paper, the films of overoxidized polypyrrole (PPyox) directed single-walled carbon nanotubes (SWNTs) have been electrochemically coated onto glassy carbon electrode (GCE). Electroactive monomer pyrrole was added into the solution containing sodium dodecyl sulfate (SDS) and SWNTs. Then, electropolymerization was proceeded at the surface of GCE, and a novel kind of conducting polymer/carbon nanotubes (CNTs) composite film with the orientation of CNTs were obtained correspondingly. Finally, this obtained polypyrrole (PPy)/SWNTs film modified GCE was oxidized at a potential of +1.8 V. It can be found that this proposed PPyox/SWNTs composite film modified GCE exhibited excellent electrocatalytic properties for some species such as nitrite, ascorbic acid (AA), dopamine (DA) and uric acid (UA), and could be used as a new sensor for practical applications. Compared with previous CNTs modified electrodes, SWNTs were oriented towards the outside of modified layer by PPyox and SDS, which made the film easily conductive. Moreover, this proposed film modified electrode was more stable, selective and applicable. © 2007 Elsevier B.V. All rights reserved. Keywords: Overoxidized polypyrrole; Carbon nanotubes; Electrocatalytic oxidation; Glassy carbon electrode

1. Introduction Since the discovery of carbon nanotubes (CNTs) in 1991 (Iijima, 1991), their unique structural, mechanical, electronic, and thermal properties have stimulated multi- and interdisciplinary research activities in various fields (Dresselhaus et al., 1996). As a consequence, CNTs can serve as excellent substrates for the development of sensor and biosensors devices (Wang and Musameh, 2003; Musameh et al., 2002; Lim et al., 2005; Baughman et al., 2002; Gong et al., 2004a,b) and/or as a modifier to promote electron transfer reactions between biomolecules with the underlying electrode (Luo et al., 2001; G. Wang et al., 2002a; J. Wang et al., 2002b; Wang et al., 2003; J.H. Chen et al., 2001a; R.J. Chen et al., 2001b; Britto et al., 1996; Davis et ∗

Corresponding author at: Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China. Tel.: +86 553 3869320; fax: +86 553 3869303. ∗∗ Corresponding author. Tel.: +86 553 3869320; fax: +86 553 3869303. E-mail address: [email protected] (Y. Li). 0956-5663/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.bios.2007.02.001

al., 1997). To exploit the potential applications in future nanodevices, it is necessary to develop versatile approaches to assemble or integrate CNTs onto solid surface. Some common methods, such as solution-casting (G. Wang et al., 2002a; J. Wang et al., 2002b; Wang et al., 2003) and layer-by-layer (LBL) selfassembly technique (Huang et al., 2005; Rouse and Lillehei, 2003; Zhang et al., 2004a,b,c; Qin et al., 2005), were developed for immobilizing CNTs on the surface of electrodes. However, it must be pointed out that in these reported works, the CNTsmodified electrodes are unstable and irreproducible. Although some creative efforts (for example, covalent modification methods, Liu et al., 2000; Profumo et al., 2006 and sol–gel method, Gong et al., 2004a,b) on the construction of the CNTs-based electrodes have almost witnessed the possibility to resolve above problems, a simple method for fabrication of stable CNTs-based electrodes is still desired for electrochemical studies and for reliable and durable electrochemical applications. Recently, conducting polymer/CNTs composites have been intensively studied to improve the conductivity, electronic transport, and electromagnetic properties for the applications in

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nanoelectronic elements and electro-optical devices (Chen et al., 2000; An et al., 2002; Zhang et al., 2004a,b,c, 2005; Ramanathan et al., 2005; Hughes et al., 2002). Some conducting polymer/CNTs composites modified electrodes have been applied to ˇ glucose (Slijki´ c et al., 2006; Callegari et al., 2004), nitrite (Guo et al., 2005) and DNA (Cheng et al., 2005) sensing. However, most conducting polymer/CNTs composites have good conductivity, but the background currents of these composites modified electrodes are much larger. If these conducting polymer/CNTs composites based on the surface of electrodes are directly used as sensors, the sensitivities of these methods are relative low, even some important species cannot be detected. So, it is necessary to solve this problem when these conducting polymer/CNTs were used as sensors. Polypyrrole (PPy), one of the most important conducting polymers, have potential applications because of its facile preparation, high conductivity and good environmental stability (Pernaut and Reynolds, 2000; Malitests et al., 1990; Hepel, 1998; J.H. Chen et al., 2001a; R.J. Chen et al., 2001b). However, most of the primary studies were made use of the conductivity of PPy. As is well known, the PPy film can be further overoxidized at higher potentials (Asavapiriyanont et al., 1984), resulting in an insulating membrane with a microporous structure. Herein, we describe a simple strategy for immobilizing single-walled carbon nanotubes (SWNTs) on the surface of glassy carbon electrode (GCE) by direct oxidation of pyrrole in 0.1 M aqueous solution of sodium dodecyl sulfate (SDS) containing a certain amount of SWNTs. The obtained PPy/SWNTs composites modified GCE transferred to a phosphate buffer solution (PBS) for electrochemical oxidation at +1.8 V (versus SCE) for 250 s. So the PPyox/SWNTs film modified GCE was obtained. The SWNTs in PPyox/SWNTs composites film could be oriented towards outside of the composite film, and it can promote the electron transfer between solution and GCE. Moreover, it is found that this PPyox directed SWNTs films modified electrode have good conductivity, stability and applicability. Finally, the sensing applications of this PPyox/SWNTs composites-modified GCE were studied carefully. The results showed that some species such as nitrite, ascorbic acid (AA), dopamine (DA) and uric acid (UA) could be electrocatalytic oxidized at the PPyox/SWNTs film modified GCE, and this proposed PPyox/SWNTs film modified GCE could be used as a sensitive sensor for the assessment of nitrite, AA, DA and AA. 2. Experimental 2.1. Chemicals Pyrrole was obtained from Aldrich and purified twice by distillation under the protection of high purity nitrogen and then kept in refrigerator before use. Single-walled carbon nanotubes (SWNTs) used in this work were purchased from Nanoport Co. Ltd. (Shenzhen, China). The as-received SWNTs were treated under sonication in 1:3 concentrated nitric–sulfuric acid at ca. 50 ◦ C for 25 min. Dopamine hydrochloride (DA) was obtained from Sigma (USA). SDS, ascorbic acid (AA), uric acid (UA) and sodium nitrite were obtained from Shanghai Chemical Co. Ltd.

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(Shanghai, China). Hexaammineruthenium (III) chloride (A.R., Ru(NH3 )6 Cl3 ) and potassium ferricyanide (A.R., K3 Fe(CN)6 ) were obtained from Alfa Products. All other reagents used were of analytical grade. Phosphate-buffered saline (PBS; 0.1 M) solutions of different pH were prepared by mixing four stock solutions of 0.1 mol/l H3 PO4 , KH2 PO4 , K2 HPO4 and K3 PO4 (Shanghai Chemical Company, Shanghai, China), and the pH values were set up for 0.1 M PBS. All aqueous solutions were prepared in doubly distilled, deionized water. High purity nitrogen was used for deaeration of the prepared aqueous solutions. 2.2. Apparatus Electrochemical experiments such as cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed on CHI 660A workstation (ChenHua Instruments Co., Shanghai, China) with a conventional three-electrode system, which consisted of a saturated calomel electrode (SCE), a Pt foil auxiliary electrode, and a working electrode. All potentials were reported versus SCE unless stated otherwise. Solutions in the electrochemical cell were deaerated by N2 bubbling before experiments and kept under a N2 atmosphere during experiments. X-ray photoelectron spectroscopy (XPS) was performed on ESCALAB MKII spectrometer (VG Co., UK). Field emission scanning electron microscope (FE-SEM) images were obtained on a JSM-6700F field emission scanning electron microanalyser (JEOL, Japan). 2.3. Preparation of PPyox/SWNTs modified electrode Glassy carbon electrode was carefully polished successively with 6, 1 and 0.05 ␮m alumina slurries. Then it was rinsed with water, and sonicated in ethanol and water for 5 min, successively. The SWNTs were dispersed in 0.1 M SDS aqueous solution and sonicated for 1 h (Islam et al., 2003). Then, PPy monomer was dissolved in this emulsion solution under ultrasonic stirring for 15 min at room temperature. The PPy/SWNTs compositesmodified GCE was made by CV from −0.2 to 0.8 V with the scan rate 0.1 V/s for eight circles in the solution with 0.1 M PPy, 0.2 mg/ml SWNTs and 0.1 M SDS. The obtained modified electrode, denoted as PPy/SWNTs/GCE, was rinsed with distilled water, and transferred to a PBS for electrochemical oxidation at +1.8 V for 250 s. The obtained electrode, denoted as PPyox/SWNTs/GCE, was gently washed with distilled water to remove any non-adsorbed species. For the sake of comparison, a PPy/GCE was prepared under the same conditions. 3. Results and discussion 3.1. Characterizations of PPyox/SWNTs/GCE The polymerization current of pyrrole in an aqueous solution containing SDS and SWNTs increases significantly from scan to scan, indicating an efficient film deposition on the electrode surface. Eight cycles were selected as the optimal condition for obtaining an appropriate thickness of the PPy/SWNTs compos-

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Fig. 1. Cyclic voltammograms of the PPy/GCE (a) and the PPy/SWNTs/GCE (b) in 0.1 M PBS (pH 7.04) at 50 mV s−1 .

ite film. The prepared PPy/SWNTs/GCE shows a typical cyclic voltammogram as presented in Fig. 1 (curve b). For comparison, the cyclic voltammetric investigation of pure PPy film was also carried out and the corresponding results are shown in Fig. 1 (curve a). From the cyclic voltammograms (Fig. 1, curves a and b), it can be seen that the peak potentials of the PPy/SWNTs/GCE are about 100 mV more negative than those of the pure PPy/GCE, indicating the anionic dopant role of the CNTs (Chen et al., 2000). Moreover, the background current of PPy/SWNTs/GCE is apparently larger than that of PPy/GCE, which indicates the PPy/SWNTs modified GCE has larger effective surface area (Bard and Faulkner, 1980). On the other hand, when these obtained PPy/SWNTs/GCE and PPy/GCE were oxidized in PBS at +1.8 V for 250 s, the background current of PPyox/SWNTs/GCE was only about 1/30 that of PPy/SWNTs/GCE (shown in Supplemental Information Fig. S1, curve c), and almost no voltammetric response could be observed on the PPyox/GCE (Fig. S1, curve b), indicating anodic polarization at +1.8 V maybe turned the PPy into an insulating PPyox with a large loss of electroactivity (Van Dyke and Martin, 1990). Compared curves c and b in Fig. S1, it could be seen that CNTs in PPyox film displayed an important role in electron transfer. The SEM images of the PPy/SWNTs composite film and pure PPy film on GCE surface were shown in Fig. 2. From Fig. 2a, it could be clearly seen that there were many onedimensional nanostructures that stood almost vertically on the deposited films. Such nanostructures were of roughly uniform size with diameters in the range of several hundreds of nanometers and height of up to several micrometers. On the other hand, these one-dimensional nanostructures could not be seen in the absence of SWNTs (Fig. 2b), which indicated that the growth of these one-dimensional nanostructures at the surface of GCE was contributed by SWNTs. Compared with the size of pure SWNTs, the diameters of these formed nanostructures were much larger. Therefore, it can be deduced that these one-dimensional nanostructures were not SWNTs themselves, but SWNTs enwrapped within the deposited substance. These vertically oriented structures might be very important to promote the electron transfer between the solution of electrochemical cell and GCE, and the PPyox/CNTs composite film modified GCE exhibited strongly electrocatalytic character to some substances.

Fig. 2. SEM images of PPy/MWNTs composite film (a) and PPy film (b) on glassy carbon electrode surface.

XPS measurements confirmed the presence of both the PPy and SDS on the GCE after electropolymerization. The results were shown in Supplemental Information (Fig. S2). Fig. S2a shows the XPS spectrum of PPy/SWNTs composite film modified GCE using SDS as supporting electrolyte. The N(1s) peak at 396.6 eV (Fig. S2b) confirms the presence of polypyrrole (Kang et al., 1993), while the S(2p) peak at 168.9 eV confirms the presence of SDS molecules (Muthuraman et al., 2005). According to previous studies (Islam et al., 2003; Kang and Taton, 2003; Moore et al., 2003), SDS molecules could be adsorbed strongly onto the SWNTs surface through van der Waals’ force and form sheaths around the tubes, which made the nanotubes disperse in the solution uniformly and stably. When PPy molecules were added into this SDS-encapsulated CNTs solution, they could enter the interiors of micelle-encapsulated carbon nanotubes and locate at the interfaces between surfactants and carbon nanotubes (Zhang et al., 2004a,b,c). Then, if a positive voltage was added to GCE, due to orientation of carbon nanotubes in the presence of an electric field (Kumar et al., 2004), these SDS-encapsulated SWNTs composite nanostructures in electrochemical cell would orient towards the surface of electrode (Kamat et al., 2004). Once SDS-encapsulated SWNTs nanocomposites ran into GCE, if a high potential was added to the modified electrode, PPy located at the interfaces between SDS and SWNTs would oxopolymerize at the surfaces of both the GCE and CNTs (Zhang et al., 2004a,b,c), and SWNTs were uniformly and stably immobilized on the surface of GCE. The amount of SWNTs used in this paper was also checked, and it was found that if the concentration of SWNTs was high,

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this nanocomposite film is not easily formed. On the other hand, if the concentration of SWNTs was low, the conductivity of this modified electrode was poor. So, 0.2 mg/ml SWNTs was chosen in this experiment. 3.2. Selectivity of PPyox/SWNTs/GCE GCE were modified by use of the above procedure in Section 2. Fe(CN)6 3− and Ru(NH3 )6 3+ were chosen as electrochemical probes because of their similar size and fast kinetics on carbon electrodes, however, of opposite charges. These ions were used to investigate the interactions between charged probes and the PPyox/SWNTs film. The CV responses of the electrode were recorded as shown in Supplemental Information (Fig. S3A). Solid curve shows a well behaved reversible redox wave of K3 [Fe(CN)6 ] containing 10 mmol/l KCl at a bare GCE (pH 7.04). However, the redox reactions were significantly inhibited by the PPyox/SWNTs film at the PPyox/SWNTs/GCE (dashed curve), indicating that the PPyox/SWNTs film was negatively charged. Fig. S3B shows the CV responses of Ru(NH3 )6 3+ . It could be seen that a well reversible redox wave could also be found at PPyox/SWNTs modified electrode (dashed curve). Moreover, the current increased obviously on the PPyox/SWNTs modified GCE. This increased CV response on PPyox/SWNTs/GCE might be attributed to the incorporation of CNTs and SDS that gave negative charges to the polymer. More Ru(NH3 )6 3+ would be attracted and accumulated on the modified surface due to the strong electrostatic interaction. So this PPyox/CNTs composite film modified electrode exhibits some selectivity though more experiments should be done to prove it. 3.3. Sensing applications of PPyox/SWNTs/GCE 3.3.1. Electrocatalytic oxidation of nitrite at PPyox/SWNTs/GCE To test the practical applications of PPyox/SWNTs/GCE, nitrite was firstly selected as one of testing species for electrochemical experiment. Fig. 3 shows the CV responses of 0.5 mM nitrite with pH 5.5 PBS at bare GCE (curve a), PPyox/GCE (curve b) and PPyox/SWNTs/GCE (curve c). As can be seen,

Fig. 3. Cyclic voltammograms of 0.5 mM nitrite with 0.1 M PBS (pH 5.5) at bare GCE (a), PPyox/GCE (b) and PPyox/SWNTs/GCE (c). Scan rate: 50 mV s−1 . Inset: Amperometric response curves of PPyox/SWNTs film modified GCE in 0.1 M PBS (pH 5.5) to successive additions of 50 ␮M nitrite at an applied potential of 0.77 V.

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the Faradic responses of nitrite at the PPyox/SWNTs/GCE were much higher than those at PPyox/GCE or bare GCE. The CV of nitrite at PPyox/SWNTs/GCE showed an about 2.5-fold increased anodic current response relative to that at PPyox/GCE, and about 5-fold to that at bare GCE. Moreover, it can also be seen that a broad CV peak of nitrite appeared at about 1.1 V at the bare GCE. However, a well-defined peak appeared at about 0.77 V at the PPyox/SWNTs/GCE. The 330 mV negative shift with much enhanced peak current indicates a strong catalytic effect of PPyox/SWNTs layer. The anodic peak currents of nitrite were proportional to the square root of scan rate in the range of 50–250 mV s−1 , showing a diffusion-controlled reaction. Amperometric response curves of PPyox/SWNTs/GCE to successive addition of 50 ␮M nitrite at an applied potential of 0.77 V were presented in Fig. 3 (inset). The nearly equal current steps for each addition of nitrite demonstrated that PPyox/SWNTs/GCE had stable and efficient catalytic activity, and it could be used as an excellently amperometric nitrite sensor. 3.3.2. Electrocatalytic oxidation of ascorbic acid, dopamine and uric acid at PPyox/SWNTs/GCE The practical applications of the proposed electrode were also tested by three biological compounds, i.e. ascorbic acid (AA), dopamine (DA) and uric acid (UA). AA, DA and UA are three important biological molecules, and they coexist in the extra cellular fluid of the central nervous system and serum. Since they have similar oxidation potentials at most solid electrodes, separate determination of these species is a great problem due to overlapped signals. In order to evaluate the sensitivity and selectivity of the present system for the quantification of AA, DA and UA, the electrochemical behavior of AA, DA and UA at PPyox/SWNTs/GCE was studied. The CV curves of AA, DA and UA at a bare GCE, the PPyox/GCE and the PPyox/SWNTs/GCE are shown in Supplemental Information (Fig. S4). Curves (a)–(c) correspond to the bare, PPyox film and PPyox/SWNTs film modified GCEs in the presence of AA (0.1 mM), DA (50 ␮M) and UA (50 ␮M), respectively. From Fig. S4, it can be seen that at the PPyox/SWNTs film modified electrodes, the anodic currents enhanced considerably. The anodic potentials of AA, DA and UA at the bare GCE are 415, 270 and 438 mV when the scan rate is 50 mV s−1 (curve c), whereas at the same scan rate, they are 95, 210 and 345 mV at the PPyox/SWNTs film modified electrodes (curve a). That is, the oxidation potentials at the PPyox/SWNTs/GCEs were reduced to 320, 60 and 93 mV for AA, DA and UA, respectively, compared with those at the bare GCEs. Moreover, a well-defined CV response was observed at the PPyox/SWNTs/GCE for AA, DA and UA with a more negative potential. These CV response changes observed indicate that the PPyox/SWNTs/GCE has an excellent catalytic activity for the electrochemical oxidation of AA, DA and UA. The electrochemical oxidation processes of AA, DA and UA in the mixture have also been investigated and the results are shown in Fig. 4A. As shown in Fig. 4A, the CV curve of a sample solution containing AA, DA and UA shows two broad and overlapped anodic peaks at 0.26 and 0.35 V at bare GCE

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current responses every few days. No apparent decreases in current responses occurred during the first day, and 7% decrease was noted after the electrode was stored for 7 days (n = 5, and R.S.D. ≤ 3%). Furthermore, only 16% decrease occurred after 1 month. The stability of the PPyox/SWNTs film modified electrode is much better than that of the previous nitrite biosensor (Strehlitz et al., 1996) or uric acid and ascorbic acid sensor (Zhang and Lin, 2001). The excellent stability of PPyox/SWNTs/GCE may be attributed to the stability of PPyox itself and the protecting effect of PPyox film against the desorption of SWNTs from GCE surface. 4. Conclusions

Fig. 4. (A) Cyclic voltammograms in a solution of 0.1 M PBS (pH 4.0) containing 0.1 mM AA + 0.1 mM DA + 0.1 mM UA at PPyox/SWNTs/GCE (solid curve) and bare GCE (dashed curve). (B) DPVs at PPyox/SWNTs/GCE in solutions containing 10, 20, 30, 40, 50 ␮M AA; 5, 10, 20, 30, 40 ␮M DA; 5, 10, 20, 40, 60 ␮M UA, respectively.

(dashed curve). So the peak potentials for AA, DA and UA are indistinguishable at a bare GCE. Therefore, it is impossible to deduce any information from the broad and overlapped anodic peak. However, at the PPyox/SWNTs/GCE, the overlapped voltammetric peak is resolved into three well-defined CV peaks (Fig. 4A, solid curve) at about 0.10, 0.21 and 0.35 V or DPV peaks (Fig. 4B) at about 0.03, 0.18 and 0.31, corresponding to the oxidation of AA, DA and UA, respectively. If the concentrations of AA, DA and UA increased synchronously on increasing the concentrations of the three compounds, the peak currents at the modified electrode increase accordingly as shown in Fig. 4B. It can be seen that the peak currents for the three analytes increase linearly with their concentrations. Under the optimal conditions, using the DPV mode, the catalytic peak current was linearly related to AA, DA and UA concentration in the range 2.0 × 10−5 to 1.0 × 10−3 , 1.0 × 10−6 to 5 × 10−5 , 2.0 × 10−6 to 1.0 × 10−4 M, with correlation coefficients of 0.992, 0.998 and 0.993, respectively. The practical detection limit was 4.6 × 10−6 , 3.8 × 10−7 and 7.4 × 10−7 M, respectively. 3.4. Stability of PPyox/SWNTs/GCE To maintain the reproducibility of the PPyox/SWNTs/GCE and eliminate adsorption, the modified electrode was cleaned with voltammetric cycles in PBS after measurements, and was stored in PBS (pH 6.0). Stabilities of the PPyox/SWNTs film modified electrode were investigated by measuring the

This paper reports the fabrication, characterization and practical applications of PPyox/SWNTs composite film modified glassy carbon electrode. The PPyox film served as a support material for directing SDS-encapsulated SWNTs immobilization. CNTs incorporated into the PPyox matrix provides a novel class of hydride materials with a high stability, good conductivity, and selectivity. This proposed modified GCE could efficiently electrocatalyze some species such as nitrite, AA, DA and UA. These favorable features of such a modified electrode offer great promise for its sensing applications. In comparison with other polypyrrole or SWNTs modified electrodes, this PPyox directed SWNTs modified electrode has two apparent features: (1) the polypyrrole modified on the surface of GCE was been oxidized, which resulted in an insulating membrane with a microporous structure, and fast electron transfer could take place through this microporous membrance; (2) PPyox could be used as a template to immobilize SWNTs, which made the modified layer more stable. Moreover, because SWNTs were oriented towards the outside of modified electrode, it could promote the electron transfer between some substances in electrochemical cell and GCE. Therefore, the substances could be electrochemically oxidized more quickly. Furthermore, we expected that the stratagem of this fabrication of modified electrode has the potential for an extension to different conducting materials, and more relative works are in progress. Acknowledgements This work was supported financially by the Natural Science Foundation of Educational Department of Anhui Province (No. 2006kj039a), the National Natural Science Foundation of China (No. 20575001), and a Project for the Author of Excellent Teacher of Anhui Province. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.bios.2007.02.001. References An, K.H., Jeon, K.K., Heo, J.K., Lim, S.C., Bae, D.J., Lee, Y.H., 2002. J. Electrochem. Soc. 149, A1058–A1062.

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