Modification of vertically aligned carbon nanotubes with RuO2 for a solid-state pH sensor

Electrochimica Acta 55 (2010) 2859–2864

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Modification of vertically aligned carbon nanotubes with RuO2 for a solid-state pH sensor Bin Xu, Wei-De Zhang ∗ Nano Science Research Center, School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, Guangdong, PR China

a r t i c l e

i n f o

Article history: Received 30 November 2009 Received in revised form 24 December 2009 Accepted 28 December 2009 Available online 11 January 2010 Keywords: pH sensor Carbon nanotubes Ruthenium oxide Nanocomposite Electrochemical impedance spectroscopy

a b s t r a c t In this work, a novel type electrode based on RuO2 nanoparticles-modified vertically aligned carbon nanotubes (RuO2 /MWCNTs) was prepared by magnetron sputtering deposition. This RuO2 /MWCNTs electrode not only shows a high capacity nature, but also possesses a good response to the pH value. The pH sensor based on the RuO2 /MWCNTs nanocomposite electrode exhibits some advantages over the conventional pH sensors. It shows good reproducibility, long-term storage stability (over 1 month) and linear response in the whole pH range (2–12) of Britton–Robinson (B–R) buffer solutions with near-Nernstian response (about −55 mV/pH). The hysteretic widths of the nanocomposite electrode are 6.4 mV, 5.1 mV and 10.2 mV in pH 7–4–7–10–7, pH 7–10–7–4–7 and pH 2–8–12–8–2 loop cycles, respectively. Moreover, the RuO2 /MWCNTs electrode displays an excellent anti-interference property and fast response time (less than 40 s). According to the electrochemical impedance measurements, the pH sensing properties of the RuO2 /MWCNTs electrode were also discussed. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Determination of pH is indispensable in a wide range of industrial production processes as well as in clinic, environmental control and biological systems [1]. Efforts have been made on developing new approaches for the determination of pH, most of which utilize potentiometric electrodes. Among the various methods, the use of glass electrode has been widely adopted due to its good sensitivity, selectivity, stability and long lifetime. However, the limitations of glass electrode, such as acid and alkaline error, high impedance, high temperature instability and mechanical fragility restrict its further applications in certain circumstances [2]. As a result, non-glass pH electrodes, especially solid-state pH sensors based on metal oxides, have gained considerable concern in terms of developing potential alternatives to glass electrode for miniaturized systems, because metal oxides are mechanically robust, less sensitive to cation interference [3]. Up to now, various metal oxides have been used to develop pH sensors such as PtO2 , OsO2 , Ta2 O5 , TiO2 , PdO, SnO2 , ZrO2 [3], IrO2 [4–6], RuO2 [7], molybdenum bronzes [8], Co2 O3 [9], WO3 [10], and PbO2 [11]. Among these metal oxides, RuO2 is one of the most promising materials and has been more widely used in pH sensors [12,13], biosensors [14] and supercapacitors [15] due to its chemical stability and high conductivity which inhibits the space charge accumulation.

∗ Corresponding author. Tel.: +86 20 87114099; fax: +86 20 87112053. E-mail address: [email protected] (W.-D. Zhang). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.12.099

On the other hand, the discovery of carbon nanotubes (CNTs) provides a new material for electrode with high performance because of their good electrical conductivity, large surface area, surface chemical flexibility, high mechanical strength and one-dimensional nanostructure [16]. Modification of CNTs with functional materials will enhance their properties or endow them with novel properties. For instance, the capacitance of the CNT electrode in 1.0 M H2 SO4 was significantly increased from 0.35 mF/cm2 to 16.94 mF/cm2 by modification with RuO2 [17]. Electrodeposition of TiO2 or MnO2 on CNTs promoted the electrocatalytic activity towards electrochemical oxidation of hydrogen peroxide and glucose, respectively [18,19]. However, to our knowledge, few articles on pH sensor based on metal oxide-modified MWCNTs electrodes have been reported previously. In the preliminary work [20], we demonstrated a pH sensor based on WO3 /MWCNTs electrode with high mechanical strength, reproducibility, stability and selectivity. However, the sensitivity of this sensor is lower than the theoretical value. In order to improve the sensitivity of the pH sensor, ruthenium oxide (RuO2 ) was selected to modify multi-walled carbon nanotubes (MWCNTs) arrays (RuO2 /MWCNTs) for pH sensor. Compared with the WO3 /MWCNTs electrode, the RuO2 /MWCNTs electrode presented a near-Nernstian response (−55 mV/pH) and shorter response time (less than 40 s) besides the advantages of WO3 /MWCNTs electrode. The enhancement mechanism of pH sensing property of the RuO2 /MWCNTs electrode is exhaustively discussed in this paper. This approach not only provides a possibility for the miniaturization of solid-state pH sensors for specific applica-

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tions, but also promotes the application of CNTs in electrochemical sensors. 2. Experimental section Phosphoric acid, acetic acid, boric acid, sodium hydroxide and other reagents used in this experiment are all of analytical grade. All solutions were prepared with high quality deionized water (18.4 M/cm). Well-aligned MWCNTs were grown on Ta substrates [21–23], which is facile for the fabrication of an MWCNTs electrode [24,25] or for further modification for composite electrodes [26,27]. In this work, coating of ruthenium oxide on the vertically aligned MWCNTs was achieved by magnetron sputtering deposition with a ruthenium target at a power of 200 W for 5 min in an Ar/O2 (3:1) atmosphere. During this procedure, the sputtered Ru was oxidized in oxygen and deposited on the MWCNTs. The Ta plate with MWCNTs modified by RuO2 was connected to the surface of a Cu electrode using conductive silver paint (Structure probe, Inc., USA). The edge of the Ta plate and Cu electrode was insulated by pasting with nail enamel. A field-emission scanning electron microscope (SEM) (JEOL JSM 6700F) was used to observe the RuO2 -modified MWCNTs. Opencircuit potential of the RuO2 /MWCNTs electrode was measured as a function of pH value of the sample solutions by using CHI 660C electrochemical workstation (Shanghai Chenhua, China). A three-electrode system was employed with RuO2 /MWCNTs electrode as working electrode, an Ag/AgCl (3 M KCl) electrode and a platinum wire served as reference electrode and counter electrode, respectively. All potentials were referred to Ag/AgCl (3 M KCl) electrode. Electrochemical impedance spectroscopy (EIS) measurements were carried out with a frequency response analyzer (PGSTAT 30, Autolab, Eco-Chemie, the Netherlands) using the above three-electrode cell. Measurements were performed with amplitude of 5 mV and frequency ranged between 100 kHz and 100 mHz. Non-linear least-squares analysis was used to simulate the impedance plane plot. 3. Results and discussion Fig. 1A shows the overall morphology of the MWCNTs modified by RuO2 . Enlarged observation (inset in Fig. 1A) on the RuO2 /MWCNTs indicates the modified MWCNTs with a larger tubular diameter near the tips due to the presence of sputtering coating, and the underneath parts of the tubes are clear. During overhead sputtering deposition, the ruthenium oxide was directly coated on the top of the sample. Meanwhile, the coated tips blocked the lower parts of the tubes from coating with the oxide. From the TEM image, one can observe the coating layer on an MWCNT, as displayed in Fig. 1B. A magnified TEM image has been added as an inset in Fig. 1B to clearly indicate the particles of RuO2 . The deposition of RuO2 on the MWCNTs was further confirmed by energy-dispersive X-ray spectrometer, as depicted in Fig. 1C. The strong peaks of Ta in EDX profile came from the metallic substrate, where the MWCNTs were attached. Performance of pH sensors is usually characterized by measuring the open-circuit potential of the electrodes in solutions with various pH values [28]. Fog and Buck reported the general mechanism of metal oxides for pH sensing and suggested that pH response could be due to ion exchange in a surface layer containing OH group [5]. Zoubov et al. described only one redox equilibrium between insoluble ruthenium oxides [29]. So, the general sensing mechanism of RuO2 -based pH sensor can be expressed as follows: RuO2 · nH2 O + H+ + e− ⇔ (n − 1)H2 O + Ru(OH)3

(1)

Fig. 1. (A) SEM, (B) TEM images and (C) EDS of the RuO2 /MWCNTs nanocomposite.

According to Nernst equation for the equilibrium, the electrode potential can be stated: 0 ERuO2 ·nH2 O/Ru(OH)3 = ERuO

2 ·nH2 O/Ru(OH)3

− −

−

a(RuO2 ·nH2 O) RT ln zF a(Ru(OH)3 )

RT 1 0 = ERuO ln 2 ·nH2 O/Ru(OH)3 F a(H+ ) 2.303 RT 0 pH = ERuO + m pH 2 ·nH2 O/Ru(OH)3 F (2) 

where E is the measured potential, E 0 is the conditional standard potential, R is the gas constant (8.314 J/K mol), T is the absolute temperature (K), z is the signed ionic charge and F is the Faraday constant (96487.3415 C/mol). At room temperature (T = 298 K),

B. Xu, W.-D. Zhang / Electrochimica Acta 55 (2010) 2859–2864

Fig. 2. pH response curves of the RuO2 /MWCNTs electrode in B–R buffer solutions (A) with different immersion time in pH 7 buffer solution, (B) the effect of time on potential and sensitivity and (C) with N2 or O2 -saturated.

the slope m should be −59.1 mV/pH. As shown in Fig. 2A, the RuO2 /MWCNTs electrode displays linear characteristics over a wide pH range from 2 to 12. A slope of about −55 mV/pH is determined for this nanocomposite electrode. Even though the sensitivity is a bit lower than theory value (−59.1 mV/pH), it is much higher than the WO3 /MWCNTs electrode (−41 mV/pH) [19] and nearly consistent with the other RuO2 film composite electrode [13,30,31].  The decrease in E 0 of the freshly prepared electrode after immersing in pH 7 buffer solution for 24 h is the typical aging phenomenon that affects metal oxide electrodes [32]. Potential drifts exceeding 200 mV have been reported for freshly prepared IrO2 electrodes [33,34]. Fig. 2A shows the pH-potential response

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curves of the RuO2 /MWCNTs electrode. After being immersed in  a pH 7 buffer solution for 24 h, the E 0 drifted about 79 mV, while the sensitivity drifted about 2.3 mV/pH. For 48 h immersion, the three calibration curves of the electrode almost coincided with one another. Fig. 2B more directly shows the effect of immersing time  on E 0 and sensitivity of the RuO2 /MWCNTs electrode. After immer sion of 48 h, the E 0 and sensitivity are the same as those with 24 h immersion of the electrode, which indicates the electrode becoming stable. In our studies, all the RuO2 /MWCNTs electrodes were immersed in pH 7 buffer solution for 48 h prior to use. The aging phenomenon has long been considered to be caused by the progress of hydration reactions at the electrode surface [32,34], the redox processes involving atmospheric oxygen [33] and the initial presence of RuO3 [35]. As we all know, ruthenium has three oxidation states: RuO2 , RuO3 and RuO4 . RuO3 and RuO4 are more volatile and have relatively low melting and boiling points [36]. According to Bell and Tagami [37], the formation of RuO3 and RuO4 at standard state or reduced oxygen pressure is thermodynamically 0 ) and unfavorable due to the standard state free energy (G298 equilibrium partial pressures (p) are 191.81 kJ/mol, 101.77 kJ/mol and 2.39 × 10−34 atm, 1.44 × 10−18 atm, respectively. So, it is almost impossible for RuO3 to be present in RuO2 /MWCNTs electrodes under the fabrication procedure and storage conditions. It also can be seen from Fig. 2C, when the electrode was placed in the nitrogen saturated and oxygen saturated buffer solutions, there was almost no difference between the calibration curves. In addition, during the CV test in 0.5 M H2 SO4 , the immersed electrode encircled larger area and had larger capacitance than the freshly prepared electrode, which could be attributed to the hydration of RuO2 [15] (data not shown). Therefore, it is reasonable to conclude that the slow sur face hydration of RuO2 gives rise to a drift in E 0 and sensitivity, which is in line with the response mechanism (Eq. (1)). However, this conclusion is different from the work reported in Ref. [13], which ascribes the emf drift at pH measurements to the H+ diffusion in RuO2 film. The difference in conclusions may result from the different composition of electrode materials. Hysteresis is a common phenomenon for glass pH electrode or metal oxide pH electrodes. When the electrode is measured many times in the same pH buffer solution, different output voltages occurred. This phenomenon is called memory effect or hysteresis. According to the depiction by Bousse et al., the hysteresis of hydrogen ion selective electrodes could be regarded as a delay of the pH response [38]. The hysteretic widths of the RuO2 /MWCNTs electrode were valued by successively measuring the open-circuit potentials of different pH buffer solutions in each cycle. As shown in Fig. 3, the hysteretic widths are 6.4 mV, 5.1 mV and 10.2 mV in pH 7–4–7–10–7, pH 7–10–7–4–7 and pH 2–8–12–8–2 loop cycles, respectively. These hysteretic widths of RuO2 /MWCNTs electrode are smaller than those of WO3 film electrode and glass pH electrode [39], which indicates the RuO2 /MWCNTs electrode has a good response to pH value. So far, there is little explanation to hysteresis phenomenon. According to the experimental results, the slow diffusion of the H+ between inner and outer surface and the slow adjustment of the hydration situation of the electrode surface are most likely to cause this phenomenon. The open-circuit potential determination in different pH solutions is quickly carried out, so the electrode surface fails to adjust instantaneously with the corresponding solution. Reproducibility and stability are critically important to pH sensors. In this study, four samples of RuO2 /MWCNTs electrode were fabricated and measured. As shown in Fig. 4A, all of the sample electrodes have wide response voltage range and a near-Nernstian response (about −55 mV/pH). Three electrodes show almost the  same E 0 (about 640 mV), while sample 3 electrode shows a little bit  higher E 0 . When sample 1 electrode was selected to be alternately measured in buffer solutions of pH 2, 4, 6, 8, 10 and 12 separately

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Fig. 3. Hysteresis width of the RuO2 /MWCNTs electrode with different loop cycles: (A) pH 7–4–7–10–7; (B) pH 7–10–7–4–7; (C) pH 2–8–12–8–2.

(shown in Table 1), the relative standard deviation determined from five measurements was about 0.06%, 0.07%, 0.19%, 0.47%, 1.47% and 4.70%, respectively. The potential drifts for all measurements were less than 3 mV, which shows this pH sensor could effectively avoid the acid and alkaline error. After being stored in air and testing everyday for 1 month, no significant change was observed in measuring pH value of the B–R buffer solutions, as depicted in Fig. 4B. All of the experimental results illustrate that the RuO2 /MWCNTs elec-

Fig. 4. pH response curves of (A) different electrodes in B–R buffer solutions, (B) under different testing time for sample 1 electrode and (C) potential response in B–R buffer solutions with different pH value.

trode holds good reproducibility, stability and long-term storage stability. Response time is also an important factor for sensors. Traditionally, it is defined as the time required for electrode to reach 95% of the equilibrium [40]. Fig. 4C shows the response of the RuO2 /MWCNTs electrode in pH 4, 8 and 12 buffer solutions, respec-

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Table 1 Open-circuit potential of the RuO2 /MWCNTs electrode in solutions with various pH values. pH value (B–R buffer solution)

2 4 6 8 10 12

Measured value of OCP for five times (mV) 1

2

3

4

5

540.2 421.3 304.0 195.9 92.3 −23.5

540.6 421.2 302.5 193.7 90.8 −23.8

540.4 420.8 302.8 194.8 92.3 −22.6

540.0 420.8 303.0 195.4 93.6 −21.4

539.8 420.6 303.1 195.9 94.4 −21.7

tively. The response time is less than 40 s in the buffer solutions of pH from 2 to 12 and it is affected by the pH value of solutions. In low pH solutions the response time is shorter than that in the high pH ones, which may result from the fact that the dynamic response of the electrode is related to the H+ diffusion [41]. The selectivity of the electrode was established by studying the effect of some common ions (Cl− , NO3 − , SO4 2− , F− , I− , Ca2+ , K+ ) on potentiometric response. According to two solutions method (TSM) [42] sponsored by IUPAC [43], the selectivity coefficient (Kij ) can be calculated using the extended Nernstian equation as follows [44]: Ki,j =

ai {exp(zj F E/RT ) − 1}

(3)

(aj )zi /zj

where ai and aj are the activities of the primary ion whose charge is zi and the interfering ion whose charge is zj , respectively, E is the difference between the electrode potentials in the solutions containing both ‘i’ and ‘j’ ions, and only ‘i’ ion, while ai remains the same in both solutions. In our studies, the selectivity coefficients (Ki,j ) of this electrode for H+ with respect to Cl− , NO3 − , SO4 2− , F− , Ca2+ , K+ and I− were calculated based on Eq. (3) by considering hydrion as the primary cation with 1.0 × 10−1 mol dm−3 of the corresponding interfering ion. As shown in Table 2, the largest selectivity coefficient (Ki,j ) comes from I− , which is less than 1.0 × 10−5 , while the selectivity coefficients are all less than 1.0 × 10−10 from other tested ions. This is because the oxidation of I− was involved in the redox of RuO2 . The results indicate that the interference from the most common ions for pH measurement was negligible. Especially for the F− , which can cause great interference on glass pH sensor, has no interference on the RuO2 /MWCNTs electrode. The RuO2 /MWCNTs electrode was also evaluated by electrochemical impedance spectroscopy (EIS) in an attempt to understand its pH sensing characteristics. EIS is an effective approach for investigating the electron transfer across the electrolyte and the surface of electrode. The Nyquist plot for the RuO2 /MWCNTs electrode in a buffer solution of pH 8 is presented in Fig. 5. One can see only one time-constant from the plot and it is typical capacitance behavior. These demonstrated that the reaction of the electrode was under kinetic control [45]. Simulation of the experimental data was carried out via Boukamp non-linear leastsquares program provided by the FRA software (Version 4.9.007). The values resulting from the fit are in good agreement with the experimental data giving the 2 < 10−2 . The equivalent circuit of the RuO2 /MWCNTs (the inset of Fig. 5) consisted of a charge trans-

Averaged value of OCP (mV)

RSD%

540.2 420.9 303.8 195.1 92.7 −22.6

0.06 0.07 0.19 0.47 1.49 4.70

fer resistance (RCT ), a constant phase element (CPE) in parallel, the electrolyte resistance (Rs ) and the T diffusion element (ZT ). The constant phase element, defined as ZCPE = Z0 (jω)−n , where Z0 and ω are constants, j = (−1)1/2 , and 0 ≤ n ≤ 1, is used instead of a capacitance to describe a non-ideal capacitive response because of surface inhomogeneities [46]. The T diffusion element (ZT ) is characteristic of another type of film which contains a fixed amount of electroactive substance. It is a useful model for diffusion when the finite diffusion (a thin film) is involved and defined as ZT = −0.5 0.5 Z0 (jω) coth[B(jω) ]. So, it is also called the “bounded Warburg”. Batteries or supercapacitors often share this behavior [47]. The finite diffusion region of EIS (Fig. 5) showed a slope slightly below 90◦ and a constant phase element can be used to simulate it. This anomalous behavior was already observed by other authors [48,49] and explained as a result of both electrode porosity and roughness at the blocking interface. The charge transfer resistance (RCT ) is assigned to the impedance related to charge transport at the Pt counter electrode and the surface of the RuO2 /MWCNTs electrode. The impedance behaviors in different pH solutions were quite similar to that presented in Fig. 5 for pH 8. For all applied pH solutions, the same type of equivalent circuit was obtained. The difference lied in the values of the RCT . The charge transfer resistant, RCT , was calculated and found to increase gradually from 1.80 k to 10.50 k with the pH values from pH 2 to 12, indicating faster reaction kinetics at higher proton concentration, as listed in Table 3. It is believed that this phenomenon was due to the decrease of hydrogen concentration difference between the electrolyte and the surface of electrode with the increase of pH which halted the transfer of H+ and caused the RCT increase. This result was well consistent with the expansion of the response time upon the increase of pH. The RuO2 /MWCNTs electrode has also been employed to measure real samples. It was applied to pH determination of various soft drinks and solutions, such as Fanta, Sprite and NaOH solution (0.001 M), the pH values were 2.86 (RSD = 1.32%), 3.37

Table 2 Ion selective coefficients of the RuO2 /MWCNTs electrode. Interfering ion

Concentration (mol/L)

Selectivity coefficient (Ki,j )

K+ Cl− Ca2+ SO4 2− F− NO3 − I−

0.1 0.1 0.1 0.1 0.1 0.1 0.1

<10−10 <10−10 <10−10 <10−11 <10−10 <10−10 <10−5

Fig. 5. Electrochemical impedance spectra of the RuO2 /MWCNTs electrode in B–R buffer solution (pH = 8). Inset is the equivalent circuit.

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Table 3 The fitting results of Rs and RCT at various pH values. pH

Rs (k)

RCT (k)

2 4 6 8 10 12

0.303 0.572 0.319 0.232 0.221 0.189

1.80 2.33 3.57 4.92 9.66 10.5

(RSD = 0.17%), and 11.08 (RSD = 0.14%), respectively. These results were almost consistent with the values of 2.74, 3.32 and 11.03 determined by the conventional glass pH electrode, which fully showed the RuO2 /MWCNTs electrode could be applied in reality. 4. Conclusion A novel solid-state pH sensor based on RuO2 -modified MWCNTs has been successfully fabricated. Structure and composition analysis elucidated the successful deposition of RuO2 thin film on the vertically aligned MWCNTs by magnetron sputtering. The MWCNTs not only served as support, but also played as a conductor as other metals do in metal/metal oxide pH sensors. In pH measurements, the RuO2 /MWCNTs electrode demonstrates high stability (over a month), good reproducibility (RSD <5%), fast response (<40 s), favorable anti-interference property, and a high sensitivity of about −55 mV/pH from pH 2 to 12. Moreover, the method also makes it possible to miniaturize pH sensors. With vertically aligned nanoscale pH electrode, the determination of the pH value in vitro and in vivo intracellular can be achieved. For example, by inserting one carbon nanotube or a bundle of carbon nanotubes electrode modified with RuO2 into kidney cell, the intracellular pH could be determined, which is our further research. Acknowledgements The authors thank Natural Science Foundation of China (No. 20773041) and the Research Fund for the Doctoral Program of Higher Education (No. 20070561008) for financial support. References [1] G.M. da Silva, S.G. Lemos, L.A. Pocrifka, P.D. Marreto, A.V. Rosario, E.C. Pereira, Anal. Chim. Acta 616 (2008) 36. [2] P. Shuk, K.V. Ramanujachary, Solid State Ionics 86–88 (1996) 1115. [3] A. Fog, R.P. Buck, Sens. Actuators 5 (1984) 137.

[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49]

T. Katsube, I. Lauks, J.N. Zemel, Sens. Actuators 2 (1982) 410. P.J. Kinlen, J.E. Heider, D.E. Hubbard, Sens. Actuators 22 (1994) 13. M. Wang, S. Yao, M. Madou, Sens. Actuators 81 (2002) 313. C. Colombo, T. Kappes, P.C. Hauser, Anal. Chim. Acta 412 (2000) 69. P. Shuk, K.V. Ramanujachary, M. Greenblatt, Solid State Ionics 86–88 (1996) 1115. Q.W. Li, G.A. Luo, Y.Q. Shu, Anal. Chim. Acta 409 (2000) 137. K. Yamamoto, G.Y. Shi, T.S. Zhou, F. Xu, M. Zhu, M. Liu, T. Kato, J.Y. Jin, L.T. Jin, Anal. Chim. Acta 480 (2003) 109. A. Eftekhari, Sens. Actuators B 88 (2003) 234. J.A. Mihell, J.K. Atkinson, Sens. Actuators B 48 (1998) 505. S. Zhuiykov, Sens. Actuators B 136 (2009) 248. T.R.L.C. Paixao, M. Bertotti, Electroanalysis 20 (2008) 1671. D. Susanti, D.S. Tsai, Y.S. Huang, A. Korotcov, W.H. Chung, J. Phys. Chem. C 111 (2007) 9530. R.H. Baughman, A.A. Zakhidov, W.A. de Heer, Science 297 (2002) 787. J.S. Ye, H.F. Cui, X. Liu, T.M. Lim, W.D. Zhang, F.S. Sheu, Small 1 (2005) 560. L.C. Jiang, W.D. Zhang, Electroanalysis 21 (2009) 988. J. Chen, W.D. Zhang, J.S. Ye, Electrochem. Commun. 10 (2008) 1268. W.D. Zhang, B. Xu, Electrochem. Commun. 11 (2009) 1038. W.D. Zhang, Y. Wen, S.M. Liu, W.C. Tjiu, G.Q. Xu, L.M. Gan, Carbon 40 (2002) 1981. W.D. Zhang, J.T.L. Thong, W.C. Tjiu, L.M. Gan, Diamond Related Mater. 11 (2002) 1638. W.D. Zhang, F. Yang, P.Y. Gu, Nanotechnology 16 (2005) 2442. J.S. Ye, Y. Wen, W.D. Zhang, L.M. Gan, G.Q. Xu, F.S. Sheu, Electrochem. Commun. 6 (2004) 66. J.S. Ye, Y. Wen, W.D. Zhang, L.M. Gan, G.Q. Xu, F.S. Sheu, Electroanalysis 15 (2003) 1693. H.F. Cui, J.S. Ye, X. Liu, W.D. Zhang, F.S. Sheu, Nanotechnology 17 (2006) 2334. J.S. Ye, Y. Wen, W.D. Zhang, H.F. Cui, G.Q. Xu, F.S. Sheu, Nanotechnology 17 (2006) 3994. D.D. Macdonald, J. Liu, D. Lee, J. Appl. Electrochem. 34 (2004) 577. N. de Zoubov, M. Pourbaix, in: M. Pourbaix (Ed.), Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon, Oxford, 1966, p. 343. Y.H. Liao, J.C. Chou, Sens. Actuators B 128 (2008) 603. H.N. McMurray, P. Douglas, D. Abbot, Sens. Actuators B 28 (1995) 9. T. Katsube, I. Lauks, J.N. Zemel, Sens. Actuators 2 (1982) 399. W. Olthuis, M.A.M. Robben, P. Bergveld, Sens. Actuators B 2 (1990) 247. M.J. Tarlov, S. Semancik, K.G. Kreider, Sens. Actuators B 1 (1990) 293. J.A. Rard, Chem. Rev. 85 (1985) 1. W. Pan, S.B. Desu, Phys. Stat. Sol. (a) 161 (1997) 201. W.E. Bell, M. Tagami, J. Phys. Chem. 67 (1963) 2432. L. Bousse, H. Van Den Vlekkert, N.F. De Rooij, Sens. Actuators B 2 (1990) 103. J.L. Chiang, S.S. Jan, J.C. Chou, Y.C. Chen, Sens. Actuators B 76 (2001) 624. P.C. Chang, H.Y. Chen, J.S. Ye, F.S. Sheu, J.G. Lu, ChemPhysChem 8 (2007) 57. D.C. Chen, C.Y. Fu, J.S. Zhen, Trans. Nonferrous Met. Soc. China 15 (2005) 855. C. Macca, Electroanalysis 15 (2003) 997. Y. Umezawa, P. Buhlmann, K. Umezawa, K. Tohda, S. Amemiya, Pure Appl. Chem. 72 (2000) 1851. K. Ren, Talanta 52 (2000) 1157. F.M. Al-Kharafi, W.A. Badawy, Electrochim. Acta 42 (1997) 579. M. Dijksma, B.A. Boukamp, B. Kamp, W.P. Van Bennekom, Langmuir 18 (2002) 3105. T. Jacobsen, K. West, Electrochim. Acta 40 (1995) 255. F. Artuso, F. Bonino, F. Decker, A. Lourenco, E. Masetti, Electrochim. Acta 47 (2002) 2231. J. Bisquert, G. Garcia-Belmonte, P. Bueno, E. Longo, L.O.S. Bulhoes, J. Electroanal. Chem. 452 (1998) 229.