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Prussian Blue electrodeposited on MWNTs–PANI hybrid composites for H2O2 detection
Talanta 72 (2007) 437–442
Prussian Blue electrodeposited on MWNTs–PANI hybrid composites for H2O2 detection Yongjin Zou a,b , Lixian Sun a,∗ , Fen Xu a a
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China b Graduate School of the Chinese Academy of Sciences, Beijing 100049, China
Received 27 June 2006; received in revised form 18 October 2006; accepted 1 November 2006 Available online 28 November 2006
Abstract A Prussian Blue (PB)/polyaniline (PANI)/multi-walled carbon nanotubes (MWNTs) composite film was fabricated by step-by-step electrodeposition on glassy carbon electrode (GCE). The electrode prepared exhibits enhanced electrocatalytic behavior and good stability for detection of H2 O2 at an applied potential of 0.0 V. The effects of MWNTs thickness, electrodeposition time of PANI and rotating rate on the current response of the composite modified electrode toward H2 O2 were optimized to obtain the maximal sensitivity. A linear range from 8 × 10−9 to 5 × 10−6 M for H2 O2 detection has been observed at the PB/PANI/MWNTs modified GCE with a correlation coefficient of 0.997. The detection limit is 5 × 10−9 M on signal-to-noise ratio of 3. To the best of our knowledge, this is the lowest detection limit for H2 O2 detection. The electrode also shows high sensitivity (526.43 A M−1 cm−2 ) for H2 O2 detection which is more than three orders of magnitude higher than the reported. © 2006 Elsevier B.V. All rights reserved. Keywords: Prussian Blue; Multi-walled carbon nanotubes; Polyaniline; Hybrid composite; H2 O2
1. Introduction With their unique electronic and mechanical properties, carbon nanotubes (either multi-walled or single-walled carbon nanotubes) are of great interest for the fabrication of new classes of advanced materials [1]. In addition, the ability of carbon nanotubes (CNTs) to promote the electron-transfer reactions of important molecules, such as cytochrome c [2], ascorbic acid [3], neurotransmitters [4], NADH [5] and H2 O2 [6,7], have made them attractive for the construction of various electrochemical sensors. Recently, conducting polymer/CNTs composites have received significant interest because the incorporation of CNTs in conducting polymers can lead to new composite materials possessing the properties of each component with a synergistic effect that would be useful in particular applications [8]. Among the various conducting polymers, polyaniline, a particular conducting polymer with a high application potential, has become the most attractive one because of its facile preparation, ∗ Corresponding author at: Materials & Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China. Tel.: +86 411 84379213; fax: +86 411 84379213. E-mail address: [email protected] (L. Sun).
0039-9140/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2006.11.001
high conductivity and good environmental stability. PANI/CNTs composites have been prepared by electropolymerization of aniline [9] or in situation chemical polymerization [10] which have improved the electrical conductivity, electrochemical capacitance and electrocatalytic properties of the polymers [11]. On the other hand, Prussian Blue, known as an “artificial peroxidase” [12], has high electrocatalytic activity, stability and selectivity for H2 O2 electroreduction. It has been extensively studied and used for H2 O2 detection [13,14]. It is superior to noble metal such as platinum or platinised electrodes which detect H2 O2 through its oxidation at anodic potential (>+0.6 V, Ag/AgCl) [15]. Many substances, such as uric acid and ascorbic acid, normally present in biologic samples, can also be electrochemically oxidized at such a potential, which may cause an interfering response in the quantization of substrate concentration. Low-potential detection of H2 O2 is also possible with peroxidase modified electrodes [16–19]. Despite the low detection limits achieved, peroxidase electrodes demonstrate saturation with the H2 O2 , which affects linear calibration range. In addition, peroxidase electrodes are usually less selective relative to oxygen. Moreover, the use of the enzyme obviously deteriorates transducer properties because of its instability and high cost.
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Monitoring of low levels of H2 O2 is of great importance for modern medicine, environmental control, and various branches of industry [20,21]. Many of the sensors so far developed show a satisfactory sensitivity for the detection relatively lower concentration of H2 O2 and the possibility to detect H2 O2 down to 10−8 M is achieved. But monitoring of lower H2 O2 levels is significance both in clinical diagnostics and environmental control [13]. Prussian Blue films have been deposited on a variety of surfaces, the most common being glassy carbon [12–14], graphite [22], Pt [23] and carbon fiber [24]. To our knowledge, there is no literature which electrodeposits PB on MWNTs substrate for H2 O2 detection. The authors of the present paper have studied glucose biosensor based on platinum catalyst and porphine [25]. In this work, a PB/PANI/MWNTs composites modified electrode has been fabricated by step-by-step electrodeposition on GCE. Cyclic voltammetry (CV) was used to investigate the electrochemical behavior to the reduction of H2 O2 of the modified electrode. Effects of electrodeposition time of PANI, MWNTs thickness and rotating rate on amperometric response of the PB/PANI/MWNTs composite modified electrode to H2 O2 have been investigated and discussed. 2. Experimental 2.1. Reagents All chemicals from commercial source were of analytical grade. Aniline (≥99.5%, Shenyang Lianbang Reagent Factory, Shengyang, China) was distilled before experiments. Multiwalled carbon nanotubes (95% 20–60 nm) purchased from Shenzhen Nanotech. Port. Co., Ltd. (Shenzhen, China) were treated with nitric acid during purification process and then filtered, rinsed with double-distilled water and dried. Doubledistilled water was used throughout the experiments. A fresh H2 O2 aqueous solution was prepared prior to use. 0.1 M phosphate buffer solution (PBS, pH 6.5), which was made from Na2 HPO4 and NaH2 PO4 , was always employed as supporting electrolyte. 2.2. Instruments Scanning electron microscopy (SEM) images were obtained by using JSM-6360LV SEM (JEOL, Japan). Electrodeposition and electrochemical characterization experiments were performed with an IM6e electrochemical workstation (ZahnerElektrik, Kronach, Germany). All electrochemical experiments were carried out with a conventional three-electrode system. The working electrode was the Bioanalytical Systems (BAS) cavity glassy carbon electrode (3-mm diameter). The rotating disk electrode experiments were performed by BAS rotator system in conjunction with an IM6e. The rotating rate is 3000 rpm when detect H2 O2 unless stated otherwise. An Ag/AgCl (saturated with NaCl) reference electrode was used for all measurements, and all the potentials were reported in this paper versus this reference electrode. A platinum wire was used as a counter electrode. Before all batch amperometric experiments, the potential of each
electrode was held at the operating value, allowing the background current to decay to a steady state value. All experiments were performed at room temperature. 2.3. Fabrication of the modified electrode The fabrication of the PB/PANI/MWNTs glassy carbon electrode was summarized as follows. One milligram of purified MWNTs was dispersed in 5 ml dimethylformamide (DMF) with the aid of ultrasonic agitation to give a 0.2 mg ml−1 black suspension. The GCE was carefully polished with 1.0 and 0.3 m a-Al2 O3 powders successively according to the literature [18], and then cleaned ultrasonically in the double distill water and ethanol for a few minutes, respectively. The well-polished GCE with a fresh surface was preheated at about 80 ◦ C for 30 min. The GCE was treated by dropping a suspension (15 l) of the MWNTs in DMF and then dried under an infrared lamp. The MWNT/GCE obtained was thoroughly rinsed with water. PANI was electrochemically prepared via cyclic voltammetry in a three-electrode cell containing an aqueous solution of 0.1 M HCl and 0.1 M aniline. The working electrode was the MWNTs/GCE. The PANI film was obtained on the working electrode by cycling between −0.2 and 0.7 V versus Ag/AgCl at a sweep rate 0.1 V s−1 for 10 cycles. After electrodeposition, the electrode was rinsed with pure water, and the next electrochemical experiment was performed. PB film was potentiostatically deposited at a potential value of 0.40 V for 40 s from aqueous solutions of 2 × 10−3 M K3 [Fe(CN)6 ] + 2 × 10−3 M FeCl3 in the supporting electrolyte 0.1 M KCl + 0.01 M HCl [26]. After deposition, the modified electrode was rinsed with water (pH 5.3) and immersed into a solution containing 0.1 M KCl + 0.01 M HCl, where the electrode potential was cycled between −0.05 and 0.35 V at a scan rate of 0.05 V s−1 , until a stable voltammetric response was obtained. 3. Results and discussion 3.1. Effect of the MWNTs thickness The amount of MWNTs modified on the glassy carbon electrode was one of the important factors to be considered. The calibration plots for H2 O2 obtained at electrode by adjusting thickness of the MWNTs were compared. The sensitivity increased with the MWNTs loading from 5 to 20 l. When the magnitude of MWNTs increased, the sensitivity increased, but when the amount of MWNTs beyond 15 l, the sensitivity decreased. This may be attributed to too much MWNTs loading on the GCE so as to make the mechanical stability of the MWNTs film rather poor. As a compromise, the amount of MWNTs magnitude of 15 l was selected in the experiments. 3.2. Electropolymerization of aniline The composites of MWNTs/PANI can be obtained from chemical polymerization in a solution containing MWNTs and aniline. In the present paper, PANI was deposited on the surface
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detection. When the rotating rate is increased, the response cur−1/2 ) is linear rent is also increased. The Levich plot (i−1 L versus ω up to 3000 rpm, and after that it reaches the kinetic limitation. The response current does not increase and keep steady which is in agreement to the literature [31]. Therefore, the rotating rate 3000 rpm was chose for H2 O2 detection. 3.5. Characterization of the PB/PANI/MWNTs modified GCE
Fig. 1. Cyclic voltammograms recorded during electrochemical deposition of polyaniline at a MWNTs/GCE in solutions containing 0.1 M aniline and 0.1 M HCl. Potentials were swept between −0.2 and 0.7 V (vs. Ag/AgCl) with a sweep rate of 0.1 V s−1 .
of MWNTs by an electrochemical potential cycling process. It was reported that the morphology of the polymer films depended on the electropolymerization process [27]. To obtain a compact and relatively thin film, the electropolymerization of PANI was performed in the solution of aniline scanning from −0.2 to 0.7 V (versus Ag/AgCl) with a sweep rate of 0.1 V s−1 for 10 full voltammetric cycles. From Fig. 1, it can be seen that there are three pairs of redox peaks indicating the presence of discrete electroactive regions in the film. Three reversible pairs of peaks can be distinguished at potentials around +0.15, +0.47 and +0.65 V. The first and the third pairs have been ascribed to the polaron and bipolaron forms of the PANI [28]. The origin of the second peak is much more complex and it has been attributed to many different intermediates and degradation products (crosslinked polymer, benzoquinone, etc.) [29]. It is similar to the reported in the literature [30]. 3.3. Optimization of electropolymerization condition The scan cycles in PANI electropolymerization was also important factors to be considered. When the polymer film is thin, the response time will be increased, whereas the modified electrode is unstable and the sensitivity is low when the film is thick. The sensitivity increases with the increasing of the scan cycles. When the scan cycles are up to 10 cycles, the best sensitivity is obtained. The aniline solution was chosen to be electropolymerized on MWNTs/GCE with a sweep rate of 0.1 V s−1 for 10 cycles when the PB/PANI/MWNTs modified GCE was prepared. 3.4. Effect of rotating rate The effect of rotating rate for H2 O2 detection was also investigated. Microelectrode can decrease the diffusion polarization effectively, but it generates low currents, which is hardly detectable with the conventional electrochemical technique. In the present work, the rotating disk electrode was used for H2 O2
Fig. 2 shows the SEM images of the modified glassy carbon electrodes at the optimized condition: (a) MWNTs, (b) PANI/MWNTs and (c) PB/PANI/MWNTs. As shown in Fig. 2a, the porous MWNTs film has large surface area which provides an ideal matrix for the distribution of PB. The SEM images also reveal that the MWNTs, with a diameter ranging from 30 to 60 nm, are well distributed on the surface and that most of the MWNTs are in the form of small bundles or single tubes. Such small bundles and single tubes assembled homogeneously on the substrate are believed to be very beneficial for the modified electrode performance because most of the well-dispersed MWNTs are electrochemically accessible. After electropolymerization of PANI film on MWNTs modified electrode, the majority of MWNTs has been entrapped in the PANI film (Fig. 2b). From Fig. 2c, it can be seen that Prussian Blue particles are dispersed on the surface of PANI/MWNTs. Fig. 3 shows cyclic voltammograms of PB/PANI/MWNTs modified GCE at various scan rates. The ratio of cathodic to anodic peak currents is nearly unity. Peak currents vary linearly with the scan rates, as shown in the inset of Fig. 3. When the scan rate is increased the Epc shifts negatively and the Epa shifts positively. When the scan rate is higher than 200 mV s−1 , the wave shape is distorted severely (Ep > 200 mV). This indicates that the electrode reaction becomes electrochemically irreversible at higher scan rates. Though PB films for H2 O2 detection is very well established, the operational stability of the PB is not solved completely because the PB film is unstable in neutral and alkaline solution. The hydroxyl ions are known to be able to break the Fe–CN–Fe bond and solve the PB [32]. Moreover, Prussian-White (PW, the reduced redox state of PB at 0.0 V) is thermodynamically unstable on electrode surfaces, and hydroxyl ions (produced in the hydrogen peroxide reduction in neutral media) may solubilize the inorganic polycrystal. Polypyrrole have been used to protect the PB film, but the electropolymerization of Polypyrrole is difficult to control. The polymer film is usually thick and the response time will be increased [33]. In this work, the PB/PANI/MWNTs modified GCE also shows good stability after scanned in 0.1 M PBS + 0.1 M KCl (pH 6.5) for 50 cycles with no peak current decrease. It is still electroactive and sufficient to catalyze the H2 O2 after 300 cycles in the PBS solution and the decrease of the signal was only 8% of the initial value. The peak currents decrease slightly after 300 cycles in PBS solution. In our opinion, the good results in the present paper achieved, similar to these observed in a previous paper, are due to the proposed method of deposition, which involves the chemical synthesis of PB in the presence of graphite. The treated MWNTs have carboxylic groups similar
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Fig. 2. SEM images of the modified GCEs: (a) MWNTs, (b) PANI/MWNTs and (c) PB/PANI/MWNTs.
to those produced with aqua regia on graphite power and the presence of these groups seems to be good to PB films [34]. On the other hand, MWNTs increase the surface dimension. The porous high surface area MWNTs matrix provides a high loading capacity for the deposition of PB particles while the PANI thin film further stabilizes the PB film. Fig. 4 shows the cyclic voltammograms obtained with the PB/PANI/MWNTs modified GCE in 0.1 M PBS + 0.1 M KCl (pH 6.5) without H2 O2 and with 5 × 10−7 M H2 O2 , respectively. In the absence of H2 O2 , the modified electrode gives no response
Fig. 3. Cyclic voltammograms at the PB/PANI/MWNTs/GC electrodes in 0.1 M PBS + 0.1 M KCl (pH 6.5) at different scan rates, from inside to outside: 20, 40, 60, 80, 100 and 120 mV s−1 . Insert is the plot cathodic and anodic peak current vs. scan rate.
and only the electrochemical behavior of PB was observed. The redox behavior of PB at the modified GCE shows a reversible electrochemical response. When added 5 × 10−7 M H2 O2 in the solution, the cyclic voltammogram changed, with a decrease in the reduction current. It indicates that electrocatalytic reduction of H2 O2 produced is preferable for H2 O2 detection. 3.6. Electrochemical performance of the modified GCE Analytical performances of the modified electrode for H2 O2 detection were investigated. Fig. 5 shows amperometric
Fig. 4. Cyclic voltammograms of the PB/PANI/MWNTs modified GCE at the scan rate of 0.05 V s−1 in 0.1 M PBS + 0.1 M KCl (pH 6.5): (a) in the absence of H2 O2 and (b) in the presence of 5 × 10−7 M H2 O2 .
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Table 1 Prussian Blue-based modified electrodes for H2 O2 detection in comparison with literature Electrode
Range of linearity (M)
Detection limit (M)
Sensitivity (A M−1 cm−2 )
Reference
PB/PANI/MWNTs PB/SPEs PB nanoelectrode arrays
8 × 10−9
5 × 10−9
526.43 0.324 0.06
This paper [32] [13]
2 × 10−6
to 10−7 to 5 × 10−5 1 × 10−8 to 10−2
10−7 10−8
ery rate of the modified electrode is 98% (0.049 ± 0.016 M), 99.2% (0.248 ± 0.062 M), 102% (0.511 ± 0.014 M) and 103% (1.23 ± 0.035 M), respectively. The average recovery is 101%. 4. Conclusions
Fig. 5. Calibration curve for H2 O2 detection in pH 6.5 0.1 M PBS + 0.1 M KCl at 0.0 V, rotating rate 3000 rpm. In the lower insert, the actual response curve for some H2 O2 additions is also shown. The addition successive is 0.02 M l−1 .
An organic/inorganic hybrid material composed of a conducting polyaniline thin film and Prussian Blue in a porous MWNTs matrix was synthesized. The composite film shows enhanced electrocatalytic activity to H2 O2 . The performances of PB/PANI/MWNTs modified electrode were characterized with cyclic voltammetry and scanning electron microscopy. Compared with the known amperometric detection of H2 O2 , the PB/PANI/MWNTs modified GCE shows better stability and lower detection limit. The modified GCE can also be used to develop into biosensor for detection of glucose and further work is in progress. Acknowledgments
responses of the PB/PANI/MWNTs modified GCE for H2 O2 detection. The modified GCE displays rapid response and an expanded linear response range. The linear response range of 8 × 10−9 to 5 × 10−6 M with a correlation coefficient of 0.997 is achieved. The response time is 5 s when the current reach 90% of the steady state. The detection limit is 5 × 10−9 M on signal-to-noise ratio of 3. To our knowledge, this is the lowest detection limit as compared with the known H2 O2 sensors [13,16–19,30]. The sensitivity is 526.43 A M−1 cm−2 which is more than three orders of magnitude higher than the reported (shown in Table 1). The high sensitivity and low detection limit may attribute the superior transducing ability of PANI [35] and the excellent performance of the composites of PANI/MWNTs. The synergistic effect among MWNTs, PANI and PB is preferable for the H2 O2 detection and improves the performances of the modified electrode. The fabrication reproducibility, investigated at 5 × 10−7 M H2 O2 , was the relative standard deviation (R.S.D.) of 5.1% for five different modified electrodes. For five replicate measurements at 5 × 10−7 M H2 O2 using a typical modified electrode, the R.S.D. was 2.5%. The typical interfering specie–ascorbic acid was investigated through adding 0.2 mM ascorbic acid to 5 × 10−7 M H2 O2 and do not produce any observable interference in the modified electrode response. It may be attributed to the low potential (0.0 V) employed for the electrocatalysis of H2 O2 reduction promoted by the PB layer [36]. The analytical applicability of the modified electrode was evaluated by determining the recoveries of 0.050, 0.250, 0.500 and 1.20 M H2 O2 by standard addition method [37]. The recov-
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Prussian Blue electrodeposited on MWNTs–PANI hybrid composites for H2O2 detection Yongjin Zou a,b , Lixian Sun a,∗ , Fen Xu a a
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China b Graduate School of the Chinese Academy of Sciences, Beijing 100049, China
Received 27 June 2006; received in revised form 18 October 2006; accepted 1 November 2006 Available online 28 November 2006
Abstract A Prussian Blue (PB)/polyaniline (PANI)/multi-walled carbon nanotubes (MWNTs) composite film was fabricated by step-by-step electrodeposition on glassy carbon electrode (GCE). The electrode prepared exhibits enhanced electrocatalytic behavior and good stability for detection of H2 O2 at an applied potential of 0.0 V. The effects of MWNTs thickness, electrodeposition time of PANI and rotating rate on the current response of the composite modified electrode toward H2 O2 were optimized to obtain the maximal sensitivity. A linear range from 8 × 10−9 to 5 × 10−6 M for H2 O2 detection has been observed at the PB/PANI/MWNTs modified GCE with a correlation coefficient of 0.997. The detection limit is 5 × 10−9 M on signal-to-noise ratio of 3. To the best of our knowledge, this is the lowest detection limit for H2 O2 detection. The electrode also shows high sensitivity (526.43 A M−1 cm−2 ) for H2 O2 detection which is more than three orders of magnitude higher than the reported. © 2006 Elsevier B.V. All rights reserved. Keywords: Prussian Blue; Multi-walled carbon nanotubes; Polyaniline; Hybrid composite; H2 O2
1. Introduction With their unique electronic and mechanical properties, carbon nanotubes (either multi-walled or single-walled carbon nanotubes) are of great interest for the fabrication of new classes of advanced materials [1]. In addition, the ability of carbon nanotubes (CNTs) to promote the electron-transfer reactions of important molecules, such as cytochrome c [2], ascorbic acid [3], neurotransmitters [4], NADH [5] and H2 O2 [6,7], have made them attractive for the construction of various electrochemical sensors. Recently, conducting polymer/CNTs composites have received significant interest because the incorporation of CNTs in conducting polymers can lead to new composite materials possessing the properties of each component with a synergistic effect that would be useful in particular applications [8]. Among the various conducting polymers, polyaniline, a particular conducting polymer with a high application potential, has become the most attractive one because of its facile preparation, ∗ Corresponding author at: Materials & Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China. Tel.: +86 411 84379213; fax: +86 411 84379213. E-mail address: [email protected] (L. Sun).
0039-9140/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2006.11.001
high conductivity and good environmental stability. PANI/CNTs composites have been prepared by electropolymerization of aniline [9] or in situation chemical polymerization [10] which have improved the electrical conductivity, electrochemical capacitance and electrocatalytic properties of the polymers [11]. On the other hand, Prussian Blue, known as an “artificial peroxidase” [12], has high electrocatalytic activity, stability and selectivity for H2 O2 electroreduction. It has been extensively studied and used for H2 O2 detection [13,14]. It is superior to noble metal such as platinum or platinised electrodes which detect H2 O2 through its oxidation at anodic potential (>+0.6 V, Ag/AgCl) [15]. Many substances, such as uric acid and ascorbic acid, normally present in biologic samples, can also be electrochemically oxidized at such a potential, which may cause an interfering response in the quantization of substrate concentration. Low-potential detection of H2 O2 is also possible with peroxidase modified electrodes [16–19]. Despite the low detection limits achieved, peroxidase electrodes demonstrate saturation with the H2 O2 , which affects linear calibration range. In addition, peroxidase electrodes are usually less selective relative to oxygen. Moreover, the use of the enzyme obviously deteriorates transducer properties because of its instability and high cost.
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Monitoring of low levels of H2 O2 is of great importance for modern medicine, environmental control, and various branches of industry [20,21]. Many of the sensors so far developed show a satisfactory sensitivity for the detection relatively lower concentration of H2 O2 and the possibility to detect H2 O2 down to 10−8 M is achieved. But monitoring of lower H2 O2 levels is significance both in clinical diagnostics and environmental control [13]. Prussian Blue films have been deposited on a variety of surfaces, the most common being glassy carbon [12–14], graphite [22], Pt [23] and carbon fiber [24]. To our knowledge, there is no literature which electrodeposits PB on MWNTs substrate for H2 O2 detection. The authors of the present paper have studied glucose biosensor based on platinum catalyst and porphine [25]. In this work, a PB/PANI/MWNTs composites modified electrode has been fabricated by step-by-step electrodeposition on GCE. Cyclic voltammetry (CV) was used to investigate the electrochemical behavior to the reduction of H2 O2 of the modified electrode. Effects of electrodeposition time of PANI, MWNTs thickness and rotating rate on amperometric response of the PB/PANI/MWNTs composite modified electrode to H2 O2 have been investigated and discussed. 2. Experimental 2.1. Reagents All chemicals from commercial source were of analytical grade. Aniline (≥99.5%, Shenyang Lianbang Reagent Factory, Shengyang, China) was distilled before experiments. Multiwalled carbon nanotubes (95% 20–60 nm) purchased from Shenzhen Nanotech. Port. Co., Ltd. (Shenzhen, China) were treated with nitric acid during purification process and then filtered, rinsed with double-distilled water and dried. Doubledistilled water was used throughout the experiments. A fresh H2 O2 aqueous solution was prepared prior to use. 0.1 M phosphate buffer solution (PBS, pH 6.5), which was made from Na2 HPO4 and NaH2 PO4 , was always employed as supporting electrolyte. 2.2. Instruments Scanning electron microscopy (SEM) images were obtained by using JSM-6360LV SEM (JEOL, Japan). Electrodeposition and electrochemical characterization experiments were performed with an IM6e electrochemical workstation (ZahnerElektrik, Kronach, Germany). All electrochemical experiments were carried out with a conventional three-electrode system. The working electrode was the Bioanalytical Systems (BAS) cavity glassy carbon electrode (3-mm diameter). The rotating disk electrode experiments were performed by BAS rotator system in conjunction with an IM6e. The rotating rate is 3000 rpm when detect H2 O2 unless stated otherwise. An Ag/AgCl (saturated with NaCl) reference electrode was used for all measurements, and all the potentials were reported in this paper versus this reference electrode. A platinum wire was used as a counter electrode. Before all batch amperometric experiments, the potential of each
electrode was held at the operating value, allowing the background current to decay to a steady state value. All experiments were performed at room temperature. 2.3. Fabrication of the modified electrode The fabrication of the PB/PANI/MWNTs glassy carbon electrode was summarized as follows. One milligram of purified MWNTs was dispersed in 5 ml dimethylformamide (DMF) with the aid of ultrasonic agitation to give a 0.2 mg ml−1 black suspension. The GCE was carefully polished with 1.0 and 0.3 m a-Al2 O3 powders successively according to the literature [18], and then cleaned ultrasonically in the double distill water and ethanol for a few minutes, respectively. The well-polished GCE with a fresh surface was preheated at about 80 ◦ C for 30 min. The GCE was treated by dropping a suspension (15 l) of the MWNTs in DMF and then dried under an infrared lamp. The MWNT/GCE obtained was thoroughly rinsed with water. PANI was electrochemically prepared via cyclic voltammetry in a three-electrode cell containing an aqueous solution of 0.1 M HCl and 0.1 M aniline. The working electrode was the MWNTs/GCE. The PANI film was obtained on the working electrode by cycling between −0.2 and 0.7 V versus Ag/AgCl at a sweep rate 0.1 V s−1 for 10 cycles. After electrodeposition, the electrode was rinsed with pure water, and the next electrochemical experiment was performed. PB film was potentiostatically deposited at a potential value of 0.40 V for 40 s from aqueous solutions of 2 × 10−3 M K3 [Fe(CN)6 ] + 2 × 10−3 M FeCl3 in the supporting electrolyte 0.1 M KCl + 0.01 M HCl [26]. After deposition, the modified electrode was rinsed with water (pH 5.3) and immersed into a solution containing 0.1 M KCl + 0.01 M HCl, where the electrode potential was cycled between −0.05 and 0.35 V at a scan rate of 0.05 V s−1 , until a stable voltammetric response was obtained. 3. Results and discussion 3.1. Effect of the MWNTs thickness The amount of MWNTs modified on the glassy carbon electrode was one of the important factors to be considered. The calibration plots for H2 O2 obtained at electrode by adjusting thickness of the MWNTs were compared. The sensitivity increased with the MWNTs loading from 5 to 20 l. When the magnitude of MWNTs increased, the sensitivity increased, but when the amount of MWNTs beyond 15 l, the sensitivity decreased. This may be attributed to too much MWNTs loading on the GCE so as to make the mechanical stability of the MWNTs film rather poor. As a compromise, the amount of MWNTs magnitude of 15 l was selected in the experiments. 3.2. Electropolymerization of aniline The composites of MWNTs/PANI can be obtained from chemical polymerization in a solution containing MWNTs and aniline. In the present paper, PANI was deposited on the surface
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detection. When the rotating rate is increased, the response cur−1/2 ) is linear rent is also increased. The Levich plot (i−1 L versus ω up to 3000 rpm, and after that it reaches the kinetic limitation. The response current does not increase and keep steady which is in agreement to the literature [31]. Therefore, the rotating rate 3000 rpm was chose for H2 O2 detection. 3.5. Characterization of the PB/PANI/MWNTs modified GCE
Fig. 1. Cyclic voltammograms recorded during electrochemical deposition of polyaniline at a MWNTs/GCE in solutions containing 0.1 M aniline and 0.1 M HCl. Potentials were swept between −0.2 and 0.7 V (vs. Ag/AgCl) with a sweep rate of 0.1 V s−1 .
of MWNTs by an electrochemical potential cycling process. It was reported that the morphology of the polymer films depended on the electropolymerization process [27]. To obtain a compact and relatively thin film, the electropolymerization of PANI was performed in the solution of aniline scanning from −0.2 to 0.7 V (versus Ag/AgCl) with a sweep rate of 0.1 V s−1 for 10 full voltammetric cycles. From Fig. 1, it can be seen that there are three pairs of redox peaks indicating the presence of discrete electroactive regions in the film. Three reversible pairs of peaks can be distinguished at potentials around +0.15, +0.47 and +0.65 V. The first and the third pairs have been ascribed to the polaron and bipolaron forms of the PANI [28]. The origin of the second peak is much more complex and it has been attributed to many different intermediates and degradation products (crosslinked polymer, benzoquinone, etc.) [29]. It is similar to the reported in the literature [30]. 3.3. Optimization of electropolymerization condition The scan cycles in PANI electropolymerization was also important factors to be considered. When the polymer film is thin, the response time will be increased, whereas the modified electrode is unstable and the sensitivity is low when the film is thick. The sensitivity increases with the increasing of the scan cycles. When the scan cycles are up to 10 cycles, the best sensitivity is obtained. The aniline solution was chosen to be electropolymerized on MWNTs/GCE with a sweep rate of 0.1 V s−1 for 10 cycles when the PB/PANI/MWNTs modified GCE was prepared. 3.4. Effect of rotating rate The effect of rotating rate for H2 O2 detection was also investigated. Microelectrode can decrease the diffusion polarization effectively, but it generates low currents, which is hardly detectable with the conventional electrochemical technique. In the present work, the rotating disk electrode was used for H2 O2
Fig. 2 shows the SEM images of the modified glassy carbon electrodes at the optimized condition: (a) MWNTs, (b) PANI/MWNTs and (c) PB/PANI/MWNTs. As shown in Fig. 2a, the porous MWNTs film has large surface area which provides an ideal matrix for the distribution of PB. The SEM images also reveal that the MWNTs, with a diameter ranging from 30 to 60 nm, are well distributed on the surface and that most of the MWNTs are in the form of small bundles or single tubes. Such small bundles and single tubes assembled homogeneously on the substrate are believed to be very beneficial for the modified electrode performance because most of the well-dispersed MWNTs are electrochemically accessible. After electropolymerization of PANI film on MWNTs modified electrode, the majority of MWNTs has been entrapped in the PANI film (Fig. 2b). From Fig. 2c, it can be seen that Prussian Blue particles are dispersed on the surface of PANI/MWNTs. Fig. 3 shows cyclic voltammograms of PB/PANI/MWNTs modified GCE at various scan rates. The ratio of cathodic to anodic peak currents is nearly unity. Peak currents vary linearly with the scan rates, as shown in the inset of Fig. 3. When the scan rate is increased the Epc shifts negatively and the Epa shifts positively. When the scan rate is higher than 200 mV s−1 , the wave shape is distorted severely (Ep > 200 mV). This indicates that the electrode reaction becomes electrochemically irreversible at higher scan rates. Though PB films for H2 O2 detection is very well established, the operational stability of the PB is not solved completely because the PB film is unstable in neutral and alkaline solution. The hydroxyl ions are known to be able to break the Fe–CN–Fe bond and solve the PB [32]. Moreover, Prussian-White (PW, the reduced redox state of PB at 0.0 V) is thermodynamically unstable on electrode surfaces, and hydroxyl ions (produced in the hydrogen peroxide reduction in neutral media) may solubilize the inorganic polycrystal. Polypyrrole have been used to protect the PB film, but the electropolymerization of Polypyrrole is difficult to control. The polymer film is usually thick and the response time will be increased [33]. In this work, the PB/PANI/MWNTs modified GCE also shows good stability after scanned in 0.1 M PBS + 0.1 M KCl (pH 6.5) for 50 cycles with no peak current decrease. It is still electroactive and sufficient to catalyze the H2 O2 after 300 cycles in the PBS solution and the decrease of the signal was only 8% of the initial value. The peak currents decrease slightly after 300 cycles in PBS solution. In our opinion, the good results in the present paper achieved, similar to these observed in a previous paper, are due to the proposed method of deposition, which involves the chemical synthesis of PB in the presence of graphite. The treated MWNTs have carboxylic groups similar
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Fig. 2. SEM images of the modified GCEs: (a) MWNTs, (b) PANI/MWNTs and (c) PB/PANI/MWNTs.
to those produced with aqua regia on graphite power and the presence of these groups seems to be good to PB films [34]. On the other hand, MWNTs increase the surface dimension. The porous high surface area MWNTs matrix provides a high loading capacity for the deposition of PB particles while the PANI thin film further stabilizes the PB film. Fig. 4 shows the cyclic voltammograms obtained with the PB/PANI/MWNTs modified GCE in 0.1 M PBS + 0.1 M KCl (pH 6.5) without H2 O2 and with 5 × 10−7 M H2 O2 , respectively. In the absence of H2 O2 , the modified electrode gives no response
Fig. 3. Cyclic voltammograms at the PB/PANI/MWNTs/GC electrodes in 0.1 M PBS + 0.1 M KCl (pH 6.5) at different scan rates, from inside to outside: 20, 40, 60, 80, 100 and 120 mV s−1 . Insert is the plot cathodic and anodic peak current vs. scan rate.
and only the electrochemical behavior of PB was observed. The redox behavior of PB at the modified GCE shows a reversible electrochemical response. When added 5 × 10−7 M H2 O2 in the solution, the cyclic voltammogram changed, with a decrease in the reduction current. It indicates that electrocatalytic reduction of H2 O2 produced is preferable for H2 O2 detection. 3.6. Electrochemical performance of the modified GCE Analytical performances of the modified electrode for H2 O2 detection were investigated. Fig. 5 shows amperometric
Fig. 4. Cyclic voltammograms of the PB/PANI/MWNTs modified GCE at the scan rate of 0.05 V s−1 in 0.1 M PBS + 0.1 M KCl (pH 6.5): (a) in the absence of H2 O2 and (b) in the presence of 5 × 10−7 M H2 O2 .
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Table 1 Prussian Blue-based modified electrodes for H2 O2 detection in comparison with literature Electrode
Range of linearity (M)
Detection limit (M)
Sensitivity (A M−1 cm−2 )
Reference
PB/PANI/MWNTs PB/SPEs PB nanoelectrode arrays
8 × 10−9
5 × 10−9
526.43 0.324 0.06
This paper [32] [13]
2 × 10−6
to 10−7 to 5 × 10−5 1 × 10−8 to 10−2
10−7 10−8
ery rate of the modified electrode is 98% (0.049 ± 0.016 M), 99.2% (0.248 ± 0.062 M), 102% (0.511 ± 0.014 M) and 103% (1.23 ± 0.035 M), respectively. The average recovery is 101%. 4. Conclusions
Fig. 5. Calibration curve for H2 O2 detection in pH 6.5 0.1 M PBS + 0.1 M KCl at 0.0 V, rotating rate 3000 rpm. In the lower insert, the actual response curve for some H2 O2 additions is also shown. The addition successive is 0.02 M l−1 .
An organic/inorganic hybrid material composed of a conducting polyaniline thin film and Prussian Blue in a porous MWNTs matrix was synthesized. The composite film shows enhanced electrocatalytic activity to H2 O2 . The performances of PB/PANI/MWNTs modified electrode were characterized with cyclic voltammetry and scanning electron microscopy. Compared with the known amperometric detection of H2 O2 , the PB/PANI/MWNTs modified GCE shows better stability and lower detection limit. The modified GCE can also be used to develop into biosensor for detection of glucose and further work is in progress. Acknowledgments
responses of the PB/PANI/MWNTs modified GCE for H2 O2 detection. The modified GCE displays rapid response and an expanded linear response range. The linear response range of 8 × 10−9 to 5 × 10−6 M with a correlation coefficient of 0.997 is achieved. The response time is 5 s when the current reach 90% of the steady state. The detection limit is 5 × 10−9 M on signal-to-noise ratio of 3. To our knowledge, this is the lowest detection limit as compared with the known H2 O2 sensors [13,16–19,30]. The sensitivity is 526.43 A M−1 cm−2 which is more than three orders of magnitude higher than the reported (shown in Table 1). The high sensitivity and low detection limit may attribute the superior transducing ability of PANI [35] and the excellent performance of the composites of PANI/MWNTs. The synergistic effect among MWNTs, PANI and PB is preferable for the H2 O2 detection and improves the performances of the modified electrode. The fabrication reproducibility, investigated at 5 × 10−7 M H2 O2 , was the relative standard deviation (R.S.D.) of 5.1% for five different modified electrodes. For five replicate measurements at 5 × 10−7 M H2 O2 using a typical modified electrode, the R.S.D. was 2.5%. The typical interfering specie–ascorbic acid was investigated through adding 0.2 mM ascorbic acid to 5 × 10−7 M H2 O2 and do not produce any observable interference in the modified electrode response. It may be attributed to the low potential (0.0 V) employed for the electrocatalysis of H2 O2 reduction promoted by the PB layer [36]. The analytical applicability of the modified electrode was evaluated by determining the recoveries of 0.050, 0.250, 0.500 and 1.20 M H2 O2 by standard addition method [37]. The recov-
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