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graphene oxide nanocomposite as a new electrode material for the selective electrochemical detection of mercury (II)
Synthetic Metals 220 (2016) 14–19
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Poly(3,4-ethylenedioxythiophene) nanorods/graphene oxide nanocomposite as a new electrode material for the selective electrochemical detection of mercury (II) Yinxiu Zuoa,b,1, Jingkun Xua,1, Xiaofei Zhua , Xuemin Duana,* , Limin Lub,* , Yansha Gaoa , Huakun Xinga , Taotao Yanga , Guo Yea , Yongfang Yub a b
Shool of Pharmacy, Jiangxi Science and Technology Normal University, Nanchang 330013, PR China College of Science, Jiangxi Agricultural University, Nanchang 330045, PR China
A R T I C L E I N F O
Article history: Received 7 January 2016 Received in revised form 17 May 2016 Accepted 20 May 2016 Available online xxx Keywords: Poly(3,4-ethylenedioxythiophene) Graphene oxide Interfacial polymerization Electrochemical detection Mercury ions
A B S T R A C T S
Development of selective methods for the detection of mercury (Hg2+) has received tremendous attention in modern chemical research due to its health hazard and persistence in environment. In this paper, the electrochemical determination of Hg2+ at trace level based on poly(3,4-ethylenedioxythiophene) nanorods/graphene oxide nanocomposite modified glassy carbon electrode (PEDOT/GO/GCE) is reported. PEDOT/GO nanocomposite has been proposed via a simple liquid–liquid interfacial polymerization approach. Scanning electron microscopy (SEM) and transmission electron microscope (TEM) were employed to characterize the morphology and structure of the as-prepared PEDOT/GO. The results revealed that PEDOT with a nanorods-like structure anchored on the surface of GO nanosheets, which could enhance the electro-active sites of the nanocomposite. Differential pulse stripping voltammetry (DPSV) was applied to determine low concentrations of Hg2+ on PEDOT/GO/GCE. Experimental conditions, including accumulation time, pH values and deposition potential were optimized. In optimal conditions, a good linear relationship was found between peak currents and the concentration of Hg2+ in 10.0 nM-3.0 mM range. The detection limit was estimated to be 2.78 nM at a signal-to-noise ratio of 3. Finally, the applicability for Hg2+ determination in tap water samples was successfully demonstrated. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Toxic heavy metal in the environment is a growing threat to humanity. Among them, mercury pollution has attracted extensive attention because of its health hazard, accumulative and persistence character in living organisms and environment [1,2]. Thus, developing a high sensitivity, ease use and low cost method for Hg2 + determination is necessary. Differential pulse stripping voltammetry (DPSV) has been deemed as a sensitive detection technology for trace-level heavy metals as it involves unique accumulation/ preconcentration of analyte species contained in the solutions, and then improve the detection sensitivity [3,4]. Moreover, the
* Corresponding authors. E-mail addresses: [email protected] (X. Duan), [email protected] (L. Lu). 1 These authors contributed equally to this work and should be considered cofirst authors. http://dx.doi.org/10.1016/j.synthmet.2016.05.022 0379-6779/ã 2016 Elsevier B.V. All rights reserved.
performance of solid electrode was greatly enhanced through the usage of chemically modified electrode. Recently, graphene oxide (GO) has aroused increasingly interests in fabricating various electrochemical sensors and biosensor because of the similar structure to graphene [5,6]. Unlike graphene, there are plenty of oxygen-containing groups on the graphene oxide (GO) sheet, which allows it disperse easily in water and some other solvents [7]. Besides, the functional groups on GO sheets can prevent carbon sheets from restacking and agglomeration, and act as the combining sites for graphene-based composite. In order to make full use of the advantages and compensate for the poor conductivity of GO, several nanomaterials are employed to combine with GO to form novel conducting composites for various applications [8,9]. Recently, conducting polymer/GO nanocomposite have attracted tremendous attentions and become a research focus, because they possess both excellent properties of conducting polymers and GO, such as high electric conductivity, long term
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environmental stability, good electrochemical activity and biocompatibility of conducting polymers [10], as well as unique electrical and chemical properties of GO [11,12]. Among various conducting polymers, PEDOT, a new polythiophene derivate, has the merits of excellent environmental stability, low band gap, excellent electrochemical activity, high electric conductivity, and transparency in the doped state [13,14]. Synergistic properties like enhanced properties in electrochemical cyclability and electrical conductivity have shown based on PEDOT and GO composite, and there are many good potential applications in many fields such as energy storage, supercapacitors or electrochemical sensors and biosensors for the detection of certain special substances. PEDOT/ GO nanocomposite were usually prepared by a noncovalent mixing/adsorption route, in situ polymerization or electrodeposition techniques [15–17]. However, these methods generally yield insoluble powders, and moreover, these composites showed irregular structures, resulting in a decrease in the electrocatalytic activity of the resulting composite. Very recently, our group and Sun group have reported simple liquid–liquid interfacial polymerization for the preparation of high quality PEDOT/GO nanocomposite [18,19]. The obtained PEDOT/GO nanocomposite exhibited good catalytic activity toward the oxidation of rutin and nitrite due to the plenty of electro-active sites of the nanocomposite. However, to the best of our knowledge, electrochemical determination of heavy metal ions using PEDOT/ GO has not been reported. In this work, PEDOT/GO composite was proposed via liquid/liquid interfacial polymerization, in which EDOT monomer and GO were dispersed in organic phase and water phase, respectively. It was found that such PEDOT/GO nanocomposite showed excellent catalytic activity toward Hg2+, leading to a selective sensor for the detection of Hg2+. The detection limit was calculated to be 2.78 nM with linear range 10.0 nM to 3.0 mM, which is well below to previous report [20–25].
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Vita Chemical Plant, China. Chloroform (CHCl3) and ferric chloride (FeCl3) were received from Shanghai Chemical Co. Ltd. (Shanghai, China). and CH3COONa and CH3COOH were purchased from Aldrich. All these regents were received in analytical grade and used without further purification. Doubly deionized water was used in the whole experiments. 2.2. Apparatus Transmission electron microscope (TEM) images were obtained from JEM-3010 transmission electron microscope (JEOL Co., Ltd., Japan). Scanning electron microscopy (SEM) images were obtained from Hitachi S–3000 N scanning electron microscope. The electrochemical measurements were carried out on a CHI660D electrochemical workstation (Shanghai, China) with conventional three-electrode system. A bare glassy carbon electrode (GCE, K = 3 mm) or modified GCE was used as the working electrode, a platinum foil electrode as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. 2.3. Preparation of the PEDOT/GO nanocomposite The PEDOT/GO nanocomposite was prepared according to our previous report [18]. To be briefly, 1 mL GO dispersion aqueous (0.5 mg mL1) was added with 1 mL FeCl3 (1 M) as oxidant. After 10 min sonication procedure, the above solution was slowly added into 2 mL of EDOT solution in CHCl3 (25 mg mL1) and an interface was produced in the middle of two layers. The received mixture was kept at 50 C under static conditions. The upper layer mixture was centrifuged after 12 h reaction and the obtained precipitate was washed with distilled water and ethanol a couple of times. The obtained nanocomposite was collected for further experiments (Scheme 1).
2. Experimental
2.4. Preparation of the modified electrode
2.1. Chemicals and reagents
Prior to modification, GCE was polished with chamois leather containing 0.05 mM alumina slurry, subsequently, the electrode was rinsed with doubly distilled water, absolute ethanol and doubly distilled water in an ultrasonic bath for 5 min each wash. Then the drop-coating technique was used to fabricate electrochemical sensor by dropping an aliquot of 5 mL of the mixture on the surface of GCE. After that, the drying step was performed under an IR heat lamp. The obtained modified electrode was called as PEDOT/GO/GCE.
Hg2+ stock solution was prepared by dissolving HgCl2 into doubly deionized water with different concentrations. Acetate buffer solution (ABS, 0.1 M, different pH) was obtained by mixing a certain amount of 0.1 M CH3COOH and 0.1 M CH3COONa. GO was received from Nanjing Xianfeng Nanomaterials Technology Co., Ltd. (Nanjing, China), 3,4-ethylenedioxythiophene (EDOT) was received from Sigma-Aldrich. HgCl2 was purchased from Shanghai
Scheme 1. Schematic illustration for preparation of PEDOT/GO/GCE and electrochemical determination of Hg2+.
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Y. Zuo et al. / Synthetic Metals 220 (2016) 14–19
Fig. 1. SEM images of GO (A) and PEDOT/GO (B). TEM image (C) of the PEDOT/GO.
2.5. Differential pulse stripping voltammetric measurement of Hg2+ at the modified electrodes The differential pulse stripping voltammetry (DPSV) was conducted in 0.1 M acetate buffer solution (ABS) (pH 5.0) in the range from 0.1 V to + 0.5 V with optimized parameters: accumulation time: 360 s, deposition potential: 0.2 V, amplitude: 50 mV, potential step: 4 mV. Prior to the next determination, the modified electrode was cleaned for 200 s at 0.6 V in ABS (pH 5.0). The results, such as experimental conditions, optimized parameters, and calibration curve, were subject to statistical analysis and were presented as the mean SD (standard deviation).
the electron transfer resistance (Ret), and the linear part at lower frequencies corresponds to the diffusion process [29]. As shown in Fig. 2, the Ret value corresponding to the bare GCE was about 280 V (a). When GO was modified onto the GCE (b), the semicircle dramatically increased relative to the bare GCE (a), indicating a high electron transfer resistance existed. However, compared with bare GCE (a), the semicircle of PEDOT/GCE (c) decreased distinctively, suggesting faster electron transfer kinetics of [Fe (CN)6]3/4 on the electrode surface. Furthermore, it can be seen that the Ret value of PEDOT/GO/GCE (d) was between PEDOT/GCE and GO/GCE, suggesting that PEDOT was successfully combined with GO.
3. Result and discussion
3.2. Electrochemical behaviors of Hg2+ at various electrodes
3.1. Characterization of PEDOT/GO nanocomposite
The electrochemical behaviors of Hg2+ on bare GCE (a), GO/GCE (b), PEDOT/GCE (c) and PEDOT/GO/GCE (d) were investigated in 0.1 M ABS. As shown in Fig. 3, no obvious signal was observed on GCE. While, weak peaks were observed on GO/GCE and PEDOT/GCE at 0.17 V and 0.25 V, respectively, which were ascribed to absorbing ability of carboxylic and hydroxyl on GO surface [30,31], and high electric conductivity of PEDOT [32,33]. By contrast, a sharp peak with increased current can be observed on PEDOT/GO/GCE at 0.18 V, which might be ascribed to the synergistic activity of PEDOT (high electric conductivity) and GO (excellent catalytic activity) and the large surface area of nanocomposite.
SEM and TEM were used to investigate the surface morphology and structure of PEDOT/GO nanocomposite. As can be observed from the SEM image of GO (shown in Fig. 1A), it manifests homogeneous, flexible and wrinkled sheets. For the PEDOT/GO (shown in Fig. 1B), it can be seen that lots of PEDOT nanorods with uniformly distributed on the surface of GO. The carboxyl, hydroxyl and epoxide on the surface and edges on GO functioned as recognition site to attract PEDOT [26–28]. The TEM image of PEDOT/GO (shown in Fig. 1C) further showed the diameters and lengths of PEDOT nanorods were in the range of 20–30 nm and 100–200 nm, respectively. The detailed information of impedance changes after modification was given through the electrochemical impedance spectroscopy (EIS) in 5 mM Fe(CN)63/4 containing 0.1 M KCl. Generally, the semicircle diameter at higher frequencies represents
Fig. 2. EIS of bare GCE (a), GO/GCE (b), PEDOT/GCE (c) and PEDOT/GO/GCE (d) in 5.0 mM K3Fe(CN)6/K4Fe(CN)6 (1:1) containing 0.1 M KCl.
3.3. Optimization of experimental conditions With the purpose of achieving high sensitivity for trace Hg2+ detection with PEDOT/GO/GCE, various experimental conditions, including accumulation time, pH and deposition potential were
Fig. 3. DPSV curves at the bare GCE (a), GO/GCE (b) PEDOT/GCE (c) PEDOT/GO/GCE (d) in the presence of 3.0 mM Hg2+ in 0.1 M ABS (pH 5.0). Deposition potential: 0.2 V, accumulation time: 360 s, pulse amplitude: 50 mV, pulse width: 50 ms.
Y. Zuo et al. / Synthetic Metals 220 (2016) 14–19
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optimized. The electrochemical analysis includes three steps: (i) accumulation process; (ii) electrochemical reductive process; (iii) electrochemical oxidative process [34]: (i) adsorption process: PEDOT/GO/GCE + Hg(II) (solution) ! PEDOT/GO/GCE Hg(II) (surface) (ii) electrochemical reductive process: PEDOT/GO/GCE Hg(II) (surface) + 2e ! PEDOT/GO/GCE Hg(0) (surface) (iii) electrochemical oxidative process: PEDOT/GO/GCE Hg(0) (surface) ! PEDOT/GO/GCE + Hg(II) (solution) + 2e 3.3.1. Optimization of accumulation time The amount of the Hg2+ absorbed on the surface of modified electrode was affected by accumulation time. Therefore, in order to improve the detection sensitivity, the influence of accumulation time on the peak current in 0.2 V was investigated. As it shown in Fig. 4, the accumulation time was studied from 60 s to 500 s. Peak currents of Hg2+ increased sharply with time increasing before 360 s. With the accumulation time further increased, the peak current increased slowly, indicating that 360 s is relatively sufficient to reach saturation on PEDOT/GO/GCE. Thus, 360 s was selected to be the optimal accumulation time for Hg2+ detection. 3.3.2. Optimization of pH As the effect of pH is vital to achieve high sensitivity, the effect of pH on the voltammetric response was investigated in the pH range from 3.0 to 6.0. As can be observed in Fig. 5, the stripping peak currents of Hg2+ increased with pH value ranging from 3.0 to 5.0. While, the peak current was decreased with the further increase of pH value. The decrease of peak current under pH 5.0 might be due to the competitive binding of proton ion and Hg2+ to the combining groups on the surface of PEDOT/GO, The decrease over the pH 5.0 was attributed to the hydrolysis of Hg2+ [35]. Therefore, pH 5.0 was selected as the optimal pH in this work. 3.3.3. Optimization of deposition potential The effect of deposition potential is very essential to achieve high sensitivity. Thus, the effect of deposition potential was investigated by applying varying potential. As can be observed in Fig. 6, the peak currents increased and reached a maximum at potential 0.2 V with deposition potential shifts from 0.2 V to
Fig. 4. DPSV carried out on the PEDOT/GO/GCE in the presence of 5.0 mM Hg2+ in 0.1 M ABS of different accumulation time (pH 4.5, deposition potential: 0.2 V).
Fig. 5. DPSV carried out on the PEDOT/GO/GCE in the presence of 5.0 mM Hg2+ in 0.1 M ABS with different pH (deposition potential: 0.2 V, accumulation time: 360 s).
0.2 V. However, the peak currents decreased with deposition potential varying from 0.2 V to 0.4 V, and decreased slightly with the potential shifted from 0.4 V to 0.5 V. The decreased peak currents might be attributed to the fact that PEDOT was partially de-doped. [36–40]. PEDOT has good electronic conductivity in its doped state. The higher doping level can result in higher electrical conductivity of PEDOT [41–43]. However, the degree of doping level decreased when negative deposition potential was applied, leading to a decreased conductivity. So, 0.2 V was the optimal accumulation potential for the Hg2+ detection. 3.4. Electrochemical determination of Hg2+ Under the optimized conditions, DPSV was used for the electrochemical detection of Hg2+ at different concentrations on the PEDOT/GO/GCE. From Fig. 7, it can be observed that the welldefined peak currents increased linearly with the increased concentration of Hg2+ in the range from 10.0 nM to 3.0 mM and the corresponding linear regression equation was represented as y (mA) = 6.7445 x + 0.0432 with the correlation coefficients of 0.9978 (inset of Fig. 7). The detection limit for Hg2+ was calculated to be 2.78 nM. Thus, the PEDOT/GO/GCE method exhibited a wide concentration range with a low detection limit for the detection of Hg2+.
Fig. 6. DPSV carried out on the PEDOT/GO/GCE in the presence of 5.0 mM Hg2+ in 0.1 M ABS with different deposition potential (pH 5.0, accumulation time: 360 s).
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Y. Zuo et al. / Synthetic Metals 220 (2016) 14–19 Table 1 Recovery measurements of the Hg2+ in the tap water samples using the modified electrode (n = 3). Samples
Tap water 1 Tap water 2 Tap water 3
Hg (II) (nM) Added
Found
10 30 50
9.7 29.6 51.4
Recovery (%)
RSD (%)
97.0 98.7 102.8
2.3 2.5 1.4
room temperature and used for detection of Hg2+ daily over consecutive 4 weeks. The results indicated the changes of signal current were less than 6.4% for Hg2+, which confirmed its excellent long-term stability. 3.6. Real sample analysis
Fig. 7. DPSV responses of the PEDOT/GO/GCE for the detection of different concentrations of Hg2+ (from a to m are 10 nM, 30 nM, 60 nM, 90 nM, 150 nM, 0.3 mM, 0.5 mM, 0.7 mM, 0.9 mM, 1.5 mM, 2.0 mM, 2.5 mM, 3.0 mM) in 0.1 M ABS (pH 5.0); Insert: The corresponding linear calibration plots of stripping peak currents for Hg2+.
3.5. Interference study, reproducibility and stability The interferences which may compete with Hg2+ on the surface of the modified electrode during deposition, were investigated to evaluate the selectivity of PEDOT/GO/GCE to Hg2+. In this experiment, 3.0 mM Hg2+ and 50-fold concentration interference ions, including Mn2+, Ca2+, Pb2+, Co2+, Mg2+, Cu2+, Ni2+ and Zn2+ were used. As can be seen from Fig. 8, these potential co-exist ions did not interfere significantly during Hg2+ determination and these ions did not show any stripping peaks in the potential window, even though the stripping peak potential of Cu2+ is close to that of Hg2+. These indicated good selectivity of Hg2+ with the existance of potential interference and confirmed its practicability in real sample analysis. The good selectivity of the modified electrode is largely attributed to the fact that Hg2+ has high affinity for the “soft” donor sulfur in thiophene rings of PEDOT [44,45]. Reproducibility is an important parameter for precise analytical measurements, hence, 3.0 mM Hg2+ was successively measured for 30 times under identical conditions. The relative standard deviation (RSD) was calculated to be 4.68%, which indicated the good reproducibility of the proposed method. Also, to evaluate the long-term stability of PEDOT/GO/GCE, it was stored in the air at
For the evaluation of its applicability in real sample is necessary, the fabricated PEDOT/GO/GCE was used for the determination of Hg2+ in tap water samples. The real samples were diluted with 0.2 M ABS (pH 5.0) in a ratio of 1:1. No obvious signals were observed during experiment procedure. Subsequently, after standard Hg2+ solutions with different concentrations were added to pretreated samples, well-defined stripping peaks can be observed in the potential window. As can be seen from Table 1, the recoveries in the range from 97.0% to 102.8% were obtained, indicated that the present electrode could be efficiently applied for Hg2+ detection in real samples. 4. Conclusions In summary, liquid–liquid interfacial polymerization method for preparing a PEDOT/GO hybrid nanocomposite modified electrode was proposed and the modified electrode was used for highly selective electrochemical sensing of Hg2+. SEM and TEM images demonstrated that PEDOT with a nanorods-like structure was attached on the surface of GO sheets, which could provide many electro-active sites. The GCE modified with PEDOT/GO showed high electrocatalytical activity toward Hg2+ and exhibited good selectivity, wide liner range and good long-term stability. The proposed method was also successfully applied to detect Hg2+ in tap water samples. Acknowledgements We are grateful to the National Natural Science Foundation of China (51272096 and 51302117), the Natural Science Foundation of Jiangxi Province (20122BAB216011 and 20151BAB203018), the Science and Technology Landing Plan of Universities in Jiangxi province (KJLD12081 and KJLD14069), State Key Laboratory of Chemical Biosensing & Chemometrics (2015010), Postdoctoral Science Foundation of China (2014M551857 and 2015T80688) and Postdoctoral Science Foundation of Jiangxi Province (2014KY14) for their financial support of this work. References
Fig. 8. Effect of interference ions on the electrochemical stripping signals of 3.0 mM Hg2+ at PEDOT/GO/GCE. Electrochemical stripping signals (1, 2, and 3) of 3.0 mM Hg2 + at the same PEDOT/GO/GCE after the investigation for interference.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Poly(3,4-ethylenedioxythiophene) nanorods/graphene oxide nanocomposite as a new electrode material for the selective electrochemical detection of mercury (II) Yinxiu Zuoa,b,1, Jingkun Xua,1, Xiaofei Zhua , Xuemin Duana,* , Limin Lub,* , Yansha Gaoa , Huakun Xinga , Taotao Yanga , Guo Yea , Yongfang Yub a b
Shool of Pharmacy, Jiangxi Science and Technology Normal University, Nanchang 330013, PR China College of Science, Jiangxi Agricultural University, Nanchang 330045, PR China
A R T I C L E I N F O
Article history: Received 7 January 2016 Received in revised form 17 May 2016 Accepted 20 May 2016 Available online xxx Keywords: Poly(3,4-ethylenedioxythiophene) Graphene oxide Interfacial polymerization Electrochemical detection Mercury ions
A B S T R A C T S
Development of selective methods for the detection of mercury (Hg2+) has received tremendous attention in modern chemical research due to its health hazard and persistence in environment. In this paper, the electrochemical determination of Hg2+ at trace level based on poly(3,4-ethylenedioxythiophene) nanorods/graphene oxide nanocomposite modified glassy carbon electrode (PEDOT/GO/GCE) is reported. PEDOT/GO nanocomposite has been proposed via a simple liquid–liquid interfacial polymerization approach. Scanning electron microscopy (SEM) and transmission electron microscope (TEM) were employed to characterize the morphology and structure of the as-prepared PEDOT/GO. The results revealed that PEDOT with a nanorods-like structure anchored on the surface of GO nanosheets, which could enhance the electro-active sites of the nanocomposite. Differential pulse stripping voltammetry (DPSV) was applied to determine low concentrations of Hg2+ on PEDOT/GO/GCE. Experimental conditions, including accumulation time, pH values and deposition potential were optimized. In optimal conditions, a good linear relationship was found between peak currents and the concentration of Hg2+ in 10.0 nM-3.0 mM range. The detection limit was estimated to be 2.78 nM at a signal-to-noise ratio of 3. Finally, the applicability for Hg2+ determination in tap water samples was successfully demonstrated. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Toxic heavy metal in the environment is a growing threat to humanity. Among them, mercury pollution has attracted extensive attention because of its health hazard, accumulative and persistence character in living organisms and environment [1,2]. Thus, developing a high sensitivity, ease use and low cost method for Hg2 + determination is necessary. Differential pulse stripping voltammetry (DPSV) has been deemed as a sensitive detection technology for trace-level heavy metals as it involves unique accumulation/ preconcentration of analyte species contained in the solutions, and then improve the detection sensitivity [3,4]. Moreover, the
* Corresponding authors. E-mail addresses: [email protected] (X. Duan), [email protected] (L. Lu). 1 These authors contributed equally to this work and should be considered cofirst authors. http://dx.doi.org/10.1016/j.synthmet.2016.05.022 0379-6779/ã 2016 Elsevier B.V. All rights reserved.
performance of solid electrode was greatly enhanced through the usage of chemically modified electrode. Recently, graphene oxide (GO) has aroused increasingly interests in fabricating various electrochemical sensors and biosensor because of the similar structure to graphene [5,6]. Unlike graphene, there are plenty of oxygen-containing groups on the graphene oxide (GO) sheet, which allows it disperse easily in water and some other solvents [7]. Besides, the functional groups on GO sheets can prevent carbon sheets from restacking and agglomeration, and act as the combining sites for graphene-based composite. In order to make full use of the advantages and compensate for the poor conductivity of GO, several nanomaterials are employed to combine with GO to form novel conducting composites for various applications [8,9]. Recently, conducting polymer/GO nanocomposite have attracted tremendous attentions and become a research focus, because they possess both excellent properties of conducting polymers and GO, such as high electric conductivity, long term
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environmental stability, good electrochemical activity and biocompatibility of conducting polymers [10], as well as unique electrical and chemical properties of GO [11,12]. Among various conducting polymers, PEDOT, a new polythiophene derivate, has the merits of excellent environmental stability, low band gap, excellent electrochemical activity, high electric conductivity, and transparency in the doped state [13,14]. Synergistic properties like enhanced properties in electrochemical cyclability and electrical conductivity have shown based on PEDOT and GO composite, and there are many good potential applications in many fields such as energy storage, supercapacitors or electrochemical sensors and biosensors for the detection of certain special substances. PEDOT/ GO nanocomposite were usually prepared by a noncovalent mixing/adsorption route, in situ polymerization or electrodeposition techniques [15–17]. However, these methods generally yield insoluble powders, and moreover, these composites showed irregular structures, resulting in a decrease in the electrocatalytic activity of the resulting composite. Very recently, our group and Sun group have reported simple liquid–liquid interfacial polymerization for the preparation of high quality PEDOT/GO nanocomposite [18,19]. The obtained PEDOT/GO nanocomposite exhibited good catalytic activity toward the oxidation of rutin and nitrite due to the plenty of electro-active sites of the nanocomposite. However, to the best of our knowledge, electrochemical determination of heavy metal ions using PEDOT/ GO has not been reported. In this work, PEDOT/GO composite was proposed via liquid/liquid interfacial polymerization, in which EDOT monomer and GO were dispersed in organic phase and water phase, respectively. It was found that such PEDOT/GO nanocomposite showed excellent catalytic activity toward Hg2+, leading to a selective sensor for the detection of Hg2+. The detection limit was calculated to be 2.78 nM with linear range 10.0 nM to 3.0 mM, which is well below to previous report [20–25].
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Vita Chemical Plant, China. Chloroform (CHCl3) and ferric chloride (FeCl3) were received from Shanghai Chemical Co. Ltd. (Shanghai, China). and CH3COONa and CH3COOH were purchased from Aldrich. All these regents were received in analytical grade and used without further purification. Doubly deionized water was used in the whole experiments. 2.2. Apparatus Transmission electron microscope (TEM) images were obtained from JEM-3010 transmission electron microscope (JEOL Co., Ltd., Japan). Scanning electron microscopy (SEM) images were obtained from Hitachi S–3000 N scanning electron microscope. The electrochemical measurements were carried out on a CHI660D electrochemical workstation (Shanghai, China) with conventional three-electrode system. A bare glassy carbon electrode (GCE, K = 3 mm) or modified GCE was used as the working electrode, a platinum foil electrode as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. 2.3. Preparation of the PEDOT/GO nanocomposite The PEDOT/GO nanocomposite was prepared according to our previous report [18]. To be briefly, 1 mL GO dispersion aqueous (0.5 mg mL1) was added with 1 mL FeCl3 (1 M) as oxidant. After 10 min sonication procedure, the above solution was slowly added into 2 mL of EDOT solution in CHCl3 (25 mg mL1) and an interface was produced in the middle of two layers. The received mixture was kept at 50 C under static conditions. The upper layer mixture was centrifuged after 12 h reaction and the obtained precipitate was washed with distilled water and ethanol a couple of times. The obtained nanocomposite was collected for further experiments (Scheme 1).
2. Experimental
2.4. Preparation of the modified electrode
2.1. Chemicals and reagents
Prior to modification, GCE was polished with chamois leather containing 0.05 mM alumina slurry, subsequently, the electrode was rinsed with doubly distilled water, absolute ethanol and doubly distilled water in an ultrasonic bath for 5 min each wash. Then the drop-coating technique was used to fabricate electrochemical sensor by dropping an aliquot of 5 mL of the mixture on the surface of GCE. After that, the drying step was performed under an IR heat lamp. The obtained modified electrode was called as PEDOT/GO/GCE.
Hg2+ stock solution was prepared by dissolving HgCl2 into doubly deionized water with different concentrations. Acetate buffer solution (ABS, 0.1 M, different pH) was obtained by mixing a certain amount of 0.1 M CH3COOH and 0.1 M CH3COONa. GO was received from Nanjing Xianfeng Nanomaterials Technology Co., Ltd. (Nanjing, China), 3,4-ethylenedioxythiophene (EDOT) was received from Sigma-Aldrich. HgCl2 was purchased from Shanghai
Scheme 1. Schematic illustration for preparation of PEDOT/GO/GCE and electrochemical determination of Hg2+.
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Fig. 1. SEM images of GO (A) and PEDOT/GO (B). TEM image (C) of the PEDOT/GO.
2.5. Differential pulse stripping voltammetric measurement of Hg2+ at the modified electrodes The differential pulse stripping voltammetry (DPSV) was conducted in 0.1 M acetate buffer solution (ABS) (pH 5.0) in the range from 0.1 V to + 0.5 V with optimized parameters: accumulation time: 360 s, deposition potential: 0.2 V, amplitude: 50 mV, potential step: 4 mV. Prior to the next determination, the modified electrode was cleaned for 200 s at 0.6 V in ABS (pH 5.0). The results, such as experimental conditions, optimized parameters, and calibration curve, were subject to statistical analysis and were presented as the mean SD (standard deviation).
the electron transfer resistance (Ret), and the linear part at lower frequencies corresponds to the diffusion process [29]. As shown in Fig. 2, the Ret value corresponding to the bare GCE was about 280 V (a). When GO was modified onto the GCE (b), the semicircle dramatically increased relative to the bare GCE (a), indicating a high electron transfer resistance existed. However, compared with bare GCE (a), the semicircle of PEDOT/GCE (c) decreased distinctively, suggesting faster electron transfer kinetics of [Fe (CN)6]3/4 on the electrode surface. Furthermore, it can be seen that the Ret value of PEDOT/GO/GCE (d) was between PEDOT/GCE and GO/GCE, suggesting that PEDOT was successfully combined with GO.
3. Result and discussion
3.2. Electrochemical behaviors of Hg2+ at various electrodes
3.1. Characterization of PEDOT/GO nanocomposite
The electrochemical behaviors of Hg2+ on bare GCE (a), GO/GCE (b), PEDOT/GCE (c) and PEDOT/GO/GCE (d) were investigated in 0.1 M ABS. As shown in Fig. 3, no obvious signal was observed on GCE. While, weak peaks were observed on GO/GCE and PEDOT/GCE at 0.17 V and 0.25 V, respectively, which were ascribed to absorbing ability of carboxylic and hydroxyl on GO surface [30,31], and high electric conductivity of PEDOT [32,33]. By contrast, a sharp peak with increased current can be observed on PEDOT/GO/GCE at 0.18 V, which might be ascribed to the synergistic activity of PEDOT (high electric conductivity) and GO (excellent catalytic activity) and the large surface area of nanocomposite.
SEM and TEM were used to investigate the surface morphology and structure of PEDOT/GO nanocomposite. As can be observed from the SEM image of GO (shown in Fig. 1A), it manifests homogeneous, flexible and wrinkled sheets. For the PEDOT/GO (shown in Fig. 1B), it can be seen that lots of PEDOT nanorods with uniformly distributed on the surface of GO. The carboxyl, hydroxyl and epoxide on the surface and edges on GO functioned as recognition site to attract PEDOT [26–28]. The TEM image of PEDOT/GO (shown in Fig. 1C) further showed the diameters and lengths of PEDOT nanorods were in the range of 20–30 nm and 100–200 nm, respectively. The detailed information of impedance changes after modification was given through the electrochemical impedance spectroscopy (EIS) in 5 mM Fe(CN)63/4 containing 0.1 M KCl. Generally, the semicircle diameter at higher frequencies represents
Fig. 2. EIS of bare GCE (a), GO/GCE (b), PEDOT/GCE (c) and PEDOT/GO/GCE (d) in 5.0 mM K3Fe(CN)6/K4Fe(CN)6 (1:1) containing 0.1 M KCl.
3.3. Optimization of experimental conditions With the purpose of achieving high sensitivity for trace Hg2+ detection with PEDOT/GO/GCE, various experimental conditions, including accumulation time, pH and deposition potential were
Fig. 3. DPSV curves at the bare GCE (a), GO/GCE (b) PEDOT/GCE (c) PEDOT/GO/GCE (d) in the presence of 3.0 mM Hg2+ in 0.1 M ABS (pH 5.0). Deposition potential: 0.2 V, accumulation time: 360 s, pulse amplitude: 50 mV, pulse width: 50 ms.
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optimized. The electrochemical analysis includes three steps: (i) accumulation process; (ii) electrochemical reductive process; (iii) electrochemical oxidative process [34]: (i) adsorption process: PEDOT/GO/GCE + Hg(II) (solution) ! PEDOT/GO/GCE Hg(II) (surface) (ii) electrochemical reductive process: PEDOT/GO/GCE Hg(II) (surface) + 2e ! PEDOT/GO/GCE Hg(0) (surface) (iii) electrochemical oxidative process: PEDOT/GO/GCE Hg(0) (surface) ! PEDOT/GO/GCE + Hg(II) (solution) + 2e 3.3.1. Optimization of accumulation time The amount of the Hg2+ absorbed on the surface of modified electrode was affected by accumulation time. Therefore, in order to improve the detection sensitivity, the influence of accumulation time on the peak current in 0.2 V was investigated. As it shown in Fig. 4, the accumulation time was studied from 60 s to 500 s. Peak currents of Hg2+ increased sharply with time increasing before 360 s. With the accumulation time further increased, the peak current increased slowly, indicating that 360 s is relatively sufficient to reach saturation on PEDOT/GO/GCE. Thus, 360 s was selected to be the optimal accumulation time for Hg2+ detection. 3.3.2. Optimization of pH As the effect of pH is vital to achieve high sensitivity, the effect of pH on the voltammetric response was investigated in the pH range from 3.0 to 6.0. As can be observed in Fig. 5, the stripping peak currents of Hg2+ increased with pH value ranging from 3.0 to 5.0. While, the peak current was decreased with the further increase of pH value. The decrease of peak current under pH 5.0 might be due to the competitive binding of proton ion and Hg2+ to the combining groups on the surface of PEDOT/GO, The decrease over the pH 5.0 was attributed to the hydrolysis of Hg2+ [35]. Therefore, pH 5.0 was selected as the optimal pH in this work. 3.3.3. Optimization of deposition potential The effect of deposition potential is very essential to achieve high sensitivity. Thus, the effect of deposition potential was investigated by applying varying potential. As can be observed in Fig. 6, the peak currents increased and reached a maximum at potential 0.2 V with deposition potential shifts from 0.2 V to
Fig. 4. DPSV carried out on the PEDOT/GO/GCE in the presence of 5.0 mM Hg2+ in 0.1 M ABS of different accumulation time (pH 4.5, deposition potential: 0.2 V).
Fig. 5. DPSV carried out on the PEDOT/GO/GCE in the presence of 5.0 mM Hg2+ in 0.1 M ABS with different pH (deposition potential: 0.2 V, accumulation time: 360 s).
0.2 V. However, the peak currents decreased with deposition potential varying from 0.2 V to 0.4 V, and decreased slightly with the potential shifted from 0.4 V to 0.5 V. The decreased peak currents might be attributed to the fact that PEDOT was partially de-doped. [36–40]. PEDOT has good electronic conductivity in its doped state. The higher doping level can result in higher electrical conductivity of PEDOT [41–43]. However, the degree of doping level decreased when negative deposition potential was applied, leading to a decreased conductivity. So, 0.2 V was the optimal accumulation potential for the Hg2+ detection. 3.4. Electrochemical determination of Hg2+ Under the optimized conditions, DPSV was used for the electrochemical detection of Hg2+ at different concentrations on the PEDOT/GO/GCE. From Fig. 7, it can be observed that the welldefined peak currents increased linearly with the increased concentration of Hg2+ in the range from 10.0 nM to 3.0 mM and the corresponding linear regression equation was represented as y (mA) = 6.7445 x + 0.0432 with the correlation coefficients of 0.9978 (inset of Fig. 7). The detection limit for Hg2+ was calculated to be 2.78 nM. Thus, the PEDOT/GO/GCE method exhibited a wide concentration range with a low detection limit for the detection of Hg2+.
Fig. 6. DPSV carried out on the PEDOT/GO/GCE in the presence of 5.0 mM Hg2+ in 0.1 M ABS with different deposition potential (pH 5.0, accumulation time: 360 s).
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Y. Zuo et al. / Synthetic Metals 220 (2016) 14–19 Table 1 Recovery measurements of the Hg2+ in the tap water samples using the modified electrode (n = 3). Samples
Tap water 1 Tap water 2 Tap water 3
Hg (II) (nM) Added
Found
10 30 50
9.7 29.6 51.4
Recovery (%)
RSD (%)
97.0 98.7 102.8
2.3 2.5 1.4
room temperature and used for detection of Hg2+ daily over consecutive 4 weeks. The results indicated the changes of signal current were less than 6.4% for Hg2+, which confirmed its excellent long-term stability. 3.6. Real sample analysis
Fig. 7. DPSV responses of the PEDOT/GO/GCE for the detection of different concentrations of Hg2+ (from a to m are 10 nM, 30 nM, 60 nM, 90 nM, 150 nM, 0.3 mM, 0.5 mM, 0.7 mM, 0.9 mM, 1.5 mM, 2.0 mM, 2.5 mM, 3.0 mM) in 0.1 M ABS (pH 5.0); Insert: The corresponding linear calibration plots of stripping peak currents for Hg2+.
3.5. Interference study, reproducibility and stability The interferences which may compete with Hg2+ on the surface of the modified electrode during deposition, were investigated to evaluate the selectivity of PEDOT/GO/GCE to Hg2+. In this experiment, 3.0 mM Hg2+ and 50-fold concentration interference ions, including Mn2+, Ca2+, Pb2+, Co2+, Mg2+, Cu2+, Ni2+ and Zn2+ were used. As can be seen from Fig. 8, these potential co-exist ions did not interfere significantly during Hg2+ determination and these ions did not show any stripping peaks in the potential window, even though the stripping peak potential of Cu2+ is close to that of Hg2+. These indicated good selectivity of Hg2+ with the existance of potential interference and confirmed its practicability in real sample analysis. The good selectivity of the modified electrode is largely attributed to the fact that Hg2+ has high affinity for the “soft” donor sulfur in thiophene rings of PEDOT [44,45]. Reproducibility is an important parameter for precise analytical measurements, hence, 3.0 mM Hg2+ was successively measured for 30 times under identical conditions. The relative standard deviation (RSD) was calculated to be 4.68%, which indicated the good reproducibility of the proposed method. Also, to evaluate the long-term stability of PEDOT/GO/GCE, it was stored in the air at
For the evaluation of its applicability in real sample is necessary, the fabricated PEDOT/GO/GCE was used for the determination of Hg2+ in tap water samples. The real samples were diluted with 0.2 M ABS (pH 5.0) in a ratio of 1:1. No obvious signals were observed during experiment procedure. Subsequently, after standard Hg2+ solutions with different concentrations were added to pretreated samples, well-defined stripping peaks can be observed in the potential window. As can be seen from Table 1, the recoveries in the range from 97.0% to 102.8% were obtained, indicated that the present electrode could be efficiently applied for Hg2+ detection in real samples. 4. Conclusions In summary, liquid–liquid interfacial polymerization method for preparing a PEDOT/GO hybrid nanocomposite modified electrode was proposed and the modified electrode was used for highly selective electrochemical sensing of Hg2+. SEM and TEM images demonstrated that PEDOT with a nanorods-like structure was attached on the surface of GO sheets, which could provide many electro-active sites. The GCE modified with PEDOT/GO showed high electrocatalytical activity toward Hg2+ and exhibited good selectivity, wide liner range and good long-term stability. The proposed method was also successfully applied to detect Hg2+ in tap water samples. Acknowledgements We are grateful to the National Natural Science Foundation of China (51272096 and 51302117), the Natural Science Foundation of Jiangxi Province (20122BAB216011 and 20151BAB203018), the Science and Technology Landing Plan of Universities in Jiangxi province (KJLD12081 and KJLD14069), State Key Laboratory of Chemical Biosensing & Chemometrics (2015010), Postdoctoral Science Foundation of China (2014M551857 and 2015T80688) and Postdoctoral Science Foundation of Jiangxi Province (2014KY14) for their financial support of this work. References
Fig. 8. Effect of interference ions on the electrochemical stripping signals of 3.0 mM Hg2+ at PEDOT/GO/GCE. Electrochemical stripping signals (1, 2, and 3) of 3.0 mM Hg2 + at the same PEDOT/GO/GCE after the investigation for interference.
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