- Email: [email protected]
BMIMPF6 nanocomposite modified electrode
Sensors and Actuators B 192 (2014) 452–458
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Simultaneous electrochemical determination of hydroquinone and catechol based on three-dimensional graphene/MWCNTs/BMIMPF6 nanocomposite modified electrode Xue Wang a , Min Wu a , Hui Li b , Qingjiang Wang a,∗ , Pingang He a , Yuzhi Fang a a b
Department of Chemistry, East China Normal University, Shanghai 200062, China School of Chemistry and Chemical Engineering, Shanghai Jiaotong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 27 August 2013 Received in revised form 29 October 2013 Accepted 7 November 2013 Available online 15 November 2013 Keywords: Graphene Multiwalled carbon nanotube 1-Butyl-3-methylimidazolium hexafluorophosphate Hydroquinone Catechol
a b s t r a c t A three-dimensional glassy carbon electrode (GCE) was fabricated from one-dimensional multiwalled carbon nanotubes (MWCNTs) and two-dimensional graphene (GR), using 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6 ) ionic liquid to improve its dispersibility and stability. This GR/MWCNTs/BMIMPF6 modified GCE was found to be a highly sensitive electrochemical sensor for the simultaneous determination of hydroquinone (1,4-dihydroxybenzene, HQ) and catechol (1,2dihydroxybenzene, CT). The GR was characterized using a transmission electron microscope. The electrochemical behaviours of HQ and CT at the GR/MWCNTs/BMIMPF6 /GCE were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The chemically modified electrode exhibited excellent electrochemical catalytic activities towards HQ and CT. Linear relationships between the oxidation peak current and concentration of HQ/CT were obtained in the ranges 0.5 M to 2.9 mM and 0.2 M to 0.66 mM with detection limits (S/N = 3) of 0.1 M and 0.06 M, respectively. This GR/MWCNTs/BMIMPF6 /GCE showed many advantages in terms of dispersibility, stability, sensitivity, facility and economy. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Hydroquinone (1,4-dihydroxybenzene, HQ) and catechol (1,2dihydroxybenzene, CT) are two isomers of dihydroxybenzene and often coexist in environmental samples as pollutants with high toxicity [1]. HQ and CT are harmful to environment including people, animals, and plants even at very low concentrations [2]. The acceptable emission of phenolic compounds according to the national standard of China (GB 8978-1996) is 0.5 mg/L (for dihydroxybenzene, 0.00454 M) [3]. Therefore, it is necessary to develop a simultaneous, simple, and rapid analytical method for the determination of dihydroxybenzene isomers. So far, numerous methods have been established for their determination, such as liquid chromatography [4], synchronous fluorescence [5], chemiluminescence [6], spectrophotometry [7], pH-based flow injection analysis [8], and electrochemical methods [9–11]. Among them, electrochemical methods have attracted increasing attentions owing to the advantages of low cost, fast response, excellent selectivity, and high sensitivity. However, a number of challenges for the simultaneous determination of HQ and CT isomers by electrochemical
∗ Corresponding author. Tel.: +86 21 54340054; fax: +86 21 54340054. E-mail address: [email protected] (Q. Wang). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.11.020
methods still exist [12]. A significant obstacle for the conventional solid electrodes is that the voltammetric peaks corresponding to the oxidation/reduction of the dihydroxybenzene isomers are largely overlapped in many cases. Second, the competitive adsorption of phenolic isomers on the electrode surface in the mixtures makes the relationship between the voltammetric response and isomers concentration nonlinear. In order to overcome these problems, many chemically modified electrodes have been prepared for simultaneous determination of HQ and CT. For instance, Saleh Ahammad et al. reported that poly(thionine)-modified glassy carbon electrode (GCE) can separate the oxidation peaks of HQ and CT; however, the detection limits of dihydroxybenzene isomers were very low [13]. Canevari et al. developed a sensitive technique for the simultaneous detection of HQ and CT in the presence of resorcinol using a SiO2 /C electrode spin-coated with a thin film of Nb2 O5 [14]. Qi et al. used multiwalled carbon nanotubes (MWCNTs)-modified GCE for the simultaneous determination of hydroquinone and catechol with good electrochemical performances [15]. MWCNTs have been widely used in electrochemistry due to their unique one-dimensional (1D) structural, electronic, and physical properties. In the field of chemically modified electrode, one of the most important characteristics of MWCNTs is their reported ability to promote electron-transfer process [16]. Graphene (GR) has attracted tremendous attention because of its unique
X. Wang et al. / Sensors and Actuators B 192 (2014) 452–458
nanostructure and extraordinary electrocatalytic properties such as high surface area, excellent conductivity, and high mechanical strength associated with its two-dimensional (2D) structure [17]. Based on its properties, GR is considered as an ideal electrode material for electrochemical and biosensing. GR electrodes have been successfully applied as biosensors for sensing glucose [18], NADH [19], hydrogen peroxide [20], etc. However, the excellent properties of GR emerge only in a planar direction. Recently, Yang et al. reported a new method to reduce the stacking of GR by introducing 1D carbon nanotubes to form a three-dimensional (3D) nanohybrid [21]. The properties of MWCNTs emerge in the axial direction while providing current density, high specific surface area, and thermal conductivity. Thus, a GR-MWCNTs hybrid that combines the unique properties of the two carbon allotropes in all directions and provides a high surface area per unit volume for increased catalyst loading could be an ideal electrode material [22]. Therefore, the long and tortuous MWCNTs bridged adjacent to GR efficiently inhibited their aggregation, thus enhancing the utilization of GR-based composites. Room temperature ionic liquids (RTILs) have attracted a lot of attention because of their unique electrochemical properties such as high ionic conductivity, low vapor pressure, low melting temperature, no requirement for additional supporting electrolytes, and thermal stability [23]. 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) , a type of RTIL, has attracted widespread attention as a nonaqueous polar, moderately hydrophobic, nonvolatile, chemically and thermally stable ionic liquid [24]. Recently, the RTIL-GR-modified electrodes have been reported for detecting many targets [25–27]. For instance, Liu et al. developed a GR/BMIMPF6 as the nanocomposite modified electrode for the simultaneous determination of HQ and CT, which exhibited a wide linear range and low detection limit [28]. In this study, a 3D carbon electrode was fabricated by combining 1D MWCNTs and 2D GRs that further advance the utilization of GR-based composites, BMIMPF6 ionic liquid may improve the dispersibility and stability of the 3D carbon electrode. This GR/MWCNTs/BMIMPF6 nanocomposite-modified electrode was formed and its application potential was evaluated for the simultaneous determination of HQ and CT. The separation of anodic peak potentials for HQ and CT was 122 mV, which makes it easier for the simultaneous determination of HQ and CT. The detection limits (S/N = 3) for HQ and CT for the GR/MWCNTs/BMIMPF6 /GCE were 0.1 M and 0.06 M, respectively. Moreover, the proposed sensor was applied for the simultaneous determination of HQ and CT in water samples with high selectivity. 2. Experimental 2.1. Apparatus and measurements All the electrochemical experiments were performed using CHI660E electrochemical workstation (Shanghai Chenhua Co., China). A standard three-electrode cell was used for all the electrochemical experiments. Either a bare 3 mm diameter GCE or a modified electrode was used as the working electrode. A platinum wire served as the counter electrode and all the potentials were measured relative to a saturated calomel electrode (SCE). The measurements were carried out in a phosphate buffer saline (PBS) at room temperature. The transmission electron microscope (TEM) images were obtained using a JEM-2100F (Jeol Ltd., Japan) operating at 200 kV.
453
hydrazine hydrate (85%), and sulfuric acid (98%) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). MWCNTs were obtained from Chengdu Organic Chemicals Co. Ltd. of Chinese Academy of Science and purified by refluxing in concentrated nitric acid for 7 h prior to their use. 1-Butyl3-methylimidazolium was purchased from Shanghai Cheng Jie Chemical Co. Ltd. (Shanghai, China). Deionized water was prepared using a Milli-Q water purification system (Millipore, Bedford, MA). Hydroquinone and catechol solutions were freshly prepared in PBS prior to use. 2.3. Electrode preparation and modification 2.3.1. Preparation of the graphene GR was synthesized according to the literature method [29]. Natural graphite powder (2 g), sodium nitrate (1.2 g), and potassium permanganate (6 g) were slowly added to 46 mL of sulfuric acid (98 wt.%) in an ice bath at 0 ◦ C. The reaction mixture was kept in a 35 ◦ C water bath for 30 min to oxidize the graphite. Next, 92 mL ultrapure water was gradually added into the mixture, while the temperature was increased to 98 ◦ C and the reaction was maintained for 40 min in order to increase the oxidation degree of the GR oxidation (GO) product. The mixture was then poured into 240 mL of ultrapure water followed by the addition of 6 mL of hydrogen peroxide solution (30%) into the suspension. After cooling to room temperature, the GO obtained was isolated from the solution by centrifugation. The crude GO was carefully washed with 5% aqueous hydrogen chloride solution and ultrapure water to remove the remnant acid and salt impurities. The wet GO was dried under vacuum (50 ◦ C). 100 mg of obtained GO powder was dispersed in 50 mL of ultrapure water and stirred for 1 h at room temperature to obtain a homogeneous dispersion. Hydrazine hydrate (0.6 mL, 85%) was added to the dispersion to reduce the GO, and the reaction mixture was kept at 95 ◦ C for 1 h, the thus obtained black graphene was washed with ultrapure water and dried in vacuum. 2.3.2. Fabrication of the GR/MWCNTs/BMIMPF6 /GCE Prior to coating, a GCE was successively polished using 1.0, 0.3, and 0.05 m ␣-alumina powder and successively rinsed thoroughly with 1:1 HNO3 solution, ethanol, and deionized water for 5 min, and dried at room temperature. The GR/MWCNTs composite was prepared by stirring a mixture of 1 mg GR, 1 mg MWCNTs, and 1 mL DMF for 1 h at room temperature [30]. Next, BMIMPF6 was added to the above solution (v/v, 1.5%) to obtain a homogeneous dispersion. Next, 10 L of the homogeneous dispersion of GR/MWCNTs/BMIMPF6 in an aqueous solution was applied to the clean GCE and allowed to dry in air at room temperature for 24 h. For comparison purposes, MWCNTs/GCE, GR/GCE, and GR/MWCNTs/GCE were prepared by the similar process. 3. Results and discussion 3.1. Characterization of GR and GR/MWCNTs The surface morphologies of the modified electrodes were characterized by TEM. Fig. 1(A) shows that GR has a folded structure with a disordered sheet, which can enhance the capability of the electrochemical detection of HQ and CT. As shown in Fig. 1(B), the long and tortuous MWCNTs bridged adjacent GR and reduced the aggregation of GR, leading to a 3D nanohybrid structure to enhance the utilization of GR.
2.2. Chemicals and solutions
3.2. Electrocatalytic oxidation behaviours of single HQ and CT
Hydroquinone, catechol, graphite powder, N,Ndimethylformamide (DMF), hydrogen peroxide solution (30%),
The electrochemical oxidation behaviours of single HQ and CT were studied by cyclic voltammetry (CV) at the
454
X. Wang et al. / Sensors and Actuators B 192 (2014) 452–458
Fig. 2. Cyclic voltammetry of bare GCE, GR/GCE, MWCNTs/GCE, GR/MWCNTs/GCE, and GR/MWCNTs/BMIMPF6 /GCE in 0.1 M PBS (pH 7.0) containing (A) 0.1 mM HQ, and (B) 0.1 mM CT. Scan rate: 50 mV s−1 .
Fig. 1. TEM image of GR and GR/MWCNTs.
bare GCE, MWCNTs/GCE, GR/GCE, GR/MWCNTs/GCE, and GR/MWCNTs/BMIMPF6 /GCE in 0.1 M PBS (pH 7.0). As shown in Fig. 2(A), the anodic peak current of HQ was 148 A at GR/MWCNTs/BMIMPF6 /GCE, which was approximately 15 times higher than that at the bare GCE, and much higher than other modified electrodes. Moreover, the anodic peak potential of HQ was 90 mV at GR/MWCNTs/BMIMPF6 /GCE, which was 100 mV more negative than that at the bare GCE. As shown in Fig. 2(B), the anodic and cathodic peak potentials for the detection of CT at GR/MWCNTs/BMIMPF6 /GCE were 186 and 102 mV, respectively, compared to 277 and 47 mV at bare GCE. The separation of anodic and cathodic peak potentials at GR/MWCNTs/BMIMPF6 /GCE and the bare GCE were 84 and 230 mV, respectively, indicating that CT had a better electrochemical catalysis at GR/MWCNTs/BMIMPF6 /GCE.
The above results indicate that the oxidation peak current of HQ and CT significantly increased at GR/MWCNTs/BMIMPF6 /GCE than the other electrodes, and GR/MWCNTs/BMIMPF6 /GCE made the peak potentials of HQ and CT shift negatively compared with bare GCE, which can be attributed to the good conductivity and catalytic activity of the immobilized GR and MWCNTs, at the same time, BMIMPF6 could improve the dispersibility and stability of GR and MWCNTs. Thus, both GR/MWCNTs film and BMIMPF6 were responsible for enhancing the electrochemical sensing. 3.3. Electrochemical behaviours of the mixed components of HQ and CT Fig. 3 showed the cyclic voltammograms of the mixture containing 0.1 mM HQ and 0.1 mM CT in 0.1 M PBS (pH 7.0) at the bare GCE, MWCNTs/GCE, GR/GCE, GR/MWCNTs/GCE, and GR/MWCNTs/BMIMPF6 /GCE. Fig. 3 shows one broad oxidation peak at the bare GCE, indicating that HQ and CT cannot be separated at the bare electrode. Two well-separated oxidation peaks were observed at MWCNTs/GCE, GR/GCE, GR/MWCNTs/GCE, and GR/MWCNTs/BMIMPF6 /GCE. However, the oxidation peak current remarkably increased at GR/MWCNTs/BMIMPF6 /GCE, and
X. Wang et al. / Sensors and Actuators B 192 (2014) 452–458
455
Fig. 3. Cyclic voltammetry of bare GCE, GR/GCE, MWCNTs/GCE, GR/MWCNTs/GCE, and GR/MWCNTs/BMIMPF6 /GCE in 0.1 M PBS (pH 7.0) containing 0.1 mM HQ and 0.1 mM CT. Scan rate: 50 mV s−1 .
therefore, HQ and CT exhibit the highest sensitivity at this electrode. The oxidation peak potentials for HQ and CT were 88 and 210 mV, respectively, thus giving a peak-to-peak separation of 122 mV. The separations were large enough to allow simultaneous selective determination of HQ and CT. The above CV experiments shown in Figs. 2 and 3 demonstrated that HQ and CT had the highest peak currents at GR/MWCNTs/BMIMPF6 /GCE than those of the other modified electrodes. This improvement in electron transfer kinetics may be attributed to the following factors. First, the small size, high loading, large surface area, and high electrocatalytic activity of both GR and MWCNTs could enhance the performance of the peroxide oxidation process of HQ and CT on the modified electrode and reduce the charge transfer resistance, thus improving its reversibility which greatly enhanced the peak currents. Second, the excellent properties of GR emerge only in the planar direction while those of MWCNTs emerge in the axial direction, thus providing current density, high specific surface area, and thermal conductivity. Because the 1D MWCNTs and 2D GR formed a 3D hierarchical structure, the MWCNTs efficiently inhibited the stacking of individual GR units and thereby enhanced the utilization of GR-based composites. Thus, GR-MWCNTs hybrid combines the unique properties of the two carbon allotropes in all directions and provides a high surface area per unit volume for increased catalyst loading. Third, BMIMPF6 could improve the dispersibility and stability of GR and MWCNTs, thus enhancing the electrochemical performance for the detection of different target molecules in electroanalytical applications. 3.4. Selection of the experimental conditions 3.4.1. Effect of pH on the electrochemical behaviour of HQ and CT The effect of pH on the electrochemical behaviour of a mixed solution of 0.1 mM HQ and 0.1 mM CT in 0.1 M PBS (pH 7.0) at GR/MWCNTs/BMIMPF6 /GCE was carefully investigated by CV at pH range 3.0–9.0 in 0.1 M PBS. It can be easily concluded from Fig. 4(A) that the maximum oxidation peak currents of HQ and CT increase with increasing pH till pH 7.0 and then decrease with further increase in pH. The pKa values of HQ and CT are 9.85 and 9.4, respectively. When the pH increases from 7.0 to 9.0, the increased amount of hydroxyl ions in the solution may decrease the adsorption capacity of the dihydroxybenzene isomers. Considering the
Fig. 4. The effects of pH on the oxidation peak current (A) of HQ and CT, the oxidation peak potential (B) of HQ and CT for the oxidation of 0.1 mM HQ and 0.1 mM CT in 0.1 M PBS (pH7.0). Scan rate: 50 mV s−1 .
determination sensitivity, pH 7.0 was selected as the optimum pH for the experiments. All further experiments were investigated in 0.1 M PBS (pH 7.0). Fig. 4(B) shows that the peak potentials for the oxidation of HQ and CT negatively shifted with increasing pH. The relationships between the peak potentials of HQ and CT and pH were also investigated. For HQ, the linear regression equation for oxidation peak potentials and pH could be expressed as Epa (V) = 0.51 − 0.059 pH (R = 0.9934). For CT, the oxidation peak potentials followed the linear regression equation Epa (V) = 0.62 − 0.061 pH (R = 0.9973). The slopes of the three regression equations for HQ and CT were 59 and 61 mV/pH, respectively, indicating that the number of protons and electrons involved in the electrochemical redox process of HQ (or CT) are equal. According to the previous literature [15], the electrochemical redox process of HQ and CT at GR/MWCNTs/BMIMPF6 /GCE is a two-electron and two-proton process. 3.4.2. Effect of scan rate on the electrochemical behaviour of HQ and CT The effect of scan rate on the electrochemical behaviour of a mixed solution of 0.1 mM HQ and 0.1 mM CT in 0.1 M PBS (pH
456
X. Wang et al. / Sensors and Actuators B 192 (2014) 452–458
Fig. 5. Cyclic voltammetry of GR/MWCNTs/BMIMPF6 /GCE in 0.1 M PBS (pH 7.0) containing 0.1 mM HQ and 0.1 mM CT at different scan rates (20, 40, 60, 80, 100, 120, 140, 160, 180, and 200 mV s−1 ).
7.0) at GR/MWCNTs/BMIMPF6 /GCE was carefully investigated by CV (Fig. 5). The anodic peak intensity continuously increased with increasing scan rate from 20 to 200 mV s−1 . For HQ (Fig. 6A), the oxidation peak currents followed the linear regression equation Ipa = 27.111/2 − 41.47 (A, mV s−1 , R = 0.9996). For CT (Fig. 6B), the regression equation was Ipa = 20.031/2 + 0.3159 (A, mV s−1 , R = 0.9991). The above data indicated that the electrode reaction of HQ and CT at GR/MWCNTs/BMIMPF6 /GCE was a typical diffusioncontrolled process [28]. 3.4.3. Effect of the amount of BMIMPF6 on the electrochemical behaviour of HQ and CT The effect of the amount of BMIMPF6 on the electrochemical behaviour of a mixed solution of 0.1 mM HQ and 0.1 mM CT in 0.1 M PBS (pH 7.0) at GR/MWCNTs/BMIMPF6 /GCE was carefully investigated by CV (Fig. 7). The relationships between the peak currents of HQ and CT and amount of BMIMPF6 for the GR/MWCNTs suspension was studied at a ratio of 0.5%, 1%, 1.5%, 2%, 2.5%, and 3% (v/v), respectively. As shown in Fig. 7, the results showed the optimum ratio to be 1.5%. A higher ratio causes a smaller current response, which may be attributed to the thicker film of GR/MWCNTs hampering the electron transfer. Although the larger ratio could increase the amount of BMIMPF6 on GR/MWCNTs, the surface aggregations of BMIMPF6 would decrease its surface area, leading to a decrease in the oxidation peak current. Thus, the ratio was fixed at 1.5% in the experiments.
Fig. 6. The oxidation peak currents of 0.1 mM HQ (A) and 0.1 mM CT (B) vs. the square root of scan rates.
3.5. Simultaneous determination of HQ and CT Under the optimized conditions, the determination of HQ and CT at GR/MWCNTs/BMIMPF6 /GCE was carried out in 0.1 M PBS (pH 7.0) by differential pulse voltammetry (DPV), where the concentration of one species changed while the other species remained constant. As shown in Fig. 7(A), the peak currents linearly increased with increasing concentration of HQ from 1 M to 2.9 mM, while keeping the concentration of CT constant at 50 M. The linear equation and detection limit are listed in Table 1. Fig. 7(B) shows the different DPVs of CT at different concentrations in the presence of 50 M HQ. The peak currents linearly increased with increasing concentration from 0.5 to 660 M; the regression equations and the detection limit are listed in Table 1. Further, no obvious interference can be
Fig. 7. The effect of the amount of BMIMPF6 on the electrochemical behaviour of a mixed solution of 0.1 mM HQ and 0.1 mM CT in 0.1 M PBS (pH 7.0) at GR/MWCNTs/BMIMPF6 /GCE was investigated by CV.
X. Wang et al. / Sensors and Actuators B 192 (2014) 452–458
457
Table 1 Practical values for simultaneous determination of HQ and CT at GR/MWCNTs/BMIMPF6 /GCE in 0.1 M PBS solution (pH 7.0). Solution: (0.1 M PBS, pH 7.0) including
Additions
Linear range (M)
Linear equation (A, M)
Regression
50 M CT
HQ
50 M HQ
CT
0.5–465 465–2900 0.2–80 80–660
Ipa = 0.8410 + 0.1140c Ipa = 36.59 + 0.04264c Ipa = −0.06668 + 0.2233c Ipa = 49.94 + 0.09719c
0.9959 0.9981 0.9965 0.9978
Limit of detection (S/N = 3) (M) 0.1 0.06
Table 2 Determination of HQ and CT in water samples. Samples
Original (mol/L)
Added (mol/L)
Total founda (mol/L)
Recoveries (%)
RSD (%)
Rain water
HQ CT
– –
5 5
4.82 5.23
96.4 104.6
3.56 2.89
River water
HQ CT
– –
5 5
5.16 5.13
103.2 102.6
4.17 2.63
–, not detected. a Average of five determinations.
observed for the determination of one species coexisting with the other species (Fig. 8). 3.6. Stability and reproducibility The modified electrode was immersed in PBS (pH 7.0) and stored in a refrigerator when not in use. After 15 days, the peak currents of HQ and CT decreased only by 2.15% for HQ and 1.89% for CT, respectively. To evaluate the reproducibility of modified electrode, ten GR/MWCNTs/BMIMPF6 /GCEs were investigated by comparing their peak currents in 0.1 mM HQ and 0.1 mM CT in 0.1 M PBS (pH 7.0). The relative standard deviation (RSD) for HQ and CT were 2.89% and 2.48%, respectively. 3.7. Real samples analysis Rain water and river water samples were tested with GR/MWCNTs/BMIMPF6 /GCE, in order to validate the proposed analysis method. The amount of HQ and CT in rain and river water samples was determined by the calibration method using DPV, and the results are listed in Table 2. As per the results, no dihydroxybenzene was detected in water samples by using the modified electrode, which suggests that the dihydroxybenzene content was below the detection limits. The quantitative recoveries for HQ and CT were 96.4–103.2% and 102.6–104.6%, respectively. This suggests that the proposed GR/MWCNTs/BMIMPF6 /GCE could be used to determine the concentration of dihydroxybenzene isomers in real water samples.
4. Conclusions
Fig. 8. Differential pulse voltammetry of GR/MWCNTs/BMIMPF6 /GCE in 0.1 M PBS (pH7.0) (A) containing 50 M CT and different concentrations of HQ from 0.5 M to 2.9 mM, and (B) containing 50 M HQ and different concentrations of CT from 0.2 to 660 M. Scan rate: 20 mV s−1 , pulse interval 200, pulse amplitude 25 mV, and pulse width 50 ms.
In this study, a simple and sensitive electrochemical analytical method for the simultaneous determination of HQ and CT was developed by employing GR/MWCNTs/BMIMPF6 modified GCE. 1D MWCNTs and 2D GR formed a 3D carbon electrode, which enhanced the utilization of GR-based composites, while BMIMPF6 improved the dispersibility and stability of the 3D carbon electrode. It may be concluded that the modified electrode has high electrocatalytic activities towards the oxidation of HQ and CT by decreasing the oxidation over-potentials and increasing the peak currents significantly. This GR/MWCNTs/BMIMPF6 /GCE exhibits a low detection limit and wide linear range, and the detection limits for HQ and CT were 0.1 M and 0.06 M, respectively. This proposed method may potentially be useful for developing an electrochemical sensor for phenolic compounds.
458
X. Wang et al. / Sensors and Actuators B 192 (2014) 452–458
Acknowledgements This work was financially supported by the Program for New Century Excellent Talents in University (NCET-08-0191) and the National Program on Development of Scientific Instruments and Equipment (2011YQ150072). References [1] Q. Guo, J. Huang, P. Chen, Y. Liu, H. Hou, T. You, Simultaneous determination of catechol and hydroquinone using electrospun carbon nanofibers modified electrode, Sens. Actuators B 163 (2012) 179–185. [2] C. Wang, R. Yuan, Y. Chai, F. Hu, Simultaneous determination of hydroquinone, catechol, resorcinol and nitrite using gold nanoparticles loaded on poly-3-amino-5-mercapto-1,2,4-triazole-MWNTs film modified electrode, Anal. Methods 4 (2012) 1626–1628. [3] H. Yin, Q. Zhang, Y. Zhou, Q. Ma, T. liu, L. Zhu, S. Ai, Electrochemical behavior of catechol, resorcinol and hydroquinone at graphene–chitosan composite film modified glassy carbon electrode and their simultaneous determination in water samples, Electrochim. Acta 56 (2011) 2748–2753. [4] A. Asan, I. Isildak, Determination of major phenolic compounds in water by reversed-phase liquid chromatography after pre-column derivatization with benzoyl chloride, J. Chromatogr. A 988 (2003) 145–149. [5] M.F. Pistonesi, M.S. Di Nezio, M.E. Centurión, M.E. Palomeque, A.G. Lista, B.S. Fernández Band, Determination of phenol, resorcinol and hydroquinone in air samples by synchronous fluorescence using partial least-squares (PLS), Talanta 69 (2006) 1265–1268. [6] H. Cui, Q. Zhang, A. Myint, X. Ge, L. Liu, Chemiluminescence of cerium(IV)–rhodamine 6G–phenolic compound system, J. Photochem. Photobiol. A 181 (2006) 238–245. [7] P. Nagaraja, R.A. Vasantha, K.R. Sunitha, A sensitive and selective spectrophotometric estimation of catechol derivatives in pharmaceutical preparations, Talanta 55 (2001) 1039–1046. [8] J.A. Garcia-Mesa, R. Mateos, Direct automatic determination of bitterness and total phenolic compounds in virgin olive oil using a pH-based flow-injection analysis system, J. Agric. Food Chem. 55 (2007) 3863–3868. [9] X. Ma, Z. Liu, C. Qiu, T. Chen, H. Ma, Simultaneous determination of hydroquinone and catechol based on glassy carbon electrode modified with gold–graphene nanocomposite, Microchim. Acta 180 (2013) 461–468. [10] T. Gan, J. Sun, K. Huang, L. Song, Y. Li, A graphene oxide–mesoporous MnO2 nanocomposite modified glassy carbon electrode as a novel and efficient voltammetric sensor for simultaneous determination of hydroquinone and catechol, Sens. Actuators B 177 (2013) 412–418. [11] L. Wang, Y. Zhang, Y. Du, D. Lu, Y. Zhang, C. Wang, Simultaneous determination of catechol and hydroquinone based on poly (diallyldimethylammonium chloride) functionalized graphene-modified glassy carbon electrode, J. Solid State Electrochem. 16 (2011) 1323–1331. [12] M.A. Ghanem, Electrocatalytic activity and simultaneous determination of catechol and hydroquinone at mesoporous platinum electrode, Electrochem. Commun. 9 (2007) 2501–2506. [13] A.J.S. Ahammad, M.M. Rahman, G. Xu, S. Kim, J. Lee, Highly sensitive and simultaneous determination of hydroquinone and catechol at poly(thionine) modified glassy carbon electrode, Electrochim. Acta 56 (2011) 5266–5272. [14] T.C. Canevari, L.T. Arenas, R. Landers, R. Custodio, Y. Gushikem, Simultaneous electroanalytical determination of hydroquinone and catechol in the presence of resorcinol at an SiO2 /C electrode spin-coated with a thin film of Nb2 O5 , Analyst 138 (2013) 315–324. [15] H. Qi, C. Zhang, Simultaneous determination of hydroquinone and catechol at a glassy carbon electrode modified with multiwall carbon nanotubes, Electroanalysis 17 (2005) 832–838. [16] L.L.C. Garcia, L.C.S. Figueiredo-Filho, G.G. Oliveira, O. Fatibello-Filho, C.E. Banks, Square-wave voltammetric determination of paraquat using a glassy carbon electrode modified with multiwalled carbon nanotubes within a dihexadecylhydrogenphosphate (DHP) film, Sens. Actuators B 181 (2013) 306–311. [17] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [18] C. Shan, H. Yang, D. Han, Q. Zhang, A. Ivaska, L. Niu, Graphene/AuNPs/chitosan nanocomposites film for glucose biosensing, Biosens. Bioelectron. 25 (2010) 1070–1074.
[19] K. Guo, K. Qian, S. Zhang, J. Kong, C. Yu, B. Liu, Bio-electrocatalysis of NADH and ethanol based on graphene sheets modified electrodes, Talanta 85 (2011) 1174–1179. [20] Y. Xia, W. Li, M. Wang, Z. Nie, C. Deng, S. Yao, A sensitive enzymeless sensor for hydrogen peroxide based on the polynucleotide-templated silver nanoclusters/graphene modified electrode, Talanta 107 (2013) 55–60. [21] S. Yang, K. Chang, Y. Lee, C.M. Ma, C. Hu, Constructing a hierarchical graphene–carbon nanotube architecture for enhancing exposure of graphene and electrochemical activity of Pt nanoclusters, Electrochem. Commun. 12 (2010) 1206–1209. [22] Rajesh, R.K. Paul, A. Mulchandani, Platinum nanoflowers decorated threedimensional graphene–carbon nanotubes hybrid with enhanced electrocatalytic activity, J. Power Sources 223 (2013) 23–29. [23] X. Sun, S. Hu, L. Li, J. Xiang, W. Sun, Sensitive electrochemical detection of hydroquinone with carbon ionogel electrode based on BMIMPF6 , J. Electroanal. Chem. 651 (2011) 94–99. [24] D. Abd El-Hady, A.K. Youssef, Hyphenation of ionic liquid albumin glassy carbon biosensor or protein label-free sensor with differential pulse stripping voltammetry for interaction studies of human serum albumin with fenoprofen enantiomers, Anal. Chim. Acta 772 (2013) 68–74. [25] Y. Jiang, Q. Zhang, F. Li, L. Niu, Glucose oxidase and graphene bionanocomposite bridged by ionic liquid unit for glucose biosensing application, Sens. Actuators B 161 (2012) 728–733. [26] N. Liu, X. Chen, Z. Ma, Ionic liquid functionalized graphene/Au nanocomposites and its application for electrochemical immunosensor, Biosens. Bioelectron. 48 (2013) 33–38. [27] C. Shan, H. Yang, D. Han, Q. Zhang, A. Ivaska, L. Niu, Electrochemical determination of NADH and ethanol based on ionic liquid-functionalized graphene, Biosens. Bioelectron. 25 (2010) 1504–1508. [28] Z. Liu, Z. Wang, Y. Cao, Y. Jing, Y. Liu, High sensitive simultaneous determination of hydroquinone and catechol based on graphene/BMIMPF6 nanocomposite modified electrode, Sens. Actuators B 157 (2011) 540–546. [29] X. Wang, M. Wu, W. Tang, Y. Zhu, L. Wang, Q. Wang, P. He, Y. Fang, Simultaneous electrochemical determination of ascorbic acid, dopamine and uric acid using a palladium nanoparticle/graphene/chitosan modified electrode, J. Electroanal. Chem. 695 (2013) 10–16. [30] Y. Zhang, R. Yuan, Y. Chai, W. Li, X. Zhong, H. Zhong, Simultaneous voltammetric determination for DA, AA and NO2 − based on graphene/polycyclodextrin/MWCNTs nanocomposite platform, Biosens. Bioelectron. 26 (2011) 3977–3980.
Biographies Xue Wang is currently studying for master’s degree in East China Normal University. Her research interest is mainly focused on electrochemical sensors. Min Wu is currently an associate professor of the Department of Chemistry, East China Normal University, China. Her current research interest is in electroanalytical chemistry. Hui Li received her PhD degree from the East China Normal University, China. She is currently a chemical engineer in the School of Chemistry and Chemical Engineering, Shanghai Jiaotong University, China. Her current research is in electroanalytical chemistry. Qingjiang Wang received his PhD degree from East China Normal University, China. He is currently a professor of the Department of Chemistry, East China Normal University, China. His main areas of interest are micro–nano separation analysis and electrochemical sensors. Pingang He received his PhD degree in 1996 from Fudan University, China. He is currently a professor of the Department of Chemistry, East China Normal University, China. His main areas of interest are biological electrochemistry and electroanalytical chemistry. Yuzhi Fang is currently a professor of the Department of Chemistry, East China Normal University, China. His main areas of interest are biological electrochemistry and electroanalytical chemistry.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Simultaneous electrochemical determination of hydroquinone and catechol based on three-dimensional graphene/MWCNTs/BMIMPF6 nanocomposite modified electrode Xue Wang a , Min Wu a , Hui Li b , Qingjiang Wang a,∗ , Pingang He a , Yuzhi Fang a a b
Department of Chemistry, East China Normal University, Shanghai 200062, China School of Chemistry and Chemical Engineering, Shanghai Jiaotong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 27 August 2013 Received in revised form 29 October 2013 Accepted 7 November 2013 Available online 15 November 2013 Keywords: Graphene Multiwalled carbon nanotube 1-Butyl-3-methylimidazolium hexafluorophosphate Hydroquinone Catechol
a b s t r a c t A three-dimensional glassy carbon electrode (GCE) was fabricated from one-dimensional multiwalled carbon nanotubes (MWCNTs) and two-dimensional graphene (GR), using 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6 ) ionic liquid to improve its dispersibility and stability. This GR/MWCNTs/BMIMPF6 modified GCE was found to be a highly sensitive electrochemical sensor for the simultaneous determination of hydroquinone (1,4-dihydroxybenzene, HQ) and catechol (1,2dihydroxybenzene, CT). The GR was characterized using a transmission electron microscope. The electrochemical behaviours of HQ and CT at the GR/MWCNTs/BMIMPF6 /GCE were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The chemically modified electrode exhibited excellent electrochemical catalytic activities towards HQ and CT. Linear relationships between the oxidation peak current and concentration of HQ/CT were obtained in the ranges 0.5 M to 2.9 mM and 0.2 M to 0.66 mM with detection limits (S/N = 3) of 0.1 M and 0.06 M, respectively. This GR/MWCNTs/BMIMPF6 /GCE showed many advantages in terms of dispersibility, stability, sensitivity, facility and economy. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Hydroquinone (1,4-dihydroxybenzene, HQ) and catechol (1,2dihydroxybenzene, CT) are two isomers of dihydroxybenzene and often coexist in environmental samples as pollutants with high toxicity [1]. HQ and CT are harmful to environment including people, animals, and plants even at very low concentrations [2]. The acceptable emission of phenolic compounds according to the national standard of China (GB 8978-1996) is 0.5 mg/L (for dihydroxybenzene, 0.00454 M) [3]. Therefore, it is necessary to develop a simultaneous, simple, and rapid analytical method for the determination of dihydroxybenzene isomers. So far, numerous methods have been established for their determination, such as liquid chromatography [4], synchronous fluorescence [5], chemiluminescence [6], spectrophotometry [7], pH-based flow injection analysis [8], and electrochemical methods [9–11]. Among them, electrochemical methods have attracted increasing attentions owing to the advantages of low cost, fast response, excellent selectivity, and high sensitivity. However, a number of challenges for the simultaneous determination of HQ and CT isomers by electrochemical
∗ Corresponding author. Tel.: +86 21 54340054; fax: +86 21 54340054. E-mail address: [email protected] (Q. Wang). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.11.020
methods still exist [12]. A significant obstacle for the conventional solid electrodes is that the voltammetric peaks corresponding to the oxidation/reduction of the dihydroxybenzene isomers are largely overlapped in many cases. Second, the competitive adsorption of phenolic isomers on the electrode surface in the mixtures makes the relationship between the voltammetric response and isomers concentration nonlinear. In order to overcome these problems, many chemically modified electrodes have been prepared for simultaneous determination of HQ and CT. For instance, Saleh Ahammad et al. reported that poly(thionine)-modified glassy carbon electrode (GCE) can separate the oxidation peaks of HQ and CT; however, the detection limits of dihydroxybenzene isomers were very low [13]. Canevari et al. developed a sensitive technique for the simultaneous detection of HQ and CT in the presence of resorcinol using a SiO2 /C electrode spin-coated with a thin film of Nb2 O5 [14]. Qi et al. used multiwalled carbon nanotubes (MWCNTs)-modified GCE for the simultaneous determination of hydroquinone and catechol with good electrochemical performances [15]. MWCNTs have been widely used in electrochemistry due to their unique one-dimensional (1D) structural, electronic, and physical properties. In the field of chemically modified electrode, one of the most important characteristics of MWCNTs is their reported ability to promote electron-transfer process [16]. Graphene (GR) has attracted tremendous attention because of its unique
X. Wang et al. / Sensors and Actuators B 192 (2014) 452–458
nanostructure and extraordinary electrocatalytic properties such as high surface area, excellent conductivity, and high mechanical strength associated with its two-dimensional (2D) structure [17]. Based on its properties, GR is considered as an ideal electrode material for electrochemical and biosensing. GR electrodes have been successfully applied as biosensors for sensing glucose [18], NADH [19], hydrogen peroxide [20], etc. However, the excellent properties of GR emerge only in a planar direction. Recently, Yang et al. reported a new method to reduce the stacking of GR by introducing 1D carbon nanotubes to form a three-dimensional (3D) nanohybrid [21]. The properties of MWCNTs emerge in the axial direction while providing current density, high specific surface area, and thermal conductivity. Thus, a GR-MWCNTs hybrid that combines the unique properties of the two carbon allotropes in all directions and provides a high surface area per unit volume for increased catalyst loading could be an ideal electrode material [22]. Therefore, the long and tortuous MWCNTs bridged adjacent to GR efficiently inhibited their aggregation, thus enhancing the utilization of GR-based composites. Room temperature ionic liquids (RTILs) have attracted a lot of attention because of their unique electrochemical properties such as high ionic conductivity, low vapor pressure, low melting temperature, no requirement for additional supporting electrolytes, and thermal stability [23]. 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) , a type of RTIL, has attracted widespread attention as a nonaqueous polar, moderately hydrophobic, nonvolatile, chemically and thermally stable ionic liquid [24]. Recently, the RTIL-GR-modified electrodes have been reported for detecting many targets [25–27]. For instance, Liu et al. developed a GR/BMIMPF6 as the nanocomposite modified electrode for the simultaneous determination of HQ and CT, which exhibited a wide linear range and low detection limit [28]. In this study, a 3D carbon electrode was fabricated by combining 1D MWCNTs and 2D GRs that further advance the utilization of GR-based composites, BMIMPF6 ionic liquid may improve the dispersibility and stability of the 3D carbon electrode. This GR/MWCNTs/BMIMPF6 nanocomposite-modified electrode was formed and its application potential was evaluated for the simultaneous determination of HQ and CT. The separation of anodic peak potentials for HQ and CT was 122 mV, which makes it easier for the simultaneous determination of HQ and CT. The detection limits (S/N = 3) for HQ and CT for the GR/MWCNTs/BMIMPF6 /GCE were 0.1 M and 0.06 M, respectively. Moreover, the proposed sensor was applied for the simultaneous determination of HQ and CT in water samples with high selectivity. 2. Experimental 2.1. Apparatus and measurements All the electrochemical experiments were performed using CHI660E electrochemical workstation (Shanghai Chenhua Co., China). A standard three-electrode cell was used for all the electrochemical experiments. Either a bare 3 mm diameter GCE or a modified electrode was used as the working electrode. A platinum wire served as the counter electrode and all the potentials were measured relative to a saturated calomel electrode (SCE). The measurements were carried out in a phosphate buffer saline (PBS) at room temperature. The transmission electron microscope (TEM) images were obtained using a JEM-2100F (Jeol Ltd., Japan) operating at 200 kV.
453
hydrazine hydrate (85%), and sulfuric acid (98%) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). MWCNTs were obtained from Chengdu Organic Chemicals Co. Ltd. of Chinese Academy of Science and purified by refluxing in concentrated nitric acid for 7 h prior to their use. 1-Butyl3-methylimidazolium was purchased from Shanghai Cheng Jie Chemical Co. Ltd. (Shanghai, China). Deionized water was prepared using a Milli-Q water purification system (Millipore, Bedford, MA). Hydroquinone and catechol solutions were freshly prepared in PBS prior to use. 2.3. Electrode preparation and modification 2.3.1. Preparation of the graphene GR was synthesized according to the literature method [29]. Natural graphite powder (2 g), sodium nitrate (1.2 g), and potassium permanganate (6 g) were slowly added to 46 mL of sulfuric acid (98 wt.%) in an ice bath at 0 ◦ C. The reaction mixture was kept in a 35 ◦ C water bath for 30 min to oxidize the graphite. Next, 92 mL ultrapure water was gradually added into the mixture, while the temperature was increased to 98 ◦ C and the reaction was maintained for 40 min in order to increase the oxidation degree of the GR oxidation (GO) product. The mixture was then poured into 240 mL of ultrapure water followed by the addition of 6 mL of hydrogen peroxide solution (30%) into the suspension. After cooling to room temperature, the GO obtained was isolated from the solution by centrifugation. The crude GO was carefully washed with 5% aqueous hydrogen chloride solution and ultrapure water to remove the remnant acid and salt impurities. The wet GO was dried under vacuum (50 ◦ C). 100 mg of obtained GO powder was dispersed in 50 mL of ultrapure water and stirred for 1 h at room temperature to obtain a homogeneous dispersion. Hydrazine hydrate (0.6 mL, 85%) was added to the dispersion to reduce the GO, and the reaction mixture was kept at 95 ◦ C for 1 h, the thus obtained black graphene was washed with ultrapure water and dried in vacuum. 2.3.2. Fabrication of the GR/MWCNTs/BMIMPF6 /GCE Prior to coating, a GCE was successively polished using 1.0, 0.3, and 0.05 m ␣-alumina powder and successively rinsed thoroughly with 1:1 HNO3 solution, ethanol, and deionized water for 5 min, and dried at room temperature. The GR/MWCNTs composite was prepared by stirring a mixture of 1 mg GR, 1 mg MWCNTs, and 1 mL DMF for 1 h at room temperature [30]. Next, BMIMPF6 was added to the above solution (v/v, 1.5%) to obtain a homogeneous dispersion. Next, 10 L of the homogeneous dispersion of GR/MWCNTs/BMIMPF6 in an aqueous solution was applied to the clean GCE and allowed to dry in air at room temperature for 24 h. For comparison purposes, MWCNTs/GCE, GR/GCE, and GR/MWCNTs/GCE were prepared by the similar process. 3. Results and discussion 3.1. Characterization of GR and GR/MWCNTs The surface morphologies of the modified electrodes were characterized by TEM. Fig. 1(A) shows that GR has a folded structure with a disordered sheet, which can enhance the capability of the electrochemical detection of HQ and CT. As shown in Fig. 1(B), the long and tortuous MWCNTs bridged adjacent GR and reduced the aggregation of GR, leading to a 3D nanohybrid structure to enhance the utilization of GR.
2.2. Chemicals and solutions
3.2. Electrocatalytic oxidation behaviours of single HQ and CT
Hydroquinone, catechol, graphite powder, N,Ndimethylformamide (DMF), hydrogen peroxide solution (30%),
The electrochemical oxidation behaviours of single HQ and CT were studied by cyclic voltammetry (CV) at the
454
X. Wang et al. / Sensors and Actuators B 192 (2014) 452–458
Fig. 2. Cyclic voltammetry of bare GCE, GR/GCE, MWCNTs/GCE, GR/MWCNTs/GCE, and GR/MWCNTs/BMIMPF6 /GCE in 0.1 M PBS (pH 7.0) containing (A) 0.1 mM HQ, and (B) 0.1 mM CT. Scan rate: 50 mV s−1 .
Fig. 1. TEM image of GR and GR/MWCNTs.
bare GCE, MWCNTs/GCE, GR/GCE, GR/MWCNTs/GCE, and GR/MWCNTs/BMIMPF6 /GCE in 0.1 M PBS (pH 7.0). As shown in Fig. 2(A), the anodic peak current of HQ was 148 A at GR/MWCNTs/BMIMPF6 /GCE, which was approximately 15 times higher than that at the bare GCE, and much higher than other modified electrodes. Moreover, the anodic peak potential of HQ was 90 mV at GR/MWCNTs/BMIMPF6 /GCE, which was 100 mV more negative than that at the bare GCE. As shown in Fig. 2(B), the anodic and cathodic peak potentials for the detection of CT at GR/MWCNTs/BMIMPF6 /GCE were 186 and 102 mV, respectively, compared to 277 and 47 mV at bare GCE. The separation of anodic and cathodic peak potentials at GR/MWCNTs/BMIMPF6 /GCE and the bare GCE were 84 and 230 mV, respectively, indicating that CT had a better electrochemical catalysis at GR/MWCNTs/BMIMPF6 /GCE.
The above results indicate that the oxidation peak current of HQ and CT significantly increased at GR/MWCNTs/BMIMPF6 /GCE than the other electrodes, and GR/MWCNTs/BMIMPF6 /GCE made the peak potentials of HQ and CT shift negatively compared with bare GCE, which can be attributed to the good conductivity and catalytic activity of the immobilized GR and MWCNTs, at the same time, BMIMPF6 could improve the dispersibility and stability of GR and MWCNTs. Thus, both GR/MWCNTs film and BMIMPF6 were responsible for enhancing the electrochemical sensing. 3.3. Electrochemical behaviours of the mixed components of HQ and CT Fig. 3 showed the cyclic voltammograms of the mixture containing 0.1 mM HQ and 0.1 mM CT in 0.1 M PBS (pH 7.0) at the bare GCE, MWCNTs/GCE, GR/GCE, GR/MWCNTs/GCE, and GR/MWCNTs/BMIMPF6 /GCE. Fig. 3 shows one broad oxidation peak at the bare GCE, indicating that HQ and CT cannot be separated at the bare electrode. Two well-separated oxidation peaks were observed at MWCNTs/GCE, GR/GCE, GR/MWCNTs/GCE, and GR/MWCNTs/BMIMPF6 /GCE. However, the oxidation peak current remarkably increased at GR/MWCNTs/BMIMPF6 /GCE, and
X. Wang et al. / Sensors and Actuators B 192 (2014) 452–458
455
Fig. 3. Cyclic voltammetry of bare GCE, GR/GCE, MWCNTs/GCE, GR/MWCNTs/GCE, and GR/MWCNTs/BMIMPF6 /GCE in 0.1 M PBS (pH 7.0) containing 0.1 mM HQ and 0.1 mM CT. Scan rate: 50 mV s−1 .
therefore, HQ and CT exhibit the highest sensitivity at this electrode. The oxidation peak potentials for HQ and CT were 88 and 210 mV, respectively, thus giving a peak-to-peak separation of 122 mV. The separations were large enough to allow simultaneous selective determination of HQ and CT. The above CV experiments shown in Figs. 2 and 3 demonstrated that HQ and CT had the highest peak currents at GR/MWCNTs/BMIMPF6 /GCE than those of the other modified electrodes. This improvement in electron transfer kinetics may be attributed to the following factors. First, the small size, high loading, large surface area, and high electrocatalytic activity of both GR and MWCNTs could enhance the performance of the peroxide oxidation process of HQ and CT on the modified electrode and reduce the charge transfer resistance, thus improving its reversibility which greatly enhanced the peak currents. Second, the excellent properties of GR emerge only in the planar direction while those of MWCNTs emerge in the axial direction, thus providing current density, high specific surface area, and thermal conductivity. Because the 1D MWCNTs and 2D GR formed a 3D hierarchical structure, the MWCNTs efficiently inhibited the stacking of individual GR units and thereby enhanced the utilization of GR-based composites. Thus, GR-MWCNTs hybrid combines the unique properties of the two carbon allotropes in all directions and provides a high surface area per unit volume for increased catalyst loading. Third, BMIMPF6 could improve the dispersibility and stability of GR and MWCNTs, thus enhancing the electrochemical performance for the detection of different target molecules in electroanalytical applications. 3.4. Selection of the experimental conditions 3.4.1. Effect of pH on the electrochemical behaviour of HQ and CT The effect of pH on the electrochemical behaviour of a mixed solution of 0.1 mM HQ and 0.1 mM CT in 0.1 M PBS (pH 7.0) at GR/MWCNTs/BMIMPF6 /GCE was carefully investigated by CV at pH range 3.0–9.0 in 0.1 M PBS. It can be easily concluded from Fig. 4(A) that the maximum oxidation peak currents of HQ and CT increase with increasing pH till pH 7.0 and then decrease with further increase in pH. The pKa values of HQ and CT are 9.85 and 9.4, respectively. When the pH increases from 7.0 to 9.0, the increased amount of hydroxyl ions in the solution may decrease the adsorption capacity of the dihydroxybenzene isomers. Considering the
Fig. 4. The effects of pH on the oxidation peak current (A) of HQ and CT, the oxidation peak potential (B) of HQ and CT for the oxidation of 0.1 mM HQ and 0.1 mM CT in 0.1 M PBS (pH7.0). Scan rate: 50 mV s−1 .
determination sensitivity, pH 7.0 was selected as the optimum pH for the experiments. All further experiments were investigated in 0.1 M PBS (pH 7.0). Fig. 4(B) shows that the peak potentials for the oxidation of HQ and CT negatively shifted with increasing pH. The relationships between the peak potentials of HQ and CT and pH were also investigated. For HQ, the linear regression equation for oxidation peak potentials and pH could be expressed as Epa (V) = 0.51 − 0.059 pH (R = 0.9934). For CT, the oxidation peak potentials followed the linear regression equation Epa (V) = 0.62 − 0.061 pH (R = 0.9973). The slopes of the three regression equations for HQ and CT were 59 and 61 mV/pH, respectively, indicating that the number of protons and electrons involved in the electrochemical redox process of HQ (or CT) are equal. According to the previous literature [15], the electrochemical redox process of HQ and CT at GR/MWCNTs/BMIMPF6 /GCE is a two-electron and two-proton process. 3.4.2. Effect of scan rate on the electrochemical behaviour of HQ and CT The effect of scan rate on the electrochemical behaviour of a mixed solution of 0.1 mM HQ and 0.1 mM CT in 0.1 M PBS (pH
456
X. Wang et al. / Sensors and Actuators B 192 (2014) 452–458
Fig. 5. Cyclic voltammetry of GR/MWCNTs/BMIMPF6 /GCE in 0.1 M PBS (pH 7.0) containing 0.1 mM HQ and 0.1 mM CT at different scan rates (20, 40, 60, 80, 100, 120, 140, 160, 180, and 200 mV s−1 ).
7.0) at GR/MWCNTs/BMIMPF6 /GCE was carefully investigated by CV (Fig. 5). The anodic peak intensity continuously increased with increasing scan rate from 20 to 200 mV s−1 . For HQ (Fig. 6A), the oxidation peak currents followed the linear regression equation Ipa = 27.111/2 − 41.47 (A, mV s−1 , R = 0.9996). For CT (Fig. 6B), the regression equation was Ipa = 20.031/2 + 0.3159 (A, mV s−1 , R = 0.9991). The above data indicated that the electrode reaction of HQ and CT at GR/MWCNTs/BMIMPF6 /GCE was a typical diffusioncontrolled process [28]. 3.4.3. Effect of the amount of BMIMPF6 on the electrochemical behaviour of HQ and CT The effect of the amount of BMIMPF6 on the electrochemical behaviour of a mixed solution of 0.1 mM HQ and 0.1 mM CT in 0.1 M PBS (pH 7.0) at GR/MWCNTs/BMIMPF6 /GCE was carefully investigated by CV (Fig. 7). The relationships between the peak currents of HQ and CT and amount of BMIMPF6 for the GR/MWCNTs suspension was studied at a ratio of 0.5%, 1%, 1.5%, 2%, 2.5%, and 3% (v/v), respectively. As shown in Fig. 7, the results showed the optimum ratio to be 1.5%. A higher ratio causes a smaller current response, which may be attributed to the thicker film of GR/MWCNTs hampering the electron transfer. Although the larger ratio could increase the amount of BMIMPF6 on GR/MWCNTs, the surface aggregations of BMIMPF6 would decrease its surface area, leading to a decrease in the oxidation peak current. Thus, the ratio was fixed at 1.5% in the experiments.
Fig. 6. The oxidation peak currents of 0.1 mM HQ (A) and 0.1 mM CT (B) vs. the square root of scan rates.
3.5. Simultaneous determination of HQ and CT Under the optimized conditions, the determination of HQ and CT at GR/MWCNTs/BMIMPF6 /GCE was carried out in 0.1 M PBS (pH 7.0) by differential pulse voltammetry (DPV), where the concentration of one species changed while the other species remained constant. As shown in Fig. 7(A), the peak currents linearly increased with increasing concentration of HQ from 1 M to 2.9 mM, while keeping the concentration of CT constant at 50 M. The linear equation and detection limit are listed in Table 1. Fig. 7(B) shows the different DPVs of CT at different concentrations in the presence of 50 M HQ. The peak currents linearly increased with increasing concentration from 0.5 to 660 M; the regression equations and the detection limit are listed in Table 1. Further, no obvious interference can be
Fig. 7. The effect of the amount of BMIMPF6 on the electrochemical behaviour of a mixed solution of 0.1 mM HQ and 0.1 mM CT in 0.1 M PBS (pH 7.0) at GR/MWCNTs/BMIMPF6 /GCE was investigated by CV.
X. Wang et al. / Sensors and Actuators B 192 (2014) 452–458
457
Table 1 Practical values for simultaneous determination of HQ and CT at GR/MWCNTs/BMIMPF6 /GCE in 0.1 M PBS solution (pH 7.0). Solution: (0.1 M PBS, pH 7.0) including
Additions
Linear range (M)
Linear equation (A, M)
Regression
50 M CT
HQ
50 M HQ
CT
0.5–465 465–2900 0.2–80 80–660
Ipa = 0.8410 + 0.1140c Ipa = 36.59 + 0.04264c Ipa = −0.06668 + 0.2233c Ipa = 49.94 + 0.09719c
0.9959 0.9981 0.9965 0.9978
Limit of detection (S/N = 3) (M) 0.1 0.06
Table 2 Determination of HQ and CT in water samples. Samples
Original (mol/L)
Added (mol/L)
Total founda (mol/L)
Recoveries (%)
RSD (%)
Rain water
HQ CT
– –
5 5
4.82 5.23
96.4 104.6
3.56 2.89
River water
HQ CT
– –
5 5
5.16 5.13
103.2 102.6
4.17 2.63
–, not detected. a Average of five determinations.
observed for the determination of one species coexisting with the other species (Fig. 8). 3.6. Stability and reproducibility The modified electrode was immersed in PBS (pH 7.0) and stored in a refrigerator when not in use. After 15 days, the peak currents of HQ and CT decreased only by 2.15% for HQ and 1.89% for CT, respectively. To evaluate the reproducibility of modified electrode, ten GR/MWCNTs/BMIMPF6 /GCEs were investigated by comparing their peak currents in 0.1 mM HQ and 0.1 mM CT in 0.1 M PBS (pH 7.0). The relative standard deviation (RSD) for HQ and CT were 2.89% and 2.48%, respectively. 3.7. Real samples analysis Rain water and river water samples were tested with GR/MWCNTs/BMIMPF6 /GCE, in order to validate the proposed analysis method. The amount of HQ and CT in rain and river water samples was determined by the calibration method using DPV, and the results are listed in Table 2. As per the results, no dihydroxybenzene was detected in water samples by using the modified electrode, which suggests that the dihydroxybenzene content was below the detection limits. The quantitative recoveries for HQ and CT were 96.4–103.2% and 102.6–104.6%, respectively. This suggests that the proposed GR/MWCNTs/BMIMPF6 /GCE could be used to determine the concentration of dihydroxybenzene isomers in real water samples.
4. Conclusions
Fig. 8. Differential pulse voltammetry of GR/MWCNTs/BMIMPF6 /GCE in 0.1 M PBS (pH7.0) (A) containing 50 M CT and different concentrations of HQ from 0.5 M to 2.9 mM, and (B) containing 50 M HQ and different concentrations of CT from 0.2 to 660 M. Scan rate: 20 mV s−1 , pulse interval 200, pulse amplitude 25 mV, and pulse width 50 ms.
In this study, a simple and sensitive electrochemical analytical method for the simultaneous determination of HQ and CT was developed by employing GR/MWCNTs/BMIMPF6 modified GCE. 1D MWCNTs and 2D GR formed a 3D carbon electrode, which enhanced the utilization of GR-based composites, while BMIMPF6 improved the dispersibility and stability of the 3D carbon electrode. It may be concluded that the modified electrode has high electrocatalytic activities towards the oxidation of HQ and CT by decreasing the oxidation over-potentials and increasing the peak currents significantly. This GR/MWCNTs/BMIMPF6 /GCE exhibits a low detection limit and wide linear range, and the detection limits for HQ and CT were 0.1 M and 0.06 M, respectively. This proposed method may potentially be useful for developing an electrochemical sensor for phenolic compounds.
458
X. Wang et al. / Sensors and Actuators B 192 (2014) 452–458
Acknowledgements This work was financially supported by the Program for New Century Excellent Talents in University (NCET-08-0191) and the National Program on Development of Scientific Instruments and Equipment (2011YQ150072). References [1] Q. Guo, J. Huang, P. Chen, Y. Liu, H. Hou, T. You, Simultaneous determination of catechol and hydroquinone using electrospun carbon nanofibers modified electrode, Sens. Actuators B 163 (2012) 179–185. [2] C. Wang, R. Yuan, Y. Chai, F. Hu, Simultaneous determination of hydroquinone, catechol, resorcinol and nitrite using gold nanoparticles loaded on poly-3-amino-5-mercapto-1,2,4-triazole-MWNTs film modified electrode, Anal. Methods 4 (2012) 1626–1628. [3] H. Yin, Q. Zhang, Y. Zhou, Q. Ma, T. liu, L. Zhu, S. Ai, Electrochemical behavior of catechol, resorcinol and hydroquinone at graphene–chitosan composite film modified glassy carbon electrode and their simultaneous determination in water samples, Electrochim. Acta 56 (2011) 2748–2753. [4] A. Asan, I. Isildak, Determination of major phenolic compounds in water by reversed-phase liquid chromatography after pre-column derivatization with benzoyl chloride, J. Chromatogr. A 988 (2003) 145–149. [5] M.F. Pistonesi, M.S. Di Nezio, M.E. Centurión, M.E. Palomeque, A.G. Lista, B.S. Fernández Band, Determination of phenol, resorcinol and hydroquinone in air samples by synchronous fluorescence using partial least-squares (PLS), Talanta 69 (2006) 1265–1268. [6] H. Cui, Q. Zhang, A. Myint, X. Ge, L. Liu, Chemiluminescence of cerium(IV)–rhodamine 6G–phenolic compound system, J. Photochem. Photobiol. A 181 (2006) 238–245. [7] P. Nagaraja, R.A. Vasantha, K.R. Sunitha, A sensitive and selective spectrophotometric estimation of catechol derivatives in pharmaceutical preparations, Talanta 55 (2001) 1039–1046. [8] J.A. Garcia-Mesa, R. Mateos, Direct automatic determination of bitterness and total phenolic compounds in virgin olive oil using a pH-based flow-injection analysis system, J. Agric. Food Chem. 55 (2007) 3863–3868. [9] X. Ma, Z. Liu, C. Qiu, T. Chen, H. Ma, Simultaneous determination of hydroquinone and catechol based on glassy carbon electrode modified with gold–graphene nanocomposite, Microchim. Acta 180 (2013) 461–468. [10] T. Gan, J. Sun, K. Huang, L. Song, Y. Li, A graphene oxide–mesoporous MnO2 nanocomposite modified glassy carbon electrode as a novel and efficient voltammetric sensor for simultaneous determination of hydroquinone and catechol, Sens. Actuators B 177 (2013) 412–418. [11] L. Wang, Y. Zhang, Y. Du, D. Lu, Y. Zhang, C. Wang, Simultaneous determination of catechol and hydroquinone based on poly (diallyldimethylammonium chloride) functionalized graphene-modified glassy carbon electrode, J. Solid State Electrochem. 16 (2011) 1323–1331. [12] M.A. Ghanem, Electrocatalytic activity and simultaneous determination of catechol and hydroquinone at mesoporous platinum electrode, Electrochem. Commun. 9 (2007) 2501–2506. [13] A.J.S. Ahammad, M.M. Rahman, G. Xu, S. Kim, J. Lee, Highly sensitive and simultaneous determination of hydroquinone and catechol at poly(thionine) modified glassy carbon electrode, Electrochim. Acta 56 (2011) 5266–5272. [14] T.C. Canevari, L.T. Arenas, R. Landers, R. Custodio, Y. Gushikem, Simultaneous electroanalytical determination of hydroquinone and catechol in the presence of resorcinol at an SiO2 /C electrode spin-coated with a thin film of Nb2 O5 , Analyst 138 (2013) 315–324. [15] H. Qi, C. Zhang, Simultaneous determination of hydroquinone and catechol at a glassy carbon electrode modified with multiwall carbon nanotubes, Electroanalysis 17 (2005) 832–838. [16] L.L.C. Garcia, L.C.S. Figueiredo-Filho, G.G. Oliveira, O. Fatibello-Filho, C.E. Banks, Square-wave voltammetric determination of paraquat using a glassy carbon electrode modified with multiwalled carbon nanotubes within a dihexadecylhydrogenphosphate (DHP) film, Sens. Actuators B 181 (2013) 306–311. [17] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [18] C. Shan, H. Yang, D. Han, Q. Zhang, A. Ivaska, L. Niu, Graphene/AuNPs/chitosan nanocomposites film for glucose biosensing, Biosens. Bioelectron. 25 (2010) 1070–1074.
[19] K. Guo, K. Qian, S. Zhang, J. Kong, C. Yu, B. Liu, Bio-electrocatalysis of NADH and ethanol based on graphene sheets modified electrodes, Talanta 85 (2011) 1174–1179. [20] Y. Xia, W. Li, M. Wang, Z. Nie, C. Deng, S. Yao, A sensitive enzymeless sensor for hydrogen peroxide based on the polynucleotide-templated silver nanoclusters/graphene modified electrode, Talanta 107 (2013) 55–60. [21] S. Yang, K. Chang, Y. Lee, C.M. Ma, C. Hu, Constructing a hierarchical graphene–carbon nanotube architecture for enhancing exposure of graphene and electrochemical activity of Pt nanoclusters, Electrochem. Commun. 12 (2010) 1206–1209. [22] Rajesh, R.K. Paul, A. Mulchandani, Platinum nanoflowers decorated threedimensional graphene–carbon nanotubes hybrid with enhanced electrocatalytic activity, J. Power Sources 223 (2013) 23–29. [23] X. Sun, S. Hu, L. Li, J. Xiang, W. Sun, Sensitive electrochemical detection of hydroquinone with carbon ionogel electrode based on BMIMPF6 , J. Electroanal. Chem. 651 (2011) 94–99. [24] D. Abd El-Hady, A.K. Youssef, Hyphenation of ionic liquid albumin glassy carbon biosensor or protein label-free sensor with differential pulse stripping voltammetry for interaction studies of human serum albumin with fenoprofen enantiomers, Anal. Chim. Acta 772 (2013) 68–74. [25] Y. Jiang, Q. Zhang, F. Li, L. Niu, Glucose oxidase and graphene bionanocomposite bridged by ionic liquid unit for glucose biosensing application, Sens. Actuators B 161 (2012) 728–733. [26] N. Liu, X. Chen, Z. Ma, Ionic liquid functionalized graphene/Au nanocomposites and its application for electrochemical immunosensor, Biosens. Bioelectron. 48 (2013) 33–38. [27] C. Shan, H. Yang, D. Han, Q. Zhang, A. Ivaska, L. Niu, Electrochemical determination of NADH and ethanol based on ionic liquid-functionalized graphene, Biosens. Bioelectron. 25 (2010) 1504–1508. [28] Z. Liu, Z. Wang, Y. Cao, Y. Jing, Y. Liu, High sensitive simultaneous determination of hydroquinone and catechol based on graphene/BMIMPF6 nanocomposite modified electrode, Sens. Actuators B 157 (2011) 540–546. [29] X. Wang, M. Wu, W. Tang, Y. Zhu, L. Wang, Q. Wang, P. He, Y. Fang, Simultaneous electrochemical determination of ascorbic acid, dopamine and uric acid using a palladium nanoparticle/graphene/chitosan modified electrode, J. Electroanal. Chem. 695 (2013) 10–16. [30] Y. Zhang, R. Yuan, Y. Chai, W. Li, X. Zhong, H. Zhong, Simultaneous voltammetric determination for DA, AA and NO2 − based on graphene/polycyclodextrin/MWCNTs nanocomposite platform, Biosens. Bioelectron. 26 (2011) 3977–3980.
Biographies Xue Wang is currently studying for master’s degree in East China Normal University. Her research interest is mainly focused on electrochemical sensors. Min Wu is currently an associate professor of the Department of Chemistry, East China Normal University, China. Her current research interest is in electroanalytical chemistry. Hui Li received her PhD degree from the East China Normal University, China. She is currently a chemical engineer in the School of Chemistry and Chemical Engineering, Shanghai Jiaotong University, China. Her current research is in electroanalytical chemistry. Qingjiang Wang received his PhD degree from East China Normal University, China. He is currently a professor of the Department of Chemistry, East China Normal University, China. His main areas of interest are micro–nano separation analysis and electrochemical sensors. Pingang He received his PhD degree in 1996 from Fudan University, China. He is currently a professor of the Department of Chemistry, East China Normal University, China. His main areas of interest are biological electrochemistry and electroanalytical chemistry. Yuzhi Fang is currently a professor of the Department of Chemistry, East China Normal University, China. His main areas of interest are biological electrochemistry and electroanalytical chemistry.