N-doped graphene modified glassy carbon electrode

Electrochimica Acta 146 (2014) 568–576

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Electrochemical determination of 4-nitrophenol at polycarbazole/N-doped graphene modified glassy carbon electrode Yuehua Zhang a,b , Lihua Wu a , Wu Lei a , Xifeng Xia a , Mingzhu Xia a , Qingli Hao a,∗ a b

School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China School of Chemistry and Chemical engineering, Nantong University, Nantong, 226007, China

a r t i c l e

i n f o

Article history: Received 7 June 2014 Received in revised form 24 August 2014 Accepted 30 August 2014 Keywords: N-doped graphene Polycarbazole Cyclic voltammetry 4-nitrophenol Detection

a b s t r a c t Polycarbazole (PCZ)/nitrogen-doped graphene (N-GE) composite was prepared by electropolymerization of carbazole on the N-GE modified glass carbon electrode (N-GE/GCE) for fabricating a novel electrochemical sensor for 4-nitrophenol (4-NP). The PCZ/N-GE shows high conductivity and well-distributed nanostructure. The redox behavior of 4-NP at a PCZ/N-GE/GCE was investigated in acetate buffer solution by cyclic voltammetry (CV), compared with the bare GCE, reduced graphene oxide (RGO), N-GE and PCZ modified GCEs. The results indicate that all modified electrodes show the enhanced reduction peak currents. However, the PCZ/N-GE/GCE exhibits the highest peak current and most positive reduction potential of 4-NP, which reflects the PCZ/N-GE composite has the best electrocatalytic activity towards 4-NP. The enhanced electrochemical performance of PCZ/N-GE and the electrocatalytic activity to 4-NP are contributed to the synergic effect of PCZ and N-GE with highly conductivity and large surface area, which can greatly facilitate the electron-transfer processes between the electrolyte and electrode. An electrochemical sensor for 4-NP was developed based on the PCZ/N-GE modified electrode under the optimized conditions. The reduction peak current was linear with the concentration of 4-NP in the range of 8 × 10-7 ∼2 × 10-5 M. The low detection limit of the sensor was estimated to be 0.062 ␮M (S/N = 3). The sensor based on PCZ/N-GE/GCE was also applied to the detection of 4-NP in real water samples. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Graphene has excited a great deal of interest from theoretical and experimental aspects in various areas since it was found in 2004 due to its unique physical and chemical properties[1], such as extraordinary electronic transport property, high mechanical property, high surface areas, and so on. These advantages make it a potential candidate applied in various fields such as sensors, supercapacitors, catalyst, battery and so on [2–5]. Chemical doping with hetero-atoms is an effective approach to tailor the electronic properties of host materials. Doping with hetero-atoms (e.g. B, N, S and Si) into graphene would disrupt the ideal sp2 hybridization of the carbon atoms and induce the significant changes in their electronic properties and chemical reactivity [6]. Nitrogen, as a neighbor of carbon in the period table, is considered to be an excellent doping element due to its comparable atomic size and five valence electrons which is available to form strong valence bonds with carbon atoms [7]. When replacing carbon atoms into the graphene

∗ Corresponding author. Tel.: +86-25-84315943; fax: +86-25-84315190. E-mail addresses: [email protected], [email protected] (Q. Hao). http://dx.doi.org/10.1016/j.electacta.2014.08.153 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

frameworks, nitrogen can modulate the band structure of graphene without impeding its good conducting behavior [8]. At present, many methods have been adopted to synthesis nitrogen-doped graphene (N-GE) using various nitrogen sources, such as chemical vapor deposition using NH3 gas [9], nitrogen plasma treatment of graphene, thermal annealing graphite oxide using melamine [10], high-power electrical annealing in NH3 [11], and hydrothermal method using NH3· H2 O or urea as the nitrogen precursor [12,13]. The N-GE materials, preparing by various approaches, exhibit good electrocatalytic activity to many molecules. Moreover, as studied in these references, the electrocatalytic activity of N-GE is mainly attributed to ‘pyridinic’ N and/or ‘pyrrolic’ N. Due to the electric, electronic and optical properties inherent to metals or semiconductors, conducting polymer polycarbazole (PCZ) and its derivatives have been extensively studied in many fields, such as light emitting diodes [14], photovoltaic devices [15], and so on. As other conducting polymers [16], PCZ can also be synthesized via electrochemical methods, which is favor for fabrication of modified electrodes used in sensor area [17], especially in electrochemical sensors [18,19]. For example, PCZ or its copolymer with p-tolylsulfonyl modified single carbon fiber microelectrodes can be used to detect the dopamine [20]. Chen et al developed a

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simple and novel approach to construct a chiral hybrid molecularly imprinted polymer (MIP) sensor for L-phenylalanine. In which the conductive PCZ in hybrid MIP could provide a very efficient matrix for signal transport between the electrode and the sensitive layer [21]. Recently, we fabricated a PCZ/graphene modified electrode via electropolymerization of carbazole on the surface of graphene modified electrode. For the excellent electrocatalytic activity towards the reduction of imidacloprid, this modified electrode was used in the determination of imidacloprid with the limit of detection 0.64 ␮M for CV and 0.37 ␮M for DPV, respectively [22]. Since the unique properties of PCZ and N-GE, we expect that the combination of PCZ with N-GE could produce a novel kind of materials with high electrochemical performance for sensors. Additionally, as the best of our knowledge, the modified electrode with PCZ and N-GE has not been reported for electrochemical sensors by now. 4-nitrophenol (4-NP), one of the most serious environmental contaminants, is widely used in the production of dyes, pesticides, explosive material and so on. Owing to its extreme toxicity on humans, animals and plants and difficult to degrade through the conventional treatment process, 4-NP with its derivatives were listed among the top 114 organic pollutants by The United States Environmental Protection Agency [23]. Therefore, the monitoring and determination of 4-NP is essential. Various methods are available for the determination of 4-NP, such as gas chromatography–tandem mass spectrometry [24], ultrahigh pressure liquid chromatography [25], liquid chromatography [26], spectrophotometric methods [27]. Many of these approaches are often very complex, expensive and time consuming. Hence, the sensitive, fast and accurate determination of 4-NP is of great importance. Electrochemical methods, owning many advantages such as simple operation, fast response, good sensitivity and in situ detection, are adopted for detection of various analytes. There are several modified electrodes used in literature for determination of 4-NP. However, the combination of PCZ and N-GE has not been reported for fabrication of 4-NP sensors. Herein, we fabricated the PCZ/N-GE composite film by two steps to modify glassy carbon electrode (GCE) for the sensitive determination of 4-NP. The electrochemical properties of the modified electrodes and the responses towards 4-NP have been investigated by using cyclic voltammetry. The electroanalytical performance for the 4-NP detection obtained using the PCZ/N-GE electrode was compared with the ones obtained at reduced graphene oxide (RGO), N-GE, PCZ and bare glassy carbon electrodes. Under the optimum experimental conditions, the modified electrode displayed effective catalytic activity in the reduction of 4-NP.

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2.2. Apparatus All electrochemical experiments were performed on a CHI660D electrochemical workstation (CH Instrumental Co., China) using a single-compartment cell with a conventional three electrodes. A bare or modified glassy carbon electrode (GCE, CHI104, d = 3 mm) was used as working electrode. A saturated calomel electrode (SCE) and a platinum wire were used as reference electrode and auxiliary electrode, respectively. If not differently stated, all potential values reported were given versus this reference electrode. Cyclic voltammetry (CV) measurements were performed between a potential window of -0.3 and -0.8 V at a scan rate of 100 mV s-1 for 4-NP determinations. Electrochemical impedance spectroscopy (EIS) was performed in the frequency region from 0.1 to 106 Hz with amplitude of 0.005 V. All electrochemical measurements were carried out at room temperature (25 ± 0.5 ◦ C). Scanning electron microscope (SEM) images were obtained with S-4800 (Hitachi Co., Ltd., Japan). Transmission electron microscope (TEM) image was carried out on a JEOL JEM2100 with energy dispersive spectroscopy. X-ray photoelectron spectroscopy (XPS) measurements were performed on a scanning X-ray microprobe PHI Quantera II (Ulvac-PHI, INC.) with a C60 gun. A high-performance liquid chromatography (HPLC) (Dalian Elite Analytical Instruments Co., Ltd., China) consisting of DAD230 Diode Array Detector, P230 High Pressure Constant Flow Pump and Hypersil ODS, C18, 5um, i.d.4.6mm*250 mm was used. 2.3. Preparation of nitrogen-doped graphene Graphite oxide (GO) was first prepared from natural graphite powder through a modified method we described previously [28]. N-GE was synthesized by the wet-chemical method with GO as the raw material and urea as the reducing and doping agent [29]. Generally, 150 mL of exfoliated GO solution with a concentration of 0.5 mg mL-1 was added in a three-necked flask. Dilute ammonia solution was used to adjust the pH value of the GO dispersion to 8.0. Then, urea (5.0 g) was added under vigorous stirring. The solution was then raised to 95 ◦ C and refluxed for 30 h. Afterwards, dilute HCl was dropped into the as-prepared N-GE dispersion to induce coagulation. Subsequent the resulting mixture was washed with anhydrous alcohol and double-distilled water by repeated centrifugation. Finally, the yielded solid N-GE sample was obtained after dried in vacuum. For comparison, reducedgraphene oxide (RGO) was synthesized according to the literature [30]. 2.4. Preparation of the modified electrodes

2. Experimental 2.1. Reagents Graphite powder was purchased from Shanghai Carbon Co., Ltd. Boron trifluoride diethyl etherate (BFEE, Sinopharm Chemical Reagent Co., Ltd.) was purified by distillation before use. 4-NP and all other chemicals were of analytical grade and used without any further purification. All the solutions were prepared using double-distilled water. Supporting electrolyte used throughout this study was acetate buffer solution (ABS), unless stated otherwise, which was prepared from 0.2 M HAc and 0.2 M NaAc solutions to obtain the required pH value. To eliminate the effect of dissolved oxygen at electrodes, all solutions were bubbled with high-purity nitrogen for ten minutes before the experiments and a flow nitrogen was maintained over the solution during the experiments.

The bare GC electrode was firstly polished with 0.3 and 0.05 ␮m aluminum oxide slurries and then ultrasonically cleaned with ethanol and double distilled water for 10 minutes to remove the physically adsorbed substance. The electrodes modified with N-GE and PCZ were prepared by a two-step method. Firstly, 0.25 mg mL-1 N-GE suspension was prepared by dispersing the certain volume of the as-produced N-GE in N, N-dimethylformamide (DMF) ultrasonically. The N-GE suspension was dropped on the surface of bare GCE, followed by evaporating the solvent in air. The modified electrode was referred to as N-GE/GCE. In the second step, the N-GE/GCE or GCE was dipped into the BFEE solution containing 10 mM carbazole, and the electrochemical polymerization of carbazole onto the electrode surface was carried out by CV from 0 to 1.4 V at the scanning rate of 100 mV s-1 , and the cycle number of polymerization was set as 8 in the further experiments unless specific remark. The modified electrode was referred to as PCZ/N-GE/GCE and PCZ/GCE, respectively.

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Fig. 1. (A) The first cycle of CV for the polymerization of 10 mM carbazole in BFEE at GCE (a) and N-GE/GCE (b). Scan rate: 100 mV s-1 . (B) CVs of N-GE/GCE recorded during electropolymerization of 10 mM carbazole in BFEE solution. Scan rate: 100 mV s-1 . The first 8 consecutive scans were depicted.

3. Results and discussion 3.1. Electrochemical preparation of PCZ/N-GE/GCE Fig. 1A shows the first cycle of CV for 10 mM carbazole polymerization in BFEE at the bare and N-GE modified GCEs. Obviously, both CV curves at two electrodes exhibit one anodic peak and two cathodic peaks. The onset oxidation potential of carbazole shifts from 0.8 V at the GCE (a) to 0.6 V at the N-GE/GCE (b). It means that, the electrochemical polymerization of carbazole at the N-GE/GCE was easier than that at the GCE, which is due to the electrocatalysis of N-GE. Moreover, the peak current at N-GE/GCE is higher than that at the bare GCE. It results from the higher specific surface area of NGE modified electrode. The PCZ film was obtained by scanning the potential between 0 and 1.4 V at a scan rate of 100 mV s-1 . The cyclic voltammograms (CVs, the first eight consecutive scans) of carbazole electropolymerization in BFEE solution are demonstrated in Fig. 1B. During the polymerization of carbazole, both cathodic and anodic peak currents increased with an increasing number of potential scans. Moreover, two couples of redox peaks (a1, c1 and a2, c2) appeared from the beginning of the second cycle. The first redox peaks at lower potentials (∼0.45 V and 0.23 V) are corresponding to the reversible redox processes of the dimmer [31]. The second redox peaks at around 1.1 V and 0.5 V can be ascribed to the formation of a new species of 3,6-polycarbazole [32]. The redox currents increased for successive potential cycles, indicating the progressive increasing amount of the polymer deposit. It also indicates that the PCZ films showed good redox activity [22]. The oxidation and

reduction potentials shifted positively and negatively, respectively. It also provides the information that the electrical resistance in the PCZ film was increasing and a higher over-potential was needed to overcome the resistance. The thickness of the PCZ film on the electrode surface was controlled by the number of potential cycles. 3.2. Characterization of PCZ/N-GE/GCE The electrochemical behaviors of the PCZ/N-GE/GCE, N-GE/GCE, RGO/GCE and bare GCE were investigated in 5.0 mM [Fe(CN)6 ]3-/4− solution by CV and EIS (Fig. 2). As seen in Fig. 2A, all CV curves exhibit one reversible redox peaks of [Fe(CN)6 ]3-/4− . However, the redox peaks show the best reversibility and the highest peak currents at PCZ/N-GE/GCE (a), compared with the N-GE/GCE (b), bare GCE (c) and RGO/GCE (d). This result indicates that, with comparison of bare GCE and RGO/GCE, the N-GE modified electrode is in favor of fast electron transfer due to the good electrochemical property of N-GE. Moreover, the coating of conducting PCZ on NGE/GCE makes the electrode more conductive and suitable as the transducer of electrochemical sensors. However, we find that, the CV response at RGO/GCE was less than that at the bare GCE, because the structure defects of RGO we used in current work led to a poor conductivity. EIS is used to investigate the construction process of the sensor. An ideal impedance spectrum is composed of a semicircle at higher frequencies and a straight line at lower frequency range [33]. The semicircle diameter of the EIS spectra reveals the electrontransfer kinetics of the redox probe at the electrode interface,

Fig. 2. CVs (A) and Nyquist plots of impedance spectra (B) of the PCZ/N-GE/GCE (a), N-GE/GCE (b), bare GCE (c) and RGO/GCE (d) in a 0.1 M KCl solution containing 5.0 mM [Fe(CN)6 ]3-/4− solution. Scan rate: 100 mV s-1 . The frequency range is from 0.1 to 106 Hz.

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Fig. 3. (A) XPS spectra of the RGO and the N-GE. (B) XPS N1s spectrum of the N-GE. The N1s peak can be split to three Lorentzian peaks at 398.9, 399.9 and 401.3 eV, which are labeled by pink, cyan and blue lines.

while the straight line is a typical characteristic of a diffusion limit step. Fig. 2B recorded the Nyquist plots of the PCZ/N-GE/GCE, N-GE/GCE, GCE and RGO/GCE using 5.0 mM [Fe(CN)6 ]3-/4− solution containing 0.1 M KCl solution as the marker ions at EIS frequency from 0.1 to 106 Hz. As seen in Fig. 2B, the impedance decreased in the order of RGO/GCE (d), GCE (c), N-GE/GCE (b) and PCZ/N-GE/GCE (a), indicating that the electron-transfer resistance decreased in the order. The relatively small semicircle diameter of the PCZ/N-GE/GCE revealed that PCZ and N-GE had an excellent ability to carry electrons over the electrode surface after modification, which resulted from the excellent conductivity of both components. The impedance changes of the modified electrode also indicated that N-GE and PCZ were successfully immobilized on the GCE. XPS is also used to confirm the successful doping of graphene with N, and the XPS data are shown in Fig. 3. As seen in Fig. 3A, the survey scan spectrum of RGO shows a predominant graphitic C1s peak at ca. 284.6 eV and an O1s peak at ca.532 eV. No obvious N peak could be detected in the spectrum of RGO. While an obvious distinct N peak at ca. 400 eV is observed for N-GE, N percentage of the N-GE reduced with urea is about 6.5%. It confirms the successful incorporation of N atoms into the graphitic layer of graphene. Fig. 3B shows the high-resolution of N1s spectra in N-GE. It is obvious seen that there are three peaks with binding energy located at about 398.9, 399.9 and 402.3 eV, which correspond to the ‘pyridinic’ N, ‘pyrrolic’ N and ‘graphitic’ N, respectively. It is reported that the ‘graphitic’ N is formed when the nitrogen atoms substitute the carbon atoms and dope into the graphene lattice [34]. The ‘pyridinic’ N and ‘pyrrolic’ N may contribute their p-electrons to participate in the form of ␲-conjugated system in the graphene. As shown in Fig. 3B, the intensities of the peaks for ‘pyridinic’ N and ‘pyrrolic’ N are much stronger than that for the ‘graphitic’ N, indicating that the N atoms are mainly in the form of ‘pyridinic’ N and ‘pyrrolic’ N. The doping of the two kinds of nitrogen atoms can increase the density of the electronic state near the Fermi level of the graphene and open the band gap of the graphene [35,36], which would make it possessing the excellent electrocatalytic activity. The morphologies of the modified electrodes with PCZ/N-GE and N-GE were examined by SEM and TEM. As shown in Fig. 4, N-GE/GCE exhibits the typical structure of wrinkled graphene sheets (Fig. 4A, 4B). However, with the presence of PCZ on the surface of N-GE, the surface of PCZ/N-GE becomes very loose and porous (Fig. 4C). Such a porous structure can provide a larger active specific surface for 4-NP analyte to diffuse from solution to the electrode surface, and is favor of the electron exchange and transfer.

3.3. Electrochemical behavior of 4-nitrophenol All 4-NP measurements were done in ABS at a pH value of 4.6. Fig. 4D shows the continuous cyclic voltammetry of the modified PCZ/N-GE/GCE electrode in 0.1 M ABS (pH 4.6) before and after addition of 1.0 × 10-4 M 4-NP. Before 4-NP was added into the ABS, there was no obvious redox peaks shown in the CV curve (dotted line). After 4-NP was added into ABS (solid line), peak 1 (R1) appeared at -0.665 V on the cathodic sweep during the first cycle from 0.6 to -0.8 V. It is an irreversible cathodic peak, which is contributed to the direct reduction of nitrophenol into hydroxylaminophenol with a four-electron and four-proton transfer process [37]. Then peak 2 (O2) was observed at +0.25 V on the anodic sweep of the first cycle. Its reversible reductive peak (R2) appeared at +0.186 V in the following cycles. Moreover, in addition to the irreversible reductive peak of R1, this couple of well-defined (O2/R2) can still maintain at around +0.2 V. It is due to the twoelectron transfer on the hydroxylamine group (-NHOH) to a nitroso group (-NO). Therefore, the electron transfer mechanism can be depicted in the following equations: R1 : -NO2 +4e- +4H+ → -NHOH + H2 O O2/R2 : -NHOH  -NO + 2H+ +2e− It is interesting that the R1 peak current gradually decreased and the peak potential shifted positively as increasing cyclic potential sweeps. This phenomenon is consistent with the previous reports [38]. Additionally, the peak current of R1 is obviously higher than that of R2. Considering the sensitivity and accuracy of determination, the R1 peak current in the first cathodic sweep was recorded for 4-NP analysis in the following studies. 3.4. Electrochemical behavior of 4-NP at various electrodes Electrochemical behaviors of 4-NP on PCZ/N-GE/GCE were investigated at different electrodes in ABS (pH 4.6). As shown in Fig. 5, the reduction peak of 4-NP appears at all electrodes. The reduction peak current of 4-NP at the bare GCE is the smallest (a), but it increases as the modification of GCE with RGO (e), PCZ (d), N-GE (b) and PCZ/N-GE (c). The peak currents of 4-NP at the PCZ/GCE, N-GE/GCE and PCZ/N-GE/GCE increase by 3.6, 5.6 and 12.4 times than that at the bare GCE, respectively. From the result of EIS, PCZ, N-GE and PCZ/N-GE exhibit higher conductivity than bare GCE. Therefore, the enhanced current densities at these electrodes may be explained by the remarkable electric conductivities of PCZ,

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Fig. 4. (A) Typical SEM image of N-GE. (B) TEM of N-GE. (C) SEM image of PCZ/N-GE. (D) Cyclic voltammetry of the modified PCZ/N-GE/GCE electrode in 0.1 M ABS before (dotted line) and after addition of 1.0 × 10-4 M 4-NP. pH:4.6. Scan rate: 100 mV s-1 .

N-GE and PCZ/N-GE facilitating the electron exchange and transfer. It further replies that N-doping plays an important role in improvement of electric properties of graphene. As for the RGO modified electrode, the enhanced current density should result from the improved electrochemical catalytic activity to 4-NP. On the other hand, compared to the reduction potential of 4-NP at the bare GCE, it positively shifted 36, 76 and 100 mV at the RGO/GCE, N-GE/GCE and PCZ/N-GE/GCE, respectively. It indicates that RGO/GCE, NGE/GCE and PCZ/N-GE/GCE show the obvious electrocatalytic activity to the reduction of 4-NP. Moreover, the electrocatalytic activities of these materials increase in the order of RGO < N-GE < PCZ/N-GE.

The enhancement of the electrochemical signal may be attributed to the remarkable conductivity and large surface area of the PCZ- and N-GE nanosheets, and the good synergistic effect between PCZ and N-GE. Furthermore, N-doping might play an important role in enhancing the electrocatalytic activity of graphene in electrochemical systems [39]. The incorporation of nitrogen atoms into the graphene layers enhanced the reactivity of the neighborly linked carbon atoms via alteration of the electronic structure [10]. The enhanced electrochemical performance of N-GE/GCE and PCZ/N-GE/GCE could also be attributed to the good catalytic ability of N atom in the N-GE. All the results demonstrate that the PCZ/N-GE/GCE exhibits the excellent electrochemical performance to the reduction of 4-NP, with comparison of other modified electrodes. Hence, PCZ/N-GE/GCE is a better choice for the electrochemical sensing of 4-NP. Therefore, PCZ/N-GE modified electrode was used for the further study. 3.5. Optimization of experimental conditions for determination of 4-NP

Fig. 5. Cyclic voltammetry of 1.0 × 10-4 M 4-NP in 0.1 M ABS (pH 4.6) at the bare GCE (a), the N-GE/GCE (b), the PCZ/N-GE/GCE (c), PCZ/GCE (d) and RGO/GCE (e). Scan rate: 100 mV s-1 .Potential window: -0.3 ∼ -0.8 V.

3.5.1. Effect of pH and electrolytes It is important for the chemically modified electrodes to choose buffer solution and the pH of buffer solution. Four different kinds of supporting electrolytes (pH 5.8) (0.1 M HAc–NaAc, 0.1 M NaH2 PO4 –Na2 HPO4 , borater buffer solution and 0.1 M citrate buffer solution) were tested in the current work. With comparison of the CV curves in four electrolytes, the reductive peak of 4-NP at the PCZ/N-GE/GCE in 0.1 M HAc–NaAc (pH 5.8) showed the largest current, the sharpest shape, and the most stable baseline. Therefore, 0.1 M HAc-NaAc was chosen as the electrolyte for further experiments.

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Fig. 6. Effect of pH of 0.1 M ABS (10 ␮L 0.25 mg mL-1 N-GE/DMF, 8 cycles PCZ) (A), the content of N-GE loading (pH 4.6, 8 cycles PCZ) (B) and the cycle number of PCZ polymerization (pH 4.6, 10 ␮L 0.25 mg mL-1 N-GE/DMF) (C) on the current response of 1 × 10-4 M 4-NP at PCZ/N-GE/GCE. Scan rate: 100 mV s-1 .

The effect of pH on the electrode reaction was investigated by CV in the pH ranging from 3.8 to 5.8. As can be seen in Fig. 6A, the cathodic peak current of 4-NP was dependent on the pH value. It increased with increasing the pH from 3.8 to 4.6, and the maximum current was obtained at pH 4.6. With further increasing pH, the peak current decreased. Therefore, pH 4.6 was selected as the optimum pH for the subsequent analytical experiments. 3.5.2. Effect of N-GE amount Fig. 6B shows the influence of the volume of N-GE/DMF suspension (0.25 mg mL-1 ) on the reduction peak current of 4-NP (1 × 10-4 M). The peak current of 4-NP increased gradually with the increment volume of N-GE/DMF suspension from 2 ␮L to 10 ␮L. The main reason is the good conductivity and the large surface area of the N-GE deposited on GCE. When further increasing the amount of the N-GE/DMF, the peak current of 4-NP decreased. It is because that the thick film of N-GE hindered the mass transport and the electron transfer. Therefore, 10 ␮L N-GE/DMF was chosen as the optimum amount modified GCE in this work. 3.5.3. Effect of PCZ film thickness The thickness of the PCZ film in fabrication of PCZ/N-GE/GCE was another key factor for the reduction of 4-NP. The amount of PCZ polymerized on the surface of N-GE/GCE was controlled by the cycle number of PCZ polymerization. Fig. 6C depicts the CVs of 4-NP in ABS (pH 4.6) at various PCZ/N-GE/GCEs with different amounts of PCZ obtained at various circle numbers of its polymerization. It can be seen that an obvious increase in the reduction peak current can be observed with increasing the cycle number of PCZ polymerization until eight circles. Further increasing the cycle number causes a decrease in reduction peak current, which is suggested that the PCZ film is overly thick. Consequently it hindered the mass transport or lowered the electron transfer. Therefore, considering the electrochemical activity and sensitivity of PCZ/N-GE/GCE to 4-NP, the optimized number of CV cycles for polymerization of PCZ was 8 in the following experiments. 3.5.4. Effect of scan rate Useful information involving electrochemical mechanism usually can be acquired from the relationship between the peak current and scan rate. CVs of PCZ/N-GE/GCE were recorded in 0.1 M ABS (pH 4.6) containing of 1 × 10-4 M 4-NP at different scan rates (Fig. 7A). The cathodic peak currents (Ipc ) increase with the increase of the scan rate from 2 to 250 mV s-1 . It can be seen that, with the increase of scan rate, the cathodic peak potential shifts more towards the negative potential. As shown in the Fig. 7B, linear relationships were obtained for the logarithmic peak currents vs. the logarithmic scan rate, and it can be expressed as: logIpc (␮A) = -0.43159+0.75199logV (mV s-1 ), R2 = 0.99936. The value of its slope (0.75) is higher than the

theoretical value of 0.5 for diffusion controlled process, but it is less than 1.0 which is theoretical value for the adsorption-controlled electrode process [40]. Therefore, the reduction of 4-NP is diffusion controlled adsorption process at the PCZ/N-GE/GCE. 3.6. Calibration curve 4-NP was detected by CV method at the PCZ/N-GE/GCE under the optimized experimental conditions, and the results were shown in Fig. 8A. It can be seen that the reduction peak current of 4-NP increases gradually with the addition of 4-NP. Fig. 8B illustrates the relationship between Ipc and the concentration of 4-NP. The peak current is proportional to the concentration of 4-NP in the range of 8 × 10-7 ∼2 × 10-5 M. The calibration equation is Ipc (␮A) = 0.6549+1.6531C (␮M) with a correlation coefficient 0.9971. The detection limit is estimated to be 0.062 ␮M (S/N = 3), lower than the recent reports as shown in Table 1. 3.7. Effective surface area of the PCZ/N-GE/GCE The effective surface area of the PCZ/N-GE/GCE and the bare GCE can be determined by the chronocoulometry technique using 5 × 10-4 M [Fe(CN)6 ]3- solution containing 0.1 M KCl as the probe molecules. The function of charge (Q) vs time (t) is given by Anson [47]. Q (t) = (2nFAcD1/2 t 1/2 )/1/2 + Qdl + Qads where F is the Faraday constant, c is the concentration of the [Fe(CN)6 ]3− solution, n is and diffusion coefficient according to the Table 1 Comparison of different chemically modified electrodes for the determination of 4-NP with PCZ/N-GE modified electrode. Modified electrode

Linear range (␮M)

Detection limit (␮M)

References

DTDa /AgNPb /CPEc Nano-gold/GCE HA-NPd /GCE MIPe /Au GEf /SPEg MWNTh /SPE GE-PANIi /GCE PCZ/N-GE/GCE

1-100 10-1000 1.0-300 0.1-140 25-620 10-620 0.2-20,20-100 0.8-20.0

0.25 8 0.6 0.1 0.6 1.3 0.065 0.062

[41] [42] [43] [44] [45] [45] [46] This work

a DTD: 6,7,9,10,17,18,19,20,21,22,-decahydro[h,r][1,4,7,11,15] rioxadiazacyalonano -decine-16,23-dione. b AgNP: Ag nanoparticle. c CPE: carbon paste electrode. d HA-NP: hydroxyapatite nanopowder. e MIP: macroporous imprinted polymer. f GE: graphene. g SPE: screen printed electrode. h MWNT: multi-wall carbon nanotube. i PANI: polyaniline.

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Fig. 7. (A) CVs of 1 × 10-4 M at PCZ/N-GE/GCE in ABS (pH 4.6) at scan rate from 2 to 250 mV s-1 (inside-outside). (B) The logarithmic peak currents with respect to the logarithmic scan rate.

Fig. 8. (A) CVs of PCZ/N-GE/GCE in 0.1 M ABS (pH 4.6) containing 4-NP with the concentration ranging from 0.8 to 20 ␮M, scan rate: 100 mV s-1 . (B) The dependence of peak currents on the concentrations of 4-NP.

literature value [48], Qdl is the double layer charge. The relationship between Q and t are shown in Fig. 9A. Fig. 9B depicts the relationship between Q and t1/2 . The effective surface area A could be calculated from the slope of the plot of the Q and t1/2 . It is 0.0105 and 0.0442 cm2 for the bare and PCZ/N-GE modified GCEs, respectively. The effective surface area of the PCZ/N-GE/GCE is four times as much as that of the bare GCE, which is consistent to the result of SEM images of N-GE and PCZ/N-GE modified electrodes. The large surface area of PCZ/N-GE modified electrode can increase the adsorption capacity of 4-NP, which further enhances the current response and sensitivity.

3.8. Reproducibility, stability and interference The repeatability of PCZ/N-GE/GCE for the established current response in 1.0 × 10-4 M 4-NP was tested. The peak current of the 4-NP was determined with five different electrodes which were fabricated in the same conditions, showing that the relative standard deviation (RSD) was 3.5%. It indicates that the electrodes in the study displayed an acceptable reproducibility. The stability of the PCZ/N-GE/GCEs was also investigated. The PCZ/N-GE/GCE was stored in ABS (pH 4.6) at 4 ◦ C in the refrigerator when not being in use. The current response to 4-NP decreased to 96.0% after 1 day,

Fig. 9. (A) Chronocoulometry curves for PCZ/N-GE/GCE (a) and the bare GCE (b) in 5 × 10-4 M [Fe(CN)6 ]3− solution containing 0.1 M KCl. Initial potential: - 0.1 V; final potential: 0.4 V; number of steps: 2; pulse width: 0.25 s; sample interval: 0.002. (B) linear relationship between Q and t1/2 for the PCZ/N-GE/GCE (a) and the bare GCE (b) (the conditions are the same as in Fig. 7).

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while 90.0% of the original current response remained after 5 days, indicating the PCZ/N-GE/GCE has acceptable stability. The influences of several inorganic and organic species on the current response of 4-NP were investigated. As shown in Table S1, it can be seen that most of inorganic species, such as Na+ , Mg2+ , K+ , Ca2+ , Cu2+ , Ni2+ , SO4 2- , Cl- , NO3 − hardly affect the determination of 4-NP with deviations below 4.5%. Both phenol and some substituted derivatives are considered to be no obvious influence on the response of 4-NP with derivations of below 5.0%. However, 2Nitrophenol and 3-Nitrophenol, because of having the same nitro groups, can be reduced at the close potential of 4-NP, which influence the response of 4-NP with derivations of 8.2% and 9.5% (Table S1). Therefore they may interfere with the determination of 4-NP. 3.9. Analytical application The PCZ/N-GE/GCE was applied to determination of 4-NP in the practical water samples (river, Changjiang river; lake water, Xuanwu lake, Nanjing, China; tap water). Before determination, the water samples were filtered to remove the solid material. The water samples pH were adjusted to 4.6 with HAc and NaOH. With CV technique and HPLC testing, 4-NP can not be found in these water samples when analyzed. Therefore the standard addition method was used for the analysis of the samples. The experiment results of the different water samples are listed in Table S2. It is found that the recoveries of samples are between 97.9% and 104.5%. The RSD values are less than 4.8%. The results obtained are in good agreement with the real addition. It replies that the PCZ/N-GE/GCE could be used for the determination of 4-NP in real samples. 4. Conclusion In this work, a novel electrochemical sensor was successfully constructed for the detection of 4-NP based on the PCZ/N-GE modified GCE which was prepared by a two-step method. The electrode fabrication was easy and simple. The modified electrode exhibited excellent electrocatalytic activity towards the reduction of 4-NP with a low detection limit of 0.062 ␮M. The proposed sensor could determine 4-NP present in real water samples with good stability and reproducibility. The excellent electrochemical performance of the sensor is owing to the synergistic effect of the combination of N doped graphene and conducting polymer of PCZ. Acknowledgements The work was supported by the National Natural Science Foundation of China (No. 21103092), Program for NCET-12-0629, the Fundamental Research Funds for the Central Universities (No. 30920130111003), the Ph.D. Programs Foundation of Ministry of Education of China (No. 20133219110018), Qing Lan Project (2012), PAPD, and Six Major Talent Summit (XNY-011), the Science and Technology Support Plan of Jiangsu Province, China (Nos. BE2011835 and BE2013126). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2014.08.153. References [1] 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.

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