Graphene quantum dots modified glassy carbon electrode via electrostatic self-assembly strategy and its application

Electrochimica Acta 190 (2016) 455–462

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Graphene quantum dots modified glassy carbon electrode via electrostatic self-assembly strategy and its application Xuan Jian, Xian Liu, Hui-min Yang, Min-min Guo, Xiu-li Song, Hong-yan Dai, Zhen-hai Liang* College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, 030024 Shanxi, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 13 October 2015 Received in revised form 6 January 2016 Accepted 6 January 2016 Available online 8 January 2016

An electrostatic self-assembly strategy for the preparation of graphene quantum dots (GQDs) attached to the surface of a glassy carbon electrode (GCE) has been developed. The GQDs were prepared by tuning the carbonization degree of citric acid and characterized by atomic force microscopy, transmission electron microscopy, Raman spectroscopy and UV–vis absorption spectroscopy. In addition, the electrochemical behaviours of hydroquinone (HQ) and catechol (CC) on the resulting modified electrodes were investigated. Under optimum conditions, the as-prepared GQDs modified electrode exhibited high electrochemical activity and good selectivity for the oxidation of HQ and CC, the linear ranges for HQ and CC were 4.0
600 mM and 6.0
400 mM respectively, and the detection limit was 0.40 mM and 0.75 mM, respectively. The modified electrode was also applied for the detection of real samples with satisfactory results. ã 2016 Elsevier Ltd. All rights reserved.

Keywords: Graphene quantum dots electrostatic self-assembly strategy electrochemical sensing hydroquinone catechol

1. Introduction Hydroquinone (HQ) and catechol (CC) are two isomers of phenolic compounds that are widely used in pharmaceuticals, cosmetics, antioxidants, photography and dyes. Unfortunately, these phenolic compounds are considered to be significant toxic airborne environmental pollutants. Therefore, the development and establishment of a reliable analytical method for the determination of HQ and CC is crucial. Various methods, such as high performance liquid chromatography (HPLC) [1], gas chromatography-mass spectrometry (GC-MS) [2], chemiluminescence [3], capillary electro-chromatography [4], and electrochemical methods [5], have been used to determine HQ and CC in different samples. Among these methods, electrochemical methods have received considerable attention for HQ and CC detection due to high sensitivity, good selectivity and simplicity. However, improvement in the selectivity poses a major challenge for the electrochemical determination of HQ and CC, because these two isomers of phenolic compounds possess similar structures, electrochemical properties and coexistence of usual interference. Several advanced materials with superior electrocatalytic activity (i.e., graphene [6], g-C3N4, carbon nanotubes composites [7] and

* Corresponding author. Tel.: + 86 351 6018193; fax: + 86 351 6018193. E-mail address: [email protected] (Z.-h. Liang). http://dx.doi.org/10.1016/j.electacta.2016.01.045 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.

several other metal-nanoparticals [8,9]) have been introduced on electrode interfaces to address these problems. Graphene quantum dots (GQDs), which are a new member of the carbon nanomaterial family, have attracted considerable scientific attention due to their unique, excellent physical and chemical properties. Owing to better quantum confinement, more edge defects, high aqueous solubility, strong photoluminescence (PL) emission, desirable fluorescence properties and low cytotoxicity, GQDs have been developed for applications in various fields, such as photocatalysis [10], biosensing [11] and chemiluminescence analysis [12]. GQDs can be prepared using two strategies, including the top-down method and the bottom-up growth process. Most top-down methods include cutting graphene oxide (GO) by a hydrothermal method [13] and graphitic exfoliation by an electrochemical route [14]. However, the bottom-up process primarily involves carbonization of some special organic precursors, especially citric acid (CA) via thermal treatment [15]. In comparison, the bottom-up growth process is widely used in the characteristics of its simple operation, low environmental pollution, low cost, better purity and aqueous solubility. Recently, much attention has been focused on electrostatic selfassembly for the preparation of electrochemical sensing interfaces due to the simple procedure and structural control [16–18]. The electrostatic self-assembly strategy is a versatile nanofabrication technique, which exhibits remarkable advantages over conventional methods in terms of versatility and simplicity. In addition,

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this method allows for molecular-level control over the structure and composition with simple benchmark operations [19]. To the best of our knowledge, the application of GQDs for electrochemical sensors has been rarely explored. In the current study, GQDs were prepared by tuning the carbonization degree of citric acid and followed by characterization using various techniques. Then, the GQDs were attached to the surface of a glassy carbon electrode (GCE) using an electrostatic self-assembly strategy. The GQDs modified electrode was used to investigate the electrochemical behaviour of HQ and CC. Benefiting from the advantages of GQDs, the GQDs modified electrode exhibited excellent performance, such as high electrochemical activity, good selectivity and high sensitivity for the simultaneous determination of HQ and CC. The main contribution of this study is the preparation of the GQDs modified electrode using an electrostatic self-assembly strategy. 2. Experimental 2.1. Chemicals and Apparatus Citric acid (CA, A.R.), Hydroquinone (HQ, A.R.) and Catechol (CC, A.R.) were purchased from the Tianjin Kemiou Chemical Reagent Co., Ltd (Tianjin, China). Poly (diallyldimethylammonium chloride) (PDDA, MW < 100 000) was purchased from Aladdin (Shanghai, China). Phosphate buffer solutions with various pH values were prepared by mixing 0.10 mol L1 Na2HPO4 and NaH2PO4. All of the other chemical reagents were of analytical grade and used without further purification. Double distilled ultrapure water with an electric resistance >18.3 MV was used for the preparation of all the solution. The electrochemical experiments were carried out with a CHI 660D (CH Instruments, Inc., Shanghai, China) electrochemical workstation and VMP2 potentiostat controlled by the EC-Lab software (Princeton, USA). A bare glassy carbon electrode (GCE) or the modified GCE were used as the working electrode (WE), a Pt wire was applied as the counter electrode (CE), and all of the potentials were referred to a saturated calomel electrode (SCE). Atomic force microscopy (AFM) was performed using a multimode nanoscope IIIa controller (Veeco, USA). Transmission electron microscopy (TEM) was performed with a Tecnai G220 (FEI, USA) operating at 200 kV. The Raman spectrum was recorded using laser confocal micro-Raman spectroscopy (LabRAM HR800, France). The light absorption properties of the samples were

recorded on an ultraviolet-visible (UV-Vis) spectrophotometer (UV-2450, Shimadzu Corporation, Japan) 2.2. Synthesis of GQDs The GQDs were synthesized using pyrolysis CA method based on a previous study [15]. Briefly, 2.0 g of CA powder was placed into a 10 mL breaker and heated to 473 K for 30 min until the CA was liquated and the colour of the CA liquid changed from colourless to orange. Then, 100 mL of a 10 mg mL1 NaOH solution was added dropwise into the obtained orange CA liquid under vigorous stirring followed by adjustment of the pH value to 7. Finally, the aqueous GQDs solution was obtained. 2.3. Preparation of GQDs modified electrode GQDs/GCE was fabricated using an electrostatic self-assembly strategy (Fig. 1). Prior to use, the bare GCE (3 mm diameter) was carefully polished by 0.3 mm and 0.05 mm Al2O3 powder slurries for several times to produce a mirror like finish. Then the electrode was ultrasonically washed with distilled water and ethanol for several minutes and dried in blowing N2 prior to use. The activated GCE (record as OH/GCE) was prepared according to the previously reported protocols [20]. The clean GCE was activated in 0.05 mol L1 H2SO4 by CV in a range from 0.0 to 2.0 V at a scan rate of 50 mV s1. Next, the OH/GCE was washed with copious amounts of water and dried in flowing N2. Then, the OH/GCE was immersed in a 0.3 wt% solution of poly (diallyldimethylammonium chloride) (PDDA, MW < 100 000) for 10 min to prepare a positively charged surface. Finally, the electrode was immersed in the abovementioned GQDs solution for 15 min, rinsed with deionized water and dried with N2 to afford the GQDs/GCE. 3. Result and discussion 3.1. Preparation and characterization of GQDs In the current study, the GQDs were prepared using the pyrolysis CA method based on a previous study [15]. Fig. 2A and 2B show the typical AFM images of the resulting GQDs. Based on these images, the as-prepared GQDs are highly dispersed on the substrate (Fig. 2A) with a typical topographic height distribution from 0.5
1.5 nm (1.1 nm average thickness), signifying that most of the GQDs typically consist of ca. 1
4 graphene layers (Fig. 2B) [21].

Fig. 1. Schematic drawing of the synthesis of GQDs from pyrolysis citric acid and electrochemical oxidize HQ and CC on GQDs/GCE.

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Fig. 2. (A) An AFM image of GQDs. (B) The height profile along the white line in picture A. (C) TEM image of as-prepared GQDs. (D) A high-resolution TEM (HRTEM) image of GQDs nanosheet (inset fringe pattern).

3.2. Characterization of GQDs modified electrode Fig. 4 shows the electrochemical behaviour of the bare GCE and GQDs/GCE in a 0.50 mol L1 [Fe(CN)6]3/4 (1:1) and 0.10 mol L1 KCl solution. Fig. 4. A displays a pair of redox peaks for GQDs/GCE with a higher redox peak current compared to that of the bare GCE. This higher electrochemical activity toward Fe(CN)6]3/4 was due to the unique electronic properties of the GQDs. A typical Nyquist

G-Band sp2 1640

Intensity (a.u.)

To further characterize the GQDs, the HRTEM image of the GQDs is shown in Fig. 2C and 2D. The results in Fig. 2C indicate that the assynthesized GQDs are uniform and monodisperse. From Fig. 2D, the as-prepared GQDs was primarily composed of a nanosheet that was approximately 8 nm, and the inset of figure indicates that the high crystallinity of the GQDs were in good agreement with the 0.242 nm (1120) lattice parameter of graphene [22]. In conclusion, these results revealed that the GQDs have been successfully prepared through incompletely carbonization of CA [15]. Raman spectroscopy was employed to characterize the structure of the GQDs, as shown in Fig. 3. The G band corresponds to the first-order scattering of the E2g mode observed for sp2 carbon domains, and the D band is associated with the disorder band of a structural defect, amorphous carbon or an edge [23]. It is obviously see that two characteristic peaks, generally named D (
1408 cm1) and G (
1640 cm1) bands, were observed for the GQDs prepared via pyrolysis CA. The relative intensity of the “disorder” D-band to the crystalline G-band (ID/IG), as well as the sp3/sp2 carbon ratio for the GQDs were calculated to be approximately 0.79, which indicates that oxygen-containing groups were introduced to the edges and onto the basal plane of the GQDs [22]. In addition, UV–vis absorption spectra were used to characterize the as-prepared GQDs (See Fig. S1). In conclusion, based on these analyses, we can reasonable assert that GQDs through pyrolysis CA was successfully obtained.

D-Band sp3 1408

ID / IG = 0.79

1200 1300 1400 1500 1600 1700 1800 1900

Raman Spectra (cm-1) Fig. 3. Raman spectrum of the as-synthesized GQDs.

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Fig. 4. (A) Cyclic voltammograms recorded at GCE (dash-dot line) and GQDs/GCE (solid line) for 0.50 mol L1 [Fe(CN)6]3/4 in 0.10 mol L1 KCl solution at scan rate of 50 mV s1. (B) Nyquist plots of EIS of bare GCE and GQDs/GCE in 0.50 mol L1 [Fe(CN)6]3/4 (1:1) + 0.10 mol L1 KCl solution at the applied ac frequency range: 0.05 Hz– 100 kHz, the insert was the equivalent circuit.

plots is shown in Fig. 4B, and in comparison to bare GCE, a significant decrease in the semicircular Nyquist plots was observed with the GQDs/GCE. Electrochemical impedance spectroscopy (EIS) can be employed to determine the interfacial changes in the electrode surface during the modification process and the equivalent circuit model used to fit the interfacial changes is displayed in the inset in Fig. 4B. This equivalent circuit includes the ohmic resistance of the electrolyte (R1), R2 is the electron-transfer resistance, double layer capacitance (C1) and Ws1 is the Warburg impedance. The Nyquist plots from EIS include a semicircular part at high frequency, which represents the electron-transfer limited process, and the diameter is equivalent to R2. The linear portion at lower frequency represents the diffusion limited electron transfer process. Based on these images, the R2 value for [Fe(CN)6]3/4 corresponding to the bare GCE was detemined to be approximately 800 V. After the GCE modification with the GQDs, the R2 value becomes about 200 V, indicating that GQDs was successfully attached to the surface of the GCE using the electrostatic selfassembly strategy. This electrode exhibited better electrochemical properties and promotion of the electron transfer process than the bare GCE. 3.3. Electrochemical behaviours of HQ and CC on GQDs/GCE The electrochemical behaviours of HQ and CC on different modified electrodes in 0.10 mol L1 PBS (pH 6.0) were investigated by cyclic voltammetry (CV). Fig. 5A shows the CV responses of 0.40 mM HQ and 0.40 mM CC with bare GCE, OH/GCE and GQDs/ GCE. Moreover, no evident oxidation-reduction peak for HQ and CC was observed at the bare GCE, indicating low electrochemical activity. After the electrode was activated in the H2SO4 solution (record as OH/GCE), two small redox peaks appeared at 181 mV and 280 mV. These peaks indicate that the surface-activated GCE can separate these two phenol isomers due to the introduction of oxygen-containing functional groups (such as OH) on the electrode surface. The oxygen containing functional groups can weaken the energy of the hydroxyl, as mentioned in a previous report [24]. However, higher electrochemical responses for HQ and CC were obtained on GQDs/GCE. A pair of obvious redox peaks corresponding to HQ and CC appeared at 194 mV and 306 mV, respectively. Furthermore, the peak-to-peak separation between HQ and CC was approximately 112 mV. Notably, the substantial increase in the background current at the GQDs/GCE, implied that the surface area of the GQDs/GCE is larger. Fig. 5B shows the differential pulse voltammetry (DPV) graphs of 0.2 mM HQ and 0.2 mM CC on bare GCE and GQDs/GCE, and the two well-defined

DPV peaks with good separation were observed from the oxidation of HQ and CC on the GQDs/GCE. However, no obvious response was observed on the bare GCE, indicating that HQ and CC can be effectively distinguished and accurately detected simultaneously at the GQDs/GCE. As expected, all of these results indicate that the GQDs/GCE possesses a higher electrochemical activity for HQ and CC. To further improve the analytical performance of this GQDs/ GCE, the electrochemical behaviour of HQ and CC were carefully investigated including the effects of pH and the potential scan rate. The effect of different pH value on the electrochemical behaviour of HQ and CC on the GQDs/GCE were investigated and the results are shown in Fig. 6. The peak currents of both HQ and CC decreased gradually with the increased of pH value from 6.0 to 8.0. In addition, the oxidation peak potential (Epa) and reduction peak potential (Epc) exhibit a linear relationship with the pH value from 6.0 to 8.0 (insert graph of Fig. 6). The regression equations are as follows: Epa (mV) = 71.72pH + 627.7, R = 0.9981, and Epc (mV) = 64.66pH + 536.1, R = 0.9998 for HQ; and Epa (mV) = 69.4pH + 709.2, R = 0.9987, and Epc (mV) = 64.4pH + 640.4, R = 0.9992 for CC, respectively. The results are close to the potential shift predicted by the Nernst equation (dEp/dpH = 2.303 mRT/nF), indicating that the electrode reactions of HQ and CC on the GQDs/GCE involve equal numbers of protons and electrons (Scheme 1). However, higher current responses and peak to peak separation of HQ and CC were obtained at a pH of 6.0. Therefore, a PBS buffer with pH 6.0 was chosen as the supporting electrolyte solution in the present work. The kinetic parameters for HQ and CC on the GQDs/GCE, were obtained by investigating the effect of the potential scan rate. As shown in Fig. 7, the peak currents of both HQ and CC (Ipa and Ipc) increased linearly with the increase in the potential scan rate from 20 mV s1 to 100 mV s1 (Fig. 7 inserts-A (i) and B (i)). For HQ, the redox peak currents follow a linear regression equation of Ipa = 10.22 v + 0.35 (mA, mV s1, R = 0.9987) and Ipc =  7.30 v  0.24 (mA, mV s1, R = 0.9983). For CC, the redox peak currents also follow a linear regression equation as follows: Ipa = 16.62 v + 0.36 (mA, mV s1, R = 0.9977) and Ipc =  11.04 v  0.23 (mA, mV s1, R = 0.9936). These results demonstrate that the electrode reaction for HQ and CC on the GQDs/GCE are surface-controlled process. Although the oxidation and reduction peak potentials of both HQ and CC were shifted positively and negatively, the formal potential of these two compounds does not shift as the potential scan rate increased. These results indicate that the electrode reaction of both HQ and CC are surface-controlled quasi-reversible processes. Based on Laviron's theory [25], for a surface-controlled

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Fig. 5. (A) CV graphs of 0.40 mM HQ and 0.40 mM CC at different electrodes (GCE, OH/GCE and GQDs/GCE) in 0.10 mol L1 PBS (pH 6.0), scan rate 50 mV s1, (B) DPV graphs of 0.20 mM HQ and 0.2 mM CC at the GQDs/GCE and bare GCE.

Fig. 6. CV of 0.4 mM HQ (A) and 0.2 mM CC (B) in different pH value from 6.0 to 8.0 PBS solution on GQDs/GCE, respectively. Insets are the dependence of Epa and Epc of HQ (i) and CC (ii) on the different pH value.

quasi-reversible process, the peak-to-peak potential separation DEp was higher than 0.20 V/n when the potential scan rate is sufficiently large, and the potential peak was linear with respect to the logarithm of the sweep rate. Based on this theory, the electron transfer coefficient (a) and the electron transfer rate constant (ks) can be calculated. The insets in Fig. 7 provide the relationship for

OH

O +

+ 2H + 2e OH

-

O

HQ HO

O +

+ 2H + 2e HO

CC

-

O

Scheme 1. Schematic representation of redox reduction of HQ and CC.

the peak potential of HQ and CC with respect to the he logarithm of sweep rate. Based on these results, a good linear relationship was obtained between Ep and log v for both HQ and CC when the scan rate is higher than 0.28 V s1. The linear equations are as follows: for HQ, Epa = 0.098 log v + 0.276 (V, V s1, R = 0.9982) and Epc = 0.080 log v + 0.061 (V, V s1, R = 0.9980), and for CC, Epa = 0.089 log v + 0.360 (V, V s1, R = 0.9982) and Epc = 0.045 log v + 0.187 (V, V s1, R = 0.9981). Therefore, the electron transfer coefficient (a) was calculated to be 0.55 and 0.66 for HQ and CC, respectively, based on the slopes of the two straight lines of Ep vs. log (v) using the following equation [25]: log

ka a ¼ log kc 1a

ð1Þ

Where ka and kc are the a slope of the straight lines for Epa vs. log (v) and Epc vs. log (v), respectively. The electron transfer rate constant (ks) can be calculated according to the following equation [25]: RT Þ logks ¼ alogð1  aÞ þ ð1  aÞloga  logð nFv ½að1  aÞnF DEp  2:3RT

ð2Þ

Where a is the electron transfer coefficient, v is the scan rate, DEp is the peak-to-peak potential separation, F is Faraday’s constant, R is the fundamental constants of physics and T is 298 K for a reaction at room temperature. n is the number of electrons involved in the reaction, and the number of electrons involved in the reaction of

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Fig. 7. CV graphs of 0.4 mM HQ (A) and 0.4 mM CC (B) in pH = 6.0 PBS solution on GQDs/GCE at different scan rates from 20 mV s1 to 100 mV s1. Insert: (i) the plot of the anodic current peak (Ipa) and cathodic peak current (Ipc) vs. the scan rates for HQ and CC, respectively. (ii) the plot of the peak potential vs. the log (v) for HQ and CC, respectively.

HQ and CC is 2 for both in the previous studies [6,26]. Thus, the value of the electron transfer rate constant (ks) was calculated to be 0.70 cm s1 and 1.58 cm s1. In comparison to ks in other previous literature reports [27–29], we observed that the resulting GQDs/ GCE demonstrated a faster electron transfer process, indicating that the GQDs can effectively promote electron transfer.

3.4. Simultaneous determination of HQ and CC The DPV technique can exhibit excellent performance for the simultaneous and quantitative determination of HQ and CC at the GQDs/GCE. As shown in Fig. 8A, the oxidation peak current gradually increased with increasing concentration of HQ from 0 to

Fig. 8. (A) DPV of HQ with different concentrations (0, 4, 10, 20, 40, 100, 200, 400 and 600 mM) in the presence of 20 mM CC. (B) DPV of CC with different concentrations (0, 6, 10, 20, 60, 100, 200 and 400 mM) in the presence of HQ. (C) (i) and (ii) show the calibration plots of the peak current vs. the concentration of CC and HQ. (D) DPV for simultaneous determination of HQ and CC with vairous concentrations (0, 20, 50, 100, 200, 500 and 600 mM).

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Table 1 Comparison of different modified electrodes for determination of HQ and CC. Some carbon nanomaterial modified electrodes

Linear range (mM) HQ

CC

HQ

CC

Graphene-CS AuNPs/Fe3O4-APTES-GO Porous graphene CNT-g-C3N4 CNCs-RGO Laser reduced graphene Pt/ZrO2-RGO Graphitic mesoporous carbon/ionic liquids comosite film Boron-doped graphene The layered tungsten sulfide-graphene Nafion/MWCNTs/CDs/MWCNTs GQDs

1.0
300 3.0
137 5.0
90 1.0
250 1.0
300 1.0
300 1.0
1000 0.1
50

1.0
400 2.0
145 5.0
120 1.0
200 1.0
400 2.0
300 1.0
400 0.1
50

0.75 1.10 0.08 0.13 0.87 0.50 0.40 0.05

0.75 0.80 0.18 0.09 0.40 0.80 0.40 0.06

[29] [30] [31] [7] [32] [33] [34] [35]

5.0
100 1.0
100 1.0
200 4.0
600

1.0
75 1.0
100 4.0
200 6.0
400

0.30 0.01 0.07 0.40

0.20 0.02 0.06 0.75

[36] [37] [38] This work

600 mM in the presence of 20 mM CC. Based on the results in Fig. 8C (ii), the oxidation peak current exhibits a good linear relationship with the concentration of HQ, and the linear regression equation is I (mA) = 3.56 + 0.051CHQ (mM), with an HQ correlation coefficient of 0.9962. The detection limit was estimated to be 0.40 mM (S/N = 3). Similarly, the oxidation current of CC gradually increased with increasing concentration of CC from 0 to 400 mM and the linear regression equation was I (mA) = 1.18 + 0.069CCC (mM), R = 0.9984 with a detection limit that was estimated to be 0.75 mM (S/N = 3), as observed in Fig. 8B and 8C(i). We also further investigated the DPV for simultaneous determination of HQ and CC with different concentrations from 0 to 600 mM on the GQDs/GCE, and the results are shown in Fig. 8D. The separation between the two peak potentials was sufficiently large to determine HQ and CC individually or synchronously. Evidently, the oxidation peak currents of HQ and CC are linearly correlated with their concentrations. All of the above experimental results indicate that the GQDs/GCE can successfully and simultaneously be employed for the selective and sensitive determination of CC and HQ in mixed systems without interference from each other. In addition, in comparison to other carbon nanomaterial modified GCEs, the GQDs/GCE exhibits comparatively high selectivity and sensitivity, as displayed in Table 1.

Detection limit (mM)

Reference

stability and reproducibility were investigated under the optimized conditions by determination of a mixed solution with 0.4 M HQ and CC in a pH = 6.0 PBS solution at the same GQDs/ GCE seven times, and the relative standard deviation (RSD) for HQ and CC were both less than 5%. When the GQDs modified electrodes were stored at 298 K for two weeks, the peak currents retained more than 95% of their initial values. These result reveals that the GQDs/GCE demonstrate an excellent stability and reproducibility. 3.6. Real samples analysis To evaluate the validity of our electrode for use with real examples, samples containing HQ and CC were prepared from a local river (Fenhe River, Taiyuan, China) and tested using quantitative analysis by the standard addition method. The results indicated that none of the target analytes were detected in the river. Therefore, a standard solution was added to the practical samples and to determine the recovery values, which ranged from 92.3 to 105%, as shown in Table 2. These findings indicate the practical applicability and good reliability of the GQDs/GCE for the simultaneous determination of HQ and CC. 4. Conclusion

3.5. Selectivity, stability and reproducibility of GQDs/GCE The selectivity of the resulting modified electrode for the determination of HQ and CC (each 0.2 M) was investigated using DPV. The experimental results demonstrate that commonexisting substances including 100-fold concentrations of inorganic ions such as Na+, Ca+, K+, Cl, NH4+, NO3, SO42, Mg2+, and Cu2+, as well 10-fold concentrations of organic compounds, such as phenol, glucose, ethanol and uric acid, do not any interfere with the determination of HQ and CC (signals change below 5%). This reslut indicates the excellent selectivity of the GQDs modified electrode. The stability and reproducibility are important parameters for evaluating the applicability of an electrochemical sensor. The Table 2 The determination result of the practice samples. Real samples

HQ HQ HQ HQ HQ

CC CC CC CC CC

Added (mM)

Found (mM)

Recovery (%)

RSD (%)

HQ

CC

HQ

CC

HQ

CC

HQ

CC

20 30 50 100 200

20 30 50 100 200

18.23 31.05 51.34 98.74 206.84

19.52 29.86 50.25 102.34 209.92

91.1 100.4 102.7 98.7 103.4

97.6 99.5 100.5 102.3 105

3.12 1.28 1.41 1.28 2.85

2.73 1.06 1.37 2.69 2.67

In summary, we have reported a novel approach for preparing GQDs/GCE via an electrostatic assembly strategy. In addition, this modified electrode was carried out for the simultaneous electrochemical determination of HQ and CC with a wide linear range, low detection limit, high sensitivity, good stability and excellent reproducibility. The results from the current study indicate that GQDs have potential applications in the field of electrochemical sensing. Acknowledgement This work is jointly funded by the National Natural Science Foundation of China and Shenhua Group Corp (Grant nos. U1261103), the National Natural Science Foundation of China (Grant nos. 20771080), and the graduate education innovation projects of Shanxi provincial Department (Grant nos. 2015BY19). The authors also acknowledge the Institute of Coal Chemistry, Chinese Academy of Sciences for technical assistance. 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.2016.01.045.

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