Analytica Chimica Acta 899 (2015) 57e65

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The Cu-MOF-199/single-walled carbon nanotubes modified electrode for simultaneous determination of hydroquinone and catechol with extended linear ranges and lower detection limits Jian Zhou a, Xi Li a, *, Linlin Yang a, Songlin Yan a, Mengmeng Wang a, Dan Cheng a, Qi Chen a, Yulin Dong a, Peng Liu a, Weiquan Cai a, Chaocan Zhang b, ** a b

School of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, Wuhan 430070, PR China School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Cu-MOF-199/SWCNTs/GCE was facilely fabricated by the electrodeposition on SWCNTs/GCE.
An electrochemical sensor for detecting HQ and CT was constructed based on this modified electrode.
The proposed electrochemical sensor showed an extended linear range and lower detection limits.
The proposed electrochemical sensor had an excellent stability and reproducibility.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 July 2015 Received in revised form 25 September 2015 Accepted 29 September 2015 Available online 8 October 2015

A novel electrochemical sensor based on Cu-MOF-199 [Cu-MOF-199 ¼ Cu3(BTC)2 (BTC ¼ 1,3,5benzenetricarboxylicacid)] and SWCNTs (single-walled carbon nanotubes) was fabricated for the simultaneous determination of hydroquinone (HQ) and catechol (CT). The modification procedure was carried out through casting SWCNTs on the bare glassy carbon electrode (GCE) and followed by the electrodeposition of Cu-MOF-199 on the SWCNTs modified electrode. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) were performed to characterize the electrochemical performance and surface characteristics of the as-prepared sensor. The composite electrode exhibited an excellent electrocatalytic activity with increased electrochemical signals towards the oxidation of HQ and CT, owing to the synergistic effect of SWCNTs and Cu-MOF-199. Under the optimized condition, the linear response range were from 0.1 to 1453 mmol L1 (RHQ ¼ 0.9999) for HQ and 0.1e1150 mmol L1 (RCT ¼ 0.9990) for CT. The detection limits for HQ and CT were as low as 0.08 and 0.1 mmol L1, respectively. Moreover, the modified electrode presented the good reproducibility and the excellent anti-interference performance. The analytical performance of the developed sensor for the simultaneous detection of HQ and CT had been evaluated in practical samples with satisfying results. © 2015 Elsevier B.V. All rights reserved.

Keywords: Cu-MOF-199 Single-walled carbon nanotubes Hydroquinone Catechol Simultaneous determination Electrocatalysis

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C. Zhang).

1. Introduction (X.

http://dx.doi.org/10.1016/j.aca.2015.09.054 0003-2670/© 2015 Elsevier B.V. All rights reserved.

Li),

[email protected]

Hydroquinone (HQ) and catechol (CT) are two isomers of

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dihydroxybenzene which widely exist in industrial wastes, such as cosmetics, dye, pesticides, and pharmaceutical industries. They are considered as crucial environmental pollutants because of their high toxicity and the degradation-resistant properties in the ecological environment [1]. Hence, the rapid and accurate detection of HQ and CT is a significant issue for the environmental analysis. However, these two isomers are often coexisting [2], and it is difficult to separate and determine them due to their similar structures and properties. Therefore, the development of a rapid, accurate, and simple analytical method for the simultaneous detection of the two dihydroxybenzene isomers is highly desirable. To date, various techniques, including spectrophotometry [3], chromatography [4], mass spectrometry [5], pH-based flow injection analysis [6], fluorescence [7] and electrochemical methods [8e14], have been applied to the detection of HQ and CT. Among those methods, the electrochemical method has attracted more attention since it posses the merits of simple operation, fast response and low cost, which offers the opportunity for portable, cheap and rapid methodologies. The simultaneous determination of HQ and CT is a special interest in electrochemistry system because those species exhibit overlapped oxidation peaks at conventional solid electrodes and are difficult to be distinguished. Thus, electrochemical detection approaches for those two substances must be developed with suitable modification materials that can provide complete resolution of their electrochemical signals, or determination the selective of at least one substance without influence from the other. For this purpose, various kinds of electrodes modified with graphene, carbon nanotubes and gold nanoparticles [15e18] have been constructed to improve the electrochemical performance of the sensing platform. Among them, metal-organic frameworks (MOFs) have received extensive interest by virtue of their exciting properties including large surface areas, ordered pore structure and multiple coordination sites. MOFs are composed of repeated metal complex units with a threedimensional crystal lattice constructed from metal ion nodes linked together by organic linkers [19]. By the rational choice of metal ions and organic ligands, the structure of MOFs with the various pore size and framework topology can be tailored to meet the requirement of the specific application [20]. These characteristics have made MOFs develop quickly in many research fields including catalysis [21,22], ion exchange [23], gas storage and separation [24] and drug delivery [25]. Recently, a few studies have demonstrated the promise application of MOFs for the electrochemical sensing. Yuan et al. [26] reported a new 2-dimensional (2D) Co-based MOF which exhibited the electrocatalytic oxidation of reduced glutathione (GSH). The Co-MOF modified carbon paste electrode also showed a wide linear range from 2.5 mmol L1 to 0.95 mmol L1 with the detection limit of 2.5 mmol L1 for the determination of GSH. Deep et al. [27] presented the surface assembly of a nanometal organic framework [Cd(atc) (H2O)2]n (atc ¼ 2-aminoterephthalic acid) on a 2-aminobenzylamine (2ABA) modified indium tin oxide (ITO) slide by sequential dipping. The pesticide sensor based on this modified electrode was designed for the electrochemical determination of parathion by the electrochemical impedance spectroscopy and its sensitivity was better or comparable to the reported sensors. However, the application of pure MOFs in electrochemical researches is still faced with some problems because of their intrinsic deficiency such as the poor electronic conductivity and instability in aqueous environment [28e30]. An efficient way to resolve the problems is combining MOFs with other functional materials which have better electronic conductivity or make MOFs more stable [31,32]. For this purpose, carbon materials stand out as a good partner to enhance the electroconductivity and the stability. Zu et al. [33] incorporated the graphite oxide into a typical MOF,

namely HKUST-1 or MOF-199 [Cu3(BTC)2 (BTC ¼ 1,3,5benzenetricarboxylicacid)], to improve hydrothermal stability and catalytic activity. Zhang et al. [34] synthesized a MOF-macroporous carbon hybrid material. The composites modified electrode showed an increase of the electrocatalytic ability for the oxidation of NADH and the reduction of H2O2 in neutral solution. Zhou et al. [35] also synthesized a Cu-bipy-BTC/MWCNTs hybrid composite. The electrode modified by the hybrid composite had a good performance for the H2O2 detection with a wide linear range. Therefore, the combination of carbon materials with MOFs for the electrochemical sensing platforms is highly desirable. Inspired by these reports, we combined the advantages of MOFs and the single-walled carbon nanotubes (SWCNTs) to prepare CuMOF-199/SWCNTs modified electrode for the simultaneous determination of HQ and CT in this work. The combination of SWCNTs and Cu-MOF-199 could not only increase the surface area but also form a conductive interconnection network which may be favorable for the charge transfer. Moreover, the incorporation of SWCNTs could also make Cu-MOF-199 more stable. The Cu-MOF-199/ SWCNTs modified electrode exhibited excellent electrochemical performance with extended linear ranges and lower detection limits for the simultaneous determination of HQ and CT. 2. Experimental 2.1. Reagents and materials Hydroquinone, catechol, Cu(NO3)2$3H2O, 1,3,5benzenetricarboxylic acid (H3BTC), N,N-dimethylformamide (DMF), ethanol and other common chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. Single-walled nanotubes (SWCNTs) were obtained from Nanjing Xianfeng Nano Science and Technology Co., Ltd, China. All the chemicals were of analytical grade and used without further purification. Aqueous solutions were prepared with doubly distilled water. The 0.2 mol L1 phosphate buffer solutions (PBS) with different pH, including 0.1 mol L1 KCl as a supporting electrolyte, were prepared by mixing the stock solutions of 0.2 mol L1 NaH2PO4 and 0.2 mol L1 Na2HPO4 and then adjusting the pH with H3PO4 or NaOH. 2.2. Apparatus and instruments All electrochemical measurements, including cyclic voltammetry (CV), differential pulse voltammetry (DPV), and electrochemical impedance spectra (EIS), were carried out on a CHI660D electrochemical workstation (Shanghai Chenhua Co., China). A standard three-electrode cell was used for all the electrochemical experiments. A bare or modified glassy carbon electrode (GCE, 3 mm in diameter) was used as working electrode. A platinum wire was served as the counter electrode. A saturated calomel electrode (SCE) was applied as the reference electrode and all potentials reported in this paper were referenced to the SCE. The modified materials were characterized by field emission scanning electron microscopy (Zeiss Ultra Plus, Germany) and Fourier transform infrared spectroscopy (Thermo Nicolet, USA). 2.3. Preparation of the modified materials 2.3.1. The preparation of the single-walled nanotubes (SWCNTs) suspension The SWCNTs suspension (0.1 mg mL1) was prepared as described previously [36]. First, SWCNTs were mixed with a mixture solution of the concentrated H2SO4/HNO3 (V98%H2SO4:V65%  HNO3 ¼ 3:1) in a ice-water bath and then refluxed for 3 h at 80 C in

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a water bath. After that, SWCNTs were filtered with microporous membrane, washed with water until the filtrate pH was 7 and dried at 80  C. Finally, SWCNTs were dispersed ultrasonically in the doubly distilled water to form 0.1 mg mL1 SWCNTs suspension. 2.3.2. Synthesis of Cu-MOF-199 Cu-MOF-199 was synthesized according to the literature method [37]. In a typical synthesis, 87.5 mg (3.6 mmol) Cu(NO3)2,3H2O were dissolved in 12 mL DMF and mixed with 42 mg (2.0 mmol) of 1,3,5-benzenetricarboxylic acid (H3BTC) which was dissolved in 12 mL ethanol. The solution was filled in a Teflon liner and kept at 120  C for 12 h. After cooling to the room temperature, the resulting precipitate was washed with DMF and dried under vacuum at 80  C. The obtained blue powder was Cu-MOF199. Finally, Cu-MOF-199 powder was dispersed ultrasonically in DMF to form 5 mmol L1 Cu-MOF-199 suspension. 2.4. The fabrication of the modified electrode Prior to the surface modification, the GCE was carefully polished with slurries of 1, 0.3 and 0.05 mm alumina powder respectively to obtain a mirror-like surface. Then the electrode was washed ultrasonically with acetone and water sequentially. Finally, the bare GCE was placed in 0.5 mol L1 H2SO4 solution to perform an electrochemical activation of the electrode. A continuous cycling of CV in the potential range from 0.35 to 1.5 V at a scan rate of 100 mV s1 was carried out until a stable voltammogram was obtained. After that, the precondition of the bare GCE was completed. Next, 5 mL SWCNTs suspension (0.1 mg mL1) was dropped on the cleaned electrode and was exposed into the air at 40  C to form the SWCNTs film. The as-prepared electrode was denoted as SWCNTs/GCE. The electropolymerization of MOFs on the surface of SWCNTs/ GCE was performed by cyclic potential scanning from 1.0e1.5 V at 100 mV s1 in the 5 mmol L1 Cu-MOF-199 solution. Thickness of the complex film was controlled by the number of scanned cycles during the electrodeposition process. The achieved electrode was denoted as Cu-MOF-199/SWCNTs/GCE. As controls, Cu-MOF-199/GCE was also fabricated with the similar electropolymerization procedure mentioned above with GCE. 2.5. Electrochemical measurements All the electrochemical experiments were carried out at room temperature. The CV experiments were carried out at an applied potential range from 0.6 to 1.0 V with different scan rates from 20 to 420 mV s1 in 0.2 mol L1 PBS (pH ¼ 6.0) containing appropriate amounts of a single or mixed analytes. The DPVs were also performed at the same potential. The impedance measurements were performed in 5.0 mmol L1 K3 [Fe(CN)6]/K4 [Fe(CN)6] solution containing 0.1 mol L1 KCl solution with the frequency between 1.0  103 and 1.0  105 Hz (signal amplitude: 5.0 mV). 3. Result and discussion 3.1. Electrodeposition of Cu-MOF-199 on the SWCNTs/GCE surface A typical CV curve for the electrodeposition of Cu-MOF-199 on the SWCNTs/GCE surface in DMF at the potential range between 1 and þ 1.5 V was illustrated in Fig. 1. 5 mmol L1 Cu-MOF-199 solution containing NaNO3 (0.1 mol L1) was employed as supporting electrolyte. The cyclic voltammogram exhibited one anodic peak at 0.48 V (I) and one reductive peak at 0.71 V (II). Peaks I and II were ascribed to oxidation of (L) CuI to (L) CuII and reduction of (L)

Fig. 1. Cyclic voltammograms of electrodeposition of 5 mmol L1 Cu-MOF-199 in DMF containing 0.1 mol L1 NaNO3 (supporting electrolyte) on SWCNTs/GCE at 50 mV s1. Inset is an amplified figure.

CuII to (L) CuI [38]. The redox peaks at 0.48 and 0.71 V increased with subsequent scans, demonstrating that Cu-MOF-199 films were continuously deposited on the electrode surface [39]. In order to explore the formation of Cu-MOF-199 film on the electrode, the FTeIR experiments were carried out. Fig. 2A showed that the FTeIR spectra of Cu-MOF-199 powder and Cu-MOF-199 electropolymerized on the surface of ITO substrate were similar. The absorption peaks at about 1640, 1558 and 1373 cm1 could be assigned to the characteristic vibrations of C]O, the peaks at about 1444 cm1 were ascribed to the CeC stretching on the benzene ring [40] and the peaks at about 665 cm1 belonged to the stretching vibrations of the CueO [41]. This demonstrated that Cu-MOF-199 was successfully loaded on the electrode surface without any decomposition. In addition, the broad peak around 3420 cm1 in the FTeIR spectrum of Cu-MOF-199 powder could be assigned to the OeH vibration of intercalated water [40]. This broad peak wasn't observed in the FTeIR spectrum of Cu-MOF-199 electropolymerized on the surface of ITO, indicating the remove of the water during the electropolymerization process. The formation of Cu-MOF-199 film on the SWCNTs/GCE surface was further verified visually by SEM. Fig. 2B showed the image of SWCNTs on the GCE surface. SWCNTs with a tubular structure formed a three-dimensional network on the electrode surface, which could enlarge the electrochemically active area. After the deposition of Cu-MOF-199 on the SWCNTs/GCE surface, the tubular structures of SWCNTs became blurring as shown in Fig. 2C, demonstrating Cu-MOF-199 well distributed on the SWCNTs/GCE. 3.2. Electrochemical behaviors of HQ and CT on the modified electrodes The electrochemical behaviors of HQ and CT on different modified electrodes were studied by CV and the results were shown in Fig. 3A, B and C. Broad peaks with small peak currents of HQ and CT at the bare GCE were obtained, indicating a low sensitivity and selectivity. When SWCNTs or Cu-MOF-199 was modified on the GCE, the peak currents of HQ and CT increased and the peak types became sharper. This demonstrated both SWCNTs and CuMOF-199 had good catalytic activity for the electrochemical oxidation of HQ and CT. After the composite modification of SWCNTs and Cu-MOF-199, the largest peak current response was observed. Furthermore, the oxidation peak currents of HQ at CuMOF-199/GCE and SWCNTs/GCE were higher than that at GCE about 1.95 and 10.76 mA respectively in Fig. 3A. The oxidation peak

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Fig. 2. (A) The FTeIR spectrum of electropolymerization of Cu-MOF-199 in DMF (curve a) and the Cu-MOF-199 powder (curve b). (B) SEM micrographs of SWCNTs/GCE. (C) SEM micrographs of Cu-MOF-199/SWCNTs/GCE.

Fig. 3. CVs of 0.5 mmol L1 HQ (A), 0.5 mmol L1 CT (B) and a mixture of 0.5 mmol L1 HQ and 0.5 mmol L1 CT (C) at the different modified electrodes in 0.2 mol L1 PBS (pH 6.0). EIS of the different modified electrodes in 5.0 mmol L1 K3 [Fe(CN)6]/K4 [Fe(CN)6] solution with 0.1 mol L1 KCl at the frequency range of 0.001 Hze100 kHz (D).

current at Cu-MOF-199/SWCNTs/GCE became much higher than that at GCE about 17.25 mA. Obviously, this was not a simple superposition because17.25 mA was greater than the sum of 1.95 mA and 10.76 mA. The oxidation of CT at these electrodes as shown in Fig. 3B was similar to that of HQ. These increased electrochemical signals of HQ and CT at Cu-MOF-199/SWCNTs/GCE indicated the synergy effect of Cu-MOF-199 and SWCNT. In addition, two well-

defined redox peaks corresponding to HQ and CT (a separation of 105 mV in oxidation peak currents) were obtained in the mixture of HQ and CT. The distinguishable electrochemical signals at Cu-MOF199/SWCNTs/GCE provided the favorable possibility for the effectively and simultaneous determination of HQ and CT. EIS is also a powerful tool to characterize the electronic conduction capacity of the electrode. The typical impedance spectrum

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(Nyquist diagram) is generally divided into two parts. One is a straight line in the low frequency region which is usually controlled by the diffusion process. The other is a semicircle in the high frequency region which has the relationship with the electron transfer resistance. The semicircle diameter of the high-frequency region is equivalent to the resistance of the charge transfer on the electrode surface (Rct) [1]. In this work, EIS was employed to depict the surface features of the modified electrode using the redox probe Fe(CN)64/3. The results were shown in Fig. 3D. The EIS curves of Fe(CN)64/3 at the bare GCE (a), Cu-MOF-199/GCE (b), SWCNTs/GCE (c) all had a semicircle respectively in high frequency region and became almost a straight line separately in the low frequency region. In the meanwhile, the semicircle is hardly visible in the EIS curve at Cu-MOF-199/SWCNTs/GCE. This demonstrated that the charge transfer on Cu-MOF-199/SWCNTs/GCE was faster than that on other modified electrodes, which was helpful for the electrochemical reactions. These data also confirmed that SWCNTs and Cu-MOF-199 had been successfully modified on the electrode surface. The excellent performance of Cu-MOF-199/SWCNTs/GCE might be contributed to the following reasons. Firstly, the threedimensional network forming from SWCNTs could enlarge the electroactive surface area, which enhancing the electrochemical responses through adsorbing more HQ and CT on the Cu-MOF-199/ SWCNTs/GCE surface. Secondly, the hydroxyl and carboxyl group in Cu-MOF-199 and SWCNTs could interact with the two isomers via H-bonding, which increases the separation of oxidation peaks of the two isomers [2]. Thirdly, the incorporation between Cu-MOF199 and SWCNTs could not only prevent the collapse of Cu-MOF199 in the process of measurement and make the structural morphology of Cu-MOF-199 more stable in aqueous media [37], but also increase the surface area and form a conductive interconnection network which may be favorable for the charge transfer. Thus, molecules can be easily absorbed to enhance the regional concentration and offer an electron-rich substrate to accelerate the electronic transfer during the electrochemical reactions on the electrode surface. Overall, the high surface area, excellent catalytic activity, and good conductivity of the synthesized Cu-MOF-199/ SWCNTs ensured the excellent performance of the modified electrode toward detection of HQ and CT. To investigated the electrochemical reaction kinetics, the effect of scan rate on the peak currents of a mixed solution of 0.5 mmol L1 HQ and CT at Cu-MOF-199/SWCNTs/GCE was also carried out by CV at scan rates ranging from 20 to 420 mV s1. As shown in Fig. S1, two pairs of symmetrical redox peaks were obtained for HQ and CT. It was obvious that both anodic peak current (Ipa) and cathodic peak current (Ipc) of HQ and CT increased with the augment of scan rates from 20 to 420 mV s1. Inset showed that there was a fine linear relationship between the oxidation peak currents of HQ or CT and the square root of scan rate (n1/2), indicating that the electrode reactions of the two dihydroxybenzenes at the Cu-MOF-199/SWCNTs/GCE were diffusion-controlled processes. The regression equations could be expressed as Ipa (mA) ¼ 3.58 n1/2 (mV s1)1/2 þ 1.18 (R2 ¼ 0.9990) for HQ and Ipa (mA) ¼ 4.83 n1/2 (mV s1)1/2 þ 0.45 (R2 ¼ 0.9994) for CT. Moreover, Epa shifted to more positive values with the increase of scan rates, suggesting that the electron transfer was quasi-reversible [2]. According to the slopes of the regression equations, the electroactive surface of the electrode modified can be obtained by the formula of Randles-Sevcik [42]. The electroactive area of Cu-MOF199/SWCNTs/GCE for HQ and CT is 0.3 and 0.4 cm2 respectively, which is much larger than the area of the bare GCE (about 0.07 cm2 calculated from the electrode diameter 3 mm). This could directly verify the enlarged electroactive area and enhanced electrochemical responses of electrode by Cu-MOF-199/SWCNTs film.

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In order to calculate the number of electrons transferred in the redox process of HQ and CT at the surface of Cu-MOF-199/SWCNTs/ GCE, the CVs of the single HQ or CT with scan rates ranging from 100 to 1000 mV s1 were carried out. The CV results were shown in Fig. S2. The electron transfer number n can be obtained by the following formula of Laviron [43]:

IP ¼

n2 F 2 AGn nFQ n ¼ 4RT 4RT

where IP is the peak current, n is the electron transfer number, F is the Faraday constant, A is the geometric area of the electrode, G is the surface coverage, n is the scan rate, R is the gas constant, T is the absolute temperature and Q is the electric quantity. The average values of n were 2.1 and 2.0 corresponding to the redox of HQ and CT, respectively, suggesting that the redox reaction of HQ or CT at the Cu-MOF-199/SWCNTs/GCE should be a two electrons and two protons process. The inset in Fig. S2 was the probable electrode reaction mechanisms of HQ and CT. 3.3. Optimization of the experimental parameters The electrochemical performance of the as-prepared modified electrode could be affected by many factors, such as the dropping numbers of SWCNTs, the amount of Cu-MOF-199, pH value of the detection solutions, and so on. Those experimental results were optimized in order to get a more sensitive biosensor for the simultaneous detection of HQ and CT. 3.3.1. Effect of the amount of SWCNTs The effect of the amount of SWCNTs on the electrochemical behaviors of HQ and CT was carefully investigated by DPV in 0.2 mol L1 PBS (pH ¼ 6.0). The amount of SWCNTs was controlled by dropping 5 mL SWCNTs suspension many times on the electrode surface while other detecting conditions were invariant. The relationship between the peak currents of HQ and CT and the dropping number were shown in Fig. 4A. The more times SWCNTs was dropped, the higher current response was obtained. However, in the actual experiments, the modified SWCNTs film could be easily shedded in different extent with the increase of detection times when SWCNTs were dropped two or more times, as shown in Fig. S3. This might be attributed to the limited strength between the electrode surface and SWCNTs thin films. Thus, considering the stability of modified electrode and material consumption, we chose dropping SWCNTs one time to in the fabrication procedure. 3.3.2. Effect of the amount of Cu-MOF-199 The amount of Cu-MOF-199 also played a momentous role in the determination of HQ and CT. The amount of Cu-MOF-199 can be availably controlled by changing the cycle numbers during the electrodeposition of Cu-MOF-199. The dependency relationship between the cycle number and the peak currents of HQ or CT was presented in Fig. 4B. It can be seen that the peak currents of HQ and CT achieved a maximum value when the cycle number was 5. After that the peak currents decreased with the increase of cycle numbers, which could be attributed that the modification film were so thick that performed higher electrical resistance for the electrons transfer [44]. In a word, a too thick or too thin film was adverse for the electrochemical response. Therefore, 5 cycles was appropriate to achieve the highest sensitivity. 3.3.3. Effect of pH The electro-oxidation behavior of HQ or CT is influenced by electrolyte acidity because the proton participates in the electrode reaction [45]. The effect of pH on the oxidation of HQ and CT at Cu-

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Fig. 4. Dependence of the peak currents of CT and HQ in PBS solution (pH ¼ 6.0) at Cu-MOF-199/SWCNTs/GCE on (A) the amount of SWCNTs; (B) the cycle number of electrochemical polymerization for Cu-MOF-199 deposition.

MOF-199/SWCNTs/GCE was also carefully investigated by DPV at pH range 3.0e9.0 in 0.2 mol L1 PBS containing 0.5 mmol L1 HQ and CT. As shown in Fig. 5A and B, it was noticeable that the anodic peak currents of HQ, as well as that of CT, increased with pH from 3.0 to 6.0. Whereafter, the peak currents gradually decreased with increasing the pH values from 6.0 to 9.0. These results might be interpreted by the following reasons. The pKa values of HQ and CT are 9.85 and 9.4 respectively and the two isomers are protic aromatic molecules, they will be easily deprotonated and turn to anions at high pH [2,46]. Meanwhile, the surface of Cu-MOF-199/ SWCNTs/GCE also has negative charge. Thus, the electrostatic repulsion between the two isomers and electrode will enhanced with an increase of pH value, which leads to the low adsorption of the two isomers on the electrode surface so as to cause low peak currents. Considering the determination sensitivity and selectivity, pH 6.0 was chosen as the optimum pH value for the detection of HQ and CT. Fig. 5C showed the relationships between the pH values and the

anodic peak potentials (Epa) of HQ or CT. It was conspicuous that the peak potentials for the oxidation of HQ and CT negatively shifted with increasing pH, which indicated a direct involvement of protons in the electrochemical oxidation process [47]. The linear equations between Epa and pH were Epa(V) ¼ 0.4307e0.05679 pH (R ¼ 0.9994) for HQ and Epa(V) ¼ 0.5364e0.05679 pH (R ¼ 0.9984) for CT, respectively. The slopes for HQ and CT were all 56.79 mV$pH1, which was close to the theoretical value of 59.0 mV$pH1, indicating that the number of protons and electrons involved in the electrochemical redox process of HQ and CT were equal. 3.4. Simultaneous determination of HQ and CT Under the optimal conditions, the simultaneous determination of HQ and CT at Cu-MOF-199/SWCNTs/GCE was investigated by DPV. The individual determination of HQ or CT in their mixtures was carried out by increasing the concentration of one isomer

Fig. 5. (A) DPV curves of 0.5 mmol L1 HQ and 0.5 mmol L1 CT in 0.2 mol L1 PBS at Cu-MOF-199/SWCNTs/GCE in the pH range of 3.0e9.0. (B) The relationship between pH and the peak currents of HQ or CT. (C) The relationship between pH and the peak potentials of HQ or CT.

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while keeping the concentration of the other isomer constant. Fig. 6A shows the DPV of a binary mixture of 0.2 mmol L1 CT and different concentrations of HQ at Cu-MOF-199/SWCNTs/GCE in 0.2 mol L1 PBS (pH 6.0). From the inset of Fig. 6A, the increase of peak currents of HQ fit the linear equation of Ipa (mA) ¼ 0.04823 c (mmol$L1) þ 2.084 (R ¼ 0.9999) when the concentrations of HQ were in the range of 0.1e1453 mmol L1. The detection limit for HQ was 0.08 mmol L1 (S/N ¼ 3). Similarly, as shown in Fig. 6B, keeping HQ concentration constant as 0.2 mmol L1, DPVs of CT with different concentrations were investigated. The oxidation peak currents of CT were also linear with the CT concentration in the range of 0.1e1150 mmol L1, and the regression equation was Ipa (mA) ¼ 0.04485 c (mmol$L1) þ 1.442 (R ¼ 0.9990). The detection limit (S/N ¼ 3) was 0.1 mmol L1 for CT. A comparison of the proposed method with other electrochemical methods for HQ and CT detection was listed in Table 1. Compared other modified electrodes, the modified electrode proposed in this work was inexpensive and easy to be fabricated. It is notable that the peak currents of CT or HQ almost keeps constant while the oxidation peak currents of HQ or CT increases with the increase of concentration, indicating that the oxidation of HQ and CT at Cu-MOF-199/SWCNTs/GCE electrode takes place independently. Furthermore, it is outstanding that the baseline of DPV image has a perfect superposition and the linear relationships between the peak current and the concentration of two isomers are nearly faultless (RHQ ¼ 0.9999, RCT ¼ 0.9990). These demonstrated that the Cu-MOF-199/SWCNTs/GCE had excellent stability for the simultaneous detection of the two isomers. Most remarkably, the measurable concentration range of HQ and CT at Cu-MOF-199/ SWCNTs/GCE was expanded mostly (0.1e1453 mmol L1 for HQ and 0.1e1150 mmol L1 for CT) with lower detection limits, indicating that Cu-MOF-199/SWCNTs/GCE had splendid sensitivity for simultaneous of two isomers. 3.5. Stability and reproducibility of the Cu-MOF-199/SWCNTs/GCE The stability and reproducibility of Cu-MOF-199/SWCNTs/GCE were also investigated by DPV and the modified electrodes exhibited nice properties in these two aspects. When the Cu-MOF199/SWCNTs/GCE was stored at room temperature for about 20 days, the peak currents of HQ and CT decreased merely 2.4% for HQ and 2.0% for CT, respectively, indicating the high stability of CuMOF-199/SWCNTs. Additionally, under the optimized conditions, the reproducibility of five independently fabricated electrodes was investigated by comparing the peak currents of the same concentration of HQ and CT. The RSDs were 3.3% for HQ and 3.1% for CT. And the repeatability of one electrode was also detected by continuous measurements. The modified electrode also exhibited a

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good reproducibility (the RSDs were 2.2% for HQ and 2.4% for CT) after five repeated measurements. 3.6. Interference of coexisting substance The major interferences in the simultaneous determination of HQ and CT (100 mmol L1) come from the coexisting substances, which may lead to an overlapping with the existing peaks [36]. The possible interferences of some ions were also investigated at the modified electrode by iet curves and the results were showed 1002 fold concentration of Naþ, Ca2þ, Fe3þ, Cl, NO 3 , SO4 and 50-fold concentration of glucose, ethanol and 10-fold concentration of resorcinol, ascorbic acid and uric acid have no evident effect on determination of two isomers, demonstrating an excellent tolerance to interference of the modified electrodes, as shown in Fig. S4. 3.7. Sample analysis In order to assess the possible applications of the proposed method for the simultaneous determination of HQ and CT, local tap water and the water from the Yangtze River was tested. No aimed analytes could be found in the real samples, which meant that dihydroxybenzenes contents were lower than the detection limits. The standard addition technique was used for the determination of HQ and CT by adding the known concentrations of HQ and CT. The result was showed in Table 2. The recovery rates were in the range 96.38e101.9% for HQ and 96.52e103.5% for CT, respectively and the RSD (below 5%) were also acceptable, indicating that the proposed method could be efficiently used for the determination of HQ and CT. 4. Conclusion In this work, a facile electrochemical strategy for the fabrication of Cu-MOF-199/SWCNTs modified electrode was demonstrated based on the cast of SWCNTs on the bare glassy carbon electrode and the electrodeposition of Cu-MOF-199 on the SWCNTs modified electrode. The combination of SWCNTs and Cu-MOF-199 endows the modified electrode with excellent electroactive property and high stability. The modified electrode was utilized for the simultaneous determination of HQ and CT and the oxidation peak potentials exhibited sharp differences between the two isomers. After the optimization of the experimental conditions, the measurable concentration ranges of HQ and CT at Cu-MOF-199/SWCNTs/GCE are expanded remarkably with lower detection limits compared to previously reported electrochemical sensors. In addition, the modified electrode also presented the good reproducibility and the excellent anti-interference performance. Finally, the modified

Fig. 6. (A) DPV curves of a binary mixture of 0.2 mmol L1 CT and different concentrations of HQ (0.1e1453 mmol L1) at Cu-MOF-199/SWCNTs/GCE in 0.2 mol L1 PBS (pH 6.0). (B) DPVs of a binary mixture of 0.2 mmol L1 HQ and different concentrations of CT (0.1e1150 mmol L1) obtained at Cu-MOF-199/SWCNTs/GCE in 0.2 mol L1 PBS (pH 6.0). The inserts are the relationships between the peak currents and concentrations.

64

J. Zhou et al. / Analytica Chimica Acta 899 (2015) 57e65

Table 1 Performance comparison of the modified electrode for HQ and CT detection with other reported electrodes. Modified materiala

PSA/PDDA-GN/GCE GR/MWCNTs/BMIMPF6/GCE PEDOT/GO/GCE PTH/GCE Graphene-chitosan/GCE Pt/ZrO2-RGO/GCE ECF-CPE CNF/GCE PteMnO2/GCE UT-CdSe/GCE pAPBA/MWCNTs/GCE ER(GO-TT-CNT)/GCE MWNTs-IL-Gel/GCE GR/GCE Cu-MOF-199/SWCNTs/GCE

Linear range (mmol L1)

Regression

HQ

CT

HQ

CT

HQ

CT

2e400 0.5e465 465e2900 2.5e200 1e120 1e300 1e1000 1e200 6e200 3e481 0.6e1500 0.5e40 0.1e100 0.2e35 1e50 0.1e1453

1e400 0.2e80 80e660 2e400 1e120 1e400 1e400 1e200 2e200 15e447 0.2e300 7e100 0.5e200 0.18e35 1e50 0.1e1150

0.9989 0.9959 0.9981 0.998 0.99 0.9966 0.9936 0.9996 0.9984 0.9892 0.998 null 0.9969 0.996 0.991 0.9999

0.9972 0.9965 0.9978 0.999 0.99 0.9989 0.9913 0.9994 0.9964 0.9827 0.999 null 0.9927 0.999 0.994 0.9990

0.39 0.1

0.22 0.06

[1] [46]

1.6 0.03 0.75 0.4 0.4 0.25 0.0272 0.011 0.2 0.035 0.067 0.015 0.08

1.6 0.025 0.75 0.4 0.2 0.1 0.0181 0.060 0.72 0.0049 0.060 0.01 0.1

[10] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] This work

Detection limit (mmol L1)

Reference

a PSA/PDDA-GN, poly(sulfosalicylic acid)/functionalized graphene; GR/MWCNTs/BMIMPF6, graphene/multiwalled carbon nanotubes/1-butyl-3-methylimidazolium hexafluorophosphate; PEDOT/GO, poly(3,4-ethylenedioxy-thiophene)/graphene oxide; PTH, poly(thionine); ECF-CPE, electrospun carbon nanofiber-modified carbon paste electrode; CNF, carbon nano-fragment; UT-CdSe, Ultrathin CdSe nanosheets; pAPBA/MWCNTs, poly(3-aminophenylboronic acid)/multi-walled carbon nanotubes; ER(GO-TTCNT), electrochemically reduced graphene oxide e multi-walled carbon nanotube e terthiophene; MWNTs-IL-Gel, multiwalled carbon nanotubes e ionic liquids gel; GR, graphene.

Table 2 Determination results for HQ and CT in local tap water and river water. Sample

Tap water

River water

Detected (mmol L1)

1 2 3 1 2 3

Added (mmol L1)

Total (mmol L1)

HQ

CT

HQ

CT

HQ

0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0

9.99 19.96 29.91 9.99 19.96 29.91

9.99 19.96 29.91 9.99 19.96 29.91

9.628 20.24 30.40 10.18 19.65 30.13

electrode was applied in tap water and river water detection with satisfying results. All the results showed that the proposed method is simple and cost-effective which provides a platform for the simultaneous detection of phenolic compounds. Acknowledgments We greatly appreciate the financial support from the National Natural Science Foundation of China (Grant No. 51273155, 21476179 and 51272201) and the Fundamental Research Funds for the Central Universities (No. 2014-Ia-030). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.aca.2015.09.054. References [1] C. Li, W. Liu, Y. Gu, S. Hao, X. Yan, Z. Zhang, M. Yang, Simultaneous determination of catechol and hydroquinone based on poly (sulfosalicylic acid)/ functionalized graphene modified electrode, J. Appl. Electrochem. 44 (2014) 1059e1067. [2] D. Song, J. Xia, F. Zhang, S. Bi, W. Xiang, Z. Wang, L. Xia, Y. Xia, Y. Li, L. Xia, Multiwall carbon nanotubes-poly (diallyldimethylammonium chloride)graphene hybrid composite film for simultaneous determination of catechol and hydroquinone, Sensors Actuators B Chem. 206 (2015) 111e118. [3] P. Nagaraja, R. Vasantha, K. Sunitha, A new sensitive and selective spectrophotometric method for the determination of catechol derivatives and its

Recovery (%) CT

± ± ± ± ± ±

0.6 0.7 0.2 0.4 0.4 1.1

10.02 20.66 29.90 10.09 19.27 29.02

± ± ± ± ± ±

0.7 0.2 1.2 0.7 0.3 0.6

RSD (%)

HQ

CT

HQ

CT

96.38 101.4 101.6 101.9 98.44 100.7

100.3 103.5 99.97 101.0 96.52 97.01

2.3 1.4 0.3 1.5 0.7 1.5

2.8 0.4 1.6 2.7 0.6 0.8

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