mesoporous carbon

Talanta 174 (2017) 527–538

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Simultaneous and sensitive electrochemical detection of dihydroxybenzene isomers with UiO-66 metal-organic framework/mesoporous carbon Min Denga, Shourui Linb, Xiangjie Boa, Liping Guoa,

MARK

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a Key Laboratory of Nanobiosensing and Nanobioanalysis at Universities of Jilin Province, Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR China b Department of Pharmacy, Changchun Medical College, Changchun 130031, PR China

A R T I C L E I N F O

A BS T RAC T

Keywords: UiO-66/MC composite Electrocatalysis Simultaneous and sensitive determination Dihydroxybenzene isomers

The zirconium-based MOF (UiO-66)/mesoporous carbon (MC) composite was synthesized using conventional hydrothermal method for the first time. The surface morphology and structure of UiO-66/MC composite were characterized by scanning electron microscopy (SEM), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). A novel electrochemical sensor based on UiO-66/MC was constructed for simultaneous and sensitive determination of dihydroxybenzene isomers (DBIs) of hydroquinone (HQ), catechol (CT) and resorcinol (RS). The proposed sensor displays excellent electrocatalytic activity toward the oxidation of HQ, CT and RS. The peak-to-peak potential separations between CT and HQ, and RS and CT are 0.130 V and 0.345 V, respectively. Under the optimized conditions, the electrochemical sensor shows a wide linear response in the concentration range of 0.5–100 μM, 0.4–100 μM and 30–400 μM with a detection limit of 0.056 μM, 0.072 μM and 3.51 μM (S/N = 3) for HQ, CT and RS, respectively. In addition, the sensor has superior sensitivity and electrochemical stability along with good reproducibility and anti-interference properties. The fabricated sensor was also applied for the determination of DBIs in the real water samples with satisfying results.

1. Introduction Hydroquinone (HQ), catechol (CT) and resorcinol (RS) are three typical dihydroxybenzene isomers (DBIs) of phenolic compounds, which usually coexist in environmental samples [1]. They are widely used in the synthesis of antioxidants, pesticides, photostabilizers, dyes, paints, cosmetics and in other synthetic chemicals manufacturing industries [2– 4]. Moreover, they have also been certified as a toxic environmental pollutant, caused by their high toxicity and low degradability in the ecological environment [5]. These phenolic compounds can quickly enter the body through contact with the skin or mucous membranes caused by poisoning symptoms. Moreover, the DBIs may induce tiredness, headache, dizziness, pale, sometimes even kidney and liver function damage, these isomers can also cause nervous system lesion [19,20,23]. In the process of its production and use, dihydroxybenzene water pollution accidents are occasionally induced by improper operation or accidental leakage, which will be bad for the environment and human health, even at very low concentration. It is high time that the researchers should develop sensitive and rapid analytical techniques for the determination of them. Up to date, a number of analytical methods

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Corresponding author. E-mail address: [email protected] (L. Guo).

http://dx.doi.org/10.1016/j.talanta.2017.06.061 Received 25 March 2017; Received in revised form 11 June 2017; Accepted 21 June 2017 Available online 22 June 2017 0039-9140/ © 2017 Elsevier B.V. All rights reserved.

have been established to determine phenolic compounds, including high-performance liquid chromatography [6], fluorescence [7–11], chemiluminescence [12], photolithography [13], spectrophotometry [14], mass spectrometry [15], capillary electrochromatography [16], and electrochemical methods [17–20]. The electrochemical detections have attracted significant attention in the simultaneous determination of DBIs on account of many intrinsic advantages such as straightforward operation, low cost, fast response, high sensitivity, good selectivity and so on. However, due to three isomers have similar stereochemical structure and contiguous redox potentials on conventional electrode, the simultaneous determination of them is nearly impossible. To overcome these shortcoming, various nanomaterials modified electrodes including metal sulfides [21], quantum dots [22], metal/metallic oxide nanoparticles [23] and carbon nanotubes [24] have been used to individual or simultaneous determination of them. Nevertheless, it is noticed that the synthesis process of above mentioned materials should be simplified and the sensitivity for the simultaneous detection of three isomers needs to be promoted. Therefore, it is an urgent need for researchers to design ingenious and convenient sensing platform for the simultaneous and sensitive determination of DBIs.

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2. Experimental section

Metal-organic frameworks (MOFs), which consist of metal ions/ clusters connected by organic linker groups, have recently emerged as a type of novel three-dimensional (3D) coordination compounds with unique properties, such as ordered crystalline structure, permanent porosity, high specific surface areas, excellent thermal stability, and tailorable chemistry. As a result of these fascinating structures and properties, MOFs have been widely used in gas capture and separation [25], catalysis [26], energy storage [27], drug delivery [28] and sensing [29,30] in the past few years. Recently, MOFs have received special attentions in electrochemical applications. Whereas, since their intrinsic weaker conductivity and electrochemical stability, the electrochemical applications of single-phase MOFs still exist certain limitation. Thus, recent research interest has gradually shifted from single-phase MOFs to MOFs-base composites. Thus, heterogeneous nanocomposites with multiple functional components, especially conductive carbon materials, have been proposed. The nanocomposites have significantly improved electronic conductivity and electrochemical stability of MOFs materials. For instance, Zhang and co-workers reported MOF-macroporous carbon hybrid material for the oxidation of hydrazine and the reduction of nitrobenzene [31]. The combination of Cu-based metal organic frameworks (Cu-MOFs) and macroporous carbon (MPC) was investigated, which show enhanced stability and good electrocatalytic ability for ascorbic acid and hemoglobin [32]. Multi-walled carbon nanotubes (MWCNTs) was implanted into manganese-based MOFs (Mn-BDC) via one-step solvothermal method, this material can be realized simultaneous detection of biomolecules in body fluids [33]. An AuNP@MOF composite modified carbon paste electrode for the determination of bisphenol A (BPA) was developed by Silva et al. [34]. The selective detection of Bisphenols at the MOF-chitosan sensing platform was reported [35]. Herein, the combination of carbon materials with MOFs for the electrochemical sensing platforms are highly desirable. Mesoporous carbon (MC), one of carbon materials, is promising as a support platform for MOFs owing to their large specific surface area and high electrical conductivity [36]. All kinds of MOF materials, we turned our attention to UiO-66 MOF due to its excellent thermal, aqueous and acid stability [37]. As a zirconiumbased MOF, the UiO-66 has a face-centered cubic crystal structure. The structure of UiO-66 composed of octahedral cages and tetrahedral cages, and these cages is provided by narrow triangular windows with a diameter close to 0.6 nm [38]. These unique features of UiO-66 have motivated researches for wide applications. But there is extremely rare work on UiO66 and their composites for the electrochemical areas. To date, the UiO-66 MOF has been utilized as the catalytic carrier of noble metals for the detection of H2O2 [39] and telomerase [40]. Nevertheless, to the best of our knowledge, it is not found that the UiO-66 MOF and their composites as an electrochemical sensing material are used for electrochemical studies. Therefore, we hope to fabricated a novel electrochemical sensing platform base on UiO-66 for the catalytic center, which not only avoids the use of noble metals but also further extends the application of UiO-66 and other MOFs. In addition, there is not found the application of UiO-66 and their composites for the highly selective and sensitive detection of isomers. In this study, we combined the advantages of zirconium-based MOF (UiO-66) and mesoporous carbon (MC) to fabricate a novel electrochemical sensing platform for the simultaneous detection of DBIs through simple and convenient solvothermal treatment of the UiO-66 precursor mixture and MC for the first time (Scheme 1). The obtained composites are defined as UiO-66/MC-x, and its morphology were characterized. The electrochemical measurements show that UiO-66/ MC-3 exhibits drastically enhanced sensitivity with regard to simultaneously determination of DBIs. The sensitivity of HQ, CT and RS is 0.360 μA μM−1, 0.142 μA μM−1 and 0.034 μA μM−1, respectively. Compared with the existing reports, the peak-to-peak potential separation for the oxidation peaks of the CT and HQ achieved a maximum. Furthermore, the developed sensor was used for the analysis of real water samples successfully.

2.1. Reagents and materials All reagents were of analytical grade and used without further processing. Zirconium chloride (ZrCl4) and terephthalic acid (H2BDC) were purchased from Sigma-Aldrich. Calcium carbonate (CaCO3), sucrose, N,N-Dimethylformamide (DMF), acetic acid, acetone, methanol, hydroquinone (HQ), catechol (CT) and resorcinol (RS) were purchased from Tianjin Guangfu Fine Chemical Research Institute. The 0.1 mol L−1 hydrochloric acid solution was prepared by diluting the 37% concentrated hydrochloric acid with deionized water. The 0.1 mol L−1 phosphate buffer solutions (PBS) with different pH were prepare by disposing the stock solution of 0.1 mol L−1 H3PO4 and then adjusting the pH with NaOH. All aqueous solutions were prepared with deionized water. 2.2. Apparatus and instruments All of the electrochemical measurements were test on a CHI1120E electrochemical workstation (CH Instruments, China) equipped with a conventional three-electrode system: a platinum plate and Ag/AgCl electrode were acted as counter and reference electrode, respectively, the modified glass carbon electrode (GCE) was applied as the working electrode, and 0.1 M PBS was served as the supporting electrolyte. All potentials in this paper were referred to Ag/AgCl, and all experiments were carried out at ambient temperature. Scanning electron microscopy (SEM) images were collected with a Philips XL-30 ESEM operating at 3.0 kV. Transmission electron microscopy (TEM) images were obtained using a JEM-2100F transmission electron microscope (JEOL, Japan) operating at 200 kV. The crystal structure was analyzed by X-ray powder diffraction (XRD) on a Rigaku D/MAX-RB diffractometer (Japan). Fourier transform infrared (FT-IR) spectroscopy was carried out with a Nicolet Magna 560 FT-IR spectrometer with a KBr plate. Nitrogen adsorption-desorption isotherms were measured on an ASAP 2020 Micromeritics (USA) at 77 K. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface area. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo ESCA LAB spectrometer (USA). 2.3. Synthesis of MC The MC was synthesized by conventional hard template method [36]. In brief, 5.0 g of commercial hydrophilic CaCO3 nanoparticles (ca. 40 nm) was dissolved in aqueous solution containing 5.0 g of sucrose and stirred at room temperature to obtained homogeneous suspension, and the suspension was dried with magnetic stirring at 80 °C. The mixture was carbonized at 800 °C in nitrogen (99.999%) for 2 h in tube oven. The hard template was then etched away by overnight soaking in solution of dilute HCl to leave behind mesoporous carbon. 2.4. Preparation of UiO-66 and UiO-66/MC composites The UiO-66 was prepared as reported previously [41]. Briefly, 0.357 g of ZrCl4 and 0.254 g of BDC were dissolved in 21 mL of DMF and 8.6 mL of acetic acid under ultrasonication to form a welldissolved mixture suspension. Subsequently, the mixture was sealed in an autoclave and placed in a preheated oven at 120 °C for 24 h, and then slowly cooled to room temperature yielding white crystals. The product was collected by centrifugation and washed with aceton three times. After that, the obtained crystal was stirred in DMF for 12 h and in methanol for 24 h at room temperature to remove unreacted linker. Finally, the product was dried under vacuum at 65 °C for 24 h prior to use. The UiO-66/MC composite materials were prepared following the same procedure as above except for adding different amount of MC powder (0.15 g, 0.31 g, 0.61 g and 0.92 g) in the well-dispersed mixture

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Scheme 1. Illustration of the construction and detection strategy of the sensor.

The XPS survey reveals that the carbon content of UiO-66/MC-3 (ca. 86.17%) is obviously higher than that of UiO-66 (ca. 70.09%). The result also displays that the composite is successfully prepared. The crystal structure features of pure UiO-66 (a), MC (b), UiO-66/ MC-1 (c), UiO-66/MC-2 (d), UiO-66/MC-3 (e) and UiO-66/MC-4 (f) samples were established through X-ray diffraction (XRD). As depicts in Fig. 3A, MC shows two broad diffraction peaks at around 23° and 43° corresponding to the (002) and (100) crystal plane of carbon, respectively, indicating a non-graphitized structure. The relative intensity and peak positions of UiO-66 in the XRD pattern are consistent with the previous report [39], confirming the formation of a desirable UiO-type crystalline framework. The XRD patterns of the as-prepared composites are in good agreement with the original pattern of UiO-66, indicating that the incorporation of MC does not disrupt or destroy the crystal structure of UiO-66. Moreover, compared to pure UiO-66, a hump in the diffraction pattern of UiO-66/MC-x appears in the range of 20–27°, which originate from the diffraction peak of MC. Furthermore, the diffraction peaks of composites are sharp and intense, confirming their highly crystalline nature. The structure of UiO-66/MC-3 was further provided by FT-IR spectroscopy. The results are shown in Fig. 3B. In the spectrum of UiO66, the characteristic peak centered at 668 cm−1 can be assigned to the Zr-O-Zr bond. The absorption bands of 1650 and 1578 cm−1, and 1506 and 1399 cm−1 are ascribed to the asymmetric and symmetric stretching vibrations of the carboxylate groups in BDC linker, respectively. To be noticed, the spectrum of UiO-66/MC-3 is largely similar to that of UiO-66, testifying that the structure of UiO-66 MOF is not changed upon loading with MC. The porosities of pure UiO-66 and UiO-66/MC-3 were measured by nitrogen adsorption-desorption isotherms. Fig. 3C shows the typical N2 adsorption-desorption isotherms at 77 K and the Barrett-Joyner-Halenda (BJH) desorption pore size distribution. From the results, MC presents a type Ⅳ isotherm with a sharp capillary condensation step at high relative pressures and an H1-type hysteresis loop, which is a characteristics of mesoporous material. The isotherm of UiO-66 belongs to typicalⅠtype curve, which is a characteristic of microporous material. Moreover, the sorption isotherm of UiO-66/MC-3 is similar to that of MC, and it is also a type Ⅳ isotherm, indicating the existence of mesopores in the material. According to the corresponding pore size distribution curve (inset of Fig. 3C), the pore sizes and the BET surface areas of MC are measured to be 4.3 nm and 546.5 m2 g−1 and those of UiO-66 are 2.1 nm and 1312.4 m2 g−1. However, the pore sizes and the BET surface areas of UiO-66/MC-3 are 2.8 nm and 747.1 m2 g−1, respectively. Such a big pore size is helpful for the as-prepared hybrid material to concentrate large amount of analytes for the sensing application. Characterization of Electrochemical impedance spectroscopy (EIS) is an efficient and facile tool for studying the electronic conductivity of

suspension. The obtained composites were referred to as UiO-66/MC-x (x = 1, 2, 3 and 4), where 1, 2, 3 and 4 represent different MC contents, respectively. 2.5. Sensor fabrication Prior to the modification, the bare GCE was polished carefully with alumina slurry of 1.0, 0.3 and 0.05 µm in turn to create a mirror-like surface, and followed by ultrasonic washing with 1:1 nitric acid, ethanol and deionized water. And then it was rinsed thoroughly with doubledistilled water and dried at room temperature. At the same time, catalyst ink was prepared by mixing 5 mg of UiO-66/MC-3 powder and 1 mL of DMF under sonication for 10 min. Then, the suspension (5 μL) was dropped on the GCE surface and dried under an infrared lamp. The proposed sensor was marked as UiO-66/MC-3/GCE. 3. Result and discussion 3.1. Characterization of UiO-66/MC composite The morphological character and microstructure of the as-prepared samples were characterized by SEM and TEM measurements. Fig. 1A depicts the SEM micrograph of UiO-66 particles, in which a uniform shape and independent octahedrons are observed. Furthermore, the particles show sharp edge and smooth surface, indicating that the sample has good crystallinity and high purity. In comparison, the SEM image of UiO-66/MC-3 exhibits the UiO-66 crystallites original octahedral shape (Fig. 1B), suggesting that incorporation of MC has no effect on the morphology of UiO-66. In addition, the detailed structures of UiO-66 and UiO-66/MC-3 were further confirmed by TEM. The corresponding TEM images of UiO-66 and UiO-66/MC-3 are shown in Fig. 1C and D, it can be seen that both UiO-66 and UiO-66/MC-3 have regular octahedron structure, which testifies that these MOF-based products have the similar basic framework as discussed in the SEM characterization. However, the surface of the UiO-66/MC-3 octahedron is not as smooth as that of the pure UiO-66, which might be benefit from the loading of MC, testifying the cooperation of UiO-66 and MC. The SEM and TEM images of MC (Fig. S1, Supporting information) show that the carbon material has large unordered and interconnected mesopores of 40–50 nm in diameter. The chemical compositions of UiO-66 and UiO-66/MC-3 were investigated by X-ray photoelectron spectroscopy (XPS). The corresponding results are described in Fig. 2. It is observed that the main peaks observed in the survey are Zr 3d, C 1 s, O 1 s, Cl 2p and N 1 s centered at 182.5, 284.6, 531.7, 199.1 and 402.4 eV, respectively (Fig. 2A). Similarly, the same peaks are also appeared at Fig. 2B. However, the content of elements in compound has made some difference. 529

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Fig. 1. SEM images of UiO-66 (A) and UiO-66/MC-3 (B). TEM images of UiO-66 (C) and UiO-66/MC-3 (D).

petitive candidate for various electrochemical applications. In order to demonstrate the electrochemical stability of UiO-66/ MC-3, cyclic voltammetry (CV) experiments (50 cycles) of DBIs in 0.1 M PBS (pH 6.0) mixture on UiO-66 and UiO-66/MC-3 were performed (Fig. 4). As can be seen from Fig. 4A, the CVs of UiO-66/ GCE indicate two broad oxidation peaks with small peak currents. The former can attributed to the overlap of the oxidation peaks of HQ and CT, and the latter to the irreversible peak of RS. After 50 cycles, the oxidation current retains about 2% of the initial current for RS. For UiO-66/MC-3/GCE (Fig. 4B), it is observed that two pairs of redox peaks and an oxidation peak simultaneously appear, and these peaks can be assigned to the electrochemical response of HQ, CT and RS,

the different electrode materials. The typical impedance spectrum is generally consisted of two parts. One is a straight line in the low frequency region which corresponds to the diffusion-limited process. The other is a semicircle in the high frequency region which relative to the electron transfer-limited process. The electronic conductivity of samples were studied by EIS. The results are described in Fig. 3D, the bare GCE exhibits a larger semicircle part at high frequency. After being modified with UiO-66 on GCE, the semicircle diameter for EIS decreases. For MC/GCE and UiO-66/MC-3/GCE, the semicircle is hardly visible. The results demonstrate that the electron transfer ability of UiO-66 is improved greatly after the modification of MC. These fascinating characteristics could make UiO-66/MC-3 become a com-

Fig. 2. XPS spectra of pure UiO-66 (A) and UiO-66/MC-3 (B).

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Fig. 3. (A) XRD patterns of pure UiO-66 (a), MC (b) and UiO-66/MC-1, 2, 3 or 4 (c–f, respectively). (B) FT-IR spectra of pure UiO-66 and UiO-66/MC-3 composite. (C) Nitrogen adsorption-desorption isotherm curve (main panel), and its pore size distribution curve (inset) of MC, pure UiO-66 and UiO-66/MC-3. (D) EIS of the different modified electrodes in 5.0 mM K3[Fe(CN)6]/K4[Fe(CN)6] solution with 0.1 M KCl at the frequency range of 0.1 Hz to 100 kHz.

Fig. 4. CV experiments (50 cycles) of DBIs in 0.1 M PBS (pH 6.0) mixture on UiO-66 (A) and UiO-66/MC-3 (B). Scan rate: 50 mV s−1.

current after 50 CV cycles. Compared to the UiO-66/GCE, the oxidation peaks of three isomers at the UiO-66/MC-3/GCE are clearly separated and the oxidation peak currents are dramatically increase.

respectively. Furthermore, the UiO-66/MC-3 modified electrode maintains about the same current as the original peak current after 50 cycles. The oxidation peak current of RS retains 93.3% of the initial

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Fig. 5. CVs of 0.1 mM HQ (A), 0.1 mM CT (B), 0.1 mM RS (C) and a mixture of DBIs (each of 0.1 mM) (D) at the different modified electrodes in 0.1 M PBS (pH 6.0). scan rate: 50 mV s−1.

Take RS for instance, after the first cycle, the oxidation peak current for UiO-66/MC-3/GCE (Ipa = 6.29 μA) is about 3.4 times higher than that of UiO-66/GCE (Ipa = 1.85 μA). The results manifest that the MC as catalytic carrier promotes the electrochemical stability and the electrocatalytic activity of pure UiO-66 MOF for the detection of DBIs.

However, the electrochemical behaviors of RS at the above electrodes are different from HQ and CT. Only a oxidation peak is obtained (Fig. 5C), which indicates that the oxidation process of RS is a totally irreversible electrode process. Compared to the oxidation peak of RS at the UiO-66/GCE, the peak current of the UiO-66/MC-3/GCE increases from 3.74 μA to 11.87 μA. The apparent enhanced peak currents may be ascribed to the synergistic effect of UiO-66 and MC. The incorporation between UiO-66 and MC could not only improve the electronic conductivity of pure MOF in the electrochemical measurement, but also promote the electrochemical stability of UiO-66 in aqueous media. Moreover, the increased currents could also result from the fact that the UiO-66/MC-3 composite has a larger pore size, which may be favorable for the mass transfer. Thus, the target molecule can be easily adsorbed to improve the regional concentration. Overall, a large pore size, excellent electrochemical stability and good conductivity of UiO66/MC-3 ensured the superior performance of the modified electrode toward detection of DBIs. In the presence of three DBIs, the CVs of the mixture solution (each of 0.1 mM) at the different modified electrodes are shown in Fig. 5D. As well seen, two pairs of redox peaks at 0.155 V, 0.115 V and 0.285 V, 0.245 V and an oxidation peak at 0.630 V simultaneously appear in the voltammogram, these peaks can be well ascribed to the electrochemical response of HQ, CT and RS. According to previous reports [17,20,49], the redox peaks of HQ and CT are largely overlapped on the modified electrodes, leading to a significant obstacle for the simultaneous

3.2. Electrochemical behaviors of DBIs To evaluate the application potential of UiO-66/MC-3 sample for electroanalysis, the electrochemical performance of DBIs on different modified electrodes were researched by CV. Fig. 5 depicts the CV responses of 0.1 mM HQ (A), 0.1 mM CT (B) and 0.1 mM RS (C) in 0.1 M PBS (pH 6.0) at a scan rate of 50 mV s−1, respectively. As seen in Fig. 5A, when the bare GCE was chosen as the working electrode, a pair of broad and symmetrical redox peaks with small peak currents appeared. For UiO-66/GCE, the electrochemical response of HQ is almost the same as that of bare GCE. For MC/GCE, although the currents increase significantly, the redox peaks are not obvious. At the UiO-66/MC-3/GCE a pair of distinct redox peaks is observed, and the oxidation peak current for the UiO-66/MC-3/GCE (Ipa = 23.34 μA) is about 17.29 times higher than that of UiO-66/GCE (Ipa = 1.35 μA). The electrochemical behaviors of CT at the aforementioned modified electrodes as shown in Fig. 5B are similar to that of HQ. Compared with the UiO-66/GCE (Ipa = 1.57 μA), the oxidation peak current of CT at UiO-66/MC-3/GCE (Ipa = 22.59 μA) is about 14.39 times higher. 532

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Fig. 6. CV curves of 0.1 mM HQ (A), 0.1 mM CT (B) and 1.0 mM RS (C) at UiO-66/MC-3/GCE in pH 6.0 PBS solutions at different scan rate from 0.01 V s−1 to 1.0 V s−1. The inset shows the dependence of the relationships of peak currents (Ip) versus scan rate (ν).

detection of DBIs. Therefore, larger potential difference of CT and HQ could promote the sensitivity of simultaneous determination DBIs. For UiO-66/MC-3/GCE, the peak-to-peak potential separation (ΔEp(CTHQ)) for the oxidation peaks of CT and HQ are larger than those reported in literature (Table S1, Supporting information), which suggests the outstanding distinguishing capability of the sensing film toward the simultaneously analysis of DBIs. The good separations of the composite may be due to the synergistic effect of UiO-66 and MC. The UiO-66/MC composite effectively increases the active sites that enhance the electrocatalytic activity. Meanwhile, the incorporation between UiO-66 and MC could not only improve the electronic conductivity of UiO-66, but also promote the pore size of material, which is conducive to the electron transfer and mass transfer of the electrochemical process. Finally, the composite possesses excellent electrochemical stability, which may be favorable for the electrochemical measurement. Based on the above reasons, the UiO-66/MC nanocomposites can efficiently separate the cyclic voltammetric peaks of HQ, CT and RS for the simultaneous detection of DBIs. As controls, the electrochemical responses of the DBIs mixture at the bare GCE, UiO-66/GCE and MC/GCE were also tested. It is found that at these three electrodes are only observed two broad and small oxidation peaks. The result clearly indicates that these electrodes are not suitable for the simultaneous determination of three isomers. Thus, it can be

Fig. 7. CVs of 0.1 mM DBIs mixtures at the different carbon content composites modified electrodes in 0.1 M PBS (pH 6.0) at a scan rate of 50 mV s−1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 8. (A) DPV curves of 0.1 mM HQ, 0.1 mM CT and 0.1 mM RS in 0.1 M PBS (pH 6.0) at UiO-66/MC-3/GCE in the pH range from 4.0 to 9.0. (B) The relationship between pH and the oxidation peak currents of HQ, CT or RS. (C) The relationship between pH and the oxidation peak potentials of HQ, CT or RS.

peak currents (Ipa, Ipc) of HQ, CT and the irreversible oxidation peak currents (Ipa) of RS show good linear relationships with scan rates (Fig. 6A–C, insets). The regression equations could be expressed as Ipa (μA) = 0.430 v (mV s−1) + 0.751 (R2 = 0.998) and Ipc (μA) = −0.430 v (mV s−1) – 0.610 (R2 = 0.998) for CT, Ipa (μA) = 0.504 v (mV s−1) – 1.830 (R2 = 0.999) and Ipc (μA) = −0.414 v (mV s−1) – 0.411 (R2 = 0.999) for HQ and Ipa (μA) = 0.009 v (mV s−1) + 1.073 (R2 = 0.996) for RS. These results illustrate that the electrochemical reactions of DBIs on the modified electrode are typical adsorption-controlled electrochemical processes. According to Laviron theory, the adsorption properties of the electroactive material on the electrode are in accordance with the Langmuir adsorption isotherm:

concluded that the loading of MC not only realize the simultaneous detection of DBIs, but also improve the sensitivity of simultaneous determination of three isomers. The individual electrochemical oxidation of HQ, CT and RS at the UiO-66/MC-3 modified electrode were also researched by DPV in 0.1 M PBS (pH 6.0) with varying their concentrations (Fig. S2, Supporting information). With an increase in the concentration of analytes, anodic peak currents of HQ, CT and RS increase. In addition, the concentrations of three isomers and the responding currents have a good leaner relationship, respectively. Therefore, the UiO-66/MC-3/ GCE can be used for quantitative determination of three isomers.

3.3. Effect of the scan rate on the electrochemical behavior of DBIs

ip =

To infer more about the reaction of HQ, CT and RS at the UiO-66/ MC-3/GCE, the effect of the scan rate on the redox peak currents of 0.1 mM HQ, CT and RS were also investigated by CV at scan rates ranging from 0.01 to 1.0 V s−1 (Fig. 6). With the increase of scan rate, the electrochemical signals of DBIs enhance gradually and the oxidation peak potentials of DBIs shift positively. Additionally, the redox

n2F2AΓν nFQν = 4RT 4RT

Where n is the number of electron, F is the Faraday constant (96485 C mol−1), Q is the peak area of a single cyclic voltammetry process (calculated as electric quantity), R the universal gas constant, T is kelvin temperature. The average values of n were 2.0, 1.99 and 1.93

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Fig. 9. DPVs of UiO-66/MC-3/GCE in 0.1 M PBS (pH 6.0) containing a mixture of DBIs. A: 50 μM CT, 100 μM RS, 0.1–300 μM HQ. B: 50 μM HQ, 100 μM RS, 0.05–200 μM HQ. C: 50 μM CT, 50 μM HQ, 30–400 μM RS. D: 1.0–80 μM HQ, 1.0–80 μM CT and 50–220 μM RS. Insets show the relationship between the oxidation peak currents and the concentrations of DBIs. DPV parameters were selected as: pulse amplitude: 50 mV, pulse width: 50 ms, scan rate: 20 mV s−1.

indicate that the oxidation peak currents of HQ, CT and RS increase with increasing pH value until this reach 6.0, and then the oxidation peak currents decrease when the pH increase further (Fig. 8B). Therefore, pH 6.0 is chosen as the optimized pH value in the subsequent experiments. Moreover, Fig. 8C shows the effect of pH on the oxidation peak potentials. The oxidation peak potentials move to the negative direction with pH increase, and vary linearly with pH of the solutions in the range of 4.0–9.0, indicating a direct participation of protons in the electrochemical reactions. The equations of linear regression are described as follows: Epa (V) = 0.506 – 0.060 pH (R2 = 0.994), Epa (V) = 0.602 – 0.057 pH (R2 = 0.997) and Epa (V) = 1.045 – 0.069 pH (R2 = 0.996) for HQ, CT and RS, respectively. These three almost parallel lines (Fig. 8C) indicate that the peak potential difference among HQ, CT and RS is constant. According to the Nernst equation:

corresponding to the redox of HQ, CT and RS, respectively, suggesting that the oxidation reaction of HQ, CT or RS at the UiO-66/MC-3/GCE should be a two electrons process. 3.4. Optimization of the experimental conditions 3.4.1. Effect of the amount of MC The effect of the amount of MC on the electrochemical behaviors of DBIs was studied by CV in 0.1 M PBS (pH 6.0). The amount of MC can be availably controlled by changing the addition of MC (0.15 g, 0.31 g, 0.61 g and 0.92 g) during the synthesis process while other conditions were invariant. The result is shown in Fig. 7, it can be seen that the peak currents and peak-to-peak separations of DBIs are achieve a maximum when the mass ratio of UiO-66 and MC was 1:2, namely, UiO-66/MC-3 sample (green curve). Thus, 0.61 g is selected for the best carbon content.

dE p dpH

3.4.2. Effect of pH In general, the electrochemical oxidation behavior of DBIs are influenced by solution acidity because the proton participates in the electrode reaction. The effect of pH on the electrochemical oxidation of DBIs mixtures (each of 0.1 mM) at the UiO-66/MC-3/GCE were investigated by DPV in a pH range of 4.0–9.0 (Fig. 8A). The results

=

2. 303mRT nF

where m and n are the number of proton and electron, respectively; m/ n is calculated to be 1.02, 0.97 and 1.17 for the HQ, CT and RS oxidation process, respectively. It means the number of proton and electron involved in the oxidation process of DBIs are equal. Thus, the electrochemical oxidation of the three isomers at the UiO-66/MC-3/ 535

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GCE involves the transfer of two electrons and two prontons. The probable detection principle of HQ, CT and RS on the UiO-66/MC-3/ GCE are described as follows:

Table 1 Performance comparison of different electrochemical sensors for the determination of dihydroxybenzene isomers. Sensing film

Analyte

Linear range (μM)

Sensitivity (μA μM−1)

LOD (μM)

Ref

CNFa

HQ CT RS

1–300 1–250 1–200

0.048 0.087 0.024

0.40 0.50 0.80

[18]

CS/MWCNTs/PDA/ AuNPsb

HQ CT RS

0.1–10 0.1–10 –

3.625 2.729 –

0.035 0.047 –

[19]

RGO/Cu-NPs

HQ CT RS

3–350 3–350 12–200

– – –

0.032 0.025 0.088

[20]

CdS/r-GO

HQ

0.090, 0.012

0.054

[21]

CT RS

0.2–60, 60– 2300 0.5–1350 1.0–500

0.033 0.040

0.09 0.23

HQ CT RS

0.5–1000 1.0–950 5.0–600

0.045 0.058 0.031

0.17 0.28 1.0

[22]

HQ

1.366, 0.785

0.05

[42]

1.668, 0.725

0.14

RS

0.5–10, 10– 70 0.5–10, 10– 70 –

–

–

HQ CT RS

5–90 5–120 5–90

– – –

0.08 0.18 2.62

[43]

PANI/MnO2e

HQ CT RS

0.2–100 0.2–100 0.2–100

0.8 0.5 0.5

0.13 0.16 0.09

[44]

Graphene-chitosan

HQ CT RS

1–300 1–400 1–550

0.056 0.059 0.025

0.75 0.75 0.75

[45]

CFGf

HQ CT RS

1–190 5–250 –

1.83 1.24 –

0.60 0.20 –

[46]

Pt/ZrO2−RGO

HQ CT RS

1–1000 1–400 –

0.026 0.017 –

0.40 0.40 –

[47]

MOF-ERGO-5

HQ CT RS

0.1–476 0.1–566 –

0.056 0.038 –

0.10 0.10 –

[48]

Cu-MOF-199/ SWCNTs

HQ CT RS

0.1–1453 0.1–1150 –

0.048 0.045 –

0.08 0.10 –

[49]

Cu3(btc)2/CSERGO

HQ CT RS

5.0–400 2.0–200 1.0–200

0.017 0.069 0.080

0.44 0.41 0.33

[50]

UiO-66/MC-3

HQ

0.5–5.0, 5.0–100 0.4–100 30–100, 100–400

0.360, 0.147

0.056

This work

0.142 0.034, 0.018

0.072 3.51

CD/r-GO

N,S-AGR

c

CT

p-rGOd

CT RS

3.5. Quantitative analysis of DBIs In terms of quantitative analysis, DPV technique possesses higher sensitivity and better resolution than CV method. So the DPV is utilized to evaluate the quantitative analysis performance of DBIs at the UiO66/MC-based sensor. 3.5.1. Selective determination of HQ, CT and RS from their mixtures The selective determination of HQ, CT and RS in their mixed components was performed by changing the concentration of one component and keeping the concentrations of the other two isomers constant. Fig. 9A depicts the DPV responses with increasing amounts of HQ at UiO-66/MC-3/GCE in the present of 50 μM CT and 100 μM RS. The peak currents of CT and RS are hardly change, when the oxidation peak currents of HQ enhance gradually with the increase concentrations. Two liner regression equations are also obtained which are calculated as Ipa (μA) = 0.360 C (μM) + 5.190 (0.5–5.0 μM, R2 = 0.994) and Ipa (μA) = 0.147 C (μM) + 6.393 (5.0–100 μM, R2 = 0.999). When signal-to-noise ratio of 3 (S/N = 3), the detection limit of HQ is estimated to be 0.056 μM. Similarly, as shown in Fig. 9B, when keeping the concentration of HQ and RS constant (50 μM and 100 μM), the oxidation peak current increased linearly with an increasing concentration of CT. From the inset of Fig. 9B, the increase of peak currents of CT fit the linear equation of Ipa (μA) = 0.142 C (μM) − 2.775 (R2 = 0.994) when the concentrations of CT are in the range of 0.4–100 μM. The detection limit for CT is 0.072 μM (S/N = 3). Fig. 9C exhibits DPVs of different concentrations of RS in 0.1 M PBS (pH 6.0) coexisting with 50 μM HQ and 50 μM CT. The inset of Fig. 9C shows that the relationship between Ipa and RS concentrations is covered by two linear ranges of 30–100 μM and 100–400 μM with regression equations of Ipa (μA) = 0.034 C (μM) − 0.489 (R2 = 0.999) and Ipa (μA) = 0.018 C (μM) + 1.117 (R2 = 0.998). The detection limit for RS is estimated to be 3.51 μM (S/N = 3). Additionally, in Fig. 9A-C, when the concentration of one analyte is increased, a slight change of peak currents for the two fixed analytes can be observed, which is primarily attributed to the competitive absorption between them. Thus, the selective and sensitive determination of HQ, CT and RS are evaluated simultaneously with UiO-66/MC-3/GCE. Table 1 summarized the comparison of analytical performance of DBIs at UiO-66/MC-3/GCE and that at the other sensors reported recently. Comparing this result with the literature values, it can be concluded that a significant improvement in sensing performance has been achieved towards the

a

Carbon nano-fragment. Chitosan/multiwalled carbon nanotubes/polydopamin/glod nanoparticles. c Nitrogen (N) and sulfur (S) co-doped activated grapheme. d Porous reduced grapheme oxide. e Polyaniline/manganese dioxide nanofibers. f Carboxyl functionalized grapheme. b

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Table 2 Determination of dihydroxybenzene isomers in real water samples at the UiO-66/MC-3/GCE (n = 3). Sample

Origina (μM)

Added (μM)

Found (μM)

Recovery (%)

RSD (%)

HQ

CT

RS

HQ

CT

RS

HQ

CT

RS

HQ

CT

RS

Tap water

–

10 40 90

10 40 90

50 100 200

10.48 39.72 91.93

9.72 41.65 91.60

47.37 99.11 199.5

104.8 99.30 102.1

97.2 104.1 101.8

97.7 99.1 99.7

1.0 0.6 0.4

1.5 2.6 0.7

2.0 2.2 1.6

Lake water

–

20 50 80

20 50 80

50 100 150

20.29 49.42 79.85

21.12 51.43 78.96

49.26 150.5 202.2

101.5 98.8 99.8

105.6 103.9 98.7

98.5 100.3 101.1

0.7 2.1 0.6

0.6 2.3 1.8

2.1 0.8 0.7

C). In addition, the effects of some organic and biological molecules (10-fold excess) such as 2-nitrophenol, 4-nitrophenol, p-aminophenol, glycine, glucose, urea acid and ascorbic acid on electrochemical response of DBIs were also investigated (Fig. S4D–F). The results indicate that these substances do not give rise to noticeable change in electrochemical signals of HQ, CT and RS. All these results suggest that the proposed sensor has a excellent selectivity and satisfactory antiinterference ability for the determination of DBIs.

detection of DBIs. Most important of all, the UiO-66/MC-3 composite modified electrode is a suitable electrode system for the simultaneous determination of DBIs. 3.5.2. Simultaneous determination of HQ, CT and RS from their mixtures Furthermore, the DPV current responses of DBIs were also determined by simultaneously increasing their concentrations under optimized conditions. As shown in Fig. 9D, with the concentrations of three isomers increase simultaneously, the oxidation peak currents will be linearly increased accordingly. The oxidation peak currents of HQ and CT increase linearly with their concentrations in the ranges from 5.0 to 50 μM. The linear equations are Ipa (μA) = 0.289 C (μM) + 5.807 (R2 = 0.995) for HQ and Ipa (μA) = 0.234 C (μM) + 4.345 (R2 = 0.999) for CT. The detection limits for HQ and CT are calculated to be 0.081 μM and 0.076 μM (S/N = 3), respectively. For RS, the regression equation is calibrated as Ipa (μA) = 0.067 C (μM) – 2.828 ( 50–150 μM, R2 = 0.996), and the detection limit of 1.521 μM is determined by S/N = 3. The results suggest that the simultaneous determination of DBIs can be achieved sensitively and selectively at the UiO-66/MC-3/GCE. All these assays indicate that the proposed sensor allows the simultaneous and sensitive detection of HQ, CT and RS without interference from each other.

3.8. Real sample analysis To evaluate feasibility of UiO-66/MC-3 modified electrode, the diluted tap water and lake water samples were spiked with DBIs and tested by the standard addition method. As shown in Table 2, the recoveries are 98.8–104.8%, 97.2–105.6% and 97.7–101.1% for HQ, CT and RS, respectively. These results exhibit that the sensor have a good feasibility and reliability for simultaneous detection of HQ, CT and RS in real water samples. 4. Conclusion In this work, a high electrochemical stability and electrocatalytic activity zirconium-based MOF of UiO-66 and mesoporous carbon (MC) composites (UiO-66/MC) were fabricated by conventional hydrothermal method for the first time. The as-prepared UiO-66/MC-3 sample not only possesses excellent electrochemical stability, but also has a larger pore size and a good conductivity, which can provide faster electron transfer and contribute to mass transfer. Therefore, the composite is utilized as novel electrode material for the simultaneous and sensitive determination of dihydroxybenzene isomers (DBIs) of hydroquinone (HQ), catechol (CT) and resorcinol (RS). Electrochemical assays reveal that the fabricated sensor exhibits outstanding performance for simultaneous determination of DBIs with a large peak potential difference. In addition, the sensor also displays excellent selectivity, stability and reproducibility. The satisfactory results are obtained for the determination of the DBIs in the real samples by the prepared sensor. As a result, successful fabrication of UiO-66/MC not only promotes the development of new porous composite materials, but also provides new idea for the design of electrochemical sensors.

3.6. Stability and reproducibility The stability and reproducibility are important parameters to evaluate the performance of the sensors. The stability of proposed sensor was discussed by stored in air at ambient temperature for 15 days, and recorded intervals of three days (Fig. S3, Supporting information). After 15 days, it is found that the sensor retains about 93.4%, 94.1% and 90.4% of the initial current for HQ, CT and RS, respectively. Moreover, the reproducibility of sensor was evaluated by preparing five parallel electrodes for the determination of 0.1 mM HQ, CT and RS under the optimized conditions. The relative standard deviations (RSDs) of the five electrodes are 2.97%, 3.23% and 5.07% for HQ, CT and RS, respectively. All results display that the sensor has good stability and reproducibility. 3.7. Interference studies

Acknowledgments

One important issue for the feasibility of a sensor is its capability to distinguish the analytes from interferents. In this work, the selectivity of the sensor towards the other species was investigated by DPV. The electrochemical responses of 0.1 mM HQ, CT and RS were measured at the UiO-66/MC-3/GCE in the presence of different interferents (Fig. S4, Supporting information). The results show that the common inorganic ions such as K+, Na+, Mg2+, Zn2+, Cu2+, Al3+, Fe3+, Cl-, NO3- and SO42- in 10-fold excess almost do not affect the response of HQ, CT and RS signals at the composite modified electrode (Fig. S4A–

Financial support from National Natural Science Foundation of China (21575021) is highly appreciated. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.talanta.2017.06.061.

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