Environmentally friendly synthesis of Co-based zeolitic imidazolate framework and its application as H2O2 sensor

Journal Pre-proofs Environmentally friendly synthesis of Co-based zeolitic imidazolate framework and its application as H2O2 sensor Yuhua Dong, Jianbin Zheng PII: DOI: Reference:

S1385-8947(19)33105-5 https://doi.org/10.1016/j.cej.2019.123690 CEJ 123690

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

5 September 2019 11 November 2019 2 December 2019

Please cite this article as: Y. Dong, J. Zheng, Environmentally friendly synthesis of Co-based zeolitic imidazolate framework and its application as H2O2 sensor, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/ j.cej.2019.123690

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Environmentally friendly synthesis of Co-based zeolitic imidazolate framework and its application as H2O2 sensor Yuhua Donga, Jianbin Zhenga*

Abstract: The work presented that by regulating the proportion between cobalt and 2-methylimidazole in distilled water, a series of Co-based imidazolate framework compounds with diverse shapes were synthesized at ambient temperature. The morphology and structural characteristics of the compounds were surveyed by scanning electron microscope, energy disperse spectra, powder X-ray diffraction and N2 absorption-desorption. It was found by cyclic voltammetry that the zeolitic imidazolate framework compound synthesized at a ratio of 1:20 had the best electrocatalytic activity to H2O2 reduction, therefore, the compound was chosen to fabricate a H2O2 electrochemical sensor. Electrochemical studies indicated that the sensor had a determination limit of 0.7 μM and short response time within 3 s. The catalytic current was linear with H2O2 concentration from 2.5 to 212.5, 212.5 to 1662.5 and 1662.5 to 6662.5 μM and the sensitivity were 12.2, 5.3 and 2.4 μA mM-1 cm-2, respectively. Furthermore, the proposed sensor showed a good anti-interference capacity, repeatability and stability. Keywords: Electroanalysis; Electrocatalysis; zeolitic imidazolate framework; H2O2 1. Introduction Assembled by the metal ions and organic chain, metal-organic frameworks (MOFs) own many inherent advantages such as porous, periodic network skeleton, rich unsaturated metal sites, tunable structure and function

[1].

However, since most

MOF materials have the disadvantage of poor stability, they are often used after calcination

[2-4].

Zeolitic imidazolate framework materials (ZIFs), one branch of

MOFs, contain not only high stability of inorganic zeolites but also the structural character of MOFs

[5,6],

which allow ZIFs to be employed as catalyst without

calcination. Tran et al [7] used ZIF-8 as a catalyst for Knoevenagel reaction. The ZIF-8 showed a high catalytic activity and there was no significant decrease in catalytic activity after repeated use. Suttipat et al

a.

*

[8]

prepared the hierarchical faujasite/ZIF-8

College of Chemistry and Materials Science, Shaanxi Provincial Key Laboratory of Electroanalytical Chemistry, Northwest University, Xi’an, Shaanxi 710069, China Corresponding author. Tel.: +86-29-88302077; Fax: +86-29-88303448; E-mail address: [email protected] (J. B. Zheng). 1

(FAU/ZIF-8) by stepwise depositing ZIF-8 on the surfaces of imidazolate modified zeolite, and the FAU/ZIF-8 exhibied outstanding catalytic properties for the aldol condensation. Wang et al

[9]

discovered that ZIF-67 not only can catalyze

3,3′,5,5′-tetramethylbenzidine but also act as a colorimetric sensor for detecting H2O2 quantitatively. Tuan et al [10] proved that ZIF-67 can catalyze the hydrolysis of NaBH4 to produce H2. As one of ZIFs, ZIF-67 own zeolite sod topologies and is formed by Co2+ as the metal node and 2-methylimidazole as the organic ligand. The specific pore of ZIF-67 is conductive to the immobilization of biomolecules and can separate substance at the molecular level. Zhao et al [11] used ZIF-67/GCE to detect tripeptide glutathione (GSH). Because of the microporous nature, ZIF-67 only allows molecules of certain sizes to pass, solving the interference problem of easily oxidized compounds which are common when detecting GSH. Moreover, the high stability of ZIF-67 makes it usable under many conditions, broadening essentially the application ranges of the as-prepared biosensors

[12].

The ZIF-67 possesses large pore size, high

stability, both redox metal active site and organic functional groups, making it is considered to be an efficient electrochemical sensing material

[13-15].

So far, most

synthetic methods of ZIF-67 involve the use of organic reagents, such as methanol [16], dimethyl sulfoxide

[17],

N,N-dimethylformamide

[18]

and N,N-diethylformamide

[19].

However, some organic reagents will occupy the pores of ZIF-67 and are difficult to remove

[20].

Besides, they will also cause environmental pollution and increase the

costs of synthesis. Therefore, it makes sense to synthesize ZIF-67 by an easy and environmentally friendly method [21,22]. Hydrogen peroxide (H2O2) act a critical role in areas of chemical engineering, food, pharmaceutical and environmental with its good performance

[23-26].

However,

the high concentration of H2O2 can pose a threat to human health, such as corroding the skin, injuring the organs, damaging the central nervous system, even causing cancer

[27-30].

What's more, the World Health Organization classified H2O2 as a

carcinogen. Hence, it is meaningful to propose an easy and efficient way for H2O2 detection. Compared to other methods for detecting H2O2, the electrochemical method stands out because of its advantages, such as simple, fast, economical and sensitive [31-34].

Vilian et al

[31]

proposed a simple method to immobilize catalase onto L-lysine

modified functionalized MWCNTs surface for constructing a H2O2 biosensor, and the proposed sensor exhibited an outstanding electrocatalytic activity to H2O2 in a short 2

time. Ensafi et al [32] assembled silver nanoparticles (Ag NPs) on the surface of porous silicon (PSi) by a simple and fast method without any reducing agent. The Ag NPs/Psi was used to fabricate an Ag NPs/PSi modified CPE for H2O2 sensoring which can rapidly and selectively detect H2O2. Tong et al [33] prepared biphasic nickel phosphide nanosheets (BNPNS) electrocatalyst by directing high temperature phosphidation of nickel foam (NF). And the prepared BNPNS/NF electrocatalyst showed a quick and sensitive electrochemical response to H2O2. Herein, we reported the synthesis of Co-based imidazolate framework compounds with different morphologies in water at room temperature. And the Co-based zeolitic imidazolate framework compounds served as an electrochemical sensing material to detect H2O2. 2.

Experimental

2.1. Reagents 2-Methylimidazole (Hmim, AR), Cobalt chloride hexahydrate (CoCl2·6H2O, AR) and H2O2 (30%, AR) were bought from Aladdin Co. Ltd. (Shanghai, China), Guangzhou Jin hua da Chemical Reagent Co. Ltd. (Guangzhou, China) and Tianjin TIANLI Chemical Reagents Co. Ltd. (Tianjin, China) respectively. The supporting electrolyte was 0.2 M Sodium hydroxide (NaOH, AR, which was obtained from Guangzhou Jin hua da Chemical Reagent Co. Ltd.). All experiments were carried out at room temperature and the applied water was double-distilled water. 2.2. Apparatus A scanning electron microscopy (SEM, JSM 6700F, JEOL, Japan) was used to get the SEM images and the energy disperses spectroscopy (EDS) spectra of Co-based imidazolate framework compounds. An X-ray diffractometer (XRD, D/MAX 3C, Rigaku, Japan) was applied to obtain the XRD patterns of five Co-based imidazolate framework compounds. A TR2 Star3020 (Micromeritics, USA) volumetric absorption analyzer was used to gain the N2 absorption/desorption curves of five Co-based imidazolate framework compounds. A CHI660E electrochemical workstation (Shanghai CH Instrument Co. Ltd., China) was employed to do the electrochemical experiments. The working electrode was the Co-based imidazolate framework compound modified glassy carbon electrode (GCE) (the reference electrode and counter electrode was saturated calomel electrode and platinum wire electrode respectively). 3

2.3. Preparation of modified glassy carbon electrode 2.3.1.

Synthesis of Co-based imidazolate framework compounds Synthesis of L-ZIF-67: After 2.2167 g Hmim dissolved in 50.0 mL water,

0.6424 g CoCl2·6H2O was poured into and the solution was stirred continuously for 7.0 h in room temperature. The product was collected by centrifugation and was recorded as L-ZIF-67 (the molar ratio of CoCl2·6H2O and Hmim was 1:10). Synthesis of S-ZIF-67: The synthesis process of S-ZIF-67 was similar to that of L-ZIF-67, and the only difference was that the CoCl2·6H2O was 0.4425 g (the molar ratio of CoCl2·6H2O and Hmim was 1:15). Synthesis of C-ZIF-67: The synthesis process of C-ZIF-67 was similar to that of L-ZIF-67, and the only difference was that the CoCl2·6H2O was 0.3244 g (the molar ratio of CoCl2·6H2O and Hmim was 1:20). Synthesis of H-ZIF-67: The synthesis process of H-ZIF-67 was similar to that of L-ZIF-67, and the only difference was that the CoCl2·6H2O was 0.2569 g (the molar ratio of CoCl2·6H2O and Hmim was 1:25). Synthesis of G-ZIF-67: The synthesis process of G-ZIF-67 was similar to that of L-ZIF-67, and the only difference was that the CoCl2·6H2O was 0.2141 g (the molar ratio of CoCl2·6H2O and Hmim was 1:30). 2.3.2.

Electrode Modification

After polishing with alumina powders, the GCE was cleaned with ethanol and water sequentially. Then 6.5 μL Co-based imidazolate framework compounds suspension (2.0 mg/mL) and 4.0 μL nafion (0.05%) were dripped on the surface of GCE successively. After the modified electrodes dried, they were applied to the succedent electrochemical experiments. 3.

Results and discussion

3.1. Characterizations of the Co-based imidazolate framework compounds The morphologies of the Co-based imidazolate framework compounds were studied by SEM and displayed on Fig.1. As can be observed in Fig.1, when the molar ratio of CoCl2·6H2O and Hmim was 1:10, 1:15, 1:20, 1:25 and 1:30 respectively, the shape of Co-based imidazolate framework compounds were similar to the shape of leaf (Fig.1 A, B), spearhead (Fig.1 D, E), crassulaceae plant (Fig.1 G, H), hydrangea (Fig.1 I, K) and garnet (Fig.1 M, N) with size of 5.8, 2.6, 1.9, 1.9 and 1.5 μm in length accordingly. Certainly, as the proportion of Hmim increased, the length of particles 4

gradually decreased and the particles became more three-dimensional (3D). In addition, the surface of the five compounds had different roughness. The SEM figures indicated that the molar ratio of Hmim and Co2+ had a huge impact on the morphological of Co-based imidazolate framework compounds. Beyond that, the EDS spectra demonstrated the existence of Co, C and N (Fig.1 C, F, I, L, O). The formation mechanism for different ZIF-67 formations can be described as follow based on the SEM characterization results and the change of pH in the reaction system (Video 1): Due to the high pKa value of Hmim, it hydrolyzed rather than deprotonated in water first [35,36] (So the initial Hmim solution was alkaline). Hmim + H2O

Hydrolysis

H2mim+ + OH-

(1)

After Co2+ added, they linked with Hmim via coordination process to form CoII-imidazole complexes (Co(Hmim)n2+ (1 ≤ n ≤ 4)) (This process hinders the hydrolysis of Hmim (equation 1), causing a drop in the pH of the reaction system). Co2+ + Hmim

Coordination

Co(Hmim)n2+ (1 ≤ n ≤ 4)

(2)

Then, free Hmim as a deprotonating agent caused Co(Hmim)n2+ to lose protons (which can be deduced from the pH(Hmim) > pH(Co(Hmim)n2+)), resulting in the formation of ZIF-67 crystal nucleus in water eventually. Co(Hmim)n2+

Deprotonation

ZIF-67

(3)

When the molar ratio of Co2+ and Hmim was 1:10, the growth of ZIF-67 crystal nucleus only stayed in the 2D layer-structured stage due to the deficiency of Hmim ligands

[37],

and the final 2D layers were assembled into leaf-like product (L-ZIF-67)

with a larger size by the hydrogen bond. As the proportion of Hmim increased, the spare Hmim acted as a deprotonating agent to break Co-Hmim bond and form Co-mim bond, which decomposed the assembled 2D layer-structured into smaller crystals. When the molar ratio of Co2+ and Hmim was 1:15, the Hmim was insufficient to fully form the Co-mim bond, which generated a smaller-sized spearheads-like product (S-ZIF-67) mixed with some large particles. When the amount of Hmim was sufficient, a large amount of smaller crystal nucleus generated and started to random assemble into a 3D structure. Finally, for different proportions of couple of Co2+ and Hmim, the crystals grew into crassulaceae plant-like (C-ZIF-67, 1:20), hydrangea-like (H-ZIF-67, 1:25) and garnet-like (G-ZIF-67, 1:30) products in 5

the direction of minimum free energy. The structure of the five Co-based imidazolate framework compounds were investigated using XRD and showed in Fig.2. In the figure, the peaks of L-ZIF-67 and S-ZIF-67 almost appeared at the same position, which were corresponded to the topology structure of both net-Co(im)2 and simulated ZIF-67 (curve a and curve b) [38]. In addition, the C-ZIF-67, H-ZIF-67 and G-ZIF-67 had the same topology structure which was identified as simulated ZIF-67 (curve c, curve d and curve e). The XRD patterns illustrated that when the percent of Hmim was low, the product had a net zeolite-like topology apart from a sod zeolite topology. Besides, as the Hmim ratio increased, the products with relatively pure sod zeolite topology were obtained. The nitrogen adsorption-desorption of the five Co-based imidazolate framework compounds were tested to investigate the types of pore (Fig.3). The L-ZIF-67, S-ZIF-67, C-ZIF-67 and H-ZIF-67 showed a mixed adsorption-desorption isotherm of a type-IV with a H3-type hysteresis loop (at near P/P0=0.85) and a type-I isotherm (curve a, curve b, curve c and curve d), indicating the existence of micropores and mesopores simultaneously. The presence of the type-IV isotherm might be caused by a large number of mesopores between particles. However, for G-ZIF-67, there was only the typical type-I adsorption-desorption isotherm, which illustrated the existence of micropores. Besides, the Barreett–Joyner–Halenda (BJH) curves of the five Co-based imidazolate framework compounds were got from the desorption branch of isotherms (Fig.S1), which indicated that all the five Co-based imidazolate framework compounds possessed pores with sizes around 10 nm. Moreover, the pore parameters of the five Co-based imidazolate framework compounds were listed in Table S1, which showed that the total pore volume and the proportion of micropores all increased with increasing Hmim proportion. 3.2. Electrochemical study 3.2.1.

Cyclic voltammetric responses of the five Co-based imidazolate

framework compounds modified GCE The electrochemical impedance spectra (EIS) of different Co-based imidazolate framework compounds modified GCE and bare GCE were taken to study the conductivity of them. As shown in Fig.S2 (A), after the Co-based imidazolate framework compounds were modified on GCE, the diameter of the semicircle increased significantly (curve a, b, d and e), indicating that the ZIF-67 modified 6

electrodes had higher charge transfers resistance value (Rct) than GCE (curve f). In addition, the C-ZIF-67/GCE possessed the smallest semicircular diameter (curve c) among the Co-based imidazolate framework compounds modified GCE, which indicated that C-ZIF-67 had the smallest Rct among them. The impedance at the high frequency limit was the solution resistance (Rs), which represented the resistance from solution, electrode materials and membrane

[39].

In Fig.S2 (A), the impedance of

different Co-based imidazolate framework compounds modified GCE and bare GCE at the high frequency limit almost coincided. However, it can be seen from the Fig.S2 (B) that there were subtle differences among them, in which the bare GCE and C-ZIF-67/GCE had relatively small Rs than other modified electrodes. The result of EIS revealed that C-ZIF-67 had the best conductivity among the five Co-based imidazolate framework compounds. The chronocoulometry was applied to research the electroactive effective surface areas of different Co-based imidazolate framework compounds modified GCE which was displayed on Fig.S3. According to the Anson formula: 𝑄𝑡 = (2𝑛𝐹𝐴𝑐𝐷

12 12

𝑡

) 𝜋1 2 + 𝑄𝑑𝑙 + 𝑄𝑎𝑑𝑠

(4)

where n is the amount of electron transferred; A denotes electroactive effective surface areas; c represents the [Fe(CN)6]3- concentration; D means the diffusion coefficient; Qdl stands for double layer charge; Qads denotes Faradic charge; F and π are their common value. According to the slopes of carve a, b, c, d and e in Fig.S3, the A

of

L-ZIF-67/GCE,

S-ZIF-67/GCE,

C-ZIF-67/GCE,

H-ZIF-67/GCE

and

G-ZIF-67/GCE were calculated to be 0.05545, 1.028, 10.21, 0.1305 and 0.09430 cm2 respectively. The result demonstrated that the C-ZIF-67/GCE had the largest electroactive effective surface areas among the five Co-based imidazolate framework compounds modified GCE. To further determine the molar ratio of Hmim to CoCl2·6H2O for the Co-based imidazolate framework compound that produced the best electrochemical performance, the cyclic voltammetric (CV) response of the five Co-based imidazolate framework compounds modified GCE to H2O2 were investigated. As can be seen from Fig.S4, all the five Co-based imidazolate framework compounds modified GCE 7

exhibited a small reduction peak in the absence of H2O2, which might arise from the redox process of Co( Ⅱ ) and Co( Ⅲ ) (curve a, b, c, d)

[40,41].

As shown in Fig.4, after

adding 3.0 mM H2O2, the current response of the five Co-based imidazolate framework compounds modified GCE were all increased in varying degrees and the peak potential of the five modified electrodes were also different, in which the C-ZIF-67/GCE showed the largest response of 27.39 μA at about -0.456 V (curve cc’). That might be because C-ZIF-67 had larger pore volume and more mesopores than L-ZIF-67, S-ZIF-67 and H-ZIF-67/GCE (see Table 1), which can accelerate the mass

transfer

process

and

promote

the

reaction

process.

Furthermore,

C-ZIF-67/GCE had the best conductivity (Fig.S2) and the largest electroactive effective surface areas (Fig.S3) among the five Co-based imidazolate framework compounds modified GCE, which gave higher electrocatalytic behavior to C-ZIF-67/GCE. It was well known that the peak potential which closed to zero would make noise and interference small to ensure good response. From the line graph of Fig.4 (B), the C-ZIF-67/GCE had the largest current response and acceptable peak potential. Therefore, the Co-based imidazolate framework compounds possessed the best electrochemical performance when the molar ratio of CoCl2·6H2O to Hmim was 1:20. 3.2.2

Electrochemical behavior of C-ZIF-67/GCE The CV responses of bare GCE and C-ZIF-67/GCE to H2O2 reduction were

investigated. As exhibited in Fig.5 (A), the CV response of bare GCE were almost unchanged before and after adding H2O2 (curve a and a’), indicating the GCE had no effect on the electrochemical reduction of H2O2. Unlike bare GCE, the peak current of C-ZIF-67/GCE increased significantly after adding H2O2 (curve b and b’) and the peak potential was closer to zero, which might because the C-ZIF-67 not only was rich in Co2+ to promote the reduction of H2O2, but also had a large number of micropores and mesopores to accelerate the mass transfer process. In addition, the current response was improved gradually by increasing of the concentration of H2O2 (Fig.5 (B)), indicating C-ZIF-67/GCE had excellent electrochemical performance to H2O2 reduction. The electrochemical behavior of C-ZIF-67/GCE to H2O2 reduction was further investigated by recording the cyclic voltammetric responses of C-ZIF-67/GCE at various sweep speed. In Fig.6 (A), with the increasing sweep speed, the resultant 8

reduction peaks current increased gradually, and the reduction peak potential deviated from 0 V gradually. In Fig.6 (B), the peaks current was directly proportional to the square root of sweep rate (v1/2) (R=0.9989), implying that the process of H2O2 reduction was controlled by diffusion. Fig.6 (C) exhibited that the reduction peak potential (Ep) was linearly related to log v with a linear regression equation of Ep = 0.5372 + 0.04649 log v (R=0.9988). It was known fromTafel formula [42]: 𝐸𝑝 =

2.3𝑅𝑇

𝑛𝛼𝐹 log 𝑣

(5)

+K

where α denotes electron transfer coefficient; n represents electron transfer number; R, T and F are their common value. Therefore, nα was calculated to be 1.27. In addition, it was usually assumed that α was 0.5 in the process of completely irreversible electrode

[43].

Hence, the n was about 2, which suggested that the H2O2

reduction involved two-electron transfer. Thus, the feasible mechanism of H2O2 reduction was described as below [44]: 2CoII-MOF + H2O2 → 2CoIII-MOF + 2OHThe amperometric responses of C-ZIF-67/GCE at various applied potentials were recorded to seek the optimum working potential. From Fig.7, it was clear that when the absolute value of the applied potential increased, the amperometric responses of C-ZIF-67/GCE raised significantly, however, the baseline also became growing unstable which might result from the more noise and interfering substances producing at high applied potentials. And there was a relatively high amperometric response and acceptable baseline fluctuations when the applied potential was -0.35 V. Hence, the optimum working potential was appointed to be -0.35 V. The sensing performance of C-ZIF-67/GCE was further surveyed by amperometry at -0.35 V. As depicted in Fig.8 (A), the current response of C-ZIF-67/GCE to H2O2 showed a stair-stepping increase. And the higher the H2O2 concentration was, the bigger the echelonment response was. Moreover, each time H2O2 was added, amperometric response increased rapidly and stabilized within 3.0 s (Fig.8 (B)), indicating that the C-ZIF-67/GCE had fast response to H2O2. But, the amperometric response showed an unacceptable fluctuation on the sixth addition of 1 mM H2O2, implying the H2O2 concentration was so high that the C-ZIF-67/GCE no longer work. The correction curve of the current of C-ZIF-67/GCE to H2O2 concentration was exhibited in Fig.8 (C). The figure showed different linear correlation in the range of 2.5-212.5 μM, 212.5-1662.5 μM and 1662.5-6662.5 μM 9

with sensitivities of 12.2, 5.3 and 2.4 μA mM-1 cm-2 respectively (the sensitivities of the proposed sensor were got by dividing the slope of the fitted curve by the geometric surface area of the GCE), in addition, all the related coefficients were above 0.9985. Beyond that, the detection limit was calculated to be 0.7 μM by the equation: LOD = 3sb/ kl, in which sb is the standard deviation of blank solution and kl represents the slope of the low concentration section (the signal to noise ratio was 3). The calibration curve showed three-segment linearity, which might be related to the surface morphology and distribution of C-ZIF-67 on the GCE surface, reaction kinetics and the mass transport process etc [45]. Table 1 listed some reported cobalt-based and MOF-based H2O2 sensors. Compared to Co3O4/MWCNTs/CPE with high sensitivity [44], the linear range of the proposed sensor was two orders of magnitude wider and the detection limit was about two-seventh of that of Co3O4/MWCNTs/CPE. Besides, the sensitivity and linear range of the proposed sensor was 2.3 times and 10 times than that of the Fe-MOF/rGO/CPE with the lowest detection limit

[50],

respectively. The good

sensing performance of the proposed sensor was probably because the C-ZIF-67 owned more metal active sites, high porosity and specific area [55,56]. Fig.9 showed the amperometric test of C-ZIF-67/GCE to H2O2 and some disrupting substances to study its capacity of resisting disturbance. From the figure, it was clear that the proposed sensor had a significant amperometric response to H2O2. However, the current response was small and negligible when the disrupting substances with same concentration were added, demonstrating that there was a superior anti-interference ability of the C-ZIF-67/GCE. The repeatability of the C-ZIF-67/GCE were researched by using the amperometry. From Fig.10 (A), it can be seen that the current response of five identical C-ZIF-67/GCE to H2O2 showed a stair-stepping increase. According to the height of each stair, the response current of the proposed sensor to 0.3 mM H2O2 can be calculated. The average height of the five stairs on each curve represents the current response of the corresponding modified electrode to 0.3 mM H2O2, which were showed in Fig.10 (B). According to the relative standard deviation (RSD) formula: RSD =

(

∑n

(xi - x)2

i=1

n-1

)

x × 100%, the RSD of the current response of the

five identical modified electrode was calculated to be 5.1%. Fig.10 (C) and Fig.10 (D) 10

revealed that the current response was reduced to 81.3% of the original value after 25 days. Fig.10 confirmed that the C-ZIF-67/GCE had satisfactory repeatability and stability. 4.

Conclusions To sum up, the Co-based imidazolate framework compounds with different

morphologies were fabricated successfully in water at room temperature. Among them, the C-ZIF-67 owned the best electrocatalytic performance for H2O2 reduction and was used to construct a H2O2 electrochemical sensor. The detection limit of the H2O2 sensor was as low as 0.7 μM and the linear range of it was up to four orders of magnitude. This article presented a green and environmentally friendly method for synthesizing Co-based imidazolate framework compounds and also can be a reference in constructing other MOF-based H2O2 sensors. Acknowledgements The authors acknowledge the funding support by the National Natural Science Foundation of China [No. 21575113] and the Natural Science Foundation of Shaanxi Province in China [No. 2017JM2036, 2018JQ2029] References [1] Zhou H C, Long J R, Yaghi O M. Introduction to metal-organic frameworks[J]. Chemical Reviews, 112 (2012) 673-674. [2] Li Y, Xu Y, Yang W, et al. MOF-Derived Metal Oxide Composites for Advanced Electrochemical Energy Storage[J]. Small, 14 (2018) 1704435. [3] Zhao J, Dong W, Zhang X, et al. FeNPs@Co3O4 hollow nanocages hybrids as effective peroxidase mimics for glucose biosensing[J]. Sensors and Actuators B: Chemical, 263 (2018) 575-584. [4] Li S, Wang L, Zhang X, et al. A Co, N co-doped hierarchically porous carbon hybrid as a highly efficient oxidase mimetic for glutathione detection[J]. Sensors and Actuators B: Chemical, 264 (2018) 312-319. [5] Park K S, Ni Z, Côté A P, et al. Exceptional chemical and thermal stability of zeolitic imidazolate frameworks[J]. Proceedings of the National Academy of Sciences, 103 (2006) 10186-10191. [6] Shi Q, Chen Z, Song Z, et al. Synthesis of ZIF-8 and ZIF-67 by steam-assisted conversion and an investigation of their tribological behaviors[J]. Angewandte Chemie International Edition, 50 (2011) 672-675. [7] Tran U P N, Le K K A, Phan N T S. Expanding applications of metal-organic frameworks: zeolite imidazolate framework ZIF-8 as an efficient heterogeneous catalyst for the knoevenagel reaction[J]. Acs Catalysis, 1 (2011) 120-127. 11

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and

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of

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peroxide[J].

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[55]Feng X, Lin S, Li M, et al. Comparative study of carbon fiber structure on the electrocatalytic performance of ZIF-67[J]. Analytica chimica acta, 984 (2017) 96-106. [56]Tang J, Jiang S, Liu Y, et al. Electrochemical determination of dopamine and uric acid using a glassy carbon electrode modified with a composite consisting of a Co(II)-based

metalorganic

framework

Microchimica Acta, 185 (2018) 486.

15

(ZIF-67)

and

graphene

oxide[J].

Fig.1. SEM images of L-ZIF-67 (A, B), S-ZIF-67 (D, E); C-ZIF-67 (G, H), H-ZIF-67 (J, K) and G-ZIF-67 (M, N); EDS spectra of L-ZIF-67 (C), S-ZIF-67 (F), C-ZIF-67 (I), H-ZIF-67 (L) and G-ZIF-67 (O)

Fig.2. XRD patterns of L-ZIF-67 (a), S-ZIF-67 b), C-ZIF-67 (c), H-ZIF-67 (d) and G-ZIF-67 (e)

16

Fig.3. N2 adsorption-desorption isotherms of L-ZIF-67 (a), S-ZIF-67 b), C-ZIF-67 (c), H-ZIF-67 (d) and G-ZIF-67 (e)

Fig.4. (A) CVs of L-ZIF-67/GCE (aa’), S-ZIF-67/GCE (bb’), C-ZIF-67/GCE (cc’), H-ZIF-67/GCE (dd’) and G-ZIF-67/GCE (ee’) to 3.0 mM H2O2. Scan rate of 100 mV s−1; (B) The line graph of peak current and potential (with 3.0 mM H2O2) vs different modified electrode

17

Fig.5. (A) CVs obtained at bare GCE (a, a’) and C-ZIF-67/GCE (b, b’) in the absence (a, b) and presence of 3.0 mM H2O2 (a’, b’); (B) CVs obtained by C-ZIF-67/GCE in the presence of different H2O2 concentrations (a to f: 0, 1.0, 2.0, 3.0, 4.0, 5.0 mM). Base solution: 0.2 M NaOH. Scan rate: 100 mV/s

Fig.6. (A) CVs obtained at C-ZIF-67/GCE in the presence of 1.0 mM H2O2 at different scan rates (a’-j’: 20, 40, 60, 80, 100, 120, 140, 160, 180 and 200 mV/s); (B) The calibration curve of peak current vs v1/2; (C) The relationship between Ep and log v. Base solution: 0.2 M NaOH

18

Fig.7. Amperometric responses of C-ZIF-67/GCE to 0.1 mM H2O2 at different working potentials: -0.25 V, -0.3 V, -0.35 V and -0.4 V. Base solution: 0.2 M NaOH

Fig.8. (A) Amperometric responses obtained at C-ZIF-67/GCE after successive injection of H2O2; (B) The partial enlargement of Fig.8 (A); (C) The plots of Amperometric responses vs H2O2 concentration. Base solution: 0.2 M NaOH. Applied potential: -0.35 V

19

Fig.9. Amperometric response of C-ZIF-67/GCE to 0.5 mM H2O2 and disturbances (a-e: glucose, ethyl alcohol, citric acid, acetaminophen, NaNO2). Base solution: 0.2 M NaOH. Applied potential: -0.35 V

Fig.10. (A) Amperometric responses obtained by five identical C-ZIF-67 modified glassy carbon electrodes to 0.3 mM H2O2; (B) The histogram of corresponding response current vs electrode number; (C) Amperometric responses obtained by C-ZIF-67/GCE to 0.3 mM H2O2 at different days; (D) The histogram of response current vs days

20

Table 1. Comparison of the proposed sensor and several reported H2O2 sensors

Linear range (μM)

Detection limit (μM)

Sensitivity (μA mM-1cm-2)

Literature

Sensors

working potential (V)

Co3O4/MWCNTs/CPE

-0.19

20-430

2.46

1002.8

[44]

CoS/GCE

-0.35

5-14820

1.5

17.4

[46]

Nafion/GOD-NanoCoPc-Gr/ GCE

-0.12

10-600

10.1

-

[47]

3D N-Co-CNT@NG/GCE

-0.04

2.0-7449

2.0

28.66

[48]

Co3O4-NWs/CF

-0.48

10-1400

1.4

230

[49]

Fe-MOF/rGO/CPE

-0.4

5-945

0.5

5.17

[50]

MIL-53-CrIIIAS/GCE

-

25-500 

3.52 

11.9

[51]

Cu-MOF/MPC-3/GCE

-0.23

10-11600

3.2

2.97

[52]

Ni(II)-MOF/CNTs/GCE

0.5

10-51600

2.1

8.2

[53]

ZIF-67/GO/GCE

-0.4

100-22900

1920

0.2314

[54]

2.5 -212.5; C-ZIF-67/GCE

-0.35

212.5-1662.5;

1662.5-6662.5

21

12; 0.7

5.3; 2.4

This work

Graphical Abstract

22

Highlights:  A facile and environmentally friendly method for synthesizing Co-based imidazolate framework compounds was provided;  The

Co-based

imidazolate

framework

morphologies were synthesized;  A novel H2O2 sensor was constructed.

23

compounds

with

five

different