In Situ Synthesis of Sandwich-like Graphene@ZIF-67 Heterostructure

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Functional Nanostructured Materials (including low-D carbon)

In Situ Synthesis of Sandwich-like Graphene@ZIF-67 Heterostructure for Highly Sensitive Nonenzymatic Glucose Sensing in Human Serums Xuerong Chen, Dan Liu, Guojun Cao, Yong Tang, and Can Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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ACS Applied Materials & Interfaces

In

Situ

Synthesis

of

Sandwich-like

Graphene@ZIF-67

Heterostructure for Highly Sensitive Nonenzymatic Glucose Sensing in Human Serums

Xuerong Chen†#, Dan Liu§, Guojun Cao‡, Yong Tang‡*, Can Wu†*

† Faculty of Materials Science & Engineering, Hubei University, Wuhan 430062, China # School of Laboratory Medicine, Hubei University of Chinese Medicine, Wuhan 430065, China § College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China ‡ Department of Hepatobiliary Surgery, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, China * Corresponding author: [email protected]; [email protected]

ABSTRACT: Metal organic frameworks (MOFs) have been extensively studied in recent years due to their tunable porosity, huge specific area and controllable structure. The rich metal centers and large specific area have endowed MOFs with excellent electrochemical activity due to the multiple valence states, but the poor electronic conductivity of MOFs seriously impede their electrocatalytic performance. Here, a polyhedral Co-based zeolite imidazole frame [Co(mim)2]n (denoted as ZIF-67, mim = 2-methylimidazole) is in situ loaded at the two sides of physically-exfoliated graphene nanosheets (GS) at room temperature, and a sandwich-like GS@ZIF-67 hybrids with 1 / 39

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order nanostructure are easily obtained. Compared with each individual component, the as-synthesized GS@ZIF-67 hybrids exhibit higher electrochemical activity toward glucose oxidation. Besides, the hierarchical nanocomposites also show better electrocatalytic performance compared with the same ratio of physical mixture of GS and ZIF-67, further demonstrating the synergistic effect between ZIF-67 and GS. Thus, a highly sensitive nonenzymatic glucose electrochemical sensor is proposed with linear range of 1 - 805.5 μM, sensitivity of 1521.1 μA Mm-1 cm-2, detection limit of 0.36 μM (S/N = 3), and excellent stability and selectivity. More importantly, the newly fabricated sensor is also successfully applied for the glucose determination in human serums with satisfactory results, suggesting its promising potential toward glucose detection in real samples.

KEYWORDS: In situ synthesis, sandwich-like nanostructure, ZIF-67, graphene nanosheets, nonenzymatic glucose sensing

1. Introduction Metal-organic frameworks (MOFs), a new type of porous materials assembled by metal ions and organic ligands, own unique properties of large specific area and adjustable structure and porosity.1,2 Due to their intrinsic structure advantages, MOFs have been widely used in the fields of gas storage/separation, energy storage and conversion, drug delivery, electrode materials and catalysis.3-5 More recently, MOFs are demonstrated to exhibit great potential in electrochemical sensors.6-11 However, the 2 / 39


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poor electron conductivity of MOFs will seriously suppress their electrochemical activity. According to data, selecting MOFs as sacrificial templates via calcination under air or N2 atmosphere to synthesize semi-conductive metal oxides or carbonaceous composites is an efficient method to improve their electrochemical sensing performance.12,13 However, how to fabricate an efficient electrochemical sensing interface with fast electron transfer on the case of the direct application of MOFs remains to be highly challenge. Up to now, the most common strategy to address the poor conductivity of MOFs is to couple MOFs with conductive materials. For example, zero-dimensional conductive Ag nanoparticles decorated Co-based MOF synthesized through a sequential deposition-reduction method,14 and one-dimensional carbon nanotubes modified Ni-based MOF by in situ solvothermal method manifested enhanced electrochemical sensing performance.15 Therefore, anchoring MOFs on twodimensional conductive graphene nanosheets by in situ growing method could be a good alternative to afford excellent electrochemical performance due to the synergistic effect between MOFs and conductive graphene nanosheets. As an indispensable biological molecule in human body, glucose generally exists in blood and plays a key role in providing energy for normal activities. However, excessive amount of blood glucose will lead to diabetes mellitus, which will seriously affect the health of humans.16 Therefore, it is highly necessary to fabricate a fast and accurate sensing platform for monitoring the level of glucose in blood. Nowadays, various

of

spectroscopy,18

detection liquid

technology,

including

chromatography,19

fluorescence,17

ultraviolet-vis

mass-spectrometry,20

capillary 3 / 39



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electrophoresis,21 gas chromatography,22 and electrochemistry23,24 have been reported for glucose detection. In contrast, electrochemical technology owns advantages of instrumental simplicity, short analytical time and simple sample preparation. In general, electrochemical

detection

of

glucose

can

be

realized

through

enzymatic

electrochemical biosensors and non-enzymatic electrochemical biosensors. Enzymatic electrochemical biosensors can afford glucose determination with excellent sensitivity and selectivity, but the high price, unstable activity and complicated immobilization process of glucose oxidase (GOD) hamper its applying in daily life.25 Hence, developing

non-noble

metal

materials-based

non-enzymatic

electrochemical

biosensors is highly desired for its low cost. In order to achieve this purpose, a serious of non-noble metal materials like metals or their alloys,26,27 metal oxides,28 metal hydroxide,29 nitrides30-33 and phosphides,34-36 have been widely investigated to develop effective non-enzymatic glucose sensors. Although great progress has been made, many of them still suffer from slow electrode kinetics, complex assembly process as well as surface poisoning.37 Therefore, it is necessary to investigate proper new functional materials with fast interfacial reaction kinetics, high sensitivity and selectivity toward glucose oxidation. Herein, a facile in situ synthesis method was proposed to prepare Co-based MOF [Co(mim)2]n (denoted as ZIF-67, mim = 2-methylimidazole) decorated physicallyexfoliated graphene nanosheets (GS@ZIF-67) heterogeneous hybrids at room temperature. Typically, the synthesis strategy involves the simple acid treatment of the physically-exfoliated graphene nanosheets, electrostatic adsorption interaction between 4 / 39


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metal Co2+ and acidified graphene nanosheets, and the self-assembles between Co2+ and 2-methylimidazolate ligands on the surface of graphene nanosheets at room temperature. As a result, the high-conducting graphene nanosheets were fully covered by a lay of polyhedral ZIF-67 nanoparticles with size about 300 nm at the two sides and sandwich-like GS@ZIF-67 heterostructure was successfully obtained. What is more important, the as-obtained sandwich-like GS@ZIF-67 hybrids exhibited excellent electrochemical sensing performance toward glucose oxidation with high selectivity. It is believed that the superior electrochemical sensing interface of the sandwich-like GS@ZIF-67 hybrids can be ascribed to the intimate contact between high electrocatalytic activity of ZIF-67 and conductive two-dimensional graphene substrate to the greatest extend, and the optimally synergistic effect between ZIF-67 and the graphene nanosheets. Thus, a highly sensitive electrochemical sensor toward glucose oxidation was fabricated basing the as-synthesized sandwich-like GS@ZIF-67 heterostructure with linear range of 1-805.5 μM, detection limit of 0.36 μM (S/N = 3), and excellent selectivity and stability. In addition, compared with pure graphene nanosheets, ZIF-67, and the simple physical mixture of graphene nanosheets and ZIF-67 (GS+ZIF-67), the in situ obtained GS@ZIF-67 hybrids also displayed better electrochemical activity. Of course, it must be noted that several kinds of graphene-based ZIF-67 hybrids have been synthesized and applied in the fields of super-capacitor and environmental governance in the previous reports. However, the current research for MOFs-graphene composites mainly focuses on the chemically-exfoliated graphene, including graphene oxide or reduced graphene oxide, and most of the hybrids present highly disordered 5 / 39



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structure.38-41 According to data, the investigation on fabrication of MOFs/physicallyexfoliated graphene hybrids and their biosensing performances exploration is rarely reported. Given the significant difference in the morphology, ability of electron transfer, surface defects, oxygen content, degree of graphitization, capacitance of electric double layer and preparation method, etc, between chemically-exfoliated graphene and physically-exfoliated graphene,42-44 the formation of hybrids with ordered structure between the MOF and physically-exfoliated graphene may be more conducive to boost the electrochemical sensing performance for their unique advantages. First, relative to chemically-exfoliated graphene, the lower capacitance of electric double layer of physically-exfoliated graphene means lower background current, which is able to improve the signal-to-noise ratio. Second, the physically-exfoliated graphene can be easily obtained under mass manufacture through a green environment-friendly preparation method, and owns less structural damages and faster electron transfer kinetic, which is more close to the pristine graphene. Third, Co2+ of ZIF-67 owns multiple valence states, which presents rich redox chemistry, the superior conductivity of physically-exfoliated graphene can effectively promote the catalytic activity of the metal centers. In addition, the ordered self-assembly of ZIF-67 on a substrate is very challenging, because the nucleation/growth speed of ZIF-67 is very fast, which often leads to the self-nucleation.45 For the first time, ZIF-67 polyhedron decorated physically-exfoliated graphene with sandwich-like heterostructure was successfully insitu synthesized at room-temperature without a binding agent. This work is a successful case to make the most of the structure advantages of carbon materials and MOFs, which 6 / 39


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could offer effective guidance for the rational design and fabrication of other MOFs/graphene composites. Besides, considering MOFs often serve as the templating to prepare various of functional materials, the as-synthesized sandwich-like graphene@ZIF-67 heterostructure may also be used as the precursor to fabricate diversified physically-exfoliated graphene-based hierarchical structure.

2. Experimental 2.1. Regents and Characterization Cobalt nitrate hexahydrate, 2-methylimidazole, polyvinyl pyrrolidone (PVP, K30), methanol, N,N-dimethylformamide (DMF), sodium hydroxide and glucose were obtainned from Sinopharm Chemical Reagent Co., Ltd. The physically-exfoliated graphene nanosheets (GS) were provided by Xiamen Kaina nano technology Co., Ltd. SEM charaterization was conducted on a Hitachi SU8010 scanning electron microscope. XRD test was operated at an Empyrean powder diffractometer (PANalytical Company, the Netherlands). XPS analysis was performed using an AXISUL TRA DLD-600W spectrometer. Transmission electron microscopy (TEM) and EDX elemental mapping images were taken on a JEM 2010 instrument. FT-IR spectra and Raman spectra were conducted on Fourier transform infrared spectrometer (Equinox-55, Bruker Company, Germany), and confocal Raman microscopy using a 532 nm laser (LabRAM HR800, Horiba JobinYvon, France), respectively. Micromeritics ASAP2420 adsorption analyzer was used to evaluate the BrunauerEmmett-Teller (BET) specific surface. Thermogravimetric analysis (TGA) curve was 7 / 39



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obtained at STA449F3 (NETZSCH, Germany) instrument in air atmosphere with a heating rate of 10 oC min−1. 2.2. Pretreatment of GS First, 300 mg of physically-exfoliated graphene nanosheets were added into 50 mL mixed acid solution containing 12.5 mL sulfuric acid and 37.5 mL nitric acid, the mixture was then refluxed at 80 oC for 3 h with continuous stirring. Subsequently, the functional GS was collected by centrifugation and washed with deionized water for several times. At last, the as-obtained graphene samples were dried at 80 oC. 2.3. Preparation of GS@ZIF-67 Typically, 20 mg pretreated GS was first added into 25.0 mL of methanol solution containing 200 mg of PVP with sonication for 0.5 h to form a uniform suspension. After that, 582 mg of Co(NO3)2·6H2O was added into the GS suspension with stirring for 1 h. Subsequently, another 25.0 mL of methanol solution containing 657 mg of 2methylimidazole was fastly poured into the above-mentioned GS suspension under stirring. After stirring for another 3 h at room temperature, the black products were filtrated, thoroughly rinsed by pure methanol and dried at 60 oC in oven. In addition, pure ZIF-67 polyhedrons were prepared according to the aforementioned procedure without adding GS. The physical mixture of GS and ZIF-67 (GS+ZIF-67) was also obtained basing the same ZIF-67/C weight ratio as the synthesized GS@ZIF-67 hybrids. 2.4. Electrochemical Measurements Electrochemical data were collected on a CHI 830C electrochemical workstation (Chenhua Instrument, Shanghai, China) using a saturated calomel electrode (SCE) as 8 / 39


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the reference electrode, platinum wire as the counter electrode, and a modified GCE as the working electrode. For the working electrode, 2 mg of prepared materials was added into 0.5 mL DMF with sonication for 10 min to obtain a homogeneous slurry. After that, 1.5 μL suspension was coated on the surface of a cleaned glassy carbon electrode (GCE) with diameter of 3 mm until the DMF solvent was completely evaporated under an infrared lamp. 0.1 M NaOH solution was used for the supporting electrolyte for glucose determination. Electrochemical impedance spectroscopy (EIS) was carried out on a CHI 660E electrochemical workstation (Chenhua Instrument, Shanghai, China) in 0.1 M NaOH solution containing 0.1 M glucose at 0.6 V in the frequency range of 0.1 Hz - 100 kHz.

3. Results and Discussion 3.1. Synthesis Strategy of GS@ZIF-67 Hybrids Scheme 1 depicts the synthesis process of the sandwich-like GS@ZIF-67 heterogeneous hybrids. First, the physically-exfoliated graphene was functionalized with carboxylic groups (-COOH) via simple acid treatment. Then, the functionalized graphene nanosheets was uniformly dispersed in methanol solution containing metal Co2+ and PVP, and few metal Co2+ was adsorbed on the surface of graphene nanosheets under electrostatic effect. The electronegative graphene-metal ions composite serves as the nucleation sites for loading MOFs.46 Subsequently, another methanol solution containing 2-methylimidazolate ligand was added into the graphene suspension, and small ZIF-67 was preferentially formed on the surface of graphene nanosheets as the 9 / 39



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heterogeneous crystal nucleation, which leads to the uniform growth of ZIF-67 polyhedrons on the GS due to the self-assembly between metal Co2+ ions and 2methylimidazolate ligand at room temperature. As a result, the sandwich-like GS@ZIF67 heterogeneous hybrids were achieved at room temperature. 3.2. Morphology and Structure Characterization of GS@ZIF-67 Hybrids The crystal structure of the as-synthesized GS@ZIF-67 hybrids was first studied by XRD patterns. As displayed in Figure 1a, all the diffraction peaks in the range of 550o were in good agreement with the simulated ZIF-67 crystals, indicating ZIF-67 crystals were successful formed with high purity. In addition, a wide diffraction peak at 26.56o in the pattern of GS@ZIF-67 can be assigned to the (002) facet of graphitic carbon, suggesting the existence of graphene nanosheets. Similarly, the XRD pattern also confirmed the existence of ZIF-67 and GS in the simple physical mixture of GS and ZIF-67 (GS+ZIF-67) (Figure S1a, Supporting Information). Further, the morphological characterization of different materials was conducted by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure S2a (Supporting Information), the acidified graphene displayed sheet-like morphology with size about 5 μm. The low-magnification (Figure 1b) and highmagnification (Figure 1c) of SEM images of GS@ZIF-67 manifested that a number of ZIF-67 crystals with a diameter of about 300 nm were uniformly coated on the both sides of GS, showing a sandwich-like heterostructure, and few ZIF-67 nanoparticles were dissociated. TEM images with different magnifications (Figure 1d and e) also confirmed graphene acted as the role of substrate material and were stuck in the middle 10 / 39


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of ZIF-67 crystals with intimate contact. It is believed that the close attachment between ZIF-67 and graphene will undoubtedly accelerate the electrode reaction kinetics of MOFs. Elementary mapping analysis was also carried out to further probe the structure information and chemical composition of the as-synthesized GS@ZIF-67 hybrids. We can see the hybrids were composed by Co, N, and C elements, and the distribution of Co was highly overlapped with the region of N. In addition, the C content in the inner region was obviously stronger than the marginal region (Figure 1f), suggesting the asprepared hybrids were composed of ZIF-67 and graphene, and the ZIF-67 nanoparticles were uniformly distributed on the graphene matrices. Besides, the morphology characterization of pure ZIF-67 crystals (Figure S2b, Supporting Information) and the simple physical mixture of graphene and ZIF-67 (GS+ZIF-67) (Figure S1b, Supporting Information) were also carried out. Pure ZIF-67 showed smooth surface with polyhedral structure and size about 300 nm, which was similar to the anchored ZIF-67 nanoparticles in GS@ZIF-67 hybrids. As to the GS+ZIF-67, polyhedral ZIF-67 nanoparticles were randomly blended with the graphene nanosheets with uneven and loose connection. Obviously, the in situ synthesis of GS@ZIF-67 hybrids displayed significant structural differences relative to the simple physical mixture (GS+ZIF-67), which may imply different electrochemical activity. Raman, FTIR, XPS, BET and TGA were carried out in succession to further probe the structural information of the as-synthesized GS@ZIF-67 hybrids. As shown in Figure 2a, the defined Raman spectra characteristics of ZIF-67 and graphitic carbon verify the successful preparation of GS@ZIF-67 hybrids. In addition, the characteristic 11 / 39



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absorption peak at 3450 cm-1 and 1623 cm-1 in the FTIR can be indexed to the stretching vibration of O-H, C=O in carboxyl group of graphene,47 while the absorption peak at 425 cm-1 can be attributed to the Co-N bond in ZIF-67,48 also indicating the existence of ZIF-67 and GS (Figure S3, Supporting Information). The full XPS spectrum of GS@ZIF-67 reveals the existence of C, N, O, and Co element (Figure 2b), which is consistent with the result of EDX element mapping analysis. According to the highresolution XPS spectrum, a pair of peaks (Co 2p1/2/2p3/2) centered at 797.1/781.5 eV can be indexed to the doublet of Co2+ (Figure S4a). In addition, peaks at 803.1 eV and 786.5 eV can be assigned to the satellite peaks, which was caused by the shake-up.48 Besides, the peak appeared at 394.6 eV can be indexed to imidazole groups in ZIF-67 (Figure S4b). The high resolution spectrum of C 1s can be fitted into four peaks at binding energy of 284.8, 286.3, 287.8, and 289.0 eV, which correspond to the C-C, CO, C=O and COOH, respectively (Figure S4c, Supporting Information). These results comply with the characteristics of ZIF-67 and the acidified carbon materials. Afterwards, the Brunauer-Emmett-Teller (BET) specific area and porous structure of GS@ZIF-67 were tested. As shown in Figure 2c, GS@ZIF-67 displayed a large BET surface area as high as 1270 m2 g-1 with type-I hysteresis loops, which definitely indicates the highly accessible microporous structure of the ZIF-67 is maintained.49 According to the TGA curves of GS, ZIF-67 and GS@ZIF-67 presented in Figure 2d, it is clear that GS@ZIF-67 shows a sharp drop at about 300 oC, which was caused by the oxidation degradation of the organic ligands. In addition, the carbonization temperature of graphene is inferred as about 450 oC for the large weight loss at this 12 / 39


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point. It should be noted that the degradation temperature of pure graphene is higher than graphene in GS@ZIF-67 hybrids, this may be related with the existence of ZIF67, which can cause the easy degradation of carbon material.50 Therefore, the weight fraction of ZIF-67 and GS in the as-synthesized GS@ZIF-67 hybrids can be calculated to be about 82.4% and 17.6% basing the TGA analysis, respectively. 3.3. Electrochemical Behavior of GS@ZIF-67 The electrochemical behavior of GS@ZIF-67 was first studied in 0.1 M NaOH by cyclic voltammetry (CV) scanning in the range from 0 V to 0.7 V. Figure 3a shows the CV curves of GS@ZIF-67 in the absence (pink line) and presence (blue line) of 2.5 mM glucose, two pair of poorly defined redox peaks can be observed. The first pair of redox peaks I/II were located at around 0.3 V and 0.20 V, while the other pair of redox peaks III/IV were appeared at about 0.58 V and 0.52 V, respectively. Previous studies reported that anodic scanning gives rise to the formation of Co(OH)x species for the electrochemical oxidation reaction in basic media for a cobalt-based material, Co(II) species were successively oxidized to Co(III) and Co(IV) species. Subsequently, the glucose was oxidized by the Co(IV) species while the Co(IV) species were reduced to Co(III) species.13,33 Therefore, it is reasonable to speculate that the redox peaks I/II and III/IV arise from the reversible conversion between [Co(II)(mim)2]n and [Co(III)(mim)2(OH)]n, [Co(III)(mim)2(OH)]n and [Co(IV)(mim)2(OH)2]n, respectively. These two reversible electrode process can be expressed as the following equations: 14 [Co(II)(mim)2]n + nOH− → [Co(III)(mim)2(OH)]n + ne− [Co (III)(mim)2(OH)]n + nOH− → [Co(IV)(mim)2(OH)2]n+ ne−

(1) (2) 13 / 39



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It should be noted that the current response of peak III was remarkably enhanced while that of peak IV was slightly decreased upon addition of glucose, and the peak III was significantly stronger than that of peak I, suggesting the oxidation of glucose was mainly

achieved

by

the

electrode

process

[Co(III)(mim)2(OH)]n/[Co(IV)

(mim)2(OH)2]n rather than [Co(II)(mim)2]n/[Co(III) (mim)2(OH)]n. According to Figure 3b, when gradually increasing the concentration of glucose ranging from 0 to 2.5 mM, the anodic current of peak III on GS@ZIF-67 was accordingly increased, further suggesting the Co-based compound Co(IV)(mim)2(OH)2 is able to catalyze the glucose. In this way, the forward electrode process between [Co(III)(mim)2(OH)]n/[Co(IV) (mim)2(OH)2]n would be greatly accelerated, which will result in an enhanced current response for peak III when increasing the concentration of glucose. At the meanwhile, the consumption of [Co(IV)(mim)2(OH)2]n for the glucose oxidation will also in turn lead to the slight decrease of peak IV. Therefore, the oxidation mechanism of glucose can be presumably proposed as the following equation:13 2[Co(IV)(mim)2(OH)2]n + nC6H12O6 → 2[Co(III)(mim)2(OH)]n + nC6H10O6 + 2nH2O By contrast, Figure S5 (Supporting Information) showed that no obvious difference was observed for the CV curves of GS in the presence and absence of glucose, suggesting the pure GS is inactive for the electrochemical oxidation of glucose. As to the ZIF-67 nanoparticles, the current response at about 0.58 V was remarkably enhanced in the presence of glucose, this can be ascribed to the accelerated electrode process between [Co(III)(mim)2(OH)]n/[Co(IV)(mim)2(OH)2]n, indicating ZIF-67 is able to catalyze glucose under a certain potential. In addition, the physical mixture of 14 / 39


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GS and ZIF-67 displayed further improved current signal, suggesting the introduction of GS can effectively facilitate the electron transfer. However, the catalytic activity of the physical mixture of GS and ZIF-67 toward glucose oxidation was still inferior to the in-situ coupling of GS@ZIF-67 hybrids, which can be deduced according to the smaller oxidation current at about 0.58 V. Compared with each individual component and the physical mixture, the as-synthesized GS@ZIF-67 hybrids exhibit the highest electrochemical activity toward glucose oxidation. One side, the formed [Co(IV) (mim)2(OH)2]n species on the surface of the hybrids during the anodic scanning can oxidize the glucose. On the other side, with the increase of glucose concentration, an enhanced current response for peak III was achieved and the electrons transfer process was greatly accelerated due to the existence of high conductivity of GS, suggesting a synergistic effect between ZIF-67 and GS, and the excellent performance was benefited from the intimate contact between ZIF-67 and GS. 3.4. Electrochemical Activity of GS@ZIF-67 toward Glucose Oxidation Figure S6 (Supporting Information) displayed the influence of applied potential on the amperometric response of 25 M glucose at GS@ZIF-67. Obviously, the highest sensitivity was achieved at 0.55 V. After then, current-time (i-t) curves of different electrode materials were collected with the addition of glucose into 0.1 M NaOH at 0.55 V. As shown in Figure 4a, it is found that no current response was observed on bare GCE and pure GS after successive addition of 25 μM glucose for three times, revealing poor electrochemical activity of bare GCE and pure GS. By contrast, obvious current steps were observed for ZIF-67, suggesting strong electrocatalytic activity of 15 / 39



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ZIF-67 toward glucose oxidation. Besides, the oxidation signals of glucose were further enhanced at GS+ZIF-67, indicating the introduction of graphene can effectively improve the electron transfer rate. More interestingly, larger current responses of glucose were achieved at GS@ZIF-67 compared with GS+ZIF-67, which was illustrated in Figure 4b, further demonstrating the synergistic effect between ZIF-67 and GS. This result is also consistent with the CV test displayed in Figure S5 (Supporting Information). In addition, the electrochemical performance of GS@ZIF67 composites toward glucose oxidation was also evaluated when untreated GS was used. Figure S7 (Supporting Information) shows the i-t curves of treated-GS@ZIF-67 and untreated-GS@ZIF-67 with successive addition of 25 μM glucose at 0.55 V in 0.1 M NaOH, we can see that larger current response of glucose was achieved at the treatedGS@ZIF-67, suggesting the simple acid treatment is conducive to improve the electrochemical activity of the hybrids, which may be attributed to the more intimate and uniform attachment between acidified graphene and ZIF-67. Besides, reaction time is also an important parameter toward the electrochemical oxidation signal of glucose, because different growth time of ZIF-67 will affect the proportion of ZIF-67 in the hybrids. So the effect of growing time of ZIF-67 in the presence of graphene on the electrochemical response toward glucose oxidation was studied. As shown in Figure S8 (Supporting Information), GS@ZIF-67 hybrids were prepared with different reaction time ranging from 1 to 9 h, and their electrochemical response toward 25 μM glucose oxidation was studied with chronoamperometry. When the growing time extended from 1 to 3 h, the current response of glucose greatly enhanced, which may be due to the 16 / 39


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gradually increased loading amount of ZIF-67. Further extending reaction time from 3 to 9 h, the current signal was gradually reduced, this may be ascribed to the excess growth of ZIF-67, which decreased the holistic ability of electron transfer. Therefore, 3 h of growing time was employed to prepare the GS@ZIF-67 hybrids. Electrochemical impedance spectroscopy (EIS) measurement was further adopted to evaluate the electron transfer rate of different electrocatalysts during the oxidation process of glucose. As shown in Figure 4c, GS@ZIF-67 only showed 0.7 KΩ of interfacial charge transfer resistance (Rct), which is small than that of GS+ZIF-67 (1.0 KΩ), ZIF-67 (1.5 KΩ), and pure GS (6.9 KΩ), indicating the fastest glucose oxidation kinetics at the interface of GS@ZIF-67. The lower charge transfer resistance (Rct) of GS@ZIF-67 can be ascribed to the intimate interaction between the nanosized ZIF-67 polyhedrons and graphene nanosheets, which is conducive to the transfer conduction. Besides, the active area of different electrode materials was also studied using K3[Fe(CN)6] as the probe. As shown in Figure S9 (Supporting Information), a pair of redox peaks were observed on the surface of different electrode materials, and the oxidation/reduction peak currents increased accompanied by increasing the scan rate from 50 to 200 mV s-1. According to data, the active area (A) of electrode materials can be calculated by Randles-Sevcik equation: Ipa = (2.69105)n3/2AD1/2Cv1/2,51 where n is the number of transferred electron, D is the diffusion coefficient of K3[Fe(CN)6], and C is the bulk concentration of K3[Fe(CN)6]. Herein, the values of A were evaluated to be 0.079, 0.046, 0.056, and 0.064 cm2 for GS, ZIF-67, GS+ZIF-67 and GS@ZIF-67, respectively, basing the slope of peak current versus the square root of scan rate (Figure 17 / 39



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4d). It is clear that GS owned the largest active area, this may be ascribed to its high surface area and excellent conductivity, which is conducive to the contact of active species in the electrolyte and interfacial electron transfer. Upon integrating GS with ZIF-67, the interfacial electron transfer ability of ZIF-67 was undoubtedly enhanced, so increased active area values were displayed for the GS+ZIF-67 and GS@ZIF-67. Besides, GS@ZIF-67 owned a larger active response area than that of GS+ZIF-67, further indicating the superiority of the as-proposed in situ synthesis strategy relative to the simple physical mixing method, which is more in favor of the electron conduction. 3.5. Nonenzymatic Glucose Sensing with GS@ZIF-67 Benefiting from its superior conductivity, large electrochemical active area, and the synergistic effect between graphene and ZIF-67, the nonenzymatic glucose sensing performance of GS@ZIF-67 was evaluated. Figure 5a displays the amperometric curve of GS@ZIF-67 to the successive addition of glucose in 0.1 M NaOH at a potential of 0.55 V. After each injecting of glucose, strong and rapid current responses were observed, and the corresponding calibration plot relating to the concentration was shown in Figure 5b. The regression equation was I (A) = 0.108 C (M) (R = 0.998) with linear range from 1 to 805.5 M. For the GCE electrode with a diameter of 3 mm, the electrode area is 0.071 cm2, so the sensitivity of the graphene@ZIF-67 hybrids toward glucose oxidation is 1521.1 μA mM-1 cm-2. In addition, the limit of detection (LOD) is calculated using the equation LOD = 3SD/slope, where SD is the standard deviation of ten parallel detection of 1 μM glucose. Here, detection limit of 0.36 μM was calculated for glucose at the graphene@ZIF-67 hybrids electrode. These values 18 / 39


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compare favorably to many reported graphene-based and other electrochemical glucose sensors, and the comparison of sensing performance of GS@ZIF-67 hybrids with other nanomaterials was exhibited in Table 1. In order to highlight the detection sensitivity of GS@ZIF-67 hybrids toward glucose, i-t curves of different electrodes for continuous addition of 1 M glucose were illustrated in Figure S10 (Supporting Information). It is clear that the strongest current steps were observed on GS@ZIF-67, while the oxidation signals of glucose were very weak on GS+ZIF-67, GS and ZIF-67, further demonstrating the structural advantage of GS@ZIF-67. The anti-interference ability is another major metrics to evaluate the sensor performance, so some probable interferences on the glucose response were assessed. Figure 5c shows the i-t curves of GS@ZIF-67 for continuous addition of 10 μM glucose, 1 μM ascorbic acid (AA), 1 μM uric acid (UA), 1 μM dopamine (DA), 1 mM KCl, 1 μM fructose, 1 μM lactose, 1 μM galactose, and 10 M glucose. Results show AA, UA and DA, KCl, fructose, lactose, galactose produce little current responses relative to glucose, indicating GS@ZIF-67 has an excellent selectivity toward glucose detection. Additionally, reproducibility of the proposed GS@ZIF-67-based sensor was further evaluated. The relative standard deviation (RSD) of five independently fabricated GS@ZIF-67 sensor is calculated to be 3.03%, and RSD of successive addition of glucose for five times with one GS@ZIF-67 electrode is only 2.27%. The low RSD values demonstrate the good reproducibility of the fabricated sensor. The long-time stability was also examined to evaluate the performance of the proposed glucose sensor. Several GS@ZIF-67 modified glassy carbon electrodes were stored in 19 / 39



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air at room temperature, and the current responses for 25 μM glucose were measured each day. The current response retains 95.5% of its original value after a week as illustrated in Figure S11 (Supporting Information), indicating good stability of the asprepared electrochemical sensor. In addition, the structural stability of GS@ZIF-67 hybrids was also evaluated after electrochemical test. The electrochemical test was conducted by chronoamperometry at a constant voltage of 0.55 V in 0.1 M NaOH containing 0.5 mM glucose for about 30 min. The morphology and structure characterization of the graphene@ZIF-67 hybrids were then conducted by the SEM image and XRD diffraction pattern after long-time electrochemical test. As shown in Figure S12 (Supporting Information), the crystal structure and morphology of the graphene@ZIF-67 hybrid were well-retained, demonstrating superior structure stability of the graphene@ZIF-67 hybrids. In an attempt to assess the possible applications of the proposed method for real samples detection, the GS@ZIF-67 was used to determine glucose concentration in human serum samples that collected from Wuhan Union Hospital. For a typical procedure, the serum samples were first centrifuged at 8000 rpm for 15 min and the supernatant was collected for further electrochemical test. After then, standard glucose solution and different amount of serum supernatant (20 L and 40 L) were successively added to 10 mL 0.1 M NaOH during the amperometric detection with 0. 55 V of working potential (Figure 5d). The real glucose levels of different human serum samples were then determined and shows satisfied result with that of glucometer (Table S1, Supporting Information), it is seen that the glucose concentration obtained from 20 / 39


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GS@ZIF-67 and glucometer is in good agreement, demonstrating the reliability of the developed glucose sensor for real sample analysis.

4. Conclusion In summary, sandwich-like GS@ZIF-67 hybrids were successfully obtained through an in situ liquid phase growth strategy. In this hybrids, polyhedral Co-based MOF (ZIF-67) nanoparticles with size about 300 nm were uniformly loaded on the surface of graphene nanosheets. Benefiting from the enhanced electron transfer ability and synergistic effect between graphene and ZIF-67, the obtained GS@ZIF-67 exhibits excellent electrochemical activity toward glucose oxidation. Thus, a sensitive and accurate electrochemical sensing platform was successfully proposed for glucose detection and the fabricated electrochemical sensor manifests satisfying detection results aiming to the real human serum samples. Taking into account the structural diversity of carbon matrix and metal-organic frameworks, the strategy may provide some guidance for preparing other carbon-based composite materials, which may be applied in the field of environmental adsorption, energy conversion, catalysis, and so on.

ASSOCIATED CONTENT Supporting Information Available The Supporting Information is available free of charge on the http://pubs.acs.org. More results of physical characterization (XRD patterns, SEM images, FT-IR spectra, 21 / 39



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High-resolution XPS fitting spectra, i-t curves, cyclic voltammograms curves and Table S1 for sample detection. Corresponding Author [email protected]; [email protected] ORCID Can Wu: 0000-0002-1665-2957 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by National Natural Science Foundation of China (No. 21804031, 81771718 and 81672919).

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Scheme 1. Illustration of the fabrication of sandwich-like GS@ZIF-67 hybrids.

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Figure 1. (a) XRD patterns of GS, ZIF-67 and GS@ZIF-67. (b) Low- and (c) highmagnified SEM images of GS@ZIF-67. (d) Low- and (e) high-magnified TEM images of GS@ZIF-67. (f) EDX element mapping images of GS@ZIF-67.

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Figure 2. (a) Raman spectra of GS, ZIF-67 and GS@ZIF-67. (b) XPS spectra of GS@ZIF-67. (c) N2 adsorption-desorption isotherms of GS-ZIF-67. (d) TGA curves of GS, ZIF-67 and GS@ZIF-67.

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Figure 3. (a) CV curves of GS@ZIF-67 in 0.1 M NaOH in the presence (blue line) and absence of (pink line) 2.5 mM glucose. (b) CV curves of GS@ZIF-67 in 0.1 M NaOH with the presence of different concentration of glucose. The scan rate is 100 mV s−1.

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Figure 4. (a) Current-time (i-t) curves of GCE, GS, ZIF-67, GS+ZIF67 and GS@ZIF67 in 0.1 M NaOH with successive addition of 25 M glucose. (b) Histogram of current responses of 25 M glucose on different electrode materials. (c) Nyquist plots of GS, ZIF-67, GS+ZIF-67 and GS@ZIF-67 in 0.1 M NaOH containing 0.1 M glucose at 0.6 V vs SCE. Frequency range: 0.1 Hz - 100 kHz. Inset is the equivalent circuit. (d) Linear plots between reduction peak currents and square root of scan rate of GS, ZIF-67, GS+ZIF-67 and GS@ZIF-67 in 0.1 M KCl containing 5 mM K3[Fe(CN)6]. Error bars represent the standard deviations of three measurements.

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Figure 5. (a) i-t curves of different concentrations of glucose at GS@ZIF-67 in 0.1 M NaOH. (b) The linear fit of peak currents to concentrations of glucose. (c) Amperometric response to the addition of 10 μM glucose, 1 μM AA, 1 μM UA, 1 μM DA, 1 mM KCl, 1 μM fructose, 1 μM lactose, 1 μM galactose, and 10 M glucose in 0.1 M NaOH at potential of 0.55 V. (d) Amperometric response of the GS@ZIF-67 upon successive additions of 20 M glucose, 20 L serum, 40 L serum, followed by addition of 20 M glucose.

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Table 1. Performance comparison toward glucose oxidation with other nanomaterials. Electrode materials

Sensitivity

Linear range

Detection limit References

(μA mM−1 cm−2)

(mM)

(μM)

Ni/ATPa/rGOb

1414.4

0.001-0.71

0.37

[52]

Co3O4/3D-rGO

112.16

up to 0.08

0.157

[53]

CoPcc/rGO

90.52

0.0167-1.60

14.6

[54]

NiO/rGO

666.71

0.005-4.2

5

[55]

Ni-MoS2/rGO

256.1

0.005-8.2

2.7

[56]

Mn3O4/N-doped rGO

1423.9

0.0025-0.5295

1

[57]

NiO nanofiber/GOd

1100

0.002-0.6

0.8

[58]

NiO/Pt/ErGOe

668.2

0.05-5.66

0.2

[59]

Co3O4/NiCo2O4/rGO

304

0.01-3.52

0.384

[60]

CoP nanorods

116.8

up to 5.5

9

[61]

Ni-MOF/Ni/NiO/carbon

367.45

0.004-5.664

0.8

[12]

Pt/Ni nanowires

920

0.002-2

1.5

[62]

CQDsf/Cu2O

298

0.02-4.3

8.4

[63]

Ni/CNT

1384.1

0.005-2

2

[64]

Graphene@ZIF-67

1521.1

0.001-0.8055

0.36

This work

aATP:

attapulgite; brGO: reduced graphene oxide; cCoPc: cobalt phthalocyanine; dGO: graphene

oxide; eErGO: electrochemically reduced graphene oxide; fCQDs: carbon quantum dots

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Table of Content (TOC) Graphic

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In Situ Synthesis of Sandwich-like Graphene@ZIF-67 Heterostructure

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