Covalent Functionalization of Graphene Oxide with a Presynthesized

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Functional Nanostructured Materials (including low-D carbon)

Covalent Functionalization of Graphene Oxide with a Pre-synthesized MetalOrganic Framework Enables a Highly Stable Electrochemical Sensing Junhong Fu, Xiuyun Wang, Tingting Wang, Jie Zhang, Song Guo, Shuo Wu, and Fenghui Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10531 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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ACS Applied Materials & Interfaces

Covalent Functionalization of Graphene Oxide with a Pre-Synthesized Metal-Organic Framework Enables a Highly Stable Electrochemical Sensing Junhong Fu‡, Xiuyun Wang†,*, Tingting Wang†, Jie Zhang†, Song Guo‡, Shuo Wu†, Fenghui Zhu† †School

‡Gold

of Chemistry, Dalian University of Technology, Dalian 116024, P.R. China.

Catalysis Research Center, State Key Laboratory of Catalysis, Dalian Institute of

Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.R. China. KEYWORDS: covalent functionalization, graphene oxide, metal-organic framework, electrochemical sensing, purine derivatives ABSTRACT: This paper reports the covalent functionalization of graphene oxide (GO) by a presynthesized metal-organic framework (MOF): NH2-MIL-101(Fe) via ultrasonication of the two components. The formation of Fe-O covalent bonding in NH2-MIL-101(Fe)-GO nanohybrid is clearly evidenced, and the covalent bonding still remains after electrochemical reduction. The morphology and structure of the nanohybrid are characterized via scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV−vis spectroscopy, Fourier-transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Raman spectroscopy. Electrode based on electrochemically reduced NH2-MIL-101(Fe)-GO shows ultrastable and high-sensitive performance in simultaneous electrochemical sensing of three purine

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metabolic derivatives (uric acid, xanthine and hypoxanthine); in particular, no signal fading is seen even after 120 times runs. The covalent bonding within the nanohybrid is obviously the key to maintain such a stability. 1.INTRODUCTION Covalent functionalization of graphene is of great interest in recent years because of the unique structure and physical properties.1-3 A large number of studies are based on graphene oxide (GO) due to the presence of oxygen-containing groups making it strongly hydrophilic and water soluble,4 thus allowing the chemical modification on GO. Metal-organic frameworks (MOFs) are a novel class of crystalline materials that possess large surface area, structure tunability and high porosity, which enable the applications in a plenty of fields.5-7 MOF covalently functionalized GO has gained tremendous attention because the synergic effects of the two materials can make contributions to enhanced or even new properties, for example, improved stability, enhanced electrical conductivity, high selectivity; meanwhile, shortcomings of the individual components can be overcome.8-9 Covalent modification of GO by MOF generally follow the strategy of in-situ growth of MOF on top of GO. Basically, the oxygenated functional groups of GO serve as nucleation points for the growth of MOF.10-11 In addition, GO pre-anchoring with a benzoic acid group is also employed.12 However, there are some disadvantages of this strategy,13 (a) the morphology of MOF is not easy to control, (b) the mechanisms of the template role of GO are unclear, (c) there are coordination binding competition to the metal sites between the GO oxygenated groups and the oxygen atoms of the MOF ligand, which might lead to the formation of free MOF. Therefore, the synergy effects between the MOF and GO might be unpredictable, so is the overall performances. Post-synthesis approach of mixing GO with pre-synthesized MOF has also been studied, but the


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interactions between the two materials are usually in the form of π−π interactions, hydrogen bonding or electrostatic interactions,14-17 which relatively decreases the stability of the composite in comparison to the direct covalent bonding by in-situ method. Covalent functionalization of GO with a pre-synthesized MOF is hence quite intriguing, which not only enables a controllable synthesis of the hybrid, also enhances the stability of the hybrid material by covalent bonding. However, covalent bonding of GO with a prepared MOF has rarely been reported so far. Scheme 1. Covalent bonding in nanohybrid-ER.

Herein, we report for the first time on a covalent functionalization of GO with a pre-synthesized NH2-MIL-101(Fe) MOF through simple ultrasonication treatment of the two materials in aqueous solution. The covalent bonding Fe-O in NH2-MIL-101(Fe)-GO (name as the nanohybrid hereinafter) is vividly evidenced. Even after electrochemical reduction of the nanohybrid, the covalent bonding still remains (Scheme 1). Electrode based on the electrochemically reduced (ER) nanohybrid (name as nanohybrid-ER hereinafter) demonstrates an excellent ultra-stable performance in simultaneously electrochemical sensing of three purine metabolic products, which can be continuously used 120 times. 2.EXPERIMENTAL SECTION


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2.1 Chemicals. Graphene oxide (GO) was synthesized from natural graphite powder by a modified Hummers’ method.18 Uric acid was purchased from J&K Excellence, xanthine and hypoxanthine were obtained from Solarbio. The 0.1 M phosphate buffer solutions (PBS) with various pH values were prepared by mixing the stock solution (0.1 M) of KH2PO4 and K2HPO4·3H2O. 2aminoterephthalic acid (≥99%) was purchased from Organics. Iron(III) chloride hexahydrate (FeCl3·6H2O) was supplied by aladdin. Nafion (5 wt%) was purchased from Alfa Aesar. 2.2 Instrumentation. Electrochemical experiments and measurements were performed at a CHI660B electrochemical workstation (Shanghai Chenhua Co. China). A conventional three electrode system was used throughout the measurements. Electrochemical measurements were carried out in a conventional electrochemical cell with modified GCEs as the working electrodes, a platinum wire as the counter electrode, and a Ag/AgCl electrode (KCl-saturated) as the reference electrode. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) was used for electrochemical analysis. CVs were performed in the potential range of -1.5 V to 0.6 V at a scan rage of 100 mV s-1. DPVs were recorded at an increment potential of 0.004 V, pulse amplitude of 0.05 V, pulse width of 0.05 s, sample width of 0.0167 s, pulse period of 0.5 s, and quiet time of 2 s. Electrochemical Impedance Spectroscopy (EIS) was executed in KCl (0.1 M) at the open circuit potential of 0.2 V and the frequency extent from 0.01 Hz to 10 kHz using [Fe(CN)6]3-/4- (5 mM) as redox probe. X-ray powder diffraction (XRD) was recorded on a PANanlytical Empyrean-100 diffractometer with Cu Kα radiation at a scanning rate of 6°·min−1. UV-vis absorption spectra were obtained on PerkinElmer Lambda 750. Fourier-transform infrared (FTIR) spectra were recorded using Bruker Vertex 70 with an accessory of Platinum ATR. Reflection FTIR Spectroscopy was carried out on a Bruker Vertex 70 with Hyperion 3000. Transmission electron microscopy (TEM) was operated


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on a Hitachi, HT7700. Energy dispersive spectrum (EDX) element mapping images were recorded on a JEOL JEM-2100F. TEM mapping Scanning electron microscopy (SEM) was conducted on a Quanta 200F. pH values were adjusted with Mettler Toledo FE 20. The X-ray photoelectron spectroscopy (XPS) was conducted on a Thermofisher ESCALAB 250Xi XPS spectrometer with monochromatic Al Kα (1486.6eV) X-ray source. Raman scattering was performed on a Bruker Optics Senterra Raman spectrometer using a 532 nm laser source. 2.3 Preparation of NH2-MIL-101(Fe). NH2-MIL-101(Fe) were synthesized according to the literature.19 Briefly, NH2-MIL-101(Fe) was prepared by a hydrothermal treatment of 2aminoterephthalate (0.225 g) and FeCl3 .6H2O (0.674 g) in DMF (15 mL) solvent at 110 ℃ for 24 hours. The as-synthesized product washed with DMF and MeOH at least 3 times, and an overnight vacuum-dry to obtain NH2-MIL-101(Fe). 2.4 Preparation of the nanohybrid. NH2-MIL-101(Fe) was prepared in a concentration of 1 mg/mL, and GO was prepared in 0.1 mg/mL both using water as solvent. NH2-MIL-101(Fe) (1 mL) and GO (4 mL) were mixed together with vigorous stirring, followed by ultrasonication (ultrasonic cleaning instrument, 59 KHz, 50 W, Shanghai Kudos Co.) for 2 hours to obtain a homogeneous dispersion of NH2-MIL-101(Fe)-GO. 2.5 Preparation of electrodes. Typically, prior to modification, bare GCE was polished to a mirror-like surface sequentially with 0.5, and 0.05 μm α-Al2O3 powder, and followed by ultrasonic washing with ethanol and deionized water, then was dried by nitrogen gas flow. The nanohybrid colloidal suspension (5 μL) was coated on the GCE surface and dried in room temperature. Then the modified GCE surface was coated with Nafion (2 μL) and dried in room temperature. Electrochemical reduction of the nanohybrid on GCE was performed by scanning the potential


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from -1.5 V to 0.6 V for at least 10 cycles at a scan rate of 100 mV s-1 in PBS (pH 6.0) until the curves went steady, and the resultant nanohybrid-ER/GCE was carefully rinsed with deionized water to remove the residual adsorbed substances. For comparison, NH2-MIL-101(Fe)/GCE and ERGO/GCE were prepared only by replacing nanohybrid dispersion with NH2-MIL-101(Fe) or GO solutions. 3.RESULTS AND DISCUSSION The nanohybrid is obtained by ultrasonication treatment of pre-synthesized NH2-MIL-101(Fe) and GO in aqueous solution. A number of ratios of NH2-MIL-101(Fe) and GO is screened (Table S1) in order to optimize the performance. It is worth mentioning that the concentration of the nanohybrid is of importance to the stability of the electrode. It is visible that GO fall off the GCE using a relatively large quantity of the nanohybrid. The X-ray powder diffraction (XRD) patterns of the nanohybrid resemble the key diffraction peaks of NH2-MIL-101(Fe) (Figure S1). The peak corresponding to GO negatively shifts by 0.6° in the nanohybrid, which means some structural changes of GO occurred under ultrasonication. Scanning electron microscopy (SEM) images of NH2-MIL-101(Fe) display a large amount of regular and independent cuboids with a diameter of about 500 nm (Figure 1a), indicative of good crystallinity of NH2-MIL-101(Fe). SEM images (Figure 1b) of the nanohybrid show that GO sheets crosses together on the surface of NH2-MIL101(Fe), and the interface between NH2-MIL-101(Fe) and GO is in intimate contact. Transmission electron microscopy (TEM) images of the nanohybrid (Figure 1c) show thin and transparent GO sheets, suggestive of a few layers of GO sheets. Additionally, NH2-MIL-101(Fe) particles remain in their original cuboid shape. High-resolution TEM elemental images of the nanohybrid show the existence of the elements C, Fe, O and N (Figure 1d-1h). These results suggest that the


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ultrasonication treatment can make the two ingredients undergo a relatively adequate interfacial interaction to form a sandwich-like structure.

Figure 1. SEM images of (a) NH2-MIL-101(Fe) and (b) the nanohybrid. (c) TEM images of the nanohybrid. (d-h) TEM elemental mapping images of the nanohybrid. (i) SEM images of nanohybrid-ER.


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Figure. 2 (a) UV-vis absorption spectra of GO and the nanohybrid. (b) IR spectra of GO, NH2MIL-101(Fe) and the nanohybrid. The XPS spectra of (c) C 1s and (d) Fe 2p of NH2-MIL-101(Fe) (A) and the nanohybrid (B). UV-vis absorbance spectroscopy of GO exhibits a broad absorption peak at 230 nm and a weak shoulder peak at 305 nm (Figure 2a), which could be attributed to π-π* transitions of C=C bonds and n-π* transition of C=O bonds, respectively.14 However, no π-π* transitions is observed for that of the nanohybrid, suggesting that the sandwich-like structure of the nanohybrid significantly reduces π−π interactions between GO layers. Besides, a large shift of the absorption peak at 305 nm of GO shifts to 380 nm of the nanohybrid occurs, suggestive of covalent interactions between GO and NH2-MIL-101(Fe).


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Infrared (IR) spectra of GO show the bands at 3410, 1721, 1363, 1230, 1050 cm-1, arising from the O-H stretch, C=O stretch, C−OH bending vibrations, alkoxy C−O vibrations, and epoxy C−O stretching, respectively (Figure 2b).20 The peak at 1619 cm-1 is related to the skeletal vibration of unoxidized graphene domains (C=C bonds). In the spectra of NH2-MIL-101(Fe), 2aminoterephthlate is reflected by vibrations of amine group ν(NH2) at 3453 and 3306 cm−1, vibrations of ν(C-N) at 1256 cm−1 and vibrations of carboxylate group ν(COO) at 1576 and 1382 cm−1. The peak at 1653 cm−1 could be ascribed to C=C bonds in the phenyl ring of 2aminoterephthlate. For the nanohybrid, the spectra largely resemble that of NH2-MIL-101(Fe), confirming that the structure of NH2-MIL-101(Fe) remains unchanged upon incorporation with GO. One feature is that vibrations of amine group ν(NH2) positively shifts to 3461 and 3328 cm−1 for the nanohybrid. The rest of the bands are negatively shifted in wavenumber compared with that of NH2-MIL-101(Fe). It is obvious that the chemical interactions between NH2-MIL-101(Fe) and GO lead to these wavenumber shifts. Of note is the characteristic peak of C=C bonds of the three materials, the peak intensity that is assigned to sp2 carbon atoms is decreased in the order of the nanohybrid < NH2-MIL-101(Fe) < GO (Figure 2b). It is suggested that the covalent attachment of NH2-MIL-101(Fe) strongly perturbs the extended aromatic character of GO by which gives rise to a decreased intensity of sp2 stretching frequency. We infer that, during ultrasonication treatment, GO might be partially converted into reduced GO and thereby shows a decreased intensity in the sp2 hybridized carbon atoms. Generally, the coexistence of sp2 carbons of GO and the aromatic ring of 2-aminoterephthlate is supposed to increase the sp2 carbon atom intensity.16 However, the peak intensity of sp2 carbon atoms in the nanohybrid is decreased. These results clearly reveal the reduced amount of π−π interactions in the nanohybrid, thus make covalent interactions the


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paramount interactions in the nanohybrid, which is in agreement with the UV-vis absorbance analysis. X-ray photoelectron spectroscopy (XPS) is used to study the chemical oxidation state and electronic structure of NH2-MIL-101(Fe) and the nanohybrid. The full survey of both displays peaks for C 1s, O 1s, N 1s and Fe 2p (Figure S2). The deconvoluted C 1s XPS spectra of NH2MIL-101(Fe) exhibit three bands at 284.4, 286.0, 288.2 eV, which are assigned to sp2 carbon (C=C), C-N and C=O bonds (Figure 2c), respectively.15 Similarly, the nanohybrid also show the same C 1s peaks (Figure 2c); however, the binding energy of C=C and C=O both shift by 0.2 eV to the high energy, and the C-N binding energy shifts by 0.7 eV to high binding energy due to the low electron density around the organic linker, which supplements the reduced π−π interaction between NH2-MIL-101(Fe) and GO.15 Furthermore, the pristine NH2-MIL-101(Fe) shows 65% C=C bonds, and 59% was calculated for NH2-MIL-101(Fe)-GO (Table S2), indicative of a decreased intensity in the sp2 hybridized carbon atoms, which is consistent with IR spectra. In addition, the Fe 2p spectra of NH2-MIL-101(Fe) and the nanohybrid show the binding energy peaks of Fe 2p1/2, Fe 2p3/2 at 724.9, 711.1 eV and 724.7, 710.9 eV (Figure 2d), respectively, corresponding to the Fe (III).

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The comparison of NH2-MIL-101(Fe) and the nanohybrid

reveals a negative shift of 0.2 eV for both Fe 2p1/2 and Fe 2p3/2 binding energies, indicative of a decrease in oxidation state of the Fe centers in MIL-101(Fe)-GO. This strongly suggests that oxygen functionality groups of GO covalently coordinate to unsaturated Fe sites in NH2-MIL101(Fe), and thus results in an alternation of Fe oxidation states. We herein propose that oxygenfunctionalities of GO coordinate to unsaturated Fe sites of NH2-MIL-101(Fe) to form Fe-O coordination bonds. The unsaturated Fe sites are probably located on surface of NH2-MIL-101(Fe) microcrystal. It is worth mentioning that NH2-MIL-101(Fe) owns Fe sites ligated by water


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molecules and DMF molecules,22 which are easy-leaving groups and can be easily replaced by groups with strong coordination ability like carboxylic group. What’s more, ligand defects in NH2MIL-101(Fe) microcrystal also give rise to some unsaturated Fe sites.23 In short, all characterizations above verify that covalent interactions via Fe-O coordination bonding in the nanohybrid, i.e. NH2-MIL-101(Fe) became an integral component of GO, not physically mixed. We foresee that the covalent bonding in the nanohybrid would greatly boost the stability in practical applications. To the best our knowledge, this is the first example of reporting covalent functionalization of GO via grafting MOF microcrystals onto GO only through ultrasonication treatment of the two components. Electrochemical reduction enables the removal of most of oxygen-functionalities of GO, thus promotes the restoration of graphene purity and enhances the conductivity.24 A pair of redox peaks is observed for GO electrochemical reduction after cyclic voltammetry (CV) measurement, also for the nanohybrid (Figure S3). However, no redox peaks are observed for NH2-MIL-101(Fe) throughout the scan process of CVs (Figure S3), indicative of a no redox activity process. It could be concluded that GO nanosheets are continually transformed to the higher conductive electrochemically reduced GO (ERGO) with the increase of the sweep cycles. Fourier-transform Infrared (FTIR) reflectance spectra of ERGO show that most of the adsorption bands of oxygen functionalities disappear (Figure S4). The intensity of carboxylic and hydroxyl groups are considerably decreased, but still some remain, which is of importance to ERGO functionalization. The peak at 1620 cm-1 remains unchanged, which is similar to that of pristine GO. Figure 1i shows the morphology of nanohybrid-ER. No agglomeration of graphene is observed, but with minor wrinkled structures, which might be the lattice defects caused by electrochemical reduction.25 And NH2-MIL-101(Fe) microcrystals are distributed between the thin and transparent graphene sheets.


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This indicates electrochemical reduction was not able to disrupt the structure of NH2-MIL-101(Fe), and the intimate interfacial contact of NH2-MIL-101(Fe) and ERGO still remains after electrochemical reduction.

Figure. 3 Raman spectra of ERGO and nanohybrid-ER. The FTIR reflectance spectra of nanohybrid-ER largely resemble that of NH2-MIL-101(Fe), but with a positive shift in wavenumber of carboxylic group of 2-aminoterephthlate comparing with that of NH2-MIL-101(Fe) (Figure S5), suggestive of a chemical interaction between the two components. Raman spectra of nanohybrid-ER show a low energy shift of the D band and a high energy shift of the G band in comparison to ERGO (Figure 3), which are consistent with the previous report that chemically reduced GO is covalently functionalized by porphyrin via an amide bond.26 We herein propose that it is covalent bonding in nanohybrid-ER via a Fe-O bond between the oxygen functionalities of ERGO and unsaturated Fe sites of NH2-MIL-101(Fe). Since the carboxylic group is more favorable to coordinate to transition metal than hydroxyl group, it is more likely that the Fe sites coordinate to carboxylic group of ERGO (Scheme 1).


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Then investigations on the electrochemical behaviors of nanohybrid-ER/GCE (obtained by casting the nanohybrid on glassy carbon electrode (GCE) and followed by electrochemical reduction) are carried out. Electrochemical Impedance Spectroscopy (EIS) is used to observe the impedance changes of the electrode surface during the modification process. The charge-transfer resistance (Rct) of nanohybrid-ER/GCE is markedly reduced in comparison to NH2-MIL101(Fe)/GCE (Figure S6), indicating that the conductivity of nanohybrid-ER/GCE is greatly enhanced due to the participation of ERGO. Randles-Sevcik equation is employed to calculate electroactive surface areas.27 The results show that the electroactive surface area of nanohybridER/GCE (0.083 cm2) is bigger than that of ERGO/GCE (0.067 cm2), and much bigger than that of NH2-MIL-101(Fe)/GCE (0.022 cm2) (Figure S7 and Table S3). CV is carried out in 0.1 M KCl solution using Fe(CN)63-/4- as a probe to study the redox activities. As can be seen in Figure S8, nanohybrid-ER/GCE not only decreases the peak-to-peak separation potential (ΔEp), also improves the redox peak current comparing with ERGO/GCE and NH2-MIL-101(Fe)/GCE, which can be attributed to the well conductivity and large electroactive surface area, consistent with the results of EIS and electroactive surface area calculations above. It is self-evident the synergy effect between NH2-MIL-101(Fe) and ERGO plays a positive role in the electrochemical redox reaction. On the other hand, these electrochemical characterizations further verifies the presence of covalent bonding in nanohybrid-ER, which results in an enhanced stability. Purines and their metabolites such as hypoxanthine (HX), xanthine (XA), uric acid (UA) participate in a wide variety of crucial metabolic pathways in the kidney, central nervous system and blood system.28 An incompetent purine metabolic pathway has been identified to be closely linked to human diseases such as gout, hyperuricemia, leukemia, pneumonia, Lesch–Nyhan and myocardial infarction.29 Quantitative measurements of purine metabolites simultaneously in blood


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or other biological samples are hence of great diagnostic use in targeting purine metabolism and may provide a novel therapeutic strategy for these diseases, because the concentration levels of them can be taken as sensitive indicators for early diagnosis of human diseases. Therefore, nanohybrid-ER/GCE (obtained by casting the nanohybrid on glassy carbon electrode (GCE) and followed by electrochemical reduction) is applied to the simultaneous electrochemical detection of purine metabolites UA, XA and HX.

Figure. 4 (a) DPVs of 20 µM of UA, XA and HX mixture solution at different electrodes in PBS (0.1 M, pH 7.0). (b) DPV responses of nanohybrid-ER/GCE for 120 times of consecutive detection of the mixture solution of UA, XA and HX (20 µM of each). In order to eliminate adsorption effects, a blank scan was performed before each measurement. Electrochemical behaviors of simultaneous measurements of UA, XA and HX at different modified electrodes are investigated by differential pulse voltammetry (DPV). As shown in Figure 4a, nanohybrid-ER/GCE obviously demonstrates a lower background current and higher response currents than the other electrodes, it is hence used to carry out further electrochemical sensing investigations. Selective determination of UA, XA and HX at the electrode provides a high


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sensitivity of 0.01 to 1 µM for UA, 0.005 to 2 µM for XA and 0.05 to 3 µM for HX (Figure. S9) with a limit of detection (LOD) of 0.008 µM for UA, 0.004 µM for XA and 0.03 µM for HX (S/N = 3), respectively. A comparison of nanohybrid-ER/GCE to reported electrodes is listed in Table S4. The electrode nanohybrid-ER/GCE exhibits very competitive advantages on the sensitivities and LOD to the detections of purine derivatives over the reported electrodes. In addition, no interference is observed in the presence of other common components in body fluids using nanohybrid-ER/GCE (Figure S10). The reproducibility is estimated by determining the current responses of the three biomolecules at three different nanohybrid-ER/GCEs, and the R.S.D. values are 2.13%, 2.45%, 3.37%, respectively. Such good reproducibility is believed to be a favorite for the physiologist and pathologist to apply this electrode. More importantly, the electrode shows an outstanding stability, even after 120 times of consecutive DPV measurements for the mixture of UA, XA and HX, the response signals for all of the three purine metabolites did not show either increase tendency or decrease tendency (Figure 4b and Figure S11). No electrode has demonstrated such a stability in the simultaneous measurements of UA, XA and HX to date. We need to stress that it is the covalent bonding in nanohybrid-ER makes the electrode maintaining such a stability. When the modified electrodes are not in use, they are exposed to normal atmosphere. The long time stability is studied by measuring their electrochemical signals every week. No obvious decreases in the responses to the three biomolecules are observed in the 2 week storage. Even after 20 days storing, the modified electrodes still retains 97% of their initial responses (Figure S12), indicating the good long-term stability of the electrodes. The practical application of the electrode is successfully demonstrated by determining UA, XA, and HX in the human serum and urine (Table S5).


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The kinetics of the electrode reactions at nanohybrid-ER/GCE is also investigated by studying the effects of scan rate on the peak currents (Figure S13). The electrode reaction of UA, XA and HX is suggestive of a typical diffusion-controlled process. The pH vs response currents studies (Figure S14) show that two electron/two-proton transfer processes are involved in the electrode reactions. These phenomena are in agreement with the density functional theory studies of the interaction of graphene with UA, XA and HX, showing that it is π–π and O–π interactions between graphene and UA, XA and HX molecules in electrochemical sensing.30 In order to further explain the stability of nanohybrid-ER/GCE, we carry out studies on the interaction between NH2-MIL-101(Fe) and GCE beyond the covalent bonding in nanohybrid-ER. The FTIR reflectance spectra of electrochemical reduced GCE show broad bands at 1040 cm-1 and 1720 cm-1 that correspond to C-O and C=O groups, respectively (Figure S15). Then NH2-MIL101(Fe) is cast on GCE, electrochemical reduction is carried out. The FTIR reflectance spectra of electrochemical reduced NH2-MIL-101(Fe) show a positive shift of COO bands in wavenumber comparing with that of NH2-MIL-101(Fe) (Figure S15). Of note is that the intensity of C=O bands of GCE is greatly decreased comparing with that of C-O bands. These results strongly suggest that C=O groups of GCE covalently bind to the Fe site of NH2-MIL-101(Fe), which further enhances the structural stability of nanohybrid-ER/GCE, thus explains why the electrode show such a stability. 4.CONCLUSION In summary, we have presented the covalent bonding in NH2-MIL-101(Fe)-GO nanohybrid that is synthesized by simple ultrasonication of the two components, and the retainment of the covalent bonding after electrochemical reduction. The morphology and structure of the nanohybrid were


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characterized by SEM, TEM, UV−vis, IR, XPS, XRD and Raman. Electrode based on the electrochemical reduced nanohybrid demonstrates a competitive sensitivity and an ultra-stability in simultaneous electrochemical detection of purines metabolites HA, XA and UA. The electrode can be used 120 time without signal attenuation, which is the most stable electrode reported for simultaneous electrochemical detection of HA, XA and UA so far. It is no doubt that the covalent bonding plays a key role in the process. Given the easy preparation process and the resultant stability, we believe covalent functionalization of GO by pre-synthesized MOF will provide further inspiration for the use in electrocatalysis and electrochemical biosensors. ASSOCIATED CONTENT Supporting Information. XRD, XPS, cyclic voltammetry, FTIR reflectance spectra, Nyquist plots, DPV, interference studies, stability studies, calculated effective surface area, comparison of reported electrodes in the determination of UA, XA and HX, practical application. The following files are available free of charge. brief description (file type, i.e., PDF) brief description (file type, i.e., PDF) AUTHOR INFORMATION Corresponding Author *Xiuyun Wang, [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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We thank the National Natural Science Foundation of China (Grant No.21205008 and 21703235), the Fundamental Research Funds for the Central Universities (Grant DUT16LK25) and Postdoctoral International Exchange Fellowship Program for the financial support. REFERENCES (1) Georgakilas, V.; Otyepka, M.; Bourlinos, A. B.; Chandra, V.; Kim, N.; Kemp, K. C.; Hobza, P.; Zboril, R.; Kim, K. S. Functionalization of Graphene: Covalent and Non-Covalent Approaches, Derivatives and Applications. Chem. Rev. 2012, 112, 6156-6214. (2) Englert, J. M.; Dotzer, C.; Yang, G.; Schmid, M.; Papp, C.; Gottfried, J. M.; Steinrück, H.P.; Spiecker, E.; Hauke, F.; Hirsch, A. Covalent Bulk Functionalization of Graphene. Nat. Chem. 2011, 3, 279-286. (3) Johns, J. E.; Hersam, M. C. Atomic Covalent Functionalization of Graphene. Acc. Chem. Res. 2013, 46, 77-86. (4) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-based Composite Materials. Nature 2006, 442, 282-286. (5) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; Keeffe, M.; Yaghi, O. M. Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application in Methane Storage. Science 2002, 295, 469-472. (6) Cui, Y.; Li, B.; He, H.; Zhou, W.; Chen, B.; Qian, G. Metal–Organic Frameworks as Platforms for Functional Materials. Acc. Chem. Res. 2016, 49, 483-493.


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Covalent Functionalization of Graphene Oxide with a Presynthesized

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