English

• 1. INTRODUCTION

  In recent years, MOFs with designable structures and open channels are well known as the promising candidates for the preparation of metal nanoparticles and hierarchically nanoporous carbon (NPC) materials^[1]. Since the first example was reported using MOFs as precursor for preparing NPC^[2], calcination of multiple MOFs under suitable condition has become an effective approach to obtain desired nanomaterials applied in a variety of fields, such as catalyst for chemical reaction^[3], ions adsorption^[4], supercapacitor^{[1a, 2, 5]}, electrochemical sensor^[6], because the porous feature from MOFs template can transfer to the resulting composite^[7]. Furthermore, the heteroatom (N, S, P and so on) from the ligands will uniformly dope into the porous carbon, which is favorable to effective regulation of the electrochemical properties^[8]. Based on these advantages, MOF-derived composites on carbon matrix have been expected to be the promising electrode materials for sensor.

  With the development of industry, mercury species pollution in wastewater has become one of the serious environmental problems in the world. The determination of mercury ions is crucial for public health. As a result, many efforts have been focused on employing the porous materials to fabricate reliable, handy, efficient and low-cost sensors for Hg²⁺ microanalysis, such as MOFs^[9] and functionalized carbon^[10]. A triphenylphosphine-modified carbon nanotube composite was constructed to act as electrode for detecting Hg²⁺ with a liner range up to 0.15 μM^[11]. Additionally, N-doped graphene modified electrode was applied for Hg²⁺ determination with maximum detection concentration of 9 μM^[12].

  On the other hand, it's well acknowledged that diabetes have already become a serious chronic disease all over the world and the concentration of glucose in human blood is an important indicator in diagnosing diabetes^[13]. Most of the traditional enzyme-based glucose sensors are easily deactivated due to the limitation of pH, temperature and humidity^[14]. Therefore, non-enzymatic glucose sensors have drawn a lot of attention and become a research hot-topic recently^[13a]. Cu NPs was encapsulated in the porous matrix of ZIF-8, which served as a non-enzymatic glucose sensor under the concentration scope lower than 0.7 mM^[15]. Another glucose sensor was fabricated based on the Cu NPs decorated nitrogen doped graphite, which worked with a linear upper limit of 1.8 µM^[16].

  As mentioned above, most investigations for electrochemical sensors are focused on decreasing the detection limit, only a little attention has been paid to extending the detection range, which may be due to the loss of linearity at the higher contraction region. In contrast, the electrochemical sensor with wide detection range can avoid further intricate condensation of sample. Hence, it's also necessary to develop sensors with wide detection range and high upper detection limit. Moreover, it's still a considerable challenge to fabricate a multifunctional and applicable sensor for detecting both organic and inorganic compounds.

  In the present work, our group successfully synthesized a novel electrode material of Cu₂O/Cu@NPC by pyrolysis of Cu-MOF under reductive atmosphere. Cu and Cu₂O NPs were observed to disperse homogeneously on the NPC, affording a promising electrochemical sensor when loaded onto the glassy carbon electrode (GCE). It shows a wide detection range and good anti-interference for sensing both of mercury ion and glucose in their aqueous solution.

  2. MATERIALS AND METHODS

  2.1 Instruments and materials

  X-ray photoelectron spectroscopy (XPS) of these materials was recorded in a Thermo Fisher ESCALAB 250 xi spectrometer (England) using AlKα radiation (1486.6 eV). Binding energies were calculated with respect to C(1s) at 284.8 eV, which were measured with a precision of ±0.05 eV. Thermogravimetric analyses were performed under a N₂ atmosphere at a heating rate of 20 ℃ min^-1 with a Perkin-Elmer TGA4000 thermogravimetric analyser. The X-ray powder diffraction patterns (PXRD) and Infrared spectra were measured with a Bruker D8 Advance diffractometer and a Bruker Tensor 27 spectrophotometer, respectively. Transmission electron microscopy (TEM) was recorded using a FEI Tecnai F20 transmission electron microscopy operating at 200 KV (FEI USA, Co., Ltd). Scanning electron microscopy (SEM) was recorded using a MIRA3 (GMH/ GMU) scanning electron microscopy at 15.0 kV (TESCAN, Co., Ltd). The electrochemical performance was measured by CS electrochemical workstation (Wuhan Kont Instrument Co., Ltd.). The three-electrode system was constituted by Ag/AgCl electrode as the reference electrode, Pt wire as the auxiliary electrode and Cu₂O/Cu@NPC/GC as the working electrode. Fructose, lactose, sucrose, dopamine, uric acid and ascorbic acid were of analytical grade provided by Shanghai Boao Biotechnology company. All chemicals were obtained from commercial sources and used without further purification.

  2.2 Synthesis of Cu₂O/Cu@NPC

  As shown in Scheme 1, the sample of Cu-MOF was synthesized using solvothermal method as our previous literature^[17]. The phase purity was confirmed by PXRD (Fig. S1). The pyrolysis of MOF was operated as follows. 100 mg crystal sample of Cu-MOF was placed in the tube furnace under flowing N₂ (92%) and H₂ (8%) at a heating rate of 5 ℃/min with the temperature increased from ambient temperature to 500 ℃ and annealed for 2 h. Then the resulting mixture was cooled to room temperature at the rate of 5 ℃/min to obtain black powder.

  Scheme 1

  

  Scheme 1.  Schematic illustration of the Cu-MOF and its derived Cu₂O/Cu NPs on the NPC matrix for GCE modification

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  3. RESULTS AND DISCUSSION

  3.1 Characterization of Cu-MOF, Cu₂O/Cu@NPC

  PXRD was preferentially carried out to explore the composition of the pyrolysis product. The experimental PXRD profile reveals three obvious peaks at 36.3°, 42.4° and 43.3°, which are assigned to Miller indices (-1-1-1), (-200) of the cubic Cu and cubic Cu₂O phase, respectively (Fig. S1). The ratio of Cu₂O and Cu NPs was calculated to be approxi-mately 1:7, according to the peak intensity of crystal planes (-1-1-1) from Cu₂O and Cu. The result of PXRD suggested that both of Cu and Cu₂O particles located in the pyrolysis product.

  SEM was used to characterize the morphology of the as-prepared material. The secondary electron (SE) and the backscattered electrons (BSE) in Fig. 1 show that the prismatic morphology of Cu-MOF was remained and nano-particles were surrounded by carbon film after pyrolysis. The size of pyrolysis product ranges from 2 to 6 μm and that of the inner uniform particles is about 200 nm (Fig. S2).

  Figure 1

  

  Figure 1.  Diagram of SE (a) and BSE (b) for Cu₂O/Cu@NPC

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  It's significant to figure out the distribution of Cu NPs and Cu₂O NPs. The elemental mapping in Fig. 2 confirmed the existence of four elements: Cu and O from nanoparticles and C and N from the carbon matrix. Notably, O was found to distribute the out layer of the prismatic block. It was supposed that Cu was generated in the inner layer under the reduction co-effect of H₂ and pyrolytic carbon. Cu₂O formed in the out layer probably due to the influence of O fragment released from the decomposed organic ligand. This conjecture was further demonstrated by SEM of Cu₂O/ Cu@NPC after acid treatment. Since Cu₂O NPs are known to be more easily dissolved in acid solution than Cu NPs, the resulting solid was subjected to a diluted acid solution and stirred for 6 hours at room temperature. SEM showed that there were no nanoparticles. Meanwhile, a lot of holes are observed on the surface of the resulting solid after acid treatment, which can be attributed to the original location of Cu₂O NPs (Fig. S3). PXRD revealed that the Cu₂O NPs were removed except Cu NPs (Fig. S4). As a result, it can be believed that the Cu₂O NPs are located on the surface and Cu NPs at the inner layer of pyrolysis product. In addition, the element analysis in Fig. S5 indicated about 2.47% N in the sample. It has been illustrated that N-doped porous carbon can be an n-type carbon dopant to improve its electrochemical performance^[18]. Moreover, it was observed that Cu species dispersed homogeneously among the carbon matrix, which was expected to act as the electron pathway during electro-chemistry reaction^[14]. More importantly, this special structure was supposed to prevent the active nano-particles from aggregation and/or degradation.

  Figure 2

  

  Figure 2.  Elemental mapping image of Cu₂O/Cu@NPC, individual Cu (orange), C (pink), O (red) and N (yellow)

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  To analyze the nature of as-synthesized carbon matrix, pyrolysis product was characterized through Raman spectroscopy. Two typical bands located at 1362 cm^-1 for D and 1552 cm^-1 for G bands (Fig. S6) ascribed to the disordered carbons and the vibration modes of two carbons movement in opposite directions in a single graphene sheet^[1a]. Besides, the FTIR spectra were carried out to analyze the functional groups of the pyrolysis product (Fig. S7). The absence of adsorption peaks of the ligand from Cu-MOF indicated the completeness of pyrolytic reaction. The Cu–O characteristic adsorption peak for Cu₂O can be found at 503 cm^-1, while the C=C vibration peaks for graphene are at 1547 and 1456 cm^-1. The spectral peak at 1715 cm^-1 of C=N confirmed the N-doping in composites.

  XPS was carried out to characterize the valence states of Cu, N, C and O from original Cu-MOF and its pyrolysis product (Fig. 3, Fig. S8-S9). As shown in Fig. 3, the band of binding energy (BE) of Cu 2P_3/2 within the sample was confirmed at 932.2 and 932.7 eV, indicating the presence of element copper and cuprous oxide. The band of BE at 934.6 eV was also detected, probably because the oxidation of external Cu₂O NPs usually occurred when exposed in the air^[3a]. The status of N 1s from Cu₂O/Cu@NPC was divided into four types: C–N (398.2 eV), tertiary amine (398.7 eV), pyridinic-N (399.4 eV) and C=N (400.3 eV)^[19]. The C 1s spectrum was also fitted by four peaks at 284.3 eV from sp³-bonded C–C, 286.0 eV from sp²-bonded C=C, 285.2 eV from C–N and 287.5 eV from C=N^{[19a, 20]}. Regarding the O 1s XPS spectrum of Cu₂O/Cu@NPC, the peaks at 530.4 and 531.1 eV were ascribed to Cu(OH)₂ and Cu₂O^{[3a, 21]}, respectively. Meanwhile, the peak at 533.0 eV was due to the adsorbed O^[22].

  Figure 3

  

  Figure 3.  XPS of Cu 2p3/2 (a), N 1s (b), C 1s (c) and O 1s (d) for Cu₂O/Cu@NPC

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  3.2 Adsorption property of Cu₂O/Cu@NPC

  It has been illustrated that hierarchically pores may form in the carbon during MOFs' pyrolysis process^[3b]. Dye adsorption experiment is a convenient and visual method to prove the porosity of materials. Herein, the capture of dye Rh B by Cu₂O/Cu@NPC was carried out at ambient temperature. As shown in Fig. S10, 8.0 mg of pyrolysis product was added into 5.0 mL of Rh B in aqueous solution (20.0 mg/L). The pink solution of Rh B faded gradually to almost colorlessness. Meanwhile, this adsorption process was also monitored by UV/vis spectroscopy, which showed a decay of the absorption band at 554 nm. The dye uptake amount was calculated to be 12.3 mg/g in 300 minutes based on the calibration plots. Although the adsorption rate was slower than some reported materials^[23], the adsorption of dye Rh B obviously indicated the porosity of pyrolysis product. Meanwhile, the Rh B can also be released from Cu₂O/ Cu/NPC (Fig. S17).

  3.3 Fabrication of Cu₂O/Cu@NPC/GCE as the electrochemical sensor

  Firstly, the GCE was sequentially polished using alumina powder with the size of 1, 0.3 and 0.05 μm. Then, 20 mg of as-synthesized powder Cu₂O/Cu@NPC was milled and added into a 0.3% Nafion solution. The uniform suspension was obtained through ultrasonic dispersion for 20 minutes, then dispensed this liquid onto the GCE surface. The infrared lamp was used to dry the electrode, and the same process of dispensing/drying was repeated for 10 times to get Cu₂O/Cu@NPC located onto a glassy carbon electrode, namely Cu₂O/Cu@NPC/GCE. For comparison, the fabrication of Cu-MOF/GCE followed the same above process.

  3.4 Amperometric response of the modified electrode to mercury ion

  Differential Pulse Voltammetry (DPV) is considered as a convenient method to determine the concentration of heavy metal ion from phosphate buffered saline (PBS)^[24]. The stability of Cu₂O/Cu@NPC in the PBS solution of Hg²⁺ was confirmed by PXRD, which showed that the framework was not destroyed (Fig. S11). Herein, DPV test was carried out through a three-electrode system. As shown in Fig. 4(a), the peak current was recorded after sequential adding 50 μL of Hg²⁺ solution (10 mM) into 50 mL of PBS (pH = 6). The relation curve between the concentration of Hg²⁺ and peak current was obtained, in which the linear equation is I = 0.0284C + 5.57 and R² = 0.9920. The wide linear range (from 10 to 70 μM) and high upper limit are found to be superior to the previously reported electrochemical sensors (Fig. S12).

  Figure 4

  

  Figure 4.  (a) Cu₂O/Cu@NPC/GCE as electrochemical sensor for mercury ions; (b) Selectivity study of electrochemical sensor for mercury ions

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  The anti-interference is an essential factor to evaluate Cu₂O/Cu@NPC/GCE as an electrochemical sensor. In order to investigate the target ion response performance in the presence of other metal ions, coexisting metal ions (Ca²⁺, Cd²⁺, Cr³⁺, Fe³⁺, K⁺, Mg²⁺, Hg²⁺) were added into 50 mL PBS to form a mixed ion (30 μM) system, and the corresponding DPV was measured, as shown in Fig. 4(b). The Cu₂O/Cu@NPC/GCE showed desirable performance as the Hg²⁺ sensor even in the presence of other interfering ions.

  3.5 Electrocatalytic performance of Cu₂O/Cu@NPC/GCE to glucose

  The electrocatalytic performance and the reaction mecha-nism of composite for glucose were also investigated by cyclic voltammetry (CV) techniques. The pure GC electrode modified by Cu₂O/Cu@NPC was examined using CV in 0.10 M NaOH solution with 20 mM glucose. The response current of oxidative peak to glucose has been shown in Fig. 5a. Pure GC electrode had no apparent oxidative peak and distinct response current exhibited a typical curve of capacitive charge-discharge. However, the response current of GCE dramatically increased with a large slope appearing in the potential window from +0.70 V to +0.90 V, which corresponds to the oxidation of glucose. The black curve showed that the current prominently raises at +0.65 V because of the decomposition of water. After adding 20 mM glucose, the Cu₂O/Cu@NPC/GCE displayed that the potential of oxidative peak of glucose had an evidently negative shift starting at +0.30 V and peak potential at approximately +0.50 V. The decrease of anodic overpotential for glucose suggested that Cu₂O/Cu@NPC/GCE had a highly electrocatalytic performance in the oxidation of glucose due to synergistic effect by the improvement of electroactive surface area and electron transfer rate.

  Figure 5

  

  Figure 5.  (a) CV curves of bare GCE in the absence and presence of 20 mM glucose and Cu₂O/Cu@NPC/GCE in the absence and presence of 20 mM glucose; (b) Amperometric response to injections of 50 μL 2 M of glucose per 50 s at different potentials from +0.50 to +0.70 V; (c) Amperometric response of GCE, Cu₂O/Cu@NPC/GCE and Cu-MOF/GCE with successive addition of 10 mM glucose per 50 s at +0.50 V; (d) Selectivity study through amperometric response to injections of 0.02 mM interferents and 2.00 mM glucose at +0.50 V

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  The possible electro-oxidation mechanism of glucose on the Cu₂O/Cu@NPC matrix surface in an alkaline solution can be assumed as follows:

  $ \mathrm{Cu}+2 \mathrm{OH}^{-}-2 \mathrm{e}^{-} \longrightarrow \mathrm{Cu}(\mathrm{OH})_2 $

  (1)

  $ \mathrm{Cu}_2 \mathrm{O}+\mathrm{OH}^{-}-\mathrm{e}^{-} \longrightarrow \mathrm{Cu}(\mathrm{OH})_2 $

  (2)

  $ \mathrm{Cu}(\mathrm{OH})_2+\mathrm{OH}^{-} \longrightarrow \mathrm{CuOOH}+\mathrm{H}_2 \mathrm{O}+\mathrm{e}^{-} $

  (3)

  $ \mathrm{CuOOH}+\text { glucose }+\mathrm{e}^{-} \longrightarrow \mathrm{CuO}+\mathrm{OH}^{-}+\text {gluconicacid } $

  (4)

  $ \mathrm{CuO}+\mathrm{OH}^{-} \longrightarrow \mathrm{CuOOH}+\mathrm{e}^{-} $

  (5)

  This mechanism means Cu₂O/Cu@NPC can be as a strong oxidant for electrocatalytic glucose, resulting in a considerable number of electrons transferring the interface between the surface of Cu₂O/Cu@NPC/GCE and glucose solution. Therefore, the visible response current appeared at the potential of glucose oxidation.

  The linear range of current response and glucose concentration was determined by the chronoamperometry. As shown in Fig. 5(b), potentiostatic polarization was performed at different potentials, and 50 μL of 2 M glucose solution was added per 50 s by micropipette. The response current of Cu₂O/Cu@NPC/GCE electrochemical sensor showed an identical current step, indicating good linear relationship. According to the insert map in Fig. 5(b), the slope of fitting curve was increased from +0.30 to +0.70 V, which explained the sensitivity improved as the potential gradually enhanced as well. Since high potential would bring about the oxidation of interfering substances generating disturbing responsive current, +0.50 V is chosen as the optimum potential.

  Fig. 5(c) showed amperometric response plots of glucose in NaOH solution at the GCE, Cu-MOF/GCE and Cu₂O/ Cu@NPC/GCE, respectively. The curve of bare GCE was almost a straight line under +0.50 V, and the Cu-MOF/GCE displayed a similar trend, i.e. responsive current was almost invisible, which indicated the catalytic sites of Cu-MOF were not well exposed in accordance with the result of SEM. In contrast, the Cu₂O/Cu@NPC/GCE showed higher current response than the previous two electrodes. The linear equation of the response current and glucose concentration of the electrode was I = 0.58C + 0.83 and the correlation coefficient R² = 0.9935. The linear range of detection for the Cu₂O/Cu@NPC/GCE glucose sensor was from 10 to 20 mM with a sensitivity of 4.6 μA·mM^-1·cm^-2 and a detection limit of 8 μM (Signal/Noise ≈ 3). The wide detection range originated from porous structure with larger surface areas, providing more electroactive sites.

  The performance of prepared Cu₂O/Cu@NPC/GCE sensor was compared with other nonenzymatic glucose sensors such as noble metals and transition metal as electrocatalyst, which was summarized in Fig. S13. It was remarkable that the sensor based on Cu₂O/Cu@NPC modified GCE exhibits wider detection range, fast response and wider line arrange compared to most other sensors.

  Anti-interference performance is also an essential factor, especially for non-enzymatic sensors. Because these species, such as dopamine (DA), ascorbic acid (AA), uric acid (UA) and other carbohydrate compounds, usually co-exist with glucose in human blood, they are easily to be oxidited. The electrochemical response of interfering species was also examined at Cu₂O/Cu@NPC/GCE, as shown in Fig. 5(d). The concentration of glucose was at least 30 times of interfering species in human blood, and the anti-interference detection was carried out by adding dropwise of 2.00 mM glucose and 0.20 mM interfering species in 0.10 M NaOH solution at a time interval of 100 s. An obvious glucose response was observed, while responses of interfering species were insignificant at +0.50 V. Small amount of DA, AA, UA and other carbohydrate compounds like fructose, lactose and sucrose would not affect the detection of glucose in serum. Therefore, the Cu₂O/Cu@NPC/GC electrode showed high selectivity for glucose determination.

  3.6 Reproducibility and stability of Cu₂O/Cu@NPC/GCE

  The reproducibility and stability of the sensor were evaluated. The three electrodes of Cu₂O/Cu@NPC/GCE were prepared in the previous way, which were investigated at +0.50 V to compare their amperometric current responses. The relative standard deviation (R.S.D.) was 8.75%, indicating that the preparation method was well reproducible. Seven successive injections of glucose on one Cu₂O/ Cu@NPC/GCE yielded an R.S.D. of 4.51%, revealing that the sensor was stable. The long-term stability of the sensor was also evaluated by measuring its current response to glucose within a 10-day period (Fig. S14). The sensor was exposed to air and its sensitivity was tested every day. The current response of Cu₂O/Cu@NPC/GCE was approximately 89.30% of its original value of the responsive current.

  4. CONCLUSION

  In summary, a hybrid composite Cu₂O/Cu@NPC with well-defined structure, in which Cu and Cu₂O NPs are uniformly dispersed on N-doped NPC has been synthesized by the pyrolysis of Cu-based MOF under reducing N₂/H₂ atmosphere. The composite exhibits good porosity and adsorption properties. Under optimized condition, a novel non-enzymatic glucose sensor of Cu₂O/Cu@NPC/GCE has been successfully fabricated, which presents wider detection range, durative stability, good reproducibility, and excellent selectivity to the improvement of surface area, facilitating the diffusion of glucose and electrocatalytic activity resulting from Cu₂O/Cu@NPC nanoparticles. These experimental results indicate that the Cu₂O/Cu@NPC/GCE has high-performance for effective nonenzymatic determination of glucose as well as mercury ion.

  
  ACKNOWLEDGEMENTS: We are grateful to Prof. Dr. Dieter Fenske, Prof. Dr. Chengyong Su and Dr. Wenting Liu for their continuous supports and assistance.
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  Scheme 1  Schematic illustration of the Cu-MOF and its derived Cu₂O/Cu NPs on the NPC matrix for GCE modification

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  Figure 1  Diagram of SE (a) and BSE (b) for Cu₂O/Cu@NPC

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  Figure 2  Elemental mapping image of Cu₂O/Cu@NPC, individual Cu (orange), C (pink), O (red) and N (yellow)

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  Figure 3  XPS of Cu 2p3/2 (a), N 1s (b), C 1s (c) and O 1s (d) for Cu₂O/Cu@NPC

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  Figure 4  (a) Cu₂O/Cu@NPC/GCE as electrochemical sensor for mercury ions; (b) Selectivity study of electrochemical sensor for mercury ions

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  Figure 5  (a) CV curves of bare GCE in the absence and presence of 20 mM glucose and Cu₂O/Cu@NPC/GCE in the absence and presence of 20 mM glucose; (b) Amperometric response to injections of 50 μL 2 M of glucose per 50 s at different potentials from +0.50 to +0.70 V; (c) Amperometric response of GCE, Cu₂O/Cu@NPC/GCE and Cu-MOF/GCE with successive addition of 10 mM glucose per 50 s at +0.50 V; (d) Selectivity study through amperometric response to injections of 0.02 mM interferents and 2.00 mM glucose at +0.50 V

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