Materials Advances This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online PAPER View Journal | View Issue A yolk–albumen–shell structure of mixed Ni–Co oxide with an ultrathin carbon shell for high- Cite this: Mater. Adv., 2020, Open Access Article. Published on 17 June 2020. Downloaded on 11/22/2022 9:12:51 AM. 1, 908 sensitivity glucose sensors† Xuan Zhang, ab Yawei Zhang,b Wei Guo,a Kai Wan, a Ting Zhang,c Jordi Arbiol, *cd Yong-Qing Zhao, e Cai-Ling Xu, e Maowen Xu *b and Jan Fransaer*a Non-enzymatic glucose sensors based on different Co–Ni–C composite materials were developed by pyrolysis of bimetallic or single metal based metal–organic frameworks (MOFs). The structure and composition of the resulting materials were explored by XRD, nitrogen adsorption/desorption isotherms, SEM, HRTEM and STEM-EELS. The electrochemical performance of the bimetallic MOF derived novel yolk–albumen–shell structure of Ni–Co@C (YASNiCo@C) stands out from these materials. The Received 22nd April 2020, YASNiCo@C electrode exhibited a sensitivity of 1964 mA cm2 mM1 with the detection limit of 0.75 mM, Accepted 16th June 2020 a linear range from 5 mM to 1000 mM and good stability for the detection of glucose. These promising DOI: 10.1039/d0ma00230e electrochemical performances prove that YASNiCo@C is a promising material for glucose sensors. More- over, the strategy outlined in this work for the design of MOF based nanomaterials can also be used rsc.li/materials-advances beyond glucose sensors. 1. Introduction efforts have been made to use enzymatic biosensors and three generations of glucose sensors have been successfully Detection of glucose is widely used in the food industry, medical developed based on glucose oxidase (GOx). The first generation diagnosis and environmental monitoring. The development of glucose sensor relied on the use of the natural oxygen high sensitivity and low-cost glucose sensors is highly desirable co-substrate and generation with indirect detection of glucose for the broad academic community and industry. Even though by detecting hydrogen peroxide.3 Due to the oxygen dependence many techniques can be used for the detection of glucose, and high operating potential of the first generation glucose electrochemical sensors dominate the market because of their sensor, the second generation glucose sensors based on the sensitivity, reproducibility and low cost. immobilization of GOx on a redox mediator were developed, Generally speaking, there are two kinds of electrochemical which solved the problem of oxygen dependence.4,5 In order to sensors of glucose: enzymatic and non-enzymatic. Since the eliminate the mediator, the third generation of glucose sensors 1960s, when the first enzyme electrode was described,1,2 many uses direct electron transport between the immobilized enzymes and the electrode surface. However, the third generation glucose a sensors still suffer from low stability, low sensitivity and small Department of Materials Engineering, KU Leuven, Leuven 3001, Belgium. E-mail: jan.fransaer@kuleuven.be linear range.6 More importantly, all these enzymatic sensors b Key Laboratory of Luminescent and Real-Time Analytical Chemistry (Southwest are affected by the intrinsic properties of the GOx, which is University), Ministry of Education, School of Materials and Energy, Southwest affected by temperature (o40 1C), pH (2–8) and humidity. University, Chongqing 400715, P. R. China. E-mail: xumaowen@swu.edu.cn These drawbacks strongly hinder the glucose sensor applica- c Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, tions in the food industry, environment monitoring, fuel cells Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain. E-mail: arbiol@icrea.cat and operating batteries. A fourth generation glucose sensor d ICREA, Pg. Lluı́s Companys 23, 08010 Barcelona, Catalonia, Spain based on non-enzymatic electrodes, constructed out of active e State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous metal-based nanoparticles, is considered as a good candidate Metal Chemistry and Resources Utilization of Gansu Province, Laboratory of for these applications. Special Function Materials and Structure Design of the Ministry of Education, Different transition metal (Cu, Ni, Fe, Co, Mn, etc.) oxides have College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, China been explored as fourth-generation glucose sensor materials † Electronic supplementary information (ESI) available. See DOI: 10.1039/ due to their high sensitivity and low cost.7–9 Among these d0ma00230e transition metals, Co and Ni show superior electrocatalytic 908 | Mater. Adv., 2020, 1, 908--917 This journal is © The Royal Society of Chemistry 2020 View Article Online Paper Materials Advances activity and high stability toward electro-oxidation of glucose in preparation of the mixed Co–Ni metal oxides for glucose sensors alkaline media.10–13 In order to increase the electrocatalytic via pyrolysis of bimetallic MOFs has been rarely reported. active surface area of these active materials, different kinds of In this work, Co–Ni bimetallic MOFs with a uniform distribu- nanostructures have been used for glucose electrocatalysis, tion of the nodal positions were synthesized. After pyrolysis, This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. such as nanowires,13 nanosheets,14 nanodisks, nanorods,15 different compositions and structures of Co–Ni metal oxide with nanotubes,16 nanoflowers17 and so on. However, the poor electrical ultrathin carbon shell materials were prepared. The resulting conductivity of these metal oxides and their tendency to aggregate materials were used to fabricate non-enzymatic electrochemical has hindered their applications. The combination of metal glucose sensor electrodes. Among these electrodes, the bimetallic oxides and carbon materials is a promising way to solve these MOF-derived materials displayed much better electrochemical problems, which could be beneficial for the activity, selectivity, catalytic performance towards glucose than materials based on and durability of such glucose sensors.18–23 For example, pure Co or Ni MOFs. Open Access Article. Published on 17 June 2020. Downloaded on 11/22/2022 9:12:51 AM. Junhong Chen et al. prepared CoO/graphene microsphere hybrid materials for glucose sensors.24 Compared to pure CoO electrodes, CoO/graphene electrodes showed around 2 times 2. Results and discussion higher current response for 0.1 mM glucose with a sensitivity of 670 mA mM1 cm2. The Co–Ni composite materials were synthesized by the pyrolysis of To increase the effective surface area and enhance the mass bimetallic Co–Ni MOFs as shown in Fig. 1. For M(HBTC)(4,40 -bipy) and ion transport of these nanomaterials, the construction of (M = Ni, Co or mixture of Co and Ni), the coordination environ- porous carbon–metal oxide nanomaterial composite materials ment of the metal centers is bound by three BTC units with one via pyrolysis of metal–organic frameworks (MOFs) has drawn bidentate carboxylic group and two monodentate carboxylic extensive attention. These materials have been used for groups, forming a layered structure with honeycomb pores. batteries,25,26 supercapacitors,27,28 solar cells29,30 and catalysis31–35 The polar positions on the nickel(II) or cobalt(II) centers are applications. Recently, this method was also used to prepare occupied by 4,4 0 -bipyridine along the c axis to form a stable 3D materials for glucose sensors. Minbo Lan et al. pyrolyzed ZIF-67 framework. The morphologies and compositions of these MOFs in different atmospheres (air and nitrogen) at 700 1C to prepare are characterized by SEM. In order to evaluate the elemental cobalt nanoparticles-porous carbon composites materials.36 distribution of Ni/Co, electron probe microanalysis (EPMA) The sensitivity of ZIF-67 derived CoO/carbon composite coupled with wavelength dispersive spectroscopy (WDS) was materials (pyrolyzed under nitrogen) showed a sensitivity of used for determination of metal element compositions, which 227 mA mM1 cm2 (10 times higher than pyrolyzed under air), has more sensitivity than energy dispersive spectroscopy (EDS).46 and a linear range of 0.1–1.1 mM with a limit of detection of As shown in Fig. 2, all the samples have similar hexagonal sheet- 6 mM. Compared with single metal-based electrodes, bimetallic like morphology. For Co–Ni bimetallic MOFs, the distribution of compounds often show higher sensitivity.37,38 For example, Ni and Co in the hexagonal sheets is fairly homogeneous. The G. Gnana Kumar et al. synthesized CuO/NiO–carbon nanocom- ratio between Co and Ni is around 1 : 1, which is in good posite materials by the pyrolysis of Cu–Ni MOFs.39 Compared agreement with the ICP results (1.06 : 1). The crystallinity of the to the pure Cu–carbon based or Ni–carbon based materials, as-prepared MOFs and the corresponding metal oxide–carbon CuO/NiO–carbon nanocomposite materials had a considerable composite materials is characterized by XRD (Fig. 3 and Fig. S1, current response for 5 mM glucose (around 1.5 times higher ESI†). All these MOFs have similar XRD patterns as they possess than CuO/C and 1.2 times higher than NiO/C nanocomposite the same crystal structure, indicating the successful incorporation materials) with a sensitivity of 587 mA mM1 cm2. However, of a second metal into the frameworks. The powder XRD patterns MOFs that have uniformly distributed metals ion of nodal show clear characteristic diffraction peaks at 2y E 7.81, 10.61, positions often suffer from the fragile frameworks due to the 15.61 and 18.51, which match well with the simulated patterns.47 introduction of second metal nodes.40–45 To the best of our The thermal stabilities of the pristine MOFs were explored knowledge, even though mixed Co–Ni metal oxides are consid- by thermal gravimetric analysis (TGA) under an argon atmo- ered as one of the most active materials for glucose sensors,37,38 sphere (Fig. 4a). The TGA curves showed two weight-loss steps. Fig. 1 Schematic illustration of the synthesis of YASNiCo@C. This journal is © The Royal Society of Chemistry 2020 Mater. Adv., 2020, 1, 908--917 | 909 View Article Online Materials Advances Paper This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 17 June 2020. Downloaded on 11/22/2022 9:12:51 AM. Fig. 2 SEM images of different MOFs and the corresponding EMPA elemental mapping images (at an acceleration voltage of 15 kV and a probe current of 15 nA): (a) Ni-MOF; (b) CoNi-MOF and (c) Co-MOF. The first step corresponds to the release of guest molecules, Meanwhile, the XRD peaks of the YASNiCo@C can be assigned to such as ethanol (b.p. = 78 1C) and DMF (b.p. = 153 1C). The the peaks of cubic NiO and cubic Co3O4 without any impurities. second step is caused by the thermal decomposition of the N2 adsorption–desorption isotherms were used to explore linker molecules. The decomposition temperatures of these the surface area of the resulting catalyst (Fig. 4b). The NiNP@C MOFs are 298 1C, 412 1C and 425 1C for Co-MOF, CoNi-MOF exhibited a surface area of 36.2 m2 g1, which is 79.2% higher and Ni-MOF, respectively. Interestingly, the CoNi MOF shows a than that of CoNP@C (20.2 m2 g1). After incorporation of Ni significantly enhanced thermal stability (more than 100 1C) into the frameworks of Co-MOFs, the bimetallic YASNiCo@C compared to the pure Co-based MOFs and close to the Ni-based catalysts exhibit an enhanced surface area (29.8 m2 g1) com- MOF. As thermal stability is important for MOF applications pared to CoNP@C. As shown in Fig. S2–S4 (ESI†), the hexagonal such as heat storage, catalytic processes at high temperatures morphology is maintained during pyrolysis. SEM observation at and so on,48 this enhanced property of bimetallic MOFs could higher magnification exhibited a porous honeycomb-structure be interesting. After pyrolysis and annealing, the XRD patterns for all of these samples, which is caused by the release of gas of MOF derived NiNP@C (Fig. S1a, ESI†) and CoNP@C (originated from the decomposition of the linkers). These (Fig. S1b, ESI†) match well with the standard peaks of cubic NiO porous honeycomb-structures can provide fast ion transport (JCPDS 65-2901) and cubic Co3O4 (JCPDS 43-1003), respectively. from the electrolyte to the surface of the active material.49–53 910 | Mater. Adv., 2020, 1, 908--917 This journal is © The Royal Society of Chemistry 2020 View Article Online Paper Materials Advances This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 17 June 2020. Downloaded on 11/22/2022 9:12:51 AM. Fig. 3 XRD pattern (using Cu Ka radiation with wave length l = 0.15405 nm and Ni filter with a step size of 0.021) of (a) pristine MOFs and (b) YASNiCo@C. Fig. 4 (a) TGA traces (under an argon atmosphere with a heating rate of 5 1C min1) of Ni-MOF, CoNi-MOF and Co-MOF. (b) N2 adsorption–desorption isotherms of NiNP@C, CoNP@C and YASNiCo@C. To further explore the structure of single nanoparticles, the more active sides for catalysis. Both Co3O4 and NiO phases structure of the MOF-derived catalysts was studied by TEM, could be observed for the YASNiCo@C sample. EELS chemical HRTEM and STEM-EELS composition maps. Fig. 5 shows that composition maps were used to confirm the elemental distri- the NiNP@C, CoNP@C and YASNiCo@C samples consisted of bution of YASNiCo@C. According to the EELS chemical com- nano-particles with irregular morphologies. According to the position maps obtained from the red square region in the HRTEM images, the NiNP@C and CoNP@C showed good HAADF STEM micrograph (Fig. 5c and Fig. S5, S6, ESI†), the crystallinity. The magnification of the region marked in yellow coexistence of Ni, Co, O and C in these samples is proven. and its corresponding power spectrum reveal that the crystal However, uneven distribution of Co and Ni can be observed structures are cubic NiO (space group FM3% M) with a = b = c = by elemental maps in the YASNiCo@C nanostructures. Co 4.2000 Å and cubic Co3O4 (space group FD3% MZ) with a = b = c = preferentially sits on the outside of the particles. A similar 8.0550 Å, respectively. Both Co3O4 and NiO phases are found in phenomenon was observed in the work of Huolin L. Xin et al.,54 the YASNiCo@C sample. The NiO nanostructures are between where the authors mentioned that it was probably caused by 10 and 30 nm which is smaller than the CoO nanostructures the different diffusion coefficients of Co and Ni in the Ni–Co–O (40–80 nm) while the Co–Ni oxide particles have a size of system.55–57 On the surface of the nanoparticles, there is an 20–40 nm, which is smaller than Co3O4 nanostructures. These ultrathin (around 2 nm thickness) C layer, leading to a yolk– results are in good agreement with XRD and BET results. The albumen–shell structure. This structure of ultrathin C coating smaller particles could translate into a higher surface area, and metal oxide particles is a possible highway for electrons to This journal is © The Royal Society of Chemistry 2020 Mater. Adv., 2020, 1, 908--917 | 911 View Article Online Materials Advances Paper This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 17 June 2020. Downloaded on 11/22/2022 9:12:51 AM. Fig. 5 HRTEM images and the corresponding power spectra (FFT) of (a) NiNP@C and (b) CoNP@C. (c) HRTEM images and EELS chemical composition maps obtained of YASNiCo@C from the area marked by the square in the STEM micrograph. Individual Ni L2,3-edges at 855 eV (red), Co L2,3-edges at 779 eV (green), O K-edge at 532 eV (blue) and C K-edge at 284 eV (yellow) as well as the composite (Ni–Co, O–C and Ni–O–C) elemental map of this nanostructure. overcome the low conductivity of the metal oxides. On the other Considering the influence of interferents and electrolyte polar- hand, in order to improve the catalytic performance of metal ization (oxygen evolution) at a higher potential, +0.55 V vs. oxide–carbon composite materials, the effective surface area of Hg/HgO was used for the rest of the tests. At +0.55 V the metal oxide@carbon composite materials should be increased response currents were recorded while different concentrations by reducing the particle size and decreasing the thickness of the of glucose solutions were successively injected into a 0.1 M carbon layer. It can be estimated that the increased surface area NaOH solution under stirring with YASNiCo@C as the catalyst and ultrathin carbon layer of YASNiCo@C could facilitate the (Fig. 6c). Based on these data, the calibration curves are electrocatalysis performance of YASNiCo@C for glucose oxidation. presented in the inset of Fig. 6c. Since the electrochemical For an analytical application, the fabricated MOF-derived oxidation of glucose on the catalyst surface, it is considered as a materials were used for the detection of glucose using an surface catalytic process. According to previous reports,58,59 the amperometric method. When 1 mM glucose was added to the Langmuir fitting equation can be used to fit the calibration 0.1 M NaOH solution, a clear increase of current (at +0.55 V vs. curve. The current (y) is correlated with the glucose concen- Hg/HgO) can be observed for all these samples (Fig. 6a), except tration (x) in the range of 5–5000 mM with a correlation for the bare carbon paper (Fig. S7, ESI†). Even though CoNP@C coefficient of 0.999 by the following equation: has a much smaller surface area than NiNP@C, the ampero- 6:5237x metric response of CoNP@C (1.11 mA cm2) is 76.2% higher y¼ (1) 2325:43 þ x than that of NiNP@C (0.63 mA cm2) for 1 mM glucose. Compared to the single metal-MOF derived materials, the Between 5 mM and 1000 mM glucose, a linear relationship can amperometric repsonse of YASNiCo@C (1.82 mA cm2) is two be approximated as: times higher than that of NiNP@C and 64.0% higher than that y = 0.07474 + 0.00198x (2) of CoNP@C for 1 mM glucose. This might be due to the 2 increase of surface area and/or the synergistic effect of bime- The amperometric response is linear with R = 0.992. In this tallic oxide. The optimal detection potential of glucose of concentration range, the sensitivity is 1964 mA cm2 mM1. The YASNiCo@C was explored by scanning the potential from detection limit of the YASNiCo@C is 0.75 mM at a signal-to- +0.45 to +0.6 V vs. Hg/HgO in a solution of 1 mM glucose. As noise ratio of 3. Compared to other Ni–Co oxide based electro- shown in Fig. 6b, the response current increases with increasing des reported in the literature, our YASNiCo@C based electrode potential. However, when the potential increased from +0.55 V to has a higher sensitivity than most of the recently reported +0.6 V vs. Hg/HgO, the response current only increased by 7.7%. bimetallic metal oxides based on Ni and Co (Table S1, ESI†). 912 | Mater. Adv., 2020, 1, 908--917 This journal is © The Royal Society of Chemistry 2020 View Article Online Paper Materials Advances This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 17 June 2020. Downloaded on 11/22/2022 9:12:51 AM. Fig. 6 (a) Amperometric responses of the NiNP@C (A), CoNP@C (B) and YASNiCo@C (C) electrodes to the addition of 1 mM glucose at +0.55 V vs. Hg/HgO. (b) Amperometric responses of the YASNiCo@C electrode at potentials from +0.45 V to +0.6 V vs. Hg/HgO to the addition of 1 mM glucose. (c) Amperometric response to successive additions of glucose obtained by the YASNiCo@C electrode at an applied potential of 0.55 V vs. Hg/HgO, while the inset shows the calibration curve. (d) Long time-stability test for the YASNiCo@C electrode and normalized sensitivity of the YASNiCo@C electrode for glucose measured every two days by amperometric measurement over 15 consecutive days. The possible electro-oxidation mechanism for glucose oxidation using 1 mM glucose. After around 1 h of continuous ampero- on the YASNiCo@C electrode surface in an alkaline solution is metric testing, the current response of the YASNiCo@C electrode consistent with previous reports as follows (M = Ni or Co):38,60 retains 95% of the initial response (Fig. 6d). The stability of the YASNiCo@C electrode was also assessed by amperometric M(OOH)3 + H2O + e " M(OH)2 + OH (3) measurement every two days during a 15 day period. After 15 days 2M(OOH)3 + glucose(C6H12O6, R1CHOHR2) - 2M(OH)2 (in ambient atmosphere), the sensitivity of the electrode still retained 94% of its initial response as shown in the inset of Fig. 6d. + gluconolactone(C6H10O6, R1CHOR2) (4) To investigate the reproducibility of the YASNiCo@C electrode, The reactions (3) occur electrochemically at the target potential three different YASNiCo@C electrodes were prepared separately to (0.55 V). After addition of glucose into the electrolyte, the oxidative detect 1 mM glucose. The inter-electrode reproducibility was about M(III) could catalyze glucose oxidation to generate gluconolactone 5.4%. In addition, the three YASNiCo@C electrodes exhibit an and then gluconolactone is further hydrolyzed to glucose acid. intra-electrode reproducibility with a relative standard deviation For the detection of glucose, there are various interferents (R.S.D.) of 2.5% (n = 3) for 1 mM glucose. All these results suggest present in human body fluids, such as ascorbic acid (AA), uric that the YASNiCo@C electrode is an excellent catalytic material for acid (UA), fructose and sucrose. Physiologically relevant concen- amperometric glucose sensors. trations of these interferents were added stepwise to the electro- lyte in the following order: glucose (1 mM), UA (60 mM), AA (20 mM), sucrose (15 mM) and fructose (1.62 mM).61–67 There is no 3. Conclusions noticable current response for these interferents (Fig. S8, ESI†). The stability of the YASNiCo@C electrode for the detection of In summary, different compositions and structures of Co–Ni glucose was determined by continuous and intermittent tests metal oxides with ultrathin carbon shell materials were This journal is © The Royal Society of Chemistry 2020 Mater. Adv., 2020, 1, 908--917 | 913 View Article Online Materials Advances Paper prepared via pyrolysis of bimetallic or single metal based as the decomposition temperature (Td). The morphologies and MOFs. Among these materials, the bimetallic MOF derived microstructures were observed on an FEI/Philips XL30 FEG yolk–albumen–shell structure of the Ni–Co@C composite microscope (SEM) at an acceleration voltage of 15 kV. Chemical materials exhibited the highest electrocatalysis performance composition analyses of MOFs were recorded by a full quanti- This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. for glucose oxidation. The good electrocatalytic properties of tative electron probe micro-analyzer system coupled with wave- the YASNiCo@C can be attributed to (1) the large surface area length dispersive spectroscopy (EPMA-WDS; JEOL JXA-8530F, and the presence of an ultrathin carbon layer, (2) a multi- Tokyo, Japan) at an acceleration voltage of 15 kV and a probe dimensional hierarchical structure and (3) the synergistic effect current of 15 nA. For the MOF samples, a 10 nm layer of Pt was of the combination of two metal oxides. These results indicate first sputtered on top of the samples before SEM and EPMA that the YASNiCo@C electrode is a promising candidate for tests. A FEI Tecnai F20 field emission gun microscope was used amperometric glucose sensors. to collect high resolution transmission electron microscopy Open Access Article. Published on 17 June 2020. Downloaded on 11/22/2022 9:12:51 AM. (HRTEM) and scanning transmission electron microscopy (STEM) images, operated at 200 kV with a point-to-point 4. Experimental section resolution of 0.19 nm, equipped with high angle annular dark Synthesis of bimetallic MOFs field (HAADF) and electron energy loss spectroscopy (EELS) detectors. The metal ratios in the CoNi-MOFs were analyzed All reagents were of analytical purity and used as received by inductively coupled plasma optical emission spectroscopy without further purification. The series of MOFs are synthe- (ICP-OES, Varian 720-ES). Before the ICP test, CoNi-MOF sized by a mild solvothermal method: 3.492 g of Ni(NO3)26H2O samples were dissolved in 0.5 M HCl. The specific surface area (Z99%, VWR International bvba, Belgium) was dissolved in and pore size distributions were investigated by nitrogen 40 mL N,N-dimethylformamide (DMF, Z99% Chem-Lab). adsorption/desorption isotherms by the Brunauer–Emmett– 0.844 g 1,3,5-benzenetricarboxylic acid (H3BTC, 98% purity, Teller method (BET, Quantachrome Instruments, USA). ABCR, Germany) and 0.768 g 4,4-dipyridyl (bipy, 98% purity, Acros Organics) were dissolved in 40 mL DMF in a second glass Electrochemical measurements bottle. After mixing of both solutions, the mixture solution was All the electrochemical properties were recorded on an Autolab heated to 50 1C in a water bath (Memmert, WNB 45) for 72 h. electrochemical workstation at room temperature in a three- The resulting products were washed with DMF and ethanol electrode system. The working electrodes were prepared by a several times, respectively. After removing the residual reactant, dip coating procedure: 10 mg MOF-derived material inks were the samples were dried in an oven under air at 60 1C overnight. dispersed into 0.5 mL ethanol (containing 20 mL 5 wt% Nafion, The resulting sample was named as Ni-BTC MOFs. Co-BTC Alfa Aesar Germany). After 30 minutes sonication, 150 mL MOFs and Co–Ni–BTC MOFs were synthesized by the same suspension ink was loaded on the surface of Toray 90 carbon method as described for Ni-BTC MOFs, except that the metal paper (Etek, USA) with a mass loading of about 2 mg cm2. sources were changed to 3.497 g Co(NO3)26H2O (Z99%, Merck, The as-prepared carbon paper electrode, platinum gauze and a Germany, for Co-BTC) or a mixture (for Co–Ni–BTC MOFs) of Hg/HgO electrode (Tianjin Aida Hengsheng Technology Co. 2.328 g Ni(NO3)26H2O and 1.164 g Co(NO3)26H2O. The metal Ltd, China) were used as the working electrode, the counter ratio of Ni and Co in the CoNi-MOFs is 1 : 1.06, which was electrode and the reference electrode, respectively. Before testing, determined by ICP-OES analysis after dissolution into 0.5 M HCl. the working electrode was activated by cycling the potential Preparation of mixed Ni–Co oxide@ultrathin carbon shell between 0 V and 0.65 V vs. Hg/HgO at a scan rate of 30 mV s1 until the CV curves are stable. All electrochemical experiments Firstly, the MOF samples were pyrolyzed at 500 1C for 8 h were carried out in 0.1 M NaOH. in a nitrogen gas atmosphere (99.998%) with a flow rate of 300 cm3 min1. The temperature inside the furnace was gradually increased to 500 1C at a heating rate of 3 1C min1 Conflicts of interest (the same rate for the cooling procedure). In order to get metal oxide, the resulting powder was further annealed at 350 1C There are no conflicts to declare. for 3 h under air. These powders were named as NiNP@C, YASNiCo@C and CoNP@C, respectively. Acknowledgements Material characterization X. Zhang and J. Fransaer acknowledge funding from FWO project All X-ray diffraction (XRD) data were recorded on a Bruker AXS (12ZV320N). Funding from Southwest University (SWU019042) is D8 diffractometer using Cu Ka radiation (wave length l = acknowledged. K. Wan is grateful to the Oversea Study Program of 0.15405 nm) and a Ni filter with a step size of 0.021 (1.0 s per Guangzhou Elite Project. M.W. Xu appreciates support from the step). Thermogravimetric analysis (TGA) tests were analyzed National Natural Science Foundation of China (No. 21773188). under an argon atmosphere on an AutoTGA 2950HR V5.4A, TA ICN2 acknowledge funding from Generalitat de Catalunya 2017 Instruments with platinum pans at a heating rate of 5 1C min1. SGR 327 and the Spanish MINECO project ENE2017-85087-C3. The onset temperature of the weight loss in the TGA was used ICN2 is supported by the Severo Ochoa program from Spanish 914 | Mater. Adv., 2020, 1, 908--917 This journal is © The Royal Society of Chemistry 2020 View Article Online Paper Materials Advances MINECO (Grant No. SEV-2017-0706) and is funded by the Heterostructure for Non-enzymatic Glucose Sensing, Chem- CERCA Programme/Generalitat de Catalunya. Part of the present ElectroChem, 2017, 4, 1135–1140. work has been performed in the framework of Universitat Autònoma 15 C. Y. Chen, M. Shi, M. W. Xue and Y. J. Hu, Synthesis of de Barcelona Materials Science PhD program. TZ has received nickel(II) coordination polymers and conversion into porous This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. funding from the CSC-UAB PhD scholarship program. NiO nanorods with excellent electrocatalytic performance for glucose detection, RSC Adv., 2017, 7, 22208–22214. 16 K. F. Huo, Y. Li, R. S. 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