Nitrogen-Doped Graphene-Encapsulated Nickel Cobalt Nitride as a

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Biological and Medical Applications of Materials and Interfaces

Nitrogen-Doped Graphene Encapsulated Nickel Cobalt Nitride as Highly Sensitive and Selective Electrode for Glucose and Hydrogen Peroxide Sensing Applications Thangasamy Deepalakshmi, Duy Thanh Tran, Nam Hoon Kim, Kil To Chong, and Joong-Hee Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15069 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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ACS Applied Materials & Interfaces

Nitrogen-Doped Graphene Encapsulated Nickel Cobalt Nitride as Highly Sensitive and Selective Electrode for Glucose and Hydrogen Peroxide Sensing Applications a

b

Thangasamy Deepalakshmi, Duy Thanh Tran, Nam Hoon Kim,b Kil To Chong,a* Joong Hee Leeb,c* a

Department of Electronic Engineering, Advanced Research Center of Electronics and

Information, Chonbuk National University, Jeonju, Jeonbuk, 54896, Republic of Korea b

Advanced Materials Institute of BIN Convergence Technology (BK21 Plus Global) &

Department of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk, 54896, Republic of Korea c

Carbon Composite Research Centre, Department of Polymer - Nano Science and Technology,

Chonbuk National University, Jeonju, Jeonbuk, 54896, Republic of Korea

KEYWORDS: sensors; glucose; hydrogen peroxide; nickel cobalt nitride; core-shell

ABSTRACT: To explore a natural non-enzymatic electrode catalyst for highly sensitive and selective molecular detection for targeting biomolecules was a very challenging task. Metal nitrides have received numerous interests as the promising electrode for glucose and hydrogen peroxide sensing applications due to its exceptional redox properties, superior electrical conductivity, and superb mechanical strength. However, the deprived electrochemical stability extremely limits the commercialization opportunities. Herein, a novel nitrogen-doped graphene encapsulated nickel cobalt nitride (NixCo3−xN/NG) core-shell nanostructures with the controllable molar ratio of Ni/Co are successfully fabricated and employed as highly sensitive and selective electrode for glucose and hydrogen peroxide (H2O2) sensing applications. The highly sensitive and selective properties of the core-shell NiCo2N/NG electrode is because of the high synergistic effect of NiCo2N core and NG shell, as evidenced by a superior glucose sensing

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performance with a short response time of 800 °C.22,

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The as-

synthesized materials exhibited high electrochemical performance; however, the expensive instruments, low yield, toxic gases, and difficulties in synthesis limit their application in electrochemical sensors and biosensors. Also, the as-obtained materials are not fully converted into MNs. This leads to a reduction in the electrical, mechanical, and thermal properties in them. Therefore, exploring an eco-friendly approach that would avoid hazardous NH3 gases is a key issue in the scientific world. The recent research activities established that NG encapsulated nickel cobalt nitride core-shell hybrids are highly efficient electrode material for supercapacitors,24 however, glucose oxidation and H2O2 reduction properties have not been reported elsewhere and yet to be evaluated. Herein, we establish a simple, scalable, eco-friendly, and single-step approach to fabricate a serious of NixCo3-xN/NG (0 ≤ x ≤ 3) through pyrolysis technique and employed as bifunctional electrode for glucose and hydrogen peroxide (H2O2) sensing applications. The structure and morphology changes and sensing performances of the core-shell NixCo3-xN/NG have been studied in detail. The impacts of nickel species are also demonstrated with significant


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enhancement from the sensing performances and vary with the molar ratio of Ni/Co in the NixCo3-xN/NG nanohybrids. The core-shell NixCo3-xN/NG nanohybrid exhibits a high synergistic effect and excellent electronic contact between the NixCo3-xN core and the NG shell, which may simplify the diffusion pathways, increase the electroactive sites, and enhance the electron transportation properties. Of these as-synthesized NixCo3-xN/NG hybrids, the core-shell NiCo2N/NG exhibits superior sensing performance towards glucose and H2O2, with a high sensitivity, wide linear detection range, ultra-low detection limit of nanomolar levels, acceptable selectivity, tremendous reproducibility, and outstanding stability. The sensing performance of NiCo2N/NG/GCE is superior to non-enzymatic sensor-based electrode/electrode materials stated in the literature. Furthermore, NiCo2N/NG/GCE sensor can also performed to detect the glucose and H2O2 in human blood serum with enhanced sensing performances. 2.

EXPERIMENTAL SECTION

2.1. Materials. Nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O, ≥ 99.99%), cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O, ≥ 99.99%), cyanamide solution (50 wt.% in H2O, NH2CN, ∼99%), β-D-glucose (≥99.5%), hydrogen peroxide (H2O2, 30 wt.%), dopamine hydrochloride (DA, ≥98%), uric acid (UA, ≥99%), and L-ascorbic acid (AA, ≥99.5%) were obtained from Aldrich Chemicals Co. (USA) and used without further purification. The phosphate buffer solution (PBS; pH ∼7.4; 0.1 M) was prepared using Na2HPO4 (≥99.0%) and NaH2PO4 (≥98%). All reagents are purchased from analytical grade and used as received. The DI water with superior conductivity (~ 5.5×10−8 S cm−1) was used in all studies. 2.2. Synthesis of core-shell NixCo3-xN/NG nanohybrid. Graphene oxide was synthesized using our previous research report.25 The core-shell NixCo3-xN/NG nanohybrid was fabricated by a simple, scalable, single-step and cost-effective pyrolysis technique. The core-shell NixCo3-xN/NG


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nanohybrid was prepared according to our previous report with necessary modifications.24 The nickel and cobalt precursors with suitable molar ratio were added into the 50 mL of GO (1:1 ratio of DI water: C2H5OH) suspension. Different molar ratios of Co and Ni precursors were used to synthesize nanohybrids with altered nickel substitution (0 ≤ x ≤ 3), for example, to fabricate the core-shell NiCo2N/NG nanohybrid, 34.89 mg of Ni(NO3)2·6H2O, 69.84 mg of Co(NO3)2·6H2O, and 5 mL of 50% NH2CN were added into the GO solution, and stirred at ∼80 °C for overnight and then freeze-dried at −50 °C for complete drying to attain a gray powder. The powder was ground well and two-step annealing process, which includes polymerization at ∼450 °C and then pyrolyzed at ∼800 °C in nitrogen atmosphere for 2 h (heating rate of 2 °C min−1) in a quenching oven. To remove the weakly bounded MNs species and metal oxide from the as-synthesized materials, the final materials were treated with 1.0 M H2SO4 at ∼80 °C overnight and then washed with ethanol and water repeatedly, then dried at ∼60 °C for overnight in a vacuum oven. The as-obtained material denoted as core-shell NiCo2N/NG nanohybrid. For comparison, the pristine NiCo2N and pristine NG were synthesized using a similar procedure without GO and mixed metal precursors, respectively. To investigate the advantages of the coreshell structure, NiCo2N decorated NG (without core-shell) phase was prepared and sensing performances were investigated for comparison. In detail, C3N4/GO was synthesized by mixing of 50 mL GO solution (1 mg mL−1) and 4 mL of 50% NH2CN stirred at ∼80 °C until the complete evaporation of the solvent. The gray powder was heat-treated at 400 °C for 2h to obtain C3N4/GO. Further, about 1 g of C3N4/GO powder was poured in 10 mL of ethanol and keep sonication for 30 min. About 34.89 mg of Ni(NO3)2·6H2O, 69.84 mg of Co(NO3)2·6H2O were dissolved in 10 mL of ethanol and added to the C3N4/GO suspension, then stirred at room temperature for 5 h. The as-obtained material was washed with ethanol and water repeatedly to


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remove the unreacted metal nanoparticles, and then dried at ∼60 °C in vacuum oven. The powder heat-treated at 800 °C in the N2 atmosphere for 2 h (heating rate: 3 °C min−1), the resultant nanohybrid denoted as D−NiCo2N/NG (i.e., D represents ‘decorated’). In addition, the NiCo2O4/graphene was synthesized via the following procedure: 34.89 mg of Ni(NO3)2·6H2O, 69.84 mg of Co(NO3)2·6H2O, and 60 mg of GO were dispersed in DI water: C2H5OH (60 mL; 1:1 ratio) and keep sonication for one hour. About 2 mL of NH4OH was slowly added to mixed metal precursors and GO mixture, then poured into a 100 mL Teflon-lined autoclave and then the reaction was carried out at ∼180 °C for 12 h, the resultant material is denoted as NiCo2O4/G nanohybrid. 2.3. Characterization techniques. The as-obtained materials were analyzed by the transmission electron microscopy (TEM, H-7650, Hitachi Ltd., Japan, 200 kV; KIST), field emission scanning electron microscopy (FE-SEM; SUPRA 40 VP; Carl Zeiss, Germany; CBNU), EDAX (SUPRA 40 VP, Carl Zeiss, Germany), ICP-AES (J-A1100; Jarrell-Ash Company, Japan), powder X-ray diffraction (XRD, Rigaku Corporation, Japan, Cu Kα radiation, λ = 0.154 nm), X-ray photoelectron spectroscopy (XPS, Theta Probe, Thermo Fisher Scientific; UK), and BET (Micromeritics ASAP 2020 at ~77 K). The thermal properties of the core-shell NiCo2N/NG nanohybrid was examined by thermal gravimetric analysis (TGA) and differential thermal gravimetric (DTA) in air atmosphere (Q50; TA Instrument, USA; heating rate: 5 °C min−1). 2.4. Electrochemical measurements. All electrochemical performance of the as-obtained materials was measured on a CH660E (CH Instruments, Inc., USA). The as-synthesized electrode materials were used as the working electrode, whereas Pt wire and Ag/AgCl were used as the counter and reference electrodes, respectively. The 0.1 M NaOH and 0.1 M PBS (pH ∼7.4) were utilized as electrolytes for glucose and H2O2 sensors, respectively. Electrochemical


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impedance spectroscopy (EIS) was performed in a frequency range of 100 to 0.1 Hz (amplitude: ∼10 mV). The successive addition of the specified concentration of glucose into 0.1 M NaOH as well as specified H2O2 concentration into 0.1 M PBS under stirring condition was carried out to obtain the chronoamperometric measurements. The current density of each addition was noted when it attained the steady state. To maintain the O2-free environment, N2 was purged to the electrolyte solution. All sensing performances were examined at an ambient condition (∼35±1 °C). 2.5. Human blood serum sample preparation. To investigate the practical ability of the fabricated sensors in a real-time application, the human blood serum samples were prepared, and the levels of glucose and H2O2 in them were investigated. The human blood serum (100 mL) was diluted in 0.1 M NaOH (25 mL) for detection of glucose, whereas the same quantity of the human blood serum was diluted with 0.1 M PBS (25 mL) for detection of H2O2. Finally, the human blood serum samples were spiked with three known concentrations each of glucose and H2O2. 3.

RESULTS AND DISCUSSION

3.1 Fabrication and structural investigations of core-shell NixCo3-xN/NG. The core-shell NixCo3-xN/NG (0 ≤ x ≤ 3) nanohybrid was fabricated by a simple, scalable, single-step and costeffective pyrolysis technique (Figure 1). When compared to the conventional methods for the synthesis of MN-based materials,22,23 our developed pyrolysis route avoids hazardous NH3 gases at the eco-friendly manner, and most importantly, could serve as a versatile technique to fabricate the ternary MNs based core-shell architectures. For comparison, D−NiCo2N/NG and NiCo2O4/G nanohybrids were fabricated to investigate the glucose oxidation and H2O2 reduction properties.


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Figure 1 Schematic representation for the synthesis of the core-shell NixCo3-xN/NG nanohybrid for glucose and H2O2 sensing applications. The surface morphology of the core-shell NiCo2N/NG was investigated by FE-SEM analysis. The SEM image shows that the ultra-fine NiCo2N (average size: ∼6−8 nm) are homogeneously encapsulated by the NG sheet (Figure 2a). Furthermore, the SEM image shows that the core-shell NiCo2N/NG networks are interconnected with each other to make unique porous frameworks, which may simplify the diffusion pathways, increase the electroactive sites, and enhance the ion/electron transportation. The high magnification SEM image of the core-shell NiCo2N/NG


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reveals that the NG sheets effectively encapsulated the NiCo2N species, establishing a hierarchical nanostructure with exclusive porous networks (Figure 2b). During the pyrolysis process, the polymeric intermediate formed at a reaction temperature of ∼400 °C, then when reaction temperature increased to ∼800 °C and the cyanamide started to decompose, and the N is successfully doping into the NiCo2 alloy NPs, as well as the graphene nanonetworks. Impressively, a small number of well-graphitized NG shells developed around the core NiCo2N species, which may prevent the agglomeration of the active NiCo2N species and improve the catalytic activity of the core-shell nanohybrid in sensing applications. Also, there was a direct bond formation between the highly dispersive NiCo2N species and the neighboring atoms (C or N), which may enhance the sensing performances. In addition, the SEM image and EDAX analysis of the pristine NG and pristine NiCo2N are depicted in Figure S1. In contrast, the SEM images of the D−NiCo2N/NG reveal that the of NiCo2N (particle size: ∼18−20 nm) nanoparticles with irregular shapes are decorated on NG sheets (Figure S2a, b). For NiCo2O4/G nanohybrid (Figure S3a, b), the SEM images clearly display that the graphene sheets are anchored by bimetallic layered NiCo2O4 nanostructures (size: ∼40−45 nm). To further examine the detailed intrinsic morphology of core-shell NiCo2N/NG nanohybrid by TEM, HR-TEM, and HAADF STEM-EDS line mapping analysis, as shown in Figure 2d−g. As depicted in Figure 2d, TEM image displays that NiCo2N species (average size: 8−10 nm) are homogenously encapsulated by NG nanonetworks, whereas NiCo2N core are encapsulated by ultra-thin NG shell in the core-shell nanohybrid. In addition, there was no interlayer space between NG shell and NiCo2N core, which further confirms that the high synergistic effect of NiCo2N core and NG shell in the core-shell NiCo2N/NG.


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Figure 2 Morphological characterizations of the core-shell NiCo2N/NG nanohybrid: (a) SEM, (b) high-resolution SEM images, (c) EDAX, (d, e) TEM images, (e, f) HR-TEM image of the core-shell NiCo2N/NG (inset: the corresponding FFT), and STEM-EDS elemental maps of coreshell NiCo2N/NG: (g) HAADF-STEM image and its corresponding Ni, Co, N, and C elemental maps. The core-shell formation in the nanohybrid originates from mixed metal precursors and GOcontaining functional groups during the hydrolysis process,26 as presented in Figures 2e and 2f. The EDAX was used to investigate the existence of Ni, Co, N, and C in the core-shell NiCo2N/NG, as shown in Figure 2c. In addition, there was no other peak perceived, demonstrating that the core-shell nanohybrid is ultra-pure in nature. The atomic weight percentage of the elements in the core-shell NiCo2N/NG was also evaluated by ICP-AES, as illustrated in Table S1. The HR-TEM image shows that the NiCo2N species is highly crystalline


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in nature (Figure 2f). As depicted in Figure 2f, the interlayer d-spacing of ∼2.489A° is corresponds to (111) plane of the active NiCo2N species. The strong interfacial contact between the NG shell and the NiCo2N core may enhance the sensitivity, selectivity and cyclic life of glucose and H2O2 sensors. The FFT pattern of the core-shell NiCo2N/NG reveals its polycrystalline nature (inset of Figure 2f). In contrast, the TEM image of D−NiCo2N/NG clearly indicates that the irregular bulk shapes of NiCo2N particles are dispersed on NG matrices (Figure S2d). Note that, the absence of core-shell nanostructure in the D−NiCo2N/NG nanohybrid is revealed by HR-TEM image, which may lead to reduce the sensing performance and stability (Figure S2e and f). Therefore, we conclude that the structure and morphological features of the nanohybrid is fully dependent on the fabrication protocol. In case of the NiCo2O4/G, the TEM and HR-TEM images clearly reveal that the bimetallic layered NiCo2O4 architecture with porous networks are anchored on graphene networks (Figure S3d−f). The STEM-EDS elemental maps of the core-shell NiCo2N/NG further proved the successful elemental doping and alloy formation, as presented in Figure 2g. The Ni, Co, N, and C elemental maps were well match with the core-shell NiCo2N/NG (Figure 2g). The NiCo alloys were significantly higher at the center, indicating that NiCo alloy NP is successfully formed and present only in the core of the nanohybrid. The STEM-EDS color mapping undoubtedly indicates that during the pyrolysis process, nitrogen elements are effectively doping into the NG as well as the active NiCo2N in the core-shell nanohybrid, representing the successful design of a ternary NiCo2N core with the NG shell in a single-step synthesis protocol. The N are perceived in the shell as well as in the core of the nanohybrid, indicating that the N elements are successfully doped into the core-shell nanohybrid. The C signals are observed from the shell and support of the nanohybrid. Therefore, the present work established a core-shell nanohybrid with


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a hierarchical nanostructure by an eco-friendly, single-step synthesis protocol, which avoided hazardous NH3 gas. The atomic force microscopy (AFM) was performed to investigate the surface thickness of as-obtained core-shell NiCo2N/NG, as presented in Figure S4. As depicted in Figure S4, the AFM image clearly displays that the core-shell NiCo2N/NG has a twodimensional nanostructure with a thickness of ∼3.3 nm. Moreover, this study reveals that the core-shell NiCo2N/NG holds a rough surface area (rms roughness ∼6.3 nm). It was concluded that this was because of the ultra-fine NiCo2N core present in the nanohybrid, which is goodagreement with SEM and TEM observations. The thermal stability of the core-shell NiCo2N/NG nanohybrid was investigated by TGA-DTA analysis, as presented in Figure S5. The major weight loss was detected in the range of 373 to 514 °C is because of the decomposition of NG.27 Furthermore, there was no more weight loss occurred at >514 °C. The TGA analysis reveals that the compositions of carbonaceous materials in the core-shell NiCo2N/NG nanohybrid is estimated to be ∼52.5%. The XRD patterns of the pristine NG, pristine NiCo2N, and core-shell NiCo2N/NG are presented in Figure S6a. The core-shell NiCo2N/NG shows diffraction peaks at ∼ 36.02°, 42.04°, 60.97°, 73.04°, and 75.81°, which corresponds to (111), (200), (220), (311), and (222) planes of NiCo2N phase, respectively. The XRD is consistent with a recent report.28 The NiCo2N exhibits an average crystallite size of about 9.1 nm, as indicated by an evaluation using Scherer’s equation. This XRD pattern result is well consistent with the TEM observation. In addition, the core-shell NiCo2N/NG exhibits a major peak around 26.32°, corresponding to the (002) plane of NG network, which indicates that the NG have successfully converted from GO during the pyrolysis at ∼800 °C.29 The perceived diffraction peaks are relatively larger, which is due to the existence of ultra-fine NiCo2N nanoparticles encapsulated by NG sheets. In contrast, the XRD


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pattern of D−NiCo2N/NG and NiCo2O4/G nanohybrids are illustrated in Figure S6b. The diffraction peaks of NiCo2O4/G nanohybrid are well matches with JCPDS card no. 73-1702. The structural properties and disorder nature of the core-shell NiCo2N/NG was examined by Raman spectroscopy and are presented in Figure S7. As depicted in Figure S7, the Raman spectrum exhibits a G band at ∼1589 cm−1, corresponding to the sp2 vibration mode for the twodimensional lattice of carbon. Whereas the D band, at ∼1361 cm−1, relates to the defected layer of the graphene networks. The core-shell NiCo2N/NG nanohybrid G band is shifted to a higher region of wave number (∼1589 cm−1), which is much higher than NG (∼1578 cm−1), indicating that there is a high synergistic effect of NG shell and NiCo2N core in the core-shell NiCo2N/NG nanohybrid.30 The core-shell NiCo2N/NG possesses a ID/IG of ∼1.03, which is lower than that of the NG (ID/IG= 1.12). This is because of the NG encapsulated NiCo2N species, and it makes hierarchical nanostructure with excellent graphitization. To further examine the electronic states and chemical composition of the core-shell NiCo2N/NG, the XPS study was performed

and are shown Figures 3a–e. Figure 3a shows the

survey XPS spectrum of the core-shell NiCo2N/NG. The Ni, Co, N, and C content in the NiCo2N/NG nanohybrid were of ∼3.74 at. %, 7.49 at. %, 11.98%, and 71.42 at. %, respectively (Table S1). The pristine NG holds only ∼8.68 at. % of the nitrogen content, which further reveals that N is successfully incorporated into the active NiCo2N core species of NiCo2N/NG nanohybrid. The XPS high resolution Ni 2p spectrum shows the intense peaks at the binding energies of ∼855.14 eV and 873.03 eV, which corresponds to Ni 2p3/2 and Ni 2p1/2, respectively (Figure 3b). Furthermore, the satellite peaks were perceived at ∼856.91 eV, and 877.02 eV. These peaks indicate the existence of a Ni3+state, which is very close to nickel nitride (Figure 3b).


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Figure 3 (a) Survey XPS spectrum of core-shell NiCo2N/NG, high-resolution XPS spectra of (b) Ni 2p, (c) Co 2p, (d) N 1s, (e) C 1s, and (f) N2 sorption isotherms of the NG, NiCo2N, and coreshell NiCo2N/NG. The intensity peaks of the Ni3+state is higher than Ni2+state is because of the homogeneous doping of nitrogen, which may prevent a further surface oxidation process in the MN species. It is well known that the Ni3+ state in the core-shell nanohybrid is more electroactive than the Ni2+ state,17 which is beneficial to the enhance sensing performance towards glucose and H2O2. On the other hand, Co2p3/2 (∼779.04 eV) and Co2p1/2 (∼794.12 eV) peaks are accompanied by two broad satellites, representing the existence of Co2+ and Co3+ states (Figure 3c), which is well consistent with a previous study.19 XPS analysis shows that the core-shell NiCo2N/NG has the chemical composition of the Ni2+, Ni3+, Co2+, and Co3+ species. The high resolution N1s XPS spectrum shows five peaks at the binding energies of ∼398.8 eV, 399.9 eV, 401.7 eV, 403.5, and 400.8 eV, corresponding to pyridinic N, pyrrolic N, graphitic N, oxidized N, and Ni Co-N,


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respectively (Figure 3d). Note that, the NiCo-N peak appeared at ∼400.8 eV in the core-shell NiCo2N/NG, and this further confirms a strong bond formation in the NiCo-N-C superstructure. Thus, it was concluded that there was a fast electron-transfer kinetics from NiCo-N to C in the core-shell nanohybrid. The high resolution C1s XPS spectrum shows the binding energies such as ∼284.66 eV (C=C), ∼285.71 eV (C-C), ∼285.91 eV (C=N), ∼286.69 eV (C−O), ∼287.23 eV (C=O), ∼288.45 eV (COOH), and ∼290.77 eV (π−π*) (Figure 3e). Furthermore, the core-shell nanohybrid shows low C1 signals at higher binding energies than that of graphene oxide, which further indicates that -OH and -COOH functional groups, as well as sp3 carbons, are eliminated from graphene oxide during the pyrolysis treatment, which is consistent with the reported literature.30 This study reveals that core-shell NiCo2N/NG possesses the mass ratio of the Ni: Co is ∼1: 2, which agrees to the stoichiometric ratio of ∼1: 2. To examine specific surface area and pore size distribution, the core-shell NiCo2N/NG was further characterized by N2 sorption isotherms. The detailed results of this characterization are presented in Figure 3f. When the relative pressure (P/P0) of the electrode materials reaches 0.45, the characteristic hysteresis loop has been developed. The surface area of core-shell NiCo2N/NG is ∼693 m2g−1, which is much higher than NiCo2N (138 m2g−1) because of the extraordinary surface area nature of the NG (798 m2g−1). Interestingly, the surface area of core-shell NiCo2N/NG is superior to D−NiCo2N/NG (354 m2g−1) and NiCo2O4/NG (258 m2g−1) (Figure S8). The deprived surface area of D−NiCo2N/NG hybrid is due to the poor synergistic effect of the bulk NiCo2N and NG, which may hinder the cyclic life of the glucose and H2O2 sensors. Remarkably, the surface area of the core-shell NiCo2N/NG is superior to recently reported metalgraphene based nanohybrid.31 In order to find the pore diameter of the core-shell NiCo2N/NG, there is a further need to do characterization by the Barrett-Joyner-Halenda (BJH) technique


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(Figure S9). The core-shell NiCo2N/NG exhibits a peak at ∼6.45 nm, which is well-consistent with the void attained from TEM, which indicates that the perceived peak is also ascribed to the core-shell NiCo2N/NG nanohybrid mesoporous nature. Impressively, the core-shell NiCo2N/NG showed large surface area and pore volume due to the hierarchical architecture and exclusive mesoporous networks. This large surface area and unique porous nature may simplify the diffusion pathways, increase the electroactive sites, and enhance the electron transportation properties kinetics during the electrochemical sensing performances. 3.2. Electrochemical detection of glucose. Glucose is a vital biomolecule required by the powerhouse of the animal cell to produce energy to perform biological activities. In past decades, diabetes mellitus (DM), the abnormal clinical condition wherein a significant increase in blood glucose, was reported to be a serious pathological issue worldwide. Periodic monitoring of blood glucose levels becomes mandatory for those taking medication for DM. Glucose with an extra quantity in human blood could cause diabetes mellitus, which is one of the major diseases preceding to disability and death all over the world.32, 33 In 1962, Clark et al. established the first enzyme-based electrochemical glucose sensors that showed excellent sensitivity and selectivity;34 however, they have some draw back such as, immobilization difficulties, high-cost, and poor electrochemical stability.35 Therefore, it is a very challenging task to synthesize costeffective, non-enzymatic electrode materials for glucose sensing. Herein, the catalytic activity of the core-shell NiCo2N/NG nanohybrid for the detection of glucose by the electrochemical oxidation method was studied. Furthermore, the core-shell NiCo2N/NG was employed as a sensor for the electrochemical determination of glucose in real sample analysis. The electrochemical responses of NG/GCE and NiCo2N/NG/GCE in aqueous 0.1 M NaOH electrolyte in absence and presence of 1.0 mM glucose at a constant scan rate of 50 mV s−1 are


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shown in Figure 4a. For NG/GCE, there is no redox peak observed (curve 1) in the absence of the glucose with poor sensing response after the addition of 1.0 mM glucose, which suggests a poor ability to sense glucose. NiCo2N/NG/GCE shows a pair of intense redox peaks at ∼0.41 and 0.31 V (curve 3) in the absence of glucose, which are attributable to redox reaction of Ni2+/Ni3+ and Co2+/Co3+ in the NiCo2N/NG hybrid.33 However, the OH− ions play a imperative role in the redox kinetics of NiCo2N, as reported in the literature.36,

37

Impressively, NiCo2N/NG/GCE

displayed sharp and higher intensity anodic as well as cathodic peaks (curve 4) in the presence of the 1.0 mM glucose than that of without glucose. This CV study confirms that NiCo2N/NG/GCE had superior sensing performance towards glucose. Such excellent sensing performance is due to the high synergistic effect of the NG shell and NiCo2N core in the core-shell nanohybrid. The glucose oxidation reaction at the NiCo2N/NG/GCE is deliberating to the reaction mechanisms proposed here as follows: NiCo2N + glucose

→

NiOOH + CoOOH + gluconolactone + H2O

(1)

This significant enrichment in anodic current density is due to the glucose oxidation.11, 33 This can be further described in detail as follows. In alkaline NaOH medium, the Ni(II) and Co(II) are electrochemically oxidized to Ni(III) and Co(III), respectively. Further, the glucose is converted into the gluconic acid by deoxidization process (i.e., Ni(III) and Co(III) again deoxidized to Ni(II) and Co(II), respectively).


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Figure 4 (a) CV scans for NG/GCE (curve 1 and 2) and NiCo2N/NG/GCE (curve 3 and 4) in 0.1 M NaOH with the absence (curve 1 and 3) and the presence (curve 2 and 4) of 1.0 mM glucose (scan rate: 50 mV s−1), (b) CV scans for NiCo2N/NG/GCE in 1.0 M glucose at sweep rates from 10 to 250 mV s−1 (inset: plot of current density vs. square root of scan rate), (c) Current-time plot of the NiCo2N/NG/GCE with several addition of glucose (inset: the amperometric response to 50 nM glucose), (d) The corresponding calibration curve for the current vs. glucose concentration, (e) Amperometric response for glucose in the occurrence of interfering biomolecules, and (f) Glucose concentration with different blood serum samples. To authorize the superior glucose sensing properties of the core-shell NiCo2N/NG, comparative electrodes of NiCo2N/GCE, D−NiCo2N/NG/GCE, and NiCo2O4/NG/GCE in aqueous 0.1 M NaOH electrolyte in the absence and the presence of 1.0 mM glucose at a constant scan rate of 50 mV s−1 (Figure S10a and b). The NiCo2N/GCE, D−NiCo2N/NG/GCE, and NiCo2O4/NG/GCE exhibit a poor current response with the addition of 1 mM glucose in 0.1 M NaOH, indicating that all electrodes have poor catalytic activity towards glucose oxidation. These results further confirm that the NiCo2N/NG/GCE acts as the best catalyst towards glucose


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oxidation. Furthermore, CV studies were performed for metal nitrides with various amounts of nickel (x = 0, 1, 1.5, 2, 3) in aqueous 0.1 M NaOH electrolyte in the absence and the presence of 1.0 mM glucose at a constant scan rate of 50 mV s−1 and the results are illustrated in Figure S11a and b. Among the as-synthesized electrodes, NixCo3-xN/NG (x = 1) (i.e., NiCo2N/NG) delivers best sensing capability than other NixCo3-xN/NG (x = 0, 1.5, 2, 3) compositions. Furthermore, the glucose sensing performance of NiCo2N/NG/GCE electrode is superior to recently reported nonenzymatic electrode materials (Table S2). The CVs of the NiCo2N/NG/GCE at different scan rates from 10 to 250 mV s−1 are presented in Figure 4b. The anodic as well as cathodic peaks shifted in the positive as well as the negative direction when the sweep rate was increased from 10 to 250 mV s−1, demonstrating the quasi-reversible electron transfer kinetics of the NiCo2N/NG/GCE. The excellent linear relationship between the scan rates and peak current density, as presented in the inset of Figure 4b, imply that there is a surface-controlled process of glucose oxidation on the NiCo2N/NG/GCE, which occurs instead of a diffusion-controlled process.38,

39

In contrast, the CVs of the NG/GCE, NiCo2N/GCE, D−NiCo2N/NG/GCE, and

NiCo2O4/NG/GCE at different scan rates from 10 to 250 mV s−1 are presented in Figure S10c−f. Also, the CV curves of the NixCo3-xN/NG/GCE (x = 0, 1.5, 2, 3) at different sweep rates from 10 to 250 mV s−1 are presented in Figure S11c−f. To investigate the optimum applied potential for the glucose sensor, the amperometric current response for the NiCo2N/NG/GCE in successive addition of the 0.5 mM glucose in alkaline 0.1 M NaOH at different potentials from 0.30 to 0.50 V was examined, and the results are presented in Figure S12. When there was an increase in the applied potential from 0.30 to 0.50 V, a significant raise in the current response was perceived. It is well known that the signal noise and background current are more important to measure the amperometric analysis. This result reveals


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that the ∼0.45 V showed more noticeable noise signals and background current when compared to other applied potentials. Thus, 0.45 V is employed as optimum applied potential in the following amperometric response to achieve high sensing performances in measuring glucose oxidation. The amperometric responses of the NiCo2N/NG/GCE to successive glucose addition in alkaline 0.1 M NaOH were determined, and these are shown in Figure 4c. Remarkably, the NiCo2N/NG/GCE has an ultra-fast response to the oxidation of glucose and can attain a steadystate within a 3 s. The calibration curves of the NiCo2N/NG/GCE sensors within low and high concentration range are presented in Figure 4d. The proposed sensor shows a wide linear range of 2.008 µM to 7.15 mM and an ultra-high sensitivity of 1803µA mM−1 cm−2. The low limit detection (LOD) of NiCo2N/NG/GCE sensor was as low as 50 nm at S/N =3. This sensing performance of the NiCo2N/NG/GCE is much higher than recently reported glucose sensors based on non-enzymatic electrocatalyst (Table S2). Selectivity is one of the imperative factors in implementing the sensors for real-time application as non-enzymatic glucose sensors. Numerous interfering biomolecules (like carbohydrates, lipids, proteins with AA, UA, lactose, fructose, lactic acid), neurotransmitters (like DA), and buffering molecules (like NaCl) are existence in human blood serum. Thus, the anti-interference study was executed by consecutive addition of the 1.0 mM of glucose, and other interfering biomolecules, such as 0.1 mM UA, 0.1 mM DA, 0.1 mM AA, 0.1 mM lactic acid, 0.1 mM lactose, 0.1 mM fructose, and 2.0 mM NaCl in alkaline 0.1 M NaOH (Figure 4e). Interestingly, the NiCo2N/NG/GCE showed a significant response to glucose but the almost negligible amperometric response to the other interferences, and there was a remarkable current response achieved again with another addition of 1.0 mM of glucose. The NiCo2N/NG/GCE sensor reproducibility test was examined by determining the current response during the


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oxidation of glucose by employing ten fabricated electrodes (Figure S13). The relative standard deviation (RSD) of about 2.6% was achieved for glucose oxidation, representing an excellent reproducibility. The cyclic life period of the NiCo2N/NG/GCE sensor is a vital factor in the realtime application for the determination of the glucose from the real sample analysis. When the tested electrode was exposed to atmospheric air for 45 days at the ambient condition, the current response to 0.5 mM of glucose was only reduced by 7.69%, which reveals that the NiCo2N/NG/GCE sensor holds outstanding stability towards glucose oxidation (Figure S14). The SEM and TEM analysis reveal that the morphology of the NiCo2N/NG still well retained after glucose oxidation (Figure S15a, c, and d). In addition, the XRD pattern clearly proved that the structure of NiCo2N/NG is well maintained after the long-term stability (Figure S15b). Thus, it has been concluded that the proposed NiCo2N/NG/GCE sensor has the promising its application toward determination of glucose in real samples. To authorize the practicability of this proposed sensor in regular sample analysis, NiCo2N/NG/GCE was employed to measure the glucose from human blood serum (Figure 4f). The recovery test outcomes are illustrated in Table S3. The standard addition method is used to investigate the quantitative analysis measurement of glucose. The glucose concentration level calculated by the proposed sensors is well matched with the value received from the commercial glucometer (Sannuo Biological Sensor Co., LTD) with RSDs of 3.2 and 4.6%. Therefore, this proposed sensor exhibits great potential for application in real human blood serum analysis. 3.3. Electrochemical detection of H2O2. The highly sensitive and selective detection of H2O2 is imperative in biomedical applications. H2O2 is one of the significant molecules needed to maintain the carbonic acid buffer system in the human body via oxygen metabolism.40 Hence, H2O2 is a known free radical, and excess production of H2O2 leads to oxidative stress and DNA


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damage, and it can cause cancer as a result of ROS induced mutation,41, 42 which was reported to cause an increase in the high mortality rate worldwide. Furthermore, it is well known that the generation of an excess amount of H2O2 in cells decreases the propagation of the cell itself.43 Therefore, the fabrication of a highly sensitive and stable electrode for monitoring the trace amounts H2O2 released from living cells is a key challenging task in the scientific community. The horseradish-peroxidase-based natural enzyme has been explored as the best catalyst for the detection of H2O2 due to its excellent sensing properties of excellent selectivity and sensitivity.44 However, the poor stability, and high cost, problematic immobilization process of the enzyme hinders its practical application. Therefore, it is a key task to find a more economical catalyst for H2O2 sensing.45 Herein, the NixCo3-xN/NG/GCE (0 ≤ x ≤ 3) was used as a sensor for the detection of H2O2 and applied for trace amount detection of H2O2 from human blood serum. The electrochemical detection of H2O2 has been explored through the electrochemical oxidation/reduction employing numerous types of electrode materials.46−48 However, detection of H2O2 via electrochemical oxidation at electrodes showed at a high overpotential of >0.7 V. At this high potential region, various electroactive interferences such as AA, DA, and UA are detected by electrode materials. Therefore, it is a great task to detect the H2O2 by an electrochemical reduction method. Herein, the catalytic activity of NiCo2N/NG/GCE for the detection of H2O2 by an electrochemical reduction method was explored. The CVs of NG/GCE and NiCo2N/NG/GCE in 0.1 M PBS in the absence (curve 1 and 3) and the presence (curve 2 and 4) of 1.0 mM H2O2 at a scan rate of 50 mV s−1 are presented in Figure 5a. Expressively, NG/GCE reveals no noticeable redox peaks observed (curve 1) in the absence of H2O2 with little current response (curve 2) in the presence of 1.0 mM H2O2. Impressively, the NiCo2N/NG/GCE exhibits a pair of redox peaks (curve 3) in absence of H2O2 and significant current response


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(curve 4) after the addition of 1.0 mM H2O2, demonstrating that our fabricated NiCo2N/NG/GCE holds exceptional electrochemical sensing performance towards H2O2 reduction. Based on the previous report,47 the H2O2 reduction process could be proposed here as follows but need further investigation to find the exact mechanism. NiCo2N (Ox) + e−

↔

NiCo2N(Red)

(2)

NiCo2N (Red) + 2 H2O2

↔

NiCo2N (Ox) + 2 H2O + O2

(3)

To validate the excellent H2O2 reduction performance of the core-shell NiCo2N/NG, the CV curves of the NiCo2N/GCE, D−NiCo2N/NG/GCE, and NiCo2O4/NG in aqueous 0.1 M NaOH electrolyte in the absence and the presence of 0.1 M PBS at a scan rate of 50 mV s−1 are shown in Figure S16a and b. Noticeably, the NiCo2N/GCE, D−NiCo2N/NG/GCE, and NiCo2O4/NG/GCE showed a slight response with the addition of H2O2 in 0.1 M PBS, representing the poor electrochemical performance towards H2O2 reduction. Whereas, the NiCo2N/NG/GCE displayed a significant reduction peak at around 0.0 V after the addition of H2O2, which further reveals that the core-shell NiCo2N/NG has superior sensing performance in H2O2 reduction. The H2O2 reduction potential of the NiCo2N/NG/GCE has a higher reduction current and more positive onset potential than that of NiCo2N/GCE (∼−0.13 V vs. Ag/AgCl), D−NiCo2N/NG/GCE (∼−0.04 V vs. Ag/AgCl), and NiCo2O4/NG (∼−0.10 V vs. Ag/AgCl). Also, the H2O2 reduction potential of the NiCo2N/NG/GCE is superior to Fe3N-Co2N/CC (∼−0.20 V vs. Hg/HgO),18 GC/rGONg@Ag (∼−0.60 V vs. SCE),49 GF/AuNS (∼−0.25 V vs. Ag/AgCl),50 and Cu3N NA/CF (∼−0.25 V vs. SCE).51 This CV study revealed that NiCo2N/NG/GCE holds an excellent electrocatalytic activity and ultra-fast electron transfer kinetics towards H2O2 reduction. This electron transfer kinetics of the NiCo2N/NG/GCE is supported by the EIS study (Figure S17), which further proved that the charge transfer resistance (Rct) is ∼17.16 Ω, which is lower than that of the


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NG/GCE (Rct ∼28.15 Ω), NiCo2N/GCE (Rct ∼39.87Ω), D−NiCo2N/NG/GCE (Rct ∼40.28 Ω), and NiCo2O4/NG/GCE (Rct ∼59.81 Ω). This shows the excellent electron transfer kinetics of the core-shell NiCo2N/NG. To further demonstrate the effect of the nickel incorporation into the core-shell nanohybrids, CV studies were performed with various amounts of nickel (x = 0, 1, 1.5, 2, 3) in

0.1 M PBS containing 1.0 mM H2O2 at a scan rate of 50 mV s−1 (Figure S18a and b).

Among the as-synthesized electrode materials, NixCo3-xN/NG (x = 1) (i.e., NiCo2N/NG) delivers higher H2O2 sensing performance than the other NixCo3-xN/NG (x = 0, 1.5, 2, 3) compositions, and reported non-enzymatic H2O2 sensing electrode materials (Table S4). Figure 5b displays the CVs with different sweep rates from 10 to 200 mV s−1 of NiCo2N/NG/GCE in 0.1 M PBS containing 1.0 mM H2O2. The peak current increased significantly and displayed well-resolved anodic as well as cathodic peaks in the range of 0.19 to 0.32 eV and 0.04 to 0.05 eV, which corresponds to H2O2 oxidation as well as reduction, respectively.52 The anodic and cathodic peak current density increases in positive as well as negative directions, demonstrating that reaction kinetics are controlled by the adsorption of H2O2.53 At working potential, the anodic as well as cathodic current linearly enhanced with square root of the scan rate, as illustrated in inset Figure 5b. The extraordinary specific surface area, unique porous network, and superior synergistic effect of the NiCo2N core and NG shell in the core-shell NiCo2N/NG nanohybrid could be responsible for such high sensing performance of the NiCo2N/NG/GCE. Also, during the electrochemical sensing performance, the electron transport kinetics may be facilitated by the extraordinary specific surface area, high electrical conductivity, and unique porous architecture of the NG. On the other hand, NG acts as an excellent core and support for successful construction of NixCo3-xN without aggregation, leading to its hierarchical framework with enhanced sensing performances. In contrast, the CVs with


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different sweep rates from 10 to 200 mV s−1 of NG/GCE, NiCo2N/GCE, D−NiCo2N/NG/GCE, and NiCo2O4/NG/GCE in 0.1 M PBS containing 1.0 mM H2O2 are presented in Figure S16c−f. Besides, the CVs of the NixCo3-xN/NG/GCE (x = 0, 1.5, 2, 3) at various sweep rates from 10 to 200 mV s−1 are illustrated in Figure S18c−f.

Figure 5 (a) CV scans for NG/GCE (curve 1 and 2) and NiCo2N/NG/GCE (curve 3 and 4) in 0.1 M NaOH with the absence (curve 1 and 3) and the presence (curve 2 and 4) of 1.0 mM H2O2 (scan rate: 50 mV s−1); (b) CV scans for NiCo2N/NG/GCE in 1.0 M H2O2 at sweep rates from 10 to 200 mV s−1 (inset: plot of current density vs. the square root of scan rate); (c) current-time plot of the NiCo2N/NG/GCE with several additions of H2O2 (inset: the magnified view); (d) calibration curve for the current vs. H2O2 concentration; (e) amperometric for H2O2 in the occurrence of interfering biomolecules, and (f) H2O2 concentration with various blood serum samples. The amperometric technique was employed to investigate the sensitivity of the NiCo2N/NG/GCE for detection of H2O2. The current-time responses of the NiCo2N/NG/GCE at a fixed potential of 0.0 V upon the successive addition of the H2O2 into the nitrogen-saturated 0.1


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M PBS (pH∼7.4) are illustrated in Figure 5c. The NiCo2N/NG/GCE attained >98% of the steadystate response within ~3 s for each addition of H2O2, which is a much faster response than those reported in the literature.46, 54 Such a quick response may be attributed to the fast adsorption of H2O2 on the NiCo2N/NG/GCE and simultaneous reduction of H2O2. And the steady-state reaction was associated with the core-shell NiCo2N/NG/GCE. The calibration curve I (µA) = 202.26(µM) + 0.2206 and I (µA) = –11.71(µM) + 3.896 exhibited a linear relationship between the current density as well as the concentration of H2O2 from 200 nM to 68.5µM (R2 = 0.9984) and 198.5µM to 3.4985 mM with a correlation coefficient of ∼0.9965, respectively (Figure 5d). The NiCo2N/NG/GCE sensor exhibited an ultra-high sensitivity of 2848.73µA mM−1cm−2, as perceived from the calibration curve. The current-time response of stepwise changes to the concentration of H2O2 at a nanomolar (nM) level is also investigated and presented in the inset of Figure 5c. The NiCo2N/NG/GCE sensor exhibits noticeable current responses from the nM level to several mM additions of H2O2 into the 0.1 M PBS. The extensive current response of the NiCo2N/NG/GCE sensor to H2O2 at the nM level is certainly in frequent use for a sensing electrode material without the help of the bio-recognizer, which indicates that the present fabricated sensors could be used in the industrial sector to detect different concentration level of H2O2 in biosystems. The LOD of the NiCo2N/NG/GCE sensor is ∼200 nM at S/N = 3, as shown in the inset of Figure 5c. This nanomolar detection limit of the NiCo2N/NG/GCE sensor makes it suitable for H2O2 detection on the cellular level. It is well known that lower concentration detection of H2O2 was conquered by the adsorption phenomenon, while higher concentration detection of H2O2 depends on the catalyst.55 Thus, the high sensitivity of H2O2 is related to the core-shell nanoarchitecture of the NiCo2N/NG. This study suggests that the ultra-small NiCo2N


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core with excellent catalytic activity and exceptional specific surface area of the NG shell also played an imperative role in the ultra-high-sensitive detection of H2O2. Selectivity is one of the major challenging factors in developing non-enzymatic H2O2 sensors. The interferents, such as AA, DA, glucose, UA, lactose, and fructose, coexist with H2O2 in human blood and body fluids.56, 57 The amperometric response of the NiCo2N/NG/GCE with successive additions of 1 mM H2O2, 0.1 mM AA, 0.1 mM DA, 0.1 mM UA, 0.1 mM glucose, 0.1 mM urea, 2 mM NaCl, and 1 mM H2O2 are shown in Figure 5e. This study reveals that the NiCo2N/NG/GCE has an amazing response to H2O2 but the negligible response to other interferents, such as AA, UA, DA, NaCl, and urea. Then it attained its current response to another addition of H2O2, representing that the NiCo2N/NG/GCE has superior selectivity with regards to H2O2. Long-term stability is another great challenge in the development of the nonenzymatic H2O2 sensor for practical application. The fabricated NiCo2N/NG/GCE was tested by the amperometric response of 1 mM of H2O2 in 0.1 M PBS at 0.0 V for 2000 s. It had about a 98.52% current response retention after 3000 s, demonstrating that the NiCo2N/NG/GCE has superior stability (Figure S19). Instead, to further evaluate its stability, the NiCo2N/NG/GCE was tested every five days in 1 mM H2O2 in 0.1 PBS and stored at < 5 °C. About 8.95% of the current density decreased after 45 days, indicating that it has outstanding stability (Figure S20). The morphology of the fabricated electrode was tested after 45 days. The core-shell NiCo2N/NG has retained the original structure after long-term stability test, which is confirmed by SEM, XRD, and TEM (Figure S21). The reproducibility of the NiCo2N/NG/GCE was examined by measuring its sensing response to 1 mM H2O2 in 0.1 mM PBS at ten numbers of NiCo2N/NG/GCEs (Figure S22). Only ∼3.1% of the RSD of current densities were retained, indicating an exceptional reproducibility.


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The NiCo2N/NG/GCE was examined with regard to various parameters, such as pH, applied potential, and temperature. To progress in the development of the biosensors for real-time application, the effect of pH is one of the significant parameters that need to be examined through in vivo studies. The response with respect to H2O2 increasing the pH from 5.8 to 8.0 is shown in Figure S23. Impressively, the highest response for the NiCo2N/NG/GCE was at the pH of 7.4, which makes it suitable for the analysis of biological samples in a real-time application. In the amperometric study, the NiCo2N/NG/GCE showed different current responses for H2O2 detection at various potentials. This study shows that the highest current response for the NiCo2N/NG/GCE is about 0.0 V. The effect of temperature on detection of H2O2 was evaluated at different temperatures between 10 to 45 °C (Figure S24). The sensing property of the NiCo2N/NG/GCE significantly increased when the temperature raised from 20 to 35 °C and maintained a steady state after that. For further real-sample analysis, all the electrochemical sensing performance was conducted at the ambient condition. These significances reveal that the NiCo2N/NG/GCE sensor fulfills the essential criteria for H2O2 detection in the biological samples. To investigate the practical applicability of the NiCo2N/NG/GCE, human blood serum samples were prepared in 0.1 M PBS and tested. The tested values in human blood serum are illustrated in Table S4. When the NiCo2N/NG/GCE sensor was evaluated in the presence of the blood serum, there was a remarkable recovery of H2O2 reduction (Figure 5f). This result demonstrates that the proposed NiCo2N/NG/GCE sensor has superior sensitivity in the realsample analysis. These measured sensing values were very close to the recovery value of hospital determination. Therefore, the NiCo2N/NG/GCE sensor could be a gifted candidate for the detection of H2O2, and it can be extremely helpful in real-time applications. After being


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employed as non-enzymatic glucose and H2O2 sensor, the morphology and structural integrity of the electrode was further proved by SEM, TEM and XRD (Figure S15 and S21). The SEM and TEM observation demonstrate that there were no obvious changes were observed in the coreshell NiCo2N/NG nanohybrid. These results propose that the NiCo2N/NG/GCE sensor could be utilized in tests of normal cells and cancer cells from the human body. 4.

CONCLUSION

A simple, scalable, eco-friendly, cost-effective, and single-step approach for the fabrication of a core-shell NiCo2N/NG nanohybrid was developed. For the first time, a core-shell NiCo2N/NG nanohybrid has been demonstrated as an efficient bifunctional electrode for glucose and hydrogen peroxide sensing applications. Firstly, core-shell NiCo2N/NG networks are interconnected with each other to provide an ion/electron transport pathway, which is beneficial to enhance sensing performance. Secondly, hierarchical core-shell superstructure minimizes the aggregation of active NiCo2N, which enhances the electroactive sites and improves the lifetime of sensors. As non-enzymatic glucose and H2O2 sensors, the core-shell NiCo2N/NG electrode demonstrates excellent sensing performances with ultra-high sensitivity, selectivity, a low-limit detection, tremendous reproducibility, and long-term stability (retention about >91% of original response after 45 days test). The sensitivity and selectivity of NiCo2N/NG/GCE sensor are superior to previous reports, and the detection limit of glucose and H2O2 can be as low as 50 nm and 200 nm, respectively. Furthermore, the NiCo2N/NG electrode showed excellent sensitivity and selectivity in real sample analysis. The present investigation not only provides a costeffective electrode for glucose and H2O2 sensors but opens a new pathway for the investigation of employing highly active ternary MN-based core-shell nanohybrids as advanced electrodes for batteries, fuel cells, water splitting, and other electrochemical biosensing applications.


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ASSOCIATED CONTENT Supporting Information Additional FE-SEM, EDAX, TEM, AFM, XRD, Raman spectroscopy, CV, amperometric response studies, reproducibility test and stability test for the glucose and H2O2 sensors, optimization of pH and temperature, and tables are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * Corresponding authors: Email address: [email protected] (Prof. Kil To Chong) Fax: +82 632702394; Tel: +82 632702478 [email protected] (Prof. Joong Hee Lee) Fax: +82 632702301; Tel: +82 632702342 Funding Sources The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by the Brain Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2017M3C7A1044815) and by the Ministry of Trade, Industry and Energy (MOTIE). REFERENCES (1) Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43-51.


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(2) Gong, F.; Wang, H.; Xu, X.; Zhou, G.; Wang, Z. S. In Situ Growth of Co0.85Se and Ni0.85Se on Conductive Substrates as High-Performance Counter Electrodes for Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2012, 134, 10953-10958. (3) Cui, Z. M.; Fu, G. T.; Li, Y. T.; Goodenough, J. B. Ni3FeN-Supported Fe3Pt Intermetallic Nanoalloy as a High-Performance Bifunctional Catalyst for Metal-Air Batteries. Angew. Chem., Int. Ed. 2017, 56, 9901-9905. (4) Chang, P. J.; Cheng, K. Y.; Chou, S. W.; Shyue, J. J.; Yang, Y. Y.; Hung, C. Y.; Lin, C. Y.; Chen, H. L.; Chou, H. L.; Chou, P. T. Tri-iodide Reduction Activity of Shape- and CompositionControlled PtFe Nanostructures as Counter Electrodes in Dye-Sensitized Solar Cells. Chem. Mater. 2016, 28, 2110-2119. (5) Wang, Y. J.; Zhao, N. N.; Fang, B. Z.; Li, H.; Bi, X. T. T.; Wang, H. J. Carbon-Supported PtBased Alloy Electrocatalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells: Particle Size, Shape, and Composition Manipulation and Their Impact to Activity. Chem. Rev. 2015, 115, 3433-3467. (6) Xu, Y.; Zhang, B. Recent Advances in Porous Pt-Based Nanostructures: Synthesis and Electrochemical Applications. Chem. Soc. Rev. 2014, 43, 2439-2450. (7) Han, L.; Yang, D. P.; Liu, A. H. Leaf-Templated Synthesis of 3D Hierarchical Porous Cobalt Oxide Nanostructure as Direct Electrochemical Biosensing Interface with Enhanced Electrocatalysis. Biosens. Bioelectron. 2015, 63, 145-152. (8) Muench, F.; Sun, L.; Kottakkat, T.; Antoni, M.; Schaefer, S.; Kunz, U.; Molina-Luna, L.; Duerrschnabel, M.; Kleebe, H. J.; Ayata, S.; Roth, C.; Ensinger, W. Free-Standing Networks of Core-Shell Metal and Metal Oxide Nanotubes for Glucose Sensing. ACS Appl. Mater. Interfaces 2017, 9, 771-781. (9) Zhang, R. Z.; Chen, W. Recent Advances in Graphene-Based Nanomaterials for Fabricating Electrochemical Hydrogen Peroxide Sensors. Biosens. Bioelectron. 2017, 89, 249-268. (10) Li, X.; Liu, X. Y. Group III Nitride Nanomaterials for Biosensing. Nanoscale 2017, 9, 73207341. (11) Dai, X. L.; Deng, W. Q.; You, C.; Shen, Z.; Xiong, X. L.; Sun, X. P. A Ni3N-Co3N Hybrid Nanowire Array Electrode for High-Performance Nonenzymatic Glucose Detection. Anal. Methods 2018, 10, 1680-1684. (12) Li, Y. Z.; Li, T. T.; Chen, W.; Song, Y. Y. Co4N Nanowires: Noble-Metal-Free Peroxidase


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Nitrogen-Doped Graphene-Encapsulated Nickel Cobalt Nitride as a

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