Hierarchical NiCo2O4 Hollow Sphere as a Peroxidase Mimetic for Colorimetric Detection of H2O2 and Glucose

In this work, the hierarchical NiCo2O4 hollow sphere synthesized via a “coordinating etching and precipitating” process was demonstrated to exhibit intrinsic peroxidase-like activity. The peroxidase-like activity of NiCo2O4, NiO, and Co3O4 hollow spheres were comparatively studied by the catalytic oxidation reaction of 3,3,5,5-tetramethylbenzidine (TMB) in presence of H2O2, and a superior peroxidase-like activity of NiCo2O4 was confirmed by stronger absorbance at 652 nm. Furthermore, the proposed sensing platform showed commendable response to H2O2 with a linear range from 10 μM to 400 μM, and a detection limit of 0.21 μM. Cooperated with GOx, the developed novel colorimetric and visual glucose-sensing platform exhibited high selectivity, favorable reproducibility, satisfactory applicability, wide linear range (from 0.1 mM to 4.5 mM), and a low detection limit of 5.31 μM. In addition, the concentration-dependent color change would offer a better and handier way for detection of H2O2 and glucose by naked eye.


Introduction
Natural enzymes have been widely studied and applied in various fields due to their high substrate specificity and brilliant catalytic activity under mild conditions [1]. However, application of these enzymes is restricted by their high cost, complicated purification and immobilization procedures, and harsh reaction conditions [2,3]. Therefore, the low-cost enzyme mimics with good stability, excellent catalytic activity, and satisfactory selectivity were highly in demand [4]. Since Gao's [5] first report on an intrinsic peroxidase-like activity of Fe 3 O 4 nanoparticles, various nanomaterials such as MFe 2 O 4 [6,7], MoS 2 [8], Co 3 O 4 [9,10], NiO [11], CuS [12], graphene oxide [13], and graphene oxide-Fe 3 O 4 composite [14] have been further investigated as peroxidase mimetics for H 2 O 2 detection. These researches reveal that the nanomaterial-based peroxidase mimetics possess the advantages of high specific surface area, controllable morphologies and sizes, and tunable catalytic properties that would be promising candidates for bioassays and clinical diagnosis [4,15,16].
NiCo 2 O 4 is a typical spinal metal oxide, with Ni ions occupying the octahedral sites while Co ions are integrally distributed to both the octahedral and tetrahedral sites [17,18]. Due to its superior catalytic activity and conductivity to that of single Co 3 O 4 or NiO [19,20], NiCo 2 O 4 has gained great attention in the areas of supercapacitor, electrocatalyst, oxidation evolution reaction, and biosensor [21,22]. It has been reported that Co 3 O 4 nanoparticles possess an intrinsic peroxidase-like according to our previous work [28]. First, 5 mg of the as-prepared Cu 2 O nanospheres, 1.13 mg of CoCl 2 ·6H 2 O, 0.57 mg of NiCl 2 ·6H 2 O, and 0.33 g of PVP were dissolved into a mixed solution (V ethanol :V water = 1:1, 10 mL), stirred, and sonicated. Then, 4 mL of Na 2 S 2 O 3 aqueous solution (1 M) was added dropwise into the mixture and kept stirring for about 10 min. The resulting precursor was collected by several rinse-centrifugation cycles with deionized (DI) water and ethanol. Finally, the precursor was calcinated in a muffle furnace at 300 • C for 4 h at a ramp rate of 1 • C·min −1 under the air condition.

Catalytic Oxidation of TMB
To investigate the peroxidase-like activity of NiCo 2 O 4 hollow sphere, catalytic oxidation reaction of TMB in sodium citrate buffer solution was performed in presence of H 2 O 2 . Five microliters of 1 mg·mL −1 NiCo 2 O 4 hollow sphere dispersion was incubated with 3 mL of pre-prepared sodium citrate buffer solution (0.1 M, pH 4.5) in the presence of H 2 O 2 (0.02 M) and TMB (0.08 mM). The photographs and UV-vis spectra were obtained after incubation at room temperature for 30 min.

Colorimetric Detection of H 2 O 2
Five microliters of 1 mg/mL NiCo 2 O 4 hollow sphere dispersion, 25 µL of 10 mM TMB, and 3 µL of H 2 O 2 in different concentrations were added to 3 mL sodium citrate buffer solution (0.1 M, pH 4.5). The UV-vis absorption spectra were recorded from 500 nm to 800 nm.

Colorimetric Detection of Glucose
Five microliters of 1 mg/mL NiCo 2 O 4 hollow sphere dispersion, 25 µL of 10 mM TMB, 5 µL of 10 mg/mL GOx, and 3 µL of glucose in different concentrations were added to 3 mL of sodium citrate buffer solution (0.1 M, pH 4.5). The UV-vis absorption spectra were recorded from 500 nm to 800 nm.

Materials Characterization
The Ni-Co hydroxide precursor was synthesized via a deliberately designed "CEP" process, and the duplication and the elimination of Cu 2 O template were completed simultaneously within 10 min at room temperature [29,30]. The final NiCo 2 O 4 hollow sphere product was obtained after a simple thermal treatment based on the above processes. As shown in Figure 1a, a high-quality Cu 2 O template with an average size of approximately 400 nm was obtained, and its rough texture on the surface was conducive to creating more chemical etching interface. From the TEM image in Figure 1b, a solid construction can be confirmed. After the etching process and thermal treatment, the obtained NiCo 2 O 4 sphere ( Figure 1c) well inherited the spherical structure of the template, indicating the controllability of the "CEP" process and the stability in structure. The "wormlike" secondary structures can be observed clearly in the magnified image in Figure 1d, and the hollow structure feature can be initially identified by the cracked sphere. Afterwards, we performed TEM characterization to further testify to the inner structure of the hierarchical sphere.
As shown in Figure 2a,b, the distinct contrast between the shells and inner space fully demonstrates the hollow construction of the hierarchical NiCo 2 O 4 sphere. The diameter of the hollow sphere is about 400 nm, which is similar to that of pre-grown Cu 2 O sphere. Furthermore, the apparent bright and dark aberrations on the ultrathin shell (<30 nm) imply the existence of nanopores, and sheetlike secondary structures of several nanometers in thickness are uniformly grown on the sphere. Notably, the ultrathin hollow sphere with affluent secondary structures could provide more efficient inner and outer surface and create more catalytically active sites. The inserted selected area electron diffraction (SAED) pattern in Figure 2b reveals a polycrystalline feature of the as-prepared material. Figure 2c displays the amplified image of the selected area marked in (b), in which the nanopore and ripple structure can be identified, and the high-resolution TEM image in Figure 2d also attests to the evident crystalline structure in the NiCo 2 O 4 hollow sphere. More detailed structure information was further determined by XRD and N 2 adsorption-desorption tests.       [31]. No other impurity peaks appear, indicating the high purity of the samples. The porosity of the hierarchical NiCo 2 O 4 hollow sphere was assessed by N 2 adsorption-desorption test as shown in Figure 3b, which gives a typical IV-type isotherm. The specific surface area estimated by Brunauer-Emmett-Teller (BET) method is 59.4 m 2 ·g −1 , and the inserted pore size distribution curve demonstrates the existence of mesopores [32]. Apparently, the porous structure of the hierarchical NiCo 2 O 4 hollow sphere is beneficial to supply more mass transport pathways and active sites. Furthermore, the DLS result shows that the surface of the as-obtained NiCo 2 O 4 hollow spheres is positively charged (about 3.0 mV) in water, which is conducive to the absorption of negatively charged GOx and improves the dispersibility of the particles in the solution. Additionally, the surface composition and valence state of the hierarchical NiCo 2 O 4 hollow sphere are also vital, since the catalytic reactions mainly happened on the surface of catalysts.  [31]. No other impurity peaks appear, indicating the high purity of the samples. The porosity of the hierarchical NiCo2O4 hollow sphere was assessed by N2 adsorptiondesorption test as shown in Figure 3b, which gives a typical IV-type isotherm. The specific surface area estimated by Brunauer-Emmett-Teller (BET) method is 59.4 m 2 ·g −1 , and the inserted pore size distribution curve demonstrates the existence of mesopores [32]. Apparently, the porous structure of the hierarchical NiCo2O4 hollow sphere is beneficial to supply more mass transport pathways and active sites. Furthermore, the DLS result shows that the surface of the as-obtained NiCo2O4 hollow spheres is positively charged (about 3.0 mV) in water, which is conducive to the absorption of negatively charged GOx and improves the dispersibility of the particles in the solution. Additionally, the surface composition and valence state of the hierarchical NiCo2O4 hollow sphere are also vital, since the catalytic reactions mainly happened on the surface of catalysts. XPS was applied to study the element and valance information of NiCo2O4 hollow sphere. The survey spectrum in Figure 4a demonstrates the existence of nickel, cobalt, oxygen, and slight carbon atoms. By using a Gaussian fitting, the high-resolution spectrum of Ni 2p exhibits two spin-orbit doublets (Ni 2+ and Ni 3+ ) and two shakeup satellites ( Figure 4b). The peaks at 854.2 eV and 871.9 eV are attributed to Ni 2+ , and the peaks at 855.8 eV and 873.5 eV can be assigned to Ni 3+ . The Co 2p spectrum can also be fitted into two spin-orbit doublets (Co 2+ and Co 3+ ) and two shakeup satellites ( Figure 4c) in the same way. These results are well in agreement with many other reports, demonstrating the Ni 2+ and Ni 3+ , Co 2+ and Co 3+ inside the NiCo2O4 hollow sphere [33]. The highresolution spectrum of O 1s exhibits three oxygen species marked as O1, O2, and O3. Based on the previous reports, O1 (with a binding energy of 529.5 eV) was a typical metal-oxygen bond, and O2 (with a binding energy of 530.7 eV) was generally attributed to defects, impurities, and some low oxygen coordination within the spinal structure [34]. As for O3 (with a binding energy of 531.6 eV), it was associated with physi-and chemisorbed water molecules [35]. The rich surface chemical composition and valence state of the NiCo2O4 hollow sphere may lead to a large number of distributional differences in local electronic cloud density, which greatly affect the catalytic activity. XPS was applied to study the element and valance information of NiCo 2 O 4 hollow sphere. The survey spectrum in Figure 4a demonstrates the existence of nickel, cobalt, oxygen, and slight carbon atoms. By using a Gaussian fitting, the high-resolution spectrum of Ni 2p exhibits two spin-orbit doublets (Ni 2+ and Ni 3+ ) and two shakeup satellites ( Figure 4b). The peaks at 854.2 eV and 871.9 eV are attributed to Ni 2+ , and the peaks at 855.8 eV and 873.5 eV can be assigned to Ni 3+ . The Co 2p spectrum can also be fitted into two spin-orbit doublets (Co 2+ and Co 3+ ) and two shakeup satellites (Figure 4c) in the same way. These results are well in agreement with many other reports, demonstrating the Ni 2+ and Ni 3+ , Co 2+ and Co 3+ inside the NiCo 2 O 4 hollow sphere [33]. The high-resolution spectrum of O 1s exhibits three oxygen species marked as O1, O2, and O3. Based on the previous reports, O1 (with a binding energy of 529.5 eV) was a typical metal-oxygen bond, and O2 (with a binding energy of 530.7 eV) was generally attributed to defects, impurities, and some low oxygen coordination within the spinal structure [34]. As for O3 (with a binding energy of 531.6 eV), it was associated with physiand chemisorbed water molecules [35]. The rich surface chemical composition and valence state of the NiCo 2 O 4 hollow sphere may lead to a large number of distributional differences in local electronic cloud density, which greatly affect the catalytic activity.

Synthesis Mechanism
Based on the above discussion, the well-designed "CEP" process has been successfully applied to the synthesis of the hierarchical porous NiCo2O4 hollow sphere. Figure 5a-d display the evolution in morphology and structure, from Cu2O solid sphere to the final hierarchical NiCo2O4 hollow sphere. Figure 5e displays the corresponding schematic illustration of the formation process. As is well known, according to the HSAB (hard and soft acids and bases) theory, Cu + has much stronger interaction with S2O3 2− (soft-soft interaction) than that of O 2− (soft-hard interaction). Thus, the presence of S2O3 2− would lead to the elimination of the Cu2O template and an increased local OH − concentration on the etching interface. Besides, accompanied by the hydrolyzation of S2O3 2− and coprecipitation of metal ions with OH − , Ni-Co hydroxide units are preferential to nucleation and growth at the etching interface to form the Ni-Co hydroxide hollow sphere (Figure 5b). It is noteworthy that both the template elimination and duplication processes can be accurately controlled by simply adjusting the ratio of water to ethanol.

Synthesis Mechanism
Based on the above discussion, the well-designed "CEP" process has been successfully applied to the synthesis of the hierarchical porous NiCo 2 O 4 hollow sphere. Figure 5a-d display the evolution in morphology and structure, from Cu 2 O solid sphere to the final hierarchical NiCo 2 O 4 hollow sphere. Figure 5e displays the corresponding schematic illustration of the formation process. As is well known, according to the HSAB (hard and soft acids and bases) theory, Cu + has much stronger interaction with

Synthesis Mechanism
Based on the above discussion, the well-designed "CEP" process has been successfully applied to the synthesis of the hierarchical porous NiCo2O4 hollow sphere. Figure 5a-d display the evolution in morphology and structure, from Cu2O solid sphere to the final hierarchical NiCo2O4 hollow sphere. Figure 5e displays the corresponding schematic illustration of the formation process. As is well known, according to the HSAB (hard and soft acids and bases) theory, Cu + has much stronger interaction with S2O3 2− (soft-soft interaction) than that of O 2− (soft-hard interaction). Thus, the presence of S2O3 2− would lead to the elimination of the Cu2O template and an increased local OH − concentration on the etching interface. Besides, accompanied by the hydrolyzation of S2O3 2− and coprecipitation of metal ions with OH − , Ni-Co hydroxide units are preferential to nucleation and growth at the etching interface to form the Ni-Co hydroxide hollow sphere (Figure 5b). It is noteworthy that both the template elimination and duplication processes can be accurately controlled by simply adjusting the ratio of water to ethanol.    Figure 6a, only with the coexistence of H 2 O 2 and NiCo 2 O 4 catalyst could beget an obvious absorption peak at 652 nm (iii) due to the oxidation of TMB, which indicates a peroxidase-like activity. As in previous reported work, both of the as-prepared NiO (iv) and Co 3 O 4 (v) catalysts exhibited peroxidase-like activity in this research. However, the corresponding absorption spectra reveals that the hierarchical NiCo 2 O 4 hollow sphere obviously possesses a stronger absorption peak than that of NiO and Co 3 O 4 under the parallel condition, suggesting an enhanced peroxidase-like activity. The possible reaction mechanism is different from that of Fe-based catalysts, which are mainly conducted by the "Fenton" reaction. During this process, TMB molecules are absorbed on the surface of the hierarchical NiCo 2 O 4 hollow sphere, and can donate lone-pair electrons from the amino groups to the catalyst [8].

Peroxidase-Like Activity of the Hierarchical NiCo 2 O 4 Hollow Sphere
That results in an increase in electron density and mobility in the NiCo 2 O 4 hollow sphere, which would facilitate the electron transfer from the NiCo 2 O 4 hollow sphere to H 2 O 2 to accelerate the oxidation rate of TMB by H 2 O 2 [9]. Furthermore, the enhanced peroxidase-like activity of the NiCo 2 O 4 catalyst should be attributed to the diversified distribution of surface electron clouds and the hierarchical porous hollow structure that could provide ample active sites for catalytic reaction. The peroxidase-like activity of the hierarchical NiCo 2 O 4 hollow sphere is also dependent on the TMB concentration and pH environment. As shown in Figure 6b, the absorption intensity at 652 nm was increased with the increasing of TMB, but no obvious increase was observed when the concentration was greater than 0.08 mM (25 µL), suggesting an optimized TMB concentration. Additionally, the pH-dependent peroxidase-like activity in Figure 6c indicates that the optimal pH should be 4.5.

Peroxidase-Like Activity of the Hierarchical NiCo2O4 Hollow Sphere
To investigate the peroxidase-like activity of the hierarchical NiCo2O4 hollow sphere, the catalysis of the peroxidase substrate TMB was performed in presence or absence of H2O2. Additionally, the peroxidase-like activities of NiO and Co3O4 hollow spheres synthesized by the same method were also studied for comparison. Thus, five controlled experiments were carried out: (i) TMB + H2O2; (ii) TMB + NiCo2O4; (iii) TMB + H2O2 + NiCo2O4; (iv) TMB + H2O2 + NiO; (v) TMB + H2O2 + Co3O4. As shown in Figure 6a, only with the coexistence of H2O2 and NiCo2O4 catalyst could beget an obvious absorption peak at 652 nm (iii) due to the oxidation of TMB, which indicates a peroxidaselike activity. As in previous reported work, both of the as-prepared NiO (iv) and Co3O4 (v) catalysts exhibited peroxidase-like activity in this research. However, the corresponding absorption spectra reveals that the hierarchical NiCo2O4 hollow sphere obviously possesses a stronger absorption peak than that of NiO and Co3O4 under the parallel condition, suggesting an enhanced peroxidase-like activity. The possible reaction mechanism is different from that of Fe-based catalysts, which are mainly conducted by the "Fenton" reaction. During this process, TMB molecules are absorbed on the surface of the hierarchical NiCo2O4 hollow sphere, and can donate lone-pair electrons from the amino groups to the catalyst [8]. That results in an increase in electron density and mobility in the NiCo2O4 hollow sphere, which would facilitate the electron transfer from the NiCo2O4 hollow sphere to H2O2 to accelerate the oxidation rate of TMB by H2O2 [9]. Furthermore, the enhanced peroxidase-like activity of the NiCo2O4 catalyst should be attributed to the diversified distribution of surface electron clouds and the hierarchical porous hollow structure that could provide ample active sites for catalytic reaction. The peroxidase-like activity of the hierarchical NiCo2O4 hollow sphere is also dependent on the TMB concentration and pH environment. As shown in Figure 6b, the absorption intensity at 652 nm was increased with the increasing of TMB, but no obvious increase was observed when the concentration was greater than 0.08 mM (25 μL), suggesting an optimized TMB concentration. Additionally, the pH-dependent peroxidase-like activity in Figure 6c indicates that the optimal pH should be 4.5.

Colorimetric Detection of H 2 O 2
Based on the above peroxidase-like activity investigation of the hierarchical NiCo 2 O 4 hollow sphere, a colorimetric method was developed for detection of H 2 O 2 . In Figure 7a, the absorbance at 652 nm for oxidized TMB increases with the concentration of H 2 O 2 . Figure 7b shows the corresponding linear calibration curve and photograph of color change. The linear range is from 0.01 mM to 0.4 mM, and the detection limit is calculated to be 0.21 µM. Notably, the concentration-dependent color change could offer a handy way for detection of H 2 O 2 by naked eye.

Colorimetric Detection of H2O2
Based on the above peroxidase-like activity investigation of the hierarchical NiCo2O4 hollow sphere, a colorimetric method was developed for detection of H2O2. In Figure 7a, the absorbance at 652 nm for oxidized TMB increases with the concentration of H2O2. Figure 7b shows the corresponding linear calibration curve and photograph of color change. The linear range is from 0.01 mM to 0.4 mM, and the detection limit is calculated to be 0.21 μM. Notably, the concentrationdependent color change could offer a handy way for detection of H2O2 by naked eye.

Colorimetric Detection of Glucose
The concentration of blood glucose is an important physiological indicator for the human body. The detection of glucose is meaningful for clinical diagnosis and treatment of disease. H2O2 is one of the main intermediate products of glucose oxidase-catalyzed reaction. To take full advantage of the peroxidase-like activity of the hierarchical NiCo2O4 hollow sphere, the colorimetric detection of glucose was also developed, and the reaction mechanism is illustrated in Figure 8. As shown in Figure  9a, the absorbance at 652 nm for oxidized TMB increases with the concentration of H2O2. The corresponding linear calibration curve and photograph of color change in Figure 9b shows that the linear range is from 0.1 mM to 4.5 mM with a detection limit of 5.31 μM. It can be seen from Table 1 that the proposed catalyst performs satisfactory combination properties toward glucose sensing compared with most non-Pt-based catalysts. Even though the present NiCo2O4-based sensor exhibits slightly lower sensing performance to that of a Pt-based sensor, but it is also a low-cost and effective detection method. Moreover, the concentrations of blood glucose in healthy and diabetic persons is generally in the range of 3-8 mM and 9-40 mM, respectively [36]. Therefore, the proposed method is applicable to glucose analysis in real serum by simple dilution.

Colorimetric Detection of Glucose
The concentration of blood glucose is an important physiological indicator for the human body. The detection of glucose is meaningful for clinical diagnosis and treatment of disease. H 2 O 2 is one of the main intermediate products of glucose oxidase-catalyzed reaction. To take full advantage of the peroxidase-like activity of the hierarchical NiCo 2 O 4 hollow sphere, the colorimetric detection of glucose was also developed, and the reaction mechanism is illustrated in Figure 8. As shown in Figure 9a, the absorbance at 652 nm for oxidized TMB increases with the concentration of H 2 O 2 . The corresponding linear calibration curve and photograph of color change in Figure 9b shows that the linear range is from 0.1 mM to 4.5 mM with a detection limit of 5.31 µM. It can be seen from Table 1 that the proposed catalyst performs satisfactory combination properties toward glucose sensing compared with most non-Pt-based catalysts. Even though the present NiCo 2 O 4 -based sensor exhibits slightly lower sensing performance to that of a Pt-based sensor, but it is also a low-cost and effective detection method. Moreover, the concentrations of blood glucose in healthy and diabetic persons is generally in the range of 3-8 mM and 9-40 mM, respectively [36]. Therefore, the proposed method is applicable to glucose analysis in real serum by simple dilution.

Selectivity, Reproducibility, and Applicability
The selectivity, reproducibility, and applicability of the proposed sensor for glucose detection were investigated. As shown in Figure 10, the presence of glucose apparently produced much stronger absorption intensity and color change than that of other analytes, indicating a good selectivity to glucose of this biosensor. The reproducibility was evaluated by three consecutive detections, resulting in an acceptable relative standard deviation (RSD) of 3.24%. For applicability, the sensor was applied to detect the human serum sample: the detected glucose concentration was 4.2 mM with an acceptable RSD of 4.5%, compared to the concentration of 4.4 mM determined by the hospital.

Selectivity, Reproducibility, and Applicability
The selectivity, reproducibility, and applicability of the proposed sensor for glucose detection were investigated. As shown in Figure 10, the presence of glucose apparently produced much stronger absorption intensity and color change than that of other analytes, indicating a good selectivity to glucose of this biosensor. The reproducibility was evaluated by three consecutive detections, resulting in an acceptable relative standard deviation (RSD) of 3.24%. For applicability, the sensor was applied to detect the human serum sample: the detected glucose concentration was 4.2 mM with an acceptable RSD of 4.5%, compared to the concentration of 4.4 mM determined by the hospital.

Selectivity, Reproducibility, and Applicability
The selectivity, reproducibility, and applicability of the proposed sensor for glucose detection were investigated. As shown in Figure 10, the presence of glucose apparently produced much stronger absorption intensity and color change than that of other analytes, indicating a good selectivity to glucose of this biosensor. The reproducibility was evaluated by three consecutive detections, resulting in an acceptable relative standard deviation (RSD) of 3.24%. For applicability, the sensor was applied to detect the human serum sample: the detected glucose concentration was 4.2 mM with an acceptable RSD of 4.5%, compared to the concentration of 4.4 mM determined by the hospital.

Conclusions
In summary, the hierarchical NiCo2O4 hollow sphere was successfully synthesized by a "coordinating etching and precipitating" process and exhibited intrinsic peroxidase-like activity. The experimental results revealed that the hierarchical NiCo2O4 hollow sphere possessed superior peroxidase-like activity to that of single NiO or Co3O4. For use as a peroxidase mimic, the proposed sensing platform showed a commendable response to H2O2 with a linear range from 10 μM to 400 μM and a detection limit of 0.21 μM. More importantly, combined with GOx, the developed glucosesensing platform exhibited high selectivity, superb reproducibility, satisfactory applicability, wide linear range from 0.1 mM to 4.5 mM, and low detection limit of 5.31 μM. Additionally, the concentration-dependent color change could offer a handy way for detection of H2O2 and glucose by naked eye. Therefore, the hierarchical NiCo2O4 hollow sphere would be a promising candidate for colorimetric detection of H2O2 and glucose.

Conclusions
In summary, the hierarchical NiCo 2 O 4 hollow sphere was successfully synthesized by a "coordinating etching and precipitating" process and exhibited intrinsic peroxidase-like activity. The experimental results revealed that the hierarchical NiCo 2 O 4 hollow sphere possessed superior peroxidase-like activity to that of single NiO or Co 3 O 4 . For use as a peroxidase mimic, the proposed sensing platform showed a commendable response to H 2 O 2 with a linear range from 10 µM to 400 µM and a detection limit of 0.21 µM. More importantly, combined with GOx, the developed glucose-sensing platform exhibited high selectivity, superb reproducibility, satisfactory applicability, wide linear range from 0.1 mM to 4.5 mM, and low detection limit of 5.31 µM. Additionally, the concentration-dependent color change could offer a handy way for detection of H 2 O 2 and glucose by naked eye. Therefore, the hierarchical NiCo 2 O 4 hollow sphere would be a promising candidate for colorimetric detection of H 2 O 2 and glucose.