High-Sensitivity Enzymatic Glucose Sensor Based on ZnO Urchin-like Nanostructure Modified with Fe3O4 Magnetic Particles

A novel and efficient enzymatic glucose sensor was fabricated based on Fe3O4 magnetic nanoparticles (Fe3O4MNPs)-modified urchin-like ZnO nanoflowers (ZnONFs). ZnONFs were hydrothermally synthesizing on a flexible PET substrate. Fe3O4MNPs were deposited on the surface of the ZnONFs by the drop-coating process. The results showed that the urchin-like ZnONFs provided strong support for enzyme adsorption. For Fe3O4MNPs, it significantly promoted the redox electron transfer from the active center of GOx to the ZnO nanoflowers beneath. More importantly, it promoted the hydrolysis of H2O2, the intermediate product of glucose catalytic reaction, and thus improved the electron yield. The sensitivity of the Nafion/GOx/Fe3O4MNPs/ZnONFs/Au/PET sensor was up to 4.52 μA·mM−1·cm−2, which was improved by 7.93 times more than the Nafion/GOx/ZnONFs/Au/PET sensors (0.57 μA·mM−1·cm−2). The detection limit and linear range were also improved. Additionally, the as-fabricated glucose sensors show strong anti-interference performance in the test environment containing organic compounds (such as urea, uric acid, and ascorbic acid) and inorganic salt (for instance, NaCl and KCl). The glucose sensor’s service life was evaluated, and it can still maintain about 80% detection performance when it was reused about 20 times. Compared with other existing sensors, the as-fabricated glucose sensor exhibits an ultrahigh sensitivity and wide detection range. In addition, the introduction of Fe3O4MNPs optimized the catalytic efficiency from the perspective of the reaction mechanism and provided potential ideas for improving the performance of other enzymatic biosensors.


Introduction
Glucose is the most widely distributed and essential monosaccharide in nature, which has significant value in many fields, such as the fermentation industry, food industry, chemical industry, or material synthesis, just to mention a few [1]. In human life and health, as an important energy source of cells, glucose has a vital impact on human health; ensuring the balance and stability of blood glucose content is essential for maintaining health. It is estimated that the number of diabetes patients in the world will reach 463 million (9.3%) in 2019, 578 million (10.2%) by 2030, and 700 million (10.9%) by 2045 [2]. Therefore, the accurate, rapid, and noninvasive detection of glucose content is an urgent problem to be solved in health care. Among the numerous methods for detecting glucose concentration, enzyme-based electrochemical glucose sensors have shown great advantages such as good highly sensitive performance, and strong anti-interference ability. In this paper, ZnONFs were synthesized by the hydrothermal method and modified with Fe 3 O 4 MNPs to construct a high-performance enzyme glucose sensor. The peroxidase-like activity of the Fe 3 O 4 MNPs and the ZnO nanoflowers' three-dimensional structure were combined to produce a synergistic effect. On the one hand, it provides the necessary conduction channel for the electrons on the electrode surface. On the other hand, it promotes the catalytic reaction of enzymes and improves the electron yield. This methodology is simple and effective. By optimizing the electrode's micro nanostructure and introducing magnetic nanoparticles to improve the enzyme reaction principle, the enzyme glucose sensor's performance was improved. It provides a new idea for the preparation of a high-performance enzymatic glucose sensor.

Apparatus and Software
The nanostructure of the Fe 3 O 4 MNPs and ZnONFs thin film was observed by Field-Emission Scanning Electron Microscope (FESEM, GeminiSEM 500, Zeiss, Oberkochen, Germany). The crystal structures of Fe 3 O 4 MNPs and ZnONFs thin film were measured by X-ray diffraction (XRD). The magnetic properties of Fe 3 O 4 nanoparticles were measured by vibrating sample magnetometer (VSM-7410, Lake Shore Cryotronics, Westerville, OH, USA), and the magnetic field range was set off -3kOe to 3kOe. The electrochemical tests such as electrochemical impedance spectroscopy, cyclic voltammogram characterization, and amperometric response were conducted on an electrochemical workstation (CHI660D, CH Instruments, Austin, TX, USA).

Preparation of the Fe 3 O 4 MNPs Modified ZnONFs Electrodes
As shown in Figure 1, there are four procedures to prepare the Fe 3 O 4 NPs modified ZnONFs enzymatic glucose sensors. Firstly, the flexible PET was deposited with Au film to enhance the conductivity of the basic substrates. Secondly, ZnONFs were synthesized by the hydrothermal method on the prepared substrates. Then, Fe 3 O 4 MNPs were modified on ZnONFs by the drop-coating method to obtain the Fe 3 O 4 MNPs/ZnONFs/Au/PET nanostructure. Finally, GOx was adsorbed on Fe 3 O 4 MNPs/ZnONFs/Au/PET substrates and coated with Nafion to cover it for fixation. The electrochemical analysis was based on the traditional three-electrode systems. The working electrode, auxiliary electrode, and reference electrode used in the experiment are as-fabricated electrodes, platinum wire, and Ag/AgCl. reference electrode used in the experiment are as-fabricated electrodes, platinum wire, and Ag/AgCl.

Pretreatment of the PET Substrates
Firstly, the PET substrate was cleaned with ethanol solution to remove impurities; then, the residual ethanol solution was flushed by DI water. The prepared substrates are placed in a super clean room at a temperature of 22 °C until the surface is completely dried. Then, Au film of thickness 50 nm was deposited on PET substrates by ion sputter to enhance the flexible substrates' conductivity. Finally, the Au/PET was cut into a rectangle of 0.5 cm × 2 cm as the substrates of the enzyme sensor's working electrodes.

Hydrothermal Preparation of ZnONFs
ZnONFs were synthesized using the hydrothermal method. Firstly, 0.35 g of hexamethylenetetramine and 0.742 g of zinc nitrate hexahydrate were dissolved into 100 mL of deionized water, and the concentration of Zn 2+ was 25 mM. Secondly, the solution's temperature from room temperature is raised to 90 °C and is kept stirring until it is completely mixed. Finally, the Au/PET substrates were quickly put into the growth solution and placed in a 90 °C water bath for 2.5 h to prepare ZnONFs. Thus, the ZnONFs structure was firmly grown on the Au/PET substrates so it forms the ZnONFs/Au/PET substrates.

Drop-Cast Coating Fe3O4MNPs on the ZnONFs/Au/PET Substrates
Fe3O4MNPs were electrostatically adsorbed on ZnONFs by a drop-coating method. First, 2.5 g of iron oxide were dissolved into 50 mL of deionized water. Then, the mixed liquids were dispersed by ultrasonic bath for 15 min to configured as Fe3O4 suspension. The prepared suspension was stored at 4 °C until being used, and the content of Fe3O4MNPs in the suspension was about 50 mg/mL. Finally, applied 10 μL of the prepared suspension on the surface of ZnONFs/Au/PET substrates and dried in the ultra-clean laboratory. Thus, the Fe3O4MNPs/ZnONFs/Au/PET substrates were achieved.

Pretreatment of the PET Substrates
Firstly, the PET substrate was cleaned with ethanol solution to remove impurities; then, the residual ethanol solution was flushed by DI water. The prepared substrates are placed in a super clean room at a temperature of 22 • C until the surface is completely dried. Then, Au film of thickness 50 nm was deposited on PET substrates by ion sputter to enhance the flexible substrates' conductivity. Finally, the Au/PET was cut into a rectangle of 0.5 cm × 2 cm as the substrates of the enzyme sensor's working electrodes.

Hydrothermal Preparation of ZnONFs
ZnONFs were synthesized using the hydrothermal method. Firstly, 0.35 g of hexamethylenetetramine and 0.742 g of zinc nitrate hexahydrate were dissolved into 100 mL of deionized water, and the concentration of Zn 2+ was 25 mM. Secondly, the solution's temperature from room temperature is raised to 90 • C and is kept stirring until it is completely mixed. Finally, the Au/PET substrates were quickly put into the growth solution and placed in a 90 • C water bath for 2.5 h to prepare ZnONFs. Thus, the ZnONFs structure was firmly grown on the Au/PET substrates so it forms the ZnONFs/Au/PET substrates.

Drop-Cast Coating Fe 3 O 4 MNPs on the ZnONFs/Au/PET Substrates
Fe 3 O 4 MNPs were electrostatically adsorbed on ZnONFs by a drop-coating method. First, 2.5 g of iron oxide were dissolved into 50 mL of deionized water. Then, the mixed liquids were dispersed by ultrasonic bath for 15 min to configured as Fe 3 O 4 suspension. The prepared suspension was stored at 4 • C until being used, and the content of Fe 3 O 4 MNPs in the suspension was about 50 mg/mL. Finally, applied 10 µL of the prepared suspension on the surface of ZnONFs/Au/PET substrates and dried in the ultra-clean laboratory. Thus, the Fe 3 O 4 MNPs/ZnONFs/Au/PET substrates were achieved.

Drop-Coated GOx on the As-Fabricated Electrodes
GOx solution (40 mg·mL −1 ) was prepared to modify the enzyme catalytic electrode. Subsequently, 10 µL of fresh GOx solution was dropped on the surface of the as-fabricated substrates and then dried naturally at 4 • C in the air to achieve the GOx/Fe 3 O 4 MNPs/ ZnONFs/Au/PET and GOx/ZnONFs/Au/PET substrates. After that, 5 µL Nafion solution was coated on the as fabricated substrates to form an ion transparent membrane on top. Thus, the Nafion/GOx/Fe 3 O 4 MNPs/ZnONFs/Au/PET and Nafion/GOx/ZnONFs/Au/PET enzymatic electrodes were achieved. The prepared electrode is related to the lead, fixed with silver conductive adhesive, and used as the working electrode to form a three-electrode system together with the reference electrode and counter electrode, to carry out further electrochemical tests. To maintain the biological activity of GOx, these as-fabricated electrodes should be stored at 4 • C.

Measurement of Electrochemical Properties of the As-Fabricated Electrodes
The electrochemical test is carried out by using the traditional three electrode system, in which the working electrode is the as-fabricated electrode in this paper, platinum wire is used as the auxiliary electrode, and Ag/AgCl is used as the reference electrode. An aerometric i-t curve was used to characterize the as-fabricated electrode's performance. In the measurement process, it is necessary to ensure the stability of the background current to achieve an accurate current response. Then, 100 µL glucose solution was added into 50 mL of PBS solution obtain the current response curve. After waiting for a stable response current, we repeated the above steps until the current response curve was saturated. Electrochemical impedance spectroscopy (EIS) was used to characterize the electron transfer ability of the electrodes. The electrochemical impedance spectroscopy of the as-fabricated glucose sensors was tested in PBS solution. The frequency is from 0.01 Hz to 100 kHz, and the bias potential is +0.1 V. The redox process was described by cyclic voltammetry. A cyclic voltammogram was performed in a 3 mM glucose solution with a scan rate of 50 mV s −1 and potential range of −0.8 V to +0.8 V. The sensitivity of the as-fabricated glucose sensor to urea, uric acid, ascorbic acid, sodium chloride, and potassium chloride was also evaluated. All the above electrochemical tests were carried out at room temperature.

The Morphology and Composition of the Fe 3 O 4 MNPs/ZnONFs/Au/PET and ZnONFs/Au/PET Substrate
The morphology of the ZnONFs/Au/PET substrate is shown in Figure 2a. Each urchin-like ZnONF is constructed with tens of ZnO nanorods, which are radially clustered together. The ZnO nanorods have a typical hexagonal wurtzite crystal structure, which suggests that the ZnONFs are well-ordered and highly crystallized. The diameter of each ZnO nanorod is about 200 nm. Moreover, the average size of ZnONFs shows good consistency, which was about 3 µm. It is worth knowing that different ZnONFs cluster and stack together to form a three-dimensional porous structure. Urchin-like spines support the ZnONFs in the same plane to form plenty of polymorphic voids. At the same time, the disordered stacking of ZnONFs forms multipath holes in the longitudinal direction. In order to comprehend the detail of The XRD patterns of the ZnONFs and Fe3O4MNPs/ZnONFs substrate are illustrated in Figure 3a. The black curve indicates that the XRD spectrums of the ZnONFs thin-film structure show evident and intense diffraction peaks of (001), (002), (102), and (110) planes, which corresponded to the crystal structure of ZnO (JCPDS cards #36-1451). The close-packed hexagonal structure and high crystallinity of ZnO are confirmed. In addition to the ZnO X-ray diffraction peaks, the Fe3O4MNPs/ZnONFs thin-film structure exhibited the characteristic diffraction peaks at 30.0°, 43.0°, 53.4°, and 56.9°, which corresponded to the (220), (400), (422), and (511) planes, respectively (shown in the red curve), which confirms the monoclinic crystal structure of Fe3O4 lattice (#75-1610). In addition, combined with the characterization data of the X-ray curve, the average crystalline size of Fe3O4MNPs can be calculated by the Debye-Schell formula shown in Equation (1).
The grain diameter perpendicular to the crystal plane is presented as Dhkl, and the Scherrer constant is k (usually 0.89). The incident wavelength of the X-ray is λ (usually 0.15418 nm), the Bragg diffraction angle is θ, and the half-maximum width of the diffraction peak is β. It is found that the average particle size of the Fe3O4MNPs is about 33.1 nm. The above analysis shows that both of the ZnONFs/Au/PET and Fe3O4MNPs/ZnONFs/Au/PET substrates have a well-crystallized ZnO nanostructure. The Fe3O4MNPs/ZnONFs thin film also shows a typical high crystalline cubic spinel structure of Fe3O4 with an average size of about 33.1 nm. All in all, X-ray diffraction confirmed the formation of the Fe3O4MNPs/ZnONFs composite film structure.   The XRD patterns of the ZnONFs and Fe3O4MNPs/ZnONFs substrate are illustrated in Figure 3a. The black curve indicates that the XRD spectrums of the ZnONFs thin-film structure show evident and intense diffraction peaks of (001), (002), (102), and (110) planes, which corresponded to the crystal structure of ZnO (JCPDS cards #36-1451). The close-packed hexagonal structure and high crystallinity of ZnO are confirmed. In addition to the ZnO X-ray diffraction peaks, the Fe3O4MNPs/ZnONFs thin-film structure exhibited the characteristic diffraction peaks at 30.0°, 43.0°, 53.4°, and 56.9°, which corresponded to the (220), (400), (422), and (511) planes, respectively (shown in the red curve), which confirms the monoclinic crystal structure of Fe3O4 lattice (#75-1610). In addition, combined with the characterization data of the X-ray curve, the average crystalline size of Fe3O4MNPs can be calculated by the Debye-Schell formula shown in Equation (1).
The grain diameter perpendicular to the crystal plane is presented as Dhkl, and the Scherrer constant is k (usually 0.89). The incident wavelength of the X-ray is λ (usually 0.15418 nm), the Bragg diffraction angle is θ, and the half-maximum width of the diffraction peak is β. It is found that the average particle size of the Fe3O4MNPs is about 33.1 nm. The above analysis shows that both of the ZnONFs/Au/PET and Fe3O4MNPs/ZnONFs/Au/PET substrates have a well-crystallized ZnO nanostructure. The Fe3O4MNPs/ZnONFs thin film also shows a typical high crystalline cubic spinel structure of Fe3O4 with an average size of about 33.1 nm. All in all, X-ray diffraction confirmed the formation of the Fe3O4MNPs/ZnONFs composite film structure.  The grain diameter perpendicular to the crystal plane is presented as Dhkl, and the Scherrer constant is k (usually 0.89). The incident wavelength of the X-ray is λ (usually 0.15418 nm), the Bragg diffraction angle is θ, and the half-maximum width of the diffraction peak is β. It is found that the average particle size of the

Characterization of EIS Curve
The Nyquist diagrams of the Nafion/GOx/ZnONFs/Au/PET and Nafion/GOx/Fe3O4MNPs/ZnONFs/Au/PET glucose sensors are shown in Figure 4a. The high-frequency characteristic of the Nyquist curve describes the electron migration characteristic. The calculation shows that the Rct of the ZnONFs/Au/PET electrodes is about 463.1 ± 9.05 kΩ (black curve), while the Rct of the Fe3O4MNPs/ZnONFs/Au/PET electrodes is about 366.1 ± 10.67 kΩ (red curve). The electron transfer resistance shows a decreasing trend, and this reveals that Fe3O4MNPs can promote electron transfer in the electrode process to a certain extent.
The electrical equivalent circuit is fitted by ZSimpWin software, which is consistent with the actual measurement results, as shown in Figure 4a as an inset picture. On a planar electrode, the electrode reaction process can be regarded as a process controlled by both electron transfer and diffusion. Rs represents the electrolyte solution resistance, Rct represents the charge transfer resistance, Cdl represents the double layer capacitance, Zd represents the Warburg impedance, and Rcoat and Ccoat are the Warburg diffusion elements corresponding respectively to the diffusion through the Nafion membrane. The electrochemical impedance spectroscopy of the as-fabricated glucose sensors was tested in PBS solution. The frequency is from 0.01 Hz to 100 kHz, and the bias potential is +0.1 V.

Cyclic Voltammogram Characterization
The Cyclic voltammograms in Figure 4b shows that both oxidation and reduction peaks can be detected at the as-fabricated electrodes. It is worth noting that the redox The electrical equivalent circuit is fitted by ZSimpWin software, which is consistent with the actual measurement results, as shown in Figure 4a as an inset picture. On a planar electrode, the electrode reaction process can be regarded as a process controlled by both electron transfer and diffusion. R s represents the electrolyte solution resistance, R ct represents the charge transfer resistance, C dl represents the double layer capacitance, Z d represents the Warburg impedance, and R coat and C coat are the Warburg diffusion elements corresponding respectively to the diffusion through the Nafion membrane. The electrochemical impedance spectroscopy of the as-fabricated glucose sensors was tested in PBS solution. The frequency is from 0.01 Hz to 100 kHz, and the bias potential is +0.1 V.

Cyclic Voltammogram Characterization
The Cyclic voltammograms in Figure 4b shows that both oxidation and reduction peaks can be detected at the as-fabricated electrodes. It is worth noting that the redox current of the Fe 3 O 4 MNPs modified working electrode was significantly higher than that of ZnONFs working electrode, which indicated that the Fe 3 O 4 MNPs play a positive role in improving the enzyme catalytic reaction and the electrochemical performance of the working electrode.
Cyclic voltammograms evaluated the electrokinetics that occurred in the glucose detection system. The changing trend of redox current was characterized by a 3 mM glucose solution concentration under the scanning rate of 10 to 1000 mV·s −1 . Figure 5a,c indicated that both Nafion/GOx/ZnONFs/Au/PET and Nafion/GOx/Fe 3 O 4 MNPs/ZnONFs/Au/PET electrodes have obvious oxidation and reduction peaks, and the oxidation-reduction peak increases gradually with the increase of scanning rate, respectively. Furthermore, the peak currents of both I pa and I pc exhibit a linear relationship with the square root of the scan rate, as shown in Figure 5b,d, which means that both enzymatic electrodes were diffusion-controlled processes. current of the Fe3O4MNPs modified working electrode was significantly higher than that of ZnONFs working electrode, which indicated that the Fe3O4MNPs play a positive role in improving the enzyme catalytic reaction and the electrochemical performance of the working electrode.
Cyclic voltammograms evaluated the electrokinetics that occurred in the glucose detection system. The changing trend of redox current was characterized by a 3 mM glucose solution concentration under the scanning rate of 10 to 1000 mV·s −1 . Figure 5a,c indicated that both Nafion/GOx/ZnONFs/Au/PET and Nafion/GOx/Fe3O4MNPs/ZnONFs/Au/PET electrodes have obvious oxidation and reduction peaks, and the oxidation-reduction peak increases gradually with the increase of scanning rate, respectively. Furthermore, the peak currents of both Ipa and Ipc exhibit a linear relationship with the square root of the scan rate, as shown in Figure 5b,d, which means that both enzymatic electrodes were diffusioncontrolled processes.  Figure 6a show that both as-fabricated electrodes are with proportionate and stable electrocatalytic activities. The amperometric response of the Nafion/GOx/Fe3O4MNPs/ZnONFs/Au/PET electrode was much higher than that of the Nafion/GOx/ZnONFs/Au/PET electrode. This indicates that the Nafion/GOx/Fe3O4MNPs/ZnONFs/Au/PET electrode is more sensitive to the concentration of glucose in this test environment. Figure 6b indicated that there is a linear relationship between the response current and the concentration of glucose solution, and the correlation coefficient is as high as R = 0.999. Figure 6c shows the redox reaction mechanism and electron transport scheme of the glucose sensing. The introduction of Fe3O4MNPs  Figure 6a show that both as-fabricated electrodes are with proportionate and stable electrocatalytic activities. The amperometric response of the Nafion/GOx/Fe 3 O 4 MNPs/ ZnONFs/Au/PET electrode was much higher than that of the Nafion/GOx/ZnONFs/Au/PET electrode. This indicates that the Nafion/GOx/Fe 3 O 4 MNPs/ZnONFs/Au/PET electrode is more sensitive to the concentration of glucose in this test environment. Figure 6b indicated that there is a linear relationship between the response current and the concentration of glucose solution, and the correlation coefficient is as high as R = 0.999. Figure 6c shows the redox reaction mechanism and electron transport scheme of the glucose sensing. The introduction of Fe 3 O 4 MNPs provides more enzyme adsorption sites on the surface of ZnONFs nanomaterials, which play a catalytic role from two aspects.

Amperometric Response
Glucose + GO (FAD) → Gluconic acid + GO (FADH )  On the one hand, as discussed in Section 3.2.1, the introduction of Fe 3 O 4 MNPs improves the electron transfer efficiency between the solution and the electrode surface and also promotes electron transfer in the reaction process. On the other hand, as shown in Equations (3) and (4), the intrinsic peroxidase-like activity of Fe 3 O 4 MNPs [25] can promote the hydrolysis of the intermediate product H 2 O 2 , which can form H 2 O and O 2 , thus generating more electrons and promoting the electron yield. More importantly, the enzyme catalytic process consumes O 2 , and the O 2 produced by hydrogen peroxide hydrolysis supplements the O 2 content in the solution, which promotes the catalytic cycle of the enzyme and accelerates the electronic production. Based on the combination of these two aspects, the introduction of Fe 3 O 4 MNPs can significantly improve the enzyme-catalyzed glucose sensor's performance.
The sensitivities of the Nafion/GOx/ZnONFs/Au/PET sensor are 0.57 µA·mM −1 ·cm −2 , and the Nafion/GOx/Fe 3 O 4 MNPs/ZnONFs/Au/PET is 4.52 µA·mM −1 ·cm −2 , which is 7.93 times higher than the other. The low detection limits are 0.105 µM and 0.089 µM, respectively. The Nafion/GOx/ZnONFs/Au/PET sensors' linear range reached 8.5 mM, and the Nafion/GOx/Fe 3 O 4 MNPs/ZnONFs/Au/PET sensor extended to 12.5 mM. Human blood glucose is maintained at a constant level of 4 to 6.5 mM, and the linear range of blood glucose monitoring should cover the range of 3-8 mM. Both as-fabricated sensors meet the requirements of human glucose monitoring. The results show that the introduction of Fe 3 O 4 magnetic nanoparticles can improve glucose sensor performance based on ZnONFs nanostructure. The performance comparison between this work and other similar literature is shown in Table 1. From the current research on the enzymatic glucose sensor, it can be seen that the main methods to improve the performance of the sensor focus on two aspects: one is to prepare nano mechanism materials with high specific surface areas, such as CNT-Mucin and ZnO nanostructure, to increase the solid-liquid contact area of an enzymatic glucose sensor, improving the electron yield of the enzyme-catalyzed reaction, and then leading to the improvement of performance. In this aspect, the ZnONFs structure prepared in this work can provide 3D morphology support, which has the same advantages as other literature. Another option is to introduces other superconducting materials, such as graphene and Pt, to improve the efficiency of electron transfer, thereby improving the performance of the sensor. Compared with the sensor of another material system, the Fe 3 O 4 NPs-modified sensor has better sensitivities. These are mainly due to the Fe 3 O 4 MNPs having a special peroxidase-like activity, which can promote the hydrolysis of H 2 O 2 to produce H 2 O and O 2 and enhance the electron yield of the enzyme catalytic process. In conclusion, to compare with other similar sensors, the as-fabricated Nafion/GOx/Fe 3 O 4 NPs/ZnONFs/Au/PET sensors also reach the equivalent or even better sensitivity.

The Anti-Interference Capability of the As-Fabricated Glucose Sensors
Considering that the glucose sensor may be used in a complex environment that contained various substances, the prepared sensor should have a certain anti-interference performance to ensure detection accuracy. It is necessary to verify the current response of the glucose sensors to other potential substances. The following procedure is performed to verify the anti-interference of the as-fabricated glucose sensor.
Glucose (3 mM), urea (U, 0.5 mM), uric acid (UA, 0.5 mM), ascorbic acid (AA, 0.1 mM), NaCl (0.5 mM), KCl (0.5 mM), and glucose (3 mM) were added in 50 mL PBS solution in turns at the experimental environment. The amperometric current was observed to obtain the ability of the as-fabricated sensors to respond to these potential substances. As shown in Figure 7a, when glucose solution was added, the oxidation currents response is obvious, and a step is formed. When U, AA, UA, NaCl, and KCl were added to PBS solution successively, the oxidation currents of the two glucose sensors were almost unchanged, and a step was formed after glucose was added later. It suggests that the Nafion/GOx/ZnONFs/Au/PET and Nafion/GOx/Fe 3 O 4 MNPs/ZnONFs/Au/PET glucose sensors are not easy to be interfered with by other substances and have high anti-interference performance for glucose detection. The following two contributions may provide a basis: the advantage of enzyme catalytic sensing is the specific reaction of glucose oxidase to glucose, and the Nafion is coated to provide a membrane to interfere with anions such as UA and AA, respectively.
As shown in Figure 7a, when glucose solution was added, the oxidation currents response is obvious, and a step is formed. When U, AA, UA, NaCl, and KCl were added to PBS solution successively, the oxidation currents of the two glucose sensors were almost unchanged, and a step was formed after glucose was added later. It suggests that the Nafion/GOx/ZnONFs/Au/PET and Nafion/GOx/Fe3O4MNPs/ZnONFs/Au/PET glucose sensors are not easy to be interfered with by other substances and have high anti-interference performance for glucose detection. The following two contributions may provide a basis: the advantage of enzyme catalytic sensing is the specific reaction of glucose oxidase to glucose, and the Nafion is coated to provide a membrane to interfere with anions such as UA and AA, respectively.  Figure 7b indicates the Nafion/GOx/Fe3O4MNPs/ZnONFs/Au/PET glucose sensors' service life. The glucose sensors were kept in a 3 mM glucose solution. The device's service life was evaluated by recording the current intensity in the response's cyclic voltammogram. The abscissa is the measurement times of the same sensor, and the ordinate is the attenuation of the sensor's current response after different measurements. It is shown that Nafion/GOx/Fe3O4MNPs/ZnONFs/Au/PET glucose sensors retained 85.4% of the initial oxidation current after being reused 16 times, respectively. After being reused 19 times, the glucose sensors' oxidation currents decreased to 81.3% of the initial values. This suggests that the sensor's measurement accuracy is maintained at about 80% of the initial state when it is reused about 20 times. When repeatedly used 28 times, the Nafion/GOx/Fe3O4MNPs/ZnONFs/Au/PET glucose sensors' performance rapidly decayed to 54.7% of the initial values, indicating that the sensor reached the limit of its service life. For the enzyme sensor, the decline of its performance is mainly due to the loss of enzyme and the attenuation of enzyme activity. The covering Nafion ion membrane is an effective way to prevent excessive loss of enzymes. In addition, it should be noted that storing the sensor in a suitable environment can also effectively ensure enzyme activity.

Conclusions
This study presents a simple approach to fabricate an efficient enzymatic glucose sensor based on urchin-like ZnO nanoflowers modified with Fe3O4 MNPs. The excellent conductivity of Fe3O4MNPs can promote the transfer of redox electrons from a GOx active center to ZnONFs. As nanoscale magnetic particles, the intrinsic peroxidase-like activity The device's service life was evaluated by recording the current intensity in the response's cyclic voltammogram. The abscissa is the measurement times of the same sensor, and the ordinate is the attenuation of the sensor's current response after different measurements. It is shown that Nafion/GOx/Fe 3 O 4 MNPs/ZnONFs/Au/PET glucose sensors retained 85.4% of the initial oxidation current after being reused 16 times, respectively. After being reused 19 times, the glucose sensors' oxidation currents decreased to 81.3% of the initial values. This suggests that the sensor's measurement accuracy is maintained at about 80% of the initial state when it is reused about 20 times. When repeatedly used 28 times, the Nafion/GOx/Fe 3 O 4 MNPs/ZnONFs/Au/PET glucose sensors' performance rapidly decayed to 54.7% of the initial values, indicating that the sensor reached the limit of its service life. For the enzyme sensor, the decline of its performance is mainly due to the loss of enzyme and the attenuation of enzyme activity. The covering Nafion ion membrane is an effective way to prevent excessive loss of enzymes. In addition, it should be noted that storing the sensor in a suitable environment can also effectively ensure enzyme activity.

Conclusions
This study presents a simple approach to fabricate an efficient enzymatic glucose sensor based on urchin-like ZnO nanoflowers modified with Fe 3 O 4 MNPs. The excellent conductivity of Fe 3 O 4 MNPs can promote the transfer of redox electrons from a GOx active center to ZnONFs. As nanoscale magnetic particles, the intrinsic peroxidase-like activity of Fe 3 O 4 MNPs can promote the hydrolysis of H 2 O 2 , the intermediate product, and generate more electrons to promote the electron yield. The sensitivity of the Nafion/Fe 3 O 4 MNPs/ZnONFs/Au/PET glucose sensor increased to 4.52 µA·mM −1 ·cm −2 , and the detection limit reached 0.089 µM. The linear range was 0.089-12.5 mM, which could match the requirement of clinical physiological blood glucose detection in the range of 3-8 mM physiological blood glucose level. The as-fabricated glucose sensors exhibited excellent selectivity in a test environment containing organic compounds and inorganic salt. The evaluation of the glucose sensors' service life demonstrated that the device's current response maintained 78.9% of its initial value after reuse 22 times. The results can improve the electrochemical biosensors' performance based on the combination of magnetic nanoparticles and nanostructured matrix materials.