3D Modeling of Silver Doped ZrO2 Coupled Graphene-Based Mesoporous Silica Quaternary Nanocomposite for a Nonenzymatic Glucose Sensing Effects

We described the novel nanocomposite of silver doped ZrO2 combined graphene-based mesoporous silica (ZrO2-Ag-G-SiO2,) in bases of low-cost and self-assembly strategy. Synthesized ZrO2-Ag-G-SiO2 were characterized through X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, Nitrogen adsorption-desorption isotherms, X-ray photoelectron spectroscopy (XPS), and Diffuse Reflectance Spectroscopy (DRS). The ZrO2-Ag-G-SiO2 as an enzyme-free glucose sensor active material toward coordinate electro-oxidation of glucose was considered through cyclic voltammetry in significant electrolytes, such as phosphate buffer (PBS) at pH 7.4 and commercial urine. Utilizing ZrO2-Ag-G-SiO2, glucose detecting may well be finished with effective electrocatalytic performance toward organically important concentrations with the current reaction of 9.0 × 10−3 mAcm−2 and 0.05 mmol/L at the lowest potential of +0.2 V, thus fulfilling the elemental prerequisites for glucose detecting within the urine. Likewise, the ZrO2-Ag-G-SiO2 electrode can be worked for glucose detecting within the interferometer substances (e.g., ascorbic corrosive, lactose, fructose, and starch) in urine at proper pH conditions. Our results highlight the potential usages for qualitative and quantitative electrochemical investigation of glucose through the ZrO2-Ag-G-SiO2 sensor for glucose detecting within the urine concentration.


Introduction
Biosensors are developed for giving symptomatic data for the patient's prosperity status. Electrochemistry, fluorescence, colorimetry, photoelectrochemistry, and chemical luminescence, have been received for glucose sensing [1,2]. Among them, the electrochemical detecting method has gotten high attention due to its high affectability, promising reaction time [3][4][5]. Glucose oxidation on the sensor is dependable for the chemisorption of the hydroxyl group onto the metal oxide and shaping the bond among the d-electron of metal and glucose atoms. The oxidation state of glucose particles is affected by the metal surface as well as metal-glucose interaction, glucose-metal bond quality, and desorption of glucose particles. By considering an imitating method of the enzyme-like component, a few metals and metal oxides like Au, Pt, Cu, Ni, Mn, Co, and Fe [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] have been studied. Limitation of detection (LOD) for the analyte, the typical nanomaterials considered [22]. Graphene has gotten around the world consideration for the improvement of biosensors because graphene-based biosensors have high electron transfer rates, high charge-carrier mobility, and are extremely significant for biomarkers owing to their extraordinary electrochemical (amperometric, voltammetry, impedimetric) response [23,24]. In addition, graphene shows a thickness of the edge-plane-like structure, giving numerous dynamic destinations for electron transfer to chemical and biological species [23]. Graphene containing zirconium oxide (ZrO 2 ) offers a way to upgrade their application by allowing flexible and ideal electrochemical properties, extraordinary potential applications within the broad fields of sensing [25][26][27][28][29][30]. The affectability and conductivity of graphene may be advance upgraded by enhancing Ag NPs owing to their high electron transfer for modifiers in biosensors [31,32]. Biocompatibility, nontoxicity, high conductivity, chemical and steadiness of SiO 2 make to idealize for utilization for adsorption, biosensors [33,34]. With these points, we developed the ZrO 2 -Ag-G-SiO 2 which was effectively synthesized by the self-assembly method. ZrO 2 , G, and SiO 2 have octahedral coordination, Ag occupies on the ZrO 2 -G-SiO 2 displays giving dynamic response for possible charge transfer to electrolyte [35][36][37].
In this consideration, ZrO 2 -Ag-G-SiO 2 nanocomposite was developed main active material for glucose sensing. ZrO 2 -Ag-G-SiO 2 was effectively synthesized by utilizing a basic, low-cost, self-assembly method, and was inspected for nonenzymatic glucose oxidation for the quick response. It shows especially high effectiveness for glucose oxidation counting a greatly low working potential of as it were 0.2 V vs Ag/AgCl. In general, ZrO 2 -Ag-G-SiO 2 affirmed a significant response without any electron facilitator, provoking a novel way for glucose sensing within the urine. The electrochemical sensing behavior of the ZrO 2 -Ag-G-SiO 2 sensor towards glucose sensing was examined utilizing amperometric techniques.

Synthesis of ZrO 2
6.5 g of Pluronic F127 was mixed up in 50 mL of ethanol and zirconium (IV) isopropoxide solution was included in 50 mL of ethanol and ethylene glycol separately with vigorous mixing and added together at 314 K with 50 mL of H 2 O. Hydrochloric acid was included to alter the pH 2.4 and kept at 314 K for 1 h and 354 K in a closed container for 24 h after that dried at 374 K and calcined at 674 K for 5 h.

Synthesis of Silver Doped ZrO 2 (ZrO 2 -Ag)
3.5 g of AgNO 3 was in 50 mL of deionized water. Then ZrO 2 was poured dropwise to solution blended till the gel came out. The gel was dried at 374 K for 3·1⁄2 h, calcined at 674 K, and after that ground to get the ZrO 2 -Ag nanoparticles.
2.4. Synthesis of ZrO 2 -Ag-G 0.33 g of graphite oxide (GO) was scattered into 300 mL of water and ultra-sonicated for 40 min. Sonicated graphene oxide exchanged poured into ZrO 2 -Ag solution and 50 mL of 1 M sodium hydroxide included into the sonicated mixture dropwise for expected pH and blended for 3 h at 374 K. The color turned into coffee color, demonstrates the effective combination of G with Ag combined ZrO 2 arrangement E.

Synthesis of ZrO 2 -Ag-G-SiO 2
For the synthesis of final nanocomposites, 1.1 g of triblock copolymer Pluronic F-127 was included to 100 mL of deionized water and 61 mL of 2 M HCl at 313 K. 4 mL of tetraethyl orthosilicate (TEOS) was included and blended at 314 K for 12·1⁄2 h and heated to 374 K for 20 h after that washed with water and ethanol and dried at 338 K overnight and the copolymer was calcination at 824 K for 3·1⁄2 h. The solution ZrO 2 -Ag-G was drop-by-drop included on 0.3 g of the silica powder and this mixture was blended with 374 K for 24 h and ultrasonicated for 1 1⁄2 h and get the powder, washed with 1.5 mL of methanol, and dried at 338 K overnight. Calcined at 974 K at 283 K/min and held at 974 K for 5 h. Dark color items were found.

Preparation of ZrO 2 -Ag-G-SiO 2 Electrode
The zrO2-Ag-G-SiO2 coated film was prepared using a routine doctor-blade method [38]. For the altered doctor-blade method, we controlled the thickness of ZrO 2 -Ag, ZrO 2 -Ag-GO, ZrO 2 -Ag-GO-SiO 2 . To begin with, synthesized fabric powder (1.1 g) was mixed with Ethylcellulose and acetone (1.5 mL) in a mortar for 15 min. After that, the prepared glues were coated on FTO glass to create a film, after being dried within the open state for 35 min. One drop greasing up oil was put onto the film surface and stabilized beneath 374 K in the dry oven for 25 min to decrease breaks.

Electrochemical Measurements
Cyclic voltammetry (CV) and estimations were performed a three-electrode electrochemical set up to check the current and voltage profiles, where ZrO 2 -Ag, ZrO 2 -Ag-G, ZrO 2 -Ag-G-SiO 2 was utilized as working electrode whereas platinum and Ag/AgCl electrode as counter and reference anodes, individually. Electrochemical properties in commercial urine were utilized with the measured pH 6.0, 6.7, and 6.5, individually. As electrolytes, 0.1 M NaOH, 0.1 M KOH, and Buffer were utilized. The following equation is used to determine the LOD [39][40][41] where SD is the standard deviation of the analyte concentration calculated from the current reaction of progressive including of glucose into the electrolyte; N is the slope of the calibration curve which demonstrates the affectability of the anode with a signal-to-noise ratio 3. Moreover, CV tests were performed from −0.3 to +0.2 V versus Ag/AgCl at a filter rate of 10 mV s −1 . All estimations were carried out by voltammetry (PG201, Potentiostat, Galvanostat, Volta lab TM , Radiometer, Aalborg, Denmark).

Characterization of the ZrO 2 -Ag-G-SiO 2 Sample
The mesoporous semiconductors were anchored on graphene nanosheets since this mesoporous conductive arrangement facilitates electron transport among nanostructure and electrolytes, hence making this a desirable stage for the design of biosensors. Figure 1 illustrates the crystalline characteristic properties of ZrO 2 -Ag, ZrO 2 -Ag-G, and ZrO 2 -Ag-G-SiO 2 samples affirmed by the X-ray diffraction (XRD) technique. Nanomaterials 2022, 11, x FOR PEER REVIEW 4 of 18 rate of 10 mV s −1 . All estimations were carried out by voltammetry (PG201, Potentiostat, Galvanostat, Volta lab TM , Radiometer, Aalborg, Denmark).

Characterization of the ZrO2-Ag-G-SiO2 Sample
The mesoporous semiconductors were anchored on graphene nanosheets since this mesoporous conductive arrangement facilitates electron transport among nanostructure and electrolytes, hence making this a desirable stage for the design of biosensors. Figure  1 illustrates the crystalline characteristic properties of ZrO2-Ag, ZrO2-Ag-G, and ZrO2-Ag-G-SiO2 samples affirmed by the X-ray diffraction (XRD) technique.   Figure 2a showed the morphology of profoundly amplified TEM image of ZrO 2 which distributed as clustered in a flower shape. Morphology of ZrO 2 was also confirmed by SEM image (Inset). Figure 2b showed the TEM image of ZrO 2 -Ag where Ag nanoparticles interconnected with ZrO 2 . Figure 2c revealed the good distribution of ZrO 2 -Ag on the Graphene surface. Figure 2d showed that the ZrO2-Ag G combined with mesoporous SiO 2 . These figures showed that ZrO 2 -Ag-G-SiO 2 were uniformly distributed. Every single TEM image is carried with a corresponding SEM image (Inset). Such flake-like nanostructured geometry leads to a rough surface of the electrode which can expectedly lead to an upgrade of the electrode performance on account of its high surface area, high surface-to-volume ratio, and exposure of more active sites on ZrO 2 -Ag-G-SiO 2 . Figure 2e Figure 2a showed the morphology of profoundly amplified TEM image of ZrO2 which distributed as clustered in a flower shape. Morphology of ZrO2 was also confirmed by SEM image (Inset). Figure 2b showed the TEM image of ZrO2-Ag where Ag nanoparticles interconnected with ZrO2. Figure 2c revealed the good distribution of ZrO2-Ag on the Graphene surface. Figure 2d showed that the ZrO2-Ag G combined with mesoporous SiO2. These figures showed that ZrO2-Ag-G-SiO2 were uniformly distributed. Every single TEM image is carried with a corresponding SEM image (Inset). Such flake-like nanostructured geometry leads to a rough surface of the electrode which can expectedly lead to an upgrade of the electrode performance on account of its high surface area, high surface-tovolume ratio, and exposure of more active sites on ZrO2-Ag-G-SiO2. Figure 2e,f showed the HRTEM image, the lattice space of 0.28 nm was given out to the interplanar of the (111) plane of the ZrO2-Ag-G-SiO2 sample and another lattice space of 0.26 nm was arranged to the interplanar of the (022) plane of the ZrO2-Ag-G-SiO2 sample.
The elemental state of the ZrO2-Ag-G-SiO2 nanoparticles was furthermore analyzed through EDS mapping.
As shown in Figure 3, the composition of ZrO2-Ag-G-SiO2 was presented to confirm the coexistence of Zr, C, Ag, and Si with the evaluated composition within the gravimetric rate of 29% Zr, 35% C, 12% Ag, and 5% Si. The elemental state of the ZrO 2 -Ag-G-SiO 2 nanoparticles was furthermore analyzed through EDS mapping.
As shown in Figure 3, the composition of ZrO 2 -Ag-G-SiO 2 was presented to confirm the coexistence of Zr, C, Ag, and Si with the evaluated composition within the gravimetric rate of 29% Zr, 35% C, 12% Ag, and 5% Si.
Raman spectroscopy was also performed to characterize the G band showing in the composite.
As shown in Figure 4a, the G band of the as-synthesized sample appeared two peaks located at 1331 and 1573 cm −1 corresponding to the (D band) and the C-C bond stretching frequency (G band), individually. For the most part, the intensity ratio of the D-and G bands (ID/IG) is utilized to evaluate the degree of disorder and the average size of sp 2 spaces. In this fact, the value of ID/IG was calculated to be 0.94. Nanomaterials 2022, 11, x FOR PEER REVIEW 6 of 18 Raman spectroscopy was also performed to characterize the G band showing in the composite.
As shown in Figure 4a, the G band of the as-synthesized sample appeared two peaks located at 1331 and 1573 cm −1 corresponding to the (D band) and the C-C bond stretching frequency (G band), individually. For the most part, the intensity ratio of the D-and G bands (ID/IG) is utilized to evaluate the degree of disorder and the average size of sp 2 spaces. In this fact, the value of ID/IG was calculated to be 0.94. The resultant absorbance of UV-DRS is depicted in Figure 4b. The optical bandgap of the ZrO 2 -Ag, ZrO 2 -Ag-G, ZrO 2 -Ag-G-SiO 2 , can be determined by the (Equation (2)): [42] (αhv) where 'α' was the molar assimilation coefficient calculated as α = (1 − R) 2 /2R, hv is the incident light frequency, 'A' is the proportionality constant, and 'E g ' is the bandgap energy of the material.
E VB = E CB + E g (4)   The resultant absorbance of UV-DRS is depicted in Figure 4b. The optical bandgap of the ZrO2-Ag, ZrO2-Ag-G, ZrO2-Ag-G-SiO2, can be determined by the (Equation (2)): [42] ( ℎ ) where 'α' was the molar assimilation coefficient calculated as α = (1 − R) 2 /2R, hʋ is the incident light frequency, 'A' is the proportionality constant, and 'Eg' is the bandgap energy of the material. Table 1 outlines the information form (αhv) 1⁄2 as a function of photon energy. Band gaps showed 3.11 eV for Ag-doped ZrO2-Ag and decrease after combining with graphene turned to 2.61 for the ZrO2-Ag-G eV. Surprisingly, the band gaps change remarkably decreased to 2.00 eV within the ZrO2-Ag-G-SiO2 after combining through Here, E VB and E CB are valence and conduction band edge potentials, individually. c is the electronegativity of the semiconductor, E e is the energy of free electrons on the hydrogen scale and E g is the bandgap energy of the semiconductor.  Figure 4c presents the N 2 adsorption-desorption isotherms of ZrO 2 -Ag-G and ZrO 2 -Ag-G-SiO 2 samples. ZrO 2 -Ag-G and ZrO 2 -Ag-G-SiO 2 samples display typical type IV isotherm, illustrating those materials had mesopores. Isotherms of samples display an H 2 type hysteresis loop at a relative pressure (P/P 0 ) between 0.6 and 0.9, showing that these materials possess large and uniformly distributed mesopores. In addition, the hysteresis loops gradually shift to higher relative pressure (P/P 0 ) from ZrO 2 -Ag-G to ZrO 2 -Ag-G-SiO 2 , proposing that these mesopores were extending with the counting mesoporous SiO 2 . A mesopore diameter as large as 5.67 nm finds out the ZrO 2 -Ag-G sample. When combining through mesoporous SiO 2 it proceeds to extend up to 8.96 nm as well as BET surface area also expanded from 8.66 to 9.17 m 2 g −1 , individually ( Table 1). The electrochemical properties of nanocomposites were correlated with the BJH and BET analysis results. From BET analysis, the total pore volume and mean pore diameter of sensor active material are reduced due to the oxidizing agent treatment. According to the summary results of BET and BJH, the surface area and total pore mass of the graphene increased with SiO 2 . The mesopore state and high surface area are the main parameters that are valuable for framing ion-transport tunnels in electrochemical reactions.
In magnetic field determination, one-level effective mass approximation (EMA) is utilized for a basic non-degenerate energy band. Bloch electrons in an energy band are treated as free electrons with the free electron mass m0 replaced by the effective mass m*. The Schrodinger equation for the function of the conduction electron in electric and magnetic fields can be shown with the following equation [43]. Figure 4d confirms the magnetic field curve of the ZrO 2 -Ag-G-SiO 2 samples measured at ambient temperature. The saturation magnetization (MS), which is determined by the plot of M versus 1/H using data at low magnetic fields, is observed to be 0.0036.5 emu g −1 to 0.0046.5 emu g −1 .
For characterizing detailed surface chemical compositions of ZrO 2 -Ag-G-SiO 2 , XPS analysis was performed.
The results are revealed in Figure 5. The complete spectrum of ZrO 2 -Ag-G-SiO 2 shows the presence of Si, Zr, C, Ag, and O atoms attributed to the effective modification. The corresponding high-resolution spectra with respect to C1s signal 284.5 eV as a reference binding energy in Figure S1b attributed to C-C, bonds of graphene. As existing in Figure S1c, Si2P peaks were found at 102.8 eV. These peaks located at 184.08 eV correspond to Zr3d in Figure  S1d. Besides, the interaction of the carbonyl group and hydroxyl group were also confirmed in O1s existing in Figure S1e with the binding energy at 531.6 eV corresponding to C-O bonds. Finally, the peaks at 367.0 eV and 373.1 eV revealed in Figure S1f correspond to Ag 3d. The overall results of the XPS study confirmed that all surface chemical compositions of ZrO 2 -Ag-G-SiO 2 were found in the as-prepared nanocomposite.
The corresponding high-resolution spectra with respect to C1s signal 284.5 eV as a reference binding energy in Figure S1b attributed to C-C, bonds of graphene. As existing in Figure S1c, Si2P peaks were found at 102.8 eV. These peaks located at 184.08 eV correspond to Zr3d in Figure S1d. Besides, the interaction of the carbonyl group and hydroxyl group were also confirmed in O1s existing in Figure S1e with the binding energy at 531.6 eV corresponding to C-O bonds. Finally, the peaks at 367.0 eV and 373.1 eV revealed in Figure S1f correspond to Ag 3d. The overall results of the XPS study confirmed that all surface chemical compositions of ZrO2-Ag-G-SiO2 were found in the as-prepared nanocomposite.

Electrocatalytic Activity of the ZrO2-Ag-G-SiO2 Electrode towards Glucose Sensing
The electrochemical tests for working electrodes, ZrO2-Ag, ZrO2-Ag-G, ZrO2-Ag-G-SiO2 were performed in a three-electrode cell system with Pt wire as counter electrode and Ag/AgCl as a reference electrode within the potential range of −0.3 to +0.3 V. Figure 6a presents the CV profile of electrochemical responses in 10.5 mL of commercial urine and different electrolytes without glucose.

Electrocatalytic Activity of the ZrO 2 -Ag-G-SiO 2 Electrode towards Glucose Sensing
The electrochemical tests for working electrodes, ZrO 2 -Ag, ZrO 2 -Ag-G, ZrO 2 -Ag-G-SiO 2 were performed in a three-electrode cell system with Pt wire as counter electrode and Ag/AgCl as a reference electrode within the potential range of −0.3 to +0.3 V. Figure 6a presents the CV profile of electrochemical responses in 10.5 mL of commercial urine and different electrolytes without glucose.
There was a poor oxidation peak observed in Figure 6a in the absence of glucose. In contrast, ZrO 2 -Ag, ZrO 2 -Ag-G, ZrO 2 -Ag-G-SiO 2 electrodes showed a well-defined oxidation peak at the potential of +0.2 V. By adding 0.05 mmol/L of glucose, a poor response was noticed with the ZrO 2 -Ag rather than ZrO 2 -Ag-G and ZrO 2 -Ag-G-SiO 2 electrode in the presence of glucose, due to the high bandgap energy of ZrO 2 . After combining with Ag nanoparticle and graphene, the bandgap energy-reduced and ZrO 2 -a supporting material for ZrO 2 , rapidly transporting electrons during the electrochemical reaction due to their good conductive property. ZrO 2 -Ag-G-SiO 2 electrode exhibited a substantial increase in anodic current density 4.0 × 10 −3 mAcm −2 as showed in Figure 6b. For varying electrolytes such as 0.1 M phosphate buffer, NaOH, KOH, significant and fast current responses 9.0 × 10 −3 mAcm −2 were observed for the ZrO 2 -Ag-G-SiO 2 electrode with the addition of 0.55 mmol/L glucose as presented in Figure 6c. The obtained result clearly recommends the oxidation peak corresponds to the electro-oxidation of glucose at the ZrO 2 -Ag-G-SiO 2 electrode [44]. Thus, a mechanism of non-enzymatic glucose sensing on the ZrO 2 -Ag-G-SiO 2 electrode is clarified in Scheme 1. There was a poor oxidation peak observed in Figure 6a in the absence of glucose. In contrast, ZrO2-Ag, ZrO2-Ag-G, ZrO2-Ag-G-SiO2 electrodes showed a well-defined oxidation peak at the potential of +0.2 V. By adding 0.05 mmol/L of glucose, a poor response was noticed with the ZrO2-Ag rather than ZrO2-Ag-G and ZrO2-Ag-G-SiO2 electrode in the presence of glucose, due to the high bandgap energy of ZrO2. After combining with Ag nanoparticle and graphene, the bandgap energy-reduced and ZrO2-Ag-G, ZrO2-Ag-G,  . For varying electrolytes such as 0.1 M phosphate buffer, NaOH, KOH, significant and fast current responses 9.0 × 10 −3 mAcm −2 were observed for the ZrO2-Ag-G-SiO2 electrode with the addition of 0.55 mmol/L glucose as presented in Figure 6c. The obtained result clearly recommends the oxidation peak corresponds to the electro-oxidation of glucose at the ZrO2-Ag-G-SiO2 electrode [44]. Thus, a mechanism of non-enzymatic glucose sensing on the ZrO2-Ag-G-SiO2 electrode is clarified in Scheme 1.

Scheme 1.
The glucose-sensing mechanism of the ZAGS sample.
To demonstrate the analytical parameters (for example sensitivity, linear range, detection limit, and response time), the amperometric response of the ZrO2-Ag-G-SiO2 electrode was performed at a fixed voltage of +0.2 V (versus Ag/AgCl) in 0.1 M PBS by stepwise adding of glucose at different concentration. A well-defined and fast response to the ZrO2-Ag-G-SiO2 electrode was observed. Figure 6d confirms the current response which was estimated to be as high as 5.0 × 10 −3 mA cm −2 at lower glucose concentration (0.05 mmol/L to 0.35 mmol/L). By adding glucose, the current response quickly reached a steady-state and attains ~98% of response within 1 s. The response current was linearly increased with increasing glucose concentration, ZrO2-Ag-G-SiO2 electrode exhibited high sensitivity in the linear range (0.05 mmol/L to 0.35 mmol/L). Figure S4 displays the ZrO2-Ag-G-SiO2 glucose sensor calibration curve and cyclic voltammogram. With a linear range of 150-350 L and a correlation coefficient (R) of 0.996, the calibration curve indicated excellent linearity. Table S1 compares the detecting characteristics of several electrochemical glucose sensors, differentiating LOD and linear range. For glucose oxidation, the ZrO2-Ag-G-SiO2 sensor has a better linear response range and detection limit. The nanostructure of ZrO2-Ag-G-SiO2 offered greater surface area and exposed active sites, which facilitated electrolyte transport from solution to all active sites [45][46][47][48][49]. To demonstrate the analytical parameters (for example sensitivity, linear range, detection limit, and response time), the amperometric response of the ZrO 2 -Ag-G-SiO 2 electrode was performed at a fixed voltage of +0.2 V (versus Ag/AgCl) in 0.1 M PBS by stepwise adding of glucose at different concentration. A well-defined and fast response to the ZrO 2 -Ag-G-SiO 2 electrode was observed. Figure 6d confirms the current response which was estimated to be as high as 5.0 × 10 −3 mA cm −2 at lower glucose concentration (0.05 mmol/L to 0.35 mmol/L). By adding glucose, the current response quickly reached a steady-state and attains~98% of response within 1 s. The response current was linearly increased with increasing glucose concentration, ZrO 2 -Ag-G-SiO 2 electrode exhibited high sensitivity in the linear range (0.05 mmol/L to 0.35 mmol/L). Figure S4 displays the ZrO 2 -Ag-G-SiO 2 glucose sensor calibration curve and cyclic voltammogram. With a linear range of 150-350 L and a correlation coefficient (R) of 0.996, the calibration curve indicated excellent linearity. Table S1 compares the detecting characteristics of several electrochemical glucose sensors, differentiating LOD and linear range. For glucose oxidation, the ZrO 2 -Ag-G-SiO 2 sensor has a better linear response range and detection limit. The nanostructure of ZrO 2 -Ag-G-SiO 2 offered greater surface area and exposed active sites, which facilitated electrolyte transport from solution to all active sites [45][46][47][48][49].
Overall, the enhanced sensing performance of the non-enzymatic glucose sensor is ascribed to the direct growth of mesoporous ZrO 2 -Ag-G-SiO 2 thin film on FTO electrodes which offers a high surface area for ZrO 2 modification, resulting in fast electron transfer during the electrochemical process of glucose oxidation occurring between electrolyte and electrode. Importantly, we have used the self-assembly method to fabricate nonenzymatic ZrO 2 -Ag-G-SiO 2 glucose-sensing electrodes which account for controllable nanostructures with great reproducibility and a cost-effective fabrication process for stable glucose sensing devices.

Selection of Electrolytes towards ZrO 2 -Ag-G-SiO 2 Electrode
Sensing of glucose by ZrO 2 -Ag-G-SiO 2 sample with different electrolytes (PBS, NaOH; KOH) and different concentrations was investigated under ambient conditions. Glucose oxidation with these electrolytes was measured in 0.1 M NaOH, phosphate buffer, and KOH by subsequent addition of 0.55 mmol/L of glucose at regular intervals and observed the current responses after every injection. Figure 7a shows that when 0.55 mmol/L of glucose adding to different concentrations of electrolytes resulted in almost the best current density towards the phosphate buffer electrolytes. The current state of the ZrO 2 -Ag-G-SiO 2 electrode greatly depends on glucose concentration and electrolyte pH (i.e., the amount of OH − ), since OHare required to neutralize the protons generated during the dehydrogenation stage of the reaction. Hence, a better outcome is confirmed towards phosphate buffer for the ZrO 2 -Ag-G-SiO 2 electrode. matic ZrO2-Ag-G-SiO2 glucose-sensing electrodes which account for controllable nanostructures with great reproducibility and a cost-effective fabrication process for stable glucose sensing devices.

Selection of Electrolytes towards ZrO2-Ag-G-SiO2 Electrode
Sensing of glucose by ZrO2-Ag-G-SiO2 sample with different electrolytes (PBS, NaOH; KOH) and different concentrations was investigated under ambient conditions. Glucose oxidation with these electrolytes was measured in 0.1 M NaOH, phosphate buffer, and KOH by subsequent addition of 0.55 mmol/L of glucose at regular intervals and observed the current responses after every injection. Figure 7a shows that when 0.55 mmol/L of glucose adding to different concentrations of electrolytes resulted in almost the best current density towards the phosphate buffer electrolytes. The current state of the ZrO2-Ag-G-SiO2 electrode greatly depends on glucose concentration and electrolyte pH (i.e., the amount of OH − ), since OHare required to neutralize the protons generated during the dehydrogenation stage of the reaction. Hence, a better outcome is confirmed towards phosphate buffer for the ZrO2-Ag-G-SiO2 electrode. As described above, the sensitivity and linear range of glucose sensing can be found by plotting peak current density against glucose concentrations as shown in Figure 7d. In 0.05 mmol/L glucose concentration, the sensor response had a sensitivity of 4.0 × 10 −3 mA cm −2 and 0.35 mmol/L glucose concentration, the sensor response had a sensitivity of 5.0 × 10 −3 mAcm −2 . Here we can see that the response range is proportional to the concentra- As described above, the sensitivity and linear range of glucose sensing can be found by plotting peak current density against glucose concentrations as shown in Figure 7d. In 0.05 mmol/L glucose concentration, the sensor response had a sensitivity of 4.0 × 10 −3 mA cm −2 and 0.35 mmol/L glucose concentration, the sensor response had a sensitivity of 5.0 × 10 −3 mA cm −2 . Here we can see that the response range is proportional to the concentration range. So, after observing this ratio we can easily reach this decision that we can measure a diabetic urine sample with this sensor for qualitative and quantitative analysis.

Anti-Interference Ability of the ZrO 2 -Ag-G-SiO 2 Sensor
The anti-interference ability of non-enzymatic-based glucose sensing devices is a major challenge, which could affect the electrode's sensing performance. To check the selectivity of ZrO 2 -Ag-G-SiO 2 electrode in the presence of interfering species (such as Vitamin C, Starch, Lactose, Fructose, NaCl, KCl, and Urea), the amperometric response of the sensing electrode was checked by adding 0.91 mmol/L glucose and each above mentioned interfering species was in same concentration in the 0.1 M PBS solution at +0.2 V (versus Ag/AgCl), shown in Figure 7b. The addition of 0.91 mmol/L glucose leads to a rapid current response, although interfering species addition exhibited negligible current responses. As shown in the histogram of each interfering species addition and current response is shown here, which confirms the negligible current responses compared to 0.91 mmol/L glucose. These results suggest the suitability of the ZrO 2 -Ag-G-SiO 2 electrode for the selective sensing of glucose in real samples. This confirmed that the ZrO 2 -Ag-G-SiO 2 electrode was selective towards glucose without being affected by interferences. This enhanced sensing performance is basically attributed to a great interaction among the nanostructure and electrode with the high surface area for catalytic sites, facilitating a suitable path for electron transport during electrochemical activity. The results obtained with the proposed method were compared with other methods for the detection of glucose (Table S1). Overall, the ZrO 2 -Ag-G-SiO 2 electrodes can be envisioned as a promising design for non-enzymatic glucose measurement in real clinical samples which may gain considerable benefits for different biomolecules sensing.

Discussion
The electrocatalytic properties of ZrO 2 -Ag-G-SiO 2 were examined toward applications involving physiological pH, such as the detection of Glucose. Considering that glucose can be oxidized to gluconolactone (Scheme 1) at a neutral pH via a two-electron electrochemical reaction [50][51][52]. However, an excellent response was observed with the ZrO 2 -Ag-G-SiO 2 sensor in the presence of glucose. This can be attributed to the excellent electrocatalytic nature of ZrO 2 , which mediates the heterogeneous chemical oxidation or reduction of the glucose, while the converted ZrO 2 can be continuously and simultaneously recovered by electrochemical oxidation or reduction due to their high surface to volume ratio [51]. Additionally, in our sensor, the Ag-G-SiO 2 works as a supporting material for ZrO 2 , rapidly transporting electrons during the electrochemical reaction due to their good conductive property. Also, the less dense morphology of the Ag-G-SiO 2 provides better permeability of the sensing matrix to the solution. The possible electrochemical reactions involved in glucose oxidation through the Zr 4+ /Zr 3+ centers of ZrO 2 are given below [51]: 2Zr 3+ → 2Zr 4+ + 2e − Therefore, electrooxidation of glucose on ZrO 2 -Ag-G-SiO 2 for the nonenzymatic detection of glucose at physiological pH was investigated.

Conclusions
We developed a simple approach for producing ZrO 2 -Ag-G-SiO 2 using the facile self-assembly method, producing a catalyst-coated and binder-free composite electrode. The ZrO 2 -Ag-G-SiO 2 exhibited a uniform and highly mesoporous network of the catalytic film. Also, multiple active sites in ZrO 2 -Ag-G-SiO 2 along with enhanced conductivity of graphene oxide improved the electrocatalytic performance of this electrode toward glucose oxidation. The ultra-high sensitivity (9.0 × 10 −3 mA cm −2 ) at a low applied potential of only 0.2 V versus Ag/AgCl, wide linear range (0.05 mmol/L-0.35 mmol/L), low sensing limit (0.05 mmol/L), with impressive qualitative and quantitative analysis, selectivity and stability make this ZrO 2 -Ag-G-SiO 2 a promising electrode to serve as a non-enzymatic glucose sensor. Based on the results, ZrO 2 -Ag-G-SiO 2 provided an excellent sensitivity in commercial urine specimens, so this biosensor is believed to have a high possibility for practical use.