Electrochemical Sensing of H2O2 by Employing a Flexible Fe3O4/Graphene/Carbon Cloth as Working Electrode

We report the synthesis of Fe3O4/graphene (Fe3O4/Gr) nanocomposite for highly selective and highly sensitive peroxide sensor application. The nanocomposites were produced by a modified co-precipitation method. Further, structural, chemical, and morphological characterization of the Fe3O4/Gr was investigated by standard characterization techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) and high-resolution TEM (HRTEM), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS). The average crystal size of Fe3O4 nanoparticles was calculated as 14.5 nm. Moreover, nanocomposite (Fe3O4/Gr) was employed to fabricate the flexible electrode using polymeric carbon fiber cloth or carbon cloth (pCFC or CC) as support. The electrochemical performance of as-fabricated Fe3O4/Gr/CC was evaluated toward H2O2 with excellent electrocatalytic activity. It was found that Fe3O4/Gr/CC-based electrodes show a good linear range, high sensitivity, and a low detection limit for H2O2 detection. The linear range for the optimized sensor was found to be in the range of 10–110 μM and limit of detection was calculated as 4.79 μM with a sensitivity of 0.037 µA μM−1 cm−2. The cost-effective materials used in this work as compared to noble metals provide satisfactory results. As well as showing high stability, the proposed biosensor is also highly reproducible.

Recently, NCs developed using carbon cloth aligned with different nanoparticles for various electrochemical applications. Carbon fiber cloth or/carbon cloth (CFC or/CC) is an NPs of Pd-Pt-Fe 3 O 4 are excellent for the detection of H 2 O 2 in the real sample as well as in the biomedical field [43]. Xu et al. employed the dual mode sensor (colorimetric sensor and electrochemical sensor) for the detection of H 2 O 2 in the linear range from 1.0 µM to 120 µM. They prepared the NCs of iron oxide-molybdenum disulfide-gold (Fe 3 O 4 -MoS 2 -Au). The limit of detection from electrochemical (EC) and colorimetric sensing was 0.08 µM and 0.109 µM, respectively [44].
Iron oxide NPs, despite their large surface area, tend to aggregate due to large magnetic dipole interaction. Therefore, iron oxide NPs-based sensors show poor sensing performance [45]. Iron oxide nanoparticles (NPs) with carbon-based compounds result in excellent biosensor applications. Due to the unique electrochemical properties of various nano allotropes of carbon reinforced with Fe 3 O 4 NPs, they show better performance for the detection of H 2 O 2 . Zhao et al. [46] have demonstrated a real sample H 2 O 2 sensor based on 3D graphene (Gr)-supported Fe 3 O 4 quantum dots. H 2 O 2 released from living cells is in very small quantities so it is a huge challenge to detect in situ detection of H 2 O 2 . The Fe 3 O 4 /3DGr NCs was found to be very efficient in situ detection with an outstanding reproducibility and very high sensitivity and selectivity of 274.15 mAM −1 cm −2 . The sensor also revealed LOD of~78 nm and a fast response time of 2.8 s. In a similar study, Yousefinejad et al. [47] have proposed a novel NCs for the detection of H 2 O 2 in nanomolar range. The proposed NCs comprise carbon dots and iron oxide (Fe 3 O 4 ) NPs. The LOD for hydrogen peroxide was determined to be 1.0 × 10 −9 M and linear range was found to be from 1.0 × 10 −8 M to1.0 × 10 −3 M.
Cai et al. have prepared the ternary NCs materials of GO (graphene oxide, Fe 3 O 4 and PG (pristine graphene) and analyzed by transmission electron microscope (TEM) which clears that the NPs of Fe 3 O 4 are arranged on the sheets of PG and GO. They employed this sensor for the detection of H 2 O 2 and dopamine and found the LOD of 90 nM for hydrogen peroxide and 180 nM for dopamine [48]. Zhao et al. have synthesized a sensor by spraying the particles of platinum on NCs of Fe 3 O 4 /rGO and checked the electrochemical performance peroxide sensing. This sensor is very fast and sensitive for the detection of H 2 O 2 and also very stable and compatible as compared to the other reported sensors. The limit of detection was 1.58 × 10 −6 M, sensitivity was 6.9 µA mM −1 [49]. Cai et al. also made the ternary materials of GO (graphene oxide), PG (pristine graphene), and Fe 3 O 4 by co-precipitation method and checked by SEM and XPS. With the help of SEM, it concludes that the ternary compound of GO, PE, and Fe 3 O 4 has outstanding performance for the detection of dopamine as compared to the binary compound of Go and Fe 3 O 4 . The finding limit of detection by this sensor was about 370 nM [50]. Hence, carbonaceous materials provide an excellent platform for biosensors due to their excellent thermal conductivity, high charge mobility and extraordinary electrochemical catalytic activity and shielding properties [51] Korea. HCl (35%) and H 2 O 2 were purchased from Sinopharm Chemical Reagent Co., Ltd., (Shanghai, China). Polymeric carbon fiber cloth was purchased from AvCarb Material Solutions (Lowell, MA, USA) and polymeric carbon fiber cloth (pCFC or CC) with a thickness of 356 microns and basis weight of 132 g/m 2 , grade of HCB with a plain weave construction.

Synthesis of Nanocomposite
Nanocomposite was synthesized by conventional chemical co-precipitation method. First, in a separate beaker, both salts of iron (Fe 2+ /Fe 3+ ) were taken in 1:2 ratios and dissolved in distilled water to obtain a clear aqueous solution. Second, ultrasonic treatment was given to as-received graphene powder in distilled water for 3 h and solid graphene sample was collected from aqueous solution using a centrifuge at 3000× g RPM. Both samples of iron oxide were transferred into the aqua treated graphene suspension and the mixtures were stirred for 30 min. A 2M NaOH aqueous solution was added into the mixture drop by drop with continuous stirring until the whole solution turns into black precipitates. Additionally, 10 mL of hydrazine hydrate was added to the solution to stop the oxidation of graphene into graphene oxide during NCs preparation. The precipitates obtained from nanocomposite were washed several times by using distilled water and dried in an oven at 70-80 • C for 4-5 h. A schematic representation of Fe 3 O 4 /Gr NCs fabrication is illustrated in Scheme 1.

Materials Used in Synthesis
To obtain Fe 3+ /Fe 2+ ions we have used 99.9% pure ferric chloride hexahydrate and ferrous sulfate heptahydrate salt which was procured from Sigma-Aldrich (St. Louis, MO, USA). Graphene nanopowder was purchased from Iljin Nano Tech, Seoul, Republic of Korea. HCl (35%) and H2O2 were purchased from Sinopharm Chemical Reagent Co., Ltd., (Shanghai, China). Polymeric carbon fiber cloth was purchased from AvCarb Material Solutions (Lowell, MA, USA) and polymeric carbon fiber cloth (pCFC or CC) with a thickness of 356 microns and basis weight of 132 g/m 2 , grade of HCB with a plain weave construction.

Synthesis of Nanocomposite
Nanocomposite was synthesized by conventional chemical co-precipitation method. First, in a separate beaker, both salts of iron (Fe 2+ /Fe 3+ ) were taken in 1:2 ratios and dissolved in distilled water to obtain a clear aqueous solution. Second, ultrasonic treatment was given to as-received graphene powder in distilled water for 3 h and solid graphene sample was collected from aqueous solution using a centrifuge at 3000× g RPM. Both samples of iron oxide were transferred into the aqua treated graphene suspension and the mixtures were stirred for 30 min. A 2M NaOH aqueous solution was added into the mixture drop by drop with continuous stirring until the whole solution turns into black precipitates. Additionally, 10 mL of hydrazine hydrate was added to the solution to stop the oxidation of graphene into graphene oxide during NCs preparation. The precipitates obtained from nanocomposite were washed several times by using distilled water and dried in an oven at 70-80 °C for 4-5 h. A schematic representation of Fe3O4/Gr NCs fabrication is illustrated in Scheme 1.

Characterization
The investigation of nanocomposite was done by X-ray diffraction using Cu Kα radiation (λ = 1.54156 Å) with D8AαS advanced X-ray diffractometer to obtain phase and crystallite size. Particle size and morphology were observed by scanning electron microscopy (FESEM, JEOL, JSM-7600F, Tokyo, Japan). The chemical bonding characteristics were explored using Fourier transform infrared (FTIR) spectroscopy (ATR-FT-IR model Nicolet IS 10). For TEM images, a transmission electron microscope (TEM/HRTEM, JEOL, JEM-2100F) was used and operated at 120 kV for nanocomposite samples. A X-ray photoelectron spectroscopy was carried out by ESCALAB250 equipment outfitted with an Al K X-ray source.

Electrode Fabrication Process
For the fabrication of working electrodes, the surface of the CC is washed with ethanol/water under sonication and dried at room temperature. The 100 mg powder of each material (GO, Gr, and Fe 3 O 4 /Gr) is dispersed individually in the 1 mL of Nafion solution (binder) in order to generate a well dispersed slurry. The prepared slurry was dropped carefully onto the CC (1 cm × 2 cm) with a marked area of 1 cm 2 and dried at room temperature. The fabricated electrodes were denoted as GO/CC, Gr/CC, and Fe 3 O 4 /Gr/CC and results were compared with bare CC. The fabricated flexible electrodes such as GO/CC, Gr/CC, and Fe 3 O 4 /Gr/CC were used as the working electrodes for the electrochemical measurements. The electrochemical tests were carried out on a potentiostat (VersaSTAT 3, Princeton Research, Princeton, NJ, USA) with a standard three electrode system. The Ag/AgCl was used as a reference electrode (filled with 3.0 M KCl) and a platinum gauge as a counter electrode.

X-ray Diffraction (XRD)
From the XRD patterns, a significant peak of graphene was observed at two theta angles of 26.49 • corresponding to the (002) plane of graphene [52], as represented in Figure 1a. The rest of the peaks are related to iron oxide which were observed at 30.

Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX)
The morphology of graphene and Fe3O4/Gr NCs was studied and represented in Figure 1. The Figure 1b shows the SEM image of graphene. The SEM image was taken with a magnification of 15,000 times under an electron beam accelerated with a 20 kV. The image shows multiple graphene sheets spread over each other. At the edge, many aggregates of graphene sheets with multi thick layers are visible. There may be some functional groups found attached on the surface of graphene sheets. It may be due to the fact that during the synthesis process some functional groups such as (−OH and −COOH) still get attached

Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX)
The morphology of graphene and Fe 3 O 4 /Gr NCs was studied and represented in Figure 1. The Figure 1b shows the SEM image of graphene. The SEM image was taken with a magnification of 15,000 times under an electron beam accelerated with a 20 kV. The image shows multiple graphene sheets spread over each other. At the edge, many aggregates of graphene sheets with multi thick layers are visible. There may be some functional groups found attached on the surface of graphene sheets. It may be due to the fact that during the synthesis process some functional groups such as (−OH and −COOH) still get attached and they can tune the electronic and chemical properties of graphene [54]. The SEM image in Figure 1c shows that at low magnification, several nanosheets of graphene can be seen decorated with Fe 3 O 4 NPs. The spherical shape of Fe 3 O 4 NPs in the form of clusters is grafted onto the nanosheets of graphene. Further analysis shows the random distribution of various clusters of Fe 3 O 4 NPs on the graphene (Figure 1d). The nanospheres of iron oxide are spread on the surface of graphene sheets individually and most of the particles are aggregated. These clusters are helpful during electrochemical reactions by providing a more reactive site and extending the link of the Fe 3 O 4 nanospheres with the analytes [55]. Additionally, SEM image, FTIR and RAMAN spectra are presented in Figure 2. nanocomposite. The elemental composition for Fe, O, and C atoms was obtained and presented in Table 1.  NCs of iron oxide and graphene were analyzed using FTIR to identify their chemical The standard method for measuring the elemental make-up and composition of materials in the scanning or transmission electron microscope (SEM/TEM) is energy dispersive Xray spectrometry (EDXS). EDS is typically used in place of the frequently abbreviated EDXS. An elemental breakdown of the Fe 3 O 4 /Gr nanocomposite is shown in the energy-dispersive X-ray spectra (Figure 1d,e). It thus validates the formation of Fe 3 O 4 /Gr nanocomposite. The elemental composition for Fe, O, and C atoms was obtained and presented in Table 1.

Fourier Transform Infrared Spectroscopy (FTIR)
NCs of iron oxide and graphene were analyzed using FTIR to identify their chemical structure. Figure 2a presents the FTIR spectra of graphene where certain peaks are visible due to the attached functional group. However, peaks at 731 cm −1 , 1492 cm −1 , and 1722 cm −1 appeared with weak intensity which are attributed to the C=C bending, C-H bending, and C=O stretching, respectively [56][57][58]. The other strong peak is observed at 2345 cm −1 which is attributed to the O=C=O and another band appears at 3684 cm −1 which is attributed to O-H stretching. However, these bands are due to the partial oxidation of graphene-to-graphene oxide [59]. But most of the peaks which are due to partial oxidation of graphene, vanished in Fe 3 O 4 /Gr nanocomposite. In Figure 2b, the FTIR spectrum displays the peaks of nanocomposites (Fe 3 O 4 /Graphene) at different wavenumbers. Absorption peak of Fe 3 O 4 /graphene nanocomposites is observed at 538 cm −1 , 1105 cm −1 , 1592 cm −1 , 2259 cm −1 , and 2833 cm −1 . First peak appeared at 538 cm −1 denoted the stretching vibration mode of Fe-O [53,60]. Two broad absorption peaks appear at 2833 cm −1 due to the stretching of C-H mode [61] and at 1105 cm −1 represents the stretching vibration of C=C mode [53,62] while another peak appear at 1592 cm −1 also denoted the stretching vibration mode of C=C [53,63,64]. The peak observed at 2259 cm −1 specifying the stretch vibration mode of C≡C [65]. Thus, corresponding peaks are related to the FTIR spectra of Fe 3 O 4 /Gr NCs and differ from the peaks of Gr/GO used as precursor in the synthesis of NCs. Figure 2c presents the RAMAN spectroscopy of graphene. RAMAN analysis is a helpful technique which provides eminence features. The peak intensities, positions, and shapes of the curve give useful information in graphene and related materials [66]. From Figure 2c, we observe three different peaks appearing at 1338 cm −1 , 1571 cm −1 , and 2688 cm −1 . The bands appearing at 1338 cm −1 and 2688 cm −1 are assigned to the D band and 2D band respectively. The band appears at 1571 cm −1 is assigned to the G band. The Id/Ig ratio was found to be less than 1 which shows that graphene has less defects [66,67]. The intensity of the 2D band is observed to be less than D and G band which explains that the graphene used in this study is the multilayer graphene (MLG) [66,67]. Figure 3 represents the TEM and HRTEM images of the Fe 3 O 4 /Gr matrix which analyzes the particle size and morphology of iron oxide nanoparticles. Figure 3a shows the spherical shaped Fe 3 O 4 nanoparticles decorated on the graphene sheets ranging from 5-10 nm in size. Figure 3b demonstrates the Fe 3 O 4 nanoparticles encircled at a point, presented in yellow color while at other points a rectangle is marked which represents the graphene sheets. Figure 3c, d is the enlargement of the point which is marked for Fe 3 O 4 and graphene sheets respectively. The interplanar lattice space calculated for the Fe 3 O 4 nanoparticles was found to be 0.25 nm while for graphene sheets it was calculated 0.34 nm.

TEM and HRTEM Analysis
spherical shaped Fe3O4 nanoparticles decorated on the graphene sheets ranging from 5-10 nm in size. Figure 3b demonstrates the Fe3O4 nanoparticles encircled at a point, presented in yellow color while at other points a rectangle is marked which represents the graphene sheets. Figure 3c, d is the enlargement of the point which is marked for Fe3O4 and graphene sheets respectively. The interplanar lattice space calculated for the Fe3O4 nanoparticles was found to be 0.25 nm while for graphene sheets it was calculated 0.34 nm.

X-ray Photoelectron Spectroscopy (XPS)
XPS survey scan of Fe 3 O 4 /Gr nanocomposites is shown in Figure 4a. The presence of peaks related to carbon, oxygen, and iron in the prepared material of Fe 3 O 4 /Gr nanocomposites confirms the nanocomposite synthesis. Figure 4b shows that there are 2p core levels of iron, such as, Fe 2p 1/2 with binding energy of 723.9 eV and Fe 2p 3/2 with binding energy of 710 eV. In Fe 3 O 4 , Fe exists in two states i.e., Fe 2+ and Fe 3+ , so the peak observed at 710.2 eV (red color) corresponds to Fe 2+ states and the peak observed at 712.6 eV (cyan color) corresponds to Fe 3+ in Fe 2p 3/2 . Moreover, at 718 eV (green color), a satellite peak was observed, indicating that there is partial oxidation of Fe 3 O 4 to Fe 2 O 3 [68,69]. Figure 4c, d shows the spectrum of 1s core level of oxygen and carbon, respectively. In Figure 4c the peak of the oxygen atom appears at 530.2 eV (red color) attributed to the Fe-O bond and at 531.2 eV (green color) attributed to O-H bonds. The scan of C 1s core level spectrum is shown in Figure 4d. The peak in C 1s is obtained at 284.7 eV (red color) corresponding to C=C sp 2 carbon atom and at 285.3 eV (green color) attributed to C-C sp 3 atom. Moreover, the peak appearing at 287.5 eV (blue color) corresponds to C-O bonds which may be due to the functionalized group attached on graphene [70][71][72]. The XPS data also show that no reaction takes place between graphene nanosheets and iron oxide nanoparticles [11].
peak of the oxygen atom appears at 530.2 eV (red color) attributed to the Fe-O bond and at 531.2 eV (green color) attributed to O-H bonds. The scan of C 1s core level spectrum is shown in Figure 4d. The peak in C 1s is obtained at 284.7 eV (red color) corresponding to C=C sp 2 carbon atom and at 285.3 eV (green color) attributed to C-C sp 3 atom. Moreover, the peak appearing at 287.5 eV (blue color) corresponds to C-O bonds which may be due to the functionalized group attached on graphene [70][71][72]. The XPS data also show that no reaction takes place between graphene nanosheets and iron oxide nanoparticles [11].

Fabrication of Fe 3 O 4 /Gr/CC and Sensing toward H 2 O 2
At room temperature, the fabrication of Fe 3 O 4 /Gr/CC flexible electrode was carried out and the schematic representation is shown in Scheme 2 and the detailed process is described in Section 2.4.

Fabrication of Fe3O4/Gr/CC and Sensing toward H2O2
At room temperature, the fabrication of Fe3O4/Gr/CC flexible electrode was carried out and the schematic representation is shown in Scheme 2 and the detailed process is described in Section 2.4.  Additionally, we looked into the electrochemical capabilities of the Fe3O4/Gr/CC for sensing of H2O2 (Figure 5c). Figure 5c displays the CV curves of the Fe3O4/Gr/CC and CC in 0.1 M PBS with a pH of 7.0 and a scan rate of 0.08 V s −1 . For the bare CC, a poor current response was observed. For the Fe3O4/Gr/CC, an improved current response toward the detection of H2O2 that may arise due to the reduction/oxidation was observed. This high Using CV, it was also investigated how different H 2 O 2 concentrations affected the electrochemical performance of Fe 3 O 4 /Gr/CC (Figure 6a). With a scan rate of 0.08 Vs −1 , we were able to get various CV curves of Fe 3 O 4 /Gr/CC in the presence of varying concentrations of H 2 O 2 from 10 µM to 110 µM (Figure 6a,b). The observations obviously demonstrated that the current response for reduction has increased at each addition of H 2 O 2 . The calibration plot for linearity check between the peak current response and concentration of H 2 O 2 revealed that this improved current response was linear (Figure 6c). The linear equation was observed as y = −3.70091E −5 x −0.00122, and R 2 = 0.995. Additionally, we have also looked at the effects of various scan speeds on the Fe3O4/Gr/CC toward H2O2 for the electrocatalytic reduction H2O2 for sensing application (Figure 7). Figure 7 displays the CVs of Fe3O4/Gr/CC that were obtained at various applied scan speeds of 20-210 mVs −1 in 0.1 M PBS (pH 7.0) containing initial concentration of H2O2. We noticed that as the scan rate increased from 20 to 210 mVs −1 , the peak current response at the reduction side increases as the scan speed increases. The obtained results show the responses of peak current at 20 to 210 mVs −1 and a linear plot can be obtained. This plot further provides the information that indicates the linear increase in the response of the peak current and suggests the process is diffusion controlled for the sensing of H2O2 [73]. We noticed that as the scan rate increased from 20 to 210 mVs −1 , the peak current response at the reduction side increases as the scan speed increases. The obtained results show the responses of peak current at 20 to 210 mVs −1 and a linear plot can be obtained. This plot further provides the information that indicates the linear increase in the response of the peak current and suggests the process is diffusion controlled for the sensing of H 2 O 2 [73].

Limit of Detection and Sensitivity of Flexible Sensor
The limit of detection (LOD) and sensitivity of fabricated Fe3O4/Gr/CC sensors can be estimated using the previous method reported [74,75]. For this, we used Equations (1) and (2), which were mentioned below, to find out the LOD and sensitivity of the Fe3O4/Gr/CC for the sensing of H2O2: Limit of detection (LOD) = 3.3 (σ/S) (1) Sensitivity = Slope/area of the working electrode (2) where σ = standard error, and S = slope. The sensitivity, LOD, and linear range of the Fe3O4/Gr/CC toward H2O2 using an electrochemical approach were recorded as 0.037 µA µM −1 cm −2 , 4.79 µM, and 10-110 μM respectively, and the plausible mechanism can be seen in Scheme 3. Scheme 3 shows the micrographic images which show the transformation of H2O2 to H2O + O2 with the electrochemical cyclic voltammetry graph. The graph shows the gradual changes in the detection slope of H2O2. The reduction/oxidation takes place at the surfaces of an electrode (Fe3O4/Gr/CC electrode) and helps in the detection of H2O2 (Scheme 3). The reduction of H2O2 carried out by Fe3O4/Gr NC via electron transfer leads to the formation of hydroxyl ions (OH − ), and after combining produced water and oxygen molecules [75].

Limit of Detection and Sensitivity of Flexible Sensor
The limit of detection (LOD) and sensitivity of fabricated Fe 3 O 4 /Gr/CC sensors can be estimated using the previous method reported [74,75]. For this, we used Equations (1) and (2), which were mentioned below, to find out the LOD and sensitivity of the spectively, and the plausible mechanism can be seen in Scheme 3. Scheme 3 shows the micrographic images which show the transformation of H2O2 to H2O + O2 with the electrochemical cyclic voltammetry graph. The graph shows the gradual changes in the detection slope of H2O2. The reduction/oxidation takes place at the surfaces of an electrode (Fe3O4/Gr/CC electrode) and helps in the detection of H2O2 (Scheme 3). The reduction of H2O2 carried out by Fe3O4/Gr NC via electron transfer leads to the formation of hydroxyl ions (OH − ), and after combining produced water and oxygen molecules [75].

Test of Analytical Parameters (Selectivity, Repeatability, Reproducibility, and Stability) for Fe 3 O 4 /Gr/CC as Flexible Sensor
One of the most essential analytical parameters for sensors is selectivity. Several bodies, including glucose (Glu), uric acid (UA), ascorbic acid (AA), dopamine (DA), nitrophenol (NP), chlorophenol (CP), nitrophenol (NP), and sodium chloride (NaCl) were applied to test the anti-interference performance of Fe 3 O 4 /Gr/CC, as shown in Figure 8a. There was no noticeable current change after the addition of interfering species, and the current response to the addition of H 2 O 2 was unaffected, confirming the exceptional selectivity. Further, CV curves were used to test the repeatability of the modified electrode (Fe 3 O 4 /Gr/CC) in order to evaluate the accuracy of the sensor. The results were repeatable, with a relative standard deviation (RSD) of 3.27% (four repeated runs at single electrode) (Figure 8b).

Test of Analytical Parameters (Selectivity, Repeatability, Reproducibility, and Stability) for Fe3O4/Gr/CC as Flexible Sensor
One of the most essential analytical parameters for sensors is selectivity. Several bodies, including glucose (Glu), uric acid (UA), ascorbic acid (AA), dopamine (DA), nitrophenol (NP), chlorophenol (CP), nitrophenol (NP), and sodium chloride (NaCl) were applied to test the anti-interference performance of Fe3O4/Gr/CC, as shown in Figure 8a. There was no noticeable current change after the addition of interfering species, and the current response to the addition of H2O2 was unaffected, confirming the exceptional selectivity. Further, CV curves were used to test the repeatability of the modified electrode (Fe3O4/Gr/CC) in order to evaluate the accuracy of the sensor. The results were repeatable, with a relative standard deviation (RSD) of 3.27% (four repeated runs at single electrode) (Figure 8b). The reproducibility of Fe3O4/Gr/CC sensor was tested by fabricating six independent electrodes and RSD of current responses for the sensing of H2O2 was revealed to be 3.04%. (Figure 8c). A stability was also assessed by evaluating the current response toward H2O2 for 10 days with the retention of 93.01-94.7% of its initial current response and advising good stability. Thus, the presented sensor has acceptable selectivity, reproducibility, and stability.
In comparison with fabricated electrodes consisting of graphene, CNTs, MOFs, noble metals, and metal oxides, we presented a rather economical sensing platform (Table 2). The reproducibility of Fe 3 O 4 /Gr/CC sensor was tested by fabricating six independent electrodes and RSD of current responses for the sensing of H 2 O2 was revealed to be 3.04%. (Figure 8c). A stability was also assessed by evaluating the current response toward H 2 O 2 for 10 days with the retention of 93.01-94.7% of its initial current response and advising good stability. Thus, the presented sensor has acceptable selectivity, reproducibility, and stability.
In comparison with fabricated electrodes consisting of graphene, CNTs, MOFs, noble metals, and metal oxides, we presented a rather economical sensing platform ( Table 2). The CC is a suitable, cost-effective substrate with flexible properties, good conductivity, and tunable size, in contrast to certain typical electrodes [30]. Designing a sensing platform based on the CC with variable sizes results in fast analyte adsorption because of its synergistic and structural features. We can see from Table 2 that low detection limit was obtained using Fe 3 O 4 /Gr/CC for the H 2 O 2 detection with a good linear range. The noble metal-containing electrodes such as Ag NPs/SnO 2 /GCE, AgNPs/PQ11/graphene, and Pt NPs/UiO-66/GCE were even found comparable in terms of LOD ( Table 2). The graphene-MWCNT/GCE, rGO-Fe 2 O 3 -GCE, CuGa 2 O 4 /GCE shows either narrow LOW or comparable as compared to Fe 3 O 4 /Gr/CC. As a result, we have a sensor with good LOD, adaptable characteristics, and a low cost.

Conclusions
The simplistic and facile fabrication of a new type of flexible Fe 3 O 4 /Gr/CC was achieved by the co-precipitation method. This modified carbon cloth-based electrode exhibited excellent physical and electrochemical properties. When a carbon fiber clothbased electrode was used as a flexible electrochemical sensor for H 2 O 2 determination, the modified electrode demonstrated high sensitivity, a wide linear range, and a low detection limit. The results show the high sensitivity of 0.037 µA µM −1 cm −2 with a limit of detection of 4.79 µM and linearity range was measured from 10 µM to 110 µM. The sensor also revealed good reproducibility and high selectivity. It is expected that the flexible and freestanding Fe 3 O 4 /Gr/CC will provide a modular approach for materials to be manufactured in the future due to its ease of preparation, compatibility with in vivo work, and availability in many potential applications.

Data Availability Statement:
The data presented in this study are available from the corresponding authors upon reasonable request.