Flexible Hydrogen Peroxide Sensors Based on Platinum Modiﬁed Free-Standing Reduced Graphene Oxide Paper

: In this article, we report a facile method to fabricate free-standing reduced graphene oxide paper (rGOP) by vacuum ﬁltration. Pt nanoparticles were electrodeposited on reduced graphene oxide paper to construct a sensitive and ﬂexible hydrogen peroxide (H 2 O 2 ) sensor. The properties of Pt/rGOP were characterized by scanning electron microscopy, energy dispersive X-ray, X-ray photoelectron spectroscopy and Raman spectroscopy. The electrochemical characterizations of the resulting sensor were carried out by cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy. The Pt/rGOP hybrid electrode had an excellent electrochemical property, with superb electrocatalytic activity, a large electrochemical active surface area, ﬂexibility and high stability. The linear ranges of the as-prepared sensor for H 2 O 2 detection were divided into two linear sections: 0.2 µ mol/L to 2.0 mmol/L and 2.0 to 8.5 mmol/L, with a detection limit of 100 nmol/L (S/N = 3) and a response time of less than 5 s. The proposed sensor has great potential to become a reliable and ﬂexible tool in biosensor and point-of-care medical devices.


Introduction
Hydrogen peroxide (H 2 O 2 ) is one of the products during enzyme catalyzing, such as glucose oxidase, which is closely related to diverse physiological and pathological consequences including aging, organ injury and cancer, has received enormous attentions in academy and industry [1][2][3][4][5][6]. There is great need to detect H 2 O 2 in environmental, industrial and clinic fields [7]. Thus, it is critical to develop a novel method for the accurate and quick detection of H 2 O 2 . During the past few decades, many techniques have been established to detect H 2 O 2 , such as fluorimetry [8], titrimetry [9], chemiluminescence [10], spectrophotometry [11], chromatography [12], colorimetry [13], electron spin resonance spectroscopy [14], electrochemical methods [15,16], and so on. Among these techniques, electrochemical methods have attracted wide attention for real-time detection of H 2 O 2 because of their high sensitivity, fast response and low cost. At present, the electrochemical sensors used for detection of H 2 O 2 are mainly classified into enzymatic and nonenzymatic sensors. Compared to enzymatic sensors, the nonenzymatic sensors composed of transition metal nanoparticles (NPs), such as Pt, Ag, Au, Ni, etc., have generated a great deal of interest, thanks to their outstanding long-term stability/sensitivity regardless of the intrinsic nature of enzymes [17][18][19][20]. their outstanding long-term stability/sensitivity regardless of the intrinsic nature of enzymes [17][18][19][20].
Carbon-based materials, such as graphene and carbon nanotubes, have become a hot spot in many research fields [20,21]. Graphene is a two-dimensional material with a layer of sp 2 C atoms tightly packed into a honeycomb lattice. It possesses extraordinary mechanical, thermal and electronic properties [22,23]. Moreover, the large surface area and good electronic properties of graphene are considered advantages of an alternative electrode material for sensors [24,25]. Until now, several methods have been used to synthesize graphene, such as chemical vapor deposition [26], electrochemical exfoliation [27], micromechanical exfoliation [22], thermal growth from silicon carbide [28], etc. Among these methods, a solution-based method is thought to be the most promising route to produce large quantities of graphene [29]. Graphene oxide (GO) can be reduced by different methods, including chemical [30][31][32], thermal [33], electrochemical [34][35][36] and microwave-assisted techniques [37,38]. So far, GO can be manufactured into free-standing reduced graphene oxide paper (rGOP), with thicknesses ranging from 1 to 30 μm [39]. Such rGOPs have preferable extension and tensile properties (32 GPa) compared to graphite foils, bulky papers and other paper-like materials [39]. The combination of the superior mechanical strength, high electrical conductivity (7200 S m −1 ), thermal stability, structural uniformity and biocompatibility enables rGOP to be an ideal material for many technological applications [40,41].
In this article, we report a nonenzymatic biosensor based on free-standing reduced graphene oxide paper modified with Pt nanoparticles (PtNPs). Free-standing reduced graphene oxide paper could be an alternative to the traditional glassy carbon electrode, as the substrate for detection of H2O2. The schematic illustration of the fabrication process for a Pt/rGOP electrode is shown in Figure  1. Test results reveal that electrodeposition of PtNPs on free-standing reduced graphene oxide paper has two good linear responses to H2O2, ranging from 0.2 μmol/L to 8.5 mmol/L, with a detection limit of 100 nmol/L (S/N = 3). The as-prepared flexible and free-standing reduced graphene oxide paper has great potential in real applications, especially for biosensor and point-of-care medical devices.

Materials
Graphene oxide (10 mg/mL) aqueous solution was purchased from ACS Materials (Shanghai, China). Filter paper (mixed cellulose membrane) with a diameter of 50 cm and a pore size 0.22 μm was purchased from Xin Ya Purification Equipment Co. Ltd. (Shanghai, China). Hydrogen

Materials
Graphene oxide (10 mg/mL) aqueous solution was purchased from ACS Materials (Shanghai, China). Filter paper (mixed cellulose membrane) with a diameter of 50 cm and a pore size 0. 22  Ascorbic acid (AA), glucose (Glu), fructose (Fru), uric acid (UA), oxalic acid (OA) and citric acid were obtained from Sigma-Aldrich (Shanghai, China). All other chemicals used were of analytical grade, without need of further purification. Phosphate buffer solution (PBS) was made of potassium dihydrogen phosphate (KH 2 PO 4 ) and disodium hydrogen phosphate (Na 2 HPO 4 ) at pH = 7.4.

Preparation of Pt NPs Modified rGOP
Graphene oxide aqueous solution (GO 10 mg/mL) was diluted with deionized (DI) water to 2 mg/mL; the free-standing graphene oxide paper (GOP) was fabricated, using a vacuum filtration method, by filtrating 20 mL of 2 mg/mL GO. The beauty of this facile method is that the thickness of the graphene oxide paper could be easily controlled by adding different volumes of GO solution. When most of the water had been filtered by a vacuum filtration machine, a round and integral layer of graphene oxide paper with a smooth surface could be obtained. The graphene oxide adhered-to filter paper was then put in the oven at 60 • C for 5 min to evaporate the remaining water. The as-prepared GOP was easily peeled off from the filter paper without any cracks. Then, GOP was chemically reduced by being immersed in 45% HI solution in a sealed cuvette at room temperature for 1 h. The HI-reduced graphene oxide paper was rinsed with DI water and ethanol several times, and the reduced graphene oxide paper was dried at room temperature.
A conventional three-electrode system was employed for electrodeposition, while platinum net and saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. Before electrodeposition, rGOP was tailored to several sheets at 7 mm × 20 mm. Electrochemical deposition of PtNPs on rGOP was performed in 0.5 mol/L H 2 SO 4 solution in the presence of 1 mmol/L H 2 PtCl 6 , added using cyclic voltammetry. The potential ranged from −0.2 V to 1.25 V with a scan rate of 50 mV/s, and the effect of different cycles (5, 10, 15, 20 and 25 cycles) was evaluated. After electrodeposition, the rGOP modified with PtNPs was rinsed thoroughly with DI water several times to remove residual electrolytes and then dried at room temperature.

Characterization and Electrochemical Measurements
The morphology of the rGOP was investigated by field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi Co. Ltd., Tokyo, Japan) at an accelerating voltage of 10 kV. The distribution of elements in crystals was characterized using an energy dispersive X-ray spectrometer (EDX) that was attached to SEM. Raman spectrum (SENTERRA, Karlsruhe, Germany) was used for showing the intrinsic structure between GOP and rGOP. The compositional analysis of the samples was checked by X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi, Thermo Scientific, Waltham, USA). Chronoamperometry, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out using an electrochemical workstation (CHI-760E CH Instruments, Shanghai, China). Both CV and EIS measurements for rGOP were performed in PBS (pH 7.4). Chronoamperometry was used to investigate the sensing performance of electrodes. Different potentials of 0, −0.05, −0.1, −0.15, −0.2, −0.25 and −0.3 V were set to carry out the optimization. Before measurement, all solutions used in electrochemical tests were bubbled with high pure nitrogen for 15 min to thoroughly remove oxygen from the solution.

Characterization of rGOP
Free-standing rGOP is fabricated through vacuum filtration, followed by chemical reduction in HI aqueous solution. It is a facile process to control the thickness of free-standing rGOP by adding different volumes of GO aqueous suspension, as shown in Figure 2. Chemical reduction by HI could effectively remove most oxygen-functional groups and the covalent bond structure in GOP. Compared to GOP, rGOP has an improved electron transfer ability, is silvery in color and shows metallic gloss on both sides. The as-prepared rGOP is highly flexible and can be randomly folded; one of them was formed into a paper crane ( Figure 2A). The morphology of rGOP is characterized by SEM with high magnification, as shown in Figure 2B,C. The images reveal that rGOP is constructed layer-by-layer by parallel rGOP nanosheets, with uniform thickness throughout its cross-section. The rGOP is intensely crumpled, and the typical wrinkled nanostructure is caused by the overlap of graphene nanosheets after chemical reduction ( Figure 2D). one of them was formed into a paper crane (Figure 2A). The morphology of rGOP is characterized by SEM with high magnification, as shown in Figure 2B,C. The images reveal that rGOP is constructed layer-by-layer by parallel rGOP nanosheets, with uniform thickness throughout its cross-section. The rGOP is intensely crumpled, and the typical wrinkled nanostructure is caused by the overlap of graphene nanosheets after chemical reduction ( Figure 2D).
To examine the surface chemical composition and electronic states of the elements, X-ray photoelectron spectroscopy (XPS) was used. The results are presented in Figure 3 for GOP and rGOP, with or without PtNPs. Figure 3A shows that there are big differences in C 1s peaks between GOP and rGOP in XPS spectra. The two peaks with a binding energy of 284.6 eV (C=C/C-C) and 286.8 eV (C-O) are clearly exhibited in the GOP spectrum. After reduction, the main peak in rGOP greatly decreases to around 284.8 eV, which is attributed to C-O. This clearly demonstrates that GOP was successfully reduced to rGOP by HI reduction method. As Figure 3B shows, the peaks with a binding energy of 74.58 eV and 71.18 eV are related to Pt 4f5/2 and Pt 4f7/2, respectively, indicating that the Pt is mainly metallic in Pt/rGOP [42]. Raman spectroscopy is an important experimental method in graphene research, which provides information about the structural defects and graphite structure of GOP and rGOP. As illustrated in Figure 4, the Raman spectra of GOP and rGOP have two obvious peaks. The peak at 1344 cm −1 is named the D band, which is related to the degree of defect in the lattice of graphene structure. The peak at 1586 cm −1 is named G band, which is assigned to the first-order scattering of the E2g mode of GO and corresponds to the in-plane vibration of sp 2 -bonded carbon atoms in a 2D hexagonal lattice [43]. The intensity ratio of G band over D band, IG/ID ratio, is widely used to estimate the density of defects in graphene structures: the smaller the value of IG/ID ratio, the higher the defect density [44]. The value of IG/ID decreased from 0.916 to 0.696 once the GOP reduced to rGOP. The decrement of IG/ID indicates that rGOP has more sp 3 electronic configurations and defects in the carbon atoms lattice structure when GOP is reduced to rGOP. After electrodeposition of PtNPs, the value of IG/ID increased from 0.696 to 0.812, which suggests that PtNPs, electrodeposited between rGOP layers, may reduce the defect density. To examine the surface chemical composition and electronic states of the elements, X-ray photoelectron spectroscopy (XPS) was used. The results are presented in Figure 3 for GOP and rGOP, with or without PtNPs. Figure 3A shows that there are big differences in C 1s peaks between GOP and rGOP in XPS spectra. The two peaks with a binding energy of 284.6 eV (C=C/C-C) and 286.8 eV (C-O) are clearly exhibited in the GOP spectrum. After reduction, the main peak in rGOP greatly decreases to around 284.8 eV, which is attributed to C-O. This clearly demonstrates that GOP was successfully reduced to rGOP by HI reduction method. As Figure 3B shows, the peaks with a binding energy of 74.58 eV and 71.18 eV are related to Pt 4f 5/2 and Pt 4f 7/2 , respectively, indicating that the Pt is mainly metallic in Pt/rGOP [42].
Raman spectroscopy is an important experimental method in graphene research, which provides information about the structural defects and graphite structure of GOP and rGOP. As illustrated in Figure 4, the Raman spectra of GOP and rGOP have two obvious peaks. The peak at 1344 cm −1 is named the D band, which is related to the degree of defect in the lattice of graphene structure. The peak at 1586 cm −1 is named G band, which is assigned to the first-order scattering of the E 2g mode of GO and corresponds to the in-plane vibration of sp 2 -bonded carbon atoms in a 2D hexagonal lattice [43]. The intensity ratio of G band over D band, I G /I D ratio, is widely used to estimate the density of defects in graphene structures: the smaller the value of I G /I D ratio, the higher the defect density [44].
The value of I G /I D decreased from 0.916 to 0.696 once the GOP reduced to rGOP. The decrement of I G /I D indicates that rGOP has more sp 3 electronic configurations and defects in the carbon atoms lattice structure when GOP is reduced to rGOP. After electrodeposition of PtNPs, the value of I G /I D increased from 0.696 to 0.812, which suggests that PtNPs, electrodeposited between rGOP layers, may reduce the defect density.   Figure 5A-E. Figure 5A shows that Pt nanoparticles were electrodeposited by 5 cycles on rGOP. SEM images ( Figure 5B-D) of Pt nanoparticles, obtained by different electrodeposition cycles, indicate that the diameter of Pt nanoparticles is less than 30 nm, and that they are homogeneously distributed on the rGOP without obvious agglomeration. Figure 5E shows that the obvious agglomeration occurred when the electrodeposition cycles were up to 25. The EDX spectrum of rGOP-PtNPs is shown in Figure 5F, which depicts the existence of C, O and Pt. C and Pt can easily be attributed to the rGOP and PtNPs. The existence of O may be attributed to the adsorbed oxygen species of rGOP when using CV method to electrodeposit PtNPs.   Figure 5A-E. Figure 5A shows that Pt nanoparticles were electrodeposited by 5 cycles on rGOP. SEM images ( Figure 5B-D) of Pt nanoparticles, obtained by different electrodeposition cycles, indicate that the diameter of Pt nanoparticles is less than 30 nm, and that they are homogeneously distributed on the rGOP without obvious agglomeration. Figure 5E shows that the obvious agglomeration occurred when the electrodeposition cycles were up to 25. The EDX spectrum of rGOP-PtNPs is shown in Figure 5F, which depicts the existence of C, O and Pt. C and Pt can easily be attributed to the rGOP and PtNPs. The existence of O may be attributed to the adsorbed oxygen species of rGOP when using CV method to electrodeposit PtNPs.  Figure 5A-E. Figure 5A shows that Pt nanoparticles were electrodeposited by 5 cycles on rGOP. SEM images ( Figure 5B-D) of Pt nanoparticles, obtained by different electrodeposition cycles, indicate that the diameter of Pt nanoparticles is less than 30 nm, and that they are homogeneously distributed on the rGOP without obvious agglomeration. Figure 5E shows that the obvious agglomeration occurred when the electrodeposition cycles were up to 25. The EDX spectrum of rGOP-PtNPs is shown in Figure 5F, which depicts the existence of C, O and Pt. C and Pt can easily

Electrochemical Characterization
CV curves of the Pt/rGOP electrode, scanned in deaerated 0.5 mol/L H2SO4 solution within a potential range from −0.2 V to 1.25 V, is shown in Figure 6A. The peak at 0.3 V is associated with the reduction of Pt oxide species. In Figure 6A, the peak of Pt species is invisible, but the analysis of previous data showed that PtNPs were successfully electrodeposited. Additionally, it is obvious that, once PtNPs were electrodeposited on the free-standing graphene paper, the current of Pt/rGOP charge and discharge became much larger than rGOP and Pt foil, indicating Pt/rGOP has the highest surface area. The electrochemical sensing performances of Pt foil and Pt/rGOP have been investigated in 0.1 mol/L PBS (pH = 7.4) containing 1.0 mmol/L H2O2 at a scan rate of 50 mV/s, as depicted in Figure 6B. Compared to the Pt foil, Pt/rGOP has a higher reduction current in the presence of H2O2, reflecting that Pt/rGOP has superior electrochemical activity. EIS is often used to gain important information about the interfacial properties of electrodes, and Figure 6C exhibits the Nyquist plots of Pt/rGOP and rGOP in the presence of 0.1 mol/L KCl containing 1 mmol/L [Fe(CN)6] −3/−4 . It is obvious that the charge-transfer resistance of the Pt/rGOP is much smaller than rGOP. This is mainly attributed to the high loading of PtNPs, with their uniform distribution and construction of an effective conduction pathway to boost the electron transfer on rGOP.

Electrochemical Characterization
CV curves of the Pt/rGOP electrode, scanned in deaerated 0.5 mol/L H 2 SO 4 solution within a potential range from −0.2 V to 1.25 V, is shown in Figure 6A. The peak at 0.3 V is associated with the reduction of Pt oxide species. In Figure 6A, the peak of Pt species is invisible, but the analysis of previous data showed that PtNPs were successfully electrodeposited. Additionally, it is obvious that, once PtNPs were electrodeposited on the free-standing graphene paper, the current of Pt/rGOP charge and discharge became much larger than rGOP and Pt foil, indicating Pt/rGOP has the highest surface area. The electrochemical sensing performances of Pt foil and Pt/rGOP have been investigated in 0.1 mol/L PBS (pH = 7.4) containing 1.0 mmol/L H 2 O 2 at a scan rate of 50 mV/s, as depicted in Figure 6B. Compared to the Pt foil, Pt/rGOP has a higher reduction current in the presence of H 2 O 2 , reflecting that Pt/rGOP has superior electrochemical activity. EIS is often used to gain important information about the interfacial properties of electrodes, and Figure 6C exhibits the Nyquist plots of Pt/rGOP and rGOP in the presence of 0.1 mol/L KCl containing 1 mmol/L [Fe(CN) 6 ] −3/−4 . It is obvious that the charge-transfer resistance of the Pt/rGOP is much smaller than rGOP. This is mainly attributed to the high loading of PtNPs, with their uniform distribution and construction of an effective conduction pathway to boost the electron transfer on rGOP.  Figure 7A shows the amperometric responses of Pt/rGOP towards H2O2 at different applied potentials (0.0 to −0.3 V). The result reveals that Pt/rGOP exhibits the highest current density once the applied potential was −0.25 V. Figure 7B depicts the amperometric response changes with the different electrodeposition cycles. The current density gradually increased, with the electrodeposition cycles increased from 5 to 20, and further to 25, and the current density substantially reduced. This is because PtNPs exhibit an agglomerating phenomenon, comparing with the specimen deposited for 20 cycles. The result demonstrates that electrodeposition for 20 cycles in rGOP could obtain the largest electroactive area.

Amperometric Response to Hydrogen Peroxide
The amperometric response of Pt/rGOP to successive additions of different concentrations of H2O2 in 0.1 mol/L PBS with potential applied at −0.25 V is given in Figure 7C. It shows the sensor has a fast response and the current is able to reach a dynamic equilibrium within 5 s after adding H2O2. The fast response indicates a favorable electron transfer between Pt/rGOP and H2O2. It is found that the amperometric responses towards H2O2 concentration is wide, from 0.2 μmol/L to 8.5 mmol/L, and the detection ranges are divided into two linear sections: 0.2 μmol/L to 2.0 mmol/L and 2.0 to 8.5 mmol/L, while the sensitivities are 67.51 μA cm −2 mM −1 and 40.21 μA cm −2 mM −1 , respectively, as shown in Figure 7D. The existence of two linear ranges could indicate different mechanisms: at low concentrations of hydrogen peroxide, the electrocatalytic mechanism is dominant. Otherwise, at high concentration of hydrogen peroxide, the direct reduction of hydrogen peroxide on the surface can play an important role in the analytical signal [45]. Moreover, the detection limit of H2O2 can be as low as 100 nmol/L, at a signal-to-noise (S/N) ratio of 3. The sensor provides a more comparable detection limit and linear range compared to previous reports, as listed in Table 1. In the meantime,  Figure 7A shows the amperometric responses of Pt/rGOP towards H 2 O 2 at different applied potentials (0.0 to −0.3 V). The result reveals that Pt/rGOP exhibits the highest current density once the applied potential was −0.25 V. Figure 7B depicts the amperometric response changes with the different electrodeposition cycles. The current density gradually increased, with the electrodeposition cycles increased from 5 to 20, and further to 25, and the current density substantially reduced. This is because PtNPs exhibit an agglomerating phenomenon, comparing with the specimen deposited for 20 cycles. The result demonstrates that electrodeposition for 20 cycles in rGOP could obtain the largest electroactive area.

Amperometric Response to Hydrogen Peroxide
The amperometric response of Pt/rGOP to successive additions of different concentrations of H 2 O 2 in 0.1 mol/L PBS with potential applied at −0.25 V is given in Figure 7C. It shows the sensor has a fast response and the current is able to reach a dynamic equilibrium within 5 s after adding H 2 O 2 . The fast response indicates a favorable electron transfer between Pt/rGOP and H 2 O 2 . It is found that the amperometric responses towards H 2 O 2 concentration is wide, from 0. as shown in Figure 7D. The existence of two linear ranges could indicate different mechanisms: at low concentrations of hydrogen peroxide, the electrocatalytic mechanism is dominant. Otherwise, at high concentration of hydrogen peroxide, the direct reduction of hydrogen peroxide on the surface can play an important role in the analytical signal [45]. Moreover, the detection limit of H 2 O 2 can be as low as 100 nmol/L, at a signal-to-noise (S/N) ratio of 3. The sensor provides a more comparable detection limit and linear range compared to previous reports, as listed in Table 1. In the meantime, Pt/rGOP exhibits the highest sensitivity among all the sensors.

Stability and Bending Tests
The reproducibility and long-term stability of the Pt/rGOP were determined by evaluating the amperometric current response towards 0.2 mmol/L H2O2 in 0.1 mol/L PBS. The amperometric current responses of 5 sensors, fabricated under the same conditions, were tested for 10 days. This

Stability and Bending Tests
The reproducibility and long-term stability of the Pt/rGOP were determined by evaluating the amperometric current response towards 0.2 mmol/L H 2 O 2 in 0.1 mol/L PBS. The amperometric current responses of 5 sensors, fabricated under the same conditions, were tested for 10 days. This revealed that Pt/rGOP electrodes exhibit a relative standard deviation (RSD) of 3.8% for 10 days, signifying that the Pt/rGOP possesses high reproducibility and stability ( Figure 8A). The flexibility is an important factor for flexible Pt/rGOP electrodes, and five Pt/rGOP electrodes were bent 180 • up to 200 times, as shown in Figure 8B. The results show that it retains nearly 92% of original response after being bent 200 times. This proves that the flexible Pt/rGOP electrodes have excellent mechanical stability for electrochemical devices to detect H 2 O 2 .

Anti-Interference Studies and Real Samples Analysis
The selectivity is also a significant analytical factor for sensors, and several common interfering species were chosen to evaluate the selectivity of the flexible Pt/rGOP electrode. Figure 8C represents the typical amperometric responses upon successive addition of 0.1 mmol/L H2O2, 0.2 mmol/L glucose (Glu), 0.2 mmol/L fructose (Fru), 0.2 mmol/L uric acid (UA), 0.2 mmol/L ascorbic acid (AA), 0.2 mmol/L oxalic acid (OA) and 0.2 mmol/L citric acid (CA) into 0.1 mol/L PBS. The result shows that these potential interfering substances do not exhibit obvious amperometric signals and proves the flexible Pt/rGOP electrode has an excellent selectivity to H2O2.
Food safety has aroused concern with the improvement of the quality of life. Residual hydrogen peroxide is often found in milk, so the sensor can be used to determine H2O2 in milk. The amperometric curve of successive addition of H2O2 is shown in the Figure 8D. The Pt/rGOP electrode does not exhibit an obvious amperometric response towards pure milk. When the milk contains H2O2, the electrode shows an obvious amperometric response, which is similar in the case of successive addition of H2O2 without milk under the same concentration, as shown in Figure 7C. The result indicates that the Pt/rGOP electrode has excellent practical application.

Anti-Interference Studies and Real Samples Analysis
The selectivity is also a significant analytical factor for sensors, and several common interfering species were chosen to evaluate the selectivity of the flexible Pt/rGOP electrode. Figure 8C  Food safety has aroused concern with the improvement of the quality of life. Residual hydrogen peroxide is often found in milk, so the sensor can be used to determine H 2 O 2 in milk. The amperometric curve of successive addition of H 2 O 2 is shown in the Figure 8D. The Pt/rGOP electrode does not exhibit an obvious amperometric response towards pure milk. When the milk contains H 2 O 2 , the electrode shows an obvious amperometric response, which is similar in the case of successive addition of H 2 O 2 without milk under the same concentration, as shown in Figure 7C. The result indicates that the Pt/rGOP electrode has excellent practical application.

Conclusions
In this work, we have developed a facile vacuum filtration method to prepare free-standing GOP, and it can be easily reduced by HI to rGOP. The as-prepared rGOP is made up of multilayer reduced graphene oxide whose thickness could be easily controlled. PtNPs were electrodeposited on rGOP and it thus remarkably increases active surface area and speeds up electron charge-transfer. The amperometric responses of Pt/rGOP towards H 2 O 2 are wider from 0.2 µmol/L to 8.5 mmol/L, and the detection ranges are divided into two linear sections: 0.2 µmol/L to 2.0 mmol/L and 2.0 to 8.5 mmol/L. The sensitivity of the sensor is much higher than previous reported ones, and the detection limit of H 2 O 2 can be as low as 100 nmol/L (S/N = 3). Furthermore, the sensor has long-term stability and nearly retains 92% of original response after being bent 200 times. This flexible sensor is an effective and fascinating tool for quantitative and qualitative monitoring of H 2 O 2 in future biomedical and environmental application.