Preparation of Nano Au & Pt Alloy Microspheres Decorated with

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Preparation of Nano Au & Pt Alloy Microspheres Decorated with Reduced Graphene Oxide for Non-enzymatic Hydrogen Peroxide Sensing Zhixue Bai, Wenhao Dong, Yipeng Ren, Cong Zhang, and Qiang Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02626 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Preparation of Nano Au & Pt Alloy Microspheres Decorated with Reduced Graphene Oxide for Non-enzymatic Hydrogen Peroxide Sensing

Zhixue Bai, Wenhao Dong, Yipeng Ren, Cong Zhang, Qiang Chen*

The Key Laboratory of Bioactive Materials, Ministry of Education, College of Life Science, Nankai University, Weijin Road No.94, Tianjin 300071, PR China

____________ *Corresponding author. Tel.: +86 22 23507273; fax: +86 22 23506122. E-mail address: [email protected] (Q. Chen).

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Abstract: The flourish of nanotechnology has brought new vitality to the research and development of electrochemical sensing materials. In this work, we successfully synthesized Nano Au & Pt alloy microspheres decorated with reduced graphene oxide (RGO/nAPAMSs) by a simple, facile and eco-friendly one-step reduction strategy for the fabrication of highly sensitive non-enzymatic H2O2 sensing interfaces. Energy-dispersive X-ray spectroscopy mapping (EDX mapping), energy-dispersive X-ray spectroscopy analyzer (EDX), transmission electron microscopy (TEM), Fourier transform infrared spectrum (FT-IR) and X-ray diffraction spectrum (XRD) were employed to characterize RGO/nAPAMSs from a microscopic perspective. The results of cyclic voltammetry and chronoamperometry exhibited excellent electrochemical behaviors toward H2O2, with a -1

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rapid response time within 5 s, remarkable sensitivity of 1117.0 µA mM cm , wide linear range of 0.005 to 4.0 mM and lower detection limit of 0.008 µM (S/N=3), which provide RGO/nAPAMS not only a promising prospect for the quantitative detection of H2O2 but also a potential application in other fields of sensors. Moreover, further analysis showed the principles of the superior H2O2 sensing performance of RGO/nAPAMSs. This discovery provides a significant contribution to future study in non-enzymatic H2O2 sensing based on Nano Pt, Nano Au noble metal electrocatalysts.

Keywords: Nano Au & Pt alloy microspheres; Reduced graphene oxide; Non-enzymatic H2O2 sensor; Nano Pt; Nano Au 2


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1. Introduction Hydrogen peroxide (H2O2), a simple but essential molecule in nature, owing to its unique chemical properties, has been widely used in clinic, pharmaceutical, environmental, food manufacturing and chemical industries1; in vivo, as one of side products of biochemical reactions, H2O2 plays a significant role in regulating various biological signaling transduction processes, responding to invading pathogens and relating to aging and disease, such as Parkinson’s, Alzheimer’s, cancer, diabetes, cardiovascular and neurodegenerative disorders2-5. Therefore, H2O2 concentration detection is of practical significance for both academic and industrial purpose. Up to now, a variety of analysis techniques, such as colorimetry, fluorescence, chemiluminescence, high-performance liquid chronmatography and electrochemistry, have been developed for monitoring H2O2, among which electroanalytical method has been widely employed in accurate determination of H2O2 due to its easy miniaturization, low-cost, high sensitivity, rapid response and the possibility for real-time detection6-14. The H2O2 electrochemical biosensors can be classified into two broad categories, enzymatic H2O2 electrochemical biosensors and non-enzymatic H2O2 electrochemical biosensors15. Since the 1990s, enzyme-based biosensors have been widely applied to detect H2O2 due to their intrinsic redox capability of redox protein16-17. However, the drawbacks, such as the limited lifetime, susceptibility to temperature and pH value, complex immobilization procedures and the high-cost of the enzymes, limit their practical

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applications in H2O2 sensing18. Consequently, many efforts have been devoted to the development of non-enzymatic H2O2 sensors to overcome these limitations19. Significantly, the sensitivity, selectivity and stability of non-enzymatic H2O2 detection strongly rely on the structure and properties of electrode materials14. Therefore, it is still highly desirable to develop the modified electrode materials with high electrocatalytic activities and excellent conductivity. Up to date, diversified noble metals, transition metals, and metallic oxides have been studied extensively as candidate high-efficiency catalyst materials for electrochemical applications, among which noble metals, such as Au, Ag, Pt and Pd, are the most widespread catalysts for chemical reactions due to their excellent electronic conductive, superior catalytic performances and high chemical stability20. Among them, Pt nanoparticles (Pt NPs) have attracted much attention owing to their superior catalytic activity and validly lessening the oxidation/reduction overvoltage in H2O2 determination, which is significant for avoiding interference from other co-existing substances1. For instance, Liu et al. reported a rapid process to synthesize porous graphenesupported PtNPs (Pt/PG)21. Besides, Au nanoparticles (Au NPs) have also been intensively applied in electrochemical sensors. For example, Fang et al. reported an electrochemical sensor based on Au nanoparticles/graphene nanosheets hybrid (AuNPs/GNs)22. Furthermore, it has been proved that metallic alloy is an effective route to further increase the electrocatalytic activity and enhance the lone-term catalytic stability due to the access to additional handles for the structures the strong electronic coupling1. The synergistic

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effect between noble metal elements can effectively increase the specific surface area, electrocatalytic activity and long-term catalytic stability of the hybrid21, 23-24. In this work, we tried to fabricate Nano Au & Pt alloy microspheres, which could play the dual roles of catalyzing redox reactions and assisting direct electron transfer from the substrate to the electrode surface. In addition, to enhance charge transporting of electrochemical sensors, depositing alloy microspheres on highly conductive substances has been proved to be an effective strategy1, 25-26

. Reduced graphene oxide (RGO), as a new carbon material consisting of single-layer-

atoms planar sheet structure with sp2-bonded carbon atoms packed in a honeycomb twodimensional lattice16,

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, has many unique characteristics, such as high conductivity,

chemical stability and biocompatibility28-30. Compared with graphene, RGO contains more lattice defects and surface functional groups, which is favorable for the immobilization on its surface31-33. By dispersing on the typical wrinkled laminated structure of RGO, the alloy microspheres could effectively avoid the aggregation problem. Moreover, owing to the metal-substance interaction, the catalytic activity of the RGO-supported alloy microspheres can be improved2.

In this study, Nano Au & Pt alloy microspheres decorated with reduced graphene oxide (RGO/nAPAMSs) had been tried to prepare by a facile, eco-friendly and controllable route. And a novel and highly sensitive non-enzymatic H2O2 sensor was successfully fabricated based on RGO/nAPAMSs/GC electrode. It was found that the prepared non-enzymatic 5


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H2O2 sensor exhibited superior electrocatalytic activity toward H2O2, with high sensitivity, low detection limit, and wide linear range. These outstanding results indicate that RGO/nAPAMSs is promising for fabricating non-enzymatic H2O2 sensors and has great application prospect for other biological analytes. Moreover, further analysis showed why RGO/nAPAMSs has extremely electrocatalytic activity, which provides new principles for the further study in non-enzymatic H2O2 sensing based on Nano Pt, Nano Au noble metal electrocatalysts. 2. Experimental 2.1 Materials Hexachloroplatinic acid (H2PtCl6), Gold chloride hydrate (HAuCl4) and Sodium borohydride (NaBH4) was obtained from sigma Aldrich Co. Graphene oxide (GO) was obtained from Nanjing XF NANO Materials Tech Co. Hydrogen peroxide (H2O2), Ethanol (C2H5OH), NaH2PO4·2H2O and Na2HPO4·12H2O were purchased from Tianjin Damao Chemical Reagent Co. (China). Fresh phosphate buffer solution (PBS, 0.1 M, pH 7.0), prepared from NaH2PO4 and Na2HPO4 was used as a supporting electrolyte. Double distilled water purified by Millipore system was used throughout the experiments. 2.2

Apparatus The EDX mapping and EDX analysis patterns were collected on an energy-dispersive

X-ray spectroscopy analyzer which was equipped on the Tecnai G2 F20 instrument (Philips

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Holland). Transmission electron microscopy (TEM) images analysis was performed on a Tecnai G2 F20 instrument. Fourier transform infrared spectrum (FT-IR) images was gathered on a TENSOR 37 (Bruker, German). The X-ray diffraction (XRD) analysis was carried out on a Rigaku D/max-rA with Cu Ka radiation (l=1.5418 Å) (Rigaku, Japan). A 283 Potentiostat-Galvanostat electrochemical workstation (EG&G PARC with M 270 software) were used to carry out all the electrochemical experiments. 2.3

Synthesis of RGO/nAPAMSs 20 mg of GO were uniformly dispersed in 20 mL doubly distilled water by sonication

for 2 h. 60 mg of H2PtCl6 and 60mg of HAuCl4 were added into 20 mL doubly distilled water, and then this solution was mixed with the suspension solution of GO and ultrasonically dispersing for 1 h. After that, 10 mL supersaturated solution of sodium borohydride was dropwise injected into the 40 mL new dispersed phase under magnetic stirring. Then, the reduction reaction was continued by being magnetic stirred at room temperature for 16 h and the resulting product was obtained by centrifugal cleaning 3 times with doubly distilled water and ethyl alcohol, respectively. The final product was dried in vacuum oven at 80 °C for 12 h. RGO, RGO/PtNPs and RGO/AuNPs were synthesized by the same principle (The details were presented in Supplementary material).

2.4

Preparation of modified electrodes

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5 mg synthesized RGO/nAPAMSs was ultrasonically dispersed in 5 mL water to prepare 1 mg/mL RGO/nAPAMSs suspension. Before modification, the GC electrode should be polished with 0.3 and 0.005 µm α-alumina powder respectively and then ultrasonically cleaned in doubly distilled water and ethanol for 5 min, sequentially. After that, 6 µL of RGO/nAPAMSs suspension was dropped down the center of GC electrode surface and dried up naturally. Then, the modified GC electrode can be used directly on the non-enzymatic H2O2 sensor for the next electrochemical experiments. 2.5

Electrochemical experiments A 283 Potentiostat-Galvanostat electrochemical workstation (EG&G PARC with

M270 software) with a conventional three-electrode system were used for the electrochemical experiments, including a coiled Pt wire (1 mm diameter) as the counter electrode, an Ag/AgCl (staturated with KCl) as the reference electrode and the modified GC electrode as the working electrode34. Highly pure nitrogen gas was used to deaerate solutions for at least 10 min before the experiments. After the testing, the electrode should be moved out from the detection system and cleaned with doubly distilled water. 2.5.1 Cyclic voltammetric (CV) experiments To characterize the electrochemical properties of the synthesized materials, the modified electrode was put into 0.1 M KCl solution containing 10 mM [Fe(CN)6]3-; To show the electroreduction ability of the synthesized materials toward hydrogen peroxide, the modified electrode was put into 0.1 M PBS (pH=7.0) with 5 mM H2O2. After the 8


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optimization, CV scanning rate was set to be 50 mV/s. 2.5.2 Amperometric experiments Amperometric measurements was employed to show the response of modified electrode

to

different

H2O2

concentrations.

The

three-electrode

system

of

electrochemical workstation was used same as CV experiments. Successive additions of H2O2 solution were added into the three-electrode cell under constant low speed magnetic stirring with certain time intervals. 3. Results and discussion 3.1

The Characterization of RGO/nAPAMSs

3.1.1 Elemental Composition Characterization Here is Figure 1. Here is Figure 2. Here is Figure 3. The elemental composition of the RGO/nAPAMSs was characterized by EDX, EDX mapping and XRD. Figure 1 shows the EDX analysis of the RGO/nAPAMSs. As shown, the nanocomposite contains elements of C, O, Cu, Au and Pt, among which the Cu element is derived from the matrix, while the C/O and Au/Pt elements are derived from the reduced graphene oxide (RGO) and Nano Au & Pt alloy microspheres (nAPAMSs), individually. This characterization indicates that nAPAMSs have been successfully modified on the RGO sheets. Furthermore, as shown in Figure 2, the EDX mappings of RGO/nAPAMSs

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showed that the C element (blue) representing the graphene sheet structure covered the entire field of view; Pt element (green) and Au elements (Red) evenly blended together and distributed on the graphene sheets with a particle size about 10 nm. The EDX and EDX mapping results reveal that the RGO / nAPAMSs material is a kind of Nano-scale alloy microsphere composites formed by the gently mixing of platinum and gold, rather than the mixture of Pt nanoparticles and Au nanoparticles, indicating the successful synthesis of RGO / nAPAMSs nanocomposite. In order to further confirm that RGO / nAPAMSs were successfully synthesized, XRD patterns of GO (a), RGO (b) and RGO / nAPAMSs (c) was shown in Figure 3. A sharp diffraction peak at 11.9° (001) and a significant diffraction peak at 23.99° (002) are the typical peaks of the GO plane structure (curve a). For RGO (curve b), there was obvious diffraction peak at 23.99° (002), while no peak at 11.9° (001), indicating that GO has been successfully reduced to RGO33. In RGO / nAPAMSs diagram (curve c), we can see that there were other four obvious diffraction peaks, among which the peak at 39.2° (111) is in good with the crystal planes of Au35 and the other tree at 44.3° (200), 64.5° (220) and 77.5° (311) are attributed to the reflections of Pt plane structure36. This XRD pattern indicates that RGO / nAPAMSs were successfully prepared which consistent with the results based on EDX and EDX mapping. 3.1.2 Optical Microscopic Characterization Here is Figure 4.

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Figure 4 shows the microscopic morphologies of RGO/nAPAMSs at different magnifications of TEM. In Figure 4A, a typical wrinkled laminated structure of RGO was shown. In Figure 4B, the spherical nAPAMSs with a particle size of 2-10 nm can be clearly seen. Figure 4C and 4D are the RGO/nAPAMSs under lower magnification, it can be seen that highly dispersed and uniformly distributed nAPAMSs have almost no aggregation, which suggested that nAPAMSs are well modified on the RGO lamellas. The modification of highly dispersed nAPAMSs greatly increases the surface area of RGO lamellas, thus giving RGO/nAPAMSs higher catalytic activity and conductivity. 3.1.3 Chemical Bond Characterization Here is Figure 5. To further confirm the existence of stable covalent bonds between nAPAMSs and RGO, FT-IR and XRD were used in this paper. Figure 5 shows the FT-IR spectra of GO (curve a) and RGO/nAPAMSs (curve b). By comparing the absorption peaks in the two spectrums, the bonds between nAPAMSs and RGO can be explained from aspect of the chemical covalent bonding. As shown in curve (a), the spectrum of GO includes 6 obvious characteristic peaks representing O-H broad peak (VO-H at 3428 cm-1), C=O (VC=O at 1720 cm-1), aromatic C=C (VC=C at 1628 cm-1), C-OH (VC-OH at 1397 cm-1), C-O-C (VC-O-C at 1218 cm-1) and C-O (VC-O at 1048 cm-1) respectively37, which illustrates the various carbonyl groups on the surface of the GO. For RGO/nAPAMSs, the peak at 3428 cm-1 and 1048 cm-1 are obviously decreased or even disappear, which represents the disappearance

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of O-H bond and C-O bond, indicating that GO is successful reduced to RGO38. At the same time, the peaks at 1720 cm-1 and 1397 cm-1 almost disappear, illustrating that the original carbonyl groups on the surface of GO are modified due to the addition of nAPAMSs1. This result confirms that nAPAMSs bond with RGO lamellas through the stable covalent bond.

3.1.4 Elctrochemical Characterization

Here is Figure 6. The electrochemical properties of RGO/nAPAMSs were characterized by cyclic voltammetry recorded in potassium ferricyanide solution. As shown in Figure 6, the cyclic voltammetry curve of RGO/nAPAMSs/GC (E) in 0.1M KCl solution containing 10 mM [Fe(CN)6]3- (scanning rate: 50 mVs-1) shows a couple of well-defined redox peaks at 283 mV and 180 mV. According to the Randles-Sevecik equation, Ip = 2.69×105AD1/2n3/2v1/2C, (Ip represents the redox peak current; A is the electroactive surface area (cm2); D represents the diffusion coefficient of the molecule in solution, whose value is (6.70±0.02) × 10-6 cm2s-1; n is the number of electron transferring in the process, in this system is 1; v is the scan rate, 50 mV s-1; C represents the concentration of the probe molecule in the solution, here is 10 mM.)

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the electroactive surface area of RGO/nAPAMSs modified glassy carbon electrode is about 0.116 cm2, which is much higher than the surface area of the electrode itself and is 1.15, 1.20, 1.25 and 1.35 times higher than those of RGO/PtNPs/GC (0.101 cm2), RGO/AuNPs/GC (0.096 cm2), RGO / GC (0.093 cm2) and bare GC electrode, respectively. The results show that the alloy microspheres composite RGO/nAPAMSs has higher electroactive surface area and better electrochemical performance, comparing with two kinds of single metal nanomaterials - RGO/PtNPs and RGO/AuNPs. These results can be attributed to the stronger synergistic effect between nAPAMSs and RGO.

3.2

Hydrogen Peroxide Electroreduction Ability of RGO/nAPAMSs Based on Electrochemical Measurements

3.2.1 Cyclic Voltammetric Here is Figure 7. Figure 7 shows the CVs of bare GC electrode (a), RGO/GC electrode (b), nAPAMSs/GC electrode (c), RGO/AuNPs/GC electrode (d), RGO/PtNPs/GC (e) and RGO/nAPAMSs/GC electrode (f) in 0.1 M PBS with 5 mM H2O2 at the scan rate of 50 mVs-1. It can be seen that, in the scanning range of -500 ~ 800 mV, the CV of bare GC electrode (a) represented no oxidation/reduction peak of H2O2, and the current response was almost zero. Comparing with bare GC electrode, the response current of RGO/GC electrode (b) increased, but there was no oxidation/reduction peak appear. The CV of nAPAMSs/GC electrode (c) represented obvious cathodic peak and relatively weak current 13


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responds comparing to the CV of RGO/nAPAMSs/GC electrode (f), which indicated that the excellent electrocatalytic activity of reduction towards H2O2 comes from nAPAMSs, instead of RGO whose major function is promoting the electronic conductive and the dispersing of nAPAMSs. The RGO/AuNPs/GC electrode (d), in this voltage range, represented response current increasing significantly with slightly oxidation/reduction peak, while the RGO/PtNPs/GC electrode (e) represented strong current response and an obvious cathodic peak at 60 mV, whose peak current reached to -155 µA. Compared to the two

single

mental

nanocomposites

RGO/AuNPs

(d)

and

RGO/PtNPs

(e),

RGO/nAPAMSs/GC electrode (f) possessed the superior cathodic peak current centered around 80 mV. The cathodic peaks can be attributed to the eletrocatalytic reduction of nAPAMSs towards H2O2. Thus, the most obvious cathodic peak of RGO/nAPAMSs/GC electrode (f) indicates that RGO/nAPAMSs has excellent electrocatalytic activity of reduction towards H2O2 and 80 mV can be chosen as the working potential of I-T test. 3.2.2 The Optimization of Scan Rate and pH Before the I-T test, the two variables - the scan rate and the pH of the H2O2 test system, are optimized first. As shown in Figure 8A, the cathodic peak current rose upon as the scan rate increased from 20 to 80 mVs-1, while the cathodic potential shifts toward negative. It was indicated that electrochemical kinetics is controlled by the adsorption of H2O2. Thus, in order to obtain the obvious current response at the same time ensure the stable scanning, 50 mVs-1 was chosen as the optimal scan rate.

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Here is Figure 8. Figure 8B shows the effect of pH value of PBS on the amperometric response of the RGO/nAPAMSs/GC electrode toward 5 mM H2O2 in 0.1 M PBS. The cathodic peak current added as the increase of pH value from 3.0 to 7.0, and then decreased when pH value rose from 7.0 to 9.0. This result demonstrated that in the H2O2 test system, the intensity of the reduction of H2O2 molecules increased when the pH value rise from 3.0 to 7.0 and decreased following decline of the pH value from 7.0 to 9.0, which means that the H2O2 molecular is unstable and easily reduced at pH 7.0. Therefore, pH 7.0 was chosen as the optimum pH value for H2O2 amperometric detection. 3.2.3 Chronoamperometry Here is Figure 9. Under the optimized conditions, chronoamperometry was carried out for testing the instant current response of the non-enzymatic H2O2 sensor based no RGO/nAPAMSs. Figure 9A shows a typical amperometric response of the RGO/nAPAMSs/GC electrode to a series of H2O2 concentration gradients added into 0.1 M PBS (pH 7.0) at 80 mV. It can be found that the RGO/nAPAMSs/GC electrode showed a fast and stable current response to the addition of H2O2 and the response time is less than 5 s, indicating that RGO/nAPAMSs nanocomposite has excellent electron-transfer ability and electrocatalytic activity. As shown in Figure 9B, the response current increased significantly with the H2O2 concentration enhancement, but the slope of the curve decreased. For RGO/nAPAMSs non-

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enzymatic H2O2 sensor, the response current was linear with H2O2 concentration and had two linear ranges. In the range of 0.005-1.0 mM, the correlation coefficient is 0.982; in the range of 1.0-4.0 mM, the correlation coefficient is 0.983. The regression equations in Figure 9B were Ip（µA）=0.93-78.19C（mM） Ip（µA）=-61.00-26.38C（mM） RGO/nAPAMSs non-enzymatic H2O2 sensor exhibited extremely high sensitivity, which is 1117.0 µA mM-1cm-2 in the range of 0.005 to 1.0 mM, 376.9µA mM-1cm-2 in the range of 1.0 to 4 mM, and quite low minimum detection limit of 0.008 µM（S/N=3）. Comparing with other studies of H2O2 sensors based on Pt or Au reported in recent years, the electrochemical performance of RGO/nAPAMSs/GC electrode is very prominent, as shown in Table 1. Here is Table 1. From the above experimental results, RGO/nAPAMSs non-enzymatic H2O2 sensor has two linear regions, with a higher sensitivity in low H2O2 concentration range and a lower sensitivity in high H2O2 concentration range. This may be caused by the different H2O2 absorption and activation behavior on RGO/nAPAMSs/GC under different H2O2 concentration39. There is no strong interaction between H2O2 molecules, and the molecules can diffuse freely in the low H2O2 concentration range; on the contrary, in the high concentration range, the stronger adsorbing interaction between H2O2 molecules leads to a

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decrease in sensitivity40. RGO/nAPAMSs/GC electrode exhibiting excellent performances in H2O2 detection may be attributed to the synergistic effect between nAPAMSs (large specific surface area, strong conductivity and electrocatalytic activity) and RGO (and high electrical conductivity), which contributed to H2O2 molecules adsorbing on the electrode surface, thereby speeding up the electron-transfer between H2O2 molecules and the surface of GC electrode. To testify its capability for practical applications, real samples analysis has also been carried out. The RGO/nAPAMSs non-enzymatic H2O2 sensor was investigated to detect H2O2 in disinfected fetal bovine serum (FBS). As the results displayed in Table 2, the recovery was in the range of 95.00-102.50% and the relative standard deviation (R.S.D.) was ranged from 2.63% to 3.45%, indicating that the sensor has the potential to be used for the determination of H2O2 in real samples. Here is Table 2. 3.2.4 Selectivity Here is Figure 10. Selectivity is a significant parameter for non-enzymatic hydrogen peroxide sensing, and Uric acid (UA), acetaminophen (AP), ascorbic acid (AA) and glucose (Glu) are four kinds of common physiological species that cause serious interference with the nonenzymatic H2O2 detection in physiological conditions. The selectivity performance of RGO/nAPAMSs/GC electrode was investigated by comparing the amperometric current

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responses to 0.1 mM uric acid (UA), 0.1 mM acetaminophen (AP), 0.1 mM ascorbic acid (AA), 0.1 mM glucose (Glu) and 0.1 mM H2O2 at the applied potential of 80 mV vs. Ag/AgCl. As shown in Figure 10, the response caused by UA, AA, AP and Glu were negligible, which indicates that RGO/nAPAMSs/GC electrode has excellent selectivity for the non-enzymatic determination of H2O2. This result confirmed that RGO/nAPAMSs can be used for highly selective electrochemical detection of H2O2 in physiological conditions. 3.2.5 Repeatability and Long-Term Stability Here is Figure 11. In addition, there are another two significant parameters to evaluate the performance of the sensor, the repeatability and long-term stability. The repeatability of the prepared H2O2 sensor was evaluated by detecting the response current to 0.5 mM H2O2 at five electrodes independently, and the average relative standard deviation (R.S.D.) was calculated to 3.2%. Moreover, by detecting the response current to 0.5 mM H2O2 solution every 5 days for one month, the long-term stability of RGO/nAPAMSs/GC electrode was evaluated. It remained 92.2% of its initial current response for H2O2 after one month (Figure 11). Both of the results demonstrated that RGO/nAPAMSs non-enzymatic H2O2 sensor has favorable repeatability and stability. 3.3

The Anatomy of RGO/nAPAMSs To further explore the role of Pt and Au individually in the highly sensitive H2O2

sensing of RGO/nAPAMSs, we also carried out H2O2 chronoamperometry testing based

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on RGO/PtNPs and RGO/AuNPs respectively. 3.3.1 Amperometric Response of RGO/PtNPs/GC elecrtode to H2O2 Here is Figure 12. Figure 12A shows the amperometric response of RGO/PtNPs to a series of H2O2 concentration gradients added into 0.1 M PBS (pH 7.0) at 60 mV (in Figure 7 (d), RGO/PtNPs represented cathodic peak at 60 mV). It can be found that the RGO/PtNPs showed a fast and stable current response to the addition of H2O2 and the response time is less than 5 s. As shown in Figure 12B, the RGO/PtNPs/GC electrode exhibited good linear relationship between the response current and the H2O2 concentration in the range of 0.0051.0 mM with the sensitivity of 1107.3 µA mM-1cm-2 and in the range of 1.0-4.0 mM with the sensitivity of 357.6µA mM-1cm-2, and had a minimum detection limit of 0.013 µM （S/N=3）, which indicates that RGO/PtNPs has strong electron transporting ability and obvious electrocatalytic activity to H2O2. 3.3.2 Amperometric Response of RGO/AuNPs/GC elecrtode to H2O2 Here is Figure 13. Comparing with the RGO/PtNPs/GC electrode, the I-T test result of RGO/AuNPs/GC electrode was unsatisfactory. Since the CV of RGO/AuNPs/GC electrode did not have an obvious cathodic peak in the scanning range of -500 ~ 800 mV, we extended the scanning range to -1000 ~ 1000 mV. In Figure 13A, it can be seen that RGO/AuNPs/GC electrode in 0.1 M PBS with 5mM H2O2 (b) has a cathodic peak at -500 mV, comparing with the CV

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of RGO/AuNPs/GC electrode in 0.1 M PBS (a). Thus, we chose -500 mV as the working potential of chronoamperometry to the electrocatalytic reduction to H2O2 of RGO/AuNPs. As shown in Figure 13B, the response to the addition of H2O2 concentration gradients was slow with response time much longer than 5 s. The sensitivity of RGO/AuNPs/GC electrode was much lower than RGO/PtNPs/GC electrode and there was no obvious linear relationship between response current and H2O2 concentration, which indicates that the electrocatalytic activity of AuNPs to H2O2 is poor. But what is worth to mention is that the maximum detection limit of RGO/AuNPs/GC electrode was up to 8 mM, which was only 4 mM as to RGO/PtNPs/GC electrode. This indicates that RGO/AuNPs has the advantage of being resistant to higher concentration of H2O2. Combining with Figure 7, these results demonstrated that RGO has strong conductivity but no electrocatalytic property of H2O2 reduction, while AuNPs has weak electrocatalytic effect on H2O2 and conductivity enhancing ability. By comparison, PtNPs has obvious electrocatalytic effect on H2O2 reduction and stronger conductivity enhancing ability. 4. Conclusion In this work, we successfully synthesized RGO/Nano Au & Pt alloy microspheres by a simple, facile and eco-friendly one-step reduction strategy for the fabrication of highly sensitive non-enzymatic H2O2 sensing interfaces. The prepared non-enzymatic H2O2 sensor exhibited excellent electrocatalytic activity toward H2O2 with high sensitivity, low detection limit, wide linear range and strong stability, which made it promising for the

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quantitative detection of H2O2. Moreover, Further analysis showed that the superior H2O2 sensing performance of RGO/nAPAMSs mainly attributed to the extremely strong electrocatalytic activity of Nano Pt ingredient and the fusion with Nano gold further promoted its electrocatalytic activity, which may be due to the synergistic effect between Nano Pt and Nano Au. But Nano Au itself do not has strong electrocatalytic activity to H2O2. This discovery provides new principles and ideas for the future study of nonenzymatic H2O2 sensing based on Pt, Au noble metal electrocatalysts. Acknowledgements The financial supports from National Natural Science Foundation of China (Grant Nos. 81671779) are acknowledged.

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(Supplement C), 195-200. Table captions Table 1. Electrochemical performances of different non-enzymatic H2O2 sensors. Table 2. Determination of H2O2 concentration in FBS samples (n=6). Figure captions Figure 1. EDX spectrum of RGO/nAPAMSs nanocomposites. Figure 2. EDX mappings of RGO/nAPAMSs nanocomposites. Figure 3. XRD patterns of GO (a), RGO (b) and RGO/nAPAMSs (c). Figure 4. TEM images of RGO (A) and RGO/nAPAMSs under different magnification (B、 C and D). Figure 5. FT-IR spectra of GO (a) and RGO/nAPAMSs (b). Figure 6. CVs of bare GCE (a), RGO/GC electrode (b), RGO/AuNPs/GC electrode (c), RGO/PtNPs/GC electrode (d), RGO/nAPAMSs/GC electrode (e) recorded in 0.1 M KCl solution containing 10 mM [Fe(CN)6]3-. Scan rate: 50 mVs-1. Figure 7. CVs of bare GC electrode (a), RGO/GC electrode (b), nAPAMSs/GC electrode (c), RGO/AuNPs/GC electrode (d), RGO/PtNPs/GC electrode (e) and RGO/nAPAMSs/GC electrode (f) in 0.1 M PBS (pH=7.0) with 5 mM H2O2. Figure 8. (A) CVs of the RGO/nAPAMSs/GC electrode in 0.1 M PBS (pH=7.0) with 5 mM H2O2 at differernt scan rate: 20, 30, 40, 50, 60, 70 and 80 mVs-1. (B) The effect of pH on the amperometric response of 0.5 mM H2O2 in 0.1 M PBS. Figure 9. (A) Amperometric response of RGO/nAPAMSs/GC electrode upon successive 27


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additions of H2O2 into 0.1 M PBS solution under stirring. The inset was the amplification of curve with the lower concentration region. (B) The calibration curve between amperometric response and H2O2 concentration. Fugure 10. Selectivity test of RGO/nAPAMSs/GC electrode in 0.1 M PBS (pH=7.0) with 0.1 mM H2O2 and other interfering substances (0.1 mM UA, 0.1 mM AP, 0.1 mM AA and 0.1 mM Glu). Figure 11. The long-term stability of the RGO/nAPAMSs non-enzymatic H2O2 sensor. Figure 12. (A) Amperometric response of RGO/PtNPs/GC electrode upon successive additions of H2O2 into 0.1 M PBS solution under stirring. The inset was the amplification of curve with the lower concentration region. (B) The calibration curve between amperometric response and H2O2 concentration. Figure 13. (A) CVs of RGO/AuNPs/GC electrode in 0.1 M PBS (a) and 0.1 M PBS with 5 mM H2O2 (b). (B) Amperometric response of RGO/PtNPs/GC electrode upon successive additions of H2O2 into 0.1 M PBS solution under stirring. The inset was the amplification of curve with the lower concentration region.

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Preparation of Nano Au & Pt Alloy Microspheres Decorated with

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