Graphene oxide was synthesized by Hummers method, starting with natural graphite (3 g, 20 μm, 99.99% purity, from Lianyungang Jinli Carbon). The remaining graphite was heated at 300˚C in oven for 2 h. to remove the water content. The dried solid was then mixed with 1.5 g of NaNO₃ and 9 g of KMnO₄, by slow adding 69 ml concentrated sulfuric acid to the mixture in a 1000 ml beaker under cooling and stirred at room temperature for 30 min. The solution would be diluted using 420 ml of water so that the temperature of the system went down to 50˚C. Slowly added 3 ml of 30% H₂O₂ to the beaker while stirring. The suspension was stirred at certain temperature and time and the solution was washed using deionized water with vacuum filtration. Finally, graphene oxide (GO) precipitates were dissolved in 30 ml of distilled water.

The GO was dispersed in the water to form a 30 ml of 0.2 mg∙ml⁻¹ by ultrasonication for 20 min. Added 0.2 M of ZnCl₂ into the GO dispersion, then added NaOH 0.1 M, 30 ml. The microwave equipment was used for the synthesis, consuming the power of 420 W for 30 s and the sample was suspended for 20 s then repeated this procedure until 10 min. Then adjusted the pH of the sample to be neutral. After ultrasonicating for 20 min, the sample reacted for 2 hours at 80˚C; dark grey precipitates were obtained by vacuum filtration. The sample was then washed using DI-water until the pH was close to 7. The precipitates were dried in tube furnace at 300˚C for 2 hours in Ar-ambient air in order to form the zinc oxide (ZnO) structure.

Raw durian peels, obtained generally from a market in Thailand, were used for the synthesis of DPA. Before carbonization, the durian peels were thoroughly cleansed using filtered water at least 2 times and the deionized water was used at the final stage. The durian peels were cut into thin slices of size 2 cm × 1 cm and combusted in tube furnace at 650˚C for 2 hours in Ar-ambient air. The DPA sheets were dry-milled using stainless ball mill for 12 h. in 50 mL Teflon-bottle. Then the solid product was sieved using steel wire mesh. The ash product obtained was denoted for the DPA powder.

Transmission electron microscope (TEM, JEOL 1200) and field emission scanning microscope (FESEM, Hitachi, s-4700) were used to investigate the morphology of the prepared rGO, ZnO/rGO and DPA composites. The GO, rGO ZnO/rGO and a%DPA:b%ZnO/rGO composites were dropped on the substrates of glass slide and dried on the hotplate, then the product film was characterized by X-ray diffraction (XRD) on Bruker D8 Advance diffractometer with Cu Kα radiation at a slowly scan rate of 0.021 degree/s to study the structural property.

The electrochemical measurement was carried out in aqueous solution and the composite samples with apparent area of 1 × 1 cm² were synthesized and used as a working electrode, a Pt wire was used as a counter electrode, a saturated calomel electrode (SCE) was employed as a reference electrode. The loading amount of GO, rGO, and a%DPA:b%ZnO/rGO composites were determined by the weight difference of the above electrode was approximately 1.5 mg. The stock solution was prepared from 0.1 to 1 mM. Amperometric detection VersaSTAT3 potentiostat with three-electrode cell was employed for electrochemical hydrazine detection.

There is homogeneous growing on the plane of rGO as seen from Figure 1(a), the SEM horizontal image, and Figure 1(b), the SEM vertical image. The microstructure and the dimension of the zinc oxide nanoparticles (ZnO) were similar to those of the nanoparticles, as seen in Figure 1(c). The synthesis was in fact carried out in deoxygenated hi-purity water to minimize oxidation [18]. Figure 1(d), the top of the surface of DPA shows a high porosity of DPA and its bottom surface is shown in Figure 1(e). The average thickness of the as-prepared rGO, which measured from the height profile of the AFM image, is about 1.04 nm, (approximately 3 layers of rGO) which corresponds to a few layers of rGO compared with the theoretical value of 0.34 nm for monolayer graphene, Figure 1(f). The morphology and the size distribution of ZnO nanoparticles were well-developed spherical shape at approximately 68.76 ± 4.08 nm, as seen in Figure 1(g).

The X-ray diffraction (XRD) patterns of rGO, ZnO, ZnO/rGO and DPA composites are shown in Figure 2 with the main peak of XRD of rGO found at 2 theta of about 25 degrees. The XRD pattern of ZnO nanoparticles is found at room temperature. After thermal treatment in ambient atmosphere it changes to ZnO/ rGO powder. Additionally, the impurities could not be observed. All possible peaks of ZnO were observed indicating the polycrystalline nature of the product. The XRD pattern of ZnO/rGO composite confirms the formation of ZnO nanoparticles through the characteristic sharp peaks at (100), (002), (101), (102), (110), (200), (103) and (121) planes of the wurtzite structure of ZnO, which is consistent with the JPCDS 36-1451 database [19].

The specific surface area (SSA) of the samples was shown in Table 1. and the kind of microstructure is determined by the selection of the substrate. The activated process and the preparation were controlled as in [20]. From Table 1, it is obvious that rGO can give very high SSA of 625.89 m²/g whereas the pure DPA holds the SSA of 309.70 m²/g. When ZnO is attached to the surface of rGO, the SSA of 70%DPA:30%ZnO/rGO is decreased to 208.40 m²/g. Babitha and coworkers explained that when ZnO NPs were developed in situ on the rGO, and the size of ZnO nanocrystals was controlled, the SSA of ZnO/rGO nanocomposite was decreased as ZnO nanocrystals could insert into rGO assembly [21].

Figure 3(a) shows the current density and the concentration of the samples: GO, rGO and ZnO/rGO. It can be seen that ZnO/rGO gives the best performance, followed by rGO and GO has the lowest. The best response of ZnO/rGO was obtained at 0.4 mM of hydrazine concentration. Figure 3(b) exhibits the current density when varying %DPA with different scan rate of 0 - 0.1 V/s. As expected, the CVs of the a%DPA:b%ZnO/rGO mixed with Nafion-117 polymer modified

Remark: The graphite powder was treated at 300˚C for 2 h in air atmosphere before analyzing.

electrode exhibit a single redox couple with the anodic peak at +0.49 V and the cathodic peak at +0.16 V versus Ag/AgCl/KCl. Figure 3(c) shows the plot of 70%DPA:30%ZnO/rGO modified electrode signal of the anodic and maximum peak current vs. the scan rate. The linear relationship in the anodic current of 70%DPA:30%ZnO/rGO modified electrode according to the scan rate expressed that the film obtained specific characteristics of the surface controlled thin-layer electrochemical behavior. The corresponding linear regression equation was I_max (mA) = 17.76 + 0.17 s, with R² = 0.9060 where s is scan rate (mV/s). Figure 3(d) shows the electrochemical activity of 70%DPA:30%ZnO/rGO. From the shape of the impedance spectrum, the electron transfer kinetics and diffusion characteristics can be determined. The EIS was conducted in 0.4 mM hydrazine monohydrate solution at the frequency range from 10 mHz to 10 kHz. The straight line showing the relationship between the current and the concentration could be observed. The high frequency was ascribed to the double-layer capacitance (CDL) in parallel with Rct at the contact interface between electrode and electrolyte solution [22]. At low frequency, the impedance plot should theoretically

be a vertical line, which is parallel to the imaginary axis. As can be seen from Figure 3(d) and Table 1, the relationship between the imaginary part and the real part of impedance is quite linear; this result can be caused by the porosity of 70%DPA:30%ZnO/rGO is higher than that of other composites under the study which reflects that the solution resistance(Rs) decreased with the increasing of porosity [23].

Figure 4(a) shows the typical amperometric curve of 70%DPA:30%ZnO/rGO composite on successive step change of hydrazine concentration. Under the pre- sent condition, the steady state current has a linear relationship with the concentration of hydrazine in the range from 0.1 to 1 mM. It can be concluded that 70%DPA:30%ZnO/rGO composite is a good candidate for the fabrication of efficient hydrazine sensors. Figure 4(b) shows the positive linear relationship between the current and the concentration of hydrazine which is I (μA) = 48.69 + 21.91C (mM) with the association coefficient (R²) of 0.9870. The responsibility of the composite material was 222.92 mA/mM∙cm². The minimum concentration of hydrazine solution that the modified electrode can measure was 0.125 mM, obtained from 3 × standard deviation/slope.