Nanowires Nickel Oxide and Nanospherical Manganese Oxide Synthesized via Low Temperature Hydrothermal Technique for Hydrogen Peroxide Sensor

Binary catalyst nickel oxides (NiO) andmanganese oxides (MnO) were prepared individually via hydrothermal route.The catalysts were characterized by scanning electron microscope (SEM), Brunauer-Emmett-Teller (BET) analysis, cyclic voltammetry (CV), and amperometry. Morphology studies revealed physical structure of nanowires nickel oxide and spherical manganese oxide with estimated length of 0.3–2.3 μm and diameter of 0.2–0.8 μm, respectively. Surface areas obtained for nickel oxide and manganese oxide were 68.9m g and 45.2m g, respectively. Cyclic voltammetry exhibits electrochemical responses corresponding to the electrode surfaces. The linear responses of the binary catalyst modified gold electrodes with NiO-MnO were observed in the concentration range from 31.8μM to 0.5mM with the detection limit of 62.5 μM.


Introduction
Hydrogen peroxide was employed in many applications including pharmaceutical, food, and industry applications.It can also be a biological marker for certain diseases like cancer, aging, and renal diseases [1][2][3][4].Hence, rapid, sensitive, and accurate determination of H 2 O 2 is extremely important.Amperometry is the most attractive techniques to detect hydrogen peroxide because of high sensitivity, low cost, and its simplicity [5].Nanomaterials gained a lot of interest among researchers due to their excellent mechanical and electrical properties.To increase the sensitivity of the sensor, nanoparticles (NPs) are the best candidate due to their large specific surface area and biocompatibility [6].Nickel oxide (NiO x ) was intensively studied because of its high electron transfer capability and good biological compatibility [7][8][9][10].However, the performance of these electrodes could be limited by surface poisoning [11].Some studies revealed that, by using multicomponent surface compositions, especially the metal composite can overcome surface poisoning problems [12,13].Manganese oxide (MnO x ) is a very attractive metallic oxide and widely used in fuel cells, sensor, catalyst, and metal coating [14][15][16][17].Moreover, MnO x was also used as a cocatalyst material in biosensor and fuel cells [18].Binary catalysts are very attractive materials for their unique properties, when compared to single catalyst.For example, Ding et al. fabricated NiO hybrid nanowires with uniformly distributed Ag and Au NPs, enhancing glucose biosensing performance.Polydopamine has been widely used as a coating agent for various inorganic and organic substrates [19,20].Dopamine will self-polymerize at alkaline pH oxidizing and forming thin polydopamine films.Some researchers are working on binary catalyst NiO-MnO for various applications [21][22][23][24].For example, Zhang et al. synthesized MnO-NiO composite oxide catalyst for lean methane combustion at low temperature.However, to the best of our knowledge, there are not many published reports on the binary catalyst NiO-MnO for hydrogen peroxide detection.Therefore, this work could be beneficial for researchers who make studies on catalyst for electrochemical sensors.We report on fabrication of polydopamine-nickel oxide-manganese oxide composite on gold electrode to detect low concentrations (31.8 M to 0.5 mM) of hydrogen peroxide.

Apparatus.
A binary catalyst was individually prepared by using Muffle Furnaces, P80-Series.The material characterizations testing was performed using Brunauer-Emmett-Teller (BET) BELSORP-mini II, scanning electron microscope (SEM), and LEO 1450VPSE.For sensor fabrication, 2-electrode system was used; both working and counter electrodes were made from gold.The electrode was obtained from Bioptik Company (Taiwan).The electrochemical tests were performed by using home-made potentiostat.For voltammetry testing, the potential was swept between −1 V and +1 V with the scan rate of 50 mV/s.The solution for voltammetry testing was 0.1 M phosphate buffer, pH 7.4.For amperometry testing, the applied potential was 0.7 V.The concentration of hydrogen peroxide was varied from 31.8 M to 0.5 mM.The product was separated from the solution by filtration process, washed with deionized water and ethanol several times, and finally dried at 120 ∘ C in the air.The powder was further heated to 400 ∘ C for 2 hours [25].Finally, the nickel oxide was obtained as shown in Figure 1(c).

Synthesis NiO and
To prepare manganese oxide (MnO), 0.2 M Mn(CH 3 COO) 2 and 0.2 M KMnO 4 were individually dissolved in deionized water.These solutions were mixed together, with the molar ratio of 1 : 1 as shown in Figure 2(a).The mixed solution was loaded into the stainless steel mould.During the hydrothermal reaction, the heating temperature setting was 125 ∘ C for 3 hours [26].The black precipitates were obtained and collected through filtration and washed with deionized water and finally dried at 120 ∘ C for 10 hours.Finally, the manganese oxide was obtained as shown in Figure 2(b).

Modified Gold Electrodes.
For single catalyst modified gold electrodes, dopamine solution was prepared by mixing 1 mg dopamine with 1 mL phosphate buffer (pH 8.4).Single catalyst solutions were individually prepared by mixing 1 mg of each nickel oxide and manganese oxide in 1 mL ethanol solution.Single catalyst modified electrodes were prepared by dropping 10 L of single catalyst suspension on the electrodes surface and dried at room temperature (first layer).This type of modified gold electrodes was used in cyclic voltammetry testing.Then, for amperometry testing, 6 L of dopamine solution was dropped onto metal oxide layer and dried at room temperature with exposure to the room light for 7 hours (second layer).
For binary catalyst modified gold electrode, 1 mg nickel oxide and 1 mg manganese oxide were mixed in 1 mL of ethanol solution.Then 10 L of this suspension was dropped on the electrode surface and dried at room temperature (first layer).This type of modified gold electrodes was used in cyclic voltammetry testing.Then, for amperometry testing, 6 L of dopamine solution was dropped onto metal oxide layer and dried at room temperature with exposure to the room light for 7 hours (second layer).
Schematic of modified electrode for amperometry testing is shown in Figure 3.

Results and Discussion
Figure 4 presents SEM images taken from the synthesized nanoparticles.Figure 4(a) shows an image of the nickel oxides.The nanowires nickel oxides are in agglomerated form.The estimated wires length is between 0.3-2.3m. Figure 4(b) shows an image of the manganese oxides.The structure of manganese oxides is in spherical shape with estimated diameters size between 0.2-0.8m.The specific surface areas of the synthesized nanoparticles were also estimated using Brunauer-Emmett-Teller (BET) analysis.The BET specific surface areas of NiO and MnO were 68.9 m 2 g −1 and 45.2 m 2 g −1 , respectively.In our work, NiO, MnO, and their mixture were used as a catalyst for hydrogen peroxide detection.Polydopamine was used as a coating material for fixing metal oxide on the electrode surface.Therefore, the modified electrode with polydopamine can be used for multiple measurement.Dopamine is a simple and easy coating material because it can undergo self-polymerization under effects of light, pH, and heat.Dopamine will undergo several transformation phases before forming a polymer.Mechanism of dopamine polymerization is reported by Jang et al. [27].Polymerization of dopamine can also be observed by the colour change from clear to darkish brown [28].
Figure 5 shows XRD patterns of manganese oxide and the nickel oxide.It was found that the nickel oxide shows very high and sharp crystalline peaks compared to the manganese oxide.The crystalline peak from another works were also matched in the same figure [29][30][31].This nickel oxide shows only the divalent form (NiO) [29].The manganese oxide was a mixture of Mn 3 O 4 and MnO 2 [30,31].The crystallinity of most oxides is related to the heating temperature during the preparation.Our nickel oxide shows higher crystallinity than manganese oxide because they were prepared at 400 ∘ C and 125 ∘ C, respectively.
The electrochemical properties of modified gold electrodes without polydopamine were investigated using cyclic voltammetry (CV) as shown in Figure 6.The testing was conducted in 10 mM phosphate buffer with 50 mV/s −1 scan rate.Both bare gold electrode and polydopamine modified gold electrode exhibit almost no voltammetric response.This implies that electron transfers at both of these electrodes surfaces are very slow.The current at the electrode with polydopamine was a little bit lower than the bare electrode.The coverage of polydopamine layer might reduce the electron transfer.However, it is necessary for immobilization of the nickel oxide and manganese oxide.After deposition of single or binary catalysts on the electrode surfaces, the voltammetric responses were greatly improved.Currents show a dramatic increase when MnO, NiO, or NiO + MnO catalysts were coated on the gold electrodes surface.
However, single and binary catalysts exhibit different electrochemical responses as shown in Figure 5. Modified electrode with single catalyst NiO showed small redox peaks at 0.50 V and it may be attributed to the following redox reaction [32]: Modified gold electrode with MnO single catalyst showed two significant reduction peaks at 0.254 V and −0.558 V.The reduction peaks are attributable to the following equations [33]: at −0.558 V, Modified gold electrode with (NiO-MnO) binary catalyst showed a stable voltammetric response without oxidationreduction peaks.Figure 7 shows amperometric response at different concentration of H 2 O 2 for the modified gold electrodes.The modified gold electrode with binary catalyst showed highest amperometric response.The highest sensing activity by modified gold electrode with binary catalyst may be attributed to an increased number of Ni 3+ active sites induced by MnO.El-Refaei et al. found that the electrocatalytic activity greatly improved when they used both MnO and NiO catalyst together for glucose oxidation.They also explained the significant role of MnO in changing catalytic activity of NiO [34].MnO will convert Ni 2+ to Ni 3+ ; therefore it will increase the charge carrier concentration leading to an increased conductivity.The proposed hydrogen peroxide sensor displays a detection range from 31.8 M to 500 M with the detection limit of 62.5 M.

Conclusions
A binary catalyst with nickel oxides nanowires and manganese oxides spherical nanoparticles was successfully synthesized via hydrothermal technique.A simple technique was used to fabricate all modified gold electrode for hydrogen peroxide detection.With a binary catalyst of NiO-MnO modified gold electrode, the sensitivity of the sensors is enhanced in comparison with single catalyst modified gold electrodes.This can be attributable to the increased number of Ni 3+ active sites induced by MnO.However, modified gold electrode with single catalyst NiO has better linear response  to hydrogen peroxide if compared to modified gold electrode with binary catalyst.
MnO.All experiments were conducted under air atmosphere.Nickel oxide and manganese oxide were prepared by hydrothermal technique.To prepare nickel oxide (NiO), 0.1 M of Ni(CH 3 COO) 2 ⋅4H 2 O were dissolved in a mixed solvent of deionized water and ethylene glycol with a ratio (1 : 1) as shown in Figure1(a).The solvent was transferred into the stainless steel close mould.The furnace was maintained at 200 ∘ C for 7 hours.Nickel hydroxide Ni(OH) 2 was obtained at this stage as shown in Figure 1(b).

Figure 3 :
Figure 3: Schematic of modified gold electrode (layer by layer).

Figure 6 :
Figure 6: Cyclic voltammetry of modified gold electrodes from −1 V to +1 V at scan rate 50 mV s −1 .All curves correspond to 3rd cycle.

Figure 7 :
Figure 7: Amperometric response for modified gold electrodes with different concentration of H 2 O 2 at applied potential of 0.7 V.