Electrochemical Deposition of Reduced Graphene Oxide Decorated with Silver and Nickel Oxide Nanoparticles for Quantification of Ascorbic Acid in Bottled Fruit Juice and Vitamin C Tablet

Because of its simplicity of evaluation, ability to detect the lowest quantities, and convenience of operation, electrochemical determination of ascorbic acid is gaining popularity. The high sensitivity, selectivity, and low cost, nanosensors have gained enourmous attention in recent years for the detection of active pharmacological compounds and food pollutants. Ni and Ni-containing compounds have a favorable affinity for a number of organic functional groups such as -thio, -hydroxyl, -carboxyl, and -amine. However, its poor conductivity hinders its electrochemical performance. Hence, the procedures for improving the conductivity of metal oxides that are incredibly good studies query to meet the desired level of detection. We devised a straightforward method for concurrently synthesizing silver and nickel oxide nanoparticles on reduced graphene oxide using electrochemical deposition method on a glassy carbon electrode (GCE). The electrodeposited materials were scratched from GCE and characterized for Raman, SEM/EDS, EIS, and XPS. The materials produced after deposition were examined also for Ascorbic Acid (AA) detection in bottled fruit juice and Vitamin C tablets yielded 0.457 μM limits of quantification and 100.61% and 99.40% average recoveries, respectively.

Ascorbic acid (AA), often known as vitamin C, is a nutrient that aids in the body's production of collagen, a protein that gives structure to blood vessels, muscles, bones, teeth, and cartilages. 1,2 It is an antioxidant that is water soluble and provides hydrogen to stop oxidation and subsequently tissue damage. 3,4 The human body is unable to produce ascorbic acid on its own. 5 Fruits and vegetables have different amounts of ascorbic acid. 6 Citrus fruits, leafy vegetables, green and red peppers are all high in ascorbic acid. 7 Patients with AIDS and cancer are frequently advised to take supplements that are high in vitamin C. 8 Ascorbic acid in food samples and vitamin C pills must be identified and measured as a consequence. The amount of ascorbic acid in medications, vegetables, fruits, and juices has been measured using a number of methods. Colorimetric techniques based on bromine/water are commonly used to determine ascorbic acid. 9 Fuorimetric method, 10 titrimetric method, 10 Dichlorophenolindophenol dye method, 11 spectroscopic methods such as high performance liquid chromatography (HPLC) 12,13 and gas chromatography (GC). 14 However, because they involve the use of several chemicals, labor-intensive analyses, and expensive methodologies, these technologies are not sustainable. 1 Due to its rapid analysis, ability to detect smaller amounts, and simplicity of application, electrochemical determination of ascorbic acid is becoming more and more popular. 15 Nano-sensors that can meet the needs of quick and accurate analysis for target analytes have gained a lot of interest in the detection of active pharmaceutical ingredients and food hazards in recent years due to their high sensitivity, selectivity, and low cost. 16,17 The conventional approach of depositing a tiny amount of one metal on top of another conductive substrate is in order to change its surface properties is known as electrodeposition, which is part of the process known as electrochemical deposition. It is a method that uses electrical current to decrease the cations of a selected material from an electrolyte and coat those materials as a thin film onto a conductive substrate surface. It is based on the principle of electrolysis. This is done to increase heat tolerance, reduce wear and friction, obtain the necessary electrical and corrosion resistance, and for aesthetic purposes. As a result, it is possible to adjust a material's surface characteristics and use this technology to investigate new materials for a number of applications. A straightforward electrodeposition device for metal ion solution is shown in Fig. 2.
Nickel oxide (NiO) and silver nanoparticles are key transition metals that might be used in batteries, 18-21 chemical sensors [22][23][24][25] and solar energy conversions 26,27 due to its enhanced catalytic, electrical and magnetic properties. As shown in scheme 1, it has been demonstrated that a number of organic functional groups, including -thio, -hydroxyl, and -amine, -phosphate, have a high affinity for NiO or Ni-containing compounds. [28][29][30] However, due to its low conductivity, the electrochemical performance is poor. 28 As a result, enhancing the conductivity of the metal oxide material is an important research subject in order to achieve the appropriate degree of detection. As a result, we developed a simple method for synthesizing silver and nickel oxide nanoparticles decorated on the surface of reduced graphene oxide using an electrochemical deposition process on a glassy carbon electrode. Hence, ascorbic acid in a bottled fruit juice and vitamin C tablets was quantified. To the best of our knowledge, the work has not been reported elsewhere. It is to note that the electrochemical deposition of graphene loaded silver and nickel oxide nanoparticles on glassy carbon electrode may boost the electrochemical sensitivity of Ni towards ascorbic acid due to the coupled synergetic effect in NiAg/rGO nanocomposite. z E-mail: tadechem@gmail.com

Methodology
Apparatuses and instruments.-Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Raman, X-ray photoelectron spectroscopy (XPS), water bath, oven, furnace, analytical balance, magnetic stirrer (different sizes), mortar and pestle, thermometer, beakers with different sizes, volumetric flasks (different size), pipettes, micropipettes, pH meter plastic test tubes centrifuge tubes 40 ml, plastic test tubes centrifuge tubes 10 ml, centrifuge, Biologic cyclic voltammetry (CV) instruments. Synthesis of graphene oxide.-Modified Hummers′ method is a chemical process that can be used to generate graphite oxide through the addition of potassium permanganate to a solution of graphite, sodium nitrate, and sulfuric acid and the GO was synthesized by modified Hummers method through oxidation of graphite powder. 31 2 g of graphite powder and 2 g of NaNO 3 were mixed in 50 mL of H 2 SO 4 (98%) in a 1000 mL volumetric flask which was kept under at ice bath (0°C-5°C) with continuous stirring and the mixture was stirred for 2 h at this temperature. 6 g of potassium permanganate was added to the suspension very slowly and carefully. The rate of addition was carefully controlled to keep the reaction temperature lower than 15°C. The ice bath was removed, and the mixture was stirred at 35°C until it became pasty brownish and was kept under stirring for 2 d and it was diluted with slow addition of 100 ml DI water. The reaction temperature was rapidly increased to 98°C with effervescence, and the color was changed to brown color. Further this solution was diluted by adding additional 200 ml of water stirred continuously. The solution was finally treated with 10 ml H 2 O 2 to terminate the reaction with the appearance of yellow color. For purification, the mixture was washed by rinsing and centrifugation using 10% HCl and then deionized (DI) water several times. After filtration and drying, the graphene oxide (GO) was obtained as a powder. The prepared GO was sonicated for 2 h, centrifuged, dried it again in the oven, and further characterized by Raman and SEM for further applications.
Preparation of phosphate buffer and standard solutions of analytes.-Phosphate buffer was prepared from 0.1 M K 2 HPO 4 .2H 2 O and KH 2 PO 4 at different pH by adjusting the pH with 0.5 M H 2 SO 4 and 0.5 M NaOH drop wise until optimum pH one is obtained. After optimum pH was obtained, different concentrations of ascorbic acid were then prepared in 0.1 M phosphate buffer of pH 2.
Electrochemical measurements.-The electrochemical behavior of standard compound of ascorbic acid was investigated using cyclic voltammetry (CV) in range of different oxidation or reduction potential. The determination of analytes concentration was carried out using differential pulse voltammetry (DPV). DPVs will be obtained by scanning the potential in the different range of frequency. Before each experiment, modified electrode will be washed with distilled water. 0.05 M NaOH solution was used as electrolyte and its CV was run with a scan rate of 50 mV s −1 until  the peak of standard analytes disappeared. The standard analytes concentrations were obtained by measuring the heights of the oxidative peak currents from DPV curve.
Calibration curve preparation.-In order to prepare a calibration curve of the standard analytes, different concentrations of standard ascorbic acid was prepared from 10 μM-700 μM using phosphate buffer at pH 2. Differential pulse voltammetry (DPV) at pulse of amplitude 25 mV, with pulse width of 25 ms as well as the potential window of 0-0.2 V to 0.4 V Vs AgCl/Cl− each of them prepared standard solutions were measured and calibration curves was drawn using the origin software and the detection limit was calculated using the following formula: LOD = k*s/m, Where k is chosen to be 2 or 3; the k value 2 corresponds to a confident level of 92.1% the k value 3 corresponds to a confident level of 98.3%, s is the standard deviation of the blank and m is the slope of calibration. 32 Real sample analysis.-The proposed method was applied for the determination of ascorbic acid in bottled fruit juice and vitamin C tablet. Vitamin C was weighed and powdered finely in a mortar. The average weight of each sample powder was added to phosphate buffer solution in 100 ml volumetric flask and shaken until it was dissolved. The solution was then be centrifuged and filtered by No 41 Whatman filter paper. The standard solution of ascorbic acid was filled with phosphate buffer and it was further diluted. The diluted solutions were expected to have a specified amount of ascorbic acid. The DPV was measured using cyclic voltammetry instruments. The calibration curve of the standard analytes was extrapolated and recovery was calculated by dividing the obtained concentration to the spiked one. Ascorbic acid in bottled fruit juice was analyzed first bottled fruit juices were obtained from supermarket then, ascorbic acid content in the bottled fruit juice samples was diluted in DI water and was followed by measuring its DPV then, its concentration was extrapolated from the calibration curve and recovery was calculated by dividing the obtained concentration to the spiked.
Reproducibility test.-The modified electrode was left at room temperature for limited period of time then it was tested for its electrochemical activity toward the analytes of interest and its reproducibility was determined by comparing with its initial activity.

Results and Discussion
The morphology and structural characterization of graphite powder and synthesized GO by Hammer modified.- Figure 1 presents the respective SEM images of the synthesized GO by Hammer modified and graphite powder. Figure 1a illustrates the graphite powder, as it is clearly indicating the stacked sheet of graphene; whereas Fig. 1b depicts the dispersed sheets of graphene oxide. This confirms the successful exfoliation of few layers of graphene oxide sheets by modified Hammer's Method. GO can be synthesized chemically from graphite powder and graphene oxide in its structure contains different functional groups attached on the basal planes and the sideways of the structure. The structures, qualities, and size distribution of the GO sheets prepared from graphite powder precursors are nearly identical. More importantly, in the case of using small graphite flakes, the GO dispersion can be directly purified without the requirement of centrifugation to remove unoxidized residuals. Considering the high yield, simplified purification procedure, and high-quality of GO, the modification of using graphite flakes with small size is an important step towards the massproduction of GO at industrial scale. Hence, Fig. 1a showed that a graphite flake is highly aggregated and stacked together. However, Fig. 1b shows a few layers of GO sheets are well dispersed after prepared by using Modified Hammer method followed by sonication. These clearly indicate that we successfully prepared few layers graphene oxide from graphite powder.
Preparation of NiAg/rGO by electrodeposition method.-80 mg GO was dispersed in 20 mL of DI water and the dispersion were ultrasonicated to form a homogeneous 4 mg ml −1 GO suspension solution. Before the electrodeposition, the 3 mm diameter glassy carbon electrode was polished with a 0.3 μm alumina powder, and sequentially was sonicated in DI water, hydrous ethanol, deionized water, and was allowed to dry under room temperature. Two steps were employed to prepare the AgNi/rGO composite substrate. The first electrodeposition of nickel oxide nanoparticles and electrochemically reduced graphene oxide simultaneously onto the GCE surface via one-step process by dipping glassy carbon electrode in the solution of 4 mg ml −1 GO, 0.1 M NiSO 4 and 0.5 M H 2 SO 4 was followed by applying a constant potential of −1.40 V (vs SCE) for 150 s. 33 The as-prepared AgNi/rGO electrode was further electrodeposited with Ag nanoparticle in the solution of 5 mM AgNO 3 and 0.5 M H 2 SO 4 with the constant potential of −1.40 V vs SCE for 150 s to get AgNi/rGO. Ag at 0.6 V vs SCE for 4 min Ni at −1.25 V vs SCE for 10 s, AgNi, Ag/rGO, Ni/rGO at −1.40 V for 4 min were deposited on the GCE by using the same procedure as AgNi/rGO and Electrode was used for electrochemical study without further treatment and optimization was done on the voltage applied, concentrations of salt precursors, and deposition time until the best electrode obtained.
In order to compare the significance of synthesis method and GO effect to NiAg nanoparticles, the AgNi, AgNi/rGO, and AgNi/rGO nanoparticles were prepared by solgel and electrodeposition methods, respectively. Figure 3a is NiAg nanoparticle prepared by solgel method and it is clearly indicates that NiAg nanoparticles are highly aggregated and relatively with large particle size. Figure 3b shows the NiAg/rGO synthesized by solgel method and it is better dispersion and small particles size compared to Fig. 3a, this indicates that GO played significant role on particle size and particles distribution. Figure 3c is NiAg/rGO prepared electrodeposition method and it is clearly indicates that particles are relatively uniform, small particles size and well dispersed compared to Figs. 3a and 3b. This shows that the synthesis methods and GO played significant role on morphology of nanomaterials.
The SEM/EDS of the electrodeposited NiAg/rGO sample after scratching from GCE surface was analyzed. Consequently, Figs. 4a and 4b are illustrate the SEM/EDS of NiAg/rGO, it clearly indicate that the existence of Ag, Ni and C elements in NiAg/rGO. The Raman spectroscopy is used to provide detail information about chemical structure, phase, polymorphy, crystalline and molecular interactions based upon light interaction with chemical bond with materials. Figure 5c shows the Raman spectroscopy of GO, rGO and AgNi/rGO, respectively and the peaks at 1356.35 cm −1 and 1603.11 cm −1 show the D-band and G-band, respectively. The intensity ratio of D-band (I D ) to G-band (I G ) (I D /I G ) for GO, rGO, and AgNi/rGO is 0.963, 0.998, and 1.03, respectively. These results indicate the formation of defective structure on carbon in rGO and AgNi/rGO. The increase in I D /I G band intensity ratio of GO, rGO and AgNi/rGO clearly indicate the defect carbon formation is increase from GO to rGO then to AgNi/rGO respectively.
The electrodeposition of NiAg/rGO modified GCE was synthesized using the potentiostatic deposition technique/chronoamperometric methods. The electric current as a function of time for potentiostatic deposition of NiAg/rGO at −1.40 V vs Ag/AgCl/Cl− on the GCE is applied and the current vs time graph shown in Fig. 5 and the inset in Fig. 5 is the photographic image of the modified glassy carbon electrode. The chronoamperometry result shows that the current rapidly increases at the start of the applied cathodic potential, and Ni 2+ , Ag + , and rGO are reduced and deposited on the surface of the GCE electrode for the first 20 s. The electrodeposition rate then decreases and the current varies between 30 mA and 8 mA, followed by bubbles developing on the electrode surface, which may represent the simultaneous H 2 reduction, as seen in the Fig. 5 inset. With a potential of −1.40 V vs Ag/AgCl/Cl − applied, Ni 2+ , Ag + , and GO are swiftly consumed in this electrodeposited process. Meanwhile, many bubbles formed as a result of hydrogen evolution influences the diffusion and reaction of Ni 2+ , Ag + , and GO on the electrode surface. As the hydrogen bubbles developed on the electrode surface and drifted away. The H 2 production may influence the morphology of the deposited Ni/Ag/rGO. The current is steady after about 20 s which signifies that the electrode's surface is saturated with Ag + , Ni 2+ , and rGO. After the applied potentials and time of deposition, finally the optimum time of deposition and applied potential was found to be 4 min and −1.40 V Vs Ag/AgCl/ Cl−, respectively. In comparison to alternative approaches, such as hydrothermal and template-directed for materials preparation, the electrodeposition is simple, inexpensive, is binder free catalyst, and time effective. However, electrodeposion technique is also full of challenges such as time and potential optimization, solvent and precursors selections, and deposited materials characterizations.
For in depth understanding on the information on the electronic structure and composition of Ag 3d, C1s, Ni 2p and O 1s are obtained by XPS analysis of NiAg/rGO after scratch it deposited NiAg/rGO from glassy carbon electrode. The high-resolution XPS of Ag 3d, C1s, Ni 2p and O 1s are shown in Figs. 6a-6d. Figure 6a depicts the XPS narrow scan of Ag 3d and peaks at 374.18, 372.67, 371.97, 368.19, 366.67, and 365.94 eV are due to Ag + and Ag 0 states, respectively indicating that the presence of both Ag + and Ag 0 states in the NiAg/rGO electrocatalyst/nano-sensor. 34 Figure 6b refers to the high resolution spectra of C 1 s in the NiAg/rGO. The peak of C-C, C-OH/C-O-C, C=O, and O=C-OH were located at 284.52, 285.08, 286.39, 287.06, and 288.57 eV, respectively. 35 Figure 6c shows nickel which exhibits 2P 3/2 that consists of the main peak at ∼853 eV, shoulder peak at ∼855 eV, and shake up satellite peak at ∼861 eV. Similarly, 2P 1/2 displays two peaks at ∼872 eV and ∼879 eV, corresponding to the main peak and the satellite peak respectively. Ni 2P 3/2 peak is located at ∼853 eV. 36 As shown in Fig. 6d, the O1s narrow scan spectrum analysis exhibit the presence of oxygen electronic states. The peak with binding energy of O 1s at 532.94 eV corresponds to the oxygen in the metal oxide, either NiO or Ag 2 O crystal lattice. It consisted of three peaks, with binding energies of 530.9, 531.8, and 533.3 eV, which can be attributed to    the "non-lattice" oxygen state inside of the metal oxide structure, C-OH, and small impurities of adsorbed water, respectively. 37 In order to identify the effect of synthesis methods on electrochemical performance/sensitivity of AA towards NiAg, we synthesized NiAg in electrodeposition and solgel methods and test their cyclic voltammetry in 0.5 mM AA. Figure 7a clearly depicts that the electrodeposited NiAg is highly sensitive towards AA whereas, solgel synthesized NiAg is negligible compared to electrodeposited one. These are due to the following reasons (1) the particles size of electrodeposited NiAg is very small compared to solgel synthesized NiAg (2) in case of electrodeposition there is no Nefion polymer binder to attach the catalyst on the surface of GCE, which played a significant role by decreasing the conductivity of the catalyst. 38 Conductivity of the materials played a substantial role in the activities of the given material. To study the effect of conductivities of the prepared materials we measured the conductivity of all electrodeposited materials (NiAg/rGO, rGO, Ag, Ni/rGO, Ag/rGO, NiAg and Ni) for comparisons in 0.5 mM AA solutions from 10 mHz to 100 KHz frequency and 10 mV applied potential and result was displayed in Fig. 7b. As it is clearly observed from Fig. 7b, Ag and rGO is highly conductive with small charge transfer resistance; whereas NiO is less conductive with large charge transfer resistance. However, it's conductivity is enhanced when it was mixed with Ag and rGO. This indicate that NiO is poor conductive but it is sensitive towards -OH and -COOH functional groups of AA. When we mixed NiO with Ag and rGO, its conductivities enhanced and its sensitivity also increased towards the detection of AA which agrees with what we proposed.
To identify the suitable pH for standard AA, the CV curves of 0.5 mM standard AA was measured from the pH 1-10. The influence of pH on the oxidation peak current and peak potential of AA at NiAg/rGO modified GCE was investigated. As it has seen in Fig. 8a, the anodic peak current surged quickly from pH 1-2 and then progressively declined up to pH 10 and pH 2 was found to be greater than the others. Therefore, in the ensuing trials, pH 2 was chosen as the optimal pH of the buffer solution, which corresponds with previously published work. 39 The effect of pH on ascorbic acids oxidative peak potential was also investigated. As a result, the shift in the oxidation peak potential direction with increasing pH implies that protons participate in the oxidation of AA at NiAg/rGO. Figure 8b clearly shows that anodic peak current increases from pH 10 to pH 2 and then decrease again and the optimum pH was found to be 2. Hence, pH 2 was taken for further electrochemical analysis.
The electrodeposited NiAg/rGO was tested against phosphate buffer solution with 0.5 mM AA and without AA and it was found that there is strong intense peak at 0.10 V and there is no any peak without AA solution which clearly indicating that NiAg/rGO creates the faradaic current due to the interaction with AA as shown Fig. 9a. On the other hand, the study of the modified glassy carbon electrodes with NiAg/rGO was carried out by means of cyclic voltammetry in the potential range of −0.2 to 0.8 V vs Ag/AgCl/Cl− with a scan rate of 50 mVs −1 using an electron mediator redox couple and potassium ferrocyanide 10 mM in 0.1 mol L −1 KCl as support electrolytes solution. Figure 9b shows the cyclic voltammogram of standard redox couple of ferrocyanide and it clearly shows that the NiAg/rGO shows good redox behavior for redox (Fe(CN) 6 4− to Fe(CN) 6 3− + e − and vice versa. The CV curves of 0.5 mM of AA in 0.1 M PBS (pH 2.0) at a scan rate of 50 mV s −1 were investigated for the following eight electrodes, GCE, rGO, Ni, Ag, NiAg, Ni/rGO, Ag/rGO, and NiAg/ rGO. As shown in Fig. 10a, there is no oxidation of AA response is observed for the bare GCE at the potential range from −0.4 to 0.4 V and the bare GCE, no obvious redox response is observed, which indicates that the bare GCE is not interacting with AA to generate faradaic current at the electrode surface in the absence of nanomaterials. Compared with bare GCE, all modified electrode exhibit well-defined oxidation CV peaks. The NiAg/rGO shows the highest oxidation peak currents, which indicates that the NiAg/rGO underwent high electrocatalytic activity toward AA oxidation, thereby further proving the NiAg/rGO are beneficial for electron transfer between the electroactive center of NiAg/rGO and GCE. All Nicontaining electrode such as Ni, NiAg Ni/rGO, and NiAg/rGO show intense oxidation peak compared to other electrode, this shows that Ni-containing compound have strong affinity towards -hydroxyl, -carboxyl functional groups of AA as indicated in Fig. 10b. The NiAg/rGO-modified GCE showed one well-defined oxidation peak at the potentials of about 0.10 V corresponding to AA oxidation. Since the oxidation peak potential of AA was shifted to a more positive and small peak current with distinct oxidation peaks were obtained for AA. For other electrodes such as rGO, Ag, and Ag/rGO it indicates that those electrodes doesn't have a strong interaction with hydroxyl and carboxyl functional groups of AA as indicated in Fig. 10c. Hence, determination of AA in phosphate buffer becomes possible with the NiAg/rGO-modified GCE. The effective electrocatalytic resolution of the peaks for AA at the NiAg/rGO-modified glassy carbon electrode may be due to the excellent behavior of the   NiAg/rGO-modified such as very high conductivity, strong adsorptive capability, and significant increment in active electrode area.
To evaluate the effect of time of deposition and deposition potential of NiAg/rGO on GCE, different potentials were taken and which are −1.40 V and −1.25 V Vs Ag/AgCl/Cl− as shown in Fig. 11a cyclic voltammetry for at −1.25 V for 10 seconds as that of Ni electrodeposition and at −1.40 V for 4 min as that of GO electrodeposition and CV was tested in 0.5 mM AA at pH 2 of phosphate buffer solution and −1.40 V Vs Ag/AgCl/Cl− for 4 min was found to be highest peak current was obtained. Therefore, electrodeposition at −1.40 V potential for 4 min was taken for further analysis. Since the concentration of GO also played a significant role on the electrochemical performance of the materials we have evaluated the GO concentration from 1 mg ml −1 to 5 mg ml −1 and optimum concentration was found to be 4 mg ml −1 as it shown in Fig. 11b.
The effect concentrations of AA with different concentration of AA which varies from 10-500 μM with scan rate of 50 mV s −1 on NiAg/rGO are shown in Fig. 12. As shown in Figs. 12a and 12b, there is a direct relationship between peak current with concentration with the regression value of R 2 = 0.97. This relationship between peak current and concentration confirmed that the catalyst NiAg/ rGO has sensitive towards AA.
The differential pulse voltametric methodology was used to demonstrate the practical applicability of the proposed sensor for quantitative measurement of AA. This is because, as compared to the cyclic voltammetric approach, the observed current signals are better defined even at low concentrations. Keeping the best differential pulse parameters in mind, differential pulse volammograms at various low doses of AA were done as shown in Fig. 13a. Peak current signals increased linearly with analyte concentrations in the examined range between 10 μM and 100 μM, and from  100−700 μM. A standard calibration curve has been created and indicated Fig. 13b based on the plot between the anodic peak current and analyte concentrations. The linear regression equation is shown below along with the correlation coefficient (R 2 ). Ipa (A) = 0.00564 + 2.358 × 10 −4 C, R 2 = 0.999 and the linear regression equation from 100-700 μM was found to be Ipa (A) = 0.851 × 10 −4 + 7.81 × 10 −4 C, R 2 = 0.980. These results depict that for higher AA concentrations, deviations from linearity have been observed. These might be due to another occurrence, such as the adsorption of AA or any of its oxidation products, or both, on the sensor/analyte interface. The limit of detection for AA was calculated by measuring the DPV of the modified electrode without AA in its nine operations along with the standard deviation from same measurements. As a result, the magnitude of standard deviation from the nine measurements is 8.6 × 10 −10 . The limit of detection was calculated by LOD = . Accordingly, the calculated value of LOD of AA is 0.457 μM.
To validate the sensors for real samples application recoveries have been done. Accordingly, different amounts of standard AA solutions were added onto the Vitamin C tablets and fruits juices samples to check for recoveries and the results are summarized in Table I. The recoveries of the different AA concentrations are in the range of 96.25%-101.46%. The average recoveries of AA concentration in Vitamin C Tablet is 99.40% and the average recoveries of fruit juices samples is 100.61% which indicates that the fabricated sensor could be used for AA determination in pharmaceutical samples and bottled fruit juices.
Stability study.-In order to assess the stability of the NiAg/rGO modified electrode, the modified electrodes were made and kept at room temperature when they were not in use. Following two weeks, the response of the current for 0.5 mM AA at pH 2 was recorded. At the end of the assessment time, it was discovered that the average response was around 89.56% of the initial one.
The effect of scan rate on the oxidation of AA was investigated by CV by varying the sweep rate from 5-200 mV s −1 in 0.1 M NaH 2 PO 4 .2H 2 O solution of pH 2 as supporting electrolyte. The anodic peak currents linearly increased with the increase of scan rate as show in Fig. 14a. Figures 14b and 14c show the linear relationship between peak current vs scan rate. Figure 14c is peak current vs square root of scan rate of AA at NiAg/rGO with the regression equation: ipa = 0.1216x + 0.11432, R 2 = 0.991. These results indicated that the electron transfer process of AA was operative under diffusion controlled manner at the NiAg/rGO. The observations that peak potential of anodic process is shifting towards positive with increasing scan rate and the linear behavior of Log I vs Log v with regression equation: ipa = 0.462 + 0.797 v 1/2 , R 2 = 0.994. Figure 14d also further presents the evidence for the non-adsorptive behavior of AA on the surface of sensor.

Conclusions
In this work a very simple electrochemical deposition technique has been used to deposit NiAg/rGO on GCE for electrochemical quantification of AA in Vitamin C tablets and bottled fruit juices. The deposited nanomaterials was scratched from GCE and characterized by Raman, SEM/EDS and XPS. The sensitivity of NiAg/ rGO was tested towards standard AA and it is very sensitive to AA compared to bare GCE, rGO, Ag, Ag/rGO, Ni, NiAg, Ni/rGO and NiAg/rGO, respectively. The NiAg/rGO produced after deposition was studied for AA detection in bottled fruit juice and Vitamin C tablets and yielded 0.457 μM limits of detection and 100.61%, 99.40% average recoveries, respectively. Hence, the proposed nanosensor is proven good sensor for AA detection in real sample analysis.