## 3 Results and Discussion
### Preparation and Characterization of ERGO-NiNP Film-Modified Electrode
Finally, based on SEM images, the described electrodeposition method enables simultaneous preparation of graphene and nickel nanocomposite materials in which the nickel metallic nanoparticles exhibited small size and were well dispersed into the graphene matrix film. The modified electrode surfaces are further characterized using cyclic voltammetry using K3 Fe(CN)\({}_{6}\)/K4 Fe(CN)\({}_{6}\) as a couple redox probe (Figure 3). The CVs display the characteristic signals of Fe(III)/Fe(II) couple. Remarkably, peak currents were increased along with smaller peak-to-peak separations after the modification process. The peak-to-peak separation values (\(\Delta\)Ep = Epa-Epc where Epa and Epc are the anodic and cathodic peak potentials, respectively) are estimated to be approximately 64 mV for both ERGO and ERGO-NiNP-modified surfaces with the respect to 120 mV of the GCE electrode. Furthermore, the electrochemically active surface areas (ECSA) calculated according to the Randles-Sevcik equation were estimated to be 0.075, 0.165 and 0.157 cm\({}^{2}\) corresponding to GCE, GCE/ERGO and GCE/ERGO-NiNPs respectively (See Electronic Supporting Information Figure S4). The value for the GCE surface is very close to the geometric value of 0.071 cm\({}^{2}\), indicating good reliability of this estimation. The modified surfaces exhibited a significant enhancement of ECSA values up to 2.2- and 2.1-fold for ERGO and ERGO-NiNP-modified surfaces with respect to GCE surface. Surprisingly, a slightly lower ECSA is obtained for ERGO-NiNP compared to ERGO. As observed via the FE-SEM images, the introduction of nickel results in more compact ERGO films, and thus potentially lower electrochemical activity than ERGO single films. This observation could be considered a reasonable explanation for the slightly lower ECSA of ERGO-NiNP-modified surfaces. These results clearly demonstrate the successful preparation of ERGO and ERGO-NiNP-modified surfaces with enhanced surface area and good conductivity of both nickel and graphene nanomaterials. Figure 2: FE-SEM images of nanomaterials electrodeposited onto ITO surface using 10 successive CV scans: (**A**) ERGO film; (**B,C**) ERGO-NiNP film at different magnifications. The electrochemical behaviours of SY on the modified electrodes were studied using CV and DPV techniques. Voltamograms display a well-defined peak at E = 0.735 V/SCE with a peak-to-peak separation \(\Delta\)Ep = Epa-Epc equal to 30 mV for both GCE/ERGO and GCE/ERGO-NiNP surfaces (Figure 4A). Thus, the number of electrons transferred in the redox reaction could be estimated at 60/\(\Delta\)Ep = 2 for a reversible redox reaction. This observation indicates that the electrochemical oxidation of SY onto graphene-modified GCE electrodes is a reversible process [57]. Compared to GCE, the modified surfaces lead to a remarkable enhancement of the SY signal, with an even better signal on ERGO-NiNPs (Figure 4A). DPV experiments exhibit a clear effect of the different surfaces (Figure 4B). The DPV signal shows a peak of 0.3 uA at 0.77 V for the bare GCE. Current intensities are significantly increased up to 3.3 and 5.4 uA for ERGO and ERGO-NiNP-modified surfaces respectively, approximately improved by 11- and 18-fold. It could be proposed that the NiNPs of nanocomposite materials induce higher affinity to adsorb SY with respect to ERGO materials, and this leads to higher DPV current responses than on ERGO-modified electrodes. Furthermore, oxidation peak potentials of SY are negatively shifted to 0.74 V/SCE for ERGO and ERGO-NiNP-modified surfaces. ERGO-NiNP nanocomposite-modified surfaces led to a significant improvement in SY sensitivity due to the combination of the excellent properties of graphene with the nickel nanoparticles in the composite that provide high surface area and conductivity. This result confirms that the incorporation of nickel metal into the ERGO matrix improves the adsorption capacity for SY and thus enhances the oxidation as observed from other investigations [54].