## 3 Results and Discussion
### Evaluation of Electrochemical Performances  
In order to assess the electrochemical performances, CVs for various modified electrodes were measured in a mixture solution of 0.5 mM [Fe(CN)\({}_{6}\)]\({}^{3-4-}\) and 0.1 M KCl (Figure 3A). A pair of quasi-reversible redox peaks occurred on all of the electrodes with \(i_{\text{pa}}\)/\(i_{\text{pc}}\approx 1.0\). At bare GCE, a pair of weak redox appeared with the anodic and cathodic peak current of 12.23 and 9.02 \(\upmu\)A, respectively. After the modification of GCE by MnO\({}_{2}\)NFs or NrGO, the redox peak currents increased by 2-fold approximately. As expected, a well-defined and sharp redox peak was observed at the MnO\({}_{2}\)NFs/NrGO/GCE, with the highest anodic and cathodic peak currents (\(i_{\text{pa}}\) = 92.41 \(\upmu\)A, \(i_{\text{pc}}\) = 87.89 \(\upmu\)A). It indicates that MnO\({}_{2}\)NFs/NrGO significantly improved electrochemical performances. It is well-known that the effective electroactive area is a critical factor that directly influences the electrochemical sensing performances. The effective electroactive areas of different electrodes were also calculated, using the _Randles-Sevcik_ equation as follows [16; 17; 20]:  
\[i_{\text{pc}}=(2.69\times 10^{5})\;n^{3/2}\;\text{D}^{1/2}\;v^{1/2}\;AC \tag{1}\]  
where \(i_{\text{pc}}\) represents the cathodic peak current (A), \(n\) represents the electron transfer number, D represents the diffusion coefficient of K\({}_{3}\)[Fe(CN)\({}_{6}\)] (7.6 \(\times\) 10\({}^{-6}\) cm\({}^{2}\) s\({}^{-1}\)[39]), \(v\) denotes the scanning rate (V s\({}^{-1}\)), A denotes the effective electroactive area (cm\({}^{2}\)), and \(C\) denotes the K\({}_{3}\)[Fe(CN)\({}_{6}\)] concentration (mol cm\({}^{-3}\)). The effective electroactive areas were estimated to be 0.0770, 0.3183, 0.3958, and 0.7496 cm\({}^{2}\) for the bare GCE, MnO\({}_{2}\)NFs/GCE, NrGO/GCE, and MnO\({}_{2}\)NFs/NrGO/GCE, respectively. The effective electroactive area of MnO\({}_{2}\)NFs/NrGO/GCE is about 9-fold higher than that of the bare GCE approximately. The results suggest that the MnO\({}_{2}\)NFs/NrGO nanocomposites significantly enlarged the effective electroactive surface area, which promoted the accumulation of target analysts and thus increased the response electrochemical signals.  
Figure 2: Scanning electron microscopy (SEM) images of MnO\({}_{2}\)NFs(**A,B**) and MnO\({}_{2}\)NFs/NrGOnanocomposites (**C,D**) at different magnifications.  
EIS has intensively been used to investigate interfacial properties of various electrochemical sensors [16; 17; 40; 41; 42]. Nyquist plots for various electrodes are plotted in Figure 3B. Obviously, Nyquist plots comprise of the semicircular at higher frequencies relating to the electron transfer-limited process, and linear portions at lower frequencies corresponding to the diffusion-controlled process. The semicircular diameter represents the charge transfer resistance (\(R_{\mathrm{ct}}\)). The \(R_{\mathrm{ct}}\) values for the bare GCE, MnO\({}_{2}\)NFs/GCE, NrGO/GCE, and MnO\({}_{2}\)NFs/NrGO/GCE are 1950, 2551, 72.47, and 25.28 ohm, respectively. After modification with MnO\({}_{2}\)NFs, the \(R_{\mathrm{ct}}\) value increased by 601 ohm because of the poor electro-conductivity of MnO\({}_{2}\). When GCE was modified with NrGO, the \(R_{\mathrm{ct}}\) significantly decreased to 74.25 ohm, which can attribute to the good electro-conductivity and high-specific surface area of NrGO [35; 36]. As expected, the lowest \(R_{\mathrm{ct}}\) value was obtained at MnO\({}_{2}\)NFs/NrGO/GCE, probably due to the existence of abundant electrocatalytic active sites that can greatly accelerate the redox reaction of [Fe(CN)\({}_{6}\)]\({}^{3-\
icefrac{{4}}{{4}}}\). The results demonstrate that the MnO\({}_{2}\)NFs/NrGO can effectively decrease the \(R_{\mathrm{ct}}\).