## 3 Results and Discussion
### Individual and Simultaneous Determination of DA and UA  
For individual detection of DA and UA at the MnO\({}_{2}\)NFs/NrGO/GCE, LSVs were measured in the potential range of 0-0.8 V in 0.1 M PBS (pH 3.93). In this case, only the concentrations of the target substance were varied, while the concentrations of the other substance were kept unchanged. As illustrated in Figure 8A,B, there was a good linear relationship between the \(i_{\text{pa}}\)(DA) and DA concentrations ranging from 0.4 uM to 10 uM. The linear equation is \(i_{\text{pa}}\)(DA) = \(1.3090\text{C}_{\text{DA}}-0.1953\) (R\({}^{2}\) = 0.989). However, the \(i_{\text{pa}}\)(DA) were positively proportional to the Napierian logarithm of DA concentrations (In\(C_{\text{DA}}\)) in the concentration range of 10 uM-100 uM (Figure 8C,D). The linear regression equation can be expressed as \(i_{\text{pa}}\)(DA) = \(19.1371\text{In}_{\text{CDA}}-32.3044\) (R\({}^{2}\) = 0.993). The LOD was estimated as 0.054 uM. Regarding the individual determination of UA, the \(i_{\text{pa}}\)(UA) were well-linear to the working concentrations, varying from 0.4 uM to 6.0 uM (Figure 9A,B), with the linear equation of \(i_{\text{pa}}\)(UA) = \(2.4934\text{C}_{\text{DA}}-0.9302\) (R\({}^{2}\) = 0.989). At the higher concentration region (6.0 uM to 100 uM), the \(i_{\text{pa}}\)(UA) were positively correlated to the Napierian logarithm of UA concentrations (In\(C_{\text{UA}}\)). Figure 6: (**A**) CVs of 10 uM DA and UA (I:1) at different scanning rates. (**B**) Linear relationship between the redox peak currents of DA and square root of scanning rates (\(n\) = 3); (**C**) Linear relationship between the anodic peak currents of UA and square root of scanning rates (\(n\) = 3). Figure 7: Effect of accumulation potential (**A**) and time (**B**) on the anodic peak currents of DA and UA. The corresponding linear equation is \(i_{\text{pa}}(\text{UA})=44.3228\text{ln}C_{\text{UA}}-65.7789\) (R\({}^{2}\) = 0.995). The LOD is 0.062 uM for the individual determination of UA. It is noteworthy that the addition of the target biomolecule does not have a notable interference on the electrochemical response signals of the other biomolecule. The results firmly imply that DA and UA can be sensitively and selectively detected at MnO\({}_{2}\)NFs/NrGO/GCE in the mixture of DA and UA. The remarkable electrocatalytic activity of MnO\({}_{2}\)NFs/NrGO enables simultaneous detection of DA and UA using the LSV method (Figure 10). Two well-separated anodic peaks belonging to the electrooxidation of DA and UA were observed on LSV curves using MnO\({}_{2}\)NFs/NrGO/GCE. Furthermore, LSV responses were resolved into two peaks at 0.420 V and 0.554 V, respectively. The results demonstrate an excellent discriminability from the two biomolecules in mixture solutions. At the lower concentration region (0.02 uM-6.0 uM), the \(i_{\text{pa}}(\text{DA})\) and \(i_{\text{pa}}(\text{UA})\) enhanced linearly with their concentrations increasing (Figure 10A, B). The linear plots can be expressed as \(i_{\text{pa}}(\text{DA})=3.0627C_{\text{DA}}-0.2848\) (R\({}^{2}\) = 0.991), and \(i_{\text{pa}}(\text{UA})=3.0627C_{\text{UA}}-0.2848\) (R\({}^{2}\) = 0.990), respectively. However, the \(i_{\text{pa}}(\text{DA})\) and \(i_{\text{pa}}(\text{UA})\) are positively proportional to the Napierian logarithm of the DA and UA concentrations (\(\text{ln}C_{\text{DA}}\) and \(\text{ln}C_{\text{UA}}\)) at the higher concentration region from 6.0 uM to 100 uM (Figure 10C, D). The corresponding linear regression equations are \(i_{\text{pa}}(\text{DA})=16.2222\text{ln}C_{\text{DA}}-12.4506\) and \(i_{\text{pa}}(\text{UA})=37.7032\text{ln}C_{\text{UA}}-48.2926\), respectively. The correlation coefficient is 0.990 for both DA and UA. The LODs are calculated to be 0.036 uM and 0.029 uM for DA and UA, respectively. All of the results indicate that the proposed MnO\({}_{2}\)NFs/NrGO/GCE featured wider linear detection ranges and a lower LOD for the electrochemical oxidation of DA and UA.