## 3 Results and Discussion
### Characterization of the Electrochemical Aptasensor
The diameter of the semicircle denotes the charge-transfer resistance R\({}_{\text{ct}}\) at the electrode [27]. The R\({}_{\text{ct}}\) values decreased after GO/SPCEs because the electron transfer rate was amplified upon modification with GO, which facilitated the redox process. The EIS data agrees with the cyclic voltammetry data (Figure 2A), thereby showing the successful modification of the SPCEs with GO. The charge transfer resistance was increased after aptamer immobilization. This might be due to the aptamer hindering the access of redox species to the transducer surface, thereby blocking electron transfer. The increase in R\({}_{\text{ct}}\) after aptamer immobilization indicates the successful immobilization of the aptamer on the electrode surface. The R\({}_{\text{ct}}\) was increased again after incubation with the target protein (GA). This is probably due to the binding of the protein to the aptamer complex hindering the electron transfer process. Figure 2: (**A**) Cyclic voltammograms of Bare SPCEs, GO-SPCEs showing successful immobilization of each layer. (**B**) EIS spectra of Bare SPCEs, GO-SPCEs, aptamer/GO–SPCEs, and GA/aptamer/GO–SPCEs in 5 mM [Fe (CN)\({}_{6}\)]\({}^{4-/3-}\). The inserted circuit graph in (**B**) is the EIS fitting model. The effect of the scan rate on the voltammetric behavior of GO/SPCEs was examined. The scan rate was increased from 10 to 300 mVs\({}^{-1}\). A linear relationship was observed after plotting the square root of the scan rate vs. the anodic (I\({}_{\text{pa}}\)) and cathodic (I\({}_{\text{DC}}\)) peak currents (Figure S1A), with R\({}^{2}\) values of 0.9992 and 0.9996, respectively (Figure S1B). This result suggested the characteristics of the corresponding thin-layer type voltammetry [28]. The plot between the logarithms of scan rates (v) and log I\({}_{\text{pa}}\) and I\({}_{\text{pc}}\) showed a linear relationship with slope values of 0.49 and 0.51, respectively. The resulting values were consistent with those for purely diffusion-controlled currents [29]. This in turn suggests that the modified SPCE surface was not fouled, and the electrochemical process was diffusion controlled. FTIR characterization was carried out for the characterization of the functionalized GO. Figure 3 illustrates the FTIR spectra of the original GO and EDC/NHS-activated GO. The bands at 1271 and 1258 cm\({}^{-1}\) represent the C-O stretching (ether) of the original GO and activated GO, respectively. The peaks at 1132 and 1101 cm\({}^{-1}\) were attributed to C-O stretching (alcohol) of the original and activated GO. C-H bending (aromatic) functional groups were observed in the 1792 and 1674 cm\({}^{-1}\) peak positions. The broad band at 2669 cm\({}^{-1}\) was attributed to O-H stretching of both GO types. The absorption peaks were more intense for activated GO than for the original GO. These factors contribute to the characteristics of GO, as reported by previous studies [30; 31; 32; 33]. New bands were introduced at 1639, 1545 and 578 cm\({}^{-1}\), corresponding to N-H stretching and C-N stretching of activated GO. This proves that the chemical modification resulted in amine groups that reacted with NHS. FTIR spectra revealed that GO functionalization of the amine-modified aptamer was complete.