## 3 Results and Discussion
### Analytical Performance of the Electrochemical Aptasensor  
The analytical performance of the aptasensor was determined by calibration with various concentrations of GA (from 1 to 10,000 \(\upmu\)g mL\({}^{-1}\)) under the optimal conditions. During the electrochemical measurements, the immobilized aptamer exhibits a free and steady assembly on the electrode surface, yielding a relatively significant current. When incubating the aptasensor with GA molecules, the aptamer changed the conformation for target binding. The captured GA-aptamer complex on the sensor surface acted as a blocking layer, hindering the electron transfer reaction, and thereby reducing the peak current. The decreasing SWV peak current signal was observed corresponding to increasing concentration of GA (Figure 5A). Electrochemical measurements were carried out before and after target binding with the aptamer. The SWV peak current change after sample incubation was calculated as \(\Delta\)I = I\({}_{0}-\)I\({}_{1}\), where \(\Delta\)I refers to the current change, and I\({}_{0}\) and I\({}_{1}\) represent the current before and after incubation with the target protein, respectively. In our experiment, a decrease in the maximum peak current was observed after GA treatment, confirming the efficient binding of GA by the aptasensor. The standard curve was plotted against the peak current change (\(\upmu\)A) with the logarithm of the GA concentration (see Figure 5B). The current change increased with increasing GA concentration. A linear relationship between the logarithm of GA concentration and the current change was observed (I = 0.0975 ln (c) + 0.0654), with a correlation coefficient (R\({}^{2}\)) of 0.9899 for GA detection over a range of 1-10,000 \(\upmu\)g mL\({}^{-1}\). The limit of detection (LOD) of GA was calculated by the formula LOD = 3\(\sigma\)/S, where \(\sigma\) is the standard deviation of the average measurement of the lowest concentration of GA and S is the slope of the regression line. Therefore, the calculated LOD of the experiment is 0.031 \(\upmu\)g mL\({}^{-1}\), which is sensitive enough to detect GA concentrations comparable with those of previous studies (Table 1). The normal range of GA was 0.2-7 mg mL\({}^{-1}\) based on the method used [40]. The developed aptasensor is not only sensitive but also exhibits a broad linearity range (from 1 \(\upmu\)g mL\({}^{-1}\) to 1 \(\times\) 10\({}^{4}\)\(\upmu\)g mL\({}^{-1}\)) in comparison with other studies (see Figure 6) Therefore, the developed aptasensor can confidently be applied for sensitive detection of both low and high GA levels in clinical samples not demanding 100-fold dilutions.  
Figure 5: (**A**) SWV response of aptamer/GO/SPCEs aptasensor after incubation with 1 \(\upmu\)g mL\({}^{-1}\) (purple), 10 \(\upmu\)g mL\({}^{-1}\) (blue), 100 \(\upmu\)g mL\({}^{-1}\) (red), 1 \(\times\) 10\({}^{3}\)\(\upmu\)g mL\({}^{-1}\) (orange), and 1 \(\times\) 10\({}^{4}\)\(\upmu\)g mL\({}^{-1}\) (green) of GA; (**B**) Calibration plot of the aptasensor showing the linear relationship between the logarithm of GA concentration 1–1 \(\times\) 10\({}^{4}\)\(\upmu\)g mL\({}^{-1}\) and corresponding maximum current (\(\Delta\)I) from SWV. Mean current value of each concentration was plotted from three independent experiments.  
bilirubin levels were not detected in the assay. In contrast, the experimental results indicate that the developed aptasensor exhibit high specificity for the detection of GA.