## 3 Results and Discussion
### Antigen Detection by Electrochemical Method  
The SWV method was utilized to identify SARS-CoV-2 electrochemically. This technique detects alternate electrochemical analysis stages, such as potassium ferrocyanide and potassium ferricyanide, with excellent sensitivity. This electroanalytic diagnostic approach relies on measuring signal dampening produced by SARS-CoV-2 viral antigen-antibody interaction. As a result, an increase in the analyte's nucleocapsid protein (NP) generates an inversely corresponding drop in the signal of an Fe(CN)\({}_{6}^{-3/-4}\) redox probe. That is, the antigen-antibody interaction prevents the oxidation-reduction probe from accessing the working electrode surface. In all measurements, device parameters were determined as follows, in accordance with the literature: operating frequency 80.0 Hz, potential amplitude 75.0 mV, and potential step 8.0 mV [41]. First, the effect of the incubation time of the modified electrode on the electrode response at very low NP concentrations of SARS-CoV-2 was evaluated (Figure 5). All experiments were repeated three times with increasing concentrations of NP between 6.4 \(\times\) 10\({}^{-14}\) g/mL and 1 \(\times\) 10\({}^{-9}\) g/mL. New electrodes were used for each measurement. The findings are given as \(\Delta\)I = I - I\({}_{0}\). Here, I represents the current measured for the working electrode after NP incubation on the electrode surface [Fe(CN)\({}_{6}\)]\({}^{3-/4-}\), and I\({}_{0}\) is the current measured for the working electrode before activating the modified electrode with NP. When the results of SWV analyses were examined, the best analytical sensitivity and linearity were obtained by incubating the antibody-modified electrode and virus antigen for 10 min. The optimum incubation time was determined as 10 min in further experiments. In Figure 6, square wave voltammetry results of modified electrodes incubated for 10 min at various concentrations of NP in 0.1 M PBS are given. Accordingly, as the NP concentration increases, the SWV response for the redox probe decreases. The reduction  
Figure 5: Graph of SWV current suppression (\(\Delta\)I) of the modified electrodes incubated in the range of 6.4 \(\times\) 10\({}^{-14}\) g/mL to 1 \(\times\) 10\({}^{-9}\) g/mL at different NP concentrations and at different times (1, 5, and 10 min). in the analytical signal by the particular interaction between antibody modification on the electrode surface and NP is assumed to be the cause of this outcome [34,48]. The binding of the nucleocapsid protein to antibodies on the biosensor surface causes the closure of the electroactive parts on the modified electrode surface and blocking of the SWV signal. As a result, reduction in measured current is considered as a great result, showing SARS-CoV-2's presence. The calibration graph of SWV signal suppression obtained at each NP concentration versus the logarithmic function of the NP concentrations is given in Figure 7 (\(n=3\)). According to this graph, the measurements show linearity with a high correlation coefficient (R2 = 0.9917) between 6.4 \(\times\) 10\({}^{-14}\) g/mL and 1 \(\times\) 10\({}^{-9}\) g/mL NP concentration range. Figure 6: Baseline corrected square wave voltammetries of modified electrodes incubated for 10 min at different NP concentrations. Figure 7: Calibration graph: linear technique of analysis of the recorded current signal suppression (\(\Delta\Pi\)) analytical curve of modified electrodes incubated in different NP concentration for 10 min plotted against the logarithmic function of NP concentrations. The limit of detection (LOD) and the limit of quantification (LOQ) of the designed electrode system were calculated by linear regression analysis of the calibration graph. Accordingly, the LOD was calculated as 5.08 \(\times\) 10\({}^{-10}\) g/mL, i.e., 508 pg/mL. The LOQ was calculated as 1.54 \(\times\) 10\({}^{-9}\) g/mL, i.e., 1.54 ng/mL.